Selective radical reactions in multiphase systems: Phase-transfer halogenations of alkanes

The present paper shows that selective radical reactions can be initiated a nd carried out in multiphase systems. This concept is applied to the select ive functionalization of unactivated aliphatic hydrocarbons, which may be l inear, branched, and (poly)cyclic, strained as well as unstrained. The phas e-transfer system avoids overfunctionalization of the products and simplifi es the workup; the selectivities are excellent and the yields are good. Thi s is the only method for direct preparative iodination of alkanes applicabl e to large scale as well. We demonstrate that the reaction systems are inde ed phase-transfer catalyzed through a systematic study of variations of the reactants, solvents, catalysts, and by measuring as well as computing the H/D kinetic isotope effects for the rate-limiting C-H abstraction step by . CHal(3) radicals which are held responsible for the observed radical react ions. In the case of . CBr3, this key intermediate could also be trapped un der otherwise very similar reaction conditions. To stimulate further work, the tolerance of some functional groups was tested as well.