Crystal structures of highly stabilized ylides: methyl(3-methoxy,2-methoxycarbonylbenzoyl)tri-phenylphosphoranylideneacetate and methyl(2-methoxycarbonyl,6-nitrobenzoyl)triphenylphosphoranylideneacetate and the salt methyl(triphenylphosphoranylidene)acetate tetrafluoroborate

At lower temperatures stabilized ylides react with unsymmetrically substitu ted phthalic anhydrides to give two acyclic adducts. When the reactions are allowed to proceed at higher temperature enol lactones are formed. Identif ication of the acyclic intermediates was necessary to understand the mechan ism of these Wittig reactions. The transient species trapped in the reactio n with trimethyloxonium tetrafluoroborate were unambiguously identified by crystallographic methods. The crystal structures of the tetrafluoroborate s alt of methyl(triphenylphosphoranylidene)-acetate (8), methyl(3-methoxy,2-m ethoxycarbonylbenzoyl)triphenylphosphoranylideneacetate (6 beta), and methy l(2-methoxycarbonyl,6-nitrobenzoyl)triphenylphosphoranylideneacetate (7 alp ha) were studied by X-ray diffraction. The ionic salt (8) is monoclinic, P2 (1)/c, a = 12.640(5), b = 13.945(9), c = 14.825(6) Angstrom, beta = 125.32( 3)degrees, Z = 4, and R = 0.065 (F > 5.4 sigma(F)). Crystal 6 beta is monoc linic, P2(1)/c, a = 16.391(16), b = 9.029(6), c = 19.835(19) Angstrom, beta = 116.60(6)degrees Z = 4, and R = 0.070 (F > 4.6 sigma(F)), while crystal 7 alpha is also monoclinic, P2(1)/c, a = 9.513(5), b = 9.361(3), c = 30.908 (13) Angstrom, beta = 98.42(3)degrees, Z = 4, and R = 0.057 (F > 5 sigma(F) ). In the BF4- salt (12), the four P-C distances are equal (1.791(5)-1.801( 7) Angstrom) with identical tetrahedral angles. For the two triphenylphosph oranylideneacetate compounds, the fourth P-C(1) bond is shorter (1.762(6)-1 .734(5) Angstrom) than the three P-C(Ph) bonds (avg. 1.809(5) Angstrom). Th e angles C(1)-P-C(Ph) are also larger (avg. 112.9(2)degrees for 6 beta and 111.9(2)degrees for 7 alpha) than the C(Ph)-P-C(Ph) angles (avg. 105.8(2)de grees for 6 beta and 106.9(2)degrees for 7 alpha). These values suggest a m ultiple nature for the P-C(1) bond. In the nitro derivative, the nitro and the ester groups are disordered equally in positions 2 and 6.