David Todd Worcester Polytechnic Institute Worcester. MA 01609
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Textbook Errors, 136:
The Reducing Action of Sodium Borohydride
Every beginnin:: urganic text mentions the US^ of sodium boruhvdride as a reducinnapent. \Vhil(, 11 is well-known rhnt this reagent is widely used io reduce ketones and aldehydes to the corresponding alcohols, virtually all texts considerably oversimplify the subject. It is understandable that textbook authors should emphasize the useful selectivity of the reagent, but unfortunatelvin their zeal to make the subject clear to the student they impart several erroneous notions about the chemistry of NaBH4 that are difficult to eradicate later. On balance it would seem wiser to present the subject accurately, even though this means the student must work a little harder to absorb it. As Lenin said: "Facts are stubborn things." There are three aspects of the use of N&Hd that are almost always presented incorrectly in texts. Theseare: I ) NaBH4 does not reduce esters. This is incorrect. Esters are slowly reduced by a large excess of NaBH4 in methanol (or ethanol) a t room temperature and in good yield. A study of this topic was reported a good many years ago (1). T h e method is, in fact, a perfectly good way to reduce ethyl benzoate to benzyl alcohol, for instance. This reduction of esters to primary alcohols, for certain special cases, was reported in another paper the same year (2). Several further examples are given in Gaylord's 1956 book (3).This reduction method was put to good use in a recent synthesis in the azaprostaglandin field (4). I n the light of what has been clearly reported in the literature, it is strange that this error has persisted as long as it has. Since the use of LiAIH4 to reduce esters is far more hazardous in the hands of beginners than is the use of NaBH4, it is rather important, particularly in undergraduate laboratories, that this facet of N&H4 chemistry become better known than it is. If an ester reduction can be carried out. albeit slowlv. by means uf a tar safer reazrnr, it would oe the rmc: teilcher who would silrrificesaferv for svt.ed.'l'hecoit ~riNaHF14 isKP, that of LiAIH4, which atleast partially offsets the additional cost of the need to use 5 or 10 moles of NaBH4 per mole of --&--
TSLSl.
While it is entirely correct to emphasize the very considerable-and useful-kinetic differknce in the reaction of NaBHd with aldehydes and ketones on the one hand, and with esters on the other, there is scarcely a reagent in organic chemistry that does not exhibit the same sort of kinetic mectrum. The Friedel-Crafts acetvlation of anisole is lo7 times faster than that of chlorobenzene; Grignard reagents will react selectivelv with a ketone " moon. in the oresence of an ester group, etc. Why discriminate against N ~ B in H this ~ re"04'7 6"'".
2 ) NaBH4 does not reduce the C=C of a,&unsaturated carbonyl compounds.
Suggestions of material suitable for this column and columns suitable for publication directly should be sent with as many details as and particularly reference to modern textbooks, to W,possible, H. Eberhardt, of with Chemistry, Georgia of Tech-
cited. In order to be presented, an error must occur in at least two independent recent standard books. 540 / Journal of Chemical Education
This is incorrect-and how this'mvth ever cot started is difficult to understand since more oken thanunot, NaBH4 reacts with this class first bv. 1.4-addition, and subseauentlv . by redurtiun i9f the saturated rarhunvl wmpound to the saturated alcuhol. Fur instance, it ha5 I~ienreourred 31that the NaBHa reduction of cyclopenten-2-one-fgives 100% cyclopentanol. Other examples are given in House ( 6 ) . It has recently been reported that the reduction of n,P-unsaturated ketones in methanol, in the presence of CeCls.6Hz0, by sodium borohydride gives almost exclusively the allylic alcohol (7). ' 3) The direct product of the NaBH4 reduction of a ketone R2C=0 in a n alcohol solvent, R'OH, is the borate ion (R2CHO)aBe corresponding to the ketone. This is incorrect. The reduction, in fact, gives directly the alcohol, the borate ion present being derived from the solvent alcohol employed (8): H BHP
+ RZC=O CHgOH A R~AoH+ B ( o c H s p
Indeed, Wigfield and Gowland point out that: "An amusing corollary of this result is that the routine hydrolysis following a borohydride reduction may be entirely unnecessary." The error that they corrected goes back to the very early work of Schlesinger and Brown. In the first paper that mentioned the reducing action of NaBH4 on a ketone the authors wrote: "A possible course of the reaction is suggested in the equation" NaBH,
+ 4(CH3)&=0
-
This was an entirelv reasonable suneestion. Aonarentlv no".. body questioned itskalidity, and certainly nobody attempted to check on it-the main reason beine that from the ooint of view of the lab chemist who just wantsVtoget a reductiin done, it does not matter which alcohol is tied uo as a borate ion. However, this is just another case in whicha suggestion, put forward with appropriate caution, became hardened into dogma. In sum, it is to be hoped that in future most teachers will subject the somewhat complex topic of sodium borohydride reduction to a cost-benefit analysis, and decide not to continue to sell it short. Finally, it must be adniitted that the statement in a new organic laboratory manual (10)-namely, that sodium borohydride is dark grey rather than white-is an inexcusable error! Literature Cited 111 Rrsuln. M. S-and Rapnport. H.. J. Or#. Chem., 28.3261 119ti3). 421 Mesthinu. J. A.and Rond. C. H.. J. O m Chem.. 28.:1128 li96:ii. I:II navl,>.a. N., ueductilln ~ i t complex h ~ ~~ ~t d a~ lnterscience i ~d ~ ~ : P New Ysrk. N.Y.. 1956.p.50:3. 44) xc,retie. P.and Rareehr. F.. ~ ~ t r a h e d r c m szs (19771. 15) H m w t ~ H.C. . and Hem. H. M . J . Or#. ('hem., 3 4 2206 !196~).
U ~ I ~lne.. S ~ ~ ~ .
err..
!W HH"u". H. W.."M~dernSynlhetieReaetiunr."R~njamin.Readin~.Ma~.,?nded., 1972,
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iloi n , d d , u . . ~ ~ ~ ~ p ~ ~ i ~ P , ~~~ ~~~ Ct ~ a . H ~~IEI o ,~I ~~ ~CI. P ~ . ~~, , ~ , , ~i c~I ~~F h~ s~ NJ..1978. p. I : I ~ .
