Hydroboration of Alkenes Catalysed by a Nickel N-Heterocyclic Carbene Complex: Reaction and Mechanistic Aspects

Article abstract of DOI:10.1002/chem.202000289

The pentamethylcyclopentadienyl N-heterocyclic carbene nickel complex [Ni(η⁵-C₅Me₅)Cl(IMes)] (IMes=1,3-dimesitylimidazol-2-ylidene) efficiently catalyses the anti-Markovnikov hydroboration of alkenes with catecholborane in

Full text of DOI:10.1002/chem.202000289

A Journal of
Accepted Article
Title: Hydroboration of alkenes catalysed by a nickel N-heterocyclic
carbene complex: reaction and mechanistic aspects
Authors: Franck Ulm, Yann Cornaton, Jean-Pierre Djukic, Michael J
Chetcuti, and Vincent Ritleng
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To be cited as: Chem. Eur. J. 10.1002/chem.202000289
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Hydroboration of alkenes catalysed by a nickel N-heterocyclic carbene
complex: reaction and mechanistic aspects
Franck Ulm,[a] Yann Cornaton,[b] Jean-Pierre Djukic,[b] Michael J. Chetcuti,*[a] and Vincent Ritleng*[a],[c]
Abstract: The pentamethylcyclopentadienyl N-heterocyclic carbene
nickel complex [Ni(η5-C5Me5)Cl(IMes)] (IMes = 1,3-dimesitylimidazol-
2-ylidene) efficiently catalyses the anti-Markovnikov hydroboration of
alkenes with catecholborane in the presence of a catalytic amount of
potassium tert-butoxide, and joins the very exclusive club of nickel
catalysts for this important transformation. Interestingly, the
regioselectivity can be reversed in some cases by using
pinacolborane instead of catecholborane. Mechanistic investigations
principally based on noble metals, such as rhodium and iridium.7
The decrease in non-renewable natural resources has indeed
become a global concern for the future, and precious transition
metals, are, just like to fossil fuels, limited and thus strategic
resources. This can be monitored by the tremendous increase of
their prices over the last decade. New directions for
homogeneous catalysis should thus not only address energy
issues by developing highly reactive catalysts, but should also
favour the use of earth-abundant materials as catalysts.
1
involving control experiments, H and 11B NMR spectroscopy, cyclic
voltammetry, piezometric measurements and DFT calculations
suggest an initial reduction of the Ni(II) precursor to a Ni(I) active
species with the concomitant release of H2. The crucial role of the
alkoxo-catecholato-borohydride species resulting from the reaction
of potassium tert-butoxide with catecholborane in the formation of an
intermediate nickel-hydride species that would then be reduced a
Ni(I) active species, is highlighted.
In this context, complexes based on first-row transition
metals,8 most notably copper,9 cobalt,10 iron,11 and manganese12
have recently emerged as alternative catalysts for this process.
In contrast, although it has shown promise for the hydroboration
of alkynes, 13 the 1,4-hydroboration of 1,3-dienes, 14 or more
recently for formal hydroborations making use of a diboron
reagent with a proton source,15 there are to our knowledge only
three reports of the nickel-catalysed hydroboration of alkenes.16
The first one involved an activated nickel powder that showed
relatively low activity and selectivity.16a In the two other
examples, well-defined nickel(II) pre-catalysts were used, which
showed excellent activity and selectivity. Hence, in 1996,
Srebnik et al. reported that the half-sandwich triphenylphosphine
nickel complex, [Ni(Cp)Cl(PPh3)] (Cp = η5-C5H5), catalyses the
selective hydroboration of trans-4-octene and styrene with
HBPin to give 4-pinacolboryloctane and 1-phenyl-2-
pinacolborylethane in high yields (Scheme 1, eq. 1).16b In
contrast, Schomaker et al. recently reported that the heteroleptic
NHC-phosphine nickel complex, [NiCl2(IMes)(PCy3)] (IMes =
1,3-dimesitylimidazol-2-ylidene), catalyses the Markovnikov-
selective hydroboration of styrenes with good efficiency in the
presence of KOt-Bu (Scheme 1, eq. 2).16c Remarkably, no
dehydroborylation product was observed in both cases, but the
reaction scopes were rather limited, and no mechanistic study
was carried out.
Introduction
Boronic acid and ester derivatives are important starting
materials in molecular synthesis and are well-known partners for
classical transformations, such as the Petasis reaction,1 Chan–
2
3
Lam coupling, conjugate addition reactions, and, most
4
impressively, Suzuki-Miyaura cross-couplings,
but their
traditional production methodology involves a metal–halogen
exchange with an organoalkali reagent and trialkylborates, 5
which is far from being satisfying from an environmental
perspective. Subsequently, new methods were developed
through the direct borylation of aryl halides, mostly catalysed by
Pd.6 More recent advances have allowed the hydroboration of
alkenes, 7
a reaction which is of great interest since the
substrates are readily available and it creates new C–B bonds
by the most straightforward and atom-thrifty synthetic route.
The main drawback of this methodology is that it is
B
O
B
O
O
[NiCpCl(PPh3)] (1 mol%)
CH2Cl2 / rt / 18 h
O
O
+
H
B
(1)
O
or
or
[a]
F. Ulm, Prof. M. J. Chetcuti, Prof. V. Ritleng
Université de Strasbourg
97%
99%
(1 equiv.)
Ecole européenne de Chimie, Polymères et Matériaux
CNRS, LIMA UMR 7042
F-67000 Strasbourg, France
E-mail: michael.chetcuti@unistra.fr, vritleng@unistra.fr
Dr. Y. Cornaton, Dr. J.-P. Djukic
Université de Strasbourg
CNRS, Institut de Chimie de Strasbourg UMR 7177
F-67000 Strasbourg, France
O
O
[NiCl2(IMes)(PCy3)] (3 mol%)
KOtBu (6 mol%)
B
O
B
O
45-89%
(13 examples)
+
(2)
H
R
toluene / 60 °C / 18 h
R
(1.2 equiv.)
[b]
[c]
Scheme 1. Previously reported nickel-catalysed hydroboration of alkenes.16b,c
Prof. V. Ritleng
Institut Universitaire de France
1 rue Descartes, F-75000 Paris, France
In parallel, efforts from our group and others towards the
diversification of pseudo-trigonal [Ni(η5-C5R5)L(NHC)](+) species
and the study of their catalytic activity in a large array of
reactions, have allowed their establishment as a highly versatile
Supporting information for this article is given via a link at the end
of the document.((Please delete this text if not appropriate))
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platform for important organic transformations.17 Most notably,
interesting activities have been obtained for the hydrosilylation of
carbonyl 18 and imine 19 derivatives. Thus, the half-sandwich
nickel complex, [Ni(Cp)Cl(IMes)] (1a), was shown to efficiently
catalyse the hydrosilylation of a vast array of aldehydes,
ketones, aldimines and ketimines (over 60 examples!) under
mild conditions in the presence of a catalytic amount of sodium
borohydride.18a,19 This precedent, together with (i) the scarcity of
nickel-catalysed alkene hydroborations,16 (ii) the electronic and
structural proximity of [Ni(Cp)Cl(PPh3)] and 1a, (iii) the renewed
interest in the fascinating chemistry of nickel,20 and (iv) the
necessity to gain a better understanding of its reaction paths in
order to be able to tame its reactivity, prompted us to the enter
[Ni] (3 mol%)
KOtBu (6 mol%)
O
O
O
O
O
B
B
+
O
+
H
B
toluene / 60 °C / 18 h
(1.2 equiv.)
AM
M
iPr
Ni
Ni
Cl
Ni
Cl
iPr
Cl
iPr
N
N
N
N
N
N
iPr
1a: 0 %
1c: 2 %
AM : M -
1b: 30 %
AM : M = 36 : 64
iPr
this challenging field with
a
series of [Ni(Cp†)L(NHC)](+)
Ni
Ni
iPr
Cl
Cl
iPr
N
N
complexes (Cp† = Cp, Cp* (η5-C5Me5); L = Cl-, NCMe, NHC =
IMes, IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), ICy
(1,3-dicyclohexylimidazol-2-ylidene) and to try to unravel the
mechanism at play.
N
N
iPr
2a: 75 %
AM : M = 40 : 60
2b: 59 %
AM : M = 50 : 50
Herein, we show that the Cp* complex, [Ni(Cp*)Cl(IMes)]
(2a), efficiently catalyses the anti-Markovnikov hydroboration of
alkenes with HBCat (catecholborane) in the presence of a
catalytic amount of KOt-Bu. Notably, the selectivity can be
reversed for some styrene derivatives by using HBPin
(pinacolborane) instead of HBCat. A combination of experiments
gives some insight to the mechanism. The latter is proposed to
go through a Ni(I)/Ni(III) cycle after initial reduction of the
Figure 1. Pre-catalysts’ screening for the hydroboration of styrene with HBPin.
The promising result obtained in 15 h at 60 °C (entry 1)
prompted us to decrease the temperature to 25 °C. This resulted
in a much better selectivity with a AM/M ratio of 26:74, but the
conversion decreased to 62 % (entry 2). Running the reaction
for 24 h at 25 °C allowed us to almost reach the conversion
observed after 15 h at 60 °C (entry 3). Carrying out the same
nickel(II) precursor by
species.
a tert-butoxo-catecholato-borohydride
Table 1. Hydroboration of styrene with HBPin or HBCat catalysed by 2a
2a (3 mol%)
KOtBu (6 mol%)
B(OR)2
Results and Discussion
B(OR)2
+
+
H
B(OR)2
toluene / 25-60 °C
15-24 h
Initial studies focussed on the hydroboration of styrene with
HBPin in CH2Cl2 at room temperature, conditions under which
the complex [Ni(Cp)Cl(PPh3)] (1 mol%) had been reported to
quantitatively convert styrene to 1-phenyl-2-pinacolborylethane
1.2 -2.5 equiv.
AM
M
Borane
(equiv.)
Temp.
(°C)
Time
(h)
Conv.
(%)[b]
AM/M
Entry
(%)[b]
in 18
h
(Scheme 1, eq. 1).16b Disappointingly however,
1
HBPin (1.2)
HBPin (1.2)
HBPin (1.2)
HBPin (1.2)
HBPin (2.5)
HBCat (1.2)
HBCat (1.8)
HBCat (2.5)
HBCat (2.5)
HBcat (2.5)
60
25
25
60
60
60
60
60
25
60
15
15
24
24
15
15
15
15
15
15
74
40:60
26:74
25:75
39:61
20:80
50:50
67:33
85:15
100:0
85:15
absolutely no conversion was observed under these conditions
with the closely related complexes, 1a and [Ni(Cp)Cl(IPr)] (1b).
Intrigued by this negative result, we reinvestigated the reaction
with [Ni(Cp)Cl(PPh3)] and were unable to reproduce Srebnik’s
result.
2
62
3
70
4
100
100
73
We thus decided to investigate the reaction in toluene at
60 °C in the presence of 3 mol% of nickel precatalyst and 6
mol% of KOt-Bu.16c Under these conditions, 1a proved again to
be unreactive (Fig. 1). A first hit was observed with 1b with 30%
conversion after 18 h and 64% Markovnikov selectivity, but the
Cp derivatives proved to be globally poorly reactive, as was also
illustrated by the very poor conversion observed with
[Ni(Cp)Cl(ICy)] (1c). A net gain of activity was observed with the
electron-richer Cp* derivatives, notably with 2a that allowed 75%
conversion with a 40:60 anti-Markonikov/Markovnikov (AM/M)
ratio. The IPr derivative 2b proved a little less active in this case,
with 59% conversion after 18 h, and showed no selectivity.
Complex 2a was thus chosen for the optimization study of the
reaction conditions (Table 1).
5
6
7
83
8
100
33
9
10[c]
100
[a] Reaction conditions: styrene (1.04 mmol), 2a (3 mol%), KOt-Bu (6
mol%) in toluene (2 mL). [b] Conversions and AM/M ratio determined by 1
NMR spectroscopy with 1,3,5-trimethoxybenzene as an internal standard;
average values of at least two runs. [c] Reaction run with NaOt-Bu or LiOt-
Bu instead of KOt-Bu.
H
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catalysis at 60 °C led to full conversion, but still with a low
selectivity (entry 4). Finally, increasing the number of
equivalents of HBPin to 2.5 allowed us to observe full
conversion as well, but surprisingly with an even better AM/M
ratio of 20:80 than at room temperature (entry 5). However, the
impossibility, at least in our hands, to cleanly separate the
isomers by column chromatography led us to explore the
reaction with HBCat. A similar result to HBPin (1.2 equiv.) at
60 °C (entry 1) was observed with HBCat, with 73 % conversion
and a 50:50 AM/M ratio after 15 h reaction (entry 6). Increasing
the number of equivalents to 1.8 and 2.5 allowed the conversion
toluene, 60 °C, 15 h), we then examined the scope of the
hydroboration of alkenes catalysed by 2a with HBCat (Table 2).
Electron-neutral and electron-rich styrenes were all fully
converted with high linear selectivity (entries 1-5). In the cases
of 4-methoxystyrene and 2-methylstyrene, the linear isomer was
actually the sole product (entries 3 and 5). Electron-poor
styrenes were hydroborated with conversions ranging from
moderate in the case 4-trifluoromethylstyrene (entry 9) to full
with 2-bromo- and 4-fluoro-styrenes (entries 7 and 8). With the
exception of 2-bromostyrene, which was converted with a poor
selectivity, the other substrates all gave good to excellent linear
selectivity. In all cases where the branched isomer was formed
as a minor product, it decomposed on the silica column,22
allowing the isolation of the pure AM isomer in yields ranging
from 38 % in the case of 4-trifluoromethylstyrene (entry 9) to
86 % for 4-methylstyrene (entry 4). The reaction is not limited to
styrenes, and vinylcyclohexane was fully converted to the linear
isomer, allowing its isolation with 60 % yield (entry 10). Cis-
stilbene was also hydroborated with good efficiency (80 %), but
the resulting product decomposed on the column (Fig. 2). In
addition, a number of substrates performed either poorly or non-
selectively. Thus, trans-stilbene and 1-phenylcyclohexene gave
no conversion at all, and 1-methylstyrene, 4-cyanostyrene,
cinnamyl chloride and 1-heptene23 gave complicated mixtures
despite good to excellent conversions (Fig. 2).
to reach 83 and 100%, respectively, but with
a reverse
regioselectivity (entries 7 and 8), as was recently observed with
an aryloxy-tethered NHC-Fe(II) catalyst.21 Indeed, in this case,
the linear isomer was the major product, and a 85:15 AM/M ratio
was obtained when 2.5 equiv. of HBCat were used (entry 8). In
light of the increased selectivity observed at room temperature
with 1.2 equiv. of borane (entries 2 and 3), we then decided to
check if a similar tendency could be observed in the presence of
2.5 equiv. of borane. The anti-Markovnikov isomer was the sole
product formed under these conditions, but the conversion was
only 33 % after 15 h (entry 9). Finally, the replacement of
potassium tert-butoxide by sodium or lithium tert-butoxide had
no consequence on the reaction (entry 10).
With these optimized conditions in hand (2.5 equiv. borane,
Table 2. Hydroboration of alkenes with HBCat catalysed by 2a[a]
2a (3 mol%)
KOtBu (6 mol%)
BCat
M
BCat
+
+
H
BCat
R
R
R
toluene / 60 °C / 15 h
2.5 equiv.
AM
Conv.: 80 %
Decomposed
Conv.: 0 %
Cl
Conv.: 58 %
unkown AM/M
due to degradation
Conv.: 0 %
Substrate
Conv.
AM/M
Yield
Entry
(%)[b]
(%)[b]
(%)[c]
1
100
85:15
85:15
100:0
80
NC
Conv.: 100 %
Multiple isomers
Conv.: 94 %
Multiple isomers
Conv.: 100 %
Multiple isomers
2[d]
3
100
100
72
85
Figure 2. Substrates that performed either poorly or non-selectively with
MeO
HBCat or that gave a product that decomposed.
4
5
6
100
100
93
89:11
100:0
85:15
86
52
40
As catalysts that can enable different regioselectivities are
invaluable for the construction of a diverse set of value-added
compounds,8 we next also briefly investigated the reaction scope
with HBPin to check if the observed reversal of selectivity with
styrene was general (Table 3). Electron-rich and electron-poor
styrenes such as 4-methoxystyrene, 2-bromostyrene and 4-
trifluoromethylstyrene were also converted to the branched
isomer with good to high regioselectivities (entries 2-4).
Surprisingly however, 2- and 4-methylstyrene gave the linear
isomer quasi-exclusively (entry 5 and 6). Although substrate
dependent regioselectivity is observed every now and then,9-12
and had, in particular, been noted with the closely related
[Ni(Cp)Cl(PPh3)] for styrene and 4-octene (Scheme 1),16b it is to
our knowledge rarely observed within a series of simple styrene
derivatives. We therefore did not pursue our substrate scope
investigation with HBPin any further.
Br
7
8
9
100
100
67
60:40
100/0
80/20
59
79
38
Br
F
F3C
10
100
100/0
60
[a] Reaction conditions: alkene (1.0 mmol), HBCat (2.5 mmol), 2a (3
mol%), KOt-Bu (6 mol%) in toluene (2 mL) at 60 °C for 15 h. [b]
Conversions and AM/M ratio determined by 1H NMR spectroscopy with
1,3,5-trimethoxybenzene as an internal standard; average values of at least
two runs. [c] Isolated yield of the AM isomer. [d] [NiCp*(NCMe)(IMes)]PF6
(3a) was used as precatalyst instead of 2a.
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Table 3. Hydroboration of styrenes with HBPin catalysed by 2a[a]
2a (3 mol%)
This point being established, we then recorded a cyclic
voltammogram of the pre-catalyst (1 mM) in CH3CN (0.1 M
NBu4PF6). As the latter tended to generate a deposit at the
surface of the carbon electrode that altered the reproducibility of
the measurements, the electrochemical characterization was
BPin
KOtBu (6 mol%)
BPin
+
+
H
BPin
toluene / 60 °C / 15 h
R
R
R
2.5 equiv.
AM
M
carried
out
with
the
cationic
derivative,
Substrate
Conv.
(%)[b]
AM/M
Yield
(%)
Entry
[Ni(Cp*)(NCMe)(IMes)]PF6 (3a), of 2a that shows a similar
(%)[b]
catalytic activity (Table 2, entry 2). At a scan rate of 200 mV.s-1,
a quasi-reversible oxidation process can be observed at E1/2ox
=
1
2
100
100
20:80
26:74
-
0.16 V vs. Fc/Fc+ (ΔEp = 154 mV), and a quasi-reversible
reduction peak is seen at E1/2red = -1.75 V (ΔEp = 190 mV) (Fig.
3). By analogy with the electrochemical data reported by
Crabtree for the Cp derivative 1a (E1/2ox = 0.03 V and E1/2red = -
1.51 V vs. Fc/Fc+, v = 100 mV.s-1),26 these values most likely
correspond to Ni(II)/Ni(III) oxidation and Ni(II)/Ni(I) reduction
processes, while the irreversible oxidation and reduction peaks
observed at -0.25 V and -1.48 V (vs. Fc/Fc+) potential would
correspond to ligand-centred redox processes. Nevertheless,
51[c]
MeO
[d]
3
4
100
100
-
94
Br
13:87
95[c]
F3C
5
6
100
100
99:1
99:1
22[e]
36[e]
the Cp* ligand has been described ‘to act as an electron density
27
buffer’,
which might mean that neither of these redox
[a] Reaction conditions: alkene (1.0 mmol), HBPin (2.5 mmol), 2a (3 mol%),
KOt-Bu (6 mol%) in toluene (2 mL) at 60 °C for 15 h. [b] Conversions and
AM/M ratio determined by 1H NMR spectroscopy with 1,3,5-
trimethoxybenzene as an internal standard; average values of at least two
runs. [c] Isolated yield of the AM/M mixture. [d] The AM:M ratio was difficult
to evaluate due to the possible presence of debromoborylation products.24
[e] The low isolated yields are due to massive decomposition during the
chromatographic purification.
processes are purely metal centred, and could explain the
counter-intuitive observation of a higher Ni(II)/Ni(III) oxidation
potential for 3a with respect to 1a.
To gain insight to the mechanism, we first checked whether
the hydroboration process was the result of a true homogeneous
catalysis by conducting the hydroboration of styrene with HBCat
in the presence of 190 equivalents of Hg relative to 2a. No
inhibition was observed, nor a change of selectivity, and thus a
process catalysed by nickel particles is unlikely.25
Owing to the propensity of nickel to easily access multiple
oxidation states,20 we then checked whether the reaction
involves a radical by performing control experiments in the
presence of radical scavengers. The addition of 1 equiv. of
TEMPO or galvinoxyl (relative to styrene) at t0 completely
inhibited the reaction (Scheme 2). Furthermore, the addition of 3
mol% TEMPO limited the conversion to 55 % after 15h reaction,
and the addition of 1 equiv. TEMPO after 7 h reaction to 76 %
(vs. 100% without radical scavenger). This suggested the
presence of radicals not only during the activation phase but
during the catalytic cycle itself.
Figure 3. Cyclic voltammogram of 3a: 1 mM 3a in a 0.1 M NBu4PF6 solution in
CH3CN with a glassy C working electrode, a Pt wire counter electrode, and a
Ag/AgCl, KCl 3 M reference electrode. Referenced to a 2mM Fc/Fc+ external
standard: E1/2 = 0.48 V vs. AgAgCl, KCl 3 M. Scan rate = 200 mV.s-1.
2a (3 mol%)
KOtBu (6 mol%)
BCat
M
BCat
+
+
H
BCat
In parallel, stoichiometric studies were conducted to try to
identify reaction intermediates. Thus, we monitored the reaction
toluene / 60 °C / 15 h
2.5 equiv.
AM
1
of 2a with 1 equiv. of HBcat and 1 equiv. of KOt-Bu by H NMR
No radical scavenger: 100% conv.
in C6D6 from 253 to 298 K. At 293 K, signals belonging to a Cp*-
Ni(IMes) species were unambiguously identified (Fig. 4). Though
a little broad, these signals are found at the same chemical
shifts as those of 2a, suggesting that most of the starting
complex is unreacted under these conditions. Nevertheless, a
singlet observed at 4.47 ppm is assigned to molecular H2.
Furthermore, another singlet, observed at -22.9 ppm and
TEMPO (1 equiv.) at t = 0: < 1% conv.
Galvinoxyl (1 equiv.) at t = 0: < 1% conv.
TEMPO (3 mol%) at t = 0: 55% conv.
TEMPO (1 equiv.) at t = 7 h: 76% conv.
Scheme 2. Effect of radical inhibitors.
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10.1002/chem.202000289
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alkoxo-catecholato-borohydride species, [H(t-BuO)BCat]- (4), at
8.2 ppm, that has been described to act as a universal reducing
agent for a number of iron, cobalt, nickel and manganese
complexes in various hydrofunctionalization reactions.12b,28,30
The addition of this preformed mixture (6 mol%) and HBCat
(2.5 equiv.) to 2a (3 mol%), previously solubilized in toluene in
the presence of styrene, resulted in a catalytic system that
demonstrated similar activity to when 4 is generated in situ
(Scheme 3), thus suggesting that this species could also act as
the pre-catalyst activator here.
O
O
Ni
KOtBu
! = )%*+#''(#
+
Cl
+
H
B
N
N
1 : 1 : 1
#!"#
!"!"#
!$$%&#''(#
O
KOtBu
+
H
B
"
$#
O
toluene / rt
-
OtBu
Figure 4. 1H NMR (C6D6, 293 K) spectrum of a 2a / HBCat / KOt-Bu (1:1:1)
mixture.
B
O
H
O
4 (6 mol%)
+
present in a ratio of ca. 5% when compared to 2a, is assigned to
a nickel-hydride species by analogy with the corresponding
signal of [Ni(Cp)H(IMes)].18a Albeit to a low extent, this suggests
that a reaction has well occurred at the nickel centre. In addition
to these observations, the proton of the starting HBCat is only
observed in very minor proportions, and the singlet belonging to
the tert-butoxide anion has shifted from 1.05 to 1.28 ppm, which
corresponds to the shift observed when the latter is reacted with
HBCat in the absence of nickel pre-catalyst, and thus suggests
that the main reaction occurred between these two species
under these conditions.
To gain further insight into this reaction, we next recorded
the 11B NMR spectrum of a 1:1 mixture of HBCat and KOt-Bu in
C6D6 at room temperature (Fig. 5). In agreement with the data
reported by the groups of S. P. Thomas28 and T. B. Clark29 for
the reaction of HBPin with NaOt-Bu, several peaks are observed,
which can be attributed to BH3 (or more likely to B2H6) at -13.3
ppm, to (t-BuO)BCat at 25.8 ppm, and most importantly to an
Ni
Cl
N
N
BCat
M
2a (3 mol%)
BCat
+
+
H
BCat
toluene / 60 °C / 15 h
2.5 equiv.
AM
100 % conv. (AM : M = 89 : 11)
Scheme 3. Addition of preformed 4 to a catalytic reaction mixture.
We were intrigued by the observation of
a strong
effervescence at the beginning of each catalytic reaction, i.e.:
immediately after the addition of HBCat, and this independently
of the presence or absence of styrene (the latter being
indifferently added before or after HBCat). So, we next
conducted a piezometric study in order to quantify the amount of
H2 produced. For that purpose, we used a thermostatically
controlled double-walled Schlenk tube that is connected to a
pressure transducer, which allows the real-time monitoring of the
pressure inside the Schlenk (Fig. S1).31 Using this experimental
set-up, we measured the production of 1 equiv. of H2 per mole of
nickel when 83 equiv. HBCat were added to a 1:2 mixture of 2a
and KOt-Bu in toluene at room temperature. Similarly, the
reaction of 2a with 2 equiv. of KOt-Bu and 2 equiv. of HBCat
produced 1 equiv. of H2 per mole of nickel. In contrast, the
reaction of 1 equiv. 2a with 1 equiv. KOt-Bu and 1 equiv. of
HBCat only produced 0.5 equiv. of H2 per mole of nickel.
OtBu
-
B
O
H
O
H
H
O
O
H
B
tBuO
B
In view of these data, we considered the possible implication
of the nickel hydride complex [NiCp*H(IMes)] (5a), that would
act as the true catalytic precursor18a and would be reduced with
concomitant H2 release to the Ni(I) active species, [NiCp*(IMes)]
(6a), whose IPr analogues (Cp and Cp*) have already been
isolated32 and shown to be catalytically competent in Suzuki
couplings.33 To try to validate this possibility, we then prepared
6a by reacting 2a with KC8 (or Na/Hg) in THF at -78 °C (Scheme
4). The generation of 6a was established by 1H NMR
Figure 5. 11B NMR (C6D6, 298 K) spectrum of a HBCat / KOt-Bu (1:1) mixture.
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10.1002/chem.202000289
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FULL PAPER
spectroscopy. Two highly deshielded singlets at 107 and 31 ppm,
integrating for 15 and 2 protons respectively (Fig. S2), could
indeed be assigned to the protons of the Cp* group and of the
imidazolylidene ring by comparison with the reported data for
[NiCp*(IPr)].32 However, despite repeated attempts, we have
been unable to isolate it pure due to its high sensitivity.
Nevertheless, we still attempted the hydroboration of styrene
with HBCat in the presence of impure 6a (3 mol%). 70 %
conversion was observed after 15 h at 60 °C, with a 60:40 AM/M
ratio (Scheme 5). Despite the reduced performance and
selectivity compared to 2a and KOt-Bu, which may be attributed
to the presence of impurities, this result, observed in the
absence of KOt-Bu, tended to confirm the implication of 6a in the
catalytic cycle.
Table 4. Reaction Gibbs energies computed at the PBE-D3(BJ)/def2-
TZVPP level of theory
ΔrG0 at 25 °C
ΔrG0 at 60 °C
Reaction
(kcal/mol)
(kcal/mol)
5a + HBCatOt-Bu- → 6a + BCatOt-Bu●-
53.66
52.38
+ H2
5a → 6a + ½ H2
13.82
-6.77
13.12
-4.94
6a + HBcat →7a
7a → 5a + Bcat●
43.85
17.80
-41.96
-26.72
-14.84
0.40
42.14
19.74
-40.29
-25.08
-16.79
-1.58
7a + styrene → 8a
Bcat● + styrene → AM●
Bcat● + styrene → M●
8a → 5a + AM●
KC8 (1.1 equiv.)
Ni
Mes
Ni
8a → 5a + M●
Cl
Mes
THF / -78 °C / 4 days
N
Mes
N
Mes
N
N
AM● + 5a → 6a + AM
M● + 5a → 6a + M
-12.82
-24.77
-12.95
-24.87
2a
6a
84 % (impure)
Scheme 4. Synthesis of 6a.
known for more than 40 years,34 and even if this reaction is
endergonic, the equilibrium should be shifted toward the
formation of 6a as a consequence of the law of mass action
since H2 mostly escapes the solution as a gas, as observed
experimentally.
BCat
M
6a (3 mol%)
BCat
+
+
H
BCat
toluene / 60 °C / 15 h
2.5 equiv.
AM
The first step of the catalytic cycle was initially supposed to
be an oxidative addition of 6a to HBCat to generate a Ni(III)
intermediate, as CV measurements suggested a Ni(I)/Ni(III)
catalytic cycle could be amenable. However, all DFT attempts to
optimize such a structure led instead to the “agostic” adduct 7a
where the H–B bond of the HBCat attractively interacts with the
Ni(I) centre in an exergonic fashion at both 25 °C (ΔrG0 = -6.77
kcal/mol) and 60 °C (ΔrG0 = -4.94 kcal.mol). Mapping the spin
density of this adduct revealed an important single electron
population on the hydrogen involved in the agostic interaction,
giving it a strong hydrogen radical character (Fig. S3).
70% conv. (AM : M = 60 : 40)
Scheme 5. Hydroboration of styrene catalysed by 6a.
From that point on, different mechanisms for the
hydroboration of styrene by HBCat catalysed by 6a were then
considered. Density functional theory (DFT) computations at the
PBE-D3(BJ)/def2-TZVPP level of theory were carried out to
discriminate between the different hypothesis that were
formulated. For this study, only reaction intermediates (local
energy minima) were considered, and no transition state (energy
From this agostic intermediate, two mechanisms have been
considered: (i) a first one where the B–H bond would undergo an
homolytic fission to generate a boron radical that will then be
captured by the styrene forming a carbon radical and 5a
(Scheme 6); or (ii) a second one where an adduct 8a between
7a and a styrene (Fig. S4) would homolytically break the B–H
bond and release the same carbon radical as in the previous
mechanism and 5a (Scheme 7). In both cases, the carbon
radical would then react with 5a to regenerate 6a and yield the
final hydroboration product. Figure 6 shows the Gibbs energy
profiles for both considered mechanisms of the catalytic cycle,
leading to both products (AM and M) at 25 °C and 60 °C. The
second step of both these mechanism, the only one by which
they differ, is endergonic but the reaction Gibbs energy is more
than twofold larger in mechanism 1 than in mechanism 2, at both
considered temperatures. The final product AM is predicted to
be more stable than its regioisomer M, in agreement with the
experimental results. The difference of stability between AM and
saddle-points)
were
searched
for.
Thus,
only
the
thermodynamics of the reactions were studied, with no insight
into the kinetics. Reaction Gibbs energies computed at this level
of theory are compiled in Table
considered in the experimental part of this work, i.e.: at 25 °C
and 60 °C.
Two reactions were considered for the formation of the Ni(I)
catalyst 6a from the Ni(II) hydride 6a: (i) one where 5a would
react with an equivalent of 4, which would act as a reductant, to
generate 6a and one equivalent of H2; (ii) the other where two
molecules of 5a would react together to generate two
equivalents of 6a and one of H2. In these two reactions, both
1 at both temperatures
reactants should lose
a hydrogen radical. Both of these
reactions have been found to be endergonic with the former
being largely less favourable than the latter at both 25 °C (ΔrG0
= 53.66 vs. 13.82 kcal/mol) and 60 °C (ΔrG0 = 52.38 vs. 13.12
kcal/mol). Examples of reactions similar to the latter have been
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Ni
Mes
O
O
N
Mes
N
H
B
O
6a
B
O
Ph
Mes
Ni
H
Ni
O
Mes
N
.
+
H
N
B
B
O
Ph
O
N
N
O
Mes
.
Mes
AM
5a
7a
O
.
B
O
Ph
.
BCat
Scheme 6. Proposed catalytic mechanism 1 (outer-sphere transfer), through
the release of a boron radical.
Figure 6. Gibbs energy profiles for the catalytic cycle following mechanism 1
in orange (synthesis of M) and green (synthesis of AM), or mechanism 2 in red
(synthesis of M) and blue (synthesis of AM) at 25 °C (solid lines) and 60 °C
(dashed lines).
Ni
O
Mes
H
B
N
Mes
N
O
O
6a
B
O
Ph
generate the corresponding nickel hydride species 5a, possibly
through a σ-bond metathesis mechanism, which contrasts with
the commonly proposed formation of a putative nickel alkoxide
intermediate that would then react with the borane16c (or
silane18b35) to generate the Ni(II) hydride intermediate. Then, in
an initiation step, two 5a could react together to generate the
catalytically active Ni(I) species 6a (Scheme 8).36 The latter
would then form an “agostic” intermediate 7a, where the B–H
bond of the borane interacts with the Ni(I). Styrene would then
form an adduct 8a with 7a before homolytically breaking the B–H
bond, hence regenerating 5a and releasing a carbon radical.
This carbon radical would finally react with 5a, to yield the
hydroboration product and regenerating 6a (Scheme 7).
Importantly, this mechanism is in agreement with our previous
observation, in the related hydrosilylation reaction of carbonyl
derivatives catalysed by 1a,18a where we noted that although a
Ni(II)–H species is most probably the true catalytic precursor,
there is no insertion of the C=O or C=C double bond into this
Ni(II)–H bond.
Mes
Ni
Ni
O
Mes
H
O
.
+
H
N
N
B
B
O
Ph
N
N
Mes
O
Mes
.
AM
5a
7a
Mes
Ni
H
Ph
N
Ph
B
N
O
O
Mes
8a
Scheme 7. Proposed catalytic mechanism 2 (inner-sphere transfer), through
the formation of the adduct 8a.
M does not vary between 25 °C and 60 °C [Δ(ΔrG0) = -3.29
kcal/mol in both cases]. Hence, the experimentally observed
difference in regioselectivity with the temperature seems to arise
from the third step of the mechanism, i.e.: the formation of the
carbon radical AM● or M●. As expected, AM● is more stable than
M●, the difference in stability being larger than between AM and
M, but this stability difference does not vary much with the
OtBu
K+
-
B
O
H
O
temperature [Δ(ΔrG0(25 °C))
Δ(ΔrG0(60 °C))
-15.21 kcal/mol]. However, considering
=
-15.24 kcal/mol vs.
4
Ni
Ni
Ni
Mes
Mes
=
1
Cl
Mes
H
- KCl
- tBuOB(OR)2
- /2 H2
Mes
N
N
N
Mes
N
N
N
mechanism 2, this step is exergonic at both temperatures for the
formation of AM●, but goes from slightly exergonic at 60 °C to
slightly endergonic at 25 °C for the formation of M●.
Considering these data, we propose the following
mechanism. The reaction of an equivalent of 4 with 2a could
Mes
2a
6a
5a
Scheme 8. Proposed mechanism for the generation of the Ni(I) active species
6a.
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fast degradation of the desired products, especially with HBCat.22 The
branched isomer, in particular, was found to be very unstable upon
filtration over the silica plug, allowing the isolation of the pure linear
isomer with HBCat and generating (in some cases) an increase of the
linear/branched ratio with HBPin.
Conclusions
In conclusion, we have shown that the [NiCp*Cl(IMes)] complex
2a is an efficient precatalyst for the hydroboration of alkenes in
the presence of a catalytic amount of KOt-Bu. With HBCat,
about ten styrene derivatives and vinylcyclohexane have been
converted to the linear product with yields ranging from
moderate to very good depending on the substrate. The use of
HBPin instead of HBCat allowed to obtain the branched product
as the major one in the cases of styrene, 4-
trifluoromethylstyrene and 4-methoxystyrene.
Mechanistic studies strongly suggest the initial generation of
a the nickel-hydride complex 5a by reaction of 2a with the in situ
formed tert-butoxo-catecholato-borohydride species [H(t-
BuO)BCat]- (4), as evidenced by 1H and 11B NMR spectroscopy.
Two molecules of the latter would then react together to
generate two equivalents of the Ni(I) species 6a and one
equivalent of H2, as suggested by combined experimental and
theoretical studies. The latter would then act as the active
Synthesis of [NiCp*(IMes)] (6a)
In a glovebox, a Schlenk tube was charged with a stirring bar and 2a
(300 mg, 0.562 mmol). The Schlenk tube was then laid in a horizontal
position in order to introduce sparkling orange shavings of KC8 (84 mg,
0.621 mmol) without touching the purple powder. In this position, the
Schlenk tube was closed and taken out of the glovebox. 2a was then
dissolved in THF without touching KC8 (10 mL). While putting the
Schlenk tube in a vertical position, the system was plunged into a bath at
-78 °C. The temperature was allowed to slowly increase to room
temperature and the heterogeneous purple medium was stirred for 4
days. After this time, the resulting dark yellow suspension was filtered
through a thin pad of Celite under argon and the filtrate was concentrated
under vacuum. The resulting brown powder was dissolved in a small
amount of pentane and stored at -28 °C overnight. The mother liquor was
removed with a syringe to yield 6a as a shining yellow-brown powder
(234 mg, 0.470 mmol, 84 %) that was dried under high vacuum. 1H NMR
(C6D6, 400.13 MHz): δ 107.31 (s v.br., 15H, C5Me5), 30.95 (s br., 2H,
NCH), 5.29 (s br., 4H, m-H), 3.57 (s, 6H, o-Me), 2.21 (s, 6H, p-Me), 1.42
(s, 6H, o-Me).
species through
intermediate 7a and
a
catalytic cycle involving an agostic
styrene adduct 8a that would
a
homolytically break the B–H bond of the borane. These
unprecedented studies with only the fourth nickel hydroboration
catalyst reported to date16 shed light both on the potential of
nickel as another base metal for this important transformation,
and on the fact that appropriately designed nickel(I) species may
be an interesting lead for future research in this domain.
Control experiments
Investigation of the mercury effect: An oven dried Schlenk tube was
charged with 2a (17 mg, 0.0318 mmol, 3 mol%), KOt-Bu (7 mg, 0.0624
mmol, 6 mol%), and toluene (2 mL), containing the internal standard
(trimethoxybenzene, 0.6 mg/mL). This was followed by the addition of Hg
(1.2 g, 5.98 mmol, 188 equiv.), HBCat (0.266 mL, 2.50 mmol, 2.5 equiv.),
and styrene (0.115 mL, 1.00 mmol, 1 equiv.). The resulting reaction
mixture was stirred in a preheated oil bath at 60 °C for 15 h. The solution
was then cooled to room temperature and an aliquot of the mixture was
removed by syringe in order to determine the conversion and the
isomeric ratio by 1H NMR spectroscopy. A conversion of 100 % with a
90:10 AM/M ratio was observed.
Experimental Section
General comments
All reactions were carried out using a glovebox or standard Schlenk
techniques under an atmosphere of dry argon. Solvents were distilled
from appropriate drying agents under argon. Unless otherwise specified,
solution NMR spectra were recorded at 298 K on Bruker Avance I 300
MHz, or Bruker Avance III HD 400 MHz spectrometers operating at
300.13 or 400.13 MHz for 1H, at 75.47 or 100.61 MHz for 13C and at
96.29 or 128.38 MHz for 11B. The chemical shifts are referenced to the
residual deuterated or 13C solvent peaks. Chemical shifts (δ) and
coupling constants (J) are expressed in ppm and Hz respectively. The
catalyst precursors 1a,37 1b,17b 1c,38 2a,39 2b,39 and 3a17f were prepared
according to published procedures.
Investigation of the radical scavenger effect: An oven dried Schlenk tube
was charged with 2a (17 mg, 0.0318 mmol, 3 mol%), KOt-Bu (7 mg,
0.0624 mmol, 6 mol%), and toluene (2 mL), containing the internal
standard (trimethoxybenzene, 0.6 mg/mL). This was followed by the
addition of HBCat (0.266 mL, 2.50 mmol, 2.5 equiv.), styrene (0.115 mL,
1.00 mmol, 1 equiv.) and TEMPO (156 mg, 1.00 mmol, 1 equiv. (at t = 0
or 7 h) or 5 mg, 0.0320 mmol, 3 mol%) or galvinoxyl (422 mg, 1.00 mmol,
1 equiv.). The resulting reaction mixture was stirred in a preheated oil
bath at 60 °C for 15 h. The solution was then cooled to room temperature
and an aliquot of the mixture was removed by syringe in order to
determine the conversion by 1H NMR spectroscopy. Complete inhibition
of the reaction was observed when 1. 0 equiv. TEMPO or galvinoxyl was
added at t = 0 h. 55% conversion was observed when 3 mol% TEMPO
was added at t = 0 h. 76% conversion was observed when 1.0 equiv.
TEMPO was added at t = 7h.
General procedure for the hydroboration of alkenes
In an oven dried Schlenk tube were added 2a (17 mg, 0.0318 mmol, 3
mol%), KOt-Bu (7 mg, 0.0624 mmol, 6 mol%), and toluene (2 mL),
containing the internal standard (trimethoxybenzene, 0.6 mg/mL). This
was followed by the addition of HBCat (0.266 mL, 2.50 mmol, 2.5 equiv.)
or HBPin (0.363 mL, 2.50 mmol, 2.5 equiv.) and of the desired alkene
(1.00 mmol, 1 equiv.). The resulting reaction mixture was stirred in a
preheated oil bath at 60 °C for 15 h. The solution was then cooled to
room temperature and an aliquot of the mixture was removed by syringe
in order to determine the conversion and the isomeric ratio by 1H NMR
spectroscopy. The rest of the solution was filtered through a plug of silica
that was washed with CH2Cl2. The resulting filtrate was dried under high
vacuum to give a pale orange solid (with HBCat) or a pale yellow oil (with
HBPin). The purification procedure was performed rapidly because of the
1H NMR follow-up of a 1:1:1 mixture of 2a/HBCat/KOt-Bu: An oven dried
Schlenk tube was charged with 2a (32 mg, 0.0599 mmol), KOt-Bu (7 mg,
0.0624 mmol) and freeze-pump-thaw degassed C6D6 (2 mL). The
resulting mixture was cooled -25 °C before the addition of HBCat (6.4 µL,
0.0598 mmol). The reaction mixture was then vigorously stirred for
several seconds prior to the transfer of an aliquot of 0.6 mL into an NMR
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10.1002/chem.202000289
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tube placed in a bath at -25 °C. 1H NMR spectra were then recorded from
253 to 298 K.
pressure-to-voltage transducer was air-tightly screwed to the neck of the
glass vessel and connected to a Pico-Log analog-logic converter/data-
logger board interfaced to a computer. This allowed the measurement of
11B NMR analysis of a 1:1 mixture of HBCat/KOt-Bu: An oven dried
Schlenk tube was charged with KOt-Bu (42 mg, 0.375 mmol) and freeze-
pump-thaw degassed C6D6 (2 mL). The resulting mixture was cooled -25
°C before the addition of HBCat (40 µL, 0.375 mmol). The reaction
mixture was then vigorously stirred for several seconds prior to the
transfer of an aliquot of 0.6 mL into an NMR tube placed in a bath at -25
°C. 11B NMR spectra were then recorded from 253 to 298 K.
the pressure of hydrogen (computed from
a
linear absolute
pressure−output voltage relationship) developed in the known overhead
volume of the sealed reactor as a function of time. The absence of leaks
was preliminarily checked by applying a moderate pressure of argon gas.
HBCat was swiftly injected into the vessel via a lateral glass valve, which
was tightly closed immediately upon injection to avoid gas leaks. Several
experiments have been performed with varying amounts of reagents: (i)
2a (48 mg, 0.0899 mmol), KOt-Bu (20 mg, 0.178 mmol, 2 equiv.) and
HBCat (0.799 mL, 7.496 mmol, 83 equiv.) in toluene (6 mL); (ii) 2a (144
mg, 0.270 mmol), KOt-Bu (61 mg, 0.544 mmol, 2 equiv.) and HBCat (60
µL, 0.563 mmol, 2 equiv.) in toluene (12 mL); (iii) 2a (144 mg, 0.270
mmol), KOt-Bu (30 mg, 0.267 mmol) and HBCat (29 µL, 0.272 mmol) in
toluene (12 mL).
Hydroboration of styrene catalysed by 2a with 4: To an oven dried
Schlenk tube were added HBCat (0.266 mL, 2.50 mmol, 2.5 equiv.) and
KOt-Bu (7 mg, 0.0624 mmol, 6 mol%) in toluene (2 mL), containing the
internal standard (trimethoxybenzene, 0.6 mg/mL) in order to form 4. This
mixture was stirred for 5 min at room temperature before adding 2a (17
mg, 0.0318 mmol, 3 mol%) and styrene (0.115 mL, 1.00 mmol). The
resulting reaction mixture was then stirred in a preheated oil bath at 60
°C for 15 h. The solution was finally cooled to room temperature and an
aliquot of the mixture was removed by syringe in order to determine the
conversion and the isomeric ratio by 1H NMR spectroscopy. A conversion
of 100 % with a 89:11 AM/M ratio was observed.
Computational details
Geometry optimizations of the different species thought to be involved in
the reaction mechanism were performed at the density functional theory
(DFT) level of theory, using the ORCA program system40 version 4.2.0.
41
The Perdew-Burke-Ernzerhof (PBE) functional
augmented with
Grimme’s D3 dispersion correction with a Becke-Johnson (BJ) damping
function42 was used for all calculations. All computations were carried out
using Karlsruhe’s second-generation default triple-ζ valence with two sets
of polarization functions (def2-TZVPP) basis set43. Solvent effects were
accounted for in all computations by using the conductor-like polarizable
continuum model (CPCM),44 simulating toluene (ε=2.4, n=1.497) as a
solvent. Geometry optimizations by the quasi Newton update procedure
with the BFGS update were carried out in all cases with integration grid
accuracy 5 to 7, an energy gradient convergence criterion of 10-4 Eh/a0,
and a tight SCF convergence criterion. All thermodynamic data have
been evaluated at 298.15 K and 333.15 K. 3D molecular structures and
isosurfaces were produced using Jmol.
Hydroboration of styrene catalysed by 6a: In an oven dried Schlenk tube
were added 6a (15 mg, 0,0301 mmol, 3 mol%) and toluene (2 mL),
containing the internal standard (trimethoxybenzene, 0.6 mg/mL). This
was followed by the addition of HBCat (0.266 mL, 2.50 mmol, 2.5 equiv.)
and styrene (0.115 mL, 1.00 mmol, 1 equiv.). The resulting reaction
mixture was stirred in a preheated oil bath at 60 °C for 15 h. The solution
was then cooled to room temperature and an aliquot of the mixture was
removed by syringe in order to determine the conversion and the
isomeric ratio by 1H NMR spectroscopy. A conversion of 70 % with a
60:40 AM/M ratio was observed.
Cyclic voltammetry
The cyclic voltammogram of complex 3a (1 mM in CH3CN) was recorded
with
a Voltalab 50 potentiostat/galvanostat (Radiometer Analytical
Acknowledgements
MDE150 polarographic stand, PST050 analytical voltammetry and
CTV101 speed control unit) controlled by the Voltamaster 4 software. A
conventional three-electrode cell (10 cm3) was used in our experiments
with a glassy carbon disk (s = 0.07 cm2) set into a Teflon rotating tube as
a working electrode, a Pt wire as a counter electrode, and a KCl (3
M)/Ag/AgCl reference electrode (+210 mV vs. NHE). Prior to each
measurement, the surface of the glassy carbon electrode was carefully
polished with 0.3 !m aluminum oxide suspension (Escil) on a silicon
carbide abrasive sheet of grit 800/2400. Thereafter, the glassy carbon
electrode was copiously washed with water and dried with paper towel
and argon gas. The electrode was installed into the voltammetry cell
along with a platinum wire counter electrode and the reference electrode.
The 1 mM solution of 3a was vigorously stirred and purged with O2-free
(Sigma Oxiclear cartridge) Ar for 15 min before the voltammetry
experiment was initiated, and maintained under an Ar atmosphere during
the measurement procedure. The voltammogram was recorded at
23(1) °C with 0.1 M NBu4PF6 as supporting and inert electrolyte. The
voltage sweep rate was set at 200 mV.s-1, and the cyclic voltammogram
was recorded from +0.5 V to -2.2 V vs. Fc/Fc+.
MJC and VR are grateful to the Université de Strasbourg and
the CNRS for their financial help. The Institut Universitaire de
France is acknowledged for its support to VR, and the Région
Grand Est for the doctoral fellowship of F. Ulm. Dr. Mourad
Elhabiri is gratefully acknowledged for his help with the cyclic
voltammetry measurements. YC and JPD thank the Centre de
Calcul de l'Université de Strasbourg for providing access to the
HPC facilities for the computational part of this work.
Keywords: nickel • hydroboration • alkene • N-heterocyclic
carbene • Ni(I) active species
[1]
N. R. Candeias, F. Montalbano, P. M. S. D. Cal, P. M. P. Gois, Chem.
Rev. 2010, 110, 6169-6193.
[2]
[3]
J. X. Qiao, P. Y. S. Lam, Synthesis 2011, 829-856.
G. Berthon-Gelloz, T. Hayashi in Boronic Acids. Rhodium- and
Palladium-Catalyzed Asymmetric Conjugate Addition of Organoboronic
Acid, 2nd Ed. (Ed.: D. G. Hall), Wiley-VCH, Weinheim, 2011, pp. 263-
314.
Piezometric studies31
[4]
[5]
A. Suzuki, Angew. Chem. Int. Ed. 2011, 50, 6722-6737.
D. G. Hall in Boronic Acids. Structures, Properties, and Preparation of
Boronic Acid Derivatives: Overview of their Reactions and Applications,
2nd Ed. (Ed.: D. G. Hall), Wiley-VCH, Weinheim, 2011, pp. 1-134.
2a, KOt-Bu and toluene were introduced in a double-walled Schlenk
vessel of 28 mL, thermostated at 20 °C under Ar. After the resulting
mixture was stirred for a few minutes, a Mykrolis A332984-007 absolute
This article is protected by copyright. All rights reserved.

10.1002/chem.202000289
Chemistry - A European Journal
FULL PAPER
[6]
[7]
a) T. Ishiyama, M. Murata, N. Miyaura, J. Org. Chem. 1995, 60, 7508-
7510; b) M. Murata, T. Oyama, S. Watanabe, Y. Masuda, J. Org. Chem.
2000, 65, 164-168.
G. C. Fu, Transition Metal-Catalyzed Hydroboration of Olefins, in
Transition Metals for Organic Synthesis: Building Blocks and Fine
Chemicals, 2nd Ed. (Eds.: M. Beller, C. Bolm), Wiley-VCH, Weinheim,
2008.
J. V. Obligacion, P. J. Chirik, Nature Rev. Chem. 2018, 2, 15-34.
a) D. Noh, H. Chea, J. Ju, J. Yun, Angew. Chem. Int. Ed. 2009, 48,
6062-6064; b) D. Noh, S. K. Yoon, J. Won, J. Y. Lee, J. Yun, Chem.
Asian J. 2011, 6, 1967-1969; c) S. C. Schmid, R. Van Hoveln, J. W.
Rigoli, J. M. Schomaker, Organometallics 2015, 34, 4164-4173; d) H.
A. Kerchner, J. Montgomery, Org. Lett. 2016, 18, 5760-5763.
Poblador-Bahamonde, S. Choppin, S. Bellemin-Laponnaz, M. J.
Chetcuti, T. Achard, V. Ritleng, Dalton Trans. 2018, 47, 17134-17145.
[19] L. P. Bheeter, M. Henrion, M. J. Chetcuti, C. Darcel, V. Ritleng, J.-B.
Sortais, Catal. Sci. Technol. 2013, 3, 3111-3116.
[20] a) S. Z. Tasker, E. A. Standley, T. F. Jamison, Nature 2014, 509, 299–
309; b) M. Henrion, V. Ritleng, M. J. Chetcuti, ACS Catal. 2015, 5,
1283–1302; c) V. P. Ananikov, ACS Catal. 2015, 5, 1964–1971; d) V.
Ritleng, M. Henrion, M. J. Chetcuti, ACS Catal. 2016, 6, 890–906.
[21] A. J. MacNair, C. R. P. Millet, G. S.Nichol, A. Ironmonger, S. P.
Thomas, ACS Catal. 2016, 6, 7217-7221.
[22] a) K. Burgess, W. A. van der Donk, S. A. Westcott, T. B. Marder, R. T.
Baker, J. C. Calabrese, J. Am. Chem. Soc. 1992, 114, 9350-9359; b) S.
A. Westcott, H. P. Blom, T. B. Marder, R. T. Baker, J. C. Calabrese,
Inorg. Chem. 1993, 32, 2175-2182.
[23] A possible explanation for the observation of a mixture of products with
1-heptene may be due alkene chain isomerization, see reference 10a.
[24] L. P. Bheeter, D. Wei, V. Dorcet, T. Roisnel, P. Ghosh, J.-B. Sortais, C.
Darcel, Eur. J. Inorg. Chem. 2015, 5226-5231.
[8]
[9]
[10] a) J. V. Obligacion, P. J. Chirik, J. Am. Chem. Soc. 2013, 135, 19107-
19110; b) L. Zhang, Z. Zuo, X. Leng, Z. Huang, Angew. Chem. Int. Ed.
2014, 53, 2696-2700; c) L. Zhang, Z. Zuo, X. Wan, Z. Huang, J. Am.
Chem. Soc. 2014, 136, 15501-15504; d) W. N. Palmer, T. Diao, I.
Pappas, P. J. Chirik, ACS Catal. 2015, 5, 622-626.
[11] a) J. Y. Wu, B. Moreau, T. Ritter, J. Am. Chem. Soc. 2009, 131, 12915-
12917; b) J. V. Obligacion, P. J. Chirik, Org. Lett. 2013, 15, 2680-2683;
c) L. Zhang, D. Peng, X. Leng, Z. Huang, Angew. Chem. Int. Ed. 2013,
52, 3676-3680; d) J. Zheng, J.-B. Sortais, C. Darcel, ChemCatChem
2014, 6, 763-766; e) J. Chen, T. Xi, Z. Lu, Org. Lett. 2014, 16, 6452-
6455.
[12] a) C. Zhang, H. Zeng, J. Wu, Z. Yin, S. Zheng, J. C. Fettinger, Angew.
Chem. Int. Ed. 2016, 55, 14369-14372: b) J. R. Carney, B. R. Dillon, L.
Campbell, S. P. Thomas, Angew. Chem. Int. Ed. 2018, 57, 10620-
10624.
[25] J. A. Widegren, R. G. Finke, J. Mol. Catal. A 2003, 198, 317-341.
[26] O. R. Luca, B. A. Thompson, M. K. Takase, R. H. Crabtree, J.
Organomet. Chem. 2013, 730, 79-83.
[27] M. N. Peñas-Defrutos, C. Bartolomé, P. Espinet, Organometallics 2018,
37, 3533-3542.
[28] J. H. Docherty, J. Peng, A. P. Dominey, S. P. Thomas, Nature Chem.
2017, 9, 595-600.
[29] I. P. Query, P. A. Squier, E. M. Larson, N. A. Isley, T. B. Clark, J. Org.
Chem. 2011, 76, 6452-6456.
[30] For relevant borohydride species, see: a) H. C. Brown, B. Nazer, J. A.
Sikorski, Organometallics 1983, 2, 634-637; b) H. Wen, L. Zhang, S.
Zhu, G. Liu, Z. Huang, ACS Catal. 2017, 7, 6419-6425.
[13] I. D. Gridnev, N. Miyaura, A. Suzuki, Organometallics 1993, 12, 589-
592.
[14] a) S. Crotti, F. Bertolini, F. Macchia, M. Pineschi, Org. Lett. 2009, 11,
3762; b) R. J. Ely, Z. Yu, J. P. Morken, Tetrahedron Lett. 2015, 56,
3402.
[15] a) K. Hirano, H. Yorimitsu, K. Oshima, Org. Lett. 2007, 9, 5031-5033; b)
J.-F. Li, Z.-Z. Wei, Y.-Q. Wang, M. Ye, Green Chem 2017, 19, 4498-
4502; c) T. Kamei, S. Nishino, T. Shimada, Tetrahedron Lett. 2018, 59,
2896-2899; d) G. Vijaykumar, M. Bhunia, S. K. Mandal, Dalton Trans.
2019, 48, 5779-5784.
[16] a) G. W. Kabalka, C. Narayana, N. K. Reddy, Synth. Commun. 1994,
24, 1019-1023; b) S. Perreira, M. Srebnik, Tetrahedron Lett. 1996, 37,
3283-3286; c) E. E. Tourney, R. Van Hoveln, C. T. Buttke, M. D.
Friedberg, I. A. Guzei, J. M. Schomaker, Organometallics 2016, 35,
3436-3439.
[17] a) T. K. Macklin, V. Snieckus, Org. Lett. 2005, 7, 2519–2522; b) R. A.
Kelly, N. M. Scott, S. Díez-González, E. D. Stevens, S. P. Nolan,
Organometallics 2005, 24, 3442–3447; c) W. Buchowicz, A. Kozioł, L. B.
Jerzykiewicz, T. Lis, S. Pasynkiewicz, A. Pęcherzewska, A.
Pietrzykowski, J. Mol. Catal. A 2006, 257, 118–123; d) D. A. Malyshev,
N. M. Scott, N. Marion, E. D. Stevens, V. P. Ananikov, I. P. Beletskaya,
S. P. Nolan, Organometallics 2006, 25, 4462–4470; e) W. Buchowicz,
W. Wojtczak, A. Pietrzykowski, A. Lupa, L. B. Jerzykiewicz, A. Makal, K.
Wozniak, Eur. J. Inorg. Chem. 2010, 648–656; f) V. Ritleng, A. M.
Oertel, M. J. Chetcuti, Dalton Trans. 2010, 39, 8153–8160; g) A. M.
Oertel, V. Ritleng, L. Burr, M. J. Chetcuti, Organometallics 2011, 30,
6685–6691; h) A. M. Oertel, V. Ritleng, M. J. Chetcuti, Organometallics
2012, 31, 2829–2840; i) M. Henrion, M. J. Chetcuti, V. Ritleng, Chem.
Commun. 2014, 50, 4624–4627; j) Y. Wei, A. Petronilho, H. Mueller-
Bunz, M. Albrecht, Organometallics 2014, 33, 5834–5844; k) Ł. Banach,
P. A. Guńka, D. Górska, M. Podlewska, J. Zachara, W. Buchowicz, Eur.
J. Inorg. Chem. 2015, 5677–5686; l) F. P. Malan, E. Singleton, P. H.
van Rooyen, M. Landman, J. Organomet. Chem. 2016, 813, 7–14; m)
Ł. Banach, P. A. Guńka; W. Buchowicz, Dalton Trans. 2016, 45, 8688–
8692; n) S. Ando, H. Matsunaga, T. Ishizuka, J. Org. Chem. 2017, 82,
1266–1272.
[31] a) M. Hamdaoui, M. Ney, V. Sarda, L. Karmazin, C. Bailly, N. Sieffert,
S. Dohm, A. Hansen, S. Grimme, J.-P. Djukic, Organometallics 2016,
35, 2207-2223; b) M. Hamdaoui, C. Desrousseaux, H. Habbita, J.-P.
Djukic, Organometallics 2017, 36, 4863-4884.
[32] S. Pelties, D. Herrmann, B. de Bruin, F. Hartl, R. Wolf, Chem.
Commun. 2014, 50, 7014-7016.
[33] J. Wu, A. Nova, D. Balcells, G. W. Brudvig, W. Dai, L. M. Guard, N.
Hazari, P.-H. Lin, R. Pokhrel, M. K. Takase, Chem. Eur. J. 2014, 20,
5327-5337.
[34] a) J. Evans, J. R. Norton, J. Am. Chem. Soc. 1974, 96, 7577-7578; b)
G. Trinquier, R. Hoffmann, Organometallics 1984, 3, 370-380; c) J. P.
Collman, J. E. Hutchison, P. S. Wagenknecht, N. S. Lewis, M. A. Lopez,
R. Guilard, J. Am. Chem. Soc. 1990, 112, 8206-8208; d) J. P. Collman,
P. S. Wagenknecht, N. S. Lewis, J. Am. Chem. Soc. 1992, 114, 5665-
5673; e) B. A. Schaefer, G. W. Margulieux, B. L. Small, P. J. Chirik,
Organometallics 2015, 34, 1307-1320; f) N. X. Gu, P. H. Oyala, J. C.
Peters, J. Am. Chem. Soc. 2018, 140, 6374-6382.
[35] B. L. Tran, M. Pink, D. J. Mindiola, Organometallics 2009, 28, 2234-
2243.
[36] The fact that this mechanism accounts for the production of half a
equivalent of H2 for
a molecule of 2a is in agreement with our
piezometric measurement in the presence of 1:1:1 mixture of 2a, KOt-
Bu and HBCat. Hence, the apparent contradiction with the
measurement of one equivalent of H2 per molecule of 2a when 2 equiv.
of KOt-Bu and HBCat or more are used may arise from an unknown
side-reaction.
[37] C. D. Abernethy, A. H. Cowley, R. A. Jones, J. Organomet. Chem.
2000, 596, 3-5.
[38] J. Yau, K. E. Hunt, L. McDougall, A. R. Kennedy, D. J. Nelson, Beilstein
J. Org. Chem. 2015, 11, 2171-2178.
[39] V. Ritleng, C. Barth, E. Brenner, S. Milosevic, M. J. Chetcuti,
Organometallics 2008, 27, 4223-4228.
[40] F. Neese, WIREs Comput. Mol. Sci. 2012, 2, 73-78.
[41] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865-
3868.
[42] a) S. Grimme, J. Anthony, S. Ehrlich, H. Krieg, J. Chem. Phys. 2010,
132, 154104; b) S. Grimme, S. Ehrlich, L. Goerigk, J. Comput. Chem.
2011, 32, 1456-1465
[18] a) L. P. Bheeter, M. Henrion, L. Brelot, C. Darcel, M. J. Chetcuti, J.-B.
Sortais, V. Ritleng, Adv. Synth. Catal. 2012, 354, 2619-2624; b) L.
Postigo, B. Royo, Adv. Synth. Catal. 2012, 354, 2613-2618; c) Y. Wei,
S.-X. Liu, H. Mueller-Bunz, M. Albrecht, ACS Catal. 2016, 6, 8192-
8200; d) M. Rocquin, V. Ritleng, S. Barroso, A. M. Martins, M. J.
Chetcuti, J. Organomet. Chem. 2016, 808, 57-62; e) F. Ulm, A. I.
[43] F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 7, 3297-3305.
[44] V. Barone, M. Cossi, J. Phys. Chem. A 1998, 102, 1995-2001.
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10.1002/chem.202000289
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Hydroboration of alkenes catalysed
by a nickel N-heterocyclic carbene
complex: reaction and mechanistic
aspects
Initial reduction of the air-stable Ni(II) precursor [NiCp*Cl(IMes)] to the
[Ni(I)Cp*(IMes)] active species provides an efficient catalyst for the regioselective
hydroboration of alkenes.
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