1,5-cyclooctadiene reaction with borane

Finally, reaction of borane with 1,5-cyclooctadiene gives a product where two of the B-H units have reacted with the two C=C units across the ring to give a bicyclic product (see Chapter 9, Section 9.7), 62. The name of this dialkylborane is 9-borabicyclo[3.3.1]nonane, which is often abbreviated 9-BBN or drawn as shown later. Nomenclature rules for bicyclic systems such as this are described in Chapter 9, Section 9.7. A. There are a total of nine atoms (nonane) but C9 has been replaced with boron (9-bora). All three of these boranes react as Lewis acids with an added alkene. 

Monochloborane-dimethyl sulfide coexists with small amounts of the borane and dichloroborane complexes, but the bromoborane-dimethyl sulfide complex appears to be almost pure. - These complexes react readily with alkenes at 25 °C and can be used for hydroborations in a variety of solvents. Dialkylha-loboranes are obtained in high yield as their dimethyl sulfide complexes (equation 24), but dimethyl sulfide is readily removed under reduced pressure if required. - The reagents are also useful for cyclic hydroborations of dienes such as cyclooctadiene (equation 25). An alternative approach to dialkylbro-moboranes involves the reaction of dialkyl(methylthio)boranes with bromine. ... 

The calculated energy barrier starting with the mono-hydroborated D8 1,5-cyclooctadiene was 8.25 kcal/mol, which is easily overcome in room temperature, and therefore, a rapid reaction is expected. Note, however, that to determine the actual activation energies, the relative energy of the reactant states also needs to be accounted for, as B-H-B bridges are generally more stable than the interacting intramolecular n-bond-borane adduct [17]. We have not calculated this here, but estimate it to be below 7 kcal/... 

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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