Cu^I‑Catalyzed Conjugate Addition of Silyl Boronic Esters: Retracing Catalytic Cycles Using Isolated Copper and Boron Enolate Intermediates

Abstract

Copper­(I)-catalyzed conjugate additions of silyl boronic esters to α,β-unsaturated aldehydes, ketones, and esters are synthetically well-established reactions. For the first time central reactive intermediates as well as the boron enolates as the primary reaction products are isolated and employed in order to deduce catalytic cycles on an experimental basis. Employing an NHC Cu^I complex as a model catalyst, it is possible to perform efficient catalytic transformations as well as to isolate and characterize the formed copper enolate complexes as the key intermediates. It is shown that for this catalytic system the nature of this enolate<i>O</i>- or <i>C</i>-enolateis crucial for the catalytic process. For α,β-unsaturated aldehydes and ketones the <i>O</i>-enolate is formed predominantly, while for α,β-unsaturated esters the <i>C</i>-enolate is the major product. Catalytic turnover is only facile for copper <i>O</i>-enolates, as they react efficiently with the silyl boronic ester under (re)­formation of the catalytically active Cu–Si species and a thermodynamically favored boric acid ester. Thus, the formation of copper <i>C</i>-enolates is inhibiting the catalytic process, and effective turnover is possible only after solvolysis by an alcohol additive. The individual catalytic processes were retraced by performing stepwise stoichiometric reactions monitored by in situ NMR spectroscopy