Ylids salt-free

Sulphur ylids also react with acyl silanes by two different, competing pathways, to give either silyl enol ethers, formed under salt-free conditions, or /1-ketosilanes, formed in the presence of soluble inorganic salts (Scheme 76)186. 

In contrast, the nonstabilized ylids (C6H5)3P=CH2 and (C6H3)3P=C(CH3)2 and even the moderately stabilized ylid (C6H5)3P=CHC6H5 react preferentially with the foemyl group of 2, even under salt-free conditions.4 Inverse addition is desirable to avoid presence of excess ylid. 

A small number of cyclopropylids are known in the literature (13, 14,) but their coordination chemistry has not been developed. For a detailed study of the effect of the cyclopropyl ring on the donor properties of ylids a series of simple species was prepared in the pure, salt-free state and characterized by multiple-resonance experiments (14, 15). 

A brominated polystyrene is reacted with sodium diphenylphosphan to form the polymeric Wittig reagent. Reaction with a halide and a base form the ylid which reacts with the carbonyl compound to the olefin. Wittig olefination can be made stereoselective (33). The formation of cis-olefins is accomplished in salt-free solu-... 

As the two components approach each other at 90° 76a, the two substituents keep out of each other s way 77 and so, when you flatten out the four-membered ring 69a, they end up on the same side 69. Part of the evidence for this mechanism is that salt-free ylids give the highest Z-selectivity.12 Normal preparations of ylids by treatment of, say, a phosphonium bromide with BuLi, have a molecule of salt (here LiBr) inevitably present. But not this way ... 

This procedure was used in the synthesis of the upper chain of a prostaglandin 82 (see chapter 6) where the stereochemistry is related to the biological activity.13 Note that the aldehyde is tied up as a hemiacetal in the starting material 81 (see chemoselectivity chapter) and that the salt-free ylid is actually a carboxylate anion made with sodium derivative of DMSO in DMSO. As you will see in the next section, not all Wittig reactions are cis selective - those of stabilised ylids are trans selective. 

The stabilization of the benzhydryl ylid is expected to be quite high since the related fluorenylid (2) can be isolated as a salt free solid. The high yield of 32 was attributed to hydrogen atom abstraction from the cyclohexene by the diphenylcarbene and subsequent coupling of di-phenylmethyl radicals 46>. 

In contrast to an earlier report, treatment of (69) with alkyl-lithium in salt-free THF at —78 °C did not give any detectable (by n.m.r.) quantities of either (70) or (71), although a red colour was observed which could have been due to a low concentration of the ylid. 

Wittig synthesis. A soln. of phenyl isothiocyanate in anhydrous benzene added dropwise to a salt-free soln. of methylenetriphenylphosphorane in the same solvent, stirred 12 hrs., the resulting intermediate ylid (Y 85%) suspended in benzene, benzaldehyde added, and stirred 48 hrs. at room temp. thiocinnamic anilide (Y 73%). F. e. s. H. J. Bestmann and S. Pfohl, Ang. Ch. 81, 750 (1969). 

Another synthesis of CJH eminating from the Corey labs involved a modification of the Schlosser modification of the Wittig Reaction. Whereas Wittig reactions between non-stablilized phosphoranes and aldehydes usually provide Z-olefins, Schlosser reported a variation of this reaction that provided -olefins. His group showed that when non-stabilized phosphoranes are reacted with aldehydes in the presence of lithium halides (as opposed to the salt-free conditions that favor Z-olefins), followed by deprotonation of the intermediate adduct, and reprotonation of the resulting j3-oxido ylid, T-olefins are obtained. Corey and Yamamoto reported a variation of this reaction in which the intermediate j3-oxido ylid was treated with formaldehyde, resulting in formation of trisubstituted allylic alcohols. For example, 102 reacted sequentially with aldehyde 103 and formaldehyde to provide 105. This material was then carried on to CJH intermediate 106, and the CJH derivative where R=H. 

Ethyl acetoacetate reacted to give salt 37 (R = Et02C, R = Me), the free ylid 38 (R = Et02C, R = Me) again was produced by exposure to... 

Path 5 has not been confirmed in the ylid, but products which can be attributed to this path are noted in the decomposition of the fluor-enylid and in alkoxy-substituted quaternary ammonium salts. Although ethylene and polymethylene are observed in the decomposition of the ylid, it has been suggested that these products are formed via a stepwise alkylation reaction. However, the ylid may be regarded as a carbenoid, with the N+(CH3)3 group behaving as a pseudohalogen, which reacts either as the free ylid or as the lithium halide complex. 

---