From Other Ylides

Ylides other than acceptor-substituted diazomethanes have only occasionally been used as carbene-complex precursors. lodonium ylides (PhI=CZ Z ) [1017,1050-1056], sulfonium ylides [673], sulfoxonium ylides [1057] and thiophenium ylides [1058,1059] react with electrophilic transition metal complexes to yield intermediates capable of undergoing C-H or N-H insertions and olefin cyclopropanations. 

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It is possible to remove a proton from the methyl group of a trialkylmethylammonium halide with strong bases. Thereby, a betaine (see Section 4.7.4) is produced with the structure R3N+—CH2. A betaine in which the positive and the negative charges are located on adjacent atoms as in R3N+—CH2 is called an ylide. The yl part of the name ylide refers to the covalent bond in the substructure N+—CH2. The ide part indicates that it also contains an ionic bond. When one wants to distinguish the ylide R3N+—CH2 from other ylides, it is called an ammonium ylide or an N ylide. 

To identify the carbonyl compound and the ylide required to produce a given alkene mentally disconnect the double bond so that one of its carbons is derived from a car bonyl group and the other is derived from an ylide Taking styrene as a representative example we see that two such disconnections are possible either benzaldehyde or formaldehyde is an appropriate precursor... 

Syntheses in which a nitrile provides atoms 1 and 2 start from an ylide (82JFC373), or a 1-amino-2-pyridone (82S974) to give compounds 45 and 46. Other two atom fragments used with l-amino-2-pyridones are amides which give compounds such as 47 (86S860). 

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

When planning reactions of thiocarbonyl compounds with electrophilic carbene complexes it should be taken into aceount that thiocarbonyl compounds can undergo uncatalyzed 1,3-dipolar cycloaddition with acceptor-substituted diazomethanes to yield 1,3,4-thiadiazoles. These can either be stable or eliminate nitrogen to yield thiiranes or other products similar to those resulting from thiocarbonyl ylides [1338]. 

As an example of a o-arabino phosphonium salt, XVIa was prepared. Simple Wittig reactions with the ylide derived from XVIa did not proceed as smoothly as with the other ylides, and for that reason salt XVIb, which proved to be much better behaved (results not shown), was also prepared. 

Ylide Generation from Other Heteroatoms and Subsequent Reactions 173... 

Two examples are given for illustration the reaction of (172) with DMAD gave 32% (171) compound (173), on the other hand, has been obtained from the ylide (174) and the enamine (175) in 36% yield. A compilation of 1,3-dipolar cycloadditions which lead to indolizines has been presented (76S209). 

Carbenoid sources other than those derived from diazo precursors for catalytic cyclopropanation reactions are currently limited. Inter- and intramolecular catalytic cyclopropanation using iodonium ylide have been reported. Simple olefins react with iodonium ylides of the type shown in equations 83 and 84, catalysed by copper catalysts, to give cyclopropane adducts in moderate yield127 128. In contrast to the intermolecular cyclopropanation, intramolecular cyclopropanation using iodonium ylides affords high yields of products (equations 85 and 86). The key intermediate 88 for the 3,5-cyclovitamin D ring A synthon 89 was prepared in 80% yield as a diastereomeric mixture (70 30) via intramolecular cyclopropanation from iodonium ylide 87 (equation 87)1 0. 

In the mode of bonding of bifunctional ylides, gold was again found to differ from other metals. The complexes [M CH(PPh2)2 2] (M = Ni, Pd, or Pt) reacted (62, 338) with Et2MeP=CH2 to yield mononuclear products... 

Under salt-free conditions, the cw-oxaphosphetanes formed from nonstabilized ylides can be kept from participating in the stereochemical drift and left intact until they decompose to give the alkene in the terminating step. This alkene is then a pure ci.s-isomer. In other words, salt-free Wittig reactions of nonstabilized ylides represent a stereoselective synthesis of cis-alkenes. 

A mixture of (E)- and (Z)-chlorostyrenes was obtained from the above ylide, with R = Ph and R = H once again, (dichloroiodo)benzene was differentiated from other chlorinating agents, since these yiides with A-chlorosuccinimide delivered the ( )-isomer with high selectivity. 

The Wittig reaction is a C,C-forming olefin synthesis from phosphonium ylides and carbonyl compounds (see also Section 4.7.4). In more than 99% of all Wittig reactions, ylides of the structure Ph3P+—CH-—X (i.e., triphenylphosphonium ylides) are used. Therein, X usually stands for H, alkyl, aryl, or C02-alkyl and seldom for other substituents. 

Reactions which formally involve the oxidation of azides have been reviewed by Boyer. Other oxidations with useful synthetic applications include two which start from nitrogen ylides. Sulfimides (50) derived from electron-deficient aromatic and heterocyclic amines are oxidized to the corresponding nitroso compounds by MCPBA. - This is a very useful method of preparation of some otherwise inaccessible nitroso compounds such as 2-nitrosopyridine and 1-nitrosoisoquinoline. They can be further oxidized, for example by ozone, to the nitro compounds. Phosphimides (51) are oxidized directly by ozone to the nitro compounds, although the nitroso compounds are intermediates. Isocyanates can also be oxidized to the corresponding nitro compounds, by dimediyldioxiraiK (1). ... 

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