Wittig mechanism

A recent review on four-membered heterocycles formed from imino-phosphoranes concentrates on the preparation and the reactivity of 2,4-diimino-l,3-diazetidine and related compounds (93JPR305). As an example, the synthesis via bisiminophosphorane 85 is described in Scheme 42. The bisiminophosphorane has both a heteroaryl and a styryl site. From a mechanistic view, the reaction of the bisiminophosphorane proceeds with aryl isocyanate formation via an aza-Wittig mechanism. Intermediate car-bodiimide formation (86) occurs directly on the iminophosphorane moiety... 

Moreover the reaction runs the normal Wittig mechanism over the four-membered oxaphosphetane ring and collapses to the alkene mixture. (For mechanistic details see Chapter 10.)... 

Vedejs, E., Marth, C. F., Ruggeri, R. Substituent effects and the Wittig mechanism the case of stereospecific oxaphosphetane... 

R = Ph, R = Et) and arylaldehydes ArCHO has been reported to yield azadienes with general formula ArC=N-C(R)=CHC02R. These azadienes appear to be very useful as starting materials for the preparation of nitrogen heterocycles. An easy one-pot synthesis of dihydropyrimidines from NH=C(Ph)N=PPh3 and a,(3-unsaturated aldehydes has been described. The reaction is supposed to proceed via an aza-Wittig mechanism followed by ring closure. ... 

If the Staudinger reaction of an alkyl azide containing a pendant carbonyl group is heated under anhydrous conditions, the iminophosphorane can nucleophilically attack the carbonyl leading to cyclic imines (via the aza-Wittig mechanism) and liberate phosphine oxide as shown below. The reaction also works intermolecularly. 

In 1970, a review entitled The Stereochemistry of the Wittig Reaction was published in Topics of Stereochemistry by M. Schlosser (1). This review was a milestone in early attempts to understand the Wittig mechanism based on a stepwise, ionic process (Scheme 1) (1,2). The reaction of ylides with aldehydes was believed to involve a partly reversible nucleophilic addition step to generate a betaine intermediate (1 or 2). Subsequent decomposition to the alkene was attributed to the formation of oxaphosphetanes 3 or 4, structures that were assumed to be intermediates of higher energy leading from the betaine to the alkene. 

Both of the above teehniques can be reinforeed by erossover experiments (method C) (12-14,20,2Ic,22,23a). An excess of CIC H CHO (ArCHO) is added to the solution below the temperature for oxaphosphetane decomposition. If stereochemical equilibration occurs exclusively by a retro-Wittig process to give the ylide 33, then an excess of the crossover aldehyde must produee the crossover products. Since 33 would be intercepted by excess ArCHO faster than it can recombine with the original aldehyde, the conversion from one oxaphosphetane diastereomer into the other (i.e., from 31 to 32) by way of any retro-Wittig mechanism will be suppressed using method C. However, it is essential to prove that the oxaphosphetane has not already decomposed prior to the addition of the crossover aldehyde. Otherwise, there is the risk of a false-negative crossover result. 

As an alternative to the manner in which the stability of the intermediate species 277 is raised by increasing electron input to phosphorus, a reduction in nucleophilic character of the original carbonyl oxygen might be contemplated. In this respect, it may be noted that a reaction between the lithium salt of diisopropyl (fluoromethyl)phosphonate and the ketones R R CO affords both the alkene (s) predicted from the Wittig mechanism, but also the alcohols (Pr 0)2p(0)CHFC(0H)R R as mixtures of diastereoisomers, distinguishable spectroscopically but not separable ... 

To understand the mechanism of the Wittig reaction we need to examine the struc... 

To understand the mechanism of the Wittig reaction, we need to exanine the structure and properties of ylides. Ylides are neutral molecules that have two oppositely... 

Active Figure 19.13 MECHANISM The mechanism of the Wittig reaction between a phosphorus ylide and an aldehyde or ketone to yield an alkene. Sign in at www.thomsonedu.com fo see a simulation based on this figure and to take a short Quiz. 

The final step in an attempted synthesis of laurene, a hydrocarbon isolated from the marine alga Laurencin glandulifera, involved the Wittig reaction shown. The product obtained, however, was not laurene but an isomer. Propose a mechanism to account for these unexpected results. 

Willstaller, Richard. 524 Winstein, Saul. 375 Wittig, Georg, F. K., 720 Wiltig reaction, 720-722 mechanism of, 720-721 uses of. 722... 

Exanrple Ester ( 2) was needed as a model compound to study mechanisms of biological ester hydrolysis. It is clearly made from acid (33) which could be made from aldehyde (34) by a Wittig reaction. We discussed the synthesis of (3b) on page 21. 

Because the mechanisms of the Peterson reaction, a synthetic alternative to the Wittig reaction, have recently been reviewed [13, 14], this section will try only to summarize recent developments and trends concerning the Peterson reaction. 

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