Hydrogen transfer stereoselectivity

Figure 2 (a) Productivity (mmol prod/gcat h) in the hydrogen transfer reaction obtained with Cu/MgO, MgO and CuO/MgO (b) Stereoselectivity obtained with Cu/MgO, MgO and CuO/MgO, respectively. 

Edegger, K., Stampfer, W., Seisser, B. et al. (2006) Regio- and stereoselective reduction of diketones and oxidation of diols by biocatalytic hydrogen transfer. European Journal of Organic Chemistry, (8), 1904—1909. 

Edegger, K., Gruber, C.C., Poessl, T.M. et al. (2006) Biocatalytic deuterium- and hydrogen-transfer using overexpressed ADH- A enhanced stereoselectivity and 2/7-labeled chiral alcohols. Chemical Communications, (22), 2402-2404. 

However, upon thermolysis of 178.d or 178.h, isomeric mixtures of the butadiene complexes 183.a and 183.b were formed. Since intramolecular hydrogen transfer within (3,3-dime thyl-773 r -allylacy iron complexes is well precedented101 (see Section VI,B), it seems likely that this process is responsible for diene complex formation. Note that only the Z-diene complex was isolated from the reaction mixture, a surprisingly stereoselective result. 

Rearrangement of allylic alcohols. RuClj in combination with NaOH is a useful catalyst for isomerization of allylic alcohols of the type RCIIOHClI=CH2 to saturated ketones, RCOCH2CH3. In the isomerization of a chiral allylic alcohol such as I, the product is optically active. The 1,3-hydrogen transfer is 37% stereoselective. 

CHEMO-, REGIO- AND STEREOSELECTIVITY IN STEROID HYDROGENATION WITH CU/AI2O3. INTRA- AND INTERMOLECULAR HYDROGEN TRANSFER REACTIONS. 

Therefore we used 4-androsten-3,17-dione 1 and 5a-androstan-3,17-dione 2 as model substrates to investigate the chemo- regio- and stereoselectivity of hydrogen transfer from different secondary alcohols, 2-propanol, 2-octanol, cyclohexanol, 1-phenyl-ethanol and diphenylmethanol in the presence of CU/AI2O3. In particular, hydrogenation of 1 allowed to determine the selectivity towards 5p isomers, whereas the percent of axial alcohol was derived from the hydrogenation of 2. These results can be compared with those obtained with the same catalyst in the presence of molecular hydrogen. 

For compounds such as 63, a more complex rearrangement replaces the decarbonyla-tion reaction [65]. This photoisomerization yields lactone 64 by a-cleavage at C-2, C-3 position (Norrish I) and hydrogen transfer from C-l to C-3 followed by a stereoselective nucleophilic attack at the carbonyl group by the terminal carbon of the electron-rich double bond and final ring closure (Scheme 34). 

The asymmetric hydrogen transfer of aryl ketones can be accomplished with ruthenium catalysts that contain amino alcohols 165 in modest to high enantioselectivities.211 With amino alcohol ligands, the optimal rate and stereoselectivities are produced from catalysts prepared in situ with [RuCl2(r)6-C6Me6)]2. 

Allylic or propargylic metal compounds are often intermediates in isomerization reactions of unsaturated systems (see Section 4.5.5) the product distribution on protonation depends very much on the substitution and the conditions of hydrolysis and most often mixtures of isomers are obtained. In contrast double bond shift generally occurs on protodesilylation of allylic silanes by acids, a reaction which takes place stereospecifically anti. Only when intramolecular delivery of the proton in a chair-like transition state is possible may a different kind of stereoselectivity be effective and good 1,3- or 1,4-induction in hydrogen transfer be observed (Scheme 83). ° ... 

Green, M. M., Boyle, B. A., Vairamani, M., Mukhopadhyay, T., Saunders, W. H., Jr., Bowen, P., Allinger, N. L. Temperature-dependent stereoselectivity and hydrogen deuterium kinetic isotope effect for -hydrogen transfer to 2-hexyloxy radical. The transition state for the Barton reaction. J. Am. Chem. Soc. 1986,108, 2381-2387. 

An alternative method whereby a fused-ring oxirene may be generated is the epoxidation of a cycloalkyne. Curci et al. have employed methyl(trifluoromethyl)dioxirane (TFD) toward that end, and obtained (215) and c -bicyclo[4.4.0]decan-2-one in ca. 7 1 ratio from cyclodecayne (213) <92TL7929>. These products obviously stem from stereoselective 1,5- and 1,6-transannular insertion pathways. The authors conclude that the products of cycloalkyne oxidations may arise directly from trapping of the oxirene intermediates by transannular hydrogen transfer, as shown in Scheme 46. 

The repair of carbohydrate radicals vithin polynucleotides can proceed with remarkable stereoselectivity, as demonstrated for hydrogen transfer from both 2-mercaptoethanol and dithiothreitol to deoxyuridin-l -yl radials within single-and double-stranded oligonucleotides (Scheme 3.3, Reactions (3.21a) and (3.21j8)) [56]. 

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