Pure & Appl. Chem., Vol. 71, No. 2, pp. 209–219, 1999. Printed in Great Britain.  1999 IUPAC 209 Geodesic polyarenes with exposed concave surfaces* Lawrence T. Scott,† Hindy E. Bronstein, Dorin V. Preda, Ronald B. M. Ansems, Matthew S. Bratcher and Stefan Hagen Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467-3860, USA Abstract: Polycyclic aromatic hydrocarbons (PAHs) in which five-membered rings have been interspersed among the six-membered rings generally display curved  systems that can be either fully closed (fullerenes) or partially open (bowl-shaped fullerene fragments), depending on the number of five-membered rings. Both classes of compounds belong to the larger family we call ‘GEODESIC POLYARENES.’ Herein, we highlight the utility of high temperature aryl radical cyclizations as a general strategy for constructing strained geodesic polyarenes. We also report the first examples of reactions at the ‘interior’ carbon atoms of neutral PAHs that are not fullerenes, e.g. carbene additions, 1,3-dipolar cycloadditions, osmylation, nucleophilic addition of MeLi, and electrophilic addition of  CHCl 2 and  CCl 3 . INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) composed entirely of fused benzene rings strive to maintain planar structures that can be regarded as subunits of two-dimensional graphite sheets, although steric factors sometimes override this tendency, as in the helicenes. Networks of trigonal carbon atoms harboring one or more imbedded five-membered rings, on the other hand, invariably show lower resistance to out-of-plane deformations and nearly always adopt nonplanar equilibrium geometries. When such networks of trigonal carbon atoms contain exactly 12 five-membered rings, closed-surface fullerenes result. Any number of five-membered rings fewer than 12 produces bowl-shaped molecules with convex and concave surfaces both exposed. Many such molecules comprise substructures of C 60 and are sometimes referred to as ‘fullerene fragments’ or ‘buckybowls’, e.g. 1 [1–3], 2 [4,5], and 3 [6]. There is no absolute requirement, however, that the ring system of such molecules map onto that of C 60 , and some do not, e.g. 4(a substructure of C 76 ) [7]. We consider all PAHs that have curved surfaces resulting from the presence of one or more fully unsaturated five-membered (or smaller) rings to be ‘GEODESIC POLYARENES’, as they are all reminiscent of the geodesic structures of Buckminster Fuller, the architect for whom [60]fullerene was first named. The fullerenes constitute one family of geodesic polyarenes, and bowl-shaped PAHs constitute another. While research on the chemistry of C 60 and the higher fullerenes has run at a furious pace in the 1990s, we have been busy developing new strategies for the synthesis of open geodesic polyarenes. Our approach takes advantage of the fact that the normal out of plane deformations of simple planar or nearly planar polyarenes become greatly amplified under conditions of flash pyrolysis. By catching such molecules in their temporarily distorted conformations, using cyclization reactions based on the intramolecular trapping of one or more strategically located carbenes [2a] or aryl radicals [5], we have been able to prepare and characterize a wide range of bowl-shaped fullerene fragments. More than a dozen of these novel aromatic hydrocarbons have now been prepared in our laboratory by this highly successful strategy (1 – 4 and Fig. 1) [2,5–9], and their chemical behavior is under active investigation. *Lecture presented at the 9th International Symposium on Novel Aromatic Compounds (ISNA-9), Hong Kong, China, 2–7 August 1998, pp. 209–302. ² Corresponding author: E-mail: lawrence.scott@BC.edu