Mechanism of Wittig reaction with cyclic anhydrides

Abstract

The reactions of stabilized phosphoranes with cyclic anhydrides give enol-lactones as final products. The initial condensation, however, leads to the formation of acyclic adducts that are observable by NMR, can be easily trapped, and, in some cases, can be isolated. A study of the mechanism of these condensations by NMR spectroscopic methods and by various trapping experiments is described and the reaction pathway is proposed.

Intra‐ and Intermolecular C(sp2)–H···O Hydrogen Bonds in a Series of Isobenzofuranone Derivatives: Manifestation and Energetics

“…Thus, Wittig reactions yielded exclusively E isomers, in 60 % yield when the reaction was carried out without a solvent20a or in 87 % yield after 18 h in chloroform at reflux 20b. Later, during a mechanistic study of this reaction, it was found that in chloroform at room temperature the E isomer was formed in 52 % yield along with a 13 % yield of the Z isomer 20c. Non‐cyclic anhydrides are known to react with stabilized phosphoranes to afford the acylation products, diketo phosphoranes,21 and the occurrence of the normal Wittig reaction and the exclusive or predominant formation of the E isomers with phthalic anhydride were explained in terms of formation of a π–π complex between the C=O group of the phosphorane and the electron‐deficient benzenic ring of phthalic anhydride 20a,20c,20d.…”

“…Later, during a mechanistic study of this reaction, it was found that in chloroform at room temperature the E isomer was formed in 52 % yield along with a 13 % yield of the Z isomer 20c. Non‐cyclic anhydrides are known to react with stabilized phosphoranes to afford the acylation products, diketo phosphoranes,21 and the occurrence of the normal Wittig reaction and the exclusive or predominant formation of the E isomers with phthalic anhydride were explained in terms of formation of a π–π complex between the C=O group of the phosphorane and the electron‐deficient benzenic ring of phthalic anhydride 20a,20c,20d. In the process of this investigation, when we realized that 3,6‐H atoms of phthalic anhydride and its analogues can form relatively strong HBs, we assumed that the H‐bonded complexes with the phosphorane can precede the Wittig reaction, and that chloroform, which is itself capable of H‐bonding, is therefore not the best choice as a solvent.…”

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

“…A study of the condensation of ylide 8 with unsubstituted phthalic anhydride, reported previously (6) …”

“…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). The enollactones are subsequently formed via a higher energy irreversible transformation ( 6 ) . Taking into account a facile reversibility of acyclic adduct(s) formation, it was not possible to determine if the initial addition products are converted to enol-lactones, or if there exists an alternate, higher energy, pathway leading directly from cyclic anhydrides to enol-lactones.…”

Palladium-Catalyzed Ylidyl-Carbonylation of Aryl Halides To Produce α-Acylphosphoranes

“…These examples highlight the importance of α-acylphosphoranes in organic synthesis. Traditionally, α-acylphosphoranes have been synthesized by acylation reaction of carboxylic acid chlorides with stabilized phosphonium ylides (Scheme , a) ,, or using carboxylic acid derivatives, ,, cyclic anhydrides, aldehydes, 1,3-dicarbonyl compounds, benzoic acid trimethylsilyl ester (TMSE), and tellurol esters . However, an efficient and simple synthesis of α-acylphosphoranes via palladium-catalyzed carbonylation under atmospheric CO pressure has received no attention.…”