Palladium-Catalyzed Intramolecular Coupling of 2-[(2-Pyrrolyl)silyl]aryl Triflates through 1,2-Silicon Migration

Abstract

Palladium-catalyzed intramolecular direct arylation of 2-[(2-pyrrolyl)silyl]aryl triflates gives 3,2'-silicon-bridged 2-arylindoles and -pyrroles in good to high yields. The reaction proceeds through cleavage of C-OTf, C-Si, and C-H bonds to result in the formation of C-C and C-Si bonds. Various kinds of functional groups such as OMe, CN, Cl, F, and SiMe(3) tolerated the conditions. The new reaction allows synthesis of functionalized Si-bridged 2-arylindoles that emit intense and efficient blue fluorescence in the solid state.

Design and synthesis of highly emissive solid fluorophores

“…[47][48] Optimization of the reaction conditions for the novel intramolecular coupling disclosed that the use of 1,2-bis(diphenylphosphino)ethane (dppe) and Et 2 NH in a large excess in the presence of Pd(OAc) 2 as a catalyst in DMA at 100 °C was the best choice so that 1a was isolated in 89% yield along with 3% yield of 8a (Scheme 2). 49 The isolated yield was much higher than that obtained in the normal intramolecular cyclization starting from 2-(3-indolyl)silylphenyl triflate. It should be noted that chlorine functionality that could serve as a clue for further modification of the cyclized products was found to tolerate the conditions (vide infra).…”

Pd‐Catalyzed Synthesis of Other Heteroatom‐Containing Heterocycles

“…), dimethylacetamide (DMA), 100 ∘ C). Unexpectedly, Si-bridged 2-phenylindoles were isolated as a major product (62% yield) along with the expected product (8%) (Scheme 4.33) [48]. Then they screened the conditions and found that the use of 1,2-bis(diphenylphosphino)ethane (dppe) and Et 2 NH in large excess in the presence of Pd(OAc) 2 as a catalyst was effective in suppressing the formation of the by-product so that Si-bridged 2-phenylindoles were isolated in 89% yield.…”

Cross‐Coupling of Heteroarenes by CH Functionalization: Recent Progress towards Direct Arylation and Heteroarylation Reactions Involving Heteroarenes Containing One Heteroatom

“…[215] Such a process, which occurred in DMA at 100 8C in the presence of a large molar excess of Et 2 NH and a PdA C H T U N G T R E N N U N G (OAc) 2 /dppe catalyst system, produced compounds 329 as the major reaction products, which were sometimes accompanied by minor amounts of compounds 330 (Scheme 167). [215] One of the key features for the success of this novel intramolecular high yielding coupling was the electron-donating ability of nitrogen in the indole/pyrrole moiety of compounds 328. In fact, the corresponding 2-silylfurans and (benzo)thiophenes 331 ( Figure 46) did not undergo silicon migration.…”

“…Subsequent 1,2-silicon migration followed by deprotonation would produce intermediate E which by reductive elimination would give compound 329 along with the catalytically active Pd(0) species (Scheme 168). [215] In the same year, Bowie Jr. and Trauner [216] reported a highly efficient and concise synthesis of the racemic form of naturally occurring (À)-rhazinal which involved as a key step the PdA C H T U N G T R E N N U N G (OAc) 2 Removal of the MOM-protecting group in the coupling product 333 by treatment with a large molar excess of BCl 3 at room temperature gave racemic rhazinal (334) (Scheme 169). [216] The alkaloid (À)-rhazinal, which was isolated in 1958 from the stem extracts of Malayan Kopsia teoi, [217] has been reported to mimic the effects of both vinblastine and Taxol by inducing a non-reversible spiralization of tubulin and by inhibiting the cold-induced disassembly of microtubules.…”