Unexpected Migratory Insertion Reactions of M(alkyl)₂ (M=Zn, Cd) and Diamidocarbenes

Abstract

The electrophilic character of free diamidocarbenes (DACs) allows them to activate inert bonds in small molecules, such as NH3 and P4 . Herein, we report that metal coordinated DACs also exhibit electrophilic reactivity, undergoing attack by Zn and Cd dialkyl precursors to afford the migratory insertion products [(6-MesDAC-R)MR] (M=Zn, Cd; R=Et, Me; Mes=mesityl). These species were formed via the spectroscopically characterised intermediates [(6-MesDAC)MR2 ], exhibiting barriers to migratory insertion which increase in the order MR2 = ZnEt2 < ZnMe2 < CdMe2 . Compound [(6-MesDAC-Me)CdMe] showed limited stability, undergoing deposition of Cd metal, by an apparent β-H elimination pathway. These results raise doubts about the suitability of diamidocarbenes as ligands in catalytic reactions involving metal species bearing nucleophilic ligands (M-R, M-H).

Cyclic(Alkyl)(Amino)Carbene (CAAC)‐Supported Zn Alkyls: Synthesis, Structure and Reactivity in Hydrosilylation Catalysis

“…At room temperature, it is stable for days in C 6 D 5 Br but decomposes within minutes in CD 2 Cl 2 to unknown species. NMR data for [ 5 ][B(C 6 F 5 ) 4 ] (C 6 D 5 Br, RT) agree with the proposed formulation, with, in particular, a 13 C NMR C carbene signal for 5 + significantly upfield shifted versus that of the neutral precursor 2 ( δ =231.7 and 267.6 ppm, respectively), which is in line with a more Lewis acidic Zn II in 5 + . As determined by X‐ray diffraction studies, salt [ 5 ][B(C 6 F 5 ) 4 ] crystallizes as discrete 5 + and B(C 6 F 5 ) 4 − ions in close contact (Figure ) with a rather short Zn(1)⋅⋅⋅F(11) distance (2.620 Å, which is 0.2 Å shorter than the sum of the van der Waals radii for Zn and F).…”

“…Overall, the ready formation of insertion product 3 from unstable CAAC adduct 2 clearly reflects the enhanced electrophilicity of the CAAC carbene atom and further illustrates the ability of CAACs to activate/functionalize various small molecules through insertion reactivity . Though CAAC insertion in Zn−alkyl bonds is not documented, a related carbene insertion reactivity was recently reported upon combining ZnR 2 dialkyls with diamidocarbenes …”

“…[9] Though CAAC insertion in ZnÀ alkyl bonds is not documented, ar elatedc arbene insertion reactivity was recently reported upon combining ZnR 2 dialkyls with diamidocarbenes. [20] CAAC-supported ZnR organocations5 + + ,6 + + and 7 + + as B(C 6 F 5 ) 4 À À salts…”

“…NMR data for [5][B(C 6 F 5 ) 4 ]( C 6 D 5 Br,R T) agree with the proposed formulation, with, in particular,a 13 CNMR C carbene signal for 5 + + significantly upfield shifted versust hat of the neutralp recursor 2 (d = 231.7 and 267.6 ppm, respectively), [8]: Zn(1)À C(1) = 2.017(4),Zn(1)ÀC(22) = 1.957 (6), C(1)ÀN(1) = 1.470(5),C (1)À C(21) = 1.507 (6), C(1)-Zn (1) which is in line with am ore Lewis acidic Zn II in 5 + + . [20] As determined by X-ray diffraction studies, salt [5][B(C 6 F 5 ) 4 ]c rystallizes as discrete 5 + + and B(C 6 F 5 ) 4 À ions in close contact ( Figure 3) with ar ather short Zn(1)···F(11) distance (2.620 ,w hich is 0.2 shorter than the sum of the van der Waals radii for Zn and F).…”

Ring expansion of a ring expanded carbene

“…For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt03046k at δ 5.30 ppm and a correlated (2H) doublet signal at δ 4.01 ppm were strongly reminiscent of the respective C-H and SiH 2 Ph signals reported by Bertrand and co-workers for compound 5, 14 which resulted from analogous room temperature oxidative addition of PhSiH 3 to the saturated imidazolidin-2-ylidene analogue of compound 1. Similar solution NMR observations have also been described as a result of the migratory insertion reactions of diamidocarbene (DAC) ligands with a variety of B-H, 15 B-B and Cu-H functionalities, 16 while cyclic (alkyl)(amino)carbenes have recently been reported to react at room temperature with the Si-H bonds of both organosilanes and hydrogen-terminated silicon surfaces. 17 On this basis we were able to ascribe compound 6 as a similar product of Si-H oxidative addition, (6-Mes-H) SiH 2 Ph (eqn (1)), a deduction that was confirmed by a subsequent X-ray diffraction analysis performed on a single crystal obtained by slow evaporation of a pentane solution.…”