Chemistry Reference
In-Depth Information
Me
(
t
-BuO)
3
W
≡
C
t
-Bu
O
O
Me
8.494
H
2
, Pd/BaSO
4
quinoline
O
O
O
O
8
5
8
6
Scheme 8.132
CH
3
H
CH
3
O
H
1. (Ph
3
SiO)
3
Mo
≡
CPh
O
2. H
2
, P2-Ni
O
H
O
O
H
O
8.498
8.497
Scheme 8.133
Alkyne metathesis can also be used in ring-closing mode, but only for the formation of large rings, as the
presence of the alkyne prevents medium-sized rings from being stable. The macrocyclic lactone musk
8.496
and its higher homologue, required for studies in perfumery were prepared by an alkyne metathesis, reduction
sequence (Scheme 8.132).
155
The macrolactone
8.496
was found to be “musky, warm and metallic”.
The more reactive molybdenum catalyst
8.484
was employed to close the macrocyclic ring in the penulti-
mate step of a protecting group free synthesis of Ecklonialactone A
8.498
(Scheme 8.133).
156
8.5 Carbene Complexes from Diazo Compounds
Diazo compounds, with or without metal catalysis, are well-known sources of carbenes. For synthetic purposes
a metal catalyst is used.
157
The diazo compounds employed are usually
- to an electron-withdrawing group,
such as an ester or a ketone, for stability. In the early days, copper powder was the catalyst of choice, but
now salts of rhodium are favoured. The chemistry that results looks very like the chemistry of free carbenes,
involving cyclopropanation of alkenes, cyclopropenation of alkynes, C-H insertion reactions and nucleophilic
trapping. As with other reactions in this chapter, free carbenes are not involved. Rhodium-carbene complexes
are responsible for the chemistry. This has enormous consequences for the synthetic applications of the
carbenes - not only does the metal tame the ferocity of the carbene, but it also allows control of the chemo-,
regio- and stereoselectivity of the reaction by the choice of ligands.
In many cases, a given substrate can follow different pathways according to the catalyst used. The rhodium
complexes used typically have either carboxylate or amide ligands. Tremendous differences in reactivity can
