Edited by GEORGE WIGER
textbook forum
California State University Carson. CA 90747
Conjugate and Nonconjugate with LiAIH4 and NaBH4 G. Richard Meyer The College of Charleston, Charleston. SC 29401 Most organic textbooks, whether hepinning o r advanced, discuss the use of complex metal hydrides as reducing agents. This discussim, especially in beginning texts, is often limited to a consideration of the action of lithium aluminum hydride and sodium honrhydride. In a n apparent attempt to simplify the topic, virtually every author who considers the phenomenon d"cr1njugate reduction" of n,/3-unsaturated carhonyl compounds with these reagents seriously misrepresents the data t o he found in the chemical literature ( 1 1. Some errors in the presentation of the reducing action of sodium borohydride have heen noted recently (21. Aunarentlv the errors with reeard t o coniueate .. . .. reduction arise from (a) a feeling that sodium horohydride is more selective than lithium aluminum hydride and ( h ) a desire to make such selectivity more or less universal. Thus, t h e most common error is t o state or implv that. while sodium horo-
tims-e.g., by changing the temperature i:i,3) or the solvent (5)or by employing inverse addition ( 4 I. Conjugate reduction can he amiided by the use 01aluminum hydride 16) or diisoI~utylaluminumhydride (7) as the reducing agent. o,o-unsaturated carhonyl compounds with strong electron withdrawing groups attached to the n-carbon atom are not considered here. I( is typical for only the carhon-carbon double bond to he r e d u c d hy both lithium aluminum hydride (8) and sodium hmohydride ( 9 )in such systems. In summarv, it is to he noted t h a t in most m.8-unsaturated systems (i.e., aldehydes and ketones) other than cinnamyl, lithium aluminum hvdride txedominantlv reduces onlv the carhonyl group, while sodium horohydride has a greater tendency t o reduce the carhan-carbon d ~ ~ u h hond le (at least the reduction uf this functional grouping
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Thus, one may read in one text t h a t lithium aluminum hydride is a ". . . powerful reducing agent. Will reduce C=C if conjugated with C=O" and in another t h a t ". . . lithium aluminum hydride often reduces double h m d s which are in conjugation with C=O honds, as well as reducing the C=O honds." One may also read t h a t sodium horohydride is more "selective" than lithium aluminum hydride and catalytic hydrogenation in that it ". . . has no effect on C=C honds" and ". . . sodium hnrahydride, NaBHd, does not reduce carboncarbon douhle honds, not even those conjugated with carhmyl groups, and is, therefore. useful for the reduction of such unsaturated carhonyl compnunds t o unsaturated alcohols." One also finds t h a t ". . . neither lithium aluminum hvdride nor sodium borohydride reduces alkenyl drluble honds" and that ". . . t h e reduction of'2-butenal. CH.,CH=CHCHO, must he carried out a t low temperatures in order to avoid reduction of the douhle bond." Unfi~rtunately,none of these statements corresponds to the experimental facts. T h e experimental data involving the reduction of o,o-unsaturated compounds (limited primarily t o aldehydes and ketones) are presented in the accompanying tal~le. T h e erroneous notion t h a t lithium aluminum hydride produces conjugate reduction is apparently based on studies of the cinnamyl system (e.g., entries 19-23), With b u t few exceptions (e.g.. entries 16 and 171, this is the only system i n is o b s r r w d with lithium olumiwhich ~ ~ n j u g ( zreduction te nurn h.sdridr. T h e tendency of sodium horohydride t o reduce the conjugated carbon-carhon douhle hond is also well documented, a s can he seen in the tahle (e.g., entries 1,s.6,8-14, 16-18, 20-23). T h e tabulated results record, except where noted. "normal" lithium aluminum hvdride idiethvl ether solvent) and sodium horohydride (alcohol solvent) reductions a t room temperature. It is eenerallv. . possible t o alter t h e product ratios tahulated hy changing the reaction condi628
Journal of Chemical Education
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101 49% of the rtaning material was recavered (PI46% of a rearrangedalcohal was formed. (q) 53% of the sfaning maternal was recovered. ir) 16% offhe staning material war recovtned. 1.11 . 31 % *I the starisno material w m recovered. Ill 18% of the sfaning material was recovered. ( 8 ~ackron.w R andzurquiyah. A . J Chem Sw'. 5280 (1965). (u) Brown. M. s.. and ~apoport.H.. J mg C k m . 28,3261 119631. Iw) Iqbal. K.. and Jackson. W. R.. J. Chrm. Soc.(CI. 616 (1968). lx) Rondestve% Jr.. C. S.. J. A m r COsm. Sac. 73, 1509 11951). (y) &Idwin. J. E.. J. Oq.Chem.. 31. 2441 (1966). 11) Wwdward. R. 6.. and Kafz. 1.J.. Tehahedhedm.5. 70 119591. Izzl Cookson. R S.. lhaacr. N. S.. and Szelke. H.. Tehshsnm. 20. 717 (1964).
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