Cobalt species, accelerators

Several reports have appeared on the effect of additives on the Pauson-Khand reaction employing an alkyne-Co2(CO)6 complex. For example, addition of phosphine oxide improves the yields of cyclopentenones 119], while addition of dimethyl sulfoxide accelerates the reaction considerably [20]. Furthermore, it has been reported that the Pauson-Khand reaction proceeds even at room temperature when a tertiary amine M-oxide, such as trimethylamine M-oxide or N-methylmorpholine M-oxide, is added to the alkyne-Co2(CO)6 complex in the presence of alkenes [21]. These results suggest that in the Pauson-Khand reaction generation of coordinatively unsaturated cobalt species by the attack of oxides on the carbonyl ligand of the alkyne-Co2(CO)6 complex [22] is the key step. With this knowledge in mind, we examined further the effect of various other additives on the reaction to obtain information on the mechanism of this rearrangement. 

Cobalt species in combination with graphitic carhon nitride (g-CN) generate a new hybrid photosystem, based on inexpensive substances, that synergically catalyse C02-to-CO conversion under mild conditions with visible light. g-CN acts as both a capture/activation substrate of CO2 and a photocatalyst. On the other hand, cobalt species act as redox promoters to accelerate both charge-carrier separation and transfer kinetics. 

In an attempt to improve on the original PKR, Pauson reported on the effects of various additives. Phosphines, ether as preformed cobalt species or simply added to the PKR, only served to reduce the rate and yield of the reaction. On the other hand, ultrasound and tributylphosphine oxide proved equally effective at accelerating the rate of reaction and increasing the overall yield of the PKR. The reaction yields were described as erratic varying irreproducibly as the reaction atmosphere was changed. The use trimethylamine-A-oxide was mentioned but no results were reported. 

Metal-Catalyzed Oxidation. Trace quantities of transition metal ions catalyze the decomposition of hydroperoxides to radical species and greatiy accelerate the rate of oxidation. Most effective are those metal ions that undergo one-electron transfer reactions, eg, copper, iron, cobalt, and manganese ions (9). The metal catalyst is an active hydroperoxide decomposer in both its higher and its lower oxidation states. In the overall reaction, two molecules of hydroperoxide decompose to peroxy and alkoxy radicals (eq. 5). 

The reactive species that iaitiate free-radical oxidatioa are preseat ia trace amouats. Exteasive studies (11) of the autoxidatioa mechanism have clearly estabUshed that the most reactive materials are thiols and disulfides, heterocycHc nitrogen compounds, diolefins, furans, and certain aromatic-olefin compounds. Because free-radical formation is accelerated by metal ions of copper, cobalt, and even iron (12), the presence of metals further compHcates the control of oxidation. It is difficult to avoid some metals, particularly iron, ia fuel systems. 

Methyl methacrylate can also be polymerized by radiation using either a cobalt-60 source or accelerated electrons at dose rates up to 3 megarads/sec. The activation energy for the electron beam polymerization is about 7.0kcal/ mole (Ref 12). Radical polymerization can also occur using diisocyanates or hydroperoxides as the initiating species (Ref 15)... 

Evidence was presented that cobalt precursors under the reaction conditions are transformed into cobalt carbonyls.31 Additives such as Lewis bases accelerate the formation of the catalyst.11 [CoH(CO)4] the key catalytic species was shown by infrared (IR) spectroscopy to be formed under hydroformylation conditions32 and was isolated in the reaction of [Co(CO)4]2 and hydrogen.33 [CoH(CO)4] dissociates carbon monoxide to create [CoH(CO)3] [Eq. (7.2)], which is capable of olefin com-plexation because of a ligand vacancy ... 

For the reaction of MOH(n 1)+ with propionic anhydride,200 the Bronsted plot of log kMOH versus the pKa of MOH2n+ follows a smooth curve if the values for HzO and OH- are included (Figure 4). However, if the line is drawn to exclude the fcHj0 value, a Bronsted /3 of ca, 0.25 is obtained. Although kMOH for [Co(NH3)5OH]2+ (3 M s 1) is some 103-fold less than k0H, this reaction will compete favourably at neutral pH with base hydrolysis. At pH 7 where the cobalt(III) complex exists almost completely as the MOH2+ species the observed first order rate constant for nucleophilic attack by OH would be ca. 10-4 s 1. AIM solution of [Co(NH3)5OH]2+ would give a value of kobs 2.5 s 1, a rate acceleration of > 104-fold. Since the effective concentration of a nucleophile in the intramolecular reaction could be ca. 102 M, rate accelerations of 10° are possible. The role of the metal ion in such reactions is to provide an effective concentration of an efficient nucleophile at low pH. 

The rates of hydrolysis of amino acid esters or amides are often accelerated a million times or so by the addition of simple metal salts. Salts of nickel(n), copper(n), zinc(n) and cobalt(m) have proved to be particularly effective for this. The last ion is non-labile and reactions are sufficiently slow to allow both detailed mechanistic studies and the isolation of intermediates, whereas in the case of the other ions ligand exchange processes are sufficiently rapid that numerous solution species are often present. Over the past thirty years the interactions of metal ions with amino acid derivatives have been investigated intensively, and the interested reader is referred to the suggestions for further reading at the end of the book for more comprehensive treatments of this interesting and important area. 

The cobalt(O) species [Co(N2)(PPh3)3] is a catalyst for the isomerization of a-olefins (284). The isomerization is actually accelerated by dinitrogen, and this is believed to be due to the fact that the dinitrogen, which is probably a stronger ligand for cobalt in these materials than the olefins, displaces a metallation equilibrium as shown in Eq. (11). 

The cobalt-catalyzed autoxidation of toluene in acetic acid at 363 K is accelerated by butan-2-one and benzaldehyde because peroxy radicals play a minor role in ratecontrolling propagation reactions. High rates of autoxidation are also obtained in the presence of Br because bromine atoms are important chain-propagating species. ... 

Cobalt-dioxygen complexes, such as those formed from Co(salen) type species do not seem to play any significant role as intermediates. The reason for this conclusion is that the most effective catalysts are not oxygen-sensitive. The accelerating effect of cobalt complexes is due to... 

The decomposition of the peroxide to hydroxide and oxygen is a key rate-limiting step in the reaction sequence. To accelerate the reduction of the peroxide species and the overall reaction rate, the air cathode is formulated using catalytic compounds which promote the reaction in step 2. These catalysts are typically metal compounds or complexes such as elemental silver, cobalt oxide, noble metals and their compounds, mixed metal compounds including rare earth metals, and transition metal macrocyclics, spinels, manganese tUoxide, phtalocyanines or perovskites." - ... 

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