Electrochemical Cleavage of Double Bonds in Conjugated Cycloalkenyl- and 1,2-Alkenobenzenes

Abstract

The electrochemical cleavage of olefinic bonds in conjugated C 5 −C 7 -cycloalkenylbenzenes 1, indene 4a, and 1,2-dihydronaphthalene ( 4b ) results from anodic oxidation of these compounds in MeOH in an undivided cell. The process includes the formation of (1,2-dimethoxycycloalkyl)benzenes 2a − c, 1,2-dimethoxyindan ( 5a ), and 1,2-dimethoxytetralin ( 5b ) as intermediates and leads to ω,ω-dimethoxy-ω-phenyl- n -C 5 −C 7 -alkanal dimethyl acetals 3a − c, [2-(dimethoxymethyl)phenyl]ethanal dimethyl acetal ( 6a ), and 3-[2-(dimethoxymethyl)phenyl]propanal dimethyl acetal ( 6b ) as major products, respectively. The best yields of 3a − c and 6a,b (58−76%) were obtained when the electrolysis was carried out at 60 °C and 6 F/mol of electricity based on 1, 4a, and 4b was passed using C-anode and Bu 4 NBF 4 as a supporting electrolyte. Compounds 2a − c and 5a,b as mixtures of cis and trans diastereomers were obtained as major products in 81−90% yield when the electrolysis was terminated after 2 F/mol of electricity based on 1, 4a, and 4b had been passed. The cis/trans ratio in 2a − c was increased with the increasing the size of cycloalkane rings in 1, 4a, and 4b and the obvious stereoselective effect of the anode nature (C or Pt) being observed in the case of dimethoxylation of 4a .