Synthesis, characterization and crystal structure of two polymorphs of trans N-benzyl-3,9,9-trimethyl-1,2,3,4,4a,9,9a,10-octahydroacridine

Abstract

Two polymorphs of the new heterocyclic compound trans-N-benzyl-3,9,9-trimethyl-1,2,3,4,4a,9,9a,10-octahydroacridine have been efficiently prepared via a one-pot BiCl3-catalyzed cationic Povarov reaction of N-benzyl aniline with a) (±)-citronellal (Polymorph I) and b) citronella essential oil from C. nardus (Polymorph II), in a highly diastereoselective fashion. The polymorphs were characterized by ATR-FTIR spectroscopy, 1H/13C NMR spectroscopy, IE and ESI-IT mass spectrometry, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and by single crystal X-ray diffraction. Polymorph I is monoclinic, space group Cc (N° 9), with unit cell parameters a = 12.0555(9) Å, b = 17.4664(13) Å, c = 17.7337(13) Å, β = 99.134(7), V = 3686.8(5) Å3, Z = 8. Polymorph II is triclinic, P1¯ (No. 2), with a = 9.8528(10) Å, b = 10.5144(11) Å, c = 10.8845(10) Å, α = 103.211(9)°, β = 114.048(9)°, γ = 101.483(9)°, V = 946.78(18) Å3, Z = 2. In polymorph I, the two independent molecules interact via weak C–H⋯C contacts to form dimers which are further connected by C–H ⋯ π interactions. In contrast, in form II the molecules interact via the same contacts but form infinite chains along the a-axis. Hirshfeld surface analysis indicates that H⋯H interactions are more important than C⋯H interactions. Theoretical calculations indicated that PM6-D3 methods produced the best agreement between the experimental and the energy minimized structure.