Chemistry Reference
In-Depth Information
R
R
R
R
Co
2
(CO)
6
often
written as
Co
2
(CO)
8
R
Co(CO)
3
R
(OC)
3
Co
7.1
7
.
2
7
3
[O]
Scheme 7.1
silyl enol ether
7.9
with almost no selectivity, because the alkynyl substituent is non-bulky.
5
The complex
7.10
, on the other hand, gave a highly diastereoselective reaction (Scheme 7.3).
Another demonstration of the size of the complexed alkyne comes from a synthesis of tautomycin
intermediates.
6
A tetrahydropyran
7.12
was found to exist predominantly with the alkynyl group axial.
This was determined from the observation of a 1 Hz coupling constant between H
a
and H
b
(Scheme 7.4).
After complexation with cobalt, a ring flip occurred to the tetrahydropyran
7.13
with a conformation having
the alkynyl complex equatorial being favoured, with an H
a
-H
b
coupling constant of 6.5 Hz.
The complexes are inert to many typical alkyne reactions: most forms of catalytic hydrogenation fail,
as does hydroboration. As a simple application, the dicobalt complex can be regarded as a protected
alkyne. Some methods to achieve simultaneous alkyne reduction and decomplexation have been reported
(Scheme 7.5).
7
Hydrogenation of the cyclic alkyne complex
7.14
using Wilkinson's catalyst gives the corre-
sponding decomplexed alkene
7.15
, accompanied by the alkene isomerization product
7.16
. While tri-
n
-butyl
tin hydride may be used for this transformation, silane reagents and sodium hypophosphite, NaH
2
PO
2
,(for
an example, see Scheme 7.7) offer better chemoselectivity.
8
123
°
127
°
118
°
116
°
121
°
117
°
116
°
118
°
121
°
121
122
°
123
°
119
°
°
118
°
117
°
116
°
137
°
139
°
7
8
°
43
°
99
°
103
°
Figure 7.1
A dicobalt-alkyne complex
7.2
(R
=
Ph). Reprinted with permission from Sly, W. G. et al.
J. Am.
Chem. Soc.
1959
,
81
, 18.
c
1959 American Chemical Society.
