Cyclophanes DOI: 10.1002/anie.200901025 New Strategies for Synthesizing Short Sections of Carbon Nanotubes** Brian D. Steinberg and Lawrence T. Scott* Dedicated to Professor Julius Rebek, Jr. on the occasion of his 65th birthday aromatic hydrocarbons · cyclophanes · macrocycles · strained molecules · synthetic methods “Bent and battered benzene rings” have captivated organic chemists since the middle of the last century. [1] Most commonly, the bending is enforced by bridging the para (or meta) positions of benzene with short chains of atoms to produce “cyclophanes”. [1, 2] The degree of bending depends inversely on the length of the tether, and these strained macrocycles sometimes contain more than one bridged benzene ring. Cyclophanes incorporating bent naphthalene units or even larger polyarenes have also been prepared. In recent months, one of the highest unclimbed pinnacles in this field was finally conquered, and shortly thereafter, the impressive record for bending was resoundingly shattered. Figure 1 depicts the new cyclophanes that have recently rocked this venerable branch of chemistry. The scientists responsible for their synthesis have noted the structural features that these compounds share with carbon nanotubes (Figure 1), and they are already contemplating ways to extend their successes toward the rational, chemical synthesis of carbon nanotubes. The first of these recent breakthroughs was reported in late 2008 by Bertozzi and co-workers. [3] As newcomers to cyclophane chemistry, this creative team conceived a fresh strategy for synthesizing macrocycles composed entirely of para-substituted benzene rings. Implementing the plan re- quired a novel solution to one particularly challenging late- stage transformation, but, ultimately, they prepared and spectroscopically characterized three members of the pre- viously unknown family of [N]cycloparaphenylenes (CPP-9, CPP-12, and CPP-18 ; N = 9, 12, and 18, respectively). [4] Besides solving one of the longest standing problems in cyclophane chemistry, this synthesis is also remarkable for its brevity: only five steps were required to assemble the fully aromatized macrocycles from 1,4-diiodobenzene (Scheme 1). The syn-selective addition of 4-iodophenyllithium to para- benzoquinone provided the key building block (1), from which the final target molecules were all assembled. A portion of 1 was converted into the corresponding diboronate (2), and Suzuki coupling of 1 with 2 gave macrocycles 3a, 3b, and 3c in a combined yield of 22%. The smallest of these cyclic oligomers actually appeared as an unexpected bonus and presumably arises from the homocoupling of 2, which gives an acyclic dimer that then cross-couples with 1. The syntheses were completed by chromatographic separation of the three macrocycles and then reductive elimination of all the methoxy groups by the action of lithium naphthalenide at Figure 1. a) [N]cycloparaphenylenes (CPP-9, CPP-12, and CPP-18, respectively); b) mapping of CPP-12 onto a section of a [12,12]carbon nanotube; c) 1,1,8,8-tetramethyl[8](2,11)teropyrenophane (4); d) mapping of the teropyrene unit in 4 onto a section of an [8,8]carbon nanotube. [*] B. D. Steinberg, Prof. Dr. L. T. Scott Department of Chemistry, Merkert Chemistry Center Boston College, Chestnut Hill, MA 02467-3860 (USA) Fax: (+ 1) 617-552-6454 E-mail: lawrence.scott@bc.edu [**] Financial support from the U.S. National Science Foundation is gratefully acknowledged. Highlights 5400  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2009, 48, 5400 – 5402