Regioselective electrochemical oxidative radical ortho-(4 + 2)/ipso-(3 + 2) cyclization

doi:10.1016/S1872-2067(23)64510-3

Abstract

Abstract:

Achieving regioselectivity in radical cyclization reactions is of central importance, yet extremely challenging. Although Baldwin’s rules provided guidance on the addition of radical species with alkenes/alkynes, the ortho-/ipso-selectivity of the cyclic reaction between radical species (especially alkyl and alkenyl radical) and aryl groups is still ambiguous. Herein, we develop an electrochemically enabled regioselective ortho-(4 + 2)/ipso-(3 + 2) cyclization of alkyl/alkenyl radicals with aryl groups, which provides a series of tetrahydronaphthalene and spirocarbocycle derivatives, exhibiting a broad substrate scope and functional group tolerance. Alkyl/alkenyl radicals are generated by Cp₂Fe-mediated electrochemical oxidative radical addition of benzylic malonates with alkenes and alkynes. The method avoids the use of chemical oxidant/base/noble metal, the pre-functionalization of substrates, and the over-oxidation of compounds. Theoretical studies reveal that the dominant factor promoting the alkene-preferred ortho-addition is the favorable interaction energy; the alkyne-preferred ipso-addition regioselectivity is controlled by the distortion energy. Notably, this strategy is regarded as an important supplement to Baldwin’s rules for radical cyclization.

Key words: Regioselectivity, Radical, Ortho-(4 + 2)/ipso-(3 + 2) cyclization, Electrochemical oxidation, Carbocyclic compound

Zhipeng Guan, Dongfeng Yang, Zhao Liu, Shuxiang Zhu, Xingxing Zhong, Huamin Wang, Xiangwei Li, Xiaotian Qi, Hong Yi, Aiwen Lei. Regioselective electrochemical oxidative radical ortho-(4 + 2)/ipso-(3 + 2) cyclization[J]. Chinese Journal of Catalysis, 2023, 52: 144-153.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(23)64510-3

https://www.cjcatal.com/EN/Y2023/V52/I9/144

Figures/Tables 6

Scheme 1. Instruction of radical cyclization reaction and our design and work.

Scheme 2. Studies of electrochemical ortho-(4 + 2) cyclization between benzylic malonates and alkenes. Reaction conditions: benzylic substrates (0.5 mmol), alkenes (0.2 mmol), Cp2Fe (0.02 mmol), nBu4NBF4 (1.0 mmol), MeOH (4 mL), DME (2 mL), carbon felt anode, Pt cathode, undivided cell, constant current = 5 mA, 3.75 h, rt, N2. Isolated yield.

Scheme 3. Studies of ipso-(3 + 2) spirocyclization benzylic malonates and alkenes. Reaction conditions: benzylic substrates (0.3 mmol), alkynes (0.2 mmol), Cp2Fe (0.02 mmol), nBu4NBF4 (1.0 mmol), H2O (100 μL), MeOH (6 mL), carbon felt anode, Pt cathode, undivided cell, constant current = 5 mA, 3.25 h, 50 °C, N2. Isolated yield.

Scheme 4. Scope of substrates of the ipso-(3 + 2) spirocyclization. Reaction Conditions: benzylic substrates (0.3 mmol), alkynes (0.2 mmol), Cp2Fe (0.02 mmol), nBu4NBF4 (1.0 mmol), H2O (100 μL), MeOH (6 mL), carbon felt anode, Pt cathode, undivided cell, constant current = 5 mA, 3.25 h, 50 °C, N2. Isolated yield.

Scheme 5. Derivatization of product and mechanistic studies.

Scheme 6. Proposed reaction mechanism.

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