Letter pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Acceptorless Dehydrogenation of Hydrocarbons by Noble-MetalFree Hybrid Catalyst System Hiromu Fuse,† Masahiro Kojima,† Harunobu Mitsunuma,† and Motomu Kanai*,† †
Graduate School of Pharmaceutical Sciences, The University of Tokyo, Tokyo 113-0033, Japan S Supporting Information *
ABSTRACT: A hybrid catalysis that comprises an acridinium photoredox catalyst, a thiophosphate organocatalyst, and a nickel catalyst-enabled acceptorless dehydrogenation of hydrocarbons is reported. The cationic nickel complex played a critical role in the reactivity. This is the first example of acceptorless dehydrogenation of hydrocarbons by base metal catalysis under mild reaction conditions of visible light irradiation at room temperature.
C
Based on previous reports of the combined use of a photoredox catalyst and a nickel catalyst in C−C bond formation,11,12 we envisioned that nickel would be a suitable element to substitute for palladium. Our mechanistic blueprint for CAD of tetrahydronaphthalene (1a) as a model substrate is illustrated in Figure 1.8,13 Visible-light irradiation of a
atalytic acceptorless dehydrogenation (CAD) of saturated alkyl motifs in hydrocarbons to produce unsaturated C− C bonds and hydrogen gas is an attractive transformation in organic synthesis, and a technological foundation for a potential future hydrogen society.1 However, because of the unfavorable enthalpy factor, the CAD of hydrocarbons generally requires harsh conditions, such as a high temperature (∼200 °C) or UV light irradiation, and noble-metal catalysts, such as iridium2 or rhodium3 complexes. Hybrid catalyst systems having a photoredox catalyst4 as a component recently emerged to facilitate CAD of organic compounds, including hydrocarbons and N-heterocycles, under milder conditions.5 Sorensen’s group pioneered the base metalcatalyzed acceptorless dehydrogenation of hydrocarbons at room temperature using a combination of decatungstate and cobaloxime catalysts.6 However, the disadvantages of this catalytic system are its requirement for near-UV light irradiation, low product yield, and insufficient substrate generality. Li reported that merging Ru(bpy)3Cl2·6H2O and cobaloxime catalysts promoted acceptorless dehydrogenation of N-heterocycles at room temperature.7 We simultaneously developed the first example of CAD of hydrocarbons that proceeded under visible-light irradiation at room temperature, devising a three-component hybrid catalysis that comprises an organo-photoredox catalyst, a hydrogen atom transfer (HAT) organocatalyst, and a palladium catalyst.8 Specifically, we identified a unique combination of acridinium photoredox catalyst 79 and thiophosphoric imide (TPI) to generate a HATactive thiyl radical. This reaction exhibited relatively broad functional group tolerance, because of the mild reaction conditions. Considering the chemical foundation underlying a hydrogen society as a long-term goal, however, a critical drawback of our reaction is the requirement for the rare and noble-metal element palladium. Thus, we were motivated to achieve CAD using a base metal catalyst10 in place of the palladium catalyst. Herein, we report a nickel-catalyzed CAD of hydrocarbons that proceeded under visible-light irradiation at room temperature. © XXXX American Chemical Society
Figure 1. Working hypothesis for CAD of 1a.
photoredox catalyst (PC+) generates a long-lived photoexcited state (*PC+). This excited photoredox catalyst oxidizes the organocatalyst (RSH) to produce a HAT-active thiyl radical (RS•)14,15 and a proton (H+). Then, the thiyl radical abstracts a benzylic hydrogen atom of substrate 1a, affording benzyl radical 3. A nickel catalyst (NiII) would intercept radical 3 to generate an organonickel species 4 bearing a high valent state (NiIII).11 The organonickel 4 is then reduced by a photoredox catalyst acting as a reductant (PC), affording organonickel 5 bearing a low oxidation state (NiII). This reduction would facilitate βhydride elimination from 5 to produce unsaturated dihydronaphthalene (6) and nickel hydride species NiII−H. Finally, hydrogen gas evolves through the reaction between NiII−H and a proton16 generated in the photooxidation step of the Received: February 17, 2018
A
DOI: 10.1021/acs.orglett.8b00583 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Table 1. Optimization of CAD of Tetrahydronaphthalene: Effects of Metal Complex Catalyst, Photoredox Catalyst, and Organocatalyst
a
entry
photoredox catalyst
organocatalyst
metal complex catalyst
2a (%)a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21b
7 7 7 7 7 7 7 7 7 7 Ru(bpy)3(PF6)2 (Ir[dF(CF3)ppy]2(dtbpy))PF6 7 7 7 7 7 − 7 7 7
TPA TPA TPA TPA TPA TPA TPA TPA TPA TPA TPA TPA TPl quinuclidine Bu4N+PhCO2− SA PA TPA TPA − TPA
NiCl2·6H2O NiCl2·6H2O+dtbpy NiBr2·dme Nil2 Ni(OAc)2·4H2O Ni(BF4)2·6H2O Ni(NTf 2)2·xH2O Fe(BF4)2·6H2O Co(BF4)2·6H2O Cu(BF4)2·xH2O Ni(NTf2)2·xH2O Ni(NTf2)2·xH2O Ni(NTf2)2·xH2O Ni(NTf2)2·xH2O Ni(NTf2)2·xH2O Ni(NTf2)2·xH2O Ni(NTf2)2·xH2O Ni(NTf2)2·xH2O − Ni(NTf2)2·xH2O Ni(NTf2)2·xH2O
