Everything about Enyne Metathesis totally explained
An
Enyne metathesis is an
organic reaction taking place between an
alkyne and an
alkene with a
metal carbene catalyst forming a
butadiene. This reaction is a variation of
olefin metathesis.
The general scheme is given by
scheme 1:
»
When the reaction is
intramolecular it's called
ring-closing enyne metathesis or
RCEYM (
scheme 2):
»
with Y representing
oxygen or
nitrogen and n an integer.
The reaction was first described in 1985 with the conversion of
biphenyl 3.1 to a
phenanthrene in
scheme 3:
»
The carbene is a
tungsten carbonyl when used in stoichoimetric amounts (1 equivalent) yields 41% of the phenanthrene
3.2 and when used in catalytic amounts phenanthrene
3.3. The
stereoselectivity of this reaction is large with the metal atom exclusively adding to one of the alkyne carbon atoms in the initial reaction step.
Reaction mechanism
The
reaction mechanism for this reaction is outlined in scheme 4:
»
In the first
catalytic cycle the
alkyne group of enyne
4.1 forms a metallacyclobutene intermediate
4.3 with carbene
4.2 with R' and R' ' any organic group required to stabilized it. In the next step the metathesis step is reversed with formation of a new double bond and a new carbenic center in
4.4. The ring-closing step takes place when this center reacts with the alkene group to a metallacyclobutane
4.5 as in a regular
olefin metathesis reaction. The butadiene group forms in the last step with expulsion of a new methylene carbene, initiating the next cycle but now with R' = H and R' ' = H.
This is the proposed "yne-then-ene" mechanism. Evidence for an "ene-then-yne" pathway is beginning to emerge, especially for ruthenium based catalytic systems.
The driving force for this conversion is the formation of a thermodynamically stable
conjugated butadiene.
Scope
Enyne metathesis reactions are accelerated by
ethylene as is demonstrated in the reaction displayed in
scheme 5:
»
In this reaction with the
Hoveyda-Grubbs Catalyst, ethylene converts the alkyne group to the corresponding
diene group prior to the reaction with the alkene group.
Further Information
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