o-Quinonoid compounds. Part 15. Rapid formyl migration in 1-formylindenes; evidence for concertedness and exclusive 1,5-sigmatropy

Abstract

1-Formyl-3-methoxymethyl-1-methylindene, (6) and 1-deuterioformyl-2-formyl-1,3-dimethylindene (15) have been prepared in optically active form. Interconversion of (6) and 1 -formyl-1 -methoxymethyl-3-methylindene (8) at 80 °C proceeds with 95% retention of optical activity over 100 half-lives. Racemisation of (15) is twice as fast as its conversion to 2-deuterioformyl-1-formyl-1,3-dimethylindene (17) showing that each act of racemisation involves formation of the symmetric 2H-indene intermediate (16) in which there is a 50% chance of CHO–CDO exchange.