Phosphoranes Stabilised

In a study of the reactions oi acyclic o-methoxyphenyl-phosphinites (45a), phosphonites (45b) and phosphites (45c) with halogens,the intermediate halogenophosphonium salts (46) are stabilised either by ligand exchange with starting material (not shown) or by elimination of methyl halides to form cyclic phosphoranes ( 47 ) 3 7. ... 

Analogous chemistry was reported [215] with thioaldehyde intermediates generated from a non-classical thermal reaction of a sulfoxide via a sulfe-nate ester (Table 3, entry 2). The reaction of cyclic tetra- or pentasulfides with stabilised phosphoranes is also a good source of thioaldehydes [216], trapped by dienes (Table 3, entry 2). 

Cyclisations of alkyllithiums onto E unsaturated phosphoranes or r-butyl esters are less stereoselective than the corresponding cyclisations onto unactivated alkenes, presumably because the additional stabilisation allows the transition state to become looser .86 With Z enoates 177, however, trans selectivity is high because of congestion in the transition state leading to the cis isomer of 178. With an alkoxy substituent (177, R = OMe), the stereoselectivity reverses, possibly due to Li-coordination. 

Although strictly outside the remit of this chapter, it is appropriate to note continued activity in the chemistry of c X -p -bonded phosphorus compounds that do not possess a lone pair of electrons at phosphorus. A monomeric metaphosphonate species (262, X=0) has been stabilised by coordination via the P=0 bond), and Harger s group has provided evidence of the intermediacy of metathiophosphonates (262, X=S) in the reactions of phosphonami-dothioic acids with alcohols.The cation (263) has been stabilised by coordination at phosphorus with 4-dimethylaminopyridine and the reactivity of bis(methylene)phosphoranes (264) and related phosphoranylidene car-benoids has been investigated. ... 

For several years, Streubel and coworkers have been exploring the chemistry of nitrilium phosphine ylides that contain low-valent organophosphorus fragments, such as complex (19) (Scheme 3). In their latest contributions to this area, they have investigated the reactions of complex (19) with phosphoranes and iminophosphoranes which have led to a number of new species containing ylidic moieties, such as the tricyclic complex (20), and 2H-l,2-azaphosphole complexes (21) and (22). Similarly, treatment of other low-valent organophosphorus complexes such as phosphinidine species (23) with stabilised phosphoranes produces new ylidic-aducts (24) and (25) (Scheme 4). ... 

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. ... 

Further progress has been reported in the chemistry of CT X, -p -bonded systems. Full details of such systems stabilised by intramolecular coordination, as in, e.g., 334, have been described. The kinetically stable system 335 has been prepared and its solid state structure determined . The P-halobis(imino)-CT -phosphoranes 336 have also been prepared, and detailed NMR studies of bis(imino) phosphoranes reported . Quin s group has continued studies of the generation and characterisation of reactive a X -systems, e.g., 337 ". Methods for the generation of monomeric metaphosphate esters in solution have been investigated. A theoretical approach has been used to probe the mechanism of the reaction between phosphanylnitrenes 338 and boranes. The thiophosphonic anhydride 339 behaves as a source of the dithioxophosphorane... 

The reaction of methylenetriphenylphosphorane with epichlorohydrin followed by addition of butyllithinm and an aldehyde in toluene provides alkylidenecyclo-butanols in moderate to good yield (Scheme 3). Similar reactions have previously been reported to afford alkylidenecyclo-propylmethanols (61) and the current report demonstrates that the reaction is highly base- and solvent-dependent. Further studies of the kinetics and mechanism of the addition of cyclopentadienylidenetriphenylphosphorane to tetrahalo-p-benzoquinones have included the reaction with tetraiodo-p-benzoquinone to give the 2,5-disubstituted quinone (62). Initial attack on sulfur is presumably involved in the reaction of two equivalents of stabilised phosphoranes with 5-arylimino-4-chloro-5H-1,2,3-dithiazoles (63) to give a mixture of N-arylcyanothioformamides (64) and, as the major product, the novel ylides (65). In one example the structure of (65) was confirmed by X-ray crystallography. 

The synthesis of 5-ylidenepyrrol-2(5H)-ones (266) from the reaction of maleimides with stabilised phosphoranes under vigorous conditions has been reported.Attempts to use similar reactions of unstabilised phosphoranes, phosphonate-, or phosphine oxide carbanions led to intractable products or recovered starting materials. The procedure developed in these studies has been applied to the synthesis of pukeleimide A (269) using the formation of the ylidenepyrrolone (268) from the maleimide (267) as a key step. ... 

Highly electrophilic P-lactams such as 36 and 38 have been shown to react with stabilised phosphoranes to yield azetidin-2-methylenes 37 and 39 [30] respectively (Scheme 13). However this reaction did not occur with less reactive P-lactams e.g. monocyclic P-lactams or cephem derivatives. The olefins 37 and 39 can be used as protected P-lactams they are much more stable to basic and nucleophilic conditions, and they readily regenerate P-lactams by ozonolysis [30]. The de-esterification of 37 and 39 by conventional methods gave the acids (R = H) which are very weak antibacterial agents or P-lactamase inhibitors. 

Another general problem of Wittig reactions with unsaturated and therefore semi-stabilised phosphoranes is the low ( /Z)-selectivity. 

Some compounds of the types indicated in Table 6.14 are believed to have been synthesised. Most of these compounds contain some trimethylsilyl, ter-2,4,6 butylphenyl or other bulky stabilising groups. Some typical syntheses are (6.620-6.623) [7,8]. Bis methylene phosphoranes were discovered by Appel and co-workers in 1982 [9]. 

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