Synthesis of 1,3-Diynes via Detelluration of Bis(ethynyl)tellurides

Neste artigo e descrita a sintese de sistemas diinicos conjugados contendo substituintes eletronatratores e eletron-doadores via a deteluracao catalizada por paladio de bis-(ariletinil)teluretos e bis-(alquiletinil)teluretos. Este procedimento foi realizado sob condicoes atmosfericas em DMF usando Pd(Oac)2 como catalisador e AgOAc como um aditivo na presenca de trietilamina. Esta rota oferece acesso eficiente a sistemas diinicos conjugados em um curto periodo de tempo. A estrutura cristalografica por difracao de raios X do telureto de bis(p-toluiletinila) e a conformacao no estado solido mostram uma cadeia supramolecular alinhada ao longo do eixo b, sustentada por interacoes CH...π.


Introduction
Organotellurium compounds play an important role in organic synthesis, and they have received considerable attention because of their potential availability and useful biological activity. 1 As reported by Bergman and co-workers 2 more than three decades ago, aryltelluriums undergo detelluration upon treatment with degassed Raney nickel to afford biaryl compounds.Although the reaction is interesting and synthetically useful, the necessity of more than a stoichiometric amount of the required metal is still a serious drawback.However, attempted transition metalcatalyzed detellurations have been unsuccessful to date. 2,3ompounds containing chains of conjugated triple bonds 4,5 are of paramount importance as versatile and useful building blocks in organic synthesis.Among these compounds, 1,3-butadiynes 6 have been prominently utilized as substructures in the formation of valuable intermediates for natural products 7 and pharmaceuticals such as antitumor, 8 antibacterial, 9 anti-inflammatory, 10 and antifungal agents. 11hese conjugated diynes also serve as the core functional group in organic molecular materials such as linearly σ-conjugated acetylenic oligomers and polymers, 12 macrocycles 13 (Figure 1), and supramolecular scaffolds. 14xidative dimerization of sp-hybridized terminal alkynes mediated by Cu(I) or Cu(II) salts under either catalytic or stoichiometric conditions is the most commonly used synthetic methodology for obtaining symmetrically substituted 1,3-butadiyne.
These approaches include Glaser's coupling, 15 Eglinton's coupling, 16 and Hay's coupling. 17In these reactions, the transmetalation of an alkynyl group to copper is proposed to generate an alkynylcopper species that undergoes subsequent oxidative dimerization to give the corresponding 1,3-butadiynes. 18

Results and Discussion
Herein, we describe a convenient protocol for the synthesis of symmetrical conjugated diynes through the palladium-catalyzed detelluration of functionalized bis(arylethynyl)tellurides and bis(alkylethynyl) tellurides at room temperature in presence of air (Scheme 1).
The approach to preparing symmetrical conjugated diynes 2a-j was based on a palladium-catalyzed detelluration reaction of functionalized bis(phenylethynyl)tellurides and bis(alkylethynyl) tellurides 1a-j.The parent precursors bis(arylethynyl)telluride 1a-j were conveniently prepared in good to moderate yields according to the procedure described by Engman and Stern . 19 initially optimized the conditions for the detelluration of functionalized bis(phenylethynyl)telluride 1.To find optimal conditions for the detelluration reaction, bis(phenylethynyl) telluride 1a was selected as a model substrate, and a variety of catalysts were screened as described in Table 1.All reactions were monitored by TLC and GC/MS.
One equiv. of AgOAc was used as an additive, along with 4 equiv. of triethylamine, and MeOH as solvent.
After the determination of the optimal catalyst for this transformation, we then studied the influence of the base.In our initial attempts, we used triethylamine, and the desired compound was obtained in 72% yield.We also attempted the same reaction with some other organic and inorganic bases such as NaOAc, Cs 2 CO 3 , K 2 CO 3 , DIPEA, pyridine and cyclohexylamine, obtaining the detelluration product in yields ranging from 27% to 67%.
To further determine the optimal conditions for the detelluration reaction, we performed the model reaction in various solvents.When MeOH, 1,4-dioxane, toluene and CH 3 CN were employed, the reaction yields were poor or moderate (52%, 44%, 38% and 58%, respectively).While  using DMSO as solvent provided a good yield (72%), DMF provided the best yield of the desired product (86%).
From these studies, it was determined that a reaction mixture containing 1.0 equiv. of bis(phenylethynyl)telluride 1a, 4 equiv. of Et 3 N, 1 equiv. of AgOAc, and 10 mol % of Pd(OAc) 2 in 5 mL of DMF at room temperature stirred under atmospheric conditions for 60 min provided the best conditions for the synthesis of conjugated diyne 2a.To demonstrate the efficiency of this detelluration reaction, we then explored its generality with a variety of bis(arylethynyl)tellurides and bis(alkylethynyl)tellurides.The results are summarized in Table 2.
After optimizing the conditions for the synthesis of symmetrical conjugated diyne 2a, it was synthesized a series of conjugated diynes (2a-j) in 21-86% yields (see Table 2 and Experimental).The reaction was carried out at room temperature.The reaction proceeded with electron-withdrawing substituents attached to the alkynyltelluride and with electron-donation substituents.All of the obtained products provided 1 H and 13 C NMR spectra that were in full agreement with their assigned structures.
On the basis of available literature 20 we propose a possible catalytic cycle for the detelluration reaction of  bis(arylethynyl)tellurides and bis(alkylethynyl)tellurides as described in Figure 2.
According to this cycle the reaction proceeds by the formation of Pd(II) complex with acetylene followed by the conversion of this intermediate into another palladium species B, which leads the formation of conjugated diyne C along the reduction of the Pd(II) complex to Pd(0).The palladium species is later oxidized in the presence of O 2 to give the initial Pd(II) species completing the cycle.
Due to our ongoing interest in tellurium structures, especially those involving p-interactions, the crystal and molecular structure of the bis-(p-tolylethynyl)telluride starting material was determined. 21The tellurium atom is located on a crystallographic twofold axis with the C-Te-C angle being 92.23 (15)°.The dihedral angle formed between the phenyl rings is 87.27 (7)° (Figure 3).
In the crystal structure, the telluride molecules are connected into supramolecular chains along the b axis via C-H...p interactions, as shown in Figure 4.

Conclusions
In summary, we demonstrated the synthesis of functionalized symmetrical 1,3-diyne systems through the palladium-catalyzed detelluration reaction of bis(arylethynyl) tellurides and bis(alkylethynyl)tellurides.The use of this methodology for the synthesis of more complex polyacetylenic compounds is currently under study in our laboratory.

Experimental
Proton nuclear magnetic resonance spectra ( 1 H NMR) were obtained at 300 MHz.Spectra were recorded in CDCl 3 solutions.Chemical shifts are reported in ppm, referenced to the solvent peak of CDCl 3 or tetramethylsilane (TMS) as the external reference.Data are reported as follows: chemical shift (d), multiplicity, coupling constant (J) in Hertz and integrated intensity.Carbon-13 nuclear magnetic resonance spectra ( 13 C NMR) were obtained at 75 MHz.Spectra were  recorded in CDCl 3 solutions.Chemical shifts are reported in ppm, referenced to the solvent peak of CDCl 3 .Abbreviations to denote the multiplicity of a particular signal are s (singlet), d (doublet), t (triplet), q (quartet), quint (quintet), sex (sextet) and m (multiplet).Column chromatography was performed using silica gel (230-400 mesh) following the methods described by Still et al. 22 Thin layer chromatography (TLC) was performed using silica gel F 254 , 0.25 mm thickness from Merck.For visualization, TLC plates were either placed under ultraviolet light, or stained with iodine vapor, or acidic vanillin.The following solvents were dried and purified by distillation from the reagents indicated: tetrahydrofuran from sodium with a benzophenone ketyl indicator.All other solvents were ACS or HPLC grade unless otherwise noted.
Air and moisture-sensitive reactions were conducted in flame-dried or oven dried glassware equipped with tightly fitted rubber septa and under a positive atmosphere of dry nitrogen or argon.Reagents and solvents were handled using standard syringe techniques.Temperatures above room temperature were maintained by use of a mineral oil bath with an electrically heated coil connected to a controller.

General procedure to homocoupling reaction
A suspension of bis(phenylethynyl)telluride (1a) (0.0824 g; 0.25 mmol), Pd(OAc) 2 (0.0056 g, 10% mol), triethylamine (0.101 g; 1 mmol) and silver acetate (0.041 g, 0.5 mmol) in 5 mL of DMF was stirred at room temperature, under air atmospheric by 60 min.The reaction mixture was diluted with ethyl acetate (25 mL), and the organic layer washed with NH 4 Cl (2 × 10 mL), dried over MgSO 4 and concentrated under reduced pressure.The crude product was purified by flash silica column chromatography using hexane as eluent and subsequently characterized.

362.78
All spectra were obtained using diphenyl ditelluride as internal standard.
b  Yield by CG-MS, the product was not isolated by column.

Figure 4 .
Figure 4. Supramolecular chain aligned along the b axis, sustained by C-H…p interactions.

Table 1 .
Effect of a catalyst on the detelluration reaction of bis(phenylethynyl)telluride 1a

Table S1 .
Chemical shifts for 127 Te