Hydrogen shift, , concerted

Quantum mechanical studies of the mechanism of gold-catalysed rearrangements of acetylenic amine-A -oxides to piperidinones or azepanones show a concerted 1,5 hydrogen shift (Scheme 112). ... 

Hydrogen shifts are often observed in thermal isomerizations of vinylaziridines. Heating of compounds 221 at 180 °C produced mixture of 3-pyrrolines 222 and hydrazones 223 (Scheme 2.54) [87]. The formation of 223 can be explained in terms either of a concerted hydrogen shift as depicted in 224 or of diradical intermediates 225, both of which would be followed by thermal isomerization of the (Z)-carbon-carbon double bonds to provide the ( ) isomers 223. 

Because of the relative inertness of functional groups other than alcohols to Dess-Martin conditions, a Dess-Martin oxidation is a good choice when an in situ reaction of the resulting aldehydes or ketones is desired. It is particularly common to use Dess-Martin periodinane in order to generate very reactive aldehydes or ketones that suffer in situ concerted reactions, such as Diels-Alder additions,67 oxy-Claisen reactions,68 pericyclic processes69 and concerted hydrogen shifts.70... 

Scheme II. Possible processes to explain the fluxionality of the hydride (Structure XI) (A) concerted hydrogen shift mechanism (B) stepwise process involving a carbene intermediate.

In connection with some other work, the optically active lactone sulphoxides (83) and (84) have been observed to undergo a rapid, thermally induced racemization in boiling chloroform. Deuterium-labelling experiments indicate that the reaction probably involves the intermediacy of the sulphenic acids, e.g. (85), that are formed by a concerted hydrogen shift. Such a sigmatropic process finds analogy in the thermal isomerization of... 

The cyclohexylpyrazole (376) and the azlrlne (377) are formed by irradiation of 3-dlazo-4-methyl-5-phenylpyrazolenine (378) in cyclohexane (Scheme 35) (77JA633). The former is the result of carbene insertion into cyclohexane followed by a [1,5] hydrogen shift, whereas the latter arises by ring cleavage of nltrene (379) or by a concerted pathway. 

The 21//,22//-tautomer 2 with hydrogens at adjacent pyrrole rings is less stable because of penetration of each hydrogen into the van der Waals sphere of the other. However, NMR studies with unsymmetrically substituted porphyrins at low temperature have allowed the observation of both tautomers 1 and 2. The kinetic parameters of tautomerism investigated by NMR measurements at different temperatures are consistent with a two-step process forming 3 from 1 via 2 rather than a concerted two-hydrogen shift which could form 3 from 1 directly. 

Originally very few types of such rearrangements were known e.g., Copes rearrangement, Claisen rearrangement and some 1, 5 hydrogen shift in some dienes, but now many others have been discovered. The common feature of such reactions is that they are concerted, uncatalysed and involve a bond migration through a cyclic transition state. 

The above selection rules, therefore, predict that [1, 5] hydrogen shifts in neutral polyenes would be thermally allowed and the reaction would be facile, but thermal [1, 3] and [1, 7] shifts must go by an antarafacial process and they will be difficult to attain because of the geometric strain. This is also confirmed by many experimental observations. Thus concerted uncatalysed [1, 3] hydrogen shifts have not been seen in the diene of the following type, [1, 5] shifts are well known. 

A third symmetry-allowed transformation in which vinylallenes partidpate involves sigmatropic hydrogen shifts. As shown in Scheme 5.45, two types of these concerted processes have so far been observed. 

The formation of cyclic nitrones (150) from pericyclic mechanism. Kinetic and computational studies have provided evidence for the involvement of a novel pseudo-pericyclic electrocyclization in the conversion of o-vinylphenyl isocyanates into quinolin-2-ones. " Such reactions have also provided evidence of torquoselectivity in a 6jt system. Hash vacuum thermolysis of triazoles (151) has been found to afford dihydroquinolines (155), presumably by generation of a-oxoketenimines (152) which can undergo a [1,5]-hydrogen shift to the o-quinoid imines (153)7(154) and subsequent electrocyclization (see Scheme 57). 

The thermal decomposition of 2-azidoacetic acid (N3CH2CO2H) in the vapour phase has been shown, by photoelectron and matrix isolation infrared spectroscopy, to involve simultaneous formation of CO2 and methanimine (CH2NH) with concerted ejection of N2.52 No evidence was found for formation of intermediate nitrene (NCH2CO2H) or the imine (HNCHCO2H) to which it could be converted by 1,2-hydrogen shift. 

The degenerate [1,5] hydrogen shift reaction of (Z)-l,3-pentadiene has been studied by a number of ab intio [67-69] and semiempirical [70-72] methods. Since the energy of concert [73] for this reaction is approximately 40 kcal/mol, there is no doubt that this reaction has a concerted mechanism with an aromatic... 

The initial disclosure of a [6 + 4] cycloaddition between tropone (1) and a 1,3-dipole involved diphe-nylnitrilimine as depicted in Scheme In this instance the [6 + 4] pathway competed rather poorly with various alternative [4 + 2] pathways and only a small quantity of the adduct in which the dipole added across the 2- and 7-positions of (1) was recovered. The three [4 + 2] adducts (45)-(d7) that were isolated from the reaction mixture presumably arose from a base-catdyzed hydrogen shift that occurred subsequent to the initial cycloaddition. Efforts to account for the divergent behavior of dienes and 1,3-dipolar species in their reactions with tropone have included invoking a dipole repulsion between the large positive charge located on the central atom of the 1,3-dipole and the partial positive charge on tropone, which must come into close proximity in the transition state of a concerted [6 + 4] cycloaddition. A ca-... 

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