Of phosphonium ylides

Alkenes (olefins) from reaction of phosphonium ylides with aldehydes or ketones... 

Since the pioneering work of H. J. Bestmann and coworkers in the 1960s, acylation of phosphonium ylides is a well known process for the preparation of P-ox-ophosphonium yhdes. The classical way (a) using acylating agents such as acyl chlorides and in situ transylidation is still useful (Scheme 4) [16,17]. 

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... 

Among the olefination reactions, those of phosphonium ylides, phosphonate anions, silylmethyl anions, and sulfone anions are discussed. This chapter also includes a section on conjugate addition of carbon nucleophiles to a, (J-unsaturated carbonyl compounds. The reactions in this chapter are among the most important and general of the carbon-carbon bond-forming reactions. 

Horner reactions of phosphonium ylide and phosphine oxide... 

With the fully functionalized heterocyclic core completed, synthetic attention next focused on introduction of the 3,5-dihydroxyheptanoic acid side-chain. This required initial conversion of the ethyl ester of 35 to the corresponding aldehyde through a two-step reduction/oxidation sequence. In that event, a low-temperature DIBAL reduction of 35 provided primary alcohol 36, which was then oxidized to aldehyde 37 with TRAP. Subsequent installation of the carbon backbone of the side-chain was accomplished using a Wittig olefination reaction with stabilized phosphonium ylide 38 resulting in exclusive formation of the desired -olefin 39. The synthesis of phosphonium ylide 38 will be examined in Scheme 12.5 (Konoike and Araki, 1994). 

Scheme 12.5. Synthesis of phosphonium ylide 38 for use in rosuvastatin synthesis.

The different evolution possibilities for the alkaline hydrolysis of phosphonium salts are shown together in the general Scheme 1. Two major parts can be distinguished on the one hand, SN(P), SN(P)mig and Ep reactions, which result from the initial attack on the phosphorus atom by hydroxide anion acting as a nucleophile and on the other, EH(X and EHp reactions (and also bearing in mind the formation of phosphonium ylides), which result from the initial attack of the hydroxide anion on the hydrogen in the a- or / -position to the phosphorus. 

Structure, bonding and spectroscopic properties of phosphonium ylides... 

The earliest theoretical studies of phosphonium ylides were EHT studies of methyl-enephosphorane62 and cyclopropylidenephosphorane63 by Hoffmann and coworkers. 

Ostoja Starzewski and Bock83 reported the photoelectron spectra of an extensive series of phosphonium ylides, focusing on the substituent effects. Their lowest ionization potential (IP) is associated with the carbanion orbital. Their main results are given in Table 9. A number of important trends can be identified and interpreted. Replacement of the methyl groups on P with phenyl groups reduces the IP of the carbanion. The phenyl group is able to stabilize the P+ charge, which reduces the ability of the phosphonium to stabilize the... 

The number of phosphonium ylides that have been characterized by NMR techniques is too large to list here or to discuss in detail. Therefore, we have restricted ourselves to some simple ylides that are directly related to the ylides discussed in the above sections. Also, we shall discuss only 13C and 31P NMR, since these techniques directly probe the nature of the P=C bond. A summary of the NMR data is given in Table 10. 

The synthesis of phosphonium ylides by reaction of ( )-2-benzylideneoxazolidine-4,5-dione with methylenetriphenylphosphoranes has been reported12 these ylide phosphoranes have the structure PhCH2CONHCOCOCR=PPh3 with R = COOMe, COOEt and COPh. 

The reactivities of phosphonium ylides and ylidones have been predicted by ab initio molecular orbital calculations22 and these results have been correlated with neutralization-reionization mass spectrometry. 

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