Changing behavior: A nucleophilic sp2 carbene‐type boryl moiety, formed upon interaction of tetraalkoxydiboranes and a Lewis base, can attack non‐activated CC bonds. Computational studies identify the interaction as the overlap between the strongly polarized BB σ bond (HOMO) and the antibonding π* orbital (LUMO) of the CC bond. Conceptually, the normally electrophilic boron becomes nucleophilic and forces the olefin to act as an electrophile.

Dedicated in memory of Lµszló Gulyµs.Enantioenriched a-chiral boron compounds were first obtained using chiral rhodium-phosphine complexes from the catalytic hydroboration of prochiral alkenes.[1] There are three reasons why metal-mediated asymmetric induction in C À B bond formation is more successful than existing methods involving interactions between the substrate and a chiral borane reagent [2] in the absence of a metal: 1) the low cost/ availability of the achiral borane reagent, 2) the milder reaction conditions, and, most importantly, 3) the possibility for optimization and maximization of the asymmetric induction by screening the chiral ligands. Considerable progress has since been made, particularly in relation to the enantioselective metal-mediated hydroboration, [3] diboration, [4] and bboration [5] of electron-deficient olefins. However, one challenge still remains to be overcome: the development of a metal-free asymmetric boron-addition reaction with achiral boron reagents.Our group has recently studied metal-mediated asymmetric conjugate borylation reactions in the presence of copper, [5c,d] palladium, [5g] nickel, [5h] and iron [5j] complexes that were modified with either chiral phosphine or carbene ligands. This field has recently been elegantly reviewed by Oestreich and co-workers, [6] and the authors conclude that asymmetric metal-free approaches to conjugate borylation might be the next pioneering step forwards. Hoveyda and coworkers recently reported an efficient metal-free b-boration of cyclic and acyclic a,b-unsaturated carbonyl groups promoted by N-heterocyclic carbenes (NHCs).[7] Mechanistic studies revealed that 10 mol % of carbene alone can activate the diboron reagent, bis(pinacolato)diboron (B 2 pin 2 ), by nucleophilic attack at one of the boron atoms (Scheme 1).After 1,4-addition of the reagent to the substrate the carbene is regenerated, this making the reaction catalytic.Attempts by Hoveyda and co-workers [7] to promote the bboration of 2-cyclohexen-1-one with PPh 3 and PCy 3 as nucleophilic reagents in the absence of a metal were unsuccessful. However, an early example by Hosomi and co-workers [8] showed that PBu 3 could induce slight conversion of benzylideneacetophenone into the b-borated ketone in the absence of the catalyst precursor CuOTf (Scheme 2).

Boron compounds have been traditionally regarded as "Lewis acids" preferring to accept electrons rather than donate them in the course of their reactions but current examples of unusual reactivity between tricoordinated boranes and electrophilic sites suggest a new conceptual context for the boryl moieties, based on their nucleophilic character which can be enhanced depending on the substituents on boron.

Bases play an important role in organocatalytic boron conjugate addition reactions. The sole use of MeOH and a base can efficiently transform acyclic and cyclic activated olefins into the corresponding β-borated products in the presence of diboron reagents. Inorganic and organic bases deprotonate MeOH in the presence of diboron reagents. It is concluded, on the basis of theoretical calculations, NMR spectroscopic data, and ESI-MS experiments, that the methoxide anion forms a Lewis acid-base adduct with the diboron reagent. The sp(2) B atom of the methoxide-diboron adduct gains a strongly nucleophilic character, and attacks the electron-deficient olefin. The methanol protonates the intermediate, generating the product and another methoxide anion. This appears to be the simplest method to activate diboron reagents and make them suitable for incorporation into target organic molecules.