Conformational control of photochemical reactions of tetrahydro-1,4-naphthoquinones

Abstract

The crystal structures of two asymmetrically substituted tetrahydro-l,4-naphthoquinones have been determined and correlated with their photochemical behaviour. T=295 K, Mo Kal, h =0.70930~. (I): 6,7-Benzo-2,4a-dimethyl-cis-4a,5,8,8a-tetrahydro- 1,4-naphthoquinone (2,4a-dimethyl-cis-4a,9a,9,10-tetrahydro-l,4-anthracenedione), C16Hi602, M r = 240.30, triclinic, P1, a=6.834(3), b=9.817(4), c=10.071(4) A, a=85.46(2), /3=73.48(3), 3,= 76-71 (3) °, V = 630.4 (5) A3,Z= 2,Dx = 1.266 gcm -3, F(000) = 256,/z = 0.73 cm -1, R = 0.042 for 2130 observed reflections. (II): 6,7-Benzo-2,3,4a-trimethylcis-4a,5 ,8,8a-tetrahydro- l ,4-naphthoquinone (2,3,4atrimethyl - cis - 4a, 9a, 9,10 - tetrahydro - 1,4-anthracenedione), C17H1802 Mr=25433, monoclinic, P2~/a, a =9.8329(17), b=8.0652(8), c= 17-3772 (32),~,, /3 = 102.69 (1)°, V = 1344.4 (4)/~3, Z = 4, Dx = 1.256 g cm -3, F(000) = 544, p. = 0.75 cm -~, R = 0.052 for 1330 observed reflections. Bond lengths and angles are close to normal values for both molecules (I) and (II). In each structure the two fused non-aromatic six-membered rings are twisted with respect to each other; however, while the solid-state conformation of compound (I) is such that the bridgehead methyl group is pseudo-axial to the cyclohexenone moiety, in compound (II) the methyl group is pseudoequatorial. Molecular-energy calculations are used to determine the energy difference between the two molecular conformations for each compound, and the different photochemical behaviour of (I) and (II) in the solid state and in solution is interpreted; the solid-state reactions are topochemically controlled, while reactions in solution are governed by the amounts and reactivities of the two conformers.