Hydrogen hetero-/homogeneous

COUPLING OF HYDROGEN HETERO-/HOMOGENEOUS CHEMISTRY AND TRANSPORT... 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

HDN with Metallophthalocyanines. The above considerations clearly show that the catalytic site geometry is important in HDN. It is difficult to control, or even to identify, the catalyst site in conventional catalysts such as the commercial Co-Mo-alumina catalyst. On the other hand, homogeneous catalysts with transition metal complexes provide a well-defined catalytic site. Unfortunately, most homogeneous catalysts are not sufficiently stable to be used at the temperatures required for the hydrogenation of hetero compounds. A class of catalysts that are thermally stable are the metallophthalocyanines,... 

The key intermediate benzothiazine 25 was accessed via displacement of fluoride in the dinitrobenzene 23 with the lithium salt of methyl thioglycolate to provide thioether 24, which after tin(II)-mediated reduction of the nitro groups and cyclization furnished benzothiazine 25 in modest yield (Scheme 6). It was noted by the authors that other methods of reduction such as hetero- or homogenous hydrogenation did not improve the yields for this step. It is also noteworthy that subsequent chemoselective reduction of the ester group to give the requisite alcohol proved problematic due to the ease of reduction of the benzothiazinone amide functionality. 

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