Ylide compounds 1.3- addition reactions

The reactions of carbenes, which are apparently unique in displaying electrophilic character in strongly basic solutions, include substitution, addition to multiple bonds, and co-ordination with lone pairs of electrons to form unstable ylides. This last reaction is of obvious relevance to a consideration of the reactions of heterocyclic compounds with carbenes and will be summarized. 

The addition reaction of enolates and enols with carbonyl compounds is of broad scope and of great synthetic importance. Essentially all of the stabilized carbanions mentioned in Section 1.1 are capable of adding to carbonyl groups, in what is known as the generalized aldol reaction. Enolates of aldehydes, ketones, esters, and amides, the carbanions of nitriles and nitro compounds, as well as phosphoms- and sulfur-stabilized carbanions and ylides undergo this reaction. In the next section we emphasize the fundamental regiochemical and stereochemical aspects of the reactions of ketones and aldehydes. 

Among the olefination reactions, those of phosphonium ylides, phosphonate anions, silylmethyl anions, and sulfone anions are discussed. This chapter also includes a section on conjugate addition of carbon nucleophiles to a, (J-unsaturated carbonyl compounds. The reactions in this chapter are among the most important and general of the carbon-carbon bond-forming reactions. 

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]. 

Addition of phosphorus ylide to carbonyl compounds Wittig reaction... 

Scheme 43 shows the details of the different steps involved in the equilibrium. The nucleophilic attack of the P(III) derivative on the acetylenic bond yields a 1,3-dipole which, after a fast protonation, frees aZ ion. If the subsequent addition of this ion occurs on the P atom (reaction a), a P(V) phosphorane is formed, but the addition of Z on the ethylenic C atom (reaction b) results in the formation of an ylide. Both of these reactions occur under kinetic control and, in both cases, X is always an OR group from the initial acetylene dicarboxylic ester. When the acetylenic compound is a diketone and X is an alkyl or aryl moiety, the C=0 group is much more electrophilic and the attack by the Z ion produces an alcoholate (reaction c), a new intermediate which can cyclize on to the P+ to form a phosphorane, or attack the a-C atom to form an ylide as in Scheme 42. Hence, reactions a and c can coexist, and are strongly dependent on the nature of the trapping reagent and of the P compound, but reaction b is blocked, whatever the reagent. This is well illustrated by the reaction of the 2-methoxytetramethylphospholane 147 on diben-zoylacetylene in the presence of methanol as trapping reagent. The proportions of the vinylphosphorane 157 and spirophosphorane 158 formed (Figure 24) are 13% and 84%, respectively.

Intramolecular Cycloadditions of Carbonyl Ylides W. Eberbach, J. Brokatzky and H. Fritz, Angew. Chem., Int. Ed. Engl., 1980, 19, 47-48. a,(3-Unsaturated Heteroatomic Compounds in 1,3-Dipolar Addition Reactions V. A. Galishev, V. N. Chistokletov and A. A. Petrov, Russ. Chem. Rev. (Engl. Transl.), 1980, 49, 880-892. 

The hemiacetal exists as an equilibrium mixture of cyclic compound 20 and its open counterpart (21), but an aldehyde addition reaction can occur only with the acyclic form A Wittig reaction of the stabilized ylide leads to an a. (3-unsaluraied ester that has the configuration with respect to the double bond. This reaction occurs under neutral conditions, so 1,4-addition of the alcohol to the a.p-unsaturated ester is avoided.6 A subsequent DIBAH reduction leads to ally lie alcohol 6 in a reaction that ordinarily shows complete 1,2-selectivit. ... 

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 . 

Unstabilized sulfonium ylides and stabilized sulfoxonium ylides show different reactions with a,P-unsaturated carbonyl compounds the former give epoxides and the latter give cyclopropanes. The epoxide formation (i.e. 1,2-addition) is kinetically favourable while cyclopropane formation (i.e. 1,4-addition, Michael addition) is energetically favourable. 

Carbenes are readily obtained by the photodecomposition of diazo-compounds. The reactions of carbenes generated in this way in rigid matrices at low temperatures have been reviewed. Addition of singlet methylene to acetonitrile affords the nitrile ylide... 

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