Phosphonium salts, carbon-phosphorus

A. Alkaline Hydrolysis. -The low kinetic isotope effect observed in the protonation of carbanions formed in phosphonium salt hydrolysis leads to the idea that there is little breaking of the phosphorus-carbon bond and correspondingly little transfer of a proton to the incipient carbanion in the transition state (87) of the rate-determining step. ... 

The phosphonium salt (116) gave the phosphorane (117) with phenyl-lithium although it has hydrogen atoms attached to carbon bonded to phosphorus. ... 

These reactions may be considered to be a method of obtaining 1,3,2,5-dioxaborataphosphoniarinanes with different substituents at carbon and phosphorus atoms of the ring. Comparing the properties of cyclic oxyalkyl-phosphines and boryloxyalkylphosphines, it should be noted that in both cases the reaction with alkyl halides results in the formation of a tertiary phosphonium salt. The reaction with electrophilic reagents such as diphe-nylchlorophosphine and diphenylchloroborane proceeded quite differently [Eq. (100)]. 

Miscellaneous Reactions.—A full report has appeared of the reactions of carbon dioxide and carbon disulphide with tervalent phosphorus aryl esters and amines the products are ureas and thioureas, respectively.74 The suggested mechanism, previously invoked for similar reactions of carboxylic acids, involves the N-phosphonium salt (97). 

This extremely air-sensitive compound, which is valence isoelectronic to an olefin, has been structurally characterized by X-ray diffraction. It has a short carbon-phosphorus double bond (1.62 A) the phosphorus and carbon atoms adopt a trigonal planar geometry with a dihedral angle of 60° (Fig. 3). This value is significantly larger than that reported for the most crowded olefin.61 Formally, this compound can be viewed as the product of a car-bene-carbenoid coupling between bis(trimethylsilyl)carbene and bis(diiso-propylamino)phosphenium triflate. Note that another route to methylene-phosphonium salt has been reported by Griitzmacher et al.62... 

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. 

In vinylic phosphonium salts, the jft-carbon is found to be substantially deshielded (A<5 = 22.4 ppm for the vinyltriphenylphosphonium bromide as compared with ethylene), although only a small inductive effect was expected. The extent of this was interpreted to be a consequence of pn-dn bonding between phosphorus and carbon39. This argument was used to identify structures when or 31PNMR spectra failed96. 

Schlosser670 proposed an equivalent mechanism at the phosphorane 64 stage, formed from the corresponding ylide and not from the phosphonium salt the driving force for the migration comes from the carbenoid nature of the carbon in the a-position to the phosphorus. This is consistent with the anionotropic nature of the migration. The mechanism is corroborated by a similar migration from the ylide 65670 (reaction 198). 

In the case of phosphonium salts, some examples of decomposition by Hofmann elimination reactions are known. The phosphonium salts which normally undergo such a decomposition possess an activated hydrogen / to the phosphorus the salts 70, with a strong electron-withdrawing Z group on the carbon / to the phosphorus (reaction 202) and the a-hydroxyalkylphosphonium salts 71 (reaction 203)... 

Table 4.51. One-Bond (J and Longer-range (2J, 3J, 4J) Carbon-Phosphorus Coupling Constants of Representative Phosphines [372], Phosphonium Salts [365], Phosphine oxides [366, 372, 373], Phosphonium Ylides [365, 373], Phosphonates [368, 372], Phosphites [374], and Phosphates [375] (in Hz). 

Thus, in alkylphosphonium salts too the C—H functions adjacent to the positively charged key atom were too acidic to let the attacking nucleophile proceed on its way towards the phosphorus center. A way out of this dilemma should consist in the application of a phosphonium salt with less acidic neighboring C—H functions. Such a salt was tetraphenylphosphonium iodide which in the reaction with phenyllithium did indeed lead to the first compound containing five covalent carbon-element bonds, pentaphenylphosphorane, (C6H5)5P (12)23). Its covalent nature was... 

The phosphorus-stabilized carbanion is an ylide (pronounced ilL-id )—a molecule that bears no overall charge but has a negatively charged carbon atom bonded to a positively charged heteroatom. Phosphorus ylides are prepared from tri-phenylphosphine and alkyl halides in a two-step process. The first step is nucleophilic attack by triphenylphosphine on an unhindered (usually primary) alkyl halide. The product is an alkyltriphenylphosphonium salt. The phosphonium salt is treated with a strong base (usually butyllithium) to abstract a proton from the carbon atom bonded to phosphorus. 

Next, consider the phosphonium salt you met at the end of Chapter 14 for use in the Wittig reaction, turning aldehydes and ketones to alkenes. It has a 2/ph of 18 Hz. There is no doubt about this structure—it is just an illustration of coupling to phosphorus. There is coupling to phosphorus in the carbon spectrum too the methyl group appears at 5c 10.6 p.p.m. with a l/pc of 57 Hz, somewhat smaller than typical Vph- We haven t yet talked about couplings to 13C we shall now do so. 

Reaction of the bromotrifluoromethane/tris(diethylamino)phosphane system with benzalde-hyde leads to the formation of the phosphonium salt 19, which decomposes at 140 C, affording the bromophenylethanc 20 and not the alcohol. Due to the ease of formation of a phosphorus compound with a P = 0 bond, in this case the intermediate carbenium ion reacts with the bromide anion, giving rise to the final reaction product. The reaction is general in character pcrfluoroalkyl bromides with different carbon-chain lengths (from 2 to 6) have been used as reactants. 

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