May, 1945
COUPLING REACTIONS OF
[CONTRIBUTION FROM THE
BUTENYLMAGNESIUM BROMIDE
DEPARTMENT OF CHEMISTRY,
84 1
UNIVERSITY OF CALIFORNIA, LOS ANGELES]
Allylic Rearrangements. XVIII. Coupling Reactions of ButenylmagnesiumBromide BY WILLIAM G. YOUNG,JOHN D. ROBERTS'AND HARRYWAX
In connection with our work on the nature of the butenyl Grigiiard reagent it was desirable to study the coupling reactions of butenylmagiiesium broinide. Henne, Climan and Turk2 have coupled butenyl chlorides in the presence of magnesium while Lespieau and Heitzmanns have reported similar experiments with crotyl bromide, but the results of these investigators are not in agreement. Furthermore, it is not clear whether or not the butenyl Grignard reagent is truly involved under conditions where the organ~metallic compouiid couples as rapidly as it forms. Indeed, valuable information might be expected from a comparison of the products resulting from the rapid coupling of two halide molecules with magnesium and from the coupling of a preparation of the Grignard reagent with a suitable halide. We have, therefore, made a study of the coupling reactions of butenylmagnesium bromide. After the conipletion of this work a paper by Levy and Cope4 appeared describing a number of similar experiments but without comment as to the significance of the results. PRODUCTS FROM Allylic, halide
THE
in these reactions. As yet no definite explanation can be offered fok the differences between the products obtained by coupling crotyl bromide with the Grignard reagent and the rapid coupling of the bromide with magnesium. It is uncertain whether or not exchange reactions between the Grignard reagent and halide molecules are involved but no products were isolated in significant amounts in the present work which could be attributed to exchange reactions alone. In conttast to the complex results obtained with allylic bromides, the coupling of butenylmagnesium halides with allylic chlorides appears to result in the predomination of but one of the possible products. Furthermore, substantially the same composition of isomers is obtained either by direct coupling of the halides or through the prior 'formation of the Grignard reagent. The fact that both crotyl and methylvinylcarbinyl chlorides give as the principal product 3-methylheptadiene-1,5 (I), leads to speculation as to which half of the diene molecule is derived in each case from the Grignard reagent and whichefrom the
TABLE I COUPLING REACTIONS OP ALLYLIC HALIDESWITH BUTENYLMAGNESIUM HALIDES 2 6-Octa-
diene, %
3-Methylheptadiene-
IA %
*3,4Dimethylhexadiene-
3-Methyl2,B-Hepta- hexadiene1.5. % diene, %
13, %
Crotyl 50 50 Trace Crotyl bromidea 10 88 2 Methylvinylcarbinyl bromide '50 35 15 .. .. .. Allyl bromide Allyl bromide .. .. .. Crotyl chlorideb Trace 92.5 7.5 Crotyl chloride 12 82 6 Methylvinylcarbinyl chlorideb 4.5 85 10.5 Methylvinylcarbinyl chloride 3 86 11 Allyl chloride .. .. .. a Equilibrium mkture of the bromides (87% primary, 13% secondary).
The compositions of the products of coupling reactions of butenylmagnesium halides with allylic halides are presented in Table I. It is evident that allylic bromides generally give quite different products than the corresponding chlorides. This may be due to a greater tendency of the bromides to undergo an ionic type6 reaction involving an electrophilic attack of magnesium on halogen. However, the operation of such a mechanism in itself would not explain the experimental results, as rearrahgefnents of both the Grignard reagent and halide molecules appear to be involved (1) Abbott Laboratories Research Fellow, 1943-1944. (2) Henne, Chanan aqd Turk, THISJOURNAL, 68,3474 (1941). (3) Lespieau and Heitzmann, Bull. SOC. rhim., 151 8, 273 (1936). (4) Levy and Cope, Tms JOVRNAL, 66, 1684 (1944). (5) Roberts, Young and Winstein, i b i d . , 6 6 2163 (1942).
.. ..
% 90
Ref.
3 This paper This paper This paper 4
58 75 50 50 68 22 78 62 .. .. 54 2 72 This paper .. .. 67 2 .. 76 This paper 92 49 This paper Grignard reagent not prepared separately.
..
.. ..
*
.. .. ..
Yield,
..
halide. Evidence on this point is furnished by the results (Table 11) of the coupling reactions of butenyl chlorides with allylmagnesium halides. The produtt from the reaction of crotyl chloride with allylmagnesium bromide was 2,6-heptadiene, while the coupling of the secondary chloride gave TDLB I1
COUPLING REACTIONS OF BUTENYL CHLORIDES WITH ALLYLUGNESIUM HALIDES
PRODUCTS
Butenyl chloride
=-rY
Primary Secondary Secondary
FROM THE
3-Methyl2,6-Hepta- hexadiene1,5, % diene, %
>94
