Trans-addition of hydrogen

Apparent trans addition of hydrogen to a carbon-carbon double bond has been the subject of intense investigations and several mechanism have been proposed the stereochemistry of hydrogenation of alkenes has been comprehensively reviewed by Siegel in 1966,66 by Molnar in 1983,67 and again by Bartok and Molnar in 1997,68 so only a brief history is related here. 

Klemm, Olson, and White discovered that the double bond in 117 may be reduced electrochemically with overall stereospecific trans addition of hydrogen to the double bond 120>. It was suggested that reduction proceeds by an initial two-electron reduction of the protonated starting material to carban-... 

In heterogeneous catalytic hydrogenations suprafacial (as) addition of hydrogen would be expected, as the transfer of hydrogen atoms from the catalyst surface to the reactant is usually assumed. However, in some Pt catalyzed reactions antarafacial (trans) addition of hydrogen is also observed. The ratio of diastereomeric products formed is determined by the chemisorption equilibrium of the surface intermediates and by the relative rates of hydrogen entrance to the different unsaturated carbon sites. Both effects are influenced by steric factors. 

The pH of the solution plays an important rate in the steric outcome of the reaction. Acidic conditions favor cis addition and basic conditions favor trans addition of hydrogen [22,67],... 

Accrombessi et al. studied the hydrogenolysis of various epoxycyclohexanes over 10% Pd-C in different solvents at 20° and 1 atm H2.25 The hydrogenolysis of both trans- and cA-l,2-epoxy-4-f-butylcyclohexanes (13 and 14) occurs to give preferentially the axial alcohols by apparent trans addition of hydrogen (eq. 13.8), as evidenced by deuterolysis. 

Digitalis lanata, where the pro-4R hydrogen of MVA occupied the pro-24S position and trans-addition of hydrogen was deduced. The demethylation steps in the biosynthesis of cholesterol have received much attention. In rat liver microsomes the removal of the 14a -methyl group of 24,25-dihydrolanosterol led to the formation of a A8,14-diene (84), and neither a A8(14)-ene nor a A8,14,24-triene were obligate intermediates in cholesterol synthesis.141,142 14a-Demethylation, which was preceded by A24,25-reduction in rat liver system, involved cytochrome P450, whilst the next step in the biosynthesis of cholesterol, AI4-reduction, and the reduction of the A7- and A5,7-sterols were completely inhibited by the drug AY-9944.142,143 In yeast, A14-reduction probably involved trans-addition of a hydride ion from NADPH and a proton from the medium at the 14a- and 15/3-positions, respectively.144... 

In the same way, reduction of an alkyl cycloprop-1-ene-l-carboxylate, e.g. 11, or the corresponding alcohol, results in overall trans addition of hydrogen to the cyclopropene double bond. ... 

Two kinds of mechanisms have been proposed for formation of ran -decalin. One suggestion is that trans isomers may be formed by direct trans addition of hydrogen at the two bridgehead positions. The other mechanism involves cis addition in a more complicated process that involves turning over of some sort of intermediate, with or without intervening desorption, following saturation of one of the bridgehead positions. Both routes have had support. 

Hydrogen is added to oleflns and aromatics by a mechanism that is predominantly, if not exclusively, cis addition. The formation of trans isomers seems definitely to involve olefin intermediates that have desorbed and turned over on the catalyst. We find no evidence for the formation of any trans isomers by the direct trans addition of hydrogen. Such a mechanism, if operative at aU, must be trivial. 

Photosynthetic microorganisms and plant cell cultures are very important sources of enzymes for the reduction of olefins151, 2981. For example, Hirata et al. found that reduction of enone 11 with Nicotiana tabacum p90 reductase and Nicotiana tabacum p44 reductase affords (S)- and (R)-alkylcyclohexanones, respectively, with excellent enantioselectivities as shown in Fig. 15-53. They also found two enone reductases from Astasia longa, a nonchlorophyllous cell line classified in Euglenales, and studied the mechanism. Both catalyzed enantiospecific trans-addition of hydrogen atoms to carvone from the si-face at the a-position and from the re-face at the p-position. 

A similar mechanism is apparently involved in the stereospecific (trans) addition of hydrogen chloride to l-alkynylphosphines (7 of... 

Probably more interesting is the examination of the catalytic cycle of hydrogenation studied by Rosales and co-workers43 which used [RuH(CO)(NCMe)2(PPh3)2]BF4 as catalyst (species (6)).44 The reversible displacement of the MeCN ligand trans to the hydride by cyclohexene is followed by an isomerization prior to rate-determining addition of hydrogen ((7) —> (8) —> (9)) (Scheme 4). 

C, C double and triple bonds C=C double bonds can be stereoselectively hydrogenated with cis or trans addition of the hydrogen atoms. 2,3-Diphenylindenone underwent a selective cis addition in 0.25 M sulfuric acid, while a trans addition was found at higher pH values (Fig. 59) [321]. For the corresponding methyl-phenyl- and dimethylindenone, the diastereoselectivity was low. 

The various TPR peaks may correspond to different active sites. One hypothesis assumed cyclization over metallic and complex (Section II,B,4) platinum sites (62e) the participation of various crystallographic sites (Section V,A) cannot be excluded either. Alternatively, the peaks may represent three different rate determining steps of stepwise aromatization such as cyclization, dehydrogenation, and trans-cis isomerization. If the corresponding peak also appears in the thermodesorption spectrum of benzene, it may be assumed that the slow step is the addition of hydrogen to one or more type of deeply dissociated surface species which may equally be formed from adsorbed benzene itself (62f) or during aromatization of various -Cg hydrocarbons. Figure 11 in Section V,A shows the character of such a species of hydrocarbon. 

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