5132
JOHN
F. L A N E
[CONTRIBUTION FROM
THE
.4ND S T E P H E N
E. ULRICH
VOl. 72
SCHOOLOF CHEMISTRY, RUTGERS UNIVERSITY j
Reactions of Organometallic Compounds with Alkyl Halides. IV. On Wurtz Syntheses with Optically Active Halides R Y JOHN E'. LANEAND STEPHEN E. ULRICII~
In earlier papers of this seriesJjh i t has been shown that the action of sodium ethyl on ( - ) 2 bromooctane gives a small amount of optically inactive 7,s-dimethyltetradecane. On the other hand, the interaction of sodium ethyl with (+)achlorooctane gives rise to a small amount of (+) 7,8-dimethyltetradecane. It was concluded3 that, while the hydrocarbon is produced in each case by the interaction of sodium s-octyl with the alkyl halide, i t is produced in the two experiments by different paths of reaction, e.g.
Exactly similar behavior obtains with the optically active (+)z-halobutanes. Thus, from [aIz5D 13.7", (+)3,4(+)2-chlorobutane, dimethylhexane, [ c Y ] ' ~ D-t- 0.58', was obtained in Sq; yield. (+)2-Bromobutane has, of course, already been shown by Wallis and A d a m 5 to give optically inactive 3,4-dimethylhexane. The results strongly support Morton's picture of the U'urtz reaction and render most unlikely the alternative view6 that this reaction proceeds via free radicals. I t is of interest, a t this point, to consider the slow data so far obtained by us and also by Letsinger7s* (-)s-CaH1TBr + Bre + dl-s-C&I,;* -t on the extent of racemization accompanying the fast dl-s-CsH~,~Na@ + d l P Z C S Q - ( S - C ~+H ~NdBr T)~ replacement of halogen in secondary alkyl halides (1) (RX) by various hydrocarbon residues derived coiicerted from reactive organometallic compounds (R1( + ) S - C ~ H , ~ C+ I d l - ~ - C s H l ~ ~ , \ j a------+ @ AT). For convenience of discussion these data (+)+meso - ( ~ - C ~ H I + , ) JNaBr (2) are summarized in Table I. In general, it is seen Now Morton4 has suggested that the initial that while replacement of chlorine is attended stage of the Wurtz synthesis involves the direct by little racemization, the replacement of bromine is attended by extensive or complete racemizaformation of a sodium alkyl tion when R 1 is alkyl and M is sodium or when 2Na + RX --+ S a x + SiR 131 KI is alkaryl and Lf is magnesium (MgBr). followed by a second stage in which this sodium When, however, R1 is alkaryl and M is sodium, or alkyl reacts with a second mole of halide t o give R 1 is allyl and Pvf either sodium or magnesium, the hydrocarbon (RR) little racemization results. NaR RX -+ RR + NaX The results obtained with alkyl sodium and (4) This second stage is, of course, precisely the kind benzyl metal compounds suggest that whether a of reaction which has been shown to give 7,s- given halide reacts according to path (1) or dimethyltetradecane from optically active 2- path ( 2 ) above depends not only on the ease of of the carbon halogen bond, but also halooctanes in the earlier studies in this s e r i e ~ . ~ ionization ,~ If it occurs in the Wurtz reactions of these and on the readiness with which the organo-metallic similar optically active halides, then comparable compound can supply the carbanion which effects optical behavior should be observed : namely, substitution. Thus, the substitution process the action of sodium on optically active 2-bromo- might be pictured as arising from the association octane should give inactive 7,8-dimethyltetra- of the organometallic compound, acting either ~ the decane, while sodium and optically active 2- a5 an ion-pair or a polar m o l e ~ u l e ,with chlorooctane should give active i,8-dinicthyl- carbon-halogen dipole 1
