Alkene Isomerization by “Sandwich” Diimine-Palladium Catalysts

Article abstract of DOI:10.1021/acs.organomet.6b00856

In contrast to traditional diimine-palladium complexes, sterically hindered “sandwich” diimine-palladium adducts act as olefin isomerization catalysts. Terminal olefins are selectively converted to 2-olefins by a sequence of migratory insertion, β-hydride elimination, and olefin displacement. The reaction is performed at 0 °C with 1 mol % of an air-stable precatalyst and tolerates functional groups such as ketones, silyl ethers, and halogens. The isomerization may be used to produce silyl enol ethers from protected allylic alcohols.

Full text of DOI:10.1021/acs.organomet.6b00856

Article
pubs.acs.org/Organometallics
Alkene Isomerization by “Sandwich” Diimine-Palladium Catalysts
Andrew L. Kocen, Kristine Klimovica, Maurice Brookhart,* and Olafs Daugulis*
Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
S
* Supporting Information
ABSTRACT: In contrast to traditional diimine-palladium complexes, sterically hindered “sandwich” diimine-palladium adducts
act as olefin isomerization catalysts. Terminal olefins are selectively converted to 2-olefins by a sequence of migratory insertion,
β-hydride elimination, and olefin displacement. The reaction is performed at 0 °C with 1 mol % of an air-stable precatalyst and
tolerates functional groups such as ketones, silyl ethers, and halogens. The isomerization may be used to produce silyl enol ethers
from protected allylic alcohols.
ate transition metal-catalyzed olefin polymerization by
bulky α-diimine-supported nickel and palladium catalysts 1
Scheme 1. Mechanistic Considerations
L
and 2 has been extensively investigated (Figure 1).1 These
Figure 1. Diimine catalysts.
complexes polymerize ethylene and α-olefins to high-molecular
weight polymers. Palladium catalysts afford hyperbranched
polymers, while the properties of material obtained by nickel
catalysts depend on the monomer concentration and temper-
ature. More recently, sandwich catalysts of type 3 and their
nickel analogues have been investigated for ethylene and α-
olefin polymerization.2
The unique feature of the diimine catalysts that allows
production of branched polymer starting from ethylene and
chain-straightened material from α-olefins is referred to as
“chain-walking”.3a Mechanistic details of this phenomenon are
shown in Scheme 1 and lead to the conclusion that, if certain
conditions are met, these complexes should be capable of acting
as efficient olefin isomerization catalysts. Migratory insertion of
palladium hydride 4 affords either linear palladium alkyl agostic
species 5 or a branched complex 7. Agostic complexes of type 5
react with an α-olefin to give alkyl palladium species of type 6,
the resting state(s) in polymerization. Alternatively, branched
agostic species of type 7 can bind olefin; migratory insertion
then affords a methyl branch in the polymer. β-Hydride
elimination from 7 can yield 8, where the double bond has
migrated into the alkyl chain. If the bound internal olefin in 8 is
displaced, 4 is regenerated together with isomerized olefin 9.
Barriers to β-hydride elimination (5 to 4 or 7 to 8) leading to
olefin hydride complexes are ca. 7 kcal/mol. The ability to
switch from a polymerization to an isomerization manifold
should be a function of the rate of chain-running and olefin
displacement (8 to 4 and 9) vs migratory insertion rates in
palladium alkyl olefin complexes such as 6. Ethylene migratory
insertion barriers in 1 (L = C2H4) are about 17 kcal/mol at −30
°C, whereas, in 3, they are 19.4 kcal/mol at −25 °C.2b,3a,b
Additional steric hindrance at the reactive center in 3 (relative
to that in 1) should result in weaker binding of bulky internal
olefin 9 with respect to the less hindered α-olefin.
Received: November 14, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.6b00856
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Consequently, under certain conditions, catalysts of type 3 may
act as olefin isomerization catalysts. Examination of the
literature shows that cationic palladium-diimine 10 acts as a
catalyst for α-olefin isomerization and oligomerization, with
TOFs of about 2100 and 200 h−1 at 25 °C, respectively
(Scheme 2).3c We speculated that palladium sandwich
Scheme 3. 1-Hexene Isomerization by 11
Scheme 2. Olefin Isomerization by Palladium-Diimine
Complexes
comparable to that for ethylene migratory insertion in
palladium sandwich-diimine complex 3.2b The experiment
shows that selective isomerization of terminal olefins to 2-
olefins by using palladium sandwich diimine complexes should
be feasible. However, use of complex 11 as a catalyst is
problematic since 11 is unstable at ambient temperatures.
Hence, a more convenient procedure was used involving
activation of stable precatalyst 12 by in situ treatment with
NaB[C6H3(CF3)2]4 and PhSiH3. This procedure is expected to
generate an active cationic palladium hydride 13 according to
earlier literature precedent (eq 1).9
complexes might act as olefin isomerization catalysts as well.
Furthermore, due to the expected weak binding of bulky
internal olefins to the palladium center crowded by extremely
bulky sandwich ligands, we expected that regioselective olefin
isomerization might be possible. We report here that terminal
olefins are selectively converted to 2-olefins by using sandwich
diimine-palladium catalysts at low temperatures. This proce-
dure allows for a simple and efficient synthesis of aldehyde-
derived silyl enol ethers from silylated allylic alcohols.
A number of olefin isomerization catalysts have been
reported in the literature.3c,4−8 Grotjahn has developed
bifunctional ruthenium catalysts that selectively isomerize 1-
alkenes to Z-2-alkenes.4 For several substrates, catalyst loadings
as low as 0.05 mol % were employed. Rhodium, iridium, and
iron catalysts have also been used for olefin isomerization.5
Several investigators report cobalt-catalyzed alkene isomer-
izations.6 Most relevant to this investigation, a number of
palladium complexes were shown to catalyze olefin isomer-
ization.3c,7 As described earlier, Bercaw has shown that cationic
palladium-diimine catalysts possessing 3,5-di-t-butylphenyl
substituents isomerize 1-hexene to a mixture of 2- and 3-
hexenes. Additionally, a substantial amount of hexene polymer-
ization is observed. The catalysis is initiated via C−H activation,
followed by Wacker oxidation, to afford an active palladium
hydride.3c Lindhardt, Skrydstrup, and Gooßen have employed
tri-t-butylphosphine-containing palladium catalysts for trans-
formation of 1-alkenes to 2-alkenes.7b,c Li and Xu have used a
palladium hydride, generated in situ from PdCl2(PPh3)2 and
(EtO)3SiH, to convert 4,4-diaryl-1-butenes to corresponding 2-
alkenes. Several other transformations include olefin double-
bond isomerization as one of the key steps.8
Results of isomerizations of a variety of olefins are
summarized in Table 1. The precatalyst 12 is activated with 3
mol % phenylsilane and 4 mol % sodium tetraarylborate.
Reactions are carried out in CDCl3 at 0 °C. 1-Hexene is
isomerized to a 2.1/1 mixture of trans- and cis-2-hexenes (entry
1). After termination of the isomerization, the reaction mixture
contains also 7% of 3-hexenes and 1.5% of 1-hexene.
Methylenecyclohexane is isomerized to trisubstituted 1-methyl-
cyclohexene (entry 2). Acyclic 2-methyl-1-pentene affords 2-
methyl-2-pentene in 85% yield (entry 3), with ca. 8% of starting
material remaining. Allylbenzene is converted to the conjugated
alkene in high yield (entry 4). Interestingly, butenylbenzene is
predominately isomerized to the nonconjugated isomer, with
only 6% overisomerization to the conjugated product (entry 5).
Hepta-1,7-diene gives 40% conversion to doubly isomerized
hepta-2,5-diene, with most of the residual material being the
monoisomerization product (entry 6). Ketone-containing
substrates are compatible with reaction conditions as well
(entry 7). Isomerization of 5-bromo-1-pentene gives 64% of 5-
bromo-2-pentene as a 1.9/1 trans/cis mixture (entry 8).
Synthetically useful silyl-protected alcohols are also reactive.
Triisopropylsilyl-protected pentenol is isomerized to the 2-
isomer in 59% yield (entry 9). t-Butyldimethylsilyl protected
methallyl alcohol affords isobutyraldehyde enol silane in 55%
yield (entry 10). Protected allyl alcohol gives propionaldehyde
enol silane in a good yield (entry 11). This reaction was
repeated on a 20 mmol scale, and the product was isolated in
59% yield. The corresponding trimethylsilyl-protected alcohols
did not give isomerization products, and free alcohols are
unreactive.
In the initial experiment, ca. 15 equiv of 1-hexene was added
to complex 11 in CD2Cl2 at −78 °C (Scheme 3). Following
warming to −5 °C, nearly complete isomerization to a mixture
of trans- and cis-2-hexenes was observed in about 1.5 h. Only
minor amounts of hexene oligomers and 3-hexene were
detected. The initial rates of 1-hexene disappearance at different
initial concentrations of the olefin at 268 K were examined and
showed a clear dependence on 1-hexene concentration (see the
Supporting Information for details). The first-order rate
constant for the isomerization of 1-hexene, k = 8·10−4 s−1,
corresponds to ΔG⧧ = 19.4 kcal mol −1. This barrier is
Mechanistically, several possibilities can lead to selective
formation of 2-olefins. The most likely scenario involves
palladium migration along the alkyl chain by one carbon, β-
hydride elimination, and internal olefin displacement (4 to 7 to
8 to 9, Scheme 1). This set of events was corroborated by
B
DOI: 10.1021/acs.organomet.6b00856
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Table 1. Pd-Catalyzed Isomerization of Alkenes: Reaction
Scope
EXPERIMENTAL SECTION
■
a
General Procedure for Isomerization of 1-Olefins to 2-
Olefins. A 2 dram vial with a screw cap (PTFE/Liner) equipped with
a magnetic stir bar was charged with NaBArF (7 mg, 8 μmol) and
olefin (0.2 mmol). Freshly prepared solutions of catalyst 12 (5 mM in
CDCl3) and PhSiH3 (60 mM in CDCl3) were cooled to 0 °C. The
precooled catalyst solution (0.4 mL, 2 μmol) was added to the vial,
followed by the addition of the precooled silane solution (0.1 mL of a
60 mM solution in CDCl3, 6 μmol) at 0 °C. Sequence of addition is
crucial for obtaining high yields. The vial was sealed, placed in a
cooling bath at 0 °C, and allowed to stir for the specified time. After
the reaction had finished, tert-butyl isocyanide (approximately 6 μL,
>20 equiv to catalyst) was added, and the mixture was shaken to
quench the reaction. The solution immediately turned from deep red
to orange. An internal standard (1,4-bis(trifluoromethyl)benzene (7.8
μL, 0.050 mmol) or pyridine (8.1 μL, 0.010 mmol)) was added to the
vial, and the whole reaction mixture was transferred to an NMR tube.
Either the allylic −CH3, allylic −CH2, or olefin signal was used for
quantification based on the best resolution in the spectra. Spectra were
taken with a delay time of 20 s. The signal for tert-butyl isocyanide
used to quench the reaction appears at 1.42 ppm.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00856.
Detailed experimental procedures and characterization
data for new compounds (PDF)
a
Reactions were quenched by adding tBuNC. Yields were determined
AUTHOR INFORMATION
Corresponding Authors
*E-mail: mbrookha@central.uh.edu (M.B.).
*E-mail: olafs@uh.edu (O.D.).
1
■
by H NMR analysis against an internal standard. See the Supporting
Information for details. 3-Hexenes (7%) also formed. Over-
isomerization product but-1-en-1-ylbenzene also formed (6%). The
b
c
d
remaining material is an isomeric mixture of hepta-1,5-dienes.
e
f
Overisomerization product 3-hexen-2-one also formed (4%). Large-
ORCID
scale (20 mmol) reaction in CH2Cl2; isolated yield.
Olafs Daugulis: 0000-0003-2642-2992
examining isomerization of 1-13C-1-decene 14 (Scheme 4).
After subjecting 14 to standard reaction conditions, 75% of
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Scheme 4. Isomerization of Labeled 1-Decene
■
This research was funded by the Welch Foundation (Chair E-
0044 to O.D., Grant E-1893 to M.B.) and NSF DMR-1507664
(to O.D). We thank Dr. Jesus Campos for initial observation of
́
hexene isomerization in Scheme 3.
REFERENCES
■
(1) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000,
100, 1169−1204. (b) Mecking, S. Angew. Chem., Int. Ed. 2001, 40,
534−540. (c) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103,
283−316. (d) Camacho, D. H.; Guan, Z. Chem. Commun. 2010, 46,
7879−7893.
1-13C-2-decene was obtained as a 1.8 E/Z isomer mixture,
together with minor amounts of internal olefins and unreacted
14. An alternative scenario that includes fast chain-running of
palladium along the aliphatic chain, followed by a displacement
of 2-olefin (in preference to terminal and 3-olefin), would result
in about 1/1 mixture of 1-13C-2-decene and 10-13C-2-decene.
Since the latter is not observed in the reaction mixture, this
mechanistic pathway can be discounted.6d
In conclusion, we have shown that sterically hindered
sandwich diimine-palladium complexes act as olefin isomer-
ization catalysts. Terminal olefins are selectively converted to 2-
olefins by a sequence of migratory insertion, β-hydride
elimination, and olefin displacement reactions. The reaction
tolerates functional groups such as ketones, silyl ethers, and
halogens. The isomerization may be used to produce silyl enol
ethers from protected allylic alcohols.
(2) (a) Zhang, D.; Nadres, E. T.; Brookhart, M.; Daugulis, O.
Organometallics 2013, 32, 5136−5143. (b) Allen, K. E.; Campos, J.;
Daugulis, O.; Brookhart, M. ACS Catal. 2015, 5, 456−464. (c) Vaidya,
T.; Klimovica, K.; LaPointe, A. M.; Keresztes, I.; Lobkovsky, E. B.;
Daugulis, O.; Coates, G. W. J. Am. Chem. Soc. 2014, 136, 7213−7216.
(3) (a) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem. Soc.
2001, 123, 11539−11555. (b) Tempel, D. J.; Johnson, L. K.; Huff, R.
L.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 2000, 122, 6686−
6700. (c) Winston, M. S.; Oblad, P. F.; Labinger, J. A.; Bercaw, J. E.
Angew. Chem., Int. Ed. 2012, 51, 9822−9824.
(4) (a) Larsen, C. R.; Erdogan, G.; Grotjahn, D. B. J. Am. Chem. Soc.
2014, 136, 1226−1229. (b) Larsen, C. R.; Grotjahn, D. B. J. Am. Chem.
Soc. 2012, 134, 10357−10360.
C
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Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(5) (a) Zhuo, L.-G.; Yao, Z.-K.; Yu, Z.-X. Org. Lett. 2013, 15, 4634−
4637. (b) Ohmura, T.; Shirai, Y.; Yamamoto, Y.; Miyaura, N. Chem.
Commun. 1998, 1337−1338. (c) Mayer, M.; Welther, A.; von
Wangelin, A. J. ChemCatChem 2011, 3, 1567−1571. (e) Sivaramak-
rishna, A.; Mushonga, P.; Rogers, I. L.; Zheng, F.; Haines, R. J.;
Nordlander, E.; Moss, J. R. Polyhedron 2008, 27, 1911−1916.
(6) (a) Crossley, S. W. M.; Barabe,
2014, 136, 16788−16791. (b) Schmidt, A.; Nodling, A. R.; Hilt, G.
́
F.; Shenvi, R. A. J. Am. Chem. Soc.
̈
Angew. Chem., Int. Ed. 2015, 54, 801−804. (c) Kobayashi, T.;
Yorimitsu, H.; Oshima, K. Chem. - Asian J. 2009, 4, 1078−1083.
(d) Chen, C.; Dugan, T. R.; Brennessel, W. W.; Weix, D. J.; Holland,
P. L. J. Am. Chem. Soc. 2014, 136, 945−955. Ni catalysis: (e) Wille, A.;
Tomm, S.; Frauenrath, H. Synthesis 1998, 1998, 305−308.
(7) (a) Davies, N. R. Aust. J. Chem. 1964, 17, 212−218. (b) Gauthier,
D.; Lindhardt, A. T.; Olsen, E. P. K.; Overgaard, J.; Skrydstrup, T. J.
Am. Chem. Soc. 2010, 132, 7998−8009. (c) Mamone, P.; Grunberg, M.
̈
F.; Fromm, A.; Khan, B. A.; Gooßen, L. J. Org. Lett. 2012, 14, 3716−
́ ́
3719. (d) Larionov, E.; Lin, L.; Guenee, L.; Mazet, C. J. Am. Chem. Soc.
2014, 136, 16882−16894. (e) Lin, L.; Romano, C.; Mazet, C. J. Am.
Chem. Soc. 2016, 138, 10344−10350. (f) Curran, K.; Risse, W.; Hamill,
M.; Saunders, P.; Muldoon, J.; de la Rosa, R. A.; Tritto, I.
Organometallics 2012, 31, 882−889. (g) Bai, X.-F.; Song, T.; Deng,
W.-H.; Wei, Y.-L.; Li, L.; Xia, C.-G.; Xu, L.-W. Synlett 2014, 25, 417−
422. (h) Lim, H. J.; Smith, C. R.; RajanBabu, T. V. J. Org. Chem. 2009,
74, 4565−4572. (i) Spallek, M. J.; Stockinger, S.; Goddard, R.; Trapp,
O. Adv. Synth. Catal. 2012, 354, 1466−1480. (j) Kochi, T.; Hamasaki,
T.; Aoyama, Y.; Kawasaki, J.; Kakiuchi, F. J. Am. Chem. Soc. 2012, 134,
16544−16547. (k) Hamasaki, T.; Aoyama, Y.; Kawasaki, J.; Kakiuchi,
F.; Kochi, T. J. Am. Chem. Soc. 2015, 137, 16163−16171.
́
(8) (a) Perez, M.; Hounjet, L. J.; Caputo, C. B.; Dobrovetsky, R.;
Stephan, D. W. J. Am. Chem. Soc. 2013, 135, 18308−18310.
(b) Werner, E. W.; Mei, T.-S.; Burckle, A. J.; Sigman, M. S. Science
2012, 338, 1455−1458. (c) Mei, T.-S.; Patel, H. H.; Sigman, M. S.
Nature 2014, 508, 340−344. (d) Wakamatsu, H.; Nishida, M.; Adachi,
N.; Mori, M. J. Org. Chem. 2000, 65, 3966−3970. (e) Millet, A.; Larini,
P.; Clot, E.; Baudoin, O. Chem. Sci. 2013, 4, 2241−2247.
(9) LaPointe, A. M.; Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1997,
119, 906−917.
D
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