Double and Single Hydroboration of Nitriles Catalyzed by a Ruthenium-Bis(silyl)xanthene Complex: Application to One-Pot Synthesis of Diarylamines and N-Arylimines

Article abstract of DOI:10.1021/acs.organomet.9b00064

A ruthenium complex bearing a bis(silyl)xanthene chelate ligand xantsil, i.e. Ru[κ³(Si,O,Si)-xantsil](CO)(PCyp₃) (1a, Cyp = cyclopentyl), was found to catalyze both double and single hydroboration reactions of nitriles with pinacolborane (HBpin) and 9-borabicyclo[3.3.1]nonane (9-BBN) to give bis(boryl)amines 2 and N-borylimines 3, respectively, in mostly >99% yields. By combination of these reactions with subsequent palladium-catalyzed deborylative C-N coupling reactions of 2 and 3 with a 1:1 mixture of bromoarenes and KO^tBu, one-pot synthetic routes to tertiary N,N-diarylamines 4 and N-arylaldimines 5 from nitriles were developed.

Full text of DOI:10.1021/acs.organomet.9b00064

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Double and Single Hydroboration of Nitriles Catalyzed by a
Ruthenium−Bis(silyl)xanthene Complex: Application to One-Pot
Synthesis of Diarylamines and N‑Arylimines
Takeo Kitano, Takashi Komuro, and Hiromi Tobita*
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
S
* Supporting Information
ABSTRACT: A ruthenium complex bearing a bis(silyl)xanthene
chelate ligand xantsil, i.e. Ru[κ3(Si,O,Si)-xantsil](CO)(PCyp3)
(1a, Cyp = cyclopentyl), was found to catalyze both double and
single hydroboration reactions of nitriles with pinacolborane
(HBpin) and 9-borabicyclo[3.3.1]nonane (9-BBN) to give
bis(boryl)amines 2 and N-borylimines 3, respectively, in mostly
>99% yields. By combination of these reactions with subsequent
palladium-catalyzed deborylative C−N coupling reactions of 2
and 3 with a 1:1 mixture of bromoarenes and KOtBu, one-pot
synthetic routes to tertiary N,N-diarylamines 4 and N-
arylaldimines 5 from nitriles were developed.
single hydroboration reactions of nitriles using HBpin and 9-
BBN, respectively.
Furthermore, because N-borylated amines and imines
eduction of CN bonds of nitriles is a useful method to
Rsynthesize amines and imines. Amines are widely used as
medicines, agrochemicals, etc.,1 whereas imines are important
obtained by double and single hydroboration reactions of
nitriles possess labile B−N bonds, they are expected to serve as
precursors for the syntheses of organic nitrogen com-
pounds.3,4,6 However, conversion reactions of these hydro-
boration products via B−N bond cleavage are still very limited,
and most of them are hydrolysis or alcoholysis reaction-
s.3a−c,g,j,n,4a−d The only known C−N bond forming reaction
from the hydroboration products is a conversion of bis(boryl)-
amines to aldimines by treatment with benzaldehyde.3a−c In
addition, Buchwald et al. previously reported that a reaction of
triaminoboranes with bromoarenes and NaOtBu catalyzed by
Pd(dba)2/P(o-tolyl)3 (a 1:2 mixture, dba = dibenzylideneace-
tone) afforded N-arylamines via C−N bond formation.6 We
applied this protocol to C(aryl)−N coupling reactions of the
hydroboration products using a palladium−phosphine catalyst
[a 1:2−4 mixture of Pd(dba)2 and 2-(dicyclohexylphosphino)-
biphenyl (CyJohnPhos)],7 and successfully prepared N,N-
diarylamines and N-arylimines by a one-pot procedure from
nitriles.
In the presence of 5 mol % of Ru−xantsil complex 1a, 4-
(trifluoromethyl)benzonitirile reacted with 2 equiv of
pinacolborane (HBpin) in cyclohexane-d12 at 40 °C to give
N,N-bis(boryl)amine 2a quantitatively via double hydro-
boration (Table 1, entry 1). The single-hydroboration product,
i.e. N-borylimine (4-CF3C6H4)CHNBpin, was not obtained
even when one equivalent of HBpin was used. Catalytic activity
of 1a toward this reaction was compared against that of related
as starting materials for organic synthesis, building blocks of
supramolecules, etc.2 Hydroboration reactions of nitriles giving
either amine or imine derivatives selectively3,4 are regarded as
efficient methods for this nitrile reduction. In the last several
years, double hydroboration reactions of nitriles with HBpin or
catecholborane (HBcat) to give N,N-bis(boryl)amines have
been investigated by use of transition-metal, rare-earth-metal,
or main-group-element compounds as catalysts.3 In sharp
contrast with these catalytic reactions, uncatalyzed reactions of
nitriles with alkyl- or arylboranes have been known to lead to
single hydroboration to give N-borylimines.4 Interestingly, to
the best of our knowledge, catalysts that are active for both of
these double and single hydroboration reactions have not been
reported.
In this work, as the catalyst for hydroboration of nitriles, we
employed a 16-electron ruthenium complex having a xanthene-
based bis(silyl) chelate ligand xantsil ((9,9-dimethylxanthene-
4,5-diyl)bis(dimethylsilyl)),5 i.e. Ru[κ3(Si,O,Si)-xantsil](CO)-
(PCyp3) (1a), which was recently developed by us.5c Strong σ-
donating ability of two silyl-ligand moieties of xantsil in 1a is
expected to make the metal center electron-rich and facilitate
the oxidative addition of hydroboranes to the Ru center.
Moreover, the Si,O,Si-type tridentate xantsil ligand of 1a can
function as a hemilabile ligand due to the weak coordination of
the xanthene oxygen. Complex 1a is therefore considered to
generate 14-electron active species with two vacant coordina-
tion sites on the metal center, which can accept both nitrile
and hydroborane molecules at the same time. In this study, we
found that 1a became a selective catalyst for both double and
Received: January 31, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00064
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Table 1. Double Hydroboration of 4-CF3C6H4CN with
Table 2. Scope of Nitriles for the Double Hydroboration
Catalyzed by 1a
a
a
HBpin Catalyzed by Complexes 1a−d
temp
time
(h)
NMR yield of 2
b c
,
entry
R
product
(°C)
(%)
>99
1
2
3
4
5
6
7
8
4-ClC6H4
Ph
2b
2c
2d
2e
2f
2g
2h
2i
40
60
60
60
60
60
60
60
60
70
60
60
4
4
12
12
12
6
>99
>99
>99
>99
>99 (63)
>99 (61)
>99
51 (43)
62 (7)
>99
4-MeC6H4
4-OMeC6H4
3-MeC6H4
2-CF3C6H4
2-ClC6H4
2-MeC6H4
2-OMeC6H4
Mes
b
entry
cat.
time (h)
NMR yield of 2a (%)
9
24
96
360
1
1
2
3
4
1a (R = Cyp)
1b (R = Cy)
1c (R = Pyrr)
1d
4
4
24
24
>99
>99
23
9
2j
2k
2l
10
11
12
tBu
>99
a
Me
2m
3
>99
Conditions: 4-CF3C6H4CN (0.15 mmol), HBpin (0.30 mmol),
a
and catalyst (7 μmol, 5 mol %) in cyclohexane-d12 (0.5 mL) at 40 °C.
Conditions: nitrile (0.14−0.15 mmol), HBpin (0.30 mmol), and
b
b
Based on 4-CF3C6H4CN.
catalyst 1a (7 μmol, 5 mol %) in cyclohexane-d12 (0.5 mL). Based on
nitrile. Isolated yields are shown in parentheses.
c
Ru−xantsil complexes, i.e. 16-electron complexes having
different phosphine ligands Ru[κ3(Si,O,Si)-xantsil](CO)(PR3)
(R = Cy (cyclohexyl) (1b)5b and Pyrr (1-pyrrolidinyl) (1c)5d)
and also an 18-electron η6-toluene complex Ru[κ2(Si,Si)-
xantsil](CO)(η6-toluene) (1d)5a (entries 2−4). The reaction
catalyzed by trialkylphosphine complex 1b for 4 h at 40 °C
afforded 2a quantitatively (entry 2), which shows that the
catalytic activity of 1b is comparable with that of 1a (entry 1).
On the other hand, when complex 1c having a less electron-
donating triaminophosphine ligand was employed as a catalyst,
even after a longer reaction time (24 h), 2a was obtained in
only 23% NMR yield (entry 3). This result implies that the
hydroboration reaction is accelerated when the metal center of
the catalyst becomes more electron-rich. η6-Toluene complex
1d was also found to catalyze this reaction to give 2a
quantitatively (entry 4), but the reaction was slower than those
with catalysts 1a and 1b.
Bis(boryl)amine 2a was prepared in a gram scale by a
reaction in THF at 60 °C for 24 h using 0.2 mol % of complex
1a as a catalyst (see eq S1 in the Supporting Information). In
this reaction, product 2a was formed nearly quantitatively and,
from the reaction mixture, 1.04 g of 2a was isolated in 83%
yield. This amount of catalyst (0.2 mol %) is smaller than those
of previously reported homogeneous catalysts for double
hydroboration of nitriles (0.5−10 mol %),3 and this fact clearly
demonstrates the high catalytic activity of 1a.
benzonitriles (five examples involving mesitylnitrile, entries 6−
10) whose CN groups are sterically hindered, and hence,
these nitriles have been rarely used as substrates for the double
hydroboration.3c,f,i−k As a result of the experiments, bis(boryl)-
amines 2g−k involving three novel compounds 2h, j, and k
were obtained in high to moderate yields (entries 6−10). For
the reaction of a certain benzonitrile derivative having a CF3,
Cl, or Me group at an ortho-position (entries 6−8), a longer
reaction time was required for complete conversion of the
nitrile compared with the reaction of that having the same
group at the para-position (entry 1 in Table 1 and entries 1
and 3 in Table 2). Nevertheless, NMR yields of products 2g
and 2i (>99% each, entries 6 and 8) are significantly improved
in comparison with the corresponding ones (2g: 50%3c and 2i:
59% (isolated yield)3f or 86%3k) reported for the reactions
using other catalysts. Furthermore, 2-OMeC6H4CN with an
electron-donating OMe group at an ortho-position and bulkier
MesCN were also doubly hydroborated, although the
reactions were slower and the NMR yields (51 and 62%) of
bis(boryl)amines 2j and 2k were lower (entries 9 and 10).
These results clearly show that the increase of steric hindrance
around a nitrile CN group retards the progress of the double
t
hydroboration. Aliphatic nitriles (R = Bu and Me) also
underwent double hydroboration to give bis(boryl)amines 2l
and 2m quantitatively (entries 11 and 12).
Scope of nitriles for the double hydroboration was explored
by use of PCyp3 complex 1a (5 mol %), which is, as has been
mentioned, one of the most active Ru−xantsil complex
catalysts 1a and 1b. In most cases, hydroboration products 2
were obtained in >99% NMR yields (Table 2). The reactions
of benzonitrile and its para- or meta-substituted derivatives
gave the corresponding bis(boryl)amines 2b−f quantitatively
(entries 1−5). The reaction rates for the benzonitrile
derivatives having an electron-withdrawing CF3 or Cl group
at the para-position (entry 1 in Table 1 and that in Table 2)
are higher than those for the corresponding compounds having
an electron-donating Me or OMe group (entries 3 and 4 in
Table 2). We also examined the reactions of ortho-substituted
To clarify whether complex 1a also functions as a catalyst for
single hydroboration of nitriles, we examined the catalytic
reaction using commercially available dialkylborane, 9-
borabicyclo[3.3.1]nonane (9-BBN). In the presence of 0.5
mol % of 1a, nitriles RCN (R = 4-CF3C6H4, Ph, 4-
OMeC6H4, and tBu) reacted with 9-BBN (0.9−1 equiv for the
monomer) in THF-d8 at room temperature ∼ 40 °C to give
single hydroboration products, i.e. N-borylimines 3, quantita-
tively (Table 3). When 2.2 equiv of 9-BBN were used for the
hydroboration of PhCN in the presence of 1a as a catalyst,
an N-borylimine·9-BBN adduct 3b·9-BBN4a was formed
quantitatively (eq 1, reaction solvent: cyclohexane-d12),8 and
further heating of the reaction mixture at 60 °C did not give
B
DOI: 10.1021/acs.organomet.9b00064
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Table 3. Single Hydroboration of Nitriles with 9-BBN
Catalyzed by 1a
Table 4. One-Pot Synthesis of N,N-Diarylamines 4 or N-
a
Arylimines 5 from Nitriles by Double or Single
a
Hydroboration and Subsequent C−N Coupling
temp
(°C)
time
(h)
NMR yield of 3
bc
,
entry
R
product
(%)
1
2
3
4
4-CF3C6H4
Ph
4-OMeC6H4
tBu
3a
3b
3c
3d
40
40
40
rt
0.5
0.5
0.5
0.25
>99 (98)
>99
>99
>99
a
Conditions: nitrile (0.14−0.15 mmol), 9-BBN (0.14 mmol for the
monomer), and catalyst 1a (0.7 μmol, 0.5 mol %) in THF-d8 (0.5
b
c
mL). Based on 9-BBN. Isolated yield of 3a is shown in parentheses.
a
Conditions for synthesis of 4: (i) nitrile (0.29 mmol), HBpin (0.63
mmol), catalyst 1a (7 μmol, 2 mol %), 40−60 °C, THF; (ii)
bromoarenes (0.61 mmol), KOtBu (0.61 mmol), Pd(dba)2 (6.1 μmol,
2 mol %), CyJohnPhos (14 μmol, 5 mol %), 60 °C, 12 h, THF. For
synthesis of 5: (iii) nitrile (0.099−0.29 mmol), 9-BBN (0.12−0.32
mmol for the monomer), catalyst 1a (0.7−1.4 μmol, 0.5−0.7 mol %),
1 h, rt, THF; (iv) bromoarenes (0.11−0.32 mmol), KOtBu (0.12−
0.32 mmol), Pd(dba)2 (2.1−6 μmol, 2 mol %), CyJohnPhos (4−14
the double hydroboration product, i.e. bis(boryl)amine
PhCH2N(BC8H14)2. This is possibly because a further reaction
of 3b with catalyst 1a is inhibited by bulkiness of the boryl
group. In comparison with the previous report for the
uncatalyzed single hydroboration reaction of nitriles with 9-
BBN,4a the reaction catalyzed by 1a proceeded under milder
conditions. For example, the reaction of 4-OMeC6H4CN
with 9-BBN in the absence of catalyst needs heating at much
higher temperature (110 °C) and for a longer time (20 h).4a
To obtain insights into mechanisms for the catalytic
hydroboration reactions, we examined a stoichiometric
reaction of 1a with 1 equiv of 4-trifluoromethylbenzonitirile
(eq S2 in the Supporting Information). The reaction
completed at room temperature within 0.25 h to give a 16-
electron nitrile complex Ru[κ2(Si,Si)-xantsil](CO)(PCyp3)-
[NC(C6H4-4-CF3)] (6) quantitatively via dissociation of the
xanthene oxygen and coordination of the nitrile. The structure
of 6 was determined by X-ray crystal structure analysis (see the
Supporting Information). On the other hand, reaction of 1a
with excess HBpin (ca. 10 equiv) at room temperature was
much slower, in which only ca. 5% of 1a reacted to give 9,9-
dimethyl-4,5-bis(dimethylsilyl)xanthene (xantsilH2) after 24 h
(eq S3 in the Supporting Information). These results imply
that the first step of the catalytic hydroboration reaction is the
formation of a nitrile complex such as 6. On the basis of these
results and taking into account a previously proposed
mechanism for ruthenium-catalyzed double hydroboration
reported by Gunanathan et al.,3d we propose a possible
mechanism for the hydroboration of nitriles catalyzed by 1a as
shown in Scheme S1 (see the Supporting Information).
b
μmol, 4−8 mol %), 60 °C, 12 h, THF. Isolated yields based on
nitriles.
Pd(dba)2 (2 mol %) as catalyst precursors,7 and base KOtBu
(2.1 equiv) in this order. The mixture was then heated at 60
°C for 12 h to give N,N-diarylamines 4, involving novel
compounds 4b and 4c, in 64−79% isolated yields. N-
Arylimines 5 were also synthesized in 53−73% isolated yields
by a similar C−N coupling reaction of single hydroboration
products 3 obtained from nitriles and 9-BBN (1.1−1.2 equiv)
in the presence of 0.5−0.7 mol % of catalyst 1a. In this
coupling reaction, 1.1−1.2 equiv each of bromoarenes and
KOtBu were used. It is noteworthy that the hydroboration and
C−N coupling reactions can be successively performed in one
pot without isolation of intermediary hydroboration products 2
and 3.9
In summary, we demonstrated that 16-electron ruthenium−
bis(silyl)xanthene chelate complex 1a functions as a highly
active catalyst for both the double and single hydroboration
reactions of nitriles to give 2 and 3 in perfect selectivity. These
products 2 and 3 were also found to be converted into tertiary
diarylamines 4 and N-arylaldimines 5 by subsequent
palladium-catalyzed deborylative C−N coupling reactions.
These hydroboration and successive C−N coupling reactions
can be conveniently done in one pot.
As novel conversion reactions of hydroboration products 2
and 3 via B−N bond cleavage, we developed palladium-
catalyzed C(aryl)−N coupling reactions shown in Table 4.6,7
After the double hydroboration of nitriles with HBpin (2.2
equiv) catalyzed by 1a (2 mol %) in THF, to the resulting
mixture containing bis(boryl)amine 2 were added bromoar-
enes (2.1 equiv), phosphine CyJohnPhos (5 mol %) and
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00064.
■
S
C
DOI: 10.1021/acs.organomet.9b00064
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Experimental procedures, proposed mechanism for
Communication
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hydroboration, crystal structure analysis, and NMR
spectra of products (PDF)
Accession Codes
CCDC 1893612 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: tobita@m.tohoku.ac.jp.
ORCID
Takashi Komuro: 0000-0003-4835-1099
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grants
JP25410058, JP15H03782, and JP16K05714 from the Japan
Society for the Promotion of Science (JSPS). We also
acknowledge the Research and Analytical Center for Giant
Molecules, Tohoku University, for mass spectroscopic
measurements and elemental analysis.
(5) (a) Tobita, H.; Hasegawa, K.; Minglana, J. J. G.; Luh, L.-S.;
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(7) Related palladium-catalyzed C(aryl)−N coupling reactions of N-
silylated amines and imines with bromoarenes via cleavage of their
Si−N bonds have been reported. See: (a) Barluenga, J.; Aznar, F.;
Valdes, C. N-Trialkylsilylimines as Coupling Partners for Pd-
Catalyzed C−N Bond-Forming Reactions: One-Step Synthesis of
Imines and Azadienes from Aryl and Alkenyl Bromides. Angew. Chem.,
Int. Ed. 2004, 43, 343−345. (b) Smith, C. J.; Early, T. R.; Holmes, A.
B.; Shute, R. E. Palladium catalysed cross-coupling reactions of
silylamines. Chem. Commun. 2004, 53, 1976−1977.
(8) During the initial stage of the catalytic single-hydroboration
reaction of PhCN with 9-BBN, adduct 3b·9-BBN was observed in
1
the reaction mixture by H and 11B NMR spectroscopy.
(9) We also confirmed that C−N coupling reactions of isolated
double and single hydroboration products 2a and 3a with
bromobenzene and KOtBu catalyzed by Pd(dba)2/CyJohnPhos gave
4a and 5a, respectively (see eqs S4 and S5 in the Supporting
Information).
̈
Anker, M. D.; Arrowsmith, M.; Hill, M. S.; Kociok-Kohn, G.; Liptrot,
D
DOI: 10.1021/acs.organomet.9b00064
Organometallics XXXX, XXX, XXX−XXX