Addition, intermolecular mechanism

For condensed species, additional broadening mechanisms from local field inhomogeneities come into play. Short-range intermolecular interactions, including solute-solvent effects in solutions, and matrix, lattice, and phonon effects in soHds, can broaden molecular transitions significantly. 

Although the high reactivity of metal-chalcogen double bonds of isolated heavy ketones is somewhat suppressed by the steric protecting groups, Tbt-substituted heavy ketones allow the examination of their intermolecular reactions with relatively small substrates. The most important feature in the reactivity of a carbonyl functionality is reversibility in reactions across its carbon-oxygen double bond (addition-elimination mechanism via a tetracoordinate intermediate) as is observed, for example, in reactions with water and alcohols. The energetic basis... 

Jug and co-workers investigated the mechanism of cycloaddition reactions of indolizines to give substituted cycl[3,2,2]azines <1998JPO201>. Intermediates in this reaction are not isolated, giving evidence for a concerted [8+2] cycloaddition, which was consistent with results of previous theoretical calculations <1984CHEC(4)443>. Calculations were performed for a number of substituted ethenes <1998JPO201>. For methyl acrylate, acrylonitrile, and ethene, the concerted [8+2] mechanism seems favored. However, from both ab initio and semi-empirical calculations of transition states they concluded that reaction with nitroethene proceeded via a two-step intermolecular electrophilic addition/cyclization route, and dimethylaminoethene via an unprecedented two-step nucleophilic addition/cyclization mechanism (Equation 1). 

There is recent evidence that stabilization to elevated temperatures (over 350°C) yields a structure with additional intermolecular cross-linking that results in improved mechanical properties in carbonized fibers [10,11], In addition, it has been noted that the addition of ammonia to the stabilizing environment accelerates stabilization [12],... 

For neutral nucleophiles (e.g. amines, alcohols, water) there is much evidence that the addition-elimination mechanism depicted in equation 1 fits very well most of the intermolecular and intramolecular nucleophilic displacements involving nitro-activated aromatic substrates1. 

Structure can be described as a four-membered ring (TS in Scheme 8). The inclusion of a second equivalent of CHsMgCl, corresponding to an intermolecular mechanism, decreases the barrier height, and the process can be considered as an assisted intermolecular mechanism the first equivalent forms the chelate structure and the second CHsMgCl carries out the nucleophilic addition to the carbonyl group. The most favorable pathway corresponds to an intermolecular mechanism via an anti attack. Analysis of the results reveals that... 

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... 

Campbell, E. S., and Mezei, M., Use of a non-pair-additive intermolecular potential function to fit quantum-mechanical data on water molecule interactions, J. Chem. Phys. 67, 2338-2344 (1977). 

The fundamental differences between these two mechanisms are that 1) the jr-allyl metal hydride mechanism involves a 1,3-hydrogen shift while the metal hydride addition-elimination mechanism involves a 1,2-hydrogen shift and 2) the hydrogen shift in the Jt-allylhydride mechanism proceeds in an intramolecular fashion while that in the metalhydride addition-elimination mechanism proceeds in an intermolecular fashion. 

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]. 

Clearly, the equilibration of the carbon atoms is due to rapid intermo-lecular silyl transfer. Of course, these results do not rule out a simultaneously occurring slow dyotropic process. The nature of the intermolecular mechanism is uncertain. Compound 43 is highly hygroscopic. Trace impurities such as moisture may cause rapid desilylation, resilylation setting in at different oxygen atom. However, addition of catalytic amounts of water or squaric acid itself has no influence on the reaction rate 62). Alternatively, desilylation may proceed according to Scheme 5. Unfortunately, studies of rate dependency on concentration are not possible for dynamic C—NMR at present. 

The fracture behavior of WPC and its fracture mechanism were investigated by Jeong [104]. As it was outlined in the previous sections, the interfacial fracture, fiber pull out, and fiber breakage are the main fracture mechanisms of AAfPC. Additionally, intermolecular cleavage may occur with cracks running across fiber, interfacial area, and polymer. The fracture surface of WPC specimens, captured by SEM are illustrated in Figure 17.8. 

Studies clarifying the mechanism of the intramolecular cyclization reactions were described in the paper. They showed that these reactions involve rate-limiting oxidative addition of the Pd(0) to the aryl halide and that both the base-induced formation and the reductive elimination from arylpalla-dium enolate intermediates were faster than the oxidative addition. A mechanism was proposed by the authors (Scheme 8.41). Lee and Hartwig [77] also demonstrated that the Pd(OAc)2/PCyj catalyst system can afford the combined intermolecular and intramolecular arylation reaction of Af-methyl-ortfro-bromoacetanilide to form 3-aryloxindoles in good yields (Scheme 8.42). 

Inclusion of an additional water molecule leading to a six-membered cycle, as in the malonaldehyde XI, provides more favorable steric conditions in XXI, XXII for proton transfer. Thus, for formamidine hydrate the calculated barrier of tautomerization XXIa XXIIb, when intermolecular mechanism is operative, (17 kcal/mol with the 3-21G and 21 kcal/mol with the 6-31G basis set) is three times lower than in the case of the intramolecular reaction Xllla Xlllb (see Sect. 9.2). 

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