Three-step catalytic reduction

Catalytic activity in the homogeneous gas-phase conversion of nitrogen oxides and carbon monoxide to nitrogen and carbon dioxide was observed to be most effective for the atomic ions Fe+, Os+, and Ir+ out of the investigation of 29 different transition metal cations M+ [462]. The overall catalytic scheme that was established in this study consists of the three catalytic cycles shown in Fig. 1.94. The catalysis occurs in two steps in which NO is first reduced to N2O. An analogous three-step catalytic reduction of NO2, in which NO2 is first reduced to NO, was also discovered. The three cycles in Fig. 1.94 were characterized with laboratory measurements of reactions of each of the three nitrogen oxides NO2, NO, and N2O with the different transition metal ions in an inductively coupled plasma/selected-ion flow tube tandem mass spectrometer [462]. 

Mechanism was first discussed of the reaction with benzylic and aryl haUdes in 1979 [8,9]. As shown in other protocols, the three-step catalytic cycle is widely accepted, that is, oxidative addition, transmetallation, and reductive elimination (Scheme 5). 

Beginning with commercially available methyl-(5)-2-hydroxymethyl propionate 11, aldehyde 12 was synthesized via a three-step protection, reduction and oxidation sequence (Scheme 4). Although a number of methods for the installation of the ClO-Cl 1 olefin were evaluated, the most reliable route utilized a Wittig olefination for the elaboration of aldehyde 12 to unsaturated ester 13. After reduction of the methyl ester, a Sharpless asymmetric epoxiation provided epoxy alcohol 14, which was then oxidized with catalytic TEMPO to provide the aldehyde 15. This route was capable of producing the desired intermediate 15 as a single diastereomer on multigram scale. 

The implications of tins equation are detailed later in the context of the basic three-step catalytic cycle for palladium-catalyzed cross-couphng reactions [llf involving (i) oxidative insertion of palladium(O) into an alkyl hahde (ii) transmetallation of the transferable group from the donor moiety onto palladium and (iii) reductive elimination of the resultant organopaUadium species to give the coupled product and regenerate the palladium(O) catalyst... 

Finally, a group from General Motors has explored the mechanistic importance of the N20 + CO reaction as an intermediate step during the reduction of NO by CO on noble metal exhaust catalysts [87,88]. Quasi-linearization of the non-linear NO + CO reaction system by identifying a critical kinetic parameter revealed that, indeed, the rate of the N20 + CO conversion as an intermediate step in the overall NO + CO conversion can be two to three orders of magnitude faster than the isolated N20 + CO reaction. This suggests that the observed suppression of N20 production at higher temperatures may be due to its fast reaction with adsorbed CO once produced, and that, contrary to the accepted wisdom, the formation of N20 and its subsequent reaction with CO can make a major contribution to the kinetics of the reduction of NO by CO in three-way catalytic converters. The validity of the theoretical results was verified by both... 

Recently, List has described a cascade reaction promoted by phosphoric acid 1 in combination with stoichiometric amounts of achiral amine, which transforms various 2,6-diketones to the corresponding ds-cyclohexylamines (Scheme 5.28) [50]. This three-step process involves initial aldolization via enamine catalysis to give conjugate iminium ion intermediate A. Next, asymmetric conjugate reduction followed by a diastereoselective 1,2 hydride addition completes the catalytic cycle. 

DPT calculations indicated that the mechanism most Hkely involves three steps electrophilic substitution, oxidation and reductive ehmination. The inactivity of the iodine complexes prompted us to investigate the counterion dependence. For the methyl-substituted complexes (Scheme 23, R = CH3) we synthesized the acetate (X = OCOCH3) 22 and the chloride complex (X = Cl) 23. The catalytic conversions are within experimental error identical to the results of the bromide complex 20. This indicates that the dissociation of a counterion is a necessary condition for the activity of the complex [59]. 

Several 1,5-dioxocanes, including the parent compound, have been prepared via the unsaturated intermediate (345), which unfortunately is only formed in very low yield from (344) <70LA(736)75). As well as (345), there is also formed the 16-membered cyclic dimer of this compound in 14% yield. Catalytic hydrogenation of (345) gives 3-methyl-1,5-dioxocane (346), whilst ozonolysis provides the ketone (347), v = 1725 cm-1, which is converted to 1,5-dioxocane (348) by a three-step reduction procedure via the alcohol and the tosylate. 

Fig. 16.30. Pd(0)-catalyzed arytation of a copper acetytide at the beginning of a three-step synthesis of an ethynyt aromatic compound. Mechanistic details of the C,C coupling Step 1 formation of a complex between the catalytically active Pd(0) complex and the arylating agent. Step 2 oxidative addition of the arylating agent and formation of a Pd(II) complex with a cr-bonded aryl moiety. Step 3 formation of a Cu-acetylide. Step 4 trans-metalation the alkynyl-Pd compound is formed from the alkynyl-Cu compound via ligand exchange. Step 5 reductive elimination to form the -complex of the arylated alkyne. Step 6 decomposition of the complex into the coupling product and the unsaturated Pd(0) species, which reenters the catalytic cycle anew with step 1.

Next step of this synthesis consisted in the conversion of alcohol (17) to pisiferic acid (1) and this has been described in Fig. (3). The alcohol (17) in hexane was treated with Pb(OAc)4 in presence of iodine at room temperature to obtain the epoxy triene (19) (51%) whose structure was confirmed by spectroscopy. Treatment of (19) with acetyl p-toluene-sulfonic in dichloromethane yielded an olefinic acetate (20) and this was hydrogenated to obtain (21). The compound (22) could be isolated from (21) on subjection to reduction, oxidation and esterification respectively. The conversion of (22) to (23) was accomplished in three steps (reduction with sodium borohydride, immediate dehydration in dichloromethane and catalytic hydrogenation). Demethylation of (23) with anhydrous aluminium bromide and ethanethiol at room temperature produced pisiferic acid (1). Similar treatment of (23) with aluminium chloride and ethanethiol in dichloromethane yielded methylpisiferate (3). 

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