Activation of aromatic CH bonds by a transition metal catalyst has received significant attention in the synthetic chemistry community. In recent years, rapid and site-selective extension of π-electron systems by C-H activation has emerged as an ideal methodology for preparing organic materials with extended π-systems. This Review focuses on recently reported π-extending C-H activation reactions directed toward new optoelectronic conjugated materials.

Conspectus The discovery and creation of new forms of carbon have always transformed the scientific landscape. For example, the discoveries of fullerenes, carbon nanotubes (CNTs), and graphenes have opened doors to the science of nanometer-sized carbon allotropes, otherwise known as nanocarbons. Since then, researchers worldwide have unveiled their outstanding physical and chemical properties, and a number of applications and technologies have arisen in not only materials science but also biological research fields. The synthesis and study of this privileged class of “single-molecule” compounds has become one of the most engaging subjects in chemistry and holds huge promise to establish new fields in molecular science. However, there have been huge gaps between established small-molecule chemistry and nanocarbon science. In the particular case of CNTs, it is still not possible to access structurally uniform CNTs. Although a wide range of synthetic methods have been reported, CNTs are generally accessed as a mixture of various structures. One logical strategy to achieve full synthetic control over CNTs is to build up from a template molecule with structural precision (the so-called “growth-from-template” strategy), where a short CNT segment molecule represents an initial synthetic target. To this end, organic synthesis techniques are our most powerful tools to synthesize short CNT segments such as carbon nanorings and carbon nanobelts. This Account highlights our 16-year campaign in the synthesis and application of carbon nanorings and carbon nanobelts. The first topic is the synthesis of carbon nanorings (cycloparaphenylenes) as substructures of CNTs. The second topic is the synthesis of armchair and zigzag carbon nanobelts, which consist solely of fully fused hexagonal rings and provide a continuous double-stranded cylindrical framework. The third topic is the synthesis of methylene-bridged cycloparaphenylene, an aromatic belt containing nonhexagonal rings, in which the cyclic paraphenylene chain is ladderized by methylene bridges. This nonalternant aromatic belt can be regarded as segments of nonconventional CNTs. During our extensive investigation, we found that the careful design of strainless macrocyclic precursors is crucial to the success of the synthesis of these curved π-conjugated nanorings and nanobelts. In the final section, some of the representative size-dependent properties of these nanorings/belts, including their HOMO–LUMO energies, strain energies, and photophysical properties, are summarized. In addition to basic properties, the utilization of these compounds as supramolecular hosts and organic materials is also briefly introduced. We hope this Account will inspire the development of new forms of nanocarbon molecules that would open doors to new fields and applications.

A novel catalytic C-H activation route to privileged dibenzo[a,e]pentalene (DBP) structures has been established. In the presence of PdCl 2 , AgOTf, and o-chloranil, a C-H/C-H annulation of arylacetylenes takes place to furnish the corresponding DBPs. A number of mechanistic experiments indicate that this new annulation occurs through alkyne-directed, ortho-selective, electrophilic aromatic C-H palladation. Not only symmetric diarylacetylenes but also unsymmetric arylacetylenes are applicable to this reaction. UV-vis absorption spectra and DFT studies on the resulting DBPs indicate a strong substituent effect on the energy levels of the HOMO and HOMOÀ1 of DBPs.

The ability to transform one functional group into another lies at the heart of organic chemistry. Such functional-group interconversions do not involve carbon-carbon bond-forming reactions and are thus seen as less efficient for the construction of complex molecules, however, these interconversions are often critical to "set up" a molecule for such a transformation. The oxidation of primary and secondary alcohols (1 and 3) to produce aldehydes (2) and ketones (4) prior to the addition of organometallic species is a prime example (Scheme 1). Although this reaction is often essential for the subsequent carbon-carbon bond-forming transformation, it does add an extra, linear step to the sequence. Thus, we imagined that performing the two steps, oxidation and addition, together would greatly simplify synthetic routes by essentially eliminating the need to carry out a preliminary oxidation before converting, for example, a primary alcohol (1) into a secondary alcohol (3), or similarly 3 into a tertiary alcohol (5).Numerous practical advantages are associated with such one-pot multistep alcohol-carbonyl interconversions, [1] but a uniform methodology has not been developed, partly because of the incompatibility of the reaction conditions. Whereas alcohol-to-carbonyl transformations are oxidative, the reverse processes such as carbonyl addition reactions are reductive in nature. Herein, we report that [Ni(cod) 2 ]/IPr (cod = 1,5-cyclooctadiene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) serves as a general catalyst for the controlled one-pot oxidation-addition of alcohols and carbonyl compounds. We demonstrate the feasibility of all possible multistep transformations in alcohol-carbonyl interconversions (Scheme 1). A one-pot nickel-catalyzed synthesis of flumecinol (a hepatic microsomal enzyme inducer) is also described.As an important progress toward controlled carbonylalcohol interconversions, we recently established that the [Ni(cod) 2 ]/IPr catalyst promotes the otherwise difficult intermolecular 1,2-addition of arylboronate esters to unactivated ketones and aldehydes.[2] Among the various arylboron reagents screened, arylboronic acid neopentyl glycol ester ArB(neo) turned out to be the most reactive. The advantage of our [Ni-IPr] catalytic system [2] over other transition-metalcatalyzed organoboron-based 1,2-additions is obvious from the viewpoint of the substrate scope. While other catalytic systems are generally only applicable to aldehydes [3] and some electronically and strain-activated ketones, [4] our [Ni-IPr] catalysis shows good reactivity not only toward aldehydes but also toward diaryl, alkyl aryl, and dialkyl ketones under mild reaction conditions.[2] The high reactivity of our [Ni-IPr] catalyst might be partly due to the unique Ni 0 /Ni II mechanism (right-hand catalytic cycle, Scheme 2).Since many transition-metal complexes are able to mediate the oxidation of alcohols to aldehydes or ketones, [5] we envisioned that our nickel catalysis could be extended to a controlled alcohol-carbonyl i...

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