Hydrogenation, catalytic stereospecific

In the catalytic hydrogenation, two new C—H a bonds are formed simultaneously from H atoms absorbed into the metal surface. Thus, catalytic hydrogenation is stereospecific, giving only the syn addition product. If the atoms are added on the same side of the molecule, the addition is known as syn addition. If the atoms are added on opposite sides of the molecule, the addition is called an anti addition. For example, 2-butene reacts with H2 in the presence of a metal catalyst to give n-butane. 

In general, a catalytic reaction may be named by adding the adjective catalytic to the standard chemical term for the reaction, for example, catalytic hydrogenation (or, if clarity demands, heterogeneous catalytic hydrogenation), catalytic hydrodesulphurisation, catalytic oxidative dehydrogenation, catalytic stereospecific polymerisation. 

The first total synthesis of trihydroxydecipiadiene (45), a member of the structurally unique decipiene diterpenes, has been reported. A route involving cycloaddition of dichloroketen and reductive dehalogenation led to (42) which was found to undergo a stereospecific aldol reaction to give (43) under carefully controlled conditions. As expected, catalytic hydrogen was stereospecific giving the desired isomer (44), which in turn was converted to (45) in several steps. 

Catalytic hydrogenation has been utilized extensively in steroid research, and the method has been found to be of great value for the selective and stereospecific reduction of various functional groups. A number of empirical correlations concerning selectivity and product stereochemistry compiled for steroid hydrogenations has been listed in a previous review. ... 

Since the stereochemical course of a catalytic hydrogenation is dependent on several factors, " an understanding of the mechanism of the reaction can help in the selection of optimal reaction conditions more reliably than mere copying of a published recipe . In the first section the factors which can influence the product stereochemistry will be discussed from a mechanistic viewpoint. In subsequent sections the hydrogenation of various functional groups in the steroid ring system will be considered. In these sections both mechanistic and empirical correlations will be utilized with the primary emphasis being placed on selective and stereospecific reactions. 

Scheme 10.27 Catalytic cycle of HppE. Dashed arrows indicate electron transport. In this scheme HPP binds to iron1". After a one-electron reduction, dioxygen binds and reoxidizes the iron center. The peroxide radical is capable of stereospecifically abstracting the (pro-R) hydrogen. Another one-electron reduction is required to reduce one peroxide oxygen to water. Epoxide formation is mediated by the resulting ironlv-oxo species.

The addition is therefore stereospecifically syn and, like catalytic hydrogenation, generally takes place from the less-hindered side of a double bond, though not much discrimination in this respect is observed where the difference in hulk effects is small.Diimide reductions are most successful with symmetrical multiple bonds (C=C, C=C, N=N) and are not useful for those inherently polar (C=N, C=N, C=0, etc.). Diimide is not stable enough for isolation at ordinary temperatures, though it has been prepared as a yellow solid at — 196°C. 

The mechanism of this catalysis has been extensively studied. The catalytic process is initiated by the coordination of the nitrogen atom of an allylamine, followed by an intramolecular stereospecific 1,3-hydrogen shift (Scheme 3).18... 

Hydrogen-bonding between the 3-oxo group of 1,4,4-trisubstituted pyrrolidine-2,3,5-triones and catalytic amounts of cinchonidine controls the stereospecific hydrogenation of the system over Pt/Al203 to yield chiral 3-hydroxy compounds (-100% yield with ee >60) [21] the nature of the (V-substituent appears to be the controlling factor for the stereoselectivity with PhCH2> Et > n-Bu > cyclo-C6H . 

Hydrogenolysis of epoxides to alcohols by catalytic hydrogenation over platinum requires acid catalysis. 1-Methylcyclohexene oxide was reduced to a mixture of cis- and /ranj-2-methylcyclohexanol [652]. Steroidal epoxides usually gave axial alcohols stereospecifically 4,5-epoxycoprostan-3a-ol afforded cholestan-3a,4/J-diol [652 ]. 

In ethyl 3-keto-2-oximino-3-phenylpropanoate catalytic hydrogenation over palladium on carbon reduced both the keto and oximino group, giving a 74% yield of ethyl ester of -phenylserine (ethyl 2-amino-3-hydroxy-3-phenylpropionate). The reduction is stereospecific and only the erythro dia-stereomer was obtained, probably via a cyclic intermediate 11097]. Similarly, hydrogenation over Raney nickel at 25-30° and 1-3 atm converted ethyl a-oximimoacetoacetate quantitatively to ethyl 2-amino-3-hydroxybutanoate [45]. 

The carbon-nitrogen double bond of A1-piperideines is susceptible to reduction. This can be useful for the stereospecific introduction of ring substituents. An illustration is the preparation of ds-2,6-disubstituted piperidines (211) by catalytic hydrogenation of the corresponding A1-piperideine (210) (77T1569). 

A very efficient, stereospecific synthesis of DL-ribose was based26 on the use of l,l-diethoxy-5-(tetrahydropyran-2-yloxy)-2-pentyn-3-ol as the substrate. Catalytic hydrogenation of this alkyne to the cts-alkene was accompanied by cyclization, to give 2-ethoxy-2,5-dihydro-5-(tetra-hydropyran-2-yloxy)furan (35). cis-Hydroxylation of the double bond in 35 was effected with potassium permanganate, yielding the ethyl DL-ribofuranoside derivative 36, which was hydrolyzed to DL-ribose. 

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