On the regioselectivity of the condensation of stabilized phosphoylids with 3-substituted phthalic anhydrides

Abstract

Condensations of stabilized phosphorane 1 with 3-substituted phthalic anhydrides were investigated. The importance of various effects influencing regio-and stereoselectivity of these reactions is discussed. It is proposed that the oxygen atom on the substituents in position 3 can act as aLewis base toward the electron-deficient phosphorus of the ylid. The resulting complexation stabilizes the transition state for the reaction at the ortho carbonyl group, thus offsetting the usual steric and "push effects, which favour attack at the meta carbonyl function.

On the mechanism of Wittig reactions with cyclic anhydrides. II.

“…Can The slightly higher proportion of the 4 a enol-lactone produced corroborates the previously postulated r-stacked transition state (1), incorporating a stabilizing complexation between the 3-methoxy substituent and an electron-deficient phosphorus atom as illustrated in Fig. 3 (1).…”

“…However, for several anhydrides studied, regioselectivity of the enol-lactones produced could not be reliably predicted on the basis of steric or electronic effects of the substituents (1)(2)(3). This was particularly evident in phthalic anhydrides with oxygencontaining groups (-OCH, or -NO,) in position 3, where regioselectivity projections based on the electronic and (or) steric effects did not correspond to the experimental results (1).…”

“…Similarly, 100% P selectivity was also observed in the reaction of 3-nitrophthalic anhydride with the fluorinated ylide Ph,PC(F)COOEt 11. The only product isolated from this condensation showed a highly deshielded doublet at 9.7 ppm, characteristic of the aromatic proton ortho to the ylidene substituent in phthalic E enol-lactones (1).…”

“…The condensation of stabilized phosphoranes with substituted cyclic anhydrides provides an excellent route to the corresponding enol-lactones (1)(2)(3)(4)(5). However, for several anhydrides studied, regioselectivity of the enol-lactones produced could not be reliably predicted on the basis of steric or electronic effects of the substituents (1)(2)(3).…”

“…Initially, we suggested that the unexpectedly high proportion of enol-lactones resulting from condensation at the carbonyl group adjacent to the oxygen-bearing substituents might be due to the stabilizing complexation between the electron deficient phosphorus atom of the ylide and the oxygen atom of the substituent (1)(2)(3). Our more recent studies of the mechanism showed that condensations of cyclic anhydrides with stabilized phosphoranes lead, in a lowenergy, reversible process, to acyclic adduct(s).…”