Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles

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Flash Vacuum Pyrolysis of Azides, Triazoles, and Tetrazoles Curt Wentrup* School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia ABSTRACT: Flash vacuum pyrolysis (FVP) of azides is an extremely valuable method of generating nitrenes and studying their thermal rearrangements. The nitrenes can in many cases be isolated in low-temperature matrices and observed spectroscopically. NH and methyl, alkyl, aralkyl, vinyl, cyano, aryl and N-heteroaryl, acyl, carbamoyl, alkoxycarbonyl, imidoyl, boryl, silyl, phosphonyl, and sulfonyl nitrenes are included. FVP of triazoloazines generates diazomethylazines and azinylcarbenes, which often rearrange to the energetically more stable arylnitrenes. N2 elimination from monocyclic 1,2,3-triazoles can generate iminocarbenes, 1H-azirines, ketenimines, and cyclization products, and 1,2,4-triazoles are precursors of nitrile ylides. Benzotriazoles are preparatively useful precursors of cyanocyclopentadienes, carbazoles, and aza-analogues. FVP of 5aryltetrazoles can result in double N2 elimination with formation of arylcarbenes or of heteroarylcarbenes, which again rearrange to arylnitrenes. Many 5-substituted and 2,5-disubstituted tetrazoles are excellent precursors of nitrile imines (propargylic, allenic, or carbenic), which are isolable at low temperatures in some cases (e.g., aryl- and silylnitrile imines) or rearrange to carbodiimides. 1,5-Disubstituted tetrazoles are precursors of imidoylnitrenes, which also rearrange to carbodiimides or add intramolecularly to aryl substituents to yield indazoles and related compounds. Where relevant for the mechanistic understanding, pyrolysis under flow conditions or in solution or the solid state will be mentioned. Results of photolysis reactions and computational chemistry complementing the FVP results will also be mentioned in several places.

CONTENTS 1. Introduction 2. The FVP Method 3. General Phenomena and Reaction Types 3.1. Chemical Activation 3.2. Nitrenes Are More Stable Than Carbenes 3.3. Types of Rearrangement 3.3.1. 1,2-H Shift 3.3.2. Aromatic Ring Expansion 3.3.3. Nitrene vs Diradical 3.3.4. Curtius-Type Rearrangements, Imidoylnitrenes, and Nitrile Imines 3.3.5. Aza-Wolff Rearrangement 4. Azides 4.1. Hydrazoic Acid (HN3) 4.2. Methyl Azide 4.3. Alkyl Azides 4.4. Benzyl Azide 4.5. Vinyl Azides 4.5.1. Vinyl Azide 4.5.2. Styryl Azides 4.6. Cyanogen Azide and Cyanonitrene 4.7. Aryl Azides, Triazoloazines, and Tetrazolylazines 4.7.1. o-Alkylphenyl Azides 4.7.2. Phenyl Azide, Phenylnitrene, and the Pyridylcarbenes 4.7.3. Carbene−Nitrene and Nitrene−Carbene Rearrangements Involving 2-Pyridylcarbenes 4.7.4. 2-Naphthylnitrene and Isoquinolylcarbenes © XXXX American Chemical Society

4.7.5. 1-Naphthylnitrene and Quinolylcarbenes 4.7.6. Phenanthrylnitrene and Phenanthridinylcarbene 4.7.7. Pyridylnitrenes, Pyridazinylcarbenes, and Pyrazinylcarbenes 4.7.8. Quinolyl- and Isoquinolylnitrenes 4.7.9. 9-Phenanthridinylnitrene and 9-Acridinylnitrene 4.7.10. 2-Pyrimidinyl- and 3-Pyridazinylnitrenes 4.7.11. 1-Phthalazinylnitrene and 2-Quinazolinylnitrenes 4.7.12. 2-Pyrazinylnitrenes and 4-Pyrimidinylnitrenes 4.7.13. 4-Quinazolinyl- and 2-Quinoxalinylnitrenes 4.7.14. Azidotriazines 4.8. Acyl Azides 4.8.1. Acyl Azides and Acylnitrenes 4.8.2. Carbamoyl Azides R2N−CO−N3 4.8.3. Alkoxycarbonyl Azides 4.9. Imidoyl Azides [RC(NR′)−N3] and Imidoylnitrenes [R(CNR′)−N] 4.10. Boryl Azides 4.11. Silyl Azides 4.12. Phosphorus Azides 4.13. Sulfonyl Azides 5. Triazoles

B B C C C D D D E E F F F F F G G G H I I I I

P Q R T W W Y Y Z AB AC AC AC AD AD AF AG AG AH AI

M Received: November 1, 2016

O A

DOI: 10.1021/acs.chemrev.6b00738 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 5.1. 1,2,4-Triazoles 5.2. 1,2,3-Triazoles 5.3. Benzotriazoles and Triazolopyridines 5.3.1. N-Unsubstituted Benzotriazoles and Triazolopyridines 5.3.2. Graebe−Ullmann Synthesis 5.3.3. N-Alkylbenzotriazoles 5.3.4. 1- And 2-Vinylbenzotriazoles 5.3.5. 1-Allylbenzotriazoles 5.3.6. 1-Acylbenzotriazoles 6. Tetrazoles 6.1. Introduction 6.2. Tetrazole 6.3. 5-Phenyltetrazole 6.4. 2-Methyl-5-phenyltetrazole and 1-Methyl-5phenyltetrazole 6.5. C-Phenyl- and C-Methyl-N-trimethylsilylnitrile Imine 6.6. 2,5-Diaryltetrazoles and Diphenylnitrile Imine 6.7. 2-Phenyl-5-ethynyl- and 2-Phenyl-5-styryltetrazoles 6.8. 2-Aryltetrazolyl-5-acroleins and 2-Aryl-5-butadienyltetrazoles 6.9. Formation of Diazoalkanes and Carbenes 6.10. Diversity of Reactivities of Aryltetrazoles 6.11. Acyltetrazoles 6.12. 5-Aminotetrazoles 6.13. Silyltetrazoles 6.14. 2-Stannyltetrazole 7. Conclusion and Outlook Author Information Corresponding Author ORCID Notes Biography Acknowledgments References

Review

activation energies around 40 kcal mol−1,7,8 which makes them very valuable sources of aryl- and heteroarylcarbenes, which in turn may rearrange to nitrenes. In this review, results of reactions behind shock waves, in flow systems, in solution, or by laser gas-phase pyrolysis, as well as photochemistry and computational chemistry, will be mentioned where relevant to amplify the information obtained from FVP experiments. Numerous thermal reactions of azides,9−11 triazoles,12−15 and tetrazoles,16,17 mostly in solution but also sometimes in the gas phase, have been reported in earlier reviews and will not be repeated here unless relevant for the discussion.

AI AI AK AK AL AM AN AO AO AP AP AQ AQ

2. THE FVP METHOD Flash vacuum pyrolysis was developed as a relatively new technique in the 1960s, but pyrolysis reactions are as old as chemistry.18 In his classic monograph, Hurd lists many examples of pyrolysis of organic compounds by passing vapors through red-hot tubes at atmospheric pressure.19 In contrast, in FVP, a sample is volatilized into a resistively heated pyrolysis tube, usually quartz, in high vacuum, usually 10−3−10−5 hPa, and the products are collected in a liquid nitrogen cold trap or condensed with Ar at 10−22 K to form a matrix.18,20−24 Detailed descriptions of several types of apparatus have been published,18 including the coupling of a FVP apparatus with matrix isolation equipment. Many examples of FVP−matrix isolation with IR, UV−vis, or electron spin resonance (ESR) spectroscopic characterization of the products will be given in this review. For the uninitiated, an excellent introduction to ESR spectroscopy of triplets is given in ref 3. Briefly, ESR spectra of triplet diradicals, carbenes, and nitrenes are characterized by the zero-field splitting (zfs) parameters D and E (which could be given in units of magnetic field or energy but are usually reported in units of cm−1 by division by hc). The value of D is a measure of delocalization of the unpaired electron(s) (lower D means more delocalization; D = 1 cm−1 for phenylnitrene is a good reference point), and E is a measure of deviation from cylindrical symmetry of the two interacting electrons (E = 0 for fully cylindrical symmetry). The contact times in the hot zone of our FVP apparatus under high vacuum are typically in the millisecond range. Many authors use lower vacua, e.g., 10−1−10−2 hPa, or a carrier gas may be used at any pressure up to atmospheric. The advantage of a vacuum as high as possible is the reduced number of collisions between molecules and hence the avoidance of secondary, bimolecular reactions. This is also achieved by using an inert carrier gas. Gas-phase laser photolysis, usually using a CO2 laser, in the presence of a carrier gas has also been described.18,25−27 With this technique wall-less reaction conditions can be achieved; i.e., reaction takes place in the irradiated gas, but the walls remain cold. This therefore provides a useful check on conventional FVP reactions, where surface reactions on the hot quartz wall can be very important, especially when molecules contain polar and/or protic groups such as OH and NH. Pulsed pyrolysis is a technique of special importance in the field of reactive intermediates.18,28,29 In this technique, the sample to be pyrolyzed is kept in a reservoir at a stagnation pressure of at least 1 atm of Ar and admitted through a pulsed valve into a very short (e.g., 10 mm) and very hot (e.g., 1000 °C or higher) capillary pyrolysis tube in high vacuum. The pyrolysis products emerging from the hot tube undergo supersonic jet expansion in the high vacuum, causing cooling

AQ AR AR AS AS AS AT AV AV AX AX AX AY AY AY AY AY AY AY

1. INTRODUCTION This review will be concerned largely with flash vacuum pyrolysis (FVP) reactions of azides, triazoles, and tetrazoles. These three classes of compounds are precursors of a wide variety of interrelated reactive intermediates, particularly nitrenes, carbenes, diradicals, nitrile ylides, and nitrile imines. The emphasis will be on reaction mechanisms and direct observation of reactive intermediates, whenever possible. The major thermal reaction of azides is the loss of N2 to generate nitrenes.1−5 Because the chemistry of nitrenes and carbenes is highly interwoven, and many carbenes undergo thermal rearrangement to nitrenes, relevant FVP reactions of diazo compounds will also be described together with that of the azides (section 4). Furthermore, diazo compounds are very often intermediates in the thermal chemistry of triazoles and tetrazoles (sections 5 and 6). However, for a more detailed treatment of heteroarylcarbenes, Sheridan’s review6 should be consulted. The activation energies for decomposition of azides and diazo compounds with nitrogen elimination are on the order of 38−40 kcal/mol. Not surprisingly, therefore, the formation of nitrenes and carbenes are very prevalent reactions. Triazoloazines and 5-aryl(heteroaryl)tetrazoles also decompose with B


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to a few degrees kelvin. When coupled with a liquid helium cryostat, the products can then be isolated in an Ar matrix for spectroscopic investigation. While spectroscopic identification of primary pyrolysis products is highly desirable, chemical proof of structure is equally important. This may be achieved carrying out an FVP reaction in the presence of a volatile trapping agent as carrier gas (e.g., at 0.1−1 hPa).18,30 If one product is obtained at one pyrolysis temperature and another at a higher temperature, it is natural to believe that the second product is formed from the first, e.g., by rearrangement. However, this is not necessarily true, and suitable control experiments are important.31 One way to demonstrate that two reactions are consecutive is to use the double pyrolysis technique, where two pyrolysis tubes at different temperatures are used in series.18,30 Kinetic monitoring of all the pyrolysis products is also helpful. It must be noted that FVP conditions are not suitable for kinetic treatments in the Arrhenius sense,18 and activation parameters derived from FVP reactions, which are sometimes found in the literature, are highly unreliable. As mentioned above, surface reactions can be very important, and the quartz tubes used in FVP are usually not seasoned. For the purpose of determining Arrhenius parameters, the very low pressure pyrolysis (VLPP) technique should be used.32 Alternatively, conventional static pyrolysis may be used, bearing in mind that the kinetics observed under the long contact time conditions of static pyrolysis do not necessarily describe the reactions of interest in FVP. The low vapor pressure of compounds with high molecular weight is a natural limitation of the feasibility of FVP experiments. To overcome this problem, a carrier gas may help entrain the vapor. In the technique of solution spray pyrolysis (SS-FVP) the dissolved sample is injected into the high vacuum through a capillary.33,34 Here, the possibility of reaction with the solvent must be considered, and the need to condense a large excess of solvent in the cold traps puts a natural limit on the amount of material that can be handled. In falling-solid flash vacuum pyrolysis (FS-FVP),18,35 the finely powdered, solid starting material is admitted in a continuous manner to the top of a vertical pyrolysis tube in high vacuum. Evaporation takes place on a loose plug of quartz wool placed in the center of the pyrolysis tube. This technique works well for preparative pyrolysis of a variety of compounds on a scale of grams with the potential for upscaling to larger quantities. The method is not designed for spectroscopy of reactive intermediates, and the possibility of surface reactions on the hot quartz must be kept in mind.

in the pyrolysis of methyl azide (CH3N3), which will be described in section 4.2. It is important to be aware of chemical activation when carrying out “control experiments” to prove or disprove that a certain reaction takes place at a certain temperature. A reaction with chemical activation will take place at a lower nominal temperature than one without, and the difference may be several hundred degrees. In order to remove some of the excess energy, a carrier gas is essential. We have found the use of N2 at a pressure of ∼1 hPa to be effective in many reactions, but it must be noted that collisional deactivation is only effective for relatively long-lived compounds. If the lifetime is much shorter than the collision frequency, it will have no effect. Thus, for example, we may deactivate cyclopentadienes and indenes formed by pyrolysis of benzotriazoles (section 5.3), but it will be more difficult to deactivate carbenes or nitrenes (section 4.7). 3.2. Nitrenes Are More Stable Than Carbenes

It has been shown on thermochemical grounds that nitrenes are thermodynamically more stable (i.e., lower in energy) than the isomeric carbenes,38 and this has been confirmed by ab initio calculations.39 Several examples of rearrangements of pyridylcarbenes to phenylnitrenes (eq 1) will be described in

section 4.The energy difference between singlet 2-pyridylcarbene (1) and singlet phenylnitrene (5) is −19 kcal/mol at the CASPT2/cc-p-VDZ//CASSCF(8,8)/cc-pVDZ level (−11 kcal/mol at the B3LYP/6-31G* level), and for the triplets it is even more, −26 kcal/mol at the B3LYP/6-31G* level (cf. Scheme 10, section 4.7.2). As a consequence of the carbene−nitrene rearrangement, FVP reactions of heteroarylcarbenes usually lead to products derived from the isomeric nitrenes (e.g., phenylnitrene in eq 1), when such a rearrangement is possible, and the thermally produced nitrenes themselves can often be observed directly, especially by ESR spectroscopy,40 but also in several cases by IR and/or UV−vis spectroscopy. Because the singlet−triplet energy splitting in arylnitrenes is generally larger than in carbenes,41,42 the nitrenes are not likely to return either to their singlet states or to the carbenes once intersystem crossing to the triplet ground states has occurred. In other words, any intermolecular rearrangements of the nitrenes should take place in the singlet states before intersystem crossing has occurred. With their smaller singlet−triplet gaps, interconversion of singlet and triplet carbenes are energetically feasible in FVP reactions, and the intermolecular rearrangements are again expected to take place on the singlet energy surfaces. For these reasons, the “reverse” rearrangement of arylnitrenes to heteroarylcarbenes is contra-thermodynamic and therefore difficult to achieve. Nevertheless, given a sufficiently high temperature, such “reversions” can be observed, albeit usually in low yields, e.g., in the case of 2,6-dimethylphenylnitrene (see section 4.7.2). There are exceptions to the rule that nitrenes are more stable than carbenes.38 For example, the lone-pair-stabilized singlet aminocarbene H2N−CH: is undoubtedly lower in energy than the highly unstable singlet methylnitrene, CH3−N.38 This was

3. GENERAL PHENOMENA AND REACTION TYPES 3.1. Chemical Activation

It is emphasized that FVP reactions are subject to chemical activation, i.e., formation of vibrationally excited (“hot”) molecules.36,37 In exothermic reactions, the energy difference between the highest transition state and the product is in principle available as chemical activation. While some of the excess energy is carried away by the N2 molecules and by collisions, the majority is still available in low-pressure FVP reactions, and this often leads to rearrangement or fragmentation of the primary products. A typical example is the decomposition of methanimine CH2NH to HCN and H2 C


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confirmed by QCISD(T) calculation,43 but importantly, the same does not hold true for the triplets, because the triplet carbene does not enjoy the same degree of lone pair stabilization, and the larger singlet−triplet gap favors the nitrene. Similarly, singlet cyanocarbene NC−CH: is predicted to be of lower energy than ethynylnitrene, HCC−N:, as a result of the lower heat of formation of a CN group relative to an ethynyl group,38 but in the triplet state, the nitrene wins out because of the larger singlet triplet splitting. Triplet phenylcyanocarbene 6 has a rather high zero-field splitting parameter D/hc of 0.6488 cm−1. Phenylcarbenes are usually around D = 0.5 (o-cyanophenylcarbene: 0.5078 cm−1), and the extra conjugation in 6 would lower the D value further, if it were a true carbene. Hence, the high D value suggests that triplet 6 has significant spin density on N as expressed in the canonical structures 36a and 36b (Figure 1).

3.3. Types of Rearrangement

3.3.1. 1,2-H Shift. 1,2-H shifts are ubiquitous in carbenes and nitrenes, for example, the rearrangement of methylcarbene to ethylene (CH3−CH: → CH2CH2) and of methylnitrene to methanimine (CH3−N: → CH2NH) (section 4.2, eq 6), and these reactions usually have very low activation energies. In contrast, nonoccurrence of 1,2-H shifts in the formal carbenes, cycloheptatrienylidenes/cycloheptatetraenes and their benzo derivatives, even in high-temperature FVP reactions, will be demonstrated in sections 4.7.1 and 4.7.4. Examples of 1.2-H or 1,2-R shifts in boryl and silyl azides will be mentioned in sections 4.10 and 4.11. 3.3.2. Aromatic Ring Expansion. The most common intramolecular reaction of arylnitrenes and heteroarylcarbenes is the ring expansion to azacycloheptatetraenes such as 3, which can be observed directly in many cases by means of matrix isolation spectroscopy, under both thermal (FVP) and photochemical conditions (eq 1). Several examples of such reactions will be described in section 4.7. It is worth noting, however, that benzo-annelation can render azacycloheptatetraenes unstable with respect to the valence-isomeric, zwitterionic, cyclic nitrile ylides, as is the case in the rearrangements of 1- and 2-naphthylnitrenes (e.g., 8, eq 2).57 The cyclic nitrile ylides such as 8 can be described as

Figure 1. Structures of triplet and singlet phenylcyanocarbene. Adapted with permission from ref 44. Copyright 2014 American Chemical Society.

bond-shift isomers of the normal, cyclic ketenimines such as 7 (for a definition of the concept of bond-shift isomerism, see ref 58). Ylides 8 are observable in matrix photolysis but not normally in FVP reactions, although the calculated energies make it likely that they are formed reversibly in FVP reactions, too. The end products of FVP reactions are often five-membered ring nitriles, e.g., cyanocyclopentadiene from phenylnitrene (eq 3), but the reactions are far from simple (section 4.7.1).

Calculations at the B3LYP/6-31G** level do not indicate the presence of a CN group at all in triplet phenylcyanocarbene (36), and it is also not observed in the experimental spectrum.44 A shallow Ph−C−C angle of ca. 160° and equidistant CC and CN bonds in conformity with structure 36a are predicted. In contrast, the singlet (16) has a distinctly bent carbene-like structure (Figure 1) with a Ph−C−C angle of ca. 118°, C−C single bonds, and a CN triple bond. Singlet phenylcyanocarbene has been generated from different precursors in solution and undergoes cyclopropanation of alkenes,45−49 but the fact that the reaction is nonstereospecific suggested that both the initially formed singlet and the groundstate triplet were involved.46 Attempts to generate phenylethynylnitrene (Ph−CC−N) led instead to chemistry derived from phenylcyanocarbene.50,51 The structure (carbene vs nitrene) of cyanocarbene52 and phenylcyanocarbene 653 has been the subject of much discussion and controversy. The singlet phenylcyanocarbene 6 is described as a bent singlet arylcarbene.53 The triplet cyanocarbene HCCN is best described as a quasi-linear molecule with a barrier to linearity on the order of 200−300 cm−1, a HCC angle of 146°−154°, and a high degree of ketenimine diradical structure, H−C• CN•.54 Indeed, the ESR spectrum of this species features a high D value of 0.8629 cm−1 (E ∼ 0).54 The IR spectrum does not feature a nitrile-type CN stretching vibration.55 The corresponding all-carbon species triplet propargylene (H− C•CC•−H) has been shown to have a symmetric (C2 or C2v) structure and a zero-field D value of 0.64 in Ar, i.e., very similar to that of 6.56 These data support the conclusion that singlet cyanocarbene is a true bent singlet carbene H−C−CN, but the triplet has a linearized diradical structure with significant nitrene character (see also section 3.3.3).

The bicyclic cyclopropenes and azirines such as 2 and 4 (eq 1) are not directly observed under FVP conditions because of their very low barriers toward ring expansion, but several bicyclic azirines have been characterized under photolysis conditions.57,59,60 The azirine formed by cyclization of 3isoquinolylnitrene is particularly long-lived, surviving in the neat state up until 110−140 K (section 4.7.8.2).61 Calculations of ground-state energies make it highly likely that the azirines are involved in many FVP reactions, too, but they will often be omitted in reaction schemes in this review. In addition to ring expansion and ring contraction, two important ring-opening reactions also occur in many cases: type I (ylidic) and type II (diradicaloid) ring opening (Scheme 1).62 Type I ring opening occurs whenever there is a meta-(1,3)relationship between a nitrene or carbene center and a ring nitrogen atom, e.g., in 3-pyridylnitrene. The reaction may take place either in the nitrene (carbene) itself or in the sevenmembered azacycloheptatetraene, and it is so facile, under both thermal and photochemical conditions, that it may be difficult D


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Scheme 1. The Two Types of Ring Openinga

field splitting parameters D of around 1 cm−1, whereas diradicals resonate at much lower field (typically ∼3000 G) and have much smaller D values63 (see the Introduction for a brief outline of ESR spectroscopy). Vinylnitrenes have a high degree of diradical character and resonate at intermediate fields. A linear correlation between nitrene zero-field splitting parameters D and calculated nitrene spin densities helps characterize nitrenes (Figure 2).63

a

Adapted with permission from ref 62. Copyright 2011 American Chemical Society.

or impossible to observe the azacycloheptatetraene intermediate. Type II ring opening in contrast occurs when there is an ortho-(1,2)-relationship between a nitrene center and a ring nitrogen atom. This reaction usually has a higher activation energy than type I ring opening, but it leads to the formation of glutacononitriles 9 in yields of about 10% by ring opening of 2pyridylnitrenes (eq 4). The ring-opened species may be either Figure 2. Correlation of the ESR zfs parameter D/hc (cm−1) with the natural spin densities ρ on the nitrene-N. Legend: 1, phenylnitrene; 4, 2-pyridylnitrene; 9, ring-opened cyanobutadienylnitrene from 2pyridylnitrene; 12, 3-pyridylnitrene; 22, 4-pyridylnitrene; 23, 2pyrazinylnitren; 28, ring-opened vinylnitrene from 2-pyrazinylnitrene; 31, 3-isoquinolylnitrene; 32, ring-opened nitrene from 3-isoquinolylnitrene; 35, 2-quinazolinylnitrene; 40, 2-pyrimidinylnitrene; 41, ringopened vinylnitrene from 2-pyrimidinylnitrene; 45, phenanthridinylnitrene; 48, ring-opened arylnitrene from phenanthridinylnitrene; 57, phenanthrylnitrene; 65, 1-isoquinolylnitrene; 67, 2-quinolylnitrene; 74, 4-quinolylnitrene; 81, 2-quinoxalinylnitrene; 82, 2-naphthylnitrene; 77, 4-biphenylylnitrene. Adapted with permission from ref 63. Copyright 2013 CSIRO Publishing.

nitrenes or diradicals, as discussed in section 3.3.3. Type II ring opening is uncommon but not unknown in 2-heteroarylcarbenes, because the carbenes rearrange to nitrenes at a higher rate. 3.3.3. Nitrene vs Diradical. The species formed upon ring opening of an α-heteroarylnitrene may exist as either a vinylnitrene (cyanobutadienylnitrene) or as a diradical. If the α-bond in the ring-opened species has double bond character due to annelation with an aromatic ring, the nitrene form will dominate. If the α-bond has single-bond character, but the βbond has double-bond character due to annelation, the diradical form will dominate (Scheme 2). The two forms can be considered as another example of bond-shift isomers. Importantly, they can be distinguished by ESR spectroscopy, where they have very different signatures, true nitrenes giving rise to transitions at high field (typically ∼7000 G) and zero-

As mentioned above, (hetero)arylnitrenes are often directly observable by ESR spectroscopy in thermal reactions by using FVP with Ar matrix isolation. The ring-opened nitrenes are usually observed in matrix photolysis reactions, but product studies clearly show that ring opening is also occurring on FVP, e.g., the formation of glutacononitriles from 2-pyridylnitrenes (eq 4). 3.3.4. Curtius-Type Rearrangements, Imidoylnitrenes, and Nitrile Imines. The Curtius rearrangement of acyl azides to isocyanates64 (eq 5) is a preparatively and technically hugely

Scheme 2. Vinylnitrenes vs Diradicals

important process, which is usually performed thermally in solution.The photochemical Curtius rearrangement is important in the study of nitrene and isocyanate intermediates under matrix-isolation conditions, but important mechanistic information can also be obtained from FVP reactions. The reactions of acyl azides, acylnitrens, and the related carbamoylnitrenes (eq 5, X = NRR′) will be described in sections 4.8 and 4.9. Although imidoylnitrenes (eq 6) are rarely observed directly, important information on their chemistry has been obtained. In E


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generation of NH in the gas-phase and for the study of its spectroscopic and chemical properties.

the absence of internal or external traps, they invariably rearrange to carbodiimides, and they constitute important links between carbodiimides and nitrile imines, which themselves are generated by either FVP or photolysis of tetrazoles (eq 6 and

4.2. Methyl Azide

Early investigations77−82 of the pyrolysis of methyl azide (CH3N3, 10) have been reviewed.36 FVP of methyl azide at 500−900 °C/10−2 hPa results in the formation of methanimine (CH2NH, 12; eq 8), which was detected by online

section 6). Nitrile imines65,66 are fascinating in their own right, and they can exist in either propargylic or allenic or even carbenic forms as described in section 6.Curtius-type rearrangements of phosphoryl- and sulfonylnitrenes will be mentioned in sections 4.12 and 4.13. 3.3.5. Aza-Wolff Rearrangement. The Wolff rearrangement of α-diazo carbonyl compounds to ketenes67 is outside the scope of this review, but an example is mentioned in section 4.7.3. The aza-Wolff rearrangement of imidoylcarbenes to ketenimines (eq 7) is described in section 5. The

microwave spectroscopy,83 millimeterwave spectroscopy,84−86 photoelectron spectroscopy,87−89 and cavity ring down spectroscopy.90,91 In addition, HCN is formed in this pyrolysis, as a minor product at 500 °C but a major one at 900 °C (eq 8). This is ascribed to chemical activation of the methanimine by ∼90 kcal mol−1 arising from the activation energy for dissociation of 10 plus the exothermicity of the reaction. Notably, temperatures above 1000 °C are required to decompose the unactivated CH2NH to HCN + H2 under these conditions. Methylnitrene 11 has a triplet ground state and a singlet− triplet energy splitting of ca. 31 kcal/mol, and the singlet is an extremely fleeting intermediate.89,92−94 Multiconfigurational ab initio calculations (CASSCF and CAS/MP2) have confirmed that methylnitrene (singlet and/or triplet) is an intermediate that undergoes isomerization to methanimine via a very low barrier (∼1.4 kcal mol−1), and a second reaction channel leads directly from singlet methylnitrene to HCN + H2 (barrier of 19.4 kcal mol−1).43,95,96 The methanimine can undergo chemically activated decomposition to HCN and H2 as well. The calculated barrier is 107 kcal/mol, so in the absence of chemical activation, this reaction will be extremely slow at temperatures below 1000 °C. The decomposition of methanimine to HNC and H2 has a much smaller calculated activation barrier of 91 kcal/molclose to the amount of chemical activation available (vide supra)which raises the question why HNC has not been observed.97 In fact, it has long been known that matrix photolysis of methylenimine at 4 K affords mainly HNC with little HCN.98−100 A likely explanation is that HNC is in fact formed in the FVP reactions, but it too is chemically activated and rearranges rapidly to HCN.43 The barrier for this process is ca. 30 kcal/mol,101,102 and the enthalpy of formation difference between HNC and HCN is close to 14 kcal/mol.103,104 This means that the thermal decomposition of methyl azide under FVP conditions can yield chemically activated CH2NH and chemically activated HNC, which finally isomerizes to HCN. The tautomerization of HNC to HCN will also be facilitated by collisions with the walls of the pyrolysis tube. In our experience, it is very difficult to detect compounds such as HO−CN and H2N−CCH in conventional FVP reactions because of facile wall-induced tautomerizations,105 and the same would apply to hot HNC.

imidoylcarbenes are mostly generated from triazoles, and 1Hazirines may be intermediates (eq 7). The high-yielding rearrangement of benzotriazole to cyanocyclopentadiene (section 5.3) is a reaction of this type.

4. AZIDES 4.1. Hydrazoic Acid (HN3)

The simplest nitrene, imidogen (NH), has been produced by pyrolysis of hydrazoic acid (HN3), in Ar behind shock waves at 600−2200 hPa and temperatures between 1200 and 1350 K with an activation energy of 36 kcal/mol.68 NH(3Σ−) was observed at 336 nm. Two reactions were identified, the major one being HN3 + Ar → NH(3Σ−) + N2 + Ar, with ΔH0 = 17.5 kcal mol−1, and the minor one being HN3 + Ar → NH(1Δ) + N2 + Ar, with ΔH0 = 53.6 kcal mol−1. Thus, the triplet formation was much preferred, and a singlet−triplet crossing probability of 10−3−10−2 was calculated. However, in a study of the collision-free CO2-laser-induced infrared multiphoton dissociation reaction of DN3, it was determined that the yields of singlet and triplet ND were on the same order of magnitude.69 Calculations at CASSCF and MCSCF-CI levels indicate that this outcome is due to a crossing of the singlet and triplet potential energy surfaces, and the mixing of singlet and triplet states at the crossing point is facilitated by spin−orbit coupling.70 Similar conclusions were derived from CASPT2/ ANO-L calculations.71 NH has also been generated by photolysis in an Ar matrix at ∼10 K.72 The lowest electronic state of NH is the triplet 3Σ−, followed by the 1Δ and 1Σ+ states. From the above data and additional investigations,73−75 a singlet−triplet splitting energy ΔEST of 36 kcal/mol is derived. The ESR spectrum has not been reported, but the zero-field splitting parameter D/hc = 1.863 cm−1 was obtained using gas-phase laser magnetic resonance spectroscopy.76 Since FVP of azides is an excellent way of generating nitrenes and isolating them in noble-gas matrices, it seems reasonable to predict that FVP of hydrazoic acid may prove useful for the

4.3. Alkyl Azides

Activation barriers of 38−40 kcal mol−1 for thermolysis of alkyl, propyl, and isopropyl azides in the gas-phase have been reported.106 Alkyl azides RR′R″CN3 afford imines R′R″CNR both thermally79 and photochemically.107 In a study of migratory aptitudes for imine formation from 1-phenyl and 1,1-diphenylethyl azides, Saunders and Caress concluded that discrete nitrenes are involved in the photochemical, but not the thermal reactions108 (the thermolyses were conducted by F


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injecting solutions of the azide on a gas chromatograph with an injector temperature of 230 °C and the column at ∼223 °C). Abramovitch and Kyba concluded the opposite, that the photochemical imine formation is a concerted reaction109,110 but that the thermal reactions in solution at 185−190 °C involve nitrenes as discrete intermediates because aromatic substitution by the nitrene was observed (in low yields) in some cases.111 Undoubtedly, reality is more complicated. The ESR spectra of photochemically generated methyl and ethyl nitrenes112,113 as well as CF3N114 have been recorded. However, thermally generated alkylnitrenes have not been observed directly (see, however, the case of CH3N above89). This is not surprising in view of the facile rearrangement to imines. In a computational study of the dissociation of methyl, ethyl, isopropyl, and tert-butyl azides at MP2/6-31G(d) and B3LYP/ 6-311+G(d) levels, the intersystem crossing point was found to lie just slightly below the transition state for the singlet reaction, and this could allow the spin-forbidden reactions to occur, 1 RN3 → 3RN + 1N2.115 The calculated energy barriers at the B3LYP level around 40−41 kcal/mol are qualitatively consistent with experimental values (36−40 kcal/mol, vide supra), but the MP2 energies are 14−15 kcal/mol too high. In another computational study of the thermal decomposition of ethyl azide at the CAS/MP2 level, the reaction mechanism was described as follows:116 CH3CH2N3 → CH3CH2N + N2; CH3CH2N → H2 + CH3CN and CH3CH2N → CH3CHNH, with the first step being rate-determining and having a somewhat too high activation barrier of 42 kcal/mol. The nitrene was predicted to be formed as either the singlet or, following intersystem crossing, the triplet. Both of these studies indicate that the barriers for formation of singlet and triplet alkyl nitrenes are nearly the same. Investigations aimed at the detection of either singlet or triplet alkyl nitrenes by pulsed pyrolysis would be of interest. The pyrolysis of several other azides with photoelectron spectroscopic monitoring of the products has been reviewed by Bock and Dammel, e.g., the formation of ethanimine from ethyl azide at 390 °C and its decomposition at higher temperatures, isopropyl azide to 2-propanimine, tert-butyl azide to N-methyl2-propanimine, fluoromethyl azide to FCN, and allyl azide to 1azabutadiene (HNCH−CHCH2), all in the temperature range 350−500 °C.117 Thermolysis of cyclopropyl azides in solution at 110−130 °C affords 1-azetines in 43−56% yield together with alkenes and nitriles (eq 9; R1 = H, CH3 or C6H5; R2−R5 = H, CH3, C6H5,

Thermolysis of 2-azidocyclobutenes in solution at 75−100 °C affords cyclopropane-1-carbonitriles in 25−80% yield (eq 11; R = COOH, ester or amide functions); the reaction can also be performed photochemically with a yield of 60−80%.123

Azidoacetone (H3C−CO−CH2N3) underwent gas-phase pyrolysis at 30−900 °C through at least two primary reaction channels, (1) to ketene CH2CO and methanimine CH2 NH and (2) to acetaldehyde and HCN, but additional, unidentified products were also formed. The product formation was probed by online photoelectron spectroscopy and matrixisolation IR spectroscopy.124 4.4. Benzyl Azide

The gas-phase pyrolysis of benzyl azide 13 affords benzalimine (C-phenylmethanimine, PhCHNH, 14) (eq 12) as observed

by gas-phase photoelectron spectroscopy.125 Early FVP studies indicated that both C-phenylmethanimine (Ph−CHNH) and N-phenylmethanimine (PhNCH2) were formed,126 but it is usually observed that migration of H is faster than that of Ph, so benzalimine should dominate. It is also indicated that benzalimine is formed on thermolysis of benzyl azide in solution,127 and it is formed on photolysis, where it was observed by matrix IR spectroscopy.128 The activation energy for decomposition of benzyl azide has been determined as ∼39 kcal mol−1,129 which is similar to the activation energies for alkyl and aryl130 azides. As in the case of methylnitrene mentioned above, it is very likely that the phenylmethanimine 14 generated by pyrolysis of benzyl azide will be chemically activated. This would result in the formation of benzonitrile 16, which in fact appears to be present in the reported spectra (eq 13).131

4.5. Vinyl Azides

4.5.1. Vinyl Azide. FVP of vinyl azide at 400−500 °C (10−1−10−3 hPa) allows the synthesis of 1-azirine,122 but importantly, ketenimine (CH2CNH), acetonitrile, and methyl isocyanide (CH3NC) are formed as well.132,133 The azirine formation was considered to be concerted, in agreement with the finding by L′abbé and Mathys that the activation energy for the formation of 1-azirines from vinyl azides was only 26−30 kcal/mol, well below the usual 38−39 kcal/mol for the formation of nitrenes in the decomposition of aryl azides.134 Because 1-azirine polymerizes at room temperature, the FVP of vinyl azide with isolation of the product in a cold trap is a convenient preparative method.122 However, many substituted azirines are stable at ambient temperature and can be

COOCH3, or halogen).118,119 1-Azetine was also generated by online chlorination and dehydrochlorination of azetidine by using the gas−solid technique of Guillemin, Denis, and their co-workers.120−122 Pyrolysis of 1-azetine at 400−500 °C caused ring opening to 2-azabutadiene (H2CN−CHCH2) (eq 10), which polymerizes at room temperature, but by performing the pyrolysis as a FVP reaction at 450 °C/10−4 hPa, the 2azabutadiene could be isolated at −78 °C and characterized by 1 H NMR spectroscopy at −60 °C.120 G


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synthesized in solution, best in dichloromethane at 150 °C for 20 min,135 or at 110 °C catalyzed with DABCO.136 Duarte and co-workers have reported a DFT study of vinyl azide decomposition with calculations of intrinsic reaction coordinates at the B3LYP/6-311++G(d,p) level.137 According to these calculations, the s-cis conformer of the azide (17cis) leads to the formation of ketenimine 18 in a single-step, again without a nitrene intermediate (eq 14), whereas the s-trans

of ketenimine to acetonitrile is a formally forbidden 1,3-H shift with a very high calculated activation barrier, and it is believed to take place via the radical pair 24 (Scheme 3).144 Photochemistry of 1-azirines (2H-azirines) can result in either ketenimine or nitrile ylide formation.146 The closed-shell singlet vinylnitrene lies some 25 kcal/mol above the OSS. The triplet nitrene is the ground state. ESR spectra of photochemically generated triplet vinylnitenes (but not vinylnitrene itself) have been reported.63,147−149 However, while aryl- and heteroarylnitrenes produced by FVP are easily matrix-isolated and observed by ESR spectroscopy,40 this has not so far been the case for vinylnitrenes. Trapping with triphenylphosphine has suggested the existence of an equilibrium between 1-azirines and vinylnitrenes at 50−60 °C.150 Epimerization of azirine 25 on FVP at 400 °C (10−5 hPa) suggests a thermal equilibrium between the azirine and the vinylnitrene in the gas phase. This also took place (with decomposition) in toluene solution at 75 °C, but not on irradiation in chloroform at −45 °C (eq 17).151

conformer 17trans was thought to undergo concerted cyclization to 1-azirine 19 (eq 15) via a transition state lying about 50 kcal/mol above the 2H-azirine, without vinylnitrene 20 as an intermediate, but the DFT study did not consider the open-shell singlet vinylnitrene.This calculated barrier is certainly too high, but the conclusion is consistent with that of L′abbé and Mathys.134 A more reasonable activation energy of ca. 30 kcal/mol for the vinyl azide → 1-azirine reaction was calculated by Nguyen et al.,138 who also pointed out that vinyl azide will be in thermal equilibrium with 4H-1,2,3-triazole 21, lying ca. 7 kcal mol below the azide, and the activation barrier for this reaction is ∼30 kcal/mol above the azide (eq 16).

Epimerization of optically active 2-methyl-3-phenyl-1-azirine (26) at 120 °C in decalin also supports the equilibration with the vinylnitrene 27. Cyclization to 2-methylindole (28) only took place at 185 °C, i.e., with a rate ca. 2000 times slower (eq 18).152

The lowest singlet state of vinylnitrene 20 is the open-shell singlet (OSS).139−145 CASPT2 calculations on the relationships between 1-azirine 19, ketenimine 18, acetonitrile 22, and vinylnitrene 20 have shown that the thermal ring opening of 19 to OSS vinylnitrene 20 has an activation barrier of ca. 33 kcal/ mol (Scheme 3).144 The OSS vinylnitrene 20 undergoes a very facile 1,2-H-shift to acetonitrile with a barrier of ca. 6.5 kcal/ mol, whereas the corresponding rearrangement to ketenimine would require ca. 33 kcal/mol (Scheme 3). The alternative ring opening of 19 to formonitrile ylide [H2CN(+)CH(−), 23] has a calculated barrier of 48 kcal/mol.144 The rearrangement

Bis-azirines have been prepared in high yield by thermolysis of diazidobutadienes in refluxing chloroform and by lowtemperature photolysis.151 4.5.2. Styryl Azides. Smolinsky reported the formation of 2-phenyl-1-azirine (31) as a major product together with 5−6% N-phenylketenimine 32 by gas-phase pyrolysis of α-azidostyrene 29 at 0.1−0.3 hPa (Scheme 4).153,154 In another study by Hassner et al., the activation parameters for decomposition of 29 in bromobenzene at 65−100 °C were determined as ΔH* = 26.8 ± 0.7 kcal mol−1 and ΔS* = −2 ± 2 cal K−1 mol−1, and it was concluded that azirine formation is probably concerted, not involving the nitrene 30.155 This was confirmed in a further study by Jordan.156 More recently, FVP of azide 29 was found to yield 2-phenyl1-azirine (31) already at 350 °C, and the amount increased with the temperature. At 650 °C azirine 31 was obtained in 82% yield together with 9% of phenylacetonitrile 33.31 On FVP at 750−900 °C, azirine 31 undergoes ring opening to regenerate the vinylnitrene 30, migration of the phenyl group to form N-phenylketenimine 32, and isomerization to phenylacetonitrile 33. This isomerization may take place directly by a phenyl shift (30 → 33) or homolytically (32 → 33) via a radical pair as described for the unsubstituted 1-azirine in Scheme 3.31

Scheme 3. CASPT2 Calculated Energies Based on CASSCF(6,5) 2σ + 3π Geometries for Ground and Transition States in Thermal Reactions of 1-Azirine 19 and OSS Vinylnitrene 20a

a

Energies in kcal/mol relative to 19. H


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Scheme 4. FVP Reactions of α-Azidostyrene 29

Scheme 5. FVP and Photolysis Reactions of β-Styryl Azide 34

Thermolysis of β-azidostyrene 34 in boiling hexadecane at 287 °C yielded indole 37 and phenylacetonitrile 33.157 Photolysis at 365 nm afforded 3-phenyl-1-azirine (36), and this compound was also obtained together with a trace of phenylacetonitrile 33 when the thermolysis was carried out at a lower temperature (100−105 °C) in ligroin. 3-Aryl-1-azirines rearrange thermally to indoles.158 Activation parameters for the rearrangement of 3-aryl-2-methyl-1-azirines at 64−185 °C were determined as ΔH* = 28−35 kcal mol−1, but ΔS* = −13 to 19 cal K−1 mol−1.158,159 Boyer et al. reported phenylacetonitrile 33 as a product of pyrolysis of β-azidostyrene 34 on the injector block of a gas chromatograph.160 FVP of β-azidostyrene 34 at 350 °C yields 3-phenyl-1-azirine (36) and indole 37 as major products. This azirine does not survive above 400 °C, when phenylacetonitrile 33 becomes the major product. Phenylacetonitrile 33 may be formed by tautomerization of ketenimine 38, but it may also form directly from the nitrene 35 by a 1,2-H shift.155 The 3-phenylketenimine 38 was not observable in the pyrolysis reactions, but photolysis of 34 in an Ar matrix yielded the azirine 36 and the ketenimine 38 as the major products (Scheme 5).31 The photochemical relationships between α- and βstyrylnitrenes, azirines, ketenimines, and nitrile ylides have also been investigated.146,161

nitrene into the alkyl chain, preferably to form dihydroindole derivatives in yields of 50−60%. The reaction occurs stereoselectively with retention of configuration, and the extent of retention was higher in the gas phase than in solution thermolysis (eq 20).168

It was proposed that it is the singlet nitrene that undergoes the C−H insertion reaction. The corresponding six-membered ring, 2,3-dimethyltetrahydroquinoline, was formed as a byproduct in 11% yield, and it is generally observed that fivemembered ring formation dominates over that of six-membered rings in nitrene cyclizations.33 2-(2-Methyl)butylaniline, the product of intermolecular H-abstraction by the nitrene, was formed in 1% yield in the gas-phase reaction and 29% in solution. The product of intramolecular abstraction of 2H from the butyl side chain was obtained in 46% yield in the gas-phase reaction and 25% in solution.169 4.7.2. Phenyl Azide, Phenylnitrene, and the Pyridylcarbenes. FVP of phenyl azide 39 at 400−500 °C generates phenylnitrene 40 in high yield. The nitrene can be isolated as such in a noble-gas matrix at cryogenic temperatures and observed directly in its triplet ground state by ESR spectroscopy.40 Preparative FVP under the mildest conditions (about 400 °C, high vacuum) yields the dimer of triplet phenylnitrene, azobenzene 41, as the major product, usually together with a small amount of aniline 42, often 75% yield). Importantly, neither the amine 202 nor the indoloquinoline 205 were detectable in the pyrolysis products, thereby showing that both the carbene and the nitrene rearrange to the ylide 204, but the carbene does not rearrange to the nitrene in this case.221

observable on matrix photolysis of the labeled azide 183A (observed at 1733 and 1711 cm−1).227 4.7.8.3. 3-Quinolylnitrenes and 4-Quinazolinylcarbenes. FVP of 3-azidoquinoline 189 at 450−600 °C yields 3cyanoindole 192 in very high yield. It is postulated that this takes place via the ring-opened nitrile ylide 191, in analogy with the ring opening of 3-pyridylnitrene. Furthermore, this nitrile ylide is directly observable in the matrix photolysis of the azide (Scheme 27).228 FVP of triazolo[4,3-c]quinazoline 193T at 600 °C gave the same product (192), again in high yield. This is explained in terms of a standard carbene ring expansion and type I ylidic ring opening 195 → 191 (Scheme 28). The diazo compound 193D and the carbene 194 are spectroscopically observable when the reaction is performed photochemically. However, in this case there was no direct evidence for a rearrangement of the carbene 194 to the nitrene 190 under either thermal or photochemical conditions.228 V


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Scheme 29. FVP of 3-Azidoquinolines and 1,2,3-Triazolo[4,3-c]quinazolines

4.7.8.4. 4-Quinolylnitrene and 2-Quinoxalinylcarbene. 3Cyanoindole 192 is again the major product of FVP of 4quinolyl azide 207 (Scheme 30). Low yields of 4-aminoquino-

ESR spectroscopy in the matrix photolysis reaction but, as usual, the carbene did not survive FVP conditions, rearranging instead to the observable nitrene 208. 1,4-Diazabenzocycloheptatetraene 209 was observable in the matrix-isolated products of FVP as well as in the photolysis.229 4.7.9. 9-Phenanthridinylnitrene and 9-Acridinylnitrene. FVP of tetrazolo[1,5-a]phenanthridine 215T generates at first the azide 215A and then the two nitriles 220 and 221. 15 N-Labeling of the nitrene-N demonstrated >70% scrambling of the label over the two nitrogen atoms in 220 and 221, thereby indicating at least partial interconversion of the nitrenes 216 and 216′ via the diazadibenzocycloheptatetraene 217 prior to ring opening (Scheme 31).225 Subsequently, 217 was observed directly by IR spectroscopy at 77 K [strong NC N absorption at 2110 cm−1 (unlabeled)] and found to be stable until −40 °C, at which temperature it dimerized to the carbodiimide dimer 222, the structure of which was determined by X-ray crystallography.223 The nitrile 221 is our only example of the formation of an N-cyanopyrrole derivative from a 2pyridylnitrene analog (cf. discussion under 2-pyridylnitrene). The nitrenes 216 and 218 as well as the diradical 219 were all observed directly by matrix-isolation ESR spectroscopy (Scheme 31).63 9-Azidoacridine 223 also yields cyanocarbazoles 220 and 221 on FVP at 700−800 °C, but all the isomeric cyanocarbazoles are formed, too (Scheme 32). This can be understood in terms of rearrangements of intermediates such as 227 and 228. It must also be kept in mind that CN-group migrations can take place in aromatic nitriles at elevated temperatures.208 The nitrene 224 was directly observable by ESR spectroscopy following FVP at 520 °C with isolation in an Ar matrix (D/hc = 0.522 cm−1, E/hc = 0.0025 cm−1) (Scheme 32). Subsequent warming of this material to room temperature afforded a mixture of 9-aminoacridine 225 and 9,9′-azoacridine 226, which were also obtained in the 700 °C pyrolysis together with the cyanocarbazoles.230 4.7.10. 2-Pyrimidinyl- and 3-Pyridazinylnitrenes. 2Azidopyrimidines 233A (absorbing at 2135 cm−1) are easily generated and observed by mild pyrolysis of tetrazolo[1,5a]pyrimidines 233T (Scheme 33). In contrast, only very weak peaks ascribed to 3-azidopyridazines 229A are observable at 2145 and 2118 cm−1 in the low-temperature IR spectra

Scheme 30

line 211 and 4,4′-azoquinoline 210 are also formed. The same products were formed by FVP at 400−600 °C of triazoloquinoxaline 212T and tetrazolylquinoxaline 213 via 2quinoxalinyldiazomethane 212D and 2-quinoxalinylcarbene 214. 4-Quinolylnitrene 208 was observable by ESR spectroscopy of the matrix-isolated pyrolysis products (490 °C) of all three precursors (D/hc = 0.882 cm−1, E/hc = 0.0020 cm−1). In addition, there was ESR evidence for the formation of a ringopened nitrene with zfs parameters typical of arylnitrenes (D/ hc = 0.992 cm−1, E = 0.00 cm−1); the phenylnitrene derivative N−C6H4−NCH−CCH, formed in a type II ring opening, is a likely candidate. 2-Quinoxalinylcarbene 214 was observed by W


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was observable upon matrix photolysis (D/hc = 1.006, E/hc = 0.003 cm−1).231 The end products of FVP of tetrazolo[1,5-b]pyridazine 229T are propargyl cyanide, tetrolonitrile, cyanoallene, and cyanocyclopropene, which are formed by ring opening of the 3pyridazinylnitrene to the observable diazo compound 231 and elimination of N2 (Scheme 33).232 In contrast to 2-pyridylnitrene, 2-pyrimidinylnitrene 234 does undergo ring contraction to form the 1-cyanopyrazole 236, which can be isolated by preparative FVP in the 400−600 °C range. A substantial amount of 2-aminopyrimidine 235 was also obtained in the preparative pyrolyses, thus suggesting that 2-pyrimidinylnitrene easily undergoes intersystem crossing to the triplet state, which abstracts hydrogen.38,231,232 2-Aminopyrimidines were also formed, to the exclusion of 1cyanopyrazoles, upon thermolysis of tetrazolo[1,5-a]pyrimidines in CDCl3 solution at 190 °C.233 However, the formation of 2-aminopyrimidine was avoided, when the FVP reaction was carried out in high vacuum in the presence of Ar for matrix isolation; the 1-cyanopyrazole then became the main product.231 In addition to these products, IR spectroscopic evidence for the ring-opened ketenimine 239 was given by a band at 2045 cm−1, and a second nitrene was observed in the ESR spectrum under matrix photolysis conditions and assigned to the open-chain nitrene 238 (D/hc = 0.875, E/hc = 0.00 cm−1). The low D value shows that it is not an arylnitrene and is in agreement with the dienylnitrene structure. This corresponds to a type II ring opening, and 238 is the likely precursor of both the ketenimine 239 and 1-cyanopyrazole 236 (Scheme 33).63,231 Analogous results were obtained with 5,7-dimethyltetrazolo[1,5-a]pyrimidine and 6,8-dimethyltetrazolo[1,5-b]pyridazine.231 Thus, the products of FVP of tetrazolo[1,5-a]pyrimidines and tetrazolo[1,5-b]pyridazines are very different under mild FVP conditions, but when the temperature of pyrolysis of tetrazolo[1,5-a]pyrimidine is increased to 700 °C, the products become the same. The 1-cyanopyrazole is still formed, but a 1,5-Shift of the CN group now leads to 3-cyano-3H-pyrazole 237, which is isolable in the dimethyl series.234 Subsequent loss of N2 results in the same diradical/carbene 232 as was formed from tetrazolo[1,5-b]pyridazine 229T and hence the same

Scheme 31

following FVP of tetrazolo[1,5-b]pyridazines 229T.231 This is in agreement with the calculated energy differences. FVP of tetrazolo[1,5-a]pyrimidine 233T at 500 °C with isolation of the product in Ar matrix yielded a strong ESR spectrum of 2-pyrimidinylnitrene 234 (D/hc = 1.217, E/hc = 0.0052 cm−1). The same was not possible for 3-pyridazinylnitrene, probably because only very little azide was present in the thermal equilibrium, but the ESR spectrum of the nitrene 230 Scheme 32

X


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Scheme 33

1-Cyanoindazole 255 was also obtained by matrix photolysis of azide 250A together with N-cyanoanthranilonitrile 256. The latter compound is also formed on FVP of tetrazolo[5,1c]quinoxaline 292T (see Scheme 40 below) but not in the FVP of tetrazolo[1,5-a]quinazoline 250. Both 1-cyanoindazole 255 and N-cyanoanthranilonitile 256 are ascribed to type II ring opening to the diradical 254, which was observed directly by ESR spectroscopy at 15 K (D/hc = 0.1187, E/hc = 0.0026 cm−1) but decayed even at this temperature, presumably by forming 255 and 256 via very low calculated activation barriers (1−2 kcal/mol).63,237 2-Quinazolinylnitrene 251 itself was observed by ESR spectrocopy (D/hc = 1.1465, E/hc = 0.0064 cm−1) and by IR and UV spectroscopy, too. Furthermore, a second (minor) arylnitrene assigned the structure 257 (D/hc = 0.9587, E/hc ≤ 0.0008 cm−1) was formed by alternate type II ring opening (Scheme 35).237 The phenyl derivative 258T similarly afforded the azide 258A at 200 °C, and 1-cyano-2-phenylindazole 262 was formed in 63% yield at 380 °C. This compound was also formed as the end product of photolysis, where all the intermediates 259− 264 were observed in matrix photolysis reactions (Scheme 36).237 4.7.12. 2-Pyrazinylnitrenes and 4-Pyrimidinylnitrenes. FVP of the tetrazolo[1,5-a]pyrazine 265 and 5,7dimethyltetrazolo[1,5-c]pyrimidine 270 affords 1-cyanoimidazoles 269 and 274 in nearly quantitative yield, so these reactions constitute useful syntheses of 1-cyanoimidazoles. The use of 15N-labeled tetrazoles, prepared from the chlorodiazines and 15N-labeled potassium azide,238 revealed the striking difference that, in the case of the pyrazine, the 15N label stays on the CN group, whereas in the case of the pyrimidine, it ends up in the ring (Scheme 37).225 The reactions were interpreted in terms of type I ring opening of the triazacycloheptatetraenes 267 and 272 to the nitrile ylides 268 and 273 (Scheme 37).239 In further studies of tetrazolopyrazines, both triazacycloheptetraenes and ring-opened nitrile ylides were observable. Thus, the chloro-substituted triazacycloheptatetraene 277a was observable at 1968 cm−1 in the matrix-IR spectrum following FVP of tertazole/azide 275T/275A at 400 °C (Scheme 38).

products, propargyl cyanide, tetolonitrile, cyanoallene, and cyanocyclopropene (Scheme 33).234 Formation of 3H-pyrazoles by sigmatropic shifts of methyl, cyano, and acyl groups are known to take place on FVP of pyrazoles.235,236 Therefore, FVP of pyrazoles leads to the same types of products as formed from 3-pyridazinylnitrenes. 4.7.11. 1-Phthalazinylnitrene and 2-Quinazolinylnitrenes. Like tetrazolo[1,5-b]pyridazine, tetrazolo[5,1-a]phthalazine 240 exists largely in the tetrazole form 240T, and only a very small amount of the azide valence tautomer 240A is generated on FVP at 450 °C (absorbing at 2121 cm−1 in Ar matrix). The calculated free energy difference between the tetrazolophthalazine and the azide is 8.6 kcal/mol, and for the pyridazines it is 5 kcal/mol at the B3LYP/6-31G* level. FVP of the tetrazolophthalazine at 550−700 °C generated the 1phthalazinylnitrene 241, observable by its UV−vis spectrum in Ar matrix. Analysis of the IR spectra indicated the operation of a phenylcarbene-type rearrangement of cyanocycloheptatetraene 244 to a mixture of cyanofulvenallenes and cyanoethynylcyclopentadienes 246−249 (Scheme 34). This was supported by calculations of the energy profile at the B3LYP/6-31G* level as well as the IR spectroscopic observation of the interconverting cyanocycloheptatetraene 244, o-cyanophenylcarbene 243, and phenylcyanocarbene 245 in the matrix photolysis (Scheme 34; see also Figure 1).44 2-Quinazolinylnitrene is isomeric with 1-phthalazinylnitrene, but there is no experimental evidence for interconversion of the two. Tetrazolo[1,5-a]quinazoline 250 exists exclusively in the tetrazole form 250T at ambient temperature, but the almost pure azide 250A is formed on sublimation at 105 °C, passing the vapor through the FVP tube at 200 °C and isolating the product in an Ar matrix. This can be ascribed to the entropy term, the calculated entropy difference being 5.74 cal mol−1 K−1 and the enthalpy difference 0.86 kcal/mol.237 FVP at 430−700 °C resulted in ring contraction to 1-cyanoindazole 255 as the main product (28% at 430 °C and 65% at 600 °C), but similarly to the case of tetrazolo[1,5-a]pyrimidines, substantial amounts of 2-aminoquinazoline 252 were also formed, 11% at 430 °C and 35% at 600 °C. Y


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Scheme 34. Phthalazinylnitrene Formation and Rearrangementa

Scheme 35

of 289 as well as an 8% yield of the indolo[2,3-b]quinoxaline 290, which corresponds to a carbazole-type cyclization of the 2quinoxalinylnitrene 288. Nitrogen elimination generates the nitrenes 282 and 288, which can undergo H-abstraction in solution to form the amines 283 and 291. Ring contraction to nitrile 289 also takes place in solution (benzene, 180 °C, sealed tube). 15N-Labeling demonstrated that the quinazoline 281* affords the nitrile 289* with the label exclusively in the imidazole ring, in analogy to the results described for tetrazolo[1,4-c]pyrimidines in Scheme 37. Matrix photolysis of both of the nitrenes at λ > 610 nm yielded the triazabenzocycloheptatetraene 285, which was clearly characterized by its IR spectrum with the strongest absorption at 2008 cm−1. The calculated energy barriers for the ring expansion of the open-shell singlet nitrenes are only 9−10 kcal/mol at the B3LYP/6-31G* level. The dimer 284 was isolated from solution thermolysis and characterized by X-ray crystallography. The fact that the indoloquinoxaline 290 is not formed from 281 implies that the two nitrenes do not interconvert under these conditions. The end product, nitrile 289, is most likely formed from the triazacycloheptatetraene 285 via the nitrile ylide 286 in both cases (type I ring opening−recyclization), although the quinoxalinylnitrene 288 may also in principle undergo direct ring contraction to 289. Calculations at the B3LYP/6-31G** level indicate that both of these processes are

a

Energies of ground states and transition state in kcal/mol at the B3LYP/6-31G* level are indicated.

Both 277a and the ylide 278a were observable following matrix photolysis. The 2-chloro-1-cyanoimidazole 279a was obtained on photolysis, but under FVP conditions (400 °C), partial isomerization to 2-chloro-4-cyanoimidazole 280a took place by means of 1,5-shifts of CN and H. This isomerization was complete at 500 °C. In the dimethyl series, 2,4-dimethyl-1cyanoimidazole 279b was obtained on both FVP (450 °C) and photolysis (Scheme 38).240 2-Pyrazinyl-, 4-pyrimidinyl-, and 2pyrimidinylnitrenes have been observed directly by ESR spectroscopy.237 The ring contractions in the tetrazolopyrazines and tetrazolo[1,5-c]pyrimidines to 1-cyanoimidazoles are so facile they can be performed in solution at 125−180 °C, or better on gas chromatography with an injector port temperature of 300 °C.233 4.7.13. 4-Quinazolinyl- and 2-Quinoxalinylnitrenes. FVP of the tetrazoloquinazoline 281T/4-azidoquinazoline 281A at 380 °C afforded the 1-cyanobenzimidazole 289 in 99% isolated yield (Scheme 39).241 Under the same conditions, the isomeric tetrazoloquinoxaline 287T/287A gave a 92% yield Z


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Scheme 39. Phenylquinazolinyl- and Quinoxalinylnitrenesa

Scheme 36

Scheme 37

a


open-shell nitrene). The barrier for the ring opening (285 → 286) is ca. 18 kcal/mol, and for nitrogen inversion and ring closure in the ylide (286 → 289), it is ca. 14 kcal/mol.241 Although the nitrile ylide 286 was not directly observable in the FVP reactions, its formation and interconvertion with the triazacycloheptatetraene 285 were demonstrated by UV−vis spectroscopy in matrix photolysis experiments.241 The parent tetrazoloquinazoline and tetrazoloquinoxaline (292T and 293T) undergo facile ring opening to the corresponding azides and yield two products, 294 and 295, in quantitative yield in a ∼45:55 ratio on preparative FVP at 450−600 °C (Schemes 40 and 41).222,241 The triazabenzocycloheptatetraene 297 and the isocyanophenylcarbodiimide 300 were observed by IR spectroscopy in the matrix photolyses (Scheme 41). By analogy with the phenyl-substituted case described above, it is assumed that both the photolysis and the

Scheme 38

Scheme 40

feasible; ring expansion (288 → 285) and ring opening (288 → 286) have almost identical barriers (∼9 kcal/mol above the S1 AA


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Scheme 41. Mechanism of Formation of 294 and 295 Calculated at the B3LYP/6-31G** Levela

Scheme 42

a


FVP of the 7-nitro-4-azidoquinazoline 302A at 400−500 °C yielded 1-cyanobenzimidazole 303 in quantitative yield. The same compound was also formed after extensive photolysis in an Ar matrix,241 but in addition, the matrix photolysis gave direct spectroscopic evidence for the triazacycloheptatetraene 304 and the Z and E ylides 305Z and 305E, as well as their interconversion at different wavelengths. Energies of ground and transition state structures at the B3LYP/6-31G** level are indicated in Scheme 42. Clearly, all the steps are energetically very facile under FVP conditions. 4.7.14. Azidotriazines. FVP reactions of azidotriazines have not been reported, but the activation energies for decomposition of 2-amino-4,6-triazido-s-triazine derivatives have been determined by thermogravimetry methods to lie in the range 24−40 kcal/mol.243 The activation energy for decomposition of 2,4,6-triazido-s-triazine in solution was determined as 34 kcal/mol, and in the solid phase it was 42 kcal/mol.244 This highly explosive compound decomposes in a high-pressure reactor at 180 °C to generate C3N4 and C3N5, and on rapid heating to 200 °C, graphitic nanoparticles and nanodiamonds are formed.245,246 Explosive decomposition in vacuum was reported to yield cyanogen [(CN)2] and N2,247 i.e., the same products as formed on FVP of cyanogen azide (NC− N3) (see section 4.6).162,163 The matrix photolysis of triazido-striazine generates the triplet, quintet, and septet mono-, di-, and trinitrenes and eventually NCN and dicyanocarbodiimide [NC−NCN−CN (C3N4)].166,248 Matrix photolysis of 2-azido-4,6-dichloroazido-s-triazine (eq 38) yields the nitrene 306, the tetraazacycloheptatetraene 307,249 the diaazo compound 308 (type I ring opening), possibly the carbene 309, and finally diisocyanodichloromethane.250,251

pyrolysis proceeded as indicated in Scheme 41, where calculations of energies of the ground and transition states are reported at the B3LYP/6-31G* level. The energies of openshell singlet nitrenes were estimated using the Cramer−Ziegler method,242 and the values of ⟨S2⟩ reported in the scheme indicate limited triplet spin contamination.241 It is noted that the quionoxalinylnitrene 299 may rearrange to the ylide 298Z on either the triplet or the singlet energy surfaces. Once formed, the nitrile ylide 298E can very easily cyclize to N-cyanobenzimidazole 295, and it can also undergo a facile 1,7-H shift to form isocyanophenylcyanamide 301, which under FVP conditions can easily overcome the ca. 30 kcal/mol isonitrile−nitrile rearrangement barrier to yield the final cyanamide 294.222,241 The formation of phenylcyanamides is only observed for the unsubstituted quinazolinyl- and quinoxalinylnitrenes because it requires the 1,7-migration of a H atom in the nitrile ylide intermediate. When this is not possible due to substitution by phenyl (Schemes 39 and 42) or chlorine (eq 37), cyanamides and/or acyclic carbodiimides are not formed or at best may be formed in very minor amounts.241

Note that the direct isomerization of the NH-carbodiimide 300 to cyanamide 301 (Scheme 41) is a “forbidden” 1,3-H shift with a very large barrier, ∼110 kcal/mol, and it will not occur in the gas phase. It can, and will, however, take place by intermolecular H-transfer in the solid or liquid state during workup or be catalyzed by wall collisions in the pyrolysis tube. AB


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reaction and characterized by the zfs parameters D = 1.623 cm−1 and E = 0.031 cm−1. In the pyrolysis reaction FCON was formed in about 50% yield together with the Curtius rearrangement product, F−NCO. The calculated activation barrier for the formation of the nitrene FCON is ∼7 kcal/mol lower than that of the concerted Curtius rearrangement (eq 40).262

4.8. Acyl Azides

FVP of the carbonyl bis-azide N3−CO−N3 at 400 °C afforded diazirinone 311 (eq 41) as a yellow solid, identified by

4.8.1. Acyl Azides and Acylnitrenes. The Curtius rearrangement, viz., the thermal rearrangement of acyl azides to isocyanates (eq 5), is a highly useful reaction synthetically,64 which can be performed both thermally and photochemically, but since the thermal reaction takes place at temperatures around 100 °C, traditionally in benzene solution, it is not usually performed by FVP. A synthesis of acyl azides and their Curtius rearrangements in a microflow reactor has been described.252 Like the analogous Wolff rearrangement of diazo-carbonyl compounds to ketenes,67 the Curtius rearrangement may be either concerted or stepwise via acylnitrenes. The consensus is that the thermal reaction is concerted, but the photochemical reaction may involve acylnitrenes, which have been observed directly.253 Experiments and calculations support the notion that acylnitrenes 310 have closed-shell singlet ground states due an interaction between the nitrene-N and the acyl oxygen, for which reason they can be described as hybrids between nitrenes (310a) and ozazirines (310b) (eq 39).

its IR spectrum. The compound is surprisingly stable with a half-life of over 1 h in the gas-phase at room temperature, decomposing to N2 and CO.263 It is remarkable that this molecule was successfully characterized after it had been pronounced “dead”, and that of all methods, it was FVP that succeeded. 4.8.2. Carbamoyl Azides R2N−CO−N3. Thermolysis of carbamoyl azides either in the gas phase or in solution generates highly reactive aminoisocyanates (R2N−NCO), which usually dimerize to cyclic aminimides (eq 42),264−266thereby preventing the isolation or even direct

observation of the isocyanates. However, FVP of dimethyl- and diphenylcarbamoyl azides in the temperature range 300−600 °C affords the aminoisocyanates, which are observable by IR spectroscopy when isolated at 77 K.267,268 Aminoisocyanate (H2N−NCO) is also obtained by FVP at 500 °C of methyl carbazate (NH2NH−COOCH3) and of 3,4-diaminofurazan, probably by rearrangement of an initially formed aminonitrile oxide H2N−CNO.268 The aminoisocyanates are also generated by photolysis of the carbamoyl azides in Ar matrices.267−270 Further photolysis of the matrix-isolated aminoisocyanates causes loss of CO and formation of 1,1-diazene (R2N+N−) (eq 43).267 By using ArF laser irradiation at 193 nm in 4 K matrices,

Cyanocarbonylnitrene NC−CO−N (310, R = CN) was invoked as an intermediate in the rearrangements of dimers and trimers of cyanogen-N-oxide to cyanoisocyanate (NC−CNO → NC−NCO) taking place under FVP conditions.254 Calculations at the G2(MP2,SVP) and MP2/6-31G* levels indicated that singlet cyanocarbonylnitrene possesses the hybrid structure 310 (R = CN) with a long N−O bond of 1.73 Å and that a closed-shell singlet nitrene without such N− O interaction is not an energy minimum. The triplet nitrene does exist as an energy minimum lying about 22 kcal/mol above the hybrid singlet.254 Further calculations at the B3LYP/ 6-31G* level also suggested the hybrid structure for 310 (R = Ph and 2-naphthyl)255−258 and at the CCSD(T)/cc-pVDZ and CBS-QB3 levels for 310 (R = CH3).259 A similar cyclic structure was calculated for singlet formylnitrene (310, R = H) lying slightly below the open-chain triplet nitrene at the G2 level.260 In earlier calculations at the STO-3G and 4-31G levels, formylnitrene and oxazirine were regarded as separate (but highly unstable) species, and a structure similar to 310 was described as a transition state.261 FVP of fluorocarbonyl azide F−CON3 at 700 °C with isolation of the product in Ar matrix allowed the isolation and IR-spectroscopic characterization of triplet fluorocarbonylnitrene (FCON), a rare example of a thermally generated, observable carbonylnitrene. The ground state is a triplet, as determined by ESR spectroscopy in a matrix photolysis

the triplet carbamoyl- and dimethylcarbamoylnitrenes (R2N− CO−N, 311-T0) were observed directly and cleanly (eq 43).270 Unlike other acylnitrenes, the carbamoylnitrenes have triplet ground states (T0), with a calculated NCO angle of ∼120°, but the closed-shell singlet excited states (311-S1) have calculated structures similar to 310 with an NCO angle of ∼95°. The AC


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FVP of N-methyoxycarbonyl-O-methylhydroxylamine at 300 °C/10−5 hPa, but only a trace of the isocyanate was obtained on FVP of methoxycarbonyl azide; instead, decomposition products CO2, formaldehyde, HNCO, and methanimine (CH2NH) were formed.268 The NCO stretching vibration of the isocyanate is at 2209 (Ne) or 2204 (Ar) cm−1 in the IR spectrum. FVP of methoxy- and ethoxycarbonyl azides with photoelectron and IR spectroscopic analysis of the products did not reveal the formation of isocyanates but instead HNCO, methanimine (CH2NH) or ethanimine (CH3CH NH), and formaldehyde or acetaldehyde, among other products. The putative four-and five-membered rings, 1,3oxazetidin-2-one and oxazolidin-2-one, formed by cyclization of the putative nitrenes were proposed as reaction intermediates but not identified.273 In view of Meth-Cohn’s reactions described above (eqs 43 and 44 and Scheme 43), such cyclizations are to be expected, and they offer a simple explanation of the formation of CO2, HNCO, RCHNH, and RCHO. The potential formation of alkoxy isocyanates was, however, not considered at all. The calculated activation barrier for the rearrangement of methoxycarbonylnitrene to methoxyisocyanate is 13−14 kcal/mol at the CCSD(T) and CBS-QB3 levels,268 so the isocyanate formation certainly needs to be considered in FVP reactions. If formed, retro-ene decomposition of alkoxycarbonyl isocyanates to aldehydes and HOCN can take place with a calculated activation energy of ∼39 kcal/mol, which would be easily accessible under FVP conditions.274 Other FVP experiments have demonstrated that cyanic acid (HOCN) isomerizes very easily to the lower-energy isocyanic acid (HNCO) during collisions with the hot wall of the quartz tube. Like the carbamoylnitrenes, but unlike the carbonylnitrenes, alkoxycarbonylnitrenes possess triplet ground states,259 which makes them observable by ESR spectroscopy. Thus, ethoxycarbonylnitrene has D = 1.603 cm−1 and E = 0.0215 cm−1.275 Triplet 4-acetylbenzoylnitrene has also been observed (D = 1.65 cm−1, E = 0.024 cm−1).276 The high D values close to those of alkylnitrenes (see Figure 2) indicate little delocalization of the nitrene electron spins, but the high E values nevertheless indicate a degree of deviation from cylindrical symmetry, presumably due to minor delocalization of an electron onto the O atom and/or a contribution from spin− orbit coupling. Alkoxycarbonylnitrenes undergo the normal nitrene additions and insertions in solution when generated either photolytically or thermally, and either singlet (concerted) or triplet (stepwise) reaction is obtained, depending on the conditions.277 The fact that the ground state is the triplet makes elevated-temperature gas-phase (e.g., solution spray) pyrolysis important as a means to enable reactions of the singlet nitrenes before they undergo intersystem crossing to the triplets.

higher-lying open shell singlets (311-S2) again have normal nitrene structures with an NCO angle of 118°.270 Since the ground states are triplets, they are observable by ESR spectroscopy. The formation of benzimidazol-2-ones in the photolyses of diarylcarbamoyl azides is ascribed to cyclization of the singlet carbamoylnitrenes.271 4.8.3. Alkoxycarbonyl Azides. Meth-Cohn has shown that intramolecular insertion of alkoxycarbonylnitrenes into C− H bonds can be effected on a preparative scale using the solution spray pyrolysis technique.33 As in the case of arylnitrenes168 (eq 20, section 4.7.1), the cyclization of 312 to 313 is stereospecific, and formation of five-membered rings is strongly preferred over six-membered ones such as 314 (eq 44).33

Under similar conditions, the addition of benzyloxycarbonylnitrenes to benzene rings leads to azepines, and this was exploited in a synthesis of 4-azaazulene 316 from azide 315 (eq 45). Here, the best yields were obtained by packing the pyrolysis tube with a mixture of copper turnings and CaO, which facilitated the elimination of HBr and CO2.33

Solution spray pyrolysis of aryloxycarbonyl azides (e.g., 317) was interpreted in terms of cyclization of the nitrene 318 onto the benzene ring to generate an intermediate (319), which under the mildest conditions (250 °C) isomerized to the isocyanate 320 (Scheme 43). At 300 °C compound 321 was obtained, which indicates the reversibility of the first steps. At 500 °C, compound 322 was obtained, probably by elimination of propene from 319 (Scheme 43).33 Matrix photolysis of alkoxycarbonyl azides affords alkoxy isocyanates.269,272 Methoxy isocyanate was also obtained by Scheme 43

4.9. Imidoyl Azides [RC(NR′)−N3] and Imidoylnitrenes [R(CNR′)−N]

The formation of imidoyl azides and imidoylnitrenes from tetrazoles will be described in this section. The chemistry of tetrazoles involving nitrile imines will be described in detail in section 6. The occurrence of tetrazoloazine−azidoazine equilibria was mentioned in several places in section 4.7. Most imidoyl azides exist in valence-tautomeric equilibrium with the corresponding tetrazoles or completely in the tetrazole form.278 However, the existence of a tetrazole−imidoyl azide (azidoazomethine imine) equilibrium is demonstrated by the AD


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sigmatropic shift of the ester group (Scheme 45).287 The relatively low yields of this FVP reaction was ascribed to the

occurrence of a Dimroth-type tetrazole−tetrazole equilibrium, often taking place in the melt or in solution at about 200 °C in 5-aminotetrazole derivatives (eq 46).279

Scheme 45

Only when highly electron-withdrawing substituents (R′ = CN or SO2CF3) are available on the imine nitrogen do these compounds exists predominately as azides at ambient temperature (Figure 9).280 In these cases, thermolysis at about 80 °C

low volatility of the tetrazoles. It would be interesting to try to overcome this problem by using the falling-solid flash vacuum pyrolysis methodology.35 Evidence for the formation of carbodiimides 334 as intermediates en route to compounds 335 in microwave-assisted solution thermolyses was also adduced.287,288 The imidoyl azide valence tautomers of the tetrazoles are not often observed in this chemistry, but they were observed in the reactions described in Scheme 46. The polymethylenetetrazoles

Figure 9. Imidoyl azides existing in the azide form.

or photolysis at 300 nm appears to generate imidoylnitrenes, which can be trapped, e.g., with cyclohexene to form aziridines,281 although prior addition of the azide to the double bond to form a triazoline has not been excluded. Stronger evidence for the involvement of imidoylnitrenes was given by the formation of azepines and aniline derivatives in reactions with benzenes.282,283 Notably, N-alkoxy- or N-aryloxycarbodiimide formation was not reported in any of these studies. Numerous thermal and photochemical reactions of tetrazoles involving imidoylnitrene rearrangements have been reviewed previously.9,16,17 Here, it is just mentioned that FVP of 2,5disubstituted tetrazoles 323 affords nitrile imines 324, which rearrange to carbodiimides 327 and/or cyclize to benzimidazoles 328 (when R = aryl) via putative imidoylnitrenes 326 and 1H-diazirines 325 (Scheme 44).284,285 Both 1H-diazirines and

Scheme 46

Scheme 44 have revealed very interesting pyrolysis chemistry. On FVP at 300 °C trimethylenetetrazole 336T undergoes ring opening to the azide 336A, which in this case is observable at 2130 cm−1 in the 77 K IR spectrum. The azide reverts to the tetrazole on warming to room temperature. At 500−800 °C it eliminates N2 and ethylene to form N-cyanomethanimine (CH2N−CN 338).289,290 It is assumed the imidoylnitrene 337 is an intermediate. The ditetrazolopyrazine derivative 339T similarly underwent ring opening to an azide 339A and, at higher temperatures, cleavage to form the same product, CH2N− CN (338).289 Interestingly, CH2NCN was also formed on FVP of 3azido-1,2,4-triazole (eq 47).291

nitrile imines, but not imidoylnitrenes, have been observed directly in photolysis reactions,286 and nitrile imines and carbodiimides have been observed in the FVP reaction285 (see also section 6). 1,5-Disubstituted tetrazoles 329 also afford the carbodiimides, always in high yields, on pyrolysis as well as photolysis (Scheme 44). For example, FVP of 1-(2-thienyl)tetrazoles 330 at 600 °C resulted in the formation of thienoimidazoles 333 ascribed to cyclization of transient imidoylnitrenes 331 followed by a 1,5-

The tetra- and pentamethylenetetrazoles underwent FVP to form the alkenylcyanamides initially, and these cyclized to Ncyanopyrrolidines by exo-trig addition of the cyanamide AE


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function to the CC double bond (eqs 48 and 49). The possible involvement of the azide valence isomers was not

Scheme 47

investigated in detail. The reactions are assumed to proceed via ring opening of the cyclic imidoylnitrenes to diradicals followed by hydrogen shifts.290FVP of the annelated polymethylenetetrazoles 340−343292−296 yielded cyclic carbodiimides 344−347 (eqs 50−53),297 which are likely formed by rearrangement of

Scheme 48

Imidoylnitrenes have triplet ground states and therefore should be observable by ESR spectroscopy. Weak, broad UV− vis absorptions around 500 nm may be due to triplet Cphenylimidoylnitrenes.298 The rearrangement reactions described above are assumed to take place in the singlet nitrenes. Calculations indicate a singlet−triplet gap on the order of 8−9 kcal/mol and low barriers for the rearrangements of the singlet nitrenes.284,299

intermediate imidoylnitrens in the same way that 1,5disubstituted tetrazoles yield open-chain carbodiimides (cf. Scheme 44).The corresponding unsaturated azocine and azonine derivatives 348 and 354 afforded cyclization products 353 and 359 via the intermediate, observable carbodiimides 350 and 356 (Scheme 47). There was spectroscopic evidence for the additional formation of the cyanamide 351, but not 357.297 A cyanamide 364 was, however, isolated together with the ring-opened product 365 following FVP of the dibenzotetrazoloazocine 360 (Scheme 48).292 It is likely that these products are formed by ring opening of either the nitrene 361 or the carbodiimide 362 to a diradical 363 (a type II ring opening; Scheme 48) analogous to the reactions of the tetraand pentamethylenetetrazoles described above and 2-pyridylnitrenes described in section 4.7.297

4.10. Boryl Azides

Early work on the thermolysis and photolysis of azides of maingroup elements has been reviewed by Bertrand et al.300 Boryl azides undergo thermal and photochemical elimination of nitrogen and a 1,2-shift of a substituent to generate iminoboranes, i.e., Ph2B−N3 → Ph−BN−Ph. These are in most cases unstable and dimerize or trimerize under the reaction conditions, yielding diboradiazetidines and borazines. These reactions have usually been performed in solution at temperatures around 200 °C.301 While many reactions may be concerted, borylnitrenes have been trapped successfully, as shown in eq 54.302 AF


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concluded that the imine Me2SiNMe was not observed but suggested that ionizations at 9.3 and 10.9 eV correspond to Me2Si(H)−NCH2, in agreement with the suggestion by Ogilvie and Perutz. It was further concluded that fragmentation occurred with formation of silaisocyanic acid (HNSi) and that the analogous germaisocyanic acid (HNGe) was obtained from trimethylgermyl azide.313 In their study of bicyclic compounds containing twisted Si N bonds, Michl and co-workers.314 expressed support for the standard belief, quoting Nguyen et al.,315 that the loss of N2 from silyl azides is concerted with the rearrangement to silanimines and that a singlet nitrene does not appear as a distinct intermediate. Triplet silylnitrenes are, however, formed on photolysis of silyl azides. Our own investigation showed that FVP of trimethylsilyl azide at 500 °C with isolation of the products in an Ar matrix at 10 K did not result in a nitrene observable by ESR spectroscopy, but UV irradiation of the matrix at 254 nm resulted in the nitrene Me3Si−N with a transition at 8179.2 G in the ESR spectrum and the zero-field splitting paramaters D/ hc = 1.54 cm−1 and E/hc = 0.00 cm−1 (H0 = 3373.4 G).316 This species was also obtained by microwave discharges in Ar or N2 (D/hc = 1.57 cm−1) but was reportedly not observed under UV irradiation due to rapid destruction.317 γ-Irradiation of trimethylsilyl azide at 77 K using a 60Co source also produced the trimethylsilylnitrene (D/hc = 1.54 cm−1, E/hc = 0.0035 cm−1), and several other methyl(phenyl)silyl azides as well as PhSiMe(N3)2 and PhSi(N3)3 also yielded ESR spectra with D/ hc values close to 1.5 under these conditions and also when the azides were irradiated in solid solution at 4 K (λ > 350 nm).318 The 14N hyperfine splitting was observable under the γirradiation conditions, but not when the azides were photolyzed at 4 K in solid solution. The silyl nitrenes were thought to abstract H from solvent, even at 40 K. Flow pyrolysis of phenyltriazidosilane resulted in loss of four N2 and formation of phenyl silaisocyanide (PhNSi), with an Si−N triple bond (eq 59).The compound was characterized by

Bettinger, Bornemann, and their co-workers generated the triplet catechol-derived borylnitrene (eq 55) by matrix photolysis and reported its IR, UV−vis, and ESR spectra, as well as its reactions with N2, CO, and O2303 and its insertion into H2.304 The corresponding tetramethylglycol-derived borylnitrene and its insertion into R−H bonds were also reported.305 As an aside, it is interesting to note that azaborine 368the BN analog of benzynehas been generated by FVP of compound 366 at 800−850 °C in the presence of N2 (eq 56).The azaborine liberated in the FVP process reacts with N2

to form the BN3 compound 367, which, isolated in an Ar matrix, can be photochemically interconverted with the free azaborine 368.306 4.11. Silyl Azides

Pyrolysis of triphenylsilyl azide in a vacuum system at 680 °C yields a cyclic dimer [Ph2SiNPh]2 and a linear polymer, suggesting the intermediacy of Ph2SiN−Ph.307 Further evidence for the formation of silaimines R2SiNR was obtained by pyrolysis of trialkyl- and triphenylsilyl azides in a nitrogen flow system at 610 °C.308 The imines underwent Wittig-type reactions with aldehyde and ketones and were also trapped with (Me2SiO)3 and (Me2SiNMe)2 in copyrolysis reactions to form eight-membered ring systems in moderate yields (eq 57).

photoelectron spectroscopy in the gas phase319 and by IR and UV spectroscopy in an Ar matrix.309,320 The presumed silanitrile intermediate PhSiN was not detectable. However, Maier and Glatthaar found that irradiation of silyl azide in Ar matrix with an ArF laser (193 nm) resulted in the formation of silacyanic acid (HSiN), whereas irradiation with a KrCl laser at 222 nm afforded silaisocyanic acid (HNSi), and the two compounds were interconvertible at these wavelengths.321

Matrix photolysis experiments have clearly revealed the formation of silanimines of the type R2SiNR′ and RSi(H) NR′ from silyl azides,309 and dimerization and trimerization of silanimines are commonly observed (e.g., eq 58).310

4.12. Phosphorus Azides

Reichle carried out vacuum pyrolysis of diphenylphosphonyl azide (Ph2P(O)N3) at 680 °C but obtained only polymeric material, presumably formed from the unstable phosphorus imine [PhP(O)N−Ph].307 Difluorophosphonylnitrene [F2P(O)N] has been generated by FVP of the azide F2P(O)N3, which exists as a 90:10 mixture of syn and anti conformers in the gas phase. The calculated barriers for decomposition of the syn and anti azides to the nitrene are 40.6 and 48.8 kcal/mol, respectively, which are significantly higher than those usually encountered for azides in the range 30−40 kcal/mol. The triplet ground state nitrene was observed by matrix IR spectroscopy (eq 60).322FPO and F3PO were also formed,

The rearrangement of a silanimine to a silylimine in the photolysis of trimethylsilyl azide was postulated by Ogilvie: Me2SiNCH3 → Me2Si(H)−NCH2.311 Perutz also concluded that the matrix photolysis of trimethylsilyl azide did not result in compounds containing SiN bonds but instead C N and Si−H bonds, possibly due to Me2Si(H)−NCH2 or, less likely, Me2Si(H)−CHNH.312 In their study of the FVP of timethylsilyl azide at 830 °C/10−2 hPa with photoelectron spectroscopic monitoring, Guimon and Pfister-Guillouzo AG


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but a Curtius-type rearrangement to FP(O)N−F was not observed. The calculated singlet−triplet splitting is ∼20 kcal/ mol, and the barrier to Curtius-type rearrangement is 15−30 kcal/mol at B3LYP and CBS-QB3 computational levels.322 A stable, crystalline, sterically hindered phosphinonitrene of the type (R2CN)2P−N has been prepared by photolysis of the bis(imidazolidin-2-iminato)azidophosphine at room temperature (eq 61).323

4.13. Sulfonyl Azides

Gas-phase pyrolysis of benzenesulfonyl azide affords azobenzene as a product in 17.5% yield.307 This suggests a rearrangement of benzenesulfonylnitrene to the sulfonylamine PhNSO2, followed by loss of SO2 and dimerization of the phenylnitrene so formed (eq 62). Photolysis in methanol solution permitted the trapping of the sulfonylamine as PhNHSO2OCH3.324

FVP of the dichloro derivative 374 at 400 °C (0.5 hPa, N2 flow) afforded the sultam 375, where one chlorine has been lost, in 34% yield, whereas thermolysis in Freon-113 at 185 °C resulted in compound 376, in which a chlorine shift has taken place (eq 69).329 The formation of pyridine derivatives in some of these reactions suggests that the sulfonylnitrenes may also add to the benzene rings with formation of azepine intermediates.

The gas-phase pyrolysis of o-tolylsulfonyl azide at 360 °C (0.05−0.2 hPa) in a stream of N2 or benzene resulted in insertion of the nitrene into the methyl group (eq 63) in up to 21% yield, and no azotoluene was formed.325 Fluorosulfonyl nitrene (FSO2N) in its triplet ground state has been produced by FVP of FSO2N3 at 800 °C in a 1.0 mm i.d., 30 mm long quartz pyrolysis tube and isolated in an Ar matrix, which permitted its complete characterization by IR and UV−vis spectroscopy.330 This nitrene is also produced by matrix photolysis.331 On irradiation at 266 nm, the matrixisolated nitrene FSO2N undergoes two parallel rearrangements to fluorosulfenyl nitrite FSO−NO and the sulfonylamine FN SO2, thus providing definitive evidence for a Curtius-type rearrangement. Furthermore, in the thermal reaction, the fluorosulfonyl radical FSO2 was formed and characterized as well.330 Similar FVP of trifluoromethylsulfoyl azide (CF3SO2N3) at 800 °C afforded the radical SO2N, whose matrix photochemistry was investigated.332 This radical was also obtained on FVP of the methyl analogue, CH3SO2N3, albeit in lower yield, and in this case methanimine (CH2NH) and SO2 were the

Several studies by Abramovitch et al. demonstrated cyclization of arylsulfonylnitrenes generated thermally in solution (eqs 64 and 65).326 FVP of β-phenethylsulfonyl azides at ∼300 °C (0.1−4 hPa) again resulted in nitrene cyclization onto the phenyl ring to yield the sultam 369 in up to 19% yield together with a variety of rearrangement products (eq 66).327In further work, sevenmembered ring sultams 371 were obtained in yields of 23−27% by FVP of aralkylsulfonyl azides 370 (eq 67).328 The mesityl derivative 372 afforded an 88% yield of the sultam 373, where a methyl group shift has occurred (eq 68). AH


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isoindoles in good yields (eq 73, where the iminocarbene may also be formulated as a 1,3-diradical or a nitrile ylide).347

major products. This suggests decomposition of CH3SO2N to CH3NSO2 and then CH3N + SO2 (cf. methylnitrene, section 4.2.) similar to the proposed decomposition of PhSO2N described in eq 62. ArF laser (193 nm) photolysis of methanesulfonyl azide (CH3SO2N3) in solid noble-gas matrices yielded the nitrene CH3SO2N in the triplet ground state. Subsequent UV photolysis at 266 nm caused rearrangement of the nitrene to the sulfonylamine CH3NSO2.333 Methoxysulfonylnitrene (CH3O−SO2−N) was similarly generated and observed upon matrix photolysis of the corresponding azide. This nitrene was probably also formed on FVP of the azide, but it underwent a common pyrolytic elimination of CH2O with formation of the simplest sulfonylamine, HNSO2.334

Hajos, Yranzo, and their co-workers investigated isomerizations occurring on FVP of 1,2,4-triazolo[3,4-c][1,2,4]benzotriazines 377 at 440−540 °C/0.02 hPa (eq 74).348

5. TRIAZOLES 5.1. 1,2,4-Triazoles

While the authors prefer a mechanism involving 1,2-diazirine intermediates, ring opening to nitrile imines 378 followed by cyclization to 379 is an attractive alternative. The reaction mechanism requires further investigation with direct observation of intermediates. FVP of the 4H-3-amido-1,2,4-triazole derivative 380 at 300− 450 °C/0.02 hPa afforded triazoloquinolone 382, which may be due to consecutive 1,5-sigmatropic shifts of the β-ketoester group followed by thermal elimination of ethanol and cyclization of the ketene 381 so formed (Scheme 49).349

A 1,5-acyl group shift in the FVP of 1-acyl-1,2,4-triazoles at 800 °C/10−2 hPa yields oxazoles in 25−96% after elimination of N2 and recyclization (eq 70).335 An analogous 1,3-acyl shift in

pyrazoles yields furans as principal products.335−339 The intermediates can be formulated as diradicals, or better as nitrile ylides, where delocalization over the carbonyl group makes them 1,5-dipoles (eq 70).However, when an α-hydrogen atom is available on the acyl moiety, FVP of 1-acyl-1,2,4triazoles constitutes a valuable synthesis of cumulenes [e.g., ketenes, ketenimines, isocyanates, and isothiocyanates, including amino- and alkoxyiso(thio)cyanates] (eq 71). Similar reactions can also be achieved with pyrazoles and imidazoles.340−346

Scheme 49

Analogous reactions of a 2-amidothiazole derivative were also reported.349 5.2. 1,2,3-Triazoles

FVP of 1,2,4-triazole at 850 °C yielded the parent nitrile ylide [HC(−)N(+)CH2] together with ketenimine and methyl isocyanide.132 Product formation was interpreted in terms of decomposition of the 1H- and 4H-1,2,4-triazoles, but it is very likely that the nonaromatic 3H-isomer may also be involved, especially in the FVP reactions (eq 72).

The valence isomerization between vinyl azide and 4H-1,2,3triazole was mentioned in section 4.5. An analogous valence isomerization of 1H-1,2,3-triazole to diazoimines is commonly observed under FVP conditions, where the diazoimines eliminate N2 to form iminocarbenes. The relationship between vinyl azides and diazoimines (eq 75) has not been well

investigated. The 4H- and 1H-1,2,3-triazoles are formally related through 1,5-H shifts, but being aromatic, the 1Htriazole is far more stable. In reality, both vinyl azides and 1H1,2,3-triazoles give rise to azirines, but by different mechanisms, as described below.

In fact, 1,5-phenyl shifts leading to population of the 3Hisomers have been shown to occur in the FVP of triphenyl1,2,4-triazoles. Ring opening, loss of N2, and cyclization afford AI


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1,2,3-Triazoles with highly electron-withdrawing substituents in the 1-position (CN, NO2, and SO2R) undergo ring opening to the diazoimines at ordinary temperatures or exist in equilibrium with these valence tautomers.350−355 In investigations of triazole chemistry, one needs to be aware of the possible occurrence of a type I Dimroth rearrangement, i.e., the ring opening and recyclization of 5-amino-1,2,3-triazole derivatives via diazoimines (eq 76).356,357

Scheme 50

1,2,3-Triazole exists almost exclusively in the 2H-form in the gas phase at room temperature, and this is therefore the form isolated in low-temperature matrices.358 However, under FVP conditions the 1H-form and even the nonaromatic 4H-form may also be populated. Initial studies of the gas-phase pyrolysis of 1,2,3-triazole at 600 °C yielded only acetonitrile,117,359 but pyrolysis over CuO at 240 °C allows the detection of 1azirine133 which, as described in section 4.5, is also a pyrolysis product of vinyl azide.122,132,133 At higher temperatures, the azirine isomerizes to acetonitrile. However, using FVP at 800 °C with matrix isolation of the products at 10 K, Maier and coworkers detected small amounts of ketenimine (CH2C NH) and nitrile ylide (CH2N+C−H) (eq 77).132,358

Scheme 51

These products were also obtained, in better yields, by FVP of 1,2,4-triazole at 850 °C, as described in eq 72 above. Matrix photolysis of 1,2,3-triazole yielded mainly the nitrile ylide. The reactions of iminocarbenes formed by FVP (and also by photolysis360) of 1H-1,2,3-triazoles have been investigated in much detail by Rees and co-workers.361 Pyrolysis of 1-methyl4,5-diphenyltriazole 383 at 600 °C/0.01 hPa yielded diphenylpropionitrile [MeC(Ph)2CN, 389] (50−60%), which is most likely formed in a free-radical rearrangement of the initial product of a Wolff-type rearrangement of the iminocarbene 384, viz., N-methyldiphenylketenimine (MeN CCPh2, 387). In addition, formation of 3-phenylisoquinoline 386 (19%) was ascribed to isomerization of the carbene to 3,4diphenyl-2-azabutadiene 385 (a 1,4-H shift) followed by sixelectron cyclization onto a phenyl group and oxidation (Scheme 50). The formation of tetraphenylsuccinonitrile 390 is good evidence for the intermediacy of free radicals 388. When different substituents are present in positions 4 and 5 of the triazole, the pyrolysis products reveal the intermediacy of a 1H-azirine 392 as a link between the two isomeric iminocarbenes: 1,4-dimethyl-5-phenyltriazole 391 and 1,5dimethyl-4-phenyltriazole 393 afforded the same product, 3methylisoquinoline 394 (Scheme 51). Similarly, the pyrolysis of 1-vinyl-1,2,3-triazoles 395 afforded pyrroles, e.g., as shown in Scheme 52. The N-unsubstituted 4,5-diphenyl-1,2,3-triazole 396 gave rise to diphenylacetonitrile and 2-phenylindole 401. The likely mechanism involves cyclization of the iminocarbene to 1Hazirine 397, isomerization to the lower-energy 2H-azirine 398, ring opening to the vinylnitrene 399, and cyclization to the

Scheme 52

7aH-indole 400 (Scheme 53), i.e., entry on an energy surface analogous to the one described for β-styryl azide in section 4.5.2. One may enquire whethere the 1H-diazirines invoked in Schemes 51−53 are true intermedites or transistion states. If substantiated, the rearrangement 397 → 398 suggests they are true intermediates. Recent calculations at the CASPT2 and B3LYP levels in the author’s research group also demonstrate that 1H-azirines exist as true intermediates, albeit in shallow energy minima. It is also worth noting that 1H-diazirines have been observed spectroscopically (see section 6.3.). Further work on the FVP of 13C-labeled 1,4- and 1,5diphenyltriazoles 402 and 403 revealed that the iminocarbene 405 derived from the 1,5-isomer underwent isomerization to the imino(phenyl)carbene 404 via the 1,2-diphenyl-1H-azirine 406 prior to cyclization to 2- and 3-phenylindoles 407 and 408 AJ


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Scheme 53

Scheme 55

and Wolff-type rearrangement to 1,3-diphenylketenimines 409 and 410 (Scheme 54).362

Sila- and germadihydro-1,2,3-triazoles decompose at or near room temperature with formation of dimers of silanimines and germanimines, Me2E = NR (E = Si or Ge).370 5.3. Benzotriazoles and Triazolopyridines

Scheme 54

5.3.1. N-Unsubstituted Benzotriazoles and Triazolopyridines. The flash vacuum pyrolysis of benzotriazole 418 is a convenient method for the preparation of cyanocyclopentadiene (cpCN) 421 in virtually quantitative yield (eq 79). This

The ability of 1,2,3-triazoles to form ketenimines on pyrolysis has been exploited in a synthesis of quinolones from the 1-aryl4-alkoxycarbonyl derivatives 411. The iminocarbene 412 formed on FVP at ∼700 °C can either cyclize to form indoles 413 or undergo a 1,2-H shift to ketenimine 414. The very facile thermal interconversion of α-oxoketenimines and α-imidoylketenes363 by means of a 1,3-shift of the MeO group now generates the ketene 415, which cyclizes to quinolone 416 in yields up to 93% (Scheme 55) The ketenimines and ketenes can be observed directly by low-temperature IR spectroscopy.364,365 The pyrolytic fragmentation of 4-oximinoisoxazol-5(4H)ones 417a and 4-oximino-1,2,3-triazol-5(4H)-ones 417b constitutes a convenient and high-yielding synthesis of fulminic acid (eq 78).366−369

compound exists largely as the fully conjugated 1-cyano isomer, although minor amounts of the 2-cyano isomer are detectable by NMR spectroscopy. Similarly, substituted cyanocyclopentadienes 424−426 are obtained on FVP of benzotriazoles 422 and 427 at 500−600 °C (eqs 80 and 81).208 The unisomerized methylcyanocyclopentadienes 424b and 426b are obtained from the corresponding 5- and 4methylbenzotriazoles 422b and 427b under mild conditions (500 °C) and with a low degree of conversion (a small amount of the cross-conjugated tautomer 425b accompanies 424b), but interconversion of the nitriles takes place on FVP at 600−700 °C and on gas chromatography.208 The pure, unisomerized 2- and 3-cyanopyrroles 430 and 432 can be obtained in nearly quantitative yields by FVP of triazolopyridines 429 and 431, respectively, at 600 °C (eqs 82 and 83).208 Likewise, naphtho[4,5-b]- and naphtho[4,5-a]triazoles 433 and 435 yield the respective cyanoindenes 434 and 436 on FVP at 500−600 °C (eqs 84 and 85), but interconversion of these nitriles does take place on FVT at 700−1000 °C due to thermally activated substituent migration, and at 1000 °C, CN group migration to the benzene ring takes place as well.208,371 As described under 1,2,3-triazoles, benzotriazoles carrying strongly electron-withdrawing substituents in the 1-position undergo easy thermal ring opening.350−355 Furthermore, 1AK


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Scheme 56

Here, too, both DFT170 and recent CASPT2 calculations in the author’s research group have confirmed benzazirine 439 as a true intermediate. A similar labeling experiment with 1Hbenzotriazole would be pointless due to its tautomeric nature. Thus, the complete reaction mechanism can now be represented as in Scheme 57. The intermediate 419 may be formulated as a diradical, as the carbene 419′ (Scheme 57), or as a resonance hybrid of the two with a strong preference for the diradical structure. It should be noted, however, that this resonance is valid for the open-shell singlet and the triplet, but the closed-shell singlet, in which two electrons are housed in a sigma-type orbital, is not able to partake in such mesomerism. Due to the open-shell nature of the lowest singlet state of 419, multideterminant computational methods are required for a proper description. The same is true for oxocyclohexadienylidenes.376−378 The ground state of 419 is the triplet diradical. The triplet 419 as well as the triplet N-methylcyclohexadienylidene and triplet N-phenyliminocyclohexadienylidene 441 formed by photolysis of 1-phenylbenzotriazole 440 have been characterized by their ESR spectra at 77−200 K.379 The spectra have the appearance of diradicals with strong half-field signals near 1500 G (419, D = 0.17 cm−1, E = 0.0025 cm−1). Both Z and E conformers of 441 were observed. The decay of the Z isomer had an activation barrier of ∼5 kcal/mol at 104−124 K. It is noteworthy that benzoxirene, i.e., the oxygen analog of 439, has not been implicated in another series of 13C-labeling experiments under FVP conditions (eq 88).376 The reason is

arylsulfonylbenzotriazoles undergo a thermal Dimroth-type interconversion, implying the intermedicy of the diazo compound (eq 86; ΔG* = 20.8 kcal/mol).372

It is likely that all the pyrolysis reactions described in eqs 79−85 also involve initial valence isomerization to diazoiminocyclohexadienes of the type 437 as the first step (eq 87).This

isomer has not actually been observed under FVP conditions, but it has been characterized in matrix photolysis experiments373−375 and absorbs in the IR at 2084 (2091) (vs) and 2065 (2072) (s) cm−1. The ring contraction of benzotriazoles to ketenimines such as 420 is highly exothermic; the initially formed ketenimine can carry ca. 50 kcal/mol excess energy in the benzotriazole case and 60 kcal/mol in the triazolopyridine case.36 The final products, cyanocyclopentadiene 421 and 3-cyanopyrrole 430, are even more highly activated, by 70 and 90 kcal/mol, respectively, and this would be more than sufficient to isomerize a cyanopyrrole or cyanocyclopentadiene. However, the effect of chemical activation under mild pyrolysis conditions is tempered because the initial products are not the nitriles but the ketenimines 420, etc., formed in a Wolff-type rearrangement (eq 87).36 The ketenimines cannot easily undergo any sigmatropic rearrangements. Therefore, the tautomerization to cyanocyclopentadienes, cyanopyrroles, etc. is likely to take place during workup and/or during collisions with the walls of the pyrolysis tube, which would help dissipate the excess energy. The ketenimine 420 has been observed directly in matrix photolysis experiments and absorbs at 2044374 or 2040 cm−1,375 in the IR spectrum. The use of 7a-13C-labeled isatin 438 (prepared from 1-13Caniline) as a precursor of the iminocarbene/1,3-diradical 419a revealed complete interconversion of the regioisomeric carbenes/diradicals 419a and 419b via 1H-benzazirine 439 prior to ring contraction to 420 and then 421 (Scheme 56).170

simply that the barrier between the oxocyclohexadienylidene diradical (open-shell singlet) and benzoxirene (∼23 kcal/mol) is significantly higher than the barrier for the Wolff rearrangement to 6-fulvenone (∼4 kcal/mol), whereas the opposite energy ordering is observed in the iminocyclohexadienylidene case. Thus, while benzoxirene formation is in principle possible, it cannot compete with the Wolff rearrangement, but benzazirene formation can compete with the hetero-Wolff rearrangement. In contrast to benzoxirene, benzothiirene is known.380 5.3.2. Graebe−Ullmann Synthesis. The Graebe−Ulmann synthesis of carbazoles 444 is carried out by pyrolysis of 1phenylbenzotriazoles 440 and substituted analogs 442 (Scheme 58).381 Originally this was performed by passing the vapors of AL


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Scheme 57

Scheme 58. Synthesis of Carbazoles from 1Arylbenzotriazoles

the triazoles through a red-hot tube at atmospheric pressure, or in sealed tubes, or by dry distillation,382,383 sometimes in admixture with CaO. Carbazoles are obtained cleanly and in excellent yields under FVP conditions. Iminocyclohexadienylidene diradicals 443 (the open-shell singlets) are likely intermediates in these reactions. By blocking the ortho-positions with methyl groups, carbazoles derived from 4aH-benzotriazoles by migration and/or loss of a methyl group as well as acridine and 1methylacridine were obtained (Scheme 59).384 The methyl group migrations are reminiscent of the CN group migrations in 4aH-cyanocarbazoles described in section 4.7.9 and Scheme 32.

FVP of 1-phenyltetrahydrobenzotriazole 445 at 590−630 °C results in cyclization to tetrahydrocarbazole 446 (17%) as well as Wolff-type rearrangement to the ketenimine 447, which was trapped with water to yield the corresponding N-phenylcyclopentanecarboxamide in 20% yield (eq 91).387

Scheme 59

5.3.3. N-Alkylbenzotriazoles. The FVP of 1-methylbenzotriazole 448 at 600 °C affords N-phenylmethanimine 449 in nearly quantitative yield. The compound trimerizes at room temperature, but the trimer is again detrimerized to the monomer on FVP (eq 92).38,385The H-shift yielding 449 is so rapid that no methylcyanocyclopentadienes 424−426 are formed in this reaction. The pyrolysis of 1-benzylbenzotriazole 451 at 400 °C in the presence of a small amount of Cu was reported by Gibson to give a low yield of phenanthridine, assumed to be formed in a Graebe−Ullmann-type cyclization.388 However, in analogy with 1-methylbenzotriazole 448, the formation of benzylidenaniline seems more likely, and this was confirmed by Khalafy and Prager.389 Indeed, Pictet and Ankersmith had previously

Cyclizations of 1-(2-pyridyl)- and 1-(1-isoquinolyl)benzotriazoles afford pyridobenzimidazoles (eqs 89 and 90).385 1-(2-Nitro-4-pyridyl)benzotriazole affords 1-nitro-γcarboline (1-nitro-3-azacarbazole), but partial loss of the nitro group also occurs, giving γ-carboline and 1-hydroxy-γ-carboline; 2-(2-nitrophenyl)benzotriazole was found to isomerize in part to 1-(2-nitrophenyl)benzotriazole.386 AM


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Scheme 61. Products of FVP of 1-Vinylbenzotriazole

pyrolyzed benzylidenaniline in a red-hot tube in a Mermet oven and obtained phenanthridine.390 This was confirmed by Storr and co-workers,391 who obtained benzylidenaniline 452 (60%) and phenanthridine 453 (11%) on FVP of 1-benzylbenzotriazole 451 at ∼750 °C, thus explaining Gibson’s reaction (eq 93).

•

5.3.4. 1- And 2-Vinylbenzotriazoles. FVP of 1-vinylbenzotriazole 454 at 700 °C/0.3 hPa yields indole 455 as the major component of a mixture of at least seven products, including 1-, 2-, and 3-phenylindoles and biphenyl, which suggests the intervention of free-radical reactions (eq 94).392

CH2CN radicals 457 (Scheme 61), which recombine to phenylacetonitriles, abstract hydrogen to form benzene and acetonitrile, and dimerize to succinonitrile and biphenyl (see also Scheme 50).144,361,395 Thus, the formation of phenylacetonitrile and biphenyl can be ascribed to homolysis of the ketenimine (Scheme 61). The same products are formed on FVP of 2-vinylbenzotriazole 459 above 650 °C, thereby suggesting the occurrence of a 1,5-sigmatropic rearrangement of 459 to 454 (eq 95).31 Pyrolytic 1,5-shifts converting 2-ethylbenzotriazole and 2-(2-nitrophenyl)benzotriazole to the corresponding 1substituted benzotriazoles have been reported.386,396

More recent FVP studies demonstrate that indole 455 and N-phenylketenimine 456 are primary products formed in concurrent, parallel reactions (Scheme 60).31,393 In contast, the Scheme 60 FVP of The 2-methylpropenyl derivative 460 afforded the ketenimine Ph−NCCMe2 461 as the major product at 650 °C, and no indole was formed. At higher temperatures, methacrylonitrile 464, 2-methylpropionitrile 463, and benzene were formed, and these compounds were also formed on pyrolysis of PhNCCMe2 461 itself (Scheme 62).31,144 Once again, the radical pair 462 is a likely intermediate FVP of N-(2-ethoxycarbonylvinyl)benzotriazole 465 at 450 °C/0.1 hPa (pyrolysis tube packed with quartz chips) similarly gave rise to the expected 3-ethoxycarbonylindole 466 and ethoxycarbonylketenimine 467, which however underwent the oxoketenimine−imidoylketene rearrangement363 to 468 followed by cyclization to 2-ethoxy-4-quinolones 469−470.397 The mechanism was supported by deuterium labeling (Scheme 63). When a phenyl substituent was added to the 1-position of the vinyl group, formation of the ketenimine and the quinolone was not possible, and only the ethyl 2-phenylindole-3-

sequential formation of N-phenylketenimine and indole in the FVP of 1-vinylbenzotriazole claimed by Katritzky, Maquestiau, and their co-workers394 has been shown to be erroneous.31 Following FVP at 900 °C, nearly all of the 1-vinylbenzotriazole was consumed, but phenylacetonitrile 458 had now replaced N-phenylketenimine. N-Phenylketenimines are known to undergo homolytic fragmentation to Ph• and AN


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Scheme 62. Formation and Pyrolysis of 1,1-Dimethyl-3phenylketenimine

An exothermic, self-sustaining decarboxylation of 1-alkoxycarbonylbenzotriazoles 474 to 1-alkylbenzotriazoles 475 occurs with vigorous evolution of CO2 on heating to 160 °C, as reported by Krollpfeiffer et al.401 Apparently it does not take place in 1-phenoxycarbonylbenzotriazole.402 Katritzky and coworkers found that 2-alkylbenzotriazoles 476 are also formed (eq 98) and suggested that the reaction is intermolecular,402 but this is hardly the case under FVP conditions. Al-Awadi et al. examined the pyrolysis of 1-ethoxycarbonylbenzotriazole 474b under static conditions at 110−200 °C, microwave irradiation, and FVP conditions (500 °C/10−2 hPa) and isolated up to six products, viz., 1- and 2-ethylbenzotriazoles 476b and 475b, 2-ethoxybenzoxazole 477, benzoxazole-2-one, tetrahydrooxazol-2-one, and biphenylene (eq 99).396 A concerted decarboxylation yielding 476b, which in a second step undergoes a 1,5-sigmatropic shift of the ethyl group to yield 475b, was proposed. Formation of 2-ethoxybenzoxazole 477 by cyclization of a carbene/1,3-diradical is analogous to the Druliner reaction (eq 97). Benzoxazole-2-one was only obtained under microwave irradiation and is most likely the result of thermal elimination of ethene from 477. FVP reactions of 1-benzoylbenzotriazole 471 and 1acetylbenzotriazole 478 yielded the N-acylketenimines 480 and the benzoxazoles 473/481 (eq 100). The N-acylketenimines 480 were characterized by low-temperature IR spectroscopy and online mass spectrometry.403

Scheme 63

carboxylate was formed in quantitative yield. Comparison with the FVP reactions of N-alkenylisoxazolones was made.397 The photolysis of 1-vinyl- and 1-cycloalkenylbenzotiazoles is an excellent method of preparation of indoles and tetrahydrocarbazoles.398,399 5.3.5. 1-Allylbenzotriazoles. 1-Allylbenzotriazoles being homologues of 1-vinylbenzotriazoles afford quinolines by cyclization of the diradical intermediates on FVP at 750 °C/ 10−2 hPa (eq 96).393

5.3.6. 1-Acylbenzotriazoles. Druliner reported the formation of 2-phenylbenzoxazole 473 in modest yield (11% in 6 h) following heating of 1-benzoylbenzotriazole 471 in diphenyl ether at 250 °C,400 presumably taking place via the carbene/diradical 472 (eq 97). Wiersum confirmed this reaction under FVP conditions.21 No yield was given, but it was mentioned that polymeric material was also formed.

Once again, C−N bond homolysis of 480 (R = CH3 or Ph) takes place on FVP in the higher-temperature regime and results in formation of the methyl- and phenylcyanocyclopentadines as well as the unsubstituted cyanocyclopentadiene AO


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421.403 The additional formation of biphenyl from 471 is a clear indication of free-radical cleavage. Similarly, FVP of 1alkoxycarbonylbenzotriazoles 474 yielded 2-alkoxybenzoxazoles 481 and 477, methyl- and ethylcyanocyclopentadienes 424− 426 and 484, and the unsubstituted cyanocyclopentadiene 421 (Scheme 64).

both on electron-impact-induced fragmentation of 1-benzoylbenzotriazoles.406,407 The group of Moyano has employed the technique of catalytic flash vacuum pyrolysis over mixed oxides in a one-pot synthesis of 7H-dibenzo[b,d]azepin-7-one from phenacyl benzotriazole (eq 103).408,409 The reaction can be considered a homologous version of the Graebe−Ullmann carbazole synthesis.

Scheme 64

The pyrolysis of 1,2,3-thiadiazoles usually results in the formation of thioketenes due to loss of N2, formation of thioacylcarbenes/1,3-diradicals and their cyclization to thiirenes, and Wolff-type rearrangements.67,410 An interesting variation is the pyrolysis of the 5-(1-benzotriazolyl)-1,2,3thiadiazole 493 at 200−300 °C/10−2−10−1 hPa, which led to the formation of compound 494, probably as a result of a rearrangement of the thioacylcarbene/1,3-diradical and transannular cyclization (Scheme 65).411 In other words, it is the thiadiazole ring, not the benzotriazole ring, that eliminates N2. Scheme 65 A proof that the ketenimine 480 (R = Ph) is also genereated photochemically was given by Ohashi and co-workers, who isolated compound 482 as a product of cycloaddition with acetone in the acetone-sensitized photolysis of 471 (eq 101).387,404

6. TETRAZOLES 6.1. Introduction

Al-Awadi and co-workers have examined the FVP (600 °C/ 0.2 hPa) and static pyrolyses (300 °C, 0.06 hPa, 15 min) of a further series of p-substituted 1-benzoylbenzotriazoles 485 with results in broad agreement with those reported above (eq 102).405 The main products under FVP conditions were the 2-

Tetrazoles exist largely as the 1H-isomers in the solid state and in solution but as the 2H-isomers in the gas phase,412,413 and hence, this is usually the form isolated in low-temperature cryogenic matrices358 (an exception is described in section 6.10). At the elevated temperatures under FVP conditions, the 1H-form may be populated, too, and the higher-energy 5Hisomer is very likely to participate as well (eq 104).285,414,415 This will be invoked in section 6.3. As mentioned in the Introduction, tetrazoles decompose with activation energies around 40 kcal/mol.7,8

Since tetrazoles are often used as precursors of azides and nitrenes, their chemistry has been described extensively in section 4. In this section the focus will be on the formation of nitrile imines and further rearrangements. In general, nitrile imines may exist in predominantly allenic (A), propargylic (P),

arylbenzoxazoles 486 in yields of 13−16%. Interestingly, small amounts of phenanthridinones 487 (1−5%) and cyanocyclopentadiene were also found. It has previously been asserted that only benzoxazoles are formed on pyrolysis, phenanthridinones on photolysis, and AP


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or carbenic (C) forms (Figure 10),416 which can be distinguished spectroscopically. The propargylic forms are

distinguished by their IR spectra, and both isomerize to the 1Hdiazirine 497 on further photolysis (Scheme 66).425 Scheme 66. Pyrolysis of 5-Phenyltetrazole

Figure 10. Structures of nitrile imines.

nitrile-like, with short CN triple bonds, long N−N single bonds, and strong IR frequencies at relatively high wavenumbers, ca. 2150−2300 cm−1. The allenic forms have longer CN bonds, shorter NN bonds, and strong IR absorptions below 2150 cm−1. The carbenic forms are stabilized by lonepair donation from substituent groups, i.e., amino, alkoxy, and halogen groups, in the same way that N-heterocyclic carbenes are stabilized. Since the carbenic forms are not cumulenes, they have very weak absorptions at relatively low frequencies, 1600− 1900 cm−1. It is emphasized that all nitrile imines can be formulated in terms of A, P, and C mesomeric structures, as well as 1,3-dipolar and diradical ones, but depending on the substituents, individual nitrile imines can be closer to one particular limiting structure. Recent experiments have shown that the lone-pair donating group in C-aminonitrile imines indeed causes a shift toward the carbenic structure, absorbing in the range 2100−1725 cm−1 with medium to very weak intensities (section 4.11).417,418 The generation of reactive intermediates and unusual molecules by photolysis of tetrazoles has been reviewed.419,420

Moreover, phenyldiazomethane 501 is also formed on FVP at and above 550 °C, and this indicates that the 5H-tautomer of the tetrazole (495c) can also be populated. The calculated gasphase barrier for the 1,5-H shift from N1 to N2 is 49 kcal/mol, and from N1 to C5 it is 55 kcal/mol at the QCISD(T)/6311+G(2d,2p) level.414 Although high, such barriers are potentially accessible under the high-temperature FVP conditions employed. A thermal isomerization of the Cphenylnitrile imine to phenyldiazomethane has not been excluded. At higher temperatures, the phenyldiazomethane yields phenylcarbene 69 and fulvenallene 73, as expected (cf. section 4.7.2).170,187

6.2. Tetrazole

FVP of tetrazole commences at ca. 500 °C (10−4−10−3 hPa), but temperatures as high as 800 °C may be necessary for complete elimination of one molecule of N2, which leads to carbodiimine (HNCNH), cyanamide (NH2CN), diazomethane (CH2N2), and nitrile imine [HC(−)N(+)NH] (eq 104).358,421 The IR spectrum of the matrix-isolated nitrile imine revealed a distinctly allenic structure (IR absorption at 2033 cm−1 in Ar matrix).358 Monosubstituted cyanamides and ketenimines exist in thermal equilibrium in the gas phase.285 In addition to the nitrogen elimination, cycloreversion of tetrazole to HN3 and HCN also takes place on FVP. Nitrile imine is also generated photochemically, and the inter-relationship between the CH2N2 isomers has been investigated.358,421,422 The existence of nitrile imine (HCNNH) was also established by neutralization− reionization mass spectrometry of the H2CN2 cation radical formed on electron ionization of 1,2,4-triazole.423

6.4. 2-Methyl-5-phenyltetrazole and 1-Methyl-5-phenyltetrazole

In analogy with the previous case, FVP of either 2-methyl-5phenyltetrazole 502 or 1-methyl-5-phenyltetrazole 503 at or above 420 °C produced N-methyl-N′-phenylcarbodiimide 504, a sensitive but isolable compound.285 While 503 can yield the carbodiimide directly via the imidoylnitrene 507, N2 elimination from 502 yields at first the nitrile imine 505, which has been observed directly in the corresponding photochemical reaction. This compound is allenic (2032 cm−1 in Ar matrix). Thermal rearrangement of the nitrile imine via the 1H-diazirine 506 then yields the carbodiimide (Scheme 67).

6.3. 5-Phenyltetrazole

FVP of 5-phenyltetrazole 495 at 260 °C yields small amounts of HN3 and benzonitrile formed by retro-1,3-dipolar cycloaddition from the 1H-tetrazole tautomer 495a.285,424 The IR specrtum of the matrix-isolated tetrazole demonstrated the coexistence of the 1H- and 2H-tautomers 495a and 495b in a ca. 1:2 ratio. FVP of both tautomers at 500−900 °C yields monophenylcarbodiimide 499, which is in thermal equilibrium with N-phenylcyanamide 500. The nitrile imine 498 has been characterized by its IR spectrum in an Ar matrix when generated photochemically285,425 and shows the remarkable property that it exists in both allenic (498A) (2073 cm−1) and propargylic (498P) (2242 cm−1) forms, which can be considered as bond-shift isomers.425 The two forms are clearly

Scheme 67

AQ


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6.5. C-Phenyl- and C-Methyl-N-trimethylsilylnitrile Imine

C-Phenyl-N-trimethylsilylnitrile imine 509 was obtained by analogous pyrolysis of tetrazole 508 at 500−550 °C, and also by matrix photolysis.285 509 is a relatively stable and distinctly propargylic nitrile imine (2245 cm−1), which can be isolated in a 77 K cold trap in the FVP experiment and then employed in 1,3-dipolar cycloaddition reactions.426 The matrix IR and UV spectra as well as the photoelectron427 and mass spectra428 of the thermally generated 509 were recorded. The nitrile imine rearranges to the carbodiimide 512 on FVP at higher temperatures or on photolysis (Scheme 68). Benzonitrile is also formed, presumably by cleavage of the nitrile imine to PhCN and Me3Si−N.

Repyrolysis of indazoles at ∼700−800 °C results in N2 elimination from the 3H-indazole tautomers with the result that fluorenes are formed, again in near-quantitative yields (eq 106).

Scheme 68

The detailed mechanism for the formation of a selection of mono- and disubstituted indazoles 522, fluorenes 524, and azaanalogues is shown in Scheme 70. It is noteworthy that all these Scheme 70. 2,5-Diaryltetrazole−Indazole−Fluorene Route

C-Methyl-N-trimethylsilylnitrile imine Me−CN+−N−− SiMe3 was generated and characterized analogously.285,427,428 6.6. 2,5-Diaryltetrazoles and Diphenylnitrile Imine

FVP of diphenyltetrazole 513 above 400 °C causes generation and cyclization of diphenylnitrile imine 514 to yield the 3H-3phenylindazole 517 followed by tautomerization to 3-phenylindazole 518 (Scheme 69). The nitrile imine is not directly Scheme 69. C,N-Diphenylnitrile Imine

observable under these conditions, but its IR spectrum is readily observed following photolysis of the tetrazole in an Ar matrix285 at 254 nm, or in a PVC matrix,429 and the UV−vis spectrum was obtained on photolysis in EPA at 77 K.430 This nitrile imine is propargylic (2242 cm−1 in Ar matrix).285,430 As usual, further photolysis rearranges the nitrile imine to the carbodiimide 516 (Scheme 69). It is noteworthy that minor amounts of nitriles are always formed in reactions generating nitrile imines by FVP. It also happens in matrix photochemistry285,428 and on electron ionization in the mass spectrometer. While cycloreversion of tetrazoles to nitriles and azides is very likely in any thermal reactions, the cleavage of nitrile imines to a nitrile and a nitrene is also a distinct possibility (eq 105).431 This issue is discussed further in section 6.10 (Scheme 80). A variety of indazoles can be synthesized from 2,5diaryltetrazoles in nearly quantitative yields in this manner.432

reactions afford excellent yields of both indazoles (and pyrazolopyridines) and fluorenes (and azafluorenes) and that the reactions are highly specific: no substituent migration has been observed. 5-(2-Furyl)-2-phenyltetrazole 525 similarly yielded 3-(2furyl)indazole 526 at 350−400 °C, and at 720 °C, loss of N2 and rearrangement to benzofulvene-8-carboxaldehyde 529 took place (Scheme 71).433 The rearrangement can be understood in terms of formation of the intermediates 527 and 528, the latter undergoing a retro-ene reaction to 529. Diarylnitrile imines can also be obtained by FVP of 1,3,4oxadiazolin-5-ones at 500 °C (yielding indazoles) or 750 °C (yielding fluorenes). The indazoles have also been obtained in AR


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and an alkene (and presumably HCN) to yield anilines 552 (Scheme 75).435 As described above, the thermal cleavage of the N−N bond in a nitrile imine to generate a nitrile and a nitrene is not unusual. It also happens photochemically285,429 in matrices and on electron impact in the mass spectrometer (eq 105).431 The triazole 553 was investigated for comparison (Scheme 76).435 FVP at 400 °C afforded three products, 556 (48%), 557 (about 30%), and 558 (about 15%). Cyclization to oxygen, giving the furan 556, is preferred (cf. eq 70, section 5.1). Rearrangement via the 1H-azirine 555 (cf. Scheme 52, section 5.2) could account for about 15% of the reaction, but since sigmatropic shifts of phenyl and cyano groups occur readily in FVP reactions of indoles and indenes (cf. Scheme 73 and Figure 4), a rearrangement (557 → 558) is a distinct possibility.

Scheme 71

6.9. Formation of Diazoalkanes and Carbenes

some cases, albeit in inferior yields, by pyrolysis of 1,5diaryltetrazoles in refluxing diphenyl ether (207 °C).432

As mentioned in section 6.3 and Scheme 66, FVP of 5phenyltetrazole at ∼500 °C generates some phenyldiazomethane and hence phenylcarbene. This is a general phenomenon, which can be used advantageously to generate arylcarbenes in the gas phase, where it must be kept in mind that nitrile imine formation is always a possibility, too, and that cycloreversion to nitriles and HN3 is a very common, competing reaction. As an example,424 FVP of 5-(p-tolyl)tetrazole 559 (Scheme 77) at 320 °C/0.01 hPa yields 7% of ptolunitrile. At 420 °C/0.01 hPa, p-tolylcarbene was generated, and it underwent the expected carbene−carbene rarrangement436 to benzocyclobutene 561 and styrene 562, which were isolated in 14% yield together with ∼10% of p-tolunitrile. The red color of the pyrolyzate indicated the presence of ptolyldiazomethane 560, but the amount was too small for isolation. By using N2 as a carrier gas to remove chemical activation, at 420 °C/1 hPa benzocyclobutene and styrene were isolated in 19% yield, p-tolunitrile in 9% yield, and ptolyldiazomethane in 9% yield. By increasing the N2 pressure to 10 hPa, benzocyclobutene and styrene were isolated in 3% yield, p-tolunitrile in 7% yield, and p-tolyldiazomethane in 13% yield. By decreasing the pressure to 10−6 hPa at the same temperature, the results were similar to those at 0.01 hPa, except that p-tolyldiazomethane was no longer detectable.424 Several examples of the use of the tetrazole route to heteroaryldiazomethanes and heteroarylcarbenes were described in section 4.7. Another example is the formation of both 1- and 2-azulenylcarbenes from the respective tetrazoles 563 and 564 by using falling-solid FVP at 800 °C (Scheme 78). The 2-azulenylcarbene rearranges to the 1-azulenylcarbene by carbene−carbene rearrangement similar to that of the arylcarbenes described in Scheme 77. Both carbenes afford the tricyclic hydrocarbon 565 as the principal product. Substantial amounts of the cyanoazulenes and HN3 were formed as well, and the azulenyldiazomethane intermediates were detectable by IR spectroscopy (Scheme 78). 437 Azulenylcarbenes have been generated and characterized in photochemical reactions by Sander and co-workers.438 A third example (Scheme 79) is the FVP of p-phenylenebistetrazole 566, which at 600 °C (0.01 hPa) gave a low yield of phenylacetylene 568, formally the result of rearrangement of the biscarbene (possibly via diazomethylphenylcarbene intermediates). Cycloreversion to terephthalonitrile and HN3 was the main reaction. A double 13C-labeling experiment demonstrated a 13C-label distribution in the phenylacetylene, in

6.7. 2-Phenyl-5-ethynyl- and 2-Phenyl-5-styryltetrazoles

2-Phenyl-5-(phenylethynyl)tetrazole 530 provides a new entry to the C15H10 energy surface. Using FS-FVP18,35 at 400−500 °C and a dynamic pressure varying between 10−3 and 10−1 hPa, cyclopenta[def ]phenanthrene 532 and cyclopenta[fg]fluorene 533 were obtained as the principal products (eq 107).These

products are explained in terms of formation of N-phenyl-Cphenylethynylnitrile imine/(phenylazo)(phenylethynyl)carbene 534, for which several mesomeric structures can be formulated, and its cyclization to indazoles 535a/535b. Pyrolytic loss of N2 from 535b generates the carbene intermediate 531. A series of cyclizations and phenylcarbenetype rearrangements via intermediates 536 and 537 lead to the final products (Scheme 72).434 Similar FVP of the 5-styryltetrazole 538 at 360 °C afforded the indazole 539 (Scheme 73) and, at 800 °C, a nearly 1:1 mixture of 3- and 2-phenylindenes 541 and 542 ascribed to cyclization of the carbene/diradical 540 (Scheme 73).434 6.8. 2-Aryltetrazolyl-5-acroleins and 2-Aryl-5-butadienyltetrazoles

Pyrolysis of compounds 543 at 250−400 °C (10−2 hPa) results in cyclization of the putative nitrile imines 544a/544b to indazolylacroleins 545b in good to high yields (60−98%) (Scheme 74).435 The reaction is completely analogous to that described in Scheme 73. The subsequent elimination of a second molecule of N2 at marginally higher temperature is also completely analogous with the reactions in Scheme 73 and leads to the formylindenes 547. Finally, CO is eliminated to form indenes 548 (Scheme 74). The chemistry of tetrazole 549 would be expected to be similar to that of 538, but instead, the putative nitrile imine intermediates 550a/550b underwent eliminations of a nitrile AS


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Scheme 72

formation of nitrile imines and carbodiimides was not investigated.

Scheme 73. From 2-Phenyl-5-styryltetrazole to Phenylindenes

6.10. Diversity of Reactivities of Aryltetrazoles

As described in the preceding sections, 5-substituted or 2,5disubstituted aryltetrazoles 569a−h undergo loss of N2 to generate N-arylnitrile imines 570 (R1−C−N+NR2 ↔ R1− CN+−N−R2) on both pyrolysis and photolysis (and on electron impact too). The nitrile imines can rearrange to carbodiimides 571 and cyanamides 572 but are sometimes also found to dissociate into nitrenes R2−N 573 and nitriles R1− CN. This can take place under photochemical285,429 and thermal285 as well as electron-impact conditions431 (see Scheme 80). As mentioned above, the rearrangement of NHcarbodiimides 571 to cyanamides 572 is not a unimolecular gas-phase reaction, but it can take place via wall collisions in FVP reactions and by intermolecular H-transfer during workup. 1,5-Disubstituted tetrazoles 574 yield imidoylnitrenes 575, which rearrange to carbodiimides 571 or cyclize onto neighboring aromatic rings, yielding benzimidazole derivatives 576 as described in section 4.9 and Scheme 80. 5-Aryltetrazoles 577 can undergo loss of N2 with formation of arydiazomethanes 578 and hence arylcarbenes 579 as desribed in the preceding section (Scheme 80). This reaction is often in competition with the formation of nitrile imines (570, R2 = H). 5-(α-N-heteroaryl)tetrazoles 580i−q are very good precursors of α-heteroarylcarbenes via the α-heteroaryldiazomethanes 581, which can be isolated and/or detected IR spectroscopically in many cases, as described in section 4. The α-heteroarylcarbenes 582 then rearrange efficiently to the more stable38,39,170 arylnitrenes 583 (Scheme 80), e.g., 4-, 3-, and 2-pyridylcarbenes to phenylnitrene (section 4.7.2),170,441 and

agreement with the carbene−carbene rearrangement, resulting in cyclization to benzocyclobutadiene 567 and finally isomerization to phenylacetylene (Scheme 79).439,440 The potential AT


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Scheme 74

4-, 3-, and 2-quinolylcarbenes to 1-naphthylnitrene (sections 4.7.4 and 4.7.5).193 Note that the nitrenes 583 formed by carbene reararngement are different from those (573) formed by fragmentation of nitrile imines (Scheme 80). A reason why the α-N-heteroaryltetrazoles behave differently can be sought in the fact that B3LYP calculations indicate that, thanks to an intramolecular hydrogen bond, the 1H-tautomers of 5-(α-N-heteroaryl)tetrazoles are by far the most stable in the gas phase, accounting for 98−99% of the ensemble of tautomers,442 whereas for other 5-aryltetrazoles, the 2Htautomers are the most stable. Thus, 5-aryltetrazoles are more likely to form nitrile imines; 5-α-heteroaryltetrazoles are more likely to form imidoylnitrenes and/or heteroaryldiazomethanes. Under FVP conditions, all the six-membered 5-(heteroaryl)tetrazoles (not just the α-isomers) investigated decompose by

Scheme 75

Scheme 76

AU


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(R2N3) from which the nitrenes (R2N) may again be formd (Scheme 80).

Scheme 77

6.11. Acyltetrazoles

Acyltetrazoles have not been investigated under FVP conditions. The FVP reactions of 1-acyl-1,2,4-triazoles were mentioned in section 5.1 (eq 70).336 2-Acyltetrazoles undergo similar elimination of N2, formation of N-acylnitrile imines, and O-to-C 1,5-electrocyclization forming 1,3,4-oxadiazoles in good yields, but this takes place under very much milder conditions, usually by thermolysis in hydrocarbon solvents (Huisgen tetrazole rearrangement).443−446 Analogous reactions of 2thioacyl- and 2-imidoyltetrazoles afford 1,3,4-thiadiazoles and 1,3,4-triazoles, respectively.447,448 5-Phenyl-2-alkoxycarbonyltetrazole similarly yields 2-alkoxy-5-phenyl-1,3,4-oxadiazoles in refluxing toluene.449,450 In the case of 2-hydrazonoyltetrazoles, the putative N-hydrazonoylnitrile imines cyclize to dihydro1,2,4,5-tetrazines, whereas the 1-hydrazonoyl- and 1-oximinoyltetrazoles afford 1,2,4-triazoles and 1,2,4-oxadiazoles via postulated imidoylnitrenes and carbodiimides.451,452 It would be of interest to investigate the propargylic/allenic/1,3-dipolar or 1,5-dipolar453 [R−C+N−NC(R′)−O−] natures of the N-acylnitrile imines with modern spectroscopic methods under matrix-isolation and/or pulsed pyrolysis/supersonic jet expansion conditions.

Scheme 78

6.12. 5-Aminotetrazoles

5-Aminotetrazole 585 and its derivatives are of interest as explosives, e.g., as air-bag inflators, and as additives for rocket propellants.454−456 There have been several experimental457−459 and computational460,461 investigations of the thermal decomposition of 5-aminotetrazole. The reaction is usually investigated by analyzing the gaseous products coming off the (flash-)heated solid. The 1H-form 585a is the structure in the solid, crystalline starting material, and the presence of the 2Hform 585b increases with increasing temperature and pressure.462 It is generally agreed that there are two initial decomposition paths, the major one yielding HN3 and cyanamide (H2N−CN) and the minor one yielding N2 and a metastable intermediate, “CH3N3”. Both the 1H- and the 2Hform can readily undergo cycloreversion to HN3 and cyanamide (Scheme 81). Several proposals have been made for the structure of the “CH3N3” formed in the minor path, and in view of the general reactivity pattern described in section 6.10, the direct formation of the carbodiimide (H2N−NCNH) 587 from the dominant 1H-tautomer 585a via the imidoylnitrene 586 is very likely. The 2H-form 585b can fragment to the nitrile imine (H2N−C−N+NH) 589, which then undergoes the usual thermal isomerization to 587. N-Aminocarbodiimides are known.463 Moreover, Nunes et al. identified 589 and 587 as the products of matrix photolysis of 585b.417 Note that the photolysis experiment starts with vacuum deposition of the gaseous starting material, which will exist almost exclusively as the 2H-tautomer 585b, thus yielding only the nitrile imine as a primary photolysis product. In the thermolyses, the cyanamide H2N−NH−CN may be a source of the secondary decomposition products459,461,462,464 HCN, N2, and H2. Methanimine (H2CNH), which was detected in the photolysis study,417 may be an additional source of HCN (HNC) and H2 (cf. section 4.2). The thermolysis of 585 in solution at 200−250 °C also yields HN3 and H2N−CN, as well as a polymer, (CH3N3)n. Attempts to trap the putative nitrile imine 589 were unsuccessful.464

Scheme 79

loss of N2 to afford heteroaryldiazomethanes, and this is thought to take place via thermally populated 5H-tetrazole tautomers (cf. eq 104) or possibly by isomerization of NHnitrile imines. In addition to all these reactions, all tetrazoles 584 may undergo cycloreversion to a nitrile (R1CN) and an azide AV


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Scheme 80. Diversity of Thermal Reactions of Aryltetrazoles

Scheme 81

1998 cm−1 as a medium-strength band.417 C-Dimethylamino-Nmethylnitrile imine 591 was generated by matrix photolysis of the tetrazole 590 by Nefedov and co-workers, and in this case, only a very weak band at 1725 cm−1 could be attributed to the CNN stretching vibration, thereby demonstrating a high degree of carbene character for this nitrile imine.418

As mentioned in section 6.1, nitrile imines may exist in predominantly propargylic, allenic, or carbenic forms, and lonepair-donating substituent groups should increase the carbenic character. Calculations indicate that C-aminonitrile imine 589 is mostly allenic (580a) with a ca. 19% carbenic contribution (589b). The CNN absorption of the nitrile imine appears at AW


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its transformation into bis(trimethylsilyl)carbodiimide 596. When the pyrolysis was performed at 700 °C, 596 was obtained directly due to the heat radiation from the pyrolysis tube. 595 was obtained cleanly by photolysis (254 nm) of 597 in an Ar matrix at 11 K. The imidoylnitrene 598 is a likely intermediate in both the pyrolysis and the photolysis.

Thermolysis of 1-trimethylsilyl-5-trimethylsilylaminotetrazole 592 at 150−160 °C affords trimethylsilyl azide (Me3Si−N3, 593), bis-trimethylsilylcarbodiimide 596, and polymeric cyanamide. It is likely that the cyanamide Me3Si−NH−CN 594 is the first-formed intermediate, which is in prototropic equilibrum with the monosilylcarbodiimide 595 (eq 108). Disproportionation then results in cyanamide and Me3Si−N CN−SiMe3 (596).465

6.14. 2-Stannyltetrazole

In contrast to the silyltetrazoles, 5-phenyl-2-stannyltetrazole 599 only underwent cycloreversion to trimethylstannyl azide 600 and PhCN on FVP at 350−400 °C/10−3 hPa (eq 111).144 A nitrile imine or carbodiimide were not detectable in the IR spectrum at 77 K, and the spectrum showed no significant changes upon warming to room temperature.

Thermal decomposition of the explosive 5,5′-azobistetrazole and 5,5′-hydrazobistetrazole in the solid state have been proposed to proceed via the bis-nitrile imines HC−N+N−N N−NN+C−H and HC−N+N−NH−NH−NN+C−H, respectively.466,467 In the former case, an ion signal at m/z 110 in the mass spectrum of the gaseous decomposition products was taken as evidence for the bis-nitrile imine (or bis-imidoylnitrene). The rearranged bis-carbodiimide HNCN−NN− NCNH would be longer-lived and more likely to survive. Not surprisingly, the final breakdown products are HCN and N2.

7. CONCLUSION AND OUTLOOK While the preparative chemistry of azides, triazoles, and tetrazoles could be considered a mature field, it is only recently that detailed investigation and direct observation of many types of nitrenes, carbenes, diradicals, nitrile imines, nitrile ylides, etc. have been performed; as a result, a deeper understanding of the stuctural and electronic properties, as well as the numerous rearrangement reactions of these species, has become possible. This has gone hand in hand with technical developments in spectroscopic techniques and computerized data handling. Flash vacuum pyrolysis combined with matrix-isolation spectroscopy is a versatile method for investigation of many of these intermediates. Numerous carbenes can also be generated in FVP reactions, but thanks largely to their smaller singlet−triplet (S−T) gaps, they are more likely than nitrenes to undergo rearrangement in the singlet state and therefore escape matrix isolation and direct observation. Nitrenes with their larger S−T gaps are more likely to relax to the less reactive triplet ground states, which allows isolation and detection. Therefore, FVP experiments often succeed in the direct detection of nitrenes, but matrix photolysis or time-resolved methods are the methods of choice for the direct observation of carbenes. Naturally, preparative studies provide a wealth of further insight into the natures of the reactive intermediates, and FVP allows the synthesis of numerous compounds that are not easily obtained in any other way. It is worth mentioning that hightemperature FVP has also led to recent developments in the synthesis of polyarenes, bowl-shaped aromatic molecules, and C60.469 The open-shell nature of many nitrenes, some carbenes, and most diradicals poses a serious challenge to computational chemists. Yet, such calculations are necessary in order to gain adequate insight into the structure and reaction mechanism. Likewise, the interplay between thermal and photochemical reactions is extremely valuable for the experimentalists, but the calculations of potential energy surfaces that are carried out usually pertain to the ground state surfaces only. Routine calculation of excited-state energy surfaces for complicated organic transformations still awaits future developments in computing capability. The experimentalist will continue to probe details of reaction mechanisms with ever-increasing sophistication. There is still a huge potential for discovery of reactive intermediates and molecules not easily obtainable by

6.13. Silyltetrazoles

The formation and characterization of the C-phenyl-Ntrimethylsilylnitrile imine 509285 was described in section 6.5. Several other N-silylnitrile imines were generated analogously, e.g., C-methyl-N-trimethylsilylnitrile imine, C-phenyl-N-dimethylsilylnitrile imine, C-phenyl-N-triphenylsilylnitrile imine, and C-phenyl-N-methyldiphenylsilylnitrile imine.428 The thermolysis of 1-trimethylsilyl-5-trimethylsilylaminotetrazole 592 was described in the preceding section. 1-Trimethylsilyltetrazole 597 decomposes at 135−140 °C with formation of trimethylsilyl azide 593, bis-trimethylsilylcarbodiimide 596, and polymeric cyanamide, which again suggests a disproportionation of an initially formed monotrimethylsilylcarbodiimide 595 (eq 109).465

Indeed, FVP of 597 at 450 °C/10−3 hPa with isolation of the product at 77 K afforded mono(trimethylsilyl)carbodiimide 595, which was observable by IR spectroscopy [νNCN 2150 (s) cm−1] (eq 110).468 Warming the solid 595 to 135 K caused

AX


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other means and for understanding their physical and chemical properties.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Curt Wentrup: 0000-0003-0874-7144 Notes

The author declares no competing financial interest. Biography Curt Wentrup was educated at the University of Copenhagen (Cand. Scient. 1966 with K. A. Jensen) and the Australian National University (Ph.D. 1969 with W. D. Crow). After postdoctoral periods with Hans Dahn (Lausanne, Switzerland), W. M. Jones (Gainesville, FL), and Maitland Jones, Jr. (Princeton, NJ), he held academic positions at the Université de Lausanne, in Switzerland, and a professorship at the Philipps-Universität Marburg, in Germany (1976−85), before returning to Australia in 1985 as professor and chair of organic chemistry and head of the organic chemistry section at The University of Queensland, where he is now emeritus professor. He is a Fellow of the Australian Academy of Science and a recipient of the Centenary Medal of the Australian Commonwealth, the David Craig Medal of the Australian Academy of Science, the A. J. Birch Medal of the Royal Australian Chemical Institute, a doctor scientiarum degree from the University of Copenhagen, and an honorary doctorate from the Université de Pau et des Pays de l’Adour, in France. He collaborates intensely with research groups in Europe (Germany, France, Portugal, Austria), Japan, and China on experimental and theoretical investigations of reactive intermediates (carbenes, nitrenes, cumulenes, nitrile imines, nitrile ylides, etc.) using flash vacuum thermolysis, photochemistry, and microwave-induced chemical reactions.

ACKNOWLEDGMENTS I am indebted to the numerous students and postdoctoral associates who have participated in the research in our laboratories and whose names are given in the references. Our recent work was supported by the Australian Reserch Council (DP0770863) and the Queensland Cyber Infrastructure Foundation (QCIF g01). REFERENCES (1) Nitrenes; Lwowski, W., Ed.; John Wiley & Sons, Inc.: New York, 1970. (2) The Chemistry of the Azido Group; Patai, S., Ed.; John Wiley & Sons, Inc.: New York, 1971. (3) Azides and Nitrenes; Scriven, E. F. V., Ed.; Academic Press: Orlando, FL, 1984. (4) Organic AzidesSynthesis and Applications; Bräse, S., Banert, K., Eds.; Johm Wiley & Sons, Ltd.: Chichester, UK, 2008. (5) Nitrenes and Nitrenium Ions; Falvey, D. E., Gudmundsdottir, A. D., Eds.; John Wiley & Sons., Inc.: Hoboken, NJ, 2013. (6) Sheridan, R. S. Heteroarylcarbenes. Chem. Rev. 2013, 113, 7179− 7208. (7) Aylward, N.; Eckhardt, U.; Winter, H.-W.; Wentrup, C. Triazoloazine − Diazomethylazine Valence Isomerization. 1,2,3Triazolo[1,5-a]pyridines and 2-Diazomethylpyridines. J. Org. Chem. 2016, 81, 667−672. (8) Manelis, G. B.; Nazin, G. M.; Rubtsov, Yu. I.; Strunin, V. A. Thermal Decomposition and Combustion of Explosives and Propellants; Taylor and Francis: New York, 2003. AY


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