Page 545 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

526 Migrations are especially facile for tetra-substituted alkenes; they occur at 50–60 C
and exhibit both regio- and stereoselectivity. The course of the reactions is dictated by
CHAPTER 5 206
the relative energy of the reversibly formed borane alkene complexes.
Polar Addition
and Elimination
Reactions
H B H
Ph 1) BH 3 -THF H B 50 °C Ph Ph 2 CH C
Ph
B
Ph Ph Ph H

Unstrained bicyclic rings have little tendency to rearrange. bis-Bicyclo[2.2.2]
octanylborane has been found to be particularly useful for formation and isomerization
of monoalkyl derivatives.

B H + CH 3 CH C(CH 3 ) 2 B C H 2 CH 2 CH(CH 3 ) 2

Ref. 207

There is evidence that boron migration occurs intramolecularly. 208 A TS that
describes this process has been located computationally (Figure 5.6). 209 It involves
an electron-deficient -complex about 20–25 kcal above the trialkylborane. These
structures are analogous to the bridged carbocations in carbocation rearrangements.
The boron can migrate to the end of the chain through a series of such structures.

H H
B H H H B H H H B H H H B H
C C C C H C C C C H CC C C H C C C C H
H H H H
H
H H H H H H B H H B H
C C C C B C C C C H C C C C H
H H H

5.7.2. Reactions of Organoboranes
Alkylboranes are very useful intermediates in organic synthesis. In this section
we discuss methods by which the boron atom can be efficiently replaced by hydroxy,

206 L. O. Bromm, H. Laaziri, F. Lhermitte, K. Harms, and P. Knochel, J. Am. Chem. Soc., 122, 10218
(2000).
207
H. C. Brown and U. S. Racherla, J. Am. Chem. Soc., 105, 6506 (1983).
208 S. E. Wood and B. Rickborn, J. Org. Chem., 48, 555 (1983).
209
N. J. R. van Eikema Hommes and P. v. R. Schleyer, J. Org. Chem., 56, 4074 (1991).