Butyldiisobutyl, triisopropyl and tri-sec.-butyl boranes

The thermal isomerization of tert. -butyl-diisobutylborane to triisobutylborane and the stepwise isomerization of triisopropyl and tri-sec.-butyl boranes to the corresponding straight-chain isomers have been studied over a range of temperature, both neat and using either the end product trialkylborane or diglyme as solvent117 . In all cases the reactions were first order and showed no solvent effect. 

The results are shown in Table 9. Based on absolute rate theory Rossi et a/.117a give AS for reaction (1) as 14.2+2.6 e.u. The mechanism proposed is 

The large negative entropies of activation for reactions (2)-(4)(AS = — 8 to — 11 e.u.) and the reversible nature of these reactions is attributed to isomerization via a rc-complex, viz. 

Of reactions (5)-(7) only reaction (7) has been directly investigated. The entropy of activation for this reaction, —3.0 e.u., probably indicates that the mechanism for the isomerization of sec.-butyl is closely related to that of iso-propyl. 

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