Organometallic compounds cycloaddition reactions

A143. H. Ulrich, Cycloaddition Reactions of Heterocumulenes. Academic Press, New York, 1967. 364 pp. Thirty pages deal with organometallic compounds. 

The intramolecular [4+3] cycloaddition reaction towards seven-membered rings involving furans and allylic cations have been reviewed <01ACR595>. A review on the use of 2-siloxyfurans as butenolide precursors has appeared <01T3221>. Organometallic compounds of furans and their benzoannulated derivatives have also been summarized <01 AHCI>. 

Complexes of iron and other metals which are prepared from a allyl or propargyl organometallic compounds and (NC)2C = C(CN)2, SO2, or related unsaturated electrophilic molecules via cycloaddition reactions are important because such complexes may find application in organic synthesis " ... 

The cycloaddition reactions of cumulenes generally produce three- to six-membered ring compounds, which often cannot be obtained in a one-step reaction. When the cumu-lene or the substrate contain metal to carbon bonds, metallacycles are readily produced. Organometalic compounds are readily obtained in the insertion reactions of the cumulenes. In the latter reactions, linear compounds are obtained. Click chemistry therefore can provide not only a vast number of cyclic compounds but also numerous linear compounds and even linear and crosslinked polymers which have commercial significance. 

Reactions.—The chemical behaviour of thioketones has mainly been investigated in four directions reaction with organometallic compounds, reaction with nucleophiles, thermal cycloadditions, and photochemical reactions. 

N-Metalated azomethine ylides generated from a-(alkylideneamino) esters can exist as tautomeric forms of the chelated ester enolate (Scheme 11.8). On the basis of the reliable stereochemical and regiochemical selectivities described below, it is clear that the N-metalated tautomeric contributor of these azomethine ylides is important. Simple extension of the above irreversible lithiation method to a-(alkylideneamino) esters is not very effective, and cycloadditions of the resulting lithiated ylides to a,p-unsaturated carbonyl compounds are not always clean reactions. When the a-(alkylideneamino) esters bear a less bulky methyl ester moiety, or when a,P-unsaturated carbonyl compounds are sterically less hindered, these species suffer from nucleophilic attack by the organometallics, or the metalated cycloadducts undergo further condensation reactions (81-85). 

In this section, some recent uses of pyrans in synthesis are described. Much of the chemistry of this class of compounds involves Diels-Alder cycloadditions, metal- and thermal-assisted reorganization reactions, and the preparation and reactions of organometallic derivatives. The chemistry of the pyrylium salts, useful precursors to substituted aromatics (B-64MI11600), will not be discussed here. 

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