Selective and nonselective aza-michael additions catalyzed by a chiral zirconium bis-diketiminate complex

Article abstract of DOI:10.1021/om400544z

Reaction of the chiral bis-diketiminate complex rac- or (R,R)-C ₆H₁₀(nacnac^Xyl)₂ZrCl₂ with AgOTf yielded the corresponding bis-triflate complex. The complex geometry changes from distorted octahedral in the dichloride complex to a pseudotetrahedral coordination involving π coordination of the diketiminate ligands. The bis-triflate complex is highly active for aza-Michael additions with turnover frequencies of 20000/h for the addition of morpholine to acrylonitrile and 1000/h for the addition of morpholine to methacrylonitrile. The enantioselectivities of the latter reaction in various solvents were low, never surpassing 19% ee. The reaction is first-order in olefin concentration and second order in amine concentration, which is explained by its participation as a base in the reaction mechanism. The presence of catalytic amounts of triethylamine slightly increases the observed rate constants and reduces the reaction order in amine to first order. Other activated alkenes such as methacrylonitrile, crotonitrile, methyl acrylate, and cyclohexenone can be employed, but no reactivity is observed toward styrene or vinyl ethers. Primary amines, secondary amines, and anilines can be employed as nucleophiles with activities correlating with their nucleophilicity, but the catalyst is unstable in the presence of alcohols.

Full text of DOI:10.1021/om400544z

Article
pubs.acs.org/Organometallics
Selective and Nonselective Aza-Michael Additions Catalyzed by a
Chiral Zirconium Bis-Diketiminate Complex
Ibrahim El-Zoghbi, Myriam Kebdani, Todd J. J. Whitehorne, and Frank Schaper*
Centre in Green Chemistry and Catalysis, Department of Chemistry, Universite
Quebec H3T 3J7, Canada
́ ́ ́
de Montreal, C. P. 6128 Succ. Centre-Ville, Montreal,
́
S
* Supporting Information
ABSTRACT: Reaction of the chiral bis-diketiminate complex rac- or
(R,R)-C6H10(nacnacXyl)2ZrCl2 with AgOTf yielded the corresponding
bis-triflate complex. The complex geometry changes from distorted
octahedral in the dichloride complex to a pseudotetrahedral
coordination involving π coordination of the diketiminate ligands.
The bis-triflate complex is highly active for aza-Michael additions with
turnover frequencies of 20000/h for the addition of morpholine to
acrylonitrile and 1000/h for the addition of morpholine to
methacrylonitrile. The enantioselectivities of the latter reaction in various solvents were low, never surpassing 19% ee. The
reaction is first-order in olefin concentration and second order in amine concentration, which is explained by its participation as a
base in the reaction mechanism. The presence of catalytic amounts of triethylamine slightly increases the observed rate constants
and reduces the reaction order in amine to first order. Other activated alkenes such as methacrylonitrile, crotonitrile, methyl
acrylate, and cyclohexenone can be employed, but no reactivity is observed toward styrene or vinyl ethers. Primary amines,
secondary amines, and anilines can be employed as nucleophiles with activities correlating with their nucleophilicity, but the
catalyst is unstable in the presence of alcohols.
with one ortho-substituted and one ortho-unsubstituted N-aryl
substituent, nacnacdipp,Ar,7l or ketiminate ligands, acnac, in which
one imino group is replaced by oxygen,7b readily form bis-
ligand complexes, but the nonsymmetric nature of the ligands
gave rise to a variety of isomers. Dulong et al. prepared
nonsymmetric, tridentate diketiminate ligands with an N-dipp
and an N-aryloxy substituent, which form the corresponding
complexes.7w Of course, no reactive coordination site remained
available in the (nacnacdipp,Ar−O)2Zr complexes obtained. A
different approach was pursued by Gong et al.: bridging of two
diketiminates by an ethylene or proypylene bridge generated a
tetradentate ligand, which afforded cis-octahedral bis-diketimi-
nate zirconium complexes (CH2)2,3(nacnacAr)2ZrCl2 with a
variety of ortho-substituted N-aryl substituents (Ar = 2,6-R2-
phenyl with R = Me, CF3, Cl, iPr).7m
INTRODUCTION
■
Although diketiminate (“nacnac”) metal complexes were first
synthesized in 1968,1 they did not have any notable impact on
the field until the late 1990s. Following Brookhart’s seminal
work on α-diimine catalysts in late-metal olefin polymer-
ization,2 it was a small step to anionic β-diketiminate ligands,
following the same design principle.3 Although not as successful
as α-diimine for olefin polymerization, the potential of β-
diketiminate ligands as spectator ligands, in particular nacnacXyl
and nacnacdipp (Xyl = 2,6-dimethylphenyl, dipp = 2,6-
diisopropylphenyl), was soon recognized.4
First reports of a zirconium diketiminate complex were made
by Lappert and co-workers in 19945 and Mittal et al. in 1995,6
soon followed by more extensive work by others7 that was
often oriented toward olefin polymerization.7d−g,l−n,q−t Bis-
diketiminate zirconium complexes, nacnac2ZrX2, were first
prepared by Collins, who reported a κ2 coordination of
diketiminate and a cis-octahedral geometry for these complex-
es.7a Mixed diketiminate−cyclopentadienyl (or indenyl) com-
plexes, (nacnac)CpZrX2, on the other hand, adopt a
pseudotetrahedral symmetry with a coordination of the
diketiminate ligand via its π system.7a It soon became apparent
that bis-diketiminate zirconium complexes did not tolerate
additional steric bulk. Substitution was limited to the para
position of the N-aryl substituent, and the ubiquitous nacnacdipp
ligand afforded only the mono-diketiminate complexes
nacnacdippZrX3(L)0,1,7b,c,i but not (nacnac)2ZrX2. Ortho sub-
stitution of the N-aryl is possible when the steric demand of the
second imine is reduced. Thus, nonsymmetric diketiminates
We recently started investigating bis-diketiminate zirconium
complexes as potential catalysts for organic transformations.
The attraction of complexes of the type nacnacR ZrX2 (1;
2
Scheme 1) as precatalysts is based on the presence of a chiral
metal center with close proximity of the N-R substituents to the
reactive coordination sites in cis positions. Initial explorations
showed that bis-diketiminate complexes with N-alkyl sub-
stituents indeed showed the desired complex geometry.7o Their
application, however, was impeded by a fast epimerization of
the metal center7a,b as well as a very low reactivity, presumably
due to steric crowding.7o Bridging of the diketiminate
Received: June 12, 2013
© XXXX American Chemical Society
A
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trile, and crotonitrile (Scheme 2, Table 1). GC/MS and NMR
analysis of all reaction mixtures showed only the presence of
Scheme 1
Scheme 2
Table 1. Aza-Michael Addition of Morpholine to
Unsaturated Nitriles
a
ligands7m,8 was then investigated to increase reactivity at the
ancillary coordination sites. The structure of bridged bis-
diketiminate complexes ( )-C6H10(nacnacXyl)2ZrX2 depends
on the size of the ancillary ligand: complexes with X = Cl (2;
Scheme 1) or Me have a trans-octahedral geometry, and with
sterically more demanding ligands (X = OEt (3), NMe2) the
complex adopts a cis-octahedral geometry.7x The deformation
caused by the cyclohexanediamine bridge increases the
reactivity of the ancillary zirconium substituents significantly
and allows easy ligand exchange. The corresponding bis-
ethoxide complex 3 (Scheme 1) proved to be highly active in
lactide polymerization, surpassing the activity of any other
group 4 catalyst by several orders of magnitude.7x Here we
report the synthesis of the Lewis acidic chiral bis-triflate
complex 4 (Scheme 1) employing the same ligand.
Zirconium triflates, in particular zirconocene triflate, have
been used to catalyze a variety of organic reactions.9 However,
for aza-Michael additions only a limited number of studies
reported the use of group 4 compounds as catalysts, in most
cases in the form of simple zirconium salts such as ZrCl4 and its
derivatives.10 We found it thus of interest to test the chiral
Lewis acid 4 in aza-Michael additions of amines to activated
olefins, such as acrylonitrile and acrylate. Enantioselective aza-
Michael additions are a challenging target,11 and we are not
aware of any applications of group 4 metal catalysts in
asymmetric aza-Michael additions. Zirconium has been used in
enantioselective conjugate additions other than aza-Michael
additions,12 and both are widely employed in asymmetric
hydroaminations of alkynes (Mg) or olefins (Zr).13 Complex 4
was thus tested in enantioselective aza-Michael additions, and
structural reasons for its rather disappointing performance will
be discussed.
reaction
entry
alkene
concentration/M time/min conversion/%
1
acrylonitrile
0.25
0.25
0.25
0.50
0.50
1.0
5
40
80
5
100
50
2
methacrylonitrile
(E/Z)-crotonitrile
methacrylonitrile
(E/Z)-crotonitrile
acrylonitrile
3
40
4
95
5
10
5
60
6
100
95
7
methacrylonitrile
(E/Z)-crotonitrile
1.0
5
8
1.0
5
90
b
b
9
acrylonitrile
1.0
80
80
80
40
b
b
10
11
methacrylonitrile
1.0
0
b
b
(E/Z)-crotonitrile
1.0
0
a
Reaction conditions: C6D6, ambient temperature, [morpholine]:
b
[alkene]:[4] = 100:100:1, yield determined by NMR. Control
experiments without addition of 4.
the anti-Markovnikov product andin the case of uncomplete
reactionsstarting material. With a catalyst loading of 1%, 4
proved to be highly efficient for the addition of morpholine to
acrylonitrile in benzene-d6, the reaction reaching full conversion
in less than 5 min. As expected, the sterically hindered alkenes
methacrylonitrile and crotonitrile were less reactive. Addition of
1 equiv (per morpholine) of NEt3 significantly reduced the
reactivity. When substance concentrations were raised from
0.25 to 1.0 M, all three olefins displayed conversions of >90%
after 5 min of reaction at room temperature (Table 1, entries
6−8). Under the same conditions, the noncatalyzed reaction of
morpholine with acrylonitrile proceeded slowly with 40%
conversion after 80 min, while the more hindered alkenes did
not react in the absence of catalyst (entries 9−11).
To test catalyst efficiency, reactions of acrylonitrile with
morpholine were run at reduced catalyst concentrations.
Efficient catalysis was still observed at 500 ppm of catalyst
loading (Table 2, entry 3, 70% after 5 min; the reaction did not
surpass 90% conversion, probably due to catalyst decom-
RESULTS AND DISCUSSION
■
Attempts to prepare mono- or bis-triflate complexes by reaction
of bis-ethoxide complex 3 with triflic acid or [PhN(H)Me2]-
[OTf] at different temperatures and reaction conditions yielded
reaction mixtures in which the main product was the
protonated ligand ( )-C6H10(nacnacXyl)2H2. Reaction of
dichloride complex 2 with 2 equiv of AgOTf, on the other
hand, cleanly yielded the bis-triflate complex
( )-C6H10(nacnacXyl)2Zr(OTf)2 (4; Scheme 1). The first and
second chlorides are exchanged with similar reactivities, and
reactions with 1 equiv of AgOTf yielded mixtures of 2, 4, and
very small amounts of a product putatively assigned as
( )-C6H10(nacnacXyl)2Zr(Cl)(OTf).
Table 2. Aza-Michael Addition of Morpholine to
Acrylonitrile at Different Catalyst Concentrations
a
b
[Zr]:[acrylonitrile]/%
reaction time/min
conversion/%
TOF
1
5
100
100
70/90
9
>20
>200
∼300
∼10
0.1
0.05
0.02
5
5/40
45
Aza-Michael Additions. Initial screening of the perform-
ance of 4 as a catalyst for aza-Michael additions was undertaken
for the addition of morpholine to acrylonitrile, methacryloni-
a
Reaction conditions: C6D6, ambient temperature, [morpholine] =
b
[acrylonitrile] = 1 M. In units of (mol of alkene/((mol of 4) min).
B
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position). Below this concentration, reactions turned sluggish
and did not reach completion even at prolonged reaction times,
consistent with catalyst poisoning by impurities in the reaction
mixture. The obtained turnover frequency of ∼300/min at 500
ppm catalyst loading should thus be considered a lower
estimate of catalyst activity. It is likely that a notable fraction of
the catalyst was deactivated by impurities already at this
concentration.
The scope of the reaction with regard to the nucleophile was
investigated with 1% 4. In all cases, NMR and GC-MS analysis
confirmed that hydroamination yielded the expected anti-
Markovnikov product and that the uncatalyzed reaction gave
less than 5% yield in 3−24 h of reaction time (Table S1,
Supporting Information). Benzylamine proved to be likewise
highly reactive, and reactions with acrylonitrile reached
completion in 5 min at room temperature (Table 3, entry 1).
crotonitrile showed low or no reactivity toward aniline under
the conditions investigated (entry 10). With acrylonitrile, 4-
methoxyaniline proved to be more reactive and 4-chloroaniline
to be less reactive than unsubstituted aniline. Thus, the
reactivity decreases in the order secondary amines > primary
amines > 4-methoxyaniline > aniline > 4-chloroaniline, in
parallel with the nucleophilicity of the amine employed.
As typically observed for aza-Michael reactions, benzylamine
and anilines yielded either only the mono-hydroamination
product (entry 4, entry 7, entry 10) or a mixture of mono- and
bis-hydroamination products which favors the former (entry 2).
Given that the mono-hydroamination products should be more
nucleophilic than the starting amines, the reason for their low
reactivity to undergo further hydroamination is unclear at the
moment. On the basis of the higher reactivity of secondary
alkyls in comparison to primary alkyls, simple steric reasons can
be excluded as an explanation for the reduced reactivity. The
observed kinetics (vide infra) also did not indicate the
possibility of product poisoning. No reactivity was observed
when phenols or ethanol were used as nucleophiles (Table 3,
entries 13−16). The presence of free bis-diketimine in the
NMR spectra indicates that the impeding factor is not the
nucleophilicity of the alcohols but protonation of the bis-
diketiminate ligand. Consequently, ethanolamine likewise did
not yield any hydroamination product (entry 17). Complex 4 is
thus not stable in the presence of alcohols.
a
Table 3. Aza-Michael Addition of Different Nucleophiles
reaction
entry
nucleophile
benzylamine
alkene
time
conversion/%
97
1
acrylonitrile
5 min
5 min
5 min
10/24 h
5 min
5/60
b
b
2
benzylamine
acrylonitrile
90
3
benzylamine
methacrylonitrile
methacrylonitrile
methacrylonitrile
acrylonitrile
20
c
cd
,
4
benzylamine
52/70
100
5
diethylamine
aniline
6
35/80
To further test the scope of the catalytic reaction and to gain
the first insights into a probable reaction mechanism, amines
were reacted with alkenes other than conjugated nitriles
(Scheme 3, Table 4). Cyclohexenone and methyl acrylate,
c
c
7
aniline
acrylonitrile
24 h
100
e
e
8
aniline + NEt3
acrylonitrile
5 min
5 min
5/24 h
5 min
5 min
24 h
5
f
f
9
aniline, 60 °C
aniline
acrylonitrile
63
c
c
10
11
12
13
14
15
16
17
methacrylonitrile
acrylonitrile
<1/9
4-methoxyaniline
4-chloroaniline
80
12
0
Scheme 3
acrylonitrile
4-methoxyphenol acrylonitrile
4-methylphenol
4-chlorophenol
ethanol
acrylonitrile
acrylonitrile
acrylonitrile
acrylonitrile
24 h
0
24 h
0
24 h
0
ethanolamine
24 h
0
a
Reaction conditions: C6D6, ambient temperature, [amine]:[alkene]:
b
[4] = 1 M:1 M:0.01 M, yield determined by NMR. [alkene] = 2 M,
10% bis-hydroamination product after 10 min. [alkene] = 2 M, only
mono-hydroamination product was observed. Laboratory-scale
reaction, isolated yield 52%. In the presence of NEt3, 1 M. Reaction
c
d
e
f
at 60 °C.
In the presence of 2 equiv of alkene (entry 2), the second
hydroamination was significantly slower and only 10% of bis-
hydroamination product was observed when all starting amine
was consumed. Reactions with methacrylonitrile proved again
to be more difficult but still proceeded reasonably quickly.
Higher conversions were reached at longer reaction times but
did not always reach completion, most likely due to catalyst
decomposition (entries 3 and 4). In line with this, no bis-
hydroamination product was observed if 2 equiv of
methacrylonitrile was reacted with benzylamine (entry 4). To
verify the reactivity of noncyclic secondary amines, diethyl-
amine was reacted with methacrylonitrile, yielding complete
conversion in 5 min. Cyclic and noncyclic secondary amines are
thus more reactive than primary amines. Reactions of aniline
with acrylonitrile proceeded more slowly than those of primary
and secondary amines. High conversions could be achieved at
longer reaction times (entry 7) or at elevated temperatures
(entry 9). As observed for morpholine, the presence of NEt3
impeded the reaction (entry 8). Methacrylonitrile and
but not methyl crotonate, reacted readily with morpholine or
diethylamine. Benzylamine and aniline were less reactive but
still reached conversion above 75% at prolonged reaction times.
(Yields of the uncatalyzed reaction were below 5% after 24 h
reaction time; see Table S1 in the Supporting Information.) In
all cases, NMR and GC/MS analysis confirmed the formation
of 1,4-addition products. Nonactivated alkenes, such as
cyclohexyl vinyl ether and styrene, did not show any reactivity,
in support of a Lewis acid mechanism. Reactions with
acetonitrile proceeded sluggishly and yielded an (unidentified)
mixture of products in low yields.
Reaction Kinetics and Mechanism. The main mecha-
nisms for hydroamination of alkenes with early transition
metals involve a [2 + 2] cycloaddition of a metal imido species,
insertion of the olefin into a metal−amide bond, or Lewis acid
activation of the alkene and outer-sphere attack of the neutral
nucleophile.13 In the case of 4, its inactivity toward non-
C
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Table 5). The reaction is thus second order with respect to
amine. Consequently, linear 1/c2 vs time plots were obtained
with equal initial concentrations of methacrylonitrile and
diethylamine (Figure S1, Supporting Information), confirming
a combined reaction order of 3 for the nucleophile and the
olefin. Assuming a first-order dependence on catalyst
concentration, the overall rate law is thus v =
k[NEt2H]2[methacrylonitrile][4] and we obtain from kinetics
at equal concentrations, excess amine, or excess olefin values for
k of 25(1), 38(5), and 25(4) min−1 M−3, respectively, which are
identical within the margin of error (Table 5).
Table 4. Aza-Michael Addition to Alkenes Other than
Unsaturated Nitriles
a
reaction
entry
nucleophile
morpholine
alkene
time
conversion/%
1
2
3
4
5
6
7
8
9
10
methyl acrylate
5 min
5/60
81
diethylamine methyl acrylate
benzylamine methyl acrylate
29/68
5/24 h
5/60
16/84
aniline
methyl acrylate
23/75
morpholine
methyl crotonate
60 min
60 min
60 min
60 min
24 h
0
diethylamine methyl crotonate
0
morpholine
morpholine
morpholine
morpholine
cyclohexenone
Cyclohexyl vinyl ether
styrene
80
0
Dependence on amine as well as olefin concentration
indicates either that coordination of methacrylonitrile to
zirconium is rate limiting, followed by fast addition of amine
in competition with dissociation, or that amine addition is rate
limiting and methacrylonitrile coordination is in a fast pre-
equilibrium far from saturation. The latter would be in
agreement with the observed slight displacements of acryloni-
trile resonances observed in 1H NMR, but the former cannot be
ruled out from the available data. A second-order dependence
on amine was slightly puzzling. The most likely activating role
of a second amine would be to act as a base which aids in the
proton transfer from N to C. However, addition of a
nonreactive amine such as triethylamine was observed to lead
to deactivation (vide supra). Closer inspection of the role of
triethylamine under pseudo-first-order conditions (excess
olefin) revealed that its presence has a slight activating effect
on the reaction if added in catalytic amounts only. In the
presence of <0.1 M triethylamine, conversions reached 50% in
0.5−1 min (Table 6, entries 2−8). In its absence, 3−5 min was
required (entry 1). Under the reaction conditions examined,
maximum activity was observed around [NEt3] = 25 mM: i.e.,
10 times the concentration of the catalyst (Figure 3). Higher
triethylamine concentrations slowly led to reduced activities
(entries 9−12), probably due to deactivation of the catalyst by
amine coordination to zirconium, although catalyst poisoning
by impurities cannot be excluded. While the increase in
reactivity with triethylamine was slight, its presence provided
additional information on the reaction mechanism. Inves-
tigation of the initial reaction rates in the presence of NEt3 (5
mM) showed a first-order dependence on diethylamine
concentration (entries 2−4, slope of 1.2 in log−log plots).
This is in contrast to the second-order dependence observed in
the absence of triethylamine and supports the notion that
diethylamine acts in an additional role, such as participating in
the proton transfer, in the absence of triethylamine. The
reaction mechanism outlined in Scheme 4 is proposed to
explain the kinetic results. The fast coordination equilibrium
between 4 and acrylonitrile observed in NMR experiments
indicates that A is likely formed in a fast, unfavorable pre-
equilibrium. Assuming that A to B is likewise reversible and that
the amine is involved in the proton transfer from B to D, a
second-order dependence on amine concentration originates
from the competition of the equilibrium backreaction (kBA) and
the proton transfer (kBC[HNEt2]) for B. The kinetic data only
indicate that a second amine is involved somewhere in the
reaction mechanism, but this is not necessarily the step with the
highest activation barrier. While participation in the proton
transfer is, in our opinion, the most likely explanation, any
other mechanism which involves two amines between the rate-
determining intermediate and the rate-determining transition
state in the catalytic cycle15 is in agreement as well with the
observed kinetics.
0
b
acetonitrile
48 h
20
a
Reaction conditions: C6D6, ambient temperature, [amine]:[alkene]:
b
[4] = 1 M:1 M:0.01 M, yield determined by NMR. Conversion of
starting material; a mixture of products was observed.
activated alkenes and the relationship between reactivity and
amine nucleophilicity argue for a Lewis acid mechanism in this
case. NMR spectra of equimolar mixtures of 4 with
acrylonitrile, morpholine, or both did not show any notable
displacement of the catalyst resonances. In 2:1 mixtures of 4
and acrylonitrile, a very slight displacement of the acrylonitrile
chemical shifts was observed (13C, 1 ppm downfield for the
terminal carbon atom and the CN group, 0.5 ppm upfield for
1
the internal carbon atom; H, slight downfield shift (<0.05
ppm) for all protons). These are in agreement with a fast,
incomplete and reversible N coordination of acrylonitrile to
4.14 No effects were observed on morpholine resonances, even
with excess 4.
The kinetics of the reaction of methacrylonitrile with
diethylamine was investigated in detail. Under pseudo-first-
order conditions (10-fold excess of amine), the reaction
showed first-order dependence on methacrylonitrile concen-
tration (Figure 1, Table 5). Reactions in the presence of excess
methacrylonitrile, on the other hand, failed to give linear
ln(conversion) vs time plots, and first-order rate constants
determined at different methacrylonitrile starting concentra-
tions were inconsistent (Figure 2). On the other hand, 1/
[NHEt2] vs time plots were highly linear and yielded rate
constants independent of starting concentrations (Figure 2,
Figure 1. Conversion vs time plot (squares) and the corresponding
linearized plot of ln c/c0 vs time (triangles) under pseudo-first-order
conditions: C6D6, ambient temperature, [NEt2H]0 = 1.3 M,
[methacrylonitrile]0 = 0.13 M, [4] = 3.1 mM (2.4%).
D
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Table 5. Observed Rate Constants Obtained under Different Conditions
a
[NEt2H]0/M
[C4H5N]0/M
[4]/mM
conversion range/%
rate law
kobs
k / min−1 M−3
1.3
1.3
0.13
0.13
3.1
3.1
24−94
46−99
v = kobs[A]
v = kobs[A]
0.18(1) min−1
0.23(5) min−1
38(5)
0.50
0.50
0.50
0.50
1.0
4.8
4.8
4.8
4.8
4.8
1.0
5.0
5.0
5.0
5.0
5.0
10
58−93
43−99
27−83
13−76
71−97
53−83
v = kobs[A]2
v = kobs[A]2
v = kobs[A]2
v = kobs[A]2
v = kobs[A]2
v = kobs[A]3
0.37(1) M−1 min−1
0.72(4) M−1 min−1
0.61(4) M−1 min−1
0.54(3) M−1 min−1
0.49(7) M−1 min−1
0.25(1) M−2 min−1
23(4)
25(1)
1.0
a
For the rate law: v = k[NEt2H]2[methacrylonitrile][4]. Errors are estimated from the standard deviation of the mean or from a linear regression
analysis.
Figure 2. Linearized first-order (squares) and second-order
(diamonds) concentration−time profiles in the presence of excess
methacrylonitrile: C6D6, ambient temperature, [NEt2H]0 = 0.50 M,
[methacrylonitrile]0 = 4.8 M, [4] = 5.0 mM (1%).
Figure 3. Pseudo-first-order rate constants in the presence of
triethylamine: C6D6, ambient temperature, [NEt2H]0 = 0.5 M,
[methacrylonitrile]0 = 5 M, [4] = 5 mM (1%).
Scheme 4
Table 6. Conversions and Observed First-Order Rate
Constants for the Addition of Diethylamine to
Methacrylonitrile in the Presence of Triethylamine
a
conversion after
1 min/%
vinitial/
b
entry [NEt3]/M
1
kobs/min
[Et2NH]/M min M−1
0.50
c
35
48
60
56
61
48
59
53
38
48
34
19
0.30(5)
0.60(2)
0.71(3)
2
0.0050
0.0049
0.0048
0.010
0.024
0.050
0.10
0.50
1.0
0.58
1.1
3
4
0.78(13)
0.75(7)
0.87(6)
0.67(7)
0.77(5)
0.36(5)
0.30(4)
0.39(7)
0.094(5)
1.5
2.1
5
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
6
7
8
9
0.24
10
11
12
0.48
1.0
2.0
a
Conditions: C6D6, ambient temperature, [methacrylonitrile]0 = 4.6
b
c
M, [4] = 5.0 mM (1%). Obtained from ln c/c0 = −kt. Second-order
reaction. Conversions <30% were fitted using a first-order rate law.
Enantioselective Reactions. The enantiopure complex
R,R-4 was prepared in a simplified procedure starting from
(R,R)-diaminocyclohexane L-tartrate and tested in the
enantioselective aza-Michael addition of morpholine to
methacrylonitrile. Several attempts to determine the obtained
ee using NMR shift reagents ((+)-Eu(tfc)3, (+)-Eu(hfc)3, or
(+)-Yb(hfc)3, tfc/hfc = 3-trifluoromethyl- or 3-heptafluoro-
propyl hydroxymethylene camphorate) were unsuccessful. Use
of chiral GC yielded the expected ee = 0% for the product
obtained with rac-4, while 2-S-methyl-3-morpholinoproprioni-
trile was obtained in low ee of 19% with R,R-4 (Scheme 5).
E
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Organometallics
Article
fected either by choice of solvent or by dilution and remained
between 10 and 19%. Reactions at low temperatures led to
incomplete conversion even at prolonged reaction times and
were not investigated further. While 4 is sufficiently stable at
room temperature to convert most substrates to high
conversions, its stability is limited at low catalyst concentrations
or low temperatures, where significantly longer reaction times
are required, and it seems unlikely that reaction conditions can
be found which would significantly increase the observed
enantioselectivities. It should be noted that Vitanova et al.
obtained enantioselectivities up to 72% ee for the intra-
molecular hydroamination of nonactivated olefins with similar
bis-diketiminate lanthanide complexes.8c The reaction mecha-
nism in this case, however, is based on olefin insertion into a
lanthanide−amide bond, distinctly different from the Lewis acid
activation mechanism proposed for 4 (vide supra).
Scheme 5
Variation of catalyst concentration or solvent (Table 7) failed
to improve the obtained enantioselectivity: in strongly
a
Table 7. Enantioselective Aza-Michael Additions with R,R-4
[NEt2H] =
conversion (after
1 h)/%
solvent
C6D6
[methacrylonitrile]/M
ee/%
0.50
95 (2 min)
19
13
1
THF
0.50
20
12
MeCN
Et2O
0.50
0.50
100
85
17
19
16
15
13
13
10
3
Solution and Solid-State Structure of 4. A possible
explanation for the unsatisfactory performance of 4 in
asymmetric reactions might be derived from the loss of C2
symmetry. Previously reported ( )-C6H10(nacnacXyl)2ZrX2
complexes (X = Cl, Me, NMe2, OEt) displayed C2-symmetric
1H and 13C NMR spectra, with one resonance for the ligand
methine group, one resonance for the methine proton of the
cyclohexyl bridge, two methyl diketiminate methyl resonances,
and two resonances for the xylyl methyl groups.7x Their solid-
state structures also displayed C2 point symmetry, in either a cis
(X = NMe2, OEt) or trans geometry (X = Cl, Me). In 1H NMR
spectra of 4, however, C2 symmetry was lost and two different
signal sets were observed for the two diketiminate moieties, as
well as four resonances for the xylyl methyl group and two
resonances for the methine group in the chiral bridge. The
fluorine NMR spectrum displayed two signals at −76.6 and
−77.5 ppm, indicating chemically different triflate anions. It was
tempting to assign these to coordinated and noncoordinated
0.25
0.125
0.050
0.50
72
30
CH2Cl2
100
70
0.25
0.125
0.050
0.50
33
17
toluene
100
84
16
10
10
10
n.d.
0.25
0.125
0.050
0.50
70
24
b
Et2O, −78 °C
30
a
Reaction conditions: ambient temperature, 1 h reaction time, 1 mol
b
% of 4. The observed conversion did not increase at prolonged
reaction times.
coordinating solvents, such as THF and acetonitrile, the
activity was drastically reduced. Diethyl ether, dichloromethane,
and toluene led to complete conversion in 1 h at reagent
concentrations of 0.5 M. Dilution led to the expected decrease
in activity (increased catalyst poisoning is also suspected for
dichloromethane). Enantioselectivities, however, were unaf-
1
triflate anions. However, lack of symmetry in the H NMR
spectrum would require either a configurationally stable five-
coordinated [C6H10(nacnacXyl)2Zr(OTf)]+ cation or a stable,
unsymmetric bridging coordination of the triflate anion, both of
Figure 4. X-ray structure of 4. Hydrogen atoms and the second, independent molecule of identical geometry are omitted for clarity. Thermal
ellipsoids are presented with a 50% probability cutoff.
F
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a
Table 8. Selected Bond Distances (Å) and Angles (deg) for 4
N1,N2,C1−5
κ2,η2
N11,N12,C41−45
κ2,η2
N3,N4,C21−45
“η5-like”
N13,N14,C61−C65
“η5-like”
coordination
Zr−N
(Zr,N,N)/(N,N,Cα,Cα)
2.307(10), 2.124(10)
68
2.271(9), 2.167(8)
72
2.166(9), 2.162(9)
68
2.169(8), 2.185(8)
68
b
Zr−Cβ
2.626(11)
2.656(11)
2.776(13)
2.805(11)
Zr−Cα
N−Cα−CMe
Zr−O6/O9/O3/O12
2.782(11), 2.596(12)
161, 143
2.852(11), 2.660(11)
168. 145
2.828(11), 2.748(13)
157. 153
2.822(10),, 2.770(10)
154, 154
2.197(8)
2.179(7)
2.235(8)
2.263(7)
a
b
Cα = C2, C4, C22, C24, C42, C44, C62, C64. Cβ = C3, C23, C43, C63. Bending of Zr out of the mean ligand plane in deg, defined by the nitrogen
and Cα atoms.
which seem unlikely. Also, a higher downfield shift would be
expected for free triflate. Both triflate resonances still fall in the
range observed for coordinated triflate anions in octahedral Zr
complexes (−73 and −79 ppm),16 while uncoordinated triflate
anions are expected around −80 ppm (e.g., [NBu4][OTf]
−78.8 ppm,17 [(Ph3P)2N][OTf] −80.5 ppm).18
Crystals of 4 suitable for an X-ray diffraction study were
obtained by diffusion of hexane into a toluene solution. Two
independent molecules are found in the asymmetric unit, which
show a noncrystallographic pseudosymmetry (glide plane
perpendicular to z; x + 0.25, y, −z) broken only by slightly
different orientations of the triflate ligands and the xylyl
substituents. The presence of pseudosymmetry resulted in the
relatively low quality of the structural data, despite sufficient
crystal quality. Nevertheless, the obtained structure clearly
revealed a coordination of both triflate anions to the zirconium
center in the solid state and an involvement of the diketiminate
π system in metal−ligand bonding (Figure 4, Table 8).
Zirconium diketiminate complexes typically show three distinct
coordination modes. Most typically observed is κ2 coordination
of the two nitrogen atoms (Scheme 6).7p The other two
bridging of two diketiminate ligands, the coordination of one
diketiminate can be clearly assigned as symmetric “η5-like”
coordination with comparable Zr−Cα distances (Δ < 0.1 Å)
and Zr−Cα−CMe angles (Δ < 5°, Table 8).7p The coordination
of the second diketiminate fragment is best described as κ2,η2
coordination, evidenced by larger differences in Zr−Cα
distances and Zr−Cα−CMe angles (∼0.2 Å and >20°).7p
While a nonsymmetric coordination of the bis-diketiminate
ligand would explain the C1-symmetric NMR spectra, neither
steric nor electronic reasons would suggest a substantive
activation barrier for interconversion between κ2,η2- and the
symmetric “η5-like” coordination (zirconium complexes with
κ2,η2-coordinated diketiminate ligands generally display sym-
metrical NMR spectra). The nonsymmetric NMR spectra of 4
are thus unlikely to be caused by the nonsymmetric κ2,η2/“η5-
like” coordination of the bis-diketiminate ligand, which we
would expect to interconvert rapidly in solution.
π coordination of the bis-diketiminate ligand also caused a
change in the general coordination geometry around Zr.
Previous C6H10(nacnacXyl)2ZrX2 complexes with X = Cl, Me,
NMe2, OEt displayed distorted-octahedral geometries, as
expected for a bis-κ2-coordinated ligand. Due to the steric
constraints introduced by the cyclohexyldiamine bridge (bite
angle of 72°), the structures are strongly distorted with
widened X−Zr−X angles of 121−125° for cis-X2 complexes (X
= NMe2, OEt).7x The structure of 4, however, is best described
as pseudotetrahedral (O3−Zr1−O6 = 83.2(3)°, O9−Zr2−O12
= 82.9(3)°, Z−Zr−Z = 138−140°, Zr−Z ≈ 2.1 Å, Z = centroid
of the diketiminate ligand), comparable to the case for
(nacnacAr)(Cp)ZrCl2 or (nacnacAr)(Ind)ZrCl2 complexes, also
showing π coordination of the diketiminate ligand (Cl−Zr−Cl
= 84−90°, Z−Zr−Z = 136−141°, Zr−Znacnac ≈ 2.1 Å, Z =
centroid of the diketiminate, Cp, or Ind ligand).7a,n,r,t,19
Comparable to substituted ansa-zirconocenes with triatomic
bridges (such as C3H6-, Me2SiOMe2Si-, or ferrocenyl-brigded
indenyl or cyclopentadienyl complexes),20 the structure of 4
deviates from C2 symmetry by placing the bridge to one side of
the complex (Figure 4). As a consequence, one Cp or N
substituent is eclipsed with the ZrX2 fragment while the other is
oriented to the side of the complex away from the ZrX2
fragment. In the resulting C1-symmetric structure, the two
triflate ligands have clearly different chemical environments,
further evidenced by differences in Zr−O distances of 0.04−
0.08 Å (Table 8). For the ansa-metallocenes mentioned above,
NMR spectra showed apparent C2 symmetry, explained by a
fast rotation of the ZrX2 fragment. Nevertheless, it is reasonable
to assume that the latter is more difficult in 4, given the more
rigid nature of the cyclohexyldiamine bridge and the higher
steric demands of xylyl and triflate substituents. The observed
C1-symmetric, pseudotetrahedral structure is thus most likely
Scheme 6
bonding modes involve coordination of the ligand π system to
the zirconium center and are commonly observed either when
the zirconium centers are electron deficient or if the second
ligand is an η5 ligand such as cyclopentadienyl. Coordination of
the π system might occur symmetrically to yield an “η5-like”
coordination of the diketiminate or an unsymmetrical κ2,η2-
coordination (Scheme 6).7p
In 4, short bond distances of Zr to the central carbon atom
Cβ of the diketiminate ligands (Zr−C3/C23/C43/C63 = 2.6−
2.8 Å, Table 4) and a high bending of Zr atom out of the
ligands’ mean planes (∼70°) clearly indicate that the
diketiminate π system is involved in the ligand−metal bonding,
in contrast to previous C6H10(nacnacXyl)2ZrX2 complexes, such
as 2 and 3 (Scheme 1)7x or other zirconium dichloride
complexes with bridged diketiminate ligands,7m which showed
bis-κ2 coordination. Despite some variation introduced by the
G
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Organometallics
Article
values using a riding model (SHELXL97).25 For further details see the
Supporting Information and Table S2.
preserved in solution and is responsible for the lack of
symmetry in NMR spectra, resulting in different coordination
environments for the triflate ligands.
( )-C6H10(nacnacXyl)2Zr(OTf)2 (4). A toluene solution of silver
triflate (77 mg, 0.30 mmol, 5.0 mL) was gradually added to a toluene
solution of 2 (100 mg, 0.15 mmol, 10 mL) at room temperature. The
precipitated AgCl was removed by filtration. The obtained yellow
filtrate was concentrated, and diffusion of hexane into the solution
yielded yellow crystals (74 mg, 75%).
CONCLUSIONS
■
Lewis acidic 4 proved to be highly active for the addition of
amines to activated olefins with a maximum turnover frequency
of 20000/h for the addition of morpholine to acrylonitrile at
ambient temperature. For sterically more hindered substrates, a
turnover frequency of 1000/h for the addition of morpholine to
methacrylonitrile compares favorably to those obtained with di-
and triphosphine Ni(II) (1−3/h),21 bis-phosphine-NHC
Pd(II) (1/h),22 bis(NHC) Pd(II) (3/h at 90 °C),23 and Ni(II)
pincer complexes (30/h−1000/h).24 Enantiopure R,R-4
showed only low enantioselectivity (ee < 20%) for the addition
of morpholine to methacrylonitrile. For the same reaction,
Togni reported Ni(II) and Pd(II) complexes with chiral
phosphine ligands which showed notably higher enantioselec-
tivities of 32−69% ee.21,22 The change of coordination mode
from C2-symmetric octahedral bis-κ2 coordination to C1-
symmetric pseudotetragonal π coordination of the bis-
diketiminate ligand in 4 is most likely responsible for the low
enantioselectivities obtained. Not only does the pseudotetrag-
onal structure show a less clear distinction between “up” and
“down” sides of the Zr(OTf)2 plane but also the two
coordination sites are chemically inequivalent with different, if
any, stereoselectivity. The overall reaction order of 4 (assuming,
in agreement with the values in Table 6, a first-order
dependence on catalyst concentration) underlines the
importance to work at high concentrations or even without
solvent to achieve high turnover frequencies. Since the
nucleophile also participates as a base in the reaction
mechanism, resulting in a partial reaction order of 2 in amine
concentration, it seems advantageous to conduct hydro-
aminations with a slight excess of amine whenever no external
base is added.
1H NMR (C6D6, 400 MHz): δ 7.05−6.64 (m, 6H, Ar), 5.65 (s, 1H,
CH(CN)), 5.02 (m, 1H, Cy CH), 4.37 (s, 1H, CH(CN)), 3.11
(m, 1H, Cy CH), 2.61 (s, 3H,(CH3)2Ar), 2.49 (s, 3H,(CH3)2Ar), 2.38
(m, 1H, Cy CH2), 2.16 (s, 3H, CH3(CN)), 1.90 (s, 3H, CH3(C
N)), 1.71 (s, 3H, CH3(CN)), 1.38−1.43 (m, 5H, Cy CH2), 1.32 (s,
3H, CH3(CN)), 1.17 (s, 3H,(CH3)2Ar), 1.05 (s, 3H,(CH3)2Ar),
0.88 (m, 3H, Cy CH2). 13C{1H} NMR (C6D6, 101 MHz): δ 164.3,
163.9, 162.4, 161.5 (CH3CN), 145.5, 144.3 (CF3SO3), 134.4, 133.7,
132.6, 132.1 (ortho (CH3)2Ar), 129.3, 129.1 (ipso (CH3)2Ar), 129.0,
128.9, 128.1, 127.9 (meta (CH3)2Ar) 126.7, 126.6 (para (CH3)2Ar),
102.6, 98.4 (CH(CN)), 73.0, 67.9 (Cy CH), 32.4, 31.2 (Cy CH2),
25.0 ((CH3)2Ar), 24.7 (Cy CH2), 24.6, 23.5, 22.1((CH3)2Ar), 20.6
(CH3CN), 20.3 (Cy CH2), 20.1, 19.2, 17.3 (CH3CN). 19F{1H}
NMR (C6D6, 400 MHz): −76.6 (s, 3F), −77.5 (s, 3F). Anal. Calcd for
C34H42F6N4O6S2Zr: C, 46.83; H, 4.85; N, 6.42; S, 7.35. Found: C,
47.01; H, 5.34; N, 6.38; S, 7.06. Mp: 169 °C.
(R,R)-C6H10(nacnacXyl)2Zr(OTf)2 (R,R-4). A solution of NaOH
(2.2 g, 55 mmol) in a mixture of water (5 mL) and brine (5 mL) was
added dropwise to a suspension of (1R,2R)-(+)-1,2-cyclohexane L-
tartrate (2.00 g, 7.58 mmol) in CH2Cl2 (50 mL). The resulting
mixture was stirred overnight. The organic layer was separated, and the
aqueous layer was extracted with CH2Cl2 (4 × 20 mL). The combined
organic phases were dried over MgSO4, filtered, and concentrated to
yield 0.60 g of (R,R)-cyclohexyldiamine as a white solid, which was
used without further purification. Preparation of the ligand, dichloride,
and bis-triflate complex followed the described procedures for the
racemic complex, with similar yields and identical spectroscopic
properties (final yield of the bis-triflate complex: 80 mg, 77%). An
analytically pure sample was obtained from C6D6 solution. Anal. Calcd
for C34H42F6N4O6S2Zr·C6D6: C, 50.24; H, 5.69; N, 5.86; S, 6.71.
Found: C, 50.19; H, 5.67; N, 5.49; S, 6.57.
General Procedure for Catalytic Hydroamination Reactions.
In a glovebox, the olefin and a stock solution with the desired amount
of 4 was combined in a J. Young NMR tube. The desired amount of
amine was added, followed by C6D6, to bring the volume to 500 μL.
Reactions were monitored by 1H NMR to determine conversion. The
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out under an
inert atmosphere using Schlenk and glovebox techniques under a
purified N2 atmosphere. Complex 2 was prepared as described
elsewhere;7x all other chemicals were purchased from common
1
obtained products were identified by H NMR and GC/MS.
2-Methyl-3-morpholinopropanenitrile. Methacrylonitrile (5.0
mmol, 0.42 mL) was added to a C6D6 solution of rac-4 or R,R-4 (0.050
mmol, 1.82 mL, 0.0274 M). Morpholine (5.0 mmol, 0.43 mL) was
added to the obtained yellow solution. The reaction mixture was
followed by 1H NMR for conversion determination, poured into a vial
containing wet hexane, and purified by flash chromatography to yield a
colorless oil (650 mg, 85%).
1
commercial suppliers. H and 13C NMR spectra were acquired on a
Bruker AMX 400 or Bruker AV 400 spectrometer. 19F NMR spectra
were acquired on a Bruker AV 400 spectrometer. Chemical shifts were
referenced to the residual signals of the deuterated solvents (C6D6, 1H
δ 7.16 ppm, 13C δ 128.38 ppm; CDCl3, H δ 7.26 ppm, 13C δ 77.00
1
1H NMR (CDCl3, 400 MHz): δ 3.67 (t, J = 6, (CH2)2O), 2.69−
2.20 (m, 1H, MeCH), 2.6 (dd, J = 17, 1 H, CH2), 2.48 (t, J = 6,
(CH2)2N), 2.38 (dd, J = 7, 1 H, CH2), 1.31 (d, J = 8, 3H, MeCH). GC
(β dex; 90°, isothermal): tR (min) 139 (R), 142 (S, major product with
R,R-4) (assigned by comparison to literature).26
ppm). Solvents were dried by passage through activated aluminum
oxide (MBraun SPS) and deoxygenated by repeated extraction with
nitrogen. C6D6 was dried over sodium and degassed by three freeze−
pump−thaw cycles. Acrylonitrile, methacrylonitrile, (Z/E)-crotonitrile,
2-cyclohexeneone, benzylamine, morpholine, aniline, and diethylamine
were distilled under reduced pressure and dried over 4A molecular
sieves. Elemental analyses were performed by the Laboratoire
3-Benzylamino-2-methylpropanenitrile. A reaction as de-
scribed above (methacrylonitrile, 5.0 mmol, 0.42 mL; 4, 0.05 mmol,
1.82 mL, 0.0274 M; benzylamine, 5.0 mmol, 0.55 mL; 24 h) yielded a
colorless oil (530 mg, 55%).
́ ́ ́ ́
d’analyse elementaire (Universite de Montreal). GC/MS analyses
were obtained using an Agilent 6890 N spectrometer. The
enantiomeric excess (ee) was determined by chiral GC (HP 6890
Series, β-dex120 (30 m × 0.25 mm × 0.25 μm, isothermal mode at 90
°C).
1H NMR (CDCl3, 400 MHz): δ 7.237.34 (m, 5H, Ar), 3.85 (s,
2H, CH2−Bn), 2.49−2.90 (m, 3H, CH2 and MeCH), 1.31 (d, J = 6,
3H, MeCH).
Determination of Reaction Kinetics. For slow reactions, the
required amount of diethylamine, methacrylonitrile, and triethylamine
(either directly or from C6D6 stock solutions) was combined with
C6D6 in a J. Young tube inside a glovebox. A stock solution with the
required amount of 4 was added last, yielding a total reaction volume
of 500 μL in most of the cases. In several cases, hexamethyldisiloxane
Diffraction data for 4 were recorded on a Bruker APEXII/
Microsource diffractometer (Cu Kα radiation) at 100 K using the
APEX2 software package. Data reduction was performed with SAINT;
the structure was solved with direct methods (SHELXS97).25 All non-
hydrogen atoms were refined anisotropically using full-matrix least
squares on F2 and hydrogen atoms refined with fixed isotropic U
H
dx.doi.org/10.1021/om400544z | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
served as an internal standard. Conversion was monitored by 1H NMR
spectroscopy at the required intervals. For faster reactions, reagents
were combined in a small vial and agitated. Samples were taken at the
desired intervals and added to vials containing wet CDCl3 for NMR
analysis (control experiments confirmed that no further reaction took
place in wet CDCl3).
R. S.; Cabrera, A. R.; Peoples, B. C.; Spannhoff, K.; Valderrama, M.;
Frohlich, R.; Kehr, G.; Erker, G. Dalton Trans. 2012, 41, 1243.
̈
(u) Dulong, F.; Bathily, O.; Thuery, P.; Ephritikhine, M.; Cantat, T.
Dalton Trans. 2012, 41, 11980. (v) Kurokawa, Y. I.; Nakao, Y.; Sakaki,
S. J. Phys. Chem. A 2012, 116, 2292. (w) Dulong, F.; Thuery, P.;
Ephritikhine, M.; Cantat, T. Organometallics 2013, 32, 1328. (x) El-
Zoghbi, I.; Whitehorne, T. J. J.; Schaper, F. Dalton Trans. 2013, 42,
9376.
ASSOCIATED CONTENT
* Supporting Information
Table S1 and Figure S1 and a CIF file giving single-crystal
diffraction data. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
(8) (a) Vitanova, D. V.; Hampel, F.; Hultzsch, K. C. J. Organomet.
Chem. 2005, 690, 5182. (b) Vitanova, D. V.; Hampel, F.; Hultzsch, K.
C. Dalton Trans. 2005, 1565. (c) Vitanova, D. V.; Hampel, F.;
Hultzsch, K. C. J. Organomet. Chem. 2011, 696, 321.
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Collins, S. J. Org. Chem. 1998, 63, 1885. (c) Qiu, R.; Xu, X.; Peng, L.;
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(f) Smitha, G.; Reddy, C. S. Catal. Commun. 2007, 8, 434. (g) Reddy,
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S
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank J. Bartholomeus, C. Trudeland, and Prof. H. Lebel
for chiral GC analyses, B. B. Mougang Djanteng, A. B. Salah,
■
́
and Prof. D. Zargarian for access to GC-MS, F. Belanger for
support with X-ray diffraction studies, and Profs. L. S.
Liebeskind and D. L. Lichtenberger for helpful comments on
reaction kinetics. Funding was provided by the NSERC, Centre
en chimie verte et catalyse (CCVC) and FQRNT (Ph.D.
stipend for I.E-Z. and T.J.J.W.).
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