On the mechanism of Ni(II)-promoted Michael-type hydroamination of acrylonitrile and its substituted derivatives

Article abstract of DOI:10.1039/c6dt02105k

Michael-type hydroamination of acrylonitrile and its substituted derivatives promoted by Ni(ii) complexes is believed to proceed via an outer-sphere nucleophilic attack on the cationic adduct of the nitrile-coordinated substrate. As a test for the validit

Full text of DOI:10.1039/c6dt02105k

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DOI: 10.1039/C6DT02105K
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ARTICLE
On the Mechanism of Ni(II)-Promoted Michael-Type
Hydroamination of Acrylonitrile and Its Substituted Derivatives†
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
S. Lapointea and D. Zargariana,*
www.rsc.org/dalton
Michael-type hydroamination of acrylonitrile and its substituted derivatives promoted by Ni(II) complexes is believed to
proceed via an outer-sphere nucleophilic attack on the cationic adduct of the nitrile-coordinated substrate. As a test for
the validity of this mechanistic postulate, we have sought to establish a correlation between the electrophilic character of
the Ni(II) center and the degree to which it can activate the substrate toward nucleophilic attack by amines. This has been
done by screening the catalytic activities of the cationic acetonitrile adducts [(R-POCOP)Ni(NCCH3)][OSO2CF3] bearing an
electron-donating or electron-withdrawing substituent R on the central aromatic ring of the POCOP ligand (R-POCOP=
P,C,P-2,6-(i-Pr2PO)2-4-R-C6H2; R= OMe (3), COOMe (4)). The catalytic activities for the addition of primary amines to
crotonitrile, methacrylonitrile, and cinnamonitrile were found to depend on the precursor and the amine used, as well as
on the reaction time. These studies were complemented by ligand exchange studies that established the relative binding
order among the main components of a typical hydroamination mixture (RCN > amine > OSO2CF3), thus supporting the
assertion that cationic nitrile adducts constitute the resting state in the catalytic manifold. We have also prepared and
characterized the cationic acrylonitrile and cinnamonitrile adducts [(R-POCOP)Ni(NCCH=CHR’)][OSO2CF3] (R’= H : R=
COOMe (7) or OMe (8); R’= Ph: R= COOMe (9) or OMe (10)) as models of the postulated catalytic intermediates in the
addition of amines to these substrates. To allow structural comparisons to the nitrile adducts, we have prepared and
characterized the ammonia adducts [(R-POCOP)Ni(NH3)][OSO2CF3] (R= H, 11, and COOMe, 12). The results of structural,
spectroscopic, and reactivity studies carried out on these compounds and their implications for the mechanism of Michael-
type hydroamination reactions promoted by the title system have been discussed.
the N-coordinated nitrile substrate toward attack by the amine
nucleophile (Scheme 1),8, 12, 13 and an inner-sphere, insertion
mechanism involving amine adducts and amido intermediates
Introduction
Michael additions on acrylonitrile can proceed in an
uncatalysed fashion with highly nucleophilic aliphatic amines,1
but a catalyst is generally required when a wider range of
amines is used. A catalyst is also required with all amines for
the analogous amination of acrylonitrile’s substituted
derivatives, including crotonitrile, methacrylonitrile, and
cinnamonitrile. The most commonly used catalysts/promoters
for these reactions are simple salts or complexes of early and
late transition metals.2-11 Although the mechanisms operating
in these systems have not been established unequivocally, two
different postulates have been proposed, an outer-sphere
mechanism involving cationic adducts that serve to activate
(
Scheme 2).14-16,
Our group has shown that cationic Ni complexes featuring
PCP- and POCOP-type pincer ligands are competent pre-
catalysts for the Michael-type regioselective hydroamination
of acrylonitrile and its substituted derivatives (Equation 1).17-20
A number of studies have suggested that these reactions
proceed by an outer-sphere mechanism involving an
intermediate featuring the acrylonitrile substrate that, by
virtue of being N-coordinated to a cationic Ni(II) center, is
activated toward an attack by the amine nucleophile.2, 17, 21
Evidence in support of this proposal includes the isolation and
a.Département de chimie, Université de Montréal, Montréal (Québec), Canada H3C
3J7
E-mail : zargarian.davit@umontreal.ca
† Electronic Supplementary Information (ESI) available. CCDC 1476211, 1476212,
1476251, 1476252 and 1476253. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/x0xx00000x
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Having developed practical synthetic routes to new
View Article Online
POCOP-based cationic Ni(II) compleDxOesI: 10f.1e0a3t9u/rCi6nDgT02r1i0n5gK-
substituents, and having established the relative stabilities and
oxidation potentials of these complexes,22 we set out to
examine the catalytic activities of some of these species as a
function of the ring substituents. The present report reports
the catalytic activities of the cationic complexes [(4-R-
Scheme 1 An outer-sphere mechanism for hydroamination of acrylonitrile catalyzed by
electrophilic metal complexes
POCOP)Ni(NCMe)][OSO2CF3] (R= OMe,
3; CO2Me, 4) in the
addition of various amines to methacrylonitrile, crotonitrile,
and cinnamonitrile. The main objective of these catalytic tests
was to examine whether or not the catalytic activities of
3 and
4
correlate with the electrophilicity of each species. We have
also examined the spectra, solid state structures, and relative
coordinating aptitudes of the different acrylonitrile,
cinnamonitrile, and ammonia adducts [(4-R-POCOP)Ni-
(NCCH=CHR’)][OSO2CF3] (R= CO2Me: R’= H (
R’= H ( ), Ph (10)) and [(4-R-POCOP)Ni(NH3)][OSO2CF3] (R= H
11), CO2Me (12)).
7), Ph (9); R= OMe:
Scheme 2 An inner-sphere mechanism for hydroamination of acrylonitrile catalyzed by
electrophilic metal complexes
8
(
structural characterization in a PCP-Ni system of the two
postulated cationic adducts, one bearing the acrylonitrile
substrate and the other bearing the product of aniline addition
to it (equation 2).17
Results and discussion
Hydroamination tests. We began our studies by screening the
catalytic reactivities of cationic acetonitrile adducts bearing an
OMe or a CO2Me substituent on the central aromatic ring of
the pincer ligand. The catalytic tests were designed to
establish whether the electronic impact of these ring
substituents would have the anticipated influence on the
hydroamination activities.
The outer-sphere mechanism invoked above would require
that factors enhancing the electrophilicity of the Ni centre
impart a favorable influence on catalytic activities, and a
limited number of investigations have confirmed such a
correlation. For instance, the more electrophilic POCOP-based
systems display greater activities compared to their PCP
counterparts.17 On the other hand, other variables such as the
nature of POCOP ligand backbone (aliphatic vs. aromatic) and
P-substituents (i-Pr vs. Ph) do not appear to exert significant
influence on catalytic reactivities. Thus, addition of aniline to
acrylonitrile is fairly equally promoted by the aromatic- and
aliphatic-backboned POCOP adducts [(POCOP)Ni(NCMe)]+,18
whereas aromatic-backboned adducts featuring i-Pr2P and
Ph2P moieties led to comparable catalytic activities.20
Another potentially favorable factor for the (pincer)Ni(II)-
promoted hydroamination of acrylonitrile and its substituted
derivatives would be the presence of electron-withdrawing
ring-substituents on the central aromatic ring of resorcinol-
based POCOP ligands. To date, only one report has examined
this issue and the results obtained were counter-intuitive: the
cationic Ni adduct bearing a Cl-substituted POCOP ligand,
[(2,6-(i-Pr2O)2-3,5-Cl2-C6H)Ni(NCMe)]+, showed a lower catalytic
activity for hydroamination of acrylonitrile relative to its
unsubstituted analogue.17 It should be mentioned, however,
that the weak catalytic performance observed in this case is
likely a reflection of the limited thermal stability of this pre-
catalyst and cannot disprove the anticipated impact of ring-
substituents on catalytic activities.
Initial results revealed that acrylonitrile is very reactive
with a variety of Ni precursors, a fact that did not allow us to
differentiate among the various precursors. Moreover,
acrylonitrile does undergo uncatalysed Michael addition with
some aliphatic amines (e.g., ca. 50% over 24 h at r.t. with
morpholine),18 thus complicating our studies. In contrast,
crotonitrile, methacrylonitrile, and cinnamonitrile are
unreactive to amines in the absence of a suitable catalyst,
proving to be much more challenging substrates for Ni-
promoted Michael-type hydroamination reactions.19 These
substrates were, therefore, selected for our studies. Ten
primary amine substrates were also selected to cover a wide
range of nucleophilic and steric properties: seven aromatic
(aniline; 2-X-aniline, X= F, Cl, Br, I; 4-NO2-aniline; 2,5-
dimethylaniline) and three aliphatic (cyclohexylamine,
octylamine, and ethanolamine).
The acetonitrile adducts [(4-R-POCOPi-Pr)Ni(NCMe)][OTf]
(R= OMe (
3), CO2Me (4)) were synthesized and purified
22
following previously reported procedures (Scheme 3
)
and
used as precursors for our catalytic tests, which were
conducted (in triplicate) on THF solutions (total volume ~1 mL)
containing one mmol each of the amine and nitrile substrates
and NEt3, plus the precursor Ni complex (0.01 mmol) and
dodecane as internal standard (0.1 mmol). The reactions were
performed by heating the samples at 50 °C for the designated
time (2 or 20 h), and the final mixtures were analyzed by
GC/MS; the conversions and yields were determined based on
a calibration curve prepared using authentic samples of the
2 | Dalton Trans., 2016, 00, 1-3
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Table 1 Single and double addition products for hydroamination reaction of crotonitrile
View Article Online
and methacrylonitrile promoted by precursors 3 and 4.
DOI: 10.1039/C6DT02105K
TONa
(yield, %)
TOFa
[h-1]
Product
Nitrile
[Ni]
3
4
28.6 ± 0.6
14.3 ± 0.3
20.0 ± 0.7
40 ± 1
15a
15b
15c
3
4
4.9 ± 0.1
9 ± 2
2.43 ± 0.03
4 ± 1
3
4
3
4
2.6 ± 0.5
1.3 ± 0.2
2.0 ± 0.1
3.9 ± 0.4
1.3 ± 0.3
0.7 ± 0.1
1.0 ± 0.1
2.0 ± 0.2
15d
15f
3
4
3
1.1 ± 0.1
4.8 ± 0.3
36.4 ± 0.6
0.56 ± 0.04
2.4 ± 0.2
Scheme 3 Synthesis of cationic nitrile adducts 7-10 and previously reported complexes
1-6. 18, 22
18.2 ± 0.3
anticipated products. The results of these tests are listed in
Table 1 and graphically plotted in Figures S28 and S29 (ESI).
(See also Tables S3-S10 in ESI for more detailed results of all
the catalytic runs.) These results lead us to the following
general observations.
First, the predominant reactivity observed in nearly all
cases consists of mono addition of one amine N-H bond across
the double bond moiety of the nitrile substrates, but double N-
H addition to give a tertiary amine was observed to a minor
15h
15i
15j
4
3
4
3
23.9 ± 0.6
56.1 ± 0.6
28.2 ± 0.8
18 ± 1
11.9 ± 0.3
28.0 ± 0.3
14.1 ± 0.4
9.1 ± 0.7
4
3
6.8 ± 0.1
21 ± 1
35 ± 1
38 ± 2 *
5.2 ± 0.7
3.4 ± 0.1
10.6 ± 0.7
17.5 ± 0.7
19 ± 1 *
4
16a
16b
16c
3
4
3
4
2.6 ± 0.4
extent in the reactions of octylamine (see Tables S5, S6, S9,
7.8 ± 0.4
3.2 ± 0.3
0
3.9 ± 0.2
1.6 ± 0.2
0
and S10 in the ESI). Crotonitrile and methacrylonitrile showed
varying activities with all amines except 2-iodoaniline and 4-
nitroaniline, whereas cinnamonitrile showed little or no
activity with all amines except aniline (vide infra).
3
4
3
4
3
4
2.4 ± 0.5
3.2 ± 0.5
7 ± 1
1.2 ± 0.2
1.6 ± 0.3
3.6 ± 0.5
Second, reaction time appears to play an important role in
the relative levels of catalytic activity for the two pre-catalysts
examined. Thus, for the catalytic additions to crotonitrile and
methacrylonitrile conducted over a short reaction time,
16d
16f
precursor
aromatic amines aniline, 2-chloroaniline being the exception,
whereas precursor was more competent for the addition of
aliphatic amines (Table 1). In contrast, precursor is the best
4
proved to be more competent with nearly all
0.6 ± 0.1
39 ± 1
0.30 ± 0.03
19.7 ± 0.6
3
16h
16i
16j
22.7 ± 0.6
34.6 ± 0.7
12 ± 1
14.6 ± 0.7
0
11.3 ± 0.3
17.3 ± 0.3
6.1 ± 0.5
7.3 ± 0.4
0
3
3
4
3
4
promoter for addition of nearly all amines when the catalytic
reactions are allowed to proceed for 20 h (Tables S3 vs S4 and
S7 vs S8). A speculative explanation for these counter-intuitive
observations involves the relative substitutional labilities of
the two (R-POCOP)Ni systems: the stronger binding of the
addition product to the Ni centre in the more electrophilic
Reaction conditions: Amine (1 mmol), crotonitrile (14a) or methacrylonitrile (14b)
(1 mmol), NEt3 (1 mmol), 3 or 4 (1 mol%), dodecane (10 mol%, internal standard)
and THF (500 µL), 50 °C, 2 h. GC/MS yields. Reactions were done in triplicate and
the yields are the average of those three experiments. * In this experiment, 5
mmol of amine was used instead of 1 mmol.
system
4 would hinder the product-substrate exchange
equilibrium that we assume to be crucial for catalytic turn-
overs; the impact of such a product inhibition would be more
pronounced over longer reaction times. It should be
emphasized, however, that this and similar conclusions remain
to be tested against precise kinetic data that can identify the
relative rates of the different steps in the catalytic manifold
and the impact of various factors on these steps.23
in the presence of catalyst
3 yields the mono addition product
in ca. 29% over 2 h compared to ca. 48% over 20 h (See Table
S3 and S4). However, in other cases we noted that longer
reaction times resulted in either no increase in yields or even a
slight decrease. For instance, the reaction of crotonitrile with
Longer reaction times also appear to improve yields in
some cases. For example, reaction of crotonitrile with aniline
2-bromoaniline in the presence of precursor
2% yields after 2 h and 20 h, respectively (See Table S4 in SI for
4 gave ca. 4% and
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The acrylonitrile and cinnamonitrile adducts [(4-R-POCOPi-
reaction yields after 20h). Such apparent (and minor)
degradations in yields are presumably due to secondary Pr)Ni(NCH=C(H)R’)][OTf] (R= CO2Me: R’= HDO(7I: 10.1039/C6DT02105K
reactions of the products, but the precise reasons for these R’= H ( ), Ph (10)) were prepared following the same synthetic
observations have not been examined. route used for accessing the analogous acetonitrile adducts
A final observation relates to the relative reactivities of the 6 22
Treatment of the charge-neutral halo analogues (R-
View Article Online
), Ph (9); R= OMe:
8
1
-
.
nitrile substrates as a function of the substituents on the POCOPi-Pr)NiX (X= Cl, Br)24,25 with AgOTf and R’C(H)=CHCN for 3
olefinic moiety: on the basis of the similar hydroamination h at rt gave the target complexes 7-10 in 63-98% yields (See
yields obtained with crotonitrile and methacrylonitrile, it Scheme 3).
would appear that the electronic/steric impact of the Me
group does not vary much as a function of its position ( vs.
These complexes were then subjected to spectroscopic and
X-ray diffraction studies, and the results were compared to


relative to the CN moiety), whereas the presence of a Ph group data obtained for previously examined analogues in search of
(in cinnamonitrile) leads to a significant loss of reactivity. evidence demonstrating substrate activation upon RCN Ni
Indeed, hydroamination of cinnamonitrile occurred only with coordination. For example, comparison of IR data for 7-10 to
aniline and in the presence of precursor (22 ± 1 % yield of 3- the corresponding data for the previously reported acetonitrile
morpholino-3-phenylpropanenitrile), no reactivity being complexes should allow us to understand how the
observed with other amines or in the presence of precursor substituents present on the POCOP central ring and on the

4
1-6
3
.
It should be emphasized, however, that even this limited substrates can affect the electrophilicity of the nickel center
reactivity is significant for such a challenging substrate, and and the substrate-Ni interaction. We have shown earlier22 that
attests to the strong activating capacity of precursor
4
.
the nature of the backbone substituent can have a significant
To sum up, the catalytic reactivities documented in Table 1 impact on the nitrile stretching frequency, ν(C≡N), with
and Figures S28 and S29 (ESI) indicate that the activities of the frequencies of up to 2329 cm-1 for the CO2Me substituted
precursors
being more efficient for the addition of the more In a similar manner, it would be interesting to know what is
nucleophilic aliphatic amines at all time intervals, and the effect of the nitrile substituent on the ν(C≡N) and how it
precursor being more efficient for the addition of the less compares to the closely related complexes reported
nucleophilic anilines over 2 h. That the more electron- previously.
withdrawing CO2Me substituent results in a greater activation Inspection of the IR data shown in Table 2 leads to the
of the more challenging substrates crotonitrile, following conclusions. First, we observe large and positive
3 and 4 are time- and amine-dependent, precursor complex 4 3.
and 2297 cm-1 for the OMe substituted complex
3
4
methacrylonitrile, and particularly the quite inert values of Δν(C≡N), implying a significant difference in the
cinnamonitrile toward nucleophilic attack by the less ν(C≡N) values for the free and nickel-bound nitrile substrates.
nucleophilic aniline is consistent with the anticipated Moreover, in all cases both ν(C≡N) and Δν(C≡N) values are
substituent effect discussed above. On the other hand, it is not greater in the adducts bearing the electron-withdrawing
clear why the same effect is not observed for the reactions of substituent. This observation is consistent with the reasoning
the more nucleophilic aliphatic amines.
that the enhanced electrophilicity of the nickel center in
CO2Me adducts , and increases the RCN Ni electron
Cationic acrylonitrile and cinnamonitrile adducts. In the donation, which in turn strengthens the C≡N bond; this follows
absence of an unambiguous correlation between substituent from the anticipation that net -donation from the nitrile lone
4
,
7
9


effects and catalytic Michael-type hydroamination activities, pair, which has partial anti-bonding character, should reinforce
we set out to study the structures and substitutional labilities the C≡N bond.26, 27 It should be noted, however, that while
of the cationic adducts as models of reaction intermediates. this trend is also observed in the cinnamonitrile analogues
9
Isolation of acrylonitrile adducts allowed us to study the and 10, curiously the difference between the ν(C≡N) values in
substitutional lability of the nitrile substituent and establish these complexes is very small: 2244 cm-1 vs 2241 cm-1.
whether amine adducts might be involved in the
Further comparison of the data for various adducts shows
hydroamination catalytic cycle. In addition, the solid structure greater Δν(C≡N) values for acetonitrile relative to acrylonitrile
of one acrylonitrile adduct was studied by X-ray diffraction and cinnamonitrile. For instance, the acetonitrile adducts show
analysis. Unfortunately, the analogous studies could not be Δν(C≡N) values of +41
conducted on the corresponding crotonitrile or corresponding acrylonitrile and cinnamonitrile adducts show
methacrylonitrile adducts, because we did not succeed in values of +21 ( ), +61 ( ), +23 (10), and +26 ( ). While the
(3) and +77 (4), whereas the
8
7
9
isolating these derivatives. While it is tempting to conclude precise reason for this discrepancy in the levels of activation of
that this finding reflects the thermal instability of these acetonitrile vs acrylonitrile and cinnamonitrile is not known at
adducts and might explain the limited reactivity of these this stage,28 this observation is reflected in the previously
substrates, this assertion is inconsistent with the successful reported reactivity of acetonitrile adducts with amine
preparation and isolation of cationic adducts with the least nucleophiles (to form amidines).19, 29
reactive substrate, cinnamonitrile. The solid state structures of
the two cinnamonitrile adducts bearing the CO2Me and OMe
ring-substituents were investigated to gain insight into its
inertness.
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Table 2 IR ν(C≡N) data for complexes 1-4 and closely related systems
Complex
ν(C≡N) (cm-1)
Δν(C≡N) [nitrile] (cm-1)c
[(H-POCOPi-Pr)Ni(NCMe)][OSO2CF3], 1 a
[(p-Me-POCOPi-Pr)Ni(NCMe)][OSO2CF3], 2a
[(p-OMe-POCOPi-Pr)Ni(NCMe)][OSO2CF3], 3a
[(p-CO2Me-POCOPi-Pr)Ni(NCMe)][OSO2CF3], 4a
[(p-Br-POCOPi-Pr)Ni(NCMe)][OSO2CF3], 5 a
2297 (2292)b
2294
+45 (+40)b
+42
2293
2329
2302
+41
+77
+50
[(m,m-t-Bu2-POCOPi-Pr)Ni(NCMe)][OSO2CF3], 6a
[(p-CO2Me-POCOPi-Pr)Ni(NCCH=CH2)][OSO2CF3], 7
[(p-OMe-POCOPi-Pr)Ni(NCCH=CH2)][OSO2CF3], 8
[(p-CO2Me-POCOPi-pr)Ni(NCCH=CHPh)][OSO2CF3], 9
[(p-OMe-POCOPi-Pr)Ni(NCCH=CHPh)][OSO2CF3], 10
[(H-POCOPiPr)Ni(NCCH=CH2)][OSO2CF3] a
2294
2291
2251
2244
2241
2257
2252
+42
+61
+21
+26
+23
(+27) b
(+22) b
a
[(H-POCsp3OPiPr)Ni(NCCH=CH2)][OSO2CF3]
a) Previously reported complexes.18 b) The ν(CN) values in parentheses were measured using KBr pellets to provide a comparison to the solid-state ATR measurements
used in the discussion. c) Δν(CN) is relative to the free nitrile stretching frequency of the nitrile in brackets (Free acetonitrile: 2252 cm-1; Free acrylonitrile: 2230 cm-1;
Free cinnamonitrile: 2218 cm-1).
however, there are statistically significantly differences in Ni-N
distances in these complexes. Overall, the solid state data
indicate that changing the nature of the Ni-bound nitrile ligand
has a significant effect only on the Ni-N distances, but not on
other distances and angles around the coordination plane.
Formation of amine-bound complexes. The above proposed
mechanistic schemes for metal-catalyzed hydroamination
reactions under discussion invoked the intermediacy of
cationic acrylonitrile adducts (outer-sphere mechanism,
Scheme
1) or amine adducts (inner-sphere mechanism,
Scheme 2), the latter converting to amido species that might
undergo olefin insertion. The observation of thermally stable
and isolable acrylonitrile and cinnamonitrile Ni adducts
provides indirect support for the validity of the outer-sphere
mechanism operating in our systems. In an effort to study the
feasibility of the alternative inner-sphere mechanism in our
(POCOP)Ni-based systems, we have examined the kinetic
accessibility of cationic amine adducts to establish whether
they can be viable intermediates in the catalytic cycle, and if so
to isolate one or more examples of such adducts and study
their structures and reactivities. NMR tube experiments were
thus conducted to monitor the reaction of the charge-neutral
triflate species, (p-OMePOCOP)Ni(OTf) with excess amine; in the
cases where amine adducts could be generated in-situ, they
were treated with acrylonitrile to measure the relative binding
forces of the two substrates. The results of these tests are
summarized below.
In light of its prevalence in our hydroamination studies,
aniline was the first amine tested. No color change was
observed when 10 equiv of aniline was added to a 0.06 M C6D6
solution of the triflate complex (p-OMePOCOP)Ni(OTf), but the
31P NMR spectrum of the mixture showed two broad signals.
The broad signal at ca. 186.5 ppm (See Figure 4a; LW1/2 = 31
Hz) is attributed to (p-OMePOCOP)Ni(OTf), while the other at ca.
189 ppm (See Figure 4a; LW1/2 = 38 Hz) is believed to arise
from the amine-coordinated complex; based on their
respective integration values, the major species is the triflate
precursor (60:40). The analogous NMR test with the more
Figure 1 Front view of the molecular diagram for complex 7. Thermal ellipsoids are
shown at the 50% probability level. Hydrogens are omitted for clarity.
Structural analyses of nitrile-bound complexes. The most
pertinent structural parameters for the three cationic nitrile
adducts complexes subjected to X-ray diffraction studies are
listed in Table 3, and the front view of the molecular diagrams
of adducts 7, 8 and 10 are shown in Figures 1 to 3; the side
views of the molecular diagrams for these complexes are
shown in Figures S1-S3, whereas details of the diffraction
studies are given in Tables S1 and S2 of the Supporting
Information (ESI).
The nickel center in structures 7, 9, and 10 adopts a square
planar geometry displaying slight distortions from the ideal
geometry, most of which are primarily due to the small bite
angle of the POCOP ligands: P-Ni-P~163-164°. The
displacement of the nickel atom out of the coordination plane
(P1-Ni-P2-Cipso) is very minor (<0.05(1) Å). We observe close to
ideal Cipso-Ni-N angles (ca. 177-179°) in all three complexes,
with little or no lifting of the nitrile moiety out of the
coordination plane. This contrasts with the parent acetonitrile
complexes
2 and 6 where we have observed a significant
deviation from the ideal square-planar geometry (Cipso-Ni-N
angles of ca. 170-172°).22 The Ni-P (ca. 2.17 Å) distances in
complexes 7, 9 and 10 are very close to the corresponding
distances found in the previously reported complexes 1-6
;
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View Article Online
DOI: 10.1039/C6DT02105K
Table 3 Bond distances (Å) and angles (deg.) for complexes 1-6 and 7, 9 and 10
Complex
Ni-C
Ni-N
N≡C
C=C b
Ni-P1
Ni-P2
P1-Ni-P2
C1-Ni-N
1 a
2 a
3 a
4 a
5 a
6 a
7
1.881(2)
1.879(3)
1.884(2)
1.880(2)
1.879(3)
1.890(2)
1.877(2)
1.878(2)
1.881(2)
1.874(2)
1.875(2)
1.881(2)
1.879(2)
1.871(3)
1.875(2)
1.873(2)
1.867(2)
1.864(2)
1.140(3)
1.142(4)
1.135(3)
1.138(2)
1.145(4)
1.140(2)
1.147(3)
1.140(2)
1.149(3)
2.1683(7)
2.1693(8)
2.1784(5)
2.1698(4)
2.1685(9)
2.1785(5)
2.1798(5)
2.1801(4)
2.1709(6)
2.1704(7)
2.1687(7)
2.1747(5)
2.1748(4)
2.171(2)
2.1640(4)
2.1777(5)
2.1758(5)
2.1740(6)
164.38(3)
163.52(3)
163.16(2)
164.45(1)
164.44(4)
163.79(2)
164.10(2)
164.49(2)
163.57(2)
175.8(1)
171.7(1)
176.84(7)
178.43(6)
176.6(2)
170.35(6)
176.62(8)
177.14(7)
178.59(9)
1.319(3)
1.333(3)
1.316(4)
9
10
a) Previously reported complexes 18, 22 b) Nitrile moiety alkene bond distances
70 ppm associated with decomposition products arising from
hydrolysis or oxidation of the phosphinite moieties.
Finally, we were surprised to find that morpholine, whose
nucleophilicity should be similar to that of piperidine and
cyclohexylamine, reacted quite differently than its
counterparts, displacing the triflate moiety only partially
(~20%). The 31P NMR spectrum of this mixture showed two
new signals, the major one (accounting for ~18 % of the P-
containing species) representing the morpholine adduct and
the minor peak remaining unidentified. The above
observations indicate that cyclohexylamine and piperidine act
as stronger nucleophiles in displacing the triflate moiety,
whereas the substitution reaction appears to be incomplete
with aniline and morpholine.
While the relative binding aptitudes of amines and the
triflate anion are interesting to note, a more important
question in the context of the hydroamination mechanism is
whether the amine substrates can compete with nitrile
substrates for binding to the cationic Ni center. To answer this
question, we reacted the in-situ generated amine adducts with
acrylonitrile and monitored the ligand substitution reaction by
31P NMR spectroscopy. Results of these tests showed that even
a small amount of acrylonitrile (less than half the equiv of
amines) led to a complete and instantaneous suppression of
the signals for amine adducts and the sharp signal at ca. 195
ppm for the corresponding acrylonitrile adduct emerged in
every case. This observation confirms the much greater
binding aptitude of nitrile substrates and rules out scenarios
involving the generation of amine adducts under the normal
condition of the hydroamination catalysis wherein the amine
and RCN substrates are normally present in a 1:1 ratio.
Figure 2 Front view of the molecular diagram for complex 9. Thermal ellipsoids are
shown at the 50% probability level. Hydrogens are omitted for clarity.
Isolation of amine adducts. Although the above results
discounted the possibility that amine adducts might form
during the hydroamination catalysis in our system, we were
nevertheless interested in isolating such species and examining
their structures and reactivities. Unfortunately, none of the
amine adducts discussed above furnished isolable products,
but an ammonia adduct was obtained serendipitously from the
reaction of (R-POCOPi-Pr)Ni(OTf) (R= H, CO2Me) with
tris(trimethylsilyl)amine (N(SiMe3)3); the same material was
also isolated from the reaction of the triflate precursor with
NH4OH. These reactions and the complete characterization of
Figure 3 Front view of the molecular diagram for complex 10. Thermal ellipsoids are
shown at the 50% probability level. Hydrogens are omitted for clarity.
nucleophilic cyclohexylamine showed a complete conversion
of the triflate complex and emergence of a new major species
showing a 31P NMR signal at ca. 188.7 ppm (see Figure 4b
;
LW1/2 = 5.4 Hz); a minor species is also present in this mixture
(at ca. 190 ppm). Complete conversion of the triflate complex
was also observed with piperidine (at ca. 188 ppm), but in this
case we observed additional peaks in the upfield region of 50-
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the products [(R-POCOPi-Pr)Ni(NH3)][OTf] (R = H (11), CO2Me should be added that a black oily residue was alVsioewfAortricmleeOdnliinne
(
12)) are described below.
these reactions, pointing to possiblDeOI:d1e0c.1o0m39p/Co6siDtTio0n2105oKf
The reaction of (H-POCOPi-Pr)Ni(OTf) with excess N(SiMe3)3 complex
in relatively dry THF (~40 ppm H2O) led to the disappearance
Structural analyses of complexes 11 and 12 (see Table 4)
of the 31P NMR signal for the starting material and a new signal showed that they are globally quite similar to analogous
emerged at 189 ppm. X-ray diffraction studies conducted on a cationic complexes 1-10 in terms of most bond distances and
crystalline solid obtained from this sample revealed the angles around the nickel atom (see Table 3), the main
formation of the ammonia adduct 11
(Figure 5); this species exception being the Ni-N distances which are longer in the
presumably arises from the hydrolysis of the N-Si bonds in the ammonia adducts (ca. 1.96 vs. 1.86-1.88
Å
). Another
corresponding N(SiMe3)3 adduct. This hydrolysis scenario was important difference is the presence in complexes 11 and 12
supported by noting that adding one equiv of N(SiMe3)3 and of hydrogen bonding between a H atom in the NH3 moiety and
only three equiv of water to the mixture of the triflate the O atom of the triflate moiety, with distances of 2.2(1) Å for
precursor gave the corresponding ammonia adduct with 71% 11 and 2.1(1) Å for 12. The positions of the hydrogen atoms on
yield over 2 h. We obtained the ammonia adducts 11 and 12 in the ammonia nitrogen atom were located using electron
85% and 90% yields, respectively, by direct reaction of 1-10 density.
equiv of NH4OH with (R-POCOPi-Pr)Ni-OTf (R = H, OMe). It.
Table 4 Bond distances (Å) and angles (deg.) for complexes 11 and 12
11
12
Ni-C
Ni-N
1.894(2)
1.961(2)
1.890(2)
1.962(2)
Ni-P1
Ni-P2
P1-Ni-P2
C1-Ni-N
2.1862(4)
2.1850(4)
162.96(2)
177.01(6)
2.1873(5)
2.1807(5)
163.37(2)
177.70(7)
Figure 5 Molecular diagram for complex 11. Thermal ellipsoids are shown at the 50%
probability level. Hydrogens are omitted for clarity
Figure 4 31P NMR (162 MHz, C6D6, rt) spectrums from the treatment of (p-OMePOCOP)Ni-
(OTf) with 10 equiv of: aniline (a), cyclohexylamine (b), piperidine (c), and morpholine
(d). Peaks marked with a star symbol (*) are due to the precursor complex.
Figure 6 Molecular diagram for complex 12. Thermal ellipsoids are shown at the 50%
probability level. Hydrogens are omitted for clarity
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The-above discussed direct pathway to the ammonia catalytic activities for the addition of aniline: complex
3
led to
View Article Online
DOI: 10.1039/C6DT02105K
adduct is very interesting as it requires a very inexpensive and
readily available nucleophile. Since it was shown previously h.
that POCOP pincer complexes decompose in basic media,30 it is
higher yields when the reactions were allowed to run over 20
Finally, the structural analyses of the substrate adducts
7,
significant that the fragile P- O linkage of the POCOP ligands
9, and 10 revealed no concrete structural evidence for the
survives the basic aqueous conditions of the reaction with anticipated activation of the nitrile-coordinated acrylonitrile or
NH4OH. The formation of this adduct also implies that cinnamonitrile substrate. These observations suggest that
ammonia is a much stronger ligand compared to water, and even if the outer-sphere postulate fairly represents the “true”
the latter is in turn stronger than the triflate moiety as mechanism of action in this system, there are many kinetic
reported in a previous study.20
subtleties that must be understood in order to rationalize all
As a final test for the feasibility of the inner-sphere observations. We have speculated that one such subtlety is
mechanism, we treated the ammonia adducts with NaH in an that the stronger nitrile-Ni binding anticipated with the more
effort to form the postulated charge-neutral Ni-amido electrophilic precursor
4 can result in a stronger binding of the
derivatives via deprotonation; these reactions failed to addition product; such a product inhibition factor would, in
generate the target Ni-NH2 species, leading instead to the turn, hinder the product-substrate exchange equilibrium that
decomposition of the starting materials, presumably because is crucial for turnover in the envisaged catalytic cycle.
of secondary reactions at the relatively fragile P-O linkage.30
The somewhat ambiguous nature of the above results
Finally, addition of excess acetonitrile to the ammonia adducts prompted us to isolate and study amine adducts that are
11 and 12 at r.t. resulted in significant broadening of the postulated to be involved in the alternative, inner-sphere
original 31P NMR signal of the ammonia adduct, giving LW1/2 of mechanism for the title reactions. Although ligand exchange
127 Hz with 10 equiv of MeCN and 187 Hz with 50 equiv. These studies showed that certain amines have sufficiently strong
observations imply that ammonia is a highly competitive binding capacities to generate observable amine adducts, we
nucleophile for binding to the Ni center.
were unable to isolate thermally stable adducts of the amines
that are active in hydroamination reactions with the nitrile
substrates in question. However, two new cationic ammonia
adducts were isolated, which allowed us to study the
structures of these rare complexes. Finally, preliminary ligand
exchange studies showed that ammonia-Ni interaction is
sufficiently robust so as not result in displacement of the NH3
moiety by large excess of acetonitrile.
Future studies will aim to examine more closely the
kinetics of the Michael-type hydroamination of acrylonitrile
with the objective of establishing the relative importance of
the various steps in the postulated reaction mechanism,
including coordination-activation of the nitrile substrate,
nucleophilic attack by amines, H+-transfer, as well as product
inhibition. Of outmost importance will be to identify
unequivocally which, if any, of these steps is the rate-
determining event in the catalytic process.
Conclusion
This study has generated a number of findings in support of
the outer-sphere reaction mechanism postulated for the Ni(II)-
promoted Michael-type hydroamination of acrylonitrile and its
substituted derivatives. For instance, ligand binding studies
showed that the cationic, nitrile-coordinated adducts of the
substrates are the dominant species under the conditions of
the catalytic reactions (1:1 mixture of amines and nitrile
substrates); this observation is consistent with the idea that
the key role of the Ni(II) center in the catalytic cycle is to
activate the substrate toward nucleophilic attack by amines.31
Indeed, cationic adducts of crotonitrile and methacrylonitrile
could be isolated and structurally characterized. The catalytic
tests undertaken also showed that addition of aniline to
crotonitrile and methacrylonitrile over a short time period (2
h) proceed with higher yields in the presence of pre-catalyst
the more electrophilic of the two cationic acetonitrile adducts
4,
Experimental section
tested. Moreover, this same pre-catalyst was the only one that General methods
could promote the addition of aniline to cinnamonitrile, the
least reactive Michael acceptor examined.
Unless otherwise indicated, all manipulations were carried
out under a nitrogen atmosphere using standard Schlenk and
glovebox techniques. The solvents were dried by passage over
activated alumina contained in MBRAUN-SPS systems and
analyzed by a Coulorimetric Karl Fischer titrator to acceptable
water content. Triethylamine was dried by distillation over
CaH2. The following reagents and NMR solvents were
purchased from Sigma-Aldrich and used without further
purification: nickel powder, bromine, ClP(i-Pr)2, methyl 3,5-
dihydroxybenzoate, silver trifluoromethanesulfonate, C6D6 and
CDCl3. 5-methoxyresorcinol was purchased from Chemsavers
and used as received.
On the other hand, some of the observations from our
catalytic tests appear to be inconsistent with the main
elements of the outer-sphere proposal. For instance, for the
addition of the more nucleophilic, aliphatic amines onto
crotonitrile and methacrylonitrile, the greater catalytic
activities were observed with complex 3, the less electrophilic
of the two pre-catalysts investigated, regardless of the time
period over which the catalytic reactions were monitored. This
finding does not correlate well with a mechanism in which
activation of the nitrile-coordinated substrate plays an
important role in determining reactivities.32 Moreover,
allowing a longer reaction time resulted in a reversal of
8 | Dalton Trans., 2016, 00, 1-3
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NMR spectra were recorded using Bruker AVII400 and 107.10 (t, JPC = 6, 2C, CArH), 107.33 (s, 1C, Ni-Cipso), 133.18 (s,
View Article Online
DOI: 10.1039/C6DT02105K
AV500 spectrometers. Chemical shift values are reported in 1C, CArCO2CH3), 141.38 (s, 1C, NCCH=CH2), 166.27 (s, 1C,
ppm (δ) and referenced internally to the residual solvent
C
ArCO2CH3), 168.94 (t, JPC = 9, 2C, (CAr-OP)2). 31P{1H} NMR (162
signals (1H and 13C: 7.26 and 77.16 ppm for CDCl3; 7.16 and MHz, C6D6) δ 192.2 (s). 19F {1H} NMR (376 MHz, CDCl3) δ -77.97
128.06 ppm for C6D6) or externally (31P, H3PO4 in D2O, δ = 0). (s). Elemental analysis was not satisfactory for this complex,
Coupling constants are reported in Hz. Due to either because it proved difficult to remove all traces of solvents.
insufficiently concentrated samples or an insufficient number
of scans collected, we were unable to 13C signals for some [(2,6-(i-Pr2OP)24-(OCH3)C6H2)Ni(NCCHCH2)][OSO2CF3] (8). The
quaternary carbons. In the case of the 13C signal for the triflate procedure described above for the preparation of
7 was used
moiety, a similar observation has been reported previosuly in for this synthesis, using (2,6-(iPr2OP)2-4-(OMe)C6H2)NiCl24 (199
closely related complexes.18 The IR spectra were recorded on mg, 0.430 mmol, 1.00 equiv) as starting material. The desired
a Bruker Alpha-P FTIR (4000-400 cm-1). The elemental analyses product was obtained as a yellow solid (271 mg, 98%).
were performed by the Laboratoire d’Analyse Élémentaire,
Département de Chimie, Université de Montréal. Synthesis P(CHC(CH3)2)2), 1.36 (q, JHH = 9, 12H, P(CHC(CH3)2)2), 2.41 (sept,
and characterization of complexes as well as the triflate JHH = 8, 4H, P(C C(CH3)2)4), 3.18 (s, 3H, OCH3), 5.61 (s(br), 1H,
complex (1-OTf) have been reported elsewhere.18, 22 Synthesis NCCH=C
H), 5.70 (s(br), 1H, NCC =CH2), 6.00 (d(br), JHH = 23,
of the neutral bromo or chloro precursors were done following 1H, NCCH=CH
). 6.19 (s, 2H, (CArH)2). 13C{1H} NMR (125 MHz,
reported procedures.18, 24
C6D6) δ 16.71 (s, 4C, P(CH( H3)2)2), 17.58 (t, JPC = 3, 4C,
Ligand exchange reactions with the charge-neutral triflate P(CH( H3)2)2), 28.58 (t, JPC = 11, 4C, P( H(CH3)2)4), 55.11 (s, 1C,
complex and different amines and acrylonitrile were probed as CArO H3), 93.50 (vt, JPC = 8, 2C, (CArHmeta)2), 107.28 (s, 1C, Ni-
follows. To an NMR tube containing a 0.086 M solution of the Cipso), 140.61 (s, 1C, NCCH= H2), 163.79 (s, 1C, CArOCH3),
170.00 (t, JPC = 9, 2C, (CAr-OP)2). 31P{1H} NMR (162 MHz, C6D6) δ
1H NMR (500 MHz, C6D6) δ 1.15 (q, JHH = 8, 12H,
1
-
6
H
H
H
H
C
C
C
v
C
C
charge-neutral triflate complex (prepared following
a
procedure published elsewhere20) was added 10 equiv of the 194.9 (s). 19F{1H} NMR (376 MHz, C6D6) δ -77.48 (s). Elemental
target amine. The 31P NMR spectrum was recorded following a analysis was not satisfactory for this complex, because it
vigorous shaking of the sample tube to ensure complete proved difficult to remove all traces of solvents
mixing. To the same solution was added 3.0 equiv of
acrylonitrile and another 31P NMR spectrum was recorded [(2,6-(i-Pr2OP)24-(CO2CH3)C6H2)Ni(NCCH=CHPh)][OSO2CF3] (9).
after more vigorous shaking.
To a Schlenk flask containing the charge-neutral chloro
Catalytic tests were performed following this general complex (2,6-(iPr2OP)2-4-(CO2Me)C6H2)NiCl24 (268 mg, 0.540
procedure. In a sealable vial was placed the amine and nitrile mmol, 1.00 equiv) in dichloromethane (15 mL) was added
substrates (1 mmol each), NEt3 (1 mmol), the internal standard silver triflate (167 mg, 0.650 mmol, 1.20 equiv) and
(dodecane, 0.1 mmol), and 0.5 mL of a 0.02 M solution of the cinnamonitrile (682 µL, 5.43 mmol, 10.0 equiv) at rt. The
Ni precursor (3 or 4) in THF, and the resulting mixture was solution was then agitated overnight and filtered to remove
heated at 50 °C for the designated time. A small aliquot of the the insoluble silver salts. Single crystals suitable for x-ray
final mixture was diluted with acetone (~100x) and analyzed by diffraction were obtained by slow evaporation in air of the
GC/MS. The conversion and yield were determined based on a dichloromethane solution. The crystals obtained were washed
calibration curve prepared using authentic samples of the with cold hexanes and crushed, giving a yellow-orange powder
anticipated products.
(252 mg, 63%).
1H NMR (400 MHz, C6D6) δ 1.08 (q, JHH = 8, 12H,
[(2,6-(i-Pr2OP)24-(CO2CH3)C6H2)Ni(NCCHCH2)][OSO2CF3] (7). To P(CHC(CH3)2)2), 1.33 (q, JHH = 8, 12H, P(CHC(CH3)2)2), 2.36 (sept,
a Schlenk flask containing the charge-neutral chloro complex JHH = 8, 4H, P(C C(CH3)2)4), 3.43 (s, 3H, CO2CH3), 6.65-7.04
=C
(C6H5)), 7.38 (s, 2H, (CArH)2). 13C NMR
1.00 equiv) in dichloromethane (15 mL) was added silver (101 MHz, CDCl3) δ 16.87 (s, 4C, P(CH( H3)2)2), 17.69 (t, JPC = 3,
triflate (71 mg, 0.28 mmol, 1.2 equiv) and acrylonitrile (5 mL) 4C, P(CH( H3)2)2), 28.80 (t, JPC = 12, 4C, P( H(CH3)2)4), 52.45 (s,
at rt. The solution was then agitated for 3 h and filtered to 1C, CArCO2CH3), 96.09 (s, 1C, NC H=CH(C6H5)), 107.24 (s, 2C,
H
H
24
(2,6-(iPr2OP)2-4-(CO2Me)C6H2)NiCl
(114 mg, 0.230 mmol, (m(br), 7H, NCC
H
C
C
C
C
remove the insoluble silver salts. Evaporation of the filtrate CArHmeta) 127.62 (s, 2C, NCCH=CH(CorthoC4H5)), 129.23 (s, 2C,
gave the desired product as a yellow solid (125 mg, 80%). NCCH=CH(CmetaC4H5)), 131.51 (s, 1C, NCCH=CH(CparaC5H5)),
Single crystals suitable for diffraction studies were obtained 133.51 (s, 1C, NCCH=CH(CipsoC5H5)), 151.41 (s, 1C,
from slow evaporation of a concentrated THF solution of the NCCH=CH(C6H5)), 166.27 (s, 1C, CArCO2CH3), 168.94 (t, JPC = 9,
complex
.
2C, (CAr-OP)2). 31P{1H} NMR (162 MHz, CDCl3) δ 194.27 (s).
1H NMR (400 MHz, CDCl3) δ 1.35 (q, JHH = 7, 12H, 19F{1H} NMR (376 MHz, CDCl3) δ -77.98 (s). Elemental analysis
P(CHC(CH3)2)2), 1.42 (q, JHH = 7, 12H, P(CHC(CH3)2)2), 2.57 (sept, was not satisfactory for this complex, because it proved
JHH = 6, 4H, P(C
12; 18, 1H, NCC
(d, JHH = 18, 1H, NCCH=CH
16.86 (s, 4C, P(CH( H3)2)2), 17.60 (t, JPC = 3, 4C, P(CH(
28.68 (t, JPC = 11, 4C, P(
H
H
C(CH3)2)4), 3.86 (s, 3H, CO2CH3), 6.15 (dd, JHH
=CH2), 6.33 (d, JHH = 12, 1H, NCCH=C H), 6.42
). 13C{1H} NMR (101 MHz, CDCl3) δ [(2,6-(i-Pr2OP)24-(OCH3)C6H2)Ni(NCCH=CHPh)][OSO2CF3] (10).
=
difficult to remove all traces of solvents
H
H
C
C
H3)2)2), The same procedure described above for the preparation of 9
C
H(CH3)2)4), 52.4 (s, 1C, CAr-CO2CH3), was used for this synthesis, using (2,6-(iPr2OP)2-4-
This journal is © The Royal Society of Chemistry 2016
Dalton Trans., 2016, 00, 1-3 | 9
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25
(OMe)C6H2)NiBr
(500 mg, 0.980 mmol, 1.00 equiv) and
1H NMR (500 MHz, C6D6): δ 1.06 (q, JHH = 7, 12H,
P(CHC(CH3)2)2), 1.28 (q, JHH = 8, 12H, P(CHC(C
View Article Online
DOI: H103.)120)329),/C26.3D1T0(2s1e0p5tK,
furnished a yellow-orange powder (486 mg, 70 %).
1H NMR (400 MHz, C6D6) δ 1.18 (q, JHH = 8, 12H, JHH = 7, 4H, P(C
H
C(CH3)2)4), 3.00 (s(br), 3H, NH3), 3.43 (s, 3H,
P(CHC(CH3)2)2), 1.38 (q, JHH = 8, 12H, P(CHC(CH3)2)2), 2.46 (sept, OCH3) 7.42 (s, 2H, CAr-Hmeta). 13C{1H} NMR (126 MHz, CDCl3) δ
JHH = 8, 4H, P(C
C
NCCH=CH(C6(Hortho)2H3)),
H
C(CH3)2)4), 3.18 (s, 3H, OCH3), 6.02 (s, 2H, 16.46 (s, 4C, P(CH(
ArHmeta), 6.35 (s(br), 1H, NCC =CH(C6H5)), 7.05 (s(br), 3H, JPC = 11, 4C, P( H(CH3)2)4), 51.72 (s, 1C, O
7.53 (m(br), 3H, 2C, CAr-Hmeta), 131.41 (s, 1C, Cipso-Ni), 133.29 (s, 1C, CArCO2CH3)
2Hpara)). 13C NMR (101 MHz, CDCl3) δ 166.13 (s, 1C, CArCO2CH3), 168.90 (t, JPC = 9, 2C, CAr-OP). 31P{1H}
H3)2)2), 17.66 (s, 4C, P(CH(
H3)2)2), 28.64 (t, NMR (202 MHz, C6D6) δ 190.94 (s). 19F {1H} NMR (376 MHz,
H(CH3)2)4), 55.68 (s, 1C, CArO H3), 93.24 (t, JPC
7, 1C, CArHmeta), 95.360 (s, 1C, NC H=CH(C6H5)), 113.05 (t, JPC
21, 1C, N
C
H3)2)2), 17.62 (s, 4C, P(CH(
CH3)2)2), 28.40 (t,
H
C
C
H3) 107.05 (t, JPC = 6,
NCCH=CH(C6H2(Hmeta
16.87 (s, 4C, P(CH(
JPC = 12, 4C, P(
)
C
C
C
C
=
=
C6D6)
δ -78.03 (s). Anal. calcd. for C21H36F3N1Ni1O7P2S1
(624.22): C, 40.41; H, 5.81; N, 2.24; S: 5.14. Found: C, 40.26; H,
C
C
CH=CH(C6H5)) 128.15 (s, 2C, NCCH=CH(CorthoC4H5)), 5.87; N, 2.16; S, 4.92.
129.22 (s, 2C, NCCH=CH(CmetaC4H5)), 131.94 (s, 1C,
NCCH=CH(CparaC5H5)), 133.27 (s, 1C, NCCH=CH(CipsoC5H5)),
163.55 (s, 1C, CArOCH3), 169.28(t, JPC = 9, 2C, (CAr-OP)2). 31P{1H}
NMR (202 MHz, CDCl3) δ 194.37 (s). 19F{1H} NMR (470 MHz,
CDCl3) δ -78.13 (s). Anal. calcd. for C38H47F3N2NiO6P2S (837.5):
C, 54.50; H, 5.66; N, 3.34; S: 3.83. Found: C, 55.04; H, 5.81; N,
3.50; S, 3.37.
Acknowledgments
The authors are grateful to: NSERC of Canada for financial
support of this work (Discovery and RTI grants to D.Z.);
Université de Montréal for providing graduate fellowships to
S.L.; Dr. Michel Simard and Ms. Francine Bélanger-Gariépy for
their valuable assistance with crystallography and many
interesting discussions; Ms. Elena Nadezhina for the elemental
analyses; Mr. Jean-Philippe Cloutier for the DFT calculations
that helped shed some light on the IR results; and reviewers of
our manuscript for many valid and insightful suggestions. S.L.
is also grateful to Centre in Green Chemistry and Catalysis for a
travel award and to all group members for many valuable
discussions and practical advice.
[(2,6-(iPr2OP)2C6H3)Ni(NH3)][OSO2CF3] (11)
Schlenk flask containing the charge-neutral triflate complex
(2,6-(iPr2OP)2C6H3)Ni(OSO2CF3) 1-OTf) (975 mg, 1.78 mmol,
. Procedure 1. To a
(
1.00 equiv) was added tris(trimethylsillyl)amine (415 mg, 1.78
mmol, 1.00 equiv) and water (96 µL, 5.3 mmol, 3.0 equiv) at rt.
The solution was then agitated for 2 h and the resulting
insoluble, black oily residue was removed. Evaporation of the
solution gave the desired product as a yellow solid (715 mg, 71
%). Single crystals suitable for x-ray diffraction were obtained
by a slow evaporation in air of an acetone solution.
Notes and references
Procedure 2. To a Schlenk flask containing the charge-
neutral bromo complex (2,6-(iPr2OP)2C6H3)NiBr (240 mg, 0.500
mmol, 1.00 equiv) in THF (15 mL) was added NH4OH (38.9 µL,
1.00 mmol, 2.00 equiv) and AgOTf (154 mg, 0.600 mmol, 1.20
equiv) at rt. The solution was then agitated for 2 h, filtered to
remove the AgBr salts, and the organic phase separated.
Evaporation of the solution gave the desired product as a
yellow solid (254 mg, 90 %).
1
2
3
4
L.-W. Xu, L. Li and C.-G. Xia, Helv. Chim. Acta, 2004, 87,
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1H NMR (400 MHz, C6D6): δ 1.10 (q, JHH = 7, 12H,
P(CHC(CH3)2)2), 1.32 (q, JHH = 8, 12H, P(CHC(CH3)2)2), 2.34 (sept,
5
6
JHH = 7, 4H, P(C
8, 2H, CAr-Hmeta), 6.80 (t, JHH = 8, 1H, CAr-Hpara). 13C{1H} NMR
(101 MHz, CDCl3) δ 16.80 (s, 4C, P(CH( H3)2)2), 17.77 (t, JPC = 3,
4C, P(CH( H3)2)2), 28.21 (t, JPC = 11, 4C, P( H(CH3)2)4), 105.73 (t,
JPC = 6, 2C, (CAr-Hmeta)2), 130.25 (s, 1C, CAr-Hpara), 168.90 (t, JPC
HC(CH3)2)4), 2.96 (s(br), 3H, NH3), 6.46 (d, JHH =
7
8
C
C
C
=
9, 2C, (CAr-OP)2). 31P{1H} NMR (162 MHz, C6D6) δ 189.42 (s). 19
F
9
{1H} NMR (376 MHz, C6D6) δ -77.97 (s). Anal. calcd. for
C19H34F3NNiO5P2S (566.18): C, 40.31; H, 6.05; N, 2.47; S: 5.66.
Found: C, 40.95; H, 6.11; N, 2.18; S, 5.13.
10
[(2,6-(iPr2OP)2-4-(CO2CH3)C6H2)Ni(NH3)][OSO2CF3] (12). The
procedure 2 described above for the preparation of 11 was
11
12
13
S. Kim, S. Kang, G. Kim and Y. Lee, J Org Chem, 2016, 81,
4048-4057.
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128, 4246-4247.
B. M. Cochran and F. E. Michael, J. Am. Chem. Soc., 2008,
130, 2786-2792.
used
(CO2Me)C6H2)NiBr
for
this
synthesis
using
(2,6-(iPr2OP)2-4-
25
(215 mg, 0.400 mmol, 1.00 equiv). The
desired product was obtained as a yellow solid (211 mg, 85 %).
Crystals suitable for x-ray diffraction were obtained by slow
evaporation from a concentrated acetone solution.
10 | Dalton Trans., 2016, 00, 1-3
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Journal Name
ARTICLE
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C. Munro-Leighton, E. D. Blue and T. B. Gunnoe, J. Am.
Chem. Soc, 2006, 128, 1446-1447.
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DOI: 10.1039/C6DT02105K
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Gunnoe, Organometallics, 2007, 26, 1483-1493.
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39, 1171-1175.
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Mol. Catal. A: Chem., 2011, 335, 1-7.
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Organometallics, 2011, 30, 5352-5364.
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2555-2562.
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2015, 34, 3520-3531.
The authors wish to thank a reviewer of the manuscript
for asking that we include this clarification here for the
readers’ benefit.
24
25
26
27
B. Vabre, F. Lindeperg and D. Zargarian, Green Chem.,
2013, 15, 3188-3194.
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Chauvin, Int. J. Quantum Chem, 2016, DOI:
10.1002/qua.25174, n/a-n/a.
28
Simple DFT calculations carried out on acetonitrile,
acrylonitrile, and cinnamonitrile have shown extensive
mixing of the C=C bending mode with the CN stretching
oscillator in acrylonitrile and cinnamonitrile, whereas no
significant mixing was observed in acetonitrile. This might
offer a partial explanation for why v(CN) values of these
substrates are impacted differently upon coordination to
the cationic Ni(II) centre. We thank a reviewer of our
manuscript for suggesting that we probe this possibility.
B. Vabre, Y. Canac, C. Lepetit, C. Duhayon, R. Chauvin and
D. Zargarian, Chemistry – A European Journal, 2015, 21,
17403-17414.
29
30
31
J. Zhang, C. M. Medley, J. A. Krause and H. Guan,
Organometallics, 2010, 29, 6393-6401.
It should be recognized here that the absence of
detectable quantities of the postulated amine adducts
does not rule out the possibility that these species can
form in small quantities and might indeed act as
intermediates in the alternative inner-sphere mechanistic
scenario. We thank a reviewer of our manuscript for
suggesting that this point be stated clearly.
32
Our working hypothesis has been, and remains, that the
attack of the amine on the olefin is rate-limiting.
However, a reviewer of our manuscript has pointed out
correctly that our observations to date do not
substantiate this hypothesis; therefore, the rate-limiting
step in this process should be considered unknown.
This journal is © The Royal Society of Chemistry 2016
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DOI: 10.1039/C6DT02105K
TOC Text for
On the Mechanism of Ni(II)-Promoted Michael-Type
Hydroamination of Acrylonitrile and Its Substituted Derivatives
S. Lapointe and D. Zargarian
Monocationic Ni(II) complexes featuring variously substituted POCOP-type pincer ligands promote the addition of
primary amines to crotonitrile, methacrylonitrile, and cinnamonitrile.