Synthesis, structure, and reactivity of (triphenylarsoranylidene)-methylcyclohepta-2,4,6-trienone derivatives: reactions with heterocumulenes and an activated acetylene

Stable arsonium ylide derivatives 7a, b bearing cyclohepta-2,4,6-trienyl and electron-withdrawing CO2Et and CN groups, respectively, have been prepared for the first time through a reaction of 2-chlorocyclohepta-2,4,6-trienone with the corresponding arsonium methylide derivatives in the presence of (BuOK)-O-t. Compounds 7a,b are isolated as stable crystalline compounds, which do not undergo hydrolysis even in acidic conditions. The X-ray crystal analysis revealed that their As-O bond distances (2.31 Angstrom for 7a, 2.39 Angstrom for 7b) lie below the sum of the van der Waals radii (3.37 Angstrom), and thus, there is appreciable bonding interaction between the arsenic and the oxygen elements. With a view to constructing a series of cyclohepta-annulated heterocycles and in order to gain a better understanding of a series of arsonium ylides, 7a, b were allowed to react with heterocumulenes such as carbon disulfide, phenyl isothiocyanate, diphenylcarbodiimido, and phenyl isocyanate, in a Wittig type reaction followed by electrocyclization or a formal [8 + 2]-type cycloaddition eliminating triphenylarsine sulfide or oxide to give 2H-cyclohepta[b] furan-2-thione, its imine, 2-phenylimino-2H-cyclohepta[b] pyrrole, and 2H-cyclohepta[b] furan-2-one. On the other hand, the reactions of 7a, b with dimethyl acetylenedicarboxylate (DMAD) give azulene derivatives.