Ylide-phosphorane equilibria

Phosphorane-Ylide Tautomerism. The lability of a protons in alkylphosphoranes is not known. Deuterium exchange of the benzyl protons is not observed, even in the presence of NaOD in D2O-CDgSOCDg. Deprotonation of h in THF by CHgLi at room temperature is fast. This red solution is shown by variable temperature 31p NMR to contain an equilibrium mixture of phosphorane 7 and ylide 8 (Scheme II). This mixture and CH3I give phsophorane 9. ... 

Another example of ylide-phosphorane equilibrium is found in the reaction of trimethyl phosphite with acetylenic ketones, in the presence of phenol as trapping reagent216 (Scheme 42). Here, too, the 155 156 ratio depends on the nature of the solvents, being 37 63 in CH2C12 and 74.5 25.5 in CC14. 

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.

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). 

The methoxyphosphorane (5 R = Me) is in rapid equilibrium with the ylide and methanol in non-polar solvents at room temperature, but with the phenoxyphosphorane (5 R = Ph) this equilibration is slow on the n.m.r. timescale under the same conditions. Methylmethoxytriphenyl-phosphorane is covalent in the crystalline state, but its solutions are tinged with the yellow of the ylide. 

A detailed study of the interaction of dimethyl phenylphosphonite (62 R = Ph) and ( )-63 (R = Pr ) initially showed that, at -50 °C only traces of unsaturated phosphinate and dimethyl phenylphosphonate are formed, and that the phosphorane 66 (R = Ph, R = Pr ) is the main produc In outline, the system of observed reactions resembles that observed for trimethyl phosphite, with the formation of the dipolar species 65 in equilibrium with the phosphorane 66 and with the ylide 67 the last acts as the immediate precursor to the methyl [(alk-2-enyl)phenyl]phosphinates 68 (R = Ph). Although hydrolysis of the phosphorane could be expected to yield the methyl [(nitroalkyl)phenyl]phosphinates... 

The extrusion of disulfide from spirophosphazene (24 X = S, Y = (CH2)3) provides a useful route to two coordinate triazaphosphole (102) <76T2039>. The related extrusion reaction of (103) includes the loss of dimethylamine (Equation (1)) <80TL1307>. In some cases there is an equilibrium between the phosphorane, for example (57) and the phospholan plus disulfide <83JOC38i5>. Spirophosphonium ylide (11) can be converted to its lithium diylide which forms a nickel complex <8lCB3l6l>. Spirophosphoranide (31 Y = CF3) has also been converted to a series of metal complexes <93PS(76)87>. 

Cyclic Phosphoranes - The reaction of dioxetanes (28a-c) with ylides (29a-d) gave phosphonium alkoxides (30a-f) in equilibrium with the pentacoordinate dioxa-2,5-phosphorinanes (31a-f).16 Hydrolysis via the hydroxyphosphoranes (32a-f) gave the phosphine oxides (33a-f). 

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