Isomerization intermolecular mechanism

Bromination can be conveniently effected by transfer of bromine from one nucleus to another. As the Friedel-Crafts isomerization of bromoaromatic compounds generally takes place through an intermolecular mechanism, the migrating bromine atom serves as a source of positive bromine, thus effecting ring brominations (161,162). 2,4,6-Tribromophenol, for example, has been prepared by bromination of phenol with dibromobenzene. 

There are basically two types of mechanisms which can explain rearrangement of linkage isomers. In the intermolecular mechanism, the ligand giving rise to linkage isomerism (L-L, with two donor centers) dissociates from and recombines with the metal M ... 

Studies with sulfated zirconia promoted with Pt309 and industrial chlorinated Pt on AI2O3 isomerization catalysts310 led to the same conclusion, namely, the intermolecular mechanism operative for M-butane isomerization. A significant difference, however, is that on the industrial catalysts extensive hydride and methyl shifts taking place in the intermediates prior to P scission do not lead to a random distribution of the labels. Instead, a binomial distribution with one and three 13C atoms is observed.310 This is indicative of the involvement of the 31 carbocationic intermediate. 

Indeed, isomerization of 1-AMN into 2-AMN was found to be much faster in the presence of 2-MN than in its absence. Furthermore, this intermolecular mechanism was confirmed by investigating the transformation of a mixture of 1-AMN with a deuterated methoxy group (OCD3) and of nondeuterated 2-MN.26... 

Fig.9.3 Mechanisms of xylene isomerization- a Intramolecular mechanism (1,2 methyl shifts) b Intermolecular mechanism via disproportionation and transalkylation steps...

Isomerization of I into II is much faster in presence than in absence of 2MN, which suggests an intermolecular mechanism for this reaction (transacylation) ... 

The transalkylation and isomerization reactions can be satisfactorily explained by the Streitwieser mechanism( ). This mechanism proposes a 1,1-diphenylethane-type intermediate. For example, para-diethylbenzene. (Figure 3) Such an intermolecular mechanism is consistent with the experimental data and does not require the assumption of a sequence of intramolecular 1,2 shifts. The decay of the polyethylbenzenes towards equilibrium is consecutive and not concurrent. The catalyst seems to be associated with the most basic center and when it reaches steady-state, the catalyst transfers to the next most basic one. There is also a concurrent intramolecular isomerization such as 1,2,4 triethylbenzene going to 1,3,5 triethylbenzene. There is hence a movement towards isomer equilibrium as well as product equilibrium. 

By analogy with thermal processes (92), photoinduced isomerization s may occur either by intramolecular or intermolecular mechanisms. Intramolecular isomerizations are further classified either as "twisting" mechanisms, which involve no metal-ligand bond breaking, or as bond-rupture mechanisms. The latter term applies to reactions of chelate compounds in which one of the ligands is for a short time partially dissociated. Intermolecular mechanisms involve species other than those that make up the reactant complex. 

An intermolecular mechanism involving a primary ligand photodissociation step has been proposed for the cis trans isomerization of the dicarbene complexes, M(C0) L2, (M = Cr,Mo,... 

The intermolecular mechanism of the isomerization reaction is further detailed by experiments with cumene labelled in the alkyl chain. 

The rate of the (uncatalysed) isomerization of cw-[Pt(PEt8)2(o-tolyl)Q] to its /ranr-isomer is decreased by adding chloride ion, which indicates a mechanism involving reversible loss of chloride ion followed by stereochemical reorganization in the three-co-ordinate intermediate. Theoretical calculations support an earlier suggestion that photochemical cis-trans isomerization of [Pt(gly)2] takes place by a torsional-twist mechanism, whereas thermal isomerization has an intermolecular mechanism. 

Perfluoro-l,4-pentadiene has been polymerized via an intra-intermolecular mechanism. Simultaneous isomerization to the 1,3-diene occurred. The latter copolymerized via the usual 1,4-addition, leading to a final polymer of structure [18] (50,163). 

The ring cleavage of 3-aryl-2-substituted-2//-azirines by molybdenum hexacarbonyl has been described earlier in regard to the synthesis of pyrroles, pyrazoles and isoxazoles. In contrast to this behavior, analogous reactions of 2-unsubstituted derivatives lead to the formation of mixtures of 2,5-diarylpyrazines (139) and isomeric 3,6- and 1,6-dihydropyrazine derivatives (140,141) (Scheme 163).47,53 It is possible that the pyrazine products are formed by an intermolecular nitrene mechanism akin to the intramolecular processes described earlier (see Scheme 22 in Section IV,A,1). 

Transition states for betaine isomerization to ylides via the intramolecular mechanism were not localized. We believe that these processes are intermolecular and involve donor solvent molecules or the second betaine molecule as a proton carrier. 

The intermolecular Heck reaction of halopyridines provides an alternative route to functionalized pyridines, circumventing the functional group compatibility problems encountered in other methods. 3-Bromopyridine has often been used as a substrate for the Heck reaction [124-126]. For example, ketone 155 was obtained from the Heck reaction of 3-bromo-2-methoxy-5-chloropyridine (153) with allylic alcohol 154 [125]. The mechanism for such a synthetically useful coupling warrants additional comments oxidative addition of 3-bromopyridine 153 to Pd(0) proceeds as usual to give the palladium intermediate 156. Subsequent insertion of allylic alcohol 154 to 156 gives intermediate 157. Reductive elimination of 157 gives enol 158, which then isomerizes to afford ketone 155 as the ultimate product This tactic is frequently used in the synthesis of ketones from allylic alcohols. 

Examples of catalytic formation of C-C bonds from sp C-H bonds are even more scarce than from sp C-H bonds and, in general, are limited to C-H bonds adjacent to heteroatoms. A remarkable iridium-catalyzed example was reported by the group of Lin [116] the intermolecular oxidative coupling of methyl ethers with TBE to form olefin complexes in the presence of (P Pr3)2lrH5 (29). In their proposed mechanism, the reactive 14e species 38 undergoes oxidative addition of the methyl C-H bond in methyl ethers followed by olefin insertion to generate the intermediate 39. p-hydride elimination affords 35, which can isomerize to products 36 and 37 (Scheme 10). The reaction proceeds under mild condition (50°C) but suffers from poor selectivity as well as low yield (TON of 12 after 24 h). 

In this article, the features and mechanism of the crystal-to-crystal reactions of 1,3-diene compounds are described on the basis of the molecular packing structure and intermolecular interactions in the crystals for starting materials and products. The dimerization and isomerization of unsaturated compounds as well as addition polymerization via a chain reaction mechanism are ideal sohd-state reactions, because they produce no leaving group during the reac-... 

The photoinduced -elimination of 1,2,3-triazole from 1-(A,A-bisacyl)amino-l,2,3-triazoles (142), itself formed from the photochemical isomerization of triazoles (141), proceeds either via an intra-or intermolecular hydrogen abstraction or electron-transfer mechanism followed by homolytic cleavage of the A,A-bond (path a) or via t -assisted )8-cleavage of the same weak bond (path b). The composition of the products suggests that in all cases a c-type 1,2,3-triazolyl radical (143) is eliminated which is further quenched by hydrogen abstraction as shown in Scheme 24 <93JHC1301>. 

Remarkable enhancements of the unimolecular c-t isomerization of c-S with p-MeO and oxidation of S with -MeO are explained by charge-spin separation in such S Unimolecular c-t isomerization of such c-S proceeds with a chain mechanism, while regioselective oxidation occurs in such S because of the spin localization. Cycloreversion of t,c,t-TPCB occurs to give a a-St 2, while the photochemical cycloreversion of TPCB and t,t,t-TPCB gives Tr-St 2 and t-St /t-St pair, respectively. Radical cations of phosphorus compounds (9 and 10 form intramolecular rr-dimer between two Nps from which Np 2 forms. Formation of intermolecular a-dimer of aromatic acetylene (11 - and 12 -) and intramolecular dimer of 13 and diarylmethanoT was observed, and the n = 3 rule is not effective for intramolecular dimer -. 

Mechanistic studies of the rearrangement activity of the ring-opening metathesis polymerization catalyst [Ru(H20)6]2+ were reported for unfunctionalized alkenes (112). The mechanism was found to be intermolecular, the alkene isomerization proceeding through an addition-elimination mechanism with a metal hydride catalytic species. This interpretation was... 

---