Chemistry Reference
In-Depth Information
[Co]
6.350
[Co]
Br
[Co]
AcO
AcO
6.348
6.349
[Co]
[Co]
6.351
[Co] = Co(dmgh)
2
NC
5
H
5
Scheme 6.129
OH
[Co]
[Co]
Ts O H
6.352
6.353
Me
3
Si
Me
3
Si
[Co]
OH
1. TEMPO,
h
ν
2. Zn, AcOH
6.354
6.355
Scheme 6.130
OH
1. Na[Co(dmgh)
2
py]
2. PPTS
[Co]
N
N
OTs
6.356
6.357
[Co]
O
OH
1. TEMPO,
h
ν
2. H
2
, Rh/Al
2
O
3
3. BH
3
N
H
H
1. EtOH,
Δ
2. Zn, AcOH
N
N
N
H
3
B
6.358
6.359
6.360
Scheme 6.131
shown that the two carbon atoms involved are not equivalent.
140
Equilibrating
-stabilized carbocations
6.351
2
-complex
6.350
.
The trapping of these cations with carbon nucleophiles has made them useful for synthesis. The carbon
nucleophiles must be stable to the mildly acidic conditions used to generate the cations. Examples are
trisubstituted alkenes, in an intramolecular fashion,
141
has been proposed as a better representation, rather than a
1
-
alkylcobalt complex
6.354
produced may be converted to an alcohol
6.355
by free radical methods as the
carbon-cobalt bond undergoes photochemical homolysis.
The intramolecular trapping by a pyrrole has been used in a short synthesis of tashiromine
6.360
,an
indolizidine alkaloid (Scheme 6.131).
143
The optically pure cobaloxime complex
6.357
, generated from the
corresponding alcohol
6.356
, was directly trapped by the pyrrole with overall retention of stereochemistry.
Reduction of the pyrrole and conversion of the carbon-cobalt bond to a carbon-oxygen bond yielded the
alkaloid
6.360
and its diastereoisomer. Another synthesis of this alkaloid may be found in Scheme 2.122.
allyl silanes (Scheme 6.130) and pyrrole.
142
The
