Quaternary salts from phosphines

Ion-trapping is based on the fast reactions between cations and trialkyl-phosphines leading to stable phosphonium salts, namely tertiary phosphonium salts when a proton is trapped (e.g. from/F.i) and quaternary salts when the reaction involves tertiary oxonium ions (e. g. IV.4 and/or IV.5) or alkoxycarbenium ions. 

Use of the hydroxymethyl group, which is easily removed as formaldehyde from quaternary salts by means of alkali or triethylamine227 (cf. page 702). Tetrakis(hydroxymethyl)phosphonium chloride, being commercially available, is a suitable starting material for preparation of symmetrical tertiary phosphines (e.g., tripropylphosphine) by alternate fission and alkylation.796... 

The principle of extraction method used to separate PTC and product is based on solubility of quaternary ammonium salt in alkaline aqueous solution. " For example, tetrabutylammonium bromide is soluble to the extent of 27% in dilute (1% NaOH) aqueous solutions, but when the solution is made more concentrated (15% NaOH), the solubility of Bu4N Br decreases to 0.07%. When the products are obtained in PTC system, they can be usually separated from PTC by distillation method. PTC catalyst in the distillation residue may sometimes be reusable. With quaternary ammonium salts as catalysts, temperatures above 100-120 C usually result in partial or total decomposition of the quaternary salts to trialkylamines and other products. Mieczynska et al. and Monflier et al. investigated the hydrogenation and hydroformylation under phase transfer catalytic conditions. They found that the yield of aldehydes obtained in hydroformylation of 1-hexene strongly depends on solvent 24% in toluene, 53-86% in toluene-water-ethanol mixture and 77-94% in water-ethanol solution. The mixture of water-ethanol as a solvent was also found to be the best for hydrogenation of 1-hexene (96% of hexane). Conversion of Ph2PCH(CH3)(COOH) phosphine into sodium salt Ph2PCH(CH3)(COONa) yields aldehyde in toluene, 92% in toluene-water and 94% in toluene-water-ethanol mixture. 

Carbonylation of cinnamyl acetate in the presence of NEt3, acetic anhydride, and catalytic amounts of a palladium phosphine complex such as Cl2Pd(PPh3)2 gives 1-naphthyl acetate in good yield (Scheme 2) Esterification of 1-naphthol, the initial product, by acetic anhydride is essential to avoid side reactions leading to a complex mixture. Cinnamyl bromide reacts similarly, but the yield is lower probably because formation of the quaternary ammonium salt from the bromide and NEts competes with the carbonylation. This reaction is applicable to the synthesis of various substituted 1-naphthyl acetates. In the reaction of cinnamyl acetates with a meta-substituent, two possible regioisomeric products are obtained, where the para-cyclization predominates. 

However, the observed remarkable selectivity for amide formation suggests that free hemiaminal intermediate in solution is not involved, since the latter would likely liberate water faster than H2. We believe that the amine reacts with the intermediate aldehyde while it is still coordinated to the metal center, leading to formation of the quaternary salt 12 (Fig. 6), which may transfer a proton to the dearomatized phosphine arm (to give intermediate 10). Subsequent (3-hydride elimination and ring closure by the amine arm would lead to the tra/is-dihydride complex 6 and product amide. Liberation of dihydrogen from 6 would complete the catalytic cycle with regeneration of catalyst 5. 

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 ... 

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). 

Whatever metal is used, homogeneous processes suffer from high cost resulting from the consumption of the catalyst, whether recycled or not. This is why two-phase catalytic processes have been developed such as hydroformylation catalyzed by rhodium complexes, which are dissolved in water thanks to hydrophilic phosphines (cf. Section 3.1.1.1) [17]. Due to the sensitivity of most dimerization catalysts to proton-active or coordinating solvents, the use of non-aqueous ionic liquids (NAILs) as catalyst solvents has been proposed. These media are typically mixtures of quaternary ammonium or phosphonium salts, such as 1,3-dialkylimi-dazolium chloride, with aluminum trichloride (cf. Section 3.1.1.2.2). They prove to be superb solvents for cationic active species such as the cationic nickel complexes which are the active species of olefin dimerization [18, 19]. The dimers. 

The final product 63 retains the Ph3P+ group from the first vinyl phosphonium salt 59 and in the example below, using a boron enolate, this was hydrolysed to the stable phosphine oxide11 66. Note the creation of a quaternary carbon atom at a spiro centre. 

General directions for the preparation of quaternary arylphosphonium salts by the cobalt salt method 236 Anhydrous cobaltous chloride (5 mmoles) is suspended in a solution of a tertiary phosphine (50 mmoles) and an aryl halide (100 mmoles) in ether (75 ml) into this is dropped a 2N-solution of phenylmagnesium bromide (50 mmoles) in ether under nitrogen. The mixture is stirred vigorously for 0.5 h, then boiled for 3 h, after which it is hydrolysed with 2N-hydrochloric acid (100 ml). Filtration removes at most a small precipitate — if present it is boiled out with water. Addition of sodium iodide to the aqueous phase precipitates the phosphonium iodide. After recrystallization from water or aqueous ethanol, yields are around 50%. 

Some recent patents describe improvements to existing processes. Addition of dialkylaluminium hydrides to conventional hydroformylation catalysts gave an increased yield of linear products from internal alkenes, Co/porphyrin catalysts were shown to hydroformylate acrylonitrile, addition of R2P(0)H gave higher conversions in undecene hydroformy-lation, acetal yield was improved by addition of quaternary ammonium salts and the stability of phosphine-modified Co catalysts was improved by addition of arylalkylsulphonate salts. ... 

Quaternary ammonium salts of RhClJ catalyze the disproportionation of 1,3-cyclohexadiene in a two-phase system. At 20 °C, the products are benzene and H2 at 70-90 °C, benzene and cyclohexane are formed. The reaction does not proceed in the absence of an H donor and is initiated by water, alcohols, silicon hydrides, or by brief treatment with H2. The assumption that rhodium hydrides are the active catalysts is supported by the detection of hydride resonances at high field in the NMR spectrum (-11 to -12 ppm).Most dehydrogenation reactions use alcohols as substrates. The liquid-phase dehydrogenation of methanol is achieved by ruthenium phosphine catalysts, generated for example from... 

Telomerization of 1,4-butadiene with water, alcohols, amines, and acids is an extremely useful reaction since it leads to the formation of practically important products. (179,180). For example, the telomer with water, 2,7-octadiene-l-ol can be further hydrogenated to 1-octanol which is a raw material for plasticizers for poly(vinyl chloride). In fact, this reaction was among the processes disclosed in the first patents on the use of TPPTS in biphasic solvent mixtures (58). The catalyst for such telomerizations usually consists of palladium(O) and an excess of TPPTS, TPPMS, or other water-soluble phosphines (eg, with quaternary ammonium substituents). The telomerization of 1,4-butadiene with water was developed into an industrial process by Kuraray Ind. (Scheme 26). Interestingly, the best ligand was the phosphonium salt shown in (Scheme 26) and the catalyst could be prepared in situ from this ligand and [Pd(OAc)2] (179). It is assumed that under the reaction conditions the corresponding tertiary phosphine can be formed to some extent and coordinates to palladium. In any case with a large excess of... 

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. 

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