Oxaphosphetane

Step 1 The ylide and the aldehyde or ketone combine to form an oxaphosphetane... 

Step 2 The oxaphosphetane dissociates to an alkene and triphenylphosphme oxide... 

The Wittig reaction is one that IS still undergoing mech anistic investigation An other possibility is that the oxaphosphetane intermedi ate IS formed by a two step process rather than the one step process shown in Figure 17 13... 

The submitters have shown that these reactions proceed by dehydro-chlorination of the acid chloride to the ketene, which is then trapped by reaction with the phosphorane. The resulting betaine decomposes to the allenic ester via an oxaphosphetane. In contrast, the reaction of acid chlorides with 2 equivalents of phosphoranes involves initial acylation of the phosphorane followed by proton elimination from the phosphonium salt. ... 

Reaction of an alkyltriphenylphosphorane in tetrahydrofuran with an aldehyde produces the oxaphosphetane B, which can be further treated with 1 eq. of 3-butyllithium to form the P-oxidophosphonium ylide C. This ylide can in turn react with another aldehyde, for instance, paraformaldehyde to give, after work-up, the trisubstituted olefin D. 

The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

Evidence for the four-membered ring intermediate—the oxaphosphetane 9—comes from P-NMR experiments betaines of type 8 have in some cases been isolated. 

The energetics of ylid formation and tlieir reaction is solution has been studied. For many years it was assumed that a diionic compound, called a betaine, is an intermediate on the pathway from the starting compounds to the oxaphosphetane, and in fact it may be so, but there is little evidence for it. ... 

Betaine precipitates have been isolated in certain Wittig reactions, but these are betaine-lithium halide adducts, and might just as well have been formed from the oxaphosphetane as from a true betaine. However, there is one report of an observed betaine lithium salt during the course of a Wittig reaction. In contrast, there is much evidence for the presence of the oxaphosphetane intermediates, at least... 

Olefination Reactions Involving Phosphonium Ylides. The synthetic potential of phosphonium ylides was developed initially by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphonium ylide with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond. The mechanism originally proposed involves an addition of the nucleophilic ylide carbon to the carbonyl group to form a dipolar intermediate (a betaine), followed by elimination of a phosphine oxide. The elimination is presumed to occur after formation of a four-membered oxaphosphetane intermediate. An alternative mechanism proposes direct formation of the oxaphosphetane by a cycloaddition reaction.236 There have been several computational studies that find the oxaphosphetane structure to be an intermediate.237 Oxaphosphetane intermediates have been observed by NMR studies at low temperature.238 Betaine intermediates have been observed only under special conditions that retard the cyclization and elimination steps.239... 

Structure 3 is the intermediate oxyanion adduct. TS2 is the structure leading to cyclization of the oxyanion to the oxaphosphetane. Structure 4a is the oxaphosphetane, and the computation shows only a small barrier for its conversion to product. 

Monocyclic Phosphoranes. - Further studies on the mechanism and stereochemistry of the Wittig reaction have been conducted by a combination of 1H, 13C and 3 P n.m.r.2k 25. The results show that at -18°C both ois and trans diastereomeric oxaphosphetans (e.g. 17 and 18) may be observed and their decomposition to alkenes monitored by n.m.r. Evidence was presented to suggest that during this process oxaphosphetan equilibration involving the siphoning of (17) into (18) occurred in competition with alkene formation. 

This accounts for the considerable discrepancy between the alkene Z/E ratio found on work-up and the initial oxaphosphetan ais/trans ratio. By approaching the problem from the starting point of the diastereomeric phosphonium salts (19) and (20), deprotonation studies and crossover experiments showed that the retro-Wittig reaction was only detectable with the erythreo isomer (19) via the cis-oxaphosphetan (17). Furthermore, it was shown that under lithium-salt-free conditions, mixtures of (19) and (20) exhibited stereochemical drift because of a synergistic effect (of undefined mechanism) between the oxaphosphetans (17) and (18) during their decomposition to alkenes. 

In a similar study McEwen has shown that during the reaction of benzaldehyde with the semi-stabilised ylids (21) and (22), the intermediate oxaphosphetans (23 and 24) are not detectable by n.m.r. 

Hydrolysis of the oxaphosphetan (25) gave the phosphine oxide (26) which was converted into (27) by treatment with a mixture of thionyl chloride and pyridine. Treatment of (25) with HC also caused ring opening to (28) which was reversed on treatment with triethylamine. The chlorophosphorane (28) lost nexaxluoroiso-propanol on heating to give (27) which was fluorinated to give (29)27. All the compounds were characterised Dy 1H, 19F and 31P n.m.r. 

The fact that only the vinyl-substituted l,2X5-oxaphosphetanes 22 but not the arylated phosphorus heterocycles 21 undergo photofragmentations is presumably due to the inability of the latter to absorb the light (X > 280 nm) supplied for carbene formation (7- 8) [e.g. 21, Ar = C6H5 e280 200 22b, d e2g0 9000 (in methanol)]I8,20). 

The generally quite stable methylenephosphorane nevertheless resembles the short-lived highly reactive methyleneoxophosphorane. The oxaphosphetane intermediate 25 formed by -I- n2]-cycloaddition, which can only be isolated in exceptional cases21, is to be seen against the stable oxaphosphetanes of type 26, which can be photolyzed if suitably substituted or thermolyzed under drastic... 

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