Methyl sorbate hydrogenation

Table 14.3 Hydrogenation of methyl sorbate to 3-hexenoic acid methyl ester with [Cr(CO)3(arene)] (45) catalysts.

By far most of the reports on addition reactions of hetero-nucleophiles to activated dienes deal with sulfur-nucleophiles17,48,80,120-137, in particular in the synthesis of 7/3-sulfur-substituted steroids which, like their carbon-substituted counterparts (Section n.A), are of interest because of their ability to inhibit the biosynthesis of estrogens80,129-137. Early investigations17,120-122 concentrated on simple acyclic Michael acceptors like methyl sorbate and 2,4-pentadienenitrile. Bravo and coworkers120 observed the formation of a 3 1 mixture of the 1,6- and 1,4-adduct in the reaction of methyl sorbate with methanethiol in basic medium (equation 39). In contrast to this, 2,4-pentadienenitrile adds various thiols regioselectively at C-5, i.e. in a 1,6-fashion (equation 40)17,121,122, and the same is true for reactions of this substrate with hydrogen sulfide (equation 41), sodium bisulfite and ethyl thioglycolate17. 

Selective 1,4-hydrogenation of 1,3-dienes to (Z)-alkenes is possible with benzenechromium tricarbonyl as a catalyst precursor. Only those conjugated dienes that can adopt a cisoid conformation, and hence chelate to the metal, are hydrogenated. Methyl sorbate (68) is reduced to methyl (Z)-3-hexenoate (70) [46,47]. The reaction... 

Several complexes having arenes, [Cr(CO)3(arene)] (arene = benzene, mesitylene, methyl benzoate, etc.) and [Cr(CO)3(cycloheptatriene)], are excellent catalysts for selective 1,4-hydrogen addition to 1,3-dienes. " 1,4-Hydrogenation of methyl sorbate (34), isoprene (36) and 1,3-pentadiene (38) in cyclohexane affords methyl cw-3-hexenoate (35), 2-methyl-2-butene (37) and cw-2-pentene (39), respectively. [Cr(CO)3(MeCN)3] is also an active catalyst under mild conditions (40 C, H2 1.4 atm), as are [Cr(CO)e] or [Cr(NBD)(CO)4] under UV irradiation. ... 

The homopolymers of 10 were branched and exhibited broad GPC molecular weight distributions. Studies of the homopolymers molecular weights from polymerization at different monomer concentrations while (1) holding the [10]/[I] ratio constant, and (2) employing different [10]/[I] ratios confirmed that major differences existed in homopolymerizations of 10 versus vinylferrocene.56 In ethylacetate the rate law was r = k [M]1 [I]0 5. Polymerizations in benzene exhibited low initiator efficiencies. The rate was three halves order in the concentration of 10, similar to that found for 8.53 Polymers incorporating 10 were able to catalyze the selective 1,4-hydrogenation of methyl sorbate, but not terminal or internal olefins.56 This resembled the catalytic behavior of styrene/r 6-(styrene)tricarbonylchromium copolymers in hydrogenation.75... 

M(CO)e has been used to stereoselectively hydrogenate methyl sorbate and trans,trans-conjugated fatty esters at ambient T and P by exposing the reaction mixture to UV irradiation (3500 A) . Under the same conditions, (heptatriene)Cr(CO)3 showed lower activity than Cr(CO)g and Mo(CO)g and (mesitylene)Mo(CO)3 showed no significant activity toward conjugated substrates. 

S-cw-configuration undergo this reaction readily. Thus, methyl sorbate is reduced to methyl 3-hexenoate over tricarbonyl(methylbenzoate)chromium, with exclusive 1,4-hydrogen addition" ... 

The polymers are effective as catalysts of stereoselective hydrogenation of methyl sorbate to c/5-3-hexenoate at 160°C in cyclohexane or tetrahydrofuran. Soluble completes have been isolated after hydrogenation. Then, the catalytic activity decreases this is probably caused by the loss of Cr(CO)3 groups. The results agree with the mechanism involving the dissociation of Cr(CO)3 groups, a considerable amount of which cannot be reassociated with polymeric phenyl groups. 

Ladder polyphenylsilsequioxane and linear polydiphenylsiloxane polymers have been used to complex Cr(C0)g to form CrCCO) complexes which proved to be better than similar complexes of CrCCO) with cross-linked polystyrene as stereoselective hydrogenation catalysts in hydrogenation of methyl sorbate to cis-3-hexenoate. 

The six-membered ring heterocyclic analog of the isoxazolidines (a dihydro-1,2-oxazine such as 2.192) is available by a Diels-AIder reaction with nitroso derivatives. Reaction of methyl sorbate (2.190) and 1-chIoro-l-nitrosocyclohexane (2.191) gave 2.192 Catalytic hydrogenation followed by hydrolysis reduced the alkene, opened the ring and converted the ester to an acid 5-amino-2-hydroxyhexanoic acid, 2.193. 

Chromium. The complexes Cr(arene)(CO)s are highly selective catalysts for the hydrogenation of dienes to monoenes. A recent study concentrated on the reduction of methyl hexa-2,4-dienoate (sorbate), and the mechanism was found to be similar to that for reaction with iron-carbonyl catalysts. It does not involve rate-determining loss of the arene to give Cr(CO>3 as the active species, as suggested earlier for a different catalytic system, but rather the arene becoming bi- or uni- rather than ter-dentate, so that the diene can be co-ordinated ... 

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