Abstract
Since the early Hückel molecular orbital (HMO) calculations in 1950, it has been well known that the odd alternant hydrocarbon (OAH), the phenalenyl (PLY) system, can exist in three redox states: closed shell cation (12π e−), mono-reduced open shell neutral radical (13π e−) and doubly reduced closed shell anion (14π e−). Switching from one redox state of PLY to another leads to a slight structural change owing to its low energy of disproportionation making the electron addition or removal process facile. To date, mono-reduced PLY based radicals have been extensively studied. However, the reactivity and application of doubly reduced PLY species have not been explored so far. In this work, we report the synthesis of the doubly reduced PLY species (14π e−) and its application towards the development of redox catalysis via switching with the mono-reduced form (13π e−) for aryl halide activation and functionalization under transition metal free conditions without any external stimuli such as heat, light or cathodic current supply.
Highlights
In 1950, Huckel molecular orbital (HMO) theory suggested the existence of non-bonding molecular orbitals (NBMOs) in odd alternant hydrocarbons (OAHs) with a positive MO coefficient in the alternative carbon atoms and a nearly zero coefficient in the rest of the carbon atoms.[1,2] A theoretical study by Fukui et al revealed that electrons occupying the NBMO are mainly responsible for the chemical reactivity of such OAHs.[3]
The present literature lacks reports on the synthesis and utilization of 14p eÀ doubly reduced phenalenyl species and their application. We have addressed such challenges by synthesizing a doubly reduced 14p eÀ PLY species and devised a redox catalytic method for the transition metal free C–C cross coupling reaction by direct C–H arylation of arenes/heteroarenes with aryl halides under ambient conditions
This conclusion is fully consistent with the total electron density plots of the three redox states of the PLY(O,O)–K compound (1a) which show that upon consecutive reduction, the negative charge gradually accumulates over the PLY moiety (Fig. 2a)
Summary
In 1950, Huckel molecular orbital (HMO) theory suggested the existence of non-bonding molecular orbitals (NBMOs) in odd alternant hydrocarbons (OAHs) with a positive MO coefficient in the alternative carbon atoms and a nearly zero coefficient in the rest of the carbon atoms (spin density plot, Chart 1a).[1,2] A theoretical study by Fukui et al revealed that electrons occupying the NBMO are mainly responsible for the chemical reactivity of such OAHs.[3]. (e) This work: development of a doubly reduced PLY moiety and its switching with mono-reduced PLY for designing redox catalysts for the transition metal-free C–C cross coupling reaction under ambient conditions through activation of aryl halides.
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