Page 366 - Mechanism and Theory in Organic Chemistry

Electrophilic Addition to Double and Triple Bonds 353

very important. The rate trends in chlorination parallel those for br~mination.~~
The limited facts available indicate that the mechanism is similar to that of
addition to olefins. Pincock and Yates have studied the addition of bromine to a
number of alkyl- and arylacetylenes in acetic acid. At low bromine concentra-
tions the reaction is second-order, first-order each in Br, and in acetylene.
Alkylacetylenes give only anti addition, indicating that a bromonium ion lies
on the reaction path. Ring-substituted phenylacetylenes, however, give both syn
and anti addition; and the logs of the rates correlate linearly with the a+ con-
stants of the substituents, giving a very large negative p value ( - 5.17). In these
compounds, open vinyl cations are apparently formed as intermediate^.^^

Hydroboration56
The addition of a boron hydride across a double or triple bond (Equ_a~i~n-7~U
is
caled hydroboration.
L

We include it here despite uncertainty about whether attack is initiated by
electrophilic boron or nucleophilic hydrogen or both sim~ltaneously.~~
In additions of diborane, the major product is formed by the attachment of
boron to the less substituted carbon. For example, addition of diborane to
1-hexene (31) gives a product that has 94 percent of the boron attached to the
terminal carbon. Similarly, diborane added to 2-methylbutene-2 (32) gives 98
percent of boron incorporation at C3.58

If, instead of diborane, a boron hydride substituted with bulky alkyl groups
is added to a double bond, the regiospecificity increases. Thus bis(3-methyl-2-
buty1)borane reacts with 1-hexene to give 99 percent terminal boron incorpora-
tion (Equation 7.18) .59
Electronic effects as well as steric effects are important in determining the
orientation of addition as is shown, for example, by the data in Table 7.8. The
regiospecificity is increased as 2-butene is substituted in the 1 position by increas-

64 See note 15(c), p. 341.
66 (a) J. A. Pincock and K. Yates, Can. J. Chem., 48, 3332 (1970) ; (b) J. A. Pincock and K. Yates,
J. Amer. Chem. SOC., 90, 5643 (1968).
68 H. C. Brown, Hydroboration, W. A. Benjamin, Menlo Park, Calif., 1962.
67 See note l(a), p. 337.
68 H. C. Brown and G. Zweifel, J. Amer. Chem. SOC., 82, 4708 (1960).
68 H. C. Brown and G. Zweifel, J. Amer. Chem. Soc., 82, 3222 (1960).