Oxidations of secondary alcohols to ketones using easily recyclable bis(trifluoroacetate) adducts of fluorous alkyl iodides, CF3(CF2)(n-1)I(OCOCF3)2.
Reactions of commercial fluorous alkyl iodides RfnI (1-Rfn; Rfn = CF3(CF2)(n-1); n = 7, 8, 10, 12) with 80% H2O2 and trifluoroacetic anhydride give RfnI(OCOCF3)2 (2-Rfn; 97-89%). These efficiently oxidize aliphatic and benzylic secondary alcohols to the corresponding ketones (92-57%) in the presence of aqueous KBr and absence of organic or fluorous solvents. Bromide ion activates the reagents and/or generates a relay oxidant such as a functional equivalent of Br+. Oxidations are much more rapid (<30 min, 2-R(f8); <70 min, 2-R(f10)) than with other iodine(III) compounds under similar conditions. The coproducts 1-Rfn can be recovered by adding 3-5 volumes of methanol to the reaction mixtures. Fluorous/methanolic liquid/liquid (1-R(f8)) or solid/liquid (1-R(f10)) biphase systems result. The recovered 1-Rfn can be reoxidized to 2-Rfn and reused. Three cycles are conducted with 1-phenyl-1-propanol and 2-R(f10). The propiophenone yields range from 92% to 83% per cycle, and after the final cycle 59-57% of the original charge of the fluorous iodide species is recovered.
Tesevic
Verona
V
Institut für Organische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Henkestrasse 42, 91054 Erlangen, Germany.
Gladysz
J A
JA