Formation and Reactions of Ylides

Interaction of an electrophilic carbene or carbenoid with R—S—R compounds often results in the formation of sulfonium ylides. If the carbene substituents are suited to effectively stabilize a negative charge, these ylides are likely to be isolable otherwiese, their intermediary occurence may become evident from products of further transformation. Ando 152 b) has given an informative review on sulfonium ylide chemistry, including their formation by photochemical or copper-catalyzed decomposition of diazocarbonyl compounds. More recent examples, including the generation and reactions of ylides obtained by metal-catalyzed decomposition of diazo compounds in the presence of thiophenes (Sect. 4.2), allyl sulfides and allyl dithioketals (Sect. 2.3.4) have already been presented. 

Fig. 4.13. Formation and reactions of carbonyl ylides from carbonyl compounds and electrophilic carbene complexes.

Other carbonyl functionality-containing nitrogen atoms have been examined for the formation and reaction of carbonyl ylides (Schemes 4.36 and 4.37). 

In addition to the formation and reactions of carbonyl ylides discussed in the previous section, carbenoids also react intramolecularly with ethereal oxygen atoms to generate oxonium intermediates. When the ether is part of a ring as in substrates 63 a-b, the intramolecular addition of rhodium carbenoids produces bicyclic oxonium intermediates, which generated [5.2.1] oxabicycles 64a-b upon rearrangement by a [2,3]-sigmatropic pathway, Eq. 44 [74]. 

Formation and Reactions of the Ylide (C l3Si)2CPMe2Cl and the Effect of Substituents on Ylide Formation... 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

Semi-empirical PM3 calculations" reveal that ylide 23 is a minimum on the potential energy surface and that both steps are exothermic. The enthalpy of the reaction of ylide formation in CH3CN was estimated to be —43 kcal/ mol and the enthalpy of reaction of the second step, 1,2-hydrogen shift, was calculated to be —12.5kcal/mol. 

Only a limited number of examples have been reported. The reactivity of sulfonium ylide 98a, prepared by the reaction of thiepine 96 and dimethyl diazomalonate (Section 13.03.6.1), was examined <20060BC2218>. The reactivity of the stabilized sulfonium ylide 98a was restricted to the highly reactive Michael acceptor, tetracya-noethylene 152 (the ylide failed to react with benzaldehyde or dicyanoethylene). Reaction of ylide 98a with tetracyanoethylene 152 led to the consumption of the ylide 98a (Equation 22). Thiepine 96 was produced in the reaction and the formation of cyclopropane 153 was suggested. 

The former Chapter Reaction of Ylides with Saturated or a,P-Unsaturated Carhonyl Compounds got rid of its three-membered ring formations and the rest strictly remodelled to lumish the new Chapter 11 Reaction of Phosphorus- or SuHur-stabilized CNucleophiles with Carhonyl Compounds Addition-induced Condensations . 

The reaction of ylides with carbodiimides usually produces linear 1 1 adducts. The adducts derived from DMSO and carbodiimides undergo a facile reaction with primary alcohols to give an aldehyde (Moffat oxidation). With phenols and carboxyhc acids, alkylation products and esters, respectively, are formed. The oxidation proceeds under mild conditions and can be applied to sensitive compounds. Primary alcohols are oxidized solely to aldehydes without the formation of even trace amounts of carboxylic acids. The carbodiimide adducts generated from DMSO or the dimethylseleniumoxide " adducts have structure 369 (X = S, Se). 

Schlosser modification of Wittig reaction The presence of soluble metal salts such as lithium salts decreases the aVfrans-selectivity. The normal Wittig reaction of non-stabilized ylides with aldehydes gives Z-alkenes. The Schlosser modification of the Wittig reaction of non-stabilized ylides furnishes -alkenes. In the presence of lithium halides oxaphosphetanes can often be observed, but betaine-lithium halide adducts are also formed. If lithium salts are added to the equilibrium, oxaphosphetane formation and elimination of... 

The photochemical generation and reactions of nitrile ylides have again been studied. Irradiation of azirine (55) in the presence of 1,4-naphthoquinone (56) gave the isoindoledione (57) via 1,3-dipolar addition of the transient nitrile ylide (SS)."" The kinetics of the reactions of nitrile ylides with electron-poor alkenes have been determined, with evidence being reported for the formation... 

This ylide chemistry is treated in a number of monographs and reviews. The formation and properties of 77 and 78 were first dealt with by Corey et al. Reactive ylides are prepared from dialkyl sulfide and dialkyl sulfoxide in a non-aqueous medium. The solvent for the epoxidation reaction may be an aqueous basic solution. No side-reactions occur in a two-phase system or under the more recent phase-transfer conditions. ... 

In a study of ylides containing bis(trifluromethyl) groups Roschenthaler et al. also reported an unusual method for the formation of monocyclic phosphoranes (37ab) by the reaction of ylide (36ab) with hexafluoroacetone. The products were characterized by H, NMR, mass spectrometry and... 

By far the most important reactions of ylides are those of the Wittig type, especially those with carbonyl compounds, and they will be dealt with first other reactions of this sort are those with other C = X functions and with nitroso-compounds. Other carbanionic reactions are then considered and a final section deals with the formation of cyclic compounds from arsonium ylides. Hydrolysis has already been discussion in Section III.B. 

The format of this chapter is similar to that used in previous volumes. The first section deals with methylene phosphoranes and their Wittig reactions, the second looks at the Horner-Wadsworth-Emmons reaction of phosphonate anions and the third the structure and reaction of lithiated phosphine oxides - an area which continues to receive particular attention. The majority of reports concerning ylides relate to their use in synthesis and in the final section some of these applications are reviewed. 

Another TS model was proposed by Schlosser and Schaub (66) as illustrated by structure 143. This model explicitly recognizes the importance of phosphorus substituents, but it invokes a dominant P-phenyl propeller effect that requires kinetic cis selectivity for all Ph3P=CHR as well as PhjP CHX reactions and kinetic trans selectivity for reactions of ylides that do not contain the propeller . The formation of trans alkenes from ylides Ph3P= CHX would have to be due to equilibration. None of these generalizations is consistent with Table 22 or with the control experiments that demonstrate kinetically controlled decomposition of a variety of oxaphosphetanes. 

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