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7.4 Titanium-Catalyzed Pauson-Khand-Type Cyclizations
In 1996, Buchwald and co-workers reported a Ti-catalyzed intramolecular coupling of 1,6-
enynes under an atmosphere of carbon monoxide, generating bicyclic cyclopentenones in
good to excellent yields, up to 95% (Scheme 7.31). 58, 59 In a different approach, Negishi
et al. showed the stoichiometric Cp 2 Zr-mediated bicyclic zirconacyclopentenes could be
transformed into the corresponding cyclopentenones in the presence of CO in up to 65%
isolated yield. 60
R
5-20 mol% Cp 2 Ti(CO) 2
R
Z
Z
O
Toluene, 90 ° C, 12- 48 h
CO (18 psig)
58-95% yield
R=H,Me,Ar
Z = O, NPh, CH 2 ,C(CO 2 Et) 2
Scheme 7.31 Ti-catalyzed PKR of 1,6-enynes under CO atmosphere.
A pioneering establishment of a catalytic enantioselective Pauson-Khand reaction was
achieved by Buchwald in 1996. 61, 62 In fact, an effective enantioselective reaction under a
CO atmosphere using a transition metal catalyst with a chiral ligand was rather problematic
at that time, because the chiral ligand could be easily dissociated from the metal center
due to the excess amount of strong CO donor, and thus part of the reaction might proceed
under an achiral environment. Notably, Buchwald tackled these difficulties by employing a
chiral Ti-complex in which the metal center and chiral scaffold were bridged with a
-bond
(Scheme 7.32). The 1,6-enynes were transformed into desired cyclopentenones in good to
excellent enantioselectivity. However, this PK reaction must be performed in a glovebox
because the low valent Ti-complex containing a Ti-C
-bond is highly sensitive to air and
moisture.
R
R
5-20 mol%
( S,S )(ebthi)Ti(CO) 2
Z
Z
O
OC
Ti
CO
*
Toluene, 90 ° C, 12 h
CO (14 p sig )
R=Ar,Me,H
Z=O,CH 2 ,C(COOEt) 2
72-96% ee
70-94% yield
( S,S )-(ebthi)Ti(CO) 2
Scheme 7.32 Enantioselective Ti-catalyzed PKR of 1,6-enynes.
In addition to C - and O -tethered enynes, Buchwald further disclosed the use of
a modified Ti-catalyst for the Pauson-Khand-type cyclization of N -tethered enynes
 
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