Sterically enhanced, selective C(CO)-C(α) bond cleavage of a ketones by rhodium porphyrin methyl

Article abstract of DOI:10.1021/om1007852

Selective carbon(CO)-carbon(α) bond activation of ketones was achieved by rhodium(III) 5,10,15,20-tetrakis-4-toylporphyrinato methyl (Rh(ttp)Me (1)) to yield the corresponding rhodium porphyrin acyls at temperatures as low as 50 °C. More hindered isopropyl ketones were much more reactive than ethyl or methyl ketones. Rh(ttp)OH (3a) was proposed to be the intermediate to cleave the C(CO)-C(α) bond.

Full text of DOI:10.1021/om1007852

Organometallics 2010, 29, 4421–4423 4421
DOI: 10.1021/om1007852
Sterically Enhanced, Selective C(CO)-C(r) Bond Cleavage of a Ketones
by Rhodium Porphyrin Methyl
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
Received August 12, 2010
Summary: Selective carbon(CO)-carbon(R) bond activation
of ketones was achieved by rhodium(III) 5,10,15,20-tetrakis-4-
toylporphyrinato methyl (Rh(ttp)Me (1)) to yield the corre-
sponding rhodium porphyrin acyls at temperatures as low as
50 °C. More hindered isopropyl ketones were much more
reactive than ethyl or methyl ketones. Rh(ttp)OH (3a) was pro-
posed to be the intermediate to cleave the C(CO)-C(R) bond.
and can undergo CCA even at 50 °C. Rhodium(III)
porphyrin hydroxide is the intermediate in cleaving the
C(CO)-C(R) bond (Scheme 1). We now communicate our
results.
Results and Discussion
Initially, Rh(ttp)Me (1) underwent selective C(CO)-C(R)
bond activation successfully with various aryl and aliphatic
ketones at 200 °C to give the corresponding rhodium por-
phyrin acyls (Table 1, eq 1). Rh(ttp)COPh (2a) was selec-
tively formed from aryl alkyl ketones (Table 1, entries 1-4),
and isopropyl ketones reacted much faster than methyl and
ethyl ketones. Acetophenone, propiophenone, and n-butyr-
ophenone reacted at a similar rate at 200 °C to give Rh-
(ttp)COPh (Table 1, entries 1-3). The reactions with acetone
and diethyl ketone gave Rh(ttp)COMe (2b) in 20% yield and
Rh(ttp)COEt (2c) in 45% yield at 200 °C in about 16 days
(Table 1, entries 5, 6). Likely, poor observed solubilities of
Rh(ttp)Me in acetone and diethyl ketone account for the low
product yields. Surprisingly, CCA of the bulkier isopropyl
ketones required a much shorter reaction time (Table 1,
entries 4, 7, 8). The most reactive diisopropyl ketone reacted
completely in only 30 min to give 86% yield of Rh(ttp)COiPr
(2d). The coproduct acetone (δ ∼1.5 ppm in C6D6) was also
observed.
The high reactivities of isopropyl ketones prompted us to
examine the reaction at mild conditions (Table 2, eq 2).
Isobutyrophenone and methyl isopropyl ketone were found
to react with Rh(ttp)Me even at 50 °C in 3 days to give a 41%
yield of Rh(ttp)COPh and a 57% yield of Rh(ttp)COMe,
respectively (Table 2, entries 1, 2). The most reactive diiso-
propyl ketone gave Rh(ttp)COiPr in 94% yield at 50 °C in
1 day (Table 2, entry 3).
Introduction
Carbon(CO)-carbon(R) bond activation (CCA) of car-
bonyl compounds via oxidative addition by rhodium(I)
complexes has been widely studied due to its importance in
organic transformation1 as well as in mechanistic under-
standings.2 Typical examples of the C(CO)-C(R) bond
activations of cyclobutanone are driven by ring strain release
with Rh(I) complexes, such as Rh(PPh3)3Cl.3 Moreover, Jun
et al. have developed chelation-assisted C(CO)-C(R) bond
activations of unstrained ketone by Rh(PPh3)3Cl with 2-ami-
no-3-picoline as a cocatalyst.4 Murakami et al. applied the
chelation strategy on the regioselective C(CO)-C(R) bond
cleavage of R-(o-hydroxylphenyl)cyclobutanone on the
hindered side by low-valent [Rh(cod)2]BF4.3c
Oxidative addition with a trivalent group 9 transition
metal complex to a carbon-carbon bond is less common,
as a M(V) (M = Co, Rh or Ir) intermediate would be
involved.5 We have previously reported the C(CO)-C(R)
bond cleavage of acetophenones by iridium(III) porphyrin
carbonyl chloride at 200 °C.6 Both Ir(ttp)H and Ir(ttp)OH
are the possible intermediates that cleave the C(CO)-C(R)
bond of acetophenones, likely through σ-bond metathesis.
Herein, we report the selective C(CO)-C(R) bond activa-
tion of various unstrained ketones and an acyl transfer to
rhodium(III) porphyrin. We have found that isopropyl
ketones are much more reactive than methyl or ethyl ketones
To further identify the organic coproduct structure, an
“intramolecular trap”, the cyclic 2,6-dimethylcyclohexa-
none, was then reacted with Rh(ttp)Me at 100 °C. To our
delight, an 85% yield of Rh(ttp)COCHMe(CH2)3COMe
(2e),8 with two characteristic carbonyl signals at 208.12
ppm (singlet) and 207.42 ppm (doublet, JRh-C =29.8 Hz)
in the 13C NMR spectrum, was obtained in 4 days (Scheme 2).
An X-ray analysis further established the product structure
(Figure 1).
*Corresponding author. E-mail: ksc@cuhk.edu.hk.
(1) Jun, C. H.; Lee, J. H. Pure Appl. Chem. 2004, 76, 577–587.
(2) Park, Y. J.; Park, J. W.; Jun, C. H. Acc. Chem. Res. 2008, 41, 222–
234.
(3) (a) Huffman, M. A.; Liebeskind, L. S.; Pennington, W. T. Orga-
nometallics 1992, 11, 255–266. (b) Murakami, M.; Takahashi, K.; Amii, H.;
Ito, Y. J. Am. Chem. Soc. 1997, 119, 9307–9308. (c) Murakami, M.; Tsuruta,
T.; Ito, Y. Angew. Chem., Int. Ed. 2000, 39, 2484–2486.
(4) (a) Jun, C. H.; Lee, H. J. Am. Chem. Soc. 1999, 121, 880–881.
(b) Ahn, J.-A.; Chang, D.-H.; Park, Y. J.; Yon, Y. R.; Loupy, A.; Jun, C. H.
Adv. Synth. Catal. 2006, 348, 55–58.
ꢀ
(5) (a) For oxidative addition (OA) of the C-H bond: Gomez, M.;
(7) Luo, Y. R. Handbook of Bond Dissociation Energies in Organic
Compounds; CRC Press: Boca Raton, FL, 2002.
(8) After CCA of the C(CO)-C(R) bond by Rh(ttp)OH, Rh(ttp)-
COCHMe(CH2)3CHOHMe presumably forms. The second carbonyl
comes from dehydrogenation of the hydroxyl group (see Scheme 3 for
details).
Robinson, D. J.; Maitlis, P. M. J. Chem. Soc., Chem. Commun. 1983,
825–826. (b) For OA of the Si-H bond: Klei, S. R.; Tilley, T. D.; Bergman,
R. G. J. Am. Chem. Soc. 2000, 122, 1816–1817.
(6) Li, B. Z.; Song, X.; Fung, H. S.; Chan, K. S. Organometallics 2010,
29, 2001–2003.
r
2010 American Chemical Society
Published on Web 09/27/2010
pubs.acs.org/Organometallics

4422 Organometallics, Vol. 29, No. 20, 2010
Fung et al.
Scheme 1. Selective C(CO)-C(r) Activation of Ketones and
Acyl Transfer by Rh(ttp)OH
Table 1. CCA of Ketones by Rh(ttp)Me
Figure 1. ORTEP presentation of the molecular structure
with numbering scheme for Rh(ttp)COCHMe(CH2)3COMe
(2e) (30% probability displacement ellipsoids).
Scheme 2. Reaction of Rh(ttp)Me with 2,6-Dimethylcyclo-
hexanone
a Acetone was observed after the reaction.
by σ-bond metathesis, to give a rhodium porphyrin acyl
and an alcohol (Scheme 3, eq iii). Further dehydrogena-
tion of the alcohol by Rh(ttp)OH affords the carbonyl
compound and Rh(ttp)H9 (Scheme 3, eq iv). The R-CHA
Table 2. CCA of Ketones by Rh(ttp)Me at 50 °C
9
complex is regenerated when Rh(ttp)H or Rh2(ttp)2
further reacts with ketones (Scheme 3, eq iia). Experi-
ments were then carried out to validate the proposed
mechanism.
Eq i. Rh(ttp)Me indeed catalyzed the self-Aldol condensa-
tion of acetophenone at 200 °C in 12 days to give a 28% yield
of 1,3,5-triphenylbenzene by 1H NMR, and water was observed
visually in 5 days (eq 3).10
a 32% yield of Rh(ttp)Me recovered. b 39% yield of Rh(ttp)Me
recovered.
Based on the above stoichiometry and the reported me-
chanism for CCA with Ir(ttp)Cl(CO),6 Scheme 3 illustrates a
proposed mechanism for ketone CCA by Rh(ttp)Me. Rh-
(ttp)Me can catalyze Aldol condensation of non-isopropyl
ketone to generate 1,3,5-triphenylbenzene and water
(Scheme 3, eq i). Concurrently, Rh(ttp)Me can react with
the R-carbon-hydrogen bond of an alkyl ketone to give an
R-carbon-hydrogen bond activation (R-CHA) complex and
methane (Scheme 3, eq iia). Rh(ttp)OH (3a) is formed upon
hydrolysis of the R-CHA complex (Scheme 3, eq iib). Then
the C(CO)-C(R) bond is cleaved by Rh(ttp)OH, presumably
Eq iia. Rh(ttp)Me activated the R-carbon-hydrogen bond
of acetophenone at 200 °C in 30 min, and Rh(ttp)CH2COPh
(9) (a) For equilibrium between Rh(por)OH, Rh(por)H, and Rh2-
(por)2, see: Fu, X.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 2623–
2631. (b) Rh(ttp)H and Rh2(ttp)2 reacted with ketone to give back R-CHA
complex; see Supporting Information for details.
(10) (a) Lewis acid-catalyzed aldol condensation of acetophenone:
Fumiaki, O.; Yuichi, I.; Yuusuke, T.; Masato, E.; Tsuneo, S. Synlett
2008, 15, 2365–2367. (b) Rhodium porphyrin-catalyzed aldol condensation
of other ketones: Aoyama, Y.; Tanaka, Y.; Yoshida, T.; Toi, H.; Ogoshi, H.
J. Organomet. Chem. 1987, 329, 251–266.

Communication
Organometallics, Vol. 29, No. 20, 2010 4423
Scheme 3. Proposed Mechanism for Ketone CCA by Rh(ttp)Me
The source of water for non-Aldolable ketones is the trace
amount of residual water in ketones. Added water should
therefore increase the reaction rate. Indeed, added water
(100 equiv) accelerates the reaction of Rh(ttp)Me and 2,6-
dimethylcyclohexanone, and the reaction time was shor-
tened from 4 days to 1 day (Scheme 2 vs eq 6).
Figure 2. Relative rate of hydrolysis of R-CHA complexes.
(4) and methane in 66% and 87% yield were observed in
benzene-d6, respectively (eq 4, Scheme 3, eq iia). A 33% yield
of Rh(ttp)H (3b) formed, likely due to the hydrolysis of
Rh(ttp)CH2COPh (Scheme 3, eq iib, see below).
We propose two possibilities for the origin of the sterically
enhanced reactivity of isopropyl ketone. First, the C(CO)-
C(iPr) bond is around 3 kcal mol-1 weaker than the C(CO)-
C(Me) bond (Table 1).7 The hydroxyl group in Rh(ttp)OH
is not very sterically demanding, so the CCA occurs faster
at a weaker carbon-carbon bond. Second, the R-CHA
complexes of isopropyl ketones are more susceptible
toward water hydrolysis to generate a higher concentra-
tion of Rh(ttp)OH (Figure 2).
In summary, we report that various unstrained ketones
undergo CCA by rhodium porphyrins at temperatures as low
as 50 °C. Bulkier isopropyl ketones are more reactive than the
others. Mechanistically, Rh(ttp)OH is suggested to be the inter-
mediate for cleaving the carbon(CO)-carbon(R) bond. To the
best of our knowledge, the results presented here show the first
example of CCA by rhodium porphyrin hydroxide.14 We are
working on further evidence to support the mechanism as well as
developing catalytic carbon(CO)-carbon(R) bond activation.
Eq iib. Rh(ttp)CH2COPh was rapidly hydrolyzed by water
(100 equiv) at 200 °C in 5 min to give a 42% yield of
Rh(ttp)H, a 13% yield of Rh2(ttp)2 (3c),9 and a 57% yield
of acetophenone, respectively (eq 5). The nearly 1:1 ratio of
the total rhodium products and PhCOMe supports the water
hydrolysis. The hydrolysis product of Rh(ttp)OH is presum-
ably thermally unstable and rapidly gives Rh2(ttp)2.11 Both
Rh(ttp)H and Rh2(ttp)2 can activate the R-carbon-hydrogen
bond of ketones.9
Eqs iii and iv. The Rh-OH insertion to the C(CO)-C(R)
bond of ketone is supported by the organic coproduct,
acetone (Table 1, entry 8). and Rh(ttp)COCHMe(CH2)3-
COMe (Scheme 2).12 A rhodium porphyrin acyl and
an alcohol are generated along with the C(CO)-C(R) bond
cleavage (Scheme 3, eq iii). Dehydrogenation of an alcohol
by Rh(ttp)OR (R = Me or H) to give Rh(ttp)H and a
carbonyl compound has been reported (Scheme 3, eq iv).13
Acknowledgment. We thank the Direct Grant of
CUHK for financial support.
Supporting Information Available: Tables and figures of
1
crystallographic data for complex 2e (CIF and PDF) and H
and 13C NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
(11) The Rh(ttp)OH analogue, Ir(ttp)OH, was proposed to undergo
reduction to give Ir2(ttp)2 and IrIII(ttp)H; see ref 6 for details.
(12) Selective formation ofRh-OH over Rh-HorRhII due to the polar
nature of ketones. For Rh-OH formation in aqueous solution, see ref 9a.
(13) (a) Collman., J. P.; Boulatov, R. Inorg. Chem. 2001, 40, 560–563.
(b) Fung, H. S.; Chan, Y. W.; Cheung, C. W.; Choi, K. S.; Lee, S. Y.; Qing,
Y. Y.; Chan, K. S. Organometallics 2009, 28, 3981–3989.
(14) Precedent CCA examples by rhodium porphyrin include: (a)
TEMPO CCA by rhodium(II) porphyrin: Chan, K. S.; Li, X. Z.; Dzik,
W. I.; de Bruin, B. J. Am. Chem. Soc. 2008, 130, 2051–2061. (b)
Cyclooctane CCA by rhodium(II) porphyrin-catalyzed Rh-H insertion:
Chan, Y. W.; Chan, K. S. J. Am. Chem. Soc. 2010, 132, 6920–6922.