Intramolecular competition experiment

Recent comparative studies (intermolecular competition experiments using two different bismuth(V) oxidants and intramolecular competition experiments with unsymmetrically substituted bismuth(V) oxidants [80, 81]) revealed that the substituents affect the triarylbismuth(V) oxidants as follows ... 

The cooperative nature of two similar or different meta-related DMG enables DoM at the in-between site thus constituting a different route to continuous 1,2,3-substitution patterns (7) [9]. The analogous meta and para-related DMG isomers provide routes, most of which have not been tested, to other patterns which are a function of their relative hierarchy qualitatively established by inter-or intramolecular competition experiments (Scheme 3) [5],... 

Dolbier and Dai 43> found through intramolecular competition experiments with 1,1-dideuterioallene and acrylonitrile at 206—225 °C that the carbon bearing deuterium atoms was found predominantly at the exocyclic position. The observed product ratio, interpreted in terms of a partitioning of an intermediate, gave Ah/ d 1.13—1.21. 

We have found through intramolecular competition experiments, reacting mixtures of allene and tetradeuterioallene with acrylonitrile at 200 °C to low conversion, a slight kinetic preference for the unlabeled allene Ah/ d = 1.06 7h No conscionable manipulation of statistical factors or postulation of probable errors can make this ratio equivalent to the 1.13—1.21 observed by Dolbier and Dai, and therefore the overall reaction... 

A ketone added to the aged solution is reduced effectively, but a carboxylic acid or ester is not reduced. This weak hydride donor is thus useful for the selective reduction of a keto acid to the corresponding hydroxy acid. Both intermolecular 2md intramolecular competition experiments with tetrakis-(N-dihydropyridyl)-aluminate showed that diaryl ketones are more reactive to this reagent than either dialkyl or aralkyl ketones. This relationship is the opposite of that found by H. C. Brown for reduction with sodium borohydride in isopropyl alcohol, where the order of reactivity is acetone > acetophenone > benzophenone. 

Intramolecular competition experiments using a radical clock have revealed that the 5- and 6-exo cyclizations of primary alkyl radicals bearing an acylsilane moiety are slightly faster than those of the 5- and 6-alkenyl radicals [600]. The rate of cyclization depends on the substituents on silicon - the presence of a phenyl group on silicon increases the rate, probably because of its electron-withdrawing nature. 

Mechanistic studies revealed that pinacolone acts both as solvent and the reactions mandatory oxidizing agent. Further, inter- and intramolecular competition experiments with isotopic-labeled ketones provided evidence for a precoordination... 

Elegant mechanistic studies revealed that pinacolone not only served as a solvent, but was also required as a sacrificial oxidizing agent (Scheme 9.14). Additionally, inter- and intramolecular competition experiments with isotopically labeled ketones provided evidence for a precoordination of the ruthenium catalyst by the oxygen of the aryl ketone. Thus, a mechanism was elaborated consisting of (a) coordination, (b)... 

Indirect evidence about the rate of cleavage of different elec-trofugal groups has come from intramolecular competition experiments. Thus in p-xylyl derivatives of type 7 (see Scheme 7), the rate constants for fragmentation according to Eq. 7 are in the order" k,(SiM ) > 10 k,(COj + H ) > 10, (H ). 

Isotope effects are also inconsistent with an electrophilic aromatic substitution. Thus, intramolecular competition experiments between phenyl and pentadeutero-phenyl groups led to the determination of intramolecular isotope effects kH/feo = 5.0 (135 °C) and = 6.7 (100°C) for this arylation [37]. This led to the proposal of a mechanism based on a proton-abstraction for this reaction [37], which is in line with the findings of other studies [25e. A related mechanism, in which acetate acts as the basic ligand, has been proposed for the cyclometallation of benzyUc amines with Pd(OAc)2 [38]. 

A palladium-catalyzed methylation of 2-arylpyridines with tibutylperoxyde, and related peroxides, was recently developed [47]. This transformation also involves a C—H bond-activation step by a Pd(II)R complex, although no isotope effect was observed in an intramolecular competition experiment, which suggests that a radical mechanism might be involved. 

Independently, elegant mechanistic studies by Echavarren provided support for the CMD mechanism in palladium-catalyzed direct arylation of arenes.Thus, an intramolecular competition experiment with substrate 85, revealed that the less nucleophilic, but more C-H acidic fluori-nated arene was preferentially functionalized (Scheme 22). 

This proposal has been supported by the observation of kinetic isotope effects (KIEs) on carrying out inter- and intramolecular competition experiments. 

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