Ylids resonance stabilized

The physical and chemical properties of the X -phosphorins 118 and 120 are comparable to those of phosphonium ylids which are resonance-stabilized by such electron-pulling groups as carbonyl or nitrile substituents Thus they can be viewed as cyclic resonance-stabilized phosphonium ylids 118 b, c, d). As expected, they do not react with carbonyl compounds giving the Wittig olefin products. However, they do react with dilute aqueous acids to form the protonated salts. Similarly, they are attacked at the C-2 or C-4 positions by alkyl-, acyl- or diazo-nium-ions Heating with water results in hydrolytic P—C cleavage, phosphine oxide and the hydrocarbon being formed. 

In the first step of the conversion catalyzed by pyruvate decarboxylase, a carbon atom from thiamine pyrophosphate adds to the carbonyl carbon of pyruvate. Decarboxylation produces the key reactive intermediate, hydroxyethyl thiamine pyrophosphate (HETPP). As shown in figure 13.5, the ionized ylid form of HETPP is resonance-stabilized by the existence of a form without charge separation. The next enzyme, dihydrolipoyltransacetylase, catalyzes the transfer of the two-carbon moiety to lipoic acid. A nucleophilic attack by HETPP on the sulfur atom attached to carbon 8 of oxidized lipoic acid displaces the electrons of the disulfide bond to the sulfur atom attached to carbon 6. The sulfur then picks up a proton from the environment as shown in figure 13.5. This simple displacement reaction is also an oxidation-reduction reaction, in which the attacking carbon atom is oxidized from the aldehyde level in HETPP to the carboxyl level in the lipoic acid derivative. The oxidized (disulfide) form of lipoic acid is converted to the reduced (mer-capto) form. The fact that the two-carbon moiety has become an acyl group is shown more clearly after dissocia-... 

The ylid is a neutral compound which is resonance stabilized by phosphorus. The phosphorus atom, being a second-row element, has unfilled d orbitals in the valence shell that can accept electrons from carbon. Consequently a major... 

The fourth type of photoirradiated cationic initiator is dialkyl-4-hydroxyphenyl-sulfonium salt 47> (Table 1). Photoexcitation of 4 gives rise to the formation of a resonance-stabilized ylid 6 and an acid HX. 

This resonance stabilization should be even more pronounced in the cyclic dithiocarbene (274), as indicated by the neutral carbene structure (274) and the ylid structures (275-280), in which the potential aromatic sextet of the 1,3-dithiolium ion is retained. Consequently, H-2 in 269 should be acidic. The analogy with the thiazolium and imidazolium ions, which have been intensively studied by Bres-... 

The photolysis of dialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenyl-sulfonium salts is different from that of triphenylsulfonium salts. The latter compounds undergo irreversible photoinduced carbon-sulfur bond cleavage the former compounds, however, react by reversible photodissociation and form resonance-stabilized ylids as shown in Fig. 5. Because of the slow thermally induced reverse reaction, only small equilibrium concentrations of the ylid and acid arc present during irradiation and the concentration will rapidly decrease when photolysis has been terminated. Therefore, in contrast to triarylsulfonium salt initiation, no dark reaction will continue after the irradiation step. 

Once the ylid has been formed, various decomposition reactions are observed. In the presence of Lewis acids it is difficult to ascertain whether the free ylid, its metalated adduct, or a dimetalated species is involved in the activated complex leading to decomposition. If the anionic portion of the ylid is stabilized by resonance, the extent of complexation with the Lewis acid is reduced. 

The photodecomposition of dialky1-4-hydroxyphenylsulfonium salts (64) gives rise to a resonance-stabilized ylid and an acid HX. Styrene oxide, 1,4-cyclohexene oxide, trioxane, and vinyl... 

Due to the presence of these resonance forms in azomethine ylids, reaction with alkenes often leads to mixtures of regioisomeric cycloadducts. Resonance stabilizing substituents are usually employed to enhance the regioselectivity. Intramolecular reactions usually favor only one mode of addition. An example of an intermolecular reaction is taken from Williams synthesis of spirotryprostatin B,372 in which 5,6-diphenylmor-pholin-2-one (473) reacted with the aldehyde shown to give a mixture of ( )- and (Z)-azomethine ylids (474). This product was generated in situ with oxindole 475, and [3-i-2]-cycloaddition product 476 was obtained in 82% yield. 

The reason for the failure of the carbonyl-oleflnation with ketones by way of the phosphorus ylid lies in the resonance stabilization of the ylid. The charge distribution is concentrated in the direction of the oxygen atom. Therefore, it also appeared fitting here to introduce a C=N-R group in place of the C=0 group in phosphorane. 

The mobility of the proton in position 2 of a quaternized molecule and the kinetics of exchange with deuterium has been studied extensively (18-20) it is increased in a basic medium (21-23). The rate of exchange is close to that obtained with the base itself, and the protonated form is supposed to be the active intermediate (236, 664). The remarkable lability of 2-H has been ascribed to a number of factors, including a possible stabilizing resonance effect with contributions of both carbene and ylid structure. This latter may result from the interaction of a d orbital at the sulfur atom with the cr orbital out of the ring at C-2 (21). 

When the nitrogen atom is substituted by a nitrophenacyl group, OH attack gives the betainic zwitterion (Scheme 13). which is soluble in organic solvents (32). The stability of the C-betainic or ylid structure has been explained as an effect of resonance of the negative charge in the molecule (33, 34). 

Heterocyclic carbanions stabilized by ylid formation, or by resonance that places the negative charge on a heteroatom, are specifically excluded. In addition, heterocyclic systems that do not depend on additional stabilization factors for their initial deprotonation, continued existence, or subsequent reaction with electrophilic substrates are discussed in less detail. 

Here, the ylid form can be stabilized by enolate formation and by d-orbital resonance, which explains the moderate value of pK. The possibility of pseudo-base formation can be excluded (75) by comparison with u.v. spectra. Thus, polarography made it possible to detect and characterize stable ylid formation in solutions several years before these ylids were isolated. 

---