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

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 non‐classical 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. [ABSTRACT FROM AUTHOR]