Phosphoranes compounds

A process observed in some pentacovalent phosphorane compounds (or intermediates) in which there is an interchange of apical and equatorial ligands. 

Many five coordinate phosphorane compounds have been examined, the keen interest arising primarily from the variation of structures which have relevance with respect to their stereochemical importance as reactive intermediates in the reaction pathways of four coordinate compounds <74JA4143>. The stereochemistry of phosphoranes vary on a structural coordinate between trigonal bipyramidal (t.b.p.) and square pyramidal (s.p.). Phosphoranes which have been found to have conformations closer to the t.b.p stereochemistry include thienyl phosphorane (33 R = H, = 2-thienyl) <83ZN(B)466>, spiro(4,4)phosphoranes (41) <77JA3332>, (47) <89PS(45)255>, (42 R = Ph,... 

In the following procedure, use is made of the basic character of epoxides. In the presence of phosphonium salts, ethylene oxide removes hydrogen halide and the alkylidenetriphenylphosphorane is produced. If a suitable carbonyl compound is present in the reaction mixture, its reaction with the in situ generated phosphorane proceeds readily to give the Wittig product. 

Akiba and coworkers reported a new type of anionic hexacoordinated phosphorus derivative exemplified by compound 29 bearing a stable three-membered dioxaphosphirane ring [48]. Phosphoranide 34, generated from P-H phosphorane 35 with KH in the presence of 18-crown-6, was exposed to dioxygen to give crystalline 29 (50%). Phosphate 29 was found to be quite stable as a solid to ordinary room light as well as to the air at ambient temperature for several months (Scheme 5). 

Finally, new tricyclic hexacoordinated phosphoranes with internal P-N coordination were synthesized by Swamy and coworkers by oxidative addition of cyclic phosphite precursors with quinones or with a combination of diols and (z-Pr)2NCl [57, 58]. Various ring sizes from five to eight membered were obtained showing the generality of the approach. A selection of compounds (47a-47e) is presented in Fig. 8. 

Finally, the lower symmetry of compounds 27 and 28, of type P(ab)2(cd), and 29, of type P(ab)2(cc), could have lead to the formation of many (racemic) di-astereomers. Interestingly, this is not the case as the configuration of the 10-P-5 phosphorane precursors is translated integrally to the P(VI) derivatives, as shown by Akiba and Kawashima [45-48]. Further manifestation of this phenomenon will be described in Sect. 3.1. A particularly striking example is the exclusive formation of 29 over 29 and 29" (Fig. 18) [48]. 

The reaction of organolithium compounds with dibiphenylenephos-phonium iodide has been extended to form the phosphoranes (115),... 

Quinquecovalent phosphoranes readily undergo ligand exchange on treatment with organolithium compounds. With alkyl-lithiums the resulting alkylphosphoranes give alkylidenephosphoranes, e.g. (2), by loss of benzene. 

A number of preparations of mixed halogenophosphoranes from tervalent phosphorus-fluorine compounds have been reported. For example, acyclic and cyclic fluorine compounds have been converted to phosphoranes, such as (36) and (37), by treatment with chlorine. Similar reactions leading to AA-dialkylaminodichlorodifluorophosphoranes (38) have been described and the stability of (38) to exchange processes commented upon. iVA-Dialkylaminotetraiodophosphoranes (39) have been prepared from AA-dialkylaminodichlorophosphines and lithium iodide, although no detailed physical evidence for the structure of these unusual compounds has yet been reported. The preparation of bis-(A-alkylamino)difluorophosphoranes (4) has been described above (see Section lA). 

The reactions of phosphorus pentachloride and diphenyltrichloro-phosphorane with amines and related compounds are reported in Chapter 9. The n.m.r. spectra of phosphine-phosphorus pentafluoride adducts have been described (see Chapter 11 for details). 

Quantitative yields of allenes were obtained from keten and the stable phosphoranes (48). Addition of similar phosphoranes occurred at the j3-position of the allenic ketones (49) to give the phosphoranes (50). The compound (50 = COPh) eliminated phosphine oxide under the... 

The seven-membered exocyclic phosphorane (111) with the fluorene-aldehyde (112) gave triphenylphosphine and the aldehyde (114) instead of the expected olefin. Compound (114) could have been formed as shown, the phosphorane functioning as a base to generate the anion (113). 

The stereoselectivity of the Wittig reaction is believed to be the result of steric effects that develop as the ylide and carbonyl compound approach one another. The three phenyl substituents on phosphorus impose large steric demands that govern the formation of the diastereomeric adducts.240 Reactions of unstabilized phosphoranes are believed to proceed through an early TS, and steric factors usually make these reactions selective for the d.v-alkcnc.241 Ultimately, however, the precise stereoselectivity is dependent on a number of variables, including reactant structure, the base used for ylide formation, the presence of other ions, solvent, and temperature.242... 

Both neutral and anionic phosphorus compounds are good nucleophiles toward alkyl halides. We encountered examples of these reactions in Chapter 2 in connection with the preparation of the valuable phosphorane and phosphonate intermediates used for Wittig reactions. 

Two series of phosphoranes (86 or 87) containing a dioxaphos-pholene ring and a 2-phospholene or 3-phospholene ring have been synthesised from trico-ordinate phospholenes (83) or (84) and 1,2-dicarbonyl compounds (85)1 7. The compounds were characterised by analysis, XH and J1P n.m.r. and, in contrast to earlier work,1 8 were found to be stable towards the retrodiene fragmentation. 

In acetic acid, or aqueous acetone with subsequent treatment with acetic anhydride, the esters (196 R=4-Me0 or 4-MeS) give rise to the expected esters(197) or (198). For the 4-chlorophenyl derivative, a mixture of the unexpected products (199, 200 Ar=4-chlorophenyl) was obtained. The unsubstituted compound (196 R=H) gave only (199) in aqueous acetone, but (200) in acetic acid. The postulated mechanism tor such a rearrangement centres around pseudophosphonium and-or phosphorane... 

Thus it has so far proved possible to isolate stable derivatives of monomeric metaphosphoric acid and of metathio- and metaselenophosphoric acid, which, understandably, generally bear tert-butyl and/or trimethylsilyl substituents u. Specifically, we know aminobisiminophosphoranes (3, Z = NR2, X = Y = NR)2,3,4), aminoiminothio (or seleno)phosphoranes (5, Z = NR2, X = NR, Y = S or Se)5), and aminoiminomethylenephosphoranes (1, R = NR2, X = NR)6>. Conspicuously, no stable phosphorus(V) three-coordinate compounds have been synthesized with oxygen as divalent ligand. 

Viewed systematically, formation of the olefins 24 on reaction of methyleneoxo-phosphorane 9 with a,P-unsaturated carbonyl compounds is to be classified as an olefination reaction. The similarity to the Wittig reaction is obvious, the differences being just a matter of degree. 

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