Alkyls catalysis, alkyl intermediates

However when X = OPh or SPh, the substitution product (4) is eventually produced reflecting the better leaving-group ability of the phenyl compared to the alkyl derivatives. The intermediate (5) has been postulated in the catalysis by sultite of the displacement by ammonia of the hydroxyethylamino group in 1-hydroxyethylamino-2-nitro-4-aminobenzene. 

Until there is a sufficient excess of ethene over [PdH(TPPTS)3] their fast reaction ensures that aU palladium is found in form of tratts-[Pd C(CO)Et (TPPTS)2]. However, at low olefin concentrations (e.g. in biphasic systems with less water-soluble olefins) [PdH(TPPTS)3] can accumulate and through its equihbrium with [Pd(TPPTS)3] (eq. 5.5) can be reduced to metallic palladium. This is why the hydroxycarbonylation of olefins proceeds optimally in the presence of Brpnsted acid cocatalyts with a weekly coordinating anion. Under optimised conditions hydrocarboxylation of propene was catalyzed by PdC + TPPTS with a TOE = 2507 h and l = 57/43 (120 °C, 50 bar CO, [P]/[Pd] = 4, P-CH3C6H4SO3H) [38], In neutral or basic solutions, or in the presence of strongly coordinatmg anions the initial hydride complex cannot be formed, furthermore, the fourth coordination site in the alkyl- and acylpaUadium intermediates may be strongly occupied, therefore no catalysis takes place. 

Accordingly, in order to understand the roles of active sites in catalysis, we should clarify not only the intermediates but also the functions of active sites in relation to elementary processes. For this reason, it may be an interesting question whether the two reactions taking place via the same kind of intermediates occur on the same or different active sites. One good example is the isomerization of olefins via alkyl intermediates and their subsequent hydrogenation. In the Horiuti and Polanyi mechanism, complete overlapping of the intermediates, as well as the reaction routes, was tacitly assumed as described in Eq. (1). In this reaction scheme, step (1) and step ( ) are the... 

A number of methods employ an imidazole-2-thione or related compound with a bifunctional reagent to build the thiazine ring in one or two steps. Treatment of (682) with acrylyl chloride directly produced (683) (64JOC1720). Substituting acrylic acid under catalysis allows isolation of the S-alkylated intermediate, which cyclized to (683) upon heating (64JOC1715). 

The appropriate interpretation may depend on the range of structures considered, the catalyst and the conditions employed. The interpretations differ in that in one, the rate of desorption is assumed to be fast relative to the conversion of adsorbed alkene to the alkyl intermediate, while in the other, desorption is assumed to be relatively slow. Catalysis by palladium may conform more closely to the reversible adsorption model than does platinum. 

Anti addition has been observed in compounds which lack an allylic hydrogen that can participate in a 1,3-hydrogen shift as above. With heptane as solvent (25 °C, 1 atm), the hydrogenation of dimethyl bi-cyclo[2.2.2]oct-2-ene-2,3-dicarboxylate (21) yields 7.1% of the anti addition product (24) over a Pt/C catalyst, but only syn addition (22) occurs over Rh/C. ° An acidic medium (small amounts of a strong acid in methanol) has little effect on catalysis by Rh, but greatly increases the fraction of anti addition catalyzed by Pd/C, as much as 60% in the presence of p-TsOH. Evidence for the participation of the solvent was assumed to indicate a proton transfer to the alkyl intermediate with inversion of configuration at the attached carbon atom. 

In 1998, Knochel reported that, in the presence of 4-trifluoromethylstyrene, [Ni(acac)2] efficiently catalyzed cross-couplings between polyfunctional arylzinc derivatives and alkyl halides possessing P-hydrogens (Equation 5.15). While the alkyl halides were limited to primary alkyl iodides, the scope of nickel catalysis was significantly expanded. The role of the electron-deficient olefin, 4-trifluoromethylstyrene, was proposed to accelerate the reductive elimination step by decreasing the electron density at the nickel center of an (alkyl) (aryl)nickel intermediate [18]. 

The final synthesis we will consider uses asymmetric catalysis to establish absolute stereochemistry. Thus, treatment of cyclohexenone (172) with dimethylzinc and catalytic Cu(I) in the presence of chiral ligand 173 proceeded with good asymmetric induction. Alkylation of the intermediate enolate using allyl acetate in the presence of Pd(0) provided 174 with 96% ee. This ketone was reduced to provide a mixture of alcohols which were separated and converted to 171 by degrading the allylic side chain to a carboxyl group, and displacing the alcohol with a nitrogen nucleophile. 

To illustrate the inner-sphere characteristics of the CH activation chemistry, an analogy can be made between CH activation by coordination of an alkane CH bond to a metal center and the known catalysis resulting from coordination of olefins via the CC double bond (note that the nature of the orbitals involved in bonding are quite different). It is well known that coordination of olefins to electrophilic metal centers can activate the olefin to nucleophilic attack and conversion to organometallic, M-C, intermediates. The M-C intermediates thus formed can then be more readily converted to functionalized products than the uncoordinated olefin. An important example of this in oxidation catalysis is the Wacker oxidation of ethylene to acetaldehyde. In this reaction, catalyzed by Pd(II) as shown in Fig. 7.14, ethylene is activated by coordination to the inner-sphere of an electrophilic Pd(II) center. This leads to attack by water and facile formation of an organometallic, palladium alkyl intermediate that is subsequently oxidized to acetaldehyde. The reduced catalyst is reoxidized by Cu(II) to complete the catalytic cycle. The Wacker reaction is very rapid and selective and it is possible to carry out the reaction is aqueous solvents. This is largely possible because of the favorable thermodynamics for coordination of olefins to transition metals that can be competitive with coordination to the water solvent. The reaction is very selective presumably because the bonds of the product (po-... 

With the catalysis of strong Lewis acids, such as tin(IV) chloride, dipyrromethenes may aiso be alkylated. A very successful porphyrin synthesis involves 5-bromo-S -bromomethyl and 5 -unsubstituted 5-methyl-dipyrromethenes. In the first alkylation step a tetrapyrrolic intermediate is formed which cyclizes to produce the porphyrin in DMSO in the presence of pyridine. This reaction sequence is useful for the synthesis of completely unsymmetrical porphyrins (K.M. Smith, 1975). 

Reactions 33 and 35 constitute the two principal reactions of alkyl hydroperoxides with metal complexes and are the most common pathway for catalysis of LPOs (2). Both manganese and cobalt are especially effective in these reactions. There is extensive evidence that the oxidation of intermediate ketones is enhanced by a manganese catalyst, probably through an enol mechanism (34,96,183—185). 

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

The most widely used process for the production of phenol is the cumene process developed and Hcensed in the United States by AHiedSignal (formerly AHied Chemical Corp.). Benzene is alkylated with propylene to produce cumene (isopropylbenzene), which is oxidized by air over a catalyst to produce cumene hydroperoxide (CHP). With acid catalysis, CHP undergoes controUed decomposition to produce phenol and acetone a-methylstyrene and acetophenone are the by-products (12) (see Cumene Phenol). Other commercial processes for making phenol include the Raschig process, using chlorobenzene as the starting material, and the toluene process, via a benzoic acid intermediate. In the United States, 35-40% of the phenol produced is used for phenoHc resins. 

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