Boron-Carbon Multiple Bonding in Open-Chain Unsaturated Organoboranes

BORON-CARBON MULTIPLE BONDING IN OPEN-CHAIN UNSATURATED ORGANOBORANES 

The detection of the structural effects of B—C rr-bonding in unsaturated open-chain organoboranes should prove more difficult than in boracyclo-polyenes for several reasons. First, as already mentioned, there is the conformational freedom of the open-chain boranes, which reduces B(2pz)—C(2pz) overlap (cf. 17 vs. 19). Second, there is the linear overlap of 34 versus the cyclic, Htickel aromatic overlap of 35. 

substituents raising the effective electronegativity of boron will be expected to enhance B—C 7r-overlap (cf. 25a and 26). Finally, there can be competition between linear conjugation (36) and cross-conjugation (24). 

Such cross-conjugation (where two of the vinyls are conjugated with each other through boron but not simultaneously with the third vinyl35) always lowers the total delocalization energy (DE) of the conjugated atomic set In simple Hiickel MO theory, for example, ( )-l,3,5-hexatriene (37) has a DE of 0.988/3, while 2-vinyl-1,3-butadiene (38) yields a DE of 0.900/3. 

The synergistic action of these four effects may explain why trivinylborane (24) shows no sign of B—C 7r-bonding but vinylboron fluorides (e.g., 25a) do. 

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