Phosphorus ylides bonding

The ylides have been classified on the basis of the heteroalom covalently bonded to the carbanion. Accordingly, they can be differentiated into nitrogen ylide (Scheme 2), sulfur ylide Scheme 3, phosphorus ylide Scheme 4, arsenic ylide Scheme 5, antimony ylide (Scheme 6), bismuth ylide (Scheme 7) and thallium ylide (Scheme 8). 

Retrosynthetic cleavage of the trans A8,9 disubstituted double bond in intermediate 11, the projected precursor of diketone 10, provides phosphorus ylide 12 and aldehyde 13 as potential precursors. In the forward sense, a Wittig reaction could conceivably achieve a convergent coupling of intermediates 12 and 13 with concomitant formation of the requisite trans C8-C9 olefin. Ordinarily, the union of a nonstabilized ylide, such as 12, with an aldehyde would be expected to afford an alkene with a cis geometry.8 Fortunately, however, the Schlosser modification of the Wittig reaction permits the construction of trans olefins from aldehydes and nonstabilized phosphorus ylides.9... 

However an unexpected new cyclic ruthenium phosphorus ylide half-sandwich complex 42 has been obtained by reaction of 41 with dichloromethane as solvent [79]. The cyclisation involves a C-Cl activation and corresponds to the incorporation of the methylene moiety in the P-C bond and to the ortho-metal-lation of one phenyl of the phosphine. An other novel unusual phosphonium ylide ruthenium complex 43 has also recently been described [80]. 

There are a few isolated cases of the addition of amines, thiols, carboxylic acids, and a phosphorus ylide to doubly bonded germanium compounds. Again, the reactions are regioselective, with the nucleophilic portion of the weak acid adding to the germanium and the proton adding to the heteroatom. 

In contrast, the corresponding phosphorus ylides show insertion of the ylidic carbon fragment into the C /C3 bond of 4,4-diacyi triafulvenes giving rise to a-acyl diacyl-methylene cyclobutenes 561269 which are isomerized thermally to the 2-methylene pyranes 560, probably via the allene 562. 

It is commonly accepted5,6,19 that unstable betaines IV (X = C) are intermediates of the cyclopropanation of olefins with the polar C=C bond by phosphorus ylides. However, only one compound of this type, viz., Me3P( + )-CH2-CMe2-C5H4( ) (1), synthesized in the reaction of dimethyl-fulvene with methylenetrimethylphosphorane, was isolated and characterized by multinuclear NMR spectroscopy.20... 

The double bond in silenes is strongly polarized. They react with phosphorus ylides, as shown by Brook and MacMillan,45 like alkenes with the strongly polar C=C bond. Therefore, it is reasonable to suggest that the reaction also occur through the betaine intermediate (12) (Scheme 6). 

As mentioned above (see Scheme 1), three main directions of the decomposition of intermediates that formed are possible when phosphorus and arsenic ylides react with compounds bearing C=X bonds 5,6,19,63,64,88 (i) elimination of R3E15=X to form olefins (Wittig type reaction) (ii) retro-Wittig type decomposition and (iii) elimination of R3E15 and formation of three-membered cycles (Corey-Chaykovsky type reaction). According to the data of Erker and coworkers,12,13,51 under kinetic control, the reaction of phosphorus ylides with thiocarbonyl compounds also affords phosphines and thiiranes, whose further transformations lead to olefins and R3PS under thermodynamic control. 

Addition of sodium tetrafluoroborate to 2d led to the carbodiphosphor-ane 77 possessing a P-H bond, which was isolated in 70% yield.31 Although 77 is thermally quite stable as a solid (mp 116°C), it slowly rearranges in solution at room temperature (1 week) into its isomeric phosphorus ylide 78. This last result was not surprising, since calculations predicted 78 to be 19 kcal/mol more stable than 77.16... 

Carbodiphosphoranes (R3P = C = PR3) are known,79 but ylides with a P-H bond are rare.80 Therefore, the spectroscopic characterization of 77 was unexpected. Even more surprising was the characterization of the carbodiphosphorane 79 featuring two P-H bonds.31 This compound, prepared by treatment of 2d with tert-butyllithium, rearranged in solution at room temperature over a period of 16 h to afford the phosphorus ylide 80 with one remaining P-H bond. This compound was also unstable and transformed completely into the diphosphinomethane 81 overnight. Note that calculations for the model compounds where R = NH2 predicted 79 to be 28 kcal/mol less stable than 80, which is also 34 kcal/mol above 81.16 The surprising stability of 79 and 80 is probably due to the presence of bulky substituents, since tetracoordinate phosphorus atoms can more readily accommodate the increased steric constraints than can their tricoordinate counterparts. 

The most useful methods for the formation of C-C bonds are based on the addition of C-nucleophiles to carbonyl compounds. Among the many variations of this basic scheme phosphorus ylides, capable of olefinating aldehydes or ketones in a single step, have proven to be exceedingly valuable reagents in organic synthesis. 

The bonding characteristics of phosphorus ylides is a matter of continuos discussion in the literature. This bonding is most frequently referred to as ylide, ° while sometimes the ylene description is also used (Scheme 2). Some characteristics of the X -P=C... 

It has been suggested that the preferential formation of ( )-alkene on Wittig reaction of amide-substituted phenyl 3-pyridyl ketones with non-stabilized phosphorus ylides which contain a carboxyl terminus is a consequence of either hydrogen bonding or salt... 

Another unique synthetic methodology for the carbene phosphine bidentate ligand is the utilization of destabilized C-amino phosphorus ylides 25 as carbene source [66]. Due to the electrostatic repulsion between the two lone pairs on the nitrogen and on the ylidic carbon, the yUdic bond is very labile. Thus, these C-amino phosphorus ylides readily act as 1,6-bidentate ligand precursors by insertion of the metal fragment between the phosphine and carbene centers. 

The synthetic potential of phosphorus ylides was initially developed by G. Wittig and his associates at the University of Heidelberg. The reaction of a phosphorus yhde with an aldehyde or ketone introduces a carbon-carbon double bond in place of the carbonyl bond ... 

Preparation of alkenes Ketone reacts with phosphorus ylide to give alkene. By dividing a target molecule at the double bond, one can decide which of the two components should best come from the carbonyl, and which from the ylide. In general, the ylide should come from an unhindered alkyl halide since triphenyl phosphine is bulky. 

Phosphorus ylide reacts rapidly with aldehydes and ketones to produce an intermediate called a betaine. Betaines are unusual since they contain negatively charged oxygen and positively charged phosphorus. Phosphorus and oxygen always form strong bonds, and these groups therefore combine... 

The phosphorus ylide complex [(Ph3PCH2)2Au]2Ag2(C104)4 445 also contains Au-Ag bonds [(2.783(2)/2.760(2) A] unsupported by any covalent bridge within a Au2Ag2 ring, but each silver atom is further bonded to two oxygen atoms from two... 

CHMe, cyclopropylidene, and CMe2 to activated double bonds.1075 Similar reactions have been performed with phosphorus ylides, 076 with pyridinium ylides,1077 and with the compounds (PhS)3CLi and Me3Si(PhS)2CLi.1078 The reactions with ylides are of course nucleophilic addition. 

Phosphorus ylides also react in a similar manner with the 0=0 bonds of ketenes,686 isocyanates,687 and certain anhydrides688 and imides,689 the N=0 of nitroso groups, and the C=N of imines.690... 

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