Electrochemical oxidation of 4-substituted N,N-dimethylaniline in the presence of silyl enol ether. Effect of the substituent on the formation of Mannich bases. II.

Abstract

. Can. J. Chem. 62, 565 (1984). The formation of Mannich bases from the electrochemical oxidation of 4-substituted N,N-dimethylaniline in the presence of silyl en01 ether was studied. The yield of bases obtained depended on the relative oxidation potentials of the starting amine compound and the base formed. An electron-donating substituent on the amine showed a large difference in the oxidation potentials and gave the highest yield of product. On the other hand, the oxidation potentials were very close with an electron-withdrawing substituent and the yield in base was relatively much lower. Furthermore, the main product in the case of an electron-withdrawing group in the presence of 1-trimethylsilyloxy-I-cyclohexene was 4-substituted 2-(2-oxocyclohexyl)-N,N-dimethylaniline. Some side products were also isolated and were identified as N,N-diketonic compounds. The first paper of this series (1) described the synthesis of Mannich bases from the electrochemical oxidation of N,Ndimethylmesidine in the presence of silyl en01 ethers. It was noticed that the reaction depended upon the relative electrochemical oxidation potentials of the silyl en01 ether, the amine, and the product. In this paper, a selection of 4-substituted N,N-dimethylanilines was chosen in order to examine the effect of the substituent on the formation of the Mannich base: Results and discussionIn order to compare the effect of the substituent on the yield of Mannich base formed, the experimental conditions for all the runs were kept constant. The only variable was the reaction oxidation potential (E). This potential was controlled at such a voltage that a current of 100 mA was attained at the beginning of the electrolysis (Fig. 1, arrows). Then, the current dropped slowly to about 50 mA or less after the passage of 2 F/mol.The silyl en01 ethers chosen for this study, l-trimethylsilyloxy-1-cyclohexene 3 and l-phenyl-l-trimethylsilyloxyethylene 4, are known from the previous study to give good yield of Mannich bases in the presence of the iminium salt of N,N-dimethylmesidine. The halfwave potentials (E,,?'s) of the two silyl en01 ethers are 1.25 V and 1.30 V vs. Ag/AgN03 respectively. The Eli2's of the amino compounds 1 are shown in Table 1 and they are all much less anodic than those of the silyl en01 ethers 3 and 4. On the other hand, the halfwave potentials of the amines 1 are, as expected, relatively more anodic for an electron-withdrawing substituent than for an electron-donating substituent. The yield of products 5 and 8 was markedly higher with an electron-donating group (Tables 1 and 2).The oxidation potentials of at least three compounds must also be considered: the silyl en01 ether, the amine 1, and the Mannich base formed. As for the silyl en01 ethers 3 and 4, their E's are somewhat much too anodic to affect the result of the

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