Page 820 - Advanced Organic Chemistry Part B - Reactions & Synthesis

796 The dialkylboranes can be prepared from thexylchloroborane. The thexyl group does
not normally migrate.
CHAPTER 9
Carbon-Carbon KBH(OR) 3
Bond-Forming Reactions BHCl + R′CH CH 2 BCH 2 CH R′ BCH CH R′
2
2
2
of Compounds of Boron,
Silicon, and Tin Cl H
A similar strategy involves initial hydroboration by BrBH . 30
2
Br
2
2
BrC CR R′CH CH B H – OMe R′CH CH 2 H
2
R′CH CH + BrBH 2 R′CH CH BHBr
2
2
2
Br R (MeO) B R
2
H +
R′CH CH 2 H
2
H R
Stereoselective syntheses of trisubstituted alkenes are based on E- and
Z-alkenyldioxaborinanes. Reaction with an alkyllithium reagent forms an “ate” adduct
that rearranges on treatment with iodine in methanol. 31
R R′
O
1) R″Li R R′ 1) R″Li R R″
R B O C C
3
3
2) I 2 , CH OH C C or 2) I 2 , CH OH C C
C C H B O
3) NaOH H R″ O 3) NaOH H R′
H R′
Both alkynes and alkenes can be obtained from adducts of terminal alkynes and
boranes. Reaction with iodine induces migration and results in the formation of the
alkylated alkyne. 32

Li +
_ I 2
B C C(CH ) CH 3 C C(CH ) CH 3
2 3
2 3
3 –78°C
100%
The mechanism involves electrophilic attack by iodine at the triple bond, which
induces migration of an alkyl group from boron. This is followed by elimination of
dialkyliodoboron.

2
Li + I 2 R B I
R B – C C R′ C C R C C R′ +R BI
2
3
R R′
30
H. C. Brown, T. Imai, and N. G. Bhat, J. Org. Chem., 51, 5277 (1986); H. C. Brown, D. Basavaiah,
and S. U. Kulkarni, J. Org. Chem., 47, 3808 (1982).
31 H. C. Brown and N. G. Bhat, J. Org. Chem., 53, 6009 (1988).
32
A. Suzuki, N. Miyaura, S. Abiko, M. Itoh, H. C. Brown, J. A. Sinclair, and M. M. Midland, J. Am.
Chem. Soc., 95, 3080 (1973); A. Suzuki, N. Miyaura, S. Abiko, M. Itoh, M. M. Midland, J. A. Sinclair,
and H. C. Brown, J. Org. Chem., 51, 4507 (1986).