Palladium equilibrium

The equilibrium is more favorable to acetone at higher temperatures. At 325°C 97% conversion is theoretically possible. The kinetics of the reaction has been studied (23). A large number of catalysts have been investigated, including copper, silver, platinum, and palladium metals, as well as sulfides of transition metals of groups 4, 5, and 6 of the periodic table. These catalysts are made with inert supports and are used at 400—600°C (24). Lower temperature reactions (315—482°C) have been successhiUy conducted using 2inc oxide-zirconium oxide combinations (25), and combinations of copper-chromium oxide and of copper and silicon dioxide (26). 

The intramolecular Heck reaction presented in Scheme 8 is also interesting and worthy of comment. Rawal s potentially general strategy for the stereocontrolled synthesis of the Strychnos alkaloids is predicated on the palladium-mediated intramolecular Heck reaction. In a concise synthesis of ( )-dehydrotubifoline [( )-40],22 Rawal et al. accomplished the conversion of compound 36 to the natural product under the conditions of Jeffery.23 In this ring-forming reaction, the a-alkenylpalladium(n) complex formed in the initial oxidative addition step engages the proximate cyclohexene double bond in a Heck cyclization, affording enamine 39 after syn /2-hydride elimination. The latter substance is a participant in a tautomeric equilibrium with imine ( )-40, which happens to be shifted substantially in favor of ( )-40. 

Assuming the composition of the hydride to be expressed by PdjH2 (which corresponds to PdH0. 7) and bearing in mind the interstitial positioning of the hydrogen in the palladium lattice, the authors postulate the existence of the following equilibrium at the surface of the j8-hydride phase... 

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. 

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

CO oxidation, an important step in automotive exhaust catalysis, is relatively simple and has been the subject of numerous fundamental studies. The reaction is catalyzed by noble metals such as platinum, palladium, rhodium, iridium, and even by gold, provided the gold particles are very small. We will assume that the oxidation on such catalysts proceeds through a mechanism in which adsorbed CO, O and CO2 are equilibrated with the gas phase, i.e. that we can use the quasi-equilibrium approximation. 

It is not clear whether the X anion remains ligated to the palladium(II) center. For example, for acetic acid, the palladium hydride was initially postulated as being HPd(OAc)L ,377,378 but more recently as HPdL +.367 To date, none of these complexes has been characterized.367 Oxidative addition of acetic acid or formic acid to a palladium(O) complex in DMF affords a cationic palladium hydride /ruw.v-I IPd(PPh3)2(DMF)+, with an acetate or a formate counter-anion. Both reactions are reversible and involve an unfavorable equilibrium so that a large excess of acid is required for the quantitative formation of the palladium hydride complex.379 This allows us to conclude that the catalytic reactions initiated by reaction of palladium(O) and acetic acid (or formic acid) proceed via a cationic palladium hydride trans-HPdfPPtHWDMF)"1", when they are performed in DMF.379... 

In contrast to kinetic models reported previously in the literature (18,19) where MO was assumed to adsorb at a single site, our preliminary data based on DRIFT results suggest that MO exists as a diadsorbed species with both the carbonyl and olefin groups being coordinated to the catalyst. This diadsorption mode for a-p unsaturated ketones and aldehydes on palladium have been previously suggested based on quantum chemical predictions (20). A two parameter empirical model (equation 4) where - rA refers to the rate of hydrogenation of MO, CA and PH refer to the concentration of MO and the hydrogen partial pressure respectively was developed. This rate expression will be incorporated in our rate-based three-phase non-equilibrium model to predict the yield and selectivity for the production of MIBK from acetone via CD. 

The poor regioselectivity of alkyne insertion in our polycychc aromatic hydrocarbon synthesis (Scheme 17) suggested to us that perhaps the palladium intermediate in that process was actually undergoing migration from one aromatic ring to the other, perhaps by a Pd(IV) hydride intermediate, to establish an equilibrium mixture of two regioisomeric arylpalladium intermediates under our reaction conditions (Scheme 18). This, indeed, appears to be true as... 

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

The quadridentate ligand (46), LH2, has been used to extract palladium from solutions containing platinum. Equilibrium studies indicated that a mono-deprotonated form of the ligand is... 

If the unsaturated hydrocarbon is a diene, both double bonds may coordinate to palladium ). (Diene)palladium(II) complexes have been isolated and characterized. For example, 2 and 3 are stable complexes in which both double bonds are coordinated to the metal10. Conjugated dienes constitute a special case and although /j4-diene complexes, e.g. 4, are postulated as intermediates, they have not yet been isolated. The butadiene complex 4 is in equilibrium with the zr-allyl complex 5 in solution, and attempts to isolate the diene complex from this mixture lead to formation of a yellow crystalline complex 511. 

The palladium-catalyzed addition of silylstannanes to 1,6-diynes invokes intramolecular cyclization, giving rise to 1,2-dialkylidene cyclopentanes (Equation (110)).269 The resulting Z,Z-l-silyl-4-stannyl-l,3-diene moiety fixed in an i -m-conformation makes the molecule axially chiral. Rapid equilibrium between the two helical forms is observed by NMR spectroscopy.270,161... 

The extraordinary power of the Ishikura palladium-catalyzed couplings of indolylborates is revealed by the several examples shown below [148,152,154,155]. The carbonylation version is discussed in Section 3.6. The formation of allenylindoles 139 vis-d-vis alkynylindoles 141 apparently depends on the equilibrium between an allenylpalladium complex and a propargylpalladium complex, and SN2-like attack on the latter by 136 to give 141 is favored by the Ph3P-ligated Pd complex. 

Since the equilibrium conversion of CO to methanol under the conditions of Table II is low, the reaction was further studied under higher pressures. The results at 535 kPa are shown in Table III. The catalysts having smaller palladium particle sizes... 

The allenyl carboxylate 35 was obtained in an enantiomerically enriched form by the palladium-catalyzed reduction of the racemic phosphate 34 using a chiral proton source [53]. The two enantiomers of the (allenyl)samarium(III) intermediate are in rapid equilibrium and thus dynamic kinetic resolution was achieved for the asymmetric preparation of (i )-35 (Scheme 3.18). 

The intermediate Jt-allyl complex is formally the palladium(II) complex of an allylic anion that can be represented by the two mesomeric forms shown in Scheme 17.2. It is important to note that this is not a fast equilibrium between two cr-allyl complexes but a stable species where palladium is simultaneously bound to both carbon-1 and carbon-3. All eight atoms of the Jt-allyl moiety are almost in the same plane. All three carbon atoms have sp2 character and the rotation between the Cl-C2 and C2-C3 bonds is blocked. As a consequence of the hindered rotation, four dia-stereomeric Jt-allyl complexes are possible. For example, in Scheme 17.2 both R and R are syn to the hydrogen on carbon-2, therefore this complex is called the syn,syn diastereomer. 

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