Chemistry Reference
In-Depth Information
O
O
F
3
B
acacRh(CO)
2
, dppb, H
2
O
+
S
S
5
2
5
0
5
5.204
Scheme 5.57
PhB(OH)
2
, acacRh(CH
2
CH
2
)
2
,
(
S
)-BINAP, H
2
O
O
O
Ph
5.206
5.203
PhBF
3
K, (COD)
2
RhPF
6
,
(
S
)-BINAP, H
2
O
Scheme 5.58
i
-Pr
O
O
[RhCl(CH
2
CH
2
)
2
]
2
,
L*, KOH
B(OH)
2
+
i
-Pr
5.201
5
.
7
5
.
8
Bn
L*=
Bn
5.209
Scheme 5.59
5.2
Insertion Reactions Involving Zirconium and Titanium
5.2.1 Hydrozirconation and Carbozirconation
In a reaction that parallels hydroboration, alkenes and alkynes undergo insertion into the metal-hydrogen
bond of Schwartz's reagent, zirconocene hydrochloride, generating alkyl zirconium complexes
5.211
(Scheme 5.60).
63
The reaction occurs at room temperature. As with hydroboration, for terminal alkenes,
the hydrogen becomes attached to the internal position and the metal at the terminal position. Unlike hy-
droboration (except when it is carried out under extreme conditions), internal alkenes give the same product
as terminal alkenes. This is because a series of rapid
-hydride elimination-reinsertion reactions occur after
formation of the initial addition product
5.210
, allowing the zirconium to migrate to the terminal position.
64
The importance of the reaction rests upon the reactivity of the resulting alkyl and alkenyl zirconium
complexes. While the complexes are comparatively stable in air, the carbon-zirconium bond may be cleaved
by simple electrophiles such as bromine and iodine, as well as by acid chlorides,
65
and by oxidation with
peroxide reagents (Scheme 5.60).
66
Epoxide opening can be achieved intramolecularly and with Lewis-acid
assistance (Scheme 5.61).
67
Carbonylation is also possible: the resulting acyl zirconium complexes
5.216
may be protonated to give aldehydes
5.217
, or oxidized in the presence of either water or an alcohol to give
a carboxylic acid
5.219
or an ester
5.218
, respectively (Scheme 5.62).
68
