Ylide formation and rearrangement in the thermal reaction of 6H-dibenzo[b,d]thiopyran and its 6-phenyl derivative with diazoalkanes

Abstract

Reaction of 6-phenyl-6H-dibenzo [b,d]thiopyran (1b) with diphenyldiazomethane (2a) in refluxing benzene affords 6,6,7-triphenyl-6,7-dihydrodibenzo[b,d]thiepin (3b) as major product, together with minor amounts of 6-diphenylmethyl-6-phenyl-6H-dibenzo[b,d]thiopyran (4b). Analogous results are obtained from the reaction of 6H-dibenzo[b,d]thiopyran (1a); however, in this case the yields of the corresponding ring-expansion and insertion products (3a) and (4a) are similar. Copper(II) sulphate-catalysed thermal decomposition of ethyl diazoacetate (2b) in the presence of (1b) yields mainly ethyl 6-phenyl-6H-dibenzo[b,d]thiopyran-6-ylacetate (4d), together with minor amounts of (probably) 6-ethoxycarbonyl-7-phenyl-6,7-dihydrodibenzo[b,d]thiepin (3d), but only the product of insertion of ethoxycarbonylcarbene into the C(6)–H bond, i.e. (4c), is observed from the reaction of (2b) with (1a). Formation of the products (3) and (4) is rationalized in terms of a thermal ylide exchange reaction of the unstable sulphonium ylides (7a–d), initially produced in competition with a Stevens-type rearrangement.

In support of the above mechanism, pyrolysis of the stable 6H-dibenzo[b,d]thiopyran-5-io(bismethoxycar-bonyl)methanide (7e) gave only the product of insertion of bismethoxycarbonylcarbene into the C(6)–H bond, i.e. the malonate (4e), whereas almost exclusive formation of the ring-expansion product (3f) occurred on pyrolysis of the 6-phenyl derivative of the ylide (7e).