Hydroformylation, aldehydes from, with mechanism

The generally accepted mechanism for the hydroformylation of olefins catalyzed by Co2(CO)g was first proposed by Heck and Breslow and is depicted in Scheme 1 for the formation of a linear aldehyde. The proposed mechanism includes the generation of HCo(CO)4 from Co2(CO)g and hydrogen as the first step, followed by the three crucial unit processes mentioned above. Instead of hydrogenolysis of the acyl-cobalt species, RCH2CH2CO-Co(CO)4, reductive cleavage of the acyl-cobalt species with HCo(CO)4 is also possible to regenerate Co2(CO)g. 

Under lower CO partial pressures a 16e RCo(CO)3 species will have a long enough lifetime to allow reverse P-Hydride Elimination (see Mechanisms of Reaction of Organometallic Complexes) and increase the possibility for alkene reinsertion to the branched alkyl species, which is slightly more favored thermodynamically. At this point, CO addition and insertion will yield a branched aldehyde, or another /3-hydride elimination can give alkene isomerization. This second mechanistic explanation is in line with more recent results from Rh/PPh3-catalyzed hydroformylation studies (see Section 2.4). 

In 1952, it was discovered by Schiller that rhodium salts generated highly active hydroformylation catalysts. It was from these early studies that rhodium was estimated to be 1000 to 10 000 times more active than cobalt. Rhodium was also found to be very selective to aldehydes, with httle hydrogenation to alcohols observed under normal catalysis conditions. It was suggested early on that HRh(CO)4 was the active catalyst species, analogous to HCo(CO)4, and the same monometallic mechanism was proposed (Scheme 6). 

It is observed from Figures 4 and 5 that the molar ratio of normal to iso aldehyde products decreased significantly with time, and it obviously varied with the agitation intensity and stirrer type. This phenomenon, closely related to the mechanisms of biphasic hydroformylation, is difficult to explain even in a qualitative manner. At the same time, significant variation of emulsification of the reaction mixture occurred in correlation with the hydroformylation system, surfactant addition, and agitation. 

The mechanism of the hydroformylation has been intensively investigated by Drent and Budzelaar [6], who analyzed the competition between alternative reactions once a Pd-acyl complex was formed from a Pd-hydride species. The reaction with a second olefin leads to ketones (hydroacylation) and polyketones (copolymerization), respectively, whereas upon hydrogenolysis of the Pd-acyl bond, an aldehyde is released and thus a catalytic hydroformylation cycle is finally closed. Because of the high hydrogenation activity of palladium complexes, the aldehydes formed may be immediately converted into the corresponding alcohols. The type of the actually observed reaction pathway is mainly determined by [6]... 

A mechanism was suggested on the basis of deuteration labeling studies and by the isolation of some intermediates (Scheme 8.17) [4]. In the first stage, the catalyst 1 is loaded with the substrate aldehyde. Under the effect of benzoic acid, the hydrido-Rh-acyl complex 2 is formed. After decarbonylation of the acyl unit, the corresponding Rh-alkyl complex 4 is obtained, which is immediately converted into the jc-olefin complex 5. Exchange of the olefin by the formyl acceptor olefin (here NBD) and subsequent hydroformylation of the latter result in the net transfer of the formyl group from one aldehyde to the other. 

The mechanism of ethylene l ydroformy1at1on with Pt/Sn catalysts has been invest gated.279 j, e diphenyl phosphine oxide complex (58) catalyses the hydroformylation of 1-hexene and even 2-hexene to give mainly linear aldehydes and alcohols best results were obtained from an in-situ mixture of PtCcodlj, Ph2P0H and dppe (1 1 1). There is no activity without Ph2P0H.280... 

The mechanism of olefin hydroformylation catalyzed by rhodium complexes has been extensively studied. For TPP as a ligand, it corresponds to Wilkinson s dissociative mechanism, which involves the four-coordinated active intermediate HRh(CO)L2 (L = TPP, Figure 14.2). Coordination of olefin with HRh(CO)L2 yields the 7t-complex 2. The insertion of coordinated olefin to the Rh-H bond leads to the formation of alkyl complexes 3a or 3b, respectively, via the anti-Markovnikov or the Markovnikov path. Subsequently, the alkyl migration to the CO affords the acyl complexes 4a or 4b, which leads to linear or branched aldehyde and HRh(CO)L2 via hydrogenolysis, eventually. The water-soluble catalyst HRh(CO)(TPPTS)3 is considered to react according to the dissociative mechanism [10]. However, the reaction occurs at the liquid phase or the gaseous-Hquid interface [11], and the activity and selectivity are remarkably different from those... 

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