Published on Web 03/03/2006
P-Hydrogen-Substituted 1,3,2-Diazaphospholenes: Molecular Hydrides Sebastian Burck,† Dietrich Gudat,*,† Martin Nieger,‡ and Wolf-Walther Du Mont¶ Contribution from the Institut fu¨r Anorganische Chemie, UniVersita¨t Stuttgart, Pfaffenwaldring 55, 70550 Stuttgart, Germany, Institut fu¨r Anorganische Chemie, UniVersita¨t Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany, and Institut fu¨r Anorganische Chemie, Technische UniVersita¨t Braunschweig, Hagenring 30, 38106 Braunschweig, Germany Received November 30, 2005; E-mail:
[email protected] Abstract: P-Hydrogen-substituted 1,3,2-diazaphospholenes 1 were prepared by an improved procedure from diazadienes and were characterized by spectroscopy and in one case by X-ray diffraction. A unique hydride-type reactivity of the P-H bonds was documented by extensive reactivity studies. Aldehydes and ketones were readily reduced to diazaphospholene derivatives of the corresponding alcohols, with alkylsubstituted ketones being converted at much lower rates than aldehydes or diaryl ketones. Reactions with the tetrachlorides of group 14 elements proceeded via hydride/chloride metathesis to give either partially chlorinated derivatives EHnCl4-n (n ) 0-3 for E ) C, Si) or HCl and phosphenium salts 16c[ECl3] (for E ) Ge, Sn) which were characterized by spectroscopic and X-ray diffraction studies. Tin dichloride was readily reduced to the element. Reactions of 1c with the P-chloro-diazaphospholene 3c and the salt 16c[OTf] allowed the first experimental detection of intermolecular exchange of a hydride, rather than a proton, between phosphine derivatives. Computational studies indicated that the hydride transfer between 1c and the cation 16c involves a transient H-bridged species with bonding properties similar to those of B2H7-. The preference for the formation of these bridged intermediates over P-P bonded phosphenium-phosphine adducts is attributed to the low electrophilicity of the diazaphospholenium cations and characterizes a novel reaction mode for phosphenium ions.
Introduction
Phosphine (PH3), primary organophosphines (RPH2), and secondary organophosphines (R2PH) are distinguished by the presence of reactive phosphorus-hydrogen bonds whose capability to undergo chemical transformations makes these species widely used reagents and synthetic intermediates.1 Many typical reactions involve formal conversion of the P-H into other P-E bonds (where E may be nearly any other metal or nonmetal) and occur via either metathetic replacement of a hydrogen or addition of a P-H bond to a carbon-carbon or carbon-heteroatom multiple bond (“hydrophosphination”). Several of these processes, e.g., the conversion into tertiary phosphines which are valuable ligands in many catalytic processes, are of considerable importance for both academic research and industrial chemistry.1 From a mechanistic standpoint, substitution reactions are frequently carried out in the presence of strong bases,1 whereas hydrophosphinations may proceed as radical processes under homolytic cleavage of the † ‡ ¶
Universita¨t Stuttgart. Universita¨t Bonn. Technische Universita¨t Braunschweig.
(1) (a) Quin, L. D. A Guide to Organophosphorus Chemistry; WileyInterscience: New York, 2000. (b) Corbridge, D. E. C. Phosphorus World 2005 (http://www.phosphorusworld.com). (c) The Chemistry of Organophosphorus Compounds, Vol. 1; Hartley F. R., Ed.; John Wiley & Sons: Chichester, 1990. 3946
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P-H bond (Scheme 1a),1,2 as ionic reactions that are catalyzed by acids or bases,1,2 or under catalysis by transition metal complexes.3 Acid-catalyzed addition reactions of phosphines are presumed to proceed by nucleophilic attack of the phosphine on the substrate which was previously activated by the acid and subsequent deprotonation of the resulting phosphonium ion;2,4 in contrast, base-induced addition and substitution reactions are initiated by deprotonation of the phosphine by a catalytic or stoichiometric amount of base to give a phosphanide (R2P-M+), which is then quenched by reaction with an electrophile or addition to a multiple bond.2,5 Both reaction mechanisms are in accord with the generally accepted concept of phosphines as weak acids (Scheme 1b).6 Since, as a consequence of the similar electronegativities of hydrogen (χAR 2.2) and phosphorus (χAR 2.06), the phosphorushydrogen bond lacks a pronounced polarity, it is conceivable that P-H-substituted phosphines can react not only as a source of radicals or as acids but also as hydride donors (Scheme 1c). An ambiguous behavior of this type is well established for (2) Elsner, G. Methoden der Organischen Chemie (Houben-Weyl), 1952-, 4th ed.; Georg Thieme Verlag: Stuttgart, 1980; Vol. 13/E1, p 122 ff. (3) Baillie, C.; Xiao, J. Curr. Org. Chem. 2003, 7, 477. (4) Rauhut, M. M.; Hechenbleikner, I.; Currier, H. A.; Schaefer, F. C.; Wystrach, V. P. J. Am. Chem. Soc. 1959, 81, 1103-1107. (5) Hoff, M. C.; Hill, P. J. Org. Chem. 1959, 24, 356. (6) Hudson, H. R. In ref 1c, p 473 ff. 10.1021/ja057827j CCC: $33.50 © 2006 American Chemical Society
P-Hydrogen-Substituted 1,3,2-Diazaphospholenes Scheme 1. Possible Ways for Cleavage of P-H Bonds
ARTICLES Scheme 3
Scheme 2
hydrogermanes, where the electronegativity situation (χAR 2.02 for Ge) is similar:7 the polarity of the Ge-H bonds is generally low, and substituent influences or even special reaction conditions can have a determinative influence and may lead to a shift from radical to either “proton” or “hydride” reactivity. Such changes can have serious consequences on the outcome of reactions and may, e.g., result in an inverted regioselectivity (“Umpolung”), as exemplified in Scheme 2 for the addition of Et3GeH and Cl3GeH to carbonyl compounds, which can yield either a germylcarbinol (reflecting “proton” character of the Ge-H bond) or a germyl ether (reflecting “hydride” character of the Ge-H bond).7 A dualism between acidic and hydride character even in the presence of a somewhat larger electronegativity difference is likewise known for the reactions of Si-H bonds in silanes (χAR 1.74 for Si).8 That substituent-induced inversion of the E-H bond polarity is, in principle, also feasible in phosphorus compounds was first established by the observation of hydride-type reactivities for some hypervalent Lewis base adducts of phosphines.9 Some time ago, we described 2-H-diazaphospholenes 1 as the first threecoordinate phosphine derivatives showing “Umpolung” of the P-H bond polarity. This behavior is due to a hyperconjugative interaction between nonbonding π-electrons in the C2N2 unit and the σ*(P-H) orbital (expressed by resonance between the canonical structures 1-1′′, Scheme 3), which induces a weakening of the P-H bond and an increase of negative charge density at the hydrogen atom.10 A preliminary survey of the chemical reactivity of 1 revealed that the hydride character became evident in the ability to react with acids under dehydrocoupling and to convert benzaldehyde into a benzyloxy-diazaphospholene 2 (Scheme 3), with concomitant reduction of the carbonyl to an alcohol function.10 Although the potential of phosphines as reducing agents has long been recognized,11 the reaction of 1 is remarkable as it does not proceedsas usualsunder deoxygenation of the substrate and conversion of the phosphine into a phosphine oxide, but rather via transfer of a hydride. Reactions involving reductive hydride transfer from phosphines to organic and main-group (7) Rivie`re, P.; Rivie`re-Baudet, M.; Satge´, J. In ComprehensiVe Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 2, p 399 ff. (8) Armitage, D. A. In ref 7, Vol. 2, p 1 ff. (9) (a) Carre´, F.; Chuit, C.; Corriu, R. J. P.; Mehdi, A.; Reye´, C. J. Organomet. Chem. 1997, 529, 59. (b) Bezombes, J.-P.; Carre´, F.; Chuit, C.; Corriu, R. J. P.; Mehdi, A.; Reye´, C. J. Organomet. Chem. 1997, 535, 81. (c) Alder, R. W.; Read, D. Angew. Chem., Int. Ed. 2000, 39, 2879. (10) Gudat, D.; Haghverdi, A.; Nieger, M. Angew. Chem., Int. Ed. 2000, 39, 3084. (11) (a) Buckler, S. A.; Doll, L.; Lind, F. K.; Epstein, M. J. Org. Chem. 1962, 27, 794. (b) Studer, A.; Amrein, S. Synthesis 2002, 835.
Scheme 4. Synthesis of Diazaphospholenesa
a R ) MeOCH CH ; ECl ) SiCl or GeCl ; R1 ) tBu (1c-6c), Mes 2 2 n 4 2 (1d-6d), 2,6-iPr2C6H3 (DIPP, 1e-6e).
element compounds have as yet not been studied systematically. However, in consideration of the present knowledge on the reactivity of 1, it is conceivable that further explorations in this field may lead to new applications for phosphines in organic and organometallic synthesis. This prospect stimulated us to evaluate the reactivity of the diazaphospholenes 1 toward various types of substrates in more detail. In this work, we will give a full account of the synthesis of 2-hydro-diazaphospholenes and a crystal structure study of one derivative, and we will report on a comprehensive investigation of the use of these species as hydride-transfer reagents toward organic carbonyl compounds, di- and tetrahalides of group 14 elements, and chlorophosphines. Results and Discussion
Synthesis and Characterization of 2-Hydrido-1,3,2-diazaphospholenes. The P-hydrogen-substituted diazaphophospholenes 1 are readily prepared via H/Cl exchange from the P-chloro-diazaphospholenes 3.10 The latter are in turn accessible from readily available diazadienes 4 in a two- or three-step procedure (Scheme 4) involving (i) reduction of the starting material to a dilithium diazadienide salt 5 and (ii) either direct metathesis of this intermediate with PCl312 or, alternatively, (12) Carmalt, C. J.; Lomeli, V.; McBurnett, B. G.; Cowley, A. H. Chem. Commun. 1997, 2095. J. AM. CHEM. SOC.
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conversion of the diazadienide into a diazasilole 13 or cyclic germylene12 and subsequent ring metathesis with PCl3. The direct substitution pathway is clearly advantageous from a practical point of view, as it involves fewer and less timeconsuming steps, but a drawback is that it provides only moderate yields of diazaphospholene (55% of 3c by metathesis from 5c12). In connection with our recent studies on the synthesis of N-heterocyclic arsenium and stibenium ions,14 we found now that the yields of chloro-diazaphospholenes 3 can be substantially improved when the dianions 5 are first quenched by protonation with Et3NHCl and the resulting R-aminoaldimines 6 are then reacted with PCl3 in the presence of NEt3 as acid scavenger. Isolation of the intermediates 5, 6 is not required so that the whole synthesis can be carried out in one pot. Using this protocol allows to prepare multigram quantities of the N-tert-butyl- and N-aryl-P-chloro-diazaphospholenes 3c-e in one step from diazadienes with oVerall yields of 70-80%. The completion of the synthesis of the target compounds 1c-e from 3c-e requires as the last step the replacement of the P-chloro-substituent by hydrogen. Although LiAlH4 and LiBEt3H have been initially employed for this purpose,10 their use suffers from the occurrence of side reactions (mainly cleavage of P-N bonds due to uncontrolled over-reduction) which reduce the yields and require extra efforts for the removal of the byproducts during workup. We have found that using a stoichiometric amount of sodium bis(methoxyethoxy)aluminum dihydride (“Red-Al”) for hydride transfer gives superior results by avoiding unspecific P-N bond cleavage and facilitating the separation of chloroaluminate salts by filtration. Following these protocols, pure P-hydrido-diazaphospholenes 1c-e are easily isolated in yields of 50-70% after distillation (1c) or recrystallization (1d,e). The pure diazaphospholenes 1c-e are light yellow liquids (1c) or solids that are highly air and moisture sensitive. They are readily soluble in hydrocarbons, ethers, and acetonitrile but decompose when dissolved in protic solvents and in chlorinated hydrocarbons (see below). The composition and identity of all compounds were verified by analytical and spectroscopic data and in the case of 1e by a single-crystal X-ray diffraction study. The spectroscopic data of 1c-e display no peculiarities and are listed in detail in the Experimental Section. The structure of crystalline 1e is composed of isolated molecules, without significant intermolecular interactions (Figure 1), whose individual features bear close similarity to those of the previously described 4-chloro-substituted diazaphospholene 1b.10 As in that case, the five-membered ring exhibits a flat “envelope” conformation in which the phosphorus atom is out of the plane containing the other four ring atoms. The position of the hydrogen atom at phosphorus was located and refined freely using isotropic thermal displacement parameters, showing that the P-H bond takes on a flagpole position. The P-H distance of 1.48(1) Å is quite similar to that in 1b (1.51(4) Å10) and likewise notably longer than known P-H bond lengths in phosphines (1.288 ( 0.09 Å 15), and the endocyclic bond (13) Denk, M. K.; Gupta, S.; Ramachandran, R. Tetrahedron Lett. 1996, 37, 9025. (14) (a) Gudat, D.; Gans-Eichler, T.; Nieger, M. Chem. Commun. 2004, 2434. (b) Gudat, D.; Gans-Eichler, T.; Nieger, M. Heteroat. Chem. 2005, 16, 327. (15) Average and standard deviation as the result of a query in the CSD database for P-H distances in three-coordinate phosphorus compounds HnY3-nP (Y ) substituent bound via a p-block element). 3948 J. AM. CHEM. SOC.
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Figure 1. Molecular structure of 1e in the crystal, ORTEP view. Thermal ellipsoids are at the 50% probability level. Selected bond lengths (in Å): P1-N5 1.691(1), P1-N2 1.712(1), P1-H1 1.48(1), N2-C3 1.408(2), N2C6 1.437(2), C3-C4 1.317(2), C4-N5 1.416(2).
distances in 1e (see caption to Figure 1) and 5 (PsN 1.709(3), 1.722(3); CsN 1.407(5), 1.410(5); CdC 1.327(5) Å10) are indistinguishable. The characteristic pattern of deviations from standard single- and double-bond distances has been interpreted as the result of hyperconjugation between the six π-electrons in the C2N2 unit and the σ*(P-H) orbital and was stated as the dominant cause for the bond weakening and increased negative charge density at the hydrogen atom that account for the unique hydride nature of the P-H bonds in diazaphospholenes.10 Reactions with Carbonyl Compounds. Although the reductive dechlorination of phosgene is a well-established route for the preparation of phosphinous and phosphonous chlorides, R2PCl and RPCl2, from primary and secondary phosphines,2 the latter react with aldehydes and ketones not by reduction of the carbonyl group but via P-C bond formation to give R-phosphino-carbinols. Having discovered10 the reduction of benzaldehyde by 1b (Scheme 3) as a remarkable exception to this behavior, we wanted to evaluate the scope and limitation of this reaction as a new approach to the reduction of organic carbonyls. For this purpose we set out to study the behavior of the diazaphospholene 1c, which was chosen as the synthetically most easily accessible derivative, toward a larger range of substrates. Additional sampling experiments revealed that 1d,e behave very much the same way as 1c, whereas the reactions of 1a,b were in some cases complicated by side reactions involving the 4-chloro substituents (see below). The reactions of 1c with aldehydes (2-methoxybenzaldehyde, butyraldehyde) and benzophenone were completed within a couple of minutes at ambient temperature (as judged by the fading of the orange color of the starting material) and proceeded, like the reaction of 1b with benzaldehyde, with reduction of the carbonyl function to yield the diamidophosphinites 8-10 (Scheme 5) along with varying amounts ( 2σ(I)) ) 0.038, wR2 ) 0.104, largest differential peak and hole 0.399 and -0.288 e Å-3. 16c[GeCl3]: orange crystals, C10H20Cl3GeN2P, M ) 378.2, crystal size 0.55 × 0.50 × 0.40 mm3, orthorhombic, space group Pnma (No. 62), a ) 12.9640(4) Å, b ) 17.3985(6) Å, c ) 7.3944(2) Å, V ) 1667.84(9) Å3, Z ) 4, F(calcd) ) 1.51 Mg m-3, F(000) ) 768, µ ) 2.40 mm-1, 7370 reflections measured, 1527 unique reflections (Rint ) 0.035) used for structure solution and refinement with 82 parameters, empirical absorption correction with multiple reflections (maximum and minimun transmission 0.4125 and 0.3831), R1 (I > 2σ(I)) ) 0.021, wR2 ) 0.051, largest differential peak and hole 0.254 and -0.271 e Å-3. 16c[SnCl3]: orange crystals, C10H20Cl3N2PSn, M ) 424.3, crystal size 0.40 × 0.35 × 0.30 mm3, orthorhombic, space group Pnma (No. 62), a ) 12.7132(3) Å, b ) 17.4583(5) Å, c ) 7.7723(2) Å, V ) 1725.07(8) Å3, Z ) 4, F(calcd) ) 1.63 Mg m-3, F(000) ) 840, µ ) 2.02 mm-1, 9453 reflections measured, 1574 unique reflections (Rint ) 0.036) used for structure solution and refinement with 82 parameters, empirical absorption correction with multiple reflections (maximum and minimum transmission 0.5682 and 0.5375), R1 (I > 2σ(I)) ) 0.020, wR2 ) 0.046, largest differential peak and hole 0.441 and -0.490 e Å-3. 1H
Acknowledgment. We thank Dr. U. Kessler, University of Bonn, for the powder diffraction measurements of tin samples.
Note Added after ASAP Publication. After this paper was published ASAP on March 3, 2006, Figure 1 was modified to properly display the thermal ellipsoids, as described in the caption. The corrected version was published ASAP on March 6, 2006. Supporting Information Available: IR and Raman data for 16c[GeCl3] and 16c[SnCl3]; complete ref 40; computational details (extended geometric and energy data) (PDF); X-ray crystallographic data (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. JA057827J (40) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467. (41) Sheldrick, G. M., University of Go¨ttingen, 1997. J. AM. CHEM. SOC.
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