Page 543 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

524 Table 5.7 includes data for some mono- and dialkylboranes that show even higher
regioselectivity than diborane itself. These derivatives are widely used in synthesis
CHAPTER 5 and are frequently referred to by the shortened names shown with the structures.
Polar Addition
and Elimination CH 3 CH 3 H
Reactions B
[(CH 3 ) 2 CHCH] 2 BH (CH 3 ) 2 CHCBH 2
CH 3
disiamylborane thexylborane 9-BBN
bis-(1,2-dimethylpropyl)borane 1,1,2-trimethylpropylborane 9-borabicyclo[3.3.1]nonane

These reagents are prepared by hydroboration of the appropriate alkene, using control
of stoichiometry to achieve the desired degree of alkylation. 9-BBN is prepared from
1,4-cyclooctadiene

CH 3
2 (CH 3 ) 2 C CHCH 3 + BH 3 [(CH 3 ) 2 CHCH) 2 BH
CH 3
(CH 3 ) 2 C C(CH 3 ) 2 + BH 3 (CH 3 ) 2 CHCBH 2
CH 3

As is true for most addition reactions, there is a preference for approach of the
borane from the less hindered face of the double bond. Since diborane itself is a
relatively small molecule, the stereoselectivity is not high for unhindered molecules.
Table 5.8 gives some data comparing the direction of approach for three cyclic alkenes.
The products in all cases result from syn addition, but the mixtures result from both
the low regioselectivity and from addition to both faces of the double bond. Even 7,7-
dimethylnorbornene shows only a modest preference for endo addition with diborane.
The selectivity is much enhanced with the bulkier reagent 9-BBN.
The haloboranes BH Cl, BH Br, BHCl , and BHBr are also useful hydroborating
2
2
2
2
reagents. 200 These compounds are somewhat more regioselective than borane itself,
but otherwise show similar reactivity. The halogen(s) can be replaced by hydride and
a second hydroboration step can then be carried out. This allows for preparation of
unsymmetrically substituted boranes.
Table 5.8. Stereoselectivity of Hydroboration with Cyclic Alkenes a
Product composition b
3-Methyl- 3-Methyl- 7,7-Dimethyl
cyclopentene cyclohexene norbornene
Hydroboration trans-2 cis-3 trans-3 cis-2 trans-2 cis-3 trans-3 exo endo
Reagent
Diborane 45 55 16 34 18 32 22 c 78 c
Disiamylborane 40 60 18 30 27 25 – –
9-BBN 25 50 25 0 20 40 40 3 97
a. Data from H. C. Brown, R. Liotta, and L. Brener, J. Am. Chem. Soc., 99, 3427 (1977), except where noted otherwise.
b. Product composition refers to the alcohols formed by subsequent oxidation.
c. H. C. Brown, J. H. Kawakami, and K.-T. Liu, J. Am. Chem. Soc., 95, 2209 (1973).
200
H. C. Brown and S. U. Kulkarni, J. Organomet. Chem., 239, 23 (1982).