RESEARCH
Bisactivated olefins cyclize in high yields Electrolytic reductive c o u p l i n g gives acyclic, tricyclic, a n d heterocyclic c o m p o u n d s
Electrolysis forms cyclic compounds
Ct
ΗςΟ-C
-CW..CH-CH.2-CH2-
CH= CH-C-OC2H5
°κ Electrolysis In aqueous
*"^>
°
%
//
OjHjP -C-GH£CH-CH-CH-C-OC2H5
tetraethyiammonium p-toiuenesulfonate and aeetonitrife
,
j,j ' . ^"% ~ * * " 2 " M.2
Electrolysis in aqueous tetraethyiammonium p-toluenesulfonate and acetonitrile Diethyl 2,3-norbornanediacetate
Θ CH %~ CHt- S ( C H 3 ) C H = CCCOOC Η , Λ ^ 2 5'2 Dimethyl offbis- >e;.hoxycarbonyl)vinyljphenethylsuifonium p-toluenesuifonate
Electrolysis In aqueous tetraethyiammonium ptoluenesulfonate and dimethyiformamide
CH(cooczH5)2 Diethyl 3-indanyl maionate
Electrohydrocyclization mechanism
intermediate anion radical may be oriented with the unpaired electron at the electrode surface
38 C&EN JAN. 2, 1967
Cyclic compounds can arise from the electrolytic reductive coupling of bisactivated olefins, according to James D. Anderson of Monsanto Co/s central research department in St. Louis. Diolefins of the general formula ROOCCH=:CH(Z)CH= CHCOOR are activated by two termi nal ester groups. R is ethyl and Ζ is usually ( C H 2 ) n . When η is 1, 2, 3, or 4, the coupling yields ring compounds; when η exceeds 4, only acyclic com pounds form. This is the first exam ple of the formation of cyclic com pounds from linear compounds by electrolytic methods [/. Org. Chem., 31, 3890, 3897 (1966)]. The reac tion is also useful for synthesizing bi cyclic and heterocyclic compounds. The electrolytic reduction of a bisactivated olefin may lead to several products—the desired cyclized prod uct, the monoolefin, the tetrahydro derivative, and an oligomer. Mr. Anderson and his coworkers, Dr. Manuel M. Baizer and Dr. John P. Petrovich, find that reduction of diethyl 4,4-diethyl-2,5-heptadiene-l,7-dioate (predominantly the trans, trans isomer) gives rise to a mixture of cis- and irans-diethyl 3,3-diethyl-l,2-cyclopropanediacetate. The reac tion is carried out in aqueous tetra ethyiammonium p-toluenesulfonate containing acetonitrile. Similarly, electrolysis of diethyl 2,6-octadiene1,8-dioate (mixture of cis, trans and trans, trans isomers) gives a 4 1 % yield of a mixture of cis- and trans-aiethyl 1,2-cyclobutanediacetates. However, electrolysis of the homol ogous compounds, diethyl 2,10-dodecadiene-l,12-dioate and diethyl 2,20docosanediene-l,22-dioate, does not lead to cyclic compounds but to mix tures of tetrahydro-derivatives and oligomers. The electrolysis of cis-diethyl 1,3cyclopentanediacrylate in aqueous acetonitrile using tetraethyiammonium p-toluenesulfonate as the electrolyte gives frans-diethyl 2,3-norbornanedi acetate, a product of transannular electrohydrocyclization ( EHC ). As an example of the synthesis of hetero cyclic systems, the E H C of l,2-bis(2ethoxycarbonylvinyloxy ) ethane gives the cyclic product 2,3-bis(ethoxycarbony lmethyl)dioxane in 89% yield. In an unusual synthesis, di-
methyl o- [ ( bis-£-ethoxycarbonyl ) vi nyl Iphenethylsulfonium p-toluenesulfonate gives diethyl 3-indanylmalonate in 12.6% yield, on electrolysis in dimethylformamide, with tetra-n-butylammonium iodide as the supporting electrolyte. EHC mechanism. Mr. Anderson and Dr. Petrovich have also studied the EHC mechanism. They propose a concerted reduction-cyclization mechanism for the electrolysis of the bisactivated olefins. The Monsanto chemists postulate that the interme diate anion-radical is oriented with the unpaired electron on the electrode surface. In their study of the mechanism of the reaction, they obtained polarograms of a series of bisactivated ole fins in dimethylformamide solution containing 0.2M tetra-n-butylammonium bromide. The cyclization reac tions showed an anodic shift of the half-wave potentials toward more pos itive values. The more positive wave was always associated with the cycli zation reaction, the more negative wave with the linear reactions. Mr. Anderson notes that when only one polarographic wave is obtained (if the Ε γ , is positive with respect to that of a reference compound), cyclization has occurred. Steric effects. Mr. Anderson and Dr. Petrovich studied molecular mod els of diolefins which form cyclopro pane and cyclopentane rings and they observed that the double bonds could be parallel. Also, the carbon atoms located beta to the carbethoxy groups could be brought to within about one carbon-carbon bond dis tance without serious steric interac tions. This may explain the ease of ring formation in these cases. With diolefins lined up to form cy clobutane and cycloheptane rings, however, a parallel alignment of the double bonds is difficult. Some steric hindrance exists when attempting to bring carbon atoms within one bond length of each other, and ring closure is more difficult. According to the Monsanto group, the EHC mechanism most probably involves a concerted one-electron re duction of both double bonds forming the cyclic anion radical on the mer cury electrode surface. The anion radical is then rapidly reduced to the corresponding dianion. The proto nation of the dianion by water yields the observed products. Mr. Anderson and Dr. Petrovich say that it is very tempting to apply the concerted reduction mechanism of bisactivated olefins to simple monoolefins. With this view in mind, the group is further studying the electrohydrodimerization reaction of simple monoolefins.
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