Protonation, of phosphines

Evidence for a major mode of catalyst deactivation in this system came from the observation of phosphonium cations (HPR3) in the reaction mixture, which could form through the pro to nation of free PR3 by the acidic dihydride complex. It is not known which species decomposes to release free PR3, but the decomposition pathway is exacerbated by the subsequent reactivity in which protonation of phosphine removes a proton from the metal dihydride, effectively removing a second metal species from the cycle. 

The conclusion regarding the site of protonation of phosphin-amides has been reached rather indirectly, from the effect of protonation on coupling across the P—N bond. Phosphinamides [128] are very labile in acid solution (Haake and Koizumi, 1970),... 

Studies on the protonation of phosphines have recently been extended by Olah and co-workers to similar systems. <1969>12)... 

The protonation of phosphine complexes ML3 with strong acids leads to cationic hydrides [LjMH]+. 

Protonation of phosphine, phosphites or phosphorous halides in strong acid forms P—H bonds. Phosphine in cone H SO or HCl is converted" to [PHJ ... 

Protonation of phosphinic acid, phosphites, or halophosphines by FSOjH or FSOjH-SbFj in SOj at low T yields phosphonium ions, as observed in H and P NMR spectra, e.g. ... 

Phosphonium salts containing a P—H bond form in condensed phases upon strong-acid protonation or in the gas phase by hydrogen-ion protonation of phosphines ... 

Protonation of phosphine oxides in aqueous sulphuric acid has been studied by and P n.m.r., and the correlation of basicity with oxide structure used to set up a new Hp-o acidity function. Chemical studies on a related matter have revealed the structure of the 1 1 adducts formed by... 

Compounds. Estimations of the degree of protonation of phosphine oxides vary, depending on whether conclusions are drawn from n.m.r. or n.m.r. spectra. A comparison of six oxides indicates that 8p reflects direct protonation to a greater extent than Sh, which depends also on the state of the molecule as a whole.A comparison of the shielding effects of oxides and sulphides (11 X = O or S) showed that the deshielding... 

In a biological system solvation effects will clearly be important. It is difficult to predict whether pKj s or gas-phase proton-affinities will be more reliable guides to the degree of protonation of phosphines in vivo. There are a variety of compartments of greatly differing polarities that are potentially available. Lipophilic phosphines would be expected to become localised in non-aqueous environments, e.g. membranes. It is possible that a phenyldiphosphine could act as a H shuttle across a membrane. The only way to probe this may be to investigate whether proton translocation occurs in a model system. 

Towards a simple Lewis base, for example the proton, phosphine is a poorer electron donor than ammonia, the larger phosphorus atom being less able to form a stable covalent bond with the acceptor atom or molecule. Phosphine is, therefore, a much weaker base than ammonia and there is no series of phosphonium salts corresponding to the ammonium salts but phosphonium halides. PH4X (X = Cl, Br, I) can be prepared by the direct combination of phosphine with the appropriate hydrogen halide. These compounds are much more easily dissociated than ammonium halides, the most stable being the iodide, but even this dissociates at 333 K PH4I = PH3 -t- HI... 

However, when either P(CgH )(CH2)2 or P(CgH )2(CH2) is used to form cis- or /n j -M(N2)2(PR3)4j M = Mo or W, respectively, followed by treatment with acid, ammonia yields of about 2 mol or 0.7 mol pet mole of complex for M = W and Mo, respectively, are produced (193,194). These and related data have been used to suggest a possible stepwise sequence for the reduction and protonation of N2 on a single molybdenum atom ia nitrogeaase (194). However, acidificatioa leads to complete destmctioa of the complex. Using both the stabilizing effect of the chelating phosphine triphos,... 

The energies of protonation of the complete series of methyl and ethyl phosphines have been calculated. pATa values for the hydroxyphenyl-... 

A differential vapour-pressure technique has been used to determine the molecular weights of phosphonic and phosphinic acids in 95% ethanol. Cryoscopic and n.m.r. studies have been made on solutions of phosphinic acids in sulphuric acid and oleum. Mass spectrometry has indicated the ready formation of phosphinylium ions after electron bombardment of phosphonic and phosphinic acids and their derivatives. However, the cryoscopic results in sulphuric acid indicated that reaction did not proceed beyond protonation, and the n.m.r. study on oleum solutions suggested that sulphonation occurred. 

A method for determining the basicities of phosphoryl compounds has been decribed which is based on 31P n.m.r. chemical shift measurement and a two phase system consisting of an organic solvent and 12H sulphuric acid.245 The gas-phase protonation of aliphatic phosphine oxides have been determined by cyclotron resonance. There was a good correlation of pK4 with Kabachnik constants and HO... 

Analogously to the phosphorylation of alcohols with phosphoric azolides the phos-phinylation is carried out by means of phosphinic azolides (CH2CI2/C2H5OC2H5). In the following case a I2/C2H5OC2H5) protonated diphenylphosphinic imidazolide is used to give the allylic diphenylphosphinic esters in good yields ... 

A number of examples have been reported documenting the use of palladium phosphine complexes as catalysts. The dialkyl species [PtL2R2] (L2 = dmpe, dppe, (PMe3)2 R = Me, CH2SiMe3) catalyze the reaction of [PhNH3]+ with activated alkenes (acrylonitrile, methyl acrylate, acrolein).176 Unfunctionalized alkenes prove unreactive. The reaction mechanism is believed to proceed via protonation of Pt-R by the ammonium salt (generating PhNH2 in turn) and the subsequent release of alkane to afford a vacant coordination site on the metal. Coordination of alkene then allows access into route A of the mechanism shown in Scheme 34. Protonation is also... 

A remarkable application of phosphines by Grey and coworkers for acid site characterization is the use of diphosphines with alkyl chain spacers of different length between the phosphine moieties. Based on careful NMR analysis and appropriate loading levels with diphosphines, the Al distribution can be determined [223, 224], The idea behind this tool is that the phosphine groups will be proto-nated, when they are close to an acid site in the zeolite structure. Protonation of both phosphine groups in one probe molecule will only occur, when the distance between the two acid sites is compatible with the molecular dimension of the diphosphine. 

Analysis of the data in Table XVIII suggests that silene formation is kinetically the most favorable process. However, according to experiment, metallated silenes are formed. This is related to the fact that in polar solvents proton transfer from the carbon atom to silicon is intermolecular, which leads to a considerable decrease in the reaction barrier. We believe that when the migration of substituents from the carbon atom to silicon is suppressed, for example, by the introduction of two alkyl radicals, the elimination of phosphines resulting in silene formation becomes the most probable process. 

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