Phosphonium zwitterions

Spontaneous Altmiating Copolymerization of Cydic Phosphorus Compounds via Phosphonium Zwitterion Intermediates... 

As shown in Scheme 1, aliphatic phosphines such as P(n-Bu)3 catalyze the addition of alcohols (2) to methyl propiolate (3) [35]. The mechanism is believed to involve an initial addition of the phosphine to the C = C moiety to give a zwitterionic allenolate (I), which then deprotonates the alcohol, yielding a vinyl phosphonium salt (II). An alkoxide addition to give an enolate (III), followed by phosphine elimination gives the product 4 and regenerates the catalyst. Several experiments suggest that when alcohols are used in excess, the catalyst rests as the original phosphine [34]. 

Reactions under quaternisation of exocyclic R2P-substituents have so far been applied exclusively to benzophospholide derivatives. The most convenient approach involves treatment of the substrate with an appropriate alkyl halide [27, 31, 35] or acrylic acid (Scheme 10) [27]. The quaternisation products formed are in general isolable without complication if pure starting materials have been employed as is normally the case for zwitterionic substrates. Anionic benzophospholides such as 29 and 30 are, in contrast, normally only accessible as crude product mixtures whose quaternisation affords mixtures of several phosphonium salts. Separation of the desired product may in these cases require lengthy work-up procedures and result in substantially lower yields [31]. 

In this chapter, specific paragraphs are not devoted to the zwitterionic structures, such as betaines, or to the applications of phosphonium salts in organic synthesis, but both topics are mentioned incidentally. 

Intermolecular and intramolecular nucleophilic substitution of an alcoholic hydroxy group by the triphenylphosphine/dialkyl azodicarboxylate redox system is widely used in the synthesis and transformation of natural products and is known in organic chemistry as the Mitsunobu reaction.1951 This reaction starts with formation of the zwitterionic phosphonium adduct 19 (Scheme 9) from triphenylphosphine and diethyl (or diisopropyl) azodicarbox-... 

Reactions of tertiary phosphines afford zwitterionic phosphonium salts by addition to C(3) ... 

The reactions of picryl fluoride with (2,3-dihydroxypropyl)-ammonium or -phosphonium compounds have been found120 to yield zwitterionic spiro-cr-adducts such as (31). Aromatic diazonium cations have been shown to add to the 9-position of cr-adducts of 9-nitroanthracene to yield neutral derivatives.121 A study has been reported of the effects of ion pairing and change of solvent on the kinetics of decomposition of some anionic cr-adducts.122... 

A mechanistic interpretation of the Wittig reaction, allowing the various experimental data collected during many years to bring into accord with theoretical demands, was reported only 25 years after its discovery4,7 9). Wittig already formulated a zwitterionic adduct of the form of a P—O-betain 52) (Scheme 2). This is better described by formula 6, since at that time almost exclusively lithium alkyls have been used for the deprotonation of the phosphonium salts. The reversibilitiy of the... 

Propagation proceeds through opening of the phosphonium ring, which is located at the end of one zwitterion, by nucleophilic attack of the carboxylate group of another zwitterion. This reaction pattern belongs to the family of the Arbusov reaction (in the rectangle). 

The key intermediate of the above redox copolymerization is a zwitterion 41. The propagation step involves opening of the phosphonium ring of a zwitterion by... 

The adduct 65 of aziridine to the acetylenic phosphonium salt 64 readily rearranges to the enamine 66, which in refluxing acetonitrile is converted into the pyrroline derivative 68 by opening of the aziridine ring and recyclization of the resulting zwitterion 67 (equation 33)54. 

Reactions of carbene complexes with phosphorous compounds are topics of current research.164-167 (l-Alkynyl)carbene complexes (CO)5M= C(OEt) —C=CR 1 (M = Cr, W R = Ph, r-Bu, SiMe3) readily add tertiary phosphanes PPhR2 (R = Ph, Me) and form stable zwitterionic phosphonium allenide complexes (CO)5M —C(OEt)=C=C(R)—P+PhR2 145, which have been characterized by X-ray diffraction (Scheme 57).55 In previous studies on Michael-type additions of nitrogen and oxygen nucleophiles to (l-alkynyl)carbene complexes 1, zwitterionic allenide complexes of type 145 have been postulated as intermediates.21,23,503 62 112,168,169 Unambiguous structural evidence of the tendency to form such intermediates is given by isolation of a stable zwitterionic adduct, such as compound 145. Zwitterionic... 

An interesting example of a Lewis acid/base reaction between neutral reactants is the formation of tris(n-butyl)phosphonium-dithiocarboxylate, ( -Bu)3P" — 82 , from tris(n-butyl)phosphane and carbon disulfide in solution. As expected, this equilibrium is strongly shifted in favour of the dipolar zwitterion with increasing solvent polarity (diethyl ether dimethyl sulfoxide) [272, 273]. 

Another remarkable example of reaction type (c) in Table 5, somewhat related to the Menschutkin reaction, is the Sn2 reaction between tris(n-butyl)phosphane and carbon disulfide, to give zwitterionic tris( -butyl)phosphonium dithiocarboxylate via a corresponding dipolar activated complex [790, 791],... 

Stabilized species in the phosphorus yhde category are normally generated in a two-step sequence beginning with the formation of the quaternary phosphonium salt. This is usually accomplished with ease by the reaction of triphenylphosphine with the appropriate haloalkane. Salt formation is followed by deprotonation at the carbon adjacent to phosphorus using an appropriate base to generate a zwitterionic species stabilized by the adjacent functionality (illustrated in equation 20). The resultant phosphorus species reacts with an introduced carbonyl compound to generate an intermediate oxaphosphatane that undergoes decomposition to produce alkene and phosphine oxide at relatively low temperatures (equation 21). 

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