Two‐Coordinate NHC‐Supported Zn<sup>II</sup> Organocations: Steric and Electronic Tunability and Use in Alkyne Hydroboration Catalysis
Refka Guermazi, David Specklin, Christophe Gourlaouen, Pierre Frémont, Samuel Dagorne
Abstract
Abstract We herein report on the synthesis, structure, and use in alkyne hydroboration catalysis of [(IPrCl)Zn−R] + and [(I t Bu)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 I t Bu carbene (I t Bu=1,3‐bis(1,1‐dimethylethyl)−1,3‐dihydro‐imidazol‐2‐ylidene). Ionization of neutral precursors [(IPrCl)ZnR 2 ] ( 1 a , R=Et; 1 b , R=Me) and [(I t Bu)ZnEt 2 ] ( 2 ) with one equivalent of [Ph 3 C][B(C 6 F 5 ) 4 ] led to with robust and stable two‐coordinate Zn II cations [(IPrCl)Zn−R] + ( 3 a , R=Et; 3 b , R=Me) and [(I t Bu)Zn−Et] + ( 4 ), respectively, all isolated as [B(C 6 F 5 ) 4 ] − salts. Further derivatization of alkyl cations 3 b and 4 by reaction with one equivalent of [B(C 6 F 5 ) 3 ] afforded cations [(IPrCl)Zn−C 6 F 5 ] + ( 5 ) and [(I t Bu)Zn−C 6 F 5 ] + ( 6 ) as [B(C 6 F 5 ) 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−C 6 F 5 ] + (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−C 6 F 5 ] + , experimental data and preliminary DFT calculations are consistent a Lewis‐acid‐type catalysis.