Crossover product

The exact mechanism has still not been completely worked out. Opinions have been expressed that it is completely intermolecular, completely intramolecular, and partially inter- and intramolecular. " One way to decide between inter- and intramolecular processes is to run the reaction of the phenolic ester in the presence of another aromatic compound, say, toluene. If some of the toluene is acylated, the reaction must be, at least in part, interraolecular. If the toluene is not acylated, the presumption is that the reaction is intramolecular, though this is not certain, for it may be that the toluene is not attacked because it is less active than the other. A number of such experiments (called crossover experiments) have been carried out sometimes crossover products have been found and sometimes not. As in 11-14, an initial complex (40) is formed between the substrate and the catalyst, so that a catalyst/substrate molar ratio of at least 1 1 is required. 

Though the mechanism has not been investigated thoroughly, it has been shown to be intramolecular by the failure to find crossover products when mixtures of boranes are used, The following scheme, involving three boron-to-carbon migrations, has been suggested. 

Evidence is the lack of a catalyst, the fact that the reaction is first order in the ether, the absence of crossover products when mixtures are heated, and the presence of the ally lie shift, which is required by this mechanism. A retro-Claisen rearrangement is... 

Many reactions have been recently reported, which involve palladium migration along an aromatic skeleton. For example, a Heck-type process led to the anticipated 3-substituted pyridyl, Heck product as well as a crossover product (Equation (157)). 

A mechanism involving a palladacycle E has been proposed to explain the formation of such a crossover product (Scheme 28).131... 

In an elegant experiment, Yu and coworkers showed that intermolecular crossover is also possible. Thus, the TMSOTf-catalyzed rearrangement of two structurally similar orthoesters 75 and 76 led to crossover products 77/78 and 79/80 (Scheme 5.20d) [66],... 

Flash vapor pyrolysis of the rf -thiophene l,l-dioxide)cobalt complexes results in extrusion of SO2 to generate (cyclobutadiene)cobalt complexes (Scheme 63)229. The absence of ligand crossover products indicates that this reaction occurs in a unimolecular fashion. Pyrolysis of the diastereomerically pure complex 240 gave the cyclobutadiene complex as an equimolar mixture of diastereomers 241a and 241b. In addition, the recovered starting material (37%) was shown to have ca 40% scramble of the diastereomeric... 

A new [2,3]-sigmatropic rearrangement of the lithium salt 192 of the Af-benzyl-0-allylhydroxylamines (191) affording Af-benzyl-A-allylhydroxylamines (193) in moderate yields was reported (equation 56). The absence of crossover products confirms the intramolecular character of the transformation and an envelope transition state is proposed. The rearrangement proceeds via a transition state where facial selectivity is determined by stereoelectronic effects. 

Thus, if a mixture of these hydrocarbons reacts with a singlet state, then the addition product will consist entirely of undeuterated (do) and deuterated(i i2) compounds. If some of the reaction occurs by the hydrogen abstraction-recombination route, then some crossed products (di and/or du compounds) will be formed. By using this technique, it has been shown that, while some of the DPC-cyclohexane adduct (34) is formed by combination of radical pairs, there is no evidence of crossover product present in the FL-cyclohexane adduct (37). The results are interpreted by assuming rapid spin state equilibration relative to the reaction of either spin state with solvent. The larger amount of singlet chemistry of FL relative to DPC can then be explained if the S-T gap is smaller in FL than in DPC. 

The SiR3 group migration seems to be an intramolecular process, since practically no crossover products were formed in the reaction of a mixture of Ru(CO)4(SiMe3)2 and Ru(CO)4(GeMe3)2 with cyclohepta-triene (17). 

Scheme 2.22 Independent pathways in the solvolysis of 3-aryl-2-butyl sulfonates (60) established by the absence of crossover product.

Nitrogen extrusion by diiron nonacarbonyl affords a mixture of two isometric carbene iron complexes (111) rather than (110). The fact that a crossover product is formed is taken as evidence for the intermediacy of selenirene and carbenes (112a,b). 

In fact, no singly labelled compounds were found NMR analysis showed that the product consisted entirely of unlabelled or doubly labelled molecules. The CO2H group remains attached to the same molecule (though not to the same atom) and the first mechanism is correct. Crossover experiments demand some sort of double labelling, which does not have to be isotopic. An example where crossover products are observed is the light-initiated isomerization of allylic sulfides. 

The crossover product, propionaldehyde-l,3-d-3- C 12, clearly demonstrated that the isomerization occurred via intermolecular 1,3-hydrogen shift. These results are consistent with a modified metal hydride addition-elimination mechanism which involves exclusive 1,3-hydrogen shift through oxygen-directed Markovnikov addition of the metal hydride to the carbon-carbon double bond (Scheme 12.2). The directing effect of functional groups on the selectivity of transition metal catalysis is well presented [9], and an analogous process appears to be operative in the isomerization of allylamines to enamines [10]. 

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