Photocatalytic carbon-carbon σ-bond anaerobic oxidation of ketones with water by rhodium(III) porphyrins

Article abstract of DOI:10.1021/om400672t

Photocatalytic carbon-carbon σ-bond oxidation of unstrained ketones by water using rhodium(III) porphyrin catalyst was accomplished. The catalysis yielded the corresponding one-carbon-less carbonyl compound and H₂ with up to 30 turnovers in bot

Full text of DOI:10.1021/om400672t

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pubs.acs.org/Organometallics
Photocatalytic Carbon−Carbon σ‑Bond Anaerobic Oxidation of
Ketones with Water by Rhodium(III) Porphyrins
Siu Yin Lee and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China
S
* Supporting Information
ABSTRACT: Photocatalytic carbon−carbon σ-bond oxida-
tion of unstrained ketones by water using rhodium(III)
porphyrin catalyst was accomplished. The catalysis yielded the
corresponding one-carbon-less carbonyl compound and H2
with up to 30 turnovers in both aliphatic and cyclic ketones
with α substituents. No carbon loss was observed in aromatic
ketone. Mechanistic studies suggest that (Ph3P)RhIII(ttp)OH (ttp = tetratolylporphyrinato dianion) is the key intermediate in
the carbon−carbon σ-bond anaerobic oxidation.
of ketone suggests that RhIII(ttp)OH generated from the
INTRODUCTION
Catalytic cleavage of carbon−carbon σ bonds has recently been
■
hydrolysis of RhIII(ttp)Me is the proposed key intermediate in
accentuated in organometallic research,1 since it has emerged as
the CCA step, in accordance with the reaction stoichiometry.
However, as the strong Rh−C(CO) bond (54 kcal mol−1)16
an important strategy for structural modification in synthetic
chemistry.2 However, catalytic carbon−carbon bond activation
(CCA) is kinetically very challenging, as carbon−hydrogen
remains intact even at the high temperature of 200 °C in the
presence of H2O, the high stability limits further development
bond activation (CHA) is usually a more competitive process,3
in thermal catalysis.15
as a result of the more abundant and more easily accessible C−
To further functionalize the thermally and hydrolytically
H bonds in hydrocarbons.4 With the aid of the robust
stable Rh−C(CO) bond, we took advantage of the known
metalloporphyrins that can endure high temperatures, certain
homolytic cleavage of (tmp)Rh−Me (tmp = tetramesitylpor-
phyrinato dianion) to generate RhII(tmp) through photolysis.17
Photolysis at λ > 350 nm provides extra energy up to 81.6 kcal
mol−1 to cleave the Rh−C(CO) bond homolytically for further
catalytic reaction. Metalloporphyrin-based photocatalysis is a
growing area due to the attractive high visible-light absorptivity
of metalloporphyrins.18 Examples include photocatalytic hydro-
gen generation,19 photocatalytic dehydrogenation from alco-
hol,20 photoreduction of carbon dioxide to carbon monoxide,21
and photooxidation of phenol22 and aromatic aldehyde.23
Catalytic oxidation of the σ carbon−carbon bond using water
is another key focus in this report. Current research on
photocatalysis using H2O as the oxygen source is well
exemplified in the hydroxylation of silyl alkane,24 epoxidation
of alkenes,25−27 and oxidation of alcohols26−29 and sul-
fides.27,29,30
stoichiometric selective CCA under thermal conditions have
been developed over the past decade.5−7
Despite the success in stoichiometric reactions, catalytic
functionalization strategies thus far are quite limited. Murakami
and Ito et al.8 developed the strain-released CCA of
cyclobutanone derivatives catalyzed by rhodium(I). Suggs9
and Jun et al.9,10 discovered the CCA of acylquinolines assisted
by the chelation of substrate on the rhodium catalyst. Chatani
et al.11 also reported the decarbonylative CCA in aromatic
ketones bearing an oxazoline or pyridine as the chelating group
on ruthenium. Other catalytic CCA examples include the
catalytic hydrogenation of C(sp2)−C(sp2) bonds in biaryls, as
illustrated by Jones et al.,12 and the rhodium-catalyzed C(sp2)−
C(sp3) bond hydrogenation or hydrosilylation of diphosphine
mesitylene derivatives documented by Milstein et al.13 The
catalytic C(sp3)−C(sp3) bond hydrogenation of [2.2]-
paracyclophane was also recently reported by the Chan
group.14 In addition to these remarkable advances, mild and
selective catalytic CCA on nonstrained substrates remains rare.
The Chan group has reported the stoichiometric CCA of 2,6-
dimethylcyclohexanone by RhIII(ttp)Me to give RhIII(ttp)-
COCHMe(CH2)3COMe (eq 1).15 The water-promoted CCA
Herein, we present the photocatalytic oxidation of the
stronger and more robust carbon−carbon bond in unstrained
ketones using rhodium(III) porphyrin catalysts and water
under anaerobic conditions to yield the corresponding oxygen-
incorporated organics and H2 (Scheme 1). H2O serves as the
oxygen source in the one-pot reaction.
Received: July 8, 2013
Published: September 19, 2013
© 2013 American Chemical Society
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dx.doi.org/10.1021/om400672t | Organometallics 2013, 32, 5391−5401

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is more stable at lower temperature to account for the
improved TONs. RhIII(ttp)OH ((ttp)Rh−OH, 46 kcal
mol−1)31 is known to be thermally unstable over 60 °C to
give RhII(ttp) and H2O2.32 A higher loading of water also
enhanced the TONs significantly (Table 1, entries 6−8). At
higher H2O concentration, the equilibrium of Rh(II)
disproportionation by H2O would be shifted to maintain a
higher concentration of (Ph3P)RhIII(ttp)OH (eq 3),33 support-
Scheme 1. Catalytic Carbon−Carbon Bond Oxidation with
H2O
RESULTS AND DISCUSSION
■
Initially, the light-induced catalytic carbon−carbon σ-bond
oxidation of 2,6-dimethylcyclohexanone by RhIII(tmp)Me was
achieved under ambient conditions to yield 2-heptanone in 9
turnovers (detected by GC/MS) using a 400 W mercury halide
lamp (eq 2).
ing its key role in catalysis. A 350 nm UV filter was applied to
eliminate the Norrish type I or II reaction in all of the
photolysis reactions.34 A control experiment was also carried
out to confirm the catalytic role of rhodium porphyrins.
Without rhodium porphyrins, no catalysis occurred (Table 1,
entry 9). After screening through different conditions, the
reaction was optimized at 6−10 °C and 500 equiv of H2O with
RhIII(ttp)Me as the catalyst.
To our delight, the catalysis is also applicable to various
ketones under the optimized conditions (Table 2). Photo-
To improve the catalytic efficiency, the effects of (1)
porphyrin ligand, (2) donor ligand loading, (3) temperature,
(4) water concentration, and (5) solvent polarity on catalysis
were studied independently. The sterically less hindered
RhIII(ttp)Me catalyzed the transformation to give higher
turnovers than for RhIII(tmp)Me (Table 1, entries 1 and 2).
The catalysis was promoted by the addition of PPh3 (Table 1,
entries 2 and 3). This suggests that the phosphine ligand
effectively trapped RhIII(ttp)OH to generate a higher effective
concentration of (Ph3P)RhIII(ttp)OH at 6−10 °C. Lower
temperatures promoted the catalytic activity of RhIII(ttp)Me
(Table 1, entries 2, 4, and 5). Presumably, (Ph3P)RhIII(ttp)OH
Table 2. Substrate Scope of Photocatalytic CCA by
RhIII(ttp)Me
Table 1. Optimization of Photocatalysis
amt of
H2O
TOF
temp
(10−4
a
entry
1
catalyst
(°C)
(equiv)
solvent
TON
12.6
s−1
)
Rh(tmp)
Me
25−30
25−30
25−30
50−60
6−10
500
500
500
500
500
50
4.4
2
Rh(ttp)
Me
21.1
11.3
15.9
29.6
7.8
7.3
b
3
Rh(ttp)
Me
3.9
4
5
6
7
8
9
Rh(ttp)
Me
5.5
a
A 400 W quartz mercury halide lamp was used. A B+W 67 mm MRC
Rh(ttp)
Me
10.3
2.7
b
UV filter was used for cutting off light rays below 350 nm. Reaction
Rh(ttp)
Me
6−10
temperature 50−60 °C.
Rh(ttp)
Me
6−10
100
1000
9.5
3.3
catalytic CCA of diisopropyl ketone successfully yielded
acetone (Table 2, entry 1). However, propane, as expected
from the reaction stoichiometry, was not observed due to the
detection difficulty of low-molecular-weight compounds. Cyclic
ketones were then employed as intramolecular traps. Photo-
catalytic activation of 2-methylcyclohexanone gave a better
yield of 2-hexanone with the selective cleavage of the more
hindered but weaker (CO)−CH(Me)R bond (∼81.3 kcal
mol−1)35 in comparison to the (CO)−CH2R bond (∼84.1
kcal mol−1)35 (Table 2, entry 2). 2,6-Dimethylcyclohexanone,
with an extra isopropyl moiety, was photolyzed to yield 2-
heptanone with a much improved turnover (Table 2, entry 3).
Rh(ttp)
Me
6−10
31.0
10.7
no catalyst
50−60
6−10
500
500
0
0
c
10
Rh(ttp)
Me
benzene
acetone
19.3
6.7
c
11
Rh(ttp)
Me
6−10
500
18.6
6.5
a
A 400 W quartz mercury halide lamp was used. A B+W 67 mm MRC
b
UV filter was used for cutting off light rays below 350 nm. No PPh3
was added. Conditions: 1000 equiv of 2,6-dimethylcyclohexanone,
concentration 2.57 M; solvent-free conditions, concentration 7.33 M.
c
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1,3-Diphenylacetone was also photocatalytically activated to
give benzaldehyde and toluene (Table 2, entry 4), though with
a lower turnover number. The high melting point of 1,3-
diphenylacetone (34 °C)36 restricted the reaction temperature
at 60 °C; the higher reaction temperature probably reduces the
effective concentration of (Ph3P)Rh(ttp)OH32 and accounts
for the lower efficiency. When the unsymmetric isobutyr-
ophenone was used, the more hindered but weaker (C
O)−iPr bond (∼81.3 kcal mol−1)35 was selectively cleaved over
the (CO)−Ph bond (∼97.2 kcal mol−1);35 however,
benzaldehyde was catalytically formed instead of benzene
(Table 2, entry 5). This is ascribed to the much slower
decarbonylation rate in benzoyl radical (PhCO), which has a
much lower rate constant of 1.5 × 10−7 s−1 at 296 K in
comparison to other acyl radicals on the order of 104 s−1.37
Attempts to develop photocatalytic CCA on aldol-conden-
sable cyclohexanone were unsuccessful. Three reasons have
been proposed to account for this observation. First, aldol
condensation can be a competitive process to form the
conjugated enone, and this consumes the starting material, as
Rh(por) can serve as a Lewis acid to catalyze the reaction.15
Second, the reverse intramolecular ring closing proceeds at a
much faster rate than any subsequent intermolecular reactions
(Scheme 2). Third, the (CO)−CH2R bond is 3 kcal mol−1
generate the acyl radical 1 and RhII(ttp) monomer to complete
the catalytic cycle. Facile decarbonylation of the acyl radical
occurs simultaneously to give the alkyl radical 2.37 2 then
undergoes hydrogen atom transfer (HAT) from the bulk 2,6-
dimethylcyclohexanone solvent to form 2-heptanone. The
decarbonylation accounts for the one-carbon-less 2-heptanone
obtained in the photolysis of 2,6-dimethylcyclohexanone. The
only exception is in the case of isobutyrophenone; the slow
decarbonylation cannot compete with HAT, so that the CO
group remains intact (Scheme 4). Finally, the tertiary alkyl
radical 3 formed after HAT then abstracts H from Rh(ttp)H to
complete the photocatalysis. Overall, there is an anaerobic
photocatalytic oxidation of ketone using H2O with the CCA
step involving PPh3-coordinated rhodium(III) porphyrin
hydroxide.40
The radical mechanism is supported by a control experiment
using (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as the
radical trap. TEMPO significantly retarded the photocatalytic
C(CO)−C(α) bond oxidation of 2,6-dimethylcyclohexanone
with only 3.5 turnovers obtained under the optimized catalytic
conditions (eq 5).
Scheme 2. Fast Intramolecular Ring Closing
After the photocatalysis, the highly air-sensitive RhII(ttp)
cannot be regenerated. The instability of RhII(ttp) is attributed
to its reaction with O2 presumably generated from the
reduction of RhIII(ttp)OH to give H2O2, which further
dissociates into H2O and O2.32 The catalyst deactivation also
accounts for the limited turnover number observed.
The key steps in the proposed mechanism were supported by
independent experiments. The disproportionation of Rh(II)
with H2O was confirmed by H218O labeling. The photolysis of
2,6-dimethylcycylohexanone with H218O catalyzed by RhII(ttp)-
Me afforded 18O-enriched 2-heptanone and 16O 2-heptanone in
a 4:1 ratio and 27 turnovers (eq 6, Figure S1 in the Supporting
stronger than the (CO)−iPr bond,35 which makes the CCA
more difficult. We have also previously reported that the
stoichiometric CCAs of unsubstituted ketones are much less
reactive than those of isopropyl ketones.15 A high temperature
of 200 °C is required to achieve CCA in diethyl ketone by
Rh(ttp)Me to give Rh(ttp)COEt in 45% yield in 16 days, while
the CCA of diisopropyl ketone occurred at 50 °C in 1 day to
give Rh(ttp)COiPr in 94% yield.15
The catalysis was also operative in benzene or acetone
solvent, though with slightly lower TOFs (Table 1, entries 10
and 11). This in turn allowed us to follow the reaction progress
using 1H NMR spectroscopy. Under the same reaction
1
conditions, H2 was observed in C6D6 by H NMR with 17
turnovers in a 1:1 ratio with 2-heptanone (eq 4). This supports
the dehydrogenation to give hydrogen.
Scheme 3 shows the proposed reaction mechanism using 2,6-
dimethylcyclohexanone as the substrate. Photolysis of
RhIII(ttp)Me cleaves the Rh−C bond homolytically to give
RhII(ttp).38 PPh3 then coordinates to RhII(ttp) monomer to
generate the more electron-rich (Ph3P)RhII(ttp),39 which then
disproportionates with H2O to give (Ph3P)RhIII(ttp)OH and
RhIII(ttp)H (eq 3).33 Selective carbon(CO)−C(α) bond
activation of 2,6-dimethylcyclohexanone by (Ph3P)RhIII(ttp)-
OH occurs through σ-bond metathesis with subsequent
dehydrogenation of the alcohol to afford the stoichiometric
CCA product RhIII(ttp)COCH(CH3)(CH2)3COCH3.15 Pho-
tolysis further cleaves the Rh−C(CO) bond homolytically to
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Scheme 3. Proposed Catalytic Cycle
Scheme 4. Slow Decarbonylation in PhCO Radical
Scheme 5. Acid-Catalyzed 16O/18O Exchange and
(Ph3P)RhIII(ttp)OH-Catalyzed D2O and 16O/18O Exchange
Information). Control experiments were carried out to rule out
direct 16O/18O exchange on 2-heptanone under both photolytic
and thermal conditions, as no 18O enriched 2-heptanone was
observed (eqs 7 and 8). This suggests that H2O is the source of
oxygen incorporated in the product.
acid) catalyze the addition of H218O to form the geminal diol
hydrates 4 and 5 and transform the trigonal carbon to a
tetrahedral configuration. The extra stability gained from
restoring the distortion of the chair conformation in the
transition state leads to a more favorable addition reaction.
The photocatalytic dehydrogenation of alcohol under
alkaline conditions with rhodium porphyrin complex has
been reported by Saito et al. to yield a ketone and H2.20 The
successful detection of H2 in the photocatalyic carbon−carbon
bond oxidation of 2,6-dimethylcyclohexanone (eq 4, Figure S2
in the Supporting Information) supports the step of alcohol
dehydrogenation into ketone.42 The generation of a stoichio-
metric amount of H2 from the photolysis of RhIII(ttp)H (eq
10) is not likely, since it proceeded much more slowly than the
The partial 18O incorporation in ketone product is attributed
to two reasons. First, the commercially available H218O used is
only of 95 atom % 18O enrichment; the presence of H216O
limited the complete incorporation. Second, under the same
reaction conditions, the rapid 16O/18O exchange of ketone with
H218O was significant. In the control experiment between
H218O and 2,6-dimethylcyclohexanone without Rh(ttp)Me
catalyst, the 16O/18O exchange occurred in 1 h to give 15%
H216O (eq 9). At the end of 8 h, the recovery yield of H218O
was 69% and H216O contributes to the remaining 31% water
content in the reaction mixture (eq 9). In the photocatalytic
CCA of 2,6-dimethylcyclohexanone catalyzed by Rh(ttp)Me, a
small amount of H216O formed from exchange acts as the
oxidant during the course of the reaction and this accounts for
the 16O-2-heptanone (20%) observed after photolysis. It should
be noted that the 18O exchanged into the carbonyl oxygen of
2,6-dimethylhexanone yields C18O and does not affect the 18O
content of 2-heptanone.
photocatalytic carbon−carbon bond oxidation of ketones. The
former required at least 4 h for completeness in benzene,43
while the latter occurred in 30 turnovers within 8 h.
Furthermore, 1.5 equiv of H2 would be observed if homolytic
cleavage of (ttp)Rh−H for H2 generation was operating
according to the reaction stoichiometry (Scheme 3, dotted
arrow). In this system, H2 and 2-heptanone were observed in a
nearly 1:1 ratio (eq 4). Therefore, this pathway can be ruled
out.
A mechanism via a geminal diol intermediate can account for
the increased reactivity of 2,6-dimethylcyclohexanone in the
16O/18O exchange in comparison to that of 2-heptanone
(Scheme 5). In six-membered cyclic ketones, the trigonal-
planar carbonyl carbon distorts the stable chair conformation of
six-membered rings, making it more favorable to exchange into
a germinal diol.41 Acid and (Ph3P)Rh(ttp)OH (also a Lewis
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dx.doi.org/10.1021/om400672t | Organometallics 2013, 32, 5391−5401