Phosphonium Salt Organocatalysis

Abstract

The field of organocatalysis is growing rapidly and attracts an increasing number of research groups. Most organocatalysts can be classified as Lewis bases, Lewis acids, Brønsted acids and Brøn-A C H T U N G T R E N N U N G sted bases. However, examples of Lewis acid organocatalysis under homogeneous conditions are comparatively rare. Phosphonium salts are easily accessible and frequently used intermediates in organic synthesis, long known phase-transfer catalysts and potential Lewis acid organocatalysts. This review covers the application of phosphonium salts as Lewis acidic catalysts for a variety of C À C, C À O and C À N bondforming reactions under homogeneous conditions. Moreover, recent developments are included which show the potential of chiral phosphonium salts as asymmetric (phase-transfer) catalysts.

Non-conventional Lewis Acids and Bases in Frustrated Lewis Pair Chemistry

“…Phosphonium cations are a different class of Lewis acids in that their Lewis acidity is derived from a vacant σ* orbital. While others have utilized chelating oxygen substituents [95] and simple alkylation to generate moderately acidity [96], generally these systems are not strongly Lewis acidic as this σ* orbital is generally too high in energy. However, we have installed electron-withdrawing substituents on phosphorus and thus lowered the energy of the σ* orbital.…”

Development of P-Spiro Chiral Aminophosphonium Salts as a New Class of Versatile Organic Molecular Catalyst

“…Chloride for a Highly Anti and Enantioselective Direct Henry Reaction Tetraaminophosphonium salts bearing primary amino groups [(RHN) 4 P ] have unique and intriguing features. Firstly, the HN P NH moiety of the cation could function as a hydrogen bonding donor toward the anion; such secondary interaction has been documented.…”

“…For instance, while quaternary phosphonium salts are readily accessible and widely applicable stoichiometric reagents in organic synthesis, their utilization as a catalyst are very limited. 4,5 This is probably because the intrinsic reactivity of phosphonium salts toward bases and nucleophiles would be a major obstacle in making them suffi ciently stable for use as catalysts. On the other hand, tetraaminophosphonium salts exhibit rather different properties from common alkyl substituted phosphonium salts.…”

Phosphonium salts and P-ylides

“…90 The reaction of TPP, DAAD and a range of N-H, C-H or S-H acids, such as 2-thiazoline-2-thiol, 2-benzoxazolinone, pyrrole-2-carboxaldehyde, benzotriazole, 5-methylbenzotriazole, 5-chlorobenzo-triazole, diethylmalonate, and acetylacetone to give the corresponding ylides (54), was shown to occur in water as a sole medium in the presence of polyethyleneglycol (PEG), b-cyclodextrin (b-CD), glycerine (Gly) or ethyleneglycol (EG), thus minimizing the cost operational hazards performing the reaction under environmentally benign conditions. 91 Under these conditions the reaction of 1,3-dimethylbarbituric or Meldrum's acid resulted in the 1,4-diionic products (55). Note that highly functionalized phosphorus ylides derived from the threecomponent reaction exist in solutions as a mixture of Z-and E-isomers of the corresponding zwitterionic species (56).The slow rotation about the partial double bond in (E)-3 and (Z)-3 geometrical isomers on the NMR timescale at room temperature allows an estimate of their mole ratio at equilibrium.…”

“…54 Werner has reviewed the data up to 2008 concerning the applications of phosphonium salts as Lewis acid catalysts for a variety of C-C, C-O and C-N bond forming reactions as well as the use of chiral P-salts as asymmetric phase-transfer catalysts. 55 New examples dealing with direct application of phosphonium salts in asymmetric organocatalysis should be also mentioned. T. Ooi and co-workers 56 described a practical method for the incorporation of a wide variety of chiral, non-racemic quaternary a-amino acids at specific sites of a peptide strand.…”