Phosphoranes isomerism

A variety of 1-azirines are available (40-90%) from the thermally induced extrusion (>100 °C) of triphenylphosphine oxide from oxazaphospholines (388) (or their acyclic betaine equivalents), which are accessible through 1,3-dipolar cycloaddition of nitrile oxides (389) to alkylidenephosphoranes (390) (66AG(E)1039). Frequently, the isomeric ketenimines (391) are isolated as by-products. The presence of electron withdrawing functionality in either or both of the addition components can influence the course of the reaction. For example, addition of benzonitrile oxide to the phosphorane ester (390 = C02Et) at... 

The 1,3,2-dioxaphosphorinans (101 R = Cl, NMe2, or OMe), derived from trans-2-hydroxymethylcyclopentanol, condensed with perfluorobiacetyl to give the expected phosphoranes.64 That from (101 R = OMe) probably had the relationship of methoxy to ring junction shown in (102) it slowly isomerized in solution at 40 °C to a 60 40 mixture with the isomer (103). The corresponding isomerizations... 

Preparative details and extensive i.r., n.m.r., and mass spectra have been described for the phosphoranes (87).48 These phosphoranes have a TBP structure, and for (87a)—(87c) their n.m.r. spectra are temperature-independent, and indicate that the fluorines bonded to phosphorus are equivalent. The authors have suggested an explanation based on rapid intramolecular isomerization, and discussed the possibility that a facile TR pathway exists for this process.46 Octahedral adduct formation between (87) and fluoride ion or trimethylphosphine has also been described,46 as shown in (88). 

Tellurophosphoranes, obtained through a transylidation reaction between tellurenyl halides and phosphoranes, react with aldehydes to give the expected vinylic tellurides as an E Z isomeric mixture (method a). One other methodology involves the treatment of equimolar amounts of phenyl tellurenyl bromide and phosphonium salts with t-BuOK followed by an aldehyde (method b). Under these lithium-salt-free conditions, (Z)-vinylic tellurides are the main products. ... 

To generate the phosphorane, Magnone [80,81], Wang et al. [82] and John and Paust [83] used respectively sodium methoxide, triethylamine/MgCh in JV,A/-dimethylacetamide and LiOH in N,N-dimethylformamide. For the isomerization step, the two first authors emploied rose Bengal as photosensitizer and the latter Erythrosine B, to give isotretinoin, Fig. (45). 

Hydroxybenzofuran-5-carbaldehydes 682 can react with carbethoxymethylenetriphenyl-phosphorane 683 to afford biologically active furanocoumarins 684 in high yield. The reaction proceeds via Wittig reaction of the phosphorane 683 with the aromatic aldehyde to form the intermediate cinnamate, which then undergoes isomerization of the double bond and lactonization to furnish the desired furanocoumarins 684 (Scheme 165) <1995T3087>. 

Isocyanates have provided one carbon atom in transformations of the general type (296) - (290). Thus, azide (343) reacts with triphenylphosphine in a Staudinger reaction giving phosphorane (344) which is then treated with isocyanates (RNCO R = benzyl or aryl) giving naphthyridines (346) (50-59% yield) via the carbodiimide (345). The cyclization (345)->(346) occurred in toluene at 160°C (92T4601). Azide (347) undergoes a similar sequence of reactions yielding a 1 1 mixture of isomeric naphthyridines (348) and (349) (44—47% yield). 

This has meant, in general, an application of the BPR mechanism f to the isomerizations of the hypothetical phosphorane intermediates. 

Scheme 3 contains several of the fundamental structures with penta-valent phosphorus, among them phosphorane itself, PH6 (21), and pentahydroxyphosphorane, P(OH)6 (26). The latter is the hydrate of phosphoric acid H3P04+H20 - P(OH)5. A hydrate of methylphos-phonic acid (CH3)(HO)4P (29), is also included as a model for its esters. Mono- and di-ionized forms, (27), (28), (30), are also given, since their stabilities in various isomeric forms provide important data concerning the role of intermediate oxyphosphoranes in the chemistry of phosphoric acid and its derivatives. A model compound of the 1,2-oxaphospholene ring (31), is provided, since this system, in the form of several derivatives, will be discussed extensively in Section VIII. 

The positional exchange of ligands in phosphorane (33) occurs by a regular, and not by an irregular, permutational isomerization process. This conclusion is supported by the data given in Table 14, and by the following arguments ... 

The caged 2 1 phosphoranes from biacetyl, (42), (43), (44), have not been studied in sufficient detail. Their relevance to the problem of the mechanism of TBP-permutational isomerizations will depend on the... 

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