Rhodium-Catalyzed Trans-Bis-Silylation Reactions of 2-Ethynyl-3-pentamethyldisilanylpyridines

Rhodium-catalyzed reactions of 2-ethynyl-3-pentamethyldisilanylpyridine derivatives (1 and 2) are reported. The reactions of compounds 1 and 2 in the presence of catalytic amounts of rhodium complexes at 110 °C gave the corresponding pyridine-fused siloles (3) and (4) through intramolecular trans-bis-silylation cyclization. The reaction of 2-bromo-3-(1,1,2,2,2-pentamethyldisilanyl)pyridine with 3-phenyl-1-propyne in the presence of PdCl2(PPh3)2-CuI catalysts afforded 1:2 bis-silylation adduct 6. DFT calculations were also performed to understand the reaction mechanism for the production of compound 3 from compound 1.


Introduction
Various synthetic methods of organosilicon compounds have been reported so far [1]. Silicon-containing compounds, due to their unique physical and chemical properties, are attractive as candidates for optical and electronic materials such as organic thin film transistors, organic light-emitting diodes, and organic photovoltaics [2][3][4][5][6][7]. In particular, the synthesis and properties of silole derivatives with low-lying LUMO have been extensively studied. The bis-silylation of unsaturated carbon compounds, which provides two siliconcarbon bonds simultaneously, has been developed [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Bis-silylation reactions using various substrates have been reported so far, but most of them are cis-additions of a silicon-silicon bond to unsaturated compounds. This severely limits the practicality of the method. It seems to be very important that trans-bis-silylation of alkynes can be carried out easily and successfully. There are several reports on the trans-bis-silylation reactions of alkynes. In 2012, Matsuda and co-workers showed the possibility of a trans-selective bissilylation reaction of the C-C triple bond in the Rh(I)-catalyzed intramolecular cyclization of specific (2-alkynylphenyl)disilanes [18]. The reaction mechanism of the rhodium-catalyzed reactions has not yet been clarified.
It is of considerable interest to us to investigate the chemical behavior of 2-ethynyl-3-pentamethyldisilanylpyridine derivatives in the presence of rhodium catalysts to synthesize pyridine-fused silole derivatives. In this paper, we report the rhodium-catalyzed reactions of 2-ethynyl-3-pentamethyldisilanylpyridine derivatives, and DFT calculations to investigate the energy and structural changes in the synthesis route from 3-(1,1,2,2,2pentamethyldisilanyl)-2-(trimethylsilylethynyl)pyridine (1) to trans-bis-silylation product 3.
Molecules 2023, 28, x FOR PEER REVIEW 2 of 12 been examined for their optical and physical properties, as well as their medical potential [25]. It is of considerable interest to us to investigate the chemical behavior of 2-ethynyl-3pentamethyldisilanylpyridine derivatives in the presence of rhodium catalysts to synthesize pyridine-fused silole derivatives. In this paper, we report the rhodium-catalyzed reactions of 2-ethynyl-3-pentamethyldisilanylpyridine derivatives, and DFT calculations to investigate the energy and structural changes in the synthesis route from 3-(1,1,2,2,2-pentamethyldisilanyl)-2-(trimethylsilylethynyl)pyridine (1) to trans-bis-silylation product 3.

Scheme 1. Synthesis of compounds 1 and 2.
We first examined the reaction of compound 1 in the presence of Di-μ-chlorotetracarbonyldirhodium(I), [RhCl(CO)2]2. The treatment of compound 1 in the presence of a catalytic amount of [RhCl(CO)2]2 in refluxing toluene for 12 h gave 1,1-dimethyl-2,3bis(trimethylsilyl)-1H-silolo(3,2-b)pyridine (3) in 46% yield (Scheme 2). Many unidentified products were detected in the reaction mixture by GLC and GPC. Compound 3 was obtained via the intramolecular trans-bis-silylation of compound 1. The structure of compound 3 was verified by spectroscopic analysis. The mass spectrum for compound 3 showed parent ions at m/z 305, corresponding to the calculated molecular weight of C15H27NSi3. The 1 H NMR spectrum for compound 3 showed signals at 0.29, 0.34, and 0.38 ppm due to the methyl protons on the silicon atoms, and three doublets of doublet signals at 6.98, 7.73, and 8.46 ppm due to the pyridyl ring protons. The 29 Si NMR spectrum for compound 3 showed signals at −9.8, −6.6, and 10.1 ppm. Scheme 1. Synthesis of compounds 1 and 2.
We first examined the reaction of compound 1 in the presence of Di-µ-chloro-tetracarbo nyldirhodium(I), [RhCl(CO) 2 ] 2 . The treatment of compound 1 in the presence of a catalytic amount of [RhCl(CO) 2 ] 2 in refluxing toluene for 12 h gave 1,1-dimethyl-2,3-bis(trimethylsilyl)-1H-silolo(3,2-b)pyridine (3) in 46% yield (Scheme 2). Many unidentified products were detected in the reaction mixture by GLC and GPC. Compound 3 was obtained via the intramolecular trans-bis-silylation of compound 1. The structure of compound 3 was verified by spectroscopic analysis. The mass spectrum for compound 3 showed parent ions at m/z 305, corresponding to the calculated molecular weight of C 15 H 27 NSi 3 . The 1 H NMR spectrum for compound 3 showed signals at 0.29, 0.34, and 0.38 ppm due to the methyl protons on the silicon atoms, and three doublets of doublet signals at 6.98, 7.73, and 8.46 ppm due to the pyridyl ring protons. The 29 Si NMR spectrum for compound 3 showed signals at −9.8, −6.6, and 10.1 ppm.
Firstly, transition states (TSs) were searched based on Scheme 4. Then, for each TS, the intrinsic reaction coordinate (IRC) [30] was evaluated for both directions (reactant and product). At the end of IRC, normal optimization was followed until the two local minima (LMs) were reached.
Firstly, transition states (TSs) were searched based on Scheme 4. Then, for each TS, the intrinsic reaction coordinate (IRC) [30] was evaluated for both directions (reactant and product). At the end of IRC, normal optimization was followed until the two local minima (LMs) were reached.
Model 0 is a combined system of compound 1 and RhCl(CO) x . The reaction proceeded in the order of Model 0 → TS 0-1 → Model 1 → TS 1-2 → Model 2 → TS 2-3 → Model 3 (compound 3 + RhCl(CO) x ). Model 3 corresponds to compound 3 with Rh complex. All the optimized structures for LMs and TSs on the trans route are shown in the Supplementary  Materials (Figures S13-S29). The energy change, along the reaction coordinate, are shown in Figure 1.
Model 0 is a combined system of compound 1 and RhCl(CO)x. The reaction proceeded in the order of Model 0 → TS 0-1 → Model 1 → TS 1-2 → Model 2 → TS 2-3 → Model 3 (compound 3 + RhCl(CO)x). Model 3 corresponds to compound 3 with Rh complex. All the optimized structures for LMs and TSs on the trans route are shown in the Supplementary  Materials (Figures S13-S29). The energy change, along the reaction coordinate, are shown in Figure 1. (compound 3 + RhCl(CO)x). Model 3 corresponds to compound 3 with Rh complex. All the optimized structures for LMs and TSs on the trans route are shown in the Supplementary  Materials (Figures S13-S29). The energy change, along the reaction coordinate, are shown in Figure 1.  Figures 1 and 2 show the SCF energy and the free energy, referring to the sum of energies of [RhCl(CO) 2 ] 2 and compound 1, where the destabilization due to decomposition or disproportionation was taken into consideration. Both of the energies changed in parallel, and the location of the rate determining step did not depend on the two criteria.
Adopting the free energy, the rate determining steps were TS 2-3 and TS 2-3B for the trans and cis routes, and their activation energies were 169 and 189 kJ mol −1 , respectively, with the RhCl(CO) model. With the RhCl(CO) 2 model, the rate determining step for the cis route was TS 2-3B with an activation energy of 170 kJ mol −1 ; however, the rate determining step for the trans route was TS 0-1 with an activation energy of 123 kJ mol −1 . Thus, both of the calculation models afforded the same result: that the trans-bis-silylation is the more stable product with respect to the activation energy, in accordance with our experimental result. Although the reactions with the RhCl(CO) model seem to proceed on the lower potential energy surfaces, the RhCl(CO) 2 mode is a more promising catalyst for two reasons. (1) The activation energies for the rate determining step were 169 and 123 kJ mol −1 with RhCl(CO) and RhCl(CO) 2 modes for the trans route. (2) The destabilization by disproportionation ([RhCl(CO) 2 ] 2 → RhCl(CO) + RhCl(CO) 3 ) was 117 and 59 kJ mol −1 for the SCF and free energies. On the other hand, the decomposition energies ([RhCl(CO) 2 ] 2 → 2RhCl(CO) 2 ) were 30 and −36 kJ mol −1 , respectively. At the free energy level, the decomposition became stabilization. The present work is the first to investigate the two catalyst models intensively.
Synthesis of Compound 2. A mixture of compound 1 (1.571 g, 5.14 mmol), potassium carbonate (0.982 g, 7.11 mmol), and methanol (50 mL) was stirred at room temperature for 2 h. The reaction mixture was concentrated under reduced pressure, and hexane (15 mL) and water (30 mL) were added to the residue. The layers were separated and the aqueous layer was extracted with hexane (4 × 15 mL). The solvent was then evaporated, and the residue was chromatographed on a silica gel column, eluted with hexane-ethyl acetate Reaction of Compound 1 in the Presence of [RhCl(nbd)] 2 Catalyst. In a 30 mL twonecked flask fitted with a reflux condenser were placed 0.300 g (0.98 mmol) of compound 1 and 0.045 g (0.098 mmol) of [RhCl(nbd)] 2 in 5 mL of dry toluene. The mixture was heated to reflux for 12 h. The solution was then hydrolyzed, and the organic layer was separated, washed with water, and dried over anhydrous magnesium sulfate. The solvent was then evaporated and the residue was chromatographed on a silica gel column, eluting with hexane-ethyl acetate (10:1), to obtain 0.018 g (6% yield) of compound 3. All spectral data for compound 3 were identical to those of an authentic sample.
Reaction of Compound 1 in the Presence of RhCl(PPh 3 ) 3 Catalyst. In a 30 mL twonecked flask fitted with a reflux condenser were placed 0.575 g (1.88 mmol) of 1 and 0.176 g (0.190 mmol) of RhCl(PPh 3 ) 3 in 5 mL of dry toluene. The mixture was heated to reflux for 12 h. The solution was then hydrolyzed, and the organic layer was separated, washed with water, and dried over anhydrous magnesium sulfate. The solvent was then evaporated and the residue was chromatographed on a silica gel column, eluting with hexane-ethyl acetate (10:1), to obtain 0.030 g (5% yield) of compound 3. The starting compound 1 was almost recovered (0.502 g). All spectral data for compound 3 were identical to those of an authentic sample.
Reaction of Compound 2 in the Presence of [RhCl(CO) 2 ] 2 Catalyst. In a 30 mL twonecked flask fitted with a reflux condenser were placed 0.134 g (0.574 mmol) of compound 1 and 0.023 g (0.059 mmol) of [RhCl(CO) 2 ] 2 in 5 mL of dry toluene. The mixture was heated to reflux for 12 h. The solution was then hydrolyzed, and the organic layer was separated, washed with water, and dried over anhydrous magnesium sulfate. The solvent was then evaporated and the residue was chromatographed on a silica gel column, eluting with hexane-ethyl acetate (10:1), to obtain 0.064 g (48% yield) of compound 4: HR-MS: calcd. for C 12  Reaction of Compound 2 in the Presence of [RhCl(nbd)] 2 Catalyst. In a 30 mL twonecked flask fitted with a reflux condenser were placed 0.130 g (0.557 mmol) of compound 1 and 0.025 g (0.054 mmol) of [RhCl(nbd)] 2 in 5 mL of dry toluene. The mixture was heated to reflux for 12 h. The solution was then hydrolyzed, and the organic layer was separated, washed with water, and dried over anhydrous magnesium sulfate. The solvent was then evaporated and the residue was chromatographed on a silica gel column, eluting with hexane-ethyl acetate (10:1), to obtain 0.007 g (5% yield) of compound 4. All spectral data for compound 4 were identical to those of an authentic sample.
Reaction of Compound 2 in the Presence of RhCl(PPh 3 ) 3 Catalyst. In a 30 mL twonecked flask fitted with a reflux condenser were placed 0.094 g (0.403 mmol) of compound 2 and 0.037 g (0.040 mmol) of RhCl(PPh 3 ) 3 in 5 mL of dry toluene. The mixture was heated to reflux for 12 h. The solution was then hydrolyzed, and the organic layer was separated, washed with water, and dried over anhydrous magnesium sulfate. The solvent was then evaporated and the residue was chromatographed on a silica gel column, eluting with hexane-ethyl acetate (10:1). The starting compound 2 was recovered (0.075 g).