Ylide sulfur-based

The reaction was further developed to form a wide range of p-lactam-based products (52). Treatment of the racemic ylide processor 186 with suitable sulfur-based thiocarboxylate or thiocarbonate dipolarophiles gave rise to the expected racemic penams 189 and 190 and penems as single regioisomers (Scheme 3.54). Once again, the use of the chiral dipolarophile 186 furnished the cycloaddition product 191 with complete enantiomeric integrity. Similarly, the use of aldehydes... 

Ylide can be viewed as a special carbanion in which the negative charge on carbon is stabilized by an adjacent positively charged heteroatom. The most common ylides are phospho-nium ylides, sulfur ylides (sulfonium and sulfoxonium ylides) and certain nitrogen-based ylides (ammonium, azomethine, pyridinium and nitrile ylides). In addition to synthetically important phosphorus, sulfur and nitrogen, ylides of tin (Sn) and iodine (I) have been developed in recent years. 

A number of elements form ylides which are analogous to the sulfur-based reagents discussed above, and these reagents display chemistry which is very similar to that obtained with the sulfur compounds. The following discussion will emphasize processes which appear to offer real advantages to better-known techniques. 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

The proposed mechanism is based on the basis of the fact that ylides (Scheme 23 and Scheme 24) undergo bond fission between the phosphorus atom and the phenyl group in TPPY as reported by Nagao et al. [51] and between the sulfur atom and the phenyl group in POSY as observed in triphenylsulfonium salts [52-55] when they are irradiated by a high-pressure mercury lamp. The phenyl radicals thus produced participate in the initiation of polymerization. 

Sulfur ylides are a classic reagent for the conversion of carbonyl compounds to epoxides. Chiral camphor-derived sulfur ylides have been used in the enantioselective synthesis of epoxy-amides <06JA2105>. Reaction of sulfonium salt 12 with an aldehyde and base provides the epoxide 13 in generally excellent yields. While the yield of the reaction was quite good across a variety of R groups, the enantioselectivity was variable. For example benzaldehyde provides 13 (R = Ph) in 97% ee while isobutyraldehyde provides 13 (R = i-Pr) with only 10% ee. These epoxy amides could be converted to a number of epoxide-opened... 

Cyclic dinuclear ylide complexes have also been prepared with sulfur ylides. The reactions of (dppm)(AuCl)2 or (dppe)(AuCl)2 with trimethylsulfoxonium tetrafluoroborate and base under phase-transfer conditions gave the products shown in Scheme 42. Auration of the dppm ligand leads to the byproducts.27... 

A very convenient asymmetric synthesis of cyclopropane or epoxide systems developed by Johnson (184) is based on the use of chiral sulfur ylides as the agents that induce optical activity. Generally, this method consists of the asymmetric addition of a chiral sulfur ylide to the C=C or C=0 bond and subsequent cyclization of the addition product to form a chiral cyclopropane or epoxide system together with chiral sulfinamide. A wide range of chiral... 

The proton in the thiazolium ring is relatively acidic (pATa about 18) and can be removed by even weak bases to generate the carbanion or ylid (see Box 11.8). An ylid (also ylide) is a species with positive and negative charges on adjacent atoms this ylid is an ammonium ylid with extra stabilization from the sulfur atom. 

A class of compounds in which a positively charged atom from group V or VI of the periodic table (c.g., N, O, S, P, As, Se) is bonded to a carbon atom having an unshared pair of electrons. Whereas there is only one canonical form for nitrogen ylides (R3N —CR2 ), because of pTT-dTT bonding, two canonical forms can be written for phosphorus Le., R3P=CR2 R3P —CR2 ) and sulfur ylides (R2S=CR2 R3S —CR2 ). A number of enzyme-catalyzed reactions have been reported to utilize ylide-based chemistry. For example, the ylide form of the... 

A sulfur ylide is formed with base which then abstracts the a proton, generating dimethylsulfide and the aldehyde or ketone. 

S02R, —02CCF3, and C6H11NHC=NC6H11, respectively. In the next step a sulfur ylide, 8, is formed from the reaction of a base with 7, but the ylide... 

Ylides based upon sulfur are the most generally useful in these cyclopropane-forming reactions.133 Early work in this area was done with the simple dimethyloxysulfonium methylide (9) derived from dimethyl sulfoxide. The even simpler dimethylsulfonium methylide (10) was studied at the same time as a reagent primarily for the conversion of carbonyl compounds into epoxides.134 Somewhat later, other types of sulfur ylides were developed, among which the nitrogen-substituted derivatives such as (11) are... 

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