Trimethylsilyl-Induced N–O Bond Cleavage in Nitrous Oxide-Derived

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Trimethylsilyl-Induced N−O Bond Cleavage in Nitrous OxideDerived Aminodiazotates Yizhu Liu, Leó nard Y. M. Eymann, Euro Solari, Farzaneh Fadaei Tirani, Rosario Scopelliti, and Kay Severin* Institut des Sciences et Ingénierie Chimiques, É cole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

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arylazides, achieving 51% yield at elevated N2O pressures.3b Cbased nucleophiles have also been examined in reactions with N2O, mostly giving the corresponding diazo or hydrazone derivatives in low yield and poor selectivity.4 N-Heterocyclic carbenes (NHCs) present an exception in this context. They form stable covalent adducts with N2O.5 These adducts can be converted cleanly into azo dyes, with the azo group originating from N2O.6 Recently, we found that lithium dialkylamides readily capture N2O to give lithium dialkylaminodiazotates such as 1 (Scheme 1b).7 These diazotates were the first well-characterized covalent adducts between N-nucleophiles and N2O.8 Subsequent reactions with Grignard reagents give trisubstituted triazenes, which represent useful reagents in synthetic organic chemistry.7 Long before the identification of dialkylaminodiazotates, the structurally related bis(trimethylsilyl)aminodiazotate (2) was described by Wiberg and co-workers.9 This silylated compound was not prepared from N2O but by the reaction of lithium tris(trimethylsilyl)hydrazinide with organic nitrites (Scheme 1c).9 In contrast to 1, the silylated 2 is thermally labile and decomposes quickly into hexamethyldisiloxane [(TMS)2O] and lithium azide (Scheme 1c).9 This result indicates that trimethylsilyl (TMS) substitution has a pronounced impact on the stability and reactivity of aminodiazotates. The results summarized above prompted us to explore whether TMS-substituted aminodiazotates could also be prepared from N2O. If this was the case, we might observe spontaneous N−O bond cleavage by silyl group migration as displayed by 2, resulting in the formation of N-rich compounds. A reactivity of this kind was indeed observed experimentally, and details are given below. Stirring a solution of potassium bis(trimethylsilyl)amide (KHMDS) in tetrahydrofuran (THF) under 1 atm of N2O at room temperature (RT) resulted in the precipitation of potassium azide (KN3) along with the formation of a colorless solution. After 12 h, the latter contained mainly (TMS)2O, as evidenced by 1H NMR monitoring of a reaction in THF-d8. KN3 was isolated in 90% yield, and its composition was confirmed by IR spectroscopy and elemental analysis. When the reaction was performed in the presence of the ionophores 18-crown-6 or 2,2,2-cryptand (1.2 equiv with respect to KHMDS), we were able to isolate the corresponding adducts (for details, see the Supporting Information, SI). The results suggest that KHMDS forms a covalent adduct with N2O in analogy to what was

ABSTRACT: The chemical activation of nitrous oxide (N2O) typically results in O-atom transfer and the extrusion of N2 gas. In contrast, reactions of Ntrimethylsilyl (TMS)-substituted amides with N2O give inorganic or organic azides, with concomitant formation of silanols or siloxanes. N-TMS-substituted amides are also able to induce N−O bond cleavage in N2O-derived dialkylaminodiazotates, generating tetrazene salts. These results indicate the potential of silyl groups in devising transformations, in which N2O acts as an N-atom donor.

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pon chemical activation, nitrous oxide (N2O) typically acts as a clean O-atom-transfer reagent with N2 as the benign byproduct.1 Utilization of N2O as a N-atom donor, on the other hand, has received much less attention. The first example dates back to 1892, when Wislicenus reported the generation of sodium azide by passing N2O over sodium amide at 150−250 °C (Scheme 1a).2 Since then, the reactivity of Scheme 1. Wislicenus Reaction (a), Generation and Reactivity of the Dialkylaminodiazotate 1 (b), and Synthesis of the TMS-Substituted Aminodiazotate 2 and Its Decomposition (c)

several other N-based nucleophiles toward N2O has been investigated.3 Unfortunately, such reactions usually proceed with low efficiency and selectivity, simultaneously generating a range of products. For example, the reaction of lithium anilinide (LiHNPh) and N2O was found to give azobenzene, phenylazide, biphenyl, and a large amount of reprotonated parental aniline.3a The same reaction was later optimized for the synthesis of © XXXX American Chemical Society

Received: August 7, 2018

A

DOI: 10.1021/acs.inorgchem.8b02129 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry observed for lithium dialkylamides (Scheme 2).7 The N2O adduct then undergoes a decomposition similar to that of the

Scheme 4. Reaction of LiN(TMS)Ph with N2O on a Preparative Scale in THF

Scheme 2. Reaction of KHMDS with N2O

diazotate 2 described by Wiberg and co-workers.9 N−O bond cleavage in the transient NNO moiety and 2-fold conversion of Si−N into Si−O bonds may start from intramolecular rearrangement to give (trimethylsilyl)azide (TMSN3) and potassium trimethylsilanolate (TMSOK), followed by intermolecular reaction between the two (Scheme 2). Such a mechanism is supported by the fact that mixing a stoichiometric amount of TMSN3 and TMSOK in THF at RT gave KN3 and TMS2O (see the SI). The reaction between KHMDS and N2O can be regarded as a solution-based version of the Wislicenus reaction under mild conditions. A main driving force of the reaction is the high oxophilicity of silicon (569 kJ/mol for a Si−O bond).10 A related chemistry is the metathesis between HMDS salts and CO2, yielding a range of products including TMSO−, TMS2O, TMSNCO, and (TMS)NCN(TMS).11 The formal 2-fold metathesis between KHMDS and N2O prompted us to investigate the reactivity of organic amides containing only one N-TMS group. Upon stirring of a solution of KN(TMS)(Dipp) (Dipp = 2,6-diisopropylphenyl) under 1 atm of N2O in THF, (2,6-diisopropylphenyl)azide (3a) was isolated in almost quantitative yield after 90 min (Scheme 3). 1H

LiHNPh and N2O reported by Meier.3a In addition, we observed the formation of dilithium (Z)-1,4-diphenyltetraaz-2-ene-1,4diide (4; 22% yield according to NMR but difficult to purify), along with some other unidentified side products. The discrepancy between the NMR- and preparative-scale reactions is likely related to different reaction kinetics in the two setups (e.g., slow diffusion of N2O into the solution in the NMR tube or stirring-accelerated side reactions). Single crystals of 4 (twinned) could be isolated, and crystallographic analysis was performed. The tetrazene dianion is a known compound with rich coordination chemistry.12 It is conventionally prepared by the reaction of LiHNPh with PhN3, followed by deprotonation with n-butyllithium.12a,b However, its solid-state structure has not been reported before. Figure 1 shows a graphic representation of the polymeric structure of 4 in the crystal.

Scheme 3. Reactions of MN(TMS)Ar with N2O

NMR monitoring showed that most of the TMS group was transferred into TMSOK (see the SI). A similar reactivity was observed by NMR spectroscopy when solutions of other MN(TMS)Ar [M = K, Li; Ar = 2,4,6-tBu3Ph (Mes*), Ph, 4MeOPh] in THF-d8 were exposed to N2O (Scheme 3). Spectra recorded after 24 h [119 h for KN(TMS)(Mes*)] showed the formation of arylazides 3b−3d in high yields. These results suggest that TMS-substituted anilinides also form aminodiazotates, which undergo TMS-induced N−O bond cleavage, in analogy to that shown in Scheme 2. When the above reactions were performed on preparative scales, more complex reaction patterns were observed. For example, reacting LiN(TMS)Ph and N2O in THF on a 4.4 mmol scale gave very little of the expected PhN3. Instead, azobenzene (10%) and HN(TMS)Ph (20%) were found (Scheme 4), which is reminiscent of the reaction between

Figure 1. Molecular structure of 4 in the crystal highlighting the asymmetric unit (a) and with through-axis (b) and along-axis (c) views.

The structural parameters of the tetrazene dianion are comparable to those in the corresponding metal complexes.12 The asymmetric unit comprises half of the molecule, which is associated with the other by the symmetry code of −x, −y + 1, z (Figure 1a). The N1−N2 (or N1#′−N2#′) bond [1.365(5) Å] is shorter than a typical N−N single bond (ca. 1.45 Å),13 while the N2−N2#′ bond [1.296(8) Å] is slightly longer than a NN double bond (ca. 1.24 Å),13 indicating charge delocalization over the tetrazene dianion moiety. Compound 4 crystallizes into polymeric chains with a dihedral angle of the neighboring tetrazene dianion planes of 58.6(3)° (Figure 1b,c). Each Li+ is tetracoordinated by three N atoms as well as one THF molecule. B

DOI: 10.1021/acs.inorgchem.8b02129 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

bond, indicating delocalization of the negative charge over the N1−N2−N3 unit. This is consistent with a shorter Li1−N1 distance [1.957(3) Å] than Li1−N4 [2.034(3) Å]. 5a also crystallizes into polymeric chains (Figure 2a) with a dihedral angle of 70.98(6)° between the alternating Z-tetrazene anion planes (Figure 2b). Each Li+ is chelated by one Z-tetrazene anion as well as bound in a η2-side-on fashion by the N2−N3 bond of the neighboring tetrazene moiety. The Li1′−N3 bond length [1.954(3) Å] is comparable to that of Li1−N1 [1.957(3) Å] in the chelate, while the Li1′−N2 bond length [2.246(3) Å] is comparable to those in a weakly activated Li+-N2 complex16 as well as a manganese(III) tetraaz-2-ene-1,4-diide complex featuring similar η2-Li+(NN) coordination.17 In summary, we have discovered that N-TMS-substituted amides covalently capture N2O, and the resulting aminodiazotates undergo spontaneous N−O bond cleavage, giving inorganic or organic azides. The former transformation can be regarded as a solution-based version of the Wislicenus reaction. The N-TMS-substituted anilinide LiN(TMS)Ph also effects N− O bond cleavage in N2O-derived dialkylaminodiazotates, generating unsymmetrical tetrazene monoanion salts. The formal metathesis between N-TMS-substituted amides and either N2O or aminodiazotates seems to take advantage of the formation of a strong Si−O bond. However, sterics as well as the electronics on the silyl substituent are also important. For example, no reactivity toward N2O was observed when both TMS groups in KHMDS were replaced with triphenylsilyl groups. Further development of the TMS-assisted N2O transformation to access value-added N-containing chemicals is underway.

The Li−N distances are in a narrow range of 2.098(9)− 2.103(9) Å, comparable to those observed for the κ2-Li(NC CN)2− motif.14 Generation of the tetrazene dianion in 4 was intriguing because it suggested that the intermediate aminodiazote can further react with the N-TMS-substituted amide (Scheme 4) apart from going through intramolecular rearrangement. We were thus interested in examining whether a similar reaction would occur with aminodiazotes without TMS groups. Because these aminodiazotates are thermally more stable, a higher yield of the tetrazene could be expected. As a first reaction, we mixed the aminodiazote derived from lithium diisopropylamide and N2O7 with an excess of LiN(TMS)Ph (2.5 equiv) in THF. After workup, we were able to isolate lithium (Z)-4,4-diisopropyl-1-phenyltetraaz-2-en-1-ide (5a) in 39% yield (Scheme 5). 5a is metastable both in solution Scheme 5. Reactions of Lithium Dialkylaminodiazotates with LiN(TMS)Ph

and in the solid state, complicating its purification and characterization. However, it can be alkylated with methyl iodide, and the resulting neutral tetrazene was identified by mass spectroscopy (see the SI). The common approach to prepare tetrazenes is the oxidative dimerization of hydrazines, and thus symmetrical tetrazenes have been described in the literature.15 To the best of our knowledge, unsymmetrical trialkylaryltetrazenes or trisubstituted tetrazene monoanion salts have not been reported so far. Structural analogues of 5a were prepared by using aminodiazotates derived from lithium dicyclohexylamide and lithium 2,2,6,6-tetramethylpiperidine. The corresponding tetrazenides 5b and 5c could be isolated in yields of 49% and 25%, respectively (Scheme 5). In addition to NMR spectroscopy analysis, we have again characterized 5b and 5c by alkylation with MeI, followed by mass spectroscopy analysis of the resulting neutral tetrazenes (see the SI). The structure of 5a in the crystal could be established by single-crystal X-ray diffraction (Figure 2). As for compound 4, a Z configuration of the tetrazene moiety was observed for 5a, promoted by κ2-N1,N4-chelation with Li+. While N3−N4 is a typical single bond [1.484(2) Å], the N1−N2 [1.310(2) Å] and N2−N3 [1.283(2) Å] bonds are slightly longer than a double

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02129. Experimental details and Figures S1−S27 (PDF) Accession Codes

CCDC 1854213−1854215 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: kay.severin@epfl.ch. ORCID

Yizhu Liu: 0000-0003-2669-9473 Kay Severin: 0000-0003-2224-7234 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.L. is an EPFL Fellow cofunded by the Marie SkłodowskaCurie program (Grant 665667). We thank Drs. Daniel Ortiz and Laure Menin for mass spectrometry measurements and Emilie Baudat for NMR measurements.

Figure 2. Molecular structure of 5a in the crystal with along-axis (a) and through-axis (b) views. C

DOI: 10.1021/acs.inorgchem.8b02129 Inorg. Chem. XXXX, XXX, XXX−XXX

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Verbindungen. XXXVIII. Reaktionen des Natrium-bis-[trimethylsilyl]amids mit Kohlenstoffchalkogeniden und Kohlenstoffhalogeniden. Z. Anorg. Allg. Chem. 1964, 333, 54−61. (b) Sita, L. R.; Babcock, J. R.; Xi, R. Facile Metathetical Exchange between Carbon Dioxide and the Divalent Group 14 Bisamides M[N(SiMe3)2]2 (M = Ge and Sn). J. Am. Chem. Soc. 1996, 118, 10912−10913. (c) Xu, M.; Jupp, A. R.; Stephan, D. W. Stoichiometric Reactions of CO2 and Indium-Silylamides and Catalytic Synthesis of Ureas. Angew. Chem., Int. Ed. 2017, 56, 14277− 14281. (12) (a) Lee, S. W.; Miller, G. A.; Campana, C. F.; Maciejewski, M. L.; Trogler, W. C. Generation of Mono- and Dianions of 1,4-Diphenyl-2tetrazene by Nonoxidative N-N bond Formation. A Novel Route to a 2Tetrazene, a Silacyclotetrazene, and the Tetrazenide Complex (1,4Diphenyltetrazenido)bis(triethylphosphine) Palladium. J. Am. Chem. Soc. 1987, 109, 5050−5051. (b) Lee, S. W.; Miller, G. A.; Campana, C. F.; Trogler, W. C. Synthesis and Molecular Structure of 1,4Diphenyltetrazenido Complexes of Bis(phosphine)nickel, -Palladium, and -Platinum. Inorg. Chem. 1988, 27, 1215−1219. (c) Preut, H.; Obloh, R. C.; Neumann, W. P. 1,3-Di-tert-butyl-2,2-di-methyl-5,8-diphenyl-1,3,5,6,7,8-hexaaza-2-sila-4-germa-spiro[3.4]oct-6-ene. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1988, 44, 574−575. (d) Miller, G. A.; Lee, S. W.; Trogler, W. C. Synthetic, Structural, and Theoretical Studies of Diphenyltetrazene Complexes of Silicon and Germanium. Organometallics 1989, 8, 738−744. (e) Lee, S. W.; Trogler, W. C. Synthesis, Structure, and Properties of Dicarbonyl Bis(phosphine) 1,4-Diphenyltetraazabutadiene Complexes of Molybdenum and Tungsten. Organometallics 1990, 9, 1470−1478. (f) Lee, S. W.; Trogler, W. C. Structure of 1,4-Di-phenyl-2-tetrazenidobis(triethylphosphine)-nickel. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1990, 46, 900−902. (g) Paek, C.; Ko, J.; Kang, S.; Carrol, P. J. Synthesis and Structure of 1,2,3,4,5-Pentamethylcyclopentadienyl-1,4-Diphenyltetraazabutadiene Complexes of Rhodium and Iridium. Bull. Korean Chem. Soc. 1994, 15, 432−436. (h) Danopoulos, A. A.; HayMotherwell, R. S.; Wilkinson, G.; Cafferkey, S. M.; Sweet, T. K. N.; Hursthouse, M. B. Reactions of Iridium and Ruthenium Complexes with Organic Azides. J. Chem. Soc., Dalton Trans. 1997, 3177−3184. (i) Hill, N. J.; Moser, D. F.; Guzei, I. A.; West, R. Reactions of Stable Silylenes with Organic Azides. Organometallics 2005, 24, 3346−3349. (13) Lide, D. R. Characteristic Bond Lengths in Free Molecules. In CRC Handbook of Chemistry and Physics, Section 9, 99th ed.; Rumble, J. R., Ed.; CRC Press, 2018. (14) (a) Scholz, J.; Richter, B.; Goddard, R.; Krüger, C. Notizen/ Notes [Li2(THF)4DAD] und [DAD−H][ZrCl5(THF)]: Neue Strukturvarianten Phenylsubstituierter 1,4-Diaza-1,3-dien-Liganden. Chem. Ber. 1993, 126, 57−61. (b) Görls, H.; Neumüller, B.; Scholz, A.; Scholz, J. Lanthanoid Complexes with [(dad)Li] LigandsNew Starting Materials for Organolanthanoid Chemistry. Angew. Chem., Int. Ed. Engl. 1995, 34, 673−676. (c) Wenzel, M.; Lindauer, D.; Beckert, R.; Boese, R.; Anders, E. The Reduction of Oxalic Amidines with Metallic Lithium: Preparation of Lithiated Bisamides [R’’N(RR’N)C = C(NRR’)NR’’]Li2 and Their Use as Intermediates in a Novel Synthesis of Tetraaminoethenes. Chem. Ber. 1996, 129, 39−44. (d) Scholz, J.; Görls, H.; Schumann, H.; Weimann, R. Reaction of Samarium 1,4Diaza-1,3-diene Complexes with Ketones: Generation of a New Versatile Tridentate Ligand via 1,3-Dipolar Cycloaddition. Organometallics 2001, 20, 4394−4402. (e) Iravani, E.; Neumü ller, B. Oligomerization of Cyclohexylisonitrile by tBuELi2 (E = P, As). Organometallics 2005, 24, 842−847. (f) Yakub, A. M.; Moskalev, M. V.; Bazyakina, N. L.; Cherkasov, A. V.; Shavyrin, A. S.; Fedushkin, I. L. Hydroamination of 2-Vinylpyridine, Styrene, and Isoprene with Pyrrolidine Catalyzed by Alkali and Alkaline-earth Metal Complexes. Russ. Chem. Bull. 2016, 65, 2887−2894. (15) Bräse, S.; Muller, T. Aryltriazenes, Aryltetrazenes, and Related Compounds. Sci. Synth. 2007, 31b, 1845−1872. (16) Ho, J.; Drake, R. J.; Stephan, D. W. [Cp 2 Zr(μPPh)]2[((THF)3Li)2(μ-N2)]: a Remarkable Salt of a Zirconocene Phosphinidene Dianion and Lithium Dication Containing Side-bound Dinitrogen. J. Am. Chem. Soc. 1993, 115, 3792−3793.

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Inorganic Chemistry (17) Vaddypally, S.; McKendry, I. G.; Tomlinson, W.; Hooper, J. P.; Zdilla, M. J. Electronic Structure of Manganese Complexes of the Redox-Non-innocent Tetrazene Ligand and Evidence for the Metal− Azide/Imido Cycloaddition Intermediate. Chem. - Eur. J. 2016, 22, 10548−10557.

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DOI: 10.1021/acs.inorgchem.8b02129 Inorg. Chem. XXXX, XXX, XXX−XXX