Quick Recap
• We’ve previously discussed σ-bond metathesis where low e- count
metal alkyls (d1 or d0) display the following reactivity and are thought to
pass through a 4-centered transition state as shown below
• Analogously Olefin Metathesis is
R1
+
R1
R2
R2
R2
[cat]
2
R1
• The reaction can be driven by pairing electron donating R1 with electron withdrawing R2 to create a stabilizing pushpull effect in the metathesis product.
• Can also be driven by removal of gaseous olefins such as ethylene
History
Early mechanistic proposals
• Bradshaw, J. Catalysis, 1967, 7, 269. Studies done on Co-Oxide/Mo-Oxides supported on alumina
(CoO·MoO2·Al2O3)
• Initial mechanistic proposals followed a pairwise mechanism with a cyclobutane intermediate. Such a mechanism is
forbidden by Woodward-Hoffman rules for 2+2 cycloadditions. Also cyclobutanes are not detected in the reaction
mixture or decomposed by the catalyst.
• Alternatively a pairwise mechanism involving a metallacyclopentane was proposed.
R1HC
R1
M
M
R2HC
R1
R2
R2
R1
2
1
R HC
CHR
M
2
R
M
M
CH2
The Actual Mechanism
• 1971 Chauvin. Proposes a sequential [2+2] cycloaddition pathway.
The Actual Mechanism
• 1975 Katz and McGinnis. Double cross over experiment ruled out pairwise mechanisms.
Typical catalyst cocktail of
MoCl2(CO)2(PPh3) and Me3Al2Cl3
• For “pairwise” mechanism we expect to see only C12 and C16 at early time
points.
• The C14 double-cross product should only form at early times due to a nonpairwise mechanism
More Mechanistic Studies
• Some people proposed that a more pairwise mechanism might still give
the C14 products seen at early timepoints by Katz if the olefin was bound
strongly to the metal and underwent multiple metathesis events before
being released.
• To disprove the “sticky olefin” hypothesis a crossover experiment where
the products of metathesis cannot themselves undergo metathesis was
required.
• The statistical mixture of 1:2:1 d0, d2 and d4 ethylene confirms again the
non-pairwise mechanism Chauvin proposed.
Some Examples of Alkene and Alkyne Metathesis Reactions
Catalyst Structures
• In general Schrock type catalysts are based on Mo or W and Grubbs catalysts are based on Ru. Generic
structures for each type are shown below. The important feature in each is the alkylidene ligand which is
required for olefin metathesis activity.
• In general Ru catalysts are more tolerant of functionality and have greater stability while the Mo systems
display greater activity.
L
NAr
Cl
Ru
Cl
Mo
X
Ar
R
X
L
Grubbs
Schrock
• A number of variants have been prepared for a wide range of applications, some of which are shown below.
PCy3
Cl
Ru
Cl
Ar
N
N
Ar
Ar
N
Cl
PCy3
Grubbs I
Cl
Ru
Cl
Ph
PCy3
Grubbs II
Ar
F 3C
Cl
Ru
Ph
N
O
i
Pr
Hoveyda-Grubbs
F 3C
CF3
NAr
O Mo
O
F3C
Schrock
Selectivity in Cross-Metathesis
• The selectivity for the formation of different metathesis products can be predicted by
the categorized olefins into four different types.
• The table below shows type classifications with several examples for a Grubbs II
catalyst.
Type I
Type II
Type III
Type IV
Rapid homodimerization
Slow homodimerization
No homodimerization
Spectators to cross‐
metathesis
Terminal olefins, allylic alcohols, esters, allyl boronate esters, allyl halides, styrenes
Styrenes w/ large ortho
substituents, acrylates, vinyl ketones, perfluoroalkyl
olefins
1,1‐disubstituted olefins, non‐bulky tri‐substituted olefins, vinyl phosphonates
Vinyl nitro olefins, tri‐
substituted allylic alcohols
(protected)
• Reactions between two olefins of Type 1 leads to a statistical
mixture of cross-metathesis products.
• Reactions between two olefins of the same type (non Type 1)
leads to non-selective cross-metathesis.
• Reactions between olefins of two different types leads to selective
cross metathesis.
• An olefin’s type classification is somewhat dependent on catalyst
identity but in general Type I olefins are unhindered and electron
rich while Types II-IV are increasingly hindered and electron
deficient.
Mechanism of Alkyne Metathesis
• Schrock has studied alkyne metathesis with
• The reaction is more poorly understood using Mo(CO)6
and phenols to generate an in situ catalyst.
high valent tungsten carbyne complexes and
• It is possible that the Mo is oxidized and then catalyzes
found it mechanistically similar to the Chauvin
the reaction via the same route as the W carbyne system
mechanism for alkene metathesis
studied by Schrock although it is unclear how high
oxidation states would be generated.
• metallacyclopentadiene and butadiene complexes
containing metal carbonyl fragments are known and have
been proposed as relevant intermediates although
supporting data is scarce.
JACS, 1981, 103, 3139
JACS, 1982, 104, 6808
J. Org. Chem. 1998, 63, 8606
Mechanism of Enyne Metathesis
• Mostly studied with Grubbs Ru catalsts
JACS, 2005, 127, 576
Some Places You Can Go For More Information
• Crabtree, Chapter 12
• Hartwig, Organotransitionmetal Chemistry, chapter 21.
• https://en.wikipedia.org/wiki/Olefin_metathesis