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

Abstract

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 Bu^tOK. 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 Å for 7a, 2.39 Å for 7b) lie below the sum of the van der Waals radii (3.37 Å), 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.