Claisen rearrangements solvent effects

The Claisen rearrangement is an electrocyclic reaction which converts an allyl vinyl ether into a y,8-unsaturated aldehyde or ketone, via a (3.3) sigmatropic shift. The rate of this reaction can be largely increased in polar solvents. Several works have addressed the study of the reaction mechanism and the electronic structure of the transition state (TS) by examining substituent and solvent effects on the rate of this reaction. 

Davidson, M. M., Hillier, I. El., Hall, R. J. and Burton, N. A. Effect of solvent on the Claisen rearrangement of allyl vinyl ether using ab initio continuum methods, J.Am.Chem.Soc., 116 (1994), 9294-9297... 

Thus, ketenes (2) can react as dienophiles with (E)-l,3-diazabuta-l,3-dienes (E)-(25) to yield either [4 + 2] cycloaducts (26) or (27) depending on the participation of the C = C or C = O moieties of the ketenes (Scheme 7). Claisen rearrangement of 3,6-dihydro-2-methylene-2//-l,3,5-oxadiazines (27) yields the p-lactams (28). Alternatively, reaction between ketenes (2) and (Z)-l,3-diaza-buta-1,3-dienes (Z)-(25) leads to the usual zwitterionic intermediates (29), whose conrotatory electrocyclation leads to p-lactams (28). No computational data including solvent effects have been reported for these reactions. 

Alkenyl(phenyl)iodine(III) compounds can also serve as starting materials in rearrangements. Allenyl(aryl)iodine(III) compounds of type 86 can be synthesized from (diacetoxyiodo) derivatives 85 and propargylsilanes [145]. It depends on the leaving group ability of the aromatic substituent on iodine in 86 as to whether the reaction proceeds via nucleophilic substitution to compounds of type 87 or by an iodonio-Claisen rearrangement to compounds 88, Scheme 37 [146,147]. The easy access to propynyl compounds 87 has been shown [148] and solvent effects in these reactions have been investigated as well [149,150]. 

The orfAo-Claisen rearrangement, a no-mechanism type of reaction, is included here since it is, effectively, an alkylation reaction though not an electrophilic substitution. Two reports have recently appeared describing the rearrangement of 2-allyloxypyridines in tertiary amine solvents or in the absence of a solvent. Rearrangement... 

The cholesteric mesophase formed by cholesteryl p-nitrobenzoate at 200 °C has been used as the solvent to effect an asymmetric synthesis lrans-but-2-enyl p-tolyl ether gave the product of an ortho-Claisen rearrangement, 2-(but-1 -en-3 -yl)-4-methylphenol. This material exhibited circular dichroism, although neither the optical yield nor the configuration of the product is yet known.262 Decarboxylation of ethylphenylmalonic acid in cholesteryl benzoate at 160 °C (cholesteric liquid-crystalline phase) also proceeded with asymmetric induction to give (R)-(—)-2-phenylbutyric acid, with 18% optical yield.263 Electric dipole moments are reported for some esters of 5a-cholest-8(14)-en-3j8-ol there is some slight correlation with melting points.264... 

For this reaction, the rate was found to vary by a factor of 34 in going from the least polar ( -tetradecane) to the most polar solvent (phenol), and by a factor of 102 in going from the gas phase to the most polar solvent [153]. The small solvent and substituent effects observed suggest that a slightly dipolar activated complex must be formed during the Claisen rearrangement of this allyl aryl ether [153]. 

As was indicated previously, the effect of solvent upon the Claisen rearrangement is usually minor unless wide variations in solvent polarity are made. The effect of solvent upon the rearrangement of allyl p-tolyl ether at two slightly different temperatures is given in Tables 33 and 34. 

Acevedo, O., Armacost, K. Claisen rearrangements Insight into solvent effects and "on water" reactivity from QM/MM simulations. J. Am. Chem. Soc. 2010,132, 1966-75. 

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