Mild and selective C(CO)-C(α) bond activation of ketones with rhodium(III) porphyrin β-hydroxyethyl

Article abstract of DOI:10.1021/om3009519

Rhodium(III) porphyrin β-hydroxyethyl, Rh^III(ttp)CH ₂CH₂OH (ttp = 5,10,15,20-tetratolylporphyrinato dianion), was found to serve as a precursor of the highly reactive Rh^III(ttp)OH for the C(CO)-C(α) bond activation (CCA) of ketones under mild and aerobic conditions of 25-50 C.

Full text of DOI:10.1021/om3009519

Article
pubs.acs.org/Organometallics
Mild and Selective C(CO)−C(α) Bond Activation of Ketones with
Rhodium(III) Porphyrin β‑Hydroxyethyl
Chung Sum Chan, 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: Rhodium(III) porphyrin β-hydroxyethyl,
RhIII(ttp)CH2CH2OH (ttp = 5,10,15,20-tetratolylporphyrinato
dianion), was found to serve as a precursor of the highly reactive
RhIII(ttp)OH for the C(CO)−C(α) bond activation (CCA) of
ketones under mild and aerobic conditions of 25−50 °C.
INTRODUCTION
■
Carbon−carbon bond activation (CCA) has attracted much
research interest in recent decades, as it is a fundamentally
important step in organic syntheses and industrial applica-
tions.1−4 Selective CCA is challenging, as the carbon−carbon
method to generate RhIII(ttp)OH would be desirable for the
bonds in organic compounds are kinetically less accessible than
facile CCA of ketones.
Indeed, mild and selective cleavage of the C(CO)−C(α)
bond of isopropyl ketones was recently achieved at 50 °C from
the surrounding carbon−hydrogen bonds. In the past, examples
of selective CCA of organic compounds were achieved with the
utilization of low-valent transition-metal complexes via
the hydrolysis of RhIII(ttp)Me to give RhIII(ttp)OH.6 However,
oxidative addition.2−4 In contrast, CCA by high-valent
the ketone substrate is limited to non-aldol-condensable
transition-metal complexes was less reported, since electroni-
isopropyl ketones, since their aldol reactions are reversible
cally less accessible high-valent transition-metal intermediates
without extensive undesirable consumption of ketones.
from oxidative addition would be involved.5
RhIII(ttp)CH2CH2OH (Figure 1) has been used as a
We recently reported that high-valent rhodium(III) and
RhIII(ttp)OH surrogate in the mild aldehydic CHA with aryl
iridium(III) porphyrins cleave the C(CO)−C(α) bonds of
and alkyl aldehydes.11 We now report our findings that
ketones with the assistance of water at 200 °C.6,7 Initially,
RhIII(ttp)CH2CH2OH can also serve as a RhIII(ttp)OH
rhodium(III) and iridium(III) porphyrins catalyze the aldol
equivalent to selectively undergo CCA of ketones under mild
condensation of ketones, giving water as a byproduct. The
and aerobic conditions.
water that forms hydrolyzes the α-carbon−hydrogen bond
activation (α-CHA) product of ketones with rhodium(III) and
RESULTS AND DISCUSSION
iridium(III) porphyrin complexes (MIII(ttp)CHRCOR′, M =
■
Atmosphere Effect. Initially, RhIII(ttp)CH2CH2OH (1a),
Rh, Ir) to generate RhIII(ttp)OH and IrIII(ttp)OH, respectively
prepared from the reductive alkylation of Rh(ttp)Cl (1b) with
(Scheme 1). RhIII(ttp)OH and IrIII(ttp)OH were thus
NaBH4/BrCH2CH2OH,11,12 reacted with diisopropyl ketone
proposed intermediates in cleaving the C(CO)−C(α) bonds
under solvent-free conditions under N2 at 25 °C in 15 min,
of ketones through σ-bond metathesis and oxidization of the
giving RhIII(ttp)COiPr (2a) and acetone in 82% and 39% yields
alkyl fragments.6,7
(GC-MS), respectively (eq 2). When the reaction was carried
In the metalloporphyrin-based CCA of ketones, some
limitations exist.6,7 (1) High reaction temperature is required
for the generation of MIII(ttp)OH (M = Rh, Ir). (2) The
reaction generally requires over 10 days. (3) Solvent-free
conditions limit the use of high-boiling substrates. (4) The
extensive and competitive aldol condensation of ketones,
catalyzed by Lewis acidic metalloporphyrins, converts a large
amount of starting material into undesired products.6−8 Hence,
out in air, RhIII(ttp)COiPr (2a) and acetone were formed in
80% and 40% yields (GC-MS), respectively (eq 2). The
presence of air did not affect the reactivity or selectivity of the
the aldol-condensable diethyl ketone gives a lower yield of
CCA product with rhodium(III) porphyrins in a slow reaction
(eq 1),6 and cyclohexanone is unsuccessful.9 (5) The proposed
intermediate RhIII(ttp)OH is highly reactive but extremely
thermally unstable toward decomposition to form [RhII(ttp)]2
Received: October 11, 2012
Published: December 19, 2012
and unknown rhodium porphyrin complexes.10 Hence, a mild
© 2012 American Chemical Society
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Scheme 1. Mechanism of CCA of Ketones with Metalloporphyrins
Table 2. CCA of Diisopropyl Ketone in Different Solvents
entry
solvent
time
yield of 2a (%)
1
2
3
4
5
6
7
a
15 min
2 d
80
b
benzene
56
Figure 1. Structure of RhIII(ttp)CH2CH2OH (1a).
b
tetrahydrofuran
acetone
2 d
53
b
2 d
34
c
reaction. Hence, the more user-friendly aerobic conditions were
used for further studies. It was noted that acetone was formed
in only half the yield of RhIII(ttp)COiPr (2a). This might be
due to the less efficient dehydrogenation of the alcohol formed
from the CCA, which will be discussed in the Proposed
acetonitrile
2 d
trace
c
dimethylformamide
dimethylacetamide
2 d
trace
c
2 d
trace
a
b
Conditions: 480 equiv of diisopropyl ketone, concentration 7.1 M. A
trace amount of 1a was observed by TLC analysis. 1a was recovered
c
Mechanism discussion.
quantitatively.
PPh3 Effect. Though the aldehydic CHA by RhIII(ttp)-
CH2CH2OH (1a) was significantly accelerated by added
PPh3,11 surprisingly, the addition of PPh3 reduced the yield
of RhIII(ttp)COiPr (2a) greatly to about 40% yield, even
though RhIII(ttp)CH2CH2OH (1a) was completely consumed
in 15 min (Table 1, entries 2 and 3). We are unclear of the
reason.
inhibit the reaction. Further studies were thus carried out under
solvent-free conditions.
CCA of Isopropyl Ketones. RhIII(ttp)CH2CH2OH (1a)
selectively cleaved the C(CO)−C(iPr) bond of various
isopropyl ketones at 25 °C to give the corresponding
rhodium(III) porphyrin acyls (2) (Table 3). The less reactive
Table 1. CCA of Diisopropyl Ketone with Addition of PPh3
Table 3. CCA of Various Isopropyl Ketones with
RhIII(ttp)CH2CH2OH (1a)
entry
amt of PPh3 (equiv)
yield of 2a (%)
1
2
3
0
80
40
38
0.1
1
Solvent Effect. To lower the ketone substrate loading and
expand the scope for solid substrates, various solvents were
screened in the reaction of RhIII(ttp)CH2CH2OH (1a) with
diisopropyl ketone (Table 2). RhIII(ttp)CH2CH2OH (1a)
reacted with 100 equiv of diisopropyl ketone in benzene and
tetrahydrofuran to give RhIII(ttp)COiPr (2a) in 56% and 53%
yields, respectively, in 2 days (Table 2, entries 2 and 3).
However, in the more polar solvent acetone, the yield of
RhIII(ttp)COiPr (2a) was reduced to 34% (Table 2, entry 4),
likely due to the observed poorer solubility of RhIII(ttp)-
CH2CH2OH (1a) in acetone at room temperature. When the
reactions were carried out in acetonitrile, dimethylformamide,
and dimethylacetamide solvents, only a trace amount of
RhIII(ttp)COiPr (2a) was obtained, and RhIII(ttp)CH2CH2OH
(1a) was recovered quantitatively (Table 2, entries 5−7).
Coordination of solvent molecules to the rhodium center may
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methyl isopropyl ketone and isobutyrophenone reacted with
RhIII(ttp)CH2CH2OH (1a) for 3 days to give RhIII(ttp)COMe
(2b) and RhIII(ttp)COPh (2c) in 28% and 20% yields,
respectively, while RhIII(ttp)CH2CH2OH (1a) was recovered
in 40% and 33% yields, respectively (Table 3, entries 2 and 3).
When RhIII(ttp)CH2CH2OH (1a) reacted with 2,6-dime-
thylcyclohexanone, RhIII(ttp)COCH(CH3)(CH2)3COCH3
(2d) was obtained in 68% yield in 4 h (Table 3, entry 4).
Cleavage occurred selectively at the C(CO)−C(iPr) bond,
placing the alkyl fragment into a carbonyl moiety. Similarly,
RhIII(ttp)CH2CH2OH (1a) reacted with the unsymmetrical 2-
methylcyclohexanone to give RhIII(ttp)CO(CH2)4COCH3
(2e) in 39% yield in 3 days (Table 3, entry 5). Selective
CCA at the more hindered C(CO)−C(alkyl) bond was
observed, as for RhIII(ttp)Me.6
CCA of Non-Isopropyl Ketones. Some aldol-condensable
ketones reacted successfully. RhIII(ttp)CH2CH2OH (1a)
reacted with diethyl ketone at 50 °C to give RhIII(ttp)COEt
(2f) in 24% yield in 2 days (Table 4, entry 1). Cyclohexanone
also reacted with RhIII(ttp)CH2CH2OH (1a) to give a 34%
yield of RhIII(ttp)CO(CH2)4CHO (2g) in 2 days (Table 4,
entry 2).
illustrates a mechanism to account for the selective C(CO)−
C(α) bond activation of ketones by RhIII(ttp)CH2CH2OH
(1a). RhIII(ttp)CH2CH2OH (1a) first undergoes reversible β-
hydroxyl elimination to give the highly reactive RhIII(ttp)OH
(eq i).11,14 RhIII(ttp)OH then cleaves the C(CO)−C(α) bond
of ketones through σ-bond metathesis, giving RhIII(ttp)COR
(2) and an alcohol (eq ii).6 The activation barrier for this CCA
step is lower due to the synchronous formation of the Rh−
C(CO) and C(alkyl)−O bonds in the four-center transition
state (TS1).6 The alcohol then undergoes dehydrogenation
catalyzed by RhIII(ttp)OH to give a carbonyl species and
RhIII(ttp)H (eq iii).6,15 RhIII(ttp)H possibly reacts with ketones
to give α- or β-CHA products (eq iv),6 which are then
hydrolyzed to regenerate RhIII(ttp)OH (eq v).6
From the proposed mechanism, the stoichiometry of the
formation of RhIII(ttp)COiPr (2a) and acetone should be 1:1 in
the CCA of diisopropyl ketone (eq 2). However, acetone was
formed in only half the yield of RhIII(ttp)COiPr (2a), probably
due to the inefficient dehydrogenation of the alcohol. Isopropyl
alcohol formed from the CCA is present in low concentration
and is less competitive with the much greater excess of ketone
substrate to react with RhIII(ttp)OH. This results in the
incomplete conversion to acetone (Scheme 2, eq iii).
Table 4. CCA of Non-Isopropyl Ketones with
RhIII(ttp)CH2CH2OH (1a)
CONCLUSION
■
In summary, the highly reactive rhodium(III) porphyrin
hydroxide was successfully generated from the readily accessible
and air-stable RhIII(ttp)CH2CH2OH. The selective CCA of
ketones was achieved by RhIII(ttp)CH2CH2OH under solvent-
free and aerobic conditions at 25−50 °C.
EXPERIMENTAL SECTION
■
Unless otherwise noted, all reagents were purchased from commercial
suppliers and directly used without further purification. Hexane was
distilled from anhydrous calcium chloride. Tetrahydrofuran (THF)
was distilled from sodium benzophenone ketyl prior to use. Thin-layer
chromatography was performed on precoated silica gel 60 F254 plates.
Silica gel (Merck, 70−230 mesh) was used for column chromatog-
raphy in air.
1H NMR and 13C NMR spectra were recorded on a Bruker Avance
III 400 spectrometer or Bruker Avance III 400Q spectrometer at 400
and 100 MHz, respectively. Chemical shifts were referenced to the
residual solvent protons in C6D6 (δ 7.15 ppm), CDCl3 (δ 7.26 ppm),
a
Observed by TLC analysis.
1
The successful CCA results of these two aldol-condensable
ketones are encouraging. RhIII(ttp)CH2CH2OH (1a) is there-
fore a much better precursor of RhIII(ttp)OH than RhIII(ttp)-
Me and RhIII(ttp)Cl, which could not cleave the C−C bond of
cyclohexanone due to extensive aldol condensation.9 The use of
mild reaction conditions and the less Lewis acidic RhIII(ttp)-
CH2CH2OH (1a) can eliminate the aldol condensation, as no
aldol condensation product was observed (Table 4, entries 1
and 2).
or tetramethylsilane (δ 0.00 ppm) in H NMR spectra as the internal
standards and referenced to CDCl3 (δ 77.16 ppm) in 13C NMR
spectra as the internal standard. Chemical shifts (δ) are reported as
parts per million (ppm) in the δ scale downfield from TMS. Coupling
constants (J) are reported in Hertz (Hz).
High-resolution mass spectra (HRMS) were recorded on a
ThermoFinnigan MAT 95 XL mass spectrometer. Fast atom
bombardment spectra were performed with 3-nitrobenzyl alcohol
(NBA) as the matrix. All samples for microanalysis were recrystallized
from CH2Cl2/MeOH and vacuum-dried at room temperature for at
least 2 days before submission.
GC-MS analysis was conducted on a GCMS-QP2010 Plus system
using a Rtx-5MS column (30 m × 0.25 mm). The details of the GC
program are as follows. The column oven temperature and injection
temperature were 35.0 and 180 °C, respectively. A split injection mode
was applied. The carrier gas used was helium with a primary pressure
of 500−900 kPa. Linear velocity was applied as the flow control mode.
The pressure, total flow, column flow, linear velocity, purge flow, and
split ratio were 32.4 kPa, 19.9 mL/min, 0.81 mL/min, 32.3 cm/s, 3.0
mL/min, and 20.0, respectively. The column oven temperature was
kept at 35.0 °C for 5 min and then elevated by a rate of 30.0 °C/min
up to 200.0 °C. The column oven temperature was then kept at 200.0
However, RhIII(ttp)CH2CH2OH (1a) reacted with acetone
at 50 °C to yield the α-CHA product , RhIII(ttp)CH2COCH3
(3) solely in 17% yield after 2 days without any CCA product
(Table 4, entry 3). This is likely due to the stronger C(CO)−
C(α) bond of acetone (BDE: C(CO)−C(Me), 84.1 kcal/mol;
C(CO)−C(Et), 82.3 kcal/mol; C(CO)−C(iPr), 81.3 kcal/
mol).13 In addition, the less hindered but more abundant α-C−
H bonds of acetone favor α-CHA. α-CHA therefore
predominates.
Proposed Mechanism. On the basis of the above findings
and previous proposed mechanisms,6,11,14,15 Scheme 2
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Scheme 2. Proposed Mechanism of CCA of Ketones by RhIII(ttp)CH2CH2OH (1a)
°C for 1 min. The ion source temperature and interface temperature
were 220 and 270 °C, respectively.
(1/1) as eluent to give RhIII(ttp)COiPr (2a; 7.5 mg, 0.0089 mmol,
40%).
RhIII(ttp)CH2CH2OH (1a)11,12 and RhIII(ttp)Cl (1b)16 were
synthesized according to the literature methods.
Reaction between RhIII(ttp)CH2CH2OH and Diisopropyl
Ketone with PPh3 (1 equiv) at 25 °C in Air. A stock benzene
solution of PPh3 (1.0 mL, 0.022 M, 0.022 mmol) was transferred to a
Teflon screw capped tube. The benzene solvent was removed by
vacuum distillation. RhIII(ttp)CH2CH2OH (1a; 18.1 mg, 0.022 mmol)
and diisopropyl ketone (1.5 mL) were then added, and the mixture
was stirred at 25 °C in air for 15 min. The excess solvent was removed
by vacuum distillation. The red residue was purified by column
chromatography on silica gel with a hexane/CH2Cl2 solvent mixture
(1/1) as eluent to give RhIII(ttp)COiPr (2a; 7.1 mg, 0.0084 mmol,
38%).
Reaction between RhIII(ttp)CH2CH2OH and Diisopropyl
Ketone at 25 °C under N2. RhIII(ttp)CH2CH2OH (1a; 18.1 mg,
0.022 mmol) and diisopropyl ketone (1.5 mL) were degassed for three
freeze−pump−thaw cycles. The reaction mixture was stirred at 25 °C
under N2 for 15 min. The excess solvent was removed by vacuum
distillation and collected by a vacuum trap. The red residue was
purified by column chromatography on silica gel eluting with a
hexane/CH2Cl2 solvent mixture (1/1) as eluent to give RhIII(ttp)-
COiPr (2a;6 15.3 mg, 0.018 mmol, 82%). The yield of the organic
coproduct acetone (39%) was measured by GC/MS using
naphthalene as the internal standard. RhIII(ttp)COiPr (2a): Rf =
Reaction between RhIII(ttp)CH2CH2OH and Diisopropyl
Ketone (100 equiv) in Benzene at 25 °C in Air. RhIII(ttp)-
CH2CH2OH (1a; 9.0 mg, 0.011 mmol) and diisopropyl ketone (160
μL, 1.1 mmol) were mixed in benzene (590 μL) and stirred at 25 °C
in air for 2 days. A trace amount of RhIII(ttp)CH2CH2OH (1a) was
observed by TLC analysis. The excess solvent was removed by vacuum
distillation. The red residue was purified by column chromatography
on silica gel with a hexane/CH2Cl2 solvent mixture (1/1) as eluent to
give RhIII(ttp)COiPr (2a; 5.2 mg, 0.006 mmol, 56%).
1
0.57 (hexane/CH2Cl2 1/1); H NMR (CDCl3, 400 MHz) δ −3.67
(sept, 1 H, J = 6.8 Hz), −2.14 (d, 6 H, J = 6.8 Hz), 2.70 (s, 12 H), 7.54
(t, 8 H, J = 8.7 Hz), 7.99 (d, 4 H, J = 7.5 Hz), 8.09 (d, 4 H, J = 7.6
Hz), 8.79 (s, 8 H).
Reaction between RhIII(ttp)CH2CH2OH and Diisopropyl
Ketone at 25 °C in Air. RhIII(ttp)CH2CH2OH (1a; 18.1 mg,
0.022 mmol) and diisopropyl ketone (1.5 mL) were stirred at 25 °C in
air for 15 min. The excess solvent was removed by vacuum distillation
and collected by vacuum trap. The red residue was purified by column
chromatography on silica gel with a hexane/CH2Cl2 solvent mixture
(1/1) as eluent to give RhIII(ttp)COiPr (2a; 14.9 mg, 0.018 mmol,
80%). The yield of the organic coproduct acetone (40%) was
measured by GC/MS using naphthalene as the internal standard.
Reaction between RhIII(ttp)CH2CH2OH and Diisopropyl
Ketone with PPh3 (0.1 equiv) at 25 °C in Air. A stock benzene
solution of PPh3 (100 μL, 0.022 M, 0.0022 mmol) was transferred to a
Teflon screw capped tube. The benzene solvent was removed by
vacuum distillation. RhIII(ttp)CH2CH2OH (1a; 18.1 mg, 0.022 mmol)
and diisopropyl ketone (1.5 mL) were then added, and the mixture
was stirred at 25 °C in air for 15 min. The excess solvent was removed
by vacuum distillation. The red residue was purified by column
chromatography on silica gel with a hexane/CH2Cl2 solvent mixture
Reaction between RhIII(ttp)CH2CH2OH and Diisopropyl
Ketone (100 equiv) in THF at 25 °C in Air. RhIII(ttp)CH2CH2OH
(1a; 9.0 mg, 0.011 mmol) and diisopropyl ketone (160 μL, 1.1 mmol)
were mixed in THF (590 μL) and stirred at 25 °C in air for 2 days. A
trace amount of RhIII(ttp)CH2CH2OH (1a) was observed by TLC
analysis. The excess solvent was removed by vacuum distillation. The
red residue was purified by column chromatography on silica gel with a
hexane/CH2Cl2 solvent mixture (1/1) as eluent to give RhIII(ttp)-
COiPr (2a; 4.9 mg, 0.006 mmol, 53%).
Reaction between RhIII(ttp)CH2CH2OH and Diisopropyl
Ketone (100 equiv) in Acetone at 25 °C in Air. RhIII(ttp)-
CH2CH2OH (1a; 9.0 mg, 0.011 mmol) and diisopropyl ketone (160
μL, 1.1 mmol) were mixed in acetone (590 μL) and stirred at 25 °C in
air for 2 days. A trace amount of RhIII(ttp)CH2CH2OH (1a) was
observed by TLC analysis. The excess solvent was removed by vacuum
distillation. The red residue was purified by column chromatography
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on silica gel with a hexane/CH2Cl2 solvent mixture (1/1) as eluent to
give RhIII(ttp)COiPr (2a; 3.2 mg, 0.004 mmol, 34%).
give RhIII(ttp)CO(CH2)4COCH3 (2e;6 7.7 mg, 0.009 mmol, 39%): Rf
= 0.04 (hexane/CH2Cl2 1/1); H NMR (CDCl3, 400 MHz) δ −3.11
1
Reaction between RhIII(ttp)CH2CH2OH and Diisopropyl
Ketone (100 equiv) in Acetonitrile at 25 °C in Air. RhIII(ttp)-
CH2CH2OH (1a; 9.0 mg, 0.011 mmol) and diisopropyl ketone (160
μL, 1.1 mmol) were mixed in acetonitrile (590 μL) and stirred at 25
°C in air for 2 days. The excess solvent was removed by vacuum
distillation. The red residue was purified by column chromatography
on silica gel with a hexane/CH2Cl2 solvent mixture (1/1) as eluent to
give a trace amount of RhIII(ttp)COiPr (2a). RhIII(ttp)CH2CH2OH
(1a) was recovered quantitatively.
(t, 2 H, J = 7.0 Hz), −1.30 (quin, 2 H, J = 7.3 Hz), −0.58 (quin, 2 H, J
= 7.5 Hz), 0.96 (t, 2 H, J = 7.6 Hz), 1.62 (s, 3 H), 2.69 (s, 12 H), 7.53
(d, 8 H, J = 7.8 Hz), 8.04 (t, 8 H, J = 8.3 Hz), 8.80 (s, 8 H).
RhIII(ttp)CH2CH2OH (1a; 1.6 mg, 0.002 mmol, 9%) was recovered.
Reaction between RhIII(ttp)CH2CH2OH and 3-Pentanone at
50 °C in Air. RhIII(ttp)CH2CH2OH (1a; 18.1 mg, 0.022 mmol) and
3-pentanone (1.5 mL) were heated at 50 °C in air for 2 days. The
excess solvent was removed by vacuum distillation. The red residue
was purified by column chromatography on silica gel with a hexane/
CH2Cl2 solvent mixture (1/1) as eluent to give RhIII(ttp)COEt (2f;6
4.4 mg, 0.005 mmol, 24%): Rf = 0.52 (hexane/CH2Cl2 1/1); 1H NMR
(CDCl3, 400 MHz) δ −3.10 (q, 2 H, J = 7.3 Hz), −1.67 (t, 3 H, J = 7.3
Hz), 2.70 (s, 12 H), 7.54 (d, 8 H, J = 7.6 Hz), 8.06 (d, 8 H, J = 6.4
Hz), 8.80 (s, 8 H). RhIII(ttp)CH2CH2OH (1a; 8.9 mg, 0.011 mmol,
49%) was recovered.
Reaction between RhIII(ttp)CH2CH2OH and Diisopropyl
Ketone (100 equiv) in Dimethylformamide at 25 °C in Air.
RhIII(ttp)CH2CH2OH (1a; 9.0 mg, 0.011 mmol) and diisopropyl
ketone (160 μL, 1.1 mmol) were mixed in dimethylformamide (590
μL) and stirred at 25 °C in air for 2 days. The excess solvent was
removed by vacuum distillation. The red residue was purified by
column chromatography on silica gel with a hexane/CH2Cl2 solvent
mixture (1/1) as eluent to give a trace amount of RhIII(ttp)COiPr
(2a). RhIII(ttp)CH2CH2OH (1a) was recovered quantitatively.
Reaction between RhIII(ttp)CH2CH2OH and Diisopropyl
Ketone (100 equiv) in Dimethylacetamide at 25 °C in Air.
RhIII(ttp)CH2CH2OH (1a; 9.0 mg, 0.011 mmol) and diisopropyl
ketone (160 μL, 1.1 mmol) were mixed in dimethylacetamide (590
μL) and stirred at 25 °C in air for 2 days. The excess solvent was
removed by vacuum distillation. The red residue was purified by
column chromatography on silica gel with a hexane/CH2Cl2 solvent
mixture (1/1) as eluent to give a trace amount of RhIII(ttp)COiPr
(2a). RhIII(ttp)CH2CH2OH (1a) was recovered quantitatively.
Reaction between RhIII(ttp)CH2CH2OH and Methyl Isopropyl
Ketone at 25 °C in Air. RhIII(ttp)CH2CH2OH (1a; 18.1 mg, 0.022
mmol) and methyl isopropyl ketone (1.5 mL) were stirred at 25 °C in
air for 3 days. The excess solvent was removed by vacuum distillation.
The red residue was purified by column chromatography on silica gel
with a hexane/CH2Cl2 solvent mixture (1/1) as eluent to give
RhIII(ttp)COMe (2b;6 5.0 mg, 0.006 mmol, 28%): Rf = 0.38 (hexane/
CH2Cl2 1/1); 1H NMR (CDCl3, 400 MHz) δ −2.79 (s, 3 H), 2.70 (s,
12 H), 7.54 (t, 8 H, J = 7.8 Hz), 8.06 (dd, 8 H, J = 3.0 Hz, 6.6 Hz),
8.80 (s, 8 H). RhIII(ttp)CH2CH2OH (1a; 7.2 mg, 0.009 mmol, 40%)
was recovered.
Reaction between RhIII(ttp)CH2CH2OH and Cyclohexanone
at 50 °C in Air. RhIII(ttp)CH2CH2OH (1a; 18.1 mg, 0.022 mmol)
and cyclohexanone (1.5 mL) were heated at 50 °C in air for 2 days. A
trace amount of RhIII(ttp)CH2CH2OH (1a) was observed by TLC
analysis. The excess solvent was removed by vacuum distillation. The
red residue was purified by column chromatography on silica gel with a
hexane/CH2Cl2 solvent mixture (1/1) as eluent to give RhIII(ttp)-
CO(CH2)4CHO (2g; 6.6 mg, 0.007 mmol, 34%): Rf = 0.31 (hexane/
1
CH2Cl2 1/1); H NMR (CDCl3, 400 MHz) δ −3.09 (t, 2 H, J = 6.9
Hz), −1.25 (quin, 2 H, J = 7.2 Hz), −0.60 (quin, 2 H, J = 7.5 Hz), 0.98
(dt, 2 H, J = 1.3, 7.5 Hz), 2.70 (s, 8 H), 7.55 (d, 8 H, J = 7.9 Hz), 8.05
(m, 8 H), 8.81 (s, 8 H), 9.03 (s, 1 H); 13C NMR (CDCl3, 100 MHz) δ
19.3, 21.7, 22.2, 42.0, 42.9, 122.9, 127.6, 131.7, 133.9, 134.2, 137.5,
1
139.2, 143.2, 201.8 (d, JRh−C = 17.2 Hz), 207.7; HRMS (FABMS)
calcd for C54H45N4O2Rh+ m/z 884.2592, found m/z 884.2595.
Reaction between RhIII(ttp)CH2CH2OH and Acetone at 50 °C
in Air. RhIII(ttp)CH2CH2OH (1a; 18.1 mg, 0.022 mmol) and acetone
(1.5 mL) were heated at 50 °C in air for 2 days. The excess solvent
was removed by vacuum distillation. The red residue was purified by
column chromatography on silica gel with a hexane/CH2Cl2 solvent
mixture (1/1) as eluent to give RhIII(ttp)CH2COCH3 (3;6 3.1 mg,
0.004 mmol, 17%): Rf = 0.15 (hexane/CH2Cl2 1/1); 1H NMR
(CDCl3, 400 MHz) δ −4.63 (d, 2 H, J = 3.8 Hz), −1.79 (s, 3 H), 2.70
(s, 12 H), 7.55 (d, 8 H, J = 7.8 Hz), 8.04 (m, 8 H), 8.79 (s, 8 H).
RhIII(ttp)CH2CH2OH (1a; 9.6 mg, 0.012 mmol, 53%) was recovered.
Reaction between RhIII(ttp)CH2CH2OH and Isobutyrophe-
none at 25 °C in Air. RhIII(ttp)CH2CH2OH (1a; 18.1 mg, 0.022
mmol) and isobutyrophenone (1.5 mL) were stirred at 25 °C in air for
3 days. The excess solvent was removed by vacuum distillation. The
red residue was purified by column chromatography on silica gel with a
hexane/CH2Cl2 solvent mixture (1/1) as eluent to give RhIII(ttp)-
COPh (2c;6 3.9 mg, 0.004 mmol, 20%): Rf = 0.43 (hexane/CH2Cl2 1/
1); 1H NMR (CDCl3, 400 MHz) δ 2.43 (d, 2 H, J = 8.1 Hz), 2.70 (s,
12 H), 5.95−6.00 (m, 2 H), 6.40 (t, 1 H, J = 7.8 Hz), 7.52−7.56 (m, 8
H), 7.95−8.01 (m, 8 H), 8.76 (s, 8 H). RhIII(ttp)CH2CH2OH (1a; 6.0
mg, 0.007 mmol, 33%) was recovered.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures giving 1H and 13C NMR spectra for 2g. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ksc@cuhk.edu.hk.
■
Reaction between RhIII(ttp)CH2CH2OH and 2,6-Dimethylcy-
clohexanone at 25 °C in Air. RhIII(ttp)CH2CH2OH (1a; 18.1 mg,
0.022 mmol) and 2,6-dimethylcyclohexanone (1.5 mL) were stirred at
25 °C in air for 4 h. The excess solvent was removed by vacuum
distillation. The red residue was purified by column chromatography
on silica gel with a hexane/CH2Cl2 solvent mixture (1/1) as eluent to
give RhIII(ttp)COCH(CH3)(CH2)3COCH3 (2d;6 13.7 mg, 0.015
mmol, 68%): Rf = 0.03 (hexane/CH2Cl2 1/1); 1H NMR (CDCl3, 400
MHz) δ −3.61 (sext, 1 H, J = 5.6 Hz), −2.34 (d, 3 H, J = 6.8 Hz),
−1.65 (q, 1 H, J = 7.8 Hz), −1.39 (m, 2 H), −0.92 (quin, 1 H, J = 8.6
Hz), 1.13 (m, 2 H), 1.72 (s, 3 H), 2.72 (s, 12 H), 7.56 (d, 8 H, J = 7.9
Hz), 8.04 (dd, 8 H, J = 2.0, 7.4 Hz), 8.10 (dd, 8 H, J = 2.1, 7.4 Hz),
8.84 (s, 8 H).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Direct Grant of CUHK (A/C 2060425) for
financial support.
■
REFERENCES
■
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