1,3-Dipolar cycloaddition of α-phenyl-N-tert-butylnitrone (PBN) to dichloro- and dibromo-malononitrile, chlorotricyanomethane and tetracyanomethane. Structure of products and kinetics of their formation

Abstract

The kinetics of cycloaddition between α-phenyl-N-tert-butylnitrone (PBN; IUPAC name: N-benzylidene-tert-butylamine N-oxide) and six activated nitriles R–CN, viz. dibromo- and dichloro-malononitrile (1a and 1b), chlorotricyanomethane (1c), tetracyanomethane (1d), trichloroacetonitrile (1e) and carbamoylchlorodicyanomethane (5), to give 2-tert-butyl-3-phenyl-5-R-2,3-dihydro-1,2,4-oxadiazoles 2a–e have been determined, as well as the similar reaction between 4-NO₂PBN and 1c. The second-order rate constants for reaction between PBN and 1 in acetonitrile fall (relative rate constant in parenthesis) in the order of 1d (5.4 × 10³) > 1c (3.1 × 10²) > 1b (8) > 1a (1) > 5 (not given) > 1e (9 × 10^–3). The rate constants are slightly larger in non-polar solvents. The rate constants of the PBN–1c and PBN–1d cycloadditions (0.15 and 2.6 dm³ mol^–1 s^–1, respectively) are of the same magnitude as those recently used to classify thioketones as ‘superdipolarophiles’.

X-Ray crystal structures are reported for 2-tert-butyl-3-phenyl-5-(dichlorocyanomethyl)-2,3-dihydro-1,2,4-oxadiazole (2b) and 2-tert-butyl-3-phenyl-5-(chlorodicyanomethyl)-2,3-dihydro-1,2,4-oxadiazole (2c).