Abstract
It has been established that an electron-deficient cationic CpE-rhodium(III) complex catalyzes the non-oxidative [2+2+2] annulation of N-(1-naphthyl)acetamide with two alkynoates via cleavage of the adjacent C–H and C–N bonds to give densely substituted phenanthrenes under mild conditions (at 40 °C under air). In this reaction, a dearomatized spiro compound was isolated, which may support the formation of a cationic spiro rhodacycle intermediate in the catalytic cycle. The use of N-(1-naphthyl)acetamide in place of acetanilide switched the reaction pathway from the oxidative [2+2+2] annulation-lactamization via C–H/C–H cleavage to the non-oxidative [2+2+2] annulation via C–H/C–N cleavage. This chemoselectivity switch may arise from stabilization of the carbocation in the above cationic spiro rhodacycle by the neighboring phenyl and acetylamino groups, resulting in the nucleophilic C–C bond formation followed by β-nitrogen elimination.
Highlights
The transition-metal-catalyzed [2+2+2] annulation of three unsaturated compounds is a useful method for the synthesis of six-membered carbocycles and heterocycles [1,2,3,4,5,6,7,8,9]
For the synthesis of naphthalene derivatives, benzynes have been employed as one of the three alkynes but the use of the benzynes suffers from the redundant precursor synthesis and harsh reaction conditions [15,16,17]
We have established that the electron-deficient cationic CpE -rhodium(III) complex is able to catalyze the non-oxidative [2+2+2] annulation of N-(1-naphthyl)acetamide (2c) with two alkynoates 3 via cleavage of Molecules 2018, 23, 3325; doi:10.3390/molecules23123325
Summary
The transition-metal-catalyzed [2+2+2] annulation of three unsaturated compounds is a useful method for the synthesis of six-membered carbocycles and heterocycles [1,2,3,4,5,6,7,8,9]. Complex, derived from 1, catalyzes the oxidative tandem [2+2+2] annulation-lactamization of acetanilide (2a) with two alkynoates 3 via cleavage of adjacent two C–H bonds at room temperature under air to give densely substituted banzo[cd]indolones 4a (Scheme 1, top) [23]. Several examples of the transition-metal-catalyzed decarboxylative and oxidative [2+2+2] annulation of benzoic acids with two alkynoates via cleavage of the adjacent C–H and C–C bonds, leading to densely substituted naphthalenes, have been reported [24,25,26,27], only a single example of the non-oxidative [2+2+2] annulation via cleavage of the adjacent C–H and C–N bonds, in which the acylamino moiety is employed as a traceless directing group, has been reported in the neutral Cp*-rhodium(III) complex-catalyzed synthesis of tetraarylnaphthalenes from N-acylanilines and two diarylacetylenes at elevated temperature (110 ◦ C) (Scheme 1, bottom) [28].
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