Synthesis and Characterization of Stereoisomers of 1,4-Dihydro-1,4-diarsinines

Novel 1,4-diarsa-2,5-cyclohexadiene, i.e., 1,4-dihydro-1,4-dimethyl-2,3,5,6-tetrakis(tert-butoxycarbonyl)-1,4-diarsinine, was synthesized by radical reaction of pentamethylcyclopentaarsine (cyclo-(MeAs)5) and di-tert-butyl acetylenedicarboxylate. Two steroisomers, cis-DHDAtBu and trans-DHDAtBu, were obtained as crystals, of which structures were determined by X-ray crystallography. The six-membered ring in cis-DHDAtBu has a boat form, and both the methyl groups on the arsenic atoms are in equatorial positions. Two different polymorphs of trans-DHDAtBu were obtained as a pale yellow platelet crystal and a colorless crystalline fiber. The colorless crystal has a boat-shaped six-membered ring, and the yellow crystal has a nearly flat chair-shaped ring. The double bonds connected with the two arsenic atoms were longer than that of the boat-shaped DHDAtBu. The UV−vis absorption spectra of cis-DHDAtBu and trans-DHDAtBu in CH2Cl2 showed long-wavelength absorption maxima at 272 and 300 nm, respectively.