Phosphorane intermediate, cyclic

In the first report on this famous reaction, Wittig expressed a clear preference for the cyclic phosphorane intermediate 22 such as had already been formulated by Staudinger and Meyer in their earlier work. This view became generally accepted onlv many years later35). In this respect, the Wittig reaction did indeed still reflect some prominent features of the underlying research program on penta-coordinated molecules, from which it now evolved rapidly to a very active life of its own 36). 

In a study of the reactions oi acyclic o-methoxyphenyl-phosphinites (45a), phosphonites (45b) and phosphites (45c) with halogens,the intermediate halogenophosphonium salts (46) are stabilised either by ligand exchange with starting material (not shown) or by elimination of methyl halides to form cyclic phosphoranes ( 47 ) 3 7. ... 

A proposal (3) that vinyl phosphate might be formed from the ketophosphonium intermediate, after rearrangement via a four-membered cyclic phosphorane 8 is therefore excluded by our results under these reaction conditions. 

In these reactions, retention can only be accommodated by formation of a phosphorane intermediate, followed by pseudorotation, before loss of the leaving group. Inversion could result either from a concerted reaction, or a stepwise process, if the phosphorane collapses before pseudorotation can occur. The stereochemical and LFER data indicate that both mechanisms occur. In a study of similar cyclic triesters (Fig. 12c and d) that combined LFER studies, stereochemical analysis, and solvent isotope effects, Rowell and Gorenstein found a trend in / g values as a function of attacking nucleophile.61 These, and the stereochemical data, are strong evidence for a mechanistic continuum from concerted to stepwise reactions of six-membered cyclic phosphate triesters.61... 

Analysis of the distortions of cyclic phosphoranes from ideal trigonal-bipyramidal geometry shows that these are towards square-pyramidal geometry, i.e. along the pathway followed in Berry pseudorotation. Ab initio calculations on the reaction of HgPO with H support a transition state or intermediate with distorted trigonal-bipyramidal geometry and equatorial 0. ... 

The theory is best illustrated by the acid hydrolysis of methylethylene-phosphate. Scheme 4 shows how pseudorotation of the phosphorane intermediate can accommodate exocychc hydrolysis. The rate enhancement of the five-membered cycUc phosphate arises from two effects of the ring. The cyclic phosphate ester is strained in spanning the tetrahedral angle of 109°, which raises the strain energy of the cyclic phosphate relative to that of the acyclic phosphate (65). In going from the phosphate ester to the intermediate, the angle that the ring is required to span is reduced to 90°,... 

The use of oxirane together with Na2HP03 to prepare (2-hydroxyethyl)phosphonic acid as its disodium salt is of historical interest. Other poorly exploited, yet interesting and potentially valuable, reactions include the combination of a trialkyl phosphite and dialkyl chlorophosphate with an oxirane to yield O-phosphorylated derivatives of (2-hydrox-yalkyl)phosphonic diesters (reaction 10)" and, following the initial reaction of an a,j8-unsaturated ketone with a phosphorus(III) triester to give the cyclic phosphorane 272, the subsequent further reaction of the latter with an aldehyde followed by hydrolysis (reaction 11)" Dialkyl acetyl phosphites are reported to react with oxirane through anionic intermediate species with the formation, albeit in low yields, of dialkyl (2-acetyloxyethyl)phos-phonates hydrolysable with concentrated HCl, to give (2-hydroxyethyl)phosphonic acid ... 

The reactions between simple phosphorus(III) esters and nitroalkenes occur very readily, but are complex, and their outcome depends on several factors, including the nature of the ester and the experimental conditions. Several processes, including addition, elimination and oxidation reactions, occur alongside one another. In some respects the reactions of phosphorus(III) esters with nitroalkenes resemble those of the same esters with a,j5-unsaturated ketones evidently equilibration occurs between dipolar adducts and cyclic quinquecovalent intermediates of the phosphorane type, and each may decompose, possibly to give different products. 

A detailed study of the desulphurization of the disulphides (301)(R,R = dialkoxy, or alkyl(alkoxy) ) by phosphorus(III) compounds, including (Mc2N)2P, Ph P, or acyclic or cyclic (based on catechol) phosphotriesters or ester amides) considered the role of phosphonium and phosphorane intermediates. The isomerization of (302) into (303), and the decomposition (desulphurization, deoxygenation, or dealkylation) into end products, are influenced by electronic and steric factors. 

Oxaphospholans.—The stable phosphoranes (41) were obtained from ylides and epichlorohydrin.35 The formation of (41 R = H) from the action of base on the salt (42) supports a mechanism via the intermediate ylides (43). The crystalline cyclic... 

The phosphoranes, e.g. (69), which are intermediates in the Arbusov reactions of the cyclic phosphite (68) with chlorine, bromine, and benzenesulphenyl chloride have been detected at low temperatures by 31P n.m.r. spectroscopy.47 A similar technique revealed the intermediate (71) in the reaction of the thiophosphate (70) with chlorine.48... 

A series of uridine 3 -alkylphosphates (308) undergo in alkaline solution a hydroxide-ion catalysed reaction to give the 2, 3 -cyclic monophosphates (309) via phosphorane-type intermediates (Scheme 35). The alkyl groups ranged from ethyl... 

Cyclic 1,3,2-dioxaphosphorinanes (P-CN = 4) react with nucleophiles, probably by initial addition of the nucleophile to the phosphorus atom, giving phosphoranes (P-C1V = 5) as intermediates (or transition states). This holds not only for P=0 derivatives but also for quaternary 1,3,2-dioxaphosphorinanium salts (81JA5894). By steric rearrangement of the phosphoranes the configuration of the P atom can be changed (retention or inversion) (see Section 1.17.5). 

Analogous chemistry was reported [215] with thioaldehyde intermediates generated from a non-classical thermal reaction of a sulfoxide via a sulfe-nate ester (Table 3, entry 2). The reaction of cyclic tetra- or pentasulfides with stabilised phosphoranes is also a good source of thioaldehydes [216], trapped by dienes (Table 3, entry 2). 

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