Chemistry Reference
In-Depth Information
HO Cl
Ph
3
PAu C l , A g OT f
O
Cl
6.157
Scheme 6.52
OMe
AuCl
3
, CuCl
2
, MeOH,
Δ
n
-C
6
H
13
n
-C
6
H
13
6.158
Scheme 6.53
Ph
3
PAuCl, AgOTf
Me
N
Ts
NHTs
6.35
6.159
Scheme 6.54
6.2 Other Metals: Silver, Gold, Platinum and Rare Earths
Silver, gold,
53
-bonds and activate them to nucleophilic attack.
Numerous examples have appeared over the last few years and asymmetric versions have also been reported.
54
The reactions encompass not only heteroatom nucleophiles, but also carbon nucleophiles.
55
When gold
chloride catalysts are used, they are often accompanied by silver salts, typically AgOTf, AgBF
4
or AgSbF
6
.
The purpose of the silver salt is to effect counter ion exchange of the chloride from the gold complex with
the more-labile ion of the silver salt (see Scheme 1.4), to give a more reactive gold complex. Reactive but
air-stable gold complexes with the weakly coordinating triflamide counter ion, LAuNTf
2
(L
and platinum coordinate well to C-C
=
phosphine or
NHC), can be prepared in a separate step and used in catalysis without the addition of silver salts.
56
6.2.1 Reactions of Alkenes
Alkenes tend to be the least-reactive class of substrates, requiring more forcing conditions and nonpolar
solvents. Nucleophilic attack generates a
1
-alkyl gold species. These show little or no tendency to undergo
-hydride elimination. Instead, protonolysis of the carbon-gold bond is observed, regenerating the catalyst
and resulting in net addition of HX across the alkene. As with the chemistry of the palladium catalysts,
nucleophilic attack tends to be at the more-substituted terminus of the alkene. Both oxygen (Schemes 6.52
and 6.53)
57,58
and nitrogen (Schemes 6.54 and 6.55)
59,60
nucleophiles can be used. Dienes may be employed as
substrates (Scheme 6.56).
61
Platinum catalysis, especially with platinum triflate, has also been used (Scheme
6.57).
62
1
-intermediate is not observed. Instead,
protonolysis of the carbon-metal bond is observed, giving overall addition without change of the metal
oxidation state. The difference is particularly apparent in the cyclization of
o
-allylaniline (Scheme 6.54) that
gives the indoline
6.159
with gold(I) catalysis. In contrast, the same substrate, with palladium catalysis with
an added oxidant, gives the indole (Scheme 6.16).
Electron-rich heteroaromatics may also act as the nucleophile (Scheme 6.58).
63
The platinum-catalysed
cyclization of indole
6.165
proceeds through intramolecular attack of the heterocycle on the
Unlike palladium chemistry,
-hydride elimination of the
2
-complex
