Hydrogenations 1,3 -Hydrogen atom shifts

There are no proven examples of 1,2-hydrogen atom shifts this can be understood in terms of the stereoelectronie requirements on the process. The same limitations are not imposed on heavier atoms (e.g. chlorine). The postulate309 that ethyl branches in reduced PVC are all derived from cbloroethyl branches formed by sequential 1,5-intramolecular hydrogen atom transfers as described for PE (Section 4,4.3.1) has been questioned.,6,6 It has been shown that many of these ethyl branches are derived from dichloroethyl groups. The latter are formed by sequential 1,2-chlorine atom shifts which follow a head addition (Section 4.3.1.2). 

Although this reaction appears to involve only two electrons, it was shown by Mulder [57] that in fact two n and two a electrons are required to account for this system. The three possible spin pairings become clear when it is realized that a pair of carbene radicals are formally involved, Figure 14. In practice, the conical intersection defined by the loop in Figure 14 is high-lying, so that often other conical intersections are more important in ethylene photochemistry. Hydrogen-atom shift products are observed [58]. This topic is further detailed in Section VI. 

Methyl 3,4,5,6-tetra-0-acetyl-2-deoxy-2-diazo-D-arabino-hexonate (84) has been irradiated in methanol and in 2-propanol.177 In methanol, the only photoproduct is the enol acetate 85 however, irradiation in 2-propanol results in formation of minor proportions (6%) of 85 and the alkene 86 (7%), but the major product is the deoxy sugar 87 (61%). The difference in reactivity of 84 in these two solvents is probably a reflection of the difference in the ability of methanol and 2-propanol to function as hydrogen donors when reacting with a carbene (see Scheme 31). In methanol, a 1,2-hydrogen atom shift to the divalent... 

Hydrogenation hydroperoxides, 692 see also Enthalpies of hydrogenation 1,2-Hydrogen atom shift, hydrogen peroxide, 97... 

Irradiation of Formula 428 gives the isomeric hydrocarbon (Formula 429) (189). This hydrogen atom shift may be related to those observed in... 

The other is a configurational mechanism, in which the covalent X-H bonds are broken as the hydrogen atoms shift across the line of centers to form the new hydrogen bond. This must also be a concerted process, which is only possible due to quantum mechanical tunneling ... 

High level ab initio calculations have been reported on the PES of singlet SiH2 and GeH2 reactions with water, methanol, ethanol, dimethyl ether and trifluoromethanol. Besides the classical route for EH2 (E = Si, Ge) insertion into X—O (X = H, C) bonds via 1,2 hydrogen atom shift reaction (equation 22), two new reaction channels (equations 23 and 24) were identified on each PES, except for reactions involving dimethyl ether. Equations 23 and 24 display routes for H2 elimination, following the initial formation of an association complex. 

Weighting scheme in LS refinement Treatment of hydrogen atoms Shift-to-error ratio, (A/(r)niax... 

Allylic alcohols are isomerized via direct interaction of the ruthenium atom with alcohol. /3-Elimination of ruthenium hydride from metal alkoxide yields a ruthe-nium-enone species C which undergoes insertion of the olefinic moiety into the Ru-H to form an oxyallylic intermediate D. As a result, the hydrogen atom shifts from the a- to y-position of the allylalcohol. Protonolysis of the oxyallylic species leads to a saturated carbonyl compound and cationic unsaturated species, [CpRu(PPh3)2] A. 

Alkynes containing methylene (—CHi—) functionalities a to the triple bond readily undergo thermal C-H activation under conditions necessary to initiate reaction with group VIII metal carbonyls (54). The allenyl clusters obtained isomerize thermally via a 1,2-hydrogen atom shift to afford the thermodynamically more favorable 1,3-dimetalloallyl clusters (Table 1) (49,55). A similar chemistry has been recognized for osmium clusters, albeit under more severe conditions. 

Analogies between free radicals and carbocations have been drawn several times, but one of the typical reactions of carbocations, the concerted 1,2-shift, does not occur in free radicals. A concerted 1,2-radical shift is allowed only when one of the components can react antarafacially. The 1,2-hydrogen atom shift is geometrically impossible, and the geometric requirements of the 1,2-alkyl radical shift are so stringent that it is not observed. 

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