Alkali-metal organomagnesiate complexes as catalysts for highly chemoselective crossed-Tishchenko reactions

Article abstract of DOI:10.1039/d0dt01114b

Five new heterobimetallic magnesiates bearing bidentate dianionic pyrrolyl ligands have been synthesized through co-complexation with alkali-metal reagents and di-n-butylmagnesium. Single-crystal X-ray structural analysis of these complexes revealed a variety of intriguing bonding modes. These alkali-metal (Li, Na, and K) organomagnesiates were utilized as catalysts for cross-coupling Tishchenko reactions with two different aldehydes. The sodium alkyl magnesiate complex {ⁿBuMg[2-(Me₃CNCH₂)C₄H₃N]Na(Et₂O)}₈(3) was identified as a competent catalyst, as it exhibited higher catalytic activities and chemoselectivity under mild conditions.

Full text of DOI:10.1039/d0dt01114b

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Pages 3073-3412
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DOI: 10.1039/D0DT01114B
ARTICLE
Alkali-metal Organomagnesiate Complexes as Catalysts for Highly
Chemselective Crossed-Tishchenko Reaction
Zhiqiang Guo,*a Tengfei Pang, b Yakong Wang,b Yongbin Zhang c and Xuehong Wei *a,c
Received 00th January 20xx,
Accepted 00th January 20xx
Five new heterobimetallic magnesiates bearing bidentate dianionic pyrrolyl ligand have been synthesized through co-
complexation with alkali-metal reagents and di-n-butylmagnesium. Single-crystal X-ray structural analysis of these
complexes revealed a variety of intriguing bonding modes. Those alkali-metal (Li, Na, K) organomagnesiates were utilized as
catalysts for cross-coupling Tishchenko reaction with two different aldehydes, and sodium alkyl magnesiate complex
{nBuMg[2-(Me3CNCH2)C4H3N]Na(Et2O)}∞ (3) was identified as a competent catalyst, and exhibited higher catalytic activities
and chemselectivity under mild conditions.
DOI: 10.1039/x0xx00000x
combine an alkali-metal with a more electronegative metal such
as Mg, Al, or Zn, have exhibited greatly enhanced performance
than their monometallic components by switching on metal-
metal cooperativity.[7] In particular, the alkali-metal alkyl
magnesiates have attracted a high degree of attention due to
their unique structure-reactivity relationships. For example,
Oshima has reported that chemoselectivity can be easily tuned
using magnesium ate complex.[8] Noyori demonstrated that
Introduction
The Tishchenko reaction, a well typical atom economic
reaction, allows the catalytic dimerization of two aldehydes to
give the corresponding esters. Since its first discovery in 1887[1],
and a further modification in 1906[2], it has been thoroughly
studied, comprehensively reviewed and many types of catalysts
and methods have been developed for this transformation.[3]
Unfortunately, this reaction has not been accepted as general
use for ester synthesis because of the low selectivity for single
cross-coupled ester formed by two different aldehydes.[4]
Although some groups have attempted to improve the
selectivity of a single cross-coupled ester from among four
possible esters (Scheme 1),[5] up until now, highly selective
crossed-Tishchenko reactions have been successfully
performed only for very special substrate combinations.[6]
Indeed, the ability to control and fundamentally alter the
selectivity of cross-coupled ester generated by two different
aldehydes still remains an unanswered challenge.
dilithium dialkylmagnesiates can promote
a very good
enantioselectivity in the alkylation of aldehydes.[9] and Mulvey
extended the scope of regioselectivity attainable in aromatic C-
H metalation chemistry by the supramolecular structure of the
bimetallic base.[10] In a word, the studies of those bimetallic
reagents hint at the enormous potential in organic synthesis.
Their utilizations have been evolved from stoichiometric
transformations to catalytic reactions.[11] Therefore, it is
desirable to further exploit their catalytic applications.
Here, we report the synthesis, structural features and
catalytic applications of alkali-metal (Li, Na, K) alkyl magnesiate
complexes containing bidentate dianionic pyrrolyl ligand with
various bonding modes, specifically in promoting catalytic
cross-coupling Tishchenko reaction.
On the other hand, the main group metal systems, which
O
O
H
H
H
H
O
O
R
R
R
R'
R'
O
A
O
B
Catalyst
AB
H
BA
R
H
R'
H
O
O
Results and discussion
Synthesis of alkali-metal (Li, Na, K) alkyl magnesiates
H
H
H
O
O
R'
R
R'
AA
BB
As shown in Scheme 2, five alkali-metal (Li, Na, K) alkyl
magnesiate complexes ( Mg[2-(Me3CNCH2)C4H3N]Li2(THF)4 (1),
Mg[2-(Me3CNCH2)C4H3N]Li2(TMEDA)2 (2), {nBuMg[2-(Me3CNCH
2)C4H3N]Na(Et2O)}∞ (3), {nBuMg[2-(Me3CNCH2)C4H3N]Na(TMED
A)}2 (4), {nBuMg[2-(Me3CNCH2)C4H3N]K(Et2O)}∞ (5)) were
readily synthesized by the equimolar reaction of di-n-
butylmagnesium with alkali metal complexes (generated in situ
through the reaction of [C4H3NH(2-CH2NHtBu)] with nBuLi, NaH
and KH) in donor solvents Et2O, THF or TMEDA at room
temperature. It's important to note that whichever the different
Scheme 1. The crossed-Tishchenko reaction.
a. Scientific Instrument Center, Shanxi University, Taiyuan, 030006, P.R. China. E-
mail: gzq@sxu.edu.cn
b. School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan,
030006, P.R. China.
c. Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, P. R China. E-
mail: xhwei@sxu.edu.cn
Electronic Supplementary Information (ESI) available: All experimental details and
characterization data. CCDC 1949195-1949197,1949200-1949201. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/x0xx00000x
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ARTICLE
Journal Name
Et2O
Et2O
View Article Online
DOI: 10.1039/D0DT01114B
N
N
Na
N
N
N
tBu
N
N
N
N
Mg
N
Na
tBu
Mg
N
N
N
N
nBu
Mg
N
tBu
n
nBu
N
N
Mg
N
N
3
tBu
Mg
N
tBu
nBu
1) NaH,THF, -78 o
2) MgBu2, Et2O, Rt
C
nBu
tBu
1
)
2
n
o
8
C
2
-
t
)
B
R
M
u
4
,
7
L
-
,
g
A
i
B
,
D
F
T
u
E
H
H
2
,
T
M
F
T
,
T
,
,
M
H
0
o
u 2
a
E
N
)
D
C
B
N
H
g
A
1
,
M
NHtBu
R
)
Et2O
2
Et2O
t
tBu
N
1
)
o C
K
g
0
H
,
2
THF
THF
,
N
F
t
)
T
H
M
H
T
F
,
,
i
B
L
-
7
u
u
THF
Li
THF
B
R
8
Mg
2
o
-
,
,
E
n
2
t
C
K
)
u
N
Mg
1
B
2 O
,
N
N
g
M
R
N
N
)
t
Figure 1. ORTEP diagram of 1 (at 25% probability level). Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (°): Mg(1)-N(2) 2.052(4),
Mg(1)-N(1) 2.087(4), O(1)-Li(1) 1.958(9), O(2)-Li(1) 1.922(9), N(1)-Li(1) 2.213(9),
N(2)-Li(1) 2.035(9), N(2)-C(6) 1.474(6), N(2)-C(5) 1.486(5); N(2)-Mg(1)-N(1)
79.26(14), N(2)-Li(1)-N(1) 76.7(3), C(1)-N(1)-Mg(1) 139.8(3), C(4)-N(1)-
Mg(1)103.8(3), C(1)-N(1)-Li(1) 128.4(4), C(4)-N(1)-Li(1) 83.1(3), Mg(1)-N(1)-Li(1)
81.7(2), Li(1)-N(2)-Mg(1) 87.0(3), C(5)-N(2)-Li(1) 91.3(3), C(5)-N(2)-Mg(1) 104.5(3).
Symmetry elements for 1: -x, y, -z+1/2.
2
tBu
tBu
Mg
N
1
nBu
tBu
n
nBu
5
Scheme 2 Synthetic routes to complexes (1-5).
amounts of Mg(nBu)2 or the addition order of the reagents in
the synthesis process of lithium organomagnesiate complexes,
only complexes (1) and (2) could be isolated. The
aforementioned five complexes were characterized by 1H NMR,
13C NMR spectroscopy and further confirmed by single-crystal
X-ray diffraction. The corresponding pyrrole ring protons and
other protons showed resonance signals at expected regions in
each complex, and the NMR spectra also supported the
presence of the carbon atom in each complex. Those are
consistent with the structure of X-ray analysis.
Structural characterization of complexes 1-5.
Crystals of lithium magnesiate (1) contains a monoclinic
crystal system with a space group of C2/c. The ORTEP drawing
of the molecular structure of (1) is shown in Fig. 1. The geometry
of the Mg center can be described as a distorted tetrahedral
arrangement, surrounded by four nitrogen atoms two
bidentate ligands to form two chelate rings with bite angles N1-
Mg1-N2 of 79.26 (14)°. The distances of Mg1-N1 [2.087(4) Å]
and Mg1-N2 [2.052(4) Å] are shorter than those of the reported
tridentate substituted pyrrolyl-magnesium compound [12]. Two
lithium atoms are also four-coordinate environment with a
distorted tetrahedral geometry, linked through two nitrogen
atoms of ligand and two THF molecules. The bond angles of N2-
Li1-N1 and O2-Li1-O1 are 76.7(3)° and 102.5(4)°, respectively.
Addition of the donor solvent TMEDA instead of THF to the
reaction solution resulted in the isolation of complex (2). And
the X-ray crystal structure determination was carried out on 2.
While the data were not of sufficient quality to present herein,
the structure and ligand arrangement were unambiguously
established. It was further confirmed by 1H NMR, 13C NMR
spectroscopy.
(a)
(b)
Figure 2 (a) ORTEP diagram of 3 (at 25% probability level). (b) its polymeric form created
through Na–π interactions. Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (°): Al(1)-N(1) 1.869(4), Al(1)-N(2) 1.908(3), Li(1)-N(3) 2.045(8),
Li(1)-N(2) 2.109(8), Li(1)-N(4) 2.183(7); N(1)-Al(1)-N(2) 87.87(17), N(3)-Li(1)-N(2)
126.4(4), N(3)-Li(1)-N(4) 86.6(3), N(2)-Li(1)-N(4) 140.0(4), C(5)-N(2)-C(6) 114.0(3), C(6)-
N(2)-Al(1) 121.0(3), C(5)-N(2)-Li(1) 111.1(3), C(6)-N(2)-Li(1) 108.2(3), Al(1)-N(2)-Li(1)
89.5(3). Symmetry elements for 3: 1/2–x, -y, 1/2+z; -x, 1/2+y, 1/2-z; 1/2+x, 1/2-y, -z.
Crystals of sodium alkyl magnesiate (3) suitable for X-ray
quality crystallized from the mixed Et2O/THF solution at room
temperature. In the solid state structure, the parent complex (3)
adopts an infinite chain structure with distinctive propagation
like the reported sodium alkyl zincate[13] (see Fig. 2). Both Na1
and Na2 are coordinated by two pyrrolyl rings in η5 mode to
form the distorted sandwich geometry. While the coordination
environment of Na1 is completed by two Et2O molecule, Na2
further π-bonded to a pyrrolyl ring of an adjacent molecule,
thus lead to a polymeric zigzag chain. The main Na-C bond
distances of complex (3) are in the range 2.644(3)-2.853(3) Å,
and the distances of Na(1) to the C1C2C3C4N1 plane and the
2 | J. Name., 2012, 00, 1-3
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ARTICLE
C12C13C14C15N3 plane are 2.521 Å and 2.608 Å, which are
within the range of the reported values for other pyrrolyl
sodium sandwich geometries[13, 14]. The Mg atom is four
coordinated by n-butyl carbon and nitrogen atoms of pyrrole
and the -NtBu fragment. And the bond lengths of Mg1–N1,
Mg1–N2 and Mg1–N4 are 2.099(3), 2.143(3) and 2.109(3) Å,
respectively. The bite angle N1–Mg1–N2 is 84.64(10)°.
After the successful synthesis of alkali-metal
View Article Online
organomagnesiate complexes 1-5, weDOemI: 1p0l.o10y3e9d/D0tDhTe0m1114aBs
catalysts for the crossed-Tishchenko reaction. In view of the fact
that heterobimetallic ate complexes could promote
chemoselective ortho-reactions, the reaction between
benzaldehyde and o-bromobenzaldehyde was selected as the
model reaction to test activities of alkali-metal
organomagnesiate complexes. As shown in Table 1, when 1a
was treated with 1 equivalent of 1b in the presence of 3 mol%
X-ray studies revealed TMEDA could deaggregate the
polymeric structure of (3) all the way down to a tetranuclear
dimer within (4) (see Fig.3). The complex (4) consists of two
geometrically different sodium centers, the Na1 center is
observed as a sandwich structure between two pyrrolyl ring π-
electron densities in an η5-fashion and further chelated by two
nitrogen atoms of TMEDA. The Na2 is coordinated by n-butyl
carbon atom, two nitrogen atoms of TMEDA and a pyrrolyl ring
of ligand in η5-fashion to form a half-sandwich geometry. The
main distances between the sodium ion Na1/Na2 and the
pyrrole ring atoms [range: 2.649(3)-3.019(3) Å)] are close to
those of the above complex (3). The distances of Na1 to two
pyrrolyl ring planes are 2.601 Å and 2.575 Å, and the distances
of Na2 to pyrrolyl ring plane is 2.497 Å, respectively. The
coordination pattern of magnesium atoms is similar to that of
complex (3). And the corresponding bond lengths of Mg1-N1,
Mg1-N4, Mg1-N2 and Mg2-N2, Mg2-N3, Mg2-N4 are 2.082(3),
2.1583, 2.171(3), 2.093(3), 2.098(3) and 2.129(3) Å,
respectively.
o
catalyst at 30 C all gave the ester 2a in moderate yield of 37-
57% in THF solvent (Table 1, entry 1-5). The catalyst (3) showed
the most activity for this reaction, then the amount of catalyst
(3) was increased to 5 mol%, the yield and selectivity to 2a is the
optimal (Table 1, entry 6-7). Different solvents, including diethyl
ether, hexane and toluene were screened, as was the solvent-
free reaction (Table 1, entry 8-11). And the neat condition was
found to be the best for this reaction, which indicated that the
solvent did not play a critical role on the selectivity and
reactivity of the reaction. The effect of reaction temperature
was also examined (Table 1, entries 12 and 13). The optimum
temperature was found to be 30 oC (Table 1, entry 11). Finally,
we tried to shorten the reaction time to one hour, the yield of
2a decreased to 64%. In addition, the control reactions were
carried out using 2 ‑ aminopyrrolyl lithium complex {[2-
(CH2NHCMe3)C4H3N]Li(THF)}2,[15] 2 ‑ aminopyrrolyl dilithium
compounds {μ-η5:η1-2-(Me3CNCH2)C4H3N]Li2(TMEDA)}2 [16] with
the same supporting ligand as catalysts (see supporting
information). The NMR results show that mono-, di-lithioamino
The X-ray crystal structure determination was also carried
out on potassium alkyl magnesiate complex (5). Unfortunately,
the collected X-ray data of complex (5) was poor and two Et2O
molecules coordinated to potassium is seriously disordered. but
the connectivity of (5) and its composition were deduced and
further confirmed by NMR. Here, the bonding parameters will
not be further discussed.
Table 1. Optimization for Crossed Tishchenko reactiona.
O
Br
CHO
OHC
Br
Cat.
O
1a
2a
1b
Entry
Catalyst
1 (3%)
2 (3%)
3 (3%)
4 (3%)
5 (3%)
3 (5%)
3 (10%)
3 (5%)
3 (5%)
3 (5%)
3 (5%)
3 (5%)
3 (5%)
3 (5%)
Solvent
THF
Tem. [oC] Yield [%]b
1
2
30
30
30
30
30
30
30
30
30
30
30
20
50
30
37
41
57
54
50
69
66
60
59
77
79
68
75
64
THF
3
THF
4
THF
5
THF
6
THF
7
THF
Figure 3. ORTEP diagram of 4 (at 25% probability level). Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (°): Mg(1)-N(1) 2.082(3), Mg(1)-N(4)
2.158(3), Mg(1)-N(2) 2.171(3), Mg(2)-N(2) 2.093(3), Mg(2)-N(3) 2.098(3), Mg(2)-N(4)
2.129(3), Na(1)-C(2) 2.698(3), Na(1)-C(13) 2.740(3), Na(1)-C(3) 2.767(4), Na(1)-N(3)
2.822(3), Na(1)-C(1) 2.846(3), Na(1)-C(12) 2.852(3), Na(1)-C(10) 2.950(4), Na(1)-C(4)
2.960(3), Na(1)-C(11) 2.985(4), Na(1)-N(1) 3.019(3), Na(2)-N(3) 2.649(3), Na(2)-C(10)
2.686(4), Na(2)-C(13) 2.773(3), Na(2)-C(11) 2.869(4), Na(2)-C(12) 2.912(4); N(1)-Mg(1)-
N(4) 105.51(11), N(1)-Mg(1)-N(2) 82.95(11), N(4)-Mg(1)-N(2) 92.98(11), N(2)-Mg(2)-N(3)
109.73(12), N(2)-Mg(2)-N(4) 96.11(12), N(3)-Mg(2)-N(4) 84.28(10), Mg(2)-N(2)-Mg(1)
85.72(11), Mg(2)-N(4)-Mg(1) 85.17(11). Symmetry elements for 4: –x, 1/2+y, 1/2-z.
8
Et2O
9
Hexane
Toluene
None
None
None
None
10
11
12
13
14c
[a] Reaction conditions: benzaldehyde (1 mmol), o-bromobenzaldehyde (1 mmol),
30 oC, 3 h. [b] Yields determined by 1H NMR spectroscopy with trichloroethylene
as an internal standard. [c] t = 1 h.
The crossed-Tishchenko reaction catalyzed by alkali-metal (Li, Na,
K) alkyl magnesiates.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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ARTICLE
Journal Name
O
O
O
pyrroyl complex could catalyze the Tishchenko reaction, but
their selectivity is lower than that of sodium alkyl magnesiate.
With the optimized reaction conditions in hand and o-
bromobenzaldehyde as a fixed model substrate, various
aldehydes were investigated for this crossed Tishchenko
reaction, and the representative results were summarized in
Scheme 3. The para-substituted benzaldehydes with electron-
donating groups (-CH3, -OCH3) are relatively inert, and the
desired cross coupling esters were obtained in 66% and 55%
yields (2b and 2c). Comparatively, the para-substituted
benzaldehydes with electron-withdrawing groups (-Cl, -NO2)
exhibit higher reactivity, giving the corresponding products in
77% and 80% yields (2d and 2e), respectively. And to our
delight, the crystals of 2a and 2e were suitable for single-crystal
analysis, their structures were further confirmed by X-ray
diffraction analysis. With this encouragement, we further
investigated the crossed Tishchenko reaction between
benzaldehyde and o-methoxybenzaldehyde, which exhibited
higher activity on the reaction outcome, giving the 87% yield
(2f). Additionally, both electron-donating and electron-
withdrawing groups were also tolerated, affording the expected
esters in good yields (2g-2j).
View Article Online
3
Complex (5 mmol%)
DOI: 10.10
O
39/D0DT01114B
H
H
(1)
30 o
C
H3CO
81% (1.96 g)
H3CO
1.36 g
1.06 g
Experimental
General Information
Unless otherwise noted, all syntheses and manipulations of
air-sensitive materials were performed under a purified
nitrogen atmosphere using the standard Schlenk techniques.
Tetrahydrofuran and diethyl ether were distilled from sodium-
benzophenone under nitrogen. Hexane is dried using sodium
potassium alloy and distilled under nitrogen prior to use. nBuLi
(2.2 M in n-hexane), Mg(nBu)2 (1.0 M in n-hexane), NaH (60%
dispersion in mineral oil) and KH (30% dispersion in mineral oil)
were purchased from Aladdin and used as received. N,N,N’,N’-
tetramethylethylenediamine (TMEDA) was also purchased from
Aladdin, dried by heating to reflux over calcium hydride and
stored with molecular sieves under nitrogen prior to use. All
aldehydes were sublimed, recrystallized or distilled before use.
The aminopyrrolyl ligand [C4H3NH(2-CH2NHtBu)] was
synthesized according to the our reported procedure.[13] 1H
NMR, 13C NMR and 7Li NMR data were recorded with 600 MHz
or 300 MHz spectrometers (BRUKER) in THF-d8/CDCl3 at 298 K.
Scheme 3. The Selective Crossed-Tishchenko Reactions Catalyzed by Complex 3.
O
R2
CHO
OHC
R2
Cat. 5%
O
Elemental analyses were performed on
instrument.
a Vario EL-III
30 oC, 3 h
R1
R1
O
Br
Br
O
Br
Preparation of Mg[2-(Me3CNCH2)C4H3N]Li2(THF)4 (1)
O
O
A n-BuLi hexane solution (3.0 mmol, 2.2 M in hexane) was
added dropwise to a THF (15 mL) solution of [C4H3NH(2-
CH2NHtBu)] (0.457 g, 3.0 mmol) at 0 °C under nitrogen. After the
reaction mixture was stirred at room temperature for 2 h,
immediately followed by addition of a solution of Mg(nBu)2 (3.0
mmol, 1.0 M in hexane). and the mixture was then stirred for
another 6 h, the white solid was isolated by filtration and
recrystallized from a saturated tetrahydrofuran and hexane
mixed solution to yield colorless crystals of 1 (0.523 g, 56%). 1H
NMR (300 MHz, d8-THF): 6.38 (s, 2H, C4H3N), 5.86 (s, 2H, C4H3N),
5.64 (s, 2H, C4H3N), 3.92 (s, 4H, CH2NBut), 3.50 (m, 16, THF), 1.64
(m, 16, THF), 0.79 (s, 18H, CH2NBut); 13C NMR (75 MHz, d8-THF):
147.83 (C4H3N), 128.67(C4H3N), 113.80 (C4H3N), 107.32 (C4H3N),
72.16 (THF), 55.99 (C(CH3)3), 50.77 (CH2NBut), 36.32 (C(CH3)3),
29.77(THF); 7Li NMR (116.6 MHz, d8-THF): 2.30; Anal. Calcd for
C34H56Li2MgN4O4: C, 65.55; H, 9.06; N, 8.99. Found: C, 65.67; H,
8.85; N, 9.07.
H
Me
2a
X-ray of
O
2a
2b
63% (54%)
71% (58%)
Br
Br
O
O
O
O
O
O2N
Cl
MeO
2d
77% (69%)
2e
80% (65%)
2c
55%(48%)
O
O
O
O
H
MeO
Me
MeO
2f
87% (72%)
O
2g
81% (75%)
O
2e
X-ray of
O
O
O
O
MeO
MeO
Cl
MeO
O2N
MeO
2h
75% (65%)
2j
74% (66%)
2i
88% (79%)
Reaction conditions: aldehyde (1 mmol), catalyst (5% mmol), 30 oC, 3 h. Yield
determined by 1H NMR using trichloroethylene as an internal standard, isolated
yields were given in parentheses.
Preparation of Mg[2-(Me3CNCH2)C4H3N]Li2(TMEDA)2 (2)
In order to verify the applicability of the current catalytic
system, the reaction was scaled up to 10 mmol scale (gram
scale), benzaldehyde (10 mmol, 1.06 g) was treated with 1
equivalent of o-methoxybenzaldehyde (10 mmol, 1.36 g) with
complex 3 (5% mmol) as catalyst at 30 oC, leading to a yield of
ester product (2f) of 81% (Eq. 1). Further detailed studies to
elucidate this selective reaction mechanism and to expand the
utility of this system are currently underway in our group.
A n-BuLi hexane solution (3.0 mmol, 2.2 M in hexane) was
added dropwise to a THF (15 mL) solution of [C4H3NH(2-
CH2NHtBu)] (0.457 g, 3.0 mmol) at 0 °C under nitrogen. After the
reaction mixture was stirred at room temperature for 2 h,
immediately followed by addition of a solution of Mg(nBu)2 (3.0
mmol, 1.0 M in hexane). and the mixture was then stirred for
another 6 h, then the TMEDA (0.45 mL, 3 mmol) was introduced
via syringe, the solution was filtered and concentrated, the
4 | J. Name., 2012, 00, 1-3
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residue was recrystallized from a saturated Hexane/THF
A solution of [C4H3NH(2-CH2NHtBu)] (0.457 g,V3ie.w0AmrticmleoOln)linine
1
DOI: 10.1039/D0DT01114B
solution to yield colorless crystals of 2 (0.534g, 63% yield). H THF (15 mL) was added slowly to a suspension of KH (0.40 g, 3.0
NMR (300 MHz, d8-THF): 6.52 (s, 2H, C4H3N), 5.99(s, 2H, C4H3N), mmol) in THF (15 mL) at −78 °C. The reaction was warmed to
5.77 (s, 2H, C4H3N), 4.04 (s, 4H, CH2NBut), 2.27 (s, 8H, TMEDA), room temperature and stirred for 3 h, immediately followed by
2.05 (s, 24, TMEDA), 0.67 (s, 18H, CH2NBut); 13C NMR (75 MHz, addition of a solution of Mg(nBu)2 (3.0 mmol, 1.0 M in hexane).
d8-THF): 143.05 (C4H3N), 124.38(C4H3N), 109.69 (C4H3N), 103.27 and the mixture was then stirred for another 6 h, the residue
(C4H3N), 58.77 (TMEDA), 51.44 (C(CH3)3), 48.86 (CH2NBut), 46.18 was recrystallized from a saturated THF solution to yield
(TMEDA), 31.89 (C(CH3)3); 7Li NMR (116.6 MHz, d8-THF): 0.535; colorless crystals of 5 (1.03g, 53.3% yield). H NMR (300 MHz,
1
Anal. Calcd for C30H54Li2MgN8: C, 63.77; H, 9.63; N, 19.83. d8-THF): 6.32 (s, 2H, C4H3N), 5.87(s, 2H, C4H3N), 5.70 (s, 2H,
Found: C, 63.48; H, 9.71; N, 19.69.
C4H3N), 4.18 (dd, J = 56.3, 12.2 Hz, 4H, CH2NBut), 3.62 (m, 4H,
THF), 3.37 (q, 4H, Et2O), 1.77 (m, 4H, THF), 1.29 (m, 6H,
Mg(CH2)3CH3) 1.15 (s, 18H, CH2NBut), 1.12 (t, 6H, Et2O), 0.89 (m,
Preparation of {nBuMg[2-(Me3CNCH2)C4H3N]Na(Et2O)}∞ (3)
A solution of [C4H3NH(2-CH2NHtBu)] (0.457 g, 3.0 mmol) in 12H, Mg(CH2)3CH3). 13C NMR (75 MHz, d8-THF): 147.03 (C4H3N),
THF (15 mL) was added slowly to a suspension of NaH (0.12 g, 122.76 (C4H3N), 107.11 (C4H3N), 101.26 (C4H3N), 68.07 (THF),
3.0 mmol) in THF (15 mL) at −78 °C. The reaction was warmed 66.50 (Et2O), 52.48 (C(CH3)3), 48.71 (CH2NBut), 32.28
to room temperature and stirred for 3 h, immediately followed (Mg(CH2)3CH3), 32.14 (C(CH3)3), 29.88 (Mg(CH2)3CH3), 26.61
by addition of a solution of Mg(nBu)2 (3.0 mmol, 1.0 M in (Mg(CH2)3CH3), 25.95 (THF), 15.85 (Et2O), 14.25(Mg(CH2)3CH3).
hexane). And the mixture was then stirred for another 6 h. The
General procedure for the selective crossed Tishchenko reaction
white solid was isolated by filtration and recrystallized from a
with alkali-metal organomagnesiate complexes as catalyst.
saturated THF/Et2O solution to yield colorless crystals of 3
1
In a glovebox, to a 50 mL sealed tube with a magnetic
stirring bar, was successively added benzaldehyde (102 uL, 1.0
mmol), o-bromobenzaldehyde (117 uL, 1.0 mmol) catalyst 3
(0.0329 g, 0.05 mmol). The mixture was stirred at 30 °C for 3 h.
The reaction was then quenched with H2O (10 mL) and
extracted with ethyl acetate (3 x 10 mL). The combined organic
layers were dried over Na2SO4 and concentrated. The yields
were determined by 1H NMR spectroscopy with
trichloroethylene (90 μL, 1 mmol) as an internal standard. Then,
the reaction mixture was purified on silica gel column
chromatography (EtOAc /petroleum ether: 1/10). The product
was characterized by NMR spectroscopy.
(0.540 g, 55% yield). H NMR (600 MHz, d8-THF): 6.39 (s, 2H,
C4H3N), 5.90(s, 2H, C4H3N), 5.72 (s, 2H, C4H3N), 4.17 (m, 4H,
CH2NBut), 3.61 (m, 4H, THF), 3.38 (q, 4H, Et2O), 1.77 (m, 4H,
THF), 1.21 (m, 6H, Mg(CH2)3CH3) 1.15 (br, 24H, CH2NBut and
Et2O), 0.89 (m, 8H, Mg(CH2)3CH3). 0.79 (m, 4H, Mg(CH2)3CH3).
13C NMR (75 MHz, d8-THF): 148.13 (C4H3N), 123.15 (C4H3N),
107.48 (C4H3N), 101.39 (C4H3N), 68.39 (THF), 66.48 (Et2O), 54.96
(C(CH3)3), 48.46 (CH2NBut), 35.06 (Mg(CH2)3CH3), 32.26
(C(CH3)3), 31.96 (Mg(CH2)3CH3), 26.54(Mg(CH2)3CH3), 25.80
(THF), 15.85 (Et2O), 14.24 (Mg(CH2)3CH3). Anal. Calcd for
C34H66Mg2N4Na2O2: C, 62.11; H, 10.12; N, 8.52. Found: C, 62.67;
H, 9.71; N, 8.34.
Preparation of {nBuMg[2-(Me3CN -CH2)C4H3N]Na(TMEDA)}2 (4)
Conclusions
A solution of [C4H3NH(2-CH2NHtBu)] (0.457 g, 3.0 mmol) in
THF (15 mL) was added slowly to a suspension of NaH (0.12 g,
3.0 mmol) in THF (15 mL) at −78 °C. The reaction was warmed
to room temperature and stirred for 3 h, immediately followed
by addition of a solution of Mg(nBu)2 (3.0 mmol, 1.0 M in
hexane). and the mixture was then stirred for another 6 h, then
the TMEDA (0.45 mL, 3 mmol) was introduced via syringe, the
solution was filtered and concentrated, the residue was
recrystallized from a saturated THF solution to yield colorless
In summary, five new heterobimetallic magnesiates bearing
bidentate dianionic pyrrolyl ligand have been synthesized
through co-complexation. X-ray crystallographic studies have
revealed the diversity of those alkali-metal magnesiate
structures, from simple monomeric motifs to infinite chain
structures, showing an important alkali-metal effect. The
lithium magnesiates adopt simple monomeric motifs with 2:2:1
(ligand/lithium/magnesium) stoichiometric ratio, while sodium
alkyl magnesiate and potassium magnesiate adopt infinite chain
structures, with linear or zigzag arrangements, respectively.
Didentate TMEDA could deaggregate the sodium polymer to a
monomer. They were utilized as catalysts for cross-coupling
Tishchenko reaction with two different aldehydes, and sodium
alkyl magnesiate complex (3) was identified as a competent
catalyst, and exhibited higher catalytic activities and
chemselectivity under mild conditions.
1
crystals of 4 (0.567 g, 51% yield). H NMR (600 MHz, d8-THF):
6.69 (s, 2H, C4H3N), 5.91 (s, 2H, C4H3N), 5.80 (s, 2H, C4H3N), 3.81
(d, J = 100.7Hz, 4H, CH2NBut), 2.30 (s, 8H, TMEDA), 2.15 (s, 24H,
TMEDA), 1.09 (m, 6H, Mg(CH2)3CH3), 1.01 (s, 18H, CH2NBut),
0.11(m, 12H, Mg(CH2)3CH3); 13C NMR (150.9 MHz, d8-THF):
142.71 (C4H3N), 131.77 (C4H3N), 129.20 (C4H3N), 107.71 (C4H3N),
59.12 (TMEDA), 51.40 (C(CH3)3), 46.38 (TMEDA), 32.28
(Mg(CH2)3CH3), 31.48 (CH2NBut), 29.36 (C(CH3)3), 19.65
(Mg(CH2)3CH3), 15.86 (Mg(CH2)3CH3), 1.54 (Mg(CH2)3CH3); Anal.
Calcd for C38H78Mg2N8Na2: C, 61.54; H, 10.60; N, 15.11. Found:
C, 61.67; H, 10.46; N, 15.04.
Conflicts of interest
The authors declare no competing financial interest.
Preparation of {nBuMg[2-(Me3CNCH2)C4H3N]K(Et2O)}∞ (5)
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Chemical Communications, 2018, 54, 1233-1236; e) F. M.
(
Acknowledgements
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Younis, S. Krieck, H. Görls andDOMI: 1.0.1W03e9s/Dte0rDhTa0u1s11e4nB,
Organometallics, 2015, 34, 3577-3585; (f) S. E. Baillie, W.
Clegg, P. García-Álvarez, E. Hevia, A. R. Kennedy, J. Klett and L.
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DeꢀTullio, A. Hernán-Gómez, Z. Livingstone, W. Clegg, A. R.
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Financial support from the Coal-based Key Scientific and
Technological Project (No. MH2014-07) and the Natural Science
Foundation of Shanxi Province (No. 201801D121041) is
gratefully acknowledged.
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6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0DT01114B
Table of Contents (TOC)
O
O
O
Cat. 5%
H
H
O
30 oC, 3 h
R2
R1
R1
R2
Cat. =
Five new heterobimetallic magnesiates bearing bidentate dianionic pyrrolyl
ligand have been synthesized through co-complexation with alkali-metal reagents and
di-n-butylmagnesium. Single-crystal X-ray structural analysis of these complexes
revealed a variety of intriguing bonding modes. Those alkali-metal (Li, Na, K)
organomagnesiates were utilized as catalysts for cross-coupling Tishchenko reaction
with two different aldehydes, and sodium alkyl magnesiate complex was identified as
a competent catalyst, and exhibited higher catalytic activities and chemselectivity
under mild conditions.