Dirhodium(II) complexes of 2-(sulfonylimino)pyrrolidine: Synthesis and application in catalytic benzylic oxidation

Article abstract of DOI:10.1002/ejoc.201200292

A new class of dirhodium(II) tetraamidinates derived from 2-(sulfonylimino)pyrrolidines has been prepared through ligand substitution by using dirhodium(II) acetate, in which (3,1) geometric isomers are formed predominantly. Among these complexes, (3,1)-Rh₂(Msip)₄, exhibits good catalytic performance in benzylic oxidation. In the presence of (3,1)-Rh₂(Msip)₄ a variety of benzylic derivatives, including strongly electron-deficient arylalkanes such as 1-ethyl-4- nitrobenzene, are readily oxidized in water by the inexpensive oxidant T-HYDRO (70 wt.-% aqueous tert-butyl hydroperoxide). Copyright

Full text of DOI:10.1002/ejoc.201200292

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SHORT COMMUNICATION
DOI: 10.1002/ejoc.201200292
Dirhodium(II) Complexes of 2-(Sulfonylimino)pyrrolidine: Synthesis and
Application in Catalytic Benzylic Oxidation
Abudureheman Wusiman,[a,b,c] Xiarepati Tusun,[a,b] and Chong-Dao Lu*[a]
Keywords: Rhodium / N ligands / Bridging ligands / Homogeneous catalysis / Benzylic oxidation
A new class of dirhodium(II) tetraamidinates derived from 2-
(sulfonylimino)pyrrolidines has been prepared through li-
gand substitution by using dirhodium(II) acetate, in which
(3,1) geometric isomers are formed predominantly. Among
these complexes, (3,1)-Rh2(Msip)4, exhibits good catalytic
performance in benzylic oxidation. In the presence of (3,1)-
Rh2(Msip)4 a variety of benzylic derivatives, including
strongly electron-deficient arylalkanes such as 1-ethyl-4-
nitrobenzene, are readily oxidized in water by the inexpen-
sive oxidant T-HYDRO® (70 wt.-% aqueous tert-butyl hydro-
peroxide).
capabilities as catalysts have yet to be recognized in the lit-
erature.[8] Moreover, to the best of our knowledge, studies
on dirhodium(II) complexes ligated by cyclic sulfonylamid-
ines have not been published.
Introduction
Benzylic oxidation has emerged as an important trans-
formative tool for the synthesis of fine chemicals.[1,2] The
combination of catalytic amounts of transition-metal com-
plexes and excess peroxide has been shown to promote the
transformation of arylalkanes into the corresponding carb-
onyl compounds under mild reaction conditions with high
chemoselectivity and broad compatibility with a variety of
sensitive functional groups.[3] Within the family of neutral
tetrabridged dirhodium(II) complexes, O,O- and N,O-li-
gated dirhodium(II) complexes have been extensively ex-
plored for their catalytic capabilities.[4] Among them, N,O-
ligated dirhodium(II) caprolactamate, Rh2(cap)4 (cap = ca-
prolactam anion), is the only catalyst so far reported to
be highly efficient in the oxidation of benzylic, allylic and
propargylic C–H bonds.[5,6] Therefore, the development of
other dirhodium(II) complexes for this catalytic oxidation
is desired. Here we report the synthesis of a new class of
N,N-ligated dirhodium(II) tetraamidinates derived from 2-
(sulfonylimino)pyrrolidines and their application in cata-
lytic benzylic oxidation. Notably, although neutral dirhodi-
um(II) complexes ligated by monoanionic N-donor biden-
tate bridging ligands have been widely explored for numer-
ous applications in electrochemistry, antitumor metall-
opharmaceuticals and supramolecular chemistry,[7] their
Results and Discussion
The motivation for designing tetrakis[2-(sulfonylimino)-
pyrrolidine]dirhodium(II) complexes is the unique struc-
tural characteristics of cis-(2,2)-Rh2(cap)4, which we
thought might afford interesting catalytic acitvity. We envi-
sioned that replacement of the oxygen atom of caprolactam
in Rh2(cap)4 by a functional group that could exert a steric
effect, an electronic effect, or both might alter the catalytic
activity of the complex (Figure 1). With this in mind we set
out to prepare tetrasubstituted dirhodium complexes
Rh2(sip)4 [sip = 2-(sulfonylimino)pyrrolidine anion] and ex-
amine their catalytic activity in the benzylic oxidation.
Figure 1. Design of dirhodium(II) tetrakis[2-(sulfonylimino)pyr-
rolidines].
[a] Xinjiang Technical Institute of Physics & Chemistry, Chinese
Academy of Sciences,
Tetrasubstituted dirhodium complexes Rh2(sip)4 (2a–c)
were prepared by heating a mixture of Rh2(OAc)4, an excess
amount of 2-(sulfonylimino)pyrrolidines[9] and chloro-
benzene in a round-bottomed flask, fitted with a Soxhlet
extraction apparatus containing a thimble charged with
Na2CO3, and kept at reduced pressure in an oil bath at
140 °C for 32 h (Scheme 1).[10] After facile purification by
silica gel chromatography, Rh2(sip)4 complexes were ob-
Urumqi 830011, China
Fax: +86-991-3838708
E-mail: clu@ms.xjb.ac.cn
[b] Graduate School of the Chinese Academy of Sciences,
Beijing 100039, China
[c] School of Chemistry & Chemical Engineering, Xinjiang Nor-
mal University,
Urumqi 830054, China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200292.
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A. Wusiman, X. Tusun, C.-D. Lu
SHORT COMMUNICATION
tained in moderate to high yields. Four geometric isomers the Rh–Rh bond length of Rh2(cap)4·2CH3CN (2.442 Å).[6]
are possible: (4,0), (3,1), (2,2)-trans, and (2,2)-cis. The (3,1)- Rh–N bond lengths in the complex are within the range of
Rh2(sip)4 isomer was produced preferentially; in this iso- 1.999–2.130 Å. Rh–Rh–N angles are in the range of 85.58–
mer, the dirhodium(II) core is surrounded by four bridging 88.36°, and they maintain an overall octahedral arrange-
2-(sulfonylimino)pyrrolidine ligands such that three exocy- ment of ligands around the Rh–Rh core. The sterically less
clic nitrogen donor atoms (the nitrogen atom connected to hindered rhodium atom, which is bonded to only one exo-
the sulfonyl group) and one endocyclic nitrogen atom (in cyclic nitrogen atom bearing a bulky tosyl group and to
the pyrrolidine ring) are bonded to one of the rhodium three less hindered endocyclic nitrogen atoms, axially bonds
atoms, while the second rhodium atom is bonded to three with the oxygen atom of H2O, with an Rh–Oax bond length
endocyclic and one exocyclic nitrogen atom. The geometric of 2.298 Å and an Rh–Rh–Oax bond angle of 176.07°.
configuration of the (3,1)-Rh2(sip)4 isomers 2a–c can be
identified by analysis of the H NMR spectra, which show
three sets of peaks in a 2:1:1 ratio, indicating a pair of iden-
tical ligands as well as two different ones. The structure
of (3,1)-Rh2(Tsip)4 (2b) was further confirmed by a single-
crystal X-ray diffraction analysis.[11]
The strongly electron-deficient arylalkane 3a, which typi-
cally exhibits low reactivity toward benzylic oxidation, was
used to examine the catalytic performance of the dirhodi-
um(II) complexes Rh2(sip)4 (Table 1). Initial experiments
demonstrated that water significantly facilitates the trans-
formation (Entry 1 vs. Entry 4, Entry 2 vs. Entry 5, Entry 3
vs. Entry 6, Entry 9 vs. Entry 11).[6] For example, when
dichloromethane was used (Entry 1), treatment of 1-ethyl-4-
nitrobenzene (3a) (1.0 equiv.) with tert-butyl hydroperoxide
(TBHP) (5.0 equiv.) at room temperature in the presence of
1 mol-% of Rh2(Msip)4 (2a) resulted in a yield of only 21%
yield based on 1H NMR analysis of the crude reaction mix-
ture. In sharp contrast, by performing the same reaction
in aqueous solution (Entry 4) furnished the corresponding
oxidized products (4a + 5a + 6a) in 90% overall yield, with
ketone 4a predominating.[12] Reactions catalyzed by bulkier
2b or 2c gave slightly lower yields (Entries 5 and 6). The
N,N-ligated dirhodium complex Rh2(dpf)4 (dpf = N,NЈ-di-
phenylformamidinate anion), in which ligands are acyclic
1
Scheme 1. Preparation of (3,1)-Rh2(sip)4 complexes.
The crystal structure of (3,1)-Rh2(Tsip)4 (2b) gives an amidines and the nitrogen atoms lack electron-withdrawing
Rh–Rh bond length of 2.448 Å, which is in the range typi- groups such as a sulfonyl group, was much less active for
cal for Rh–Rh bonds in tetraamidinate compounds (2.389– this transformation (Entry 7). Comparable yields were ob-
2.570 Å).[7] More interestingly, this length is very close to tained by using Rh2(cap)4 under aqueous reaction condi-
Table 1. Oxidation of 1-ethyl-4-nitrobenzene with dirhodium(II) complexes.[a]
Entry
Catalyst [mol-%]
Oxidant
Solvent
Temp [°C]
Yield [%][b] of 4a/5a/6a/7a
1[c]
2[c]
3[c]
4
5
6
7
8
9
10
11[d]
12[e]
13
2a [1.0]
2b [1.0]
T-DCM
T-DCM
T-DCM
DCM
DCM
DCM
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
room temp.
room temp.
room temp.
room temp.
room temp.
room temp.
room temp.
room temp.
60
13:5:2:0
17:4:2:0
17:5:3:2
82:2:6:0
63:3:9:0
60:3:6:2
24:4:2:8
76:3:6:0
51:2:11:10
36:20:0:0
30:3:22:3
Ͻ5:0:0:0
–
2c [1.0]
2a [1.0]
T-HYDRO
T-HYDRO
T-HYDRO
T-HYDRO
T-HYDRO
T-HYDRO
T-HYDRO
T-HYDRO
H2O2
2b [1.0]
2c [1.0]
Rh2(dpf)4 [1.0]
Rh2(cap)4 [1.0]
2a [0.1]
Rh2(cap)4 [0.1]
2a [0.1]
60
60
40
2a [1.0]
none
T-HYDRO
room temp.
[a] Reactions were performed with substrate (1.0 equiv.) and TBHP (4.0 equiv.) in air in a sealed tube for 20 h unless otherwise noted.
1
T-DCM = TBHP in CH2Cl2, T-HYDRO = 70 wt.-% aqueous TBHP. [b] Yield determined by H NMR analysis of the crude reaction
mixture; see Supporting Information for details. [c] 50 mol-% NaHCO3 and 5.0 equiv. of TBHP were used according to the conditions
reported in ref.[5] [d] Reaction was performed under argon. [e] 5.0 equiv. of 30% H2O2 were used.
2
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Dirhodium(II) Complexes of 2-(Sulfonylimino)pyrrolidine
Table 2. Rh2(Msip)4-catalyzed benzylic oxidation.
[a] Isolated yields for reactions in which Method A was carried out by using 2a (1 mol-%), substrate (1.0 equiv.) and T-HYDRO® (70 wt.-
% aqueous tert-butyl hydroperoxide, 4.0 equiv.) in H2O (1.0 mL) at room temp. for 20 h. [b] Yields in parentheses were obtained when
Method B was carried out by using 2a (1 mol-%), substrate (1.0 equiv.), NaHCO3 (50 mol-%) and TBHP-decane (5.0 equiv.) in DCE
(1,2-dichloroethane) at 40 °C for 20 h. These are the same reaction conditions as in ref.[5], except that the catalyst Rh2(cap)4 in ref.[5] was
replaced by 2a here. [c] Methyl 4-(1-hydroxyethyl)benzoate was obtained in 6% yield. [d] 5 vol.-% methanol in water was used as solvent.
[e] A small amount (Ͻ5%) of 4-methoxybenzaldehyde was observed. [f] 4-bromobenzaldehyde was obtained in 17% isolated yield. [g]
Products α-tetralone (4i) and the dione 4iЈ were obtained in a 1:1 ratio. [h] Substrates were dissolved in 0.4 mL of DCM. [i] Isolated as
a mixture of C1/C4 isomers (9:1). [j] Reaction on a 1 g scale gave benzophenone (4k) in 92% yield. [k] The products anthraquinone (4m),
anthrone (4mЈ) and anthracene (4mЈЈ) were obtained in a ratio of 6:2:1. [l] A small amount of 4-methoxybenzoic acid (Ͻ5%) was
observed. [m] A small amount of tert-butyl 4-nitrobenzoate (Ͻ5%) was observed. [n] The reaction was performed at room temp. for 20 h
and then at 60 °C for 5 h.
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A. Wusiman, X. Tusun, C.-D. Lu
SHORT COMMUNICATION
tions,whichgave85%totalyield(Entry 8).Rh2(Msip)4andRh2- and a low-energy absorption peak to appear at 969 nm in
(cap)4 performed similarly at 0.1 mol-% catalyst loading the UV/Vis spectrum. Cyclic voltammetry measurements
when catalyzing the oxidation of 1-ethyl-4-nitrobenzene at showed that Rh2(Msip)4 (2a) undergoes reversible one-elec-
60 °C, except that the product distributions differed to some tron oxidation at E1/2 = 570 mV vs. Ag/AgCl,[15] which is
extent (Entry 9 vs. Entry 10). The formation of ketone 4a, much higher than the oxidation potential of Rh2(cap)4
benzyl alcohol 5a, benzyl tert-butylperoxide 6a and benzyl (E1/2 = 55 mV vs. Ag/AgCl).[6a] The higher oxidation poten-
hydroperoxide 7a were observed in different ratios when the tial of 2a associated with its high catalytic efficiency sug-
reaction was conducted under different reaction conditions. gests that an extreme propensity for one-electron oxidation
Analogues of the above reaction products obtained during is not a prerequisite for Rh2(Msip)4 in catalytic benzylic
benzylic oxidation by using TBHP and CrO3 as a catalyst oxidation.
were reported by Muzart.[13] Finally, the experiments in
Table 1 show that H2O2 is not a suitable oxidant for this
transformation (Entry 12) and that no reaction occurs in
the absence of dirhodium(II) catalyst (Entry 13).
Conclusions
We have developed a new class of N,N-ligated dirhodi-
um(II) tetraamidinates derived from 2-(sulfonylimino)pyr-
rolidines, and we have demonstrated their ability to catalyze
benzylic oxidation. One of these complexes, (3,1)-
Rh2(Msip)4, is highly efficient at promoting oxidation of a
variety of benzylic derivatives by T-HYDRO® in water.
This study describes the first successful catalytic application
of neutral N,N-ligated dirhodium(II) complexes.
With these preliminary results in hand (Table 1, Entry 4),
the scope of catalytic benzylic oxidation was investigated by
employing Rh2(Msip)4 (2a) as catalyst (Table 2). Oxidation
of arylalkanes 3a–3j (Entries 1–5), including those contain-
ing primary benzylic positions (Entry 3), proceeded
smoothly and produced the corresponding oxo derivatives
in moderate to good yields. For diarylmethane derivatives
3k–3m, which are much more activated substrates, excellent
yields were generally obtained (Entries 6–8). Preparation of
benzophenone (4k) on a 1 g scale was achieved by using
this protocol without obvious loss of yield (Entry 6: 92%
for a 1 g scale vs. 95% for a 50 mg scale). When substrates
3i and 3m, which possess two benzylic positions, were ex-
posed to stronger reaction conditions (2 mol-% 2a, 8 equiv.
T-HYDRO, room temp., 20 h), diones 4iЈ and 4m formed
in 80% and 92% yield, respectively. This protocol was then
expanded to the oxidation of benzyl chloride and methyl
phenylacetate, leading to the formation of benzoic acid (3n)
(Entry 9) and methyl oxo(phenyl)acetate (3o) (Entry 10),
respectively. Furthermore, acyclic and cyclic benzyl ethers
3p–3t were oxidized to give the corresponding esters (En-
Experimental Section
General Procedure for the Synthesis of Dirhodium(II) Tetrakis[2-
(sulfonylimino)pyrrolidines] [Rh2(sip)4 2a–c]: Dirhodium(II) acetate
(0.20 mmol), N-(pyrrolidinylidene)sulfonamide (2.4 mmol), and
chlorobenzene (12 mL) were added to a 25 mL flask. The flask was
fitted with a Soxhlet extraction apparatus contaning a thimble with
4.0 g oven-dried Na2CO3 and sand in a 3.5:1.5 ratio. The reaction
mixture was incubated in an oil bath at 140 °C and refluxed under
reduced pressure (140–210 Torr; water aspirator) for 32 h, and then
cooled to room temperature and concentrated in vacuo. The re-
sulting residue was purified by silica gel column chromatography
to give the desired product.
1
General Procedure for the Oxidation of 1-Ethyl-4-nitrobenzene with
Dirhodium(II) Complexes: See Table 1. A mixture of 1-ethyl-4-nitro-
benzene (0.30 mmol) and dirhodium(II) catalyst (1.0 mol-% or
0.1 mol-%) in the indicated solvent (1.0 mL) was stirred vigorously
in a screw-cap vial (reactions at 60 °C were performed in a sealed
tube). To the mixture was added 4.0 or 5.0 equiv. of TBHP-HY-
DRO®, TBHP-DCM or TBHP-decane in one portion through a
syringe. The vial was tightly capped, and the reaction mixture was
stirred at the indicated temperature for 20 h. Then the reaction mix-
ture was extracted twice with ethyl acetate. The organic extracts
were combined, dried with anhydrous MgSO4, filtered and then
concentrated under reduced pressure. The residues were used for
tries 11–13). Notably, in the case of 3s and 3t, H NMR
analysis of the reaction mixtures revealed 15–20% yields of
the corresponding benzyl tert-butyl peroxides, which were
generated by incorporation of tert-butyl peroxide into the
substrates. In contrast, oxidation of N-tosyltetrahydroiso-
quinoline afforded peroxide 4u as the major product (86%
yield) instead of the corresponding amide (Entry 14). We
also subjected some substrates to anhydrous reaction condi-
tions described in ref.[5] using Rh2(Msip)4 as catalyst instead
of Rh2(cap)4. We obtained yields comparable to those pre-
viously reported with Rh2(cap)4 (see yields in parentheses
obtained with Method B, Table 2). Contrary to what was
1
crude H NMR analysis.
observed in most other reactions, the catalytic oxidation of General Procedure for the Rh2(Msip)4-Catalyzed Benzylic Oxidation
in Water: See Table 2. The substrate (0.30 mmol) and catalyst 2a
(1.0 mol-%) were stirred vigorously in water (1.0 mL) at room tem-
perature in a screw-cap vial. To the reaction mixture was added
4.0 equiv. of TBHP (70 wt.-% in H2O, 1.2 mmol) in one portion
through a syringe. The vial was tightly capped, and the reaction
mixture was stirred at room temperature for 20 h. Then the reaction
mixture was extracted twice with ethyl acetate. The organic extracts
were combined, dried with anhydrous MgSO4, filtered, and then
concentrated under reduced pressure to give the crude product,
which was purified by silica gel column chromatography (petroleum
benzyl methyl ether (3p) and methyl 4-nitrobenzyl ether (3r)
gave higher yields when the reactions were performed in
dichloroethane rather than in water (Entry 11; 4p and
4r).[6d,14]
The Rh2(Msip)4-catalyzed benzylic oxidation is most
likely to involve the same dirhodium(5+) species that has
been well documented in the Rh2(cap)4-catalyzed proto-
col.[5,6] Consistent with the presence of a one-electron oxid-
ized dirhodium core (Rh25+), addition of TBHP caused the
Rh2(Msip)4 solution to change from light blue to pink red ether/ethyl acetate) to afford the desired product.
4
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Dirhodium(II) Complexes of 2-(Sulfonylimino)pyrrolidine
[7]
[8]
H. T. Chifotides, K. R. Dunbar in Multiple Bonds Between
Metal Atoms, 3rd ed. (Eds.: F. A. Cotton, C. Murillo, R. A.
Walton), Springer-Science & Business Media, Inc., New York,
2005, chapter 12, pp. 465–589.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details, characterization data and 1H and 13C
1
NMR spectra of all new compounds, as well as H NMR spectra
of the crude reaction mixtures (Table 1).
To the best of our knowledge, the catalytic application of neu-
tral N,N-ligated dirhodium(II) complexes has not been de-
scribed. For silylformylation of alkynes catalyzed by the tetra-
cationic complex [Rh2(MeCN)2(Naft)4](BF4)4 (Naft = μ-1,8-
naphthyridine), see: a) M. Basato, A. Biffis, G. Martinati, C.
Tubaro, C. Graiff, T. Antonio, A. A. Laura, A. M. Caporusso,
J. Organomet. Chem. 2006, 691, 3464–3471; b) A. Biffis, L.
Conte, C. Tubaro, M. Basato, L. A. Aronica, A. Cuzzola,
A. M. Caporusso, J. Organomet. Chem. 2010, 695, 792–798.
For the synthesis of five-membered cyclic sulfonylamidines,
also called 2-(sulfonylimino)pyrrolidines, see: M. Yao, C.-D.
Lu, Org. Lett. 2011, 13, 2782–2785 and references cited therein.
Attempts to synthesize the Rh2(sip)4 complexes derived from
2-[(4-nitrophenyl)sulfonylimino]pyrrolidine and 2-(trifluorome-
thanesulfonylimino)pyrrolidine under the standard conditions
proved unsuccessful. Moreover, the reactions of six- and seven-
membered cyclic sulfonylamidines with dirhodium(II) acetate
were inefficient.
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (20972182 and 21172251), the “Western Light” pro-
gram (XBBS200820) and the Chinese Academy of Sciences.
[9]
[1] a) R. A. Sheldon, J. K. Kochi in Metal-Catalyzed Oxidation of
Organic Compounds, Academic Press, New York, 1981; b)
Modern Oxidation Methods, 2nd ed. (Ed.: J.-E. Bäckvall),
Wiley, Weinheim, 2010.
[2] For transition-metal-free benzylic oxidations, see: T. Dohi, N.
Takenaga, A. Goto, H. Fujioka, Y. Kita, J. Org. Chem. 2008,
73, 7365–7368 and references cited therein.
[3] a) M. Nakanishi, C. Bolm, Adv. Synth. Catal. 2007, 349, 861–
864 and references cited therein; b) Y. Bonvin, E. Callens, I.
Larrosa, D. A. Henderson, J. Oldham, A. J. Burton, A. G. M.
Barrett, Org. Lett. 2005, 7, 4549–4552 and references cited
therein.
[4] a) M. P. Doyle, R. Duffy, M. Ratnikov, L. Zhou, Chem. Rev.
2010, 110, 704–724; b) H. M. L. Davies, R. E. J. Beckwith,
Chem. Rev. 2003, 103, 2861–2904; c) M. P. Doyle, J. Org. Chem.
2006, 71, 9253–9260.
[10]
[11]
CCDC-859713 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[12] For several recent examples of catalytic benzylic oxidations in
water by using tBuOOH and water-soluble transition-metal
catalysts, see: C. S. Yi, K.-H. Kwon, D. W. Lee, Org. Lett. 2009,
11, 1567–1569 and references cited therein.
[13] J. Muzart, A. N’Ait Ajjou, J. Mol. Catal. 1994, 92, 277.
[14] M. P. Doyle, A. J. Catino, H. Choi, J. M. Nichols, U. S. Pat.
93638, 2009.
[15] Cyclic voltammetry experiments showed that 2a–c undergo re-
versible one-electron oxidation at E1/2 = 570 mV, 546 mV and
524 mV vs. Ag/AgCl, respectively. The UV/Vis spectra of 2a–c
show low-energy absorption peaks at 969 nm, 935 nm and
942 nm, respectively, upon addition of TBHP. See the Support-
ing Information for details.
[5] For benzylic oxidations, see: A. J. Catino, J. M. Nichols, H.
Choi, S. Gottipamula, M. P. Doyle, Org. Lett. 2005, 7, 5167–
5170.
[6] For the crystal structure of Rh2(cap)4, see: J. M. Nichols,
Ph. D. dissertation, University of Maryland, 2008; for allylic
and propargylic oxidations, see: a) A. J. Catino, R. E. Forslund,
M. P. Doyle, J. Am. Chem. Soc. 2004, 126, 13622–13623; b)
H. Choi, M. P. Doyle, Org. Lett. 2007, 9, 5349–5352; c) E. C.
McLaughlin, H. Choi, K. Wang, G. Chiou, M. P. Doyle, J. Org.
Chem. 2009, 74, 730–738; d) E. C. McLaughlin, M. P. Doyle,
J. Org. Chem. 2008, 73, 4317–4319; for amine oxidations, see:
H. Choi, M. P. Doyle, Chem. Commun. 2007, 745–747.
Received: March 9, 2012
Published Online: ᭿
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A. Wusiman, X. Tusun, C.-D. Lu
SHORT COMMUNICATION
Dirhodium(II) Complexes
A. Wusiman, X. Tusun, C.-D. Lu* .... 1–6
Dirhodium(II) Complexes of 2-(Sulfonyl-
imino)pyrrolidine: Synthesis and Appli-
cation in Catalytic Benzylic Oxidation
The synthesis of dirhodium(II) tetraamid-
inates derived from 2-(sulfonylimino)pyr-
rolidines and their use in the catalytic
benzylic oxidation are described. A variety
of benzylic derivatives, including strongly
electron-deficient arylalkanes such as 1-
ethyl-4-nitrobenzene, can be readily oxid-
ized by T-HYDRO® in the presence of
1.0 mol-% (3,1)-Rh2(Msip)4 in water under
mild reaction conditions.
Keywords: Rhodium / N ligands / Bridging
ligands / Homogeneous catalysis / Benzylic
oxidation
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