Chemistry Reference
In-Depth Information
reaction proceeded via the silylated phosphoric acid as the active catalyst. The resulting
silane activates the aziridine by coordination of its carbonyl group, and subsequent
attack of azide provides an intermediate and regenerates the phosphoric acid (Scheme
3.48 ).
O
O
R
CF
3
CF
3
PA
5a
or
5b
(10 mol %)
R
NH
N
TMS
N
3
R
DCE, rt
R
N
3
CF
3
9 examples
49-97% yield
70-95% ee
CF
3
O
OTMS
R
R
2
R
NH
N
R
2
R'O
P
O
R'O
N
3
R
N
3
TMSN
3
R
OH
HN
3
HN
3
R'O
O
R'O
P
O
R'O
P
OTMS
R'O
O
SiMe
3
O
R
R
2
N
R
Scheme 3.48.
Even though chiral phosphoric acids were successfully applied to various asymmetric
processes, their utility has been limited to more basic nitrogen-based electrophiles such
as imines and aziridines, because of the relatively low acidity of phosphoric acid. Yama-
moto and coworkers designed stronger chiral Brønsted acids in efforts to expand the
utility of chiral phosphoric acid-catalyzed reactions to a broad range of organic trans-
formations (Fig. 3.9 ).
Nakashima and Yamamoto prepared
N
- trifl yl phosphoramide PA
6a
, bearing a
strong electron-withdrawing group, NTf, and applied this catalyst to the D-A reactions
of α , β-unsaturated ketone with electron-rich dienes to give cyclohexene derivatives with
high enantioselectivities [84]. By introducing a strong electron-withdrawing group (NTf)
into =O in phosphoric acid, great enhancement in reactivity of the acid was observed.
This is the fi rst example of carbonyl activation with phosphoric acid derivatives (Scheme
3.49 ).
