410
P. S. SKELL, R. G. ALLENAND G. K. HELMKAMP
words, for the permanganate oxidation of aldehydes, substituents have a greater effect upon the withdrawal of two electrons, by the oxidant, than upon the removal Of the proton which also accompanies the reaction. From this work, one must infer that the acceleration of an oxidation-reduction by elec-
[COSTRIBUTION FROM
THE
Vol, 82
tron-releasing substituents is not a valid argument against an ester mechanism. Achowledgment.-The authors gratefully acknowledge the support of this work by The Petroleum Research Fund and the U.S. Rubber Co. CAMBRIDGE 38, MASS.
THE PENSSVLVASIA STATE USIVERSITYASD THE PHYSICAL SCIENCES, UNIVERSITY OF CALIFORXIA AT RIVERSIDE]
DEPARTMEST OF CHEMISTRY OF
DIVISIOXOF
Specific Rotations of Pure 2-Bromobutanes. Stereochemistry of the 2-Butanol to 2-Eromobutane Conversion BY
P.s. S K E L L , ' K. G. .ALLEN2
.\ND
G. K.
HELXK;i>IP3
RECEIVED X a u 28, 1959 There is no satisfactory method for demonstrating the optical purity or for resolving alkyl halides with halogen a t the D is now demonstratcd t o be 28';; asymmetric center. T h e purest 2-bromobutane reported in the literature ( [ C Y ] ~ 28.8') racemized. Introduction of one deuterium in the 3-position creates a second asymmetric center which serves as an internal standard of configuration, enabling one t o define t h e stereochemical course of reactions without resorting to resolutions. erythro- and threo-3-deuterio-2-butanols are converted t o the bromides with phosphorus tribromide. Comparison of these products with pure erythro- and tizreo-3-deuterio-2-bromobutanes indicates the alcohol t o bromide conversion occurs without skeletal rearrangement and produces n product which is 85-90';l, inverted a t the carbinol carbon corresponding t o 20-30Si, racemization, the rcinainder having the same configuration as the alcohol. Thus it becomes possible to estimate [CY]25D for optically pure 2-brotnobutane (39.4" 1, erythro-3-deuterio-2-brornobutane (38.9') and threo-3-deuterio-2-bromobutant (39.9'). Racemization is minimized by low reaction temperatures, 2-broinobutane with [ a ]P t +32.09" ~ being produced a t
-IS0.
Introduction Although the specific rotations of the completely resolved haloalkanes are necessary for interpretations of mechanism studies employing these enantiomorphs, there are probably no satisfactory values recorded in the literature. The 2-bromobutanes have been prepared from the resolved 2butanols by reaction with phosphorus tribromide or hydrogen bromide, and by brominating 2butylmercuric bromide in pyridine yielding preparations with [CY]%D2S.G0,4 26.1 ',' 28.45°,8 88.Go,' etc., these being the highest values which appear in the literature. Unpublished experimental results,* which will be presented in detail elsewhere, suggested that the pure enantiomers probably have a value of [CY]'~D36 '. Since further resolution of 2-bromobutanc is not feasible a t present, an alternative means of gaining information about optical 'purity was conceived. Substitution of a deuterium atom for u ~ a~ one of the hydrogens of the -C&- i n t r o dCtS second asymmetric center which serves as an internal standard of configuration. Thus it becomes possible to determine the extent of isonieriza(1) T h i s research was supported b y t h e United S t a t e s .4ir Force through t h e Air Force Office of Scientific Research of t h e Air Research & Development C o m m a n d , under contract Xo. X F 49(638)-457. ( 2 ) A portion of a thesis submitted in partial fulfillment of t h e requirements for t h e P h . D . degree. A fellonship provided by t h e Allied Chemical Corporation is gratefully acknowledged. (3) T h i s research w a s supported in p a r t by a g r a n t from T h e Petroleum Research F u n d administered by t h e American Chemical Society. Grateful acknowledgment is hereby made to t h e donors oi this fund. (4) P. A. Levene a n d R . X a r k e r , J . B i d . C h r i i t . , 91, 403 (1831). (6) R. Letsinger, THISJ W R X A L , 7 0 , 406 (1948). ((i) G. K . H e l m k a m l ~C. , 11. Juel a n d 11. S h a r m a n , J . Oi,g. C h e i i l . , 21, 844 (l95c;) ( 7 ) F. R. Jensen, 1,. 1). \'v'Iii~)ple, 11. K . \\'edegarrtner and J . A . Landgrebe, THISJ O U R N A L , 81, 12fir3 (1959). (8) G . P. Bean and P. S. Skell.
tion during conversion of 3-deuterio-2-butanol to 3-deuterio-2-bromobutane without resorting to resolutions. For example, if erythro-3-deuterio-Ybutanol is converted to the bromide, the major product might be threo-3-deuterio-2-broniobutane, and the extent of isomerization during the conversion would be indicated by the erytlzro-3-deuterio2-bromobutane content of the product. Since these substances are diastereomerically related, the full array of available physical properties may be used to distinguish these isomers. Pure samples of d,l-ergthro- and d,l-threo-3deuterio-2-bromobutanes are available by radical chain additions of deuterium bromide to tranaand czs-%butenes, respe~tively.~These bromides can also be prepared from the corresponding thrroand cuythro-3-deuteri0-2-butanols, which are available by lithium aluniinuin deuteride reduction of meso- and d,l-2,3-epoxyb~tanes.~ Analysis for Diastereomers.-Unfortunately the non-congruous infrared absorption bands (up t o 13 fi) of erythro- and threo-3-deuterio-2-bromobut a m s are not sufficiently resolved for convenient determination of extent of intercontamination However, since alkaline dehydrohalogenations of these substances t o the butenes are stereospecific trans pro~esses,~ this reaction was used for two independent methods of analysis. A. Dehydrohalogenation of the erythro isomer yields trans-%butene and 2-deuterio-cis-2-butene, and the threo isomer yields cis-2-butene and 2deuterio-trans-2-butene. A mixture of erythro and threo yields the four 2-butenes. The cisand trans-olefins were separated by vapor phase chromatography. The percentage C4HBin each of them was detcrniincd by quantitative infrared analysis aiid the C,H;D content by difference i ' 0 I'
5
> k < l l .LIICI I
\lien
'111~5 1 0 1 I R V A I
81, i i 8 1
(l'lill\
Jan. 20, 1960
B. A primary kinetic isotope effect accounts for the different ratios of 1-, cis-2- and trans-2-butenes obtained from the 2-bromobutanes. Thus, the ratio of these olefins differs considerably for undeuterated 2-bromobutane and the erythroand threo-3-deuterioisomers. In Table I is recorded the ratio of the olefins from these three substances under identical dehydrohalogenation conditions, determined by vapor phase chromatography of the olefinic product. For mixtures of threo- and erythro-3-deuterio-2-bromobutanes linear interpolations of the observed percentage compositions leads directly to the isomer ratio for the bromides. The isomer ratio for the bromides was calculated independently from the cis- and trans, and 1butene percentages, and the three values were in good accord. However, the compositions calculated from the trans-2-butene percentages are considered the most reliable because (a) the percentage of this olefin shows the greatest sensitivity to bromide isomer ratio, and (b) this olefin could be determined with greatest accuracy. The results reported below are based on the trans-2-butene percentages. To further check this analytical procedure, known mixtures of threo and erythro compounds were analyzed (Table I), thus demonstrating satisfactory accuracy. TABLE I PERCENTAGE COMPOSITION OF THE BUTENESPRODUCED BY DEHYDROHALOGEXATION IN 1 M POTASSIUM ETHOXIDE IN ETHANOL AT 70" CH3CH2CHBrCHP threo-CHaCHD CHBr CHI" erythro-CHaCHDCHBrCH?
411
%BUTANOL TO 2-BROMOBUTANE CONVERSION
alcohols to the halides was highly sensitive to reaction conditions. Values for [a]25Drange from 26 to 32', but careful control of reaction temperature yielded products with variations of about 0,5y0, The radical-chain photochemical addition of DBr to the 2-butenes has been d e ~ c r i b e d . ~ Results The preparations of alcohols and bromides used in this work are summarized in Chart I. An effort was made to make all reaction variables the same throughout the preparations and the analyses. Infared spectra revealed isomer intercontamination for Ia, I b and 11. I t was possible to estimate from the spectra of I a and I b that 2.6% of undeuterated 2-brornobutane was present. Dehydrohalogenation of the 3-deuterio-2-bromobutanes, prepared from the alcohols according to Chart I, yielded 1-, cis- and tra?zs-2-butenes in the percentages recorded in the last three lines of Table I . Since the undeuterated 2-bromobutane in the deuterated compounds was similar in the samples prepared from the alcohols (2.6%) and the olefin-DBr adducts ( 1.GY0),9 these differences were ignored in arriving a t threo :erytiiro ratios. By the linear interpolation described for analytical method B, the compositions of I a and I b and I1 were calculated (Table 11). TABLE I1 COMPOSITION O F erythro- A S D threo-3-DEuTERIO-2-BROMOBUTANEP P R E P A R E D Uia THE ALCOHOLS
PERCENTAGE
1-Butene
cis-2Butene
luaits-2Butene
Bromide
thuro-CaHeDBr
19.9 24.8 35.2
21.8 9.8 33.4
58.3 65.4 31.4
Sample Ia Sample I b SamDle I1
88 3
Known mixtures of erykhro and Mixture A' Obsd. 25.9 Calcd. 26.3 Mixture E d Obsd. 33.6 Calcd. 33.6
threo 13.1 12.9 31.0 30.9
61.0 60.9 35.4 35.5
Unknown mixtures of erythro and tltreo Sample Ia' (high threo content) 26.0 11.8 62.2 Sample Ib5 (high threo content) 26.7 11.8 61.5 Sample IIO (high erythro content) 3 4 . 4 30.7 34.9 I n a different set of experiments the olefin ratios were nearly identical for 2-bromobutane 2-deuterio-%bromobutane. The sample of threo contained 1.6% undeuterated 2-brotnobutane, and the same contamiiiation mas assumed for the evythro.6 12.870 erythro, 85.6y0 threo, 1.6% undeutcmted. 85.67, erythm, 12.8Cj0 threo, 1.670undeuterated. Sample I A and IB contained 2.6% undeuterated 2bromobutane, and the same contamination was assumed for sample 11.
+
Hi.1 10 8
erythuoCaHeDBr
sec-CaIIoBr
2.6 2.6
9.1 11.0 86 G
12.6)
Annlyses of knowxi ri:i.
