Reactivity studies of rhodium porphyrin radical with diazo compounds

Article abstract of DOI:10.1021/om060833j

Rh(tmp) reacted with two diazo compounds to give rhodium(III) porphyrin alkyls in high yields under mild conditions. Mechanistic studies showed that Rh(tmp) was coordinated with a diazo compound, which then underwent a rapid hydrogen atom abstraction via carbon-hydrogen bond activation to give Rh-(tmp)H. Rh(tmp)H subsequently reacted with a second molecule of the diazo compound in the rate-determining step to give Rh(tmp) alkyl and N₂. The rate law revealed that rate = k[Rh(tmp)][EDA]² for ethyl diazoacetate.

Full text of DOI:10.1021/om060833j

Organometallics 2007, 26, 679-684
679
Reactivity Studies of Rhodium Porphyrin Radical with Diazo
Compounds
Lirong Zhang and Kin Shing Chan*
Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, New Territories,
Hong Kong, People’s Republic of China
ReceiVed September 12, 2006
Rh(tmp) reacted with two diazo compounds to give rhodium(III) porphyrin alkyls in high yields under
mild conditions. Mechanistic studies showed that Rh(tmp) was coordinated with a diazo compound, which
then underwent a rapid hydrogen atom abstraction via carbon-hydrogen bond activation to give Rh-
(tmp)H. Rh(tmp)H subsequently reacted with a second molecule of the diazo compound in the rate-
determining step to give Rh(tmp) alkyl and N2. The rate law revealed that rate ) k[Rh(tmp)][EDA]2 for
ethyl diazoacetate.
Introduction
Rhodium(II) tetramesitylporphyrin, Rh(tmp) (Figure 1), is a
metallo radical and exhibits rich chemistry in both coordination
chemistry and bond activations.
Rh(tmp), with its very bulky substituents, is monomeric in
solution.1,2 Mononuclear Rh(II) complexes possess structural
and electronic features that make them generally susceptible to
ligand substitution reactions. Rh(II) porphyrins are observed to
undergo ligand association reactions to form the five-coordinate
complexes LRh(tmp).3 The series of coordination complexes
Rh(tmp)L (L ) NEt3, NHEt2, py, 2,6-Me2py, PEt3, PPh3, AsPh3,
Figure 1. Structure of Rh(tmp).
CNR) have been reported.4 The adducts Rh(tmp)L dispropor-
+
tionate especially readily to form Rh(tmp)- and Rh(tmp)L2
,
propenation of acetylenes,14 carbene insertion into C-H,15 Si-
H,16 N-H,17 and O-H18 bonds, and formation of ylides.19,20
Catalytic Rh-mediated polymerizations of carbenes, generated
when L is a σ strong donor ligand (e.g., py), because L repels
and destabilizes the unpaired electron in the dz orbital.5
2
The versatile reactivities of Rh(II) have been exemplified in
the activation of carbon-hydrogen bonds of the hydrocarbons
H-CH2R,6,7 removal of an allylic hydrogen atom from methyl
methacrylate,8 abstractions of halogen atoms from a wide variety
of organic halides,9 activation of carbon-nitrogen bonds in alkyl
isonitriles,10 and aliphatic sp3-sp3 C-C bond activation of
nitroxides11 and ketones.12
(10) Poszmik, G.; Carroll, P. J.; Wayland, B. B. Organometallics 1993,
12, 3410-3417.
(11) Carbon-carbon bond activation only occurred at 70 °C after 4 h,
but not at room temperature. See: Tse, M. K.; Chan, K. S. Dalton Trans.
2001, 510-511.
(12) Zhang, L.; Chan, K. S. J. Organomet. Chem. 2006, 691, 3782-
3787.
(13) (a) Krumper, J. R.; Gerisch, M.; Suh, J. M.; Bergman, R. G.; Tilley,
T. D. J. Org. Chem. 2003, 68, 9705-9710. (b) Chen, Y.; Fields, K. B.;
Zhang, X. P. J. Am. Chem. Soc. 2004, 126, 14718-14719. (c) Adjabeng,
G. M.; Gerritsma, D. A.; Bhanabhai, H.; Frampton, C. S.; Capretta, A.
Organometallics 2006, 25, 32-34.
(14) (a) Protopopova, M. N.; Doyle, M. P. J. Am. Chem. Soc. 1992, 114,
2755-2757. (b) Doyle, M. P.; Protopopova, M.; Muller, P.; Ene, D.;
Shapiro, E. A. J. Am. Chem. Soc. 1994, 116, 8492-8498.
(15) (a) Doyle, M. P.; Dyatkin, A. B. J. Org. Chem. 1995, 60, 3035-
3038. (b) Doyle, M. P.; Morgan, J. P.; Fettinger, J. C.; Zavalij, P. Y.; Colyer,
J. T.; Timmons, D. J.; Carducci, M. D. J. Org. Chem. 2005, 70, 5291-
5301. (c) Choi, M. K.-W.; Yu, W.-Y.; Che, C.-M. Org. Lett. 2005, 7, 1081-
1084. (d) Candeias, N. R.; Gois, P. M. P.; Afonso, C. A. M. J. Org. Chem.
2006, 71, 5489-5497. (e) Dong, C.; Deng, G.; Wang, J. J. Org. Chem.
2006, 71, 5560-5564. (f) Hilt, G.; Galbiati, F. Org. Lett. 2006, 8, 2195-
2198.
(16) (a) Davies, H. M. L.; Hansen, T.; Rutberg, J.; Bruzinski, P. R.
Tetrahedron Lett. 1997, 38, 1741-1744. (b) Bagheri, V.; Doyle, M. P.;
Taunton, J.; Claxton, E. E. J. Org. Chem. 1988, 53, 6158-6160.
(17) (a) Galardon, E.; Maux, P. L.; Simonneaux, G. J. Chem. Soc., Perkin
Trans. 1 1997, 2455-2456. (b) Galardon, E.; Maux, P. L.; Simonneaux,
G. Tetrahedron Lett. 2000, 56, 615-621. (c) Morilla, M. E.; Diaz-Requejo,
M. M.; Belderrain, T. R.; Nicasio, M. C.; Trofimenko, S.; Perez, P. J. Chem.
Commun. 2002, 2998-2999. (d) Davis, F. A.; Wu, Y.; Xu, H.; Zhang, J.
Org. Lett. 2004, 6, 4523-4525. (e) Matsushita, H.; Lee, S.-H.; Yoshida,
K.; Clapham. B.; Koch, G.; Zimmermann, J.; Janda, K. D. Org. Lett. 2004,
6, 4627-4629.
Diazo complexes have been used as carbene sources in the
transition-metal-catalyzed cyclopropanation of olefins,13 cyclo-
* To whom correspondence should be addressed. E-mail:
ksc@cuhk.edu.hk.
(1) (a) Wayland, B. B. Polyhedron 1988, 7, 1545-1555. (b) Wayland,
B. B.; Sherry, A. E. Inorg. Chem. 1992, 31, 148-150.
(2) Wayland, B. B.; Coffin, V. L. Farnos, M. D. Inorg. Chem. 1988, 27,
2745-2747.
(3) Dunbar, K. R.; Haefner, S. C. Organometallics 1992, 11, 1431-
1433.
(4) Wayland, B. B.; Sherry, A. E. Bunn, A. G. J. Am. Chem. Soc. 1993,
115, 7675-7684.
(5) Wayland, B. B.; Sherry, A. E. Bunn, A. G. J. Am. Chem. Soc. 1993,
115, 7675-7684.
(6) Sherry, A. E.; Wayland, B. B. J. Am. Chem. Soc. 1990, 112, 1259-
1261.
(7) (a) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991,
113, 5305-5311. (b) Del Rossi, K. J.; Zhang, X. X.; Wayland, B. B. J.
Organomet. Chem. 1995, 504, 47-56. (c) Del Rossi, K. J.; Wayland, B.
B. J. Chem. Soc., Chem. Commun. 1986, 1653-1655.
(8) Wayland, B. B.; Poszmik, G.; Fryd, M. Organometallics 1992, 11,
3534-3542.
(9) Anderson, J. E.; Yao, C.-L.; Kadish, K. M. Inorg. Chem. 1986, 25,
718-719.
10.1021/om060833j CCC: $37.00 © 2007 American Chemical Society
Publication on Web 01/03/2007

680 Organometallics, Vol. 26, No. 3, 2007
Zhang and Chan
Scheme 1. Possible Reaction Pathways
from the reaction of ethyl diazoacetate with a rhodium(I)
complex to form highly stereoregular polymers with a high
molecular mass, have been reported recently.21 A detailed
investigation of the mechanism of cyclopropanation of alkenes
by rhodium(III) porphyrin complexes has been carried out, with
an emphasis on the nature and characterization of intermediates
and predominant catalytic species in the catalytic cycle.22
However, until now, the metalloporphyrin carbene complex
intermediate has not been obtained and has only been observed
and supported by indirect evidence. It is likely that the high
activity of the metal carbene intermediate hampers its isolation.
Due to our continuing interest in the reactivity studies of
rhodium porphyrin radical,11,12,23 we sought to synthesize
rhodium porphyrin carbene complexes from the reactions of Rh-
(tmp) with diazo complexes using well-documented methods
in both porphyrin and non-porphyrin systems.24 Rhodium
carbene complexes of porphyrin are structurally interesting, as
two resonance forms can exist: the rhodium-carbon double
bond (C1) or the R-rhodium carbon centered radical (C2) (eq
1). Such resonance forms have been reported for the the cobalt
Et was unexpectedly isolated in 75% yield. Likewise, Me3-
SiCHN2 dissolved in diethyl ether also gave Rh(tmp)CH2SiMe3
in 70% yield.
Independent Synthesis. The structures of the two products
were further confirmed by authentic samples prepared by
independent synthesis through reduction of Rh(tmp)Cl with
NaBH4 followed by alkylation with the appropriate alkyl
chlorides (eq 3).
(1) NaBH4, EtOH, N2,
50-60 °C to room temp, 1 h
Rh(tmp)Cl
8 Rh(tmp)R
(3)
(2) RCl, 0 °C to room temp, 1 h
R ) CH2CO2Et, 85%; R ) CH2Si(CH3)3, 63%
Scheme 1 shows two possible reaction pathways, A and B,
to account for the incorporation of an extra hydrogen atom into
the product from the hydrogen source S-H. In pathway A,
Rh(tmp) abstracts a hydrogen atom to give Rh(tmp)H (step AI),
which then reacts with a diazo compound to give Rh(tmp)CH2-
CO2Et (step AII). The second step, AII, was supported by the
reaction of Rh(tmp)H with ethyl diazoacetate to give Rh(tmp)-
CH2CO2Et in 76% yield at room temperature after 15 min (eq
4). In pathway B, the rhodium porphyrin carbon-centered radical
analogue.25 Now we report the unexpected formation of rhodium
porphyrin alkyls (Rh(tmp)CH2R) from the reactions of Rh(tmp)
with diazo compounds. Mechanistic studies, including kinetic
experiments, were also carried out to gain a better understanding
of the reaction pathways.
Results and Discussion
Reactions of Rh(tmp) and Diazo Complexes. When Rh-
(tmp) in benzene solution was mixed with ethyl diazoacetate
(EDA) at room temperature, gas evolution, presumably N2, was
observed immediately (eq 2). After 30 min, Rh(tmp)CH2CO2-
benzene
Rh(tmp)CH COOEt
Rh(tmp)H + EDA room temp, 15 min8
2
76%
(4)
C2 is formed from the reaction of Rh(tmp) with a diazo
compound,25 with the extrusion of N2. The intermediate C2 then
abstracts a hydrogen atom to give Rh(tmp) alkyl.
Mechanistic experiments were carried out to distinguish the
two pathways. The key is to support or rule out the hydrogen
atom abstraction step: the C-H activation by Rh(tmp) (step
AI) or hydrogen atom abstraction by R-rhodium alkyl radical
(step BII).
benzene
RCHN
2a, 3a
Rh(tmp) +
room temp, 30 min8
2
Rh(tmp)CH2R
2b (75%), 3b (70%)
R ) COOEt (2a,b), Si(CH3)3 (3a,b)
+ N2 (2)
There are two possible hydrogen sources: (1) the C-H bonds
of diazo alkanes and (2) solvents. (1) Since EDA contains alkyl
C-H bonds which can be easily activated by Rh(tmp),6,7 other
aryl diazo compounds were prepared to examine the possibility.
While Ph2CdN226 reacted with Rh(tmp), a new doublet at -1.99
(18) (a) Hayashi, T.; Kato, T.; Kaneko, T.; Asai, T.; Ogoshi, H. J.
Organomet. Chem. 1994, 473, 323-327. (b) Ferris, L.; Haigh, D.; Moody,
C. J. Tetrahedron Lett. 1996, 37, 107-110. (c) Simal, F.; Demonceau, A.;
Noels, A. F. Tetrahedron Lett. 1999, 40, 63-66. (d) Maier, T. C.; Fu, G.
C. J. Am. Chem. Soc. 2006, 128, 4594-4595.
(19) Doyle, M. P.; Ene, D. G.; Forbes, D. C.; Tedrow, J. S. Tetrahedron
Lett. 1997, 38, 4367-4370.
1
ppm (J ) 3.3 Hz), characteristic of Rh(tmp) alkyl, in the H
(20) (a) Doyle, M. P.; McKervey, M. A. Modern Catalytic Methods for
Organic Synthesis with Diazo Compounds; Wiley: New York, 1998. (b)
Doyle, M. P.; Forbes, D. C. Chem. ReV. 1998, 98, 911-936.
(21) Hetterscheid, D. G. H.; Hendriksen, C.; Dzik, W.; Smits, J. M. M.;
van Eck, E. R. H.; Rowan, A. E.; Busico, V.; Vacatello, M.; Castelli, V. V.
A.; Segre, A.; Jellema, E.; Bloemberg, T. G.; de Bruin, B. J. Am. Chem.
Soc. 2006, 128, 9746-9752.
(22) (a) Brown, K. C.; Kodadek, T. J. Am. Chem. Soc. 1992, 114, 8336-
8338. (b) Bartley, D. W.; Kodadek, T. J. Am. Chem. Soc. 1993, 115, 1656-
1660. (c) Maxwell, J. Kodadek, T. Organometallics 1991, 10, 4-6. (d)
Maxwell, J. L.; Brown, K. C.; Bartley, D. W.; Kodadek, T. Science 1992,
256, 1544-1547.
(23) Chan, K. S.; Zhang, L.; Fung, C. W. Organometallics 2004, 23,
6097-6098.
(24) Lee, H. M.; Bianchini, C. Organometallics 2000, 19, 1833-1840.
(25) Ikeno, T.; Iwakura, I.; Yamada, T. J. Am. Chem. Soc. 2002, 124,
15152-15153.
NMR spectrum of the crude reaction mixture was observed.
However, no pure compounds were isolated, presumably due
to the steric hindrance of two phenyl rings in causing instability
of the product during chromatographic isolation. When PhC-
(O)CdN2C(O)Ph27 was used as the substrate, a complex mixture
formed. Therefore, the hydrogen source from diazo alkanes is
not completely ruled out. Presumably, the coordinating diazo
substrate can facilitate the C-H activation for both alkyl and
aryl C-H bonds. (2) The C-H bonds in the solvent are also a
(26) Kumar, S.; Murray, R. W.; J. Am. Chem. Soc. 1984, 106, 1040-
1045.
(27) Popic, V. V.; Korneev, S. M.; Nikolaev, V. A.; Korobitsyna, E. K.
Synthesis 1991, 3, 195-197.

ReactiVity Studies of Rhodium Porphyrin Radical
Organometallics, Vol. 26, No. 3, 2007 681
Table 1. Trapping Experiments for the Reaction of Rh(tmp) with EDA
BDE,28 kcal mol-1
disfavored
pathway
entry
trap
TEMPO
triethylsilane-d
toluene-d8
methanol-d4
D2O
C-H
O-H
Si-H
product [amt, %]
1
2
3
4
5
Rh(tmp)CH2CO2Et (2b) [65]
Rh(tmp)CH2CO2Et (2b) [73]
Rh(tmp)CH2CO2Et (2b) [72]
Rh(tmp)CH2CO2Et (2b) [70]
Rh(tmp)CHDCO2Et (2c) [70]
95
B
B
B
B
89
96
105
119
10 equiv. EDA and 100 equiv. trap reference to Rh(tmp) were added.
viable source. For Me3SiCHN2, the commercially available
reagent was supplied in ether solution. Ether is a coordinating
substrate with reactive weak R-C-H bonds (BDECH CH(H)OC H
was obtained in 70% yield without any Rh(tmp)CD2OD being
observed. We rationalized that the unsuccessful deuterium
incorporation may be due to the coordination of MeOH being
weaker than that of EDA as well as the C-H bond being
3
2 5
) 93 kcal mol-1 28
and can serve as a hydrogen atom donor.
)
Since Rh(tmp) was stable in benzene at 80 °C for 2 days,7a the
C-H bonds in benzene solvent and porphyrin ligand were not
possible hydrogen atom sources for pathway AI.
stronger (BDECH (H)OH ) 96 kcal mol-1 28
)
than that of EDA
2
(BDEN CHCO CH(H)CH ) 93 kcal mol-1).28
2
2
3
(5) D2O. Finally, inspired by the reported conversion of
RhII(tspp) (tspp ) tetra p-sulfonatophenyl porphyrinate) by D2O
into RhIII(tspp)D(D2O) by Wayland,31 we added D2O to the
reaction of Rh(tmp) with EDA; Rh(tmp)CHDCO2Et (2c) was
obtained in 70% yield and characterized by 1H NMR, 2D NMR,
and HRMS. In the 1H NMR spectrum of 2c, the triplet at -4.22
ppm (1 H, JH-D ) 3.9 Hz)32 indicated that one proton had been
deuteriated. 2D NMR supported the formation of 2c with a peak
at -8.90 Hz (referenced to the residual deuterium in CH2Cl2).
D2O was the deuterium atom source, and the product was
proposed to be obtained through pathway A. Three possible
pathways to give the product were proposed. (1) Rh(tmp)D was
first formed from the reaction of Rh(tmp) and D2O and then
reacted with EDA to give Rh(tmp)CHDCO2Et. In this pathway,
the yield of Rh(tmp)CHDCO2Et should be less than 50%, since
equal amounts of Rh(tmp)D and Rh(tmp)OD should be formed.
However, a 70% yield of Rh(tmp)CHDCO2Et was produced.
This suggested that the formation of Rh(tmp)D from Rh(tmp)-
(D2O) was unlikely. (2) Deuterium exchange between Rh(tmp)-
CH2CO2Et (2b) and D2O was ruled out, since no deuterated
product was observed by stirring Rh(tmp)CH2CO2Et with D2O
for about 5 days. The 1:1 ratio of H to D in Rh(tmp)CHDCO2-
Et further ruled out the exchange. (3) Rh(tmp)H was formed
from the reaction of Rh(tmp) with EDA. Rh(tmp)H then
underwent rapid deuterium exchange with D2O to give Rh-
(tmp)D, which reacted with another EDA molecule to give Rh-
(tmp)CHDCO2Et with H/D in a 1:1 ratio. This possibility is
most consistent with the experimental results.
These experiments suggest that the diazo compounds and
ether solvent are both possible hydrogen sources. Further
trapping experiments are necessary.
It is difficult to find a suitable trap, since Rh(tmp) can activate
even the inert C-H bond in methane at room temperature.6,7
Nevertheless, we attempted to trap the proposed intermediate
C2 in the reactions of Rh(tmp) with EDA.
(1) TEMPO. Since TEMPO was stable toward Rh(tmp) at
room temperature,11 it was used to trap C2. However, Rh(tmp)-
CH2CO2Et was isolated in about 65% yield. It is likely that, if
C2 is formed, it is too bulky to undergo coupling. Therefore,
no definitive conclusion could be drawn (Table 1, entry 1).
(2) Silane. Since the triethylsilane did not react with Rh(tmp)
at room temperature for about 6 days, Et3SiD was used to trap
the intermediate C2. However, only Rh(tmp)CH2CO2Et was
isolated and no deuterated product was observed. The higher
Si-H bond energy of silane (BDESi-H ) 95 kcal mol-1 28
compared to the C-H bond energy of EDA (BDECH CH-(H)CO Et
)
as
3
2
) 93 kcal mol-1 28
accounts for the failed trapping, as the
)
hydrogen abstraction is not exothermic.
(3) Toluene-d8. Due to the failed trapping of C2 in the above
two experiments, toluene-d8 was used, despite its known C-H
activation with Rh(tmp) to give Rh(tmp)Bn and Rh(tmp)H
reported by Wayland.29 However, Rh(tmp)CH2CO2Et was
isolated without any deuterium incorporation in 72% yield with
toluene-d8 added. The results suggested that C2 was not likely
generated, since C2 can abstract a hydrogen atom from the
weaker benzylic C-H bond of toluene (BDEC H CH -H ) 89
On the basis of the results of the above experiments, we
reasoned that pathway A is operating. Rh(tmp)H is most readily
formed from coordinating the hydrogen atom donor of a diazo
compound to facilitate chelation-assisted C-H activation. The
intermediate Rh(tmp)H further reacts with EDA to give
Rh(tmp)CH2CO2Et.
Kinetic Studies of the Reaction of Rh(tmp) with Ethyl
Diazoacetate. Since the reaction of Rh(tmp) with ethyl diaz-
oacetate (EDA) is high-yielding and clean, kinetic experiments
were carried out to gain further mechanistic understandings. The
kinetic measurements of reaction 5 were performed spectrally
6
5
2
kcal mol-1).28 Furthermore, the absence of Rh(tmp)Bn product
revealed that Rh(tmp) reacted much more quickly with EDA
than with toluene. The formation of Rh(tmp)H from EDA by
hydrogen abstraction may be a viable process, in view of the
better coordination of EDA to facilitate hydrogen atom abstrac-
tion.
The above three traps used failed to trap C2 in pathway B.
The alternative pathway may be operating, and coordinating
traps were used to support pathway A.
(4) Methanol-d4. As Rh(tmp) has been reported to react with
MeOH at the C-H bond at room temperature,30 we reasoned
that addition of CD3OD, a coordinating hydrogen atom donor,
can generate Rh(tmp)D and subsequently Rh(tmp)CHDCO2Et
in the reaction with EDA. However, only Rh(tmp)CH2CO2Et
Rh(tmp) + EDA f Rh(tmp)CH2CO2Et + N2
(5)
by monitoring the absorbance changes accompanying the
reaction at 522 nm under the conditions 25.0 °C and initial
concentrations (1.48-7.42) × 10-5 M Rh(tmp) and (0.56-3.91)
(28) Luo, Y. -R. Handbook of Bond Dissociation Energies in Organic
Compounds; CRC Press: Boca Raton, FL, 2003.
(29) Wayland, B. B.; Ba, S.-J.; Sherry, A. E. J. Am. Chem. Soc. 1991,
113, 5305-5311.
(30) Cui, W.-H.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 8266-
8274.
(31) Fu, X.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 2623-2631.
(32) For the magnitude of the hydrogen-deuterium coupling constant
1
in H NMR, see: http://drx.ch.huji.ac.il/nmr/techniques/1d/row1/h.html.

682 Organometallics, Vol. 26, No. 3, 2007
Zhang and Chan
attaining a fast equilibrium of the coordination of Rh(tmp) by
EDA. A facile hydrogen atom abstraction from chelation-
assisted C-H activation occurred with a coordinated EDA to
give Rh(tmp)H and the organic radical D. We are not certain
about the true identity of D and its fate. Hydrogen atom transfer
with the traps shown in Table 2 might have occurred for the
benzylic hydrogen of toluene. The rate-determining irreversible
step shown in reaction 9 occurred with the formation of the
rhodium porphyrin alkyl and extrusion of N2. Two pathways
for the insertion of Rh(tmp)H into EDA to give Rh(tmp)CH2-
CO2Et (2b) are possible. First, a concerted process operated.
Rh(tmp)H was initially coordinated with EDA in a productive
cis manner. Extrusion of N2 then gave Rh(tmp)CH2CO2Et (2b).
Alternatively, an ionic process could occur. Rh(tmp)H, being a
weak acid, ionized into Rh(tmp)- and H+ (the pKa value of
Rh(tpp)H is equal to 11 with tpp ) tetraphenylporphyrinate).33
EDA then reacted with H+ to give N2+CH2CO2Et (EDAH+),
which further underwent nucleophilic attack by Rh(tmp)- to
give N2 and Rh(tmp)CH2CO2Et (2b) (eqs 10-12). Such a
Figure 2. Pseudo-first-order rate plot for Rh(tmp) and EDA at
25.0 °C.
Rh(tmp)H h Rh(tmp)- + H+
H+ + EDA h HEDA+
(10)
(11)
Figure 3. Plot of kobs′ vs [EDA]2.
Table 2. Rate Constants of the Reaction of Rh(tmp) and
Rh(tmp)- + HEDA+ f Rh(tmp)CH2CO2Et + N2
EDA at 25.0 °C
(12)
105[Rh(tmp)],
M
103[EDA],
M
103kobs
,
entry
103kobs
′
M-2 s-1
rate ) k3[Rh(tmp)H][EDA]
1
2
3
4
5
6
7
1.48
3.71
7.42
3.71
3.71
3.71
3.71
1.12
1.12
1.12
2.23
0.56
2.79
3.91
1.45 ( 0.02
1.15 ( 0.01
1.49 ( 0.02
5.32 ( 0.06
0.30 ( 0.01
8.15 ( 0.03
15.35 ( 0.02
1.16 ( 0.02
0.92 ( 0.01
1.19 ( 0.02
1.07 ( 0.01
0.96 ( 0.03
1.05 ( 0.004
1.00 ( 0.001
) k3K2[Rh(tmp)(EDA)][EDA]
) k3K2K1[Rh(tmp)][EDA]2
K1 ) k1/k-1, K2 ) k2/k-2
kobs ) k3K2K1
(13)
× 10-3 M EDA. A time scan was carried out for at least 4
half-lives. A linear pseudo-first-order plot with slope equal to
kobs′ is obtained to yield a reaction first order in Rh(tmp),
independent of the initial concentrations of Rh(tmp) (Figure 2;
Table 2, entries 1-3). Pseudo-first-order rate constants kobs′ were
plotted against [EDA]2 to yield the linear plot shown in Figure
3. The rate constants of kobs, derived from the slopes of these
plots, are summarized in Table 2. Kinetic measurements
indicated that the reaction of Rh(tmp) with EDA was an overall
third-order reaction with first order in Rh(tmp) and second order
in EDA, as shown in eq 6.
mechanism has been proposed for the reaction of CH2N2 with
RCOOH to give RCOOMe.34 The above mechanistic studies
could not distinguish between these two possibilities. We favor
the concerted process for its simplicity (eq 9). This mechanism
readily accounts for the second-order dependence of EDA. The
first EDA molecule acted as a hydrogen atom donor, and the
second molecule acted as an alkylating agent.
Conclusion
In conclusion, Rh(tmp) reacted with diazo complexes to form
rhodium(III) porphyrin alkyls. The reaction of Rh(tmp) with
rate ) kobs[Rh(tmp)][EDA]2
(6)
(33) Nelson, A. P.; Dimagno, S. G. J. Am. Chem. Soc. 2000, 122, 8569-
8570.
(34) Smith, M. B.; March, J. March’s AdVanced Organic Chemistry:
Reactions, Mechanisms, and Structure, 5th ed.; Wiley: New York, 2001;
Chapter 10.
Proposed Mechanism. According to the rate law, the
mechanism was derived as shown in eqs 7-12, with reaction 7

ReactiVity Studies of Rhodium Porphyrin Radical
Organometallics, Vol. 26, No. 3, 2007 683
and was further stirred for 1 h at room temperature. The reaction
mixture was worked up by addition of CH2Cl2 and H2O. The crude
product was extracted with CH2Cl2 (200 mL), and the extract was
washed with H2O (25 mL × 3), dried over anhydrous MgSO4,
filtered, and rotary evaporated to dryness. After purification by
column chromatography on silica gel, with a solvent mixture of
hexane and CH2Cl2 (2:1 to 1:1) as eluent, a bright red solid of
Rh(tmp)CH2CO2Et (31 mg, 0.032 mmol, 85%) was obtained, which
was further purified by recrystallization from CH2Cl2/MeOH.
Preparation of ((Trimethylsilyl)methyl)(5,10,15,20-tetramesi-
tylporphyrinato)rhodium(III), [Rh(tmp)CH2SiMe3] (3b).35 A red
suspension of Rh(tmp)Cl (50 mg, 0.038 mmol) in EtOH (50 mL)
and a solution of NaBH4 (7 mg, 0.19 mmol) in 0.5 M NaOH (2
mL) were purged with N2 separately for about 15 min. The NaBH4
solution was added to the suspension of Rh(tmp)Cl via a cannula.
The reaction mixture was heated at 55 °C for 1 h under N2. After
the mixture was cooled to room temperature, ClCH2SiMe3 was
added via a syringe. A bright red suspension formed immediately
and was further stirred for 1 h at room temperature. The reaction
mixture was worked up by addition of CH2Cl2 and H2O. The crude
product was extracted with CH2Cl2 (200 mL), and the extract was
washed with H2O (25 mL × 3), dried over anhydrous MgSO4,
filtered, and rotary evaporated to dryness. After purification by
column chromatography on silica gel with a solvent mixture of
hexane and CH2Cl2 (10:1 to 5:1) as eluent, a bright red solid of
Rh(tmp)CH2SiMe3 (23 mg, 0.024 mmol, 63%) was obtained, which
was further purified by recrystallization from CH2Cl2/MeOH.
Reaction of Rh(tmp)H and Ethyl Diazoacetate. EDA was
added to Rh(tmp)H (20.0 mg, 0.023 mmol) benzene solution and
stirred at room temperature for 15 min under N2 in the absence of
light. The crude product was purified by chromatography on silica
gel, with a solvent mixture of hexane and CH2Cl2 (2:1 to 1:1) as
eluent. A red solid of Rh(tmp)CH2CO2Et (2b) (14.9 mg, 0.015
mmol, 68%) was obtained.
Trapping Experiments using TEMPO. A trapping experiment
using TEMPO is described as the typical procedure. TEMPO (136.6
mg, 100 equiv) was added to a benzene solution of Rh(tmp) (0.0088
mmol). Then EDA (9.0 µL, 10 equiv) was added to the mixture
and the solution was stirred for 0.5 h at room temperature. After
the solvent was removed, the residue was purified by column
chromatography. A red solid of 2b (mg, mmol, 65%) was obtained.
Trapping Experiments using Triethylsilane-d. Et3Si-D (142
µL, 100 equiv) was used. A red solid of 2b (mg, mmol, 73%) was
obtained.
Trapping Experiments using Toluene-d8. C6D5CD3 (94 µL,
100 equiv) was used. A red solid of 2b (mg, mmol, 72%) was
obtained.
Trapping Experiments using Methanol-d4. CD3OD (36 µL,
100 equiv) was used. A red solid of 2b (mg, mmol, 70%) was
obtained.
EDA exhibited an overall third-order reaction. The proposed
mechanism involved the chelation-assisted hydrogen atom
abstraction from a diazo compound by Rh(tmp) to form Rh-
(tmp)H as the intermediate. The subsequent formation of a
rhodium(III) porphyrin alkyl with nitrogen extrusion from the
reaction of Rh(tmp)H with another molecule of the diazo
compound was the rate-determining step.
Experimental Section
Unless otherwise noted, all chemicals were obtained from
commercial suppliers and used before purification. Hexane for
chromatography was distilled from anhydrous calcium chloride.
Thin-layer chromatography was performed on precoated silica gel
60 F254 plates.
1H NMR spectra were recorded on a Bruker DPX-300 instrument
(300 MHz). Chemical shifts were reported with reference to the
residual solvent protons in CDCl3 (δ 7.24 ppm) or with tetra-
methylsilane (δ 0.00 ppm) as the internal standard. Coupling
constants (J) were reported in hertz (Hz). Mass spectra were
recorded on a Finnigan MAT 95XS mass spectrometer (FAB-MS
and ESI-MS). Samples for elemental analysis were prepared by
vacuum drying (0.005 mmHg) at 50-60 °C for 2 days.
Reaction of Rh(tmp) and Ethyl Diazoacetate. Ethyl diazoac-
etate (0.009 mL, 0.088 mmol) was added via a microsyringe to a
solution of Rh(tmp) (0.0088 mmol) in benzene (4.0 mL), and the
reaction mixture was stirred at room temperature for 30 min under
N2 in the absence of light. The crude product was purified by
chromatography on silica gel, with a hexane-CH2Cl2 solvent
mixture (2:1 to 1:1) as eluent. A red solid of Rh(tmp)CH2CO2Et
(2b; 6.4 mg, 0.0066 mmol, 75%) was obtained. Rf ) 0.47 (1:1
hexane-CH2Cl2). 1H NMR (C6D6, 300 MHz): δ -4.22 (d, 2 H, J
) 4.2 Hz), -0.08 (t, 3 H, J ) 7.2 Hz), 2.17 (s, 12 H), 2.22 (s, 12
H), 2.26 (q, 2 H, J ) 7.2 Hz), 2.45 (s, 12 H), 7.22 (s, 4 H), 8.82
(s, 8 H). 13C NMR (C6D6, 100 MHz): -3.73 (d, JRh-C ) 40 Hz),
13.59, 21.45, 21.91, 22.04, 57.57, 120.75, 131.17, 137.71, 138.83,
139.20, 139.59, 143.32, 171.28. HRMS (FAB): calcd for
(C60H59N4O2Rh)+, m/z 970.3688. found, m/z 970.3687. Anal. Calcd
for C60H59N4O2Rh: C, 74.21; H, 6.12; N, 5.77. Found: C, 74.01;
H, 6.23; N, 6.11.
Reaction of Rh(tmp) and (Trimethylsilyl)diazomethane. (Tri-
methylsilyl)diazomethane (0.044 mL, 2.0 M in ether, 0.088 mmol)
was added via a microsyringe to a Rh(tmp) (0.0088 mmol) benzene
solution (4.0 mL), and the reaction mixture was stirred at room
temperature for 30 min under N2 in the absence of light. The crude
product was purified by chromatography on silica gel, with a solvent
mixture of hexane and CH2Cl2 (10:1 to 5:1) as eluent. A red solid
of Rh(tmp)CH2Si(CH3)3 (3b; 6.0 mg, 0.0062 mmol, 70%) was
1
obtained. Rf ) 0.62 (5:1 hexane-CH2Cl2). H NMR (C6D6, 300
MHz): δ -5.28 (d, 2 H, J ) 2.7 Hz), -2.09 (s, 9 H), 1.89 (s, 12
H), 2.18 (s, 12 H), 2.44 (s, 12 H), 7.14 (s, 4 H), 8.74 (s, 8 H). 13
C
Reaction of Rh(tmp) and Ethyl Diazoacetate with D2O added.
D2O (18 µL) was added via a microsyringe to a benzene solution
of Rh(tmp) (0.0088 mmol) and the reaction mixture was stirred at
r.t. for 15 min. Then EDA (0.009 mL, 0.088 mmol) was added via
a microsyringe to the reaction mixture and was stirred at r.t. for 30
min under N2 in the absence of light. The crude product was purified
by chromatography on silica gel, with a solvent mixture of hexane
and CH2Cl2 (2:1 to 1:1) as eluent. A red solid of Rh(tmp)CHDCO2-
Et (2c; 5.9 mg, 0.0062 mmol, 70%) was obtained. Rf ) 0.47 (1:1
hexane-CH2Cl2). 1H NMR (C6D6, 300 MHz): δ -4.22 (∼t, 1 H,
J ) 3.9 Hz), -0.07 (t, 3 H, J ) 7.2 Hz), 2.04 (s, 12 H), 2.08 (s,
12 H), 2.22 (q, 2 H, J ) 7.2 Hz), 2.46 (s, 12 H), 8.78 (s, 8 H). 2D
NMR (CH2Cl2, 60 MHz): δ -8.90. HRMS (FAB): calcd for
(C60H58N4O2DRh)+, m/z 971.3750; found, m/z 971.3772.
Reaction of Rh(tmp)H and Ethyl Diazoacetate with D2O
Added. D2O (46 µL) was added via a microsyringe to a benzene
solution of Rh(tmp)H (20.0 mg, 0.023 mmol), and the reaction
NMR (C6D6, 100 MHz) -6.68 (d, JRh-C ) 40 Hz), -1.85, 21.84,
22.21, 22.79, 121.31, 131.75, 138.11, 139.20, 139.26, 139.81,
143.97. HRMS (FAB): calcd for (C60H63N4SiRh)+, m/z 970.3125;
found, m/z 970.3129. Anal. Calcd for C60H63N4SiRh: C, 74.20; H,
6.54; N, 5.77. Found: C, 73.91; H, 6.53; N, 5.59.
Preparation of Ethyl (5,10,15,20-Tetramesitylporphyrinato)-
rhodium(III) Acetate, [Rh(tmp)CH2CO2Et] (2b).35 A red suspen-
sion of Rh(tmp)Cl (50 mg, 0.038 mmol) in EtOH (50 mL) and a
solution of NaBH4 (7 mg, 0.19 mmol) in 0.5 M NaOH (2 mL)
were purged with N2 separately for about 15 min. The NaBH4
solution was added to the suspension of Rh(tmp)Cl via a cannula.
The reaction mixture was heated at 55 °C for 1 h under N2. After
the mixture was cooled to room temperature, ClCH2CO2Et was
added via a syringe. A bright red suspension formed immediately
(35) Wayland, B. B.; Sherry, A. E.; Poszmik, G.: Bunn, A. G. J. Am.
Chem. Soc. 1992, 114, 1673-1681.

684 Organometallics, Vol. 26, No. 3, 2007
Zhang and Chan
mixture was stirred at room temperature for 15 min. Then EDA
(0.027 mL, 0.23 mmol) was added via a microsyringe to the reaction
mixture and was stirred at room temperature for 30 min under N2
in the absence of light. The crude product was purified by
chromatography on silica gel, with a solvent mixture of hexane
and CH2Cl2 (2:1 to 1:1) as eluent. A red solid of Rh(tmp)CHDCO2-
Et (2c; 14.9 mg, 0.015 mmol, 68%) was obtained.
Deuterium Exchange of Rh(tmp)H and D2O. D2O (12 µL)
was added to a benzene solution of Rh(tmp)H (5.0 mg, 0.0046
mmol), and the mixture was stirred for 15 min. Then the benzene
was removed and benzene-d6 was added, and the solution was
transferred to a NMR tube under N2. The NMR spectrum was
recorded, and the signal at high field of Rh(tmp)H disappeared.
Deuterium exchange had occurred.
benzene in a 2.00 mL volumetric flask, and this solution was then
transferred to a flask fitted with a Teflon stopcock and degassed.
After photolysis for 10 h at 6-10 °C, a stock solution of Rh(tmp)
(1.11 × 10-3 M) was produced. The substrate EDA (0.1674 M)
was prepared and degassed. A measured amount of the stock
solution of Rh(tmp) was added with benzene in a cuvet-Schlenk
UV cell and was thermally equilibrated at 25 °C for 15 min. Then
the EDA solution in benzene was added to the mixture quickly,
and the sample was time-scanned by UV spectra at 522 nm.
Acknowledgment. We thank the Research Grants Council
of Hong Kong of the SAR of China for financial support (No.
400104).
Supporting Information Available: NMR and HRMS spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Kinetic Studies of the Reaction between [Rh(tmp)] and Ethyl
Diazoacetate. Kinetic studies were carried out at a temperature of
25.0 ( 0.2 °C maintained by a constant temperature controlling
bath. Rh(tmp)CH3 (2.5 mg, 0.00225 mmol) was dissolved in
OM060833J