Alkyl Grignard cross-coupling of aryl phosphates catalyzed by new, highly active ionic iron(II) complexes containing a phosphine ligand and an imidazolium cation

Article abstract of DOI:10.1039/c6dt02995g

A novel family of ionic iron(ii) complexes of the general formula [HL][Fe(PR′₃)X₃] (HL = 1,3-bis(2,6-diisopropylphenyl)imidazolium cation, HIPr, R′ = Ph, X = Cl, 2; HL = HIPr, R′ = Cy, X = Cl, 3; HL = HIPr, R′ = Ph, X = Br, 4; HL = HIPr, R′ = Cy, X = Br, 5; HL = 1,3-bis(2,4,6-trimethylphenyl)imidazolium cation, HIMes, R′ = Cy, X = Br, 6) was easily prepared via a stepwise approach in 88%-92% yields. In addition, an ionic iron(ii) complex, [HIPr][Fe(C₄H₈O)Cl₃] (1), has been isolated from the reaction of FeCl₂(THF)_1.5 with one equiv. of [HIPr]Cl in 90% yield and it can further react with one equiv. of PPh₃ or PCy₃, affording the corresponding target iron(ii) complex 2 or 3, respectively. All these complexes were characterized by elemental analysis, electrospray ionization mass spectrometry (ESI-MS), ¹H NMR spectroscopy and X-ray crystallography. These air-insensitive complexes 2-6 showed high catalytic activities in the cross-coupling of aryl phosphates with primary and secondary alkyl Grignard reagents with a broad substrate scope, wherein [HIPr][Fe(PCy₃)Br₃] (5) was the most effective. Complex 5 also catalyzes the reductive cross-coupling of aryl phosphates with unactivated alkyl bromides in the presence of magnesium turnings and LiCl, as well as the corresponding one-pot acylation/cross-coupling sequence under mild conditions.

Full text of DOI:10.1039/c6dt02995g

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ARTICLE
Alkyl Grignard cross-coupling of aryl phosphates catalyzed by
new, highly active ionic iron(II) complexes containing a
phosphine ligand and an imidazolium cation
Received 00th January 20xx,
Accepted 00th January 20xx
Zhuang Li,a Ling Liu,a Hongꢀmei Sun,*a Qi Shena and Yong Zhanga
DOI: 10.1039/x0xx00000x
Abstract: A novel family of ionic iron(II) complexes of general formula [HL][Fe(PR’3)X3] (HL = 1,3ꢀbis(2,6ꢀ
diisopropylphenyl)imidazolium cation, HIPr, R’ = Ph, X = Cl, 2; HL = HIPr, R’ = Cy, X = Cl, 3; HL = HIPr, R’ = Ph, X =
Br, 4; HL = HIPr, R’ = Cy, X = Br, 5; HL = 1,3ꢀbis(2,4,6ꢀtrimethylphenyl)imidazolium cation, HIMes, R’ = Cy, X = Br, 6)
www.rsc.org/
were easily prepared via
a stepwise approach in 88%−92% yields. In addition, an ionic iron(II) complex,
[HIPr][Fe(C4H8O)Cl3] (1), has been isolated from the reaction of FeCl2(THF)1.5 with one equiv of [HIPr]Cl in 90% yield and
can further react with one equiv of PPh3 or PCy3, affording the corresponding target iron(II) complex 2 or 3, respectively.
All these complexes were characterized by elemental analysis, electrospray ionization mass spectrometry (ESIꢀMS), 1H
NMR spectroscopy and Xꢀray crystallography. These airꢀinsensitive complexes 2ꢀ6 showed high catalytic activities in the
crossꢀcoupling of aryl phosphates with primary and secondary alkyl Grignard reagents with broad substrate scope, wherein
[HIPr][Fe(PCy3)Br3] (5) is the most effective. Complex 5 also catalyzes the reductive crossꢀcoupling of aryl phosphates with
unactivated alkyl bromides in the presence of magnesium turnings and LiCl, as well as the corresponding oneꢀpot
acylation/crossꢀcoupling sequence under mild conditions.
In addition to the aboveꢀmentioned substrates, phosphates
represent another kind of attractive electrophile because they
are the most abundant esters in living organisms. Moreover,
Introduction
Transition metalꢀcatalyzed Grignard crossꢀcoupling reactions
are a powerful tool in organic synthesis to construct C–C
bonds.1 In the past decades, significant efforts have been made
to development more efficient catalytic systems as well as more
economic and ecoꢀfriendly coupling partners for this
transformation.
phosphates boast simple and inexpensive preparation, and are
more environmentally benign and stable than other kinds of
phenolic derivatives.5 In this context, alkenyl phosphates6aꢀb and
terminal conjugated dienyl phosphates6c have been found to
react readily with Grignard reagents in the presence of both
Fe(acac)3 and an appropriate additive. Pyrimidinꢀ2ꢀyl
phosphates have also been successfully applied to Grignard
coupling using a catalyst system composed of Fe(acac)3 and
biphosphine ligands.4f,6d However, aryl phosphates, which are
derived from unactivated phenols, remain challenging
substrates in ironꢀcatalyzed Grignard crossꢀcoupling reactions.7
Furthermore, only a few examples of other transition metalꢀ
catalyzed crossꢀcoupling reactions involving an aryl phosphate
have been reported,7,8 although nickelꢀcatalyzed Grignard
crossꢀcoupling of aryl phosphates has been known since the
beginning of crossꢀcoupling reactions.7a The main difficulty is
that aryl phosphates possess a less reactive aromatic CꢀO bond
compared with that of alkenyl phosphates.2a Therefore, an
alternative ironꢀbased catalyst is of great interest to address this
challenge.
Recently, ironꢀcatalyzed Grignard crossꢀcoupling using
phenolic derivatives as potential electrophiles has been
highlighted as an appealing reaction from a sustainable point of
view.2 This is not only ascribed to the fact that iron is one of the
most abundant, inexpensive, nonꢀtoxic, and environmentally
benign metals on earth,3 but also to its combination with C
−O
electrophiles, which possess many advantages compared with
the corresponding halides.2 In this context, several kinds of
C−
O
electrophiles, including aryl/alkenyl triflates,4aꢀd
aryl/alkenyl tosylates,4aꢀb,4f aryl/alkenyl pivalates,4e 2ꢀpyrone
derivatives,4i aryl carbamates,4g aryl sulfamates,4gꢀh and alkenyl
acetates,4j have been shown to work well, and generally
associated with the use of simple iron salts, i.e. FeCln and
Fe(acac)3, as the catalysts in the presence of appropriate ligands.
Based on the fact that bulky electronꢀdonating ligands, i.e.
,
7c,7e
PCy3
or
[HIPr]Cl
[1,3ꢀbis(2,6ꢀ
a.The Key Laboratory of Organic Synthesis of Jiangsu Province, College of
Chemistry, Chemical Engineering and Materials Science, Soochow University,
Suzhou 215123, People’s Republic of China. E-mail: sunhm@suda.edu.cn.
diisopropylphenyl)imidazolium chloride, which may free an
N
ꢀ
heterocyclic carbene (NHC) ligand IPr under basic
conditions],7d are usually required in nickelꢀbased catalytic
systems to overcome this issue,2a we decided to explore the
possibility of constructing highly efficient ironꢀbased catalysts
†
Electronic Supplementary Information (ESI) available: crystallographic data and
spectral data (1H NMR and 13C NMR). CCDC reference numbers 1485930, 1485924,
1485927, 1485926, 1485929, 1485931 for respectively. For ESI and
crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x.
1–6,
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by simultaneously employing two types of ligands. In fact, this complex
2, by contrast, is more tolerant to air, and can remain
strategy has already been successful in the design of palladiumꢀ unchanged for at least one hour in open air.
9
and nickel10ꢀbased catalysts for several crossꢀcoupling
Detailed examination revealed that complex 2 could be easily
reactions, offering the distinct features of the reversible synthesized by a stepwise approach in ca. 91% yield (Scheme 1,
dissociation of a labile phosphine ligand as well as the tight eqn 2). The notable advantage of the stepwise approach is the
binding of a robust NHC ligand to the central metal, which can needlessness to isolate and purify the monoimidazolium cationꢀ
aid both the oxidative addition and reductive elimination steps supported iron(II) complex
in the generally accepted mechanism as well as stabilizing the ligand leads to the formation of complex
1
. Similar protocol involving PCy3
as colorless crystals
3
metal center at different stages of the catalytic cycle.2a However, in ca. 92% yield. The exchange of the anion from Cl to Br also
its application in iron catalysis is still poorly explored,11c even works smoothly by the addition of excess amounts of NaBr
if the research on the chemistry of NHC complexes of iron has during the procedure (Scheme 1, eqn 3), affording the desired
received substantial attention over the past decade.11
Considering the available synthesis of NHC ligands by in 91%, 92% and 88%, respectively. The air tolerance of 3
ionic iron(II) complexes
4
−
6
as pale yellow crystals in yields of
is
2, which provides a great convenience
−
6
situ dehydrohalogenation of the corresponding imidazolium salt closely similar to that of
with various bases12 and our previous work on mixed for their handle and application in catalysis. However, the color
phosphine/imidazolium cation nickel(II) complexes,13 we report of their THF solutions changed slowly from colorless to yellow
here the facile synthesis of a new type of ionic iron(II) (for
complexes containing a phosphine ligand and an imidazolium expositing to open air.
cation ( ), and their catalytic potential in the crossꢀcoupling Further attempts to synthesize the corresponding neutral
of aryl phosphates with primary and secondary alkyl Grignard iron(II) complex containing both a phosphine ligand and an
2 and 3) or color deepened (for 4−6) in one hour upon
2ꢀ6
reagents and the corresponding reductive crossꢀcoupling.
NHC ligand by dehydrohalogenation of 4 or 5 with various
bases, such as nBuLi10aꢀb or KOtBu,10c were investigated.
Typically, the addition of one equivalent of base to a THF
Results and discussion
solution of
4 or 5 induced a color change to pale gray
immediately. Unfortunately, our attempts to isolate the target
Synthesis and characterization of ionic iron(II) complexes
iron(II) complex Fe(PPh3)(IPr)Br2 were unsuccessful, and only
14
pure ionic iron(II) complex [Fe(IPr)Br3](HIPr)
identified in this study.15
⋅C7H8 was
Figure 1. Molecular structure of
1 with thermal ellipsoids at the 30% probability
Scheme 1. Synthesis of ionic iron(II) complexes 1–6
level. Hydrogens and isopropyl groups are omitted for clarity. Selected bond
lengths (Å) and angles (°): Fe(1)ꢀO(1) 2.071(3), Fe(1)ꢀCl(1) 2.2857(13), Fe(1)ꢀ
Cl(2) 2.2774(12), Fe(1)ꢀCl(3) 2.2684(10); O(1)ꢀFe(1)ꢀCl(1) 101.89(10), O(1)ꢀ
Fe(1)ꢀCl(2) 100.38(8), O(1)ꢀFe(1)ꢀCl(3), 105.08(10), Cl(1)ꢀFe(1)ꢀCl(2) 114.61(4),
Cl(1)ꢀFe(1)ꢀCl(3) 113.21(5), Cl(2)ꢀFe(1)ꢀCl(3) 118.61(4).
The synthesis of target ionic iron(II) complexes
summarized in Scheme 1. According to published
procedure,14 FeCl2(THF)1.5 reacted smoothly with one
equivalent of 1,3ꢀbis(2,6ꢀdiisopropylphenyl)imidazolium
2–6 is
a
chloride ([HIPr]Cl) in THF at room temperature (Scheme 1,
eqn 1). Although both FeCl2(THF)1.5 and [HIPr]Cl were
insoluble in THF, the mixture changed from a suspended state
to a colorless transparent liquid when their THF suspensions
were mixed. After this workup, iron(II) complex
[HIPr][Fe(C4H8O)Cl3] (
ca. 93% yield. Subsequent reaction of
PPh3 occurred quickly in THF at room temperature, resulting in
the target ionic iron(II) complex [HIPr][Fe(PPh3)Cl3] ( ) as
colorless crystals in ca. 90% yield. Of note, complex is very
1
) was obtained as colorless crystals in
1
with one equivalent of
Figure 2. Molecular structure of
2 with thermal ellipsoids at the 30% probability
level. Hydrogens and isopropyl groups are omitted for clarity. Selected bond
lengths (Å) and angles (°): Fe(1)ꢀP(1) 2.4604(8), Fe(1)ꢀCl(1) 2.3020(10), Fe(1)ꢀ
Cl(2) 2.2692(9), Fe(1)ꢀCl(3) 2.2698(10); P(1)ꢀFe(1)ꢀCl(1) 102.33(4), P(1)ꢀFe(1)ꢀ
2
1
sensitive to air at room temperature in solid state, and its color Cl(2) 107.04(3), P(1)ꢀFe(1)ꢀCl(3) 119.50(4), Cl(1)ꢀFe(1)ꢀCl(2) 111.01(5), Cl(1)ꢀ
Fe(1)ꢀCl(3) 119.50(4), Cl(2)ꢀFe(1)ꢀCl(3) 114.97(5).
changed quickly from colorless to yellowish in air. Its derived
2 | Dalton Trans., 2012, 00, 1-3
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ARTICLE
These ionic iron(II) complexes
1−6 have been fully
characterized by elemental analysis, electrospray ionization
mass spectrometry (ESIꢀMS), 1H NMR spectroscopy and Xꢀray
crystallography. The positive ion ESIꢀMS of 1–6 were used to
establish the presence of the imidazolium cation, and in all
cases a peak with an intensity of almost 100% indicative of the
imidazolium cation was observed. The 1H NMR spectra of
complexes 1–6 exhibited characteristic resonances similar to
those of the corresponding imidazolium salts, except for the
slight differences and broadening in the chemical shifts,
together with one broad peak assigned to the signals of the
Figure 3. Molecular structure of
3 with thermal ellipsoids at the 30% probability
level. Hydrogens and isopropyl groups are omitted for clarity. Selected bond
lengths (Å) and angles (°): Fe(1)ꢀP(1) 2.457(2), Fe(1)ꢀCl(1) 2.301(2), Fe(1)ꢀCl(2)
2.2777(16), Fe(1)ꢀCl`(2) 2.2777(16); P(1)ꢀFe(1)ꢀCl(1) 100.58(8), P(1)ꢀFe(1)ꢀ
Cl(2) 110.33(5), P(1)ꢀFe(1)ꢀCl(2)` 110.33(5), Cl(1)ꢀFe(1)ꢀCl(2) 111.69(6), Cl(1)ꢀ
Fe(1)ꢀCl(2)` 111.69(6), Cl(2)ꢀFe(1)ꢀCl(2)` 111.70(10).
protons of tetrahydrofuran appeared at 4.22 ppm for
1 and
multiple broad and/or weak peaks assigned to the signals of the
protons of phosphine ligands appeared at ranges of 0.37–10.54
. The 13P NMR spectra of
ppm for 2–6 2–6 exhibited no signal
in CD3OCD3, indicating the direct bonding of the phosphine
ligand to the paramagnetic iron(II) center.16 The more direct
evidence of the formation of
1 and desired ionic iron(II)
complexes comes from Xꢀray structure determination.
2–6
The crystallographic and measurement data are listed in
Tables S1 and S2 (see ESI†). The molecular structures of these
Figure 4. Molecular structures of
4 with thermal ellipsoids at the 30% probability
level. Hydrogens and isopropyl groups are omitted for clarity. Selected bond
lengths (Å) and angles (°): Fe(1)ꢀP(1) 2.463(2), Fe(1)ꢀBr(1) 2.4327(16), Fe(1)ꢀ
Br(2) 2.3769(14), Fe(1)ꢀBr(3) 2.4082(14); P(1)ꢀFe(1)ꢀBr(1) 102.24(7), P(1)ꢀ
Fe(1)ꢀBr(2) 109.58(6), P(1)ꢀFe(1)ꢀBr(3) 99.57(6), Br(1)ꢀFe(1)ꢀBr(2) 111.27(6),
Br(1)ꢀFe(1)ꢀBr(3) 119.83(5), Br(2)ꢀFe(1)ꢀBr(3) 112.66(7).
complexes are shown in Figures
and angles.
1–6, along with key distances
To date, complex
ionic iron(II) complex bearing one imidazolium cation.17 As
shown in Figure 1, the solidꢀstate structure of consists of one
1 is the second structural characterized
1
[Fe(C4H8O)Cl3]− anion and one [HIPr]+ cation. The iron center
is coordinated by three Cl atoms and one O atom from solvated
THF molecule in a distorted tetrahedral geometry with angles at
iron in the range 100.38(8)–118.61(4)°. The FeꢀCl18 and FeꢀO14
bond lengths are very close to other ionic iron(II) complexes
reported in literatures. The mean value of the FeꢀCl bond length
is 2.277 Å. The bond distances and angles within the
imidazolium ring and side chain differed only slightly,
corresponding well with the related imidazolium salt [HIPr]Cl
reported previously.19c
Figure 5. Molecular structures of
5 with thermal ellipsoids at the 30% probability
level. Hydrogens and isopropyl groups are omitted for clarity. Selected bond
lengths (Å) and angles (°): Fe(1)ꢀP(1) 2.4566(14), Fe(1)ꢀBr(1) 2.4393(9), Fe(1)ꢀ
Br(2) 2.4062(10), Fe(1)ꢀBr(3) 2.4473(9); P(1)ꢀFe(1)ꢀBr(1) 108.31(4), P(1)ꢀFe(1)ꢀ
Br(2) 114.80(4), P(1)ꢀFe(1)ꢀBr(3) 97.54(4), Br(1)ꢀFe(1)ꢀBr(2) 110.67(3), Br(1)ꢀ
Fe(1)ꢀBr(3) 111.15(4), Br(2)ꢀFe(1)ꢀBr(3) 113.67(4).
For complexes
contains one [Fe(PR3)X3]− anion and one imidazolium [HL]+
cation. Similar to that of
, the structure of [HIPr]+ or [HIMes]+
2−6, each of the five molecular structures
1
cation do not change significantly on the reaction of the
corresponding imiazolium salt with simple iron(II) salt and the
phosphine ligand, and is also comparable to those of previously
reported imidazolium salts, i.e. [HIPr]Cl and [HIMes]Cl,19
respectively. In all five [Fe(PR3)X3]− anions, each iron atom is
coordinated by three halogen atoms and one phosphorus atom
in a distorted tetrahedral geometry with angles at iron in the
range 102.33(4)–119.50(4)° for
99.57(6) 119.83(5)° for , 97.54(4)
102.89(6) 114.05(4)° for . The Fe–Cl, Fe–Br, and Fe–P bond
lengths were found to lie in the ranges of 2.2692(9)–2.3020(10)
Å, 2.3769(14) 2.4473(9) Å, and 2.441(2) 2.463(2) Å,
respectively. The mean values of the Fe–Cl bond lengths are
2.280 Å for and 2.285 Å for , whereas the mean values of
the Fe–Br bond lengths are 2.406 Å, 2.431 Å and 2.427 Å for
, respectively. The Fe–P bond length of 2.457(2) Å in is
almost the same as that of 2.4566(14) Å in , even if they bear
2
, 100.58(8)
−
111.70(10)° for
3,
−
−
4
6
−
114.80(4)° for
5
, and
−
−
Figure 6. Molecular structures of
6 with thermal ellipsoids at the 30% probability
level. Hydrogens are omitted for clarity. Selected bond lengths (Å) and angles (°):
Fe(1)ꢀP(1) 2.441(2), Fe(1)ꢀBr(1) 2.4428(13), Fe(1)ꢀBr(2) 2.4272(11), Fe(1)ꢀBr(3)
2.4095(12); P(1)ꢀFe(1)ꢀBr(1) 108.65(6), P(1)ꢀFe(1)ꢀBr(2) 106.04(5), P(1)ꢀFe(1)ꢀ
Br(3) 102.89(6), Br(1)ꢀFe(1)ꢀBr(2) 112.16(5), Br(1)ꢀFe(1)ꢀBr(3) 112.28(5), Br(2)ꢀ
Fe(1)ꢀBr(3) 114.05(4).
2
3
4
−
6
3
5
different halide atoms. The similar trend in the variation of Fe–
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P bond lengths [2.4604(8) and 2.463(2) Å in
respectively] is observed in the structures of and
values are unexceptional and lie within the ranges found for was 80% when the loading of
other complexes containing [Fe(PR3)Cl3]− anion18aꢀd or other However, replacing nBuMgCl with nBu2Zn resulted in the yield
ionic iron(II) bromides.14,16f
of 3aa being less than 5%. In comparison, and
2
and
4,
electronꢀrich complex
5
, [HIPr][Fe(PCy3)Br3], is the most
2
4
. These effective, affording 3aa in 90% yield. Furthermore, the yield
was lowered to 5.0 mol %.
5
1
Nevertheless, a detailed structural analysis reveals that there [HIPr][Fe(C4H8O)Br3] showed moderate catalytic activity in
are some structural differences among these complexes. For the crossꢀcoupling, respectively furnishing yields of 3aa of
example, the complex bearing a PCy3 ligand has a slightly 65% (entry 5) and 70% (entry 6). The addition of PCy3 (10
longer FeꢀX bond and a slightly shorter Fe–P bond (compare
with , and with ), which mostly be consistent both with the reaction. A mixture of FeCl2/[HIPr]Cl/PCy3 afforded 3aa in a
larger steric bulkiness and the stronger
3
mol %) induced some acceleration of the crossꢀcoupling
2
5
4
σ
ꢀdonor ability of the 80% yield (entry 8). By the way, replacing FeCl2 with FeF2 led
PCy3 ligand relative to the PPh3 ligand.13b With respect to the to a lower yield of 65% for desired product (entry 9), even if
imidazolium cation, the complex containing a [HIPr]+ cation FeF2 had successfully been employed in Grignard crossꢀ
has a longer Fe
(compare with
[HIPr]+ cation relative to [HIMes]+ cation.19
As expected, in structures of and
the imidazolium cation and the iron(II)ꢀcontaining anion is held the combination of an electronꢀrich PCy3 ligand with a bulky
in place by extensive C ⋅⋅⋅X interactions (general in contact HIPr cation in an iron(II) complex makes the complex a more
below 3 Å),14,17 together with extensive C
between imidazolium cation and solvated THF molecule for
and . The shortest hydrogen bond of C ⋅⋅⋅X interaction
is found in , stemming from H(19) to Cl(1) (2.513 Å).
−
P bond as well as a slightly longer Fe
), mostly due to the larger steric bulkiness of complexes were Fe(PPh3)2Cl2 and Fe(PCy3)2Cl2 (entries 10 and
11). Similarly, simple iron salts FeCl2 and FeBr2 also provided
described above, 3aa in low yields (entries 12 and 13). These results suggest that
−
Br bond coupling reactions with NHC ligands.4h,21 The least effective
5
6
1
,
2
,
4
,
5
6
−
H
−H
⋅⋅⋅O interactions reactive catalyst in subsequent crossꢀcoupling,22 similar to the
1,
results obtained for palladiumꢀ9 and nickelꢀ10 based systems.
3,
5
6
−H
Table 2. Representative Aryl–Alkyl Couplingsa
2
O
5 (10 mol %)
Catalysis of ionic iron(II) complexes
+
Ar
R
(X = Br, Cl)
Ar OP(OEt)2
RMgX
THF
( )
( )
7
( )
5
Table 1. IronꢀCatalyzed CrossꢀCoupling of 1a with 2aa
3
3ab, 93% (87%)b
3ac, 86%
3aa, 80%
3ad, 80%
3ae, < 5%
( )
( )
3
( )
8
6 O
Ph
Catalyst
Conversion
Entry
Yield 3aa (%)
(10 mol %)
(%)
99
99
99
99
99
99
99
99
99
97
99
99
99
3aj, 40%c
3ag, 46%
3ah, 93%
3af, 50%
3ai, 65%
1
2
3
2
76
( )
( )
8
O
8
O
3
4
84
82
O
O
4
5
90(80)b(< 5%)c
3ak, 44%c
3al, 64%c
3am, 53%
3an, 40%
5
6
1
81
65(76)d
72(80)d
80
6
7
[HIPr][Fe(C4H8O)Br3]
FeCl2, [HIPr]Cl, PCy3
FeF2, [HIPr]Cl, PCy3
Fe(PPh3)2Cl2
Fe(PCy3)2Cl2
FeCl2
8e
9e
10
11
12
13
( )5
3bc, 92%
( )
7
3ao
30% (4.5:1)c,d
3bo
36% (3:1)c,d
65
3bb, 90%
3ap, < 5%
( )
21
( )
( )
5
5
5
( )
N
43
5
N
34
MeO
N
Ph
FeBr2
40
3ec, 78%
( )
3fc, 55%c
3dm, 72%
3cc, 75%
3dc, 75%
( )
a
Reaction conditions: 1ꢀnaphthyl diethyl phosphate (0.5 mmol), nBuMgCl (2
( )
5
( )
5
( )
5
5
5
mmol), THF (total volume: 6 mL), 85 °C, 8 h, GC yields using nꢀhexadecane as
internal standard; an average of two runs. b 5 (5 mol %). c Dibutylzinc (1 mmol). d
PCy3 (10 mol %) was added. e [HIPr]Cl (10 mol%) and PCy3 (10 mol %).
O
F3C
Ph
3gc, 30%c
O
CF3
3ic, 42%c
3hc, 52%c
( )
3kc, 0%c
3jc, 30%c
( )
O
The catalytic performances of complexes
investigated by reaction of 1a with ꢀbutylmagnesium chloride
2a. In general, quantitative transformation of 1a was observed
under the optimal conditions. As shown in Table 1, are
capable of catalyzing the alkylation of 1a, producing 3aa in
1–6 were then
5
5
n
3mc, 0%c
3lc, 0%c
3aq, 22%
2–6
a
Reaction conditions: 5 (10 mol %), aryl phosphates (0.5 mmol), RMgCl (2
mmol), THF (total volume: 6 mL), 85 °C, 8 h, isolated yields. b RMgBr (2 mmol).
c RMgCl (2.5 mmol). d Branched/linear ratio in parentheses.
76%
23%
−
−
90% yields along with naphthalene byproduct in
9% yields.20 Among these catalysts, the bulky and more
4 | Dalton Trans., 2012, 00, 1-3
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Table 2 presents the scope of this crossꢀcoupling reaction Grignard reagents afforded the corresponding desired products
using , as reflected by a number of representative aryl in 90% (3bb) and 92% (3bc) yields, respectively. Conversely, a
phosphates with diverse alkylation patterns. The nature of the lower yield of 36% was obtained with 2ꢀbuylmagnesium
nucleophile strongly influences the reaction. For example, 1a chloride (3bo). A strongly electronꢀdonating OMe group on the
coupled well with primary alkyl Grignard reagents with aryl ring of phosphate does not greatly affect the product yield
carbon atoms to give the desired products (3aa 3ad) in 3cc). Notably, 2ꢀpyridyl and 3ꢀpyridyl phosphates coupled
73% 93% yields. In contrast, methylmagnesium chloride failed efficiently with primary or secondary Grignard reagents under
to give the desired product 3ae, which possibly indicates the in standard reaction conditions to give the coupling products in
situ generation of lowꢀvalent iron species via 72% 78% yields (3dc 3dm and 3ec). In addition, 4ꢀbiphenyl
elimination.4b,4e In addition, the counteranion of the Grignard phosphate, a rare used C
O electrophile, also gave a moderate
5
1
≥
2
−
(
−
β
−
H
−
,
−
reagent also has some effect on the reaction. For example,
octylMgCl provided a slightly higher yield of 3ab than
n
n
ꢀ
ꢀ
yield of the desired product (3fc) by increasing the dosage of
Grignard reagent, meanwhile its orthoꢀanalogue gave a lower
octylMgBr. Notably, steric bulk at the
spacer in the Grignard reagents 3af and 3ag was tolerated. reactive nonꢀfused phenyl substrates, such as 4ꢀ
Functional groups in the Grignard reagents such as phenyl trifluoromethylphenyl, 3ꢀtrifluoromethylphenyl, and 3ꢀ
3ah), vinyl (3ai), ether (3aj), and ketal (3ak) survived in the methoxyphenyl phosphates, are capable of giving the coupling
reaction. In addition, a furanꢀcontaining Grignard reagent gave products in 35% 52% yields (3hc 3ic and 3jc). However, nonꢀ
3al in a yield of 64%. Secondary cyclic Grignard reagents, i.e. fused phenyl phosphates possessing electronꢀrich substituents at
cyclohexyl and cyclopentylmagnesium chloride, coupled either orthoꢀ or paraꢀposition (3kc 3lc and 3mc) were inert in
βꢀposition of the alkyl yield of 30% (3gc). Under the same reaction conditions, less
(
−
,
,
,
somewhat slowly to give the corresponding coupling products this study, which are quite similar to those reported previously
in 53% (3am) and 40% (3an) yield. In comparison, 2ꢀ for a Fe(acac)3ꢀcatalyzed aryl tosylatesꢀinvolved approach.4a
butylmagnesium chloride provided the desired 3ao in a lower The crossꢀcoupling of 3a with 4ꢀmethoxyphenylmagnesium
yield of 30% with a moderate branch selectivity. Nevertheless, chloride provided the desired 3aq in a low yield of 22%.
these latter three results reveal the potential of
5
in C−
C bond
Recently, ironꢀcatalyzed reductive crossꢀcoupling reactions
formation via the crossꢀcoupling between secondary Csp3 between two electrophiles have received increasing attention as
organometallic species and aryl Csp2 centers, which is still a an attractive alternative strategy to traditional Grignard
challenging research topic in transition metalꢀcatalyzed crossꢀ reactions.24 This new reaction pattern features the in situ
coupling reactions.23 However, the attempt to extend the generation of Grignard reagents, and thus exhibits lower hazard
substrate to an isopropyl Grignard reagent was unsuccessful potential, better operational simplicity and cost savings
(
3ap), as Shi et al 4e reported. To date, only two papers have compared with conventional Grignard reactions.24a In this
.
described the use of isopropylmagnesium chloride4g or context, von Wangelin and coꢀworkers achieved the reductive
bromide6d in ironꢀcatalyzed crossꢀcouplings with CꢀO crossꢀcoupling reaction of allyl phosphates and aryl bromides
electrophiles.
with a FeCl3/TMEDA/Mg/LiCl system 24b
. However, the related
crossꢀcoupling of aryl phosphates and alkyl halides has not
been explored. Thus, we decided to investigate the possibility
Table 3. Reductive CrossꢀCouplings of Aryl Phosphates with Alkyl Bromidesa
of extension of the crossꢀcoupling of
1
and
2 to a reductive
version. To our delight, iron(II) complex
5
is also capable of
catalyzing the reductive crossꢀcoupling of aryl phosphates with
alkyl bromides in the presence of stoichiometric Mg and LiCl.25
As seen from Table 3, the desired products were obtained in
yields very close to those achieved from the traditional
reactions (see Table 2). Nevertheless, it is worth noting that
alkyl chlorides showed low activity toward the reductive crossꢀ
coupling (3ac), possibly because of their relatively low activity
in the initiation stage of Grignard reagent formation compared
with alkyl bromides.
a Reaction conditions: 5 (10 mol %), aryl phosphates (0.5 mmol), RBr (2.5 mmol),
magnesium turnings (2.6 mmol), LiCl (2.6 mmol), isolated yields. RCl (2.5
mmol). c RBr (3.0 mmol).
Scheme 2. OneꢀPot Acylation/CrossꢀCoupling Sequence
b
Scheme 2 presents another powerful version of the crossꢀ
coupling reaction described herein. Because the pivalylation of
phenolic substrates typically proceeds quantitatively with
minimal byproduct formation,26 a oneꢀpot acylation/crossꢀ
The scope of the methodology also includes a variety of aryl
phosphates that are compatible with the crossꢀcoupling reaction.
For 2ꢀnaphthyl phosphate, the reaction with nꢀoctyl and nꢀhexyl
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coupling sequence from 1ꢀnaphthol was successful, affording homogeneous colorless solution. The reaction solution was
the desired 3aa in 73% yield.
filtered, and evaporated to dryness. The residue was washed
with hexane (3 × 10.0 mL), extracted with THF (3 × 10.0 mL),
and crystallized from concentrated THF at 0 °C. The product
was precipitated as colorless crystals in a yield of 93% (1.04 g),
1
Conclusions
mp 170 °C. Anal. calcd for C31H45Cl3FeN2O: C, 59.68; H, 7.27;
In summary, we synthesized a new type of ionic iron(II)
1
N, 4.49. Found: C, 59.66; H, 7.30; N, 4.45. H NMR (
δ
, 400
complex containing both
a phosphine ligand and an
MHz, (CD3)2CO): 1.24 (s, 24H, CH(CH3)2), 2.55 (s, 4H,
CH(CH3)2), 4.22 (br s, O(CH2CH2)2), 7.47 (s, 4H, ꢀC6H3),
7.62 (s, 2H, ꢀC6H3), 8.18 (s, 2H, NCHCHN), 9.66 (s, 1H,
NCHN). MS (ESI+): m/z 389.2954 [C27H37N2]+ (100%).
[HIPr][Fe(PPh3)Cl3] ). Procedure A: a Schlenk flask was
charged with [HIPr][Fe(C4H8O)Cl3] (0.12 g, 1.93 mmol), THF
(40.0 mL), and a stirring bar. To the colorless solution was then
added PPh3 (0.510 g, 1.93 mmol). After stirred for 2 h at room
temperature, the reaction solution was filtered, and evaporated
to dryness. The residue was washed with hexane (3 × 10.0 mL),
extracted with THF (3 × 10.0 mL), and crystallized from
concentrated THF at 0 °C. The product was precipitated as
imidazolium cation. We demonstrated their great catalytic
potential in the crossꢀcoupling of aryl phosphates with alkyl
Grignard reagents, as well as in both the corresponding
reductive crossꢀcoupling and the oneꢀpot acylation/crossꢀ
coupling sequence. Thus, this work disclosed the great potential
to utilize an aryl phosphate as an electrophile in ironꢀcatalyzed
CꢀC bond formation. Since a variety of phosphine ligands as
well as imidazolium cations are available, this work also
provides an attractively alternative strategy for building highly
active and, yet somewhat robust catalysts of iron. Investigations
aimed at further fineꢀtuning their catalytic activity by surveying
wellꢀmatched pairs of phosphine ligands and imidazolium
cations, probing mechanistic aspects of these findings, and
m
p
(
2
colorless crystals in a yield of 90% (1.42 g), mp 185
°C. Anal.
calcd for C45H52Cl3FeN2P: C, 66.39; H, 6.44; N, 3.44. Found:
synthesizing the corresponding mixed phosphine/
heterocyclic carbene iron(II) complexes are on going.
Nꢀ
1
C, 66.37; H, 6.40; N, 3.41. H NMR (
1.22 (s, 24H, CH(CH3)2), 2.54 (s, 4H, CH(CH3)2), 5.33 (br s,
P(C6H5)3), 6.66 (br s, P(C6H5)3), 7.44 (s, 4H, ꢀC6H3), 7.58 (s,
δ, 400 MHz, (CD3)2CO):
m
2H, pꢀC6H3), 8.14 (s, 2H, NCHCHN), 9.27 (br s, P(C6H5)3),
Experimental section
9.65 (s, 1H, NCHN). MS (ESI+): m/z 389.2954 [C27H37N2]+
(100%).
General procedures
All manipulations were performed under pure argon with
exclusion of air and moisture using standard Schlenk
techniques. Solvents were distilled from Na/benzophenone
ketyl under pure argon prior to use. All the substrates, reagents
and materials were purchased from Sigma Aldrich, Alfa Aesar
Procedure B: a Schlenk flask was charged with [HIPr]Cl
(0.82 g, 1.93 mmol), THF (40 mL), and a stirring bar. To this
suspension solution was added a THF (10 mL) suspension of
FeCl2(THF)1.5 (0.45 g, 1.93 mmol). The reaction mixture was
stirred for 4 h at room temperature. To the resulting colorless
solution was then added PPh3 (0.510 g, 1.93 mmol). The
reaction mixture was stirred for 2 h at room temperature. After
workup, the product was precipitated as colorless crystals in a
yield of 91% (1.43 g).
and
imidazolium
trimethylphenyl)imidazolium
TCI
Chemicals.
chloride
1,3ꢀBis(2,6ꢀdiisopropylphenyl)ꢀ
([HIPr]Cl),19c
1,3ꢀbis(2,4,6ꢀ
chloride
([HIMes]Cl),19c
[HIPr][Fe(C4H8O)Br3],14 Fe(PPh3)2Cl2 and Fe(PCy3)2Cl2
were prepared by published methods. Elemental analysis was
performed by direct combustion on a CarloꢀErba EAꢀ1110
instrument. NMR spectra were measured on a Varian Unity
INOVA 400 or VNMRS 300 MHz spectrometer at 25 °C. The
melt points were determined on a Diamond DSC (Perkin
Elmer) using powder samples under N2 atmosphere (50
27
28
[HIPr][Fe(PCy3)Cl3]
the procedure B of except that PCy3 was used instead of PPh3,
the product was precipitated as colorless crystals in a yield of
92% (1.45 g), mp 172 C. Anal. calcd for C45H70Cl3FeN2P: C,
64.94; H, 8.48; N, 3.37. Found: C, 64.93; H, 8.46; N, 3.36. H
NMR ( , 400 MHz, (CD3)2CO): 0.37 (br s, P(C6H11)3), 0.95–
(3). Following a procedure similar to
2
3
°
1
δ
mL/min). The system was heated from 50 to 350 °C at 20 °C
/min. Gas chromatographic (GC) analysis was performed on a
Varian CPꢀ3800 instruments equipped with an FID detector and
a capillary column AT.OVꢀ101 (30 m × 0.32 mm i.d., 0.10 ꢀm
film). The oven temperature was held at 80 °C for 2 min,
increased to 280 °C at 10 °C/min, and held for 2 min. High
resolution mass spectra were obtained using GCTꢀTOF
instrument with ESI or CI source.
0.97 (m, P(C6H11)3), 1.34 (s, 24H, CH(CH3)2), 1.60 (br s,
P(C6H11)3), 2.73 (s, 4H, CH(CH3)2), 3.95 (br s, P(C6H11)3), 4.16
(s, P(C6H11)3), 5.65 (br s, P(C6H11)3), 7.53 (s, 4H,
7.63 (s, 2H, ꢀC6H3), 8.47 (s, 2H, NCHCHN), 9.79 (s, 1H,
NCHN). MS (ESI+): m/z 389.2966 [C27H37N2]+ (100%).
[HIPr][Fe(PPh3)Br3] ). A Schlenk flask was charged
mꢀC6H3),
p
(
4
with [HIPr]Cl (0.848 g, 2.03 mmol), THF (40 mL), NaBr
(0.627 g, 6.09 mmol) and a stirring bar. To this suspension was
added a THF (10 mL) solution of FeBr2 (0.439 g, 2.03 mmol).
The reaction mixture was stirred for 12 h at the reflux
temperature of THF. To this cooled suspension was then added
PPh3 (0.532 g, 2.03 mmol). After stirred for 2 h at room
temperature, the reaction solution was filtered, and evaporated
to dryness. The residue was washed with hexane (3 × 10.0 mL),
extracted with THF (3 × 10.0 mL), and crystallized from
[HIPr][Fe(C4H8O)Cl3]
(1). Following a procedure similar
to the synthetic procedure of [HIPr][Fe(C4H8O)Br3]14,
a
Schlenk flask was charged with [HIPr]Cl (0.76 g, 1.80 mmol),
THF (40.0 mL), and a stirring bar. To this suspension was
added a THF (10 mL) suspension of FeCl2(THF)1.5 (0.42 g,
1.80 mmol). The reaction mixture was stirred for 4 h at room
temperature, during which time the mixture became
a
6 | Dalton Trans., 2012, 00, 1-3
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concentrated THF at 0 °C. The product was precipitated as pale bath. To this stirred mixture, alkyl bromides (2.5 mmol) were
yellow crystals in a yield of 91% (1.74 g), mp 193 C. Anal. added at 0 °C via syringe. The reaction mixture was then stirred
calcd for C45H52Br3FeN2P: C, 57.05; H, 5.53; N, 2.96. Found: for 10 h at 25 °C. The reaction was quenched by addition of
°
1
C, 57.03; H, 5.50; N, 2.94. H NMR (
1.29–1.35 (m, 24H, CH(CH3)2), 2.64 (s, 4H, CH(CH3)2), 4.38 extracted with ethyl acetate (3 × 3.0 mL) and dried over
(br s, P(C6H5)3), 7.12 (br s, P(C6H5)3), 7.52 (s, 4H, ꢀC6H3), Na2SO4. The GC yield of the desired product is determined by
7.66 (s, 2H, ꢀC6H3), 8.29 (s, 2H, NCHCHN), 9.73 (s, 1H, GC analysis, using ꢀhexadecane as an internal standard.
δ, 400 MHz, (CD3)2CO): saturated ammonium chloride solution, the mixture was
m
p
n
NCHN), 10.54 (br s, P(C6H5)3). MS (ESI+): m/z 389.2959 Purification of the crude mixture by flash column
[C27H37N2]+ (100%).
chromatography using petroleum ether (60ꢀ90 °C) as eluent
[HIPr][Fe(PCy3)Br3]
(
5
). Following a procedure similar to gave the isolated yield of desired coupling product. The identity
that of
was precipitated as pale yellow crystals in a yield of 92%
(2.21 g), mp 180 C. Anal. calcd for C45H70Br3FeN2P: C, 55.97; General Procedure for the One-Pot Acylation/Cross-Coupling
H, 7.31; N, 2.90. Found: C, 55.95; H, 7.30; N, 2.89. H NMR Sequence.
, 400 MHz, (CD3)2CO): 0.46 (br s, P(C6H11)3), 1.32–1.36 (m,
4 except that PCy3 was used instead of PPh3, the product of the product was confirmed by NMR spectroscopy and TLC.
5
°
1
(
δ
A Schlenk flask was charged with 1ꢀnaphthol (0.07 g, 0.5
mmol), THF (4 mL) and a stirring bar. To the solution was
added dropwise diethyl chlorophosphate (108 ꢁL, 0.75 mmol)
at 0 °C via syringe with stirring. The mixture was stirred at 0
°C for 2 min. To this stirred mixture was then added dropwise
triethylamine (112 ꢁL, 0.8 mmol) at 0 °C via syringe. The
reaction mixture was stirred for 2 h in an oil bath at 65 °C.
24H, CH(CH3)2), 1.70 (br s, P(C6H11)3), 1.86 (br s, P(C6H11)3),
2.77 (m, 4H, CH(CH3)2), 3.76 (s, P(C6H11)3), 4.48 (br s,
P(C6H11)3), 5.78 (br s, P(C6H11)3), 7.54–7.56 (m, 4H,
7.66–7.68 (m, 2H, ꢀC6H3), 8.91 (s, 2H, NCHCHN), 9.91 (s,
1H, NCHN). MS (ESI+): m/z 389.2959 [C27H37N2]+ (100%).
[HIMes][Fe(PCy3)Br3] ). Following a procedure similar to
that of except that [HIMes]Cl was used instead of [HIPr]Cl,
the product was precipitated as pale yellow crystals in a yield
of 88% (1.18 g), mp 179 C. Anal. calcd for C39H58Br3FeN2P:
C, 53.14; H, 6.63; N, 3.18. Found: C, 53.10; H, 6.66; N, 3.16.
1H NMR (
, 400 MHz, (CD3)2CO): 0.48 (br s, P(C6H11)3), 0.93
(br s, P(C6H11)3), 1.32 (br s, P(C6H11)3), 1.70 (br s, P(C6H11)3),
2.33 (s, 12H, CH3), 2.46 (s, 6H, CH3), 3.77 (s, P(C6H11)3), 4.42
(br s, P(C6H11)3), 5.71 (br s, P(C6H11)3), 7.30 (s, 4H,
8.49 (s, 2H, NCHCHN), 9.50 (s, 1H, NCHN). MS (ESI+): m/z
305.2002 [C21H25N2]+ (100%).
General Procedure for the Cross-Coupling of Aryl Phosphates
with Alkyl Grignard Reagents.
mꢀC6H3),
p
(
6
4
After cooling the mixture to 0 °C (ice water bath),
butylmagnesium chloride (2.5 mL, 1.0 M in THF) and complex
(0.05 g, 0.05 mmol) were added sequentially. The reaction
nꢀ
6
°
5
mixture was stirred for 8 h in an oil bath at 85 °C. The reaction
was quenched by addition of saturated ammonium chloride
solution, the mixture was extracted with ethyl acetate (3 × 3.0
mL) and dried over Na2SO4. The GC yield of the desired
δ
mꢀC6H2),
product is determined by GC analysis, using nꢀhexadecane as
an internal standard. Purification of the crude mixture by flash
column chromatography using petroleum ether (60ꢀ90 °C) as
eluent gave the isolated yield of desired coupling product. The
identity of the product was confirmed by NMR spectroscopy
A Schlenk flask was charged with complex
5 (0.05 g, 0.05 and TLC.
mmol), aryl phosphates (0.5 mmol), THF (3 mL) and a stirring
bar. The mixture was stirred at 0 °C for 2 min. To this stirred
mixture was added the solution of alkyl Grignard reagents (2.0
mL, 1.0 M in THF) at 0 °C via syringe. The resulting solution
turned black immediately and was then stirred for 8 h in an oil
bath at 85 °C. The reaction was quenched by addition of
saturated ammonium chloride solution, the mixture was
extracted with ethyl acetate (3 × 3.0 mL) and dried over
Na2SO4. The GC yield of the desired product is determined by
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Grants 21472134 and 21172164), the Key Laboratory of
Organic Chemistry of Jiangsu Province, and the Priority
Academic Program Development of Jiangsu Higher Education
Institutions (PAPD) for financial support.
GC analysis, using
nꢀhexadecane as an internal standard.
Purification of the crude mixture by flash column
chromatography using petroleum ether (60ꢀ90 °C) as eluent
gave the isolated yield of desired coupling product. The identity
of the product was confirmed by NMR spectroscopy and TLC.
Notes and references
1
(
a
) N. Miyaura, CrossꢁCoupling Reactions: A Practical
Guide, Springer: Berlin, 2002. ( ) A. de Meijere and F.
b
Diederich, Metalꢁcatalyzed Crossꢁcoupling Reactions, Wileyꢀ
VCH, Weinheim, 2004.
General Procedure for the Cross-Coupling of Aryl Phosphates with
Alkyl Bromides.
2
(
a
) A. Fürstner and R. Martin, Chem. Lett., 2005, 34,
624−629; ( ) B. J. Li, D. G. Yu, C. L. Sun and Z. J. Shi,
Chem. –Eur. J., 2011, 17, 1728−1759; ( ) T. Mesganaw and
N. K. Garg, Org. Process Res. Dev., 2013, 17, 29−39; ( ) W.
N. Li and Z. L.Wang, RSC Adv., 2013, 3, 25565−25575; ( ) J.
Cornella, C. Zarate and R. Martin, Chem. Soc. Rev., 2014, 43,
8081−8097; ( ) B. Su, Z. C. Cao and Z. J. Shi, Acc. Chem.
b
c
A Schlenk flask was charged with complex
5 (0.05 g, 0.05
d
mmol), magnesium turnings (62.4 mg, 2.6 mmol), aryl
phosphates (0.5 mmol) and a stirring bar. To this flask was
added than 5.2 mL of a solution of LiCl (0.5 M in THF) via
syringe. The mixture was cooled down to 0 °C in an iceꢀwater
e
f
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8 | Dalton Trans., 2012, 00, 1-3
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DOI: 10.1039/C6DT02995G
ARTICLE
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through the mechanism similar to that of aryl halides
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,
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DOI: 10.1039/C6DT02995G
Alkyl Grignard cross-coupling of aryl phosphates catalyzed by
new, highly active ionic iron(II) complexes containing a phosphine
ligand and an imidazolium cation
Zhuang Li, Ling Liu, Hong-mei Sun,* Qi Shen and Yong Zhang
Ionic iron(II) complexes, [HL][Fe(PR3)X3], showed high catalytic activities in alkyl Grignard
cross-coupling of aryl phosphates, and the corresponding reductive cross-coupling.