Reactions of Al/P, Ga/P and P-H functionalized frustrated Lewis pairs with azides and a diazomethane - Formation of adducts and capture of nitrenes

Uhl W., Backs J., Hepp A., Keweloh L., Layh M., Pleschka D., Possart J., Wollschläger A.

Abstract

The Al- and Ga-based frustrated Lewis pairs (FLPs) Mes2P-C(MR2)=CH-R' (1, M = Al, R = tBu; 2, M = Al, R = CH2tBu; 3, M = Ga, R = tBu) and the unique P-H functionalized FLP Mes(H)P-CH(AlR2)=C(H)-tBu [4, R = CH(SiMe3)2] were treated with a variety of azides R'-N = N = N [R' = tBu, SiMe3, Ph, CH2Ph, C6H4(4-Cl), C6H4(4-CF3), C6H4(4-Me), CH2C6H4(4-Cl), CH2C6H4(4-tBu), C6H4(2-CH=CHPh)] in order to study systematically the influence of the substituents at nitrogen, phosphorus and the metal atoms on the reaction courses and the thermal stability of the products. Azide adducts (5-8) were isolated in which the terminal nitrogen atoms of the azides (Nγ) were bound to the phosphorus and the respective metal atoms resulting in four-membered PCMN heterocycles as the sole structural motif despite the wide range of substituents and the variation in the metal atoms of the FLPs. Thermal activation of selected azide adducts led to the elimination of N2 and the formation of the nitrene adducts 9-11 in which formally a transient, highly reactive nitrene N-R with an electron sextet nitrogen atom is trapped by the FLPs. For the first time FLPs were treated with a diazomethane, Me3Si-C(H)=N=N. Reactions with 1 and 2 afforded the adducts [Mes2P-C(AlR2)=CH-Ph](μ-N2CH-SiMe3) 12 (R = tBu, CH2tBu) which had structures and spectroscopic properties similar to those of the corresponding azides. These compounds are thermally stable and do not eliminate dinitrogen upon warming or irradiation. Protonation of 12a with HCl in Et2O resulted in cleavage of the Al-N bond and formation of the zwitterionic phosphonium salt Mes2P[NH-N=C(H)-SiMe3]-C(AlCltBu2)=C(H)-Ph 13 with an intramolecular N-H⋯Cl hydrogen bond.