Two-Coordinate NHC-Supported Zn-II Organocations: Steric and Electronic Tunability and Use in Alkyne Hydroboration Catalysis

We herein report on the synthesis, structure, and use in alkyne hydroboration catalysis of [(IPrCl)Zn-R](+) and [((IBu)-Bu-t)Zn-R](+) cations bearing IPrCl (IPrCl-1,3-bis[2,6-bis(1-meth- ylethyl)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 Zn-II cations [(IPrCl)Zn-R](+) (3a, R-Et; 3 b, R-Me) and [(ItBu)Zn-Et](+) (4), respectively, all isolated as [B(C6F5)(4)](-) salts. Further derivatization of alkyl cations 3b and 4 by reaction with one equivalent of [B(C6F5)(3)] afforded cations [(IPrCl)Zn-C6F5](+) (5) and [(ItBu)Zn-C6F5](+) (6) as [Zn-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 3a-b, 5 and [(IPrCl)Zn-C6F5](+) (5 mol degrees o) slowly catalyze the selective cishydroboration 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.