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

• Published in: Chemistry : a European journal 2019-06, Vol.25 (34), p.8061-8069
• Main Authors: Bruyere, Jean‐Charles, Specklin, David, Gourlaouen, Christophe, Lapenta, Rosita, Veiros, Luis F., Grassi, Alfonso, Milione, Stefano, Ruhlmann, Laurent, Boudon, Corinne, Dagorne, Samuel
• Format: Article
• Language: English
• Subjects: Alkynes; carbenes; Carbon dioxide; Catalysis; Cations; Chemical Sciences; Chemical synthesis; Chemistry; Coordination chemistry; Hydrosilylation; Lability; Lewis acid; Lewis acids; Mathematical analysis; Organic chemistry; Reactivity; Salts; Species; Zinc; zinc alkyls
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201900961

The reactivity of ZnII dialkyl species ZnMe2 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 ZnII Lewis acidic organocations. CAAC adduct of ZnMe2 (2), formed from a 1:1 mixture of 1 and ZnMe2, is unstable at room temperature and readily undergoes a CAAC carbene insertion into the Zn−Me bond to produce the ZnX2‐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)ZnC6F5+ (7+), both isolated as B(C6F5)4− salts, showing the ability of CAAC for the stabilization of reactive [ZnMe]+ and [ZnC6F5]+ moieties. Due to the lability of the CAAC−ZnMe2 bond, the formation of bis(CAAC) adduct (CAAC)2ZnMe+ cation (6+) was also observed and the corresponding salt [6][B(C6F5)4] 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 CO2 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 ZnII organocations. New and robust! The reactivity of a cyclic(alkyl)(amino)carbene (CAAC) derivative with ZnMe2 was investigated. Combining a CAAC with ZnMe2 lead to unexpected reactivity (Me− transfer, see Figure), but also afford robust (CAAC)ZnR+ and (CAAC)2ZnR+ Lewis acidic cations, some of which could be applied in alkene, alkyne and CO2 hydrosilylation catalysis.