1582
SIDNEY 1 ).ROSS,
MANUEL FINKELSTEIN A N D ICAYMOND (1. I'ETERSEN
I t seems therefore that except for Hg, the group I1 organometal halides exist as R2M.MX2complexes rather than as RMX. The reason for this is not immediately obvious. Further work in the Zn and Ba systems is now being conducted. Acknowledgments.-The author wishes to thank Dr. Hershel Hausman and the staff and crew of the Ohio State University Cyclotron for the bombard-
VOl. 82
ment, and Dr. Byron Branson of the Robert A. Taft Sanitary Engineering Center of the U. S. Public Health Service for the use of counting equipment, and to acknowledge the financial support in the form of a Frederick G. Cottrell Grant from the Research Corporation. He also wishes to express his gratitude for stimulating conversdtions with Dr. George S Handler, Tufts University. CISCINSATI21, Orrio
[COSTRIHUTION FROM THE RESEARCH LABORATORIES OF T H E SPRAGUE I':LECIKICCo ]
The Electrochemical Degradation of Quaternary Ammonium Salts. 11. The Mechanism of the Coupling Reaction BY SIDNEY D. Ross, MANUELFINKELSTEIN AXD RAYMOND C. PETERSEN RECEIVED JULY 2, 1939 T h e cathodic decomposition in X,N-dimethylacetamide or diinethylforrnamide of quaternary ainnioniuin nitrates containing benzyl, fluorenyl, cinnamyl, P-methoxybenzyl or a-phenylethyl groups yields the pwpling products of these radicals. T h e reaction involves the generation and dimerization of free radicals, formed in a one-electron transfer at tile cathode. This is indicated by the fact t h a t d-cu-pheiiplethyltrimethylainmoniumnitrate gives only inactive products.
In a previous report from this Laboratory,' i t was shown that the electrolysis of aqueous solutions of selected quaternary ammonium salts resulted in the cleavage of one of the alkyl groups attached to nitrogen to forin a hydrocarbon and a tertiary amine. For one salt, benzyldimethylanilinium trifluoroacetate, the electrolyses were carried out in both watcr and dimethylacetamide. In watcr the products were toluene and dimethylaniline but, in thc latter solvent, the same amine and bibenzyl were obtained. It is our present purpose to show that coupling products are generally obtained when the reaction is effected in an appropriate non-aqueous solvent of high dielectric constant and, further, to cite evidence which indicates that these reactions involve the formation and subsequent reactions of free radical intermediates a t the electrode. The results of six preparative experiments are summarized in Table I. The solvent in these experiments was either dimethylacetamide or dimethylformamide. No effort was made to ensure the complete decomposition of the quaternary ammoiiiuni ions in these electrolyses, and conditions for maximizing the yields were not explored. These results, therefore, have only qualitative significance and do not permit any clear decision as to the possible preparative value of this reaction. TABLE I CATHODIC COUPLING REACTIOSS Yield, Quaternary ammonium nitrate
Product
Bibenzyl 1~enz~ltrieth~'Ianiinonium Bibenzyl Benz)ldirncth?-laniliiiium 4,4'-Dimethoxybip-hlethox~-benzyltri~r~ct hylbenzyl ammonium Dicinnamyl Cinnarnyldinieth>lanilinium Difluorenyl T:lui~re~i~-ltrimeth~lammonium ~-Plien?-leth~-ltrimethylam2,3-I)iphenylbut~rnc nionium
%
31.9 35.0
l5,O 6.0 26.1 30 0
(1) R I . Finkelstein, 11. C . Petersen and S. D. Ross, THISJUUKNAI.,
81, 2361 ( l g i i l ) .
The problem of mechanism for thcse reactions resolves itself into the question of whether the cathodic reaction involves a onc -electron change and the formation of radical intermediates or, alternatively, a two-electron transfer and carbanion intermediates. The two possibilities may hc formulated as
++
R1>;(;
KP
4
I
+2e
,,A
+
Ib,
> X P
+
Iy
:
It2,'
,
R1
L----+
Rz
"
The coupling product could result either from dimerization of the radicals or a nucleophilic displacement reaction by the carbanion on the quaternary ammonium ion.2 Some qualitative evidence which favors the freeradical mechanism has been presented,' but definitive proof is lacking. An experiment with an optically active quaternary ammonium salt mould permit a clear choice between the alternativcs provided that the cleavage occurred at the asymmetric carbon atom and the coupled product was not formed in only the meso form. a-Phenylethyltrimethylammonium nitrate proved suitable for this purpose. It could be obtained optically acti gave both meso- and dl-2,3-diphe1iylbutanc 011 electrolysis. In the electrochemical decomposition of active a-phenylethyltrimethylammonium nitrate, that portion of the hydrocarbon product which was not meso would be expected to be totally inactive, if free radicals are the intermediates. Since the radicals, if formed, are formed singly rather than in pairs, this is a more straightforward case than the decomposition of a diacyl peroxide where (2) hfechanisms involving hydrogen abstraction from the quaternary ammonium ion by the carbanion, Rp: -, have not been C U I I sidered, since the hydrocarbon RaH was not found in t h e p r ( ~ I u c l i under the present experimental conditions. Hydrogen abstractiim is, in fact. a inure proljal,!e reaction f o r these species than is displ;rcctneiit, and this may be c u n d c r c d ;is cvirlence against the c a r l ~ ~ n i o r i niechanism.
April 5, 1960
ELECTROCHEMICAL DEGRADATION OF QUATERNARY AMMONIUM SALTS
a cyclic decomposition mechanism or cage effects can result in the formation of some optically active coupled product. In fact, some active 2,3-diphenylbutane was obtained by Greene from the reaction of active hydratropoyl chloride with sodium peroxide.3 The ionic mechanism, in contrast, demands that some optically active 2,3-diphenylbutane be formed. This is true whether or not the carbanion formed a t the electrode retains its configuration, since the final product is formed by a displacement reaction on the asymmetric carbon atom of the quaternary ammonium ion. This reaction would proceed with inversion rather than racemization of this latter carbon atom. d-a-Phenylethylamine ( [nIz5D+31.3') was converted to d-a-phenylethyltrimethylammoniumiodide ([cx]'~D +12.1'), and the iodide was transformed t o the nitrate ([n]"D +22.6"). Electrolysis of the nitrate in dimethylformamide resulted in a 30% yield of a mixture of meso- and dl-2,3-diphenylbutane. The bulk of the meso-hydrocarbon was separated from the mixture by crystallization from methanol. The remaining product, a liquid a t room temperature, was shown both by vapor phase chromatography and infrared analysis to consist of trace amounts of acetophenone, less than 15% of the meso isomer and the remainder, dl2,3-diphenylbutane. This latter fraction was completely devoid of optical a ~ t i v i t y . ~ Two experiments attest to the optical stability of a-phenylethyltrimethylammonium nitrate under the experimental conditions. The d-salt (15% solution in dimethylformamide) had [a]"D equal to +lS.OO initially and [ a ] ' ' D equal to +17.7" after standing in solution a t room temperature for 50 hr. Since the electrolysis was carried out in an icewater-bath for 17.8 hr., it is concluded that there is no appreciable racemization in the absence of an electric field. In a second experiment, the Z-salt ( [ c u ] * ~ D- 18.4') was electrolyzed partially, and the undecomposed salt was recovered. Although the salt was not obtained completely pure and could not be crystallized, its activity was only slightly lower ( [CX]~'D- 16.9"). The hydrocarbon product was isolated and proved to be identical in composition to that obtained in the more complete electrolysis. This result is in accord with the free-radical mechanism. A one-electron transfer a t the cathode results ultimately in the formation of a-phenylethyl radicals which then dimerize to give the observed final products. Since the radicals involved would not be expected to maintain a tetrahedral configuration, the diphenylbutane obtained would show no optical activity. Equally important, this optical result is inconsistent with a carbanion mechanism. This experiment, therefore, permits a choice between the two classes of mechanisms. It seemed of interest to compare the composition of the 2,3-diphenylbutane product obtained by a (3) F. D. Greene, TXISJOURNAL, 11, 4869 (1955). (4) 2,3-Diphenylbutanehas been resolved (H. H . Richardson, E. J. Underhill, A. G. Brook and G. P Wright, ibid., 69, 937 (1947)). The specific rotation of the dd-isomer was -94.9O while that of the 11-2,3diphenylbutane was +99,8". Since these specific rotations are high, even small amounts of active product in our experiment would he detectable, and the optical course of the electrochemical reaction is unequivocal.
1583
known free-radical process. The decomposition of 1-azo-bis-1-phenylethane was chosen for this purpose,Band the reaction was studied both in dimethylformamide and in the absence of a solvent. The results are summarized in Table 11. The most precise analytical result is for the decomposition of the azo compound in the absence of solvent, and it is possible that the other two results are identical with it, within the limits of experiment accuracy. At any rate, there are no large differences in the product compositions. TABLE I1 Reaction
