Iridium-Mediated Borylation of Benzylic C-H Bonds by Borohydride

摘要

The transition-metal-mediated conversion of C—H to CHB bonds is an exciting recent development in the functionaliza-tion of both saturated and unsaturated hydrocarbons. In part this reflects the fact that the resulting borylated compounds (boronic esters or acids) are attractive substrates for further chemistry through a range of established protocols.C—H to C-B conversion in arenes/heteroarenes catalyzed by [{Ir(cod)X}2]/4,4'-di-tert-butylbipyridine systems (X = Cl, OMe, indenyl; cod = cyclooctadiene) has been particularly well developed, in some cases achieving selectivity for substitution patterns which have proved difficult to access using classical synthetic methods. Typically these borylation protocols utilize HBpin or B2pin2 as the boron reagent of choice (pin = pinacolato, OCMe2CMe2O), with Ir~(III)-tris(Bpin) complexes thought to be key catalytic intermediates. C—B bond formation proceeds through either M—B/C—H σ bond metathesis or through distinct C— H oxidative addition/B—C reductive elimination steps in an Ir~(III)/Ir~V cycle. An alternative mechanism implicating a mono(Bpin) complex and a Rh~I/Rh~(III) catalytic cycle has been proposed for benzylic borylation using HBpin.