Interconversion and reactions of in- and out-isomers of a triarylphosphine-containing cyclophane

The conformational isomerizations of an in;cyclophane containing a triarylp hosphine poised above a basal aromatic ring (compound 1) were studied by va rious experimental and computational methods. When the in-isomer 1 was heat ed at temperatures great enough to invert the triarylphosphine, the corresp onding out-isomer (2) was not observed, but this isomer was trapped by heat ing with sulfur to give an out-phosphine sulfide (4). The X-ray structures of 1 and 4 were determined, and 1 was found to adopt an unusual, compact co nformation, rather than the more extended conformation observed in a relate d system. The sulfuration of 1 is 10000 times slower than the corresponding sulfuration of triphenylphosphine, and the rate-determining seep for the f ormer reaction appears to be the phosphine inversion (Delta G double dagger (inv) = 34.8 kcal/mol). Desulfurization of 4 with hexachlorodisilane at roo m temperature smoothly returned the in-cyclophane 1, with no intermediate 2 detected, indicating that the reverse phosphine inversion has a low barrie r. The relative energies of the various conformations of these cyclophanes were evaluated by a wide range of computational methods, with comparisons t o experimental geometries and stabilities where possible. Molecular mechani cs and semiempirical MO calculations proved inadequate for these systems, b ut ab initio and hybrid density functional methods gave reasonable results.