Mild and selective c(co)c(α) bond cleavage of ketones by rhodium(iii) porphyrins: Scope and mechanism

Article abstract of DOI:10.1021/om200788p

Rhodium(III) porphyrins were found to undergo selective C(CO)C(α) bond activation (CCA) of ketones promoted by water at temperatures as low as 50 °C. The acyl group of the ketone was transferred to the rhodium center, and the alkyl fragment was oxidized to a carbonyl moiety accordingly. The hydroxyl group of water is transferred to the rhodium porphyrin through hydrolysis of the kinetic α- carbonhydrogen bond activation (α-CHA) product to give RhIII(ttp)OH (ttp = 5,10,15,20-tetratolylporphyrinato dianion), which subsequently cleaves the C(CO)C(α) bond of ketone.

Full text of DOI:10.1021/om200788p

Article
pubs.acs.org/Organometallics
Mild and Selective C(CO)−C(α) Bond Cleavage of Ketones
by Rhodium(III) Porphyrins: Scope and Mechanism
Hong Sang Fung, Bao Zhu Li, 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) porphyrins were found to undergo selective
C(CO)−C(α) bond activation (CCA) of ketones promoted by water at
temperatures as low as 50 °C. The acyl group of the ketone was transferred
to the rhodium center, and the alkyl fragment was oxidized to a carbonyl
moiety accordingly. The hydroxyl group of water is transferred to the rhodium porphyrin through hydrolysis of the kinetic α-
carbon−hydrogen bond activation (α-CHA) product to give RhIII(ttp)OH (ttp = 5,10,15,20-tetratolylporphyrinato dianion),
which subsequently cleaves the C(CO)−C(α) bond of ketone.
acetophenone, acetone, methyl ethyl ketone, methyl isopropyl
ketone, and diethyl ketone can undergo CCA successfully.
We have communicated that RhIII(ttp)Me (Figure 1) can
react with a wider range of ketones.6 Specifically, the more
INTRODUCTION
■
Carbon−carbon bond activation (CCA) by a transition-metal
complex is fundamentally very important. However, many of
the examples are limited to low valent transition-metal
complexes through oxidative addition.1 High valent transition-
metal complexes are much less known to cleave a C−C bond
through oxidative addition as an uncommon, high valent
transition-metal intermediate would be involved. Therefore,
CCA with a high valent transition-metal complex is
mechanistically intriguing. In the past decade, examples of
CCA with high valent group 9 transition-metal complexes have
been reported. Brookhart and Bergman et al. reported the
cleavage of the R−CN bond by the cationic rhodium(III)
complex through migration of the alkyl group to the rhodium
center without involving an oxidative addition.2
Figure 1. Structure of RhIII(ttp)Me.
We have reported several CCA examples by group 9
metal(III) porphyrins.3,4 Oxidative addition with a trivalent
group 9 transition-metal complex to a carbon−carbon bond is
even more challenging, as an M(V) (M = Co, Rh, or Ir) and
sterically hindered intermediate with three ligands on the same
side of the porphyrin plane in a cis manner would be involved.
The proposed RhII(ttp)-catalyzed RhIII(ttp)-H (ttp =
5,10,15,20-tetratolylporphyrinato dianion) insertion into the
C−C bond of cyclooctane3 and the direct insertion of
RhIII(tmp)-OH (tmp = 5,10,15,20-tetramesitylporphyrinato
dianion) into the C(α)−C(β) bond of ethers4 accounts for
the CCA without the involvement of the high valent RhV(por)
species.
hindered isopropyl ketones can react and even react much
faster than methyl and ethyl ketones. In this article, the full
scope and mechanism of the selective C(CO)−C(α) bond
activation of ketones by rhodium(III) porphyrins are reported
(Scheme 1).
Scheme 1. Selective C(CO)−C(α) Activation of Ketones and
Acyl Transfer by RhIII(ttp)OH
Recently, iridium(III) porphyrins were reported to assist the
cleavage of the carbon(CO)−carbon(α) bond of ketones with
water at 200 °C.5 Mechanistically, the water, formed from the
iridium(III) porphyrin-catalyzed Aldol condensation of ketone,
serves as the key oxygen source and hydrolyzes the α-carbon−
hydrogen activation (α-CHA) complex to give IrIII(ttp)OH.
The C(CO)−C(α) bonds of the RCOR′ were then cleaved in a
nonregioselective manner to give a mixture of IrIII(ttp)COR′,
IrIII(ttp)R, IrIII(ttp)COR, and IrIII(ttp)R′. The reaction,
however, suffers from a narrow substrate scope. Only
RESULTS AND DISCUSSION
■
CCA with RhIII(ttp)X (X = OTf, Cl, and Me). Initially,
RhIII(ttp)OTf 1a, prepared in situ from RhIII(ttp)Cl 1b and
AgOTf, catalyzed the Aldol condensation of acetophenone to
Received: August 25, 2011
Published: January 5, 2012
© 2012 American Chemical Society
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Table 2. Reaction of Acetophenones with RhIII(por)Me
form a 58 800% yield of 1,3,5-triphenylbenzene 2 (with respect
to rhodium) (Table 1, entry 1) without any carbon−carbon
Table 1. CCA of Acetophenone with Various Rhodium
Porphyrins
entry
RhIII(por)Me
time (d)
3a−3c (% yield)
1
2
3
RhIII(tp‑OMepp)Me 1d
RhIII(ttp)Me 1c
7
15
6
3b 95
3a 93
3c 94
RhIII(tp‑CF pp)Me 1e
3
acetophenone at 200 °C to give 95% yield of RhIII(tp‑OMepp)-
entry
RhIII(ttp)X
time (d)
3a (% yield)
2 (% yield)
COPh 3b and 94% yield of RhIII(tp‑CF pp)COPh 3c in 7 and
a
1
2
3
RhIII(ttp)OTf 1a
RhIII(ttp)Cl 1b
RhIII(ttp)Me 1c
1
22
15
0
89
93
58 800
3
6 days, respectively (Table 2). Although the methoxyl and
trifluoromethyl substitutents promoted the reaction rate
slightly, RhIII(ttp)Me was used for further studies due to its
synthetic availability and the reactivity comparison with other
group 9 metallotetraarylporphyrins.
CCA with para-Substituted Acetophenones. We then
examined the α-C−H acidity effect of the acetophenones as
analogous CCA was promoted by the more acidic para-
substituted acetophenones with the iridium(III) porphyrin.5
However, para-substituted acetophenones, despite their differ-
ences in acidities, reacted in similar yields with RhIII(ttp)Me
(Table 3). para-Methylacetophenone and para-fluoroacetophenone
3600 (12 d)
700 (12 d)
bond activation (CCA) product observed in solvent-free
conditions. Only a solidified reaction mixture resulted.
However, the CCA of acetophenone occurred successfully at
the PhCO−Me bond with the less Lewis acidic RhIII(ttp)Cl 1b
and RhIII(ttp)Me 1c. The benzoyl group was transferred to the
rhodium center to give RhIII(ttp)COPh 3a in 89% and 93%
yields, respectively, in addition to 3600% and 700%7 yields of
1,3,5-triphenylbenzene (Table 1, entries 2 and 3). The benzoyl
transfer over the acetyl transfer is attributed to the selective
cleavage of the weaker CO−Me bond (85.0 kcal mol−1)8 rather
than the stronger CO−Ph bond (97.2 kcal mol−1).8
Table 3. Reaction of para-Substituted Acetophenones with
RhIII(ttp)Me
CCA of Acetophenone at 150 °C. When RhIII(ttp)Me
and acetophenone were heated at 150 °C for 26 days, only an
81% yield of RhIII(ttp)CH2COPh 4a was obtained (eq 1) as a
entry
FG
time (d)
3a, 3d, 3e (% yield)
1
2
Me
H
6
3d 72
3a 93
result of carbon−hydrogen bond activation. Figure 2 shows an
X-ray structure of RhIII(ttp)CH2COPh. The cleavage of the
15
reacted with RhIII(ttp)Me to give 72% yield of RhIII(ttp)-
COC6H4-p-Me 3d and 80% yield of RhIII(ttp)COC6H4-p-F 3e
in 6 and 12 days, respectively. The reaction rate was the fastest
for para-methylacetophenone.
Scope of the Reaction. Besides acetophenone (Table 4,
entry 1), various aromatic and aliphatic ketones also reacted
successfully with RhIII(ttp)Me and RhIII(ttp)Cl to give the
corresponding rhodium acyl complexes at 200 °C (Table 4).
RhIII(ttp)Me, in general, reacted more efficiently than
RhIII(ttp)Cl.
For the phenyl alkyl ketones, propiophenone and n-
butyrophenone reacted with RhIII(ttp)Me at 200 °C to give
44% and 43% yield of RhIII(ttp)COPh in 15 and 19 days,
respectively (Table 4, entries 2 and 3). Unexpectedly, the more
sterically hindered isobutyrophenone reacted much faster to give
97% yield of RhIII(ttp)COPh in only 1 day (Table 4, entry 4).
For aliphatic ketones, acetone and diethyl ketone poorly dis-
solved RhIII(ttp)Me and thus reacted inefficiently to give 20%
yield of RhIII(ttp)COMe 3f and 45% yield of RhIII(ttp)COEt 3g
in 17 and 16 days, respectively (Table 4, entries 5 and 6). Diiso-
propyl ketone was the most reactive and required only 30 min to
give 86% yield of RhIII(ttp)COiPr 3h (Table 4, entry 7).
The regioselective CCA at the more sterically hindered
CO−iPr bond was achieved when RhIII(ttp)Me was reacted
with methyl isopropyl ketone to give 95% yield of RhIII(ttp)COMe in
1 day (Table 4, entry 8). The higher reactivity of the CO−iPr bond
Figure 2. ORTEP presentation of the molecular structure with numbering
scheme for RhIII(ttp)CH2COPh 4a (30% probability displacement ellipsoids).
kinetically less accessible C(CO)−C(α) bond thus requires a
higher temperature.
CCA with Various Porphyrin Ligands. The electronic
effect of the porphyrin ligand was then examined in order to
improve the CCA rate (Table 2). Both para-substituted
methoxy and trifluoromethyl phenyl on the porphyrin ligand
slightly increased the rate of the CCA of acetophenone as
RhIII(tp‑OMepp)Me 1d and RhIII(tp‑CF pp)Me 1e reacted with
3
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Table 4. RhIII(ttp)Me vs RhIII(ttp)Cl toward Aromatic and
Aliphatic Ketones
CCA of Isopropyl Ketones at 50 °C. The higher
reactivities of the bulky isopropyl ketones led us to examine
the reaction at milder reaction conditions (Table 5). To our
a
Table 5. Reaction of Isopropyl Ketones with RhIII(ttp)Me at
a b
,
50 °C
a
b
32% yield of 1c recovered. 39% yield of 1c recovered.
delight, isobutyrophenone and methyl isopropyl ketone reacted
with RhIII(ttp)Me at 50 °C to give 41% yield of RhIII(ttp)COPh
and 57% yield of RhIII(ttp)COMe in 3 days, respectively (Table 5,
entries 1 and 2). Diisopropyl ketone, being the most reactive
substrate, required only 1 day to give 94% yield of RhIII(ttp)COiPr
(Table 5, entry 3).
Fate of the Alkyl Fragment. While the acyl group of
ketone is transferred to the rhodium center, the alkyl fragment
is oxidized to a carbonyl compound. A quantitative amount of
acetone (δ = 1.55 ppm in C6D6) was observed in the reaction
1
of neat diisopropyl ketone with RhIII(ttp)Me by H NMR
(Table 4, entry 7).
a
1
Acetone was obtained in quantitative yield by H NMR.
To further identify the organic coproduct structure, an
“intramolecular trap”, the cyclic 2,6-dimethylcyclohexanone,
was then reacted with RhIII(ttp)Me at 100 °C. Delightfully, 85%
yield of RhIII(ttp)COCHMe(CH2)3COMe 3i was obtained in 4
(81.3 kcal mol−1)8 than the CO−Me (84.1 kcal mol−1)8 or CO−Et
(83.0 kcal mol−1)8 bond is probably due to the weaker CO−iPr bond
strength (Figure 3).
1
days (eq 2). The H NMR spectrum of RhIII(ttp)COCHMe-
(CH2)3COMe shows a singlet at 1.72 ppm with three protons,
suggesting the presence of an acetyl group. The two types of acyl
carbon signals were clearly identified in its 13C NMR spectrum
with peaks at 208.12 ppm (COMe; singlet) and 207.42 ppm
(RhCO; doublet, 1JRh−C = 29.8 Hz). The CCA product from the
2,6-dimethylcyclohexanone, therefore, clearly shows that the
carbonyl group is transferred to the rhodium center and the
alkyl fragment is oxidized to a carbonyl moiety. Figure 4 shows an
X-ray structure of RhIII(ttp)COCHMe(CH2)3COMe.
Figure 3. Relative reactivity of methyl, ethyl, and isopropyl ketones.
The more Lewis acidic RhIII(ttp)Cl underwent the
competitive Aldol condensation more extensively and thus is
less productive in giving the CCA product. Propiophenone, n-
butyrophenone, and isobutyrophenone reacted with RhIII(ttp)
Cl at 200 °C to give 61, 21, and 12% yield of RhIII(ttp)COPh in
21, 20, and 1 days, respectively (Table 4, entries 2−4).
RhIII(ttp)Cl also poorly dissolved in aliphatic ketones, and
the CCA yields were low. Only 34% yield of RhIII(ttp)COMe,
23% yield of RhIII(ttp)COEt, and 24% yield of RhIII(ttp)COiPr
were obtained from the reaction with acetone, diethyl ketone,
and diisopropyl ketone with RhIII(ttp)Cl at 200 °C for 26, 20,
and 1 days, respectively (Table 4, entries 5−7).
Source of Oxygen. As the Aldol condensation occurred
before the CCA of acetophenone, water, therefore, is a possible
oxygen source for the CCA. To examine the possible
accelerating effect of water, the reaction of RhIII(ttp)Me and
the nonenolizable diisopropyl ketone with the addition of water
was examined (Table 6). Without any water added, the reaction
proceeded to give 95% yield of RhIII(ttp)COiPr in 1 day (Table 6,
entry 1). The trace amount of water residue (max 0.2% in acetone
Regioselective CCA also occurred at the CO−iPr bond when
RhIII(ttp)Cl was reacted with the methyl isopropyl ketone as
14% yield of RhIII(ttp)COMe was obtained in 8 days (Table 4,
entry 8).
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Figure 5. ORTEP presentation of the molecular structure with
numbering scheme for RhIII(ttp)CO(CH2)4COMe 3j (30% proba-
bility displacement ellipsoids).
Figure 4. ORTEP presentation of the molecular structure with
numbering scheme for RhIII(ttp)COCHMe(CH2)3COMe 3i (30%
probability displacement ellipsoids).
respectively.5 To find out the selectivity of bond cleavage in the
rhodium analogues, the reaction of 2-methylcyclohexanone
with RhIII(ttp)Me at 100 °C was conducted (eq 9; Scheme 2).
Only the expected product RhIII(ttp)CO(CH2)4COMe was
observed. The other possible CCA product, RhIII(ttp)CHMe-
(CH2)4COOH, and its hydrolysis product, heptanoic acid9 (see
the section, Hydrolysis of the α-CHA Product, below), were
not detected. The ketone CCA by rhodium porphyrin,
therefore, is regioselective.
Table 6. Water Effect on CCA of Diisopropyl Ketone
entry
H2O (equiv)
time
3h (% yield)
1
2
3
4
0
50
1 d
3 h
3 h
3 h
95
92
95
93
Reaction Mechanism. On the basis of the stoichiometry of
the reaction, Aldol condensation, and the α-CHA product,
Scheme 3 shows a proposed mechanism for the ketone CCA by
RhIII(ttp)X (X = Me or Cl). RhIII(ttp)X first catalyzes the Aldol
condensation of ketone to generate the corresponding product
(1,3,5-triphenylbenzene in acetophenone case) and water
(Scheme 3, eq i). Concurrently, RhIII(ttp)X can react with
the α-carbon−hydrogen bond of an alkyl ketone to give an α-
carbon−hydrogen bond activation (α-CHA) complex and
methane (or HCl) (Scheme 3, eq iia). RhIII(ttp)OH is formed
upon hydrolysis of the α-CHA complex (Scheme 3, eq iib).
The C(CO)−C(α) bond is then cleaved by RhIII(ttp)OH,
presumably by σ-bond metathesis, to give a rhodium porphyrin
acyl and an alcohol (Scheme 3, eq iii). Further dehydrogenation
of the alcohol by RhIII(ttp)OH affords the carbonyl compound
and RhIII(ttp)H (Scheme 3, eqs iva and ivb). The α-CHA
100
200
solvent) in the reaction mixture is likely the water source. When
50 equiv of water was added, the reaction time was shortened
from 1 day to 3 h (Table 6, entry 1 vs 2). Further additions of 100
and 200 equiv of water did not improve the rate (Table 6, entries
3 and 4). The rate-accelerating effect of water and the reaction
stoichiometry suggest that the CCA occurs through the Rh(ttp)−
OH insertion across the C(CO)−C(α) bond, which has been
reported in the iridium analogue.5
Regioselectivity of CCA. There are two regioselectivity
issues associated with the ketone CCA: (i) selective cleavage of
unsymmetric ketone and (ii) selective formation of rhodium
porphyrin acyl.
Selective Cleavage of Unsymmetric Ketone. The sole
formation of RhIII(ttp)COMe with methyl isopropyl ketone
reveals that the C(CO)−C(α) bond cleavage is selective at the
more-hindered CO−iPr bond (Table 4, entry 8). Likewise,
regioselective CCA at the CO−iPr bond occurred in the
unsymmetric 2-methylcyclohexanone to give RhIII(ttp)CO-
(CH2)4COMe 3j only (carbonyl signal in 13C NMR: 208.11
complex is regenerated when RhIII(ttp)H or RhII (ttp)2 further
2
reacts with ketones (Scheme 3, eq iia). Experiments were then
carried out to validate the proposed mechanism.
(Scheme 3, eq i) Aldol Condensation Catalyzed by
Rh(ttp)X (X = Me or Cl). At a high temperature of 200 °C, a
series of Aldol condensations of acetophenone occurred to give
1,3,5-triphenylbenzene and water (Table 1, Scheme 3), and two
layers of immiscible liquids (water and acetophenone) were
observed. This type of Lewis acidic rhodium(III) porphyrin
catalyzed Aldol condensation is precedented. Ogoshi et al. have
reported that the Lewis acidic RhIII(oep)ClO4 catalyzes the
Aldol condensation of cyclohexanone at 50 °C.10 Furthermore,
the formation of 1,3,5-triphenylbenzene from the Aldol
condensation of acetophenone in the presence of an electro-
philic catalyst BiIII(OTf)3 has been reported.11
ppm (COMe; singlet) and 202.01 ppm (RhCO; doublet, 1JRh−C
=
30.0 Hz)) in 67% yield (eq 3). Figure 5 shows an X-ray structure
of RhIII(ttp)CO(CH2)4COMe.
(Scheme 3, eq iia) Carbon−Hydrogen Bond Activa-
tion with Rh(ttp)X (X = Me or Cl). RhIII(ttp)X (X = Me or
Cl) undergoes a kinetically more facile CHA than CCA with
acetophenone. RhIII(ttp)Me reacted with neat acetophenone to
give RhIII(ttp)CH2COPh in quantitative yield in a short
reaction time of 3 days at 200 °C (eq 10). After 15 days,
Selective Formation of Rhodium Porphyrin Acyl. Pre-
viously, the C(CO)−C(α) bond of acetone was cleaved by
IrIII(ttp)(CO)BF4 in a nonregioselective manner at 200 °C to
give IrIII(ttp)Me and IrIII(ttp)COMe in 40% and 19% yields,
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Scheme 2. Possible CCA Products with 2-Methylcyclohexanone
Scheme 3. Proposed Mechanism of the Rhodium(III) Porphyrin-Assisted C(CO)−C(α) Bond Activation of Ketone
RhIII(ttp)COPh was formed in 93% yield (eq 4). Therefore, the
α-CHA is kinetically more facile than CCA.
RhIII(ttp)CH2CHMeCOPh 4c, was only observed in a trace
1
amount in the crude reaction mixture by H NMR: δ −5.14
(ddt, 1H, 1JRh−H = 3.0 Hz, 3JHa−Hb = 7.5 Hz, 2JHa−Ha′ = 15.1 Hz,
Ha), −4.53 (ddt, 1H, 1JRh−H = 3.5 Hz, 3JHa′−Hb = 7.9 Hz, 2JHa′−Ha
=
12.4 Hz, Ha′), −2.99 (m, 1H, Hb), −2.24 (d, 3H, Hc)) and
HRMS (Calcd: m/z 918.2796. Found: m/z 918.2799.).
Presumably, the low yielding of RhIII(ttp)CH2CHMeCOPh is
due to its high reactivity toward hydrolysis (see the section,
Hydrolysis of the α-CHA Product, below).
For propiophenone, the CHA is also more facile but takes
place at the β-C−H bond. RhIII(ttp)CH2CH2COPh 4b in 68%
yield was obtained in 3 days when RhIII(ttp)Me was reacted
with propiophenone at 200 °C (eq 5). Figure 6 shows an X-ray
The more Lewis acidic RhIII(ttp)Cl presumably activates
the α-C−H bond through a heterolysis of the RhIII(ttp)−
Cl, followed by cationic activation as reported by Ogoshi et al.10
For the less Lewis acidic RhIII(ttp)Me, heterolysis of the Rh−Me
bond is less likely. Probably, in the presence of water, hydrolysis of
RhIII(ttp)Me with water to give RhIII(ttp)OH13 and subsequent
α-CHA with RhIII(ttp)OH account for the α-CHA product.
Indeed, RhIII(ttp)Me underwent hydrolysis with water in
benzene-d6 in 30 min to give 34% yield of RhII (ttp)2 1f and a
2
trace amount of RhIII(ttp)H 1g (eq 6). The formation of 11%
yield of methane suggests that RhIII(ttp)OH is likely formed.
Presumably, the oxygen atom of water attacks the rhodium
center to give RhIII(ttp)OH and the methyl ligand leaves as
methane upon protonation. The highly reactive RhIII(ttp)OH
once formed quickly yields RhII (ttp)2 and H2O2. The
2
independent reductive dimerization of RhIII(ttp)OH to give
RhII (ttp)2 and H2O2 at 120 °C has been reported.14 The trace
Figure 6. ORTEP presentation of the molecular structure with
numbering scheme for RhIII(ttp)CH2CH2COPh 4b (30% probability
displacement ellipsoids).
2
amount of RhIII(ttp)H is due to the disproportionation of
RhII (ttp)2 with H2O to give RhIII(ttp)OH and RhIII(ttp)H.4,15
2
The analogous equilibria between RhI(tspp), RhII(tspp), and
RhIII(tspp) in an aqueous medium have been published by
Wayland et al. recently.16
structure of RhIII(ttp)CH2CH2COPh. While the direct β-CHA
is reasonable, a rapid consecutive α-CHA/1,2-isomerization to
give RhIII(ttp)CH2CH2COPh is also possible. The 1,2-isomer-
ization of rhodium porphyrin alkyl through β-hydride
elimination/reinsertion is precedented.12
The RhIII(ttp)H and RhII (ttp)2 formed are then kinetically
2
When RhIII(ttp)Me was reacted with isobutyrophenone at
200 °C in a short reaction time of 1 h, the β-CHA product,
trapped by the α-C−H bond to give the CHA product. Indeed,
RhIII(ttp)H and RhII (ttp)2 activated the α-C−H bond of
2
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acetophenone to give RhIII(ttp)CH2COPh at 200 °C in a short
period of 30 min (eq 7).
equilibrium and react with ketone rapidly to give the CHA
product, which would be eventually hydrolyzed into RhIII(ttp)-
OH for CCA (eq 7).
When RhIII(ttp)H or RhII (ttp)2 were used to react with 2,6-
2
dimethylcyclohexanone, the Rh−O insertion into the C(CO)−
C(α) bond still occurred to give RhIII(ttp)COCHMe-
(CH2)3COMe in moderate yield (Table 7). It implies that
Alternatively, direct α-C−H bond activation with RhIII(ttp)-
Me by σ-bond metathesis is viable. Indeed, RhIII(ttp)Me
reacted with acetophenone in benzene-d6 in 30 min to give
methane and RhIII(ttp)CH2COPh in 87% yield and 66% yield,
respectively (eq 8). This type of σ-bond metathesis has been
shown to occur in the benzylic C−H bond activation of toluene
by RhIII(ttp)Me.17 The direct CHA is faster than the hydrolysis,
and both pathways can operate in parallel.
Table 7. Reaction of 2,6-Dimethylcyclohexanone with Rh(I),
Rh(II), and Rh(III)
entry
Rh(ttp)X
time (d)
3i (% yield)
1
2
3
RhIII(ttp)Me 1c
RhII2(ttp)2 1f
RhIII(ttp)H 1g
4
5
5
85
46
40
(Scheme 3, eq iib) Hydrolysis of the α-CHA Product.
Direct transformation of the CHA product to the CCA product
by a formal σ-bond metathesis18 is ruled out by the reaction
stoichiometry. Only the C−O bond formation at the alkyl
fragment was observed, without any C−C bond formation
product. Therefore, the Rh−O insertion into the C(CO)−
C(α) bond suggests that the CHA product hydrolyzes to give
RhIII(ttp)OH first.
RhIII(ttp)H and RhII (ttp)2 are transformed to RhIII(ttp)OH,
2
likely through equilibrium, in a polar environment before the
CCA occurs. However, RhI(ttp)− did not give any observable
product, probably due to rapid decomposition at high
temperature.
The chemoselective C(CO)−C(α) bond activation of ketone
by RhIII(ttp)OH can be rationalized in the following two
points: (i) favorable RhIII(ttp)OH formation in polar solvent
and (ii) weak Rh−OH bond.
Indeed, RhIII(ttp)CH2COPh and RhIII(ttp)CH2CH2COPh
were found to undergo hydrolysis more rapidly than RhIII(ttp)-
Me in the presence of 100 equiv of water in benzene-d6 in
5 min at 200 °C (eqs 9 and 10 vs eq 6). The formation of
acetophenone (57%) and propiophenone (29%) supports the
hydrolytic cleavage of RhIII(ttp)R into RhIII(ttp)OH and RH
rather than RhIII(ttp)H and R−OH.
Favorable Rh(ttp)OH Formation in Polar Solvent. The
reaction of RhIII(ttp)Me and diisopropyl ketone in benzene,
THF, and acetone at 50 °C (Table 8) showed that the rate and
yield of the CCA were enhanced in a polar solvent. The
reaction with diisopropyl ketone in acetone (100 equiv, Table 8,
entry 5) gave a high yield of 90% of RhIII(ttp)COiPr with only
6% of RhIII(ttp)Me remained unreacted. The reaction in
benzene solvent only gave a lower yield of 77% of
RhIII(ttp)COiPr (Table 8, entry 1) and recovered RhIII(ttp)Me
in 18% yield. These results are consistent with the proposed
RhIII(ttp)OH intermediate. A higher concentration of
RhIII(ttp)OH can be generated in a polar medium and thus a
faster rate of the reaction.
Relative Rate of Hydrolysis. The faster hydrolysis rate of
the CHA product to generate RhIII(ttp)OH promotes the rate
of CCA. The low yield of RhIII(ttp)CH2CHMeCOPh is
attributed to its rapid hydrolysis rate relative to that of
RhIII(ttp)CH2COPh and RhIII(ttp)CH2CH2COPh (Figure 7).
Weak Rh−OH Bond. The RhIII(ttp)−OH bond
(46 kcal mol−1)21 is around 14 kcal mol−1 weaker than the
RhIII(ttp)−H bond (60 kcal mol−1).8 RhIII(ttp)−H can cleave
the C−C bond of cyclooctane in a nonpolar medium,3 whereas
the more reactive RhIII(ttp)−OH cleaves the C(CO)−C(α)
bond of ketone in a polar medium.
RhIII(ttp)OH cleaves the C(CO)−C(α) bond directly
through σ-bond metathesis. The activation barrier for the
cleavage of the strong C(CO)−C(α) bond (85.0 kcal mol−1)8 is
lower due to the synchronous and simultaneous formation of
the Rh−C and C−O bonds. Indeed, benzylic C−H bond
activation of toluene by RhIII(ttp)H and RhIII(ttp)Me through
σ-bond metathesis is precedented.17 The four centered
metallocycle transition state is possible even though one of
the corners is a sterically hindered metalloporphyrin. For the
ketone CCA, although one of the corners is replaced by a more
bulky sp3 carbon (the C(α) of ketone), the less hindered sp2
carbon of the ketone renders the four centered metallocycle
Figure 7. Relative reactivities of the CHA products toward hydrolysis.
The more basic α-carbon in RhIII(ttp)CH2CHMeCOPh favors
the faster hydrolysis. This is a key factor for the high CCA
reactivity of isopropyl ketones over methy and ethyl ketones.
(Scheme 3, eq iii) Carbon(CO)−Carbon(α) Bond
Oxidation with RhIII(ttp)OH. The RhIII(ttp)R hydrolysis
product, RhIII(ttp)OH, is the only intermediate that cleaves the
C(CO)−C(α) bond, as supported by the reaction stoichiom-
etry. Other rhodium porphyrin species, such as RhIII(ttp)H and
RhII (ttp)2, though, exist in the reaction medium through
2
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Table 8. Solvent Effects toward Isobutyryl Transfer of Diisopropyl Ketone
entry
solvent
donor number19 (kcal mol−1)
0.1
dielectric constant20
2.28
iPrCOiPr (equiv)
time (d)
1c (% recovery yield)
3h (% yield)
1
2
3
4
5
6
benzene
benzene
THF
100
200
100
200
100
200
2
1
2
1
2
1
18
0
77
87
75
89
90
95
20
17
7.52
24
0
THF
acetone
acetone
21.01
6
0
were solved by direct methods and subsequent Fourier difference
techniques and refined anisotropically for all non-hydrogen atoms by
full-matrix least-squares calculations on F2 using the SHELXTL
program package. All hydrogen atoms were geometrically fixed using
the riding model. Rh(ttp)Cl 1b,25 Rh(ttp)Me 1c,26 Rh(tp‑OMepp)Me
1d:26 1H NMR (CDCl3, 400 MHz): δ −5.82 (d, 3H, 2JRh−H = 2.8 Hz),
4.09 (s, 12H), 7.26 (m, 8H, overlapped with solvent residue), 8.04
(dd, 4H, J = 2.8, 7.9 Hz), 8.10 (dd, 4H, J = 2.7, 7.9 Hz), 8.75 (s, 8H).
transition state feasible. Furthermore, the hydroxyl group of
RhIII(ttp)OH is relatively small, which makes the four centered
metallocycle transition state less sterically sensitive. While the
covalent radius of oxygen (0.66 Å)22 doubles that of hydrogen
(0.31 Å)22 and is only slightly smaller than that of sp3 carbon
(0.76 Å),22 the divalent hydroxyl oxygen is much less hindered
than the tetravalent sp3 carbon.
(Scheme 3, eqs iva and ivb) Dehydrogenation of
Alcohol. The alcohol generated from the Rh−OH insertion to
the C(CO)−C(α) bond is further oxidized to a carbonyl
moiety in the presence of a catalytic amount of RhIII(ttp)OH
(Scheme 4). Such a mechanism has been proposed by
Collman23 and Fu.24
1
13C NMR (CDCl3, 175 MHz): δ −11.7 (d, JRh−C = 27.4 Hz), 55.7,
112.2, 112.3, 122.0, 131.5, 134.7, 135.0, 135.1, 143.5, 159.3. HRMS
(FABMS): Calcd for [C49H39N4O4Rh]+: m/z 850.2021. Found: m/z
850.2010. Rh(tp‑CF pp)Me 1e:26 1H NMR (CDCl3, 400 MHz): δ
3
2
−5.81 (d, 3H, JRh−H = 2.1 Hz), 8.03 (t, 8H, J = 5.6 MHz), 8.27 (d,
4H, J = 7.8 Hz), 8.32 (d, 4H, J = 7.6 Hz), 8.67 (s, 8H). 13C NMR
1
(CDCl3, 175 MHz): δ −10.7 (d, JRh−C = 25.7 Hz), 121.3, 122.3,
1
123.9, 124.0, 127.0, 130.4 (q, JF−C = 32.4 Hz), 131.9, 134.1, 134.2,
Scheme 4. Dehydrogenation of Alcohol by RhIII(ttp)OH
159.3, 143.0, 145.6. HRMS (FABMS): Calcd for [C49H27N4F12Rh]+:
m/z 1002.1094. Found: m/z 1002.1086.). Rh2(ttp)2 1f,27 Rh(ttp)H
1g,27 and Rh(ttp)−Na+ 1h28 were synthesized according to the
literature methods.
Reaction Between Rh(ttp)OTf and Acetophenone at 200 °C
for 1 Day. Rh(ttp)OTf 1a, freshly prepared from Rh(ttp)Cl 1b
(3.1 mg, 0.004 mmol) and AgOTf (2.0 mg, 0.008 mmol), and
acetophenone (1.5 mL) were heated at 200 °C under N2 for 1 day. A
pale yellow solid of 1,3,5-triphenylbenzene 2 (58 800% wrt Rh, 720.1
mg, 2.4 mmol) was isolated by crystallization from ethanol. 1,3,5-
Triphenylbenzene 2. 1H NMR (400 MHz, CDCl3): δ 7.40 (t, 3H, J =
7.6 Hz), 7.49 (t, 6H, J = 7.6 Hz), 7.71 (d, 6H, J = 8.4 Hz), 7.79(s, 3H).
Reaction between Rh(ttp)Cl and Acetophenone at 200 °C
for 22 Days. Rh(ttp)Cl 1b (17.5 mg, 0.022 mmol) and acetophenone
(1.5 mL) were heated at 200 °C under N2 for 22 days. A red product
Rh(ttp)COPh 3a29a (17.2 mg, 0.020 mmol, 89%) with Rf = 0.43
(hexane/CH2Cl2 = 1:1) was purified and collected by column
In summary, RhIII(ttp)Cl and RhIII(ttp)Me were found to
oxidatively cleave the C(CO)−C(α) bond of ketone with water
at a temperature as low as 50 °C. The acyl group was
transferred to the rhodium center while the alkyl fragment was
oxidized to a carbonyl moiety.
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. Thin-layer chromatography
was performed on precoated silica gel 60 F254 plates. Silica gel (Merck,
70−230 mesh) was used for column chromatography. All reactions
were carried out in a Telfon screw-capped tube under N2 with the
mixture degassed for three freeze−thaw-pump cycles and wrapped
with aluminum foil to prevent undesired photochemical reactions. The
crude mixture was dried under high vacuum. The products were
further purified by silica gel column chromatography eluting with a
solvent mixture of hexane/CH2Cl2.
1
chromatography. Rh(ttp)COPh 3a. H NMR (300 MHz, CDCl3): δ
2.43 (dd, 2H, J = 8.1 Hz), 2.70 (s, 12H), 5.95−6.00 (m, 2H), 6.40
(t, 1H, J = 7.8 Hz), 7.52−7.56 (m, 8H), 7.95−8.01 (m, 8H), 8.76
(s, 8H). A 3600% yield of 1,3,5-triphenylbenzene 2 (wrt Rh) was
observed when an aliquot of the reaction mixture at 12 days was
1
analyzed by H NMR spectroscopy.
Reaction between Rh(ttp)Me and Acetophenone at 200 °C
for 3 Days. Rh(ttp)Me 1c (17.3 mg, 0.022 mmol) and acetophenone
(1.5 mL) were heated at 200 °C under N2 for 3 days. A red product
Rh(ttp)CH2COPh 4a (19.6 mg, 0.022 mmol, 100%) with Rf = 0.09
(hexane/CH2Cl2 = 1:1) was purified and collected by column
1H NMR and 13C NMR spectra were recorded on a Bruker AV-400
at 400 and 100 MHz or a Bruker AV-700 at 700 and 175 MHz,
respectively. Chemical shifts were referenced with the residual solvent
protons in chloroform-d (δ = 7.26 ppm) or tetramethylsilane (δ = 0.00
ppm) and benzene-d6 (δ = 7.15 ppm) in 1H NMR spectra, and CDCl3
(δ = 77.16 ppm) in 13C NMR spectra as the internal standards.
Chemical shifts (δ) were reported as parts per million (ppm) in (δ)
scale downfield from TMS. Coupling constants (J) were 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 single crystals were immersed in Paraton-N
oil and sealed under N2 in thin-walled glass capillaries. Data were
collected at 123, 293, or 296 K on a Bruker SMART 1000 CCD
diffractometer using Mo Kα radiation. An empirical absorption
correction was applied using the SADABS program. All structures
1
chromatography. Rh(ttp)CH2COPh 4a. H NMR (CDCl3, 400 MHz):
δ −4.14 (d, 2H, J = 3.6 Hz), 2.70 (s, 12H), 4.59 (d, 2H, J = 7.6 Hz),
6.61 (t, 2H, J = 7.7 Hz), 7.06 (t, 1H, J = 7.3 Hz), 7.53 (t, 8H, J = 8.4
Hz), 7.91 (dd, 4H, J = 1.3, 7.4 Hz), 8.01 (dd, 4H, J = 1.5, 7.5 Hz), 8.70
1
(s, 8H). 13C NMR (CDCl3, 100 MHz): δ 1.0 (d, JRh−C = 29.7 Hz),
21.7, 123.1, 125.1, 126.9, 127.5, 127.6, 130.8, 131.9, 133.8, 134.2,
137.3, 139.1, 143.2, 196.8. HRMS (FABMS): Calcd for
C50H41N4ORh+: m/z 890.2486. Found: m/z 890.2472.
Reaction between Rh(ttp)Me and Acetophenone at 200 °C
for 15 Days. Rh(ttp)Me 1c (17.4 mg, 0.022 mmol) and
acetophenone (1.5 mL) were heated at 200 °C under N2 for 15
days. A red product Rh(ttp)COPh 3a (17.9 mg, 0.021 mmol, 93%)
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was purified and collected by column chromatography. A 700% yield
of 1,3,5-triphenylbenzene 2 (wrt Rh) was observed when an aliquot of
the reaction mixture at 12 days was analyzed by 1H NMR
spectroscopy.
NMR (CDCl3, 400 MHz): δ −4.76 (dt, 2H, J = 3.0, 8.4 Hz), −3.08
(t, 2H, J = 8.3 Hz), 2.70 (s, 12H), 5.86 (d, 2H, J = 7.6 Hz), 6.89
(t, 2H, J = 7.7 Hz), 7.15 (t, 1H, J = 7.4 Hz), 7.52 (d, 4H, J = 8.0 Hz),
7.55 (d, 4H, J = 8.2 Hz), 7.97 (d, 4H, J = 7.6 Hz), 8.10 (d, 4H, J = 7.7
1
Reaction between Rh(ttp)Me and Acetophenone at 150 °C
for 26 days. Rh(ttp)Me 1c (17.4 mg, 0.022 mmol) and
acetophenone (1.5 mL) were heated at 150 °C under N2 for 26
days. A red product Rh(ttp)CH2COPh 4a (15.9 mg, 0.018 mmol,
81%) was purified and collected by column chromatography.
Reaction between Rh2(ttp)2 and Acetophenone at 200 °C
for 8 Days. Rh2(ttp)2 1f, freshly prepared from Rh(ttp)Cl 1b
(17.8 mg, 0.022 mmol), and acetophenone (1.5 mL) were heated at
200 °C under N2. Rh(ttp)CH2COPh 4a formed quantitatively
according to TLC analysis in 30 min. After 7 days, a red product
Rh(ttp)COPh 3a (18.7 mg, 0.021 mmol, 97%) was purified and
collected by column chromatography.
Reaction between Rh(ttp)H and Acetophenone at 200 °C for
7 Days. Rh(ttp)H 1g, freshly prepared from Rh(ttp)Cl 1b (17.8 mg,
0.022 mmol), and acetophenone (1.5 mL) were heated at 200 °C
under N2. Rh(ttp)CH2COPh 4a formed quantitatively according to
TLC analysis in 30 min. After 7 days, a red product Rh(ttp)COPh 3a
(19.1 mg, 0.022 mmol, 99%) was purified and collected by column
chromatography.
Hz), 8.77 (s, 8H). 13C NMR (CDCl3, 100 MHz): δ 5.5 (d, JRh−C
=
29.6 Hz), 21.7, 36.2, 122.7, 127.1, 127.6, 127.8, 131.8, 132.2, 133.8,
134.1, 137.4, 139.2, 143.4, 195.5. HRMS (FABMS): Calcd for
C57H45N4ORh+: m/z 904.2643. Found: m/z 904.2627.
Reaction between Rh(ttp)Me and Propiophenone at 200 °C
for 15 Days. Rh(ttp)Me 1c (17.5 mg, 0.022 mmol) and
propiophenone (1.5 mL) were heated at 200 °C under N2 for 15
days. A red product Rh(ttp)COPh 3a (8.5 mg, 0.010 mmol, 44%) was
purified and collected by column chromatography.
Reaction between Rh(ttp)Cl and n-Butyrophenone at 200 °C
for 20 Days. Rh(ttp)Cl 1b (17.5 mg, 0.022 mmol) and n-
butyrophenone (1.5 mL) were heated at 200 °C under N2 for 20
days. A red product Rh(ttp)COPh 3a (4.1 mg, 0.005 mmol, 21%) was
purified and collected by column chromatography.
Reaction between Rh(ttp)Me and n-Butyrophenone at
200 °C for 19 Days. Rh(ttp)Me 1c (17.6 mg, 0.022 mmol) and n-
butyrophenone (1.5 mL) were heated at 200 °C under N2 for 19 days.
A red product Rh(ttp)COPh 3a (8.3 mg, 0.009 mmol, 43%) was
purified and collected by column chromatography.
Reaction between Rh(tp‑OMepp)Me and Acetophenone at
200 °C for 7 Days. Rh(tp‑OMepp)Me 1d (18.7 mg, 0.022 mmol) and
acetophenone (1.5 mL) were heated at 200 °C under N2 for 7 days. A
red product Rh(tp‑OMepp)COPh 3b (19.7 mg, 0.021 mmol, 95%) with
Rf = 0.19 (hexane/CH2Cl2 = 1:1) was purified and collected by
Reaction between Rh(ttp)Cl and Isobutyrophenone at
200 °C for 1 Day. Rh(ttp)Cl 1b (17.5 mg, 0.022 mmol) and
isobutyrophenone (1.5 mL) were heated at 200 °C under N2 for 1 day.
A red product Rh(ttp)COPh 3a (2.3 mg, 0.003 mmol, 12%) was
purified and collected by column chromatography.
Reaction between Rh(ttp)Me and Isobutyrophenone at 200 °C
for 1 Day. Rh(ttp)Me 1c (17.5 mg, 0.022 mmol) and isobutyr-
ophenone (1.5 mL) were heated at 200 °C under N2 for 1 day. A red
product Rh(ttp)COPh 3a (18.7 mg, 0.021 mmol, 97%) was purified
and collected by column chromatography.
Reaction between Rh(ttp)Me and Isobutyrophenone at 50
°C for 3 Days. Rh(ttp)Me 1c (17.6 mg, 0.022 mmol) and
isobutyrophenone (1.5 mL) were heated at 50 °C under N2 for 3
days. A red product Rh(ttp)COPh 3a (7.9 mg, 0.018 mmol, 41%)
was purified and collected by column chromatography. Rh(ttp)Me
(5.6 mg, 0.007 mmol, 32%) was recovered.
1
column chromatography. Rh(tp‑OMepp)COPh 3b. H NMR (CDCl3,
400 MHz): δ 2.44 (d, 2H, J = 7.3 Hz), 4.08 (s, 12H), 5.98 (t, 2H, J =
7.7 Hz), 6.43 (t, 1H, J = 7.4 Hz), 7.24 (d, 4H, J = 3.3 Hz), 7.28 (d, 4H,
J = 2.5 Hz), 7.99 (dd, 4H, J = 1.5, 8.3 Hz), 8.07 (dd, 4H, J = 1.7, 8.2
Hz), 8.77 (s, 8H). 13C NMR (CDCl3, 100 MHz): δ 55.7, 112.3, 112.3,
116.7, 122.5, 125.5, 125.8, 131.7, 134.6, 135.0, 135.2, 143.4, 159.4,
1
200.9 (d, JRh−C = 30.8 Hz). HRMS (FABMS): Calcd for
C55H41N4O5Rh+: m/z 940.2127. Found: m/z 940.2152.
Reaction between Rh(tp‑CF pp)Me and Acetophenone at
3
200 °C for 6 Days. Rh(tp‑CF pp)Me 1e (22.1 mg, 0.022 mmol) and
3
acetophenone (1.5 mL) were heated at 200 °C under N2 for 6 days. A
Reaction between Rh(ttp)Cl and Acetone at 200 °C for 26
Days. Rh(ttp)Cl 1b (17.5 mg, 0.022 mmol) and acetone (1.5 mL)
were heated at 200 °C under N2 for 26 days. A red product
Rh(ttp)COMe 3f29b (6.1 mg, 0.007 mmol, 34%) with Rf = 0.38
(hexane/CH2Cl2 = 1:1) was purified and collected by column
red product Rh(tp‑CF pp)COPh 3c (22.6 mg, 0.021 mmol, 94%) with
3
Rf = 0.49 (hexane/CH2Cl2 = 1:1) was purified and collected by
1
column chromatography. Rh(tp‑CF pp)COPh 3c. H NMR (CDCl3,
3
400 MHz): δ 2.47 (d, 2H, J = 7.6 Hz), 6.03 (t, 2H, J = 7.6 Hz), 6.50 (t,
1H, J = 7.4 Hz), 8.06 (t, 8H, J = 6.24 Hz), 8.23 (d, 4H, J = 7.6 Hz),
8.32 (d, 4H, J = 8.0 Hz), 8.73 (s, 8H). 13C NMR (CDCl3, 100 MHz):
1
chromatography. Rh(ttp)COMe 3f. H NMR (CDCl3, 300 MHz): δ
−2.79 (s, 3H), 2.70 (s, 12H), 7.54 (t, 8H, J = 7.8 Hz), 8.06 (dd, 8H,
J = 3, 6.6 Hz), 8.01 (s, 8H).
Reaction between Rh(ttp)Me and Acetone at 200 °C for 17
Days. Rh(ttp)Me 1c (17.5 mg, 0.022 mmol) and acetone (1.5 mL)
were heated at 200 °C under N2 for 17 days. A red product
Rh(ttp)COMe 3f (3.6 mg, 0.004 mmol, 20%) was purified and
collected by column chromatography.
1
δ 116.6, 121.7, 124.0, 125.8, 126.0, 126.2, 130.5 (q, JF−C = 32.4 Hz),
132.0, 134.1, 134.3, 142.9, 145.5, 199.3 (d, 1JRh−C = 30.82 Hz). HRMS
(FABMS): Calcd for [C55H29N4F12ORh + H]+: m/z 1093.1278.
Found: m/z 1093.1270.
Reaction between Rh(ttp)Me and p-Methylacetophenone at
200 °C for 6 Days. Rh(ttp)Me 1c (17.4 mg, 0.022 mmol) and p-
methylacetophenone (1.5 mL) were heated at 200 °C under N2 for 6
days. A red product Rh(ttp)COC6H4-p-Me 3d (14.1 mg, 0.016 mmol,
72%) was purified and collected by column chromatography.
Reaction between Rh(ttp)Me and p-Fluoroacetophenone at
200 °C for 12 Days. Rh(ttp)Me 1c (17.3 mg, 0.022 mmol) and p-
fluoroacetophenone (1.5 mL) were heated at 200 °C under N2 for 12
days. A red product Rh(ttp)COC6H4-p-F 3e (15.7 mg, 0.018 mmol,
80%) was purified and collected by column chromatography.
Reaction between Rh(ttp)Cl and Propiophenone at 200 °C
for 21 Days. Rh(ttp)Cl 1b (17.4 mg, 0.022 mmol) and
propiophenone (1.5 mL) were heated at 200 °C under N2 for 21
days. A red product Rh(ttp)COPh 3a (11.8 mg, 0.013 mmol, 61%)
was purified and collected by column chromatography.
Reaction between Rh(ttp)Cl and Diethyl Ketone at 200 °C
for 20 Days. Rh(ttp)Cl 1b (17.5 mg, 0.022 mmol) and diethyl ketone
(1.5 mL) were heated at 200 °C under N2 for 20 days. A red product
Rh(ttp)COEt 3g29a (4.2 mg, 0.005 mmol, 23%) with Rf = 0.52
(hexane/CH2Cl2 = 1:1) was purified and collected by column
1
chromatography. Rh(ttp)COEt 3g. H NMR (300 MHz, CDCl3): δ
−3.14 (q, 2H, J = 7.5 Hz), −1.69 (t, 3H, J = 7.2 Hz), 2.70 (s, 12H),
7.26 (d, 8H, J = 7.8 Hz), 8.05 (d, 8H, J = 6.3 Hz), 8.80 (s, 8H).
Reaction between Rh(ttp)Me and Diethyl Ketone at 200 °C
for 16 Days. Rh(ttp)Me 1c (17.4 mg, 0.022 mmol) and diethyl
ketone (1.5 mL) were heated at 200 °C under N2 for 16 days. A red
product Rh(ttp)COEt 3g (8.2 mg, 0.010 mmol, 45%) was purified and
collected by column chromatography.
Reaction between Rh(ttp)Me and Propiophenone at 200 °C
for 3 Days. Rh(ttp)Me 1c (17.3 mg, 0.022 mmol) and
propiophenone (1.5 mL) were heated at 200 °C under N2 for 3
days. A red product Rh(ttp)CH2CH2COPh 4b (13.5 mg, 0.015 mmol,
68%) with Rf = 0.12 (hexane/CH2Cl2 = 1:1) was purified and
Reaction between Rh(ttp)Cl and Diisopropyl Ketone at 200 °C
for 1 Day. Rh(ttp)Cl 1b (17.5 mg, 0.022 mmol) and diisopropyl
ketone (1.5 mL) were heated at 200 °C under N2 for 1 day. A red
product Rh(ttp)COiPr 3h (4.4 mg, 0.005 mmol, 24%) with Rf = 0.57
(hexane/CH2Cl2 = 1:1) was purified and collected by column
1
1
collected by column chromatography. Rh(ttp)CH2CH2COPh 4b. H
chromatography. Rh(ttp)COiPr 3h. H NMR (CDCl3, 400 MHz): δ
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−3.68 (hept, 1H, J = 6.8 Hz), −2.14 (d, 6H, J = 6.8 Hz), 2.69 (s,
12H), 7.53 (t, 8H, J = 8.4 Hz), 7.99 (d, 4H, J = 7.4 Hz), 8.09 (d, 4H, J
= 7.6 Hz), 8.79 (s, 8H). 13C NMR (CDCl3, 100 MHz): δ 12.4, 19.3,
21.6, 29.4, 31.5, 42.6, 48.9, 122.8, 127.4, 127.5, 131.6, 133.7, 134.1,
Reaction between Rh(ttp)Me, Diisopropyl Ketone (200 equiv),
and Water (200 equiv) in Acetone at 50 °C for 3 h. Rh(ttp)Me 1c
(8.7 mg, 0.011 mmol), water (40 μL, 2.2 mmol), and diisopropyl
ketone (320 μL, 2.2 mmol) were heated in acetone (430 μL) at 50 °C
under N2 for 3 h. A red product Rh(ttp)COiPr 3h (8.6 mg, 0.010
mmol, 93%) was purified and collected by column chromatography.
Reaction between Rh(ttp)Cl and Methyl Isopropyl Ketone at
200 °C for 8 Days. Rh(ttp)Cl 1b (17.6 mg, 0.022 mmol) and methyl
isopropyl ketone (1.5 mL) were heated at 200 °C under N2 for 8 days.
A red product Rh(ttp)COMe 3f (2.5 mg, 0.003 mmol, 14%) was
purified and collected by column chromatography.
Reaction between Rh(ttp)Me and Methyl Isopropyl Ketone
at 200 °C for 1 Day. Rh(ttp)Me 1c (17.6 mg, 0.022 mmol) and
methyl isopropyl ketone (1.5 mL) were heated at 200 °C under N2 for
1 day. A red product Rh(ttp)COMe 3f (17.0 mg, 0.021 mmol, 95%)
was purified and collected by column chromatography.
1
137.3, 139.1, 143.3, 207.4 (d, JRh−C = 29.8 Hz), 208.1. HRMS
(FABMS): Calcd for C50H41N4ORh+: m/z 843.2565. Found: m/z
843.2559.
Reaction between Rh(ttp)Me and Diisopropyl Ketone at 200 °C
for 30 min. Rh(ttp)Me 1c (17.5 mg, 0.022 mmol) and diisopropyl
ketone (1.5 mL) were heated at 200 °C under N2 for 30 min. At the
end of the reaction, an aliquot of the reaction mixture was added to
benzene-d6 for 1H NMR analysis. Acetone (δ = 1.5 ppm) was observed
in quantitative yield. A red product Rh(ttp)COiPr 3h (15.9 mg, 0.019
mmol, 86%) was purified and collected by column chromatography.
Reaction between Rh(ttp)Me and Diisopropyl Ketone at 50 °C
for 1 day. Rh(ttp)Me 1c (17.5 mg, 0.022 mmol) and diisopropyl
ketone (1.5 mL) were heated at 50 °C under N2 for 1 day. A red
product Rh(ttp)COiPr 3h (17.4 mg, 0.021 mmol, 94%) was purified
and collected by column chromatography.
Reaction between Rh(ttp)Me and Diisopropyl Ketone (100 equiv)
in Benzene at 50 °C for 2 Days. Rh(ttp)Me 1c (8.7 mg, 0.011 mmol)
and diisopropyl ketone (160 μL, 1.1 mmol) were heated in benzene
(590 μL) at 50 °C under N2 for 2 days. A red product Rh(ttp)COiPr
3h (7.1 mg, 0.008 mmol, 77%) was purified and collected by column
chromatography. Rh(ttp)Me 1c (1.6 mg, 0.002 mmol, 18%) was
recovered.
Reaction between Rh(ttp)Me and Diisopropyl Ketone (200
equiv) in Benzene at 50 °C for 1 Day. Rh(ttp)Me 1c (8.6 mg,
0.011 mmol) and diisopropyl ketone (320 μL, 2.2 mmol) were heated
in benzene (430 μL) at 50 °C under N2 for 1 day. A red product
Rh(ttp)COiPr 3h (8.1 mg, 0.010 mmol, 87%) was purified and
collected by column chromatography.
Reaction between Rh(ttp)Me and Diisopropyl Ketone (100
equiv) in THF at 50 °C for 2 Days. Rh(ttp)Me 1c (8.7 mg, 0.011
mmol) and diisopropyl ketone (160 μL, 1.1 mmol) were heated in
THF (590 μL) at 50 °C under N2 for 2 days. A red product
Rh(ttp)COiPr 3h (7.0 mg, 0.008 mmol, 75%) was purified and
collected by column chromatography. Rh(ttp)Me 1c (2.1 mg, 0.003 mmol,
24%) was recovered.
Reaction between Rh(ttp)Me and Diisopropyl Ketone (200 equiv)
in THF at 50 °C for 1 Day. Rh(ttp)Me 1c (8.7 mg, 0.011 mmol) and
diisopropyl ketone (320 μL, 2.2 mmol) were heated in THF (430 μL)
at 50 °C under N2 for 1 day. A red product Rh(ttp)COiPr 3h (8.3 mg,
0.010 mmol, 89%) was purified and collected by column
chromatography.
Reaction between Rh(ttp)Me and Diisopropyl Ketone (100 equiv)
in Acetone at 50 °C for 2 Days. Rh(ttp)Me 1c (8.7 mg, 0.011 mmol)
and diisopropyl ketone (160 μL, 1.1 mmol) were heated in acetone
(590 μL) at 50 °C under N2 for 2 days. A red product Rh(ttp)COiPr
3h (8.3 mg, 0.010 mmol, 90%) was purified and collected by column
chromatography. Rh(ttp)Me (0.5 mg, 0.001 mmol, 6%) was
recovered.
Reaction between Rh(ttp)Me and Diisopropyl Ketone (200 equiv)
in THF at 50 °C for 1 Day. Rh(ttp)Me 1c (8.7 mg, 0.011 mmol)
and diisopropyl ketone (320 μL, 2.2 mmol) were heated in acetone
(430 μL) at 50 °C under N2 for 1 day. A red product Rh(ttp)COiPr
3h (8.8 mg, 0.010 mmol, 95%) was purified and collected by column
chromatography.
Reaction between Rh(ttp)Me, Diisopropyl Ketone (200 equiv),
and Water (50 equiv) in Acetone at 50 °C for 3 h. Rh(ttp)Me 1c
(8.7 mg, 0.011 mmol), water (10 μL, 0.55 mmol), and diisopropyl
ketone (320 μL, 2.2 mmol) were heated in acetone (430 μL) at 50 °C
under N2 for 3 h. A red product Rh(ttp)COiPr 3h (8.5 mg, 0.010
mmol, 92%) was purified and collected by column chromatography.
Reaction between Rh(ttp)Me, Diisopropyl Ketone (200 equiv),
and Water (100 equiv) in Acetone at 50 °C for 3 h. Rh(ttp)Me 1c
(8.7 mg, 0.011 mmol), water (20 μL, 1.1 mmol), and diisopropyl
ketone (320 μL, 2.2 mmol) were heated in acetone (430 μL) at 50 °C
under N2 for 3 h. A red product Rh(ttp)COiPr 3h (8.8 mg, 0.010
mmol, 95%) was purified and collected by column chromatography.
Reaction between Rh(ttp)Me and Methyl Isopropyl Ketone
at 50 °C for 3 Days. Rh(ttp)Me 1c (17.6 mg, 0.022 mmol) and
methyl isopropyl ketone (1.5 mL) were heated at 50 °C under N2 for
3 days. A red product Rh(ttp)COMe 3f (10.2 mg, 0.013 mmol, 57%)
was purified and collected by column chromatography. Rh(ttp)Me 1c
(6.8 mg, 0.009 mmol, 39%) was recovered.
Reaction between Rh(ttp)Me and 2,6-Dimethylcyclohexa-
none at 100 °C for 4 Days. Rh(ttp)Me 1c (17.5 mg, 0.022 mmol)
and 2,6-dimethylcyclohexanone (1.5 mL) were heated at 100 °C under
N2 for 4 days. A red product Rh(ttp)COCHMe(CH2)3COMe 3i (17.1
mg, 0.019 mmol, 85%) with Rf = 0.03 (hexane/CH2Cl2 = 1:1) was
purified and collected by column chromatography. Rh(ttp)Me (2.4
mg, 0.003 mmol, 14%) was recovered. Rh(ttp)COCHMe-
1
(CH2)3COMe 3i. H NMR (CDCl3, 400 MHz): δ −3.61 (sext, 1H,
J = 5.6 Hz), −2.34 (d, 3H, J = 6.8 Hz), −1.65 (q, 1H, J = 7.8 Hz),
−1.39 (m, 2H), −0.92 (quint, 1H, J = 8.6 Hz), 1.13 (m, 2H), 1.72 (s,
3H), 2.72 (s, 12H), 7.56 (d, 8H, J = 7.9 Hz), 8.04 (dd, 8H, J = 2.0, 7.4
Hz), 8.10 (dd, 8H, J = 2.1, 7.4 Hz), 8.84 (s, 8H). 13C NMR (CDCl3,
100 MHz): δ 15.6, 21.7, 29.8, 43.5, 122.8, 127.5, 127.6, 131.6, 133.7,
1
134.3, 137.4, 139.3, 143.3, 207.4 (d, JRh−C = 29.8 Hz), 208.1. HRMS
(FABMS): Calcd for C56H49N4O2Rh+: m/z 912.2905. Found: m/z
912.2924.
Reaction between Rh2(ttp)2 and 2,6-Dimethylcyclohexa-
none at 100 °C for 5 Days. Rh2(ttp)2 1f, freshly prepared from
Rh(ttp)Cl 1b (17.4 mg, 0.022 mmol), and 2,6-dimethylcyclohexanone
(1.5 mL) were heated at 100 °C under N2 for 5 days. A red product
Rh(ttp)COCHMe(CH2)3COMe 3i (8.2 mg, 0.010 mmol, 40%) was
purified and collected by column chromatography.
Reaction between Rh(ttp)H and 2,6-Dimethylcyclohexa-
none at 100 °C for 5 Days. Rh(ttp)H 1g, freshly prepared from
Rh(ttp)Cl 1b (17.5 mg, 0.022 mmol), and 2,6-dimethylcyclohexanone
(1.5 mL) were heated at 100 °C under N2 for 5 days. A red product
Rh(ttp)COCHMe(CH2)3COMe 3i (8.0 mg, 0.009 mmol, 40%) was
purified and collected by column chromatography.
Reaction between Rh(ttp)−Na+ and 2,6-Dimethylcyclohex-
anone at 100 °C for 5 Days. Rh(ttp)−Na+ 1h, freshly prepared from
Rh(ttp)Cl 1b (17.4 mg, 0.022 mmol), and 2,6-dimethylcyclohexanone
(1.5 mL) were heated at 100 °C under N2 for 5 days. The reaction
mixture gradually turned dark, and no product was isolated.
Reaction between Rh(ttp)Me and 2-Methylcyclohexanone
at 100 °C for 7 Days. Rh(ttp)Me 1c (17.6 mg, 0.022 mmol) and 2-
methylcyclohexanone (1.5 mL) were heated at 100 °C under N2 for 7
days. A red product Rh(ttp)CO(CH2)4COMe 3j (13.2 mg, 0.015 mmol,
67%) with Rf = 0.04 (hexane/CH2Cl2 = 1:1) was purified and collected
by column chromatography. Rh(ttp)Me (5.3 mg, 0.007 mmol, 30%)
was recovered. Rh(ttp)CO(CH2)4COMe 3j. 1H NMR (CDCl3,
400 MHz): δ −3.11 (t, 2H, J = 7.0 Hz), −1.30 (quin, 2H, J = 7.3
Hz), −0.58 (quin, 2H, J = 7.5 Hz), 0.96 (t, 2H, J = 7.6 Hz), 1.62
(s, 3H), 2.69 (s, 12H), 7.53 (d, 8H, J = 7.8 Hz), 8.04 (t, 8H, J = 8.3
Hz), 8.80 (s, 8H). 13C NMR (CDCl3, 100 MHz): δ 21.0, 21.7, 22.2,
29.6, 41.7, 43.1, 122.8, 127.6, 131.7, 133.9, 134.2, 137.5, 139.2, 143.2,
1
202.0 (d, JRh−C = 30.0 Hz), 208.1. HRMS (FABMS): Calcd for
[C55H47N4O2Rh + H]+: m/z 899.2827. Found: m/z 899.2854.
Carbon−Hydrogen Bond Activation of Acetophenone by
Rh(ttp)Me. Rh(ttp)Me 1c (4.0 mg, 0.005 mmol), acetophenone (60 uL,
578
dx.doi.org/10.1021/om200788p | Organometallics 2012, 31, 570−579

Organometallics
Article
0.5 mmol), and benzene-d6 (500 uL) were added to an NMR tube
with a Telfon screw cap. The reaction mixture was degassed for three
freeze−thaw−pump cycles and flame-sealed. The tube was heated at
200 °C in the dark for 30 min and analyzed by 1H NMR spectroscopy.
A 66% yield of Rh(ttp)CH2COPh 4a, 33% yield of Rh(ttp)H 1g, and
87% yield of methane were observed in 30 min.
Hydrolysis of Rh(ttp)Me. Rh(ttp)Me 1c (4.0 mg, 0.005 mmol),
water (10 uL, 0.5 mmol), and benzene-d6 (500 uL) were added to an
NMR tube with a Telfon screw cap. The reaction mixture was
degassed for three freeze−thaw−pump cycles and flame-sealed. The
tube was heated at 200 °C in the dark for 30 min and analyzed by 1H
NMR spectroscopy. A 34% yield of Rh2(ttp)2 1g, a trace amount of
Rh(ttp)H 1f, and 11% of methane were observed in 30 min with a
60% yield of Rh(ttp)Me 1c remained.
Hydrolysis of Rh(ttp)CH2COPh. Rh(ttp)CH2COPh 4a (4.0 mg,
0.004 mmol), H2O (100 equiv, 8.0 uL, 0.45 mmol), and benzene-d6
(500 uL) were added to an NMR tube with a Telfon screw cap. The
reaction mixture was degassed for three freeze−thaw−pump cycles
and flame-sealed. The tube was heated at 200 °C in the dark for 5 min
and analyzed by 1H NMR spectroscopy. A 42% yield of Rh(ttp)H 1g,
13% yield of Rh2(ttp)2 1f, 57% yield of acetophenone, and 42%
recovery yield of Rh(ttp)CH2COPh 4b were obtained in 5 min.
Hydrolysis of Rh(ttp)CH2CH2COPh. Rh(ttp)CH2CH2COPh 4b
(4.0 mg, 0.004 mmol), H2O (100 equiv, 8.0 uL, 0.45 mmol), and
benzene-d6 (500 uL) were added to an NMR tube with a Telfon screw
cap. The reaction mixture was degassed for three freeze−thaw−pump
cycles and flame-sealed. The tube was heated at 200 °C in the dark for
5 min and analyzed by 1H NMR spectroscopy. A 9% yield of Rh(ttp)H
1g, 18% yield of Rh2(ttp)2 1f, 29% yield of propiophenone, and 71%
recovery yield of Rh(ttp)CH2CH2COPh 4b were obtained in 5 min.
(9) The absence of heptanoic acid was confirmed with an authentic
sample by GC/MS.
(10) (a) Aoyama, Y.; Yoshida, T.; Ogoshi, H. Tetrahedron Lett. 1985,
26, 6107−6108. (b) Aoyama, Y.; Tanaka, Y.; Yoshida, T.; Toi, H.;
Ogoshi, H. J. Organomet. Chem. 1987, 329, 251−266.
(11) Fumiaki, O.; Yuichi, I.; Yuusuke, T.; Masato, E.; Tsuneo, S.
Synlett 2008, 15, 2365−2367.
(12) (a) Mak, K. W.; Chan, K. S. J. Am. Chem. Soc. 1998, 120, 9686−
9687. (b) Mak, K. W.; Xue, F.; Mak, T. C. W.; Chan, K. S. J. Chem.
Soc., Dalton Trans. 1999, 3333−3334.
(13) Hanson, S. K.; Heinekey, D. M.; Goldberg, K. I. Organometallics
2008, 27, 1454−1463.
(14) Choi, K. S.; Lai, T. H.; Lee, S. Y.; Chan, K. S. Organometallics
2011, 30, 2633−2635.
(15) Wayland, B. B.; Balkus, K. J. Jr.; Farnos, M. D. Organometallics
1989, 8, 950−955.
(16) Fu, X.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 2626−2631.
(17) Choi, K. S.; Chiu, P. F.; Chan, K. S. Organometallics 2010, 29,
624−629.
(18) Vidal, V.; Tholier, A.; Thivolle-Cazat, J.; Basset, J. M. Science
1997, 276, 99−102.
(19) Wypych, G. Handbook of Solvents; ChemTec Publishing:
Toronto, 2000.
(20) Lide, D. R., Ed. Handbook of Chemistry and Physics, 84th ed.;
CRC Press: Boca Raton, FL, 2004.
(21) Wayland et al. reported that the bond strength of RhIII(tmp)−
OMe is 46 kcal mol−1. It is assumed that the RhIII(por)−OH bond is
similar in strength. See: Sarkar, S.; Li, S.; Wayland, B. B. J. Am. Chem.
Soc. 2010, 132, 13569−13571.
(22) Cordero, B.; Gom
Echeverría, J.; Cremades, E.; Barragan
2008, 2832−2838.
́
ez, V.; Platero-Prats, A. E.; Reves
́
, M.;
́
, F.; Alvarez, S. Dalton Trans.
ASSOCIATED CONTENT
■
(23) Collman, J. P.; Boulatov, R. Inorg. Chem. 2001, 40, 560−563.
(24) Liu, L.; Yu, M.; Wayland, B. B.; Fu, X. Chem. Commun. 2010, 46,
6353−6355.
S
* Supporting Information
Tables and figures of crystallographic data for complexes 3b, 3c,
1
3f, 3h−3j, 4a, and 4b (CIF and PDF), and H and 13C NMR
(25) (a) Zhou, X.; Li, Q.; Mak, T. C. W.; Chan, K. S. Inorg. Chim.
Acta 1998, 270, 551−554. (b) Zhou, X.; Wang, R. J.; Xue, F.; Mak, T.
C. W.; Chan, K. S. J. Organomet. Chem. 1999, 580, 22−25.
(26) (a) Chan, K. S.; Mak, K. W.; Tse, M. K.; Yeung, S. K.; Li, B. Z.;
Chan, Y. W. J. Organomet. Chem. 2008, 693, 399−407. (b) Fung, H. S.;
Chan, Y. W.; Cheung, C. W.; Choi., K. S.; Lee, S. Y.; Qian, Y. Y.; Chan,
K. S. Organometallics 2009, 28, 3981−3989.
spectra. 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.
(27) Wayland, B. B.; Van Voorhees, S. L.; Wilker, C. Inorg. Chem.
1986, 25, 4039−4042.
(28) (a) Tse, A. K. S.; Wu, B. M.; Mak, T. C. W.; Chan, K. S.
J. Organomet. Chem. 1998, 568, 257−261. (b) Dolphin, D.; Halko, D.
J.; Johnson, E. Inorg. Chem. 1981, 20, 4348−4351.
(29) (a) Chan, K. S.; Lau, C. M. Organometallics 2006, 25, 260−265.
(b) Wayland, B. B.; Van Voorhees, S. L.; Wilker, C. Inorg. Chem. 1986,
25, 4039−4042.
ACKNOWLEDGMENTS
■
We are grateful to the Research Grants Council of Hong Kong
of the SAR of China for financial support (No. 400308).
REFERENCES
■
(1) Murakami, M.; Ito, Y. Cleavage of Carbon-Carbon Single Bonds
by Transition Metals. In Topics in Organometallic Chemistry; Murai, S.,
Ed.; Springer-Verlag: Berlin, 1999; Vol. 3, 97−129.
(2) Taw, F. L.; Mueller, A. H.; Bergman, R. G.; Brookhart, M. J. Am.
Chem. Soc. 2003, 125, 9808−9813.
(3) Chan, Y. W.; Chan, K. S. J. Am. Chem. Soc. 2010, 132, 6920−
6922.
(4) Lee, S. Y.; Lai, T. H.; Choi, K. S.; Chan, K. S. Organometallics
2011, 30, 3691−3693.
(5) Li, B. Z.; Fung, H. S.; Song, X.; Chan, K. S. Organometallics 2011,
30, 1984−1990.
(6) Fung, H. S.; Li, B. Z.; Chan, K. S. Organometallics 2010, 29,
4421−4423.
(7) We would like to make an amendment to our previous report in
ref 6. RhIII(ttp)Me catalyzed the aldol condensation of acetophenone
at 200 °C to give 4% yield (wrt PhCOMe) or 700% yield (wrt Rh) of
1,3,5-triphenylbenzene in 12 days, not 28% yield (wrt PhCOMe).
(8) Luo, Y. R. Comprehensive Handbook of Chemical Bond Energies;
CRC Press: Boca Raton, FL, 2007.
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dx.doi.org/10.1021/om200788p | Organometallics 2012, 31, 570−579