P-Ylides Phosphoranes

Scheme 43 shows the details of the different steps involved in the equilibrium. The nucleophilic attack of the P(III) derivative on the acetylenic bond yields a 1,3-dipole which, after a fast protonation, frees aZ ion. If the subsequent addition of this ion occurs on the P atom (reaction a), a P(V) phosphorane is formed, but the addition of Z on the ethylenic C atom (reaction b) results in the formation of an ylide. Both of these reactions occur under kinetic control and, in both cases, X is always an OR group from the initial acetylene dicarboxylic ester. When the acetylenic compound is a diketone and X is an alkyl or aryl moiety, the C=0 group is much more electrophilic and the attack by the Z ion produces an alcoholate (reaction c), a new intermediate which can cyclize on to the P+ to form a phosphorane, or attack the a-C atom to form an ylide as in Scheme 42. Hence, reactions a and c can coexist, and are strongly dependent on the nature of the trapping reagent and of the P compound, but reaction b is blocked, whatever the reagent. This is well illustrated by the reaction of the 2-methoxytetramethylphospholane 147 on diben-zoylacetylene in the presence of methanol as trapping reagent. The proportions of the vinylphosphorane 157 and spirophosphorane 158 formed (Figure 24) are 13% and 84%, respectively.

A mechanism for this reaction involving nucleophilic attack of the ylide on the cyanide group and formation of the P=N linkage via a four-centred intermediate was formulated. The structure of this phosphazene was confirmed by its synthesis from the vinyl azide, Ph(N3)C=CHPh, and triphenylphosphine. Phosphoranes stabilized by electron-withdrawing... 

P oxygen transfer, presumably via (9). The structure of (10) was confirmed by X.-ray analysis and hydrolysis to give (11). An X-ray analysis of isopropylidenetri(isopropyl)phosphorane (12) suggests that the ylide P-C bond is relatively longer, and that the... 

The inverse reaction to that described above and leading to the formation of an ylide by transformation of a phosphorane by thermodynamic evolution has also been observed212-214 (Scheme 40) the phosphorane 151, formed at — 50 °C by addition of P(OMe)3 to methyl acetylenedicarboxylate in the presence of MeOH to trap the 1,3-dipole, is rapidly transformed, at —20 °C, into the ylide 152. 

In the mid-1960s, Lucken and Mazeline77,78 reported two important EPR studies that shed a great deal of light on the nature of the P =C bond in ylides. While ylides themselves have no EPR signal, Lucken and Mazeline instead prepared two phosphorane radical cations by irradiating the appropriate crystalline precursor phosphonium salts, shown in Scheme 1. 

The first step is a Wittig reaction18 in which the ketone is converted to the terminal olefin by reaction with a phosphorus ylide (also called a phosphorane). Phosphoranes are resonance-stabilized by overlap between the carbon p-orbital and one of the d-orbitals of the phosphorus. 

The bonding in certain tricoordinate phosphorus compounds (ct , 5-P) is also termed hypervalent (DA Scheme 5). In these structures the lone pair of the planar phosphorus is effectively involved in the delocalization (IIB Scheme 5). Bis-meth-ylene-phosphorane (and its N, O, and S analogues) are typical examples for such bonding (Scheme 5). Like in the case of the ylides, no -orbital participation is needed to describe the bonding in these compounds. Rather, the delocalized 3c-4e model can again be used [33], which also accounts for the positive charge at the central phosphorus atom, in accordance with the ylidic resonance structure (IIC Scheme 5). 

Like ylides, bis-methylene-phosphoranes can also be built into 7i-systems in different ways (see Scheme 6), with one of the P=E bonds in an exocyclic position, or with the entire 3c-4e 7i-system incorporated into the cyclic delocalization. 

Application of the Wittig reaction in the carbohydrate field is accompanied by certain difficulties. A correct choice of the initial sugar components is the main problem, owing to the basicity of phosphoranes and, especially, to the drastically basic conditions employed with phosphonium ylides (2a). It is not surprising, therefore, that protected (acetalated and aeetylated) aldehydo sugars and resonance-stabilized phosphoranes were used at first,3-5 although partially protected, and even unprotected, aldoses were shown to be amenable to the reaction with various resonance-stabilized phosphoranes, thanks to the presence of the carbonyl form in the mobile equilibrium. The latter reactions, however, are extremely complicated (see Section IV, p. 284). 

This reaction has been further modified by intercepting the betaine ylide with electrophiles other than a proton to give trisubstituted alkenes in which the electrophile is introduced at what was the phosphorane a-carbon. This reaction is referred to as the a-substitution plus carbonyl olefination via P-oxodphosphorus ylides (SCOOPY) reaction.20... 

The quaternization of one phosphorus atom in the cation of 91 causes a further shortening of the P-P bond to 2.095(1) A accompanied by an equalizing of both PC contacts to 1.737(4) and 1.747(4) A. Again a short CN bond [1.322(4) A] points to an extensive Jt-delocalization as expressed by the canonical formulas 91-91". Thus, the cations also show the structural features of a phosphinylidene [Pg.729]

Wittig reactions of a-alkoxy aldehydes and sugar lactols, such as pentose ketal (48), with stabilised ylides usually proceed with low ( )-selectivity. However, Harcken and Martin have discovered that treatment of these aldehydes with (methoxycarbonylmethylene)tributyl phosphorane (49) and a catalytic quantity of benzoic acid produces the heptenonate (50) with a E Z ratio of 95 5. The stereoselectivity of the reactions between aldehydes and spirophosphoranes (51) has been examined and the phosphoranes found to favour the formation of (Z)-a,p-unsaturated aldehydes and amides. ... 

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