Cobalt based catalytic cycle

The mechanism we have proposed (13) for the cobalt-based catalytic cycle is shown (slightly modified) in Chart I. For the reasons described immediately above, the transition state is presumed to occur between the left- and right-hand sides of eq. 8. 

There are two main differences between cobalt- and rhodium-based catalytic cycles. In the cobalt-catalyzed cycle it is the nucleophilic attack by 4.7 on CH3I rather than the oxidative addition of CH3I on any unsaturated 16-electron species that initiates the catalytic cycle. Second, in the rhodium cycle reductive elimination generates acetyl iodide, whereas in the cobalt cycle it is the attack by I on 4.10 that produces acetyl iodide. Thus oxidative addition and reductive elimination steps are not involved in the cobalt cycle, but play crucial roles in the rhodium cycle. 

In 2003, Cenini and coworkers reported (tetraarylporphyrin)cobalt(II) complexes 326 as efficient catalysts (1 mol%) for cyclopropanations. In the absence of air, styrenes 321 underwent an efficient cyclopropanation with ethyl diazoacetate 322 giving cyclopropanes 324 in 65-99% yield with 3-5 1 trans/cis ratios (Fig. 77) [348]. Simple olefins and more hindered diazoesters did not react. With diazoacetate and hydrocarbons, such as cyclohexane or benzene, C-H insertion took place furnishing cyclohexyl- or phenylacetate. In line with Ikeno s proposal the cyclopropanation reaction was considerably slowed down in the presence of TEMPO, though not completely inhibited. Based on a kinetic analysis a two-electron catalytic cycle with a bridged carbene unit was formulated, however. 

Carbonylation with iron carbonyls parallels that of cobalt carbonyls. Benzylic chlorides and bromides are carbonylated with Fe(CO)5 in the presence of base. Esters are realized when carbonylation is performed in alcohols under 1 atm of CO with catalytic amounts of iron pentacarbonyl415. Under phase transfer conditions, two predominant routes are available. With catalytic amounts of iron under a CO atmosphere and strongly basic conditions, the carboxylic acids are realized in reasonable yields415,416, whereas mild bases [Ca(OH)2l, stoichiometric amounts of iron carbonyl and the omission of CO give dibenzyl ketones417. In at least a few cases, it is possible to prepare unsymmetrical methyl benzyl ketones418, des Abbayes and coworkers have observed the formation of acyltetracarbonyl anion (52) under the reaction conditions, and have proposed the catalytic cycle in Scheme 8 for the ketone formation418. 

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... 

Some mechanistic information is available on ruthenium-based homogeneous Fischer-Tropsch reactions. By in situ IR spectroscopy, in the absence of any promoter, only Ru(CO)5 is observed. An important difference between the cobalt and the rhodium system on the one hand and ruthenium on the other is that in the latter case no ethylene glycol or higher alcohols are obtained. In other words, in the catalytic cycle the hydroxymethyl route is avoided. 

The catalytic cycle for the cobalt-based hydroformylation is shown in Fig. 5.7. Most cobalt salts under the reaction conditions of hydroformylation are converted into an equilibrium mixture of Co2(CO)8 and HCo(CO)4. The latter undergoes CO dissociation to give 5.20, a catalytically active 16-electron intermediate. Propylene coordination followed by olefin insertion into the metal-hydrogen bond in a Markovnikov or anti-Markovnikov fashion gives the branched or the linear metal alkyl complex 5.24 or 5.22, respectively. These... 

It has been shown that ligands such as phosphines can control the course of homologation reaction to a large extent. Tims, it seems likely that the Ugands are coordinated to the metal center during the catalytic cycle. Based on this assumption and on the observation that the best results were obtained with a cobalt-phosphine-iodide ratio of 1 2 2, Roper and Loevenich proposed the mechanism shown in Scheme 2. 

We have found that cobalt bromide catalysis is based on a reaction involving peroxy radical and Co(II) leading to hydroperoxide formation and Co(UI) in a rate determining step (eq. 1) which is then followed by a rapid catalytic reduction of the generated Co(III) by bromide ions in the presence of hydrocarbon (eq. 2-4). The overall scheme involves two catalytic cycles and is given by equations (l)-(4) below. 

Cobalt-based catalysts were the first to be employed. Under the conditions of the reaction (370-470 K, 100 400 bar), Co2(CO)g reacts with H2 to give HCo(CO)4. The latter is usually represented in catalytic cycles as the precursor to the coordinatively unsaturated (i.e. active) species HCo(CO)3. As equation 27.20 shows, hydroformylation can generate a mixture of linear and branched aldehydes, and the catalytic cycle in Figure 27.11 accounts for both products. All steps (except for the final release of the aldehyde) are reversible. To interpret the catalytic cycle, start with HCo(CO)3 at the top of Figure 27.11. Addition of the... 

Catalytic cycle for the carbonylation of epoxides catalyzed by the aluminum/cobalt catalyst. L = Lewis base (solvent, epoxide, lactone). 

This suggests that a catalytic cycle through Co2(CO)8 cannot be important at least not at high olefin and low cobalt concentration. A contradictory conclusion (137) is based on misinterpretation. For details, see Ref (130). 

Carbonylation and decarbonylation reactions of alkyl complexes in catalytic cycles have been reviewed . A full account of the carbonylation and homologation of formic and other carboxylic acid esters catalysed by Ru/CO/I systems at 200 C and 150-200 atm CO/H2 has appeared. In a novel reaction, cyclobutanones are converted to disiloxycyclopentenes with hydrosilane and CO in the presence of cobalt carbonyl (reaction 4) . The oxidative addition of Mel to [Rh(CO)2l2] in aprotic solvents (MeOH, CHCI3, THF, MeOAc), the rate determining step in carbonylation of methyl acetate and methyl halides, is promoted by iodides, such as Bu jN+I", and bases (eg 1-methylimidazole) . A further kinetic study of rhodium catalysed methanol carbonylation has appeared . The carbonylation of methanol by catalysts prepared by deposition of Rh complexes on silica alumina or zeolites is comparable with the homogeneous analogue . 

Ruthenium compounds are widely used as catalysts for hydrogen transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. For example, BackvaU and coworkers [40] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt-Schiff s base complex. The coupled catalytic cycle is shown in Figure 5.8. A low-valent ruthenium complex reacts with the alcohol to afford the aldehyde or ketone product and a ruthenium dihydride. The latter undergoes hydrogen transfer to the benzoquinone... 

Cobalt-catalyzed C—H functionalization reactions are currently classified into two main categories based on their hypothetical catalytic cycles (i) hydroarylation of alkynes and olefins and (ii) C—H/electrophile coupling. A separate category involving arylzincation of alkynes has also been proposed. This chapter explores cobalt-catalyzed hydroarylation and C—H electrophile coupling since 2007. 

The dechlorination of chlorinated alkenes could also be performed by porphyrin cobalt complex such as 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin cobalt ((TCPP)-Co). This cobalt complex, structurally similar to vitamin B12, was found to have superior aqueous-phase dechlorination activity on chlorinated ethylenes relative to vitamin Bi2. Based on fully detailed parameters dependence, the authors suggest the catalytic cycle below.This methodology has been used to synthesize C-labeled air-DCE from TCE (Scheme 33). ... 

---