Dramatic Steric Distortions and Electronic Demands in 1,3,5-Tris(dialkylamino)-2,4,6-trinitrobenzene: Study of a Severely Warped Benzene

The static and dynamic stereochemistry of 1,3,5-tris(dialkylamino)-2,4,6-trinitrobenzene is described by a combination of crystallographic, VT-NMR, and semiempirical calculational methods. Crystal data for the N,N-diethyl derivative 1b: C₁₈H₃₀N₆O₆, M = 426.6 g*mol^-l; monoclinic (space group P2₁/c); a = 10.005 (2) Å, b = 21.452 (5) Å, c = 10.544 (2) Å; β = 104.3 (2)°; and V = 2193.8 (8) ų; d_calc= 1.29 g*cm^-3; Z = 4. This highly substituted benzene distorts to a boat form in the ground state. The bow and stern angles of the N,N-diethyl derivative are 42° and 27°, respectively. This severe out-of-plane distortion is interpreted as a result of a cooperative nonbonded steric repulsion which is reinforced by the demands of a strong push-pull conjugation. Thus, due to a dominant polar resonance form, the molecular skeleton is more quinoid or radialene-like than benzenoid. Evidence for the need of both the electronic and steric factors in order to obtain such a distorted structure is presented. Molecules in this class are found to be fluxional in solution and the boat-C_s and twist-C₂ conformers are seen to interconvert rapidly by means of a pseudorotation pathway. The barrier to interconversion is found by VT-NMR to be 9.1 kcal/mol. Semiempirical calculations (AM1) do a good job of modeling both the ground-state geometry as well as the salient features of the pseudorotation hypersurface for these unusual molecules. On the basis of these calculations, the barrier along the pseudorotation pathway appears to stem from the VDW repulsions that occur as adjacent exocyclic groups pass by one another.