4 The Pauson–Khand Reaction and Silylformylation
4.1 The Pauson–Khand Reaction
The Pauson–Khand reaction is the coupling of an alkyne, alkene and CO to yield a
cyclopentenone in an overall [2 + 2 + 1] cycloaddition reaction (Scheme 12) and has
proven to be a valuable tool in the total synthesis of various natural products
[60, 61]. The reaction was first catalysed by the bimetallic dicobalt complex
[Co 2 (CO) 8 ] [62–64], although a number of variations with monometallic cobalt
complexes and with different metals have also been reported [61]. The currently
accepted mechanism of the [Co 2 (CO) 8 ]-catalysed Pauson–Khand reaction involves
initial coordination of the alkyne perpendicular to the Co–Co axis through two
orthogonal π-bonding interactions to give complex 27. This is followed by coordination of the alkene and then insertion of the alkene into a Co–C bond. Migratory
insertion of a CO ligand and reductive elimination of the product liberates the
[Co 2 (CO) 6 ] fragment, which is trapped by a second equivalent of alkyne to renew
Au(PR3)
24
(PR 3 )Au
(IPr)Au
Au(IPr)
SbF 6
SbF 6
Ph
Ph
Me
+
Ph
Me
Ph
CD 2 Cl 2 , 25
o C
CD 2 Cl 2 , 25
o C
(IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)
b)
a)
c)
25
O
Au(PPh3)
Au(PPh3)
(Ph3P)Au
BF 4
n
Me
Me
Me
n
CHCl 3 , 60
o C
26
Scheme 11 (a) A μ
2
-η
1
:η
2
-acetylide bimetallic Au catalyst (24) used in the azide-alkyne cycloaddition, (b) a μ
2
-η
2 gem-diaurated catalyst (25) used for the cycloaddition of dialkynes, and (c) a
trimetallic gold complex (26) used in the cycloisomerisation of allenynes
116
M.J. Page et al.