Ylids, phosphonium

A ylid or ylide is a substance in which a carbanion is directly attached to a heteroatom carrying a high degree of positive charge X+-C.  

Phosphonium ylids (known also as phosphine alkylenes) contain the group -P -C (6.405). Phosphorus ylids have been known since 1894, but most interest in these compounds has developed only over the past 40 years. 

The commonest phosphonium ylids are the triphenyl phosphonium methylides (known also as triphenylphosphine methylenes, methylene triphenylphosphoranes or triphenylphosphonium meth-anides). These may be represented as a hybrid of the forms (6.406) and can be regarded as carban-ions whose ionic character is modified by the adjacent positive charge. In addition, dtt-pjt bonding is to be expected and the contribution of the right-hand structure will increase as the n character of the bond is increased. 

Dipole moments provide evidence for varying degrees of double-bond character of the phosphorus-carbon linkage, depending upon the compound under study. Thus the experimental value of p for (6.408) lies half way between that calculated for forms (a) and (b), suggesting roughly equal contributions of each form. 

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In almost all compounds that have pn-dn bonds, the central atom is connected to four atoms or three atoms and an unshared pair and the bonding is approximately tetrahedral. The pn-dn bond, therefore, does not greatly change the geometry of the molecule in contrast to the normal tc bond, which changes an atom from tetrahedral to trigonal. Calculations show that nonstabilized phosphonium ylids have nonplanar ylidic carbon geometries, whereas stabilized ylids have planar ylidic carbons. ... 

This is an extremely useful reaction for the synthesis of alkenes. It involves the addition of a phosphonium ylid, e.g. (136), also known as a phosphorane, to the carbonyl group of an aldehyde or ketone the ylid is indeed a carbanion having an adjacent hetero atom. Such species are generated by the reaction of an alkyl halide, RR CHX (137), on a trialkyl- or triaryl-phosphine (138)—very often Ph3P—to yield a phosphonium salt (139), followed by abstraction of a proton from it by a very strong base, e.g. PhLi ... 

The physical and chemical properties of the X -phosphorins 118 and 120 are comparable to those of phosphonium ylids which are resonance-stabilized by such electron-pulling groups as carbonyl or nitrile substituents Thus they can be viewed as cyclic resonance-stabilized phosphonium ylids 118 b, c, d). As expected, they do not react with carbonyl compounds giving the Wittig olefin products. However, they do react with dilute aqueous acids to form the protonated salts. Similarly, they are attacked at the C-2 or C-4 positions by alkyl-, acyl- or diazo-nium-ions Heating with water results in hydrolytic P—C cleavage, phosphine oxide and the hydrocarbon being formed. 

Treatment of the salt, [PhsBiCFhCOR1] BF4 with base generates triphenylbis-muthonium 2-oxoalkylide (84 R1 = Buf, Ph). This reacts with 1,2-dicarbonyls to give 2,3-diacyloxiranes (85 from acyclic reactants, MeCOCOR2, R2 = Me, OEt) or 2-acyl-3-hydroxytropones [e.g. (86), from the tetrachloro-o-quinone].123 Both reaction types are of considerable synthetic utility, and both are in marked contrast to the routes followed by the corresponding phosphonium ylids. 

Several aromatic acyl silanes and the fascinating but unstable pink carbonyl bis(trimethylsilane) have been prepared in reasonably good yields by the oxidation of phosphonium ylids (Scheme 17)13. 

Allylic phosphonium ylids have long had a valued place In the synthesis of 1,3-dienes, and similarly a-heterosubstituted ylids have been used in the preparation of vinyl derivatives ( 1). This paper is concerned with the preparation and ylid chemistry of a series of a-alkoxyallylic phosphorus derivatives which possess both the structural features alluded to above. 

There are many examples of Wittig-type reactions in the literature, in which the behavior of phosphonium ylids, phosphine oxide ylids, and phosphonate ylids is significantly different. 

The optical brightener Palanil 50—it makes your clothes look whiter than white and your T-shirts fluoresce in UV light—can be disconnected by the Wittig strategy to two molecules of the phosphonium ylid 51 and one of the dialdehyde 52. The availability of the dialdehyde, used in the manufacture of terylene, makes this route preferable to the alternative. 

The simplest sulfur ylids are formed from sulfonium salts 69 by deprotonation in base. These ylids react with carbonyl compounds to give epoxides.18 Nucleophilic attack on the carbonyl group 70 is followed by elimination 71 of dimethylsulfide 72 and formation of the epoxide 73. You should compare diagram 71 with diagram 23 in chapter 15. The phosphonium ylid reacted by formation of a P-0 bond and an alkene in the Wittig reaction. The sulfonium compound reacts by formation of a C-O bond 71 as the S-O bond is much weaker than the P-0 bond. The sulfonium salt 69 can be reformed by reaction of 72 with Mel. 

An acidic proton posed a potential problem during E. J. Corey s synthesis of bergamotene (a component of the fragrance of Earl Grey tea). You met the Wittig reaction in Chapter 14, and phosphonium ylids are another type of basic,... 

In Chapter 31 we discussed the Wittig reaction of phosphonium ylids with carbonyl compounds. Sulfonium ylids react with carbonyl compounds too, but in quite a different way—compare these two reactions. 

Treatment of the triflate 9 with the sodio derivative of r-butyl dimethoxphosphorylacetate in dimethylformamide (DMF), at room temperature for 20 h in the presence of a crown ether, gives the epimeric phosphonates 10 in 81% yield [9]. Cleavage of the glycosidic bond by hydrogenolysis then affords the hemiacetals 11 which, with sodium hydride in tetra-hydrofuran (THF), give the C-6 phosphonium ylid, which reacts with C-1 in the aldehydo form to afford the alkene 12 in 73% (from the glycoside 10 Scheme 2). The cyclization... 

The second compovtnd has an a,P-unsaturated ester so we are into aldol-style chemistry and here Klin the Wittig looks good, mainly because of the chemoselectivity needed in the aldol step. The =Oi ate) equivalent must react with an aldehyde but not with a ketone and the phosphonium ylid... 

The last reaction is, of course, a Wittig reaction so the first must be nucleophilic attack by the anion of the imide on the cyclopropane with the phosphonium ylid as the leaving group. 

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