Carbonylative mechanism

When dicobalt octacarbonyl, [Co(CO)4]2, is the catalyst, the species that actually adds to the double bond is tricarbonylhydrocobalt, HCo(CO)3. Carbonylation, RCo(CO)3- -CO—>RCo(CO)4, takes place, followed by a rearrangement and a reduction of the C—Co bond, similar to steps 4 and 5 of the nickel carbonyl mechanism shown in 15-30. The reducing agent in the reduction step is tetra-carbonylhydrocobalt HCo(CO)4, ° or, under some conditions, H2. When HCo(CO)4 was the agent used to hydroformylate styrene, the observation of CIDNP indicated that the mechanism is different, and involves free radicals. Alcohols can be obtained by allowing the reduction to continue after all the carbon monoxide is... 

Figure 7.8. Cobalt carbonyl mechanism after Mirbach [1]...

This final reaction step of the carbonylation mechanism is the primary distinguishing feature of each carbonylation process. A sufficient concentration of water or acetic acid in the reactor is therefore necessary to achieve high acetic acid or acetic anhydride formation rates respectively. 

Credence for this general carbonylation mechanism is supported by IR model studies in various solvents of key steps in the proposed reaction pathway [5b, 6-8, 9c,e]. These investigations include isolation and characterization of the acyl carbonyl complex as the dimer [10] and most recently spectroscopic evidence of the methyl intermediate in the presence of excess CH3I [9c, 9e]. The carbonylation rate is independent of the type of rhodium compound charged to the reaction as long as sufficient CH3I and CO are available. This supports the concept of the generation of a common active catalyst under reaction conditions [11, 12]. 

For a long time, the indirect carbonylation reaction was believed to proceed via that modified direct carbonylation mechanism. In the early 1970s, such a belief was also supported by the demonstration that the described scavenger reaction, known to be feasible with free isocyanates, could be applied as well to isocyanates complexed on various metal centers [54, 55]... 

Under both acidic and basic conditions, a carbonyl compound is electrophilic at the carbonyl carbon and nucleophilic at the a-carbon. Similarly, under both acidic and basic conditions, an a,/3-unsaturated carbonyl compound is electrophilic at the carbonyl and /3-carbons and nucleophilic at the a-carbon after a nucleophile has already added to the /3-carbon. Carbonyl mechanisms differ under acidic and basic conditions only in the protonation and deprotonation steps under acidic conditions, nucleophilic addition is always preceded by protonation of the electrophile and followed by deprotonation of the nucleophile. 

Reactions.—Carbonyls. Mechanism. Additional evidence is appearing that 1,2-oxaphosphetans are formed directly from ylides and carbonyl compounds. Further studies on the kinetics of the reactions of phenacylidenetriphenyl-phosphoranes with substituted benzaldehydes in non-polar aprotic solvents and in glycols support the concept of a highly oriented transition state of low polarity. n.m.r. spectroscopy on solutions in which ylides and carbonyl compounds had been allowed to react at — 70 °C showed the presence of only... 

VII. Other Glycosides which are Degraded by the beZa-Alkoxy Carbonyl Mechanism. 323... 

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