Giving Hydrides of Lead

By a suitable choice of conditions (metal hydrides or metal/ammonia) ketones at the 1-, 2-, 4-, 6-, 7-, 11-, 12- and 20-positions in 5a-H steroids can be reduced to give each of the possible epimeric alcohols in reasonable yield. Hov/ever, the 3- and 17-ketones are normally reduced to give predominantly their -(equatorial) alcohols. Use of an iridium complex as catalyst leads to a high yield of 3a-alcohol, but the 17a-ol still remains elusive by direct reduction. 

An especially important case is the enantioselective hydrogenation of a-amidoacrylic acids, which leads to a-aminoacids.29 A particularly detailed study has been carried out on the mechanism of reduction of methyl Z-a-acetamidocinnamate by a rhodium catalyst with a chiral diphosphine ligand DIPAMP.30 It has been concluded that the reactant can bind reversibly to the catalyst to give either of two complexes. Addition of hydrogen at rhodium then leads to a reactive rhodium hydride and eventually to product. Interestingly, the addition of hydrogen occurs most rapidly in the minor isomeric complex, and the enantioselectivity is due to this kinetic preference. 

The reaction of methylenesulphones with allyl halides in the presence of quaternary ammonium salts produces the 1-allyl derivatives [52], unlike the corresponding reaction in the absence of the catalyst in which the SN- product is formed (Scheme 6.5). In contrast, alkylation of resonance stabilized anions derived from allyl sulphones produces complex mixtures [51] (Scheme 6.6). Encumbered allyl sulphones (e.g. 2-methylprop-2-enyl sulphones) tend to give the normal monoalkyl-ated products. Methylene groups, which are activated by two benzenesulphonyl substituents, are readily monoalkylated hydride reduction leads to the dithioacetal and subsequent hydrolysis affords the aldehyde [61]. 

The working mechanism involves a [2 + 2] cycloaddition between the Ru=C bond of ruthenium vinylidene and olefin to form the metallacyclobutane 92, which subsequently undergoes P-hydride elimination leading to the 7i-allyl hydride complex 93 and reductive elimination to furnish the conjugated trienes 89 (Scheme 6.31), and eventually to give the observed aromatic product 90. 

Pinocarveol has been prepared by the autoxidation of a-pinene,5 by the oxidation of /S-pinene with lead tetraacetate,6 and by isomerization of a-pinene oxide with diisobutylalumi-num,7 lithium aluminum hydride,8 activated alumina,9 potassium ferf-butoxide in dimethylsulfoxide,10 and lithium diethylamide.11 The present method is preferred for the preparation of pinocarveol, since the others give mixtures of products. It also illustrates a general method for converting 1-methylcy-cloalkene oxides into the corresponding exocyclic methylene alcohols.11 The reaction is easy to perform, and the yields are generally high. 

Five-carbon-atom-chain extensions have been achieved with three kinds of unsaturated compounds 1,4-dienes, 2,4-dienoic acids or esters, and N-2,4-dienylamines. The 1,4-dienes are generally useful with aromatic halides, but give mixtures of isomeric amines with most vinylic halides because of elimination and readdition of palladium hydride in both possible directions to produce two different 7r-allylic intermediates. Exceptions occur in cases where symmetrical compounds are being formed and where both directions of elimination lead to the same product (8). For example ... 

Protonation of bis(dithiolene) complexes often results in the protons being added to the metal of the dithiolene complexes. Based on the similar electrochemical behavior to other metal hydrides, Vlcek and Vlcek (66) concluded that protonation of M(mnt)2]3 (M = Rh, Co) leads to addition of a proton at the metal, giving hydride complexes [M(H)(mnt)2]2. ... 

Just as further protonation of a cationic hydride can lead to Hj evolution and formation of a dication, futher protonation of a neutral hydride can lead to Hj loss and formation of a monocation. Although CF3SO3H in THF gives HMn(CO)j from [Mn(CO)5]- ... 

---