Novel and efficient Ni-mediated pinacol coupling of carbonyl compounds

Article abstract of DOI:10.1016/j.tet.2004.01.073

It was firstly found that the Rieke Ni generated in situ was able to promote the pinacol coupling of various carbonyls efficiently. Based on this information, another catalytically effective, cheaper and more convenient NiCl₂(Cat.)/Mg/TMSCl system was designed and developed further successfully. The interesting single-electron transfer (SET) mechanisms for the coupling reactions were proposed. Additionally, the DL/meso diastereoselectivity and some additive effects were also explained in terms of the proposed mechanisms.

Full text of DOI:10.1016/j.tet.2004.01.073

Tetrahedron 60 (2004) 2851–2855
Novel and efficient Ni-mediated pinacol coupling of
carbonyl compounds
*
Lei Shi, Chun-An Fan, Yong-Qiang Tu, Min Wang and Fu-Min Zhang
Department of Chemistry and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China
Received 23 October 2003; revised 14 January 2004; accepted 18 January 2004
Abstract—It was firstly found that the Rieke Ni generated in situ was able to promote the pinacol coupling of various carbonyls efficiently.
Based on this information, another catalytically effective, cheaper and more convenient NiCl (Cat.)/Mg/TMSCl system was designed and
2
developed further successfully. The interesting single-electron transfer (SET) mechanisms for the coupling reactions were proposed.
Additionally, the DL/meso diastereoselectivity and some additive effects were also explained in terms of the proposed mechanisms.
q 2004 Elsevier Ltd. All rights reserved.
1
. Introduction
through the reaction of stoichiometric or catalytic amount of
NiCl2 with Li or Mg/TMSCl, respectively. Although
increasing number of applications with organonickel agents
One of the significant structure units in nature is the
pinacols, which have wide occurrence in many biologically
active molecules, such as Taxol, (2)-grayanotoxin III
and so on. Recently, pinacols have been also used as the
8
c,10
have been reported in organic synthesis,
to our knowl-
1
a
1b
edge, the two Ni-mediated coupling systems we developed
have not been reported before. The major valuable features
of both two coupling systems involved the broader
application scope of substrates, the more convenient
experiment operation and the low-cost of the Ni-mediated
coupling reagents. Herein, we would like to report our
results on the two systems.
2
chiral ligands, auxiliaries as well as versatile synthetic
3
intermediates. Generally, the pinacol units could be
4
constructed through several approaches. Among them,
the particularly important and most widely used is the
coupling of carbonyl compounds, which can be induced
5
6
photochemically and electrochemically, or by use of
7
various metals or their salts. For the latter, the currently
used metals included Sm, Ti and so on, however some
limitations are still present. For example, some systems
2. Results and discussions
7
r
(e.g., TiCl /Li(Hg) ) were only able to give low yield of
4
corresponding pinacol products because of their strong
The Rieke Ni we examined was simply prepared by stirring
a mixture of NiCl (1.0 equiv.), Li (small cuts, 2.1 equiv.)
and the naphthalene (0.3 equiv.) in THF under Ar at room
7
l
2
reducing abilities; some metal coupling agents (e.g., Nb,
7
n
7p
7q
Yb, Bi, U ) were too expensive due to their rare
deposits; and the experimental conditions in some systems
1
1
temperature for 1 h. The pinacol coupling was performed
with various substrates and the Rieke Ni generated in situ
following a general procedure (see Section 4.3). The results
were listed in Table 1, from which we can see that the Rieke
Ni-mediated pinacol coupling was effective, in good to
excellent yields as well as with the preferential DL-dia-
stereoselectivity, to a wide range of substrates including
aromatic aldehydes (entries 1–7), aliphatic aldehydes
7
a
(e.g., Na ) were too rigorous to operate conveniently. In
8
particular, the reported low-valent Ni coupling agents,
Ni(COD)2 and NiCl /Li/Arene(Cat.), either was only
2
limited to an intramolecular pinacol coupling or merely
gave the pinacols as minor by-product. In general, these
couplings mentioned above were only applicable to a
9
relatively narrow scope of substrates. Therefore, it is still of
(entries 10 and 11), aromatic ketones (entries 12–14) and
major importance to develop a mild, extensive and effective
pinacol coupling sequence. In our efforts to this subject, we
discovered two interesting pinacol coupling protocols for
carbonyls, which were promoted by Ni generated in situ
aliphatic ketone (entry 15). It was notable that the Rieke Ni
system exhibited the tolerance for some heteroatomic
substituents (entries 3, 5 and 6) on substituted benzalde-
1
1b
hydes.
In addition the property and position of the
substituents (entries 2–6) showed no significant effect on
Keywords: Pinacol; Coupling; Carbonyl compounds.
*
this coupling. However, when the substrate bears –NO or
2
–OH group (entries 8 and 9), no expected pinacol products,
Corresponding author. Tel.: þ86-931-8912410; fax: þ86-931-8912582;
e-mail address: tuyq@lzu.edu.cn
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.01.073

2852
L. Shi et al. / Tetrahedron 60 (2004) 2851–2855
1
2
Table 1. Rieke Ni-mediated pinacol coupling reaction
but only simple reduction product R (R )CHOH was
formed.
To further examine the property of Rieke Ni-mediated
pinacol coupling and also investigate the reaction
mechanism, some additives, e.g. Lewis acid or base, or
1
4
2
a
b
Entry Substrate
R
R
Product Yield DL/meso
%)
those with the –OH, –NO or ether group were chosen to
2
(
probe this reaction using PhCHO 1a as a model substrate
following a general procedure (see Section 4.4). From the
results listed in Table 2, we found that when Lewis base
(2.0 equiv.) was added in this reaction (entries 1 and 2), an
improved distereoselectivity (DL/meso<3/1) was observed
in comparison with that of the corresponding entry 1 (DL/
meso<2/1) in Table 1 despite of their lower yields.
Furthermore, when catalytic amount of Lewis acid was
added (entries 3–5), both DL-selectivity (DL/meso<3/1) and
yield were improved. These results demonstrated that both
Lewis acid and base were in favour of the DL-selectivity in
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
Ph
2-MeC
H
2a
2b
2c
2d
2e
2f
82 67:33
92 74:26
76 59:41
94 73:27
88 72:28
76 73:27
72 74:26
72
6
H
H
6 4
H
H
H
H
H
H
H
H
H
H
2-ClC
3-MeC
3-MeOC
4-(Me
1-Naphthyl
4-NO
3-MeO,4-HOC
n-C
6
H
4
6
H
4
2
N)C
6
H
4
2g
c
2 6 4
C H
c
6
H
4
54
0
1
2
3
4
5
3
H
7
2j
2k
2l
66 65:35
62 75:25
96 79:21
99 73:27
70
Cyclohexyl
Ph
Ph
Ph
Me
n-Bu 2m
Ph
Me
this Rieke Ni-mediated coupling. However, if H O, 18-
2
2n
2o
crown-6, or 1,3-dinitrobenzene was used as an additive
(entries 6–8), only the simple reduction product PhCH OH
was given without the formation of desired pinacol product.
PhCH
2
CH
2
64 79:21
2
a
b
c
All analytical data of the diols are identical with those literature reported.
1
Measured by H NMR spectroscopy.
1
Only the reduction product R (R )CHOH was obtained.
2
Based on the above experimental results together with
1
2
corresponding literatures, a plausible single-electron
transfer (SET) mechanism was proposed as indicated in
Scheme 1. The initial radical R-1 was generated via a SET
process between Rieke Ni and R (R )CO. Next two
Table 2. Additive effects on the Rieke Ni-mediated pinacol coupling
reaction
L
S
possible key coupling species A-1 and B-1 led to the
formation of corresponding DL- and meso-products. Owing
to the high oxyphilic ability and the high coordination
number of the low-valent Ni, the formation of A-1 with the
coordinative Ni–O interaction was favoured than B-1
a
b
Entry
Additive
Ratio
Yield (%)
DL/meso
1
2
3
4
5
6
7
8
TMEDA
HMPA
ZnCl
AlCl
TiCl
2
2
40
45
86
80
85
73:27
78:22
76:24
74:26
73:27
without such interaction. Furthermore, two bulky R groups
L
in A-1 tended to display a favourable anticlinal confor-
mation which resulted in the formation of DL-product, than
the synperiplanar one which yielded the meso-product.
Consequently, the Rieke Ni-mediated coupling could
exhibit the more preferential DL-selectivity.
2
0.2
0.2
0.2
3
4
c
0.1
d
H
18-crown-6
1,3-Dinitrobenzene
2
O
73
d
2
2
86
d
92
a
The mole ratio between the additive and PhCHO 1a.
Measured by H NMR spectroscopy.
On the basis of the proposed reaction mechanism, some
additives (entries 6–8 of Table 2) and substrates behavior
(entries 8 and 9 of Table 1) in the coupling reaction could be
b
c
d
1
The volume ratio between H
The main products of entries 6–8 are PhCH
2
O and THF.
2
OH.
Scheme 1. Proposed mechanisms for Rieke Ni-promoted coupling pinacol reaction.

L. Shi et al. / Tetrahedron 60 (2004) 2851–2855
2853
understood. Because the –OH, –NO2 or ether group
presented in the additives or substrates readily quench the
radical R-1, the coupling process through A-1 and B-1 was
inhibited to give the simple reduction product. Moreover,
some properties of the coupling reaction in the presence of
Lewis acid or base additives could be explained with the
favourable A-1. Firstly, the improvement of the DL-
selectivity caused by Lewis base (LB) additives (entries 1
The preparation of NiCl (Cat.)/Mg/TMSCl system and the
2
catalytic coupling experiment were performed in the
following procedure: the catalytic amount of NiCl2
(0.05 equiv.) was first mixed with Mg (2.0 equiv.) in THF
1
2
to give a suspension, to which a solution of R (R )CO
(1.0 equiv.) and TMSCl (2.0 equiv.) was then added
dropwise. The pinacol coupling took place readily (see
Section 4.5). The results were tabulated in Table 3, which
indicated that this catalytic system was effective, to a broad
range of substrates including aromatic aldehydes (entries
1–9), aliphatic aldehyde (entry 10) and aromatic ketone
(entry 11) to give pinacols in moderate to good yields.
Further inspection of Table 3 showed some more important
information. For example, in comparison with the corre-
sponding Rieke Ni-mediated couplings in Table 1, the
meso-selectivity for some substrates, such as 1e, 1f, 1j and
1l, was improved in different degree. In particularly,
substrate 1f (entry 8) gave exclusively the meso-product,
0
and 2 of Table 2) could be understood from B and B
resulting from their coordination with the low-valent Ni (as
shown in Scheme 2). Due to the steric encumbrance
imposed by the coordinating LB additives on the nickel
center, the rotation of C–O single bond in B, leading to the
0
formation of more hindered B , was restricted to some
extent, so to improve DL-selectivity. Similarly, the additive
effect of Lewis acid (LA) (entries 3–5 of Table 2) could also
be explained in terms of the proposed Scheme 3. Because
their coordination with the oxygen gave a more sterically
crowded environment, the formation of A was favourable
1
4
which is of particular importance in organic synthesis. It
was also important that an intermolecular cross-pinacol
0
than that of A and consequently improved DL-selectivity.
1
5
coupling was successfully conducted to produce the cross-
coupling product 2al in 52% yield (entry 12).
2
Table 3. NiCl (Cat.)/Mg/TMSCl-mediated pinacol coupling reaction
1
R
2
R
a
Product Yield (%) DL/meso
b
Entry Substrate
1
2
3
4
5
6
7
8
9
1
1
1
1a
1p
1q
1r
1e
1s
1t
1f
1u
1j
1l
Ph
2-BrC H
H
H
H
H
H
H
H
H
H
H
2a
2p
2q
2r
2e
2s
2t
2f
2u
2j
70
72
72
77
79
77
71
67
66
61
51
78:22
53:48
64:36
57:43
52:48
45:55
55:45
1:99
69:31
59:41
50:50
50:50
Scheme 2. The effect of Lewis base in Rieke Ni system.
6
4
2-MeOC
3-ClC
3-MeOC
4-ClC
4-MeOC
4-(Me)
2-Furyl
6
H
H
H
4
4
4
6 4
H
6
6 4
H
6
2
NC
6
H
4
0
1
2
3 7
n-C H
Ph
Me 2l
2al
c
d
1aþ1l
52
a
The analytical data of the diols are all identical with those previously
reported.
b
c
d
1
Measured by H NMR spectroscopy.
Zn was co-reductant.
2
2
1
A cross-pinacol coupling product 2al (left R ¼right R ¼Ph, left R ¼Me,
1
Scheme 3. The effect of Lewis acid in Rieke Ni system.
right R ¼H) was isolated, together with the minor diol 2a (40%).
Encouraged by the above information that the Ni generated
in situ can efficiently induce the pinacol coupling of
carbonyl compounds via an interesting SET process, and
also on considering that few reports on the Ni-mediated
catalytic pinacol coupling were documented, we sub-
sequently designed and developed another novel effective
The Ni-catalyzed mechanism in this pinacol coupling was
proposed in Scheme 4 on the basis of the above results and
1
3
the related literatures. The initial step would involve the
OX
reduction of oxidative state Ni (Ni ) by Mg to generate the
RED
active reductive state Ni (Ni
), which subsequently
catalytical system, the NiCl (Cat.)/Mg/TMSCl, which has
2
coordinated with the carbonyl of substrate R (R )CO to
L S
1
3
not been mentioned before to our knowledge. Although a
Mg/TMSCl system reported previously did not work at all
form a radical R-2 (M¼Ni). Next two different coupling
ways may be involved. If the Ni–O bond in the initially
formed R-2 (M¼Ni) was cleaved by TMSCl before the
pinacol coupling, the formation of B-2 (M¼TMS) would be
favourable than that of A-2 (M¼TMS) because of the less
hindrance in B-2 (M¼TMS) with the antiperiplanar
conformation, and the subsequent coupling reaction would
result in the formation of more meso-product. The other way
7
j
for such kind of coupling, our experimental results below
demonstrated the NiCl (Cat.)/Mg/TMSCl system we devel-
2
oped was effective to couple of aldehydes and ketone to the
pinacol. The major values of this system involved the
catalytic effectiveness, the use of cheap Mg metal and the
convenient operation.

2854
L. Shi et al. / Tetrahedron 60 (2004) 2851–2855
Scheme 4. Proposed mechanism for Ni-catalyzed pinacol coupling reaction.
may be that the initially formed R-2 (M¼Ni) directly
OX
(0.3 equiv.) and anhydrous NiCl (1.0 equiv.) were stirred in
2
underwent a pinacol coupling through A-2 (M¼Ni) and B-2
freshly distilled THF for 1–3 h at room temperature under
argon atmosphere, then a black slurry was obtained and
ready for use.
(
M¼Ni), after which the Ni was replaced by TMS for the
next catalytic cycle This was just like the Rieke Ni system as
shown in Scheme 1 resulted in the formation of more DL-
product. In this catalytic system, we may determine the
reaction type was the former, the latter or the combination of
the two, according to the DL/meso selectivity which was
highly dependent upon the substrate structure.
4.3. General procedure A: the Rieke Ni-mediated
pinacol coupling
To a slurry of Rieke Ni (1.0 equiv.) prepared in situ was
added the substrate 1 (0.5 equiv.) at room temperature under
Ar. The reaction mixture was stirred and monitored by TLC
until the substrate was consumed completely. Then the
reaction was quenched with an aqueous solution (3 M) of
3. Conclusions
In conclusion, we have developed two novel and effective
Ni-mediated pinacol coupling systems which were applic-
able to a broad substrate scope. Particularly we believe the
Na
layer was separated and the aqueous phase was extracted
with CH Cl . The combined organic layers were washed
with the brine and dried over anhydrous Na SO . Evapor-
2 2 3 2 2
S O followed by the addition of CH Cl . The organic
2
2
optimized NiCl (Cat.)/Mg/TMSCl system would bring
2
2
4
much application in practical organic synthesis. The further
investigation is ongoing in our group.
ation of the solvent and column chromatography of the
crude pinacol product on silica gel (petroleum/ethyl acetate
4/1!2/1) afforded the desired diol 2.
4. Experimental
4.4. General procedure B: the Rieke Ni-mediated pinacol
coupling with the additives
4
.1. General
To a stirred suspension of Rieke Ni (1.0 equiv.) and the
additive (0.1 or 1.0 equiv.) in freshly distilled THF was
added PhCHO (0.5 equiv.) at room temperature under Ar.
The reaction mixture was further stirred efficiently. When
TLC analysis indicated the PhCHO was consumed com-
pletely, the reaction was quenched. The following workup
was similar to that of the general procedure A.
All reactions were carried out under an atmosphere of argon.
THF was dried and freshly distilled over sodium/benzo-
phenone before use. All reagents were commercial and used
without further purification. All reactions were monitored
by thin-layer chromatography (TLC) on gel F254 plates. The
silica gel (200–300 meshes) for column chromatography
was from the Qingdao Marine Chemical Factory in China,
1
and the distillation range of petroleum is 60–90 8C. H and
1
4.5. General procedure C: the NiCl
mediated pinacol coupling
(Cat.)/Mg/TMSCl-
2
3
C NMR spectra were recorded in CDCl solution on the
3
Avance DRX-200 instruments, and spectral data are
reported in ppm relative to tetramethylsilane (TMS) as
internal standard. High-resolution mass spectral analysis
(HRMS) data were measured on the Bruker ApexII by
means of ESI technique.
A mixture of NiCl (0.05 equiv.) and activated Mg powder
2
(2.0 equiv.) in freshly distilled THF was stirred for 10 min at
room temperature under Ar. Then a solution of substrate 1
(1.0 equiv.) and TMSCl (2.0 equiv.) was added dropwise to
the above suspension, and the reaction mixture was stirred
efficiently. After the substrate consumed by the check of
Spectroscopic data for these pinacol coupling adducts has
1
6
been reported previously.
.2. General procedure for the preparation of Rieke Ni
In a typical process, Li (small cuts, 2.1 equiv.), naphthalene
TLC, Et O and an aqueous solution (1.5 M) of HCl were
2
added to the resulting mixture. The organic layer was
separated followed by the extraction of the aqueous layer
with Et O. The combined organic extracts were washed
4
2
with saturated NaHCO solution and brine, and dried over
3

L. Shi et al. / Tetrahedron 60 (2004) 2851–2855
2855
anhydrous Na SO . After removing the solvent the residue
2
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