Computational Study of B(4)H(10) Addition to Ethene. Baskets from Boranes.
Three pathways leading from B(4)H(10) + C(2)H(4) to (CH(2)CH(2))B(4)H(8) are examined at the [MP4/6-311+G(d,p)]//MP2/6-31G(d) + ZPC level. The preferred, "addition" path involves transient [B(4)H(8)], [B(4)H(8)(C(2)H(4))], and a cage-opened [(CH(2)CH(2))B(4)H(8)] intermediate with a rate-determining barrier of 33.0 kcal/mol (for [B(4)H(8)] formation). "Indirect hydroboration" of ethene via [B(4)H(8)] is disfavored by entropy and can be excluded. "Direct hydroboration" via a higher-energy [B(4)H(10)] isomer can be competitive; the overall energetic barrier for this process is only 26.9 kcal/mol, but the hydroboration step is disfavored by entropy to such an extent that the "addition" path is the main route at the experimental temperatures. Direct hydroboration affords the intermediate Et-B(4)H(9) which is likely to be involved in side reactions leading to byproducts such as Et-(CH(2)CH(2))B(4)H(7).
Bühl
Michael
M
Department of Chemistry, Auburn University, Auburn, Alabama 36849.
McKee
Michael L.
ML