N-heterocyclic carbene dirhodium(II) complexes as catalysts for allylic and benzylic oxidations

Article abstract of DOI:10.1002/ejoc.201201300

The experimental conditions (solvent, base, temperature and oxidant) for allylic and benzylic oxidation reactions catalyzed by dirhodium(II)/N- heterocyclic carbene (NHC) complexes were optimized for the first time in this work. The oxidations of cyclohexene and fluorene were used as model reactions. Two optimized experimental conditions for both types of oxidations were found, which resulted in their ketone [aerobic conditions, 40 °C, 1 equiv. tBuOOH (TBHP)] or tert-butyl peroxide derivatives (anaerobic conditions, 25 °C, 2 equiv. TBHP). The dirhodium(II) complexes undergo a single-electron reversible oxidation by cyclic voltammetry in CH₂Cl₂, which is assigned to the Rh₂⁴⁺/Rh₂⁵⁺ redox couple at an oxidation potential that is lowered upon sequential axial coordination of further ligands to the Rh-Rh centre. The oxidation potential is discussed in terms of the electron-donor character of the NHC ligand, which was shown to act as an effective electron-releaser to the Rh₂ ⁴⁺ centre, and the electrochemical behaviour is compared to the observed catalytic activity. An N-heterocyclic carbene (NHC) ligand at the axial position of Rh₂(carboxylate)₄ complexes triggers considerable changes in the electrochemical behaviour of the formed complexes, which also catalyze allylic and benzylic oxidations. Copyright

Full text of DOI:10.1002/ejoc.201201300

FULL PAPER
DOI: 10.1002/ejoc.201201300
N-Heterocyclic Carbene Dirhodium(II) Complexes as Catalysts for Allylic and
Benzylic Oxidations
Jaime A. S. Coelho,[a,b] Alexandre F. Trindade,*[a,b] Riccardo Wanke,[c]
Bruno G. M. Rocha,[c] Luis F. Veiros,[c] Pedro M. P. Gois,*[a] Armando J. L. Pombeiro,[c] and
Carlos A. M. Afonso*[a,b]
Keywords: Electrochemistry / Rhodium / Carbene ligands / Oxidation / Density functional calculations
The experimental conditions (solvent, base, temperature and
oxidant) for allylic and benzylic oxidation reactions catalyzed
by dirhodium(II)/N-heterocyclic carbene (NHC) complexes
were optimized for the first time in this work. The oxidations
of cyclohexene and fluorene were used as model reactions.
Two optimized experimental conditions for both types of oxi-
dations were found, which resulted in their ketone [aerobic
TBHP). The dirhodium(II) complexes undergo a single-elec-
tron reversible oxidation by cyclic voltammetry in CH2Cl2,
5+
which is assigned to the Rh24+/Rh2 redox couple at an oxi-
dation potential that is lowered upon sequential axial coordi-
nation of further ligands to the Rh–Rh centre. The oxidation
potential is discussed in terms of the electron-donor charac-
ter of the NHC ligand, which was shown to act as an effective
conditions, 40 °C, 1 equiv. tBuOOH (TBHP)] or tert-butyl per- electron-releaser to the Rh24+ centre, and the electrochemical
oxide derivatives (anaerobic conditions, 25 °C, 2 equiv.
behaviour is compared to the observed catalytic activity.
vs. the saturated calomel electrode (SCE),[12a] which corre-
Introduction
5+
sponds to the Rh24+/Rh2
redox couple. In contrast,
Allylic and benzylic oxidations are important and syn-
thetically useful transformations in organic chemistry and
give access to allylic alcohols, acetates and ketones from the
respective alkenes.[1] Classically, stoichiometric quantities of
oxidants such as selenium dioxide[2] and potassium dichro-
mate[3] were used in these transformations. However, during
the last decades several methods have been disclosed that
apply transition-metal catalysts in benzylic and allylic oxi-
dations. Catalysts containing gold,[4] bismuth,[5] iron,[6] ru-
thenium,[7] copper,[1c,8] palladium,[1b,9] cobalt[10] and man-
ganese have been used.[11]
Rh2(OAc)4 (2) has limited activity in the allylic oxidation
of olefins,[13] possibly because, in contrast to Rh2(cap)4, it
undergoes an oxidation that is not fully reversible at the
much higher potential of 1.17 V vs. SCE (Scheme 1).[12a,14]
Recently, Doyle et al. very successfully demonstrated that
Rh2(cap)4 (1, cap = caprolactamate) could be used as a cat-
alyst for allylic, benzylic and propargylic oxidations with
tert-butyl hydroperoxide (TBHP).[12] The success of this cat-
alyst is believed to rely on its reversible oxidation at 0.01 V
Scheme 1. Doyle’s catalyst 1 vs. Rh2(OAc)4 2.
[a] i-Med.UL, Faculdade de Farmácia da Universidade de Lisboa,
Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal
Fax: +351-21-7946476
The mechanistic pathway for the Rh2(cap)4-mediated all-
ylic oxidation of cyclohexene as a model substrate is out-
lined in Scheme 2.[12d] The authors suggested that an initial
oxidation of 1 by TBHP occurs to form the oxidized dirho-
dium(II,III) intermediate [Rh2(cap)4(OH) was isolated] and
the tert-butoxyl radical, which, in turn, abstracts a hydrogen
atom from TBHP at a rate that is much faster than hydro-
gen atom abstraction from the allylic position of the alk-
ene.[15] The newly formed tert-butylperoxyl radical selec-
tively abstracts a hydrogen atom from the hydrocarbon sub-
strate: a process that is well documented and is widely ac-
E-mail: alexandretrindade@ff.ul.pt
pedrogois@ff.ul.pt
carlosafonso@ff.ul.pt
Homepage: http://www.imed.ul.pt/portal/index.php?option=
com_content&view=article&id=71&Itemid=79
[b] CQFM, Centro de Química-Física Molecular and IN-Institute
of Nanosciences and Nanotechnology, Instituto Superior
Técnico,
1049-001 Lisboa, Portugal
[c] Centro de Química Estrutural, Complexo I, Instituto Superior
Técnico, Universidade Técnica de Lisboa,
Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201300.
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P. M. P. Gois, C. A. M. Afonso et al.
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Scheme 2. Proposed mechanism for allylic oxidation with dirhodium caprolactamate.[12d]
cepted.[16] The capture of the allyl radical by the tert-butyl-
Table 1. Oxidation potentials [V] of dirhodium(II)–NHC complex-
es.[a]
peroxyl radical forms the mixed peroxide that is susceptible
to tert-butoxyl radical catalyzed disproportionation. The
allylic peroxide intermediates could be intercepted in the
oxidation of 1-acetylcyclohexene and when resubmitted to
the reaction provided the respective enones. The latter dis-
proportion reaction of such intermediates was found to be
highly dependent on the presence of base.
[b]
II
III
Entry Complex
IE1/2
E
1/2
E
1/2
1
2
3
4
5
6
Rh2(OAc)4(IPr) 3
+1.01
+1.00
+1.09
+0.18
+0.41
+0.90
–
–
–
–
–
–
Rh2(OAc)4(SIPr) 4
Rh2(OAc)4(ClIPr) 5
Rh2(OAc)4(IPr)2 6
Rh2(OAc)4(ClIPr)2 7
Rh2(tfa)4(IPr)2 8
+1.01
+1.03
+1.18[c]
+1.42
–
–
In 2007, we discovered that is possible to tune the reac-
tivity of dirhodium(II) complexes by attaching N-heterocy- [a] Values (Ϯ 0.02 V) measured by cyclic voltammetry and quoted
relative to SCE by using the [Fe(η5-C5H5)2]0/+ (E1/2 = 0.525 V vs.
clic carbene (NHC) ligands onto the axial position of the
bimetallic complex. This strategy has enabled us to perform
SCE)[24] redox couple in 0.2 m [Bu4N][BF4]/CH2Cl2 solution. [b]
Theoretical half-wave oxidation potentials were estimated as +1.14
(3), +0.36 (6), +0.44 (7) and +0.93 (8). For further details see Sup-
porting Information. [c] Not fully reversible.
the arylation of aldehydes with boronic acids and the intra-
molecular C–H insertion of diazoacetamides.[17] Sub-
sequently, Jang et al. observed that the axial coordination
of an NHC ligand [1,3-bis(2,6-diisopropylphenyl)imidazol-
2-ylidene, IPr] to 2 resulted in a change of the electrochemi-
cal properties.[18] Although 2 displays an irreversible oxi-
dation (in 0.1 m [Bu4N]ClO4/THF), the NHC adduct
Rh2(OAc)4(IPr) 3 shows a single-electron quasireversible
oxidation at E1/2 ≈ 0.80 V vs. Ag, which is assigned to the
5+
Rh24+/Rh2 redox couple. The reversibility of the oxi-
dation allowed considerable improvements in the catalytic
capability for allylic oxidations. They described that the all-
ylic oxidation of cyclic alkenes yielded the respective enones
in 18–57% with 1 mol-% 3, 50 mol-% K2CO3 and 100 mol-
% of TBHP at room temperature.[18] However, no infor-
mation regarding the selectivity of the reactions was pro-
vided or discussed. Therefore, we became interested in
evaluating the newly developed complexes as catalysts for
oxidation reactions as well as their impact in the reaction
selectivity. Herein, an electrochemical characterization of
such complexes and their catalytic evaluation in allylic and
benzylic oxidation reactions is provided.
Results and Discussion
Electrochemistry
The electrochemical behaviour of various dirhodium(II)–
NHC complexes was studied by cyclic voltammetry (CV).
Generally, they show a regular profile with one, two or
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NHC Rhodium Complexes for Allylic and Benzylic Oxidations
three reversible or quasireversible oxidation waves within 0.41 V; Table 1, Entries 4 and 5, respectively). However, the
the accessible range of potentials in dichloromethane solu- second oxidation (IIE1/2) is not significantly affected by the
tions [Table 1, Figure 1 for Rh2(OAc)4(IPr)2 (6)]. We chose nature of the carbene ligand and appears at 1.01–1.03 V.
dichloromethane because it is a non-coordinating solvent.
Complex 8 with four tfa bridging ligands is oxidized at
The measurements made by Jang et al.18] were conducted a potential (IE1/2 = 0.90 V) that is much higher than those
in tetrahydrofuran (THF), for which they could consider of the related dicarbene complexes with acetate bridges
Rh2(OAc)4(IPr)(THF) and Rh2(OAc)4(THF)2 species.[19]
(IE1/2 = 0.18 and 0.41 V for 6 and 7, respectively), in accord-
ance with the electron-withdrawing nature of the CF3
groups in 8. Furthermore, although all other complexes
present reversible oxidation waves, compound 8 displays
oxidation waves that are not fully reversible.
A controlled potential electrolysis (CPE) at the potential
of the first oxidation wave of 5 in dichloromethane at room
temperature indicates the occurrence of a one-electron oxi-
5+
dation process, which is attributed to the Rh24+/Rh2 re-
dox couple. This result corroborates the findings of Doyle
et al.[12a] for the dirhodium(II) caprolactamate species
Rh2(cap)4.
An interesting feature of our complexes concerns the
marked shift (by ca. 0.7–0.8 V) of the oxidation potential
to a less anodic value upon coordination of a second axial
NHC ligand (compare the dicarbene complexes 6 and 7
with the corresponding monocarbenes 3 and 5, Table 1).
This contrasts with the above-mentioned complex 2, for
Figure 1. Cyclic voltammogram of a 1 mm solution of Rh2-
(OAc)4(IPr)2 6 in 0.2 m [NBu4] [BF4]/CH2Cl2 at a platinum disc
electrode (d = 0.5 mm) (v = 0.2 Vs–1).
The involvement of a single-electron transfer in the oxi- which the addition of one axial IPr ligand to give 3 does
dation processes of the complexes has been confirmed by not lead to an appreciable shift of oxidation potential, but,
controlled potential electrolysis at the first oxidation wave instead, to the gain of reversibility.[18] Nevertheless, a shift
of complex 3 and by the similarity of the current functions already occurs for Rh2(cap)4 upon addition of an axial
(ipC–1ν–1/2, where ip = peak current, C = concentration, ν = NHC.[18]
scan rate) of the oxidation waves and that of complex 3.
To gain a further insight into the effect of a second axial
Rh2(OAc)4(IPr) 3, Rh2(OAc)4(SIPr) [4, SIPr = 1,3- ligand on the oxidation potential of our complexes and for
bis(2,6-diisopropylphenyl)imidazolin-2-ylidene]
and comparative purposes with NHC ligands, we monitored the
Rh2(OAc)4(ClIPr) [5, ClIPr = 4,5-dichloro-1,3-bis(2,6-di- cyclic voltammogram of a CH2Cl2 solution of the mono-
isopropylphenyl)imidazol-2-ylidene, Table 1, Entries 1–3], carbene complex 3 upon the addition of an extra ligand
which bear only one NHC ligand coordinated in the axial source (pyridine, NCMe, pyrazole, cyclohexyl isocyanide or
I
position, show single reversible oxidation waves at E1/2 [Et4N]I). In all cases, the initial reversible oxidation wave of
from 1.00 to 1.09 V vs. SCE. The values do not differ con- 3 at 1.01 V is progressively replaced by another one at a
siderably, although the trend is consistent with the electron- lower potential (Figure 2, for the pyridine addition), which
donor/acceptor character of the carbene ligand.[20] Com- conceivably concerns the formation of the corresponding
plex 5, which contains two inductively electron-withdrawing
I
chlorine atoms, displays the highest E1/2 value. In our case,
all the monocarbene complexes display oxidation potential
values that are lower than that of Rh2(OAc)4 (1.17 V vs.
SCE).[12a,14] Hence, the axial coordination of the NHC li-
gand to one of the Rh atoms promotes the oxidation of the
complex. A more pronounced cathodic shift results from
the further axial coordination of a second NHC ligand to
the other Rh atom [see below, dicarbene complexes
Rh2(OAc)4(IPr)2 (6) and Rh2(OAc)4(ClIPr)2 (7)].
The dicarbene complexes 6, 7 and Rh2(tfa)4(IPr)2 (8,
Table 1, Entries 4–6, tfa = trifluoroacetate) with two axial
NHC ligands show two reversible oxidation waves (IE1/2
II
and
E
in the ranges 0.18–0.90 and 1.01–1.18 V vs. SCE,
1/2
respectively) and, in one case (6, Figure 1), a third quasire-
III
versible oxidation wave at a higher potential (
E
=
1/2
1.42 V). As mentioned above for complexes 3 and 5, com-
Figure 2. Cyclic voltammograms of a 1 mm solution of 3 (a), 3 +
0.5 equiv. (b) or 3 + 1 equiv. (c) of pyridine in 0.2 m [NBu4][BF4]/
I
plex 6 displays a considerably lower E1/2 value than the
that of the chloro-substituted complex 7 (IE1/2 = 0.18 vs. CH2Cl2 at a platinum disc electrode (d = 0.5 mm, v = 0.2 Vs–1).
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P. M. P. Gois, C. A. M. Afonso et al.
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coordinatively saturated Rh2(OAc)4(IPr)(L) complex followed by hexane and then acetone (Figure 3). The degree
(Scheme 3).
of cyclohexenone formation also followed this tendency. By
taking into account the relative polarity values of these
three solvents, the lower conversion obtained in acetone
could be explained by the coordinative properties of this
solvent, which could act as a competitive inhibitor[26] by
axial coordination.
Scheme 3. Proposed ligand (L) addition to the monocarbene unsat-
urated complex 3.
With the exception of the iodo adduct (E = 0.24 V, L =
I), these oxidation potential values [0.68 (L = pyridine),
0.80 (L = NCMe, CNCy) and 0.81 V (L = pyrazole)] are
much higher than that (0.18 V) of the NHC adduct 6, which
suggests that the NHC ligand at the dirhodium(II) centre
behaves as a markedly stronger electron donor than pyr-
idine, NCMe, pyrazole and CNCy ligands. This contrasts
with previous reports that indicate that related N-hetero-
cyclic carbenes at a tetracoordinate (1,4,7-trithiacyclonon-
Figure 3. Cyclohexene conversion for different solvents [reaction
ane)CuI complex[21] or at half-sandwich carbonyl cyclopen- conditions: 3 (1 mol-%), K2CO3 (50 mol-%), solvent (0.27 m), r.t.].
tadienyl-iron(II) complexes[22] have an electron-releasing
Our reactions were followed by GC to analyze the forma-
ability similar to that of pyridine.
tion of cyclohexenone and disappearance of cyclohexene.
A value of the electrochemical EL Lever parameter (a
The appearance of two other compounds in the GC chro-
measure of the net electron-donor character of a ligand, the
stronger this property the lower the value of EL)[23] for the
studied NHC ligands of ca. +0.30 V vs. NHE (normal hy-
drogen electrode) was proposed[21–22] on the basis of the
redox potential of the limited number of investigated com-
plexes. Although our results do not allow us to estimate
with confidence the EL parameter of IPr, in view of the
limited number of complexes and the irreversibility of the
oxidation wave of the iodide adduct Rh2(OAc)4(IPr)(I)
(which does not allow us to have access to the thermo-
dynamic potential), they suggest that, at the dinuclear RhII
centre of our study, the NHC ligand appears to exhibit an
EL value much lower (stronger-electron releasing character)
than the above (ca. 0.30 V) and which is comparable to that
of iodide (EL = –0.24 V vs. NHE).[23a]
matograms was intriguing; the compounds were always
present and their ratio relative to cyclohexenone varied with
time. In fact, these two peaks were identified to be the all-
ylic alcohol 9c and the allylic tert-butyl peroxide 9b deriva-
tives of cyclohexene (Scheme 4). In Doyle’s studies allylic
tert-butyl peroxides such as 9b were identified as intermedi-
ates,[12a,12c,12d] but the authors focused their attention on
the preparation of enones. In the present work, the allylic
tert-butyl peroxide 9b is the major product, which moti-
vated further studies to improve the formation of this type
of product. These compounds can be mildly reduced[27] to
provide the allylic alcohol product that is in general only
obtained by using copper[1c] and palladium catalysis.[9b–9d]
To support our conclusion, it is should be mentioned
that aminocarbene ligands are known to display EL values
[23b,23c]
of +0.15 to –0.21 V
and that NHCs can be viewed
as a particular case of diaminocarbenes and, thus, bear two
electron-donor amino groups.
Nevertheless, the above interpretation should be taken
rather cautiously before further studies on a wide range of
NHC–dirhodium complexes are undertaken.
Scheme 4. Allylic oxidation reaction model.
After optimization of the conditions,[28] we observed that
by applying 1 mol-% of 3, 50 mol-% K2CO3 and 200 mol-%
of TBHP at room temperature, we could obtain the highest
Allylic Oxidation
As we were able to expand the library of RhII–NHC selectivity for the formation of allylic peroxide 9b
complexes,[17c–17e,25] we decided to study them in allylic oxi- (Scheme 4). During the optimization, higher selectivity for
dation reactions. An initial simple solvent effect study, un- peroxide 9b was observed when the reaction was conducted
der the conditions described by Jang et al.,[18] indicated that in an anaerobic atmosphere (Table 2, Entries 1 and 2). Ac-
dichloromethane gives higher conversion of cyclohexene, cording to the mechanism proposed by Doyle et al., molec-
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NHC Rhodium Complexes for Allylic and Benzylic Oxidations
ular oxygen can act as the oxidant for the generation of the those of previously reported catalytic systems (CuI,[30]
[7b]
allylic alcohol 9c.[12a] These results illustrate the impact of RuCl3
and 1) under these conditions.
oxygen on the formation of allylic alcohol 9c as proposed
(as its yield is higher in aerobic conditions).
The first observation that can be drawn from Table 3 is
that all dirhodium(II)–NHC complexes displayed identical
selectivity to 3 (Table 3, Entries 3–7). The free carbene or
dirhodium tetraacetate provided no catalysis and lower
catalytic capability, respectively (Table 3, Entries 10 and 2).
Owing to their coordinative saturation, catalytic activity
would not be expected for dicarbene dirhodium(II) com-
plexes. As all the mono-NHC complexes displayed identical
performance, Rh2(OAc)4(ClIPr)2 7 was selected to be tested.
The observed activity of 7 (Table 3, Entry 6) would be al-
lowed by the decoordination of one axial NHC ligand as
observed before for aldehyde arylation.[17] In fact, it was
experimentally observed that a solution of 7 undergoes a
change of colour from orange to purple upon addition of
the oxidant, which indicates the decoordination of one
NHC ligand. Furthermore, in spite of its low reversibility
and high first oxidation potential in the CV experiments,
complex 8 surprisingly proved to be almost as efficient as 3
(Table 3, Entry 7). Under our conditions, the copper and
ruthenium catalysts provided high selectivity for the forma-
tion of the peroxide 9b but the yields did not surpass 13%
(Table 3, Entries 8 and 9).
Table 2. Optimization of allylic oxidation catalyzed by dirhodi-
um(II)–NHC complex.[a]
Entry
x
T
[°C]
Atmosphere
t
9
Yield
[%][c]
Selectivity [%][d]
[h] [%][b]
9a
9b
9c
1
2
3
4
5
1
1
2
4
1
25 aerobic[e]
25 anaerobic[f]
25 anaerobic[f]
25 anaerobic[f]
40 aerobic[e]
27
24
23
24
24
32
61
29
9
29
19
30
36
46
23
11
25
34
52
57
86
71
60
21
20
3
4
6
27
20
[a] All the reactions were carried out with 0.37 mmol of 9 and cata-
lyzed with 1 mol-% of 3. [b] Yield of unreacted cyclohexene deter-
mined by GC analysis of reaction mixture. [c] Yield of products
9a–c determined by GC analysis of reaction mixture. [d] Selectivity
determined by GC analysis of reaction mixture. [e] Round bottom
flask under air. [f] In absence of atmosphere.
Table 3. Catalyst evaluation under reaction conditions I.[a]
Entry Catalyst
t
[h]
9
Yield
Selectivity [%][d]
[%][b] [%][c]
9a
9b
9c
To make analysis of the reaction mixture simple and to
avoid contact with air, the anaerobic reactions were con-
ducted in 1.5 mL vials for GC automated systems sealed
with a septum. The utilization of higher amounts of TBHP
(4 equiv.) in an anaerobic atmosphere resulted in higher
yields of enone 9a, and the allylic alcohol still only formed
in trace amounts (Table 2, Entries 2–4). This increase of cy-
clohexenone selectivity suggested that the oxidant can play
a role in the degradation of allylic peroxide 9b as proposed
by Doyle et al. (see Scheme 2).[12a,12d]
1[e]
2
3
Rh2(cap)4 1
Rh2(OAc)4 2
Rh2(OAc)4(IPr) 3
24
24
23
42
67
29
43
15
30
54
10
25
18
90
71
28
0
4
(24)[f] (25)[f] (37)[f] (20)[f] (77)[f] (3)[f]
4
5
6
7
8
9
10
Rh2(OAc)4(SIPr) 4
Rh2(OAc)4(ClIPr) 5
23
24
33
31
38
25
82
69
95
37
30
25
28
7
24
25
16
21
0
72
69
81
77
100
95
–
4
5
3
2
0
0
–
Rh2(OAc)4(ClIPr)2 7 24
Rh2(tfa)4(IPr)2 8
CuI
RuCl3
25
24
24
24
13
trace
5
–
IPr and KOtBu
Increased reaction temperatures resulted in a shift of
selectivity to the enone. Taking into consideration the pre-
vious results, the reactions run in dichloromethane at reflux
were performed in aerobic conditions (Table 2, Entry 5). A
[a] All reactions were carried out with 0.37 mmol of 9 and catalyzed
by 1 mol-% of catalyst under conditions I: room temperature,
2 equiv. TBHP in GC vial, absence of atmosphere. [b] Yield of un-
reacted cyclohexene determined by GC analysis of the reaction
further increase of the TBHP concentration (to 2 equiv.) re- mixture. [c] Yield of products 9a–c determined by GC analysis of
reaction mixture. [d] Selectivity determined by GC analysis of reac-
sulted in a lower amount of allylic alcohol formed in favour
tion mixture. [e] Reaction performed in round-bottomed flask. [f]
Scaled up with 1.85 mmol of cyclohexene.
of the enone. This shift of selectivity could be due to ther-
mal degradation of the allylic peroxide, which led us to
study the stability of the aforementioned peroxide. We
Notably, there was a shift of selectivity toward the enone
found that peroxide 9b is stable in dichloromethane at re- when 1 was applied as catalyst (Table 3, Entry 1). This re-
flux and even the addition of 0.5 equiv. of K2CO3 did not sult was obtained for the reaction performed in a flask, be-
lead to any degradation after 20 h (especially towards the cause the reaction with Doyle’s catalyst resulted in vigorous
cyclohexenone 9a as reported previously for the dialkyl per- oxygen liberation, which is attributed to the decomposition
oxides[29]).
of di-tert-butyltetraoxide formed from radical coupling of
At the end of this study, we developed two protocols that two TBHP radicals (Scheme 2).[12a,12d] In the case of all the
can provide better yields of the allylic peroxide 9b (condi- RhII–NHC catalysts applied herein, oxygen liberation is
tions I: room temperature, 2 equiv. TBHP in GC vial, ab- very modest, which suggests that under these conditions the
sence of atmosphere) or to the enone 9a (conditions II: concentration of tert-butylperoxide radical is low. This re-
40 °C, 1 equiv. TBHP, round-bottomed flask under aerobic sult also corroborates the decreased reactivity observed and
conditions). The catalytic performances of the dirhodi- the higher potential values of RhII carboxylate/NHC com-
um(II)–NHC catalysts 3, 5, 7 and 8 were compared with plexes. The less hazardous allylic oxidation provided by our
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P. M. P. Gois, C. A. M. Afonso et al.
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catalyst together with the selectivity inversion can be seen 11b is an intermediate. Furthermore, the conversion of this
as advantages of our protocol. peroxide into the respective ketone is highly dependent on
At 40 °C, all catalysts provided a shift of selectivity the temperature. An identical behaviour was observed for
towards the formation of enone (Table 4). Under these con- 12 (Table 5, Entries 3 and 4). As observed in fluorene oxi-
ditions, the best catalyst was 8 and, in general, after only dation, 13 gave almost exclusively the ketone 13a (Table 5,
24 h, a yield of ca. 54% is achieved and it does not increase Entries 5 and 6). For the less reactive 14, low conversions
significantly upon extended heating (to 72 h). Once again, and yields of acetophenone 14a were observed for both
copper iodide and ruthenium chloride failed to be competi- conditions I and II. In this case, the use of higher amounts
tive under reaction conditions II.
of TBHP (conditions III: a total of 3.5 equiv.), added three
times, provided yields of up to 65% (Table 5, Entry 9 vs.
Entries 7 and 8). This result is quite interesting because 1
provides only 20% yield with a procedure in which a total
of 10 equiv. of TBHP were added two times (3 h separation
between additions).[12b]
Table 4. Catalyst evaluation under reaction conditions II.[a]
Entry Catalyst
t
[h]
9
Yield
Selectivity [%][d]
[%][b] [%][c]
9a
9b
9c
1
2
3
4
5
6
Rh2(OAc)4(IPr) 3
24
25
25
24
24
24
20
9
1
19
35
30
46
45
29
54
31
21
52
55
44
52
26
30
21
17
26
19
21
31
27
28
31
29
53
39
Rh2(OAc)4(ClIPr) 5
Rh2(OAc)4(ClIPr)2 7
Rh2(tfa)4(ClIPr)2 8
CuI
Table 5. Oxidation of representative benzylic substrates.[a]
RuCl3
[a] All reactions were carried out with 0.37 mmol of 9 and catalyzed
by 1 mol-% of catalyst under conditions II: 40 °C, 1 equiv. TBHP,
round-bottomed flask under aerobic conditions. [b] Yield of unre-
acted cyclohexene determined by GC analysis of reaction mixture.
[c] Yield of products 9a–c determined by GC analysis of reaction
mixture. [d] Selectivity determined by GC analysis of reaction mix-
ture.
Benzylic Oxidation
En-
try
Products [%][c]
Substrate
Cond. 11–14
Motivated by the allylic oxidation results, we decided to
extend the study to benzylic oxidation. Hence, fluorene (10)
was chosen as a model substrate for the optimization of the
reaction conditions (see Supporting Information). For this
substrate, both methods described in the previous section
provided high yields of fluorenone (Scheme 5). Interest-
ingly, 1 did not surpass 50% yield (even after 72 h) when
submitted to conditions II. This result highlights the
requirement of Doyle’s catalyst for a higher amount of
TBHP (5 equiv.).[12b] In this study, the formation of benzylic
fluorene peroxide was never detected.
[%][b]
a
b
c
d
1
2
3
4
5
tetraline 11
tetraline 11
indan 12
indan 12
diphenylmethane 13
I
II
I
II
I
26
20
19
13
39
29
47
16
22
Ͻ 1
12
7
1
4
Ͻ 1
47
Ͻ 1
4
48
(35[d]
37
)
6
diphenylmethane 13
II
53
4
(37[d]
)
7
8
9
ethylbenzene 14
ethylbenzene 14
ethylbenzene 14
I
II
III
65
68
33
34[d]
31[d]
65[d]
[a] All reactions were carried out with 0.37 mmol of substrate with
3; conditions I: room temperature, 2 equiv. TBHP in GC vial, ab-
sence of atmosphere; conditions II: 40 °C, 1 equiv. TBHP, round-
bottomed flask under aerobic conditions; conditions III: same as
conditions II except the total amount of TBHP added (3.5 equiv.
total: 1 equiv. initial, 1 equiv. after 6 h and 1.5 equiv. after 30 h).
[b] Yield of unreacted substrate 11–14 determined by GC analysis
of the reaction mixture. [c] Isolated yield after purification by pre-
parative thin layer chromatography. [d] Yield of product 13a and
14a determined by GC analysis of reaction mixture.
Scheme 5. Benzylic oxidation: Rh2(cap)4 1 vs. Rh2(OAc)4(IPr) 3.
Conclusions
These two protocols were extended to oxidation of repre-
sentative benzylic substrates tetraline (11), indan (12), di-
The dirhodium(II)–NHC complexes studied herein
phenylmethane (13) and ethylbenzene (14, Table 5). In the proved to be active catalysts in allylic and benzylic oxi-
case of 11, 1-tetralone (11a) and 1-tetralinyl tert-butyl per- dations. Two distinct experimental conditions were opti-
oxide (11b) formed with 29 and 22% yield, respectively mized: conditions I, in which 2 equiv. of oxidant are used
(conditions I, Table 5, Entry 1). Under conditions II, 11 at room temperature in the absence of air; and conditions
gave almost exclusively 11a (47% yield), which suggests that II, in which 1 equiv. of oxidant is applied at 40 °C in an
1476
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Eur. J. Org. Chem. 2013, 1471–1478

NHC Rhodium Complexes for Allylic and Benzylic Oxidations
was connected to a silver wire pseudoreference electrode. The solu-
tions were saturated with N2 by bubbling this gas through them
before each run, and the oxidation potentials of the complexes were
measured by CV in the presence of ferrocene as an internal stan-
dard. The redox potential values are initially quoted relative to the
saturated calomel electrode (SCE) by using the [Fe(η5-C5H5)2]0/+
aerobic atmosphere. It was found that the allylic oxidation
reaction was highly dependent on the reaction conditions
(base, oxidant, oxygen and temperature) and it was possible
to obtain a protocol with which the catalysts developed
herein provided a distinct catalytic behaviour compared
with Doyle’s catalyst. Furthermore, it was also observed
that the reactions run with Rh2(OAc)4(IPr) are less exother-
mic and liberate lower amounts of oxygen, which made
them potentially less hazardous than those run with
Rh2(cap)4.
ox
(E1/2 = 0.525 V vs. SCE) redox couple in a 0.2 m [Bu4N][BF4]/
CH2Cl2 solution.[24]
All calculations were performed by using the Gaussian 03 software
package.[32] Pure and hybrid DFT functionals were used without
symmetry constraints, as described in the Supporting Information.
It was confirmed that the first axial coordination in rho-
dium(II) complexes lowers the oxidation potential of the
first oxidation wave of these complexes, which enhances
their catalytic ability towards the allylic and benzylic oxi-
dation reactions. Their good catalytic activity is consistent
General Procedure for Oxidation Reactions under Conditions I: A
1.5 mL vial equipped with a stirring bar under an argon atmo-
sphere was charged with catalyst (1 mol-%), base (0.183 mmol,
50 mol-%), dodecane (0.176 mmol, 48 mol-%), substrate
(0.365 mmol, 100 mol-%) and anhydrous dichloromethane (1 mL).
A sample (1 μL) was analyzed by GC (t = 0). The reaction was
started by adding TBHP (0.760 mmol, 2 equiv.) to the mixture in
two portions by syringe and more dichloromethane (0.5 mL) was
added to fill the vial to the top. The vial was then closed. The
reaction progress was followed by GC. After 72 h, the products
were isolated by TLC of the crude reaction mixture (hexanes/
AcOEt).
5+
with the reversible Rh24+/Rh2 oxidation waves observed
by cyclic voltammetry. The coordination of a second axial
ligand resulted in a further marked decrease of the oxi-
dation potential, which allowed the observation of two re-
versible oxidations in the cyclic voltammetric experiments.
A comparison of the effects of the axial coordination of
various ligands on the oxidation potential indicates that the
NHC (IPr) behaves as a much stronger electron donor to
General Procedure for Oxidation Reactions under Conditions I: A
5 mL round-bottomed flask equipped with a stirring bar filled with
air was charged with catalyst (1 mol-%), base (0.183 mmol, 50 mol-
%), dodecane (0.176 mmol, 48 mol-%), substrate (0.365 mmol,
100 mol-%) and anhydrous dichloromethane (1.5 mL). A sample
(1 μL) was analyzed by GC (t = 0). The reaction was started by
adding TBHP (0.365 mmol, 1 equiv.) to the mixture in one portion
by syringe. The flask was then equipped with a condenser and
warmed to 40 °C in an oil bath. The reaction progress was followed
by GC. After 72 h, the products were isolated by TLC of the crude
reaction mixture (hexanes/AcOEt).
4+
the Rh2 centre than pyridine, acetonitrile or isocyanide.
Experimental Section
Materials and Methods: Dirhodium(II) tetraacetate, tert-butyl hy-
droperoxide (5–6 m solution in decane), dodecane, potassium car-
bonate and sodium hydrogen carbonate were obtained from Ald-
rich. Substrates were obtained from commercial sources. Anhy-
drous dichloromethane was freshly distilled from calcium hydride
prior to use. Potassium carbonate was crushed and stored in desic-
cators. Dirhodium(II) tetraacetate was used as obtained. Di-
rhodium(II) caprolactamate was prepared by following the method
described by Doyle.[31] Rh2(OAc)4(IPr), Rh2(OAc)4(SIPr) and
Rh2(OAc)4(IPr)2 were prepared according to published work from
this laboratory.[17d] Rh2(OAc)4(ClPr), Rh2(tfa)4(IPr) and Rh2-
(OAc)4(ClIPr)2 complexes were also prepared according to pub-
lished work from this laboratory.[17e] Preparative thin-layer
chromatography plates were prepared with silica gel 60 GF254
Merck (ref. 1.07730.1000). NMR spectra were recorded at room
temperature with a Bruker AMX 300 spectrometer with CDCl3 as
solvent and (CH3)4Si as internal standard. GC analysis was per-
formed with a Shimadzu 2014 gas chromatograph equipped with a
Restek RTX-5 column (7 mϫ0.32 mmϫ0.25 μm); injector:
250 °C; detector (FID): 270 °C; split ratio = 10, column flow (He):
1 mL/min; oven 35 °C for 1 min, ramp 50 °C/min to 100 °C and
100 °C for 4min. Dodecane was used as internal standard. The elec-
trochemical experiments were carried out with an EG&G PAR
273A potentiostat/galvanostat connected to a personal computer
through a general purpose interface bus (GPIB) interface. Cyclic
voltammetry (CV) studies were undertaken with a two-compart-
ment three-electrode cell, with platinum disk working (d = 0.5 mm)
and counter electrodes. A Luggin capillary connected to a silver-
wire pseudoreference electrode was used to control the working
electrode potential. Controlled potential electrolysis (CPE) was car-
ried out with a two-compartment three-electrode cell with platinum
gauze working and counter electrodes in compartments separated
by a glass frit; a Luggin capillary, probing the working electrode,
General Procedure for Oxidation Reactions under Conditions III:
Same procedure as for conditions II, except the total amount of
TBHP added during the reaction [1.28 mmol, 3.5 equiv. total:
1 equiv. (initial) + 1 equiv. (6 h) + 1.5 equiv. (30 h)].
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, characterization data, computa-
tional details with the corresponding reference list and including a
full account of the DFT functionals screened, and tables with the
atomic coordinates of all optimized species.
Acknowledgments
The authors thank Fundação do Ministério de Ciência e Tecnolo-
gia (FCT) (POCI 2010), Fundo Europeu de Desenvolvimento Re-
gional (FEDER) (grant numbers SFRH/BD/73971/2010, SFRH/
BPD/73932/2010, SFRH/BPD/70953/2010, PTDC/QUI-QUI/
099389/2008, PTDC/QUI-QUI/101187/2008, PTDC/QUI-QUI/
102150/2008, Pest-OE/QUI/UI0100/2011, PEst-OE/SAU/UI4013/
2011, REDE/1518/REM/2005) and Santander-UTL for financial
support. The authors thank the Portuguese NMR Network (IST-
UTL Center) for providing access to the NMR facility.
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Received: October 1, 2012
Published Online: February 18, 2013
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Eur. J. Org. Chem. 2013, 1471–1478