Hydrogen structures intramolecular reactions

The ene reaction has proved to be particularly powerful in synthesis when carried out intramolecularly. The usual increase in rate for an intramolecular reaction allows relatively unreactive partners to combine. Thus the diene 6.13 gives largely (14 1) the cis disubstituted cyclopentane 6.15 by way of a transition structure 6.14. It is important to recognize that the selective formation of the ci j-disubstituted cyclopentane has nothing to do with the rules for pericyclic reactions. It is a consequence of the lower energy when the trimethylene chain spans the two double bonds in such a way as to leave the hydrogen atoms on the same side of the folded bicyclic structure. This... 

On a molar basis, most organic compounds contain similar amounts of hydrogen and carbon, and processes involving transfer of hydrogen between covalently bound sites rank in importance in organic chemistry second only to those involving the carbon-carbon bond itself. Most commonly, hydrogen is transferred as a proton between atoms with available electron pairs (l), i.e. Bronsted acid/base reactions. The alternative closed shell process, hydride transfer or shift, involves motion of a proton with a pair of electrons between electron deficient sites (2). These processes have four and two electrons respectively to distribute over the three atomic centres in their transition structures. It is the latter process, particularly when the heavy atoms are both first row elements, which is the subject of this review. The terms transfer and shift are used here only to differentiate intermolecu-lar and intramolecular reactions. 

Hydrogen abstraction by triplet carbonyl compounds has been the most widely studied excited state reaction in terms of structural variations in reactants. Consequently, the most detailed structure-reactivity relationships in photochemistry have been developed for hydrogen abstraction. These correlations derive from studies of both bimolecular reaction and intramolecular reactions. The effects of C—H bond strength and the inductive and steric effects of substituents have been analyzed. The only really quantitative comparisons between singlets and triplets and between n,n and 71,71 states have been provided by studies of photoinduced hydrogen abstractions. 

State [5] of the latter reaction. Reaction (48) might be believed to model properly the effect of the oo-bromo substituent on the nucleophilicity of the COj group. Unfortunately, even in this case the model does not work properly. In fact, the effect on the nucleophilicity is not only related to the effect of the bromine (relative to hydrogen) on the chemical potential of the initial state, but also on that of the transition state. Now it is clear that the situation met when the CO 2 nucleophile reacts with an external electrophile bears no relation to that met in the intramolecular reaction, where the bromine acts as a substituent in the initial state, but is the leaving group in the transition state. In other words, the useful distinction between a reacting and a non-reacting part of the molecule, on which many discussions of structural effects on reactivity... 

A number of recent reviews exist about intermolecular and intramolecular reactions of the iV-acyl-iminium intermediate. Moreover, detailed accounts of the application in alkaloid synthesis have recently appeared. This chapter deals with reactions of species (1) with nucleophilic alkenes (and alkynes). Other synthetically useful nucleophiles like aromatic rings, active methylene compounds and organome-tallics will not be discussed here. In (1) R, and R are hydrogen or carbon substituents, and R may also be a hetero substituent, such as alkylamino or alkoxy. This chapter differs from previous reviews, as the material is ordered here on the basis of the structural features of the A -acyliminium intermediate. Major emphasis is placed on recent developments and stereochemical details. 

Corrections for H-atom tunnelling are applied for the intramolecular hydrogen atom transfer reactions of the transition state structures using Wigner 2" order correction [198]. In this study the rate constants of three of our calculated transition states, TSPH OOH and TSC C4DO and TSCDCC DO identified in Table 6.3, are corrected for H-atom tunnelling. 

A major mass loss was observed above 400 °C (-59%) and is due to breakdown of the polymer backbone leading to the formation of ammonia, carbon monoxide, carbon dioxide, hydrogen, hydrogen cyanide gas, methane and other higher hydrocarbons. A char residue of 15% at 600 °C and 7% at 800 °C was obtained and, may be due to the formation of a crosslinked structure by reaction of the imine intermediate (inter as well as intramolecular) (Equation 5.16) ... 

It is reported that unless special precautions are taken for the vacuum distillation of crude -ionylideneacetaldehyde extensive rearrangement is observed and the tricyclic ketone (327) may be isolated in about 50% yield. " The formation of (327) can be rationalized by a scheme involving successive E-Z isomerization, [1, 5] shift of aldehyde hydrogen, and intramolecular cycloaddition of the unsaturated ketene so formed to the cyclohexene part of the molecule. A second product is formed in the course of the reaction and the compound (328) is a tentative suggestion for its structure since a second mode for the ketene-cyclohexene addition is possible. 

The intramolecular hydrogen atom abstraction by a polymer peroxy radical (POO ) can take place unless six-membered (or larger) rings are formed in the transition state. It is obvious that polymer segment conformation will determine the probability of formation of a transition complex of the optimum structure. Since structural relaxation is slow in a polymer matrix and the local mobility depends on the local conformation of a segment of macromolecule, the kinetics of intramolecular reactions will be influenced by the segment conformation of the macromolecule. 

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