Homogeneous hydrogenation esters

In benzene or similar solvents, tris(triphenylphosphine)halogenorhodium(I) complexes, RhX[P(C6H5)3]3, are extremely efficient catalysts for the homogeneous hydrogenation of nonconjugated olefins and acetylenes at ambient temperature and pressures of 1 atmosphere (6). Functional groups (keto-, nitro-, ester, and so on) are not reduced under these conditions. 

The homogeneous hydrogenation of soybean oil methyl ester Olefin disproportionation... 

Preparation of formamides from COz and a non-tertiary amine by homogeneous hydrogenation has been well studied and is extremely efficient (Eq. (12)). Essentially complete conversions and complete selectivity can be obtained (Table 17.3). This process seems more likely to be industrialized than the syntheses of formic acid or formate esters by C02 hydrogenation. The selectivity is excellent, in contrast to the case for alkyl formates, because the amine base which would stabilize the formic acid is used up in the synthesis of the formamide consequently little or no formic acid contaminates the product. The only byproducts that are likely to crop up in industrial application are the methylamines by overreduction of the formamide. This has been observed [96], but not with such high conversion that it could constitute a synthetic route to methylamines. 

Homogeneous Hydrogenation of Unsaturated Fatty Esters by Fe(CO), and Diene-Fe(CO)3... 

To elucidate the mechanism of homogeneous hydrogenation catalyzed by Fe(CO)s, kinetic studies were carried out with mixtures of unsaturated fatty esters containing a radioactive label. A C-labeled methyl octadecadienoate-Fe(CO)3 complex was prepared to serve as a catalytic intermediate. Hydrogenation of methyl oleate (m-9-octa-decenoate) and palmitoleate (cis-9-hexadecenoate) and of their mixtures with methyl linoleate was also studied to determine the selectivity of this system, the function of the diene-Fe(CO)3 complex, and the mechanism of homogeneous isomerization. Mixtures of reaction intermediates with a label helped achieve unique simulation of the kinetic data with an analog computer. 

Unusual reaction orders are found in product-promoted or reactant-inhibited ("autocatalytic") reactions, the former with positive apparent order with respect to a product, the latter with negative apparent order with respect to a reactant (see Section 8.9). An example of a product-promoted reaction is acid-catalyzed ester hydrolysis. An example of a reactant-inhibited reaction has already been encountered, namely, olefin hydroformylation, whose order with respect to CO is negative (see eqn 6.12 in Section 6.3). Such behavior is also not uncommon in heterogeneous catalysis (see Section 9.3.2) and enzyme catalysis ("substrate-inhibited" reactions in biochemistry lingo, Section 8.3). A reaction having an order with respect to a silent partner—CO in a homogeneous hydrogenation—will be examined in some detail later in this chapter (see Examples 7.3 and 7.4). 

DIRECTED HOMOGENEOUS HYDROGENATION METHYL anti-3-HYDROXY-2-METHYLPEHTANOATE (Pentanolc acid, 3-hydro>[Pg.64]

Mercaptoacetic acid adds very readily to nitriles and, according to Condo and his co-workers85 can be used for direct identification of nitriles it gives the adducts in 80-90% yield when a solution of the nitrile and mercaptoacetic acid in anhydrous ether is saturated at 0° with dry hydrogen chloride. Hydrogen sulfide and thiols add to cyanic esters, giving respectively thiocarbamic esters and (alkylthio)formimidic esters homogeneously, under mild conditions, and in almost quantitative yield 86... 

AA 24. Mohrig, J.R. Dabora, S.L. Foster, T.F. Schultz, S.C. Stereospecific Synthesis of 2-Deuterio-3-hydroxybutanoate Esters. Regiochemistry and Stereochemistry of Homogeneous Hydrogenation with Wilkinsons Catalyst J. Org. Chem. 1984, 49, 5179-5182... 

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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