[CONTRIBUTIOK FROM
THE
GEORGEHERBERT JONES LABORATORY O F THE UNIVERSITY O F CHICAGO]
Factors Influencing the Course and Mechanism of Grignard Reactions. XXII. The Reaction of Grignard Reagents with Alkyl Halides and Ketones in the Presence of Manganous Salts M. S. KHARASCH, J. W. HAYCOCK, IT7, NUDESBERG,
AND
P. 0. TAIVNE’J-l
Received November 19, 1955 Manganous salts have no effect on the course of the reaction of methylmagnesium bromide, phenylmagnesium bromide, or benzylmagnesium bromide with alkyl halides and with ketones. Alkyl and cycloalkyl Grignard reagents (except methylmagnesium bromide) react by a chain mechanism with other alkyl or cycloalkyl halides (or with ketones) in the presence of manganous chloride. A hypothesis is suggested for these observations. A satisfactory method is described for the preparation of hexamethylethane (40%) from ethylmagnesium bromide and tert-butyl bromide in the presence of manganous chloride.
Manganese salts have no effect on the course of yield (90%) of benzhydrol, an equally high yield reaction of methylmagnesium bromide with alkyl (90%) of benzopinacol is obtained in the presence halides. Thus, whereas methylmagnesium bromide of small amounts (2 mole-(%) of manganous salts.5 reacts with methvl bromide in the presence of small The present work was undertaken in an effort amounts of a cobaltous halide (2-5 mole-%) to give to elucidate the effect of manganous salt in the methane (80 f 5%), ethane (10 f 2%), and reaction of Grignard reagents with alkyl halides ethylene (10 h 2%), no reaction takes place in the and with ketones. presence of manganous halides (5 mole-%). Neither Reaction of Grignard reagents with organic halides does methylmagnesium bromide react with ethyl and with ketones in the presence of manganous salts. bromide in the presence of manganous salts (Table The results cited in Table I indicate that although I). Furthermore, whereas benzophenone2 and iso- methylmagnesium bromide does not react with phorone3 react nith methylmagnesium bromide in ethyl bromide in the presence of manganous the presence of cobalt halides (2-5 mole-%) to give chloride (5 mole-%), a wide variety of other benzopinacol and 1,1,3,3,3’,3’-hexamethyl-bicyclo- Grignard reagents do react, under these conditions, he~yl-55’-dione,~respectively, only the normal by a chain mechanism. Schematicallv, this chain addition products (diphenylmethyl carbinol and reaction may be represented as follows in the case of AI-1,3,5,5-tetramethylcyclohexene-3-ol, respec- ethylmagnesium bromide, ethyl bromide and tively) are formed in the presence of manganous manganous chloride (5 mole-%). salts. 1. CzH5MgBr + MnClz +CzH5MnCl MgBrCl Thus, manganous salts have no effect on the 2. C2H5MnC1--+ CZHS. (MnC1). course of the reaction of methylmagnesium halides 3. CzHsBr + (MnCl). +C2H5-+ MnClBr with alkyl halides or with ketones. On the other 4. 2CzHs. +CzHe + CzHd hand, small amounts of manganous salts (2 mole-%) The yield of gas (ethane and ethylene) is almost altered completely the course of the reaction when a quantitative on the basis of equations 1 to 4. different Grignard, namely isobutylmagnesium The reactions of the other Grignard reagents bromide, was used. Thus, while isobutylmagnesium given in Table I with ethyl bromide and manganous bromide reacts with benzophenone to give a high chloride (5 mole-%) follow a course similar to that of ethylmagnesium bromide, except for the tend( 1 ) The effect of cobalt chloride in the react.ion between ency of some of the radicals formed as indicated benzylmagnesium bromide and alkyl halide is incorporated in the doctorate dissertation of Mr. Tawney submitted t o in equation 2 to undergo in part disproportionation, the faculty of the Division of the Physical Sciences of The and in part dimerization. I n other words, except for methylmagnesium University of Chicago in 1942. (2) Kharasch and Lambert, J . Am. Chem. SOC.,6 3 , 2315 bromide, arylmagnesium bromides, benzylmag(1941). nesium bromide, the other Grignard reagents react ( 3 ) Kharasch and Tawney, J . Am. Chem. SOC.,6 3 , 2308 with alkyl halides in the presence of small amounts (1941). (4) The structure assigned t o this compound by Kharasch of manganous salts in a manner similar to the and Tawney [ J . Am. Chem. Soc., 6 3 , 2308 (1941)j was reaction of Grignard reagents with alkyl halides in based (see discussion in that paper) on the disclosure of Hess the presence of cobaltous salts. This difference is and Munderloh [Ber., 51, 377 (1918)], who prepared the compound by reduction of isophorone with sodium amalgam. readily understood on the basis of the reducing Unfortunately, we have overlooked a later paper by Baker power of the organic-anion (R:) in R:MnCl. [J. Chem. SOC.,663 (1926)l who proved conclusively that Anions of greater reducing power than the methyl
+
the compound prepared by Hess and Munderloh was not 3,3 ’,5,5,5 ’,5 ’-hexamethyldicyclohex-2,2-dien-l, 1-diol but was 1,1‘,3,3,3’,3’-hexamethyldicyclohexyl-5,5’-dione.
+
~
(5) Kharasch, Kleiger, Martin, and Mayo, J . Am. Chem.
Soc , 63, 2305 (1941).
322
MARCH
1956
THE COURSE AND MECHANISM OF GRIGNARD REACTIONS.
323
XXII
TABLE I HALIUES IN ( 5 mole-%)
T H E I
