Rapid microwave-enhanced, solventless desilylation on potassium fluoride doped alumina

A microwave-enhanced desilylation on potassium fluoride doped alumina in the absence of solvents has been developed. The reaction produces the corresponding hydrocarbon products in good to excellent yields .


Introduction
Terminal alkynes are very important in synthetic organic chemistry.They are widely used as reactants in Sonogashira, 1 Glaser, 2 hydroboration, 3 haloboration, 4 diboration 5 , and various coupling reactions. 6The general route to terminal alkynes involves a Sonogashira−Hagihara coupling of (trimethylsilyl)ethyne with aryl halides in the presence of base with a Cu(I)/Pd(0) cocatalyst followed by de-protection of the silyl group carried out using bases, such as potassium carbonate, potassium hydroxide or potassium fluoride in methanol solution. 7The reagents and solvents often pose waste handing problems.
We have found alumina to be a particularly useful reagent in organic synthesis because it can be modified in a variety of ways that enhance its reactivity.It also obviates a number of environmental problems. 8For example, using a commercially available alumina potassium fluoride mixture to which we added palladium powder, we were able to carry out Suzuki and Sonogashira coupling reactions on a wide variety of aromatic moieties without the use of solvent. 9icrowave irradiation of organic reaction has gained in popularity in recent years since it was found to accelerate a wide variety of transformations. 10Early experiment utilized solvents with high dielectric constants, which permitted rapid heating of reaction solutions.In recent years, a number of reports have appeared in which reactants are coated onto surfaces which themselves absorb little or no microwave energy; in these instances, the reactive species absorb the microwave energy but the temperature of the reaction mixture tends to rise only modestly.This results in relatively large energy savings as well as making it possible to carry out the reaction in simple glassware, such as open beakers and flasks. 11e now wish to report a microwave-enhanced desilylation reaction on potassium fluoride doped alumina in the absence of solvents which produces the corresponding hydrocarbon products in excellent yields.

Results and Discussion
The desilylation reaction is extremely facile for silylated−alkynes, Figure 1.The results are summarized in the Table .The data indicate that, under microwave irradiation in solvent free conditions, desilylation of 2-subsitituted-1-(trimethylsilyl)ethynes readily occurs to yield the corresponding terminal alkynes in excellent yields in the presence of potassium fluoride doped alumina.Substituents on the benzene ring are unaffected by the cleavage reaction.The reaction does not occur in the absence of potassium fluoride (entry a).Microwave irradiation accelerates the reaction and shortens the reaction time from several hours to 1 minute (entry a).The reaction can be utilized to desilylate aromatic and heterocyclic (entries k and m) but not aliphatic derivatives (entry l).

Experimental Section
General Procedures.Melting points were recorded on a MEL-TEMP melting point apparatus and are uncorrected.All 1 H-and 13 C-NMR spectra were recorded on a 250 MHz Bruker AC 250 spectrometer at 25 °C using CDCl3 as solvent.Chemical shift are given δ value with reference to tetramethylsilane (TMS) as internal standard.Coupling constants are given in Hz and without sign.GC/MS data were obtained by using a Hewlett−Packard 6890 series GC equipped with a 5973 mass selective detector.HRMS were obtained using a VG analytical ZAB-SEQ4F mass spectrometer.A commercially available Sharp Model R-4A38, 1000 watts microwave oven was used.KF/Al 2 O 3 (40% by weight) was purchased from Aldrich Chemical Co.Products were purified by flash chromatography on 230−400 mesh ASTM 60 Å silica gel.
General procedure for desilylation.Synthesis of phenylacetylene 1-Phenyl-2-(trimethylsilyl)acetylene (0.870 g, 5.00 mmol) was added to KF/Al 2 O 3 (3.00g, 40% by weight) contained in a 25 mL round-bottomed flask.The mixture was stirred at room temperature to ensure efficient mixing.The flask was then fitted with a septum which had been punctured by an 18 gauge needle (to serve as a pressure relief valve), placed in the microwave oven and irradiated at 100% power for 1 minute.After cooling, hexane (10 mL) was added and the slurry stirred at room temperature to ensure product removal from the surface.The mixture was vacuum filtered and the product purified by flash chromatography (hexane as eluent) to yield phenylacetylene.Phenylacetylene.Oil;