Triphenylphosphonium ylides

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

If direct homolysis occurs in the case of tetraphenylphosphonium tetrafloroborate, triphenylphosphonium ylide was expected to function as a photoinitiator of radical polymerization because of its similar structure. Therefore, another milestone was reached by Kondo and colleagues [50] who investigated the use of triphenylphosphonium ethoxycarbonylmethylide (TPPY) (Scheme 22) as an effective photoinitiator for the polym-... 

Triphenylphosphonium ylide reacts with the silylene complex 93 which has a highly electrophilic silicon center, to give the corresponding cationic adduct 94 [115]. The lengthening of the PC bond indicates a loss of the double bond character of the ylide and corresponds to the formation of a tetrahedral silicon center with four covalent bonds (Scheme 28). 

Acid- and base-promoted methods have also been used in the syntheses of pyrazoles. Hydrogenation of methyl 2-Cbz(hydrazine)-3-hydroxy-4,4-dimethoxybutanoate 11 followed by cyclization in the presence of trifluoroacetic acid afforded the first asymmetric synthesis of the (4S,5.R)-5-carbomethoxy-4-hydroxy-A2-pyrazoline 12 <00TL8795>. Reaction of 2-nitrobenzyl triphenylphosphonium ylide (13) with aiyl isocyanates afforded 2-aryl-2H-indazoles 14 <00TL9893>. Base-promoted reaction of nitrobenzenes 15 with aryl imines 16 afforded aryl pyrazoles 17 . 

Liu and Schlosser87 have recently shown convincingly by 13C NMR that in solution there is free rotation about the P=C bond in unstabilized triphenylphosphonium ylides. This was because they failed to find any evidence for non-equivalence of the ipso-carbon signals down to temperatures as low as — 105 °C. 

Aldehyde or ketone Triphenylphosphonium ylide Alkene Triphenylphosphine oxide... 

Alkyltriphenylphosphonium salt Base Triphenylphosphonium ylide Conjugate acid... 

Tab. 11.1 Triphenylphosphonium Ylides Nomenclature, Preparation, and Stereoselectivity of Their Wittig Reactions...

The Wittig reaction is a C,C-forming olefin synthesis from phosphonium ylides and carbonyl compounds (see also Section 4.7.4). In more than 99% of all Wittig reactions, ylides of the structure Ph3P+—CH-—X (i.e., triphenylphosphonium ylides) are used. Therein, X usually stands for H, alkyl, aryl, or C02-alkyl and seldom for other substituents. 

Reaction of 2-nitrobenzyl triphenylphosphonium ylide 205 with aryl isocyanates affords 2-aryl-2//-indazoles 206 (Scheme 122) <2000TL9893>. 

More-detailed examination of the reaction between alkylidene(and arylmethylene)triphenylphosphonium ylides and aldehydo sugars was conducted9 8 -30 in our laboratory with 2,3 4,5-di-0-cycIohexylidene-aldehydo-D-xylose (and -L-arabinose), resulting in the preparation of l-C-alkyl-3,4 5,6-di-0-cycIohexylidene-1,2-dideoxy-D-xylo(and L-ara-hino)-hex-l-enitol (27a-27d and 28a-28d, respectivelyB([Pg.236]

Reaction of 2-nitrobenzyl triphenylphosphonium ylide 745 with aryl isocyanates afforded 2-aryl-2//-indazoles 746 (Equation 159) <2000TL9893>. Reductive cyclization of o-nitroketoximes 747 in the presence of catalytic iron dimer in dioxane under a carbon monoxide atmosphere furnished l//-indazoles 748 (Equation 160) <2004H(63)373>. 

Wittig reactions of diethyl oxalate with a variety of arylmethylene-triphenylphosphonium ylides under mild conditions provide a highly (Z)-stereoselective synthesis of ethyl 3-aryl-2-ethoxyacrylates (69). Wittig reactions of the a-keto amino acid derivative (70) with stabilized, semi-stabilized and reactive ylides have been used to prepare the synthetically... 

In addition to sulfur ylides, examples are known of cyclopropanations using other ylides containing group V or group VI elements. Among these, triphenylphosphonium ylides have most frequently been studied. Some examples are collected in Table 19. The products obtained in entries 1-3 and 10 of Table 19 are suitable precursors for chrysanthemic acid. An enantioselec-tive synthesis of cyclopropanes was achieved by using electron-deficient alkenes substituted with an optically pure oxazolidine or acetonide function. When the alkene is stabilized by a ketone or nitro function instead of an ester function, the yield dropped considerably. 

Table 19. Synthesis of Cyclopropanes Using Triphenylphosphonium Ylides...

Triphenylphosphonium ylides attached a to a cyclopropane react with aldehydes and ketones under standard Wittig conditions to give 1-cyclopropylalk-l-enes in good to excellent... 

Probably the best known of all the many reactions which result in the cleavage of a phosphorus-carbon bond is one which is based on the Wittig reaction. The Wittig reaction consists in the interaction of a triphenylphosphonium ylide and an aldehyde or ketone, the outcome of the reaction being the formation of a tertiary phosphine oxide together with an... 

The formation of (cyclopent-2-enyl)phosphonic diesters from (buta-l,3-dienyl)phos-phonic diesters by the action of a dmso-derived ylide or a triphenylphosphonium ylide is depicted in Scheme 50 (Z = CN, COOEt or S02Me). The role of esters of ethenyl- and buta-l,3-dienyl)phosphonic acids in organic synthesis has been surveyed ... 

Addition reactions of (l-methoxyalkyl)triphenylphosphonium ylides, derived from the corresponding phosphonium salts (82) and n-BuLi, to aldehydes at — 78 °C followed by quenching the reaction mixture with aqueous NH4CI at the same temperature alforded a-hydroxyketones instead of the expected enol ethers.This is the first example of phosphonium ylides acting as an acyl anion equivalent. Flash vacuum pyrolysis (FVP) in a conventional flow system at 10 (84), prepared in a few steps... 

So far no simple monocyclic ylide has been prepared but there is evidence that diazocyclononatetraene (XIX, X = N2) is formed as a transitory intermediate 183]. The triphenylphosphonium ylide (XX,... 

Amongst theoretical calculations for these systems is included a quantum chemical study of the 7c-eIectron delocalization in triphenylphosphonium ylides, leading to an evaluation of the criteria for reactivity and aromaticity in the Wittig reaction. It was predicted that the as yet unknown ylide (1 X = PPhg) will not take part in the Wittig reaction. ... 

The B3P86/6-31G mechanistic results for the Wittig reaction of acetaldehyde in vacuo and in tetrahydrofuran (THF) solution with an unsubstituted trimethylphospho-nium or triphenylphosphonium ylide have been compared to those obtained at the same levels for the reaction of triphenylphosphonium ylide with a bulky chiral aldehyde, (2S, 3/ )-2,4-dimethyl-3-pyrrol-l-yl-pentanal. ° Betaine-type intermediates were found not located in vacuo when trimethylphosphonium yUde was used, while only a gauche betaine is obtained using triphenylphosphonium ylide. Conversely, in THF, the concerted and stepwise mechanisms are both represented and show TSl/TSb barriers, which are negligibly small for unsubstituted triphenylphosphonium yUde. 

Starting materials are either cyclopentanone and the triphenylphosphonium ylide derived from bromoethane or acetaldehyde and the triphenylphosphonium ylide derived from chlorocydopentane. Either route is satisfactory. 

The complex with triphenylphosphonium ylides 405 undergoes orthometallation of the ligand to form the complex with a five-membered platinacyclopentane 406 (Equation (102)). ... 

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