Substituent effect on the transition from ionic to covalent bonding in triphenylphosphonium ylide derivatives: reactivity of 3-methyl-2,2,2-triphenyl-2H-cyclohepta[d][1,2 lambda(5)]oxaphosphole with heterocumulenes

X-Ray crystal analysis of 3-methyl-2,2,2-triphenyl-2H-cyclohepta[d][1,2lambda(5)]oxaphosphole [and the triphenylphosphonium ylide] 1a and its parent compound 1b has been carried out. The P-31 and C-13 NMR spectral studies of 1a, b and their derivatives 1c, d, and the correlation of their chemical shifts with P1-O1 bond lengths obtained by X-ray analyses for 1a-d were investigated to clarify that compounds 1a-d exist as resonance hybrids of a P-O bonding oxaphosphole structure (structure A) and a phosphonium ylide structure (structures B and C). The contribution of the structure A decreases gradually in the order of 1a > 1c > 1b > 1d in the solid state. On the basis of a linear correlation between P1-O1 bond lengths and theta(sum) (the sum of bond angles between the equatorial bonds), it is clarified that an increase of the P1-O1 bonding character causes change in the configuration of the phosphorus atom from a tetrahedral to a trigonal bipyramidal arrangement. In connection with these studies, inspection of the structure of a related compound, 2,2,2-triphenyl-6,11-methano-2H-cycloundeca[d][1,2lambda(5)]oxaphosphole structure 2, was also carried out. With a view to clarifying the reactivity of 1a in connection with a contribution of canonical structures A, B, and C, compound 1a was allowed to react with phenyl isocyanate, diphenylcarbodiimide, and phenyl isothiocyanate to give heteroazulenes. The reaction pathways depending on the contribution of canonical structures A, B, and C are discussed.