About the structure of phosphorus ylids: Electron distribution and geometry

Abstract

ABSTRACT

Equilibrium Acidities and Homolytic Bond Dissociation Enthalpies of the Acidic C−H Bonds inP-(Para-substituted benzyl)triphenylphosphonium Cations and Related Cations

“…But introduction of an additional phenyl group ( P -cinnamyltriphenylphosphonium cation) increases the acidity by only 2.9 p K HA units, which is less than half of those for the introduction of an α-phenyl group into acetone (6.7 p K HA units), acetophenone (7.0 p K HA units), and acetonitrile (9.4 p K HA units) 13b. On the basis of 1 H, 13 C, and 31 P NMR chemical shift measurements for several phosphonium ylides, Schlosser et al has concluded that the resonance structure 1a contributes approximately three times more to the overall electronic distribution than the resonance structure 1b as shown in Scheme . In other words, the negative charge in 1 was mainly localized on α-carbon atom rather than equally localized at α- and γ-carbon atoms.…”

“…However, the carbanions of the ylides derived from nonstabilized triphenylphosphonium cations such as P -methyltriphenylphosphonium and P -cyclopropyltriphenylphosphonium cations were found to be pyramidal . The chemical shifts of 1 H, 13 C, and 31 P NMR studies of several phosphonium ylides showed that the dominant structure is the dipolar P + −C - zwitterion rather than the PC double bond (Scheme ) . Furthermore, molecular orbital calculations cast doubt on the d π −p π resonance interaction (PC) .…”

Theoretical study of the mechanism of the Wittig reaction: Ab initio and MNDO-PM3 treatment of the reaction of unstabilized, semistabilized, and stabilized ylides with acetaldehyde

“…In this regard, the work of Schlosser et al [29] is of importance. By use of SCHEME 3 Possible resonance structures of the third transition (ts3) of the Wittig reaction of unstabilized and stabilized ylides.…”

The “Off‐Shore” Construction of Optionally Substituted 4‐Trifluoromethyl‐2‐quinolinones

“…Trifluoroacetic anhydride, previously successfully employed in the reaction with N,3-dilithiated N-pivaloyl-3,4-dimethoxyaniline [10] gave only poor results. The oxacarbamates 1a (71 %), 1b (68 %), and 1c (51 %) thus obtained were transformed into the cinnamic ester derivatives 2a (79 %), 2b (76 %), and 2c (74 %) by simple Wittig reaction using 1-methoxy-2-(triphenylphosphonio)ethene oxide [11] [triphenylphosphonio-α-(methoxycarbonyl)methanide, (methoxycarbonylmethylene)triphenylphosphorane, [12] methyl (triphenylphosphoranylidene)acetate]. Treatment with acid or base afforded eventually the 4-trifluoromethyl-2(1H)-quinolinones 3a (73 %), 3b (74 %), and 3c (69 %) (Scheme 3).…”

“…Trifluoroacetic anhydride, previously successfully employed in the reaction with N,3-dilithiated N-pivaloyl-3,4-dimethoxyaniline [10] gave only poor results. The oxacarbamates 1a (71 %), 1b (68 %), and 1c (51 %) thus obtained were transformed into the cinnamic ester derivatives 2a (79 %), 2b (76 %), and 2c (74 %) by simple Wittig reaction using 1-methoxy-2-(triphenylphosphonio)ethene oxide [11] [triphenylphosphonio-α-(methoxycarbonyl)methanide, (methoxycarbonylmethylene)triphenylphosphorane, [12] Air-insensitive resonance-stabilized ester ylides such as the α-methyl-, [13,14] α-phenyl-, [15] α-cyano-, [16] α-formyl-, [17] and α-methoxy-substituted [18,19] (methoxycarbonylmethylene)triphenylphosphoranes and the α-fluoro-, [20] αchloro-, [21] α-bromo-, [22,23] and α-iodo-substituted [24,25] (ethoxycarbonylmethylene)triphenylphosphoranes can be readily prepared and stored. Employed in the reaction sequence outlined above, they enable the introduction of a variety of substituents into the 3-position of the final quinolinone.…”