Phosphines from phosphonium salts

The synthesis of chiral phosphines from resolved phosphonium salts or phosphine oxides is an intrinsically limited approach. The groups attached to phosphorus must be present prior to resolution and, furthermore, the preparation of phosphine oxides and phosphines from phosphonium salts by chemical or electrochemical cleavage reactions requires that one of the groups bonded to phosphorus be substantially easier to cleave than the other three. 

Potassium cyanide dimethyl sulfoxide Tert. phosphines from phosphonium salts... 

Mixed tert. amines and phosphines as well as mixed quaternary ammonium and phosphonium salts can be conveniently prepared by successive alkylations and reductions because of the varying ease with which the groups are removed. The benzyl group is most easily removed. F. e., also with a polished Pb-cathode, s. L. Horner and A. Mentrup, A. 646, 49, 65 (1961) optically active phosphines from phosphonium salts s. Tetrah. Let. 1961, 161 optically active arsines cf. Tetrah. Let. 1962, 203. 

Preparation.—It has been shown that the stability of the ylide solution formed from the reaction of dihalogenodifluoromethanes with tertiary phosphines is due to a dynamic equilibrium which lies on the side of phosphine and phosphonium salt (Scheme 1). Ylide solutions generated from chlorodifluoroacetate are unstable since no such equilibrium is possible.1... 

Carbon-13 shifts of representative phosphines [364], phosphonium salts [365], phospho-nium ylides [365, 366], diphosphines [367], phosphonates [368], phosphorous and phosphoric acid derivatives [369] are summarized in Table 4.49. I3C shift data of some group V organoelement compounds are compared in Table 4.50. It turns out that a sp3 carbon nuclei of phosphines and arsines are shielded (0-25 ppm) relative to those of amines (30-60 ppm), as expected from the heavy atom effect, sp2 carbons of CC double bonds behave correspondingly, as shown for the triphenyl derivatives in Table 4.50, with... 

In some instances, relatively mild nucleophilic agents are capable of generating tertiary phosphines from phosphonium or quasiphosphonium salts (see Section 4.4). Reagents such as KCN, aUcoxide salts, or even tertiary amines are capable of attacking certain substituents to remove them from phosphorus leaving the tertiary phosphine with retention of configuration at phosphorus. ... 

Phosphonium salts are typically stable crystalline soHds that have high water solubiUty. Uses include biocides, flame retardants, the phase-transfer catalysts (98). Although their thermal stabiUty is quite high, tertiary phosphines can be obtained from pyrolysis of quaternary phosphonium haUdes. The hydroxides undergo thermal degradation to phosphine oxides as follows ... 

This material may be converted directly to a phosphonium salt 1.40 g. (0.0054 mole) of the crude iodide is dissolved in 20 ml. of benzene, and 1.42 g, (0.0054 mole) of triphenylphosphine [Phosphine, triphenyl-] is added. On standing, 2.5 g. (77%) of the triphenylphosphonium salt precipitates as a colorless 1 1 complex with benzene, m.p. 135-137°. Recrystallization from methanol-benzene raises the melting point to 140-142°. Analysis calculated for C28H29PI CeH6 C, 68.23 H, 5.39. Found C, 68.15 H, 5.28. 

In the Wittig reaction an aldehyde or ketone is treated with a phosphorus ylid (also called a phosphorane) to give an alkene. Phosphorus ylids are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from the phosphine and an alkyl halide (10-44) ... 

Wittig reactions are versatile and useful for preparing alkenes, under mild conditions, where the position of the double bond is known unambiguously. The reaction involves the facile formation of a phosphonium salt from an alkyl halide and a phosphine. In the presence of base this loses HX to form an ylide (Scheme 1.15). This highly polar ylide reacts with a carbonyl compound to give an alkene and a stoichiometric amount of a phosphine oxide, usually triphenylphosphine oxide. 

This is an extremely useful reaction for the synthesis of alkenes. It involves the addition of a phosphonium ylid, e.g. (136), also known as a phosphorane, to the carbonyl group of an aldehyde or ketone the ylid is indeed a carbanion having an adjacent hetero atom. Such species are generated by the reaction of an alkyl halide, RR CHX (137), on a trialkyl- or triaryl-phosphine (138)—very often Ph3P—to yield a phosphonium salt (139), followed by abstraction of a proton from it by a very strong base, e.g. PhLi ... 

The cyclic phosphonium salts 140,141,143,145, and 146 so obtained are evidence for the mechanism of the oxaphospholic cyclization and especially for the main role of the tertiary carbocation formation during the process. The additional data which support this assumption, come from the investigation of the same reaction, but with different substrate, i.e., dimethyl(l,2-hexadienyl)phosphine oxide 147. In this case, the reaction mechanism involved formation of secondary carbocation that gives oxaphosphole product 148 only in 10% yield (Scheme 60) [124],... 

More recently a variation of this mechanism was reported by Novak [37], The mechanism involves nucleophilic attack at co-ordinated phosphines and it explains the exchange of aryl groups at the phosphine centres with the intermediacy of metal aryl moieties. After the nucleophilic attack the phosphine may dissociate from the metal as a phosphonium salt. To obtain a catalytic cycle the phosphonium salt adds oxidatively to the zerovalent palladium complex (Figure 2.38). Note where the electrons go . 

In a different approach [11] to access pure products, the use of strong oleum (65% SO3) for sulfonation of PPh3 resulted in quantitative formation of TPPTS oxide. This was converted to the ethyl suhbester through the reaction of an intermediate silver sulfonate salt (isolated) with iodoethane. Reduction with SiHCls in toluene/THF afforded tris(3-ethylsulfonatophenyl)phosphine which was finally converted to pure 3 with NaBr in wet acetone. In four steps the overall yield was 40% (for PPhs) which compares fairly with other procedures to obtain pure TPPTS. Since phosphine oxides are readily available from easily formed quaternary phosphonium salts this method potentially allows preparation of a variety of sulfonated phosphines (e.g. (CH3)P(C6H4-3-S03Na)2). 

From Sodium Azide and Vinylphosphonium Salts Phosphonium salts of the types 4 and 5 react with sodium azide in aqueous solution to give v-triazoles in high yield. The proposed mechanism (Scheme 13) involves nucleophilic attack at the carbon jS-tothe phosphorus, followed by cyclization with displacement of triphenyl-phosphine. 

The salts were prepared from hydroxy phosphine oxides or phosphinates as depicted in Scheme 42 after 1 h. The reactions were carried out with trifluorometh-anesulfonic anhydride in the presence of 4A molecular sieves, and it was shown by P-NMR due to downfield shifts that the phosphonium salts were formed. However, the salts could not be isolated and were prepared in situ for NMR studies and for the application in catalysis. 

Tetrakis( l-hydroxyalkyl)phosphonium salts were prepared from phosphine and various aliphatic aldehydes using similar reaction conditions... 

The difluoromethylene ylides react with carbonyl derivatives (aldehyde, lactone) to afford gem-difluoromethylene compounds.They are generated starting from halogenodifluoromethane with triphenylphosphine (or trisaminophosphine) or starting from zinc and a phosphonium salt (or a phosphine oxide)." ° ... 

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