PAPER: On the mechanism of Ti-TADDOLate-catalyzed asymmetric Diels-Alder reactions

Gothelf, K. V.; Jørgensen, K. A.

J. Org. Chem. 1995, 60, 6847–6851, doi: 10.1021/jo00126a039

Abstract

The mechanism of the diastereo- and enantioselective Diels-Alder reaction of cyclopentadiene with
N-acyloxazolidinones catalyzed by titanium a,a,a’,a’-tetraaryl-1,3-dioxolane-4,5-dimethanolate 0%
TADDOLate) complexes has been studied. On the basis of a series of experimental reactions
catalyzed by a Ti-TADDOLate-N-cinnamoyloxazolidinone complex, which has been characterized
by X-ray crystallography, it is found that both stoichiometric and catalytic reactions of this complex
with cyclopentadiene lead to the same endo:exo ratio and enantiomeric excess of the Diels-Alder
product as the Ti-TADDOLate-catalyzed reaction. The Diels-Alder reaction of Merent N-
acyloxazolidinones with cyclopentadiene in the presence of modified Ti-TADDOLate complexes
has also been investigated, and it is found that the endo:exo ratio is very dependent on chloride
and tosylato ligands bound to the titanium atom. In the presence of chloride ligands attached to
the titanium atom, the endo diastereomer of the Diels-Alder adduct is the major product formed,
whereas a significant decrease in the amount of the endo Diels-Alder adduct is found when the
more bulky tosylato ligands are bound to the titanium atom. The enantiomeric excess of the Diels-
Alder product decreases also when the chloride ligands are exchanged with the tosylato ligands.
On the basis of the experimental results, the mechanism of the Ti-TADDOLate-catalyzed Diels-
Alder reaction of cyclopentadiene with N-acyloxazolidinones is discussed. It is proposed that the
intermediate which accounts for the stereochemical outcome of the reaction is the one with the
four oxygen atoms of the TADDOL and the N-acyloxazolidinone ligands bound to the titanium
atom in the equatorial plane and the two axial positions occupied by the chloride or tosylato ligands.

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