Hydrido catalytic cycle

Hydrido complexes of all three elements, and covering a range of formal oxidation states, are important because of their roles in homogeneous catalysis either as the catalysts themselves or as intermediates in the catalytic cycles. 

A further complication is evident in the spectroscopic studies of the reacting iridium solutions, namely, a competing catalytic water gas shift reaction involving hydrido-iridium(III) species. Choice of reaction conditions determines the proportion of the iridium occupied in this catalytic cycle. 

The organometallic compound 44 should decompose via /J-hydridc elimination to olefin 45a and hydrido titanium species 46 in the absence of acid. The catalytic cycle is interrupted due to the consumption of Cp2TiCl. In the presence of Coll HCl, protonation of the Ti - H and Ti - O bonds in 46 and... 

The rr-bonded coupling product dissociates from the hydrido-Pd(II) complex in step 7, and another hydrido-Pd(II) complex is formed (Figure 13.26). It loses trifluo-romethane-sulfonic acid in step 8 of the catalytic cycle. Thereby, the same valence-unsaturated Pd(0) complex that initiated the reaction in step 1 is formed, so this complex is now available to begin another cycle. 

Based on the known reactions of cobalt carbonyls, the catalytic cycle shown in Fig. 4.7 has been suggested. The hydrido carbonyl 4.11 is converted to 4.19, a formyl species. Formyl complexes are considered to play a key role in all... 

The principal modes of conversion to hydrido complexes, oxidative addition, homolysis, and heterolysis, are shown in eqnations (40 42), respectively. Oxidative addition of hydrogen to a transition metal complex at some stage in the catalytic cycle is essential to its activity. This requires that the metal has two accessible oxidation states corresponding to d" and electronic configurations or d" and for dinuclear oxidative addition. Hence, only those metals whose complexes meet this criterion may act as catalysts. 

Quite stable catalytic reaction solutions were obtained in THF with the starting pressure for ethylene of 6-6.5 MPa at a reaction temperature of 120 °C. Under these conditions and with the ratios piperidine/rhodium of 100 1 and 1000 1 in 36 and 72 h, yields of 70 and 50 % ethylpiperidine were reached, which correspond to TONs of 2 and 7 mol amine/(mol Rh) per h, respectively. Total conversion is also possible if the reaction time is prolonged further. As a side reaction, ethylene dimerization to butene was observed. This indicates the formation of a hydrido rhodium(III) complex in the hydroamination reaction, as formulated in Scheme 3, route (b). Hydrido rhodium(III) complexes are known as catalysts for ethylene dimerization [19], and if the reductive elimination of ethylpiperidine from the hydrido-y9-aminoethyl rhodium(III) complex is the rate-limiting step in the catalytic cycle of hydroamination, a competitive catalysis of the ethylene dimerization seems possible. In the context of these mechanistic considerations, an increase of the catalytic activity for hydroamination requires as much facilitation of the reductive elimination step as possible. 

The final step of the catalytic cycle, base-assisted reductive elimination, has been addressed by Deeth et al. [14]. In their calculations, the authors investigated palladium complexes with the chelating diaminomethane H2N(CH2)NH2 and di-phosphinomethane H2P(CH2)PH2 ligands. Within this system, they found that the postulated hydrido-olefin complex, which is usually formed by p-H elimination of the y9-agostic insertion product, is in fact not a stable minimum structure in this particular case (eq. (11)) [14]. 

These observations are summarized in a mechanistic proposal shown in Scheme 1. The palladium hydrido species (compare Eq. 2) is able to add equally to the double bonds in positions 9 and 12. The alkene inserts into the Pd H bond, yielding the corresponding alkyl complexes. After reaction with hydrogen the monounsaturated fatty acids (C18 1) are set free, and the palladium hydride species is formed again, thus starting a new catalytic cycle. 

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