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Multiple Metal-Carbon Bonds for Catalytic Metathesis Reactions
Nobel Lecture December 8, 2005
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Metal-carbon double and triple bonds in which the transition metal is in a "low oxidation state"
were discovered by E. O. Fischer.
CO
Cr
CO
CO
OC
COC
OCH3
CO
W
CO
CO
OC
CBrδ +δ -
1964 1973"carbyne""carbene"
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Beta hydride elimination in an ethyl complex
M
C
C
H
H
H
L1
L2
L3M
H
L1
L2
L3+
HC
HC
H
H
HH
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Known Group 4 Peralkyl Complexes (M = Ti, Zr, Hf) in 1973.
4
All alkyls lack one or more hydrogen atoms on the atom β with respect to the metal.
M
Me3SiCH2
Me3SiCH2
CH2
CH2SiMe3
Si
Me
Me
Me
M
Me3CCH2
Me3CCH2
CH2
CH2CMe3
C
Me
MeMe
αββ
α
MC6H5CH2
C6H5CH2
CH2
CH2C6H5
H
HH
H
H
βα
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The first relatively stable permethyl complex
WMeMe
Me Me
Me
Me
WCl6 + 6 AlMe3
pentane
4 5 6 7 8Ti V Cr Mn FeZr Nb Mo Tc RuHf Ta W Re Os
A. J. Shortland and G. Wilkinson J. Chem. Soc., Dalton Trans. 1973, 872.
“*Note added in proof. Hexamethylrhenium (K. Mertis and G. Wilkinson) and pentamethyl[t]antalum (R. Schrock, DuPont, Wilmington, private
communication) have recently been synthesized.”
Geoffrey Wilkinson, Nobel Lecture, December 11, 1973
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Synthesis of tantalum pentaalkyls
Ta
C
H3C CH3
CH3H3C
H H H
2 LiMe
TaCl5 + 1.5 ZnMe2
pentane
(Juvinall)- 1.5 ZnMe2
ether
TaMe3Cl2
Decomposes above 0 °Cbimolecularly
TaCl5 + 5 Me3SiCH2MgCl 1/2 Ta
CTa
C
CH2SiMe3Me3SiCH2
Me3SiCH2 CH2SiMe3
SiMe3
SiMe3
"It is assumed that a penta-alkyl complex cannot exist for steric reasons."(Mowat, W.; Wilkinson, G. J. Chem. Soc, Dalton Trans. 1973, 10, 1120.)
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Neopentyls yield a stable product of α hydrogen abstraction.
Ta(CH2CMe3)3Cl2
2 LiCH2CMe3
Ta
t-BuCH2
t-BuCH2
Ct-BuCH2
H
CMe3- CMe4
J. Am. Chem. Soc. 1974, 96, 6796Distills in a good vacuum at 75°C.
(t-BuCH2)3Ta
CH2-t-Bu
C
t-Bu
HH
(t-BuCH2)3Ta
CH2-t-Bu
C
t-Bu
HH
(t-BuCH2)3Ta CH
t-Buδ-
δ-
δ+δ+
- CMe4
α hydrogenabstraction
(deprotonation)α hydrogen activation
δ + δ -δ +
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Alkylidenes can be deprotonated to yieldtantalum-carbon triple bonds.
Ta
t-BuCH2
t-BuCH2
Ct-BuCH2
H
CMe3
LiButyl
- ButylHTa
t-BuCH2
t-BuCH2
Ct-BuCH2
CMe3
Li+-δ+
δ+
δ-
Guggenberger, L. J.; Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 2935.
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Alkylidenes decompose bimolecularly.
TaC
Me H
H
Cp2Ta
CH2
TaCp2
CH2Me
Me
Ta CH2
Me
CH2L
TaMe
+
L = CO, C2H4, PR3
δ+δ-
2
18 electrons
L
base- H+
Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6577.[Cp2TaMe3]+
Bimolecular decomposition of alkylidenes, especially methylenes,is difficult to prevent, especially in electron deficient species.
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Olefin metathesis and the Chauvin mechanism (1971)
2 RCH=CHR' RCH=CHR + R'CH=CHR'
+ RCH=CHR' - R'CH=CHR'
M=CHR M
R
R
R'
+ RCH=CHR' - RCH=CHR
M=CHR'
H
H
H
M = Mo, W, or Re
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Alkyne metathesis and the metalacyclobutadiene mechanism
2 RC≡CR' R'C≡CR' + RC≡CR
M
R
R
R'
RC CR'M CR
RC CRM CR'M
R
R
R'
-
(suggested by T. Katz; 1975)
M = Mo, W
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Reaction of tantalum alkylidenes with olefins.
+ 4 olefinsTaClCl CH-t-Bu
TaClCl CH2
CHR
2 RCH=CH2
β H CH2=CHRCH2-t-Bu +CH3CHR=CH-t-Bu
RCH=CHCH2-t-Bu +RCH2CH=CH-t-Bu
β HCpCl2Ta
CHR
CH2
CH-t-Bu
CpCl2Ta
CH2
CHR
CH-t-Bu
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Modification of Nb and Ta yields metathesis catalysts
M(CH-t-Bu)L2Cl3 + H2C=CHR
M = Nb or Ta
L = PMe3
4 products of rearrangementof metallacyclobutanes
M(CH-t-Bu)(O-t-Bu)2Cl(PMe3) + olefins also metathesis products
(~35 turnovers for cis-2-pentene)
Alkoxides "prevent reduction" and "promote metathesis."
J. Molec. Catal. 1980, 8, 73; J. Am. Chem. Soc. 1981, 103, 1440.
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An oxo neopentylidene complex of tungsten
W
C
Cl
O
L
H
ClL
t-Bu
Tat-BuO O-t-But-BuO O-t-Bu
Cl
++
L = a phosphine, e.g. PEt3
Ta
L
Cl
ClC
t-Bu
Cl
LW
O
t-BuO O-t-But-BuO O-t-Bu
H
W
C
Cl
O
L
H
ClL
t-Bu
W
C
Cl
O
L
H
ClL
R
RCH=CHR
- t-BuCH=CHR
(AlCl3 cat)
Even R = H
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A sterically demanding diisopropylphenyl imidogroup might be a desirable "ancillary" ligand.
X = N
i-Pr
i-Pr
X
WCH-t-BuRO
RO
The OR group should be a sterically demanding tertiary alkoxide.
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A sterically demanding diisopropylphenyl imidogroup might be a desirable "ancillary" ligand.
N
WCH-t-Bu(CF3)2MeCO
(CF3)2MeCO
i-Pr i-Pr
Hexafluoro-t-butoxide was chosen as a highly electron withdrawing alkoxide.
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Synthesis of a tungsten neopentylidyne complex
Cl
W
OMe
ClMeO
MeO ClW
C
CH2 CH2
CH2
t-Bu
t-Bu t-But-Bu
+ 6 ClMgCH2-t-Bu
Volatile yellow crystals. Thermally stable, distillingat 75°C in a good vacuum.
(1978)
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Tungsten-carbon triple bonds and alkyne metathesis
purple crystals
3 HCl in dmeW
C
CH2 CH2
CH2
t-Bu
t-But-Bu
t-Bu
W
C
O
Cl
Cl
t-Bu
Cl
O
Me
MeW
C
OO
O
t-Bu
t-But-Bu
t-Bu- 3 CMe4
3 LiO-t-Bu
The tri-t-butoxide compound is a powerful catalyst for the alkyne metathesis reaction.
2 RC≡CR' R'C≡CR' + RC≡CR
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Metal-metal bonds and "metathesis" reactions.
W C
t-BuO
t-BuOt-BuO
W W
O-t-Bu
O-t-BuO-t-But-BuO
t-BuOt-BuO
+ R-C C-R R2
(1982)
X3W WX3
R-C C-R
X3W WX3
RR
X3W WX3
R
R
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Synthesis of a tungsten imido alkylidene complex
O W
N
OCl
C-t-BuCl
Me
Me
Ar HEt3N catalyst
O W
N
OCl
CCl
Me
Me
Ar
t-Bu
H
2 LiORW
N
RO
CRO
Ar
t-Bu
H
OR = O-t-Bu, OCMe2(CF3), OCMe(CF3)2,
and various bulky phenoxides
("14 electron" species)
W(NAr)(CH-t-Bu)(OR)2 species are "well-defined"catalysts for the metathesis of olefins and the
activity can be varied systematically by varying OR. N
i-Pr
i-Pr
NAr =
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Structure of syn-W(NAr)(CH-t-Bu)(O-t-Bu)2
145°
169°
1.87Å
1.76Å
000
00000
000
000
000
00
00
0000
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Tungstenacyclobutanes can be isolated, but can be too stable toward loss of olefin.
000
000
000
000
000
000
00
Molybdacyclobutane intermediates lose an olefin more readily.
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Two isomers (anti and syn) are available in any system through rotation about the M=C bond.
anti
N
M
C
t-Bu
RO
RO
H
R'
R' ka/s
ks/a
syn (usually favored)
N
M
C
H
RO
RO
t-Bu
R'
R'
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Olefin metathesis variations
Control!
RCM
R
- CH2=CH2
ROMP (Ring-Opening MetathesisPolymerization)
+ RCH=CH2
ROM/CM
x
HH H
H
H H
H H HH H
HH
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Polymerization of bistrifluoromethylnorbornadiene via enantiomorphic site control.
CF3
CF3
CMe3
Si Me
Si MeMe
Me
Ph
Ph
Mo
N
OO
R R
H
x
HHHH H HCF3 CF3 CF3 CF3 CF3
CF3HHHH
CF3 CF3 CF3 CF3
all cis and isotactic through enantiomeric site control when R = CH3
When R = CH(CH3)2 the polymer structure has a relatively random(71% cis) structure.
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Alkynes are polymerized to yield polyenes.
CO2EtCO2Et
CO2EtEtO2C
tail-to-tailhead-to-tail
EtO2C CO2Et
or
H H
H
H H H
Soluble, highly conjugated (purple), and relatively air-stable; both rings observed in polymer made with Mo(NAr)(ORF6)2 catalyst.
EE
EE
EE
EE
EE
EE
EE
**
>95% 5-membered rings produced with Mo(NAr)(O-t-Bu)2 catalyst.26
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Ring-closing metathesis with Mo catalyst (4-5 mol%)
(Catalyst = Mo(NAr)(CHCMe2Ph)[OCMe(CF3)2]2)
Me Me
OPh
O
Ph
O PhMeO Ph
Me
O
Me
Me
Me
Ph O Ph
Me Me
15 min, 92%
- C2H4
15 min, 92%
- propylene
180 min, 93%
- 2 butene N
O
Me
PhN
O
Me
Ph - C2H4
2 hr at 50°, 81%
Fu, G. C.; Grubbs, R. H. J. Am.Chem. Soc. 1992, 114, 5426; 7324.
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Synthesis of Fluvirucin-B1
NH
O
EtMe
O
Et
OOAc
MeOAc
N(H)COCF3
Me
O
HN
Et Et
OO
HO
H2N OH
Me
O
HN
Et Et
O
OAcO
OAcMe
N(H)COCF3
Me
Mo cat
92% yield
C6H6
- CH2=CH2
Fluvirucin-B1
A. F. Houri, Z. M. Xu, D. A. Cogan, A. H. Hoveyda, J. Am. Chem. Soc. 1995, 117, 2943.
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Mo or W catalyzed alkyne metathesis reactions are useful in organic chemistry.
O
W(CCMe3)(OCMe3)3
- MeC CMe
W cat =
W cat
O
Hydrog.
O
+ H2
H H
Civetone
(Fürstner)
Olefins do not appear to react with M-C triple bonds.
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Other examples of alkyne metathesis in organic synthesis
O
O
HO
OO
OH
OO OH
S
NNNH2N
H
Motuporamine CPGE2-1,15-lactone Epothilone C
MeO
MeO
O
O
O
MeO
MeO
O
O
O
W cat
80%
HO
HO
O
O
O
1. TsOH
2. 9-I-9- BBN- 2-butyne
S-(+)-citreofuran
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An enantiomerically pure Mo catalyst
Alexander, J. B.; La, D. S.; Cefalo, D. R.; Hoveyda, A.; Schrock, R. R.J. Am. Chem. Soc. 1998, 120, 4041.
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Asymmetric catalyst design; a modular approach
Imido Groups
N
Ni-Pr i-Pr
Mo
Mo
NMe Me
Mo
Cl Cl
Diolates
OO
t-Bu
t-Bu
MoOO
TRIP
TRIP
Mo
OO
CHPh2
CHPh2
MoOO
Mes
Mes
Mo
OO
Mes
Mes
Mo
OO
t-Bu
t-Bu
MoN
Mo
24 catalysts!32
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Asymmetric Ring-Closing Metathesis (ARCM)
i-Pr
i-Pr
i-Pri-Pr
i-Pr
O
OMo
N
Ph
Me
Me
R
R
O
i-Pr
O
Me Me
Me MeO
Me HR
Me
Mo
N
PhMe
Me
Me Me
OO
O
Me2Si
Me Me
O
Me2Si
Me
Me
H
R = i-Pr
2 mol % cat
no solvent,22 °C, 5 min
99% ee, 93%
2 mol % cat
no solvent,60 °C, 4 h
>99% ee, 98%
- propylene
- ethylene
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Ring-Opening / Ring-Closing Metathesis
O
OO
H
HO
Mo
N
PhMe
Me
i-Pr i-Pr
OO
5 mol %
>99% ee, 76%
C6H6
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Ring-Opening / Cross Metathesis
OMOM OMOM
X
Mo
N
PhMe
Me
i-Pr i-Pr
OO
5 mol %
95% yield, >98% ee, >98% trans
C6H6
+
X
X = H, OMe, CF3
D. S. La; J. G. Ford; E. S. Sattely; P. J. Bonitatebus; R. R. Schrock;A. H. Hoveyda J. Am. Chem. Soc. 1999, 121, 11603.
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Nitrogen-Containing substrates
O
OMo
N R
R
Me
MePh
5 mol %
NPh
PhN22 °C
>98% ee, 90% yield
- ethylene
20 min
R = Me
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Enantioselective synthesis of a tertiary ether in a drug
O O
R
R
RR
R
O
OMo
N
Ph
Me
Me
R
R
O
R
R = i-Pr
5 mol %
C6H6, 50 °C
95% ee, 95% yield
O
HN
SO2
NCF3Me
OH
O
Me
tipranavir (HIV protease inhibitor)
Cefalo, D. R.; Kiely, A. F.; Wuchrer, M.; Jamieson, J. Y.; Schrock, R. R.; Hoveyda, A. H.
J. Am. Chem. Soc. 2001, 123, 3139.
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A bis amido alkylidene catalyst precursor.
CHPh2
OH
OH
CHPh2Mo
N
CH-t-BuPh2N
Ph2N
Ar
- 2 Ph2NH
CHPh2
CHPh2
Mo
N
CH-t-BuO
O
Ar
In situAmido ligands deactivate the metal
toward metathesis reactions.
OO
H
5 mol% [Mo]
C6D6, 22 oCIn situ catalyst prepared withgives same ee (93-94%) as
the isolated catalyst.
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Dineopentyl species were examined as bisalkoxide catalyst precursors.
OR = OCH(CF3)2 , OAdamantyl, OCMe3, or OAr
Mot-Bu
t-Bu
t-Bu
N
Ar
+ ROH
- CMe4
Ar = 2,6-i-Pr2C6H3
Mot-Bu
O
R
t-Bu
N
Ar
Preliminary results suggest that monoalkoxides are at least as active as bisalkoxides!
(Surprising since dineopentyl species are essentially inactive.)
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"Well-defined" catalysts can be prepared on a silica surface.
Si
O
Silica (SiO2)
MoCH2
t-Bu
Ct-Bu
N
Ar
H
Ar = 2,6-i-Pr2C6H3
CH2
MoCH2
t-Bu
Ct-Bu
N
Ar
Ht-Bu
- CMe4
Si
OH
Silica (SiO2)
+
Bimolecular decomposition of alkylidenes is not possible.
Frédéric Blanc, Anne Baudouin, Christophe Copéret, Jean Thivolle-Cazat, Jean-Marie Basset, Anne Lesage, Lyndon Emsley, Amritanshu Sinha, Richard R. Schrock, Angew. Chem. Int. Ed., in press.
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Well-defined catalysts can be prepared on a silica surfaceusing other "clean" neopentyl sources.
M
N
CCH2
CH2
R
t-Bu
t-But-BuH
Re
C
CCH2
CH2
t-Bu
t-Bu
t-But-BuH
Re(VII)
Ta
C
CH2 CH2
CH2
t-Bu
t-Bu t-But-Bu
H
Ta(V)M
C
CH2 CH2
CH2
t-Bu
t-Bu t-But-Bu
M(VI)
(M = Mo, W)
Cóperet, C.; Chabanas, M.; Saint-Arroman, R. P.; Basset, J.-M. Angew. Chem. Int. Ed. 2003, 42, 156. 41
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The principles of high oxidation state alkylidene and alkylidyne chemistry extend to Re(VII)
W
N
CO
O
R
t-Bu
RRH
Re
C
CO
O
t-Bu
t-Bu
RR
H
Re(VII)W(VI)
Olefins react with the Re=CHR bond selectively, not the Re≡C-t-Bu bond.These Re species are active olefin metathesis catalysts.
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Present and future challenges
1. Prevent catalyst decomposition completely and/orfind ways to regenerate catalysts from decomposition products.
2. Find ways to generate and evaluate all catalystsin situ from one precursor.
3. Synthesize new catalysts and aim for additionalselectivity and efficiency in metathesis reactions.
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"Unsupported" M=M bonds are formed in bisalkoxide systems
N
W
(CF3)Me2CO(CF3)Me2CO
Me Me
N
WOCMe2(CF3)
OCMe2(CF3)
MeMe
N
W
(CF3)Me2CO(CF3)Me2CO
CH-t-Bu
MeCH=CHEt
Me Me
A W=W species (W=W = 2.49 Å)that does not contain
bridging groups.
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