Claisen rearrangements carbenes

A proposed reaction pathway is shown in Scheme 7.29, where either the aromatic carbon or oxygen atom of naphthol may work as a nucleophile. Thus, the first step is the nucleophilic attack of the carbon atom of 1 -position of 2-naphthol on the C. atom of an allenylidene complex A to give a vinylidene complex B, which is then transformed into an alkenyl complex C by nucleophilic attack of the oxygen atom of a hydroxy group upon the Co, atom of B. Another possibility is the nucleophilic attack ofthe oxygen of 2-naphthol upon the Co, atom of the complex A. In this case, the initial attack of the naphthol oxygen results in the formation of a ruthenium-carbene complex, which subsequently leads to the complex B via the Claisen rearrangement of the carbene complex. 

Claisen condensation equivalent, 10, 174 Claisen rearrangement equivalent, 10, 176 enolate alkylation equivalent, 10, 171 Mannich reaction equivalent, 10, 174 as strategic reaction, 10, 171 intermolecular carbene insertion, C-H activation-Cope rearrangement characteristics, 10, 177 as strategic reaction, 10, 178 tandem aldol reaction-siloxy-Cope rearrangement equivalent, 10, 181... 

The chemistry of allyloxy(methoxy)carbene (55) depends on the temperature at which it is generated.55 At 110°C, the carbene homolytically dissociated yielding mainly allylic ester. At 50 °C, the carbene dimerized and underwent a Claisen rearrangement to (56). 

Another series of publications from Ken s group compared kinetic isotope effects, computed for different possible transition structures for a variety of reactions, with the experimental values, either obtained from the literature or measured by Singleton s group at Texas A M. These comparisons established the most important features of the transition states for several classic organic reactions — Diels-Alder cycloadditions, Cope and Claisen rearrangements, peracid epoxidations, carbene and triazolinedione cycloadditions and, most recently, osmium tetroxide bis-hydroxylations. Due to Ken s research, the three-dimensional structures of many transition states have become nearly as well-understood as the structures of stable molecules. 

Very few pericyclic reactions of carbene complexes have been studied that are not in the cycloaddition class. The two examples that are known involve ene reactions and Claisen rearrangements. Both of these reactions have been recently studied and thus future developments in this area are anticipated. Ene reactions have been observed in the the reactions of alkynyl carbene complexes and enol ethers, where a competition can exist with [2 + 2] cycloadditions. Ene products are the major components firom the reaction of silyl enol ethers and [2 + 2] cycloadducts are normally the exclusive products with alkyl enol ethers (Section 9.2.2.1). As indicated in equation (7), methyl cyclohexenyl ether gives the [2 -t- 2] adduct (84a) as the major product along with a minor amount of the ene product (83a). The t-butyldimethylsilyl enol ether of cyclohexanone gives the ene product 9 1 over the [2 + 2] cycloadduct. The reason for this effect of silicon is not known at this time but if the reaction is stepwise, this result is one that would be expected on the basis of the silicon-stabilizing effect on the P-oxonium ion. 

Volume 9 deals with the majority of addition and elimination reactions involving aliphatic compounds. Chapter 1 covers electrophilic addition processes, mainly of water, acids and halogens to olefins and acetylenes, and Chapter 2 the addition of unsaturated compounds to each other (the Diels-Alder reaction and other cycloadditions). This is followed by a full discussion of a-, y- and S-eliminations (mainly olefin and alkyne forming) and fragmentation reactions. In Chapter 4 carbene and carbenoid reactions, and in Chapter 5 alkene isomerisation (including prototropic and anionotropic, and Cope and Claisen rearrangements), are discussed. 

The proposed mechanism involves an initial addition reaction to give the adduct 115 which undergoes an unusual polyhetero Claisen rearrangement to give 116. 116 then eliminates iodobenzene to generate the carbene 117 which then cycloaromatises to the selenazole 118 <01JHC50. >. 

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