Phosphoranes as intermediates

Mechanistic Problems Phosphoranes as Intermediates or Transition States in Reactions of... 

The present effort is intended to provide an update of the earlier edition, bringing to the chemist in concise form advances in the approaches to C-P bond formation previously discussed, as well as several other aspects of C-P bond formation. These latter aspects include the generation of organophosphorus compounds from elemental phosphorus (of particular industrial interest for purposes of cost containment) advances in the preparation of phosphoranes, including the use of transient oxophosphoranes as intermediates in organophosphorus compound syntheses and new approaches toward the preparation of compounds with aromatic and vinylic carbon-phosphorus bonds. 

Another preparation of oxadiazoles proceeds from N-acylaminoimino-phosphorane as well. With diphenylketene, an intermediate ketenimine (155) is converted to 2-diphenylmethyl-l,3,4-oxadiazole 156 (Scheme 61) [91PS(63)283]. 

In addition to the possibility of detecting their presence as intermediates during certain reactions, some hydroxyphosphoranes can be isolated as salts or can be detected as unstable intermediates in the course of a reaction178 thus, the phosphorane 102 was prepared179 and the structure of its triethylammonium salt determined by XRD180. 

Cyclic 1,3,2-dioxaphosphorinanes (P-CN = 4) react with nucleophiles, probably by initial addition of the nucleophile to the phosphorus atom, giving phosphoranes (P-C1V = 5) as intermediates (or transition states). This holds not only for P=0 derivatives but also for quaternary 1,3,2-dioxaphosphorinanium salts (81JA5894). By steric rearrangement of the phosphoranes the configuration of the P atom can be changed (retention or inversion) (see Section 1.17.5). 

The most interesting properties of phosphoranes, i.e. their role as intermediates or transition states of nucleophilic addition reactions of four-coordinate phosphorus compounds and their intramolecular rearrangements according to BPR or TR, have already been fully considered. The synthetic potential of stable phosphoranes has been reviewed by Burger in great detail (B-79MI11702), and only some special aspects need be mentioned in this chapter. 

Phosphoranide anions 1 - i.e. species based on pentacoordina-ted phosphorus having a lone pair as one of its five substituents -were proposed by Wittig and Maercker as early as 1967, to act as intermediates or transition states in nucleophilic substitutions at tricoordinated phosphorus ( p. Shortly afterwards in 1969, Hellwinkel brought indirect evidence for the formation of phosphoranide 3 in equilibrium with the carbanion 4, since the action of a base on phosphorane 2 gave, after acidic treatment, a mixture of 5 and 6 in proportions that depend on the experimental conditions (i). 

Oxo-2,3-dihydro-1//-indazol-2-yl)niethylene](triphenyl)phosphoranes 9 undergo thermal and/or acid-catalyzed rearrangement to yield quinazolin-4(l//)-ones which more likely exist as 4(3/f)-tautomers 10 and triphenylphosphane. Compounds 9 are, in some cases, isolated as intermediates in a thermal rearrangement of [(aryldiazenyl)methylene](triphenyl)phosphoranes (cf. p72). 

Whereas base-catalyzed epimerization of 27 is faster than hydrolysis base-catalyzed hydrolysis of optically active 25 gives the corresponding P-phenyl phosphetane 1-oxide with retention of configuration The initial phosphoranes A or A, which result from a attack of hydroxide on (R)- or (S)-25, respectively, can in principle equilibrate with five diastereomeric phosphoranes, as shown in Fig. 10 for (7 )-25 However, A, C, and E are relatively high energy intermediates since a Z-butyl group is located in the a position As discussed above, racemization prior to hydrolysis requires that loss of benzyl is slow compared to pseudorotation and addition-elimination... 

Attack on Oxygen. Cyclic phosphites or phosphonites (38) react with a-keto-acids to give cyclic acyloxyphosphoranes (39), which have previously been implicated as intermediates in the hydrolysis of phosphoenolpyruvate initial attack on keto-oxygen has been suggested, as shown. Acyclic phosphites gave the Michaelis-Arbusov product (40), presumably because they lack the five-membered ring to stabilize the phosphorane (39). 

Reactions of phenyl azide with a tautomeric mixture of (15) and (16) also lead to the phosphazenes (17) as intermediates, tentatively identified as such by n.m.r. spectroscopy. Intermediate (17) undergoes intramolecular addition of the N—H bond across the phosphazenyl linkage to form a phosphorane (18). Novel polymers (19), containing phosphazenyl side-groups, have been obtained by the azide route. 

The lack of direct evidence for TBP intermediates has allowed the proposal of alternative pentacoordinate intermediates and of hexacoordinate intermediates in nucleophilic substitution reactions at tetrahedral phosphorus. Pentacoordinate phosphorus species (spirocyclic phosphoranes) have been observed in which the most stable form adopts the square pyramidal (SP) geometry (Howard et ai, 1973). Such SP species have been postulated as intermediates in substitution reactions at tetrahedral phosphorus (Boudreau et ai, 1975). 

Phosphinylidene thioxophosphoranes are obtained by direct addition of S to diphosphenes, and methylene(thioxo)phosphoranes occur as intermediates in the formation of thiaphosphirene sulphides from methylene phosphoranes (9.642). Only a few of these compounds have so far been found stable enough for isolation. 

Further work has also appeared on the synthesis and reactivity of three-coordinate, pentacovalent (a X ) phosphorus compounds. New examples include the extended phosphacumulene (19 " " and the phosphavinylidene(oxo)phosphorane (197), a diphosphaallene featuring both and -phosphorus atoms." The involvement of a X -species as intermediates in phosphorylation reactions" and in nucleophilic attack at phosphoryl centres" has also received further attention. 

The mechanism of the reaction of tributyl [(trimethylsilyl)methylene]phosphorane with benzaldehyde and its para-substituted analogues has been examined. It has been found that the electronic nature of the para-substituents strongly influences the stereochemical and kinetic outcome of the Peterson oleflnation, whereas temperature substantially affects their Hammett correlation. This indicates that the Peterson oleflnation is a multistep reaction involving the formation of at least an oxyan-ion/betaine and a carbanion as intermediates. The E selectivity seems to result from the silicon-oxygen interaction and interactions of steric substituents in competing erythro-and tereo-betaines. 

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