Synthesis of Thiophene-Fused Siloles through Rhodium-Catalyzed Trans -Bis-Silylation

: Rhodium-catalyzed reactions of 3-ethynyl-2-pentamethyldisilanylthiophene derivatives ( 1a – 1c ) have been reported. At 110 ◦ C, compounds 1a – 1c reacted in the presence of a rhodium complex catalyst, yielding thiophene-fused siloles ( 2a – 2c ) through intramolecular trans -bis-silylation. To understand the production of 2a from 1a , the mechanism was investigated using density functional theory (DFT


Introduction
Organosilicon compounds have been frequently utilized in v organic synthesis, materials, and pharmaceuticals, owing to the chemical properties [1][2][3][4][5].Among the synthetic methods for orga bis-silylation reaction has emerged, enabling the simultaneous fo carbon bonds.In most bis-silylation reactions, cis-adducts are stere 20]; however, trans-adduct formation reactions have been rece Matsuda et al. demonstrated the possibility of trans-selective bis-s bond via intramolecular cyclization using a Rh(I) catalyst with (2-a [21].In 2022, we reported the first intramolecular trans-bis-silylat ence of PdCl2(PPh3)2-CuI as the catalyst [22].In 2023, we discovere with condensed pyridine rings through intramolecular trans-bis-s presence of a rhodium complex catalyst [23].Silole derivatives ha tion as novel functional materials with excellent electronic and p for various applications such as organic light-emitting diodes (O vices, and semiconductors [24][25][26][27][28][29]. Zhao et al. reported the intermolecular trans-bis-silylation of a Pd catalyst and 8-(2-substituted-1,1,2,2-tetramethyldisilanyl)qui tively form trans-bis-silylated alkenes [30], as well as the diverg kynoates using TMDQ in a synergistic Pd/Lewis acid catalytic sys Furthermore, much attention has been paid to the synthesis of thiophene-based materials, which display potential for use in electrochromic devices, field-effect transistors, OLEDs, and organic photovoltaics [32].Therefore, investigating the synthesis of siloles using thiophenes is of considerable interest.In this paper, we report the synthesis of siloles with condensed thiophene rings using a rhodium complex catalyst in an intramolecular trans-bis-silylation reaction.Furthermore, we elucidated the reaction mechanism through computational calculations using density functional theory (DFT).This paper represents the first report of synthesizing thiophene-fused siloles via transition-metal-catalyzed bissilylation reactions.

Procedures
Preparation of 3-Iodo-2-(1,1,2,2,2-Pentamethyldisilanyl)thiophene: 2,3-Diiodothiophene (9.646 g, 28.7 mmol) was added to 30 mL of dry THF in a 200-mL three-necked flask fitted with a stirrer, reflux condenser, and dropping funnel.Thereafter, a THF solution comprising 18 mL (36.0 mmol) of 2.0 M ethyl magnesium chloride was added dropwise at room temperature.The mixture was stirred for 1.5 h at room temperature and 5.885 g (35.3 mmol) of chloropentamethyldisilane was added.The resulting mixture was stirred for 6 h and then treated with distilled water.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 and eluted with hexane to obtain 7.268 g (70% yield) of 3-iodo-  29 Si NMR δ(CDCl 3 ) −20.3, −17.9.
Synthesis of Compound 1a: In a 100-mL three-necked flask equipped with a stirrer, reflux condenser, and dropping funnel, 3-iodo-2-(1,1,2,2,2-pentamethyldisilanyl)thiophene (1.751 g, 5.14 mmol), bis(triphenylphosphine)dichloropalladium (0.181 g, 0.258 mmol), and copper(I) iodide (0.049 g, 0.257 mmol) were combined with 15 mL of dry triethylamine.Following this, ethynylbenzene (1.063 g, 10.4 mmol) was slowly added to the mixture at room temperature via dropwise addition.The resulting mixture was refluxed for 12 h.Afterward, distilled water was added to the mixture, and the organic layer was isolated, washed with water, and dried using anhydrous magnesium sulfate.Evapora-tion of the solvent followed, and the remaining residue was subjected to chromatography on a silica gel column, eluting with hexane-ethyl acetate (50:1).This process yielded 0.702 g (43% yield) of compound 1a: HR-MS: calcd.for C 17  dichloropalladium, and 0.029 g (0.152 mmol) of copper(I) iodide were combined with 10 mL of dry triethylamine in a 100-mL three-necked flask equipped with a stirrer, reflux condenser, and dropping funnel.While at room temperature, 0.701 g (6.04 mmol) of 4ethynyltoluene was slowly added dropwise to this mixture.The resulting mixture was then heated under reflux for 12 h.Afterward, distilled water was added to the mixture, and the organic layer was isolated, washed with water, and dried using anhydrous magnesium sulfate.Subsequently, the solvent was evaporated, and the remaining residue was subjected to chromatography on a silica gel column using a hexane-ethyl acetate (50:1) elution system, yielding 0.324 g (33% yield) of compound 1b: HR-MS: calcd.for C 18   29 Si NMR δ(CDCl 3 ) −22.7, −17.5.
Reaction of Compound 1a in the Presence of RhCl(PPh 3 ) 3 Catalyst: A total of 0.222 g (0.707 mmol) of compound 1a and 0.065 g (0.0703 mmol) of RhCl(PPh 3 ) 3 were combined with dry toluene (1.5 mL) in a 30 mL two-necked flask equipped with a reflux condenser.The mixture underwent reflux heating for 12 h.Afterward, distilled water was added to the mixture, followed by separation of the organic layer, washing with water, and drying using anhydrous magnesium sulfate.The solvent was then evaporated, resulting in a residue that underwent chromatography on a silica gel column.Elution was performed using hexane-ethyl acetate (50:1) as the solvent, ultimately leading to the recovery of the initial compound 1a.
Reaction of Compound 3 in the Presence of [RhCl(nbd) 2 ] 2 Catalyst: Dry toluene (1.5 mL), along with compound 3 (0.065 g, 0.221 mmol) and [RhCl(nbd) 2 ] 2 (0.010 g, 0.0217 mmol), were combined in a 30 mL two-necked flask equipped with a reflux condenser.The mixture was refluxed for 12 h.Following this, the addition of distilled water to the mixture led to the isolation of the organic layer, which was washed with water and dried using anhydrous magnesium sulfate.Evaporation of the solvent was performed, and the remaining residue underwent chromatography on a silica gel column, eluting with hexane.As a result, compound 3 was obtained once again.

Synthesis of Compound 4:
In a 100-mL three-necked flask equipped with a stirrer, reflux condenser, and dropping funnel, 1.019 g (2.99 mmol) of 3-iodo-2-(1,1,2,2,2pentamethyldisilanyl)thiophene, 0.105 g (0.150 mmol) of bis(triphenylphosphine) dichloropalladium, and 0.029 g (0.152 mmol) of copper(I) iodide were combined with 10 mL of dry triethylamine.Subsequently, ethynyltrimethylsilane (0.648 g, 6.60 mmol) was added dropwise at room temperature.The resulting mixture was heated at reflux for 12 h.Following this, distilled water was added to the mixture, and the organic layer was separated, washed with water, and dried using anhydrous magnesium sulfate.Evaporation of the solvent followed, and the remaining residue was subjected to chromatography on a silica gel column, eluting with hexane-ethyl acetate (50:1), yielding 0.213 g (23% yield) of compound 4: HR-MS: calcd.for C 14 H 26 Si 3 S (M + ): 310.1063, found: 310.1066.MS m/z 310 (M + ); 1  Reaction of Compound 4 in the Presence of [RhCl(nbd) 2 ] 2 Catalyst: In a 30 mL two-necked flask equipped with a reflux condenser, compound 4 (0.101 g, 0.325 mmol) and 0.015 g (0.0325 mmol) of [RhCl(nbd) 2 ] 2 were introduced into dry toluene (1.5 mL).The resulting mixture was then subjected to reflux for 12 h.Afterward, distilled water was added to the mixture, leading to the separation of the organic layer, which was subsequently washed with water and dried using anhydrous magnesium sulfate.Evaporation of the solvent followed, and the residue underwent chromatography on a silica gel column, being eluted with hexane.Ultimately, the initial compound 4 was recovered.

Synthesis of Compound 6:
In a 100 mL two-necked flask equipped with a reflux condenser, 1.007 g (2.96 mmol) of 3-iodo-2-(1,1,2,2,2-pentamethyldisilanyl)thiophene, 0.104 g (0.148 mmol) of bis(triphenylphosphine)dichloropalladium, and 0.029 g (0.152 mmol) of copper(I) iodide were combined with 10 mL of dry triethylamine.To this mixture, 1ethynylcyclohexene (0.628 g, 5.92 mmol) was slowly added dropwise at room temperature, followed by heating at reflux for 12 h.After the reflux, distilled water was added to the mixture, and the resulting organic layer was separated, washed with water, and dried over anhydrous magnesium sulfate.Evaporation of the solvent ensued, and the remaining residue was subjected to chromatography on a silica gel column, eluting with hexaneethyl acetate (50:1), yielding 0.598 g (63% yield) of compound 6: HR-MS, calculated for Reaction of Compound 6 in the Presence of [RhCl(nbd) 2 ] 2 Catalyst: In a 30 mL two-necked flask equipped with a reflux condenser, compound 6 (0.110 g, 0.345 mmol) and 0.016 g (0.0347 mmol) of [RhCl(nbd) 2 ] 2 were introduced into dry toluene (1.5 mL).The resulting mixture was heated under reflux for 1 h.Post-reflux, distilled water was added to the mixture, leading to the separation of the organic layer, which was subsequently washed with water and dried using anhydrous magnesium sulfate.Evaporation of the solvent followed, and the remaining residue underwent chromatography on a silica gel column, being eluted with hexane.Compound 6 was consumed in the reaction; however, multiple products were detected in the reaction mixture via gas-liquid chromatography (GLC) and gel permeation chromatography (GPC).No derivatives of thiophene-fused silole were identified in the reaction mixture.
Reaction of 3-Iodo-2-(1,1,2,2,2-pentamethyldisilanyl)thiophene with 1-Octyne in the Presence of Palladium and Copper Catalysts: In a 100 mL two-necked flask equipped with a reflux condenser, 1.005 g (2.95 mmol) of 3-iodo-3-(1,1,2,2,2-pentamethyldisilanyl)thiophene, 0.104 g (0.148 mmol) of bis(triphenylphosphine)dichloropalladium, and 0.028 g (0.147 mmol) of copper(I) iodide were combined with 10 mL of dry triethylamine.Gradually, 1-octyne (0.651 g, 5.91 mmol) was added dropwise to this mixture at room temperature, and the resulting mixture was heated at reflux for 12 h.Multiple products were identified in the reaction mixture via GLC and GPC.Afterward, distilled water was added to the mixture, and the organic layer was separated, washed with water, and dried over anhydrous magnesium sulfate.Evaporation of the solvent followed, and the remaining residue was subjected to chromatography on a silica gel column, eluting with hexaneethyl acetate (10:1).
We first examined the reaction of 1a in the presence of the dicarbonyl(chloro)rhodium(I) dimer [RhCl(CO) 2 ] 2 [23].The treatment of 1a in the presence of a catalytic amount of [RhCl(CO) 2 ] 2 in refluxing toluene produced no products within 12 h.Starting compound 1a was recovered quantitatively.Next, we performed the reaction of 1a with [RhCl(nbd)] 2 (nbd = norborna-2,5-diene).When 1a was heated to reflux in toluene in the presence of [RhCl(nbd)] 2 , 6,6-dimethyl-5-phenyl-4-(trimethylsilyl)-6H-silolo[2,3b]thiophene (2a) was obtained in 43% yield (Scheme 2).Compound 2a was obtained via the intramolecular trans-bis-silylation of compound 1a.The structure of 2a was verified using spectroscopic analysis.The mass spectrum for 2a shows parent ions at m/z 314, which correspond to the calculated molecular weight of C 17 H 22 Si 2 S. The 1 H NMR spectrum for 2a shows signals at 0.01 and 0.31 ppm due to the methyl protons on the silicon atoms, and two doublets of doublet signals at 7.31 and 7.64 ppm due to the thienyl ring protons and phenyl ring protons.The 29 Si NMR spectrum for 2a shows signals at −6.8 and 0.1 ppm.Scheme 1. Synthesis of compounds 1a-1c.The reactions of 3 and 4 in the presence of [RhCl(nbd)] 2 did not afford 6,6-dimethyl-4-(trimethylsilyl)-6H-silolo[2,3-b]thiophene derivatives.The starting compounds, 3 and 4, were recovered.This may be caused by the presence of bulky substituents such as tert-butyl and trime9thylsilyl that would prevent the formation of silole derivatives via intramolecular trans-bis-silylation.
Scheme 4 illustrates a possible mechanistic interpretation of the reaction.The formation of 2 can be best explained by its reaction with rhodacyclopropene.The reaction of compound 1 with RhCl(nbd) activates the Si-Si bond to produce intermediate-1 (IM-1) via transition state-0 (TS-0) (rhodacyclopropene).A structural change from IM-1 to IM-2 is due to a migration of the spectator Cl anion, and TS-1 exists between the two local minima (LMs).During this change, the Rh-C (in C≡C) bond distances were decreased from 4.75 Å and 4.28 Å to 2.40 Å and 2.54 Å. (See Figures S44 and S46 in Supplementary Materials).The trimethylsilyl group on the rhodium atom then migrates to the carbon atom bonded to the thiophene ring through TS-2, and IM-3 is formed.Finally, the dimethylsilyl group bonds to the carbon atom adjacent to R (via TS-3), and elimination of the rhodium species affords 2. Scheme 4. Proposed mechanism for production of 2.

Theoretical Study
DFT calculations were conducted to clarify the transformation mechanism from reactant 1 to product 2, as illustrated in Scheme 4. The computational analysis utilized the Gaussian09 software package [33] employing Becke's three-parameter Lee-Yang-Parr hybrid functional [34].Los Alamos effective core potentials [35] in combination with Dunning-Huzinaga full double-basis sets [36] were applied for the Rhodium atom augmented by single 4f function, while the H, C, N, O, Si, and Cl atoms were treated with the 6-31+G(d,p) basis sets.
First, the transition states were identified and characterized.Subsequently, the intrinsic reaction coordinate (IRC) [37] was examined for both the reactant and product directions corresponding to each TS.At the end of the IRC, normal optimization procedures were applied until two LMs were achieved.
RhCl(nbd) was considered the active catalyst species formed by the decomposition of [RhCl(nbd)] 2 .The sum of the energies of 1 and RhCl(nbd) was adopted as the reference energy.The reaction proceeds in the following order: [1 + RhCl(nbd)] → TS-0 → IM-1 → TS-1 → IM-2 → TS-2 → IM-3 → TS-3 → [2 + RhCl(nbd)].All optimized structures are shown in the Supplementary Materials (Figures S41-S50).For all TSs and IMs, the Gibbs free energies were evaluated at 383.15 K. Figure 1 shows the SCF energy and the free energy changes along the reaction coordinate referring to the reference energy.Both energies change in parallel, and the free energy lies on the higher potential energy surface.The rate-determining step was TS-2, and the activation energies measured from the lowest LM were 159 kJ mol −1 and 176 kJ mol −1 for the SCF and free energies, respectively.Thus, the conversion of 1 into 2 was demonstrated using DFT calculations.
Similar reactions with 3-(alkylethynyl)-2-(1,1,2,2,2pentamethyldisilanyl)thiophenes were unsuccessful.This paper reports a new method for the synthesis of thiophene-fused siloles.Furthermore, the [RhCl(nbd)] 2 used in the catalyzed reactions differs from the rhodium complex used in the synthesis of pyridinecondensed siloles reported previously [23], providing new insights into the reaction mechanism in computational chemistry.