Stable 2-thianaphthalenes: synthesis and reactions with electrophiles

Abstract

The stable and crystalline 2-thianaphthalene derivatives 2-methyl-2-thianaphthalene-1-carbonitrile (8) and 1-benzoyl-2-methyl-2-thianaphthalene (9) have been synthesized in high yields by proton abstraction from the corresponding benzothiopyrylium salts (6) and (7) with triethylamine in ethanol. The ylidic nature of the 2-thianaphthalenes was established on the basis of spectral and chemical evidence. The reaction of the 2-thianaphthalene (8) with dimethyl acetylenedicarboxylate (DMAD) in ethanol afforded the naphthalene derivative (12), while the same reaction using benzene as a solvent although giving no naphthalene derivative, afforded the dihydrocyclopropa[a]naphthalene derivative (15) and the 5H-benzocycloheptene derivative (16). Moreover, the above reaction conducted in sulpholane solution gave, in addition to (15) and (16), a 2 : 1 adduct (17). The structure of the latter was confirmed by X-ray analysis. Treatment of compound (8) in benzene with methyl propiolate (MP) gave only a 2 : 1 adduct, (20). The reaction of the 2-thianaphthalene (9) with DMAD afforded only a dihydrocyclopropa[a]naphthalene derivative (14), whose structure was determined by X-ray crystallography. Parallel reactions with other electrophiles such as tetracyanoethylene, diphenylcyclopropenone, and diethyl azodicarboxylate gave a number of unexpected products for which reaction mechanisms are proposed.