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

The reactivity of Zn dialkyl species ZnMe with a cyclic(alkyl)(amino)carbene, 1-[2,6-bis(1-methylethyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene (CAAC, 1), was studied and extended to the preparation of robust CAAC-supported Zn Lewis acidic organocations. CAAC adduct of ZnMe (2), formed from a 1:1 mixture of 1 and ZnMe , is unstable at room temperature and readily undergoes a CAAC carbene insertion into the Zn-Me bond to produce the ZnX -type species (CAAC-Me)ZnMe (3), a reactivity further supported by DFT calculations. Despite its limited stability, adduct 2 was cleanly ionized to robust two-coordinate (CAAC)ZnMe cation (5 ) and derived into (CAAC)ZnC F (7 ), both isolated as B(C F ) salts, showing the ability of CAAC for the stabilization of reactive [ZnMe] and [ZnC F ] moieties. Due to the lability of the CAAC-ZnMe bond, the formation of bis(CAAC) adduct (CAAC) ZnMe cation (6 ) was also observed and the corresponding salt [6][B(C F ) ] was structurally characterized. As estimated from experimental and calculations data, cations 5 and 7 are highly Lewis acidic species and the stronger Lewis acid 7 effectively mediates alkene, alkyne and CO hydrosilylation catalysis. All supporting data hints at Lewis acid type activation-functionalization processes. Despite a lower energy LUMO in 5 and 7 , their observed reactivity is comparable to those of N-heterocyclic carbene (NHC) analogues, in line with charge-controlled reactions for carbene-stabilized Zn organocations.