Page 426 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Lewis acid strength and hardness of the lithium cation. Both LiBH and Ca BH are 399
4 2
4
more reactive than sodium borohydride. This enhanced reactivity is due to the greater
2+
+
+
Lewis acid strength of Li and Ca , compared with Na . Both of these reagents can SECTION 5.3
reduce esters and lactones efficiently. Group III
Hydride-Donor Reagents
CH OH
CO 2 C 2 H 5 2
Ca(BH )
4 2
CN CN
70%
Ref. 64

LiBH 4
C H CHCH CH CH OH
7 15
2
2
2
C H O O
7 15
OH 45%
Ref. 65
Zinc borohydride, which is also a useful reagent, 66 is prepared by reaction of ZnCl 2
2+
with NaBH in THF. Owing to the stronger Lewis acid character of Zn ,Zn BH
4
4 2
is more reactive than NaBH toward esters and amides and reduces them to alcohols
4
and amines, respectively. 67 Zn BH reduces carboxylic acids to primary alcohols. 68
4 2
The reagent also smoothly reduces -aminoacids to -aminoalcohols. 69
PhCHCO H + Zn(BH ) PhCHCH OH
2
2
4 2
NH 2 NH 2
87%
Sodium borohydride is sometimes used in conjunction with CeCl (Luche’s reagent). 70
3
The active reductants under these conditions are thought to be alkoxyborohydrides.
Sodium cyanoborohydride is a useful derivative of sodium borohydride. 71 The
electron-attracting cyano substituent reduces reactivity and only iminium groups are
rapidly reduced by this reagent.
Alkylborohydrides are also used as reducing agents. These compounds have
greater steric demands than the borohydride ion and therefore are more stereoselective
in situations in which steric factors come into play. 72 These compounds are prepared
by reaction of trialkylboranes with lithium, sodium, or potassium hydride. 73 Several
® 74
of the compounds are available commercially under the trade name Selectrides .
64 H. C. Brown, S. Narasimhan, and Y. M. Choi, J. Org. Chem.
65 K. Soai and S. Ookawa, J. Org. Chem., 51, 4000 (1986).
66
S. Narasimhan and R. Balakumar, Aldrichimica Acta, 31, 19 (1998).
67 S. Narasimhan, S. Madhavan, R. Balakumar, and S. Swamalakshmi, Synth. Commun., 27, 391 (1997).
68 S. Narasimhan, S. Madhavan, and K. G. Prasad, J. Org. Chem., 60, 5314 (1995); B. C. Ranue and
A. R. Das, J. Chem. Soc., Perkin Trans. 1, 1561 (1992).
69
S. Narasimhan, S. Madhavan, and K. G. Prasad, Synth. Commun., 26, 703 (1996).
70 A. C. Gemal and J.-L. Luche, J. Am. Chem. Soc., 103, 5454 (1981).
71 C. F. Lane, Synthesis, 135 (1975).
72
H. C. Brown and S. Krishnamurthy, J. Am. Chem. Soc., 94, 7159 (1972); S. Krishnamurthy and
H. C. Brown, J. Am. Chem. Soc., 98, 3383 (1976).
73 H. C. Brown, S. Krishnamurthy, and J. L. Hubbard, J. Am. Chem. Soc., 100, 3343 (1978).
74
Selectride is a trade name of the Aldrich Chemical Company.