π-Complexes Derived from Non-classical Diboriranes: Side-on vs. End-on Carbonylative Ring Expansion

Abstract

Unlike cyclopropanes, the analogous B2C species (diboriranes) tend to adopt non-classical Hückel-aromatic structures with bridging moieties R between the boron atoms. The coordination of the thus generated cyclic 2e π-system to transition metals is completely unexplored. We here report that complexation of nonclassical diboriranes cyclo-μ-RB2Dur2CPh (R=H, SnMe3; Dur=2,3,5,6-tetramethylphenyl) to Fe(CO)3 fragments allows for the carbonylative ring expansion of the B2C ring to either four- or five-membered rings depending on the nature of the BRB 3-center-2-electron bond (3c2e): The H-bridged diborirane (R=H) initially reacts with Fe2(CO)9 to the allylic π-complex with an agostic BH/Fe interaction. Subsequent formal hydroboration of CO from excess Fe2(CO)9 results in the side-on ring expansion under formation of a five-membered B2C2O ring, coordinated to the Fe(CO)3 moiety. In contrast, in case of the stannyl-bridged diborirane (R=SnMe3) under the same conditions, CO is added end-on to the B B bond with the carbon terminus formally inserting into the B2Sn 3c2e-bond. The two carbonylative ring expansion products can also be described as nido and closo clusters, respectively, according to the Wade-Mingos rules.