2.3- dimethyl-2-butene, reaction with borane

When the alkene precursor is hindered (highly substituted), it may be possible to isolate monoalkylboranes or dialkylboranes. Three alkylboranes should be discussed because they are also used in reactions with alkenes. When 2-methyl-2-butene reacts with borane, the product is the dialkylborane 60, and its common name is disiamylborane (from the common name di-secoradoo -isoamylborane). Similar reaction of borane with 2,3-dimethyl-2-butene leads to a monoalkylborane, 61. The common name of this product is thexylborane (from tertiary-he lborane). 

The calculations discussed so far are for reaction of monomeric BH3 with alkenes in the gas phase. In solution the borane is most likely to be a dimer or, in ether solvents such as THF, a borane-solvent complex. It is difficult to study the kinetics of borane addition in solution because the reaction is complicated by three addition steps (one for each B-H bond), three redistribution equilibria (in which borane and the alkyl boranes exchange substituents), and five different monomer-dimer equilibria involving all the species with at least one B—H bond. In the hydroboration of 2,3-dimethyl-2-butene with diborane in THF, the reacting species is most likely a borane-THF complex. The reaction was foxmd to be second order overall, first order in alkene and first order in BH3-THF. The Eg was foimd to be 9.2kcal/mol, while the activation entropy was —27 eu. These results stand in contrast to the value of 2 kcal/mol determined for AH for the reaction of BH3 with ethene in the gas phase. °... 

For many purposes, borane does a great job in terms of both reactivity (high) and selectivity (not always so high). However, there are processes where a less reactive and more selective borane will be preferred, and numerous compounds are available. If borane is reacted with 2,3-dimethyl-2-butene, only a single addition takes place, because of the steric hindrance that would result from a further addition. The product is termed thexylborane (11.27). Similarly, disiamylborane (SiajBH, 11.28) is prepared from 2-methyl-2-butene. The number of times the borane reacts is clearly a function of steric hindrance. 9-Borabicyclo[3.3.1]nonane (9-BBNH, 11.29) is prepared from 1,5-cyclooctadiene, and catecholborane (HBcat, 11.30) from catechol (1,2-benzenediol) and borane. All of these have been used to improve selectivity for specific reactions—in general, the more hindered the borane, the more selective the reaction. 

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