Intramolecular hydrogen-shift

Early experiments [1] using (+)-apopinene and deuterium showed, however, that in the isomerized molecules the deuterium content was very low and the isomerization was much faster than deuterium incorporation into the allylic position. Therefore it seemed probable that isomerization takes place through an intramolecular hydrogen shift. A sigmatropic 1,3-hydrogen shift was suggested, in which the allylic endo-H shifted top shift) [2]. 

Ab-initio Study of the Intramolecular Hydrogen Shift in Nitromethane and its Acid-dissociated Anion... 

Table 1. Optimized geometries for the 1,3 -intramolecular hydrogen shift of nitromethane and its acid-dissociated anion... 

AN AB INITIO STUDY OF THE INTRAMOLECULAR HYDROGEN SHIFT IN NITROMETHANE... 

The rearrangment of nitromethane to aei-nitromethane via the postulated 1,3-intramolecular hydrogen shift is a high barrier reaction (barrier height of 310 kJ/mol), in agreement with the predietion based on the higher tension of four-membered ring and orbital symmetry considerations. 

Concerning the first reaction pathway reported in Scheme 2.17, o-QM undergoes an intramolecular hydrogen shift to produce HPC. 

This reaction proceeds via an intramolecular hydrogen shift, that is, insertion of the Si—P ir-bond into the C—H bond of the ortho isopropyl group of one isityl substituent. 

Another photochemically induced domino process consisting of three steps was employed for the formation of 1,2-disubstituted cyclopentanes 5-39, as described by Tietze and coworkers. Irradiation of a mixture of 5-36, dimethyl malonate and catalytic amounts of the Lewis acid Me2AlCl in a Pyrex flask caused a Norish Type I cleavage of 5-36, followed by an intramolecular hydrogen shift to give the acyclic... 

Fig. 17. 75.4-MHz 13C MAS spectra showing the dynamics of the benzenium ion 18. The spectrum at 77 K shows a static benzenium ion with four isotropic shifts. The other features in that spectrum are spinning sidebands. The benzenium ion 18 shows a single isotropic peak (147 ppm) and an axially symmetric tensor at 298 K because of rapid intramolecular hydrogen shifts.

The relative contribution of the two mechanisms to the actual isomerization process depends on the metals and the experimental conditions. Comprehensive studies of the isomerization of n-butenes on Group VIII metals demonstrated179-181 that the Horiuti-Polanyi mechanism, the dissociative mechanism with the involvement of Jt-allyl intermediates, and direct intramolecular hydrogen shift may all contribute to double-bond migration. The Horiuti-Polanyi mechanism and a direct 1,3 sigma-tropic shift without deuterium incorporation may be operative in cis-trans isomerization. 

Vinylidenes have been transferred from a variety of precursors to olefins to produce methylenecyclopropanes . Because of ready intramolecular hydrogen shifts to give terminal acetylenes, the addition of vinylidene to olefins is rather limited to 2,2-disubstituted species. The methodologies so far developed include (1) gem-dibromides, 7, with MeLi, (2) vinyl halides " or vinyl triflates, 8 " with r-BuOK, (3) the fluoride ion promoted decomposition of vinylsilanes, 92 2,243 4 thermolysis of mercuric derivatives, 10, at 250 (5) decomposition of vinylazo compounds, 11, at 25 (6) the alkaline... 

Organic peracids behave as a source of alkyl radicals and serve as a good model for studying their chemistry. The OH transfer by alkyl radicals and the stereoelectronic control of the reaction were investigated in detail. Inter- and the intramolecular hydrogen shifts by alkyl radicals were also studied. These results were used in order to undertake selective hydroxylation of hydrocarbons. Finally we turned our attention to the reactivity of electrophilic radicals towards peracids. 

For but-l-ene isomerization it was proposed that the isomerization reaction takes place by at least two different mechanisms an intramolecular hydrogen shift, and an associative mechanism... 

Table 4 also summarizes the calculated activation barriers of all the typical reactions (in Scheme 17) during the induction period over catalyst models similar to 4g, 4g-l, and 4g-2 except that both Si atoms within each model were fully fluorinated. Fluorination of the silica support for the F-modified Phillips catalyst showed negligible influence on ethylene dimerization to 1-butene and metathesis to propylene [160], However, the energy barrier was increased significantly in reaction 5 of Scheme 17, in which 1-hexene was formed from the chromacycloheptane species through a one-step intramolecular hydrogen shift. Fluorination showed a positive effect on ring expansion in reaction 4 of Scheme 17. 

An ab initio study of the intramolecular hydrogen shift in nitromethane and its acid-dissociated anion 7. Tao... 

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