Catalytic cycle, acetylene

The above-postulated overall mechanism considers two alternative pathways depending on the nature of the acetylene derivative. Region A outlines a proposal in which the formation of the a-complex intermediate is supported by the fact that the treatment of aliphatic terminal acetylenes with FeCl3 led to 2-chloro-l-alkenes or methyl ketones (Scheme 12). The catalytic cycle outlined in region B invoked the formation of the oxetene. Any attempt to control the final balance of the obtained... 

Scheme 14 Top Plausible catalytic cycle as supported by deuterium labeling. Bottom ESI mass spectrum of a reaction mixture aliquot diluted 5000-fold in methanol from the hydrogen-mediated coupling of gaseous acetylene to an a-ketoester (Ar = p-N02Ph)...

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

Insertion of the alkyne into the Pd-H bond is the first step in the proposed catalytic cycle (Scheme 8), followed by insertion of the alkene and /3-hydride elimination to yield either the 1,4-diene (Alder-ene) or 1,3-diene product. The results of a deuterium-labeling experiment performed by Trost et al.46 support this mechanism. 1H NMR studies revealed 13% deuterium incorporation in the place of Ha, presumably due to exchange of the acetylenic proton, and 32% deuterium incorporation in the place of Hb (Scheme 9). An alternative Pd(n)-Pd(iv) mechanism involving palladocycle 47 (Scheme 10) has been suggested for Alder-ene processes not involving a hydridopalladium species.47 While the palladium acetate and hydridopalladium acetate systems both lead to comparable products, support for the existence of a unique mechanism for each catalyst is derived from the observation that in some cases the efficacies of the catalysts differ dramatically.46... 

Sonogashira has proposed a catalytic cycle (Figure 4) which shows 1) the reduction of the palladium complex, 2) coordination of the aryl halide and acetylene with the palladium (0) complex and 3) the reductive elimination of the substituted aryl acetylene and regeneration of the active catalyst.(10)... 

Remarkably, the catalytic cycle is not controlled by the presence of phosphine ligands, but it is controlled by the organo group Y at the cobalt the neutral ligand L is displaced by the substrates in the initial step. Oxidative addition of two acetylenes results in a cobaltacycle that reacts with the nitrile to give the pyridine derivative with regeneration of the active [YCo] species. 

This hydride then may add an acetylene molecule to form the vinyl derivative. A carbon monoxide insertion will produce the acrylyl nickel compound which can yield acrylate esters by either of two routes. Direct alcoholysis of the acyl nickel group may take place, as occurs with acylcobalt compounds (42) or, an acyl halide (or other acyl derivative, e.g., acyl alkanoate) may be eliminated. Alcoholysis of the acyl halide would then complete the catalytic cycle (35). 

The phenylacetylene formed would then re-enter the catalytic cycle for the second arylation. Cul is believed to assist the formation of the alkynylpalladium complexes. [Pd(PPh3)4] (103) and [PdCl2(diphos)] in the presence of Cul also catalyze the reaction, which requires only normal temperature and acetylene pressure. 

The general catalytic cycle in the Pd(0)/Cu(I)-promoted aryl-acetylene coupling reaction is shown in Figure 8.3 [3],... 

The catalytic cycle, though reasonable, is hypothetical. The proposed reactions and complexes have precedents in organometallic chemistry. The rate of the overall reaction is first order with respect to methyl acetylene and independent (zero order) of acid concentration as long as sufficient acid and 4.31 are present. Indirect but strong evidence for the proposed mechanism comes from structural modifications of the ligand and effects of such modifications... 

The proposed catalytic cycles for the mono- and biscarbonylation of acetylene arc reported in Schemes I and 2. 

Vinyl sulfides have been prepared by the catalytic addition of the S—H bond of thiols (85) to terminal alkynes (86) under solvent-free conditions using the nickel complex Ni(acac)2 (47). High alkyne conversions (up to 99%) were achieved after 30 min at 40 °C in favor of the corresponding Markovnikov products (87) (equation 23). Other metal acetylacetonate complexes were examined for this reaction, but none showed any improvement over the nickel catalyst. Mechanistic details suggest that alkyne insertion into the Ni—S bond is important to the catalytic cycle and that nanosized structural units comprised of [Ni(SAr)2] represent the active form of the catalyst. Isothiocyanates and vinyl sulfides have been produced in related Rh(acac)(H2C=CH2)2 (6) and VO(acac)2 (35) catalyzed sulfenylation reactions of aryl cyanides and aryl acetylenes, respectively. 

R = H) [26]. However, the turnover number was very limited (about 100). The strong incentive for further developments lies in the fact that both HCN and acetylene are cheap bulk chemicals in industry. The introduction of boron into the carbocyclic ligand attached to the cobalt enhances the catalytic activity considerably, probably via the suppression of the protolytic 1,4-addition of HCN to the olefinic cobaltacycle the resulting cyaino-substituted 1,3- dienes cannot be displaced from the cobalt center by acetylene and the catalytic cycle is stopped (eq. (4)). 

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