Non Chiral Pauson-Khand Reaction - The Pauson-Khand Reaction - page 54

The structures of the true catalysts were unclear, but the above-mentioned additives could
work as ligands for the cobalt complex by displacement of the CO ligand. Sugihara and
Yamaguchi disclosed a different type of additive: in the presence of a catalytic amount
of 1,2-dimethoxyethane or water, the Pauson-Khand reaction proceeded (Scheme 3.14). 14
A pressurized condition of carbon monoxide (7 atm) was required, but 2-3 mol % of
Co 2 (CO) 8 realized a good to high yield. As for the intermolecular reaction, mono-substituted
acetylenes were good substrates.

Co 2 (CO) 8 (2-3 mol%)

R 1

R 1

DME (8-12 mol%) or H 2 O

R 2

E 2 C

E 2 C O

Toluene, 120 °C

()

( )

n n

R 3

R 2

CO (7 atm)

R 3

E= CO 2 Me

DME H 2 O

R 1 = H, R 2 = H, R 3 = H (n = 1):
R 1 = Ph, R 2 = H, R 3 = H (n = 1):
R 1 = H, R 2 = H, R 3 = Me (n = 1):
R 1 = H, R 2 = Me, R 3 = H (n = 1):
R 1 = H, R 2 = H, R 3 = H (n = 2):

94% 97%
87% 90%
82% 85%
86% 88%
85% 87%

O

R

R

The same as above

+

DME
98% 97%
100% 98%
92% 90%

H 2 O

With norbornene:

R = Ph:
R = n-C 8 H 17 :
R = Ph:

With norbornadiene

Scheme 3.14

Perez-Castells realized a catalytic reaction in the presence of molecular sieves (zeolite)
without CO gas: molecular sieve 4A was pretreated under CO atmosphere at 200 ◦ Cforthe
absorption of carbon monoxide, and was used as a CO source for the Co 2 (CO) 8 -catalyzed
reaction (Scheme 3.15). 15 The stereospecific reaction proceeded, and the enyne with Z -
olefin moiety provided carbonylated cycloadduct, where the geometry of the methoxy
group and hydrogen at the ring-fused position was trans . Ortho -phenylene tethered 1,7-
enyne with a hydroxy group was also a good substrate.

3.5 Catalytic Reaction Using in-situ Generated Low-Valent
Cobalt Complex

The catalytic Pauson-Khand reaction is generally difficult to achieve, probably because
of the instability of the low-valent cobalt carbonyl complex. Actually, several catalytic
examples were reported, where low-valent cobalt carbonyl complexes were prepared
in-situ from relatively stable cobalt complexes.