Stabilized ylides

A confirmation of the possible role of sulfur in the ylide stabilization by d-high rate of decarboxylation of 5-thiazoIecarboxylates (433). 

A comparative study on ylide stability as a function of the heteroatom type was carried out by Doering et al. [3,4]. They concluded that the phosphorus and sulfur ylides are the most stable ones. The participation of three-dimensional orbitals in the covalency determines the resonance stabilization of the phosphorus and sulfur ylides [5-8]. The nitrogen ylides are less stable from this point of view. The only stabilization factor involves electrostatic interactions between the two charges localized on adjacent nitrogen and carbon atoms [9]. 

Finally the new ylides stabilized with sulfinyl but alkoxy carbonyl groups also behave in a more complex manner than for the simple sulfinyl ylide and the loss of both PhjPO and PhjP is observed [131]. Mixtures of products are obtained among which are alkenyl sulfides as major products but also sulfides and thioesters (Scheme 35). 

The ease with which this occurs is determined by the other groups attached to the carbon (cf. 946). The resulting ylide can be isolated only in special cases (e.g. 947-950) ylide stability increases with increasing possibility for spreading the negative charge (cf. 948 - 949 950). 

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

Ylide type non-stabilized ylide semi-stabilized ylide stabilized ylide... 

Stereochemical studies on the ring-opened azomethine ylide 1,3-dipoles thermally or photolytically generated from sterically defined aziridines are important in order to learn the geometry of transient ylide species. Though the aziridine route has a limitation in that an appropriate ylide-stabilizing... 

Contrary to these high stereoselective anti-ylide generations, similar ylide generated by this route are trapped as the cycloadducts of anti-ylides, for tetrahydro-jS-carboline-4-carboxylic acid, and alanine give mixtures of the maleimide cycloadducts derived from anti- and syn-ylides 108, 242, 243 (84CC180 87CC47). This difference in ylide stabilization has not been well interpreted. 

Table VIII summarizes the cycloaddition reactions of azomethine ylides with maleimides and maleic anhydrides. The upper part of the table involves endo-selective cycloadditions and the lower part nonselective reactions. Endo selectivity is closely related to the substitution pattern of the azomethine ylides used. The azomethine ylides that show high endo selectivity to these cyclic olefins bear an ylide-stabilizing substituent of the carbonyl or cyano type (EWG) at one carbon and an aryl, heteroaryl, acyl, or 1-alkenyl substituent (R ) at the other. These key substituents must be present for high endo selectivity, although the substituent (R) on the ylide nitrogen is equally important.

The azomethine ylides that undergo highly stereoselective cycloadditions to dimethyl fumarate are relatively limited. They bear an ylide-stabilizing substituent on one carbon and an aryl moiety on the other. Here again, high stereoselectivity of N-metallated dipoles is noted. 

Similar anti-azomethine ylides 76 (EWG = COOR, R = MeO) bearing an ylide-stabilizing ester moiety are involved in the diastereoselective cycloadditions to 3,4-dihydroisoquinolines and N-(arylmethylene)methylamines to provide stereoselective imidazolidine-fused cycloadducts 259 and 260 (82LA924, 82LA2146). As imines and ylides 76 bear extremely different LUMO and HOMO coefficients on each termini, uniformly high regioselec-tion is not surprising. endo-Approach of the dihydroisoquinolines to the... 

Oxosulfonium ylides as well as sulfonium ylides stabilized by an electron-withdrawing group, such as an acyl , an alkoxycarbonyP , a cyano " and one of their vinylogues satisfactorily cyclopropanate a variety of Michael acceptors (e.g., equation 81) . In these reactions, the 1,2-addition is usually a minor pathway. Ylides stabilized by two... 

Thus arsonium ylides stabilized by acyl groups alkoxycarbonyl... 

Ketodiazoester 263 is decomposed by rhodium(II) acetate with formation of polycyclic ketolactone 264 in small yield (10%). The reaction involves the intermediate formation of an oxonium ylide stabilized by proton migration and elimination of propene (88T4363). 

Other examples of cyclopropanation with stabilized 5-ylides, i.e. alkyl- or aryl-(dimeth-ylamino)oxosulfonium alkanides/ " are presented in Table 15. 5-Ylides stabilized by an anion-stabilizing group at the nucleophilic carbon atom have also been successfully utilized for the cyclopropanation of Michael acceptors. The stabilizing group was a ketone/ " ester/carboxylate anion/ phosphonate diester/ or a cyano function (Table 16). 

The reaction of selenium ylides stabilized by a carbonyl function with a,jS-unsaturated ketones afforded cyclopropanes substituted with two ketone functions, e. g. formation of 19 ° (see also refs 161 and 162). 

Kondo, K., Liu. Y, and Tunemoto, D., Preparation and reaction of sulphonium ylide stabilizes by a phosphinyl substituent, J. Chem. Soc., Perkin Trans. 1, 1279, 1974. 

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