A Gold-Mediated Strategy for the Synthesis of Cycloparaphenylenes and Related Nanohoops

[n]Cycloparaphenylenes (CPPs), a distinctive family of π-conjugated nanohoops, have attracted great interest owing to their highly strained cyclic structures and unique photophysical and electrochemical properties. For a long time, [n]CPPs were considered “dream molecules” due to the difficulty of bending benzene rings into cyclic architectures. However, between 2008 and 2010, independent breakthroughs by several research groups finally realized their synthesis, and since then a wide variety of [n]CPPs and derivatives have been reported. We have recently established an innovative approach to CPPs and related nanohoops utilizing macrocyclic gold complexes as key intermediates. This gold-mediated strategy offers unique advantages: (i) efficient access to [n]CPPs with ring sizes that are multiples of three; (ii) the recyclability of the digold complex; enabling cost-effective and environment-friendly synthesis; (iii) the use of readily available arylboronic acids as starting material; (iv) the ability to introduce bulky substituents owing to the open coordination environment of Au(I) -diphosphine unit; (v) incorporation of coordinating units such as bipyridines; and (vi) mild reaction conditions that tolerate chemically sensitive substituents. Importantly, the discovery of dynamic Au-C σ-bond exchange provided mechanistic insight into the high cyclization efficiency, enabling the development of a “reorganization method,” in which two different macrocyclic gold complexes can be mixed and subsequently reductively eliminated to construct nanohoops incorporating two distinct organic linkers. Collectively, these results highlight the versatility of gold-mediated CPP synthesis as a powerful platform for constructing structurally diverse and functionalized π-conjugated macrocycles.