Palladium hydrogenations

F. A. Lewis, TIu - Palladium-Hydrogen System, Academic Press, London, 1967, 178 pp. 

Hydrogenation of carbonyls, or incipient carbonyls such as phenols (86), in lower alcohol solvents may result in the formation of ethers. The ether arises through formation of acetals or ketals with subsequent hydrogenolysis. The reaction has been made the basis of certain ether syntheses (45,97). Reaction of alcohols with carbonyls may be promoted by trace contamination, such as iron in platinum oxide (22,53), but it is also a property of the hydrogenation catalyst itself. So strong is the tendency of palladium-hydrogen to promote acetal formation that acetals may form even in basic media (61). 

Platinum may be more useful than palladium in reduction of nitro compounds containing functions easily reduced by palladium. Hydrogenation of I over 5% Pd-on-C was nonselective with hydrogenolysis of the benzyl ethers competing with nitro hydrog ation, but over PtO in ethanol 2 was obtained in 96% yield (4). 

The above generalities apply particularly to palladium. Hydrogenation over platinum or rhodium are far less sensitive to the influence of steric crowding. Reduction of 1-t-butylnaphthalene over platinum, rhodium, and palladium resulted in values of /ci//c2 of 0.42, 0.71, and 0.024, respectively. Also, unlike mononuclear aromatics, palladium reduces substituted naphthalenes at substantially higher rates than does either platinum or rhodium. For example, the rate constants, k x 10 in mol sec" g catalyst", in acetic acid at 50 C and 1 atm, were (for 1,8-diisopropylnaphthalene) Pd (142), Pt(l8.4), and Rh(7.1)(25). 

Alkynes can be reduced to yield alkenes and alkanes. Complete reduction of the triple bond over a palladium hydrogenation catalyst yields an alkane partial reduction by catalytic hydrogenation over a Lindlar catalyst yields a cis alkene. Reduction of (he alkyne with lithium in ammonia yields a trans alkene. 

Nevertheless it does not change the principle of the mechanism proposed by Scholten and Konvalinka, i.e. the ability to act catalytically of only the superficial palladium centers released from the vicinity of the interstitial hydrogen. Bearing in mind the dynamic character of the equilibrium in a palladium-hydrogen system as a whole is to regard such centers as being mobile in the surface layer of the hydride. 

Garcia, J.A. and Mandelis, A., Study of the thin-film palladium/hydrogen system by an optical transmittance method, Review Scientific Instruments, 67(11), 3981,1996. 

The hydrostannation of alkenes and alkynes can be catalyzed by a number of transition metals (Ni, Pd, Pt, Mo, etc.), but most studies have involved palladium.106 The mechanism that is most commonly accepted is shown for an alkyne in Scheme 1 the model for an alkene is similar. This mechanism accounts for the observations that the reactions normally involve cir-addition, that the R3Sn group enters so as to avoid steric hindrance by the largest substituent group on the alkene or alkyne, and that the regioselectivity depends on the polarization of the palladium-hydrogen bond in the sense Pd -H15-. 

Other methods have appeared, but have been less frequently used, e.g. chromium chloride in acetone 140- 143 (y/yfe infra) and dissolving alkali-metal reductions.144,145 Hydrogenolysis with palladium/hydrogen has also given the monoreduced cyclobutanone.99,100... 

Palladium-Hydrogen The Classical Metal-Hydrogen System... 

One of the most useful features of metal-hydrogen systems are their pressure-composition-temperature data, P-C-T. Such relationships for palladium-hydrogen are shown in Figure 1. For compositions and temperatures within the envelope, two solid phases coexist, as required by the phase rule. The lower hydrogen-content a-phase represents solution of hydrogen into the metal, and the higher hydrogen-content jS-phase is the hydride. Both a and (3 are... 

Figure 1. Pressure-composition-temperature relationships for palladium-hydrogen (43)...

The nc value for palladium-hydrogen is 0.25 from magnetic susceptibility measurements (52) with Tc = 564°K (52), and from P-C-T data, de Ribaupierre and Manchester (26) estimate nc = 0.29 and Tc = 566°K. The Bragg-Williams approximation gives a reasonable WfiH value by using an average value of nc, the critical temperature, and an analytical expression for ie(n) determined... 

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