Two‐Coordinate NHC‐Supported ZnIIOrganocations: 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 ZnIIcations [(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 cations3 band4by 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 cation6displaying a limited stability in solution. The molecular structures of cations3 b,4and5were confirmed through X‐ray diffraction studies. Among stable cations, Fluoride ion affinity (FIA) estimations agree with cation5being the most Lewis acidic in thus far reported [(IPrCl)Zn−R]+cations. In the presence of pinacol borane and 1‐octyne, cations3 a–b,5and [(IPrCl)Zn−C6F5]+(5 mol%) slowly catalyze the selectivecis‐hydroboration of 1‐octyne to the vinylborane productA. Cation5also mediates 2‐hexyne hydroboration to afford a mixture of hydroboration productsBandC. In the case of hydroboration catalysis mediated by cations5and [(IPrCl)Zn−C6F5]+, experimental data and preliminary DFT calculations are consistent a Lewis‐acid‐type catalysis.