Phase transfer catalyzed intramolecular cycloalkylation of phenylacetonitrile with α,ω-dibromoalkanes in supercritical ethane

An intramolecular cycloalkylation of phenylacetonitrile (PAN) with different α,ω-dibromoalkanes in supercritical fluids under phase transfer catalysis (PTC) conditions is reported. In particular, the cycloalkylation of PAN with an excess of 1,4-dibromobutane has been studied in the presence of tetrabutylammonium bromide and potassium carbonate as a base in supercritical ethane at 70 °C and 11.0 MPa. The reaction was complete in less than 6 h yielding selectively the desired cycloadduct in the presence of a PTC, but conversion in the absence of catalyst was low. A cycloalkylation methodology has been developed using several dibromoalkanes, viz., 1,2-dibromoethane, 1,3-dibromopropane, 1,4-dibromobutane, 1,5-dibromopentane, 1,6-dibromohexane and 1,10-dibromodecane. When the cycloalkylation was performed in the presence of supercritical CO2, mono- and di-alkylated products were observed in addition to the cycloadduct. To compare the effect of SC ethane solvent, reactions were also carried out using different conventional solvents such as hexane, tert-butyl methyl ether, ethyl acetate and chlorobenzene. Use of SC ethane gave considerably better yields of products than the other solvents.