Iron-Catalyzed Coupling Reactions of Vinylic Chalcogenides with Grignard Reagents

Este trabalho descreve um novo método para a reação de acoplamento entre selenetos e teluretos vinílicos e reagentes de Grignard, catalisada por Fe(acac) 3 e à temperatura ambiente. A reação ocorre com retenção da configuração, fornecendo os respectivos alquenos em bons a excelentes rendimentos. Este método também é eficiente para a reação de acoplamento de calcogenetos bisvinílicos com reagentes de Grignard.

Transition-metal-catalyzed C-C bond coupling reactions are very important in many areas of science. 43,44ost current methods require expensive transitionmetal catalysts and ligands.6][47][48][49][50][51] In continuation to our interest on the synthesis and synthetic applications of vinylic chalcogenides [52][53][54][55][56][57][58] we decided to study the feasibility of their use in cross coupling reaction with Grignard species catalyzed by iron.To the best of our knowledge, iron catalysts have never been used for the coupling of vinylic selenides and tellurides as electrophiles.The possible use of iron catalysts would represent a great improvement over the high cost of palladium precursors and from the concerns about the toxicity of nickel salts.In light of the above comments it is of interest to design a simple, efficient, and versatile method for the stereoselective coupling of vinylic selenides and tellurides with Grignard reagents catalyzed by iron, aiming to the construction of olefins with defined stereochemistry.

Results and Discussion
Here we describe our results on the reaction of vinylic selenides and tellurides with Grignard reagents under the catalysis of iron species.The first reactions were performed Vol.21, No. 11, 2010   using [Fe(acac) 3 ], (acac: acetylacetonate), since it is a very stable, cheap and easy to prepare catalyst. 59The initial studies were focused on the development of an optimum set of reaction conditions for the coupling reactions of vinylic chalcogenides with Grignard reagents.We chose (E)-phenyl(styryl)tellane (1d) and a THF solution of n-octyl magnesium bromide as starting materials, searching for the best solvent and the amount of Fe(acac) 3 , performing the reactions at room temperature (Tables 1 and 2).
At first, we found that by using 10 mol% of Fe(acac) 3 , in benzene, the (E)-dec-1-enylbenzene (3e) was obtained in 12% yield after stirring for 3 h (Table 1, entry 1).We also evaluated other solvents, such as THF and NMP (Table 1, entries 2 and 3).The best yield was obtained when a mixture of THF/NMP (3:1) was used as solvent, and the product 3e was obtained in 82% yield, after stirring for 2 h at room temperature (Table 1, entry 4).We observed a beneficial effect of the addition of triethylamine as a ligand, in accordance with our own previous observations on the coupling reactions of vinylic chalcogenides catalyzed by nickel. 27,33Also, the addition of amine ligands with improved activity on iron cross-coupling have also been observed. 60en, the effect of the amount of the catalyst was evaluated.When the reaction was carried out without catalyst no product was obtained (Table 2, entry 1).The use of 5 mol% of Fe(acac) 3 produced 3e in 48% yield (Table 2, entry 2).The use of larger amounts of catalyst had no effect on the yield of reaction and the required time to completion of reaction was the same (Table 2, entry 4).Thus, the best conditions for the coupling reaction were the use of Fe(acac) 3 (10 mol%), THF/NMP (3:1, 4 mL), Et 3 N (0.5 mL), vinylic chalcogenide (0.2 mmol) and Grignard reagents (1 mmol), Scheme 1.
With these optimized conditions in hand, we next extended to some other examples to find out the scope and limitations of the present method.As can be seen in Tables 3 and 4, these conditions showed to be quite general, and a series of vinylic selenides and tellurides gave the desired products in good yields.Interestingly, the reaction proved to be also efficient for the coupling reaction between divinylic chalcogenides and Grignard reagents.The use of divinylic selenides and tellurides in cross-coupling reactions is particularly interesting since both organyls linked to selenium and tellurium atoms could a Reaction condition: (E)-phenyl(styryl)tellane (1d, 0.2 mmol), n-octyl magnesium bromide (1 mmol), Et 3 N (0.5 mL), Fe(acac) 3 (10 mol%) and solvent (4 mL).b Isolated yield.eventually be transferred. 27,33The iron catalyzed crosscoupling reaction between divinylic chalcogenides and Grignard reagents were carried out at room temperature under standard established conditions and the alkenes 3 were obtained in moderate to good yields after stirring for 1.5-3 h (Table 3).The only difference in this case was the need to increase the amount of the iron catalyst to 20 mol% in order to have good yields.
As can be seen in Table 4, the yields from the vinylic chalcogenides were higher than from divinylic chalchogenides.The highest yield was 89% of the coupling product 3g using n-butyl magnesium bromide and (E)-(4methylstyryl)(phenyl)telluride (Table 4, entry 16).For all the studied examples, the alkenes 3 were obtained in good to excellent yields (73-89%) after stirring at room temperature for 1 to 3 h.The use of aromatic Grignard reagents afforded the corresponding alkenes in lower yields than aliphatic Grignard reagents, and the formation of biphenyl was a competitive reaction.In all cases, no mixture of isomers were observed, the stereochemistry of products was the same as the starting material due to a complete retention of configuration in this type of reaction.
][63][64] Analysis of the 1 H NMR and 13 C NMR spectra showed that all alkene compounds presented data in full agreement with their assigned structures.The stereochemistry of the obtained alkenes was easily established.As an example, the 1 H NMR spectrum of compound 3d showed a double triplet at 6.21 ppm with coupling constants of 16.0 and 6.6 Hz, typical of trans relationship between hydrogens.The other vinylic hydrogen resonates at 6.37 ppm as a doublet, with a coupling constant of 16.0 Hz.The other signs are in the region between 7.5-7.0ppm (aromatic hydrogens) and 2.5-0.5 ppm (aliphatic hydrogens).

Conclusions
In summary, we have been able to demonstrate that Fe(acac) 3 is an effective catalyst for the cross-coupling reaction between vinylic selenides and tellurides with Grignard reagents at room temperature.Our method is simple, general and also proved to be efficient for the coupling reaction of divinylic selenides and tellurides with Grignard reagents, with complete retention of configuration.All coupling products were obtained in good to excellent yields.

General
All 1 H and 13 C NMR spectra were recorded at 200 MHz and 50 MHz, respectively, on a Bruker DPX 200 instrument, using CDCl 3 as solvent.Chemical shifts (d) are expressed in ppm downfield from tetramethylsilane or CHCl 3 as internal standard, and J values are given in Hz.Merck's silica gel (230-400 mesh) was used for flash chromatography.All reactions were performed in flame-dried glassware under a positive pressure of argon.Air and moisture sensitive reagents and solvents were transferred via syringe, and were introduced into reaction vessels through a rubber septum.All chemicals were of reagents grade and, if necessary, were purified in the usual manner prior use.

General procedure for the coupling reaction of vinylic selenides and tellurides 1a-j
In a 25 mL round-bottom flask under argon was added the solvent (THF/NMP 3:1, 4 mL), the vinylic chalcogenide (0.2 mmol), Et 3 N (0.5 mL) and Fe(acac) 3 (7 mg, 10 mol%).The mixture was stirred and after 5 min the Grignard reagent (1 mmol) was added dropwise.The reaction was followed by TLC and stirred at room temperature for the time indicated on Table 4.After completion of the reaction, aqueous NH 4 Cl (sat) (15 mL) was added and the reaction mixture was extracted with ethyl acetate (3 × 15 mL), the organic layer was washed with water and dried over MgSO 4 .After filtration, the solvent was removed under reduced pressure and the residue was purified by column chromatography on silica gel (ethyl acetate-hexanes, 1:99).

General procedure for the coupling reaction of divinyl chalcogenides 4a-d and compound 1k
In a 25 mL round-bottom flask under argon was added the solvent (THF/NMP 3:1, 4 mL), the divinylic chalcogenide (0.2 mmol), Et 3 N (0.5 mL) and Fe(acac) 3 (14 mg, 20 mol %).The mixture was stirred and after 5 min the Grignard reagent (2 mmol) was added dropwise.The reaction was followed by TLC and stirred at room temperature for the time indicated in Table 3 or Table 4.After completion of the reaction, aqueous NH 4 Cl (sat) (15 mL) was added and the reaction mixture was extracted with ethyl acetate (3 × 15 mL), the organic layer was washed with water and dried over MgSO 4 .After filtration, the solvent was removed under reduced pressure and the residue was purified by column chromatography on silica gel (ethyl acetate-hexanes, 1:99).

Table 1 .
Optimization of reaction conditions: effect of solvent a

Table 4 .
Fe-Catalyzed cross-coupling of vinylic selenides and tellurides with Grignard reagents a Isolated yield.