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In addition to using aldehyde as the CO surrogate, the Kwong group recently disclosed
that benzyl formate could serve as a CO source for a cooperative dual catalysis. 32 The
decarbonylation of formate by the Rh complex and subsequent carbonylation of the enyne
offered the carbonylative cycloadduct in up to 94% ee (Scheme 7.14).
O
Bn
[Rh]
O
H
Decarbonylation
process
ROH
[Rh]
CO
CO transfer
process
2.5 mol% [Rh(cod)Cl] 2
5 mol% ( S )-xylyl-BINA P
Dioxane, 120 °C, 3 d
Benzyl formate (5 eq.)
R
X
X
R'
R
Carbonylation
process
*
R'
O
X = O, NTs, C(COOEt) 2
Up to 94% ee
Scheme 7.14 The Rh-catalyzed PKR with formate as CO source.
They proposed the cooperative catalytic cycle as shown in Scheme 7.15. The formate
coordinates to the Rh center and subsequent C-H oxidative addition gives the Rh(III)
complex. The decarbonylation of the Rh-acyl complex yields the Rh-carbonyl species. The
Rh-CO complex enters a carbonylation process of the enyne to generate cyclopentenone as
the usual PKR mechanism. In addition to the suggested mechanism, they also performed 13 C
labelled experiments to further confirm the source of CO moiety. 32
P Cl
P
P
Cl
*
*
Cl
*
Rh H
CO OR
Rh
H
Rh
H
P
P
P
OR
O
O
O
OR
ROH
RO
H
Decarbonylation process
P
P
*
*
Rh
Cl
CO
Rh
Cl
P
P
O
Ph
H
Carbonylation process
[2+2+1] cyclization
O
Ph
O
P
P
Cl
Cl
P
Cl
*
*
*
CO
CO
O
Rh
Rh
Ph
Rh
Ph
P
P
Ph
P
O
O
O
Scheme 7.15 The proposed mechanism of PKR with formate as CO source.
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