Catalyzed Hydrogenolysis

HYDROGENOLYSIS OF SATURATED HYDROCARBONS 11.5.1. Metal-Catalyzed Hydrogenolysis 

The approach of finding correlation between adsorption properties and hydroge-nolysis led to the interpretation of cracking patterns. Later, the realization of relationships between hydrogenolysis and other metal-catalyzed reactions (isomerization) resulted in a much better understanding of the characteristics of hydrogenolysis reactions. 

Higher hydrocarbon molecules allow study of the unique cracking pattern of metals. These studies are usually carried out at low conversion to observe only primary hydrogenolysis. Nickel exhibits high selectivity to cleave terminal C—C bonds leading to demethylation that is, it cleaves only bonds that involve at least one primary carbon atom. For example, in the transformation of n-hexane, only methane and n-pentane are formed (180°C, Ni-on-silica catalyst, 0.3% conversion), whereas 2-methylpentane and 3-methylpentane yield methane, n-pentane, and isopentane.260 In the transformation of 2-methylpentane, the n-pentane isopentane ratio is close to 2, which corresponds to the statistical value. Under more forcing conditions, successive demethylations lead eventually to methane as the only product. 

As mentioned before, nickel usually catalyzes demethylation. For example, besides methane, mainly neopentane (as well as much less ethane and isobutane) is formed when neohexane undergoes hydrogenolysis on nickel.251 264 In contrast, 

Forge and Anderson studied so-called archetypal molecules to establish the cleavage mode of individual metals.251,265-267 Ethane is an archetype of C2-unit hydro-genolysis (i.e., the cleavage of primary-secondary and secondary-secondary bonds). Neopentane, in turn, is an archetypal molecule for iso-unit hydrogenolysis (i.e., the hydrogenolysis of bonds with at least one tertiary or quaternary carbon atom). 

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The Pd-catalyzed hydrogenolysis of allylic compounds by various hydrides gives alkenes. From terminal allylic compounds, either 1-alkenes or 2-alkenes are formed depending on the hydride sources [360a]. 

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

Diaryl disulfides and diselenides add to alkynes to afford the (Z)-l, 2-bis(ar-ylthio)alkenes 193 and (Z)-l,2-bis(arylseleno)alkenes 194. Under CO pressure, carbonylative addition takes place to give thio esters and the selenoketones 195[I07], The selenoketones are converted into the /J-seleno-a, 3-unsaturated aldehydes 196 by Pd-catalyzed hydrogenolysis with HSnBu3[108,109],... 

Pd(0)-catalyzed hydrogenolysis of vinylepoxides offers an attractive regio- and dia-stereoselective route to homoallylic alcohols (Scheme 9.36) [104, 155, 156]. Thus, hydrogenolysis of ( ) olefin 88 affords syn isomer 89 with inversion of configuration at the allylic carbon, while subjection of (Z) isomer 90 to identical reaction conditions results in the anti isomer 91. The outcomes of these reactions are ex-... 

The subjects of this chapter are the exploration of the scope and hmitations of the new Pd-Sn catalyzed hydrogenolysis route for the synthesis of thiols via 2-(perfluoroalkyl)ethane thiocyanate, the characterization of the surprisingly active and robust Pd-Sn catalysts, and the attempted correlation of the characterization of the catalysts with observed onset of hydrogenolysis reactivity and snrprisingly long lifetime in the presence of known catalyst poisons. ... 

We report the discovery of a new Pd-Sn/C catalyzed hydrogenolysis reaction to produce thiols in high yield (10), Eqnation 3. 

We report the discovery of a new Pd-Sn catalyzed hydrogenolysis reaction to produce thiol product in high yields. The relationship between catalyst activity and surface characterization (chemisorption, ESCA, and in situ temperature-dependent XRD,) has aided om understanding of the reasons why these catalysts are sulfur resistant, extremely active, and activated at certain temperatures and pressures. The predominant mode of deactivation appears to be the formation of Pd-CN species rather than the formation of Pd-S species on the surface of the catalyst. 

Selective removal of the iodine from fluorinated compounds was performed by 5% Pd/C catalyzed hydrogenolysis in the presence of triethylamine or sodium acetate.467 Ra-Ni and 1% NaOH were used for the cleavage of the C-I bond.468 The adsorption of chloroiodomethane was studied on a Pt(lll) surface. Dissociation began with C—I bond cleavage at about 150 K. Co-adsorbed deuterium atoms weaken the bonding between the starting compound and the surface and decrease the amount of dissociated molecules.469... 

The aqueous Co(CN)52- solutions under H2 have been found to catalyze hydrogenolysis of C4-unsaturated alcohols to butenes but, more remarkably, with acetylenic alcohols besides hydrogenated products secondary nitriles are also formed by addition of HCN (stoichiometric with respect to cobalt) (195) ... 

Thus, it has been shown that, in SOMC-catalyzed hydrogenolysis on an oxide at low temperature, side phenomena due to adsorption or chain walking could occur. These results with linear alkanes and polymers bring a better understanding of the catalytic activity of the ZrH catalyst. 

The Pd-catalyzed hydrogenolysis of the benzylated telomers 9 to obtain surfactants also leads to the reduction of the C=C bonds with formation of 12 (Fig. 7). 

The potassium salt of 7-methyl-3-phenyl-2,4-dioxo-l,2,3,4-tetrahydropyrido[2,3- pyrimidine-5-carboxylate 153 was condensed with l-bromo-3-chloropropane or 1,4-dibromobutane in anhydrous DMF at room temperature to afford the A-l-alkyl derivatives 154 and 155, respectively <1994FA499>. The corresponding A-l-piperazinylalkyl derivatives of 153 were also prepared in more than 40% yield. Debenzylation of 6-benzyl-3-methylpyrido[4,3-,7 pyrimidine 156 by Pd/C-catalyzed hydrogenolysis in AcOH gave the analogue 157 <1994JHC1569>. 

The Pd-catalyzed hydrogenolysis of acyl chlorides with hydrogen to give aldehydes is called the Rosenmund reduction. Rosenmund reduction catalyzed by supported Pd is explained by the formation of an acylpalladium complex and its hydrogenolysis[744]. Aldehydes can be obtained using other hydrides. For example, the Pd-catalyzed reaction of acyl halides with tin hydride gives aldehydes[745]. This is the tin form of Rosenmund reduction. Aldehydes are formed by the reaction of the thio esters 873 with hydrosi anes[746,747]. 

Benzyl alcohol linkers, such as those described in Section 3.1.1.1, can also be cleaved by palladium-catalyzed hydrogenolysis. Carboxylic acids have, for example, been obtained by hydrogenolysis of insoluble benzyl esters with Pd(OAc)2/DMF/H2 [89,161]. Resin-bound benzylic carbamates [162,163] and amides [164] can also be released by treatment with Pd(OAc)2 in DMF in the presence of a hydrogen source, such as 1,4-cyclohexadiene or ammonium formate. These reactions are quite surprising, because they require the formation of metallic palladium within the gelated beads. 

Acid-catalyzed cracking and platinum-catalyzed hydrogenolysis proceed simultaneously over dual-function catalysts. The distribution of the scission products is determined by the relative strengths of the acidic and metal-type catalytic components. 

The benzyl group has been widely used for the protection of hydroxyl functions in carbohydrate and nucleotide chemistry (C.M. McCloskey, 1957 C.B. Reese, 1965 B.E. Griffin, 1966). A common benzylation procedure involves heating with neat benzyl chloride and strong bases. A milder procedure is the reaction in DMF solution at room temperature with the aid of silver oxide (E. Reinefeld, 1971). Benzyl ethers are not affected by hydroxides and are stable towards oxidants (e.g. periodate, lead tetraacetate), LiAlH, and weak acids. They are, however, readily cleaved in neutral solution at room temperature by palladium-catalyzed hydrogenolysis (S. Tejima, 1963) or by sodium in liquid ammonia or alcohols (E.J. Reist, 1964). 

Under alkaline conditions isolating lignin degradation products which are essentially of a phenyl.ethyl rather than a phenylpropyl nature is structurally important and requires a lignin structure by which the 7-carbon may be removed as a result of a 0-7 carbon-carbon cleavage reaction, either by direct alkaline hydrolysis or alkali-catalyzed hydrogenolysis. 

The palladium catalyzed hydrogenolysis of 3-amino 4-aryl azetidin-2-ones, Fig. 11, constitutes an excellent and reliable strategy to access a-amino acid derived peptides. The discovery, development, and synthetic opportunities of this approach have been reviewed by Ojima [145-148], and will not be covered here. Only the... 

Scheme showing proposed pathways for reductive dehalogenation in Fe0-H2O systems (A) direct electron transfer from iron metal at the metal surface (B) reduction by Fe2+, which results from corrosion of metal (C) catalyzed hydrogenolysis by the H2 that is formed by reduction of H20 during anaerobic corrosion. Stoichiometries are shown. (From Matheson, L.J. and Trat-nyek, P.G., Environ. Sci. Technol., 28, 2045-2053, 1994. With permission.)... 

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