Chemistry Reference
In-Depth Information
O
PtCl
2
, THF, H
2
O
O
6.162
6.163
6.164
Scheme 6.78
O
OEt
PtCl
2
, THF, H
2
O
OEt
O
OEt
6.165
6.166
88:12
6.167
Scheme 6.79
MeOH, PtCl
2
, Na
2
SO
4
MeO OMe
6.168
6.169
Scheme 6.80
OH
OH
O
O
t
-Bu
2
(o-biph)AuCl, AgOTf
HO
6.170
6.171
Scheme 6.81
Alkynes often give the product of the addition of two nucleophiles, which may be the same, or different
species.
84
Platinum(II) chloride has been found to be a useful catalyst for the hydration of alkynes to give
ketones, avoiding the use of mercury, the classical catalyst for this reaction (Scheme 6.78).
85
With simple,
unsymmetrical alkynes, regioselectivity is poor, but the presence of a substituent can bias the regioselectivity
strongly in favour of one isomer (Scheme 6.79).
86
Replacement of water by simple alcohols leads to ketals
(Scheme 6.80).
87
The mechanism for ketal formation must involve attack by one molecule of alcohol to
form a vinyl ether, followed by a second attack. The first attack clearly involves nucleophilic attack on an
2
-platinum complex to give a vinyl ether. In principle, the second attack may be also platinum catalysed, or
catalysed by traces of Brønsted acids present. A similar situation occurs in Wacker chemistry (Scheme 6.9). In
this case, it was found that adding a bulky base, 2,6-di-
t
-butylpyridine, did not suppress the reaction. This fact
indicates that the second step is also platinum catalysed. Both gold (I)
88
and platinum(IV)
89
have also been
used in the synthesis of cyclic ketals (Schemes 6.81 and 6.82), while gold catalysis converted propargylic
alcohol into a cyclic bis-acetal
6.174
(Scheme 6.83).
90
Other nucleophiles, or combinations of nucleophiles,
can also be employed in double additions (Scheme 6.84).
91
Electron-rich heterocycles can also participate in
such reactions. This is mechanistically distinct from C-H activation (Chapter 3) as it is more closely related
to an electrophilic attack on the pyrrole (Scheme 6.85).
92
A number of catalysts have been employed for the addition of carboxylates to alkynes (Scheme 6.86). For
additions to terminal alkynes, addition can be to either the internal and terminal carbon, depending on the
catalyst.
93-95
