Iron and manganese catalysts for the selective functionalization of arene C(sp2)-H bonds by carbene insertion

Article abstract of DOI:10.1002/anie.201601750

The first examples of the direct functionalization of non-activated aryl sp² C-H bonds with ethyl diazoacetate (N₂CHCO₂Et) catalyzed by Mn- or Fe-based complexes in a completely selective manner are reported, with no formation of the frequently observed cycloheptatriene derivatives through competing Buchner reaction. The best catalysts are Fe^II or Mn^II complexes bearing the tetradentate pytacn ligand (pytacn= 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane). When using alkylbenzenes, the alkylic C(sp³)-H bonds of the substituents remained unmodified, thus the reaction being also selective toward functionalization of sp² C-H bonds. Exclusive catalysis: Iron- and-manganese-based catalysts selectively functionalize the C(sp²)-H bonds of benzene or alkylbenzenes through the formal insertion of the CHCO₂Et group from N₂CHCO₂Et (see scheme). When using alkylbenzenes, the alkylic C(sp³)-H bonds of the substituents remain unmodified.

Full text of DOI:10.1002/anie.201601750

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International Edition: DOI: 10.1002/anie.201601750
German Edition: DOI: 10.1002/ange.201601750
À
C H Bond Functionalization
Iron and Manganese Catalysts for the Selective Functionalization of
2
À
Arene C(sp ) H Bonds by Carbene Insertion
Ana Conde+, Gerard Sabenya+, Mònica Rodríguez+, Verònica Postils, Josep M. Luis,
M. Mar Díaz-Requejo,* Miquel Costas,* and Pedro J. PØrez*
In memory of Roberto Sµnchez-Delgado
Abstract: The first examples of the direct functionalization of
non-activated aryl sp2 C H bonds with ethyl diazoacetate
À
(N2CHCO2Et) catalyzed by Mn- or Fe-based complexes in
a completely selective manner are reported, with no formation
of the frequently observed cycloheptatriene derivatives through
competing Buchner reaction. The best catalysts are FeII or MnII
complexes bearing the tetradentate pytacn ligand (pytacn =
1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane).
3
À
When using alkylbenzenes, the alkylic C(sp ) H bonds of the
substituents remained unmodified, thus the reaction being also
2
À
selective toward functionalization of sp C H bonds.
T
he metal-catalyzed transfer of carbene units from diazo
compounds constitutes a powerful tool in organic synthesis. A
number of substrates ranging from olefins or alkynes to
alcohols or amines have been functionalized[1,2] upon respec-
Scheme 1. The catalytic functionalization of carbon–hydrogen bonds
with diazocompounds.
À
tive addition of the carbene moiety to the unsaturated C C
À
bond or the insertion of such unit into the X H bond.
adiene which spontaneously opens producing the cyclohepta-
triene. The opposite transformation, i.e. the retro-Buchner or
Echavarren reaction has been described.[7] A second favored
pathway involves the metal-catalyzed homocoupling of two
carbene units (Scheme 1D), a non-desired transformation
that is very often observed within these catalytic systems.[8]
The formal insertion[9] of a carbene group from a diazo
Carbon(sp3)-hydrogen bonds have also been modified by this
methodology (Scheme 1A), including not only those of the
less reactive CnH2n+2 alkanes,[3] but also of saturated poly-
olefins[4a] or even of the rather inert C H bonds of meth-
À
ane.[4b]
2
À
The use of this strategy onto the aromatic C(sp ) H bonds
À
of benzene (Scheme 1B) has not been developed that much.
This is the result of the existence of competing reactions when
exposing benzene to a diazo compound in the presence of
a metal-based catalyst. One of them is the so-called Buchner
reaction (Scheme 1C),[5,6] that occurs by formal addition of
compound in the C H bond of benzene was independently
reported by the groups of Schechter[10] and Livant,[11] using
Rh2(OAc)4 as the catalyst, at refluxing temperature, and with
the diazo compounds shown in Scheme 2. However, the use of
=
the carbene unit to the arene C C bond yielding a norcar-
[*] Dr. A. Conde,[+] Dr. M. M. Díaz-Requejo, Prof. Dr. P. J. PØrez
Laboratorio de Catµlisis HomogØnea, Unidad Asociada al CSIC,
CIQSO-Centro de Investigación en Química Sostenible, Departa-
mento de Química, Universidad de Huelva
Campus de El Carmen s/n, 21007 Huelva (Spain)
E-mail: mmdiaz@dqcm.uhu.es
perez@dqcm.uhu.es
G. Sabenya,[+] M. Rodríguez,[+] V. Postils, Dr. J. M. Luis, Dr. M. Costas
Institut de Química Computacional i Catàlisi (IQCC) and Departa-
ment de Química, Universitat de Girona
Campus Montilivi, 17071 Girona (Spain)
E-mail: miquel.costas@udg.edu
[+] These authors contributed equally to this work.
2
À
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201601750.
Scheme 2. The previous examples of the metal-catalyzed C(sp ) H
functionalization of benzene by carbene insertion from diazo com-
pounds and the novel Fe- and Mn-based catalytic systems.
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the commercially available as well as industrially relevant
ethyl diazoacetate for this reaction was not described until
2005 when one of our groups reported on the potential of
a gold-based catalyst for this transformation,[12] that gave
mixtures of the insertion (I) and addition (A) products, with
yields in ca. 3:1 ratio (Scheme 2). Further catalyst develop-
ment showed that other diazo compounds of types N2C-
(R)CO2Et (R = Me, SiMe3) could be employed;[13] in all cases
the same mixture of products was obtained in similar ratios.
The incorporation of substituents to the benzene rings favors
this reaction, with several examples being described using
phenols[14] or anilines.[15] Although not directly related to this
chemistry, it is worth mentioning the use of directing groups in
Rh/Ir-catalyzed transformations that involve the aromatic
[16]
À
C H functionalization at a given step.
In our search for more selective catalysts for direct
À
benzene C H bond functionalization by carbene insertion,
we have focused in complexes of FeII and MnII containing
polydentate amino-based ligands.[17] We have found that these
complexes, with the appropriate additive, induce the com-
pletely selective insertion of the C(H)CO2Et group into the
Scheme 3. Top: the probe reaction of benzene and ethyl diazoacetate
(EDA). Bottom: the complexes employed as catalysts in this work.
À
C H bonds of benzene and alkyl-benzenes with no formation
of cycloheptatriene byproducts (Scheme 2).
We first explored the potential of simple FeII and MnII
salts in the reaction of ethyl diazoacetate and benzene at 808C
(no reaction was observed at room temperature), with two
EDA and the formation of mixtures of the insertion (I) and
addition (A) products (Scheme 3). The MnII salt also pro-
moted the carbene transfer with low yields (entry 3) and EDA
was not completely consumed, but an unprecedented excel-
lent chemoselectivity for the insertion product was observed.
After these encouraging results we screened a series of FeII
and MnII complexes of general formula LMX2, where L
represents one of the polydentate ligands shown in Scheme 3,
and X = Cl or OTf. As inferred from entries 4–9, in experi-
ments carried out with (L1)FeX2 pre-catalysts, the amount of
F
equiv of NaBAr4 (BAr4F = tetrakis(bis-3,5-trifluoromethyl-
phenyl)borate) employed as Cl/OTf scavenger (Table 1,
entries 1 and 2), and dichloromethane as co-solvent. Interest-
ingly, the iron salts induced the consumption of the initial
Table 1: Catalytic behavior of complexes LMX2 in the reaction of benzene
and ethyl diazoacetate.[a]
Entry
L
M
X
equiv
NaBAr4
Yield
[%][b]
I:A[c,d]
F
NaBAr4 influenced the yield (a control experiment with
F
NaBAr4F as the sole additive did not induce any reaction) that
moved from 65 to 86% (EDA-based, see the Supporting
Information). On the other hand, the addition of just one
1
2
3
4
5
6
7
8
–
–
–
Fe
Fe
Mn
Fe
Fe
Fe
Fe
Fe
Fe
Mn
Fe
Mn
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Cl
2
2
2
2
2
4
4
8
8
8
8
8
8
8
8
8
8
8
8
8
55
52
20
65
69
73
75
86
83
76
77
73
47
85
18
57
67
68
70
n.r
42:58
OTf
OTf
Cl
OTf
Cl
69:31
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
–
equiv of NaBArF did not induce the reaction, most of the
4
L1
L1
L1
L1
L1
L1
L1
L2
L2
L3
L4
L5
L6
L7
L8
L9
L10
EDA remained unreacted after several hours. But the most
remarkable aspect observed is the unprecedented observation
of complete selectivity toward the insertion product as the
sole derivative formed from benzene with the series of Fe or
Mn tridentate or tetradentate ligand-containing catalyst
precursor (entries 4–19). In all cases, the products derived
from the formal carbene CHCO2Et homocoupling, diethyl
fumarate and maleate, were not formed in detectable
amounts. In addition, virtually identical product yields and
selectivities were obtained by using (L1)FeCl2 and (L1)Fe-
(OTf)2 precatalysts strongly suggesting that identical active
species are generated in both cases. Use of the Mn precatalyst
(L1)Mn(OTf)2 (entry 10) also produced the selective forma-
tion of the insertion product, but with a slightly lower yield.
Manipulation of the steric (entries 11 and 12) and electronic
(entries 13–15) properties of the pyridine ring in the catalysts
did not improve their activity. Use of other robust tetraden-
tate ligands (L8 and L9, entries 18 and 19) that have been
recently proven quite active in iron and manganese catalyzed
OTf
Cl
9
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
10
11
12
13
14
15
16
17
18
19
20
[a] Reactions carried out at 808C with 0.005 mmol of catalyst, n equiv of
NaBAr4F and 20 equiv of ethyl diazoacetate in 1.5 mL of benzene +
1.5 mL of CH2Cl2. Reaction time: 12 h. [b] Initial EDA converted into
insertion (I) + addition (A) products. Determined by GC; some ethyl
glycolate from adventitious water was detected as byproduct. [c] Deter-
mined by NMR.
[17e,f]
À
C H functionalization reactions
also produced good
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yields of the insertion product but still lower than (L1)Fe-
(OTf)2. At variance with those results, no reaction was
observed when an iron complex with a pentadentate ligand
(L10, entry 20)[17g] was used. To our knowledge, the degree of
selectivity into the insertion product obtained with this novel
catalytic system finds no precedent in the literature for the
reaction of benzene and EDA.
formal insertion has also been reported by Schwartz and co-
workers.[21]
With the aim of gaining mechanistic information, we have
performed a series of competition experiments with substi-
tuted benzenes with both the Fe- and Mn-containing catalysts
(L1)M(OTf)2 [Eq. (1)] from which, using Hammettꢀs equa-
The scope of the reaction has been extended to several
i
t
monoalkyl-substituted benzenes (R = Me, Et, Bu, Bu, Cy,
Figure 1a–d and j) using (L1)Fe(OTf)2 as the catalyst. In all
2
À
cases the exclusive functionalization of the C(sp ) H bond
was observed, with none of the products derived from the
insertion of the carbene group into the C(sp3)-H of the
substituent being detected (a certain amount of ethyl
glycolate from adventitious water was also obtained, see
Supporting Information). Di- (Figure 1 f) and tri-substituted
benzenes (Figure 1g,h) verified the same behavior. Therefore
our system is not only selective within benzene reactivity
3
À
(insertion over addition) but also relative to the C(sp ) H
bonds in the substituents, at variance with Wooꢀs iron-based
system where such C(sp3)-H functionalization was
observed.[18] Naphthalene also displayed functionalization
toward the insertion product, albeit this substrate has shown
a tendency to provide cyclopropanation derivatives.[19] The
related tetrahydronaphtalene was also modified in the
aromatic ring in an exclusive manner. Finally, the substrates
containing electron-withdrawing groups (Cl, CF3) disfavored
this transformation. It is worth pointing out that the acid-
catalyzed conversion of the insertion into addition products
described by McKervey and co-workers[20] has been discarded
on the basis of control experiments (see the Supporting
Information). The related acid-catalyzed intramolecular
tion, we have obtained respective values of 1 of À2.74(0.37)
and À2.81(0.15). These values can be associated with the
involvement of electrophilic metallocarbene intermediates. In
the aforementioned work from Woo and co-workers,[19] values
within the range À1.11 to À0.82 were found for the iron-based
catalysts. Additionally, kinetic isotopic experiments [Eq. (2)]
have also been carried out with equimolar amounts of
benzene and d6-benzene, an inverse isotopic effect being
F
observed with (L1)M(OTf)2 + 8 NaBAr4 : kH/kD was found as
0.95 Æ 0.01 for Fe and 0.80 Æ 0.02 for Mn.
The aforementioned feasible presence of iron-carbene
intermediates opens the question of the nature of the carbene
transfer, for which either concerted or step-wise pathways
could be envisaged with these metals. We have carried out an
experiment with the radical probe cyclopropylbenzene
[Eq. (3)], lacking the observance of any product derived
from the cyclopropane ring opening, that could assess that
a step-wise mechanism for the carbene transfer reaction
involving long-lived substrate carbon-radical intermediates
can be discarded. The relative selectivity of ortho, meta and
para derivatives obtained with the array of substrates tested
(Figure 1) also provides some information about the nature of
this transformation. The behavior of the mono-substituted
alkyl arenes in Figure 1a–d demonstrates that the steric effect
has only influence with the very bulky t-Bu substituent. For
trisubstituted substrates, this system is capable of accessing
À
the arene C H bond of 1,3,5-triethylbenzene (Figure 1h), at
variance with a recent work on arene C H borylation,[22] that
À
was unproductive for this substrate. Additionally, anisole gave
predominantly the ortho and para isomers, whereas the alkyl-
substituted benzenes R-C6H5 (R = Me, Et, iBu, tBu) were less
selective toward those positions. This performance resembles
to a certain extend that of an electrophilic aromatic sub-
stitution.
Figure 1. Functionalization of other arenes with (L1)Fe(OTf)2 + 8
NaBAr4F and EDA, showing isolated yields and o:m:p distribution of
products. The Cl and CF3 derivatives could only be detected in solution
by GC due to the low conversion.
Thus, any mechanistic explanation for the functionaliza-
À
tion of the arene C H bond must account for the following
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experimental facts: a) Extrusion of one Cl or OTf from
a subsequent 1,2-hydrogen shift. This would liberate the
complexes bearing two of these anions as ligands does not
provide an effective catalytic species; b) the complex
(L10)FeX2, bearing a pentacoordinate ligand and just one
OTf-coordinated ligand does not catalyze (Table 1, entry 20)
product and regenerate I1 to start the catalytic cycle. DFT
studies are currently underway to provide more insight into
this transformation and the selectivities observed.
In conclusion, we have found that the complexes (L1)M-
(OTf)2 (M = Fe, Mn) efficiently catalyze the reaction of
benzene and other alkyl-benzenes with ethyl diazoacetate,
with two yet undescribed features. These are the first
examples based on these metals for this transformation,
since the scarce precedents are based on rhodium or gold.
Second, but equally important, the catalytic system can be
tuned to avoid the formation of any of the products derived
from competing transformations: 1) the Buchner reaction
(ring expansion into cycloheptatriene); 2) the carbene
the reaction even with 8 equivalents of NaBArF ; c) an
4
electrophilic metal-carbene intermediate must exist as an
intermediate on the basis of the data from the Hammettꢀs
plot; d) competition reactions with benzene and d6-benzene
show an inverse isotopic effect, which should be related to
a sp2–sp3 hybridization in the vicinity of the transition state; e)
distribution of products show that steric effect is not high, and
thus coordination of the arene to the metal center does not
appear likely, with the pattern typical for an aromatic
electrophilic substitution being observed; and f) the use of
a carbon-radical trap bearing a cyclopropyl group did not
show the expected ring opening for long-lived, radical-
involving reactions.
homocoupling; and 3) when alkylbenzenes were employed,
3
À
the insertion into C(sp ) H bonds. Thus, this catalytic system
presents an exceptional selectivity favoring the carbene
2
À
insertion into the C(sp ) H bonds.
With the above pieces of information, we have built the
mechanistic proposal shown in Scheme 4. The dependence of
product yields with the amount of the halide scavenger, and
the lack of catalytic activity for (L10)Fe(OTf)2 (Table 1,
entry 20) suggests the formation of the dicationic species I1
that binds the diazo compound in a dihapto manner yielding
I2. While a diazo reagent can coordinate a metal center in
several ways, we have only represented that showing the
interaction of the carbon atom to the metal center since it is
the productive one regarding the formation of the metal-
locarbene species MC (other species might be in equilibria
with I2 but are not relevant at this stage). The coordination of
the carbonyl oxygen would explain that the arene does not
coordinate, in agreement with some of the above facts. The
arene would then react with the carbene ligand by means of
an outer sphere mechanism, leading to a Wheland-type
intermediate (I3). Such proposal is in good agreement with
the observation of the inverse isotopic effect as well as
a distribution of products typical for an electrophilic aromatic
substitution. This interaction resembles that proposed by
Doyle, Padwa and co-workers[23] for the rhodium-based
intramolecular catalytic formal insertion of a carbene group
Acknowledgements
Support for this work was provided by the MINECO
(CTQ2014-52769-C3-R-1, CTQ2014-62234-EXP, CTQ2015-
70795-P, CTQ2014-54306-P, and CTQ2014-52525P), and the
Junta de Andalucía (P10-FQM-06292). A.C. thanks Junta de
Andalucía for a research contract. M.C. acknowledges an
ICREA Academia Award, 2014 SGR 862 from Generalitat
de Catalunya, and ERC-239910.
À
Keywords: arenes · carbene · C H activation · iron · manganese
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6530–6534
Angew. Chem. 2016, 128, 6640–6644
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2
À
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Received: February 19, 2016
Published online: April 18, 2016
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