Visible Light Induced 1,2-aryl Migration Of α,α-diaryl Allylic Alcohols

Alkene difunctionalization, which can introduce two functional groups across a double bond, exemplifies a class of reactions with significant synthetic potential. As a result, several valuable strategies for difunctionalization of alkenes have been successfully developed to prepare versatile and fascinating compounds. Recently, aryl allylic alcohols were found to be viable substrates for difunctionalization of alkenes by transition-metal catalysis or oxidative radical addition under heating, which afforded α-aryl-β-alkylated ketones via 1,2-aryl migration.Nowadays, the application of visible light photoredox catalysis has emerged as a novel and powerful tool for chemical transformations in organic chemistry, owing to its attractive features of environmentally friendliness, excellent functional group tolerance, and high reactivity. The pioneering work that utilized α-bromo esters in various visible light-induced synthetic transformations, have attracted the attention of numerous organic chemists.Herein the methodology of using aryl allylic alcohols as substrates, α-bromo diethyl malonate as oxidative quenching reagent in the presence of photoredox catalyst to achieve the difunctionalization of alkenes via 1,2-aryl migration was investigated. After screening several reaction conditions, the optimal reaction conditions were established on the treatment of substrates(0.1 mmol, 0.1 M in DMSO) with α-bromo diethyl malonate(0.2 mmol) in the presence of fac-Ir(ppy)3(0.005 mmol) as catalyst and 2,6-lutidine as additive(0.2 mmol) under irradiation with 1 W blue LEDs in an atmosphere of N2. Under the optimal reaction conditions, 9 symmetrical diaryl allylic alcohols, 14 unsymmetrical diaryl allylic alcohols, 2 aryl alkyl allylic alcohols and 1 double-bond-substituted diaryl allylic alcohol were examined. Good to high yield and moderate selectivity show the advantages of extensive scope, excellent functional group tolerance.Furthermore, the possible mechanism was verified by control experiment of radical capture agent – TEMPO. The radical resulted from oxidatively quenched α-bromo diethyl malonate underwent intermolecular radical addition to aryl allylic alcohol. Then an intramolecular cyclization occured on the aromatic ring to generate spiro[2,5]octadienyl radical. Subsequently, this radical recovered its stable aryl structure and furnished the benzyl radical through 1,2-aryl migration. At last, benzyl radical yielded the desired product along with the loss of a proton.