Synthesis of Quinoidal Fused Oligosiloles by Rhodium-Catalyzed Stitching Reaction and Theoretical Investigation of Their Properties

Photoinduced Dual C-F Bond Activation of Hexafluorobenzene Mediated by Boron Atom

The reaction of laser-ablated boron atoms with hexafluorobenzene (C6 F6 ) was investigated in neon and argon matrices, and the products are identified by matrix isolation infrared spectroscopy and quantum-chemical calculations. The reaction is triggered by a boron atom insertion into one C-F bond of hexafluorobenzene on annealing, forming a fluoropentafluorophenyl boryl radical (A). UV-Vis light irradiation of fluoropentafluorophenyl boryl radical causes generation of a 2-difluoroboryl-tetrafluorophenyl radical (B) via a second C-F bond activation. A perfluoroborepinyl radical (C) is also observed upon deposition and under UV-Vis light irradiation. This finding reveals the new example of a dual C-F bond activation of hexafluorobenzene mediated by a nonmetal and provides a possible route for synthesis of new perfluorinated organo-boron compounds.

Dehydrogenative Annulation of Silylated 1H-Indoles with Alkynes via Silyl Migration

We investigated the dehydrogenative annulation of silylated 1H-indole derivatives with alkynes to synthesize a silole-fused indole. The addition of the in situ generated silylium ion to alkynes was followed by the sila-Friedel-Crafts reaction via silyl migration, realizing regioselective dehydrogenative annulation controlled by the steric bulkiness of a base. The optical properties of the obtained siloloindoles indicated fluorescence of which the intensity depends on the location of the fused silole.

Transition-Metal-Free, Site-Selective C-F Arylation of Polyfluoroarenes via Electrophotocatalysis

Direct C_Ar-F arylation is effective and sustainable for synthesis of polyfluorobiaryls with different degrees of fluorination, which are important motifs in medical and material chemistry. However, with no aid of transition metals, the engagement of C_Ar-F bond activation has proved difficult. Herein, an unprecedented transition-metal-free strategy is reported for site-selective C_Ar-F arylation of polyfluoroarenes with simple (het)arenes. By merging N,N-bis(2,6-diisopropylphenyl)perylene-3,4,9,10-bis(dicarboximide)-catalyzed electrophotocatalytic reduction and anodic nitroxyl radical oxidation in an electrophotocatalytic cell, various polyfluoroaromatics (2F-6F and 8F), especially inactive partially fluorinated aromatics, undergo sacrificial-reagents-free C-F bond arylation with high regioselectivity, and the yields are comparable to those for reported transition-metal catalysis. This atom- and step-economic protocol features a paired electrocatalysis with organic mediators in both cathodic and anodic processes. The broad substrate scope and good functional-group compatibility highlight the merits of this operationally simple strategy. Moreover, the easy gram-scale synthesis and late-stage functionalization collectively advocate for the practical value, which would promote the vigorous development of fluorine chemistry.

Rhodium-Catalyzed Stitching Reactions for the Facile Synthesis of Extended π-Conjugated Compounds

π-Conjugated compounds have been extensively investigated as potentially useful organic materials, however, accessible structures are currently still limited, mainly due to the lack of efficient synthetic methods. In this regard, a new synthetic strategy termed the ‘stitching reaction’ has been recently devised that allows facile access to novel types of bridged π-conjugated compounds in a convergent manner under rhodium catalysis. In addition, this synthetic strategy has been successfully applied to the preparation of π-conjugated polymers that cannot be synthesized via existing methods. Herein an overview that includes the physical properties of newly synthesized π-conjugated compounds is provided.

1 Introduction

2 Synthesis of Novel Fused Oligosiloles by the Stitching Reaction and Their Properties

3 Synthesis of Hydrocarbon-Based Fused π-Conjugated Compounds by the Stitching Reaction

4 Synthesis of New π-Conjugated Polymers by Stitching Polymerization

5 Conclusion

Rhodium-Catalyzed Stitching Polymerization of Alkynylsilylacetylenes

Polymers possessing a silicon-bridged π-conjugated repeating unit constitute an important class of compounds for their potential utility as optoelectronic materials. Herein we developed a rhodium-catalyzed stitching polymerization of nonconjugated and readily prepared alkynylsilylacetylenes for the synthesis of new π-conjugated polymers with ladder-type silicon-bridged repeating units. The polymerization proceeded smoothly by employing a Rh/tfb complex as the catalyst, and not only diynes but also triynes and tetraynes could be polymerized in a stitching manner to give polymers that are inaccessible by existing methods. The solubility of the polymers in different types of solvents could be controlled by introducing appropriate functional groups on the silicon atoms, and sequence-controlled functionalized polyacetylenes could be accessed by protodesilylation of the stitched polymers. Physical properties of the obtained polymers were also investigated to understand their characteristic features.

Intermolecular Three-Component Synthesis of Fluorene Derivatives by a Rhodium-Catalyzed Stitching Reaction/Remote Nucleophilic Substitution Sequence

A three-component synthesis of multisubstituted fluorene derivatives has been developed by devising a rhodium-catalyzed stitching reaction/remote nucleophilic substitution sequence. A variety of nucleophiles can be installed in the second step including both heteroatom and carbon nucleophiles. An efficient synthesis of 5H-benzo[a]fluoren-5-ones has also been realized using N-(2-alkynyl)benzoylpyrrole as the reaction partner through a new reaction pathway.

Visible-Light-Induced Selective Defluoroborylation of Polyfluoroarenes, gem-Difluoroalkenes, and Trifluoromethylalkenes

Fluorinated organoboranes serve as versatile synthetic precursors for the preparation of value-added fluorinated organic compounds. Recent progress has been mainly focused on the transition-metal catalyzed defluoroborylation. Herein, we report a photocatalytic defluoroborylation platform through direct B-H activation of N-heterocyclic carbene boranes, through the synergistic merger of a photoredox catalyst and a hydrogen atom transfer catalyst. This atom-economic and operationally simple protocol has enabled defluoroborylation of an extremely broad scope of multifluorinated substrates including polyfluoroarenes, gem-difluoroalkenes, and trifluoromethylalkenes in a highly selective fashion. Intriguingly, the defluoroborylation protocol can be transition-metal free, and the regioselectivity obtained is complementary to the reported transition-metal-catalysis in many cases.

Rhodium-Catalyzed Stitching Polymerization of 1,5-Hexadiynes and Related Oligoalkynes

A new mode of polymerization, rhodium-catalyzed stitching polymerization, has been developed for the synthesis of π-conjugated polymers with bridged repeating units from nonconjugated 1,5-hexadiynes containing both terminal and internal alkyne moieties as monomers. The polymerization proceeded smoothly with a high degree of stitching efficiency under mild conditions, and 1,5,9-decatriyne and 1,5,9,13-tetradecatetrayne monomers could also be employed. The present polymerization strategy would be particularly beneficial for the synthesis of polymers consisting of a repeating unit that is difficult to prepare as a stable monomer because it does not require the use of a preformed bridged π-conjugated monomer.

Helicenes Built from Silacyclopentadienes via Ring-by-Ring Knitting of the Helical Framework

Despite the apparent diversity of the protocols developed for the synthesis of helicenes, they essentially follow the same strategy: the closure of one, or several, internal rings in a key step. Herein, we report the synthesis of a new family of the heterohelicenes consisting of fused silacyclopentadiene rings formed via a facile and novel process. The treatment of oligo(alkynilydenesilylene) precursors of type H₂ C=CH-(SiMe₂ -C≡C)_n -R (n=3-7), bearing a vinyl group on the terminal silicon atom, with 9-borabicyclononane leads first to 1,2-hydroboration of the terminal double bond which then continues with a cascade of intramolecular 1,1-carboboration reactions accompanied with the closure of a new silole ring after each step affording the target silahelicenes with, currently, up to seven condensed silole rings and with excellent yields. According XRD analysis, the seven fused silole rings of the heptacyclic compound 11 b form an almost complete turn of a helix. The presented one-pot sequence of reactions is the first example of ring-by-ring knitting of a helical framework starting from easily available linear precursors.

Synthesis of carbonyl-bridged dibenzofulvalenes and related compounds by rhodium-catalyzed stitching reaction

Various carbonyl-bridged dibenzofulvalenes were synthesized by a sequence of rhodium-catalyzed stitching reaction and post-functionalization, and other bridged dibenzofulvalenes could also be synthesized using the present stitching reaction.

Theoretical study on the photophysical properties of boron-fused double helicenes

Seven boron-fused double helicenes containing different electron-donors or acceptors and their combinations were investigated to shed light on the effective way to enhance the second-order NLO response.