Pentaindenocorannulene and Tetraindenocorannulene: New Aromatic Hydrocarbon π Systems with Curvatures Surpassing That of C60
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Abstract
Short syntheses and X-ray crystal structures are reported for two new geodesic polyarenes, pentaindenocorannulene (1, C50H20) and tetraindenocorannulene (2, C44H18). These extended aromatic π systems constitute the largest curved subunits of C60 ever prepared. In agreement with theoretical predictions, the trigonal carbon atoms at the cores of these new hydrocarbons suffer even greater pyramidalization than that exhibited by the carbon atoms of C60 (average POAV angles = 12.6, 12.1, and 11.6° for 1, 2, and C60, respectively). These syntheses of such highly curved π systems by solution phase methods, starting from compounds that were derived from planar starting materials also by solution phase methods, demonstrate unequivocally that rational chemical syntheses of fullerenes, carbon nanotubes, and related carbon-rich molecules should all be possible using exclusively solution chemical methods! Recourse to high-temperature gas-phase pyrolysis should not be necessary. The synthesis of 1 begins with a 5-fold Suzuki−Miyaura coupling of 1,3,5,7,9-pentachlorocorannulene with 2-chlorophenylboronic acid, giving 1,3,5,7,9-pentakis(2-chlorophenyl)corannulene in 48% isolated yield (86% average yield per C−C coupling). Subsequent conversion to pentaindenocorannulene (1) was achieved in 35% isolated yield (81% average yield per C−C coupling) by a 5-fold, Pd(0)-catalyzed, intramolecular arylation reaction. The synthesis of tetraindenocorannulene (2) follows the same course but starts from 1,2,5,6-tetrabromocorannulene. Flash vacuum pyrolysis of the intermediate 1,2,5,6-tetrakis(2-chlorophenyl)corannulene also produces tetraindenocorannulene (2). The title compounds are both deep orange, thermally robust, air stable, crystalline solids (mp >400 °C) with rich UV−vis spectra that have long absorption tails in the visible region.
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