Oxaphosphetanes fragmentation

Oxaphosphetane Fragmentation, Last Step of Wittig and Horner-Wadsworth-Emmons Reactions... 

Like the arylmethyleneoxophosphoranes 9, the acylmethyleneoxophosphoranes 39 have so far resisted isolation as such. Flash pyrolysis12) of (diazophenacyl)-diphenylphosphine oxide (36a)27) is just as unsuccessful as that of 7 the sole product isolated is diphenylaeetylene which is presumably formed via 39 or the cyclic isomer l,2Xs-oxaphosphetane by subsequent fragmentation of 23. 

The photochemical fragmentation of vinyl-substituted 1,2k5-oxaphosphetanes, representing a step of a photochemical variant of the Wittig reaction with methyl-eneoxophosphoranes, has been examined as a model in the case of 22b20). Photolysis of this compound in methanol affords the 1,3-diene 24b as well as the highly reactive dioxophosphorane 23 which is trapped by the solvent subsequent esterification of the half-ester 62, formed as a primary product, with diazomethane to give the diester 63 was undertaken solely for preparative reasons 20). 

In view of the reaction behavior of l,2 i.5-oxaphosphetanes (22), treated above, it appears fitting to reconsider the mechanism of the hydroxylion induced fragmentation of p-bromophosphinic acid 6443). It was assumed that formation of the phosphinate 65 is followed by that of the four-membered heterocycle 66, which spontaneously decomposes to benzalacetophenone and phenyldioxophosphorane the latter then adds water to give the phosphonie acid 43 ... 

Single alkene diastereomers are accessible through a Wittig-Homer reaction only if it is performed in two steps (Figure 11.10). A 1 1 mixture of the phosphorylated lithium alkoxides syn- and anti-D is still formed but if the mixture is protonated at this point, the resulting phosphorylated alcohol diastereomers C can usually be separated without difficulty. The suitable diastereomer will be deprotonated with potassium-ferf-butoxide in the second step and then be converted into the stereouniform trans- or cis-alkene E via stereospecific oxaphosphetane formation and fragmentation. 

The first step is a simple Wittig reaction with an unstabilized ylid (Chapter 31), which we expect to favour the Z-alkene. It does but, as is common with Wittig reactions, an E/Z mixture is formed but not separated as both isomers eventually give the same compound. The reaction is kinetically controlled and the decomposition of the oxaphosphetane intermediate is in some ways like a fragmentation. 

Generally, oxaphosphetanes are thermodynamically unstable and fragment into alkenes and triphenylphosphine oxide. This elimination step is stereospecific with oxygen and phosphorus departing in a 5jn-periplanar mode to produce (Z)-alkenes, the driving force being formation of the very stable P = O bond (130-140 kcal/mol, 544-586 kJ/mol bond dissociation energy). 

Octet rule as is the carbon atom. This intermediate, which is called an oxaphosphetane, then fragments to form the desired compound. Write down the mechanism for this step. 

A more serious complication for the betaine mechanism arose when it was found that oxaphosphetanes are more stable than betaines. The earliest evidence was encountered by Ramirez et al. (17), who found that certain phosphines react with two equivalents of hexafluoroacetone to give 1,3,2-dioxaphospholane derivatives 23 (Scheme 6). These compounds rearrange into unusually stable oxaphosphetanes 26 via fragmentation to 24, followed by a proton shift to generate an intermediate ylide 25 (17). Shortly thereafter, Vedejs and Snoble (18) used P nuclear magnetic resonance (NMR) methods to show that more typical Wittig reactions of nonstabilized ylides PhjP CHR also produce oxaphosphetanes and that these intermediates can be easily observed at temperatures below 0°. Since betaines did not accumulate in any of the experiments, their conversion to oxaphosphetanes could not be rate... 

A.ii. (ElZ) Isomers in Wittig Reactions. When a phosphorus ylid reacts with an aldehyde or ketone, the alkene that is formed is a mixture of ( ) and (Z) isomers. l Initial postulations suggested that the stereochemistry of the alkene products was controlled by the stereochemistry of the betaine, which rapidly collapsed to an oxaphosphetane. Subsequent fragmentation to the alkene occurred via a syn-elimination pathway An example of this postulate is shown by addition of ylid 524 to 2-butanone to generate betaine 525 and the anti-oxaphosphetane, 527. Syn elimination led to the (El-alkene, 529. The reaction mixture also contained betaine 526, the precursor to the syn oxaphosphetane (528) which led to the (Z) alkene, 530. 

The Wittig Reaction (Section 16.6) Treating an aldehyde or a ketone with a tri-phenylphosphonium ylide gives a betaine intermediate, which rearranges to an oxaphosphetane intermediate, which in turn fragments to give triphenylphosphine oxide and an alkene. 

The mechanism originally proposed is one of nucleophilic addition of the ylide carbon to the carbonyl group to yield a dipolar intermediate (a betaine), followed by elimination of phosphine oxide. The elimination might be concerted, or it might take place via a four-membered oxaphosphetane intermediate." Alternatively, this oxaphosphetane may be formed directly by a cycloaddition reaction of the ylide and the carbonyl compound, completely bypassing the betaine as an intermediate." " The formation of oxaphosphetanes at low temperature, and their fragmentation on warming, has been clearly demonstrated, while the evidence for authentic betaine intermediates has been questioned." ... 

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