title: Radical carbonylation with [11C]carbon monoxide promoted by oxygen-centered radicals: experimental and DFT studies of the mechanism.
abstract: Photoinitiated carbonylation of alkyl iodides with [11C]carbon monoxide (11C t1/2=20.4 min) is enhanced by ketones that have lowest-lying excited triplet state of npi* character. For example, adding 5 mol % of acetophenone increases radiochemical yields from 3 to 59% in brief 6-min long reactions. Similar or higher yields were achieved by adding di-tert-butyl peroxide. Since radicaloid npi* exited-state ketones and tert-butoxyl radicals have similar reactivity, the photosensitization proceeds most likely via a H-atom transfer mechanism rather than via energy transfer. We propose a mechanism that can account for the enhancement as well as for the formation of observed byproducts. The energy profile obtained by DFT calculations support the feasibility of the mechanism, and observed experimental differences in reactivity could be well rationalized by the calculated data. NBO calculations were performed to further analyze the obtained energetics. Various [carbonyl-11C]esters and some [carbonyl-11C]amides were synthesized in good radiochemical yields from primary and secondary alkyl iodides illustrating the utility of dialkyl peroxides to accelerate the carbonylations. These findings have potential in elaborating new synthetic protocols for the production of 11C-labeled tracers for positron emission tomography.
authors.affiliation: Department of Biochemistry and Organic Chemistry, Uppsala University, Box 576, SE-75123 Uppsala, Sweden. oleksiy.itsenko@biorg.uu.se
authors.forename: Oleksiy
authors.initials: O
authors.lastname: Itsenko
authors.forename: Daniel
authors.initials: D
authors.lastname: Norberg
authors.forename: Torben
authors.initials: T
authors.lastname: Rasmussen
authors.forename: Bengt
authors.initials: B
authors.lastname: Långström
authors.forename: Chryssostomos
authors.initials: C
authors.lastname: Chatgilialoglu
mesh_terms: D002248:Carbon Monoxide / Q000737:chemistry*; D002250:Carbon Radioisotopes / Q000737:chemistry; D004952:Esters / Q000138:chemical synthesis; D005609:Free Radicals / Q000737:chemistry*; D007553:Isotope Labeling; D007659:Ketones / Q000737:chemistry; D008958:Models, Molecular; D008968:Molecular Conformation; D016011:Normal Distribution; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D010545:Peroxides / Q000737:chemistry
chemical_list: D002250:Carbon Radioisotopes; D004952:Esters; D005609:Free Radicals; D007659:Ketones; D010545:Peroxides; D002248:Carbon Monoxide; D010100:Oxygen