Synthesis and Characterization of In-Plane and Out-of-Plane Enone−Lewis Acid Complexes:  Implications for Diels−Alder Reactions¹

Abstract

In order to determine the reactive geometries of enone-TiCl 4 complexes in Diels-Alder reactions compounds 1a‚TiCl 4 and 1b‚TiCl 4 were prepared. A crystal structure of 1a‚TiCl 4 confirmed that the titanium in this complex has the desired out-of-plane geometry, with a TisOdCsC dihedral angle of 57.6°, a value remarkably similar to dihedral angles found in simple carbocyclic compounds. Difference NOE studies support a similar geometry in solution. The magnitudes of key 1 H and 13 C NMR chemical shift changes upon complex formation indicate that the out-of-plane TiCl 4 in 1a‚TiCl 4 is a stronger Lewis acid than the in-plane TiCl 4 of 1e‚TiCl 4 . These results are further supported by the changes in carbonyl stretching frequencies upon complex formation, with a ∆ν CdO for 1a‚TiCl 4 which is 24 cm -1 larger than that for 1e‚TiCl 4 . Expectations of heightened reactivity for 1a‚TiCl 4 as compared to 1e‚TiCl 4 have been confirmed; the former complex undergoes Diels-Alder reactions with cyclopentadiene 15 times more rapidly than the latter, despite being disfavored on a thermodynamic basis. These results suggest that the commonly held assumption of an in-plane reactive geometry for titanium-based Lewis acid-mediated Diels-Alder reactions may be in error.(8) Corcoran, R. C.; Ma, J.

ChemInform Abstract: Origin of Diastereoselectivity in the Thermal (4 + 2) Cycloadditions of Dienophiles Derived from Oppolzer′s Sultams: Steric vs. Stereoelectronic Influences.

“…The first term, involving the electron densities ( 4 ) in the atomic orbitals a and b, as well as the resonance (p) and overlap (S) integrals, is the first-order closed-shell repulsion term and is usually very similar for each of two possible pathways. We restrained ourselves to the uncatalyzed cycloadditions reported in the literature, in view of the difficulty in obtaining fully optimized parameters for the metals chelated to unusual moieties such as N-SO, and N-C(0) as well as to avoid discussion of the relative reactivities of complexes, whose geometries can be linear [24], bent in the chelating plane [25], or bent out of the chelating plane [26]. The second term, obviously important when ions or polar molecules react together, is the Coulomb repulsion or attraction and includes the total charge (Q) on each atom k and I, separated, with the local dielectric constant ( E ) , by the distance R. The third term is the second-order perturbation term calculated from the energies of molecular orbitals r and s, and from the coefficients of atomic orbitals a and b in molecular orbitals r and s, respectively, which are located on the different molecules.…”

Synthesis and [4+2] Cycloaddition of (2R,2′R)‐N,N′‐Fumaroylbis[fenchane‐8,2‐sultam] (=(2E)‐1,4‐Bis[(3aS,6S,7aR)‐1,4,5,6,7,7a‐hexahydro‐7,7‐dimethyl‐2,2‐dioxido‐3H‐3a,6‐methano‐2,1‐benzothiazol‐1‐yl]but‐2‐ene‐1,4‐dione) to Cyclopentadiene

“…16 ) The influence of either H-bond-donor solvents or other chelating and nonchelating Lewis acids under catalyzed and stoichiometric conditions shall be reported in due course. 17 ) For reactive out-of-plane complexation of the enone by Lewis acids in the Diels ± Alder reaction, see [29]. 18 (À)-1e, by statistically increasing the anti-s-cis conformation in a competitive non-C 2 C(a)-si directing environment.…”

Le Triflate de Bismuth(III) Comme Catalyseur Dans la Reaction de Carbonyl-Diels-Alder

“…Parmi ces méthodes, on connaît les micro-ondes, 1,2 les ultrasons, 3,4 la pression, 5 les réactions dans l'eau 6,7 et surtout les acides de Lewis. 8,9 Des réactions hétérogènes avec des argiles 10 des zéolithes, 11 ZnCl 2 ou TiCl 4 supporté sur silice, 12,13 se sont avérées trés efficaces.…”