Catalytic Recycling of a Th-H Bond via Single or Double Hydroboration of Inactivated Imines or Nitriles

Article abstract of DOI:10.1021/acscatal.9b01399

The catalytic activity of the metallacycle thorium amide [(Me₃Si)₂N]₂Th[κ²-(N,C)-CH₂Si(CH₃)₂N(SiMe₃)] (Th1) is presented for the selective dihydroboration of nitriles

Full text of DOI:10.1021/acscatal.9b01399

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Article
Catalytic Recycling of a Th-H Bond via Singly or
Doubly Hydroboration of Inactivated Imines or Nitriles
Sayantani Saha, and Moris S. Eisen
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 23 May 2019
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Catalytic Recycling of a Th-H Bond via Singly or Doubly
Hydroboration of Inactivated Imines or Nitriles
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Sayantani Saha and Moris S. Eisen*
Schulich Faculty of Chemistry, Technion – Israel Institute of Technology. Haifa City, 32000, Israel.
ABSTRACT: The catalytic activity of the metallacycle thorium amide [(Me3Si)2N]2Th[κ2–(N,C)–CH2Si(CH3)2N(SiMe3)] (Th1) is
presented for the selective dihydroboration of nitriles (–C≡N) with pinacolborane (HBpin). Using significantly low catalyst loading
(0.1 mol%), the dihydroborated amines were achieved by the hydroboration of the –C≡N triple bond attached with aromatic,
aliphatic and heteroatom backbones with high turnover frequency (TOF) as compared to all the reported homogeneous metal
catalysts in this reaction. In addition, for aldimines (–C=N−), the hydroboration precatalyst Th2 has been synthesized by the
protonolysis of a seven-membered N‑heterocyclic iminato ligand (LH) and Th1. Th2 crystal structure and its performance in the
synthesis of hydroborated secondary amine is also here presented. Detailed kinetic studies, thermodynamic and stoichiometric
experiments provided us with cumulative evidence supporting the proposed mechanism for the aforementioned reactions.
KEYWORDS. Organoactinide catalyst, Thorium, Nitrile, Imine, Hydroboration.
Mo(IV) complex that catalyzed the nitrile hydroboration was
disclosed by Nikonov’s group (Scheme 1f),13g and the
lanthanide N–heterocyclic carbene–Er(III) complex was
reported by Wang for the hydroboration of nitrile substrates
(Scheme 1h).13h
Introduction
Primary and secondary amines and their derivatives are
extensively present in natural products, drugs, agrochemicals,
polymers, dyes, textiles and in many more industrial and
academic encounters.1 Direct base–promoted N–alkylation of
amines with alkyl halides or alcohols,2 alkylative aminations,3
reductive aminations of carbonyl compounds,4 hydroamination
of unsaturated hydrocarbons with amines,5 are the most
conventional techniques to synthesize amines. Still, expensive
starting materials, generation of huge organic wastes, and
exhaustive alkylation impede, in many cases, the utility of the
above procedures.6 Hence, in the last few decades, metals or
metal-non–metal moieties catalyzed hydrogenation of nitriles
PPh3
N
N
N
N
PPh2
RuII
CoI
N2
N
Cl
N
N
Dipp
N
Cl
Cl
H
RuII
RuII
N
N
CoII
O
N
Dipp
Cl
PPh2
O
H
Dipp = 2,6-iPr2C6H3
PPh3
1d. 2.5 mol%,
60 oC, 24 h,
> 99% conv.
1b. 1 mol%,
60 oC, 15 - 36 h,
74 - 99% conv.
1a. 5 mol%,
45 oC, 18 h,
1c. 2.5 mol%,
70 oC, 16 h,
52 - 94% conv.
52 - 85% conv.
N
N
OH2
O
Mes
Cl
Mes
ErIII
N
PMe3
O
O
O
Me3P
Me3P
N
N
N
NiII
Dipp
MoIV
Dipp
MgII
Bu
Me3Si
O
N
using molecular hydrogen (H2) has become
a very
PMe3
N
Cl
O
Me3Si
OH2
N
advantageous method to produce primary amines.7 However,
the low selectivity,8 necessity of high H2 pressure,9 copious
reagents,10 formations of inorganic by–products10 again cripple
the wide usefulness of these methods. Alternatively, the
catalytic hydroelementation process such as hydroboration or
hydrosilylation came to limelight for mild and selective
reduction of nitriles to provide primary amines.11 Although the
catalytic hydrosilylation commonly affords a mixture of
monosilylated imine and disilylamine products, the catalytic
hydroboration produces diboronated amines as the sole
product, that are easily converted into the corresponding
primary amines.12 Lately, the catalytic hydroboration of
nitriles has started to be explored as a strategic method for the
synthesis of amines.13 The complexes that are known, until
now, able to perform the double hydroboration of nitriles are
summarized in Scheme 1. Szymczak13a and Gunanathan13b
reported a proton–switchable bifunctional Ru(II) complex and
[RuII(p–cymene)Cl2]2, respectively, to catalyze nitrile
hydroboration (Scheme 1a and 1b). Fout13c and Trovitch13d
reported Co(I) and Co(II) complexes that catalyzed nitrile
hydroboration, respectively (Scheme 1c and 1d). Shimada and
co-workers demonstrated that bis(acetylacetonato)nickel(II)
afforded the synthesis of diboronated amines (Scheme 1e).13e
Hill and co-workers disclosed an Mg(II)–catalyzed
hydroboration of nitriles (Scheme 1g).13f The interesting
1h. 2 mol%,
110 oC, 6 h,
1g. 10 mol%,
60 oC, 0.5-30 h,
75 - 99% conv.
1f. 5 mol%,
22 oC, 12-48 h,
32 - 99% conv.
1e. 5 mol%,
RT, 18 h,
> 99% conv.
> 90% conv.
Scheme 1 Precatalysts reported in the literature for nitrile
hydroboration.
The synthesis of secondary amines that can be obtained from
the catalytic hydrogenation of double bonds of imines,
−C=N−, turned out to be an effective method in recent days.14
The reduction of imines can be achieved using metal hydride
reagents, such as BH3, LiAlH4 or NaBH4, but these reagents
remain somewhat unappealing due to poor reaction yields and
selectivity.15 Instead, alkaline earth metals and transition
metals (Ca, Ru, Rh, Ir, Pd, Pt, Au) that catalyzed the
hydrogenation of imines has been extensively explored.16 Like
in the case of nitriles, the hydroboration of imines is also
earning eminence over the traditional methods of direct
hydrogenation due to the mild reaction conditions and the
higher selectivity in the synthesis of chiral amines. In 1995,
bidentate phosphine–gold(I) complexes with catecholborane
catalyzed the hydroboration of imines and it was the first
disclosure by Baker et al.17 Afterward, few reports have been
published on the hydroboration of imines catalyzed by alkaline
earth metals, transition metals, and some Lewis bases.18,13b
Despite having strong potentials, organoactinide compounds
have not been greatly exploited in the field of catalysis19 in
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comparison to stoichiometric reactions20. Recently, we have
developed some thorium precatalysts for the regioselective
and chemoselective hydroboration of pyridines, heterocyclic
systems, aldehydes/ketones, and carbodiimides.21
Table 1 Reaction optimization.a
Ent Substr Boranes Cat.
Time Temp. Yieldb
ry
ate
(equiv.) (mol%)
(h)
(°C)
%
HBpin
Th1 (1)
(2.2)
In this work we disclosure the unexpected extraordinary
catalytic activity of the metallacycle thorium amide complex
1
PhCN
4
25
100
[(Me3Si)2N]2Th[κ2–(N,C)–CH2Si(CH3)2N(SiMe3)]
(Th1)
HBpin
(2.2)
HBpin
Th1 (1)
(2.2)
HBpin
(2.2)
HBpin
(2.2)
HBcat
(2.2)
HBcat
(2.2)
9-BBN
(2.2)
2
3
PhCN
PhCN
PhCN
PhCN
PhCN
PhCN
PhCN
PhCN
−
24
0.25
3
25
80
80
80
80
80
80
80
25
80
80
80
25
80
−
100
95
100
100
−
(Scheme 2) in the hydroboration of nitriles using
pinacolborane (HBpin) for the synthesis primary amines with
the highest turnover frequency (TOF), among the
homogeneous catalysts, reported in the literature. Likewise,
the embedment reaction of an N–heterocyclic iminato ligand
(LH) with complex Th1 affords the synthesis of the
corresponding actinide complex Th2 (Scheme 2), which was
found to be an outstanding precatalyst for the hydroboration of
imines. In addition to the scope of the reactions, detailed
stoichiometric and kinetic studies have been performed to
investigate the reaction mechanism for both nitriles and imines
hydroborations.
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Th1
(0.1)
Th1
(0.1)
Th1
(0.1)
4
5
4
6
16
24
24
24
24
36
3
7
−
Th1
(0.1)
8
15
5
9-BBN
(2.2)
9
−
N"
ThIV
N
N
O
"N
N
PhCH
=NPh
PhCH
=NPh
HBpin
(1.1)
HBpin
(1.1)
HBpin
(2.2)
HBpin
(1.1)
HBpin
(1.1)
HBpin
(2.2)
Me
ThIV
10
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12
13
14
15
Th1 (1)
Th1 (1)
Th2 (1)
Th2 (1)
Th2 (1)
Th2 (1)
−
N
Si
Me
"N
"N
N"
Me3Si
Th1
N" = N(SiMe3)2
Th2
52
90
95
−
Nitrile Hydroboration
Imine Hydroboration
PhCN
Scheme 2 Thorium precatalysts used in
this work.
PhCH
=NPh
PhCH
=NPh
PhCH
=NPh
24
24
8
100
aReaction condition: Th1/Th2, PhCN/PhCH=NPh, and boranes
in C6D6 inside a J–Young teflon valve-sealed NMR tubes at
variable temperature; bBy 1H NMR analysis.
Results and Discussions
Catalytic Hydroboration of Nitriles. To evaluate the
catalytic efficiency of the Th1 complex for the hydroboration
of nitriles, initial exploratory experiments were carried out
using benzonitrile (1 mmol), pinacolborane (HBpin, 2.2
mmol) and the precatalyst Th1 (0.01 mmol) inside a J-Young
Teflon valve-sealed NMR tube at room temperature in C6D6.
Full conversion (100%) to the corresponding dihydroborated
amine product 2a was observed after 4 h (entry 1, Table 1). It
is important to indicate that no hydroborated product was
observed in the absence of the actinide complex Th1 (entry 2,
Table 1). Raising the reaction temperature to 80 °C expedited
the rate of reaction and 100% conversion was detected within
15 min (entry 3, Table 1). The reduction of the catalyst
loading of the model reaction from 1 to 0.1 mol% provided
100% and 95% conversion after 4 h (TOF: 250 h-1) and 3 h
(TOF: 317 h-1), respectively at 80 °C (entries 4 and 5, Table
1). The activity of the thorium complex Th1 for the nitrile
hydroboration was also examined in the presence of other
hydroboranes, such as catecholborane (HBcat) and 9-
borabicyclo(3.3.1)nonane (9-BBN). The reaction of
benzonitrile (1 equiv) and HBcat (2.2 equiv) in the presence of
0.1 mol% Th1 afforded 100% conversion after 16 h (entry 6,
Table 1) and no hydroborated product was obtained in the
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absence of the actinide catalyst (entry 7, Table 1). In
Table 2 Th1 catalyzed hydroboration of nitriles with
comparison, the reaction of benzonitrile (1 equiv) and 9-BBN
(2.2 equiv.) 15% and 5% of the double hydroborated products
was obtained after 24 h in presence of 0.1 mol% Th1 or with
no catalyst, respectively (entries 8 and 9, Table 1), indicating
that 9-BBN can perform the reaction, however in a sluggish
mode, unless larger amount of the precatalyst is used.
HBpin.a
O
H
H
R
B
O
Th1 (0.1 mol%)
C6D6, 80 C
N
O
O
+
R
N
B
B
H
O
O
Hence, we continue to explore the substrate scope for the
actinide complex Th1 under the optimized reaction conditions
(Table 2). The quantitative yields of the dihydroborated
amines obtained from the corresponding nitriles were obtained
by following the reaction via 1H NMR analysis (see
supplementary material). However, quantitative amounts of
the double hydroborated amines can be obtained, in a similar
fashion, using benzonitrile (0.1 mol), HBpin (0.22 mol), and
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9
O
B
O
O
B
O
H H
H H
H H
B
O
O
N
N
N
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B
B
B
O
O
O
O
O
O
O
2a (3 h, 95%)
TOF: 317 h-1
2b (3 h, 98%)
TOF: 327 h-1
2c (2.5 h, 100%)
TOF: 400 h-1
o
Th1 (1 × 10−4 mmol) at 80 C in C6H6 (60 mL) in a Schlenk's
O
B
O
O
O
O
O
B
O
H
H
H H
N
B
H H
N
B
N
vessel after 3 h. As presented in Table 2, electron-rich
aromatic nitriles bearing electron-donating groups such as p-
Me (entry 2b) and p-OMe gave 98% (entry 2c) and 100%
conversion after 3 h (TOF: 327 h-1) and 2.5 h (TOF: 400 h-1),
respectively. The rate of hydroboration of the −C≡N bond
decorated with polycyclic aromatic rings, such as 1-
naphthonitrile (entry 2d) is sluggish in comparison to
benzonitrile. Under the optimized conditions, the reaction
stopped after 65% conversion, whereas with 0.5 mol% catalyst
loading, 75% (TOF: 25 h-1) conversion was obtained after 6 h.
Electron-deficient aryl nitriles are generally less reactive with
HBpin than the corresponding electron rich counterparts.
Hence, under the optimized reaction conditions, 1,4-
dicyanobenzene (entry 2e) afforded 88% after 6 h (TOF: 147
h-1). Similarly, the reactions of HBpin
with 4-fluorobenzonitrile (entry 2g) progressed less efficiently
in comparison to 4-bromobenzonitrile (entry 2f) affording the
corresponding dihydroborated product with 94% yield after 5
h (TOF: 192 h-1) and 3 h (TOF: 310 h-1), respectively. In the
reaction of 4-(trifluoromethyl)benzonitrile (entry 2h) and
HBpin, 90% yield is obtained after 7 h (TOF: 150 h-1). The
incorporation of a nitro group (−NO2) in the aromatic ring
further diminished the rate of the reaction by manifolds. Since
the hydroboration reaction of 4-nitrobenzonitrile (entry 2i)
was too slow with 0.1 mol% catalyst loading, the reaction was
performed with 0.5 mol% of the thorium complex Th1, and
100% conversion was afforded after 10 h (TOF: 20 h-1). The
reaction proceeded selectively, at the nitrile position, leaving
the nitro group unreacted (Figure S31). Intriguingly, no
product was observed for picolinonitrile (entry 2j); the lack of
reactivity for the pyridine analog it might be attributed to a
catalyst deactivation via metal coordination to the pyridine
nitrogen,22 however similar systems with furan and thiophene
moieties, did not show such behavior (vide infra). Performing
the reaction with 4-pyridinecarbonitrile, 100% conversion was
obtained after 4 h (TOF: 250 h-1) (entry 2k) (Figure S33).
Moreover, the hydroboration of the −C≡N group attached to
other heteroaromatic rings, such as, 5-methyl-2-furonitrile
(entry 2l), 3-thiophenecarbonitrile (entry 2m) and 1,5-
dimethyl-2-pyrrolecarbonitrile (entry 2n) proceeded efficiently
to give the corresponding dihydroborated amines in 75% (6 h,
TOF: 125 h-1), 75% (6 h, TOF: 125 h-1) and 98% (4 h, TOF:
245 h-1) overall yields, respectively. The scope of the
substrates was expanded by performing the reaction with more
challenging inactive aliphatic nitriles. As shown in the
literature, the known catalysts are less reactive for aliphatic
B
O
N
O
B
B
B
Br
H
H
O
O
O
O
O
O
O
2ec (6 h, 88%)
TOF: 147 h-1
2f (3 h, 93%)
TOF: 310 h-1
2db (6 h, 75%)
TOF: 25 h-1
O
O
B
O
O
B
O
H H
H H
H H
B
O
N
B
N
B
N
B
F
O2N
F3C
O
O
O
O
O
O
2g (5 h, 94%)
TOF: 192 h-1
2ib (10 h, 100%)
TOF: 20 h-1
2h (6 h, 90%)
TOF: 150 h-1
O
B
O
O
O
B
H H
H H
H H
B
O
O
N
B
N
N
B
O
N
N
B
O
O
O
O
O
O
2l (6 h, 75%)
TOF: 125 h-1
2k (4 h, 100%)
TOF: 250 h-1
2j (24 h, n. r.)
O
B
H H
O
B
O
O
B
O
H H
H H
O
N
N
N
B
S
N
B
O
B
O
O
O
O
O
2m (6 h, 75%)
TOF: 125 h-1
2n (4 h, 98%)
TOF: 245 h-1
2o (2 h, 100%)
TOF: 500 h-1
O
B
O
O
B
O
O
B
O
H H
H H
H H
N
B
N
B
O
O
N
B
O
O
O
O
2rb (8 h, 80%)
TOF: 20 h-1
2q (2 h, 100%)
TOF: 500 h-1
2p (2 h, 100%)
TOF: 500 h-1
O
B
O
H H
O
O
B
O
O
B
B
N
Br
O
O
N
B
N
B
O
N
B
O
Si
O
O
O
B
O
O
O
O
O
O
O
2uc (8 h, 90%)
TOF: 112 h-1
2s (8 h, 92%)
TOF: 115 h-1
2t (8 h, 95%)
TOF: 120 h-1
aReaction condition: nitriles (1 mmol), HBpin (2.2 mmol), and
precatalyst Th1 (0.001 mmol) at 80 oC in C6D6 (0.6 mL); yields
were determined by 1H NMR spectroscopy of the crude reaction
b
c
mixture. 0.005 mmol Th1 were used. 0.002 mmol Th1 and
4.4 mmol HBpin were used.
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substrates.23 However, in our case, the thorium precatalyst,
Th1, showed remarkable turnover frequencies for the
corresponding aliphatic substrates, comparable to those
obtained for the aromatic nitriles. The reactions of HBpin with
cyclohexanecarbonitrile (entry 2o), isobutyronitrile (entry 2p)
and propionitrile (entry 2q) provided 100% conversion within
2 h (TOF: 500 h-1). Other types of aliphatic nitriles including
benzyl cyanide (entry 2r), bromopentanenitrile (entry 2s), 3-
cyanopropyltriethoxysilane (entry 2t), 5- and 2-cyanoethyl
ether (entry 2u) afforded the dihydroborated products in good
yields, 80% (TOF: 20 h-1), 90% (TOF: 115 h-1), 95% (TOF:
120 h-1) and 90% (TOF: 112 h-1), respectively. To the best of
our knowledge, our precatalyst Th1 exhibited the highest
catalytic activity as compared with any other homogeneous
catalysts reported in the literature for the nitrile
dihydroboration reaction.
9
In Th2 the Th−Nimido bond [Th1–N3 2.216(3)] is significantly
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shorter than the Th−Namido bonds [Th1–N4 2.333(3), Th1–N5
2.351(3), Th1–N6 2.375(3)] and this also supports the
observations reported literature26. In the presence of 1 mol% of
Th2, the hydroboration of benzonitrile afforded 90%
o
conversion after 3 h at 80 C (entry 12, Table 1). Although in
nitrile hydroboration the efficacy of Th2 is not as good as
Th1, in the imine hydroboration Th2 showed much higher
efficiency as compared to Th1. So, we continued our studies
with the complex Th2 for the hydroboration of imines. In the
presence of 1 mol% of Th2, 95% of the hydroborated product,
3a was afforded at 80 oC after 24 h (entry 13, Table 1).
Attempts to carry out the aforementioned reaction at room
temperature (25 oC) shows no reduced product (entry 14,
Table 1). However, a twofold increase in the amount of HBpin
showed a significant influence in the rate of the reaction;
Catalytic Hydroboration of Imines. We commenced the
imine hydroboration studies by examining the catalytic
activity of Th1. with N,1-diphenylmethanimine, PhCH=NPh.
Initial evaluation revealed that Th1 (1 mol%) afforded only
o
52% of the reduced product 3a at 80 C after 36 h (entries 10
and 11, Table 1). After some catalytic screening, we
subsequently changed the course study to study the
performance of the new complex Th2 (Scheme 2) in the imine
hydroboration. Th2 was prepared by the rapid protonolysis
reaction of Th1 with the neutral seven-membered N-
heterocyclic iminato ligand LH (Scheme S2) following to our
recent disclosed protocol.24 The solid-state structure of the
precatalyst Th2 was confirmed by single-crystal X-ray
diffraction (Figure 1). The X-ray structural analysis revealed
that the central Th(IV) is in a distorted tetrahedral geometry
consisting of one seven-membered N‑heterocyclic iminato
ligand (L) and three N-silylated amido [−N{Si(CH3)3}2]
groups. The ligand (L) binds with the metal in a monodentate
fashion, and the remaining three positions are occupied by
amido ligands, completing the tetrahedral environment. The
N3−C1ipso bond distance is 1.266(5) Å for Th2, is analogous to
the corresponding bond lengths in related imidazolin-2-
iminato complexes indicating that no mesomeric structures
donating electron density to the metal is operative from that
double bond.20b However, the Th−N3 bond distance in
complex Th2 of 2.216(3) Å indicates that the seven-
membered N‑heterocyclic iminato ligand (L) is attached to the
metal as a metalla-cumulene. This mode of bonding is
supported by the small bent Th1−N3−C1 angle 164.0(3)°
allowing a substantial π donation from the ligand to the metal,
generating a double bond between the metal and the imine
nitrogen. The X-ray structural analysis revealed alike
structures like those reported for other actinide imidazoline
iminato complexes.21b,24b,24e,25
o
100% conversion was obtained within 8 h at 80 C (entry 15,
Table 1).
Substituted aldimines (3b-3l, Table 3) were subjected to the
hydroboration reaction in the presence of 1.1 equivalent of
o
HBpin at 80 C to study the substrate scope capabilities. The
quantitative yields of the corresponding hydroborated amines
were obtained by following the reaction using 1H NMR
spectroscopy. Both aromatic and aliphatic imines afforded the
hydroborated secondary amines in good yields (90-100%) in a
maximum of 24 h. Overall, the imines comprising electron-
donating functionalities increased the rate of the reaction in
comparison to the imines decorated with electron-withdrawing
groups.
Hydroboration
of
imine
[tert-butyl
(phenylmethylene)carbamate] with BOC as a protecting group
afforded secondary amine product 3m with 80% conversion
after 24 h. The catalytic performance of the Th2 complex is
comparable with other homogeneous metal catalysts reported
in the literature for aldimine hydroboration.18 Interestingly, the
precatalyst Th2 showed no catalytic activity in the
hydroboration of ketimines, presumably due to the steric
hindrance around the metal complex after complexation.
Mechanistic Studies for Nitrile Hydroboration. To gain a
more detailed insight into the nitrile hydroboration catalytic
cycle and into the activation of the complex Th1,
stoichiometric, kinetic and temperature dependence studies
were performed. The stoichiometric reaction between Th1 and
one equiv of HBpin caused the immediate opening of the 4-
membered cyclic structure of Th1 with the disappearance of
1
its characteristic peaks in the H NMR spectrum and afforded
the complex spectrum (Figure S70). In addition, no
hexamethyldisilazane or pinacolborahexamethyldisilazane
were detected in the GC-MS or MALDI spectroscopy,
indicating that the pinacolborane has been added to the
complex. The simultaneous appearance of a broad peak at δ
5.56 ppm was assigned as the corresponding hydride (Th-H)
peak in intermediate [A] (vide infra). As expected, the signal
at δ 5.56 ppm, is not observed for the reaction of Th1 with
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Figure 1 Molecular structure of complex
Th2, with thermal ellipsoids set at the 50%

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hydroborated imine moiety never leaves the complex
Table 3 Th2 catalyzed hydroboration of imines with
coordination sphere, the reaction of Th1, (1 equiv) + HBpin (1
equiv) and PhCN (1 equiv) was performed, yielding a mixture
of the hydroborated imine and the double hydroborated amine
(Figures S76). This important result indicates that the
hydroborated imine is indeed eliminated and is hydroborated
more rapidly than the corresponding nitrile.
Kinetic measurements indicate that the empirical rate law,
observed for the hydroboration of PhCH2CN catalyzed by
complex Th1 exhibits a first-order dependence on the catalyst,
a second-order dependence on HBpin and no dependence on
nitrile concentration (Eq. 1) (Figures 2a-c). For the full kinetic
equation treatment, please see the Supplementary Information
(SI).
HBpin.a
O
B
R1
R
R1
+
O
N
N
1 (1 mol%)
O
O
R
B
H
H
C6D6, 80 C
O
O
B
O
B
B
O
O
O
N
N
N
9
H
H
H
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3b (24 h, 97%)
3a (24 h, 95%)
3c (12 h, 100%)
O
B
O
B
O
B
O
N
O
O
N
N
H
H
H
F3C
F
δp⁄δt= kobs × [Th1]1 [HBpin]2 [PhCH2CN]0
(1)
Br
3e (18 h, 100%)
3f (24 h, 82%)
3d (12 h, 100%)
O
F
O
O
Moreover, activation parameters for the hydroboration of
PhCH2CN by complex Th1 were determined from the Eyring
and Arrhenius plots with ΔS‡, ΔH‡, and Ea values of −47.25
(1.23) e.u., 6.99 (0.44) kcal/mol and 7.66 (0.44) kcal/mol,
respectively (Figures 2d, S92–94). The ΔH≠ and ΔS≠ data are
B
B
N
O
B
O
O
N
O
N
H
H
H
3g (24 h, 90%)
3i (24 h, 85%)
3h (18 h, 98%)
O
B
consistent with
a concurrent bond-cleavage and bond-
O
B
O
B
formation events and a very organized four-membered ring
rate-determining step, respectively.
Deuterium isotope studies show that the reaction exhibits a
KIE (kH/kD) of 5.4 (0.08) (Figure S91), indicating that the Th-
N(Bpin)(CH2R)/H-Bpin σ-bond metathesis is the operative
turnover-limiting step.
O
N
O
N
O
N
H
H
H
O
3j (24 h, 92%)
3k (12 h, 100%)
3l (12 h, 100%)
O
O
B
O
O
N
Based on these studies, a plausible hydroboration mechanism
for nitriles is proposed which consists of two catalytic cycles
(Scheme 3). The activation of the reaction cycles is first
achieved by the quick insertion of the HBpin into the thorium
metallacycle Th1 giving rise to the active thorium hydride
complex [A]. Cycle 1 starts with the extremely fast insertion
of the Th-H motif of complex [A] into the –C≡N moiety
allowing the formation of the intermediate complex [B].
However, as shown in the stoichiometric reactions, in the
presence of HBpin, the subsequent Th-N=C(H)R/H-B σ-bond
metathesis regenerates the active complex [A] with the
concomitant formation of the intermediate imine
(PhCH=NBpin). Simultaneously, the activation of the second
cycle is obtained by the rapid insertion of the Th-H moiety of
complex [A] into the –C=N–Bpin bong giving rise to
intermediate [C] instantly, as this C=N bond is highly
activated by the borane moiety (no imine product is detected).
The reaction of complex [C] with HBpin, via a Th-N/H-Bpin
σ-bond metathesis through the transition state {[C]-[A]}#
produced as the rate determining step the target
dihydroborated amine product, and concurrently regenerates
the active complex [A] to complete the full catalytic cycle.
Since this final hydroboration step is the turnover-limiting
H
3m (24 h, 80%)
aReaction condition: imines (1 mmol), HBpin (1.1 mmol),
o
and precatalyst Th2 (0.01 mmol) at 80 C in C6D6 (0.6 mL);
yields were determined by 1H NMR spectroscopy of the
crude reaction mixture.
DBPin in the 1H NMR (Figure S71). Furthermore, the addition
of two equiv. of DBpin in the mixture of {Th1 + HBpin (1
equiv.)} prompts the gradually disappearance of the signal at δ
5.56 ppm in 24 h (Figure S74). As can be observed, the Th−H
signal is slowly exchanging with deuterium affording the
corresponding Th−D intermediate. Clearly, this is not a free
Th-H moiety, since these hydrides appear downfield (between
16-19 ppm);27 but indicates a bridged hydride between the B
and the Th metal center.13f The IR signal at 960 cm-1 observed
in the reaction mixture of {Th1 + HBpin (1 equiv.)} was
assigned as the bridged Th-H-B moiety (Figure S77).27 The
addition of 2 equiv. of DBpin to the reaction mixture of [{Th1
+ HBpin (1 equiv.)} decreases the intensity of the 960 cm-1
signal and gives rise to a new signal at 687 cm-1 allotted as Th-
D-B moiety (Figure S78).27 It is important to point out that
further addition of HBpin (up to 5 equiv.), to the reaction
mixture, does not cause any additional changes as it is
step, we cannot eliminate
a rapid nitrile coordination
equilibrium giving the intermediate [C′].
1
observed in the H NMR spectra (Figures S72). The addition
of one equivalent of PhCN to a mixture of [Th1 (1 equiv) +
HBpin (3 equiv)] immediately gives the dihydroborated
product 1a. In the 1H NMR, there is no signal for the
formation of the intermediate imine (PhCH=NBpin) (Figure
S75). To be able to distinguish between a mechanism in which
the hydroborated imine is eliminated and rapidly added to a
Th-H moiety as compared to a mechanism in which the
a)
b)
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c)

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exchange equilibrium between the hydride at the metal and at
the HBpin resulting in an additional fine splitting at the
hydride signals of the HBPin that are split by the boron.
Stoichiometric reactions of Th2, imine (PhC=NPh) and HBpin
(1:5:5) at 80 oC give directly the hydroborated product and one
equiv of PinB–N(SiMe3)2 (Figure S83). Kinetic measurements
indicate that the empirical rate law, observed for the
hydroboration of PhC=NPh catalyzed by complex Th2
exhibits a first-order dependence on the catalyst, HBpin and
no dependence on imine concentration (Eq. 2) (Figures 3a-c).
A thorough analysis of the kinetic equation can be found in the
Supporting Information (SI).
9
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22
23
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27
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δp⁄δt = kobs × [Th2]1 [HBpin]1 [PhC=NPh]0
(2)
Activation parameters for the hydroboration of PhC=NPh
were determined from the Eyring and Arrhenius plots with
ΔS‡, ΔH‡, and Ea values of −36.46 (0.8) e.u., 13.42 (1.06)
kcal/mol and 14.05 (0.77) kcal/mol, respectively (Figures 3d,
S102–104). The ΔH‡ and ΔS‡ data are consistent with a
concerted bond-cleavage and bond-formation processes and
with
a
very organized four-centered transition state,
respectively.
Deuterium isotope studies show that the reaction exhibits a
KIE (kH/kD) of 2.85 (0.07) (Figure S101), indicating that the
Th-N(SiMe3)2/H-Bpin σ-bond metathesis is the operative
turnover-limiting step.
Based on these studies, a plausible hydroboration mechanism
is proposed for the reaction of Th2 with PhCH=NPh and
HBPin, in Scheme 4. The activation of the reaction cycle is
firstly achieved by the Th-N(SiMe3)2/H-Bpin σ-bond
metathesis forming the elusive intermediate [A′] through the
transition state {[Th2]-[A′]}‡. It is important to mention that
BPin
PinB
N
H
H
HBpin
R
N
[Th1]
HBpin
"N
O
B
R
1
H
K2
since no hydride singlet (Th-H) was detected in the H NMR,
[N]
"N
Th
K1
O
R
K5
neither in the exchange magnetization transfer 2D experiments,
we cannot discard, at least theoretically and to some extent
that the complex [Th2] and [A′] are in equilibrium. The rapid
equilibrium reaction of [A′] with the –C=N- bond of the imine
affords the intermediate [B′], that reacts with an additional
molecule of HBpin, via a subsequent Th-N/H-Bpin σ-bond
metathesis, producing the target hydroborated secondary
amine product, and simultaneously regenerating the active
species [A′] to complete the catalytic cycle.
K-2
N
Me3Si
BPin
N
BPin
Si
K-6
K6
H
H
[Th]
N
[Th]
[C]
N
[Th]
N
[Th]
H
N" = N(SiMe3)2
[A]
R
H
H
[A]
R
R
N
R
H
[C']
[N]

[B]
H
Bpin
K-4
K3
BPin
H
H
K4
[Th]
N
BPin
N
BPin
HBpin
R
N
{[C]-[A]}
R
R
[I]
H
[I]
H
Scheme 3 Proposed mechanism of hydroboration of
nitriles.
b)
a)
Mechanistic Studies for Imine hydroboration. To gain a
deeper understanding of the imine hydroboration catalytic
cycle and in the activation pathways of complex Th2,
stoichiometric kinetic and temperature dependence
measurements were performed. Stoichiometric reactions
between Th2 and variable ratios of the imine (PhC=NPh),
starting from 1 equiv to 5 equiv (Figure S79), at room
o
temperature or at 80 C, demonstrated that the complex Th2,
1
by itself, does not react with the imine. In addition, in the H
NMR spectra of the reaction mixture of Th2 and HBpin (1 –
10 equiv) at room temperature; no reaction is observed.
However, after heating the same reaction mixture to 80 C for
6 h, two new signals were found at δ 1.02 and 0.33 ppm,
corresponding to the formation of pinB–N(SiMe3)2 (Figure
S80–S82).20c
All our attempts to find and characterize the corresponding
hydride or deuteride were not successful, indicating a rapid
d)
c)
o
Figure 3 Plot of the initial reaction rate δp⁄δt against the
concentration of a) Th2, b) HBpin, c) PhC=NPh for the reaction
ACS Paragon Plus Environment
of PhC=NPh and HBpin catalysed by Th2; d) Eyring plot of
ln(k/T) vs 1/T for the reaction of PhC=NPh and HBpin catalysed
by Th2.

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(1) (a) Eller, K.; Henkes, E.; Rossbacher, R.; Höke, H. Aliphatic
2
3
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46
47
48
49
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52
53
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59
60
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Williams, M. T. Analysis of the Reactions Used for the Preparation of
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Microwave‐Promoted Selective Mono‐N‐Alkylation of Anilines with
Tertiary Amines by Heterogeneous Catalysis. Chem.–Eur. J. 2011,
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promoted mono-N-Alkylation of Aromatic Amines in Water: A New
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Schmidt, S. E.; Jung, K. W. Cesium Hydroxide Promoted
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Reductive Amination of Aldehydes via Dynamic Kinetic Resolution.
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Alkylative Amination of Aldehydes via Carbon-Carbon Bond
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#
ThIV N(SiMe3)2
BPin
H
T.S. = {[Th2]-[A']}#
PinB N(SiMe3)2
O
HBpin
K1
[Th]
H
B
[Th2]
K-1
O
[A']
K2
R
HBpin
K3
K-2
R'
N
R'
PinB
R'
N
[Th]
N
R
H
R
H
[B']
Scheme 4 Proposed mechanism of hydroboration
of imines.
Conclusions
In conclusion, the rapid hydroboration of nitriles with HBpin
to the corresponding dihydroborated amines (pro-primary
amines) promoted by the thorium amide complex (Th1) has
been established. Th1 was found to be a highly effective and
versatile precatalyst with a broad substrate scope and
according to our knowledge, it displayed the highest TOF
among all the catalysts reported in the literature. In addition,
the N-heterocyclic iminato thorium complex (Th2) was found
as an excellent catalyst for the hydroboration of aldimines
with HBpin to give the hydroborated amines (pre-secondary
amines). Further investigation is dedicated towards the
modification of the complex Th2 to extend his application to
ketimines. Efforts to explore chiral counterpart of Th2 are also
underway for asymmetric imine hydroboration and will be
reported in due course.
(5) (a) Stubbert, B. D.; Marks, T. J. Constrained Geometry
Organoactinides as Versatile Catalysts for the Intramolecular
Hydroamination/Cyclization of Primary and Secondary Amines
Having Diverse Tethered C−C Unsaturation. J. Am. Chem. Soc. 2007,
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ASSOCIATED CONTENT
(6) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and
Practice. Oxford University Press, Oxford, 1998.
The Supporting Information is available free of charge on the
ACS Publications website. Experimental details, characterization
data, kinetics, thermodynamic data, labeling experiments,
Crystallographic Data and Pertinent Refinement Parameters for
the complex Th2.
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Couturier, J.–L. Ruthenium–Benzylidenes and Ruthenium–
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AUTHOR INFORMATION
Corresponding Author
*E-mail: chmoris@technion.ac.il. Phone: +972–4–8292680
(M.S.E.).
ACKNOWLEDGMENT
This work was supported by the Israel Science Foundation
administered by the Israel Academy of Science and Humanities
under Contract No. 184/18. S. S. thanks the ‘Nancy and Stephen
Grand Technion Energy Fellowship’.
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ACS Catalysis
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Insert Table of Contents artwork here
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Nitrile Hydroboration
N"
O
H
N
B
H
ThIV
Si
Me
"N
6
7
8
9
B
O
Me
Th1 (0.1 mol%)
N
O
N
+
O
Me3Si
0
B
H
C6D6, 80
C
O
O
Th1
2.2 equiv.
Highest Activity
Highest TOF
Diverse Substrate Scope
N" = N(SiMe3)2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Imine Hydroboration
O
N
N
O
B
N
O
Th2 (1 mol%)
N
N
O
+
O
0
B
H
H
C6D6, 80
C
ThIV
"N
N"
Th2
"N
1.2 equiv.
ACS Paragon Plus Environment