Metal Catalysts with Reducible Substrates

In principle any except tetrasubstituted olefins could be labeled by catalytic exchange, but in practice it is difficult to find experimental conditions under which significant tritium incorporation can be catalyzed without reducing the double bond. However, if the olefinic starting material is separable from the reduced product, it is sometimes possible to find reaction conditions that disfavour complete reduction, then isolate the unreduced olefin which has become labeled. Two examples with which this has been accomplished are pleuromutilin (71) and cyclosporin A (72) °. Treatment with a deficiency of tritium gas for 60-90 min in the presence of 10% Pd/C in ethyl acetate (71) or DMF (72 followed by removal of reduced product by HPLC, provided the compounds, labeled as indicated, at specific activities of 10 and 19Ci/mmol, respectively. Unfortunately, it proved to be difficult to separate the cis/trans mixture formed with 72, which stimulated the development of a tritide labeling approach (see Chapter 4, Section 4.3.1). 

Deuterium exchange into starting material recovered from asymmetric reductions of fraui-2-methyl-2-pentenoic acid with cinchonidine-modified Pd/Al203 catalysts has been observed  

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The diversity of the substrates, catalysts, and reducing methods made it difficult to organize the material of this chapter. Thus, we have chosen an arrangement related to that used by Kaesz and Saillant [3] in their review on transition-metal hydrides - that is, we have classified the subject according to the applied reducing agents. Additional sections were devoted to the newer biomimetic and electrochemical reductions. Special attention was paid mainly to those methods which are of preparative value. Stoichiometric hydrogenations and model reactions will be discussed only in connection with the mechanisms. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

The cyclic substrate 32 and other disubstituted olefins such as 35a were oxidized in sc C02 to give the corresponding epoxides with reasonable rates (>95% conversion in less than 18 h) and excellent selectivities (>98%) under otherwise similar reaction conditions (Loeker and Leitner, 2000). It is important to note, however, that no addition of a metal catalyst was required in the supercritical reaction medium. Detailed control experiments revealed that the stainless steel of the reactor walls served as efficient initiator for the epoxidation under these conditions. Terminal olefins 35b,c were oxidized with somewhat reduced rates and either epoxidation or vinylic oxidation occurred as the major reaction pathway depending on the substrate (eq. 5.11). Apart from providing the first examples for efficient and highly selective oxidation with 02 in sc C02 (earlier attempts Birnbaum et al., 1999 Loeker et al., 1998 Wu et ah, 1997), this study points to the possible importance of wall effects during catalytic reactions in this medium (see also Christian et ah, 1999 Suppes et ah, 1989). 

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