Activation of aldehydic carbon-hydrogen bonds under aerobic conditions by masked rhodium(III) porphyrin cation

Article abstract of DOI:10.1021/om070135e

Rh^III(ttp)CH₂CH₂OH activated the aldehydic carbon-hydrogen bonds of functionalized aryl and enolizable aldehydes to give high yields of Rh(ttp)COR at 50 °C under both anaerobic and aerobic conditions. The Rh(ttp)(C₂H₄)OH intermediate was proposed to form via β-hydroxy elimination. The reactions exhibited rate and yield enhancement upon the addition of Ph₃P, suggesting ligand-promoted β-elimination. The nonlinear free energy relationship of the Hammett plot suggested a multistepwise reaction with the rate-determining step (binding or activation) dependent on the electronic effect of para substituents of aryl aldehydes.

Full text of DOI:10.1021/om070135e

Organometallics 2007, 26, 1981-1985
1981
Activation of Aldehydic Carbon-Hydrogen Bonds under Aerobic
Conditions by Masked Rhodium(III) Porphyrin Cation
Kin Shing Chan,* Cheuk Man Lau, Siu Kwan Yeung, and Tsz Ho Lai
Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, New Territories, Hong Kong,
People’s Republic of China
ReceiVed February 11, 2007
RhIII(ttp)CH2CH2OH activated the aldehydic carbon-hydrogen bonds of functionalized aryl and
enolizable aldehydes to give high yields of Rh(ttp)COR at 50 °C under both anaerobic and aerobic
conditions. The Rh(ttp)(C2H4)OH intermediate was proposed to form via â-hydroxy elimination. The
reactions exhibited rate and yield enhancement upon the addition of Ph3P, suggesting ligand-promoted
â-elimination. The nonlinear free energy relationship of the Hammett plot suggested a multistepwise
reaction with the rate-determining step (binding or activation) dependent on the electronic effect of para
substituents of aryl aldehydes.
Carbon-hydrogen bond activation (CHA) is an important
area of research in organometallic chemistry. Examples of
carbon-hydrogen bond activation by high-valent late-transition-
metal complexes, such as rhodium(III)2-4 and iridium(III),5,6
have been much less reported than their lower valent metal
complexes. These reaction systems raise interesting mechanistic
issues. A few mechanistic possibilities exist, such as oxidative
addition,7 σ-bond metathesis,8 and (base-promoted) heterolysis,9
and these are often not easily defined. Only in the successful
isolation of high-valent metal(V) intermediates can the oxidative
addition be firmly established.7 Furthermore, the reactivities and
selectivities of some of the cationic complexes depend on the
nature of counteranions.2,10 Therefore, further examples and
studies would aid to explore the rich chemistry in high-valent
late-transition-metal complexes in bond activation.
We have reported earlier that the high-valent RhIII(ttp)Cl
(ttp ) tetra-4-tolylporphyrinate) and alkyls undergo selective
aldehydic CHA with both aryl and aliphatic aldehydes.11 This
newly discovered five-coordinated square-pyramidal Rh(III)
system for CHA, however, requires a fairly high temperature
of 200 °C and suffers from the disadvantage of solvent-free
conditions, with excess reagent serving as the solvent. Functional
group compatibility is also limited. Rh(ttp)OTf,2,11 with a more
labile counteranion, has also been found to react more quickly
than Rh(ttp)Cl. A cationic rhodium porphyrin intermediate for
the CHA has been suggested, and the roles of anions appear to
be important.11
Our recent proposed â-amino elimination12 of Rh(ttp)CH2-
CH2NH2 to give a Rh(ttp) cationic intermediate at 80 °C has
led us to develop a mild, convenient synthesis of a reactive
cationic rhodium porphyrin via â-heteroatom elimination.13
Therefore, we have taken advantage of the reported â-hydroxyl
elimination reaction of Rh(oep)CH2CH2OH (oep ) octaeth-
ylporphyrinate) to give Rh(oep)(CH2CH2)OH by Wayland14 as
a convenient masked cationic rhodium porphyrin (eq 1). We
* To whom correspondence should be addressed. E-mail: ksc@
cuhk.edu.hk.
(1) (a) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879-2932.
(b) Rukebgm V.; Sirlin, C.; Pfeller, M. Chem. ReV. 2002, 102, 1731-1789.
(c) Labinger, J. A.; Bercaw, J. E. Nature 2002, 507, 507-514. (d) Crabtree,
R. H. J. Organomet. Chem. 2004, 689, 4083-4091. (e) Lersch, M.; Tilset,
M. Chem. ReV. 2005, 105, 2471-2526.
â-heteroatom elimination8
LRh(por)CH2CH2X
(2) Aoyama, Y.; Yoshida, T.; Sakurai, K. I.; Ogoshi, H. Organometallics
1986, 5, 168-173.
I
(3) Liu, X. Y.; W. J.; Bhalla, G.; Periana, R. A. Organometallics 2004,
23, 3584-3586.
LRh(por)(CH2CH2)X
(1)
(4) Corkey, B. K.; Taw, F. L.; Bergman, R. G.; Brookhart, M. Polyhedron
2004, 23, 2943-2954.
II
L ) Ph3P, none; por ) oep, ttp; X ) NH2,OH
(5) (a) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970-1973.
(b) Tellers, D. M.; Yung, C. M.; Arndtsen, B. A.; Adamson, D. R.; Bergman,
R. G. J. Am. Chem. Soc. 2002, 124, 1400-1410.
now report that Rh(ttp)CH2CH2OH underwent successful se-
quential â-hydroxyl elimination and selective aldehydic CHA
with aryl and alkyl aldehydes under mild, aerobic, and ligand-
promoted conditions in THF with broad functional group
compatibility.
(6) (a) Periana, R. A.; Liu, X. Y.; Bhalla, G. Chem. Commun. 2002,
3000-3001. (b) Bhalla, G.; Periana, R. A. Angew. Chem., Int. Ed. 2005,
44, 1540 -1543.
(7) (a) An example of an Ir(V) intermediate has been reported in the
silane activation: Klei, S. R.: Tilley, T. D.: Bergman, R. G. J. Am. Chem.
Soc. 2000, 122, 1816-1817. (b) Examples of activation of aldehydes by a
cationic methyl Rh(III) complex has been reported but the mechanism is
not defined.4
Results and Discussion
(8) Thompson, M. E.; Baxteer, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203-219.
(9) (a) Harkin, S. B.; Peters, J. C. Organometallics 2002, 21, 1753-
1755. (b) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 8870-
8888. (c) Liang, L. C.; Lin, J.-M.; Lee, W.-Y. Chem. Commun. 2005, 2462-
2464.
(10) Effect of the counteranion in classical oxidative addition of
aldehyde and MeI: Goikhman, R.; Milstein, D. Angew. Chem., Int. Ed.
2001, 40, 1119-1122.
Discovery of the Reaction Protocol. Rh(ttp)CH2CH2OH (2)
was conveniently prepared in a high yield of 90% by the reaction
(11) Chan, K. S.; Lau, C. M. Organometallics 2006, 25, 260-265.
(12) Steinborn, D. Angew. Chem., Int. Ed. 1992, 31, 401-421.
(13) Yeung, S. K.; Chan, K. S. Organometallics 2005, 24, 2561-2563.
(14) Wayland, B. B.; Van Voorhees, S. L.; Del Rossi, K. J. J. Am. Chem.
Soc. 1987, 109, 6513-6515.
10.1021/om070135e CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/13/2007

1982 Organometallics, Vol. 26, No. 8, 2007
Chan et al.
Table 1. Effect of Concentration of Aldehyde and Air in
CHA
Table 3. CHA of Aliphatic Aldehydes by Rh(ttp)CH2CH2OH
(2)
entry
FG
time (h)
atmosphere
yield (%)
entry
R
time (h)
yield (%)
1
2
3
4
5
6
H
H
H
F
F
F
72
48
60
72
60
24
N2
N2
air
N2
N2
air
15a (3)
62b (3)
81b (3)
28a (4)
65b (4)
74b (4)
1
2
3
4
C2H5
0.5
6
36
48
80 (12)
90 (12)a
noneb
tBu
nonea,b
a 1.2 equiv of PPh3. b No reaction.
a 10 equiv of ArCHO. b 100 equiv of ArCHO.
functional group compatibility is likely due, at least partially,
to the lower temperature required to generate reactive cationic
rhodium porphyrin complexes.
Table 2. Effect of Phosphine Ligand in Aerobic CHA
entry
FG
time (h)a
yield (%)a
Effect of Ph3P. We envisoned that the â-heteroatom elimina-
tion could be promoted by the addition of a ligand to give a
more stabilized rhodium porphyrin cation (II in eq 1). We were
delighted to find out that addition of Ph3P (1.2 equiv) enhanced
the rates with little difference in product yields of the aerobic
aldehydic CHA in most substrates (Table 2, entries 1-3 and
6). The products were isolated after column chromatography
to be free of Ph3P coordination, presumably due to the strong
trans effect of ArCO groups. Any aerobic oxidation of Ph3P
into Ph3PdO was not detrimental, as Ph3PdO was a less
effective ligand. Added Ph3PdO was found to give Rh(ttp)-
COPh either more slowly or to lower the yields under nitrogen
(46 h, 62%) and in air (16 h, 67%).
1
2
3
4
5
6
7
8
9
H
F
Cl
CF3
CN
Me
5 [60]
7 [24]
16 [48]
48 [36]
60 [48]
7 [24]
36 [36]
24 [24]
72 [72]
72
76 [81] (3)
65 [74] (4)
82 [67] (5)
35 [44] (6)
46 [46] (7)
75 [51] (8)
71 [51] (9)
83 [83] (10)
54 [42] (11)
none
tBu
OMeb
NMe2
NO2
Br
10
11
72
none
a Average of at least of two runs. Results without Ph3P are given in
brackets. b Yield was obtained at 80 °C.
Aliphatic Aldehyde. Apart from aromatic aldehydes, Rh-
(ttp)CH2CH2OH 2 also reacted successfully with non-hindered
aliphatic aldehydes (100 equiv) in aerobic conditions (Table 3;
eq 4). Propanal reacted with Rh(ttp)CH2CH2OH (2) to give a
of Rh(ttp)Cl (1) with NaBH4/BrCH2CH2OH. Initially, Rh(ttp)-
(CH2CH2)OH in THF reacted with PhCHO (10 equiv) in 72 h
at 50 °C under N2 to give Rh(ttp)COPh (3) in 15% yield
(Table 1, entry 1; eq 2). Though the yield was low, the reaction
THF
Rh(ttp)CH CH OH
+ RCHO 50 °C, time8 RCORh(ttp)
2
2
2
(4)
good yield of the aldehydic CHA product EtCORh(ttp) (12; 80%
yield) within 30 min. No CHA at the R-CH of the carbonyl
group was observed, in contrast with the reaction of enolizable
carbonyl with Rh(oep)ClO4 (oep ) octaethylporphyrinato).15
Therefore, propanal was more reactive than PhCHO. It is likely
that the sterically less hindered propanal can bind to the rhodium
easily and thus facilitates the reaction rate. When 1.2 equiv of
PPh3 was added, the reaction time increased from 0.5 to 6 h.
Presumably, after the addition of Ph3P, (ttp)Rh(Ph3P)(CH2d
CH2)OH (13) is formed but is less reactive toward a more
electron-rich aliphatic aldehyde. Unfortunately, the sterically
was more successful than the high-temperature aldehydic CHA
with Rh(ttp)Cl. Furthermore, no reduction product of Rh(ttp)-
CH2Ph was observed.11 A more selective protocol was thus
identified. When PhCHO was increased from 10 to 100 equiv,
a higher yield of 3 was obtained in 62% (Table 1, entry 2).
Presumably, a higher concentration of PhCHO favored the
selective formation of 3 with less side product formed. More
notably, when the reaction was run in air, an even higher yield
of 3, 81%, was obtained in a slightly longer time of 60 h
(Table 1, entry 3).11 The same concentration and air effects were
also observed in the reactions with 4-fluorobenzaldehyde
(Table 1, entries 4-6).
t
very hindered BuCHO did not react at all.
Mechanistic Studies. In order to gain some mechanistic
understandings of the electronic effect of CHA, the Hammett
plot was constructed from a series of competition experiments
using a 1:1 mixture of excess 4-substituted benzaldehyde and
PhCHO with Rh(ttp)CH2CH2OH (2) at 50 °C in THF in aerobic
conditions (Table 4; eq 5). All reactions were monitored by
With the new CHA reaction protocol conducted in air, Rh-
(ttp)CH2CH2OH (2) is more reactive and more functional group
compatible than Rh(ttp)Cl (Table 2; eq 3).11 With 100 equiv of
ArCHO used, most reactions with aryl aldehydes required less
than 72 h at 50 °C to give 42-81% of Rh(ttp)COAr (Table 2).
In contrast with the case for Rh(ttp)Cl, no CHA at the benzylic,
tBu, and methoxy positions was observed (Table 2, entries 6-8).
4-Chloro-, 4-cyano-, and 4-dimethylamino-substituted benzal-
dehydes all smoothly reacted (Table 2, entries 3, 5, and 9).
Anisaldehyde, though, required a higher temperature of
80 °C and did not undergo any carbon-oxygen cleavage to give
Rh(ttp)Me. Other functional groups such as nitro and bromo
were not compatible (Table 2, entries 10 and 11). This broader
TLC analysis to ensure complete consumption of Rh(ttp)CH2-
(15) (a) Aoyama, Y.; Yamagishi, A.; Tanaka, Y.; Toi, H.; Ogoshi, H. J.
Am. Chem. Soc. 1987, 109, 4735-4737. (b) Aoyama, Y.; Tanaka, Y.;
Yoshida, H.; Toi, H.; Ogoshi, H. J. Organomet. Chem. 1987, 329, 251-
266.

ActiVation of Aldehydic Carbon-Hydrogen Bonds
Organometallics, Vol. 26, No. 8, 2007 1983
Table 4. Competition Experiments of Aldehydic CHA
a
entry
p-FG
σp
log k (HFG/HH)
1
2
3
4
5
6
7
N(Me)2
tBu
Me
F
Cl
CF3
CN
-0.32
-0.15
-0.14
0.15
0.34
0.53
-0.2183
-0.2001
-0.1348
-0.0529
0.3099
0.0946
-0.1978
0.71
a σp: para-substituent constant.
CH2OH 2. Furthermore, the relative ratios of the products did
not change with time. Figure 1 shows that a nonlinear free
energy relationship was observed from the Hammett plot using
the substituent constant σp.16 As the para substituent became
more electron withdrawing, the rate increased (Me to F and Cl).
More electron poor substituents (Cl to CF3 and CN), however,
lowered the rate with an inversion of rate being observed.
Therefore, the rate-determining step of the reactions likely
changes with the para substituents of aryl aldehydes.
Scheme 1 illustrates a plausible mechanism for the CHA
reactions. Rh(ttp)CH2CH2OH initially forms the coordination
complex (Ph3P)Rh(ttp)CH2CH2OH (13) with Ph3P (eq 6). Then
13 undergoes â-hydroxyl elimination to give the Rh-ethene
complex 14.14 In the presence of Ph3P, a higher equilibrium
concentration of Rh(ttp)OH would result. The rate would then
be enhanced. Ligand substitution of 14 with an aldehyde occurs
to displace the ethene, and its rate and binding constants are
facilitated by an electron-rich aldehyde to form the more stable
complex 15. Either the aldehydic oxygen or the CO π bond
can donate an electrons to the cationic rhodium center. Both
are facilitated by electron-donating substitutents at the aldehydes.
The resultant Rh(ttp) aldehyde complex 15 forms the acyl-
rhodium porphyrin product via aldehydic CHA, which may be
faster for an electron-poor aldehyde if oxidative addition is
operating. The inversion in the Hammett plot likely arises due
to the change of rate-determining step from the aldehydic CHA
step (k4) for an electron-donating aldehyde to the binding step
(k3) for an electron-withdrawing aldehyde.17
Figure 1. Hammett plot of CHA of aryl aldehydes with Rh(ttp)-
CH2CH2OH (2).
involving a covalent rhodium porphyrin hydroxide18 is also
reasonable. 14 could also exist as a covalent, cis (Ph3P)Rh-
(ttp)OH-ethene complex and react with an aldehyde to give a
seven-coordinated cis-(Ph3P)Rh(ttp)OH-aldehyde complex
(cis OH and aldehyde) before yielding the acyl product.
Regardless of the ionic or covalent nature of the Rh-OH
bond, the hydroxide group is very important,18-19 as Rh(ttp)-
OTf did not react with PhCHO at 50 °C in 2 days.11 It is likely
that the CHA step is likely a heterolysis9 or its closely related
σ-bond metathesis.8 Oxidative addition4 appears to be less likely,
since a less common Rh(V) intermediate is required. Further-
more, Rh(ttp)OTf with a less coordinating anion would be
expected to undergo oxidative addition more quickly, due to
steric preference.
Rh(ttp)H is not likely to be an intermediate, even though Rh-
(ttp)H reacts with PhCHO at 200 °C to give Rh(ttp)COPh in
35% yield.11 First, â-hydride elimination of Rh(ttp)CH2CH2-
OH to give Rh(ttp)H is likely noncompetitive at 50 °C,
especially in the presence of added ligand.20 Addition of a ligand
such as Ph3P or pyridine has been reported to inhibit the rate
of 1,2-rearrangement of Rh(por) (por ) porphyrinate) alkyls
via the suppressed â-hydride elimination.21 Furthermore, Rh-
(ttp)H reacted with neat PhCHO to give only a trace of Rh-
(ttp)COPh at 50 °C in 48 h. The reactivity of Rh(ttp)H is
therefore too low to be a viable intermediate to directly react
with PhCHO at this temperature.
Scheme 1
k1
Rh(ttp)CH CH OH
(L)Rh(ttp)CH CH OH
2 2
+ Ly z
(6)
(7)
2
2
k-1
2
13
(L)Rh(ttp)CH CH OH yk2z
[(L)(ttp)Rh(C2H4)]+OH-
2
2
k-2
14
[(L)(ttp)Rh(C2H4)]+OH- +
ArCHO yk3z
+
- + C2H4 (8)
We do not fully understand the effect of air on rate and yield
enhancement. As RhII(tmp) (tmp ) tetrakismesitylporphryinate)
did not react with PhCHO in benzene, metalloradical activation
by Rh(II) porphyrin is less probable. One possible rationalization
could be the (co)presence of a minor pathway to generate small
amounts of Rh(ttp)H by â-hydride elimination from Rh(ttp)-
CH2CH2OH. Because Rh(ttp)H does not react with PhCHO at
50 °C, it reacts with oxygen to give the more reactive (Ph3P)-
[(L)Rh(ttp)(ArCHO)] OH
k-3
15
k4
[(L)Rh(ttp)(ArCHO)]+OH- 98 ArCORh(ttp) + L + H2O
(9)
L ) PPh3
In the absence of structural data, the detailed structures of
the intermediates remain speculative. An alternative mechanism
(18) Fu, X.; Li, S.; Wayland, B. B. J. Am. Chem. Soc. 2006, 128, 8947-
8954.
(19) For an iridium methoxy complex undergoing CHA: Tenn, W. J.,
III; Young, J. J. H.; Bhalla, G.; Oxgaard, J.; Goddard, W. A., III; Periana,
R. A. J. Am. Chem. Soc. 2005, 127, 14172-14173.
(20) Zhao, H.; Ariafard, A.; Lin, Z. Organometallics 2006, 25, 812-
819.
(16) (a) Issaacs, N. S. Physical Organic Chemistry; ELBS, Longman:
Avon, U.K., 1987. (b) A larger deviation in the linear region from
experimental points was observed using σp+, indicating little resonance
interaction.
(17) The opposing electronic effect of binding and reaction steps in
organometallic reactions has been documented: (a) Halpern, J.; Okamato.
T. Inorg. Chim. Acta 1984, 89, L53-L54. (b) Landis, C.; Halpern, J. J.
Am. Chem. Soc. 1987, 109, 1746-1754.
(21) Mak, K. M.; Chan, K. S. J. Am. Chem. Soc. 1998, 120, 9686-
9687.

1984 Organometallics, Vol. 26, No. 8, 2007
Chan et al.
60 h to yield 3 (8.5 mg, 0.0097 mmol, 81%). Rf ) 0.42 (1/1 hexane/
CH2Cl2). 1H NMR (300 MHz, CDCl3): δ 2.43 (d, 2 H, J )
7.5 Hz), 2.70 (s, 12 H), 5.98 (t, 2 H, J ) 7.6 Hz), 6.41 (t, 1 H,
J ) 7.2 Hz), 7.53 (d, 8 H, J ) 8.2 Hz), 7.95 (dd, 4 H, J ) 2.1, 8.1
Hz), 8.04 (dd, 4 H, J ) 2.1, 8.1 Hz), 8.76 (s, 8 H). HRMS
(FABMS): calcd for (C55H41N4ORh)+, m/z 876.2330; found, m/z
876.2303. IR (KBr, cm-1): ν(CdO) 1716 (s). Anal. Calcd for
C55H41N4ORh: C, 75.34; H, 4.71; N, 6.39. Found: C, 75.09; H,
4.61; N, 6.37.
Rh(ttp)OH or (Ph3P)Rh(ttp)OOH (versus (Ph3P)Rh(ttp)(C2H4)-
OH)22,23 for subsequent CHA.
In summary, we have discovered that Rh(ttp)CH2CH2OH
served as a masked rhodium porphyrin hydroxide and underwent
selective aldehydic CHA with aryl and nonbulky alkyl aldehydes
under mild conditions. Further studies are ongoing.
Experimental Section
Unless otherwise noted, all reagents were purchased from
commercial suppliers and used without purification. Hexane for
chromatography was distilled from anhydrous calcium chloride.
Tetrahyrofuran (THF) was distilled from sodium benzophenone
ketyl prior to use. Benzonitrile was distilled from anhydrous P2O5.
Benzaldehyde was purified by vacuum distillation. Thin-layer
chromatography was performed on precoated silica gel 60 F254
plates. Silica gel (Merck, 70-230 and 230-400 mesh) was used
for column chromatography. Rh(ttp)Cl11,24 and Rh(ttp)R25 were
prepared according to the literature.
1H NMR spectra were recorded on a Bruker DPX 300 (300 MHz)
spectrometer. Spectra were referenced internally to the residual
proton resonance in C6D6 (δ 7.15 ppm) and CDCl3 (δ 7.26 ppm)
or with tetramethylsilane (TMS, δ 0.00 ppm) as the internal
standard. Chemical shifts (δ) are reported in parts per million (ppm).
13C NMR spectra were recorded on a Bruker DPX 300 (75 MHz)
or Varian Innova 400 (100 MHz) spectrometer and referenced to
CDCl3 (δ 77.15 ppm). Coupling constants (J) are reported in hertz
(Hz). High-resolution mass spectra (HRMS) were recorded on a
Thermofinnigan MAT 95 XL instrument (FABMS).
Method B. Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) and 1.2
equiv of PPh3 (3.8 mg, 0.014 mmol) were mixed with 100 equiv
of benzaldehyde (124 µL, 1.20 mmol) in THF (1.0 mL) at 50 °C
for 5 h to yield 3 (8.0 mg, 0.0091 mmol, 76%).
(5,10,15,20-Tetratolylporphyrinato)(4-fluorobenzoyl)rhodium-
(III), 4-FC6H5CORh(ttp) (4).11 Method A. Rh(ttp)CH2CH2OH
(10.0 mg, 0.012 mmol) was mixed with 100 equiv of 4-fluoroben-
zaldehyde (130 µL, 1.20 mmol) in THF (1.0 mL) at 50 °C for 24
h to yield 4 (8.0 mg, 0.0089 mmol, 74%). Rf ) 0.54 (1/1 hexane/
1
CH2Cl2). H NMR (300 MHz, CDCl3): δ 2.40 (dd, 2 H, J ) 3.2,
8.7 Hz), 2.71 (s, 12 H), 5.65 (t, 2 H, J ) 8.7 Hz), 7.53 (d, 4 H,
J ) 6.0 Hz), 7.55 (d, 4 H, J ) 6.3 Hz), 7.95 (dd, 4 H, J ) 1.8,
6.0 Hz), 8.03 (dd, 4 H, J ) 1.8, 6.0 Hz), 8.77 (s, 8 H). 13C NMR
1
(75 MHz, CDCl3): δ 21.69, 112.21 (d, JC-F ) 21.6 Hz), 114.51
(d, 1JRh-C ) 20.9 Hz), 118.93 (d, 2JC-F ) 10.8 Hz), 122.90, 127.62,
131.28, 131.77, 133.96, 134.12, 137.48, 139.17, 143.17. HRMS
(FABMS): calcd for (C55H40N4OFRh)+, m/z 894.2236; found, m/z
894.2256. IR (KBr, cm-1): ν(CdO) 1711 (s).
Method B. Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) and 1.2
equiv of PPh3 (3.8 mg, 0.014 mmol) were mixed with 4-fluoroben-
zaldehyde (130 µL, 1.20 mmol) in THF (1.0 mL) at 50 °C for 7 h
to yield 4 (7.0 mg, 0.0078 mmol, 65%).
Preparation of Starting Materials. (5,10,15,20-Tetratolylpor-
phyrinato)(2-hydroxyethyl)rhodium(III), Rh(ttp)CH2CH2OH
(2). A suspension of Rh(ttp)Cl11 (100 mg, 0.11 mmol) in THF
(50 mL) and a solution of NaBH4 (40 mg, 1.08 mmol) in aqueous
NaOH (0.1 M, 2 mL) were purged with N2 for 15 min. The solution
of NaBH4 was added slowly to the suspension of Rh(ttp)Cl via a
cannula. The solution mixture was heated at 50 °C under N2 for 1
h to give a brown suspension. The solution was then cooled to
0 °C under N2 and BrCH2CH2OH (0.8 mL, 10.8 mmol) was added
via a syringe. A reddish orange suspension was formed. The reaction
mixture was then worked up by extraction with CH2Cl2/H2O. The
combined organic extract was dried (anhydrous MgSO4), filtered,
and rotary evaporated off to dryness. The reddish orange residue
was purified by column chromatography over silica gel (250-400
mesh), with a mixture of hexane and CH2Cl2 (1/1) as eluent. The
major orange fraction was collected to give a reddish orange solid
(2; 86 mg, 0.099 mmol, 90%), which was further purified by
recrystallization from CH2Cl2/CH3OH. Rf ) 0.34 (1/1 hexane/CH2-
Cl2). 1H NMR (CDCl3, 300 MHz): δ -4.97 (dt, 2 H, 2JRh-H ) 3.0
(5,10,15,20-Tetratolylporphyrinato)(4-chlorobenzoyl)rhodium-
(III), 4-ClC6H5CORh(ttp) (5).11 Method A. Rh(ttp)CH2CH2OH
(10.0 mg, 0.012 mmol) was mixed with 100 equiv of 4-chloroben-
zaldehyde (172 mg, 1.20 mmol) in THF (1.0 mL) at 50 °C for
48 h. The solvent was then removed, and 5 was isolated by column
chromatography on silica gel with hexane/CH2Cl2 (4/1). A red solid
of 5 was obtained (8.5 mg, 0.0080 mmol, 67%). Rf ) 0.70 (1/1
1
hexane/CH2Cl2). H NMR (300 MHz, CDCl3): δ 2.33 (d, 2 H, J
) 8.1 Hz), 2.71 (s, 12 H), 5.95 (d, 2 H, J ) 8.1 Hz), 7.53 (d,
4 H, J ) 6.6 Hz), 7.55 (d, 4 H, J ) 6.6 Hz), 7.93 (dd, 4 H, J )
2.1, 7.5 Hz), 8.04 (dd, 4 H, J ) 2.1, 7.5 Hz), 8.79 (s, 8 H). HRMS
(FABMS): calcd for (C55H40N4OClRh)+, m/z; found, m/z. IR
(KBr, cm-1): ν(CdO) 1703 (s). Anal. Calcd for C55H40ClN4ORh:
C, 72.49; H, 4.42; N, 6.15; Found C, 72.49; H, 4.45; N, 6.15.
Method B. Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) and 1.2
equiv of PPh3 (3.8 mg, 0.014 mmol) were mixed with 4-chloroben-
zaldehyde (172 mg, 1.20 mmol) in THF (1.0 mL) at 50 °C for
16 h to yield 5 as a red solid (9.0 mg, 0.0098 mmol, 82%).
Hz, 3JH-H
)
7.0 Hz), -2.74 (t,
1
H,
J
)
6.5 Hz), -2.20 to -2.14 (m, 2 H), 2.70 (s, 12 H), 7.52 (d, 8 H, J
) 8.4 Hz), 7.99 (dd, 4 H, J ) 2.1, 8.4 Hz), 8.05 (dd, 4 H, J ) 2.1,
8.4 Hz), 8.75 (s, 8 H). HRMS (ESI): calcd for (C50H41N4ORh)+,
m/z 816.2330; found, m/z 816.2343. Anal. Calcd for C50H41N4-
ORh: C, 73.52; H, 5.06; N, 6.86. Found: C, 73.03; H, 5.15; N,
6.71.
Reactions of Aromatic Aldehydes with Rh(ttp)CH2CH2OH
(2). (5,10,15,20-Tetratolylporphyrinato)(benzoyl)rhodium(III),
C6H5CORh(ttp) (3). General Procedure. Method A. Rh(ttp)CH2-
CH2OH (10.0 mg, 0.012 mmol) was mixed with 100 equiv of
benzaldehyde (124 µL, 1.20 mmol) in THF (1.0 mL) at 50 °C for
(5,10,15,20-Tetratolylporphyrinato)(4-R,R,R-trifluoromethyl-
benzoyl)rhodium(III), 4-CF3C6H5CORh(ttp) (6).11 Method A.
Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) was mixed with
100 equiv of 4-R,R,R-trifluoromethylbenzaldehyde (167 µL,
1.20 mmol) in THF (1.0 mL) at 50 °C for 36 h to yield 6 (5.7 mg,
1
0.0053 mmol, 44%). Rf ) 0.54 (1/1 hexane/CH2Cl2). H NMR
(300 MHz, CDCl3): δ 2.48 (d, 2 H, J ) 8.1 Hz), 2.71 (s, 12 H),
6.24 (d, 2 H, J ) 8.1 Hz), 7.53 (d, 8 H, J ) 6.6 Hz), 7.92 (dd,
4 H, J ) 2.1, 6.6 Hz), 8.04 (dd, 4 H, J ) 2.1, 6.6 Hz), 8.79 (s, 8
H). HRMS (FABMS): calcd for (C56H40N4OF3Rh)+, m/z 944.2204;
found, m/z 944.2220. IR (KBr, cm-1): ν(CdO) 1691 (s). Anal.
Calcd for C56H40N4F3ORh: C, 71.19; H, 4.27; N, 5.93. Found: C,
70.84; H 4.22; N, 5.74.
(22) Denney, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.; Goldberg,
K. A. J. Am. Chem. Soc. 2006, 128, 2508-2509.
(23) Ahijado, M.; Braun, T.; Noveski, D.; Kocher, N.; Neumann, B.;
Stalke, D.; Stammler, H.-G. Angew. Chem., Int. Ed. 2005, 44, 6947-6951.
(24) Tse, A. K. S.; Wu, B.; Mak, T. C. W.; Chan, K. S. J. Organomet.
Chem. 1998, 568, 257-261.
Method B. Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) and 1.2
equiv of PPh3 (3.8 mg, 0.014 mmol) were mixed with 4-R,R,R-
trifluoromethylbenzaldehyde (167 µL, 1.20 mmol) in THF (1.0 mL)
at 50 °C for 48 h to yield 6 (4.0 mg, 0.0042 mmol, 35%).
(25) Ogoshi, J.; Setsune, J. I.; Yoshida, Z. J. Organomet. Chem. 1978,
159, 317-318.

ActiVation of Aldehydic Carbon-Hydrogen Bonds
Organometallics, Vol. 26, No. 8, 2007 1985
(5,10,15,20-Tetratolylporphyrinato)(4-cyanobenzoyl)rhodium-
(III), 4-CNC6H5CORh(ttp) (7). Method A. Rh(ttp)CH2CH2OH
(10.0 mg, 0.012 mmol) was mixed with 100 equiv of 4-cyanoben-
zaldehyde (161 mg, 1.20 mmol) in THF (1.0 mL) at 50 °C for 48
h. The solvent was then removed, and 4-CNC6H5CORh(ttp) (7)
was isolated by column chromatography on silica gel with hexane/
CH2Cl2 (4/1) as eluent. A red solid was obtained (5.0 mg, 0.0054
mmol, 46%). Rf ) 0.42 (1/1 hexane/CH2Cl2). 1H NMR (300 MHz,
CDCl3): δ 2.44 (d, 2 H, J ) 8.7 Hz), 2.71 (s, 12 H), 6.29 (d, 2 H,
J ) 8.7 Hz), 7.54 (d, 4 H, J ) 8.1 Hz), 7.57 (d, 4H, J ) 8.4 Hz),
7.91 (dd, 4 H, J ) 2.3, 7.8 Hz), 8.03 (dd, 4 H, J ) 2.3, 7.8 Hz),
8.80 (s, 8 H). 13C NMR (75 MHz, CDCl3): δ 21.68, 109.40, 117.39,
118.07 (1JRh-C ) 18.8 Hz), 123.04, 127.74, 129.73, 131.74, 133.98,
134.06, 137.68, 138.90, 139.07, 134.11. HRMS (FABMS): calcd
for (C56H40N5ORh)+, m/z 901.2288; found, m/z 901.2282. IR
(KBr, cm-1): ν(CdO) 1639 (s), ν(CtN) 2305 (s).
Method B. Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) and 1.2
equiv of PPh3 (3.8 mg, 0.014 mmol) were mixed with anisaldehyde
(115 µL, 1.20 mmol) in THF (1.0 mL) at 80 °C for 24 h to yield
10 (9.0 mg, 0.010 mmol, 83%).
(5,10,15,20-Tetratolylporphyrinato)(4-(Dimethylamino)ben-
zoyl)rhodium(III), 4-(NMe2)C6H5CORh(ttp) (11). Method A.
Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) was mixed with 100
equiv of 4-(dimethylamino)benzaldehyde (182 mg, 1.20 mmol) in
THF (1.0 mL) at 50 °C for 3 days. The solvent was then removed
and 4-(NMe2)C6H5CORh(ttp) (11) was isolated by column chro-
matography on silica gel with hexane/CH2Cl2 (5/1) as eluent,
followed by CH2Cl2. A brick red solid was obtained (5.0 mg,
1
0.0050 mmol, 42%). Rf ) 0.30 (1/1 hexane/CH2Cl2). H NMR
(300 MHz, CDCl3): δ 2.44 (d, 2 H, J ) 8.5 Hz), 2.60 (s, 6 H),
2.70 (s, 12 H), 5.24 (d, 2 H, J ) 8.7 Hz), 7.51 (d, 4 H, J ) 5.7
Hz), 7.53 (d, 4 H, J ) 5.7 Hz), 7.95 (dd, 4 H, J ) 2.1, 7.2 Hz),
8.06 (dd, 4 H, J ) 2.1, 7.2 Hz), 8.74 (s, 8 H). 13C NMR
Method B. Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) and 1.2
equiv of PPh3 (3.8 mg, 0.014 mmol) were mixed with 4-cyanoben-
zaldehyde (161 mg, 1.20 mmol) in THF (1.0 mL) at 50 °C for
60 h to yield 7 as a red solid (5.0 mg, 0.0055 mmol, 46%).
1
(100 MHz, CD8O): δ 24.43, 43.09, 111.78 (δ, JRh-C ) 7.1 Hz),
112.58, 122.19, 127.74, 125.63, 130.74, 130.87, 133.07, 134.61,
137.37, 137.99, 140.67, 143.72, 146.73. HRMS (FABMS): calcd
for (C57H46N5ORh)+, m/z 919.2752; found, m/z 919.2763.
(5,10,15,20-Tetratolylporphyrinato)(4-methylbenzoyl)rhodium-
(III), 4-CH3C6H5CORh(ttp) (8).11 Method A. Rh(ttp)CH2CH2-
OH (10.0 mg, 0.012 mmol) was mixed with 100 equiv of
4-methylbenzaldehyde (144 µL, 1.20 mmol) in THF (1.0 mL) at
50 °C for 24 h. 8 was obtained (5.4 mg, 0.0061 mmol, 51%). Rf )
Method B. Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) and 1.2
equiv of PPh3 (3.8 mg, 0.014 mmol) were mixed with mixed
4-(dimethylamino)benzaldehyde (182 mg, 1.20 mmol) in THF
(1.0 mL) at 50 °C for 3 days to yield 15 (6.0 mg, 0.0065 mmol,
54%).
1
0.45 (1/1 hexane/CH2Cl2). H NMR (300 MHz, CDCl3): δ 1.87
(s, 3 H), 2.34 (d, 2 H, J ) 7.9 Hz), 2.70 (s, 12 H), 5.75 (d, 2 H,
J ) 7.9 Hz), 7.53 (d, 8 H, J ) 7.8 Hz), 7.92 (dd, 4 H, J ) 1.8,
6.9 Hz), 8.05 (dd, 4 H, J ) 1.8, 6.9 Hz), 8.75 (s, 8 H). HRMS
(FABMS): calcd for (C56H43N4ORh)+, m/z 890.2486; found, m/z
890.2472. IR (KBr, cm-1): ν(CdO) 1704 (s). Anal. Calcd for
C56H43N4ORh: C, 75.50; H, 4.87; N, 6.29. Found: C, 75.13; H,
4.91; N, 6.50.
Method B. Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) and 1.2
equiv of PPh3 (3.8 mg, 0.014 mmol) were mixed with 4-methyl-
benzaldehyde (144 µL, 1.20 mmol) in THF (1.0 mL) at 50 °C for
7 h to yield 8 (8.0 mg, 0.0090 mmol, 75%).
(5,10,15,20-Tetratolylporphyrinato)(ethylformyl)rhodium-
(III), CH3CH2CORh(ttp) (12).11 Method A. Rh(ttp)CH2CH2OH
(10 mg, 0.012 mmol) was mixed 100 equiv of propanal (87 µL,
1.20 mmol) in THF (2.0 mL) at 50 °C for 30 min. The solvent was
then removed and 12 was isolated by column chromatography on
silica gel with hexane/CH2Cl2 (4/1) as eluent, followed by CH2-
Cl2. A brick red solid was obtained (8.0 mg, 0.0096 mmol, 80%
yield). Rf ) 0.52 (1/1 hexane/CH2Cl2). 1H NMR (300 MHz,
CDCl3): δ -3.14 (q, 2 H, J ) 7.5 Hz), -1.69 (t, 3 H, J )
7.2 Hz), 2.70 (s, 12 H), 7.26 (d, 8 H, J ) 7.8 Hz), 8.05 (d, 8 H,
J ) 6.3 Hz), 8.80 (s, 8 H). HRMS (FABMS): calcd for (C51H41N4-
ORh)+, m/z 828.2335; found, m/z 828.2330. IR (KBr, cm-1): ν-
(CdO) 1717 (s). Anal. Calcd for C51H41N4ORh: C, 73.91; H, 4.99;
N, 6.76. Found: C, 73.88; H, 5.04; N, 6.71.
(5,10,15,20-Tetratolylporphyrinato)(4-tert-butylbenzoyl)rhod-
ium(III), 4-tBuC6H5CORh(ttp) (9).11 Method A. Rh(ttp)CH2CH2-
OH (10.0 mg, 0.012 mmol) was mixed with 100 equiv of 4-tert-
butylbenzaldehyde (150 µL, 1.20 mmol) in THF (1.0 mL) at
50 °C for 24 h to yield 9 (5.4 mg, 0.0061 mmol, 51%). Rf ) 0.43
Method B. Rh(ttp)CH2CH2OH (10 mg, 0.012 mmol) and 1.2
equiv of PPh3 (3.8 mg, 0.014 mmol) were mixed with 100 equiv
of propanal (87 µL, 1.20 mmol) in THF (1.0 mL) at 50 °C for 6 h
to yield 12 (6 mg, 0.011 mmol, 90% yield).
1
(1/1 hexane/CH2Cl2). H NMR (300 MHz, CDCl3): δ 0.95 (s,
9 H), 2.42 (d, 2 H, J ) 7.8 Hz), 2.70 (s, 12 H), 5.97 (d, 2 H, J )
7.8 Hz), 7.96 (d, 8 H, J ) 7.8 Hz), 7.96 (dd, 4 H, J ) 1.8, 6.9 Hz),
8.04 (dd, 4 H, J ) 1.8, 6.9 Hz), 8.74 (s, 8 H). HRMS (FABMS):
calcd for (C59H49N4ORh)+, m/z 932.2956; found, m/z 932.2974.
IR (KBr, cm-1): ν(CdO) 1710 (s). Anal. Calcd for C59H49N4-
ORh: C, 75.96; H, 5.29; N, 6.00. Found: C, 75.96; H, 5.49; N,
5.60.
Method B. Rh(ttp)CH2CH2OH (10.0 mg, 0.012 mmol) and 1.2
equiv of PPh3 (3.8 mg, 0.014 mmol) were mixed with 4-methyl-
benzaldehyde (150 µL, 1.20 mmol) in THF (1.0 mL) at 50 °C for
7 h to yield 9 (8.0 mg, 0.0085 mmol, 71% yield).
(5,10,15,20-Tetratolylporphyrinato)(4-methoxylbenzoyl)rhod-
ium(III), 4- MeOC6H5CORh(ttp) (10).11 Method A. Rh(ttp)CH2-
CH2OH (10.0 mg, 0.012 mmol) and 1.2 equiv of PPh3 (3.8 mg,
0.014 mmol) were mixed with anisaldehyde (115 µL,
1.20 mmol) in THF (1.0 mL) at 80 °C for 24 h to yield 10
(9.0 mg, 0.010 mmol, 83%). as a red solid. Rf ) 0.52 (1/1 hexane/
CH2Cl2). 1H NMR (300 MHz, CDCl3): δ 2.42 (d, 2 H, J )
8.5 Hz), 2.70 (s, 12 H), 3.45 (s, 3 H), 5.47 (d, 2 H, J ) 8.5 Hz),
7.53 (d, 8 H, J ) 8.1 Hz), 7.95 (d, 4 H, J ) 6.9 Hz), 8.04 (d, 4 H,
J ) 6.9 Hz), 8.76 (s, 8 H). HRMS (FABMS): calcd for
(C56H43N4O2Rh)+, m/z 906.2422; found, m/z 906.2436. IR (KBr,
cm-1): ν(CdO) 1704 (s). Anal. Calcd for C56H43N4ORh: C, 74.17;
H, 4.78; N, 6.17. Found: C, 73.92; H, 4.70; N, 5.93.
Competition Reactions between Benzaldehyde and Para-
Substituted Benzaldehydes using Rh(ttp)CH2CH2OH. A typical
procedure was described as follows. Rh(ttp)CH2CH2OH (10.0 mg,
0.012 mmol) and 1.2 equiv of PPh3 (3.8 mg, 0.014 mmol) were
mixed with 100 equiv of benzaldehyde and 100 equiv of para-
substituted benzaldehyde in THF (1.0 mL) at 50 °C. The reaction
was monitored by the TLC analysis. After the reaction was
complete, the 1H NMR spectrum of the crude reaction mixture was
taken. The relative rate was then calculated from the integration of
the â-protons of the porphyrin signals in the 1H NMR by duplicate
runs.
Acknowledgment. We thank the Research Grants Council
of the HKSAR, People’s Republic of China (No. CUHK 425/
01P), for financial support.
Supporting Information Available: NMR spectra for com-
plexes 4, 7, and 11. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM070135E