Carbon monoxide trapping reagents

Addition of nucleophiles to a carbon monoxide ligand of pentacarbonyliron provides anionic acyliron intermediates which can be trapped by electrophiles (H+ or R—X) to furnish aldehydes or ketones [18]. However, carbonyl insertion into alkyl halides using iron carbonyl complexes is more efficiently achieved with disodium tetracarbonylferrate (Collman s reagent) and provides unsymmetrical ketones (Scheme 1.2) [19, 20]. Collman s reagent is extremely sensitive towards air and moisture, but offers a great synthetic potential as carbonyl transfer reagent. It can be prepared by an in situ procedure starting from Fe(CO)5 and Na-naphthalene [20]. 

Irradiation of Fischer carbene complexes generates, by insertion of carbon monoxide, a metal-bound ketene intermediate. Photolytic reactions of carbene complexes are synthetically attractive, in that the reaction conditions are mild and the reactions of ketene intermediates with a variety of reagents is of significant scope. A low concentration of metal-bound ketene is probably obtained and in the absence of a nucleophile, the starting material can usually be recovered even after prolonged irradiation. The ketene intermediates are readily trapped with nucleophiles for example, dipeptides are formed in excellent yield and with very high diastereoselectivity upon irradiation of optically active carbenes in the presence of natural or urmatural a-amino acids (Scheme 28). Dipeptides and PEG-supported amino acids and dipeptides can also be used as nucleophiles. 

Despite the presence of a formally divalent carbon atom, CO is not in fact a particularly reactive molecule and much of its chemistry depends on the use of either extreme conditions, energetic reagents or some form of catalysis. Perhaps the simplest examples of such catalysis are found in the reactions of carbon monoxide with protic reagents such as alcohols or secondary amines, affording esters or amides of formic acid. These reactions are catalyzed by alkoxide or amide anions, respectively, and, as shown in Scheme 1, the key step is nucleophilic attack on CO by the catalyst to give a strongly basic alkoxyacyl or aminoacyl anion which is immediately trapped by proton transfer from the alcohol or amine, so generating the catalytic species. 

Conditions have been defined for the preparation of acylsilanes by trapping acyl anions, formed from alkyl-lithium reagents and carbon monoxide, at low temperature with trimethylsilyl chloride. Acylsilanes have found use as hindered aldehydes, improving selectivity in the addition of ambident nucleophiles. ... 

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