Transborylation as a general turnover strategy for main-group catalysis

Abstract

The use of organoboron reagents was once widespread in chemical synthesis due to the range and versatility of reactivity offered. These transformations include asymmetric reduction, hydroboration of alkenes and alkynes, reduction of ketones and aldehydes, allylation reactions, and the reactivity of boron enolates. However, due to the efficiency offered by transition metal catalysts the stoichiometric reactions of organoboron reagents have fallen out of widespread use, finding value only in niche applications.

Transborylation is a sub-class of σ-bond metathesis and allows redox neutral catalytic turnover. This offers an alternative to the repeatedly used mechanisms of catalysis: oxidative addition and reductive elimination. Transborylation has offered a new strategy for the catalytic turnover of organoboron species, thus allowing these previously stoichiometric reagents to be rendered catalytic.

Thus far, transborylation has been applied to catalytic hydroboration, reduction and C−H borylation chemistry using boronic esters, giving products in excellent yields and functional group tolerance. However, only a fraction of the stoichiometric reactions of organoboron species have been rendered catalytic by this turnover pathway.

An investigation into boron-catalysed reduction and functionalisations reactions of enolates is reported in this thesis. Further application of this catalytic enolate chemistry allowed the reaction with electrophiles in one-step, giving α-cyanoketones. This chemistry was further developed to reactions inaccessible by stoichiometric methods, multi-fold boron catalysis, gave access to aldol-type products from an enone-ester coupling. Transborylation as a turnover strategy was also applied to allylation reactions where the catalytic allylation of ketones and indoles with allenes was achieved. Enantioselective variants of the allylation of ketones was attained using an enantioenriched boron catalyst. Further examples of transborylation in asymmetric synthesis was developed through a catalytic Midland reduction giving enantioenriched alcohols from ketones.