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

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 (Nγγ) 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.

Al/P‐ and Ga/P‐Based Frustrated Lewis Pairs and Electronically Unsaturated Substrates: Ring Cleavage and Ring Closure, C−C and C−N Bond Formation

“…The Al−N and Ga−N bonds (196.4(2) and 208.0(2) pm) are expectedly (bonding situation, charge) shorter than those of 6 and 7 . The P−N bond lengths are with 163.8 pm on average longer than in iminophosphoranes, but similar to distances observed for azide adducts of these FLPs or nitrene complexes obtained from these adducts by release of N 2 . The N atoms are further bound to a terminal alkenyl group (C=C: 134 pm) which has the unchanged cis ‐arrangement of both phenyl substituents.…”

“…These compounds have been applied in various secondary processes such as the coordination of carbonyl compounds, azides, hydrogen halides, chalcogens, boron trihalides, transition metals, and more, or they were applicable as catalysts in the activation of alkali‐metal hydrides or the dehydrogenation of boron–amine adducts . The facile synthetic procedure allowed the generation of functionalized compounds that had, beside a protecting mesityl substituent, an alkynyl group or a H atom ( 3 ) attached to phosphorus. The latter showed a fascinating reactivity with the transfer of the phosphorus‐bound H atom to substrates which were activated by coordination to the FLP backbone through its typical donor–acceptor functionality .…”

“…The facile synthetic procedure allowed the generation of functionalized compounds that had, beside a protecting mesityl substituent, an alkynyl group or a H atom ( 3 ) attached to phosphorus. The latter showed a fascinating reactivity with the transfer of the phosphorus‐bound H atom to substrates which were activated by coordination to the FLP backbone through its typical donor–acceptor functionality . Ga atoms are softer Lewis acids compared with Al atoms and should allow a modulation of the FLP properties.…”

A Neutral Germanium/Phosphorus Frustrated Lewis Pair and Its Contrasting Reactivity Compared to Its Silicon Analogue

“…Its terminal nitrogen atom becomes coordinated between the silicon and phosphorus atoms of 1 . This binding type is the same as for Uhl's FLP ( t Bu) 2 AlC(CHPh)PMes 2 in its Me 3 SiCHNN adduct and related to the potential metal‐free activation of dinitrogen described by Stephan and Melen . The four 1 H NMR signals of 9 include a singlet at 0.05 ppm for the TMS group, two doublets at 0.87 ppm for the tert ‐butyl groups, a doublet at 2.07 ppm for the methylene and a singlet at 7.91 ppm for the N 2 CH group.…”

Cooperative activation of azides by an Al/N-based active Lewis pair – unexpected insertion of nitrogen atoms into C–Si bonds and formation of AlCN3 heterocycles

“…Organic azides, R′N 3 , represent a particularly interesting class of substrates, which react with monomolecular B/P or Al/P FLPs to yield heterocycles with one (B) [9,[19][20][21][22], two (C) [23] or three (D) [24] N atoms of the azide groups included in the rings (Scheme 1). A related reaction of [Ph 3 C][B(C 6 F 5 ) 4 ] and F 5 C 6 N 3 led in the presence of P(o-tol) 3 and Ph 3 SiH to [(Ph 3 Si)(F 5 C 6 )N-N=N-P(o-tol) 3 ][B(C 6 F 5 ) 4 ] as an acyclic analogue of compound D [25].…”