Ligand-enhanced aliphatic carbon-carbon bond activation of nitroxides by rhodium(II) porphyrin

Article abstract of DOI:10.1021/om1000869

Rh(tmp) underwent Ph₃P-enhanced aliphatic carbon-carbon bond activation with various nitroxides. (Ph₃P)Rh(tmp), rapidly formed from Rh(tmp) and Ph₃P, enhanced the rate, selectivity, and yield in comparison to Rh(tmp). From kinetic studies, the rate of reaction showed a first-order dependence on both Rh(tmp) and TEMPO (TEMPO = 2,2,6,6- tetramethylpiperidine-1-oxyl) and saturation kinetics on Ph₃P. The rate enhancement of (Ph₃P)Rh(tmp) over Rh(tmp) was estimated to be about 11 at 70 °C.

Full text of DOI:10.1021/om1000869

2850 Organometallics 2010, 29, 2850–2856
DOI: 10.1021/om1000869
Ligand-Enhanced Aliphatic Carbon-Carbon Bond Activation
of Nitroxides by Rhodium(II) Porphyrin
Kin Shing Chan,* Xin Zhu Li, and Siu Yin Lee
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong, People’s Republic of China
Received February 3, 2010
Rh(tmp) underwent Ph3P-enhanced aliphatic carbon-carbon bond activation with various nitr-
oxides. (Ph3P)Rh(tmp), rapidlyformedfrom Rh(tmp) and Ph3P, enhanced the rate, selectivity, and yield
in comparison to Rh(tmp). From kinetic studies, the rate of reaction showeda first-order dependence on
both Rh(tmp) and TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl) and saturation kinetics
on Ph3P. The rate enhancement of (Ph3P)Rh(tmp) over Rh(tmp) was estimated to be about 11 at 70 °C.
Introduction
Intermolecular aliphatic carbon-carbon bond activations
(ACCA) are of fundamental interest and industrial applica-
tions.1 ACCA by transition-metal complexes in a homoge-
neous medium is challenging (eq 1), as a sterically more
accessible but stronger carbon-hydrogen bond is preferen-
tially activated over the sterically less accessible but weaker
carbon-carbon bond in a hydrocarbon substrate. Discoveries
and mechanistic understandings of ACCA would be desirable
to aid selective activations and catalytic functionalization of
carbon-carbon bonds, especially those of alkanes.
Figure 1. Structure of Rh(tmp).
M
CðalkylÞ- CðalkylÞ
s
CðalkylÞ- M
ð1Þ
f
phyrinate) (Figure 1) undergoes unique ACCA with a variety
of organic substrates such as nitroxides,5-8 nitrile,9 and
carbonyl compounds10,11 to form Rh(tmp) alkyls. These
reactions are proposed to undergo metalloradical attack on
the carbon centers, most likely via prior coordination of the
heteroatom to the Rh center.8 The Ph3P ligand can promote
both the rates and yields of reactions with nitriles.8 To gain
further understanding ofthe promoting roles ofPh3P, wehave
undertaken more comprehensive studies of the ACCA of
nitroxides and Rh(tmp). We now report results showing the
scope of nitroxides, the Ph3P-controlled regioselectivity, and
the quantitative Ph3P rate enhancement by kinetic studies.
Various approaches have been adopted for successful
ACCA. Notable examples of intermolecular CCA by non-
porphyrin-type transition-metal complexes take advantage
of energy gain from aromatization in cleaving the alkyl-
cyclopentadienyl derivatives to give cyclopentadienyl metal
complexes2 and the energy release in cleaving strained hydro-
carbons of cubane3 and cyclopropane.4 These reactions are
proposed to occur via classical oxidative addition mechan-
isms of the metal complexes.
For metalloporphyrin complexes, we have reported that
the metalloradical RhII(tmp) (1; tmp = tetrakismesitylpor-
*To whom correspondence should be addressed. E-mail: ksc@
cuhk.edu.hk.
Results and Discussion
(1) (a) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38,
870–883. (b) Murakami, M.; Ito, Y. In Topics in Organometallic Chemistry;
Murai, S., Ed.; Springer: Berlin, 1999; Vol. 3, pp 97-129. (c) Crabtree,
R. H. Chem. Rev. 1985, 85, 245–269. (d) Park, Y. J.; Park, J.-W.; Jun, C.-H.
Acc. Chem. Res. 2008, 41, 222–234.
(2) (a) Eilbracht, P.; Dahler, P. Chem. Ber. 1980, 113, 542–554.
(b) Benfield, F. W. S.; Green, M. L. H. J. Chem. Soc., Dalton Trans.
1974, 1324–1331.
Syntheses of Nitroxides. The nitroxides, 1,1,3,3-tetra-
alkylisoindolin-2-oxyls, were synthesized according to the
(7) Mak, K. W.; Yeung, S. K.; Chan, K. S. Organometallics 2002, 21,
2362–2364.
(8) Chan, K. S.; Mak, K. W.; Tse, M. K.; Yeung, S. K.; Li, B. Z.;
Chan, Y. W. J. Organomet. Chem. 2008, 693, 399–407.
(9) Chan, K. S.; Li, X, Z.; Zhang, L.; Fung, C. W. Organometallics
2007, 26, 2679–2687.
(3) Cassar, L. G.; Eaton, P. E.; Halpern, J. J. Am. Chem. Soc. 1970,
92, 3515–3518.
(4) Anstey, M. R.; Yung, C. M.; Bu, J.; Bergman, R. G. J. Am. Chem.
Soc. 2007, 129, 776–777.
(10) Zhang, L.; Chan, K. S. J. Organomet. Chem. 2007, 692, 2021–
2027.
(11) Zhang, L.; Chan, K. S. J. Organomet. Chem. 2006, 691, 3782–
3787.
(5) Tse, M. K.; Chan, K. S. Dalton Trans. 2001, 510–511.
(6) Chan, K. S.; Li, X. Z.; Dzik, W. I.; De Bruin, B. J. Am. Chem. Soc.
2008, 130, 2051–2061.
r
pubs.acs.org/Organometallics
Published on Web 06/09/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 13, 2010 2851
Scheme 1. Synthesis of Nitroxides
Table 1. Ligand Effect of CCA of Nitroxides with Rh(tmp)
nitroxide f
RhðtmpÞ
RhðtmpÞR
ð2Þ
benzene w=o Ph3P
temp/ time/
°C
total
yield/%
entry nitroxide
h
Ph3Pa
product, yield/%b
1
2
3
TMINO
TEINO
70
70
110
4
4
60
nonec Rh(tmp)Me, 73
Ph3P Rh(tmp)Me, 83
noned Rh(tmp)Me, 9
Rh(tmp)Et, 44
73
83
53
4
110
110
110
110
110
110
110
110
60
60
60
60
60
60
60
48
Ph3P Rh(tmp)Me, 27
Rh(tmp)Et, 38
none Rh(tmp)Me, 44
Rh(tmp)Pr, 11
Ph3P Rh(tmp)Me, 18
Rh(tmp)Pr, trace
none Rh(tmp)Me, 10e
Rh(tmp)Bu, 22
65
55
18
32
17
31
19
0
5
TPINO
TBINO
TPPINO
TPhINO
6
7
8
Ph3P Rh(tmp)Me, 17
Rh(tmp)Bu, trace
9
none Rh(tmp)CH2Ph, 19
Rh(tmp)(CH2)3Ph, 12
Ph3P Rh(tmp)CH2Ph, 12
Rh(tmp)(CH2)3Ph, 7
none Rh(tmp)Ph
10
11
a 1 equiv. b At least from duplicate runs. c Reference 10. d Reference 10
reported only Rh(tmp)Et. e Trace amounts of Rh(tmp)Et and Rh-
(tmp)Pr formed.
literature method (Scheme 1).8 N-Benzylphthalimide (2)
was obtained quantitatively by refluxing phthalic anhy-
dride and benzylamine in acetic acid for 1 h.12 Alkylation
of 2 with Grignard reagents in refluxing toluene under N2
gave the corresponding 2-benzyl-1,1,3,3tetraalkylisoindo-
lines 3a-f in moderate yields. Debenzylation of 3a-d
catalyzed by 10% Pd/C under H2 in HOAc produced the
1,1,3,3-tetraalkylisoindolines 4a-d in good yields.11 Oxi-
dation of 4a-d by m-CPBA in CH2Cl2 at room tempera-
ture yielded the corresponding 1,1,3,3-tetramethyliso-
indolin-2-oxyls 5a-d in good yields.12 5e,f were obtained
by direct oxidation of 4e,f with m-CPBA at room tem-
perature for 4 days in moderate yields.13
Scope of Nitroxides. We have reported some preliminary
results of the nitroxide scope of ACCA with Rh(tmp)5,6 and
with RhIII(por)(alkyls)-TEMPO (por = porphyrinate) as
precursors for RhII(por) (Scheme 1).7,8 We then investigated
the further scope of the nitroxide substrates in ACCA with
Rh(tmp), and Table 1 gives the results of the reactions of
Rh(tmp) with and without Ph3P added with various nitr-
oxides (2 equiv) (eq 2). We only isolated the ACCA pro-
duct of Rh(tmp)R, as the competitive, concurrent aliphatic
carbon-hydrogen bond activation (ACHA) would yield
RhIII(tmp)H, which is quickly converted back to Rh(tmp)
by excess nitroxide.6 The structures of Rh(tmp)R were
further confirmed by independent synthesis through reduc-
tive alkylation of Rh(tmp)I with NaBH4/alkyl halides. For
the alkyl-substituted nitroxides examined, the ACCA oc-
curred smoothly. TMINO was the most reactive, and the
reaction required 4 h at 70 °C to give Rh(tmp)Me in 73%
yield. The very weak and sterically least hindered benzylic-
methyl bond accounts for the highest reactivity. For other
more hindered alkyl nitroxides, a longer reaction time of
60 h and a higher temperature at 110 °C were required.
Lower total product yields were also observed. Likely the
steric hindrance of nitroxides disfavors the reaction and
product yields. The total ACCA yields also decreased with
increasing sterics of nitroxides in the order TMINO >
TEINO > TPINO > TBINO ∼ TPPINO (Table 1, entries
1, 3, 5, 7, and 9). No reaction occurred with TPhINO
(Table 1, entry 11), as no Rh(tmp)Ph product was observed.
Rh(tmp) appeared to form an adduct with TPhINO, but
attempted isolation was unsuccessful. This demonstrates
that the ACCA is selective without any aromatic-aliphatic
CCA. For non-methyl-substituted nitroxides, competitive
ACCA is possible but the regioselectivity of ACCA was not
high. Both the terminal Me-methylene and the alkyl-
methine bonds were cleaved. For the less hindered TEINO
(Table 1, entry 3), the internal ethyl-methine bond clea-
vage was preferred and can be accounted for by the weaker
ethyl-R-nitrogen-substituted benzylic bond. For the more
hindered nitroxide TPINO (Table 1, entry 5), the less
hindered methyl-methylene bond cleavage was favored
and can be understood by steric factors being more
important. For TBINO and TPPINO (Table 1, entries 7
and 9), the regioselectivity in bond cleavage was too low
and both bond strength and steric factors are similarly
important.
Ph3P Effect. As (Ph3P)Rh(tmp) has been shown to form
rapidly from Rh(tmp) and Ph3P and to promote the
ACCA,6,9,10 we then investigated the ACCA by Rh(tmp) with
addition of 1 equiv of Ph3P.10,11 To our delight, (Ph3P)Rh-
(tmp) enhanced the total product yields by about 10% in the
reactions with the less hindered nitroxides TMINO and
TEINO (Table 1, entries 2 and 4). The enhancements are
likely due to the increased electron richness of (Ph3P)-
RhII(tmp), which favors oxidative addition to give RhIII-
(tmp)Me/Et. For bulky nitroxides, the total product yields
decreased, however, by 15-37% (Table 1, entries 6, 8, and 10).
(12) Griffiths, P. G.; Moad, G.; Rizzardo, E.; Solomon, D. H. Aust. J.
Chem. 1983, 36, 397–401.
(13) (a) Braslau, R.; Chaplinski, V. J. Org. Chem. 1998, 63, 9857–
9864. (b) Braslau, R.; Naik, N.; Zipse, H. J. Am. Chem. Soc. 2000, 122,
8421–8434.

2852 Organometallics, Vol. 29, No. 13, 2010
Chan et al.
Figure 2. Coordination-induced pulling effect.
The total product yields also decreased in the same trend in
Rh(tmp) with increasing sterics of nitroxides. The ACCA was
regioselective for the terminal methyl(alkyl)-methylene bond
cleavage with much less internal alkyl-methine bond clea-
vage. Probably, steric factors now play a more important role.
The attempted trapping of Rh(tmp)(Ph3P)(TPhINO) was not
very successful, as the unstable adduct formed was only
characterized by 1H NMR.
To account for the increasing sterics of the transition state
in the reaction with (Ph3P)Rh(tmp), a stereochemical model
was proposed (Figure 2). As a larger second-row transition-
metal ion, rhodium has a large ionic radius. The rhodium
center in Rh(tmp) likely is located above the porphyrin plane
and therefore is sterically more accessible in the convex face
to react with a nitroxide without suffering too much hin-
drance from the o-methyl groups in the mesityl substituents
(Figure 2a). On the other hand, the rhodium ion in (Ph3P)-
Rh(tmp) is pulled back into the porphyrin plane upon
coordination with Ph3P (Figure 2b). Consequently, (Ph3P)-
Rh(tmp) is a more bulky metalloradical. Such a “fifth ligand
pulling down” effect has been observed in the crystal struc-
tures of cobalt porphyrin complexes. The distance from the
Figure 3. First-order fitting of rate with (Ph3)PRh(tmp). Inset:
spectral change.
Table 2. Observed Rate Constants with (Ph3)PRh(tmp)
entrya
105[Rh(tmp)] (M)
104k0 (s-1
)
error of 105k0 (s-1
)
1
2
3
4.191
2.795
1.397
8.36
7.98
8.27
0.976
1.674
1.551
a Conditions: [TEMPO] = 4.140 ꢀ 10-4 M, [PPh3] = 3.735 ꢀ 10-2 M.
Table 3. Saturation Kinetics Data on Ph3P
entrya
103[PPh3] (M)
103k0 (s-1
)
error of 105k0 (s-1
)
cobalt ion to the mean plane of the four nitrogens in the
1
2
3
4
1.012
2.024
3.036
8.096
0.958
1.526
2.695
2.848
0.976
1.660
1.210
1.801
14
˚
˚
porphyrin core increases from 0 A in Co(oep) to 0.123 A
toward the pyridine nitrogen in Co(oep)(DMAP) (DMAP =
4-(dimethylamino)pyridine).15 Therefore, the ACCA is highly
sensitive toward to the sterics in the transition states.
To measure the quantitative electronic promoting effect of
Ph3P on the ACCA, we carried out kinetic studies of the
reaction of (Ph3P)Rh(tmp) with the least hindered and
commercially available nitroxide TEMPO (eq 3). The term-
inal methyl-methine bond cleavage in TEMPO is the sole
ACCA observed and allows a direct comparison with the
reported kinetics of the ACCA of TEMPO with Rh(tmp).6
a Conditions: [TEMPO] = 1.656 ꢀ 10-3 M, [Rh(tmp)] = 5.589 ꢀ
10-5 M.
an unfavorable interaction of the o-methyls in the mesityl
ring with Ph3P.
Kinetics. Kinetic studies were carried out to gain further
mechanistic insight into the ACCA. Kinetic measurements
of the reaction of Rh(tmp) and TEMPO with excess Ph3P
were carried out spectrally at 522 nm at 40-70 °C (Figure 3),
with initial concentrations (1.40-4.19) ꢀ 10-5 M Rh(tmp),
(8.28-49.68) ꢀ 10-4 M TEMPO, and 3.74 ꢀ 10-2 M Ph3P
and with TEMPO always at least 10-fold excess with regard
to Rh(tmp). The measurements were carried out for at least
4 half-lives. The kinetics conformed to first-order reactions
in both Rh(tmp) and TEMPO (observed rate: k[Rh(tmp)]-
[TEMPO]). A pseudo-first-order fitting plot of the absor-
bance change was obtained, as shown in Figure 3, and
yielded the pseudo-first-order observed rate constant k0 with
its value remaining unchanged by a tripling of [Rh(tmp)]
(Table 2). Therefore, [Rh(tmp)] is in first order. The rate
exhibited saturation kinetics with Ph3P (Table 3, Figure 4) and
suggested a rapid pre-equilibrium of Ph3P with Rh(tmp) to give
(Ph3P)Rh(tmp). The pseudo-first-order rate constant k (k0 = k
[TEMPO]) was derived from the linear slope of the plot of k0
against [TEMPO] (Table 4, Figure 5) and yielded k (70 °C) =
0.24 ( 0.01 L mol -1 s-1. The rates were further measured from
40 to 70 °C, and the data are given in Table 5. An Eyring plot
Stoichiometry. Indeed, Rh(tmp) in the presence of Ph3P
(1 equiv) reacted cleanly and rapidly with TEMPO at 70 °C in
4 h to give Rh(tmp)Me in 75% isolated yield, similar to the
76% yield obtained in the absence of Ph3P. TEMPO re-
mained stable in the presence of 1 equiv of Ph3P at 120 °C
over 3 days. Oxidation of Ph3P therefore did not occur. No
(Ph3P)Rh(tmp)Me product was observed even in the crude
reaction mixture by 1H NMR analysis, which is likely due to
(14) Scheidt, W. R.; Turoska-Tyrk, I. Inorg. Chem. 1994, 33, 1314–
1318.
(15) Summer, J. S.; Petersen, J. L.; Stolzenberg, A. M. J. Am. Chem.
Soc. 1994, 116, 7189–7195.

Article
Organometallics, Vol. 29, No. 13, 2010 2853
Table 5. Temperature Effect on Rate Constants
T
T
1/T ꢀ
104k0
(s-1
error
k (L
ln
(k/T)
entrya (°C) (K) 103 (K-1
)
)
of 106k0 (s-1) mol-1 s-1
)
1
2
3
4
5
6
40.0 313
50.0 323
55.0 328
60.0 333
65.0 338
70.0 343
3.19
3.10
3.05
3.00
2.96
2.92
4.89
11.5
13.5
20.7
32.7
49.1
1.702
3.895
0.538
1.157
3.719
3.633
0.197
0.464
0.543
0.832
1.315
1.975
-7.371
-6.547
-6.404
-5.992
-5.549
-5.157
a Conditions: [Rh(tmp)] = 5.59 ꢀ 10-5 M, [TEMPO] = 2.48 ꢀ 10-3
M, [PPh3] = 3.74 ꢀ 10-3 M.
Figure 4. Saturation kinetics on Ph3P.
Table 4. TEMPO Effect on Rate
entrya
104[TEMPO] (M)
103k0 (s-1
)
error of 105k0 (s-1
)
1
2
3
4
5
8.280
16.56
24.84
33.12
49.68
1.590
2.776
4.907
8.358
12.740
3.819
2.127
3.633
1.823
5.118
a Conditions: [Rh(tmp)] = 5.589 ꢀ 10-5 M, [PPh3] = 3.735 ꢀ 10-3 M.
Figure 6. Eyring plot of the reaction of (Ph3P)Rh(tmp) with
TEMPO.
involvement of the seven-coordinate Rh complex 90, but this
mechanism is kinetically indistinguishable and does not
affect the evaluation of rate enhancement ratio. Parallel and
rate-determining aliphatic carbon-hydrogen bond activation
and ACCA then occur to yield Rh(tmp)H and Rh(tmp)Me,
respectively, after rapid Ph3P dissociation. Then Rh(tmp)H is
rapidly recycled back to Rh(tmp) by excess TEMPO for
further bond activation reactions. In the reaction with Ph3P
added, both Rh(tmp) and (Ph3P)Rh(tmp) exist in solution.
Therefore, the rate constant is composed of two parts: the
Ph3P-free and Ph3P-bound Rh(tmp) pathways. Derivation of
the rate law yields eq 4, where [Rh]T =[Rh(tmp)]total, L =
Ph3P, and T = TEMPO (see the Supporting Information).
The enhancement ratio of (PPh3)Rh(tmp) to Rh(tmp) for the
ACCA of TEMPO at 70 °C is evaluated to be 11.
Figure 5. First-order plot on TEMPO.
(Figure 6) gave the corresponding observed free energy, enthal-
py, and entropy of activations: ΔGobsq = 21.0 ( 2.3 kcal mol-1
rate ¼ fðk3 þ k4ÞK1K2 þ
½RhꢁT
,
ΔHobsq= 17.1 ( 1.2 kcal mol-1, and ΔSobsq = -11.5 ( 3.4
ðk8 þ k9ÞK5K6K7½Lꢁg
½Tꢁ
ð4Þ
cal K-1 mol-1
.
1 þ K5½Lꢁ
The quantitative evaluation of the Ph3P-enhanced rate is
based on a simplified mechanism extracted from the previous
report.6 Only the radical processes for the Ph3P-free and
Ph3P-enhanced pathways are considered, as shown in
Scheme 2, without invoking electron transfer pathways.
Rh(tmp) (1) initially forms Rh(tmp)(Ph3P) (8), which then
binds with TEMPO (6) to give the adduct trans-η1-[Rh(tmp)-
(T)(Ph3P)] (9), where the oxygen atom in TEMPO is end-
bound to the Rh center. It then isomerizes to η2-[Rh(tmp)-
(T)(Ph3P)] (90), in which the oxygen atom is side-bound to the
Rh center. Alternatively, 8 can form 90 directly without the
In summary, we have discovered the ACCA of a variety of
nitroxides with Rh(tmp) to give Rh(tmp) alkyls. For the
unhindered nitroxides TMINO and TEINO, addition of
Ph3P gave (Ph3P)Rh(tmp) and enhanced the reaction rate
and total product yields due to increased electron richness.
For hindered nitroxides, the more bulky Rh(tmp)(Ph3P)
gave lower total product yields and highly disfavored ACCA
at the more hindered carbon-carbon bonds. In the reaction
with TEMPO, the rate enhancement of (Ph3P)Rh(tmp) over
Rh(tmp) was estimated to be about 11 at 70 °C.

2854 Organometallics, Vol. 29, No. 13, 2010
Scheme 2. Proposed Mechanism for Aliphatic Carbon-Carbon Bond Activation
Chan et al.
Experimental Section
porated off. The residue was purified by chromatography on
basic aluminum oxide with hexane as eluent. A pale brown solid
was obtained (1.23 g, 3.26 mmol, 33%). Rf = 0.53 (hexane). Mp:
78.9-79.3 °C. 1H NMR (CDCl3, 300 MHz): δ 0.67 (t, 12 H, J =
7.2 Hz), 0.86-0.99 (m, 4 H), 1.26-1.42 (m, 8 H), 1.66-1.76 (m,
4 H), 3.93 (s, 2 H), 6.93-6.97 (m, 2 H), 7.06-7.26 (m, 5 H),
7.32-7.34 (m, 2 H). 13C NMR (CDCl3, 75 MHz): δ 14.90, 18.27,
41.19, 46.94, 70.88, 123.43, 125.65, 126.59, 128.01, 129.25, 142.59,
145.26. HRMS (EI): calcd for C27H39Nþ m/z 376.2999, found
m/z 376.3007.
Preparation of 1,1,3,3-Tetrapropylisoindoline (4c). 2-Benzyl-
1,1,3,3-tetramethylisoindoline (3c; 550 mg, 2.1 mmol) and 10%
Pd/C (0.05 g, 0.05 mmol) were suspended in glacial acetic acid
(10 mL) with a magnetic stirrer bar. The mixture was placed in a
high-pressure reactor. The reactor was filled (60 psi) and
released (20 psi) with hydrogen for three cycles, and hydrogen
was finally filled to 60 psi. The whole mixture was stirred at
room temperature for 3 days. The crude product was then
neutralized with NaOH to pH 9 and extracted with ether (3 ꢀ
60 mL). The organic extract was dried over MgSO4, filtered, and
evaporated to dryness. The crude product was purified by
chromatography on basic aluminum oxide with a solvent mix-
ture of ethyl acetate and hexane (1:9) as eluent. A pale yellow oil
was obtained (0.74 g, 2.58 mmol, 83%). Rf = 0.73 (1:9 ethyl
acetate/hexane). 1H NMR (CDCl3, 300 MHz): δ 0.85-0.90
(t, 12 H, J = 7.4 Hz), 1.14-1.29 (m, 4 H), 1.34-1.46 (m, 4 H),
1.52-1.74 (m, 8 H), 7.03-7.08 (m, 2 H), 7.16-7.20 (m, 2 H). 13C
NMR (CDCl3, 75 MHz): δ 14.92, 17.94, 44.74, 68.15, 122.55,
126.60, 147.89. HRMS (EI): calcd for C20H33Nþ m/z 286.2529,
found m/z 286.2539.
Preparation of 1,1,3,3-Tetrapropylisoindolin-2-oxyl [TPINO]
(5c). To a solution of 1,1,3,3-tetrapropylisoindoline (4c) (0.10 g,
0.36 mmol) in CH2Cl2 (5 mL) was added m-CPBA (85%, 0.11 g,
0.54 mmol). The other procedure was the same as the prepara-
tion of 1,1,3,3-tetramethylisoindolin-2-oxyl (10a). A yellow oil
was obtained (0.11 g, 0.35 mmol, 96%). Rf = 0.6 (1:9 ethyl
acetate/hexane). HRMS (FAB): calcd for (C20H32NOþ) m/z
302.2478, found m/z 302.2482. Anal. Calcd for C20H32NO: C,
79.72; H, 10.66; N, 4.63. Found: C, 80.13; H, 10.94; N, 4.58.
Preparation of 2-Benzyl-1,1,3,3-tetrabutylisoindoline (3d). BuI
(8.7 mL, 75.8 mmol) was used. The procedure was the same as
the preparation of 2-benzyl-1,1,3,3-tetramethylisoindoline (3c).
A pale yellow oil of 2-benzyl-1,1,3,3- tetrabutylisoindoline was
obtained (1.45 g, 3.4 mmol, 27%). Rf = 0.67 (hexane). 1H NMR
(CDCl3, 300 MHz): δ 0.78 (t, 12 H, J = 7.1 Hz), 0.85-0.91 (m, 4
H), 0.95-1.17 (m, 8 H), 1.26-1.52 (m, 8 H), 1.79 (m, 4 H), 4.00
(s, 2 H), 7.01-7.06 (m, 2 H), 7.15-7.30 (m, 5 H), 7.40-7.42 (m, 2
H). 13C NMR (CDCl3, 75 MHz): δ 14.95, 24.27, 27.99, 39.35,
47.59, 71.43, 124.08, 126.31, 127.24, 128.59, 129.99, 143.23,
145.92. HRMS (EI): calcd for C31H47Nþ m/z 432.3625, found
m/z 432.3632.
Unless otherwise noted, all reagents were purchased from
commercial suppliers and directly used without further purifica-
tion. Hexane was distilled from anhydrous calcium chloride.
Benzene and toluene were distilled from sodium. Thin-layer
chromatography was performed on precoated silica gel 60 F254
plates. Silica gel (Merck, 70-230 mesh) was used for column
chromatography.
1H NMR and 13C NMR spectra were recorded on a Bruker
DPX-300 instrument at 300 and 75 MHz, respectively, or on a
Bruker AvanceIII 400 instrument at 400 MHz. Chemical
shifts were referenced to the residual solvent protons in C6D6
(δ 7.15 ppm), CDCl3 (δ 7.26 ppm), or tetramethylsilane (δ =
0.00 ppm) in 1H NMR spectra and CDCl3 (δ 77.16 ppm) in 13
C
NMR spectra as the internal standards. Chemical shifts (δ) are
reported as parts per million (ppm) in the δ scale downfield from
TMS. Coupling constants (J) are reported in hertz (Hz). High-
resolution mass spectra (HRMS) were recorded on a Thermo-
Finnigan MAT 95 XL mass spectrometer. Fast atom bombard-
ment spectra were performed with 3-nitrobenzyl alcohol (NBA)
as the matrix. All samples for combustion analyses were recrys-
tallized from CH2Cl2/MeOH and vacuum-dried at room tem-
perature for at least 2 days before submission.
Rh(tmp),6,16 TMINO,7 and TEINO7 were synthesized accord-
ing tothe literature procedure. Benzene was distilled from sodium
under nitrogen. 2,2,6,6-Tetramethylpiperidin-1-oxyl (TEMPO)
was obtained from Aldrich and purified by vacuum sublimation.
UV-visible spectral studies were performed on a Hitachi
U-3300 spectrophotometer equipped with a Neslab RTE-110
temperature circulator for temperature control with a mixture
of ethylene glycol and water (1/1 v/v) used as the circulating
liquid. The temperature was measured by a Fluka thermometer
connected to a K-type thermal couple wire placed in an adjacent
blank UV cell filled with benzene. Spectral and rate data were
analyzed with OriginPro 7.5 of OriginLab Corporation.
Preparation of 2-Benzyl-1,1,3,3-tetrapropylisoindoline (3c).
Magnesium powder (1.53 g, 63 mmol) was added to a three-
neck flask. Then it was heated under vacuum for 1 h. After the
system was cooled to room temperature, N2 was passed through
the reactor from the bottom. Then a condenser was connected
and a bubbler was added on the top of the condenser. After
0.5 h, N2 was removed to the top of the condenser. Dried Et2O
(36 mL) was added to the flask with a syringe. nPrBr (5.47 mL,
60 mmol) was slowly dropped into the flask with a syringe. Then
the mixture was refluxed for 3 h. The gray mixture was con-
centrated under high vacuum until the suspension did not boil at
80 °C. A solution of N-benzylphthalimide (2) (2.37 g, 10 mmol)
in dry toluene (16 mL) was added dropwise with stirring through
a cannula under N2. Then the reaction mixture was gently
refluxed for 18 h. The brown mixture was diluted with hexane
(20 mL), filtered through Celite, and washed with hexane. The
yellow organic filtrate turned purple, and the solvent was eva-
Preparation of 1,1,3,3-Tetrabutylisoindoline (4d). 2-Benzyl-
1,1,3,3-tetrabutylisoindoline (3d; 1.32 g, 3.04 mmol) was used.
The procedure was the same as the preparation of 4c. A pale
yellow oil was obtained (960 mg, 2.81 mmol, 92%). Rf = 0.77
(9:1 hexane/ethyl acetate). 1H NMR (CDCl3, 300 MHz): δ 0.84
(16) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991,
112, 5305–5311.

Article
Organometallics, Vol. 29, No. 13, 2010 2855
(t, 12 H, J = 7.1 Hz), 1.13-1.42 (m, 16 H), 1.55-1.75 (m, 8 H),
7.05-7.09 (m, 2 H); 7.17-7.21 (m, 2 H). 13C NMR (CDCl3, 75
MHz): δ 14.82, 24.10, 27.49, 42.53, 68.62, 123.10, 127.19,
148.64. HRMS (EI): calcd for C24H41Nþ m/z 342.3155, found
m/z 342.3150. Anal. Calcd for C24H41N: C, 83.90; H, 12.03; N,
4.07. Found: C, 84.00; H, 12.23; N, 4.02.
Preparationof1,1,3,3-Tetrabutylisoindolin-2-oxyl(5d, TBINO).
m-CPBA (779 mg, 3.84 mmol) was added to a solution of 1,1,3,3-
tetrabutylisoindoline (4d; 879 mg, 2.56 mmol) in CH2Cl2 (18 mL).
The rest of the procedure was the same as the preparation of
1,1,3,3-tetramethylisoindolin-2-oxyl (4c). A yellow oil with high
viscosity (906 mg, 2.53 mmol, 99%) was obtained after rotary
evaporation. Rf = 0.81 (9:1 hexane/ethyl acetate). HRMS (FAB):
calcd for C24H40NOþ m/z 358.3104, found m/z 358.3119. Anal.
Calcd for C24H40NO: C, 80.39; H, 11.24; N, 3.91. Found: C,
80.41; H, 11.38; N, 3.89.
or 110 °C under N2 in the absence of light. After the mixture was
cooled to room temperature, the solvent was removed under
vacuum. The residue was purified by chromatography on silica
gel with a solvent mixture of hexane and CH2Cl2 ranging from
10:1 to 7:1 as eluent to give orange solids of Rh(tmp)R with 1H
NMR spectra identical with those of an authentic sample.
ACCA between Rh(tmp) and TMINO (4a). Degassed TMINO
(0.055 mmol, 10.5 mg) was added to a benzene solution of
Rh(tmp) (0.0088 mmol, 4.0 mL) under N2 with stirring. The
mixture was heated under N2 in the absence of light for 4 h. Then
the reaction mixture was cooled to room temperature, and the
solvent was removed under vacuum. The residue was purified by
chromatography on silica gel with a solvent mixture of hexane
and CH2Cl2 ranging from 10:1 to 7:1 as eluent to give Rh-
(tmp)Me (0.0064 mmol, 5.8 mg) in 73% yield and was identified
by 1H NMR spectroscopy.
Preparation of 2-Benzyl-1,1,3,3-tetrakis(3-phenylpropyl)isoindo-
line (3e). Ph(CH2)3Br (11.7 mL, 75.8 mmol) was used. The
procedure was the same as the preparation of 2-benzyl-1,1,3,3-
tetramethylisoindoline (3c). A pale yellow oil of 2-benzyl-1,1,3,3-
tetrakis(3-phenylpropyl)isoindoline with high viscosity was ob-
tained (1.42 g, 2.1 mmol, 16%). Rf = 0.23 (1:9 ethyl acetate/
ACCA between Rh(tmp) and TEINO (4b). Degassed TEINO
(0.055 mmol, 13.5 mg) was used as substrate. The rest of the
procedure was same as the reaction between Rh(tmp) and
TMINO. Yields of 9% of Rh(tmp)Me (0.0008 mmol, 0.7 mg)
and 44% of Rh(tmp)Et5 (0.0039 mmol, 3.5 mg) were isolated
1
and identified by H NMR spectroscopy. Rh(tmp)Et:8 Rf =
0.44 (1:5 CH2Cl2/hexane); 1H NMR (C6D6) δ -4.31 (dq, 2 H,
2JRhH = 3.0 Hz, 3JHH = 7.5 Hz), -3.83 (dt, 3 H, 3JRhH = 1.5
Hz, 3JHH = 7.5 Hz), 1.88 (s, 12 H), 2.12 (s, 12 H), 2.44 (s, 12 H),
6.93 (s, 4 H), 7.19 (s, 4 H), 8.75 (s, 8 H).
1
hexane). H NMR (CDCl3, 300 MHz): δ 0.83-0.91 (m, 2 H),
1.25-1.36 (m, 4 H), 1.43-1.53 (m, 4 H), 1.65-1.71 (m, 4 H), 1.81
(m, 4 H), 2.34-2.42 (m, 4H), 2.56 (t, 2H, J = 7.8 Hz), 3.90 (s, 2 H),
6.91-6.96 (m, 2 H), 7.03-7.06 (m, 6 H), 7.08-7.31 (m, 21 H). 13
C
NMR (CDCl3, 75 MHz): δ 27.32, 37.19, 38.99, 47.39, 71.24,
123.99, 126.28, 126.63, 127.33, 128.73, 128.93, 129.06, 129.95,
142.68, 143.35, 145.23. HRMS (EI): calcd for C51H55Nþ m/z
680.4251, found m/z 680.4214. Anal. Calcd for C51H55N: C,
89.82; H, 8.13; N, 2.05. Found: C, 89.93; H, 8.11; N, 2.04.
Preparation of 1,1,3,3-Tetrakis(3-phenylpropyl)isoindolin-2-
oxyl (5e, TPPINO). m-CPBA (549 mg, 2.70 mmol) was added
to a solution of 2-benzyl-1,1,3,3-tetrakis(3-phenylpropyl)iso-
indoline (3e; 1.23 g, 1.80 mmol) in CH2Cl2 (20 mL), The solution
changed to yellow after 4 days, and a suspension formed. The
rest of the procedure was the same as the preparation of 4c. A
yellow oil with high viscosity (595 mg, 0.981 mmol, 54%) was
obtained after rotary evaporation. Rf = 0.47 (9:1 hexane/ethyl
acetate). HRMS (FAB): calcd for C44H48NOþ m/z 606.3730,
found m/z 606.3726. Anal. Calcd for C44H48NO: C, 87.08; H,
7.97; N, 2.31. Found: C, 86.70; H, 7.98; N, 2.24.
ACCA between Rh(tmp) and TPINO (4c). Degassed TPINO
(0.055 mmol, 16.6 mg) was used as substrate. The rest of the
procedure was same as the reaction between Rh(tmp) and
TMINO. Yields of 14% of Rh(tmp)Me (0.0012 mmol, 1.1 mg)
and 41% of Rh(tmp)nPr (0.0036 mmol, 3.3 mg) were isolated
and identified by 1H NMR spectroscopy. Rh(tmp)nPr:8 1H
2
NMR (C6D6, 300 MHz) δ -4.44 (dt, 2 H, JRhH = 3.0 Hz,
3JHH = 7.5 Hz), -3.88 (sextet, 2 H, J = 7.5 Hz), -1.42 (t, 3 H,
J = 7.4 Hz), 1.90 (s, 12 H), 2.18 (s, 12 H), 2.44 (s, 12 H), 7.11 (s,
4H), 7.20 (s, 4 H), 8.76 (s, 12 H).
ACCA between Rh(tmp) and TBINO (4d). Degassed TBINO
(0.055 mmol, 19.7 mg) was used as substrate. The rest of the
procedure was same as the reaction between Rh(tmp) and
TMINO. Yields of 10% of Rh(tmp)Me (0.0009 mmol, 0.8 mg)
and 22% of Rh(tmp)nBu (0.0019 mmol, 1.8 mg) were isolated
and identified by 1H NMR spectroscopy.
Preparation of 2-Benzyl-1,1,3,3-tetraphenylisoindoline (3f).
PhBr (8.0 mL, 75.8 mmol) was used. The procedure was the
same as the preparation of 2-benzyl-1,1,3,3-tetramethylisoindo-
line (3c). A white solid of 2-benzyl-1,1,3,3-tetraphenylisoindo-
Preparation of (5,10,15,20-Tetramesitylporphyrinato)butylrho-
dium(III) (Rh(tmp)nBu).7 A red suspension of Rh(tmp)I (0.035 g,
0.034 mmol) in EtOH (20 mL) and a solution of NaBH4 (0.005 g,
0.123 mmol) in aqueous NaOH (1.4 mL, 0.5 M) were purged with
N2 separately for about 15 min. The solution of NaBH4 was
added slowly to the suspension of Rh(tmp)I via a cannula over
a period of 30 min. The mixture was heated to 55 °C for 3 h.
line was obtained (1.25 g, 2.43 mmol, 19%). Mp: 209.9-210.2 °C.
1
Rf = 0.15 (1:9 ethyl acetate/hexane). H NMR (CDCl , 300
3
MHz): δ 4.13 (s, 2 H), 6.25 (d, 2 H, J = 6.9 Hz), 6.83-6.85 (m,
3 H), 7.05-7.26 (m, 24 H). 13C NMR (CDCl3, 75 MHz): δ 49.27,
82.44, 125.48, 126.79, 127.38, 128.00, 129.94, 130.79, 141.52,
145.90, 146.38. HRMS (EI): calcd for C39H31Nþ m/z 513.2451,
found m/z 513.2431.
n
Then the brown mixture was cooled to 0 °C under N2. BuBr
(1.12 mmol) was added with a syringe. The mixture was warmed
to room temperature and stirred for 3 h. The orange mixture was
worked up by addition with CH2Cl2/H2O. Then it was extracted
with CH2Cl2 (3 ꢀ 50 mL), washed with H2O, dried over MgSO4,
filtered, and evaporated off to dryness. The mixture was purified
by silica gel chromatography with a solvent mixture of CH2Cl2
and hexane (1:5) as eluent. Then the solid obtained was recrys-
tallized from CH2Cl2/hexane. A red solid was obtained (45 mg,
Preparation of 1,1,3,3-Tetraphenylisoindolin-2-oxyl [TPhINO]
(5f). m-CPBA (177 mg, 1.03 mmol) was added to a solution of
2-benzyl-1,1,3,3-tetraphenylisoindoline (3f; 117 mg, 0.23 mmol)
in CH2Cl2 (15 mL). The solution turned yellow after 4 days with
some suspension formed. The rest of the procedure was the same
as the preparation of 1,1,3,3-tetramethylisoindoline-2-oxyl (4c).
A yellow solid (71.1 mg, 0.162 mmol, 71%) was obtained after
rotary evaporation. The crude product was recrystallized from
ethyl acetate/hexane. A white crystal was obtained. Mp: 248.3-
248.5 °C. Rf = 0.41 (10:1 hexane/ethyl acetate). HRMS (FAB):
calcd for C32H24NOþ m/z 439.1931, found m/z 439.1924. Anal.
Calcd for C32H24NO: C, 87.64; H, 5.52; N, 3.19. Found: C, 87.80;
H, 5.53; N, 3.23.
1
0.049 mmol, 49%). Rf = 0.47 (1:5 CH2Cl2/hexane). H NMR
(C6D6, 300 MHz): δ -4.45 (dt, 2 H, 2JRhH = 3.0 Hz, 3JHH = 7.8
Hz), -3.94 (quintet, 2 H, J = 7.2 Hz), -1.14 (sextet, 2 H, J = 7.5
Hz), -0.67 (t, 3H, J = 7.2 Hz), 1.91 (s, 12 H), 2.18 (s, 12 H), 2.44
(s, 12 H), 7.12 (s, 4 H), 7.20 (s, 4 H), 8.75 (s, 8 H). HRMS (FAB):
calcd for C60H61N4Rhþ m/z 941.4024, found 941.3954. Anal.
Calcd for C60H61N4Rh: C, 76.58; H, 6.53; N, 5.95. Found: C,
76.02; H, 6.64; N, 5.96.
General Procedure for ACCA of Rh(tmp) with Nitroxides.5 To
a Teflon screw-head stoppered flask, a benzene solution of
Rh(tmp) (0.0088 mmol, 4.0 mL) was added with a nitroxide
(0.055 mmol). Then the reaction mixture was then heated to 70
ACCA Study between Rh(tmp) and TPPINO (4e). Degassed
TPPINO (0.055 mmol, 33.4 mg) was used as substrate. The rest
of the procedure was same as the reaction between Rh(tmp)
and TMINO. Yields of 12% of Rh(tmp)CH2Ph (0.0011 mmol,

2856 Organometallics, Vol. 29, No. 13, 2010
Chan et al.
1
1.0 mg) and 19% of Rh(tmp)(CH2)3Ph (0.0017 mmol, 1.7 mg)
were isolated and identified by 1H NMR spectroscopy.
identified by H NMR spectroscopy. A trace amount of Rh-
(tmp)nBu was observed by crude 1H NMR spectroscopy.
ACCA between Rh(tmp) and TPPINO (10e) with 1 Equiv of
PPh3. Degassed TPPINO (0.055 mmol, 33.4 mg) was used as
substrate. The rest of the procedure was same as the reaction
between Rh(tmp) and TMINO with 1 equiv of PPh3. Yields of
12% of Rh(tmp)CH2Ph (0.0011 mmol, 1.0 mg) and 7% of
Rh(tmp)(CH2)3Ph (0.0006 mmol, 0.6 mg) were isolated and
identified by 1H NMR spectroscopy.
Preparation of (5,10,15,20-Tetramesitylporphyrinato)benzylrho-
dium(III) (Rh(tmp)CH2Ph). The procedure was the same as the
preparation of Rh(tmp)nBu. A red solid was obtained (81 mg,
1
0.083 mmol, 84%). Rf = 0.34 (1:5 CH2Cl2/hexane). H NMR
(C6D6, 300 MHz): δ -3.15 (d, 2 H, 2JRhH = 3.6 Hz), 1.93 (s, 12
H), 2.06 (s, 12 H), 2.45 (s, 12 H), 3.66 (d, 2 H, J = 7.5 Hz), 5.76 (t, 2
H, J = 7.5 Hz), 6.22 (t, 1H, J = 7.5 Hz), 6.90 (s, 4 H), 7.42 (s, 4H),
8.72 (s, 8 H). 13C NMR (C6D6, 75 MHz): 14.29, 21.93, 22.45,
22.80, 120.00, 123.56, 125.44, 126.66, 127.80, 127.86, 130.82,
137.53, 138.58, 138.73, 139.57, 142.76. HRMS (FAB): calcd for
C63H59N4Rhþ m/z 974.3789, found 974.3839.
Attempted Trapping CCA of Rh(tmp) by TPhINO. PPh3
solution (10 μL, 1.10 M in benzene) was added to a benzene
solution of Rh(tmp) (0.0088 mmol in 4.0 mL of benzene) under
N2 with stirring. Then TPhINO (0.055 mmol) was added to the
mixture under N2. The mixture was stirred at room temperature
under N2 in the absence of light for 2 days. The crude product was
purifiedbychromatography onsilica gelwitha solvent mixture of
hexane and CH2Cl2 ranging from10:1 to7:1as eluenttogive a red
solid of suspected PPh3 Rh(tmp) TPhINO (9.90 mg, 0.006
Preparation of (5,10,15,20-Tetramesitylporphyrinato)(phenyl-
propyl)rhodium(III) (Rh(tmp)(CH2)3Ph). The procedure was the
same as the preparation of Rh(tmp)nBu. A red solid was obtained
(78 mg, 0.078 mmol, 79%). Rf = 0.34 (1:5 CH2Cl2/hexane). 1H
NMR (C6D6, 300 MHz): δ -4.46 (dt, 2 H, 2JRhH = 3.0 Hz, 3 JHH
= 7.2 Hz), -3.64 (quintet, 2 H, J = 7.8 Hz), 0.08 (t, 2 H, J = 7.5
Hz), 1.85 (s, 12 H), 2.16 (s, 12 H), 2.45 (s, 12 H), 5.57 (d, 2 H, J =
7.0 Hz), 6.55-6.67 (m, 3 H), 7.13 (s, 4 H), 7.19 (s, 4 H), 8.72 (s, 8
H). HRMS(FAB): calcdfor C65H63N4Rhþ m/z 1002.4102, found
1002.4122. Anal. Calcd for C65H63N4Rh: C, 77.83; H, 6.33; N,
5.58. Found: C, 77.56; H, 6.30; N, 5.69.
3
3
1
mmol, 71%). Rf = 0.11 (3:97 ethyl acetate/hexane). H NMR
(C6D6, 300 MHz): δ 1.29 (s, 12 H), 2.20 (s, 12 H), 2.40 (s, 12 H),
4.35-4.41 (m, 6 H), 6.16-6.22 (m, 6 H), 6.38-6.39 (m, 6 H), 6.89
(s, 4 H), 6.96-7.07 (m, 12 H), 7.25 (s, 8 H), 7.70-7.76 (m, 8 H),
8.76 (s, 8 H). HRMS (FAB or ESI): only a peak of [Rh(tmp)]þ
(m/z 884) was observed. The product decomposed to a green
compound at room temperature under N2 after 3 days.
ACCA between Rh(tmp) and TPhINO (4f). Degassed TPhI-
NO (0.055 mmol, 24.2 mg) was used as substrate. The rest of the
procedure was same as the reaction between Rh(tmp) and
TMINO. After heating to 110 °C for 48 h, no CCA product
was found by TLC. Then the solvent was removed under
ACCA of Rh(tmp) and TEMPO with PPh3 Added. PPh3 (2.4
mg, 0.0091 mmol) was added to a solution of [RhII(tmp)] (0.009
mmol) under N2, and the mixture was stirred for 30 min at room
temperature. Then TEMPO (6; 2.8 mg, 0.018 mmol) was added
and the reaction mixture was stirred at 70 °C for 4 h in the
absence of light under N2. The crude product was purified by
chromatography on silica gel using a solvent mixture of hexane
and CH2Cl2 (3:1). Red solids of [RhIII(tmp)CH3] (11; 6.0 mg,
0.0068 mmol, 75%) were obtained. Rf = 0.76 (1:1 hexane/
CH2Cl2). The 1H NMR spectrum was identical with that of an
authentic sample.
Kinetic Studies on Reaction of Rh(tmp) (1) and TEMPO (6)
with PPh3 Added. Benzene was freshly distilled over sodium,
degassed by three freeze-pump-thaw cycles, and refilled with N2.
Sublimed TEMPO (6; 0.0970 g) and PPh3 (0.0326 g) were added
independently to two 5.00 mL volumetric flasks, respectively, and
were brought up to the volume with freshly distilled benzene. The
solutions were transferred to two Teflon screw-head stoppered
flasks separately. They were degassed three times by freeze-
pump-thaw cycles and refilled with N2. Stock benzene solu-
tions of TEMPO (0.1242 M) and PPh3 (2.486 ꢀ 10-2 M) were
prepared.
Kinetic Studies. Kinetic runs were initially carried out at 70.0 (
0.2 °C. UV-visible time scans were carried out with various initial
concentrations of [RhII(tmp)] or TEMPO separately on a UV-vis
spectrometer. The stock benzene solution of TEMPO (0.1242 M)
was transferred carefully to the side arm of a Teflon-stoppered
Schlenk UV cell with a gastight syringe under N2. Stock benzene
solutions of Rh(tmp) (1.819 ꢀ 10-3 M) and PPh3 (2.486 ꢀ 10-2 M)
together with dried benzene were then transferred under N2 to the
bottom of the same cell via gastight syringes. The solutions were
stirred for 30 min at 25.0 ( 0.2 °C inside the sample compartment
under N2. All stock solutions in the UV cell were mixed quickly just
before the absorbance was measured. The reaction was monitored
at 522 nm for at least 4-5 half-lives. The temperatures of kinetic
runs were also carried out from 40.0 to 70.0 °C.
1
vacuum. Through checking by crude H NMR spectroscopy,
no CCA product was found.
General Procedure for ACCA of Rh(tmp) and Nitroxides with
Ph3P. The benzene solution of Rh(tmp) (0.0088 mmol, 4.0 mL)
was prepared as above. Then a stock benzene solution of PPh3
(10 μL, 1.10 M) was added to the benzene solution of Rh(tmp)
under N2 with stirring. A nitroxide (0.055 mmol) was added to
the mixture under N2. The mixture was heated to 70-110 °C
under N2 in the absence of light. Other steps were as similar to
those for the ACCA between Rh(tmp) and nitroxides.
ACCA between Rh(tmp) and TMINO (10a) with 1 Equiv of
PPh3. A benzene solution of PPh3 (10 μL, 1.1 M) was added to
the benzene solution of Rh(tmp) (0.0088 mmol, 4.0 mL) under
N2 with stirring. Then degassed TMINO (0.055 mmol, 10.5 mg)
was added to the mixture under N2. The mixture was heated
under N2 in the absence of light. The reaction mixture was
monitored by TLC. After reaction, the mixture was cooled to
room temperature. Then solvent was removed under vacuum.
The residue was purified by chromatography on silica gel with a
solvent mixture of hexane and CH2Cl2 ranging from 10:1 to 7:1
as eluent to give Rh(tmp)Me (0.0073 mmol, 6.6 mg) in 83%
yield, which was identified by 1H NMR spectroscopy.
ACCA between Rh(tmp) and TEINO (10b) with 1 Equiv of
PPh3. Degassed TEINO (0.055 mmol, 13.5 mg) was used as
substrate. The rest of the procedure was same as the reaction
between Rh(tmp) and TMINO with 1 equiv of PPh3. Yields of
27% of Rh(tmp)Me (0.0024 mmol, 2.1 mg) and 38% of Rh-
(tmp)Et (0.0033 mmol, 3.1 mg) were isolated and identified by
1H NMR spectroscopy.
ACCA between Rh(tmp) and TPINO (10c) with 1 Equiv of
PPh3. Degassed TPINO (0.055 mmol, 16.6 mg) was used as
substrate. The rest of the procedure was same as the reaction
between Rh(tmp) and TMINO with 1 equiv of PPh3. A yield of
18% of Rh(tmp)Me (0.0016 mmol, 1.4 mg) was isolated and
Acknowledgment. We thank the Research Grants Council
of Hong Kong (CUHK 400307) for financial support.
1
identified by H NMR spectroscopy. A trace amount of Rh-
(tmp)nPr was observed by crude 1H NMR spectroscopy.
ACCA between Rh(tmp) and TBINO (10d) with 1 Equiv of
PPh3. Degassed TBINO (0.055 mmol, 19.7 mg) was used as
substrate. The rest of the procedure was same as the reaction
between Rh(tmp) and TMINO with 1 equiv of PPh3. A yield of
17% of Rh(tmp)Me (0.0015 mmol, 1.3 mg) was isolated and
Supporting Information Available: Text giving kinetic ana-
lyses for the rate-enhanced reaction of TEMPO with (Ph3P)-
Rh(tmp) and figures giving spectra for selected compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.