Novel reactivity of N-bridged diiron phthalocyanine in the activation of C-H bonds: Hydroacylation of olefins as an example of the efficient formation of C-C bonds

Article abstract of DOI:10.1002/chem.201100650

Bridge over troubled iron: An N-bridged diiron tetra-tert- butylphthalocyanine complex, usually employed for oxidation reactions, also catalyzes the addition of acetaldehyde to olefins (see scheme) to provide methylketones with a high selectivity (up to 92%) and high turnover numbers (3600-5700). Copyright

Full text of DOI:10.1002/chem.201100650

DOI: 10.1002/chem.201100650
Novel Reactivity of N-Bridged Diiron Phthalocyanine in the Activation of
ꢀ
C H Bonds: Hydroacylation of Olefins as an Example of the Efficient
ꢀ
Formation of C C Bonds
Leonardo X. Alvarez, Evgeny V. Kudrik, and Alexander B. Sorokin*[a]
Iron phthalocyanine complexes are versatile catalysts, in
particular for selective oxidations and bleaching reactions,[1]
and they are readily available in terms of cost and accessibil-
ity at a large scale.[2] Our research group has recently intro-
duced N-bridged diiron phthalocyanines as efficient catalysts
for oxidation.[3] Preparation and spectroscopic studies of m-
nitrido bridged bimetallic complexes with phthalocyanine
and porphyrin ligands have been described in the litera-
the generation of acyl radicals which attack the double bond
of olefins.[11] However, this method is the most successful
with electron-poor olefins as substrates.[11]
There is an increasing demand for more general and sus-
tainable approaches avoiding the use of organic solvents
and large amounts of toxic catalysts.[11d,e] In addition, the re-
placement of expensive noble metal catalysts with more
available and nontoxic catalysts is highly desirable. In this
context, iron catalysts are especially attractive. Herein we
report on the addition of acetaldehyde to unactivated cyclic
or linear alkenes using low iron catalyst loading
(0.01 mol%) performed in the absence of organic solvents
(Scheme 1). This finding significantly increases the scope of
application of these emerging binuclear catalysts.
ture.[4] In particular, these complexes containing Fe
ACTHNUTRGENNU(G +3.5)ACHTUNGTERN(NUGN m-
N)FeACHTUNGTRENNUNG(+3.5) unit were efficient in the mild oxidation of
methane,[3a,b] benzene,[3c] and aromatic compounds.[3e] m-Oxo
diiron phthalocyanine complexes were also shown to be effi-
cient oxidation catalysts.[5] All these examples report on the
ꢀ
ꢀ
transformation of various C H bonds to C O and C=O
ꢀ
functionalities. The selective formation of C C bonds
ꢀ
through the activation of C H bonds is another challenging
goal in synthetic chemistry. In the course of our research we
have discovered an unexpected reactivity of m-nitrido-bis-
ACHTUNGTRENNUNG[tetra(tert-butyl)phthalocyaninatoiron] (1) which can also be
ꢀ
used as a catalyst for C C bond forming reactions. This
novel approach is particularly useful for the hydroacylation
of olefins, which is an important synthetic transformation
that results in 100% atom efficiency as the addition of an al-
dehyde to the double bond of an olefin affords a ketone
containing all the atoms of both substrates. Ketones are im-
portant organic compounds widely used in the chemical in-
dustry as final products and as key synthetic precursors.
These carbonyl compounds can be prepared by the oxida-
tion of secondary alcohols,[6] by the Friedel–Crafts acyla-
tion,[7] by the reaction of acyl derivatives with organo-metal-
lic reagents,[8] or by hydroacylation of alkenes or alkynes
with aldehydes.[9] The latter approach is mainly promoted by
rhodium or ruthenium complexes often applied in large
amounts (up to 10–30 mol%).[9a] This reaction has often
been limited by the tendency of the intermediate species to
lose carbon oxide.[10] An alternative approach is based on
Scheme 1. Hydroacylation of olefins by acetaldehyde in the presence of
N-bridged diiron tetra-tert-butylphthalocyanine catalyst 1. The structure
of one position isomer is depicted.
For the initial experiments we have chosen cyclohexene
and acetaldehyde as model substrates and tBuOOH as the
oxidant for generation of acyl radicals. It should be noted
that iron complexes in combination with peroxides usually
react with olefins to produce epoxidation and/or allylic oxi-
dation products. When this reaction was performed in an
inert atmosphere in the presence of 1, along with the expect-
ed products from the allylic oxidation of the alkene we have
obtained the ketone produced from the addition of the alde-
hyde to the olefinic double bond and the product of the oxi-
dative coupling of two cyclohexenyl moieties (Table 1). The
ratio of the formed products depended on the reaction con-
ditions. When stoichiometric amounts of acetaldehyde and
[a] Dr. L. X. Alvarez, Dr. E. V. Kudrik, Dr. A. B. Sorokin
Institut de Recherches sur la Catalyse et l’Environnement de Lyon
IRCELYON, UMR 5256
CNRS - Universitꢀ Lyon 1, 2, av. A. Einstein, 69626
Villeurbanne Cedex (France)
Fax : (+33)472-445-399
E-mail: alexander.sorokin@ircelyon.univ-lyon1.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100650.
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Chem. Eur. J. 2011, 17, 9298 – 9301

COMMUNICATION
Table 1. Hydroacylation of cyclohexene catalyzed by 1.[a]
Scheme 2. Products of cyclohexene oxidation in the absence of catalyst 1.
Entry
CH3CHO
[equiv]
Cyclohexene
conversion [%]
Product ratio
lation reactivity. When FeSO4·7H2O or monomer iron tetra-
tert-butylphthalocyanine were used instead of 1, a product
composition typical for usual radical oxidation with no for-
mation of addition product 2 was obtained (Scheme 2).
The scope of the 1-tBuOOH system was evaluated in the
functionalization of various olefins including linear sub-
strates and substrates bearing functional groups (Table 2).
G
2
3
4
1[b]
2
3
1.00
1.00
0.65
3.00
10.0
10.0
58
64
69
45
48
69
52
57
65
79
92
92
24
21
24
7
3
6
24
22
11
14
5
4
5
6[c]
2
[a] Reaction conditions: cyclohexene (10 mmol), acetaldehyde (10–
100 mmol), tBuOOH (1.5 mmol, 70% aqueous solution), catalyst
(1 mmol, 0.01 mol%), high-pressure reactor, Ar, 608C, 24 h. [b] Reaction
time: 8 h. [c] 0.1 mol% of catalyst 1.
1
Table 2. Hydroacylation of different olefins catalyzed by 1.[a]
Entry Substrate
Product
Conversion [%] Selectivity [%]
1[b]
69
92
cyclohexene were mixed and placed in presence of 1 (0.01
mol%) and tBuOOH (0.15 equiv; 70% aqueous solution)
with respect to olefin under argon at 608C, the formation of
products 2, 3, and 4 in the ratio of 2:1:1 was observed after
8 h of reaction (Table 1, entry 1). This ratio changes to
about 3:1:1 when the reaction time increases (Table 1,
entry 2). The similar selectivity was observed with
0.65 equivalents of CH3CHO (Table 1, entry 3). The ob-
served product composition suggests that the 1-tBuOOH
system reacts with both CH3CHO and cyclohexene to pro-
duce radical intermediates. To favor the preferential forma-
tion of the acetyl radical and its further addition to the
olefin, we performed the reaction using an excess of the al-
dehyde. When CH3CHO was used in 3-fold excess, the addi-
tion product 2 was obtained in 79% selectivity (Table 1,
entry 4). The further increase of CH3CHO amount to
10 equivalents led to 92% selectivity to ketone 2 although
the conversion of cyclohexene was 48% (Table 1, entry 5).
Noteworthy, a very high turnover number of 4420 was ach-
ieved under these conditions. The catalyst was soluble in the
organic medium (water content ca. 1%), thus allowing ho-
mogeneous reactions. The stability of the catalyst during re-
action was evaluated by UV/Vis spectroscopy (see the Sup-
porting Information). Remarkarbly, 91% and 80% of 1 was
still left in the reaction mixture after 6 h and 24 h, respec-
tively. With 0.1 mol% of catalyst loading the conversion was
raised to almost 70% keeping the high selectivity to 2
(Table 1, entry 6). In the absence of tBuOOH no formation
of any product was observed after 24 h. When the reaction
was performed without catalyst, the principal products were
the peroxide 3 and the dimer 4 along with small amounts of
cyclohexenone 5, cyclohexenol 6, and epoxide 7 (Scheme 2).
The similar product composition with no formation of 2 was
obtained when H2O2 was used as the oxidant. This implies
that interaction of 1 with H2O2 and with tBuOOH occurs ac-
cording to different mechanisms. Importantly, the presence
of N-bridged diiron complex 1 is essential for the hydroacy-
2[c]
3[d]
86
71
52
80
4
71
51
5
6
61
62
90
76
[a] Reaction conditions: olefin (10 mmol), acetaldehyde (100 mmol),
tBuOOH (1.5 mmol, 70% aqueous solution), catalyst 1 (1 mmol, 0.01 mol%),
high-pressure reactor, argon, 608C, 24 h. [b] 0.1 mol% catalyst loading.
[c] Isolated mixture consisting of a 95/5 mixture of anti-Markovnikov and
Markoknikov isomers. [d] Isolated mixture consisting of a 83/17 mixture of
the isomers.
The catalytic system was competent for hydroacylation of
both cyclic and linear alkenes (Table 2, entries 1 and 2). In
the case of octene-1 and 2-cyclohexen-1-one only the forma-
tion of the anti-Markovnikov product was observed. Using
allylbenzene and 2-propen-1-ol, anti-Markovnikov and Mar-
kovnikov products were obtained in 95:5 and 83:17 ratios,
respectively (Table 2, entries 3 and 6). This method can also
be applied to olefins containing easily oxidizable positions
like in allylbenzene, 2-allylphenol, and 2-cyclohexen-1-one
(Table 2, entries 3–5). In all cases neither benzylic nor allylic
oxidation products were detected. No oxidation of allylphe-
nol to corresponding quinone was observed. Olefins contain-
ing a readily oxidizable hydroxyl group were also hydroacy-
lated without the need of any protecting group as was evi-
denced in the case of allyl alcohol (Table 1, entry 6). Again,
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A. B. Sorokin et al.
ꢀ
very high turnover numbers for the formation of methyl ke-
tones have been achieved (3600–5700). Unreacted CH3CHO
left in the reaction mixture after reaction can easily be recy-
cled. The hydroacylation reaction was also performed in a
large scale using cyclohexene (0.1 mol, 8.2 g) and acetalde-
hyde (1 mol). The product 2 was isolated from the reaction
mixture by distillation with a 58% yield.
Both cyclohexene (bond dissociation energy of allylic C
H bond=81.6 kcalmolꢀ1)[13] and acetaldehyde (BDE of al-
ꢀ1 [10a]
ꢀ
C
can react with tBuO
dehyde C H bond=86.0 kcalmol )
to form allylic and acyl radicals, respectively. We propose
that FeIVNFeIV=O species should also be competent for re-
acting with these substrates to form radicals. To obtain a
high selectivity in hydroacylation reaction one should favor
the formation of the CH3C=O radical and limit the forma-
tion of the cyclohexenyl radical leading to product 4 and the
products of allylic oxidation. By increasing the aldehyde/
olefin ratio to 10:1 we favor the hydroacylation reaction.
The tentative mechanism for hydroacylation is proposed in
Scheme 4 (charges of the complexes are omitted for clarity).
Since the anti-Markovnikov addition to double bond is
typical for radical intermediates,[11] it is reasonable to sug-
gest the involvement of acyl radical in this reaction. The
composition of the side products is also in agreement with
this proposal. However, it should be noted that simple iron
salts and mononuclear iron phthalocyanine able to initiate a
radical chemistry were not active in the hydroacylation reac-
tion. To rationalize the remarkable catalytic activity of 1 in
hydroacylation we assume that the binuclear complex 1
reacts with tBuOOH to form the peroxo complex (1-
OOtBu)ꢀ. The formation of this complex was evidenced by
ESI-MS in our previous work.[3d] The kinetics of the forma-
tion of the peroxo complex was studied by UV/Vis spectros-
copy under pseudo-first order conditions in acetone at 258C.
The values of kobs were obtained from the single-exponential
fitting of the absorbance–time curves. The dependence of
kobs on [tBuOOH] showed saturation behavior, but could be
linearized in Lineweaver–Burk coordinates (see the Sup-
porting Information). The saturation kinetics suggests a two-
step process involving fast reversible binding of tBuOOH to
iron followed by the formation of the (1-OOtBu)ꢀ peroxo
complex. The values of the equilibrium constant K1 =904ꢁ
40mꢀ1 and the rate constant k1 =0.092ꢁ0.005 sꢀ1 were deter-
mined from the equation:
Scheme 4. Proposed mechanism for hydroacylation of olefins catalyzed
by complex 1.
Compound (1-OOtBu)ꢀ, formed from 1 and tBuOOH, un-
dergoes a homolytic O O bond cleavage to give two species
ꢀ
able to react with aldehyde with formation of acyl radical.
The acyl radical reacts with the olefin to form the addition
radical A. The hydrogen abstraction from the aldehyde by
the radical A to afford the hydroacylation product is ineffi-
cient.[11] We propose that this key step can be performed by
kobs ¼ k1K1 ½tBuOOHꢂ=ð1þK1 ½tBuOOHꢂÞ
FeIVNFeIII OH complex resulting from the reaction of
ꢀ
Similarly, we have proposed the formation of the hydro-
peroxo complex (1-OOH)ꢀ during activation of H2O2.[3a]
FeIVNFeIV=O with aldehyde. Thus, the hydroacylation prod-
uct is formed and the FeIVNFeIV=O species is regenerated
for further reaction with aldehyde. Consequently, diiron oxo
species formed from the binuclear complex 1 participate in
two key reactions: the generation of acyl radical from alde-
hyde and the formation of hydroacylation products from the
addition radical A (Scheme 4). This reaction sequence forms
a catalytic cycle with no need of other reagents. Note that
iron salts and mononuclear iron phthalocyanine able to ini-
tiate radical chemistry showed no hydroacylation activity.
Owing to this unique reactivity of 1, the tBuOOH oxidant
can be used in catalytic amounts (15 mol% to olefin) just to
initiate the process which provides 48–86% conversions of
olefins. In other words, tBuOOH is used for the formation
of FeIVNFeIV=O species which acts in the combination with
Several lines of spectroscopic and reactivity evidence indi-
ꢀ
ꢀ
cate a heterolytic cleavage of the O O bond in (1-OOH)
to form a high valent diiron oxo species.[3a,c,12] Because the
reactivities of the 1-H2O2 and 1-tBuOOH systems toward
cyclohexene in the presence of acetaldehyde are different,
the (1-OOtBu)ꢀ complex should not probably undergo the
ꢀ
heterolytic cleavage of the O O bond. Alternatively, the
ꢀ
ꢀ
O O bond in (1-OOtBu) can be cleaved in the homolytic
way to form tBuO· radical and [FeIVNFeIV=O]ꢀ species.
These mechanistic considerations are shown in Scheme 3.
FeIVNFeIII OH to perform two steps of the catalytic cycle.
The proposed mechanism is also consistent with the small
catalyst loading sufficient for the reaction.
ꢀ
In summary, we have discovered that N-bridged diiron
phthalocyanine, which is a powerful oxidation catalyst,[3] is
ꢀ
Scheme 3. Proposed mechanisms for activation of peroxides by 1.
also capable of performing an efficient C C bond forma-
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ꢀ
Diiron Phthalocyanine in the Activation of C H Bonds
COMMUNICATION
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tion. This finding allowed the development of a simple and
practical method for the synthesis of methylketones from
olefins and acetaldehyde in the presence of a readily avail-
able and nontoxic iron catalyst. This protocol that shows
100% atom efficiency is suitable for the hydroacylation of
linear and cyclic aliphatic olefins, alkenyl substituted aro-
matic compounds, and olefins containing readily oxidizable
functions under neat conditions. The access to functionalized
methylketones without the need of protecting/deprotecting
procedures increases the synthetic utility of this method. In
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activity of the N-bridged diiron species a tentative mecha-
nism has been proposed. Thus, N-bridged diiron phthalocya-
nine complexes can be used not only for demanding oxida-
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Experimental Section
General procedure for oxidation reactions: A 20 mL Ace-glass high-pres-
sure reactor was charged with olefin (10 mmol), catalyst (1.6 mg, 0.01%
mol), and acetaldehyde (5.6 mL, 100 mmol) under argon. This hermeti-
cally closed reactor allows to carry out reactions in a safe way at 2–3 bars
without the escaping of low-boiling compounds. After mixing for 5 min
at room temperature, the reactions were started by the addition of a
70% aqueous solution of TBHP (200 mL, 1.5 mmol) and the vessel was
heated at 608C for 24 h under argon. The reaction products were identi-
fied by GC-MS and NMR techniques. The conversions and product
yields were determined by GC using authentic products and pentadecane
as internal standard. The products were isolated by distillation or by
column chromatography (silica, pentane eluent). The purity of isolated
1
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Acknowledgements
We are grateful to Agence National de Recherche (ANR, France, grant
ANR-08-BLANC-0183–01) for financial support of this research.
ꢀ
ꢀ
Keywords: C C coupling · C H activation · homogeneous
catalysis · iron · phthalocyanines
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Received: March 1, 2011
Revised: May 25, 2011
Published online: July 20, 2011
Chem. Eur. J. 2011, 17, 9298 – 9301
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www.chemeurj.org
9301