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A new synthetic route to highly fluorescent benzofuranylmethylidenebenzoxasiloles through cationic rhodium(I)/binap complex‐catalyzed cascade cycloisomerization of bis(2‐ethynylphenol)silanes has been developed involving 1,2‐silicon and 1,3‐carbon (alkyne) migrations followed by oxycyclization. The present synthesis requires only three steps, starting from commercially available dichlorodiisopropylsilane, which is markedly shorter than our previous synthesis (eight steps starting from commercially available chlorodiisopropylsilane). Theoretical calculations elucidated the mechanism of the above cascade cycloisomerization. This reaction is initiated by the formation of a rhodium vinylidene not through direct 1,2‐silicon migration but rather through an unprecedented stepwise 1,5‐silicon migration followed by C−Si bond‐forming cyclization from a dearomatized allenylrhodium complex. Subsequent 1,3‐carbon (alkyne) migration leading to a η3‐allenyl/propargyl‐rhodium complex followed by oxycyclization through π‐bond (alkyne) activation with the cationic rhodium(I) complex affords the benzofuranylmethylidenebenzoxasilole product. The structure–fluorescence property relationships of the thus obtained benzofuranylmethylidenebenzoxasiloles were investigated, which revealed that good fluorescence quantum yields were generated in the solution state (φF=69–87 %) by introduction of electron‐donating alkyl and phenyl groups on two phenoxy groups. In the powder state, 4‐methyl‐ and 4‐methoxy‐phenoxy derivatives exhibited efficient blue fluorescence (φF=52 % and 46 %, respectively). Especially, the 4‐methylphenoxy derivative was thermally stable, and exhibited strong narrow‐band fluorescence in the film state (blue, φF=95 %) and redshifted strong narrow‐band fluorescence (green, φF=90 %) in the crystalline state as a result of the formation of an offset π‐stacked dimer; the latter was confirmed by X‐ray crystallographic analysis and by theoretical calculations. [ABSTRACT FROM AUTHOR]

Abstract: It has been established that an electron‐deficient (η5‐cyclopentadienyl)rhodium(III) [CpERhIII] complex is capable of catalyzing the decarboxylative and oxidative [2+2+2] annulation of benzoic acids with alkynes to produce substituted naphthalenes at room temperature. The appropriate choice of the additive and the solvent is crucial for this transformation. This catalyst system allowed use of oxygen as a terminal oxidant and broadened the substrate scope including both aromatic and aliphatic alkynes. In this catalysis, the electron deficient nature of the CpERhIII catalyst would cause the strong rhodium‐π interaction, which accelerates the decarboxylation as well as the C−H bond cleavage. [ABSTRACT FROM AUTHOR]

It is shown that a cationic rhodium(I)/biphep complex catalyzes the cycloisomerization of 2-(alkynylsilylethynyl)phenols, leading to alkynylmethylidene-benzoxasiloles through concomitant silicon and carbon migration. This unprecedented cycloisomerization presumably proceeds via the formation of rhodium vinylidenes through 1,2-silicon migration, followed by 1,3-carbon (alkyne) migration via the formation of hypervalent silicon centers. [ABSTRACT FROM AUTHOR]

It has been established that a cationic rhodium(I)/BIPHEP complex catalyzes the chemo- and regioselective intermolecular cross-cyclotrimerization of silylacetylenes with unsymmetrical electron-deficient internal alkynes. Chemoselectivity was highly dependent on steric bulk of the unsymmetrical electron-deficient internal alkynes used. Two molecules of sterically less demanding alkynoates and alkynone reacted with one molecule of the silylacetylene, on the contrary, one molecule of a sterically demanding alkynoate reacted with two molecules of the silylacetylene. [ABSTRACT FROM AUTHOR]

Rhodium-catalyzed enantioselective desymmetrizing intramolecular hydrosilylation of symmetrically disubstituted hydrosilanes is described. The original axially chiral phenanthroline ligand ( S)-BinThro (Binol-derived phenanthroline) was found to work as an effective chiral catalyst for this transformation. A chiral silicon stereogenic center is one of the chiral motifs gaining much attention in asymmetric syntheses and the present protocol provides cyclic five-membered organosilanes incorporating chiral silicon centers with high enantioselectivities (up to 91 % ee). The putative active RhI catalyst takes the form of an N, N, O-tridentate coordination complex, as determined by several complementary experiments. [ABSTRACT FROM AUTHOR]