Solvent-free iodination of organic molecules using the I2/urea–H2O2 reagent system

Abstract

Introduction of iodine under solvent-free conditions into several aromatic compounds activated toward electrophilic functionalisation was found to proceed efficiently using elemental iodine in the presence of a solid oxidiser, the urea–H₂O₂ (UHP) adduct. Two types of iodo-functionalisation through an electrophilic process were observed: iodination of an aromatic ring, and side-chain iodo-functionalisation in the case of arylalkyl ketones. Two reaction routes were established based on the required substrate : iodine : oxidiser ratio for the most efficient iodo-transformation, and the role of UHP was elucidated in each route. The first, requiring a 1 : 0.5 : 0.6 stoichiometric ratio of substrate to iodine to UHP, followed the atom economy concept in regard to iodine and was valid in the case of aniline (1a), 4-t-Bu-phenol (3), 1,2-dimethoxy benzene (5a), 1,3-dimethoxy benzene (5b), 1,2,3-trimethoxy benzene (7a), 1,2,4-trimethoxy benzene (7b), 1,3,5-trimethoxy benzene (7c), 1-indanone (11a) and 1-tetralone (11b). The second reaction route, where a 1 : 1 : 1 stoichiometric ratio of substrate : I₂ : UHP was needed for efficient iodination, was suitable for side-chain iodo-functionalisations of acetophenone (1c) and methoxy-substituted acetophenones. Moreover, addition of iodine to 1-octene (13a) and some phenylacetylenic derivatives (15a, 15b) was found to proceed efficiently without the presence of any oxidiser and solvent at room temperature.