Kurt V. Gothelf and Karl Anker Jørgensen
In order to investigate the structure of the reactive
intermediate in the titanium-catalysed 1,3-dipolar cycloaddition, a
series of reactions catalysed by TiX(PriO)3 and
TiX(PriO)-TADDOLate [X = Cl, Br,
CF3SO3 (OTf)] complexes have been performed. The
1,3-dipolar cycloaddition between benzylidenephenylamine
N-oxide and
3-[(E)-but-2′-enoyl]-1,3-oxazolidin-2-one is catalysed by
20 mol% TiCl(PriO)3 to give primarily the
exo-isoxazolidine. If the chloride ligand of the catalyst is
substituted with more bulky ligands such as bromide and
trifluoromethanesulfonate, the selectivity of the reaction changes to
give primarily the endo-isoxazolidine. The same change in
diastereoselectivity from exo to endo is also observed
by the application of TiX(PriO)-TADDOLate
(X = Cl, Br, OTf) complexes as catalysts for the analogous
reaction. On the basis of these results, NMR spectral investigations and
MM2 models, the most reactive intermediate in the
TiCl(PriO)3 catalysed cycloaddition is proposed to
be an octahedral complex in which the chloride ligand is located in the
axial position relative to the plane defined by the two carbonyl oxygen
atoms from 3-[(E)-but-2′-enoyl]-1,3-oxazolidin-2-one and
two alkoxide ligands. The structure of the reactive intermediate in the
TiIV-TADDOLate-catalysed 1,3-dipolar cycloaddition
and Diels–Alder reactions is also briefly discussed on the basis
of the results obtained.