Page 367 - Advanced Organic Chemistry Part B - Reactions & Synthesis

These reagents are prepared by hydroboration of the appropriate alkene, using control 339
of stoichiometry to terminate the hydroboration at the desired degree of alkylation.
SECTION 4.5
CH 3 Addition at Double
Bonds via Organoborane
2 (CH ) C CHCH + BH 3 (CH ) CHCH 2 BH Intermediates
3 2
3
3 2
CH 3
) CHC
) C C(CH ) + BH (CH 3 2 BH 2
(CH 3 2 3 2
CH 3
BH
+ BH 3
Hydroboration is a stereospecific syn addition that occurs through a four-center
TS with simultaneous bonding to boron and hydrogen. The new C−B and C−H bonds
are thus both formed from the same face of the double bond. In molecular orbital terms,
the addition is viewed as taking place by interaction of the filled alkene
orbital with
the empty p orbital on boron, accompanied by concerted C−H bond formation. 158

H B
H B B H B
H

As is true for most reagents, there is a preference for approach of the borane from the
less hindered face of the alkene. Because diborane itself is a relatively small molecule,
the stereoselectivity is not high for unhindered alkenes. Table 4.4 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 modest preference for endo addition with diborane. The selectivity is enhanced
with the bulkier reagent 9-BBN.

Table 4.4. Stereoselectivity of Hydroboration of Cyclic Alkenes a
Product composition b

3-Methyl 4-Methyl 7,7-Dimethylbi-
cyclopentene cyclohexene cyclo[2.2.1]heptene
trans-2 cis-3 trans-3 cis-2 trans-2 cis-3 trans-3 exo endo
Diborane 45 55 16 34 18 32 22 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 otherwise noted.
b. Product composition refers to methylcycloalkanols formed by oxidation.
c. H. C. Brown, J. H. Kawakami, and K.-T. Liu, J. Am. Chem. Soc., 95, 2209 (1973).

158
D. J. Pasto, B. Lepeska, and T.-C. Cheng, J. Am. Chem. Soc., 94, 6083 (1972); P. R. Jones, J. Org.
Chem., 37, 1886 (1972); S. Nagase, K. N. Ray, and K. Morokuma, J. Am. Chem. Soc., 102, 4536
(1980); X. Wang, Y. Li, Y.-D. Wu, M. N. Paddon-Row, N. G. Rondan, and K. N. Houk, J. Org. Chem.,
55, 2601 (1990); N. J. R. van Eikema Hommes and P. v. R. Schleyer, J. Org. Chem., 56, 4074 (1991).