Hydrogen bonded intermediates

The reaction of benzoxazine in die presence of 2,6-xylenol does not occur until 135 C, presumably because die hydrogen-bonded intermediate depicted for the 2,4-xylenol reaction (Fig. 7.19) cannot occur. All three types of linkages are obtained in diis case. Para-para methylene-linked 2,6-xylenol dimers, obtained from the reaction of 2,6-xylenol with formaldehyde, formed in the decomposition of the benzoxazine (or with other by-products of that process) dominate. Possible side products from benzoxazine decomposition include formaldehyde and CH2=NH, either of which may provide the source of methylene linkages. Hie amount of ortho-para linkages formed by reaction of 2,6-xylenol with benzoxazine is low. Ortho-ortho methylene-linked products presumably form by a decomposition pathway from benzoxazine (as in Fig. 7.18). 

The computational results show that transition structures derived from hydroperoxo Re complexes lie slightly higher in energy than those obtained for the corresponding peroxo complexes, nevertheless their involvement in the epoxidation reaction cannot be excluded. However, for neither MoVI nor Revn evidence Get alone preference) for hydroperoxo reaction pathways is as clear as for TiIV complexes. Of course, more complex mechanisms involving intermolecular proton transfer and/or hydrogen bonded intermediates may change this picture to some extent. 

DR. NORTON An excellent attempt to observe such hydrogen bonding was made recently by Fachinetti, et al. [Calderazzo, F. Fachinetti, G. Marchetti, F. Zanazzi, P. F. J. Chem. Soc., Chem. Commun. 1981, 181]. They took hydridocobalttetracarbonyl and triethylamine, and crystallized out a species which one can only describe as the tetracarbonylcobaltate of protonated triethylamine. They proposed some type of interaction between the hydrogen and a face of the cobalt tetrahedral complex, but it was clear that the interaction was almost entirely with nitrogens. The conclusion I would draw is that the complex appears to proceed directly to full protonation of the amine without any observable evidence for a hydrogen bonded intermediate. 

Substituent effects on the endocyclic cleavage of glycosides by trimethylaluminium have been explained in terms of a cyclic C-H O hydrogen-bonded intermediate. ... 

A kinetic smdy has been reported of substituent effects on the reactions of 2-phenoxy- and 2-(4-nitrophenoxy)-3-nitro-5-X-thiophenes with benzylamine and with A-methylbenzylamine in benzene as solvent. The intramolecularly hydrogen-bonded intermediate (14) is postulated. Reactions of the 5-unsubstimted thiophenes (X = H) are not base-catalysed, indicating that nucleophilic attack is rate limiting, and the more basic secondary amine shows higher reactivity than the primary... 

A cyclic, hydrogen-bonded intermediate (Fig. 11) is believed to be a precursor to dimer formation, as there are now a considerable number of solid-state structures of (hydroxy)(aqua) complexes that contain this unit (81-85). 

With solid acid catalysts, it is possible that the protonation of the carbonyl compound does not lead to a fully ionic form, but that by interaction with surface protons, a hydrogen-bonded intermediate can be formed. Such a mechanism, not differing in the other features from scheme (H), was proposed for acetaldehyde self-condensation over a... 

In mechanisms B and C, on the contrary, the iminium ion is first attacked by the carboxylate, which forms the hydrogen-bonded intermediate 20. Then substitu-... 

Role of Hydrogen-Bonded Intermediates and Transition States... 

The Eigen model requires two different hydrogen-bonded intermediates eorresponding to the two alkene produets that are formed. Eaeh intermediate leads to a different transition state, structures 2 and 3 drawn in Scheme 2. The former, 2, leads ultimately to 1-methylcyclopentene as the final product. This transition state has no overall molecular symmetry, but retains the methyl rotor as well as free rotation about the carbon-nitrogen single bond for a net symmetry number a = 9. Because transition state 2 is chiral, the reaction path degeneracy is equal to (18/ ) = 4. 

Fig. 15.10. The two pathways for motion from the [HO-CHs-F] central barrier to the CH3OH + F reaction products. Most of the trajectories follow the direct dissociation path. A small amount, roughly 10%, form the CHsOH-F hydrogen-bonded intermediate and follow an indirect path. Adapted from Ref. [121].

The hydrogen-bonded intermediate complex of a hindered phenolic antioxidant with a stable aminoxyl radical (TEMPO), used as a model for a hydrocarbon oxidant, has been isolated and its structure determined, as confirmation of the radical scavenging mechanism (79). 

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