Chemistry Reference
In-Depth Information
aRu-C(sp
2
) bond rather than Ru-N bond predominantly occurred to give azaruthenacyclo-
hexenediones, followed by a reductive elimination to release the desired maleimides and
regeneration of the Ru-catalyst.
O
"CO"
RNC
O
Me
[Ru]
Ph
N
R'
R''
Ph
O
O
O
Ph
Ph
Ph
Me
Ph
Ph
N
N
N
[Ru]
N
[Ru]
O
O
[Ru]
[Ru]
Ph
Me
Ph
Me
5 : 1 Ratio
Me
O
Ph
O
Major
Scheme 7.37
Proposed mechanism of Ru-catalyzed co-cyclization.
In 2008, Mastrorilli and Braunstein reported tri- and tetra-nuclear heterometallic Ru
clusters with a formula of NEt
4
[RuCo
3
(CO)
12
] which was effective as a precatalyst for
an intramolecular Pauson-Khand-type reaction of diethyl (allylpropargyl)malonate.
69
The
best result was obtained in the presence of 2 mol% precatalyst under 8 bar of CO at 70
◦
C.
The desired cyclopentenones were afforded in up to 93% isolated yield.
7.6 Nickel- and Palladium-Catalyzed Pauson-Khand-Type Cyclizations
The nickel-assisted Pauson-Khand-type [2
1] cyclization of enynes was first achieved
by Buchwald and co-workers in 1996.
70
They demonstrated that a stoichiometric amount
of Ni(0)-complex was required to convert an enyne and isocyanide into the corresponding
iminocyclopentene (Scheme 7.38). Turning the supporting ligand from phosphine to highly
electron-donating bidentate nitrogen ligands, in which
rac
-diimine
L
was superior in this
transformation gave the target product in 80% yield.
+
2
+
Ph
Me
Me
NC
Ph
Ni(cod)
2
/L
O
O
N
L=
+
Me
Benzene, 8 h
room temp.
N
N
Me
(9-Anth)
(9-Anth)
Isocyanide
80% yield
Scheme 7.38
Ni-assisted cyclization of 1,6-enyne and isocyanide.