Cross-over experiments

Both the olefin stereochemistry observed and the results of cross-over experiments (added m-chlorobenzaldehyde) confirm the faster rate of reversibility (to ylide and aldehyde) of cis-(34) compared to trans-(34) and indicate that this difference is greater than earlier work had suggested. Even more interesting is the observation of a synergistic effect (leading to excessively enhanced amounts of (E)-alkene) when deliberately prepared mixtures of ervthro- and threo-(33) are decomposed. The volumes of activation... 

In general, the intestinal perfusion technique has proved a powerful research tool, despite these shortcomings. Its primary application has been in the estimation of Peft the model also lends itself to comparing permeability differences from one site to another along the intestine, an essential prerequisite for accurately classifying controlled release products [59]. Cross-over experiments tend... 

Cross-over experiments suggest that the homocoupling reaction occurs between an alkenyl telluride and an alkenyl Pd species, which is formed via the migration of an alkenyl moiety from Te to Pd. 

Irradiation of the 1,2,3-benzotrithiole 2-oxide (41) with a high-pressure mercury lamp afforded the corresponding 1-oxide (111) <93TL673>. This rearrangement, which was also observed for a number of related benzotrithiole 2-oxides, was shown to be an intramolecular process by cross-over experiments using 0-labeled materials. 

The reactions on Rh/Ir usually proceed via oxidative addition to give hydrido (alkynyl) complexes, which then undergo 1,3-H shifts to form the vinylidene complexes. In general, a unimolecular mechanism has been considered to be operative. Recent studies of RhCl(PPr 2R)2 (R = C=NCBu =CHNMe) complexes have shown a remarkable acceleration of the isomerization, with the =C=CHBu complex being formed within seconds [32]. Suitable cross-over experiments showed that a bimolecular mechanism, earlier suggested by some experimental and computational results [33], did not operate. 

A simple and straightforward way to distinguish between a direct insertion process and one going through free radicals is by a cross-over experiment. Cyclohexane and cyclohexane-di2 are often used as the probe for crossover, and hence the reactive multiplicity of the carbenes in question (Scheme 9.8). 

Schotten-Baumann benzoylation or use of an acid chloride in pyridine rapidly causes O-acylalion giving 4-acyloxythiocoumarins (37) in high yield. These derivatives undergo the Fries rearrangement to give the 3-acyl derivatives (38), either by the action of hot pyridine (quantitatively), aluminum chloride, polyphosphoric acid, or even partially by sublimation. Cross-over experiments indicate that a mixture of intra- and intermolecular mechanisms is involved, the products 38 also being available directly from 4-hydroxythiocoumarin. 

A-Dialkylaminomethylbenzotriazoles usually exist in the crystalline state solely as the N(l) isomers, but in solution they form equilibrium mixtures of the N(l) (209, 210) and N(2) (211) isomers <75JCS(Pl)l 181, 87JCS(Pi)2673). The N(l) and N(2) isomers are of nearly equal stability in nonpolar solvents and in the gas phase (2 1 ratio on statistical grounds). Polar solvents favor the 1- and 3-substituted forms over the 2-substituted, and conversely substituents at positions four and seven favor the 2-substituted form. The interconversion of these N(l) and N(2) isomers proceeds intermolecularly, as demonstrated by cross-over experiments, by a dissociation-recombination mechanism involving the formation of intermediate iminium cations and the benzotriazole anion. 

The Skraup-Doebner-Von Miller synthesis of quinolines - involving condensation of an aniline with an ,/i-unsalurated ketone - has been investigated using 13C-labelled ketones in cross-over experiments a complex fragmentation-recombination mechanism involving imine intermediates is indicated.17... 

Reaction with letramethoxyetkylene. In a reaction that is first order with respect to each of the reacUnts, tetramethoxyethylenc (I) and tosyl azide (2) yield the imido-carbunate (3). These data, as well as cross-over experiments with isotopically labeled (I), exclude any dissociation of (1) into two molecules of dimethoxycarbcnc under the conditions applied. ... 

The migration can be either intramolecular or intermolecular, and these can be distinguished by cross-over experiments. Aryl groups tend to migrate more easily than alkyl groups. 

In order to account for the origin of the enantioselectivity and diastereoselectivity of benzylidene transfer, it is necessary know whether the sulfur ylide reactions are under kinetic or thermodynamic control. From cross-over experiments it was found that the addition of benzylsulfonium ylide to aldehydes was remarkably finely balanced (Scheme 9) [28]. The trans-epoxide was derived directly from irreversible formation of the anti-betaine 4 and the cis-epoxide was derived from partial reversible formation of the syn-betaine 5. The higher transselectivity observed in reactions with aromatic aldehydes compared to aliphatic aldehydes was due to greater reversibility in the formation of the syn-betaine. 

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