Do Spiroarsoranes Exhibit Polytopal Equilibrium in Solution?

The spiroarsoranes 5-phenyl-1,6-dioxa-4,9-diaza-5?5-arsaspiro[4.4]nonane (6), (3R,8R)-3,8-dimethyl-5-phenyl-1,6-dioxa-4,9-diaza-5?5-arsaspiro[4.4]nonane (7), (2S,7S)-2,7-dimethyl-5-phenyl-1,6-dioxa-4,9-diaza-5?5-arsaspiro[4.4]nonane (8), and (3S,8S)-3,8-dimethyl-(2R,7R)-2,5,7-triphenyl-1,6-dioxa-4,9-diaza-5?5-arsaspiro[4.4]nonane (9) were prepared by reaction of phenylarsonic acid and the correspondig amino alcohol. The presence of polytopal ? ? equilibrium in 6?9 was demonstrated by HPLC and NMR studies. NBO computations at the MP2/6-31+G(2d,2p) level indicate that methyl substitution in C2 or C3 of the oxazarsolane ring determines the predominance of ? or ? stereoisomers. GIAO B3LYP/6-311++G(2d,2p) computations were used to assign experimental 1H and 13C NMR spectra.