4590
w. M. SCHUBERT, B. s. K.lBINOVIrCH,
[CONTRIBUTIONFROM
THE
N. K.LARSON AND
v. LI.SI>%
DEPARTMENT O F CHEMISTRY AND CHEMICAL ENGINEERING, UNIVERSITY
VOl. 74 OF
WASHINGTON]
The Stereochemistry of the Debromination of Vicinal Dibromides by Metals1 BY W. M. SCHUBERT, B.S. RABINOVITCH, N. R. LARSON AND V. A, SIMS RECEIVEDAPRIL5, 1952 The debromination of mesa- and dl-2,3-dibromobutane with sodium in liquid ammonia gave the same mixture of cis- and yielded slightly different mixtures trans-2-butene, whereas mesa- and 50% vzeso-50% dl-1,2-dibromo-1,2-dideuteroethane of cis- and trans-1,2-dideuteroethylene. Magnesium in tetrahydrofuran, as well as zinc in water, yielded exclusively the “trans” debromination products with these dihalides. Both meso- and dl- forms of a,cu’-dibromobibenzyl and sodium CY,CY’dibromosuccinate gave essentially only the trans-olefin with magnesium or zinc. Conditions for debromination did not bring about isomerization of any of the olefin products. Debrominations with the divalent metals are discussed in terms of an ionic mechanism; those with sodium, in terms of a free radical mechanism.
It has been reported b y Young, Jasaitis and Levanas2 that the action of zinc on meso-4,5-dibromooctane yields pure trans-4-octene, whereas cil-4,5-dibromooctane yields cis-4-octene. I n the present work, the results of which are summarized in Table I, i t has been found that magnesium in tetrahydrofuran and zinc in water gives exclusively “trans” elimination of bromine from meso- and dl2,3-dibrornobutane and from 957, meso-5%70 dL1,%dibromo-1,2-dideuteroethane. On the other hand, with sodium in solution in liquid ammonia meso- and dl-2,3-dibromobutane gave a mixture of 2-butenes of essentially the same composition. Under the same conditions, a mixture of 955% meso-5% dl- 1,2-dibromo-1,2-dideuteroethylene and a 50% meso-50yo dl-mixture gave slightly different percentages of cis- and truns-1,Zdideuteroethylene (Table I).
although Downing and Wright5 were unable to confirm this observation. However, it was shown in the present work that &-stilbene remains unchanged under the conditions prevailing in the debrominations with magnesium or zinc. The debromination with zinc and water of mesoand dl-sodium a , n’-dibromosuccinate gave results similar to those reported for the debromination of meso- and dl-ethyl a,a ’-dibromosuccinate.” It was also found that sodium maleate is stable under the debromination conditions and hence could not have been an intermediate. The conflicting facts concerning the debromination by metals can be resolved by assuming an ionic mechanism for debromination by the divalent metals, ~iiagnesium and zinc, and a free radical rtiechaiiisni for debromination by the inonovalent inctal, sodium. The reaction with
TABLE I SUMMARY OF DEBROMINATION RESULTS Mg in CIHUO
Dibromide
95% meso-5% dl-CHDBrCHDBS 507, meso-50% dl-CHDBrCHDBr“ meso-CH3CHBrCHCHaBru dl-CH,CHBrCHCHaBr” meso-C6H.&HBrCHBrCgH~ dl-CaHSCHBrCHBsC6Ha meso-( CHBrCO0Na)t dl-( CHBrCO0Sa)z a Estimated total experimental error of 13C;o. solvent.
96.0, 9i3.0 ....... 9 3 . 3 , 94. X 7 , 6 , 1 .ii 98, loob 93, sx,90”
luaus-Olefin i n product, 211in Hs0
u
Qti. 0 , 9 4 . .......... 0 3 . 2 , 96,4
3 .3, 5 , 0, 3 , ( 1
