Synthesis of Pyridine-Fused Siloles by Palladium-Catalyzed Intramolecular Bis-Silylation

Silole derivatives are attracting significant attention as new functional materials with excellent electronic and photophysical properties. Thus, the development of synthesis methods to afford such derivatives is highly desirable. Herein, the synthesis of pyridine-fused siloles under the conditions of the Sonogashira coupling reaction is described. The reactions of 2-bromo-3-(pentamethyldisilanyl)pyridine (1) with ethynylbenzene derivatives in the presence of PdCl2(PPh3)2-CuI as a catalyst afforded the corresponding pyridine-fused siloles (2a–2c) through intramolecular trans-bis-silylation. DFT calculations were also performed to understand the reaction mechanism. This paper is the first to report on the successful use of palladium catalysts in the trans-bis-silylation of alkynes with disilanes.


■ INTRODUCTION
Silicon-containing compounds are used in various fields ranging from synthetic organic chemistry to functional materials and pharmaceutical synthesis. Thus, new synthetic methods for such compounds have been developed. 1−5 Many articles have reported on the transition metal-catalyzed bissilylation of alkynes with disilane. 6−13 Some examples of cisbis-silylation of internal alkynes with acyclic disilanes have also been reported (Scheme 1). For example, Ozawa et al. reported the bis-silylation of Me 3 SiSiF 2 Ph with internal alkynes using a Pd catalyst. 14 Spencer et al. investigated the intermolecular bissilylation of internal alkynes with hexamethyldisilane using the [(NHC) 2 Pd(SiMe 3 ) 2 ] (NHC: N-heterocyclic carbene) complex as a precatalyst. 15 Zhao et al. reported nickel-catalyzed bissilylation of internal alkynes with an unsymmetrical coordinat-ing disilane, 8-(2-substituted-1,1,2,2-tetramethyldisilanyl)quinoline. 6 Some examples of intramolecular versions have also been described, which allow access to heterocycles with different ring sizes. In 2012, Matsuda and Ichioka developed a selective synthesis of four or five silylated rings. 16 They investigated whether regioselectivity depends on the catalyst used and found that when (2-alkynylphenyl)disilane was catalyzed by a Pd/isocyanide system, cis-bis-silylation occurred to form fourmembered ring compounds. In contrast, rhodium-catalyzed reactions of (2-alkynylphenyl)disilane afforded five-membered ring compounds, that is, 3-silyl-1-benzosiloles, via trans-bissilylation (Scheme 2).
Recently, we reported the synthesis of pyridine-fused siloles using ruthenium-catalyzed hydrosilylation and their optical properties. 17 Silole derivatives are attracting attention as new functional materials with excellent electronic and photophysical properties for various applications, such as organic light-emitting diodes (OLEDs), photovoltaic devices, and semiconductors. 18−23 Herein, we report the synthesis of pyridine-fused siloles under the conditions of the Sonogashira coupling reaction. 24 These reactions are expected to proceed through the trans-bis-silylation of 3-(1,1,2,2,2-pentamethyldisilanyl)-2-(arylethynyl)pyridine. Notably, palladium catalysts have never been utilized in the trans-bis-silylation of alkynes with disilanes before.

■ RESULTS AND DISCUSSION
The reaction of 2,3-dibromopyridine with i-PrMgCl at room temperature followed by quenching with chloropentamethyldisilane afforded 2-bromo-3-(pentamethyldisilanyl)pyridine (1) in 70% yield. 25 To obtain 2-phenylethynyl-3-(pentamethyldisilanyl)pyridine, 1 was reacted with phenylacetylene under the Sonogashira coupling reaction conditions. When a mixture of 1 and ethynylbenzene in the presence of a PdCl 2 (PPh 3 ) 2 -CuI catalyst was heated to reflux in triethylamine, an unexpected product, 1,1-dimethyl-2-phenyl-3-(trimethylsilyl)-1H-silolo(3,2-b)pyridine (2a), was obtained in 15% isolated yield, along with intermolecular bis-silylation product 3a (8% yield) from the reaction of 2-phenylethynyl-3-(pentamethyldisilanyl)pyridine with phenylacetylene (Scheme 3). Compound 2a was produced via intramolecular trans-bissilylation. Many unidentified products were also detected in the reaction mixture by gas liquid chromatography (GLC) and gel permeation chromatography (GPC). Song et al. reported an unsymmetrical disilane by attaching a coordinating group to one of the two silicon atoms, which was successfully applied to the Pd-catalyzed bis-silylation of terminal alkynes. 26 Compound 2a was isolated by column chromatography, and its structure was verified by spectrometric analyses. The mass spectrum for 2a shows a parent ion at m/z 309 corresponding to the calculated molecular weight of C 18 H 23 Si 2 N. The 1 H NMR spectrum for 2a shows singlets at 0.04 and 0.29 ppm, representing methyl protons on the silicon atoms, three doublets of doublet signals at 7.00, 7.76, and 8.49 ppm, ascribed to the pyridyl protons, and signals corresponding to the phenyl protons. The 29 Si NMR spectrum for 2a shows signals at −6.2 and 3.7 ppm.
We carried out the Pd(PPh 3 ) 2 Cl 2 -catalyzed reaction of 1 with ethynylbenzene in the absence of CuI. Although many unidentified products were detected in the reaction mixture by GLC and GPC, compound 2a was not detected in the reaction mixture. We believe that a copper catalyst is necessary for coupling reactions.
In the proposed reaction mechanism to obtain 2, cyclic products were formed with the catalyst, as shown in Scheme 4. First, compound A is produced by the Sonogashira coupling reaction of 1 with ethynylbenzene derivatives, followed by oxidative addition of Si−Si bonds to produce Pd complex B. Complex B further undergoes Si−C bond formation to deliver intermediate C. Complex C then undergoes ring expansion to the five-membered cyclic complex D, followed by reductive elimination to afford bis-silylated product 2 and regenerate the Pd catalyst.
The Pd catalyst was modeled using the Pd(PPh 3 ) complex, which formed when three PPh 3 ligands detach from Pd(PPh 3 ) 4 owing to steric hindrance. Two reaction mechanisms were expected: (1) a parallel mechanism from 6 to cis-adduct 7 and trans-adduct 2a and (2) a sequential mechanism in which cisadduct 7 is first formed from 6, and then 7 is converted to 2a. However, only the latter reaction route could be determined by the DFT calculations. Figure 1 shows the energy change along the reaction coordinate which consists of seven transition states (TSs) and eight local minima (LMs). LM0 is 6, LM4 is cis-adduct 7, and LM7 is trans-adduct 2a.
Individual structures are shown in the Supporting Information. The formation of TS6 is the rate-determining step with an activation energy of 161 kJ mol −1 . Although the activation energy was higher for cis-to-trans conversion than that for the formation of cis-adduct 7, trans-adduct 2a was more stable by 91.4 kJ mol −1 . If the activation energy of 161 kJ mol −1 was not a significant barrier under the present reaction conditions, the difference between the stability would be the driving force for the formation of 2a.
We also carried out DFT calculations for the production of trans-adducts 2a in the absence of a ligand on the palladium atom. The results revealed that the TS was the ratedetermining step with an activation energy of 215.3 kJ mol −1 . ■ METHODS General Procedure. All reactions of 1 were carried out under an inert atmosphere using dry nitrogen. NMR spectra were recorded on a JMN-ECS400 spectrometer. Lowresolution mass spectrometry was performed on a JEOL JMS-700 mass spectrometer. High-resolution mass spectrometry (HR-MS) was performed on a JEOL JMS-700 mass spectrometer and a Thermo Scientific LTQ Orbitrap XL hybrid Fourier-transform mass spectrometer using electrospray ionization. Column chromatography was performed using a silica gel column (Wakogel C-300; Wako Pure Chemical Industries). p a r a t i o n o f 2 -B r o m o -3 -( 1 , 1 , 2 , 2 , 2 -Pentamethyldisilanyl)Pyridine (1). In a 300 mL threenecked flask fitted with a stirrer, reflux condenser, and dropping funnel, 10.008 g (42.2 mmol) of 2,3-dibromopyridine was added to 100 mL of dry tetrahydrofuran (THF). Next, a THF solution comprising 21.1 mL (42.2 mmol) of 2.0 M isopropyl magnesium chloride was added dropwise at room temperature. The mixture was then stirred for 1 h at room temperature, and 7.047 g (42.2 mmol) of chloropentamethyldisilane was added. The resulting mixture was stirred for 6 h. It was then hydrolyzed, after which the organic layer was separated, washed with water, and dried over anhydrous magnesium sulfate. The solvent was evaporated, and the residue was chromatographed on a silica gel column eluting with hexane to obtain 8.474 g (70% yield) of 2-bromo-3-