Page 449 - Advanced Organic Chemistry Part B - Reactions & Synthesis

422 Scheme 5.6. (Continued)

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O OSiH(Ph) 2 O
5 mol % CuCl
5 mol % NaOt Bu Ph 3 SiF 2 R′
R′X
5 mol % S-p-tol-BINAP
R Ph 2 SiH 2 R R
e.e. > 90%
dr > 15:1

When necessary, the trans:cis ratio can be improved by base-catalyzed equilibration.

5.3.4. Reduction of Other Functional Groups by Hydride Donors
Although reductions of the common carbonyl and carboxylic acid derivatives are
the most prevalent uses of hydride donors, these reagents can reduce a number of
other groups in ways that are of synthetic utility. Halogen and sulfonate leaving groups
can undergo replacement by hydride. Both aluminum and boron hydrides exhibit this
reactivity, and lithium trialkylborohydrides are especially reactive. 160 The reduction is
particularly rapid and efficient in polar aprotic solvents such as DMSO, DMF, and
HMPA. Table 5.6 gives some indication of the reaction conditions. The normal factors
in susceptibility to nucleophilic attack govern reactivity with I > Br > Cl being the
order in terms of the leaving group and benzyl ∼ allyl > primary > secondary >
tertiary in terms of the substitution site. 161 For primary alkyl groups, it is likely that
the reaction proceeds by an S 2 mechanism. However, the range of halides that can
N
be reduced includes aryl halides and bridgehead halides, which cannot react by the
S 2 mechanism. 162 The loss of stereochemical integrity in the reduction of vinyl
N
halides suggests the involvement of radical intermediates. 163 Formation and subsequent

160
S. Krishnamurthy and H. C. Brown, J. Org. Chem., 45, 849 (1980).
161 S. Krishnamurthy and H. C. Brown, J. Org. Chem., 47, 276 (1982).
162 C. W. Jefford, D. Kirkpatrick, and F. Delay, J. Am. Chem. Soc., 94, 8905 (1972).
163
S. K. Chung, J. Org. Chem., 45, 3513 (1980).