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][1,2λ5]oxaphosphole with heterocumulenes

摘要

X-Ray crystal analysis of 3-methyl-2,2,2-triphenyl-2H-cyclohepta[d][1,2λ5n]oxaphosphole [and thentriphenylphosphonium ylide] 1a and its parent compound 1b has been carried out. The n31nP and n13nC NMR spectralnstudies of 1a,b and their derivatives 1c,d, and the correlation of their chemical shifts with P1–O1 bond lengthsnobtained by X-ray analyses for 1a–d were investigated to clarify that compounds 1a–d exist as resonance hybrids ofna P–O bonding oxaphosphole structure (structure A) and a phosphonium ylide structure (structures B and C). Thencontribution of the structure A decreases gradually in the order of 1a > 1c > 1b > 1d in the solid state. On the basisnof a linear correlation between P1–O1 bond lengths and θsum (the sum of bond angles between the equatorial bonds),nit is clarified that an increase of the P1–O1 bonding character causes change in the configuration of the phosphorusnatom from a tetrahedral to a trigonal bipyramidal arrangement. In connection with these studies, inspection of thenstructure of a related compound, 2,2,2-triphenyl-6,11-methano-2H-cycloundeca[d][1,2λ5n]oxaphosphole structure 2,nwas also carried out. With a view to clarifying the reactivity of 1a in connection with a contribution of canonicalnstructures A, B, and C, compound 1a was allowed to react with phenyl isocyanate, diphenylcarbodiimide, and phenylnisothiocyanate to give heteroazulenes. The reaction pathways depending on the contribution of canonical structuresnA, B, and C are discussed.