Reactions of nitroxides with metalloporphyrin alkyls bearing beta hydrogens: Aliphatic carbon-carbon bond activation by metal centered radicals

Article abstract of DOI:10.1016/j.jorganchem.2007.11.009

Nitroxide-induced beta-hydrogen atom abstraction and beta-elimination of rhodium porphyrin alkyls have been observed. Rhodium(II) porphyrin radical were proposed intermediates to form first and subsequently reacted via aliphatic carbon-carbon bond activation with alkyl substituted nitroxides to yield rhodium porphyrin alkyl complexes.

Full text of DOI:10.1016/j.jorganchem.2007.11.009

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 399–407
www.elsevier.com/locate/jorganchem
Reactions of nitroxides with metalloporphyrin alkyls bearing
beta hydrogens: Aliphatic carbon–carbon bond activation
by metal centered radicals
Kin Shing Chan *, Kin Wah Mak, Man Kin Tse, Siu Kwan Yeung,
Bao Zhu Li, Yun Wai Chan
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China
Received 28 September 2007; received in revised form 2 November 2007; accepted 6 November 2007
Available online 13 November 2007
Abstract
Nitroxide-induced beta-hydrogen atom abstraction and beta-elimination of rhodium porphyrin alkyls have been observed.
Rhodium(II) porphyrin radical were proposed intermediates to form first and subsequently reacted via aliphatic carbon–carbon bond
activation with alkyl substituted nitroxides to yield rhodium porphyrin alkyl complexes.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Rhodium porphyrin; Nitroxides; Carbon–carbon bond activation
1. Introduction
as innocent traps in the reactions with organometallic com-
plexes. James and Dolphin have remarked in the report on
the reactivity of Ru(oep)(CH3)2 and Ru(oep)(C6H5)2
(oep = octaethylporphyrinate): ‘‘the rate of decomposition
of Ru(oep)(CH3)2 was found to be dependent on the con-
centration of TEMPO which also appears to react with
Ru(oep)CH3” [6]. While nitroxyls are well-known ligands
for a variety of transition metal complexes [1,2], TEMPO
has been reported to react with Ru(oep)CH3 to yield
Ru(oep)CO [10]. We have also reported recently that
rhodium(II) porphyrin reacts with alkyl nitroxides via
carbon–carbon bond activation (CCA) to yield alkyl
rhodium porphyrins [11,12]. During the investigation of
the mechanism of 1,2-rearrangements of rhodium porphy-
rin alkyls [13], we have observed that TEMPO did not act
as a radical trap but reacted directly with rhodium porphy-
rin alkyls bearing beta hydrogens via a novel beta-hydro-
gen abstraction/elimination process to generate rhodium
porphyrin radical which then underwent aliphatic CCA
with nitroxides [11]. We now disclose further details on
the reactions of rhodium porphyrin alkyls with nitroxides
and the subsequent carbon–carbon bond activation.
2,2,6,6-Tetramethylpiperidinoxy (TEMPO) and related
nitroxide radicals have been shown to react with organo-
metallic and inorganic complexes in various modes. They
can act as: (1) coordinating ligands [1,2], (2) trapping
agents for alkyl radicals generated from homolysis of
metal–alkyls [3–6], (3) reagents in reactions with metal
hydrides [7–9], (4) hydrogen atom abstractors with
metal–alkyls [10,11], and (5) alkyl transfer agents with
metal radicals [11,12].
The importance of nitroxide radicals in organometallic
chemistry is exemplified particularly as efficient alkyl radi-
cal traps in the estimations of bond dissociation energies of
metal–alkyl bonds by kinetic methods. Various bond
energy estimations have thus been accomplished in vitamin
B12 and related models [3,4], cyclopentadienyl metal–alkyl
complexes [3b], and ruthenium porphyrin alkyl complexes
[5,6]. These radicals can behave as reagents rather than
*
Corresponding author. Tel.: +852 26096376; fax: +852 26035057.
E-mail address: ksc@cuhk.edu.hk (K.S. Chan).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.11.009

400
K.S. Chan et al. / Journal of Organometallic Chemistry 693 (2008) 399–407
2. Results and discussion
R
R
X
X
Rhodium porphyrin chlorides were prepared form the
metalation of porphyrins with RhCl3 in refluxing PhCN
(Fig. 1, Eq. (1)). High yields of (PhCN)Rh(por)Cl were
obtained if the reactions were monitored carefully to avoid
further carbon–hydrogen bond activation of PhCN in giv-
ing meta-cyanophenyl rhodium porphyrins [14] and were
further heated under high vaccum to remove the weakly
coordinating PhCN. Then, alkyl rhodium porphyrin com-
plexes were synthesized by the reactions of the correspond-
ing rhodium porphyrin chlorides with NaBH4/RI according
to the literature procedure (Fig. 1, Eq. (1), Table 1) [13–15].
The alkyl nitroxides were prepared according to the lit-
erature methods (Scheme 1 and Eq. (3)). N-benzylphthalai-
mide (7) reacted with Grignard reagents to yield amines
8a–c. Subsequent debenzylation produced the methyl and
ethyl amines 9a–c. Oxidation of the resultant methylamines
9a, b with sodium tungstate gave TMINOs 10a, b (tetram-
ethylisoindolylnitroxyl) in good yields [16]. The method
was not effective for the preparation of 10c. Instead, 9c
was successfully oxidized with m-CPBA to generate TEI-
NO 10c (tetraethylisoindolylnitroxyl) (Eq. (3)) [17].
N
Rh
N
R1
N
X
X
X
N
X
X
X
R
R
R
X
Rh(por)
Rh(ttp)
Rh(btpp)
Rh(bocp)
Me
H
H
tBu
tBu Cl
Fig. 1. Structures of rhodium porphyrins.
Table 1
Compound numbering and synthesis of Rh(por)R
Entry
Rh(por)Cla
Rh(por)R
% Yield
1
2
3
4
5
6
7
8
Rh(ttp)Cl (1)
Rh(ttp)Cl (1)
Rh(ttp)Cl (1)
Rh(ttp)Cl (1)
Rh(ttp)Cl (1)
Rh(bttp)Cl (2)
Rh(bttp)Cl (2)
Rh(bocp)Cl (3)
Rh(bocp)Cl (3)
Rh(bocp)Cl (3)
Rh(bocp)Cl (3)
Rh(bocp)Cl (3)
Rh(bocp)Cl (3)
Rh(bocp)Cl (3)
Rh(ttp)CH3 (4a)
91
59
54
78
79
92
74
97
48
69
85
64
55
71
Rh(ttp)CH2CH3 (4b)
Rh(ttp)CH2CH2CH3 (4c)
Rh(ttp)CH2CH2Ph (4d)
Rh(ttp)CH2CH2CN (4e)
Rh(bttp)CH3 (5a)
RhCl3
(PhCN)Rh(por)Cl
H2por
ð1Þ
ð2Þ
PhCN
1-3 84-94%
Rh(bttp)CH2CH2Ph (5b)
Rh(bocp)CH3 (6a)
1. NaBH4, EtOH,
50 oC, 1 h
9
Rh(bocp)CD3 (6a–d3)
Rh(bocp)CH2CH3 (6b)
Rh(bocp)CH2CH2Ph (6c)
Rh(bocp) CH2CH2Ph (6c*)
Rh(bocp)CH(CH3)Ph (6d)
Rh(bocp)CH2CH2CN (6e)
Rh(por)R
Rh(por)Cl
Et
10
11
12
13
14
a
2. RBr or RI
0 C, 15 min
4-6
o
Et
Et
m-C PBA,
r.t., 5 min.
Et
N O.
NH
As mono PhCN adduct.
ð3Þ
ð4Þ
CH2Cl2
Et
9c
Et
Et
Et
10c 89%
O
R
RMgI,
toluene,
10 % Pd/C,
60psi H2
R
H3C
H3C
CH3
CH3
N
O.
NBz
NBz
80-110 oC
HOAc,
3 h, r.t.
4 h
R
TEMPO
R
O
7
Rh(por)R
Rh(por)CH3
8a R = CH 3, 4 0%
8b R = CD 3, 15%
8c R = Et, 40%
C6D6
Table 2 lists the results of the thermal reactions of TEMPO
and rhodium porphyrin alkys (Eq. (4)). In the presence of
excess TEMPO, Rh(por)CH3 was formed from the ther-
molysis of Rh(por)CH2CH2R. Complexes with electron
rich porphyrins of ttp and bttp reacted slower than that
of bocp. The phenylethyl complexes were the most reactive
while Rh(ttp)Et was the least reactive. No simple electronic
effect exists for the beta-substituent of phenyl, cyanao, and
methyl groups. The reactivity appears to decrease with
increasing bond strengths of beta C–H bonds [18] (Table
2, entries 1, 7 and 8).
R
R
R
Na2WO4.2H2O
H2O2, r.t.
R
.
NH
N O
MeOH/CH3CN,
12 h
R
R
R
R
9a R = CH3, 78%
9b R = CD3, 98%
9c R = Et, 99%
10a R = C H3, 98%
10b R = CD3, 71%
10c R = Et, 0%
Scheme 1. Preparation of nitroxides.
The reactions likely occurred via two consecutive steps
with the formation of fairly long-lived intermediates.
Rh(bocp)CH2CH2Ph 6c reacted with TEMPO (5 equiv.)
completely within the first hour of the reaction without
any Rh(bocp)CH3 6a formed as determined by TLC anal-
ysis. Then after a total reaction time of 7 h, Rh(bocp)CH3
6a was formed in 45% isolated yield. Increasing the amount

K.S. Chan et al. / Journal of Organometallic Chemistry 693 (2008) 399–407
401
Table 2
Summary of Thermolysis of Rh(por)R with TEMPO
Entry
Complex
TEMPO (equiv.)
Temp (°C)
Time for disappearance
of complex
Total reaction time
% Yield of Rh(por)Me
1
2
3
4
5
6
7
8
9
10
a
Rh(ttp)CH2CH2Ph (4d)
Rh(bttp)CH2CH2Ph (5b)
Rh(bocp)CH2CH2Ph (6c)
Rh(bocp)CH2CH2Ph (6c)
Rh(bocp)CH2CH2Ph (6c*)
Rh(bocp)CH(CH3)Ph (6d)
Rh(ttp)CH2CH2CH3 (4c)
Rh(ttp)CH2CH2 (4b)
5
5
5
15
15
5
10
10
10
10
80
80
80
80
80
120
120
120
120
120
48 h
48 h
15 min
15 min
15 min
3 d
6 d
6 d
7 h
90 min
90 min
3 d
4a 70b
5a 25b
6a 45b
6a 81a
6a 81a
6a 70b
4a 74b
4a 10d (51e)
4a 86b
6a 86b
7d
7 d
N.A.c
12 h
14db
24 h
24 h
Rh(ttp)CH2CH2CN (4e)
Rh(bocp)CH2CH2CN (6e)
12 h
Isolated yield from column chromatography.
b
c
1
Estimated yields from H NMR integration.
Only 19% of starting material converted.
Based on 100% of starting material.
Based on 19% staring material converted.
d
e
of TEMPO to 15 equiv. did not increase the rate of disap-
pearance of 6c but accelerated the rate of the formation
and the yield of product 6a (90 min and 81%). Since start-
ing materials disappeared completely in most cases before
Rh(por)CH3 formed, a relatively stable rhodium porphyrin
intermediate must be involved. More electron rich porphy-
rin rhodium alkyls such as 4d and 5b reacted with excess
TEMPO similarly. The rate of disappearance of 3 was also
found to be much faster than that of 1,2-rearangement to
Rh(bocp)CH(CH3)Ph (10 h at 80 °C) [13a].
Furthermore, the source of Me group in the products did
not originate from the alkyl fragment of the starting
(por)Rh–alkyl complexes.
We propose that the Me group in the product was trans-
ferred from one of the methyl groups in TEMPO via a car-
bon–carbon cleavage [12]. Labeling experiments were
carried out to test the validity of the proposal. When TMI-
NO–(CD3)4 was reacted with Rh(bocp)CH2CH2Ph,
Rh(bocp)CD3 was isolated 76% yield (Table 2, entries 1,
Eq. (6)). Furthermore, ethyl–methine bond cleavage was
also observed in the slower reaction of TEINO with
Rh(bocp)CH2CH2Ph and Rh(bocp)Et was isolated in
90% yield. Indeed, this type of carbon–carbon bond activa-
tion in TMINO and TEINO with rhodium(II) porphyrin
radical has been reported [12]. Therefore, the origin of
alkyl group was identified to come form the alkyl group
of nitroxides.
TEMPO
C6D6,
Ph
- 20 %
+
Rh(bocp)*CH2CH2Ph
Rh(bocp)CH3
*
ð5Þ
80 oC, 1 h
6a
6c*
In order to elucidate the mechanism of the formation of
Rh(por)CH3 complexes especially the source of Me group
in the product, the 13C enriched phenylethyl rhodium com-
plex 6c* was reacted with TEMPO (Eq. (5)). The rhodium
methyl group in 6a was found to be un-enriched with 13C
The generation of rhodium(II) porphyrin radical from
the reactions of Rh(por)CH2CH2R with alkyl nitroxides
could not be ascertained with pre-coordination of nitrox-
ide. Attempted detection of such coordination in a mixture
of nitroxide and Rh(por) CH2CH2R was not successful as
no change in its visible spectrum was observed. Presum-
ably, the strong trans-alkyl group does not favor TEMPO
coordination. Therefore, the formation of Rh(por)R from
the reaction of alkyl nitroxides with Rh(por)CH2CH2R
involes a two step reaction: Firstly, rhodium(II) porphyrin
radicals are generated and secondly, RhII(por) radicals
undergo CCA with nitroxides to produce Rh(por)alkyl
1
and appeared as a broad singlet at ꢀ4.84 ppm in the H
NMR spectrum of the reaction mixture without any
1JC–H coupling observed. Therefore, the rhodium methyl
group did not come from the a-carbon of the phenylethyl
ligand of 6c*.
The proton coupled 13C NMR spectrum of the reaction
mixture also showed two enhanced triplets at 73.7 and
113.8 ppm with the 1JC–H equal to 148 and 157 Hz, respec-
tively. At least two products were formed. The resonance at
113.8 ppm was assigned to the terminal sp2 carbon of sty-
rene and was quantified in 20% yield by 1H NMR spectros-
1
(Scheme 2) [8]. Kinetic studies by H NMR proved to be
difficult as the accuracy of integration was hampered by
broadening caused by the excess TEMPO added.
1
copy (cf. d(C6H5CH@CH2) = 112.3 ppm, JC–H(CH2@
CH2) = 156 Hz in CDCl3) [10]. The low yield of styrene
was accounted by the partial oligomerization catalyzed
by TEMPO [19]. However, the signal at 73.7 ppm could
not be unambiguously assigned and is unlikely to be TEM-
PO–*CH2CH2Ph. Therefore, the CH2CH2Ph fragment
remained intact without any carbon–carbon bond cleav-
age. Only the Rh–C bond was cleaved in the reaction.
R
R
N O.
ð6Þ
R
R
Rh(por)R'
Rh(por)R
C6D6

402
K.S. Chan et al. / Journal of Organometallic Chemistry 693 (2008) 399–407
Table 3
Mech. I
Summary of reactions of Rh(por)CH2CH2Ph with nitroxides at 80 °C
.
.
Rh(por)
+
CH2CH2Ph
Rh(por)CH2CH2R
Entry Complex
Nitroxide
(5 equiv.)
Total
reaction
time
Rh(por)R
% yield
Mech. II
R
+
(por)Rh H
(por)RhII
Rh(por)CH2CH2R
1
2
3
4
Rh(ttp)CH2CH2Ph
(4d)
Rh(bocp)CH2CH2Ph TMINO
(6c)
Rh(bocp)CH2CH2Ph TMINO–
(6c)
TMINO
6 d
14 h
18 h
54 h
4a 70
+
TEMPO-H
+
(por)Rh H TEMPO
6a 76
Mech. III
R
6a–d3 76
6b 90
(por)RhII
H
Med
+
HO
N
O N
+
+
Rh(bocp)CH2CH2Ph TEINO
R
Rh(por)
(6c)
TEMPO-H
decreasing C–H bond strengths (bond disscoication energy
(B.D.E.) PhCH2–H = 88 kcal/mol, 18Me(H)(CN)C–H =
90 kcal/mol [18], Me(Me)CH2–H = 95 kcal/mol [18],
MeCH2–H = 98 kcal/mol [18], TEMPO–H = 70 kcal/mol
[18]). The hydrogen abstraction step may be synchronous
with the homolysis of Rh–C bond to generate a Rh(por)
radical. Alternatively, a sufficiently long-lived carbon-cen-
tered radical stabilized by the beta-Rh(por) could exist
before Rh–C homolysis [20–22a]. The detailed nature of
the transfer of methyl or ethyl group from the nitroxides
to Rh(II) porphyrins is not very clear at this stage. It is
likely that Rh(II)-nitroxide adduct can initially form as
TEMPO–transition metal complexes are reported in the
literature [22,23]. Then the alkyl group can transfer in an
SH2 or SN2 like manner to give the rhodium porphyrin
alkyls. Further studies are required to understand the
details of the carbon carbon bond cleavage (see Table 3).
CH3
H3C
CCA
(por)RhII
.
H3C
Rh(por)CH3
+
O N
CH3
Scheme 2. Proposed mechanism for Rh(por)CH3 from Rh(por)-
CH2CH2R.
Three mechanistic possibilities exist for the generation of
Rh(por) radicals: (I) homolysis of a rhodium alkyl bond,
(II) beta-hydride elimination of Rh(por)CH2CH2R to
Rh(por)H – hydrogen abstraction with TEMPO [11] and
(III) beta-hydrogen abstraction of Rh(por)CH2CH2R with
TEMPO – Rh-alkyl homolysis.
Mechanism I is disfavored based on the unlikely homol-
ysis of the rather strong Rh–C bond at 120 °C (B.D.E.
ꢁ60 kcal/mol) [12,13] as well as the absence of
PhCH2CH2–TEMPO in the presence of excess, efficient
spin trap of TEMPO. Though mechanism II is apparently
supported by the established intermediate of Rh(por)H,
which is generated via beta-hydride elimination in the
1,2-rearrangements of Rh(por)CH2CH2R [13] and is
known to react with TEMPO to give Rh(por) radical,7
the rates of disappearance of the starting materials were,
however, much faster in the presence of TEMPO. In the
absence of TEMPO, the 1,2-rearrangements of
Rh(bocp)CH2CH2Ph and Rh(ttp)CH2CH2Ph took 10 h
[13a] and 144 h [13b] at 80 °C, respectively. Similarly, the
3. Summary
In conclusion, we have discovered a non-radical trap-
ping pathway for reactions of nitroxides with rhodium por-
phyrin alkyls bearing beta hydrogens. Rhodium(II)
porphyrins are formed by the beta-hydrogen elimination
of the alkyl group. Subsequently rhodium porphyrins can
coordinate with excess nitroxides to form Rh(II)-nitroxides
adducts. Then these adducts undergo carbon–carbon bond
activation at the methine–alkyl bonds of the nitroxides to
yield rhodium porphyrin alkyls. In view of the roles of nitr-
oxides as reagents, the innocent nature of TEMPO and
related nitroxides as radical traps in the determination of
bond energies of metal–alkyls bearing b-hydrogens, espe-
cially stronger ones, needs to be cautioned.
1,2-rearrangements
of
Rh(bocp)CH2CH2CN
and
Rh(ttp)CH2CH2CN at 140 °C required 9 and 24 days,
respectively, and the rearrangement of Rh(ttp)Pr into
Rh(ttp)CH(Me)Me took 52 h at 120 °C. All these rear-
rangements are much slower than the reactions with
TEMPO. Furthermore, the formation of 1,2-rearrange-
ment products as the intermediates is unlikely since
Rh(bocp)CH(CH3)Ph (6e) is also much less reactive than
Rh(bocp)CH2CH2Ph in the reaction with TEMPO (Table
2, entry 6). Therefore, Rh(por)CH2CH2Ph and other alkyls
must react with TEMPO directly. Mechanism III remains
the most probable. Abstraction of the fairly weak C–H
bond a to Ph, CN, and Me groups in these rhodium com-
plexes is energetically feasible at the reaction temperature
from 80 to 120 °C and the reaction rates increased with
4. Experimental
All materials were obtained from commercial suppliers
and used without further purification unless otherwise
specified. Rhodium trichloride (RhCl3 ꢂ xH2O) was
obtained from Johnson Matthey. Tetrahydrofuran (THF)
was freshly distilled from sodium benzophenone ketyl
under N2. Benzene was distilled from sodium. Dichloro-
methane and acetonitrile were distilled from calcium

K.S. Chan et al. / Journal of Organometallic Chemistry 693 (2008) 399–407
403
hydride. Benzene-d6 was vacuum distilled from sodium,
degassed twice by freeze–thaw–pump cycle and stored in
a Teflon screwhead stoppered flask. 2,2,6,6-Tetramethyl-
1-piperidinyloxy (TEMPO) was sublimed under high
vacuum.
424.0 (5.39), 535.0 (4.34), 570.5 (3.76); HRMS (ESI): Calc.
for (C48H36N4RhCl)+ (after vacuum removal of PhCN):
m/z 806.1678. Found: m/z 806.1669. Anal. Calc. for
C48H36N4RhCl (after vacuum removal of PhCN): C,
61.46; H, 4.13; N, 5.73. Found: C, 61.72; H, 4.61; N, 5.86%.
Thin layer chromatography was performed on pre-
coated silica gel plates. Column chromatography was per-
formed on silica gel (70–230 and 230–400 mesh) or
neutral aluminum oxide (activity I, 70–230 mesh).
1H NMR spectra were recorded on either a Bruker WM-
250 (250 MHz), Bruker DPX-300 (300 MHz) or a Bruker
AMX-500 (500 MHz) spectrometer. Chemical shifts were
referenced with the residual solvent protons in C6D6
(d 7.15 ppm), CDCl3 (d 7.24 ppm) or with tetramethylsil-
ane (d 0.00 ppm) as the internal standard. Chemical shifts
(d) were reported as part per million (ppm) in d scale down-
field from TMS. Coupling constants (J) were reported in
Hertz (Hz). 13C NMR spectra were recorded on either a
Bruker WM-250 (62.89 MHz), Bruker DPX-300
(75.47 MHz) or a Bruker AMX-500 (125.77 MHz) spec-
trometer. Chemical shifts were referenced to the solvent
peak of C6D6 (d 128.0 ppm) or CDCl3 (d 77.0 ppm). Cou-
pling constants (J) were reported in Hertz (Hz).
Mass spectra were recorded on either a VG7070F mass
spectrometer (E.I. 70 ev), Hewlett–Packard 5989B mass
spectrometer (FAB-MS and E.I. 70 ev), Bruker APEX
47e FT-ICR mass spectrometer (FAB-MS and ESI-MS)
or MAT-95L spectrometer (FAB-MS). Fast atom bom-
bardment spectra were obtained using 3-nitrobenzyl alco-
hol (NBA) as the matrix. Electrospray ionization spectra
were obtained with a solvent mixture of acetone with 3%
of acetic acid.
4.2. Preparation of chloro[5,10,15,20-tetrakis(p-tert-
butylphenyl)porphyrinato](benzonitrile)rhodium(III)
[Rh(bttp)(Cl)(PhCN)] (2)
A
solution mixture of p-tert-butyltoluene (44.8 g,
300 mmol), NBS (81.9 g, 460 mmol) and benzoylperoxide
(0.27 g, 1.1 mmol) in CCl4 (200 mL) was refluxed in air
for 4 h. The reaction mixture was cooled to rt and was
filtered. The filtrate was rotary evaporated and the yellow
oily liquid obtained was added to a solution mixture of
hexamethylenetetramine (115 g, 820 mmol) in H2O
(100 mL) and EtOH (100 mL). The mixture was refluxed
in air for 4 h. Concentrated HCl (50 mL) was added and
the mixture was refluxed for another 30 min. The reaction
mixture was cooled to room temperature and was worked
up by extraction with diethyl ether. The combined organic
extract was dried (MgSO4) and filtered. After rotary evap-
oration, a yellow oily liquid of p-tert-butylbenzaldehyde
1
(41.5 g, 256 mmol, 86%) was obtained. H NMR (CDCl3,
250 MHz) d 1.34 (s, 9H), 7.53 (d, 2H, J = 8.4 Hz), 7.79
(d, 2H, J = 8.4 Hz), 9.96 (s, 1H).
Pyrrole (23.5 mL, 0.34 mol) was added dropwise to a
refluxing solution of p-tert-butylbenzaldehyde (55 g,
0.34 mol) in propionic acid (1.25 L). The resulting mixture
was refluxed in air for another 30 min. The resulting black
solution was cooled to room temperature, and MeOH
(1.5 L) was added to crystallize the porphyrin. The mixture
was filtered and the purple solid obtained was washed with
MeOH. After purified by recrystallization from CHCl3/
MeOH, purple crystals of 5,10,15,20-tetrakis(p-tert-butyl-
phenyl)porphyrin [22] [H2(bttp)] (9.55 g, 0.011 mol, 13%)
was obtained. Rf = 0.71 (hexane/CH2Cl2 = 1:1); 1H
NMR (CDCl3, 250 MHz) d ꢀ 2.76 (s, 2H), 1.60 (s, 36H),
7.75 (d, 8H, J = 8.2 Hz), 8.14 (d, 8H, J = 8.2 Hz), 8.86 (s,
8H); 13C NMR (CDCl3, 125.8 MHz) d 31.8, 34.9, 121.1,
123.2, 123.7, 128.2, 131.1, 132.2, 133.3, 134.1, 134.6,
139.4, 142.6, 150.3; UV–Vis (CH2Cl2), kmax, nm (loge)
419.0 (5.67), 517.0 (4.23), 553.0 (4.02), 591.0 (3.73), 648.0
(3.78); HRMS (FAB): Calc. for (C60H62 N4)+: m/z
838.4969. Found: m/z 838.4962.
IR spectra were obtained on a FT-IR spectrophotome-
ter. Samples were prepared either as neat film on KBr
plates or as a KBr pressed disk.
4.1. Chloro(5,10,15,20-tetratoylporphyrinato)(benzo-
nitrile)rhodium(III) [Rh(ttp)Cl(PhCN)] (1)
A
solution of H2ttp (350 mg, 0.522 mmol) and
RhCl3 ꢂ xH2O (206 mg, 0.78 mmol) in benzonitrile
(30 mL) was refluxed in air for 2 h until the solution was
changed from purple to bright red in color. The solvent
was then removed under high vacuum and the crude prod-
uct was purified by column chromatography over silica gel
(70–230 mesh) eluting with a solvent mixture of hexane/
CH2Cl2 (1:4). The major red band was collected. After
removal of solvent by rotary evaporation, a red solid
(401 mg, 0.44 mmol, 84%) was obtained and was further
purified by recrystallization from CH2Cl2/CH3OH.
2 was synthesized from H2(bttp) (300 mg, 0.358 mmol)
and RhCl3 ꢂ xH2O (200 mg, 0.752 mmol). Red solid
(311 mg, 0.319 mmol, 89%) was obtained and was further
purified by recrystallization from CH2Cl2/CH3OH.
1
1
Rf = 0.29 (CH2Cl2); H NMR (CDCl3, 300 MHz) d 2.70
Rf = 0.50 (CH2Cl2); H NMR (CDCl3, 250 MHz) d 1.60
(s, 12H), 5.33 (d, 2H, J = 7.5 Hz), 6.63 (m, 2H), 6.95 (t,
1H, J = 7.5 Hz), 7.50–7.56 (m, 8H), 8.07 (d, 4H,
J = 7.5 Hz), 8.20 (d, 4H, J = 7.5 Hz) 8.94 (s, 8H);
13C NMR (CDCl3, 62.9 MHz) d 21.49, 121.05, 127.04,
127.50, 128.25, 131.16, 132.16, 133.26, 134.14, 134.76,
137.10, 139.51, 142.66; UV–Vis (CH2Cl2), kmax, nm (loge)
(s, 36H), 5.31 (d, 2H, J = 7.7 Hz), 6.60–6.66 (m, 2H),
6.97 (t, 1H, J = 7.8 Hz), 7.68–7.76 (m, 8H), 8.08 (d, 4H,
J = 8.0 Hz), 8.24 (d, 4H, J = 8.0 Hz), 8.93 (s, 8H); 13C
NMR (CDCl3, 125.8 MHz) d 31.8, 34.9, 121.1, 123.2,
123.7, 128.2, 131.1, 132.2, 133.3, 134.1, 134.6, 139.4,
142.6, 150.3; UV–Vis (CH2Cl2), kmax, nm (loge) 425.0

404
K.S. Chan et al. / Journal of Organometallic Chemistry 693 (2008) 399–407
(5.30), 535.5 (4.25), 571.0 (3.74); HRMS (FAB): Calc. for
butylphenyl)porphyrin [H2(bocp)] (0.85 g, 0.76 mmol,
88%) were obtained. Rf = 0.26 (hexane/CH2Cl2 = 2:1); H
1
(C60H60N4RhCl)+: m/z 974.3556. Found: m/z 974.3560.
NMR (CDCl3, 250 MHz) d 1.52 (s, 36H), 7.74 (d, 8H,
J = 7.8 Hz), 8.06 (d, 8H, J = 7.8 Hz); UV–Vis (CH2Cl2),
4.3. Preparation of chloro[2,3,7,8,12,13,17,18-octachloro-5,
10,15,20-tetrakis(p-tert-butylphenyl)porphyrinato](benzo-
nitrile)rhodium(III) [Rh(bocp)(Cl)(PhCN)] (3)
[13a,24]
k
max, nm (loge) 359.5 (4.28), 457.0 (5.22), 555.5 (3.86),
612.0 (4.04), 723þ.0 (3.78). LRMS (FAB): Calc. for
ðC60H3554Cl637Cl2N4Þ : m=z 1114. Found: m/z 1114.
[Rh(bocp)(Cl)(PhCN)]3 was synthesized from H2(bocp)
(500 mg, 0.45 mmol) and RhCl3 ꢂ xH2O (142 mg,
0.54 mmol). The crude product was purified by column
chromatography over silica gel (70–230 mesh) eluting with
a solvent mixture of hexane/CH2Cl2 (2:3). The fast moving
green fraction was discarded and the major red fraction
that eluted off with CH2Cl2 was collected. After rotary
evaporation, a reddish brown solid (573 mg, 0.423 mmol,
94%) was obtained. The product was further purified by
recrystallization from CH2Cl2/CH3OH. Rf = 0.62 (hex-
ane/CH2Cl2 = 2:3); 1H NMR (CDCl3, 250 MHz) d 1.55
(s, 36H), 5.69 (d, 2H, J = 7.7 Hz), 6.77–6.83 (m, 2H),
7.12 (t, 1H, J = 7.3 Hz), 7.61–7.76 (m, 8H), 8.01 (m, 8H);
UV–Vis (CH2Cl2), kmax, nm (loge) 373.0 (4.36), 449.0
(5.18), 561.5 (4.24); IR (KBr, cmꢀ1) n 2862, 2267, 1596;
HRMS (FAB): Calc. for ðC60H3552Cl737Cl2N4RhÞþ :
m=z 1250:0379. Found: m/z 1250.0376. Anal. Calc. for
C60H52Cl9N4Rh: C, 57.60; H, 4.19; N, 4.48. Found: C,
57.43; H, 4.36; N, 4.62%.
A
suspension of H2(bttp) (1.0 g, 1.2 mmol) and
Ni(OAc)2 ꢂ 4H2O (0.6 g, 2.4 mmol) in DMF (100 mL) was
refluxed for 1.5 h. The color of the suspension changed
from purple to reddish purple. The reaction mixture was
cooled to room temperature and was worked up by extrac-
tion with CHCl3/H2O. The combined organic extract was
rotary evaporated and the reddish purple residue obtained
was purified by recrystallization with CHCl3/MeOH to
give a reddish purple crystalline solid of 5,10,15,20-tetra-
kis(p-tert-butylphenyl)porphyrinato nickel(II) Ni(bttp)
(0.94 g, 1.1 mmol, 87%). Rf = 0.51 (hexane/CH2Cl2 = 2:1);
1H NMR (CDCl3, 250 MHz) d 1.55 (s, 36H), 7.68 (d, 8H,
J = 8.2 Hz), 7.94 (d, 8H, J = 8.2 Hz), 8.77 (s, 8H); UV–Vis
(CH2Cl2), kmax, nm (log e) 414.0 (5.47), 528.0 (4.31), 614.0
(3.04).
Ni(bttp) (0.94 g, 1.1 mmol) and NCS (1.68 g,
12.6 mmol) were dissolved in o-dichlorobenzene (100 mL)
and the mixture was heated at 140 °C. The reaction was
monitored by UV–Vis spectroscopy until the Soret band
shifted from 416 to 442 nm (about 2.5 h). The solvent
was then removed under high vacuum. The dark red resi-
due was purified by flash column chromatography over
neutral alumina using CHCl3 as the eluent. The major
red band was collected. After removal of solvent by rotary
evaporation, a dark red solid of 2,3,7,8,12,13,17,18-octa-
chloro-5,10,15,20-tetrakis(p-tert-butylphenyl)porphyrinto
nickel(II) [Ni(bocp)] (1.0 g, 0.86 mmol, 82%) was obtained.
Rf = 0.17 (hexane/CH2Cl2 = 2:1); 1H NMR (CDCl3,
250 MHz) d 1.49 (s, 36H), 7.64 (d, 8H, J = 8.3 Hz), 7.78
(d, 8H, J = 8.3 Hz); UV–Vis (CH2Cl2), kmax, nm (loge)
442.5 (5.42), 554.5 þ(4.31). HRMS (FAB): Calc. for
4.4. Preparation of (5,10,15,20-tetratoylporphyrinato)2-
ethylrhodium(III)[Rh(ttp)CH2CH3] (4b)
A suspension of Rh(ttp)Cl (100 mg, 0.110 mmol) in
EtOH (50 mL) and
a solution of NaBH4 (41 mg,
0.120 mmol) in aq NaOH (0.1 M, 3 mL) were purged with
N2 for 15 min separately. The solution of NaBH4 was
added slowly to the suspension of Rh(ttp)Cl via a cannular.
The solution mixture was heated at 50 °C under N2 for 1 h
to give a brown suspension. The solution was then cooled
to 30 °C under N2 and ethyl bromide (119 mg, 1.20 mmol)
was added. A reddish orange suspension was formed. After
stirred at room temperature for another 15 min under N2,
the reaction mixture was worked up by extraction with
CH2Cl2/H2O. The combined organic extract was dried
(MgSO4), filtered and rotatory evaporated. The reddish
orange residue was purified by column chromatography
over silica gel (250–400 mesh) using a solvent mixture of
hexane/CH2Cl2 (4:1) as the eluent. The major orange frac-
tion was collected and gave a reddish orange solid (45 mg,
ðC60H3552Cl367Cl2N4NiÞ : m=z 1170:0989.
Found:
m/z
1170.0979.
Concentrated sulfuric acid (100 mL) was added to a
solution of Ni(bocp) (1.0 g, 0.86 mmol) in CH2Cl2
(150 mL). The mixture was stirred at room temperature
for 30 min. The resulting green suspension was then poured
onto ice cubes and the mixture was extracted with CH2Cl2/
H2O. The combined green organic extract was neutralized
with Na2CO3, washed with saturated NaCl solution, dried
(MgSO4), and filtered. The solvent was then removed by
rotary evaporation. The greenish blue crude product
obtained was purified by column chromatography over sil-
ica gel (100–200 mesh) eluting with a solvent mixture of
hexane/CH2Cl2 (2:1). The fast moving brown fraction
was discarded. The column was then eluted with CH2Cl2
and the major green band was collected. After removal
of solvent by rotary evaporation, a greenish blue solid
of 2,3,7,8,12,13,17,18-octachloro-5,10,15,20-tetrakis(p-tert-
0.063 mmol, 59%) as the product after rotary evaporation.
2
1H NMR (CDCl3, 300 MHz) d ꢀ 4.86 (dq, 2H, JRh–H
=
3
3.0 Hz, JH–H = 6.0 Hz), ꢀ4.45 (t, 3H, J = 6.0 Hz), 2.69
(s, 12H), 7.53 (t, 8H, J = 6.3 Hz), 8.03 (d, 4H,
J = 7.2 Hz), 8.08 (d, 4H, J = 7.5 Hz), 8.71 (s, 8H); 13C
NMR (CDCl3, 75 MHz) d 10.4 (d, J = 27.0 Hz), 13.2,
21.7, 122.5, 127.5, 131.5, 133.8, 134.1, 137.3, 139.5, 143.4.
LRMS (FAB): Calc. for (C50H41N4Rh)+: m/z 800. Found:
m/z 800. Anal. Calc. for C50H41N4Rh: C, 74.99; H, 5.16; N,
6.99. Found: C, 75.11; H, 5.24; N, 6.80%.

K.S. Chan et al. / Journal of Organometallic Chemistry 693 (2008) 399–407
405
4.5. Preparation of [2,3,7,8,12,13,17,18-octachloro-5,10,15,
20-tetrakis(p-tert-butylphenyl)porphyrinato]phenethyl-
1-13C-rhodium(III)[Rh(bocp)*CH2CH2Ph] (*6c)
6.60 (t, 1H, J = 7.2 Hz), 7.67–7.74 (m, 8 H), 7.86 (d, 4H,
J = 7.5 Hz), 7.94 (d, 4H, J = 7.5 Hz); 13C NMR (C6D6,
1
125.8 MHz) d 19.04 (13C enriched, d, JRh-C = 26.9 Hz);
LRMS
(FAB):
ðC13C1H61N4Rh35Cl37Cl2Þþm=z 1321.
67
6
Lithium aluminum hydride (140 mg, 3.66 mmol) in
freshly distilled THF (20 mL) was stirred at room temper-
ature under N2 for 30 min. The gray suspension was then
cooled to 0 °C and phenylacetic-carboxy-13C acid
(250 mg, 1.83 mmol) in THF (5 mL) was added dropwise.
The resulting mixture was stirred at room temperature
for about 16 h under N2. Diethyl ether (saturated with
H2O) was then added dropwise to destroy with excess
LAH. The gray suspension was filtered and the filtrate
was dried (MgSO4) and filtered. After removal of solvent,
the colorless oily liquid of 2-phenylethanol-1-13C
(Ph–CH2–13CH2–OH) was obtained without further purifi-
Found: m/z 1322 (M+ + 1).
4.6. Preparation of 2-benzyl-1,1,3,3-tetramethylisoindoline
(8a)
A solution of MeMgI, prepared from methyl iodide
(4.8 mL, 60 mmol) and magnesium powder (1.52 g,
62.5 mmol) in anhydrous diethyl ether (35 mL) under
N2, was concentrated by high vacuum until the suspen-
sion did not boil at 80 °C. The residue was allowed to
cool to 60 °C, and a solution of N-benzylphthalimide
(7) [25] (2.37 g, 10 mmol) in toluene (30 mL) was added
dropwise with stirring through a cannula under N2. Sol-
vents were partially removed by high vacuum until the
reaction mixture was slightly refluxed at 110 °C. The
reaction mixture was heated at 110 °C for 4 h. The sol-
vents were then further removed until ꢁ15 mL was left.
The slurry reaction mixture was diluted with hexane
(25 mL). It was then filtered through celite and washed
with hexane (3 ꢃ 5 mL). The yellow organic filtrate
turned to a purple suspension after standing in air for
2 h. After rotary evaporation, the purple crude product
was chromatographed through alumina (basic, grade I,
70–230 mesh) eluting with hexane to give a white solid
of 2-benzyl-1,1,3,3-tetramethylisoindoline (8a) (1.05 g,
40%). Rf = 0.14 (hexane); 1H NMR (CDCl3, 300 MHz)
d 1.30 (s, 12H), 3.99 (s, 2H), 7.09–7.16 (m, 2H), 7.19–
7.31 (m, 5 H), 7.43–7.48 (m, 2H). GCD (column pro-
gram: initial temperature 100 °C, duration 2 min; incre-
ment rate 20 °C/min; final temperature 280 °C, duration
15 min) tR 9.96 min; LRMS (EI): Calc. for (C19H23N)+:
m/z 265. Found: m/z 265.
1
cation. Rf = 0.16 (hexane); H NMR (CDCl3, 500 MHz) d
3
2.83–2.87 (m, 2H), 3.81 (dt, 2H, JH–H = 7.0 Hz,
1JC–H = 150.5 Hz), 4.99 (bs, 1 H), 7.32–7.20 (m, 5 H);
13C
NMR
(CDCl3,
125.8 MHz)
d
39.7
(d,
1JC–C = 36.0 Hz), 64.0 (13C enriched), 126.9, 129.0, 129.6,
139.9.
Triphenylphosphine (580 mg, 2.2 mmol) was dissolved
in freshly distilled CH2Cl2 (15 mL) at 0 °C in a round bot-
tom flask fitted with a drying tube. Bromine (350 mg,
2.2 mmol) in CH2Cl2 (5 mL) was added dropwise. The
mixture was stirred at rt for 1 h and then cooled to
0 °C. The crude reaction product of 2-phenylethanol-
1-13C was dissolved in CH2Cl2 (5 mL) and the solution
was added dropwise to the reaction mixture. The mixture
was then stirred at room temperature for another 90 min
and then worked up by washing with saturated aqueous
Na2S2O3 aq, NaHCO3 aq, H2O and brine successively.
The organic extract was dried (MgSO4) and filtered. Hex-
ane was added to the filtrate and the precipitated triphen-
ylphosphine oxide was filtered. The filtrate was rotary
evaporated and purified by silica gel column chromatog-
raphy using hexane as the eluent. The first fast moving
colorless fraction was discarded and the following color-
less fraction [Rf = 0.36 (hexane)] was collected. After
removal of solvent, a colorless oily liquid (182.4 mg,
0.98 mmol, 54% from phenylacetic-carboxy-13C acid)
4.7. Preparation of 1,1,3,3-tetramethylisoindoline (9a) [17]
2-Benzyl-1,1,3,3-tetramethylisoindoline (8a) (550 mg,
2.1 mmol) and 10% Pd/C (50 mg) were suspended in HOAc
(10 mL) in a 25 mL conical flask with a magnetic stirrer
bar. The reaction flask was placed in a high pressure reac-
tor. The reactor was charged with hydrogen (60 psi) and
released (20 psi) for three cycles and was finally pressurized
with hydrogen at 60 psi. After stirring for 3 h at room tem-
perature, the reaction mixture was then neutralized with
NaOH to pH 9 and extracted with ether (3 ꢃ 10 mL).
The organic extract was then dried over MgSO4, filtered
and rotary evaporated to dryness. The crude oily product
was purified by chromatography over a short alumina col-
umn eluting with hexane:ethyl acetate (3:1) to yield color-
less crystals (286 mg, 78%), Rf = 0.06 (hexane:ethyl
acetate = 3:1); 1H NMR (CDCl3, 300 MHz) d 1.48 (s,
12H), 2.13 (br s, 1H), 7.10–7.15 (m, 2H); 7.23–7.27 (m,
2H); 13C NMR (CDCl3, 75.5 MHz) d 31.79, 63.02,
121.42, 127.17, 148.42.
was collected. Rf = 0.36 (hexane); 1H NMR (CDCl3,
3
500 MHz) d 3.14–3.19 (m, 2H), 3.57 (dt, 2H, JH–H
=
7.5 Hz, JC–H = 153 Hz), 7.20–7.34 (m, 5 H); 13C NMR
1
(CDCl3, 125.8 Hz) d 32.9 (13C enriched), 39.4 (1JC–C
=
35.0 Hz), 126.9, 128.6, 138.9; LRMS (EI): Anal. Calc.
m/z (rel int) 187 (M+ + 2, 100), 185 (M+, 78).
*6c was synthesized from 2-phenylethyl-1-13C bromide
(100 mg, 0.074 mmol) and 10 (55 mg, 0.30 mmol). A red-
dish orange solid (62.6 mg, 0.047 mmol, 64%) was obtained
after purified by column chromatography. The product
was further purified by recrystallization from CH2Cl2/
CH3OH. Rf = 0.59 (hexane/CH2Cl2 = 5:1); 1H NMR
3
(CDCl3, 500 MHz) d ꢀ 3.99 (dt, 2H, JH–H = 6.6 Hz,
1JC–H = 143.2 Hz), ꢀ2.53 to (ꢀ2.48) (m, 2H), 1.52
(s, 36H), 5.06 (d, 2H, J = 7.7 Hz), 6.46–6.50 (m, 2H),

406
K.S. Chan et al. / Journal of Organometallic Chemistry 693 (2008) 399–407
4.8. Preparation of 1,1,3,3-tetramethylisoindolin-2-oxyl
(10a) [25]
Acknowledgement
We thank the Research Grants Council of Hong Kong
of the SAR of China for financial support (No. 400104).
To the pale yellow solution of 1,1,3,3-Tetramethyliso-
indoline (9a) (148 mg, 0.84 mmol) in MeOH:CH3CN
(14:1) (2 mL), NaHCO3 (56 mg, 0.67 mmol) and
Appendix A. Supplementary material
Na2WO: 2H2O (8 mg, 0.025 mmol) were added to a sus-
4
pension [16]. 30% H2O2 (ꢁ0.3 mL) was then added and
the suspension turned to pink in color. After 12 h, a
bright yellow solution formed and 30% H2O2 (ꢁ0.3 mL)
was added. The reaction mixture was stirred for 2 days.
The product was extracted from hexane (3 ꢃ 5 mL).
The organic layer was washed with H2O, brine, dried
over MgSO4, filtered and rotary evaporated to dryness
to give a yellow solid (157 mg, 98%). Rf = 0.63 (hex-
ane:ethyl acetate = 3:1); GCD (column program: initial
temperature 100 °C, duration 2 min; increment rate
20 °C/min; final temperature 280 °C, duration 15 min)
tR 6.51 min; LRMS (EI): Calc. for (C12H16NO)+: m/z
190. Found: m/z 190.
Supplementary data associated with this article for the
compound characterization can be found. Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.jorganchem.2007.11.009.
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was
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(hex-
1
ane:CH2Cl2 = 5:1); H NMR (CDCl3, 300 MHz) d ꢀ4.87
2
(d, JRhH = 2.7 Hz, 3H), 1.53 (s, 36H), 7.68 (d,
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´
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