Selenostannylation of Arynes Produced by Silylaryl Triflates under Mild Reaction Conditions

H and C NMR spectra were obtained on a Varian INOVA 300 spectrometer (H at 300 MHz and C at 75 MHz) and on a Bruker DRX-500 spectrometer (H at 500 MHz and C at 125 MHz). The spectra were taken in deuterated solvents and the chemical shifts were given in ppm using residual undeuterated solvents or tetramethylsilane (TMS) as internal standard. Se NMR spectra were taken on a Bruker DRX-500 spectrometer at 95 MHz using CDCl 3 as solvent. The chemical shifts were given in ppm using diphenyl diselenide (PhSeSePh) diluted in CDCl 3 (1 mol L) as an external standard (d 463 ppm at 25C). Low-resolution mass spectra were obtained on a Shimadzu CG-17A/ CGMS-QP5050A instrument. Elemental analyses were obtained on a Perkin-Elmer CHN 2400 equipment. Near-IR spectra were obtained on a Bomem MB-100 spectrometer. The melting point value obtained is uncorrected. Reagents and solvents were used as obtained commercially or when necessary were purified and/or dried using procedures described in the literature. THF was distilled under nitrogen from sodium/benzophenone and CH 3 CN was distilled from calcium hydride. Both solvents were distilled prior to use. Oxygen and moisture sensitive materials were manipulated under a nitrogen atmosphere. Column chromatography and preparative thin layer chromatography separations were carried out using silica gel 60. Tributyl(phenylselanyl) stannane (1), 2-(trimethylsilyl)phenyl triflate (2a), 4,5-dimethyl-2-(trimethylsilyl)phenyl triflate (2b), 4-fluoro2-(trimethylsilyl)phenyl triflate (2c), 1-(trimethylsilyl)-2naphtyl triflate (2d), and 2-(trimethylsilyl)-1-naphtyl triflate (2e) were prepared according to literature procedures. General procedure for selenostannylation of arynes


Introduction
Benzyne is an important reactive intermediate in synthetic organic chemistry which has found application in several total syntheses 1 and in the preparation of functional materials. 2][5][6][7][8] Among them we pay particular attention to the thermal decompositions of benzenediazonium-2-carboxylate 4 and diphenyliodonium-2-carboxylate, 5 the elimination reaction resulted from oxidative addition of magnesium in 2-bromofluorobenzene, 6 the iodine-magnesium exchange reaction followed by elimination occurred in 2-iodophenyl 4-chlorobenzenesulfonate, 7 and the oxidation of 1-aminobenzotriazole in the presence of lead tetraacetate. 84][5][6][7][8] In this scenario, 2-(trimethylsilyl)aryl triflates 9 and phenyl [2-(trimethylsilyl)  aryl]iodonium triflates 10 arise as promising reagents to form arynes under mild reaction conditions. 9,10Once 2-(trimethylsilyl)phenyl triflate was transformed in a commercially available reagent and its derivatives can be easily prepared from phenols, 9 2-(trimethylsilyl)aryl triflates have emerged as remarkable reagents for generating arynes recently employed in several insertion reactions into sigma bonds. 11Because of the highly electrophilic character of arynes that comes from their low-energy LUMOs, even neutral nucleophiles of diminished nucleophilicity readily attacks arynes to produce zwitterions, which act as key intermediates in the insertion reactions into, for example, C-C, 12 N-H, 13,14 O-H, 14,15 Se-Se, 16,17 Te-Te 17 and S-Sn 18 sigma bonds.These transformations lead to the formation of polysubstituted arenes and benzoannulated compounds, which would be hardly synthesized in just one step by other methods.
In this work, we report our results involving the insertion reaction of arynes generated from 2-(trimethylsilyl)aryl triflates under mild reaction conditions into Se-Sn sigma bond of tributyl(phenylselanyl)stannane, 19 as an attractive alternative for the synthesis of tributyl[(phenylselanyl)aryl] stannanes 20 in good yields.
The organometallic compounds produced by the selenostannylation process can be considered versatile intermediates in synthetic organic chemistry, finding application in cross-coupling reactions, transmetallations and iodolyses. 18In order to give some support to this fact, Vol.21, No. 11, 2010   we subjected one of the aromatic selenostannanes produced in the course of our investigation to the iododestannylation reaction and the result obtained was enclosed herein.
In order to examine the effect of the fluoride ion sources on the transformation, we carried out the reaction of tributyl(phenylselanyl)stannane (1) with 1.5 equiv. of 2-(trimethylsilyl)phenyl triflate (2a), 1.5 equiv. of KF and 1.5 equiv. of 18-crown-6 ether in THF at 0 o C for 24 h and obtained tributyl [2-(phenylselanyl)phenyl]stannane (3a) in a moderate yield of 52% (Table 1, entry 2).By performing the reaction of the stannyl selenide 1 with 1.5 equiv. of the benzyne precursor 2a in the presence of 1.8 equiv. of tetrabutylammonium fluoride (TBAF) in THF at room temperature for 3 h, the insertion product 3a was not produced (entry 3).No further attempts were made to optimize this TBAF procedure.Afterwards, we treated tributyl(phenylselanyl)stannane (1) with 2 equiv. of 2-(trimethylsilyl)phenyl triflate (2a), 2 equiv. of KF and 2 equiv. of 18-crown-6 ether in THF at 0 o C for 24 h to give tributyl [2-(phenylselanyl)phenyl]stannane (3a) in a good 62% yield (entry 4).When the same reaction was carried out at room temperature, compound 3a was obtained in a slightly lower yield of 57% (entry 5).No significant improvement in the isolated yield of tributyl [2-(phenylselanyl)phenyl]stannane (3a) was obtained when we allowed the stannyl selenide 1 to react with 2.5 equiv. of the benzyne precursor 2a in the presence of 2.5 equiv. of KF and 2.5 equiv. of 18-crown-6 ether at 0 o C for 24 h (entry 6).As can be seen in Table 1, entry 7, compound 3a was not obtained and the starting materials 1 and 2a were recovered when the reaction was carried out in the absence of KF.This experiment shows that the success of our reaction depends dramatically on the presence of a fluoride ions source.
Employing the optimal conditions shown in Table 1, entry 4, 21 we examine the scope and limitations of the process (Table 2).The reaction between tributyl(phenylselanyl) stannane ( 1) and the electron-rich aryne precursor 2b gave the aromatic selenostannane 3b in a 60% isolated yield (entry 2).To better understand the regioselectivity of the reaction when unsymmetrical arynes are employed, we treated tributyl(phenylselanyl)stannane (1) with the 4-fluoro aryne precursor 2c and obtained a mixture of isomers 3c and 3c' in a 2:1 ratio with a total yield of 34% (entry 3).In this case, the electron-withdrawing group promoted a more sluggish transformation leading to a regioisomeric mixture of products in a moderate yield.In addition, the major product 3c was obtained by the attack of the stannyl selenide 1 at the electronically most favorable position of the fluorinated aryne formed from compound 2c.This result indicates that our reaction involves the formation of benzyne and its derivatives as intermediates.Trying to give support to the formation of arynes as intermediates in this selenostannylation process, we allowed the reaction of tributyl(phenylselanyl)stannane (1) with the silylaryl triflates 2d and 2e, and the same aromatic selenostannane 3d was produced in 55 and 54% yields, respectively (Table 2, entries 4 and 5).The formation of the same insertion product 3d from the silylaryl triflates 2d (entry 4) and 2e (entry 5) suggests the generation of 1,2-naphthalyne as a common intermediate for the transformations.Besides, we have to mention that the exclusive formation of the regioisomer 3d in both reactions (Table 2, entries 4 and 5) can be rationalized by steric factors related to the attack of the stannyl selenide 1 to the 1,2-naphthalyne intermediate. 22he products from the selenostannylation of arynes (3a-d) are versatile reagents in organic synthesis, which can find a Reaction conditions: 0.3 mmol of the stannyl selenide 1, the indicated amount of the benzyne precursor 2a, the indicated amount of base, and 3 mL of solvent were stirred at the temperature indicated for 24 h.b This reaction was stirred for 3 h.application in cross-coupling reations, homocouplings and in iodolyses.Accordingly, we subjected the stannane 3a to the reaction with iodine and obtained the diphenylselenide iodinated 4 in a good isolated yield of 67% (Scheme 1).
The structures of compounds 3a-d and 4 were assigned on the basis of a variety of spectroscopic techniques, namely, according to their IR, 1 H, and 13 C NMR spectra.The compounds 3a-d provided elemental analyses that agree with the proposed structures.Compound 4 presented LRMS that agrees with its structure and the mixture of regioisomers 3c and 3c', and compound 3d had their 77 Se NMR spectra recorded.

Conclusions
In summary, a simple procedure which involves mild reaction conditions for the selenostannylation of arynes was developed and aromatic selenostannanes were produced in good yields.This attractive process provides an alternative for the preparation of organometallic compounds which are hardly synthesized in just one step by other methods and should find applications in the construction of molecules with interesting biological properties and in the preparation of functional materials.

General procedure for selenostannylation of arynes
To a vial (10 mL) were added the tributyl(phenylselanyl) stannne 1 (0.3 mmol), the appropriate aryne precursor 2a-e (0.6 mmol), 18-crown-6 ether (158 mg, 0.6 mmol), and THF (3 mL).The mixture was cooled to 0 ºC and potassium fluoride (34.8 mg, 0.6 mmol) was added.The vial was sealed using a cap, and the reaction mixture was stirred for 24 h at 0 ºC.Afterwards, the reaction was added to a saturated solution of ammonium chloride (30 mL), which was extracted with ethyl acetate (3 × 30 mL).The organic phase was dried over MgSO 4 .After filtration, the solvent was evaporated under reduced pressure.The residue was purified by preparative TLC using silica gel as stationary phase and hexane as eluent unless otherwise indicated, affording the desired products 3a-d.