Carbonic acid cobalt complex

O3BF4RU2C15H13, Ruthenium(l+), jLplatinum chain complex, 21 153, 154... 

Combinatorial chemistry and solid-phase synthesis have evolved in the last decade to become one of the most important techniques to save time for drug discovery. To reach its full potential, the solid-phase synthesis has to incorporate many versatile organometallic reactions developed over recent several decades. The first example of the Nicholas reaction on solid phase was reported by Kann and his co-workers in 2002, which involves the reaction of polymer-bound cobalt complexes 51 with various carbon-centered nucleophiles in the presence of a Lewis acid to... 

Reductive decarboxylation of (20) yields C02, H+, and a Co(I) species at a measurable rate (94). In the presence of CO, the starting cobalt complex is regenerated, and a catalytic system for the oxidation of CO by ferricyanide is established. It is significant that in this system the metal-carbonyl bond is formed when the cobalt is in a reduced state. It is the subsequent oxidation of the cobalt by electron transfer that activated the carbonyl to attack by water or hydroxide. That this activation results in a weaker metal-carbonyl bond is evident since the Co(III)-carbonyl may be hydrolyzed in acidic solution with loss of the carbon monoxide ligand (94). 

Co(in) complexes promote similar reactions. When four of the six octahedral positions are occupied by amine ligands and two cis positions are available for further reactions, it is possible to study not only the hydrolysis itself, but the steric preferences of the complexes. In general, these compounds catalyze the hydrolysis of N-terminal amino acids from peptides, and the amino acid that is removed remains as part of the complex. The reactions apparently proceed by coordination of the free amine to cobalt, followed either by coordination of the carbonyl to cobalt and subsequent reaction with OH or H2O from the solution (path A in Figure 12-15) or reaction of the carbonyl carbon with coordinated hydroxide (path B). As a result, the N-terminal amino acid is removed from the peptide and left as part of the cobalt complex in which the a-amino nitrogen and the carbonyl oxygen are bonded to the cobalt. Esters and amides are also hydrolyzed by the same mechanism, with the relative importance of the two pathways dependent on the specific compoimds used. 

In general, the acetylenic triple bond is highly reactive toward hydrogenation, hydroboration, and hydration in the presence of acid catalyst. Protection of a triple bond in disubstituted acetylenic compounds is possible by complex formation with octacarbonyl dicobalt [Co2(CO)g Eq. (64) 163]. The cobalt complex that forms at ordinary temperatures is stable to reduction reactions (diborane, diimides, Grignards) and to high-temperature catalytic reactions with carbon dioxide. Regeneration of the triple bond is accomplished with ferric nitrate [164], ammonium ceric nitrate [165] or trimethylamine oxide [166]. 

Co2(CO)g, TFA, CH2CI2, rt, 30 min, 88-quant, yield.These conditions also cleave propargyl esters. The cobalt complex helps to stabilize a positive charge, thus facilitating cleavage of the carbamate and the carbonate with acid. 

A relatively new approach involves the functionalization of phosphines or other ligands with weakly basic or acidic functionalities. An early study reports on a modified cobalt catalyst [12]. This cobalt carbonyl complex containing the P(CH2CH2NEt2)3 ligand was claimed to be extractable into dilute carbonic acid and could be re-extracted into an organic phase by simply reducing the C02 pressure. 

OjSCjH4, Acetic acid, 2-mercaptocobalt complex, 21 21 03CH2, Carbonic acid platinum chain complex, 21 153, 154 cobalt complex, 21 120 OjPCjH, Trimethyl phosphite iron complex, 21 93... 

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