Vinylation of Aldehydes Using Mn/Cr Alloy and a N4-Ligand/NiII-Catalyst

Article abstract of DOI:10.1002/chem.201500024

We report an efficient and practical protocol for the Cr/Ni-catalyzed vinylation of aldehydes, based on the use of Mn/Cr alloy (ca. 10% Cr) and TMSCl. No additional Cr salts need to be added. In the presence of NiCl₂ (0.3 mol%) and a bis(ketimino)-2,2′-bipyridine as N₄-chelating ligand (1 mol%), the vinylations proceed smoothly at room temperature. The presence of catalytic amounts of MeOH and LiOAc as additives was found to further promote the efficiency of the catalytic system, even in the absence of the ligand. Detailed reaction monitoring revealed that LiOAc accelerates the product alcohol silylation, thus increasing the turnover rate.

Full text of DOI:10.1002/chem.201500024

DOI: 10.1002/chem.201500024
Communication
&
CꢀC Coupling
Vinylation of Aldehydes Using Mn/Cr Alloy and a N4-Ligand/NiII-
Catalyst
Wacharee Harnying* and Albrecht Berkessel*[a]
original NHK procedure has serious drawbacks, such as the re-
Abstract: We report an efficient and practical protocol for
quirement for larger than stoichiometric amounts of toxic CrII
the Cr/Ni-catalyzed vinylation of aldehydes, based on the
salts (usually in the range of 400–1600 mol%), although it is
use of Mn/Cr alloy (ca. 10% Cr) and TMSCl. No additional
catalytic in nickel.
Cr salts need to be added. In the presence of NiCl2
A catalytic version of the NHK reaction, using about
(0.3 mol%) and a bis(ketimino)-2,2’-bipyridine as N4-chelat-
15 mol% of CrCl2 (and ca. 3 mol% of NiCl2), was reported by
ing ligand (1 mol%), the vinylations proceed smoothly at
Fꢀrstner et al. in 1996 (Scheme 1).[3] The salient features are the
room temperature. The presence of catalytic amounts of
use of nontoxic, commercial Mn powder as the stoichiometric
MeOH and LiOAc as additives was found to further pro-
reductant for recycling CrIII to the active CrII species, and trime-
mote the efficiency of the catalytic system, even in the ab-
thylsilyl chloride (TMSCl) as the dissociating agent for the inter-
sence of the ligand. Detailed reaction monitoring revealed
mediate chromium(III) alkoxides.[4] Notably, the regeneration of
that LiOAc accelerates the product alcohol silylation, thus
CrII by reduction of CrIII with Al,[5] by electro-reduction,[6] and by
increasing the turnover rate.
the
organic
reductant
tetrakis(dimethylamino)ethylene
(TDAE)[7] was also reported by others. The use of [Cp2ZrCl2] as
an effective dissociating agent instead of TMSCl was later de-
veloped by Kishi et al.[8] With appropriate ligands, a reduction
of catalyst loading from typically 10–20 mol% Cr and 1–
5 mol% Ni to 1–2 mol% Ni/Cr catalysts was achieved with ac-
ceptable rates.[9] However, the rather high cost of [Cp2ZrCl2] is
prohibitive for larger scale syntheses and industrial applica-
tions. To date, only two types of chiral ligands, that is, endo,en-
do-2,5-diaminonorbornane (DIANANE)–salens[10] and oxazoline/
sulfonamides,[8a,11] have successfully been applied to the asym-
metric Cr/Ni-catalyzed vinylation of aldehydes, generating
enantiomerically enriched chiral allylic alcohols. In practice, the
methods available still suffer from limited substrate scope, low
predictability of rates and enantioselectivity, high catalyst load-
ings, and relatively complex, difficult to access ligand struc-
tures.
The Cr/Ni-mediated addition of alkenyl and aryl halides/triflates
to aldehydes, the so called Nozaki–Hiyama–Kishi (NHK) reac-
tion discovered in 1986,[1] is a versatile CꢀC bond-forming reac-
tion that proceeds under mild conditions (Scheme 1). It fea-
tures excellent chemoselectivity towards aldehydes and com-
patibility with a large variety of functionalities.[2] However, the
As a part of our ongoing efforts to improve the efficiency of
this catalytic process, we recently reported our investigations
of the catalytic roles of nickel and chromium, by using electro-
chemistry and spectroscopic (UV/Vis) methods.[12] In short, we
could confirm that a low-valent Ni species (NiI or Ni0) is neces-
sary for vinyl halide cleavage, generating an unstable Ni–vinyl
species that is prone to rapid decomposition. Importantly, this
Ni–vinyl species is stabilized in the presence of Cr, potentially
by formation of a bimetallic complex. Furthermore, the genera-
tion of low-valent Ni occurs at significantly higher (i.e., milder)
potential when Cr is present, and the type of ligand present in
the Cr complex affects the efficiency of the NiII reduction. This
last result points to an additional catalytic role of Cr when ex-
ternal chemical reductants are used.
Scheme 1. Stoichiometric and catalytic NHK reactions, and the proposed
mechanisms.
[a] Dr. W. Harnying, Prof. Dr. A. Berkessel
Department of Chemistry, Cologne University
Greinstrasse 4, 50939 Cologne (Germany)
E-mail: berkessel@uni-koeln.de
Considering Fꢀrstner’s catalytic system (Mn/TMSCl/cat.
CrCl2–NiCl2),[3] several limiting factors for the coupling of alken-
yl and aryl halides (triflates) with aldehydes need to be men-
tioned.
Homepage: http://www.berkessel.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500024.
Chem. Eur. J. 2015, 21, 1 – 6
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Communication
1) As for the stoichiometric process,[13] two equivalents of
vinyl halide are required for satisfactory product yields, as
nickel-catalyzed dimerization of the vinyl halides to form
dienes was frequently observed.
Table 1. Effect of ligands and additives on the coupling reaction of 1a
and 2a.[a]
2) The stoichiometric process proceeds smoothly in DMF or
DMSO at room temperature, whereas the catalytic process
requires 508C in DME/DMF (20:3), as it is too slow at lower
temperatures.
3) At least 15 mol% of CrCl2 is required for efficient conver-
sion. At lower catalyst loadings, the coupling proceeds only
to a certain degree, but not to completion.
Reductant
(1.5 equiv)
L
MeOH
[mol%]
Additive
(0.5 equiv)
Conv
[%][b]
3aa
4) Anhydrous CrCl2 is rather expensive, hygroscopic, and is
oxidized rapidly in air, resulting in green CrIII. The CrCl2 used
should be colorless; pale-green batches notoriously give
lower yields. If the reaction indeed proceeded with an effi-
cient CrII$CrIII regeneration process, one would expect the
catalysis to proceed regardless of the oxidation state of the
chromium source.[14]
[%][b]
1
2
3
4
5
6
7
8
9[c]
10
11
12
13
14
15[d]
16
17
18
19
20
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn93Cr7
Mn0 +Cr0
Mn0 +CrIII
Mn0
–
–
–
–
–
–
2
2
–
2
2
2
–
2
2
2
2
2
2
2
2
–
–
–
–
29
61
37
8
16
34
3
5
2
L1
L2
L3
L4
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
–
–
28
100
78
76
47
74
86
78
86
13
53
16
97
26
94
48
LiOAc
–
LiOAc
LiOAc
TMSOAc
LiCl
AcOH
LiOAc
LiOAc
LiOAc
LiOAc
LiOAc
LiOAc
LiOAc
LiOAc
86
50
42
1
54
69
50
69
1
24
0
75[e]
7
5) The total turnover is limited by the silylation step. If the si-
lylation is incomplete, the product is obtained as a mixture
of allyl alcohol and its silyl ether.
Based on the above-mentioned considerations, we sought
to employ bimetallic Mn/Cr alloy instead of the “classical” com-
bination of monometallic Mn powder and Cr salt. We expected
the Mn/Cr alloy to act as a single electrochemical entity, thus
avoiding the uncertainties of electron transfer to Cr in hetero-
genous mixtures. In this communication, we report the devel-
opment of an efficient and practical catalytic vinylation pro-
cess, based on bimetallic Mn/Cr alloy and a catalytic amount
of N4-tetradentate ligand/NiCl2, operating at very low catalyst
loading.
Mn89Cr11
Mn89Cr11
Mn89Cr11
Mn89Cr11
L2
L3
L4
80
4
[a] Reactions were carried out using 1a (0.26 mmol), 2a (1.3 equiv) in
DMF (0.26 mL). [b] Determined by GC using biphenyl as the internal stan-
dard. [c] In the absence of NiCl2·glyme. [d] CrCl3·3THF (15 mol%) was
used. [e] A trace of homocoupling of 2a was observed.
Our investigation started by monitoring the coupling of 1a
and 2a in the presence of NiCl2·glyme (0.3 mol%) and ligand
(1 mol%) using bimetallic Mn89Cr11 alloy[15] (1.5 equiv, that is,
1.3 equiv Mn and 17 mol% Cr) as the stoichiometric reductant
and TMSCl as the dissociating agent in anhydrous DMF at am-
bient temperature for 16 h (Table 1). The yield of the alcohol
3aa was determined after desilylation. To this end, we devised
a method for TMS-deprotection that allowed the whole pro-
cess to be performed as a one-pot transformation, simply by
treatment of the reaction mixture with silica-supported sulfuric
acid (H2SO4·SiO2).[16] For aliphatic aldehydes as substrates, some
silyl enol ether is typically formed as side product. Our desilyla-
tion protocol also converts this silyl enol ether back to the cor-
responding aldehyde. The conversion can be determined relia-
bly without desilylation after stirring of the crude sample with
florisil in ethyl acetate.
that were even lower than in the absence of ligand (Table 1,
entries 3–5 vs. 1). This is noteworthy, as 4,4’-di-tert-butyl-2,2’-bi-
pyridine (L4) was reported by Kishi et al. to afford CrIII and NiII
catalysts effective in the coupling of vinyl halides and aldehy-
des.[11c]
We next performed extensive screening of additives in the
presence of L1 (1 mol%) as ligand and intriguingly found that
both MeOH (2 mol%) and LiOAc (0.5 equiv) in DMF had a dra-
matic effect on reactivity,[18] affording 3aa in 86% yield with
full conversion (Table 1, entries 6 vs. 7 and 8). Changing the
solvent from DMF to THF or DME resulted in lower yields of
3aa (70–76%), still at full aldehyde conversion. Only a trace of
product was formed in MeCN.
In the absence of ligand, the reaction proceeded to 29%
conversion after 16 h, providing the allylic alcohol 3aa in 16%
yield (Table 1, entry 1). Note that the undesired homocoupling
of 2a was not observed. For the ligand screening, we focused
on N4-chelating ligands, as there appears to be no report on
the use of such N4-tetradentate ligands in coupling reactions
of this type. Among the ligands tested, the bis(imino)bipyri-
dine L1[17] provided promising results (Table 1, entry 2). Note
that the use of the related ligands L2–L4 gave coupling yields
We could pinpoint that the catalytic effect of MeOH (EtOH is
equally effective)[19] is due to the in situ generation of HCl, by
reaction with TMSCl. When MeOH was replaced by HCl(g)/diox-
ane (2 mol%), the product 3aa was obtained in identical yield.
We suggest that a small quantity of HCl may be required to ac-
tivate the metal surface, and/or to promote the in situ genera-
tion of Ni/Cr–ligand complex. Ni complex formation is of
course crucially important: When the reaction was conducted
with L1 (1 mol%) and in the absence of NiCl2·glyme, only
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Communication
a trace of product resulted, along with a large quantity of the
aldehyde silyl enol ether (Table 1, entry 9). The replacement of
in situ NiCl2·glyme/L1 (0.3:1 mol%, Table 1, entry 6)[20] by pre-
[17]
[9a]
formed L1·NiCl2 or L4·NiCl2 complexes (0.3 mol%, identical
reaction conditions) in the presence of LiOAc, afforded 3aa in
75% (95% conversion) and 47% (72% conversion) yield, re-
spectively.
We observed that about 1.5 equivalents of free TMSCl were
required for efficient coupling, and no coupling at all occurred
in the absence of TMSCl. It may be argued that in the reaction
mixture, LiOAc and TMSCl could generate TMSOAc and LiCl.
Examination of the effects of TMSOAc, LiCl, and AcOH as addi-
tives in place of LiOAc clearly confirmed the superior beneficial
effect of LiOAc (Table 1, entries 10–12). The replacement of
Mn89Cr11 by Mn93Cr7 resulted in lower yield of 3aa (Table 1,
entry 13). Using a physical mixture of pure Mn0 (1.5 equiv) and
Cr0 (20 mol%) powders did not promote the reaction at all,
demonstrating the synergistic effect resulting from atom-scale
mixing of the two metals in the bimetallic particles (Table 1,
entry 14). In addition, the use of a mixture of Mn0 and
CrCl3·3THF (15 mol%) drastically reduced the yield of 3aa to
24%, along with the pronounced formation of the silyl enol
ether of 1a (Table 1, entry15). Note that no coupling product
was formed in the absence of Cr (Table 1, entry 16). Remarka-
bly, the addition of MeOH and LiOAc also significantly promot-
ed the coupling reaction even in the absence of ligand
(Table 1, compare entries 17 vs 1) and when the ligand L1 was
replaced by L3 (Table 1, entries 19 vs 4), the product 3aa was
obtained in high yield of 75–80%. In contrast, the quaterpyri-
dine L2 and bipyridine L4 gave only trace amounts of 3aa,
even in the presence of additives (Table 1, entries 18 and 20).
To gain further insight into the catalytic effect of LiOAc, we
monitored the formation of the coupling products 3aa and
4aa as a function of time (Figure 1). For this purpose, aliquots
of the reaction mixture were withdrawn, stirred with florisil in
EtOAc, filtered, and analyzed by GC. The results are summar-
ized in Figure 1, the conversion of starting aldehyde 1a and
yields of 3aa and 4aa are plotted as a function of reaction
time.
Figure 1. Effect of LiOAc on the coupling reaction of 1a and 2a, concentra-
tion versus time profiles.
and then decreased to a persistent level of approximately 4%.
Apparently, the LiOAc additive facilitates the silylation step, po-
tentially by cleavage of chromium–alkoxide intermediates, thus
increasing overall reaction rate.
With the newly developed catalytic protocol in hand, we set
out to study the substrate scope with regard to various alde-
hydes and vinyl/aryl iodides/triflate (Table 2).[21] The coupling of
2a with the aliphatic aldehydes 1a–c proceeded smoothly
under our catalytic conditions, giving the corresponding allylic
alcohols in very good isolated yields of 75-85% (Table 2, en-
tries 1–3). With the sterically demanding aldehyde 1d, a moder-
ate yield of 3da was obtained (Table 2, entry 4). Moderate
yields were also obtained with aromatic aldehydes 1e and f
(Table 2, entries 5 and 6), For aromatic aldehydes, competing
pinacol coupling is a major side reaction. In line with this, elec-
tron-poor aromatic aldehyde 1g gave pinacol coupling exclu-
sively (Table 2, entry 7). The reaction of 2b, the triflate ana-
logue of 2a, proceeded better in the absence of LiOAc or LiCl,
affording 3aa in 26% at 43% conversion after 24 h (23% yield
when no L was added). Gratifyingly, by increasing the amount
of NiCl2 to 2 mol% and in the absence of ligand, 3aa was ob-
tained in 76% yield with full conversion and only a trace of 2b
dimerization was observed (Table 2, entry 8).
In the absence of LiOAc (Figure 1A), the graph clearly shows
that in the initial phase of the reaction (up to 9 h, 56% conver-
sion), up to 24% of the alcohol 3aa (red line) was present in
an amount almost equal to its silyl ether 4aa (green line).
Later on, the amount of silyl ether 4aa increased further, while
that of the alcohol 3aa first decreased and then remained
nearly unchanged over the further course of reaction. After
24 h (not shown in graph), a mixture of 3aa (11%) and 4aa
(45%) was obtained at an aldehyde conversion of 86%. The
concentration/time profiles of 3aa/4aa point to initial libera-
tion of the alcohol product 3aa from a metal–alkoxide inter-
mediate, and subsequent transformation to its silyl ether 4aa.
In the presence of 50 mol% LiOAc (Figure 1B), the reaction
proceeds significantly faster, and completion is reached within
about 12 h. Most importantly, the silyl ether 4aa was present
as the major product over the whole course of the reaction,
and increased steadily. In contrast, the amount of free alcohol
(3aa, red line) went through a maximum (13%) at about 6 h
The coupling of the aliphatic aldehyde 1a with iodobenzene
(2c) did not reach completion under standard conditions.
However, increasing the amount of LiOAc to 1.0 equivalent sig-
nificantly speeded up the reaction, and the benzyl alcohols 3
were obtained in yields of 50–80% (Table 2, entries 9–13), to-
gether with trace amounts of biphenyl. The reactivity of 1-io-
docyclopentene (2e) and of the 1-iodohexenes (E)-2 f and (Z)-
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Further mechanistic studies, for example, on the question of
surface versus homogeneous reaction, are underway.
Table 2. The coupling reaction of aldehydes 1 and vinyl/aryl iodides/tri-
flate 2 in the presence of L1, affording the alcohols 3.[a]
Experimental Section
General procedure:[23] A solution of NiCl2·glyme in DMF (0.19 mL,
0.04m (freshly prepared), 7.5 mmol, 0.003 equiv) and MeOH (2.0 mL,
50 mmol, 0.02 equiv) was added to a suspension of Mn89Cr11
powder (205 mg, 3.75 mmol, 1.50 equiv), L1·0.5THF (15.0 mg,
25 mmol, 0.01 equiv), and LiOAc (0.5/1.0 equiv) in dry DMF (2.0 mL)
under an argon atmosphere (note that the reaction is air sensitive).
The mixture was stirred for 15 min, followed by the addition of
TMSCl (2.1/2.7 equiv). After stirring for 5 min, resulting in a very
dark brown mixture, 2 (1.2 equiv) and aldehyde 1 (2.5 mmol,
1.0 equiv) were added successively. The reaction mixture was
stirred at ambient temperature for 15/24 h under Ar. The resulting
very dark brown mixture was then diluted with EtOAc (2 mL).
H2SO4·SiO2 (ca. 250–300 mg) was added and the mixture was
stirred for about 30 min for TMS deprotection. After completion
(by TLC), silica gel (ca. 0.5 g) and EtOAc (20 mL) were added and
the mixture was stirred vigorously for 30 min. This should result in
a green powder in solution (if not, use spatula to delump and stir
longer). The resulting mixture was subjected to suction filtration
(washing with EtOAc). After concentration of the filtrate in vacuo,
the residue was purified by column chromatography on silica gel
(EtOAc/cyclohexane) to give the alcohol 3.
RꢀCHO
R1X
LiOAc
[equiv]
t
[h]
3
Yield
[%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13[e]
14[e]
15[e]
16[e]
1a (PhCH2CH2-)
1b (H15C7-)
1c (cyHex-)
1d (tBu-)
1e (Ph-)
1 f (4-OMe-Ph-)
1g (4-CN-Ph-)
1a (PhCH2CH2-)
1a (PhCH2CH2-)
1b (H15C7-)
1c (cyHex-)
1e (Ph-)
1a (PhCH2CH2-)
1a (PhCH2CH2-)
1a (PhCH2CH2-)
1a (PhCH2CH2-)
2a
2a
2a
2a
2a
2a
2a
2b
2c
2c
2c
2c
2d
2e
2 f
2g
0.5
0.5
0.5
0.5
0.5
0.5
0.5
–
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
15
15
15
15
15
15
15
24
24
24
24
24
24
24
24
24
3aa
3ba
3ca
3da
3ea
3 fa
3ga
3aa
3ac
3bc
3cc
3ec
3ad
3ae
3af
85 (74)
75
78
55
65
45
0[c]
26 (76[d]
80 (74)
70
65
50
52[f] (34)
56 (51)
50 (35)
30 (12)
)
3ag
[a] Reactions were carried out using 1 (2.5 mmol), 2 (1.2 equiv), Mn89Cr11
(1.5 equiv), NiCl2·glyme (0.3 mol%), L1 (1 mol%), MeOH (2 mol%), LiOAc
(0.5/1.0 equiv), TMSCl (2.1/2.7 equiv) in DMF (2.2 mL) at room tempera-
ture. [b] Isolated yield after column chromatography. In parenthesis,
when no ligand was added. [c] Only pinacol coupling product was ob-
served. [d] 2 mol% of NiCl2·glyme was used. [e] The reaction did not
reach completion, both 1 and 2 remained. [f] 4,4’-Bis(ethoxycarbonyl)di-
phenyl was obtained in 5% based on 2d.
Acknowledgements
This work was supported by the Alexander von Humboldt-
Foundation and by the Fonds der Chemischen Industrie.
Keywords: CꢀC coupling · chromium · catalysis · N ligands ·
nickel
2g in the coupling with 1a was found to be lower than that
of 2a and 2c. After 24 h, the coupling products 3ae–ag were
obtained in moderate yield only (Table 2, entries 14–16), with
some 1a and 2 remaining. Extension of the reaction times im-
proved the product yields only slightly. Notably, no Z/E isomer-
ization was detected in the reactions of 2 f and 2g (Table 2, en-
tries 15, 16). Upon increasing NiCl2 to 0.5 mol% (L1 1.5 mol%),
however, partial Z!E isomerization was observed with 2g,[23]
yielding a 11:1 Z/E product mixture in 38% yield at 74% con-
version after 18 h.
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In summary, we have developed an efficient and practical
protocol for the coupling of vinyl/aryl iodides/triflate and alde-
hydes. The use of the bimetallic Mn/Cr alloy apparently pro-
vides lower reduction potential, and no additional Cr catalyst is
required. As a consequence, aliphatic aldehydes are particularly
suitable substrates, whereas aromatic aldehydes tend to under-
go pinacol coupling. High catalytic efficiency is achieved at
very low NiII/L1 catalyst loading, in the presence of MeOH and
LiOAc as additives. The reaction also proceeded well in the ab-
sence of ligand or with commercially available tetraphenylpor-
phyrin (L3), albeit with slightly lower yield. This protocol can
be considered as an alternative for large scale applications. To
the best of our knowledge, we report here the first case of
using a task-specific alloy in catalytic organometallic synthesis.
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2012, 14, 86–89.
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Communication
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1065; b) A. Berkessel, M. Schroeder, C. A. Sklorz, S. Tabanella, N. Vogl, J.
Lex, J.-M. Neudçrfl, J. Org. Chem. 2004, 69, 3050–3056; c) I. Paterson, H.
Bergmann, D. Menche, A. Berkessel, Org. Lett. 2004, 6, 1293–1295.
[11] a) H.-W. Choi, K. Nakajima, D. Demeke, F.-A. Kang, H.-S. Jun, Z.-K. Wan, Y.
Kishi, Org. Lett. 2002, 4, 4435–4438; b) K. Namba, S. Cui, J. Wang, Y.
Kishi, Org. Lett. 2005, 7, 5417–5419; c) H. Guo, C.-G. Dong, D.-S. Kim, D.
Urabe, J. Wang, J. T. Kim, X. Liu, T. Sasaki, Z. Kishi, J. Am. Chem. Soc.
2009, 131, 15387–15393; d) X. Liu, J. A. Henderson, T. Sasaki, Y. Kishi, J.
Am. Chem. Soc. 2009, 131, 16678–16680; e) X. Lui, X. Li, Y. Chen, Y. Hu,
Y. Kishi, J. Am. Chem. Soc. 2012, 134, 6136–6139.
[12] W. Harnying, A. Kaiser, A. Klein, A. Berkessel, Chem. Eur. J. 2011, 17,
4765–4773.
[13] K. Takai, K. Sakogawa, Y. Kataoka, K. Oshima, K. Utimoto, Org. Synth.
1995, 72, 180–186.
[14] It was also reported in reference [5] for the Al/Cr/Ni system that the al-
kenylation of aldehyde was not promoted when CrCl2 was replaced by
commercial anhydrous CrCl3. Interestingly, upon addition of Zn powder
(30 mol%), the coupling reaction could be achieved, albeit with lower
yield.
[18] 1 to 5 mol% MeOH gave similar results. Other additives tested included
NaOAc, NaOBz (Bz=benzoyl), LiOBz, LiI, LiCl, and were found to be less
effective. The use of Et3N·HCl and LiCl as additives was found to pro-
mote the CrII/NiII-mediated coupling reaction, see ref. [11a].
[19] The activating effect of alcohol was first observed by using a highly ef-
fective batch of L1, which turned out to be contaminated with a small
amount of EtOH. As we could not prepared an absolutely solvent-free
ligand, L1·0.5THF was prepared and used in our study.
[20] The optimal NiCl2/L ratio was found to be 1:3, a lower amount of
ligand resulted in lower conversion and lower product yield.
[21] Independent of the vinyl halide used, the silyl enol ethers of enolizable
aldehydes were observed as byproducts in all cases. When the product
allylic alcohols 3 are isolated as their silyl ethers 4, potential contamina-
tion by the silyl enol ether of aldehyde has to be kept in mind, as the
chromatographic behavior of the product silyl ethers and aldehyde silyl
enol ethers are typically similar. In practice, the pure allylic alcohol
product is best isolated after desilylation.
[22] Generally, the extent of Z!E isomerization can be suppressed by low-
ering the Ni/Cr ratio, as demonstrated by Kishi et al., in the course of
an extensive study on geometrical isomerization effected by Ni cata-
lysts during the Cr/Ni-mediated coupling, although no yields were
given, see reference [11f].
[15] The formula Mn89Cr11 indicates that the Mn/Cr alloy contains 11% (by
weight) of chromium—see Supporting Information for the preparation
of the alloy. Commercialization of the alloy is in progress.
[16] For the preparation of H2SO4·SiO2, see: Z. Yan, L. Jun, S. Zhicai, Chin. J.
Chem. 2010, 28, 1736.
[23] The Mn/Cr powder was slowly dissolved during the reaction progress
and only a trace of metal powder remained upon completion. Effecive
stirring (ca. 750 1 min, oval shaped stir bar) of the reaction mixture is
essential for promoting the coupling.
[17] It has been reported that L1 acts as a tetradentate ligand, preferentially
forming six-coordinate NiII complexes, see: G. A. Griffith, M. J. Al-Khatib,
K. Patel, K. Singh, G. A. Solan, Dalton Trans. 2009, 185–196.
Received: January 4, 2015
Published online on && &&, 0000
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
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ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
Fantastic four—Mn/Cr-alloy+Ni/N4:
When a Mn/Cr alloy (ca. 10% Cr) is used
as the stoichiometric reductant, the cat-
alytic vinylation of aldehydes proceeds
efficiently in the presence of a Ni cata-
lyst (see scheme: 0.3 mol% NiII, w/wo 1
mol% of N4-ligand). No additional Cr
catalyst is required. There is no wasteful
homocoupling of vinyl iodides to
&
CꢀC Coupling
W. Harnying,* A. Berkessel*
&& – &&
Vinylation of Aldehydes Using Mn/Cr
Alloy and a N4-Ligand/NiII-Catalyst
dienes, and the product allylic alcohol is
isolated without aqueous workup.
&
&
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
6
ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!