As-Ylides

Such ylides are unstable and react with carbonyl compounds to give both the Wittig product (p. 545) as well as AsPh3 and an epoxide. However, this very reactivity is sometimes an advantage since As ylides often react with carbonyl compounds that are unresponsive to P ylides. Substituted quaternary arsonium compounds are also a useful source of heterocyclic organoarsanes, e.g. thermolysis of 4-(1,7-dibromoheptyl)trimethylarsonium bromide to l-arsabicyclo[3.3.0]octane ... 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

The pyridine ylide method depends on the effective trapping of the carbene by pyridine. At high pyridine concentrations every carbene produced by the laser pulse will be trapped as ylide, and the ylide s absorbance (Ay) will saturate or reach a plateau (A °). The magnitude of A ° will vary with carbene structure it will decrease with both the increasing ease of carbene rearrangement and the intervention of RIES during carbene generation.3... 

The bonding characteristics of phosphorus ylides is a matter of continuos discussion in the literature. This bonding is most frequently referred to as ylide, ° while sometimes the ylene description is also used (Scheme 2). Some characteristics of the X -P=C... 

Azomethine ylides undergo a formal [3 + 2] cycloaddition with carbonyl compounds to provide oxazolines. Thioimidates, in particular, are effective as ylide precursors. For example, Kohra and co-workers reported that the thioimidate 260, upon activation with cesium fluoride, reacts with aromatic aldehydes and diaryl-ketones to provide oxazolines 261 in modest to good yields. Aliphatic aldehydes and simple ketones are unreactive (Scheme 8.72). 

The synthesis of alkenes through the Wittig reaction has generated an impressive understanding of the chemistry of organophosphorus compounds. The generated car-banions stabilized by a phosphoryl moiety can be considered as ylide anions, and the... 

The parent nonclassical annelated thiophenes represented by structure (Id X = S, Y = O, NR, S, Se) are unknown however, there is considerable information on their characteristic reactions, mainly as ylides in the case of those derivatives which have been either generated in solution or isolated as stable solid compounds. 

Current results indicate that stabilized arsonium ylides such as phenacylide, carbomethoxymethylide, cyanomethylide, fluorenylide, and cyclopentadienylide afford only olefinic products upon reaction with carbonyl compounds. Nonstabilized ylides such as ethylide afford almost exclusively epoxides or rearranged products thereof. However, semi-stabilized arsonium ylides, such as the benzylides, afford approximately equimolar amounts of olefin and epoxide. Obviously, the nature of the carbanion moiety of the arsonium ylide greatly affects the course of the reaction. It is reasonable to suppose that a two-step mechanism is involved in the reaction of heteronium (P, S, and As) ylides with carbonyl compounds (56). 

The systematic naming of these substances is cumbersome, but they have come to be known as ylides. The genesis of this name may seem obscure, but it is an attempt to reconcile the presence of a C-X a bond, which is covalent and nonpolar as in alky/ derivatives, as well as an ionic bond as in metal h Udes. Hence, the combination yl-ide.2... 

Current IUPAC and Chemical Abstracts nomenclature has been employed in this index with the former given preference. Substitutive nomenclature has been given preference over radicofunc-tional, additive, subtractive, conjunctive or replacement nomenclature, except where this becomes unwieldy. With many bicyclic and polycyclic compounds bearing heteroatoms, standard bicyclic or polycyclic oxa, aza, and thia replacement nomenclature has often been used. With certain functional groups, where the names are rather complex and probably not familiar to most organic chemists, such as ylides, those compounds have simply been named as sulfur, tellurium and arsonic ylides. Metal catbenes have been treated similarly. With more complex functionality and many heterocycles, the Beilstein Commander Crossfire nomenclature system has been used with certain modifications. 

In intermolecular cyclopropanations [100], it was found better to use a-bromoesters and amides as ylide precursors and a,/ -unsaturated ketones and esters as electron-deficient alkenes - rather than using a-haloketones as the ylide precursor. (For experimental details see Chapter 14.11.4). The reaction gives access to a range of 1,2-dicarbonyl-substituted cyclopropanes (see Fig. 10.5). The al-kene could have an aryl-, alkyl- or indole-substituted ketone, and a-substitution was also tolerated. Notably, Weinreb amides could be used as the ylide precursor and the product subsequently transformed into a diketocyclopropane. Both enan-... 

In the [7 + l]-cycloreaction of the diazopyrazole (245) with diazoalkanes, the diazoalkane reacts ylide-like as a 1-nucleophile-1-electrophile the carbon atom first behaves as a nucleophile and then as an electrophile (equation 46). Other 1-nucleophile-l-electrophiles such as ylides of phosphorus, nitrogen and sulfur react analogously with (245) (79TL1567). 

It is interesting to compare the endo selectivities of two closely related types of dipoles. The ylides 47, derived by an imine-azomethine ylide tautomerism of the imines of a-amino esters (Section II,C), are typical dipoles that exhibit high endo selectivity in cycloadditions to cyclic olefins. On the other hand, N-substituted derivatives 79, derived by the deprotonation route (Section II,D) from N-substituted a-amino esters and aldehydes, are very poor in endo selectivity. The N-unsubstituted dipoles 47 are involved in the cycloadditions as ( , )-ylides (or syn-ylides), while the N-substituted dipoles 79 as ( ,Z)-ylidic forms (or anti -ylides). However, the difference in ylide geometry is not the only reason for the lowered endo selectivity of 79, since the anti forms of heteroaromatic N-ylides undergo exclusive endo-selective cycloadditions with cyclic cis-olefins (83H1907 84CL465 85BCJ3137). 

In many cases of intramolecular cycloadditions an enhanced reactivity of dipoles is observed. Accordingly, intramolecular cycloadditions have a synthetic advantage that nonactivated olefin dipolarophiles with alkyl sub-stituent(s), which are very inert in the intermolecular versions, may serve as ylide traps. Both regio- and stereoselectivities are quite different from those observed in the intermolecular reactions. They mainly depend upon the length of the connecting chain that combines the dipole and dipolarophile and upon substituents on the chain. 

While triphenylalkylphosphonium salts as ylide precursors unambiguously lead to alkylidenetriphe-nylphosphoranes, in the case of tetraalkylphosphonium salts ylide formation may take place principally at different a-positions relative to the phosphorus. " Furthermore an intramolecular transylidation process causes the ligands at the phosphorus in methylenetrialkylphosphoranes to exist alternately as alkyl or as alkylidene substituents, thus giving rise to the formation of isomeric ylides. 

Oxiranes from Carbonyl Compounds with CH2-equivalents (CH2N2, LiCH2X, S, Se, and As Ylides) 129... 

Alternation of bond orders has been used to show that isothiazole 1-oxide and the 1,1-dioxide are not aromatic, but are better described as ylides. The degree of bond fixation in the oxides... 

Some remarkably persistent carbenes have been synthesized. This was achieved either by substitution with sterically demanding groups that hinder dimerization (triplet carbenes)399 400 or by substitution with strong Jt-electron donors (singlet carbenes)401 (Scheme 5.5). The latter are probably better described as ylides (zwitterions). 

More recently, a-diazophosphines have been prepared on irradiation a-phosphinocarbenes are formed. Although phosphinocarbenes always possess multiple bond character, they can rearrange either as carbenes or as ylides. 

---