3-eliminations involving cyclic transition

If the reaction between cinnamate 1 and hydroxylamines occurred through a concerted mechanism, the process should take place through a cyclic (five-mem-bered ring) transition state, similar to the ones proposed for [3+2] dipolar cycloadditions or retro-Cope eliminations. The cyclic transition state for the concerted addition of deuterated iV-methylhydroxylamine to ethyl cinnamate 1 is represented in Scheme 8.7 (atoms involved in the cyclization colored blue). As the nitrogen atom and the deuterium approach the C=C bond from the same side, the addition is syn, leading to intermediate 12. A rapid proton shift would give compound 3 which can be either isolated or cyclized in the presence of a Lewis acid to the isoxazolidi-none 4 by intramolecular transesterification. The stereochemistry of the products 3 and 4 is determined during the concerted addition step. 

Several types of compound undergo elimination on heating, with no other reagent present. Reactions of this type are often run in the gas phase. The mechanisms are obviously different from those already discussed, since all those require a base (which may be the solvent) in one of the steps, and there is no base or solvent present in pyrolytic elimination. Two mechanisms have been found to operate. One involves a cyclic transition state, which may be four, five, or six membered. Examples of each size are... 

The other SiH4 decomposition pathway involves Hj elimination via a three-center cyclic transition state (Roenigk et ai, 1987) ... 

Another important family of elimination reactions has as the common mechanistic feature cyclic transition states in which an intramolecular proton transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.16 depicts examples of the most important of these reaction types. These reactions are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic transition states dictate that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are referred to as thermal syn eliminations. 

A unimolecular elimination involving a semi-polar five-membered cyclic transition state (81) (Scheme 9, R1 = Ph, R2 = R3 = H) appears to account for the formation of benzaldehyde, CO, and H2O on eliminative fragmentation of mandelic acid in the gas phase.44 The same type of transition state has been proposed for gas-phase pyrolysis of ROCH2COOH (R = MeO, EtO, and Ph O) with corresponding formation of ROH, CO, and formaldehyde the rate of reaction is little dependent on R.45... 

Phenoxypropanoic acid, 3-(phenylthio)propanoic acid, 4-phenylbutanoic acid and the corresponding ethyl and methyl esters have been pyrolysed between 520 and 682 K.10 Analysis of the pyrolysates showed the elimination products to be acrylic acid and the corresponding arene. The thermal gas-phase elimination kinetics and product analysis have been found compatible with a thermal retro-Michael reaction pathway involving a four-membered cyclic transition state. 

The kinetics of the gas-phase elimination of 3-hydroxy-3-methylbutan-2-one have been investigated in a static system, seasoned with allyl bromide, and in the presence of the free chain radical inhibitor toluene.14 The reaction was found to be homogeneous, unimolecular and to follow a first-order rate law. The products of elimination are acetone and acetaldehyde. Theoretical estimations suggest a molecular mechanism involving a concerted non-synchronous four-membered cyclic transition state process. 

Cope elimination occurs under milder conditions than Hofmann elimination. It is particularly useful when a sensitive or reactive alkene must be synthesized by the elimination of an amine. Because the Cope elimination involves a cyclic transition state, it occurs with syn stereochemistry. 

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