Chemistry Reference
In-Depth Information
O
CpRu(PPh
3
)
2
Cl,
NH
4
PF
6
OH
+
OAc
OAc
8.178
8.177
O
O
1. LiOH
2. DMSO,
1. OsO
4
, NMO
2. TsOH
Δ
OAc
8
1
7
8
Scheme 8.48
Cp
Cp
(OC)
3
W
(OC)
3
W
OH
OH
BF
3.
OEt
2
OMe
OMe
8.181
8.182
MeO
C
p
C
p
(OC)
3
W
O
O
•
MeMgBr
OH
(OC)
3
W
MeO
8.185
8
8
8
1
4
Scheme 8.49
complex
8.173
. It is, however, suggested that this nucleophilic attack is facilitated by prior coordination of
the allyl alcohol double bond to ruthenium. After nucleophilic attack, the C-O bond of complex
8.175
is
activated towards cleavage to form a
-allyl complex
8.176
. Reductive elimination and dissociation yields
the product
8.171
and regenerates the catalyst. Propargylic alcohols, however, follow a different pathway.
51
This reaction was employed to synthesize the perfumery compound, rosefuran
8.180
(Scheme 8.48) by the
ruthenium-catalysed combination of alkyne
8.177
with an allylic alcohol.
52
Dihydroxylation of the product
8.178
was followed by acid-catalysed dehydration. Hydrolysis of the ester and thermal elimination of water
gave the natural product
8.180
. Another synthesis of rosefuran can be found in Scheme 2.15.
Vinylidene complexes may also be formed by the reaction of
1
-alkynyl complexes with electrophiles
(Scheme 8.49).
53
Again, if an alcohol is present, a carbene complex will be formed. In this case, the carbene
complex
8.184
was converted to a
gem
-dimethyl group by reaction with a Grignard reagent.
8.3 Metathesis Reactions Involving Carbene Complexes
What is metathesis? The term is very general and it describes an outcome, rather than any specific process.
The term metathesis implies nothing about the mechanism followed. Metathesis refers to any process in
which a pair of groups bonded together swap partners with another pair, thus, A-B reacts with X-Y to give
A-X and B-Y (Scheme 8.50). The bonds being exchanged may be single, double, triple or even ionic. A
Wittig reaction may be regarded as a metathesis reaction.
