Palladium-catalyzed aerobic dehydrogenation of substituted cyclohexanones to phenols

Article abstract of DOI:10.1126/science.1204183

Aromatic molecules are key constituents of many pharmaceuticals, electronic materials, and commodity plastics. The utility of these molecules directly reflects the identity and pattern of substituents on the aromatic ring. Here, we report a palladium(II) catalyst system, incorporating an unconventional ortho-dimethylaminopyridine ligand, for the conversion of substituted cyclohexanones to the corresponding phenols. The reaction proceeds via successive dehydrogenation of two saturated carbon-carbon bonds of the six-membered ring and uses molecular oxygen as the hydrogen acceptor. This reactivity demonstrates a versatile and efficient strategy for the synthesis of substituted aromatic molecules with fundamentally different selectivity constraints from the numerous known synthetic methods that rely on substitution of a preexisting aromatic ring.

Full text of DOI:10.1126/science.1204183

Palladium-Catalyzed Aerobic Dehydrogenation of Substituted
Cyclohexanones to Phenols
Yusuke Izawa et al.
Science 333, 209 (2011);
DOI: 10.1126/science.1204183
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References and Notes
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Es between high- and low-chalcocite phases re-
sults from the different Cu arrangements in these
two phases and the related extra Ewald energy at
the interface. This is similar to the case of the inter-
face between wurtzite (WZ) and zincblende (ZB),
where the different second nearest-neighbor atomic
positions cause different Ewald energies in these
two phases. It has been found that the interface
energy between WZ and ZB per surface unit cell
is similar to the energy difference per unit cell
(28). Thus, if we take this approximation that the
interface energy between low- and high-chalcocite
is the same as the internal energy difference be-
tween these two phases, (e1 – e2) = 40 meV per
unit cell (23), and assume that there are N Cu2S
unit formulae inside a spherical core, we have Es =
(36p)1/3N2/3(e1 – e2). Note that C = Cunit N, and
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Acknowledgments: We thank U. Dahmen at National Center
for Electron Microscopy (NCEM) and E. Borrero and
C. Dellago at University of Vienna for helpful discussions.
The authors are grateful to the support of Helios Solar
Energy Research Center (SERC) and NCEM, which are
funded by the director of the Office of Science, Office of
Basic Energy Sciences (BES), Materials Science and
Engineering Division of the U.S. Department of Energy
(DOE) under contract no. DE-AC02-05CH11231. TEM
experiments were performed using TEAM0.5 microscope
at NCEM. X-ray experiments were performed at the
Stanford Synchrotron Radiation Laboratory, a DOE-BES
user facility. J.B.R. was funded by a fellowship from Intel
Corporation, and T.A.M. and A.L. were supported by the
DOE BES Materials Sciences and Engineering Division.
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J.B.R. prepared nanorod samples, took ensemble x-ray
data, and edited the paper; B.S. contributed to nanorod
sample preparation and edited the paper; M.F.T., A.L.,
and T.A.M. provided x-ray analysis infrastructure; L.-W.W.
provided theoretical calculation and edited the paper; and
C.K. contributed to image analysis and edited the paper.
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Supporting Online Material
www.sciencemag.org/cgi/content/full/333/6039/206/DC1
Materials and Methods
SOM Text
Figs. S1 to S4
Table S1
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Movies S1 to S6
23 February 2011; accepted 25 May 2011
10.1126/science.1204713
two-step arene C–H borylation/oxidation pro-
cedure for the introduction of a hydroxyl group
into an aromatic ring, guided by steric rather
than electronic effects (2). Recent advances in
palladium-catalyzed aerobic oxidation reactions
(3–5) suggested to us that diverse phenol deriv-
atives, including those with meta substitution,
could be accessed by dehydrogenation of cyclo-
hexanones via sequential Pd-mediated C–H
activation/b-hydride elimination steps, followed by
tautomerization of the resulting dienone product
(Fig. 1A). This strategy is appealing because
PdII–hydride intermediates formed in this mech-
anism could be oxidized by molecular oxygen
(6, 7), thereby enabling the overall process to be
catalytic in Pd with water as the sole by-product
(Fig. 1B). Successful catalysts for this class of
reactions could find broad utility owing to the nu-
merous straightforward chemical reactions that
provide access to substituted cyclohexanones, in-
cluding enolate arylation and alkylation meth-
Palladium-Catalyzed Aerobic
Dehydrogenation of Substituted
Cyclohexanones to Phenols
Yusuke Izawa, Doris Pun, Shannon S. Stahl*
Aromatic molecules are key constituents of many pharmaceuticals, electronic materials, and
commodity plastics. The utility of these molecules directly reflects the identity and pattern of
substituents on the aromatic ring. Here, we report a palladium(II) catalyst system, incorporating an
unconventional ortho-dimethylaminopyridine ligand, for the conversion of substituted
cyclohexanones to the corresponding phenols. The reaction proceeds via successive
dehydrogenation of two saturated carbon-carbon bonds of the six-membered ring and uses
molecular oxygen as the hydrogen acceptor. This reactivity demonstrates a versatile and
efficient strategy for the synthesis of substituted aromatic molecules with fundamentally
different selectivity constraints from the numerous known synthetic methods that rely on
substitution of a preexisting aromatic ring.
henols are common precursors and core resents a key challenge in the preparation of these ods, conjugate addition to cyclohexenones, and
structures of industrial chemicals rang- molecules (1). Electrophilic aromatic substitutions Robinson annulation and Diels-Alder reactions
ing from pharmaceuticals to polymers. The are classical chemical reactions that remain among (Fig. 1C).
P
introduction of chemical functional groups with the most versatile methods for the synthesis of
The preparation of phenols from ketone pre-
specific patterns around the aromatic ring rep- substituted phenols; however, strong electronic cursors have been explored previously (8–16).
directing effects associated with these reactions Condensation reactions of acyclic ketones, for
limit their utility to the preparation of ortho- and example, with b-ketoaldehydes or b-diketones,
Department of Chemistry, University of Wisconsin–Madison,
para-substituted derivatives. This limitation has enable direct access to substituted phenols (8),
inspired extensive efforts to identify complemen- but low product yields, limited access to starting
tary routes to substituted phenols, such as a recent materials, and/or formation of isomeric products
1101 University Avenue, Madison, WI 53706, USA.
*To whom correspondence should be addressed. E-mail:
stahl@chem.wisc.edu
www.sciencemag.org SCIENCE VOL 333 8 JULY 2011
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REPORTS
have restricted the utility of these procedures. and thereby afford a more electron-deficient pyr- mide or iodide afforded only low yields of the
Methods for formation of phenols via dehydro- idine ligand. The position of the dimethylamino desired phenol (28% and 16%, respectively;
genation of cyclohexenones have been pursued group on the pyridine ligand is important, as not shown in Fig. 3). Diels-Alder cycloaddition
(8–13), but reactions of this type typically use revealed by the inferior results obtained with and Robinson annulation represent classical
undesirable stoichiometric reagents, such as DDQ 4-(N,N-dimethylamino)pyridine in place of the and widely used synthetic organic reactions that
(2,3-dichloro-5,6-dicyano-1,4-benzoquinone) (17); 2-substituted ligand (entries 19 and 20). Com- provide efficient routes to cyclohexenones bear-
use stepwise procedures, such as bromination/ mon heterogeneous palladium catalysts failed ing multiple substituents on the six-membered
dehydrobromination; or require harsh reaction to afford the desired m-cresol (entries 21 to 24). ring. The aerobic oxidation method described
conditions (≥200°C) that limit functional group
The optimized catalytic conditions (Fig. 2, en- here provides an attractive alternative to the use
compatibility. In contrast, no effective methods try 18) proved to be effective in the preparation of of stoichiometric reagents, such as DDQ, which
for dehydrogenation of substituted cyclohexanones a number of substituted phenol derivatives (Fig. 3). have been used previously in the dehydrogena-
exist, with relevant precedents almost exclusively Varying the position of the methyl substituent on tion of cyclohexenones (17), and such substrates
limited to reactions of unsubstituted cyclohexa- the cyclohexanone had little effect on the out- underwent very effective dehydrogenation under
none and yields of ≤30% (mostly <5%) (12–16). come of the reaction; the corresponding ortho-, the optimized catalytic conditions, including those
Iridium complexes bearing tridentate “pincer” meta-, and para-cresols were each obtained in bearing alkyl, aryl, and/or ester substituents (en-
ligands are among the most effective catalysts good yield (Fig. 3 entries 1 to 3). Aryl-substituted tries 12 to 17).
for the dehydrogenation of saturated hydrocar- phenols, including a number of meta-substituted
The 3,5-disubstituted cyclohexenones were pur-
bons (18, 19). These reactions are typically car- derivatives, were accessed via the dehydrogena- sued further, because the corresponding phenol
ried out in the presence of a sacrificial hydrogen tion of the corresponding arylcyclohexanone de- derivatives exhibit important biological activity
acceptor, such as norbornene or tert-butylethylene, rivatives (entries 4 to 11). The 3-arylcyclohexanones and products of this type cannot be readily prepared
and a recent investigation of the reaction of an were readily obtained via conjugate addition of by classical aromatic-substitution, metal-catalyzed
Ir-pincer complex with cyclohexanone resulted in aryl boronic acids to cyclohexenone, and both cross-coupling, or related synthetic methods. As a
stoichiometric dehydrogenation. Catalytic turn- electron-donating and electron-withdrawing groups representative example, O-terphenylcarbamate 5
over was inhibited by the formation of an Ir- were tolerated in the dehydrogenation reaction. was recently identified as a potent in vitro alloste-
phenoxide product, (pincer)Ir(H)(OPh) (20).
In order to explore prospects for the proposed
Pd-catalyzed dehydrogenation methods, we inves-
tigated the reactivity of 3-methylcyclohexenone
(1a) and 3-methylcyclohexanone (1b) under 1 atm
of O2 with various PdII catalysts. Preliminary anal-
ysis of PdII sources, solvents, and reaction condi-
tions revealed that 3-methylphenol (meta-cresol)
could be obtained in modest yields from 1a
and 1b (28 to 52%) with 3 mol % Pd(OAc)2 or
Pd(TFA)2 (OAc is an acetate group and TFA is
trifluoroacetate) in dimethylsulfoxide (DMSO)
as the solvent under 1 atm of O2 at 80°C (Fig. 2,
entries 1 and 2) (21, 22). Brønsted bases, such as
sodium acetate, and traditional monodentate and
bidentate nitrogen ligands (pyridine, bipyridine,
and phenanthroline) failed to increase the yield of
the phenol product (entries 3 to 6; for expanded
screening results, see table S2). The use of 4,5-
diazafluorenone (23) led to a substantial increase
in the yield of m-cresol from 1a (78%, entry 7);
however, conversion to phenol from the satu-
rated cyclohexanone 1b was still unsatisfactory.
Both key steps in the substrate oxidation sequence,
C–H activation and b-hydride elimination (Fig.
1A), should benefit from a more electrophilic
catalyst, and recent success with 2-fluoropyridine
(2Fpy) as an electron-deficient ligand in aerobic
oxidative coupling reactions (24) prompted us to
evaluate ligands of this type in the dehydrogenation
reactions (entries 8 to 15). The use of 2Fpy as a
ligand led to a modest improvement in the yield
of 2 from 1b (44%, entry 10); however, the best
results were obtained when 2-(N,N-dimethylamino)
pyridine (2NMe2py) was used in combination with
p-toluenesulfonic acid (TsOH) (entry 18). We
speculate that the improved results obtained
with the 2NMe2py/TsOH combination relative to
those with 2NMe2 py alone (compare entries 15
and 18) reflect the ability of TsOH to protonate
Substrates with aryl groups bearing a para bro- ric inhibitor of the human luteinizing hormone
Fig. 1. Strategy for the synthesis of phenols via aerobic oxidative dehydrogenation of cyclo-
hexanone derivatives. (A) Stepwise sequence for Pd-mediated dehydrogenation of cyclohexanone.
(B) Catalytic mechanism whereby cyclohexanone dehydrogenation can be achieved with O2 as the
terminal oxidant. (C) Representative synthetic methods that afford facile access to substituted
the tertiary amine of the coordinated pyridine cyclohexanone derivatives. L, a ligand; M, metal; R, organic substituent; X, halide or pseudohalide.
210
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REPORTS
receptor, which has been implicated in fertility using 3,5-dibromophenol as the starting mate- of the biaryl intermediate 6 and the desired 3,5-
and ovarian cancer (25). This compound was pre- rial; however, introduction of the unsymmet- diarylphenol 4 (30% and 62% yields, respectively;
pared by traditional Suzuki coupling methods rical aryl substitution pattern results in low yields Fig. 4). In contrast, the 3,5-diarylcyclohexenone
Fig. 2. Catalyst optimization for
aerobic oxidative dehydrogenation
of cyclohexanone derivatives 1a
and 1b. Reaction conditions are
as follows: cyclic ketone (1.0 mmol),
PdX2 (0.03 mmol), ligand/TsOH
(mol % indicated), DMSO (0.4 ml),
80°C, 1 atm O2, 24hours. Et, ethyl
group; Me, methyl group; TsOH,
p-toluenesulfonic acid. Yields de-
termined by gas chromatography.
Fig. 3. Palladium-catalyzed aerobic dehydrogenation of cyclic ketones. Reaction conditions are as follows: cyclic ketone (1.0 mmol), Pd(TFA)2 (0.03 or 0.05 mmol),
2-(N,N-dimethylamino)pyridine (0.06 or 0.10 mmol), TsOH (0.12 or 0.20 mmol), DMSO (0.4 ml), 1 atm O2, 80°C, 24 hours. Isolated product yields are reported.
www.sciencemag.org SCIENCE VOL 333 8 JULY 2011
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REPORTS
Fig. 4. Application of palladium-catalyzed aerobic oxidative dehydrogenation in the preparation of a terphenyl-derived allosteric inhibitor of the luteinizing
hormone receptor. Ph, phenyl group.
derivative 3 is obtained readily from very inex-
pensive starting materials (4-methylacetophenone,
benzaldehyde, and acetone) via sequential aldol
condensation and Robinson annulation. Subse-
quent Pd-catalyzed dehydrogenation of 3 af-
forded phenol 4 in excellent yield.
Preliminary mechanistic analysis of these de-
hydrogenation reactions was performed by mon-
itoring the conversion of cyclohexanone to phenol
by gas chromatography. The kinetic time course
revealed the formation and disappearance of the
partially dehydrogenated intermediate, cyclohex-
enone (Fig. 5A). This result is consistent with the
overall catalytic mechanism in Fig. 1B, in which
the substrate dissociates from the catalyst after
each dehydrogenation step. A fit of the kinetic
data based on a simple sequential reaction model
reveals that the two dehydrogenation steps have
similar rate constants, k1 = 0.12(T0.02) hour−1 and
k2 = 0.33(T0.04) hour−1 (Fig. 5B). The dehydro-
genation of cyclohexenone, monitored indepen-
dently, exhibits a rate constant somewhat lower
than that obtained from the fit of the sequential
reaction, k2' = 0.19(T0.02) hour−1. Accurate in-
terpretation of these results will require further
investigation; however, the higher concentration of
cyclohexenone in the latter reaction may slow the
catalytic turnover via alkene coordination to Pd.
The joint application of catalyst screening
and mechanistic studies will play an important
role in extending these reactions to different pro-
duct classes. For example, catalysts that can
effect the first step substantially more rapidly
than the second step (k1:k2 > 10:1) would enable
selective formation of the enone products rather
than phenols. Moreover, it should be possible to
develop efficient catalysts for dehydrogenative
aromatization of other substrate classes, such as
substituted cyclohexenes (26–28). Substrates of
this type are readily accessed via Diels-Alder
cycloaddition reactions, and their dehydrogen-
ation could proceed via sequential allylic C–H
activation/b-hydride elimination steps (Fig. 5C).
Toward this end, preliminary studies reveal that
Fig. 5. (A) Kinetic profile of PdII-catalyzed aerobic dehydrogenation of cyclohexanone and cyclohexenone,
showing the formation and decay of cyclohexenone as an intermediate in the reaction. Reaction condi-
tions are as follows: cyclohexanone (0.5 mmol), Pd(TFA)2 (0.025 mmol), 2-(N,N-dimethylamino)pyridine
(0.05 mmol), TsOH (0.10 mmol), DMSO (0.5 ml), 1 atm O2, 80°C. Error bars represent standard devia-
tions from five independent measurements. (B) Comparison of the rate constants obtained for the dehy-
drogenation steps in the sequential conversion of cyclohexanone to phenol and in the direct dehydrogenation
of cyclohexenone. (C) General strategy and a specific example of Pd-catalyzed aerobic dehydrogenation
of cyclohexenes.
Methods for selective dehydrogenation of sat- spective utility of such methods in the synthesis of
cyclohexene derivative 7 undergoes efficient urated carbon-carbon bonds represent an im- substituted aromatic molecules. These reactions
dehydrogenation to the trisubstituted arene 8 in portant class of C–H functionalization (18, 29), and achieve high conversions and product yields, they
near-quantitative yield.
the reactions presented above highlight the pro- are capable of using O2 as the hydrogen acceptor,
212
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REPORTS
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Supporting Online Material
www.sciencemag.org/cgi/content/full/science.1204183/DC1
Materials and Methods
Figs. S1 to S20
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during magma eruption. Melt inclusions have
been used for decades to determine pre-eruptive
volatile contents of terrestrial magmas from sub-
duction zones (9, 10), hotspots (11, 12), and mid-
ocean ridges (13, 14), as well as volatile contents
of martian magmas (15, 16). Using standard pet-
rographic methods, we identified nine inclusion-
bearing olivine crystals (Fig. 1) and analyzed melt
inclusions hosted within (17). The measured wa-
ter contents of the melt inclusions range from
615 ppm to a maximum of 1410 ppm (Fig. 2);
these water contents are up to 100 times as high
as the water content of the matrix glass surround-
ing the olivine crystals (6 to 30 ppm H2O) and
the centers of individual volcanic glass beads
from the same sample (4). The melt inclusions
also contain high concentrations of fluorine (50
to 78 ppm), sulfur (612 to 877 ppm), and chlorine
(1.5 to 3.0 ppm) that are 2 to 100 times as high as
High Pre-Eruptive Water Contents
Preserved in Lunar Melt Inclusions
2
2
Erik H. Hauri,1 Thomas Weinreich, Alberto E. Saal,
*
Malcolm C. Rutherford,2 James A. Van Orman3
The Moon has long been thought to be highly depleted in volatiles such as water, and indeed
published direct measurements of water in lunar volcanic glasses have never exceeded 50 parts per
million (ppm). Here, we report in situ measurements of water in lunar melt inclusions; these
samples of primitive lunar magma, by virtue of being trapped within olivine crystals before
volcanic eruption, did not experience posteruptive degassing. The lunar melt inclusions contain
615 to 1410 ppm water and high correlated amounts of fluorine (50 to 78 ppm), sulfur (612 to
877 ppm), and chlorine (1.5 to 3.0 ppm). These volatile contents are very similar to primitive
terrestrial mid-ocean ridge basalts and indicate that some parts of the lunar interior contain as
much water as Earth’s upper mantle.
he Moon is thought to have formed in a lunar magmas at concentrations up to 46 parts per those of the matrix glasses and individual glass
giant impact collision between a Mars- million (ppm) (4), and water contents of lunar beads from this sample (Fig. 3). Volatile contents
sized object and an early-formed proto- apatite grains from mare basalts are consistent corrected for postentrapment crystallization are
T
Earth (1). Though all of the inner planets, with similarly minor amounts of water in prim- on average 21% lower than the measured con-
including Earth, are depleted in water and other itive lunar magmas (5–7). These results indicate centrations (17) and represent the best estimate
volatiles when compared with primitive meteor- that the Moon is not a perfectly anhydrous plan- of the pre-eruptive concentrations of volatiles
ites, the more extreme depletion of volatiles in etary body and suggest that some fraction of the in the 74220 magma.
lunar volcanic rocks has long been taken as key Moon’s observed depletion in highly volatile el-
There are few descriptions in the literature of
evidence for a giant impact that resulted in high- ements may be the result of magmatic degassing melt inclusions contained within olivine from
temperature catastrophic degassing of the mate- during the eruption of lunar magmas into the near- lunar samples, but these existing observations
rial that formed the Moon (2, 3). However, recent vacuum of the Moon’s surface.
provide important context for the volatile abun-
work on rapidly quenched lunar volcanic glasses To bypass the process of volcanic degassing, dances we have observed. Roedder and Weiblen
has detected the presence of water dissolved in we conducted a search for lunar melt inclusions (18–20) noted the presence of silicate melt in-
in Apollo 17 sample 74220, a lunar soil contain- clusions in the first samples returned from the
ing ~99% high-Ti volcanic glass beads, the so- Apollo 11, Apollo 12, and Luna 16 missions and
called orange glass with 9 to 12 weight % (wt %) reported that many primary melt inclusions con-
TiO2 (8). Melt inclusions are small samples of tained a vapor bubble, requiring dissolved vola-
magma trapped within crystals that grow in the tiles to have been present in the melt at the time
magma before eruption. By virtue of their en- it was trapped within the host crystal. Klein and
closure within their host crystals, melt inclusions Rutherford (21) and Weitz et al. (22) found sul-
are protected from loss of volatiles by degassing fur contents of 600 to 800 ppm, similar to our
1Department of Terrestrial Magnetism, Carnegie Institution
of Washington, Washington, DC 20015, USA. 2Department of
Geological Sciences, Brown University, Providence, RI 02912,
USA. 3Department of Geological Sciences, Case Western Re-
serve University, Cleveland, OH 44106, USA.
*To whom correspondence should be addressed. E-mail:
ehauri@ciw.edu
www.sciencemag.org SCIENCE VOL 333 8 JULY 2011
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