Intramolecular rearrangement mechanism

The previous section outlined how rates and modes of rearrangement reactions can be determined by DNMR techniques. This section will describe in some detail the various intramolecular rearrangement mechanisms and modes which tris chelate complexes can undergo. A discussion of mechanisms must, of course, include a consideration of the experimental techniques used. Classically for kinetically slow ... 

On the basis of crossover experiments, Woerpel and Clark attributed the formation of alkynes to an intramolecular rearrangement mechanism (Scheme 7.33).101 For silver-promoted alkyne formation, insertion of the acetylene into the Ag-Si bond produced 116. Then (3 elimination of bromide occurred more rapidly than silacy-clopropene formation to generate silyl-substituted alkyne 117. Substitution of L with bromide then afforded the observed product (113a). 

The deuterium labelling established that the y, 5-unsaturated, nitrile 241 equilibrates at room temperature with the iV-allylketene imine 242 through an intramolecular rearrangement mechanism. Deuterium has been applied in the study of the novel palladium(0)-catalysed cyclization of 2,7-octadienyl carbonate containing an allylsilane moiety, 243, to product 244 (in 89%) and some 245 in the presence of phosphite 246 (equation 99). Intramolecular KlEs (Ah/Ad =3.0 and 3.5) have been observed in a bicyclic olefin formation (monoterpinene biosynthesis from [1- H,4- H2]- and [10- H2]-geranyl pyrophosphates) catalysed by pinene synthases from sage (Salvia officinalis). ... 

The mechanism of the Fries reaction is not known with certainty. One mechanism regards it as a true intramolecular rearrangement in which the acyl group migrates directly from the oxygen atom to the carbon atoms of the ring. Another scheme postulates that the ester is cleaved by the reagent... 

An intramolecular rearrangement of the conjugate acid of the triazene compound to form the oc-complex without an additional molecule of amine would correspond to a thermal [l,3]-sigmatropic rearrangement. However, such a mechanism can be ruled out on the grounds of the antarafacial pathway required from orbital symmetry considerations (Woodward-Hoffmann rules). 

As has been indicated, since there is a ring isotope effect there must be a degree of C-H bond breaking in the transition state of the rate-determining stage. Clearly further work is required in this system before a definitive mechanism can be established for the intramolecular rearrangement. 

Evidence for this mechanism is that optically active PhCHDCHs labeled in the ring with C and treated with GaBr3 in the presence of benzene gave ethylbenzene containing no deuterium and two deuteriums and that the rate of loss of radioactivity was about equal to the rate of loss of optical activity." The mechanism of intramolecular rearrangement is not very clear. The 1,2 shifts of this kind have been proposed " ... 

This mechanism is exactly analogous to the allylic rearrangement mechanism for nucleophilic substitution (p. 421). The UV spectra of allylbenzene and 1-propenylbenzene in solutions containing NH2 are identical, which shows that the same carbanion is present in both cases, as required by this mechanism. The acid BH protonates the position that will give the more stable product, though the ratio of the two possible products can vary with the identity of BH". It has been shown that base-catalyzed double-bond shifts are partially intramolecular, at least in some cases. The intramolecularity has been ascribed to a conducted tour mechanism (p. 766) in which the base leads the proton from one carbanionic site to the other ... 

Isocyanides, when heated in the gas phase or in nonpolar solvents, undergo a 1,2-intramolecular rearrangement to nitriles RNC — RCN. In polar solvents the mechanism is different. 

The sequence of reactions (58) and (59) corresponds to the mechanism proposed for chain transfer with benzene (see p. 142, footnote). The experimental result of Price and Read does not, however, rule out the possibility that the inhibitor radical (e.g., IV, or a possible successor formed by intramolecular rearrangement) may occasionally add monomer, thus giving rise to limited copolymerization of the retarder. 

One proposed mechanism involved an intramolecular rearrangement, while a second involved a free radical chain mechanism composed of the following sequence of elementary reactions ... 

Disproportionation mechanisms have been proposed for protonahon reachons and intramolecular rearrangements (see Sects. 4.3.2 and 4.3.3) [54]. The prominent feature is that follow-up processes at the level of the dianion can already take place at potentials corresponding to radical anion formation. In order to evaluate data for disproportionation reactions it is necessary to know the value of the disproporhonahon equilibrium constant. 

In the intramolecular mechanism, the ligand L —Lj rearranges its mode of coordination in M(L — Lj) without severence from the metal M. Intramolecular rearrangements dominate the field. 

Isomerizations are important unimolecular reactions that result in the intramolecular rearrangement of atoms, and their rate parameters are of the same order of magnitude as other unimolecular reactions. Consequently, they can have significant impact on product distributions in high-temperature processes. A large number of different types of isomerization reactions seem to be possible, in which stable as well as radical species serve as reactants (Benson, 1976). Unfortunately, with the exception of cis-trans isomerizations, accurate kinetic information is scarce for many of these reactions. This is, in part, caused by experimental difficulties associated with the detection of isomers and with the presence of parallel reactions. However, with computational quantum mechanics theoretical estimations of barrier heights in isomerizations are now possible. 

Treatment of cycloadduct 287, obtained from a dibenzo thiazinylium intermediate and a diene, with base affords dibenzo[d,/]pyrrolo[2,l-lr][l,3]thia-zepine 290 in 58% yield. The proposed mechanism involves formation of ylide intermediate 288 which undergoes intramolecular rearrangement into dihydro derivative 289 and spontaneous oxidative aromatization (Scheme 62 (1999TL95)). 

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