Catalisis

Molecules 2000, 5, 993-997

molecules
ISSN 1420-3049 © 2000 by MDPI http://www.mdpi.org

Lewis Acid Catalysis of the Diels–Alder Reaction Using Niobium and Tantalum Chlorides in the Presence ofCoordinating Ligands
Joshua Howarth* and Kevin Gillespie School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland Tel.: +353 1 7045312, Fax: +353 1 7045503, E-mail:howarthj@ccmail.dcu.ie Received: 29 June 2000 / Accepted: 1 August 2000 / Published: 5 August 2000

Abstract: The Jacobsen and pybox type ligands effect chiral induction in the presence of niobium(V)chloride and tantalum(V) chloride in the Lewis acid catalysed Diels–Alder reaction. Keywords: niobium, tantalum, bis-oxazoline, pybox.

Introduction Chiral induction of the Diels–Alder reaction using Lewisacid catalysis in the presence of chiral ligands has met with a great deal of success [1]. Bis-oxazolines have been used by Corey in the presence of Mg and Fe [2, 3], and by Evans, along with pybox(Figure 1), in the presence of Cu [4]. Having an interest in chiral Lewis acids based on niobium and tantalum, we decided to investigate the potential for asymmetric induction of the bis-oxazoline andpybox classes of ligand in the presence of niobium(V)chloride or tantalum(V)chloride and compare these results with those of the R,R and S,S Jacobsen ligand (Figure 2).
1 R , R = H; R , R = Pr O N R1 i 1 4 2 3

5 = S, S Jacobssen ligand 6 = R, R Jacobssen ligand H N
t

N N R
3

O

2 R , R = H; R , R = Bu
i 4

1

4

2

3

H N HO
t t

R

2

R

3 R , R = phenyl; R , R=H 4 R , R = H; R , R =
1 4 2 3 3 t

1

4

2

Bu Bu

OH

Bu

Bu

Figure 1 Figure 1.

benzyl

Figure Figure 2. 2

Molecules 2000, 5



The reaction we used toinvestigate the potential asymmetric induction was the Diels-Alder reaction between cyclopentadiene and methacrolein or crotonaldehyde, Scheme 1.
O R
1 2 1

R

+
R
1 2 2

R CHO

+

R CHO R
1...