Facile alkylation of cobalt(III) porphyrins by organosilicon compounds

Article abstract of DOI:10.1021/om970174y

(σ-Alkyl)cobalt(III) porphyrins were easily synthesized and isolated as stable crystalline compounds by the reaction of halogenocobalt(III) porphyrins with silyl enol ethers and ketene silyl acetals. The yields depended on the solvent (CH₂Cl

Full text of DOI:10.1021/om970174y

Organometallics 1997, 16, 3679-3683
3679
F a cile Alk yla tion of Coba lt(III) P or p h yr in s by
Or ga n osilicon Com p ou n d s
J un-ya Watanabe and J un-ichiro Setsune*
Department of Chemistry, Faculty of Science, and Graduate School of Science and Technology,
Kobe University, Nada, Kobe 657, J apan
Received March 3, 1997X
(σ-Alkyl)cobalt(III) porphyrins were easily synthesized and isolated as stable crystalline
compounds by the reaction of halogenocobalt(III) porphyrins with silyl enol ethers and ketene
silyl acetals. The yields depended on the solvent (CH2Cl2 > MeOH), the substituent of the
porphyrin periphery (meso-tetraphenylporphyrin > octaethylporphyrin), and the axial
halogeno ligand of cobalt(III) porphyrins (F g Cl . Br ≈ I ≈ ClO4). 1-((Trimethylsilyl)oxy)-
1,3-butadiene afforded (σ-4-oxobut-2-enyl)cobalt(III) porphyrins, which showed very broad
1H NMR signals at 25 °C. The temperature-dependent dynamic behavior of this complex
was explained in terms of the rotation around the C(R)-C(â) bond but not of the σ-π
rearrangement in the 4-oxobut-2-enyl ligand.
Sch em e 1
In tr od u ction
It has been known that cobalt(III) porphyrins undergo
nucleophilic attack by Grignard reagents to generate (σ-
alkyl)cobalt(III) porphyrins (Scheme 1).1 Vinyl ethers
are also nucleophilic enough to be capable of attacking
a cobalt(III) center leading to (σ-formylmethyl)cobalt-
(III) derivatives via cobalt(III) alkene π-complex inter-
mediates (Scheme 1).2 Although the latter method is
easily performed even in hydrophilic reaction media, it
has never been widely applied, due to the limitation on
availability of a variety of vinyl ether derivatives.3 Silyl
enol ethers, ketene silyl acetals, and allylsilanes have
recently found wide application as protected carbon
nucleophiles in organic synthesis.4 Since these orga-
nosilicon compounds are electron-rich alkenes and thus
regarded as analogues of vinyl ethers, they are expected
to react with cobalt(III) porphyrins via π-complex
intermediates. Furthermore, since a variety of orga-
nosilicon compounds are readily available compared
with vinyl ether derivatives, they would extend the
scope of the reaction of cobalt(III) porphyrins with
electron-rich alkenes as a method for the Co-C bond
formation. We now wish to describe the details of the
reaction of halogenocobalt(III) porphyrins with organo-
silicon compounds (Scheme 2).
* To whom correspondence should be addressed. E-mail:
setsunej@kobe-u.ac.jp.
Resu lts a n d Discu ssion
X Abstract published in Advance ACS Abstracts, J uly 15, 1997.
(1) (a) Clarke, D. A.; Dolphin, D.; Grigg, R.; J ohnson, A. W.; Pinnock,
H. A. J . Chem. Soc. 1968, 881. (b) Schrauzer, G. N. Angew. Chem.
1976, 88, 465. (c) Dodd, D.; J ohnson, M. D. J . Organomet. Chem. 1973,
52, 1. (d) Perree-Fauvet, M.; Gaudemer, A.; Boucly, P.; Devynck, J .
J . Organomet. Chem. 1976, 120, 439. (e) Ogoshi, H.; Watanabe, E.;
Koketsu, N.; Yoshida, Z. Bull. Chem. Soc. J pn. 1976, 49, 2529.
(2) (a) Silverman, R. B.; Dolphin, D. J . Am. Chem. Soc. 1976, 98,
4626. (b) Sugimoto, H.; Ueda, N.; Mori, M. Bull. Chem. Soc. J pn. 1981,
54, 3425.
When TPPCoIII-Cl (1a ; 0.030 mmol) (TPP ) meso-
tetraphenylporphyrin dianion) was reacted with 1-phenyl-
1-((trimethylsilyl)oxy)ethene (0.20 mmol) in methanol
(15 mL) for 50 min, TPPCoIII-CH2COPh (3a ) was
obtained in 72% yield. Table 1 summarizes the result
of the reaction of Co(III) porphyrins with various
organosilicon compounds. The progress of the reaction
was indicated by the formation of a precipitate of (σ-
alkyl)cobalt(III) porphyrins, and thus the use of metha-
nol as a reaction medium makes the product isolation
procedure quite simple. The carbonyl stretching mode
of 3a was observed at 1634 cm-1 in the IR spectrum.
This much lower wavenumber than the ordinary ν-
(CdO) values is characteristic of (σ-alkyl)metal(III)
complexes with a carbonyl group at the â-position with
respect to the metal and is indicative of the overlap
between the filled metal d orbital and the CdO anti-
bonding orbital.5
(3) (a) Watanabe, W. H.; Conlon, L. E. J . Am. Chem. Soc. 1957, 79,
2828. (b) Ireland, R. E.; Dawson, D. J . Org. Synth. 1974, 54, 71.
(4) (a) Colvin, E. W. Silicon Reagents in Organic Synthesis; Academic
Press: London, 1988. (b) Paterson, I.; Mcleod, M. Tetrahedron Lett.
1995, 36, 9065. (c) Saraswathy, V.; Sankararaman, S. J . Org. Chem.
1995, 60, 5024. (d) Suzuki, H.; Aoyagi, S.; Kibayashi, C. J . Org. Chem.
1995, 60, 6114. (e) Tokunoh, R.; Tomiyama, H.; Sodeoka, K.; Shibasaki,
M. Tetrahedron Lett. 1996, 37, 2449. (f) Kato, M.; Hasegawa, E.
Tetrahedron Lett. 1996, 37, 3483. (g) Karady, S.; Amato, J .; Reamer,
R.; Weinstock, L. Tetrahedron Lett. 1996, 37, 8277. (h) Rathove, R.;
Kochi, J . J . Org. Chem. 1996, 61, 627. (i) Zoretic, P.; Wang, M.; Zhang,
Y.; Shen, Z.; Ribeiro, A. J . Org. Chem. 1996, 61, 1806. (j) Padwa, A.;
Brodney, M.; Marino, J .; Sheehan, S. J . Org. Chem. 1997, 62, 78. (k)
Bergmeiyer, S.; Seth, P. Tetrahedron Lett. 1995, 36, 3793. (l) Osumi,
K.; Sugimura, H. Tetrahedron Lett. 1995, 36, 5789. (m) Linderman,
R.; Chen, K. J . Org. Chem. 1996, 61, 2441. (n) Horiuchi, Y.; Oshima,
K.; Utimoto, K. J . Org. Chem. 1996, 61, 4483. (o) Asao, N.; Yoshikawa,
E.; Yamamoto, Y. J . Org. Chem. 1996, 61, 4874. (p) Ofial, A.; Mayr,
H. J . Org. Chem. 1996, 61, 5823.
(5) (a) Setsune, J .; Watanabe, J . Chem. Lett. 1994, 2253. (b) Ogoshi,
H.; Setsune, J .; Nanbo, Y.; Yoshida, Z. J . Organomet. Chem. 1978, 159,
329.
S0276-7333(97)00174-X CCC: $14.00 © 1997 American Chemical Society

3680 Organometallics, Vol. 16, No. 16, 1997
Ta ble 1. Rea ction of (P or )CoIII-X w ith R2HCdCR1(OSiMe3)
Watanabe and Setsune
run no.
Por
X
R1
R2
solv
time (min)
prod.
yield (%)a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
TPP
Cl
Cl
Cl
Ph
Ph
Ph
OEt
CHdCH2
CHdCH2
H
H
H
H
H
H
MeOH
CH2Cl2
MeOH
MeOH
MeOH
CH2Cl2
MeOH
MeOH
CH2Cl2
MeOH
CH2Cl2
CH2Cl2
MeOH
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
50
2
50
50
50
2
50
50
2
10
2
2
50
2
2
2
3a
3a
3a
3b
3c
3c
72
85
56
81
44
77
0b
39c
86
67
87
0b
0
-
ClO4
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
F
Br
I
ClO4
Cl
Cl
-CH2CH2CH2-
OMe
OMe
H
Me
Me
CHdCH2
3d
3d
3f
3f
H
CHdCH2
-CH2CH2CHdCH-
OEP
Ph
Ph
Ph
Ph
Ph
Ph
CHdCH2
H
H
H
H
H
H
H
H
4a
4a
71
74
0
0
0
2
2
2
2
-
4c
4f
82
71
CHdCH2
a
b
Isolated yields, except for run 8. The isolated product was TPPCoII. c TPPCoIII-OCH(Me)CO2Me (3e) was also obtained in 19%
NMR yield.
Sch em e 2
Sch em e 3
alkyl protons of 3d and 3e appear as substantially the
same splitting pattern but with very different ring
current effects, as shown in Table 2. These data are
consistent with the structure for 3e, in which one oxygen
atom is inserted into the CoIII-C bond of 3d . Although
it is well-known that insertion of dioxygen into a MIII-C
bond generates (σ-alkylperoxo)metal(III) complexes,
especially in the cases of secondary and tertiary σ-alkyl
groups,6 this hypothetical (σ-1-(carbomethoxyethyl)-
peroxo)cobalt(III) structure (B) is not consistent with
1
The H NMR signals due to the axial organo group of
3a appear at much higher magnetic fields owing to the
ring current effect as shown in Table 2. 1-Ethoxy-1-
((trimethylsilyl)oxy)ethene and 2-((trimethylsilyl)oxy)-
1,3-butadiene similarly afforded the corresponding (σ-
(2-oxoalkyl)cobalt(III) porphyrins TPPCoIII-CH2CO2Et
(3b) and TPPCoIII-CH2COCHdCH2 (3c), in 81% and
44% yields, respectively. These (σ-alkyl)cobalt(III) por-
phyrins were thermally stable enough to be purified by
column chromatography on silica gel with dichlo-
romethane as an eluent. Reaction of 1a with 1-meth-
oxy-1-((trimethylsilyl)oxy)-1-propene afforded a mixture
of TPPCoIII-CH(Me)CO2Me (3d ; 39%) and a cobalt(III)
porphyrin byproduct, TPPCoCoIII-OCH(Me)CO2Me (3e;
19%) (Scheme 3). These products could not be separated
because they decomposed during column chromatogra-
1
the H NMR data observed for 3e. It is reported that
the axial CH2 proton signal shifts from -2.55 ppm to
-0.13 ppm upon going from TPPCoIII-CH2Ph to TPP-
CoIII-OOCH2Ph.7 Therefore, the observed chemical
shift (-3.40 ppm) due to the methine proton of 3e is
consistent not with the CoIII-OOCH(Me)CO2Me struc-
ture but with the CoIII-OCH(Me)CO2Me structure,
taking into account the chemical shift (-3.33 ppm) of
the methine proton of 3d . As for the chemical shift of
the methine proton of 3e in comparison with that of 3d ,
the stronger shielding effect due to the porphyrin ring
current on the axial â-position than on the axial
(6) Arasasingham, R. D.; Balch, A. L.; Cornman, C. R.; Latos-
Grazynski, L. J . Am. Chem. Soc. 1989, 111, 4357.
(7) Kendrick, M. J .; Al-Akhbar, W. Inorg. Chem. 1987, 26, 3972.
1
phy on silica gel. The H NMR signals due to the axial

Facile Alkylation of Cobalt(III) Porphyrins
Organometallics, Vol. 16, No. 16, 1997 3681
Ta ble 2. 1H NMR Da ta for (σ-a lk yl)CoIII P or p h yr in s (3a -f a n d 4a ,c,f)a
axial alkyl ligandb
complex
R
â (â′)
γ (γ′)
δ
ꢀ
ú
TPPCoIII-CH2COPh (3a )
TPPCoIII-CH2CO2Et (3b)
TPPCoIII-CH2COCHdCH2 (3c)
-3.13 (s, 2H)
-3.80 (s, 2H)
-3.52 (s, 2H)
4.59 (d, 2H)
1.96 (q, 2H)
3.02 (d, 1H)c
3.98 (d, 1H)d
1.73 (s, 3H)
6.62 (t, 2H)
0.21 (t, 3H)
7.10 (t, 1H)
2.25 (dd, 1H)
TPPCoIII-CH(Me)CO2Me (3d )
TPPCoIII-OCH(Me)CO2Me (3e)
TPPCoIII-CH2CHdCHCHO (3f)
OEPCoIII-CH2COPH (4a )
-3.33 (q, 1H)
-5.37 (d, 3H)
-3.40 (q, 1H)
1.10 (dt, 1H)f
-0.83 (d, 3H)
3.06 (br, 1H)
2.62 (s, 3H)
6.50 (t, 2H)
-2.99 (d, 2H)e
-3.72 (s, 2H)
-4.12 (s, 2H)
7.37 (br, 1H)
4.33 (d, 2H)
2.65 (dd, 1H)c
3.70 (dd, 1H)d
7.28 (d, 1H)j
6.95 (t, 1H)
OEPCoIII-CH2COCHdCH2 (4c)
∼2g (m, 1H)
OEPCoIII-CH2CHdCHCHO (4f)
-3.65 (d, 2H)h
0.45 (dt, 1H)i
2.57 (dd, 1H)
a
b
Conditions: 250 MHz, δ value relative to tetramethylsilane, measured in CDCl3 in 25 °C. R, â, γ, δ, ꢀ, and ú are carbon positions
c
d
e
f
g
relative to the cobalt center. J trans ) 17.5 Hz. J cis ) 10.5 Hz. J vic ) 9.5 Hz. J trans ) 15.0 Hz. The chemical shift could not be exactly
determined because of the overlapping with signals due to the porphyrin peripheral methyl protons. J vic ) 9.3 Hz. J trans ) 14.8 Hz.
h
i
Sch em e 4
R-position can cancel out the deshielding effect of the
oxygen atom inserted into the CoIII-C bond. Formation
of a similar product has been observed in the reaction
of OEPCoII (OEP ) octaethylporphyrin dianion) with
propionaldehyde in the presence of NaBH4 and O2,
which generated a mixture of (σ-1-formylethyl)cobalt-
(III) and (σ-1-formylethoxy)cobalt(III) porphyrins.5a The
complex 3d was obtained in a much better yield (86%)
in dichloromethane than in methanol (Table 1; runs 8
and 9). Moreover, the byproduct 3e was not observed
1
in the H NMR spectrum. The conversion of 3d to 3e
was promoted when 3d was allowed to stand in CDCl3
under aerobic conditions. Even if dioxygen was present,
the formation of 3e was greatly suppressed in the dark.
Therefore, the oxygenation of 3d to 3e takes place by
way of the (σ-1-carbomethoxyethyl)peroxo)cobalt(III)
intermediate (B), which is generated through the light-
driven O2 insertion into the CoIII-C bond of 3d . The
generation of 3e in methanol (Table 1; run 8) would be
attributed to the long reaction time. Although the
preparation of 3d was done under aerobic conditions and
in room light, a short reaction time (2 min) in dichlo-
romethane precluded the oxygenation of 3d to 3e.
Although 1a reacted with 1-((trimethylsilyl)oxy)cyclo-
pentene and 1-((trimethylsilyl)oxy)-1,3-cyclohexadiene,
the isolated product was TPPCoII. This suggests that
the initially formed organocobalt(III) porphyrins would
have smaller a CoIII-C bond dissociation energy due to
the steric constraint around the CoIII-C σ-bond and
thus decomposed rapidly to TPPCoII and an alkyl
radical. Reaction of 1a with allyltrimethylsilane also
afforded TPPCoII.
14-18). This trend suggests that the reaction is driven
by the bonding energy of a Si-X bond (X ) F > Cl > Br
> I > ClO4) of Me3Si-X generated in the reaction.
The above observations point to the dual role of an
organosilicon compound, that is, the nucleophilic attack
on the cobalt as an electron-rich olefin and the abstrac-
tion of the axial halide ion as a Lewis acid. Scheme 2
illustrates such a push-pull intermediate (A).
Interestingly, the reaction of 1a with 1-((trimethyl-
silyl)oxy)-1,3-butadiene gave a good yield of TPPCoIII
-
CH2CHdCHCHO (3f) with no indication of the forma-
tion of TPPCoIII-CH(CHdCH2)CHO (Scheme 4). The
(σ-allyl)cobalt(III) complex 3f could not be purified by
chromatography because of its labile nature, and thus
small signals derived from some impurities are seen in
the 1H NMR spectrum (see Figure 1). The 1H NMR
signals due to the axial organo group of 3f were assigned
according to the H-H COSY experiment as summarized
in Table 2. The spin-spin coupling constant (J ) 15
Hz) between two olefinic protons is consistent with the
trans-olefinic structure of the axial organo group. The
complex 3f showed temperature-dependent 1H NMR
spectra, as shown in Figure 1. While signals due to the
porphyrin peripheral protons (7-9 ppm) are extremely
broad and are unresolved at room temperature, they
sharpen with lowering temperature to give an ordinary
spectral pattern of (σ-alkyl)cobalt(III) porphyrins at -60
°C. This temperature-dependency is not attributed to
paramagnetism, because the chemical shifts for the
protons in the axial organo group are characteristic of
the diamagnetic Co(III) complexes and do not change
throughout the temperature range examined (+40 to
-60 °C). It has been known that allylcobaloximes show
The reaction is much faster in dichloromethane than
in methanol to give better yields of (σ-2-oxoalkyl)cobalt-
(III) porphyrins (Table 1; runs (1, 2), (5, 6), (8, 9), (10,
11), (13, 14)). This is probably because methanol
competes with alkenes for the axial coordination site of
Co(III). A TPP ligand is superior to an OEP ligand in
this reaction, probably because the electron-withdraw-
ing effect of the former makes the cobalt center more
reactive toward nucleophilic reagents. Thus, OEPCoIII-
Cl (2a ) did not react with 1-phenyl-1-((trimethylsilyl)-
oxy)ethylene in methanol, in contrast to the case of 1a
(Table 1, runs 1 and 13). When dichloromethane was
used as a solvent in this reaction, however, OEPCoIII
CH2COPh (4a ) was obtained in 71% yield.
-
The axial anionic ligand of cobalt(III) porphyrins plays
a key role in this reaction. The yield of 4a decreased
in the order F g Cl . Br ≈ I ≈ ClO4 (Table 1; runs

3682 Organometallics, Vol. 16, No. 16, 1997
Watanabe and Setsune
F igu r e 2. UV-vis spectra of 3f, 3a , 1a , and TPPCoII in
CH2Cl2.
shifts due to the axial organo group of 3f are consistent
with those expected on the basis of the ring current term
in a σ-allyl structure, but not with those in a π-allyl
structure. Thus, interconversion between a σ-allyl and
a π-allyl structure is unlikely to occur with temperature
change in the present case.
The ν(CdO) frequency at 1672 cm-1 observed for 3f
is lower than that of crotonaldehyde (1700 cm-1).10 The
UV-vis spectra of 3f and 4f are similar to those of
halogenocobalt(III) porphyrins 1a and 2a (see Figure
2), whereas the UV-vis spectra of 3a -d , and 4a ,c are
identical with those of the corresponding Co(II) porphy-
rins as is the case with ordinary (σ-alkyl)cobalt(III)
porphyrins. This indicates that the ligand field supplied
by the axial organo group is weaker in 3f and 4f than
in ordinary (σ-alkyl)cobalt(III) porphyrins. These spec-
troscopic properties are suggestive of the charge transfer
from the Co-C(R) σ-bond to the carbonyl group through
the σ-π conjugation in the axial organo group of 3f and
4f. In order for this σ-π conjugation to work well, the
parallel conformation (C) is preferred to the perpen-
dicular conformation (D) as depicted in Scheme 4,
because overlap of the σ-orbital of the Co-C(R) bond
and the π-orbitals of the enone moiety is maximal in
the former conformation. The dihedral angle, Co-
C(R)-C(â)-C(γ) is ca. 90 and 270° with the enone
moiety being kept planar in the parallel conformations
C and C′, and 180° in the perpendicular conformation
D in Scheme 4.11
F igu r e 1. Variable-temperature 1H NMR (250 MHz)
spectra of TPPCoIII-CH2CHdCHCHO (3f) in CDCl3 at 40,
20, 0, -20, -40, and -60 °C. Signals due to some impuri-
ties: δ 9.5, 6.9, 6.1, 2.0 (CH3CHdCHCHO), 7.23 (CHCl3)
5.24 (CH2Cl2), 1.51 (H2O) 0.15, 0.10 (Me3Si).
Sch em e 5
The unusual dynamic behavior observed in the 1H
NMR is explainable in terms of a hypothetical equilib-
rium process between C and C′. At low temperatures,
the axial organo group is confined to either of the two
parallel conformations, C and C′, while the porphyrin
ring is rotating around the Co-C(R) bond quickly on
the NMR time scale. In fact, eight â-pyrrole protons of
the porphyrin ring are identical at -60 °C (see Figure
1). As the temperature rises, the enone moiety begins
to rotate around the C(R)-C(â) bond, changing its
position between two parallel conformations, C and C′.
When the equilibrium process between C and C′ is
occurring as fast as the rotation of the porphyrin ring,
the enone moiety appears to move slowly with respect
to the position of the porphyrin protons. In this case,
temperature-dependent NMR spectra in solution.8 Sig-
nificant changes in the 1H NMR signals due to the axial
allyl group have been observed to provide evidence in
support for the occurrence of σ-π rearrangement be-
tween a (σ-allyl)cobaloxime (E) and a (π-allyl)cobaloxime
(F) (Scheme 5).9 In contrast, the signals due to the axial
alkyl ligand of 3f are almost unchanged over the
temperature range, except that the formyl proton at δ
7.37 (25 °C) shifts to higher fields and gets broader as
1
the temperature goes down. The observed H chemical
(10) Lide, D. R.; Milne, G. W. A. Handbook of Data on Organic
Compounds, 3rd ed.; CRC Press: Boca Raton, FL, 1994; Vol. II.
(11) According to the semiempirical MO calculation (see Experi-
mental Section), an energy minimum was observed at the conformation
where the dihedral angle Co-C(R)-C(â)-C(γ) is 96.8°. This parallel
conformation E is more stable by 5 kcal/mol than the perpendicular
conformation F.
(8) Cooksey, C. J .; Dodd, D.; Gatford, C.; J ohnson, M. D.; Lewis, G.
J .; Titchmarsh, D. M. J . Chem. Soc., Perkin Trans. 2 1972, 655.
(9) (a) Dodd, D.; J ohnson, M. D. J . Am. Chem. Soc. 1974, 96, 2279.
(b) Cooksey, C. J .; Dodd, D.; J ohnson, M. D.; Lockman, B. L. J . Chem.
Soc., Dalton Trans. 1978, 1814.

Facile Alkylation of Cobalt(III) Porphyrins
Organometallics, Vol. 16, No. 16, 1997 3683
on silica gel with dichloromethane as an eluent and recrystal-
lized from dichloromethane-methanol.
the magnetic environment of the protons at the por-
phyrin periphery fluctuates, owing to the conformational
change between C and C′. This fluctuation would not
be averaged out when the equilibrium process between
C and C′ is occurring as fast as the rotation of the
porphyrin ring, because the enone moiety appears to
move slowly with respect to the position of the protons
at the porphyrin periphery. This explains the broaden-
ing of the proton signals due to the porphyrin periphery
with increasing temperature. On the other hand, the
magnetic environment of the axial organo group does
not change so much before and after the conformational
change of the enone group, because of the symmetrical
disklike shape of the porphyrin ring. Thus, the confor-
mational change between C and C′ makes no difference
in the magnetic environment of the axial ligand protons,
even if the porphyrin ring appears to rotate slowly with
respect to the rate of the conformational change. This
is also because of the symmetrical disklike shape of the
porphyrin ring. Therefore, the 1H signals due to the
axial organo group are thought to be almost indepen-
dent of the temperature.
In conclusion (σ-alkyl)cobalt(III) porphyrins with a
carbonyl group at the â-position of the alkyl ligand have
been prepared conveniently through the reaction of
halogenocobalt(III) porphyrins with organosilicon re-
agents. On the basis of the effects, on the reactivity, of
solvents, axial halogeno ligands, and substituents of the
porphyrin periphery, a push-pull mechanism as shown
in Scheme 2 is favored. This reaction may alternatively
be explained in terms of the coupling reaction of Co(II)
and an alkyl radical induced by the single electron
transfer from an organosilicon compound to cobalt(III)
porphyrin. The redox potentials of cobalt(III) porphy-
rins (∼0.7 V vs. SCE for TPPCo in dichloromethane)2b
and these organosilicon compounds (∼1.3 V vs. SCE in
acetonitrile),12 however, would be inconsistent with this
mechanism.
3a : 1H NMR (CDCl3) δ 8.80 (s, 8H, â-PyH), 8.07 (br, 8H,
o-PhH), 7.68-7.78 (m, 12H, m- and p-PhH), see Table 2 for
the axial alkyl protons; UV-vis (CH2Cl2) 525.5, 410.0 nm; IR
(KBr) 1634 cm-1 [ν(CdO)]. Anal. Calcd for C52H35N4OCo: C,
78.98; H, 4.46; N, 7.08. Found: C, 78.93; H, 4.47; N, 7.10.
3b: 1H NMR (CDCl3) δ 8.88 (s, 8H, â-PyH), 8.13 (br, 8H,
o-PhH), 7.69-7.78 (m, 12H, m- and p-PhH), see Table 2 for
the axial alkyl protons; UV-vis (CH2Cl2) 527.0, 408.0 nm; IR
(KBr) 1696 cm-1 [ν(CdO)]. Anal. Calcd for C48H35N4O2Co: C,
75.98; H, 4.65; N, 7.39. Found: C, 75.79; H, 4.50; N, 7.50.
3c: 1H NMR (CDCl3) δ 8.86 (s, 8H, â-PyH), 8.14 (br, 8H,
o-PhH), 7.74 (m, 12H, m- and p-PhH), see Table 2 for the axial
alkyl protons; UV-vis (CH2Cl2) 526.0, 409.0; IR (KBr) 1656
cm-1 [ν(CdO)]. Anal. Calcd for C48H33N4OCo‚H2O: C, 75.98;
H, 4.65; N, 7.39. Found: C, 76.44; H, 4.61; N, 7.43.
3d : 1H NMR (degassed CDCl3) δ 9.02 (s, 8H, â-PyH), 8.87
(m, 8H, o-PhH), 7.73 (m, 12H, m- and p-PhH), see Table 2 for
the axial alkyl protons; UV-vis (CH2Cl2) 527.0, 409.0 nm; IR
(KBr) 1704 cm-1 [ν(CdO)]. Anal. Calcd for C48H35N4O2Co: C,
75.98; H, 4.65; N, 7.39. Found: C, 76.15; H, 4.63; N, 7.40.
3f: 1H NMR (CDCl3) δ 9.4-8.7 (br, 8H, â-PyH), 8.7-7.6 (br,
20H, PhH), see Table 2 for the axial alkyl protons; UV-vis
(CH2Cl2) 538.5, 423.0 nm; IR (KBr) 1672 cm-1 [ν(CdO)]. Anal.
Calcd for C48H33N4OCo‚H2O: C, 75.98; H, 4.65; N, 7.39.
Found: C, 76.46; H, 4.63; N, 7.49.
4a : 1H NMR (CDCl3) δ 10.08 (s, 4H, meso H), 3.98 (m, 16H,
Et), 1.87 (t, 24H, Et), see Table 2 for the axial alkyl protons;
UV-vis (CH2Cl2) 551.0, 516.0, 393.5 nm; IR (KBr) 1638 cm-1
[ν(CdO)]. Anal. Calcd for C44H51N4OCo: C, 74.35; H, 7.23;
N, 7.88. Found: C, 74.82; H, 7.32; N, 7.81.
4c: 1H NMR (CDCl3) δ 10.20 (s, 4H, meso H), 4.03 (m, 16H,
Et), 1.88 (t, 24H, Et), see Table 2 for the axial alkyl protons;
UV-vis (CH2Cl2) 550.0, 517.0, 392.5. IR (KBr), cm-1: 1658
[ν(CdO)]. Anal. Calcd for C40H49N4OCo: C, 72.71; H, 7.47;
N, 8.48. Found: C, 73.25; H, 7.70; N, 8.56.
4f: 1H NMR (CDCl3) δ 10.5-9.5 (br, 4H, meso H), 5.0-3.0
(br, 16H, Et), 2.5-1.5 (br, 24H, Et), see Table 2 for the axial
alkyl protons; UV-vis (CH2Cl2) 555.5, 523.5, 405.5 nm; IR
(KBr) 1706 cm-1 [ν(CdO)]. Anal. Calcd for C40H49N4OCo‚
H2O: C, 70.77; H, 7.57; N, 8.26. Found: C, 71.54; H, 7.25; N,
8.34.
Sem iem p ir ica l MO Ca lcu la tion . Theoretical calculations
were performed with the SPARTAN package version 4.0
(Wavefunction, Inc., Irvine, CA). A preliminary structure
refinement using molecular mechanics based on the SYBYL
force field was applied to a model in which peripheral sub-
stituents of the complex 3f were replaced by hydrogens.
Geometry optimization by the MNDO/d MO calculation using
a PM3 (tm) MO method13 generated the conformation C as an
energy minimum structure (heat of formation -1518 kcal/mol).
After the dihedral angle Co-C(R)-C(â)-C(γ) of the confor-
mation C was constrained to 180°, optimization was similarly
carried out to afford the conformation D (heat of formation
-1513 kcal/mol.
Exp er im en ta l Section
1H NMR spectra were measured on a Bruker AC-250 (250
MHz) spectrometer, and the chemical shifts are referenced to
tetramethylsilane as an internal standard. UV-vis spectra
were measured on a Shimazu UV-2400PC instrument. IR
spectra were obtained as KBr disks on a Hitachi I-2000
spectrometer. Dichloromethane was dried by distilling from
calcium hydride and stored over 4 Å molecular sieves. Metha-
nol was simply distilled before use.
Gen er a l Meth od . A mixture of TPPCo(III)-Cl (0.030
mmol), an organosilicon compound (0.20 mmol), and methanol
(15 mL) was stirred in a 100 mL Erlenmeyer flask. Then, (σ-
alkyl)cobalt(III) porphyrin began precipitating within 10 min.
After the solution was stirred for 50 min, the solid was
collected by filtration and washed with methanol. When
dichloromethane (7 mL) was used as a solvent for this reaction,
the reaction mixture was evaporated with a rotary evaporator,
and the residue was washed with methanol. Except for 3d -f
and 4f, the product was purified by column chromatography
Ack n ow led gm en t. This work was partially sup-
ported by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science, Sports, and Culture
of J apan. We are grateful to Prof. H. J . Callot (Univer-
site´ Louis Pasteur) for his valuable comments and to
Ms. M. Nishinaka (Kobe University) for microanalysis.
OM970174Y
(12) Fukuzumi, S.; Fujita, M.; Otera, J .; Fujita, Y. J . Am. Chem.
Soc. 1992, 114, 10271.
(13) Stewart, J . J . P. J . Comput. Chem. 1989, 10, 209.