Synthesis of (1-silyl)allylboronates by KOtBu-catalyzed ring-opening gem-silylborylation of cyclopropenes.

A KOtBu-catalyzed ring-opening gem-silylborylation of cyclopropenes with silylboronates has been developed for the synthesis of (1-silyl)allylboronates, a useful class of compounds in organic synthesis. The reaction proceeds with high selectivity under mild conditions, and the reaction mechanism has been theoretically investigated using DFT calculations.

Photoredox-Enabled Dearomative [2π + 2σ] Cycloaddition of Phenols.

Dearomative photocycloaddition of monocyclic arenes is an appealing strategy for comprehending the concept of "escape from flatland". This brings the replacement of readily available planar aromatic hydrocarbon units with a 3D fused bicyclic core with sp3-enriched carbon units. Herein, we outline an intermolecular approach for the dearomative photocycloaddition of phenols. In order to circumvent the ground-state aromaticity and to construct conformationally restrained building blocks, bicyclo[1.1.0]butanes were chosen as coupling partners. This dearomative approach renders straightforward access to a bicyclo[2.1.1]hexane unit fused to a cyclic enone moiety, which further contributed as a synthetic linchpin for postmodifications. Mechanistic experiment advocates for a plausible onset from both the reactants, depending on the redox potential.

Palladium-Catalyzed Skeletal Rearrangement of Substituted 2-Silylaryl Triflates via 1,5-C-Pd/C-Si Bond Exchange.

A palladium-catalyzed skeletal rearrangement of 2-(2-allylarylsilyl)aryl triflates has been developed to give highly fused tetrahydrophenanthrosilole derivatives via unprecedented 1,5-C-Pd/C-Si bond exchange. The reaction pathways can be switched toward 4-membered ring-forming C(sp2 )-H alkylation by tuning the reaction conditions to give completely different products, fused dihydrodibenzosilepin derivatives, from the same starting materials. The inspection of the reaction conditions revealed the importance of carboxylates in promoting the C-Pd/C-Si bond exchange.

Copper-Catalyzed Regio- and Stereoselective Formal Hydro(borylmethylsilyl)ation of Internal Alkynes via Alkenyl-to-Alkyl 1,4-Copper Migration.

Catalytic reactions involving 1,n-metal migration from carbon to carbon enable a nonclassical way of constructing organic molecular skeletons, rapidly providing complex molecules from relatively simple precursors. By utilization of this attractive feature, a new and efficient synthesis of alkenylsilylmethylboronates has been developed by formal hydro(borylmethylsilyl)ation of unsymmetric internal alkynes with silylboronates under copper catalysis. The reaction proceeds regioselectively and involves an unprecedented alkenyl-to-alkyl 1,4-copper migration. The reaction mechanism has been investigated by a series of kinetic, NMR, and deuterium-labeling experiments.

Palladium-catalyzed synthesis of benzosilacyclobutenes via position-selective C(sp3)-H arylation.

A palladium-catalyzed synthesis of benzosilacyclobutenes has been developed via position-selective C(sp3)-H bond activation, including those having substituents at the methylene carbon on the 4-membered silacycle. The obtained products could be engaged in the palladium- or nickel-catalyzed ring-expansion reactions to give compounds possessing 6-membered silacycles.

Copper-Catalyzed Synthesis of 3-Silyl-1-silacyclopent-2-enes via Regio- and anti-Selective Addition of Silylboronates to Silicon-Containing Internal Alkynes.

A copper-catalyzed disilylative cyclization of silicon-containing internal alkynes with silylboronates has been developed for the synthesis of 3-silyl-1-silacyclopent-2-enes. The reaction proceeded regio- and anti-selectively under simple and mild conditions by employing a combination of nucleophilic silicon donors and electrophilic silicon acceptors. The reaction could also be extended to the synthesis of a 1-germacyclopent-2-ene and a silicon-centered spirocyclic compound by using appropriate alkyne substrates.

A Kinetically Stabilized Nitrogen-Doped Triangulene Cation: Stable and NIR Fluorescent Diradical Cation with Triplet Ground State.

A kinetically-stabilized nitrogen-doped triangulene cation derivative has been synthesized and isolated as the stable diradical with a triplet ground state that exhibits near-infrared emission. As was the case for a triangulene derivative we previously synthesized, the triplet ground state with a large singlet-triplet energy gap was experimentally confirmed by magnetic measurements. In contrast to the triangulene derivative, the nitrogen-doped triangulene cation derivative is highly stable even in solution under air and exhibits near-infrared absorption and emission because the alternancy symmetry of triangulene is broken by the nitrogen cation. Breaking the alternancy symmetry of triplet alternant hydrocarbon diradicals by a nitrogen cation would therefore be an effective strategy to create stable diradicals possessing magnetic properties similar to the parent hydrocarbons but with different electrochemical and photophysical properties.

Palladium-catalyzed synthesis of 4-sila-4H-benzo[d][1,3]oxazines by intramolecular Hiyama coupling.

A palladium-catalyzed synthesis of 4-sila-4H-benzo[d][1,3]oxazines, silicon-switched analogs of biologically relevant 4H-benzo[d][1,3]oxazines, was developed by the intramolecular Hiyama coupling of 3-amido-2-(arylsilyl)aryl triflates. The present reaction is an unusual way of utilizing the Hiyama coupling, enabling the synthesis of value-added organosilanes as the main products. The intramolecular nature of transmetalation with inversion of the stereochemistry at the silicon center was revealed by the mechanistic investigation, and an asymmetric variant of this process was also demonstrated to give silicon-stereogenic 4-sila-4H-benzo[d][1,3]oxazines with relatively high enantioselectivity.

Synthesis of Poly(arylenevinylene)s by Rhodium-Catalyzed Stitching Polymerization/Alkene Isomerization.

Poly(arylenevinylene)s constitute an important class of π-conjugated polymers for their potential utility as optoelectronic materials. Herein, we developed a sequence of rhodium-catalyzed stitching polymerization of 1,2-dialkynyl(hetero)arenes and aromatization-driven alkene isomerization for the synthesis of new poly(arylenevinylene)s. The polymerization and subsequent alkene isomerization proceeded smoothly with high degree of stitching efficiency by employing a Rh/tfb complex as the catalyst, and not only diynes but also triynes and tetraynes could be polymerized to give poly(arylenevinylene)s that are not easily accessible by existing synthetic methods. The polymers obtained by the present method were thermally stable, and their optical properties could be varied depending on the repeating unit structure.

Synthesis and Isolation of a Kekulé Hydrocarbon with a Triplet Ground State.

Design, synthesis, and isolation of a Kekulé hydrocarbon with a triplet ground state is described. Its triplet ground state was unambiguously confirmed by ESR experiments, and the structure and fundamental physical properties were also revealed. The key feature of the molecular design is the decrease in the bonding interaction in the singlet state by aromatic stabilization of benzene rings and the increase of the exchange interaction of unpaired electrons which are favorable for the triplet state. These results contribute to the development of hydrocarbon-based organic magnetic materials.

Anionic stitching polymerization of styryl(vinyl)silanes for the synthesis of sila-cyclic olefin polymers.

Anionic stitching polymerization of styryl(vinyl)silanes has been developed for the synthesis of a new type of silicon- and carbon-containing polymer possessing fused sila-bicyclic structures in the main chain. The obtained polymers were found to be thermally stable with relatively high glass-transition temperatures and highly transparent in the visible light region.

Synthesis and Isolation of a Kinetically Stabilized Crystalline Triangulene.

The synthesis and isolation of hydrocarbons with a triplet ground state in crystalline forms have been sought in materials science. Triangulene is one of the most famous triplet-ground-state benzenoid hydrocarbons. Its unique electronic structure and highly symmetric structure have prompted many scientists to synthesize and isolate triangulene and its derivatives, but all attempts so far to isolate them as crystals have been unsuccessful. Herein we report the synthesis and isolation of a kinetically stabilized crystalline triangulene for the first time. The key to success is the introduction of bulky substituents onto the reactive zigzag edges. Its highly symmetric structure was confirmed by X-ray crystallography, and its fundamental properties, including the triplet ground state, were revealed. The achievement here will open the door for the synthesis and isolation of other hydrocarbons with higher spin multiplicity.

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.

Copper-Catalyzed Synthesis of Tetrasubstituted Alkenes via Regio- and anti-Selective Addition of Silylboronates to Internal Alkynes.

As a new and complementary method for the synthesis of structurally defined tetrasubstituted alkenes, a copper-catalyzed regio- and anti-selective addition of silylboronates to unsymmetric internal alkynes has been developed. A variety of unactivated alkynes can be employed with high selectivity under simple and mild conditions, and the resulting products have been further functionalized by utilizing silyl and boryl groups on the alkene.

Palladium-Catalyzed Synthesis of Dibenzosilepin Derivatives via 1,n-Palladium Migration Coupled with anti-Carbopalladation of Alkyne.

Catalytic reactions involving 1,n-metal migration represent a powerful method for the synthesis of complex molecules from simple precursors through the activation of C-H bonds. By utilizing this attractive feature, herein we devised a palladium-catalyzed synthesis of new members of 5H-dibenzo[b,f]silepins, a class of underexplored but potentially useful silicon-bridged π-conjugated compounds. The reaction sequence is composed of 1,n-palladium migrations and unusual anti-carbopalladation of alkynes, which was realized by the proper choice of ligand for palladium. A series of deuterium-labeling experiments were conducted toward an understanding of the reaction mechanism to propose plausible catalytic cycles. The newly obtained 5H-dibenzo[b,f]silepins exhibited tunable optical and electronic properties, demonstrating the power and importance of developing a new synthetic method based on 1,n-metal migration processes.

Phosphinative cyclopropanation of allyl phosphates with lithium phosphides.

A new cyclopropanation reaction of allyl phosphates with lithium phosphides has been developed to give cyclopropylphosphines through the formation of both a C-P bond and a cyclopropane ring at the same time, and high selectivity toward cyclopropanation over allylic substitution has been realized by conducting the reaction in the presence of HMPA.

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.

Palladium-Catalyzed Synthesis of Benzophenanthrosilines by C-H/C-H Coupling through 1,4-Palladium Migration/Alkene Stereoisomerization.

A new and efficient synthesis of 8H-benzo[e]phenanthro[1,10-bc]silines from 2-((2-(arylethynyl)aryl)silyl)aryl triflates under palladium catalysis has been developed. The reaction mechanism was experimentally investigated and a catalytic cycle involving C-H/C-H coupling through a new mode of 1,4-palladium migration with concomitant alkene stereoisomerization is proposed.

Convergent Synthesis of Fluorene Derivatives by a Rhodium-Catalyzed Stitching Reaction/Alkene Isomerization Sequence.

A convergent synthetic method for the synthesis of fluorene derivatives has been developed by devising a rhodium-catalyzed stitching reaction/alkene isomerization sequence. The reactions proceed smoothly under mild conditions for a variety of substrate combinations, and extended π-conjugation systems are also readily accessible by utilizing this synthetic method. Optical properties of the obtained fluorene derivatives have also been examined.

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.