Hydrogen-transfer step

As an explanation of the preferred formation of pyrrolidines as compared to lower and higher membered heterocyclic rings, the necessity of a nearly linear arrangement of the involved centers in the hydrogen transfer step and a minimum of nonclassical strain in a cyclic 6-membered chair-like intermediate was postulated although the experimental evidence is not conclusive. 

In outline of what follows we will begin by brief reference to previous work on coal liquefaction. The present approach will then be motivated from considerations of coal structure and hydro-gen-donor activity. A theoretical section follows in the form of a pericyclic hypothesis for the coal liquefaction mechanism, with focus on the hydrogen transfer step. Experiments suggested by the theory are then discussed, with presentation of preliminary results for hydrogen transfer among model substrates as well as for the liquefaction of an Illinois No. 6 coal to hexane-, benzene-, and pyridine-solubles by selected hydrogen donors. 

An illustration of how the overall pericyclic mechanism might apply to the decomposition of 1,2 diphenylethane, a model substrate, in the presence of A1-dihydronaphthalene, a model hydrogen-donor, has recently been given (17). In the present work, attention is focussed on the hydrogen-transfer step. 

Details of the various steps which will depend on the substrates and donors involved, are usually not well understood. Prior coordination of the donor followed by that of the substrate, equivalent to a hydride route (Section II,A), is also possible (494, 496). Formation of intermediate dihydrides from a donor (e.g., from an alcohol via oxidative addition to give a hydrido-alkoxide, and then /8-hydrogen transfer) has also been invoked (491, 492, 496, 499, 500) in mechanistic terms, the hydrogenations then become equivalent to using molecular hydrogen for the reductions. The /3-hydrogen transfer step is usually considered rate-determining (494, 496). 

As with pseudo-a-cleavage, the suppression of SHT and DHT by N-alkylation reflects the reduced tendency of ionized secondary and especially tertiary amines to undergo the initial hydrogen transfer steps to form DIs44. This common trend also supports the contention that ionized amines generally isomerize via DIs. 

Scheme 4. Tishchenko reaction written in terms of insertion and j8-hydrogen transfer steps...

Interestingly, when a fi-substituted alcohol is used in the Barton-McCombie reaction and if a [3-elimination process occurs faster than the hydrogen transfer step, then the formation of a double bond is observed. We have just seen such an example with a dixanthate (see Section 3.1.3). Many others are known as in [3-hydroxy sulfides [231] and [3-hydroxysulfones [232,233] in a modified Julia synthesis of olefins. 

The role of alkali metal cations in the [ RuCL(p-cymene) 2l-pseudo-dipepLide-catalysed enantioselective transfer hydrogenation of ketones with propan-2-ol has been examined. Lithium salts were shown to increase the enantioselectivity of the reaction when 2-PrONa or 2-PrOK was used as the base. An alternative reaction mechanism for the pseudo-dipeptide-based systems, in which the alkali metal cation is an important player in the ligand-assisted hydrogen-transfer step, has been proposed.370... 

Jardine and McQuillin (31) believed that hydrogen transfer [steps (c) and (d) of Scheme 1 ] was rate-controlling in their work, whereas Hussey et al. (27) emphasize that hydrogen diffusion through the solution can be rate-controlling as found by Maurel and Tellier (29). 

An alternative to the Maitlis scheme was provided by Gaube and Klein (37) (Scheme 5). In this mechanism too, CH2" is the key intermediate that is inserted into the chain. The growing chain is an alkylidene species. The sequence is terminated by desorption of an alkene this step competes with the hydrogen transfer step that generates the alkenyl intermediate necessary for chain growth. Neither this mechanism nor the Maitlis mechanism has yet been investigated computationally. 

We observed that vinyl radicals show exquisite reactivity for efficient and chemoselective H-abstraction [64]. The resulting translocated radical can then undergo various types of inter- or intramolecular transformations. Interestingly, we have also shown that the hydrogen transfer step can serve as a driving force for the unfavorable 4-exo-dig mode of cychzation [65]. 

The kinetic isotope effect (kw/ko = 7.1) observed when AcrH2 is replaced by AcrD2 is ascribed to this hydrogen-transfer step [218]. 

Support for the bound free-radical hypothesis (i.e., Scheme 3) comes from a variety of sources and has been summarized in several comprehensive reviews [5, 6, 26-30]. Early evidence was obtained from electron spin resonance (ESR) [31] and ultraviolet (UV) [32] spectroscopic results. Additionally, labeling experiments provided evidence of hydrogen transfer between the substrate of a variety of Bi2-dependent enzymes and the coenzyme [33, 34]. Although objections have been raised to the hydrogen-transfer step [29] on the basis that the abstractions are proposed to take place at relatively unactivated positions, calculations of the thermodynamics of the hydrogen-transfer steps support this mechanistic proposal [21, 35]. 

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