Page 450 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Table 5.6. Reaction Conditions for Reductive Replacement of Halogen and Sulfonate 423
Groups by Hydride Donors
SECTION 5.3
Approximate conditions for complete reduction
Group III
Hydride-Donor Reagents
Hydride donor Halides Sulfonates

NaBH 3 CN a C 12 H 23 I, HMPA, 25 C 4h C 12 H 23 O 3 SC 7 H 7 , HMPA, 70 C, 8 h

NaBH 4 b C 12 H 23 Br, DMSO, 85 C, 1.5 h C 12 H 23 O 3 SC 7 H 7 , DMSO, 85 C, 2 h

LiAlH c d C 8 H 17 Br, THF, 25 C, 1 h C 8 H 17 O 3 SC 7 H 7 , DME, 25 C, 6 h
4
LiB C 2 H 5 3 H c C 8 H 17 Br, THF, 25 C, 3 h

a. R. O. Hutchins, D. Kandasamy, C. A. Maryanoff, D. Masilamani, and B. E. Maryanoff, J. Org. Chem., 42, 82 (1977).
b. R. O. Hutchins, D. Kandasamy, F. Dux, III, C. A. Maryanoff, D. Rotstein, B. Goldsmith, W. Burgoyne, F. Cistone,
J. Dalessandro, and J. Puglis, J. Org. Chem., 43, 2259 (1978).
c. S. Krishnamurthy and H. C. Brown, J. Org. Chem., 45, 849 (1980).
d. S. Krishnamurthy, J. Org. Chem., 45, 2550 (1980).
dissociation of a radical anion by one-electron transfer is a likely mechanism for
reductive dehalogenation of compounds that cannot react by an S 2 mechanism.
N
R X +e – R X – ·
– · –
R X R· + X
R· + H – R H +e –

One experimental test for the involvement of radical intermediates is to study
5-hexenyl systems and look for the characteristic cyclization to cyclopentane deriva-
tives (see Part A, Section 11.2.3). When 5-hexenyl bromide or iodide reacts with
LiAlH , no cyclization products are observed. However, the more hindered 2,2-
4
dimethyl-5-hexenyl iodide gives mainly cyclic product. 164
24°C
) CH I
CH 2 CH(CH 2 3 2 +LiAlH 4 CH 2 CH(CH ) CH 3
2 3
1 h
94%
CH
CH 3 3 CH CH
24°C 3 3
CH 2 CH(CH ) CCH I +LiAlH 4 CH 2 CH(CH ) CCH 3 +
2 2
2 2
2
1 h CH 3
CH 3 CH 3 3% 81%
Some cyclization also occurs with the bromide, but not with the chloride or the tosylate.
The secondary iodide, 6-iodo-1-heptene, gives a mixture of cyclic and acyclic product
in THF. 165
CH 3
LiAlH 4 CH 3
CH 2 CH(CH ) CHCH 3 CH 2 CH(CH ) CH CH 3 +
2 3
2 3
2
THF
I 21% 72%, 3.7:1cis:trans
164 E. C. Ashby, R. N. DePriest, A. B. Goel, B. Wenderoth, and T. N. Pham, J. Org. Chem., 49, 3545
(1984).
165
E. C. Ashby, T. N. Pham, and A. Amrollah-Madjadabadi, J. Org. Chem., 56, 1596 (1991).