Two‐Coordinate NHC‐Supported ZnII Organocations: Steric and Electronic Tunability and Use in Alkyne Hydroboration Catalysis

Abstract

AbstractWe herein report on the synthesis, structure, and use in alkyne hydroboration catalysis of [(IPrCl)Zn−R]+ and [(ItBu)Zn−R]+ cations bearing IPrCl (IPrCl=1,3‐bis[2,6‐bis(1‐methylethyl)phenyl]‐4,5‐dichloro‐1,3‐dihydro‐imidazol‐2‐ylidene) and the sterically demanding ItBu carbene (ItBu=1,3‐bis(1,1‐dimethylethyl)−1,3‐dihydro‐imidazol‐2‐ylidene). Ionization of neutral precursors [(IPrCl)ZnR2] (1 a, R=Et; 1 b, R=Me) and [(ItBu)ZnEt2] (2) with one equivalent of [Ph3C][B(C6F5)4] led to with robust and stable two‐coordinate ZnII cations [(IPrCl)Zn−R]+ (3 a, R=Et; 3 b, R=Me) and [(ItBu)Zn−Et]+ (4), respectively, all isolated as [B(C6F5)4]− salts. Further derivatization of alkyl cations 3 b and 4 by reaction with one equivalent of [B(C6F5)3] afforded cations [(IPrCl)Zn−C6F5]+ (5) and [(ItBu)Zn−C6F5]+ (6) as [B(C6F5)4]− salts, with cation 6 displaying a limited stability in solution. The molecular structures of cations 3 b, 4 and 5 were confirmed through X‐ray diffraction studies. Among stable cations, Fluoride ion affinity (FIA) estimations agree with cation 5 being the most Lewis acidic in thus far reported [(IPrCl)Zn−R]+ cations. In the presence of pinacol borane and 1‐octyne, cations 3 a–b, 5 and [(IPrCl)Zn−C6F5]+ (5 mol%) slowly catalyze the selective cis‐hydroboration of 1‐octyne to the vinylborane product A. Cation 5 also mediates 2‐hexyne hydroboration to afford a mixture of hydroboration products B and C. In the case of hydroboration catalysis mediated by cations 5 and [(IPrCl)Zn−C6F5]+, experimental data and preliminary DFT calculations are consistent a Lewis‐acid‐type catalysis.