Abstract
Hydrogen atom abstraction (HAT) from C(sp3)–H bonds of naturally abundant alkanes for alkyl radical generation represents a promising yet underexplored strategy in the alkylation reaction designs since involving stoichiometric oxidants, excessive alkane loading, and limited scope are common drawbacks. Here we report a photo-induced and chemical oxidant-free cross-dehydrogenative coupling (CDC) between alkanes and heteroarenes using catalytic chloride and cobalt catalyst. Couplings of strong C(sp3)–H bond-containing substrates and complex heteroarenes, have been achieved with satisfactory yields. This dual catalytic platform features the in situ engendered chlorine radical for alkyl radical generation and exploits the cobaloxime catalyst to enable the hydrogen evolution for catalytic turnover. The practical value of this protocol was demonstrated by the gram-scale synthesis of alkylated heteroarene with merely 3 equiv. alkane loading.
Highlights
Hydrogen atom abstraction (HAT) from C(sp3)–H bonds of naturally abundant alkanes for alkyl radical generation represents a promising yet underexplored strategy in the alkylation reaction designs since involving stoichiometric oxidants, excessive alkane loading, and limited scope are common drawbacks
Few strategies have been disclosed for the efficient usage of Cl·, including (a) the direct singleelectron transfer (SET) from Cl− to photocatalyst under photothermal conditions[27,28,29,30]; (b) the ligand-to-metal charge transfer (LMCT), which has been employed for the coupling of alkanes and organohalides by metallophotoredox catalysis[31,32,33,34,35,36]; (c) the photolysis of in situ generated Cl2 via electrooxidation of HCl37; (d) the bimolecular homolytic substitution (SH2) between chloroborate and an oxy radical for the alkane borylation[38]
It was hypothesised that the excited heteroarenes I could oxidise the Cl− under irradiation to generate the Cl· for the aliphatic C–H abstraction from alkane 2
Summary
Hydrogen atom abstraction (HAT) from C(sp3)–H bonds of naturally abundant alkanes for alkyl radical generation represents a promising yet underexplored strategy in the alkylation reaction designs since involving stoichiometric oxidants, excessive alkane loading, and limited scope are common drawbacks. The unfavourable chloride-to-chlorine oxidation (E = 1.36 V vs NHE)[23] and untamed reactivity of Cl· compared with other halide analogues[17,18,24,25,26] make chlorine radical-promoted alkylation rarely explored In this endeavour, few strategies have been disclosed for the efficient usage of Cl·, including (a) the direct singleelectron transfer (SET) from Cl− to photocatalyst under photothermal conditions[27,28,29,30]; (b) the ligand-to-metal charge transfer (LMCT), which has been employed for the coupling of alkanes and organohalides by metallophotoredox catalysis[31,32,33,34,35,36]; (c) the photolysis of in situ generated Cl2 via electrooxidation of HCl37; (d) the bimolecular homolytic substitution (SH2) between chloroborate and an oxy radical for the alkane borylation[38]. Owing to the strong hydrogen atom affinity of Cl·, a wide range of inactivated and activated C(sp3)–H bonds could be arylated with good functional group tolerance and substrate diversity, and notably, with the strategic introduction of the cobaloxime catalyst, we formulate a chemical oxidant-free heteroarene alkylation protocol by releasing H2
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