Facile catalytic hydrosilylation of pyridines

Article abstract of DOI:10.1002/anie.201006135

Not only surprisingly facile, a hydrosilylation of pyridines under the catalysis of [Cp(iPr₃P)Ru(NCCH₃)₂]⁺ has the advantages that it is 1,4-regioselective and reversible. The products can be transformed in a variety of ways (see scheme). The related complex [CpRu(NCCH₃)₃]⁺ catalyzes the two-hydrogen-atom reduction of phenanthroline by HSiMe₂Ph/water.

Full text of DOI:10.1002/anie.201006135

Communications
DOI: 10.1002/anie.201006135
Synthetic Methods
Facile Catalytic Hydrosilylation of Pyridines**
Dmitry V. Gutsulyak, Art van der Est, and Georgii I. Nikonov*
Dedicated to Professor Dmitry A. Lemenovskii on the occasion of his 65th birthday
The catalytic hydrosilylation of imines has emerged as an
important method for the preparation of amines, whereas the
hydrosilylation of other nitrogen-containing substrates
remains a significant synthetic challenge.[1,2] Only a handful
of catalytic methods for the hydrosilylation of nitriles to
amines,[3] nitriles to imines,[3b,4,5] amides to amines,[6,7] and
nitroarenes to anilines[8] are known. The addition of silanes to
pyridines is even rarer. The synthesis of silylated 1,2- and/or
1,4-dihydropyridines is of importance as a potential method
for the preparation of partially reduced pyridines, which are
often used as selective reducing agents[9] and could be
valuable building blocks for organic synthesis.[10] Harrod
and co-workers reported the only example to date of a
homogeneous hydrosilylation of pyridines.[11,12] The use of a
titanium catalyst at 808C gave silylated dihydropyridines;
however, these reactions were compromised by the concom-
itant hydrogenation of the products to tetrahydropyridines. In
related studies, Crabtree and co-workers developed a reduc-
tion of quinolines by silanes in the presence of [Rh(nbd)-
(PPh3)2]PF6 (nbd = norbornadiene), which catalyzes the gen-
eration of the highly active product SiH4 from H3SiPh.[13] The
Table 1: Catalytic hydrosilylation of pyridines with HSiMe2Ph.[a]
Entry
Substrate
Conv. [%][b]
t [h]
Product(s)
1
86[c]
0.5
2
3
4
82
100
47
3
0.25
3
5
6
0
0
3
–
24
–
–
7
0
24
À
stoichiometric insertion of pyridine into an Fe Si bond to give
an h3-1-silyl-1,4-dihydropyridinyl complex was described by
Tobita and co-workers.[14]
Herein, we report a surprisingly facile catalytic hydro-
silylation of pyridines with the additional advantages that it is
1) 1,4-regioselective and 2) reversible. We also report the
coupling of the reaction products with organic substrates,
including the preparation of amides from acid chlorides and a
novel zinc-mediated hydrosilylation of imines.
We reported previously that the cationic ruthenium
complex [Cp(iPr3P)Ru(NCCH3)2]+ (1) catalyzes a variety of
hydrosilylation reactions,[15] including the first chemoselective
monohydrosilylation of nitriles.[5c] Encouraged by this success,
we chose to investigate the addition of silanes to pyridines.
Simple pyridines were subjected to catalytic hydrosilylation
by HSiMe2Ph in dichloromethane in the presence of 1
(5% mol; Table 1). The special feature of this reaction is
that it occurs rapidly even at room temperature and, unlike
8
50[d]
0.5
Mixture of partially
reduced pyridines
9[e]
100
100
0.5
0.5
Mixture of partially
reduced pyridines
10[e]
11
50
68
98
0.25
12[5c]
13[e]
14
3.5
[*] Dipl.-Chem. D. V. Gutsulyak, Prof. Dr. A. van der Est,
Dr. G. I. Nikonov
Chemistry Department, Brock University
500 Glenridge Avenue, St. Catharines, ON L2S 3A1 (Canada)
Fax: (+1)905-682-9020
[a] General procedure: 1 (5% mol) was added to a solution of the
substrate and the silane in CH2Cl2. [b] Conversion was determined on the
1
basis of H NMR spectroscopy. [c] The conversion was 96% when the
E-mail: gnikonov@brocku.ca
reaction was carried out under solvent-free conditions. [d] The con-
version was 70% when the reaction was carried out under solvent-free
conditions. [e] The reaction was carried out with 2 equivalents of the
silane.
[**] This research was supported by the NSERC. We are very grateful to
the CFI/OIT for a generous equipment grant.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006135.
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Angew. Chem. Int. Ed. 2011, 50, 1384 –1387

the previously reported titanium-catalyzed hydrosilylation,
affords selectively the product of 1,4-addition, the N-silyl
dihydropyridine 2 (Table 1, entry 1). The selective formation
of this product is of significance, as conventional reduction
methods usually give mixtures of 1,2- and 1,4-dehydropyr-
idines.[10]
Substitution in the 3- and 5-positions had no effect on the
success of this reaction (Table 1, entry 2), and a halogen in the
3-position was tolerated (entry 3). In contrast, attempted
hydrosilylation of 2-bromopyridine afforded a mixture of
pyridine and the silylated 1,4-dihydropyridine with low
overall conversion (Table 1, entry 4). This product composi-
tion suggests that the substrate initially undergoes dehaloge-
nation, which is followed by hydrosilylation. In general,
substitution in the 2-, 4-, and 6-positions inhibited the reaction
completely (Table 1, entries 5–7). These results suggest that
the steric accessibility of the nitrogen center and the
4-position is important. The lack of reactivity observed for
4-methylpyridine (Table 1, entry 7) also highlights the pref-
erence of this system for 1,4-reduction over the common
1,2-reduction. The challenging substrate quinoline was suc-
cessfully converted into a mixture of 1,4- and 1,2-hydro-
silylated products in moderate yield (Table 1, entry 8). This
reaction proceeded in better yield (70%) when it was carried
out under solvent-free conditions.
To check the compatibility of this reaction with the
presence of functional groups, we attempted the hydrosilyla-
tion of 3-acetyl- and 3-formylpyridine; however, a mixture of
products was observed in both cases (Table 1, entries 9 and
10). In contrast, the hydrosilylation of 4-acetylpyridine
cleanly gave a pinacol derivative (Table 1, entry 11). This
result suggests the possibility of a radical pathway. A cyano
group was hydrosilylated preferentially to give the corre-
sponding 3-silyliminoformyl pyridine in high yield (Table 1,
entry 12). Finally, the bishydrosilylation of triazine provided a
1,2,3,4-tetrahydro derivative in high yield (Table 1, entry 13).
Even more surprisingly, we discovered that the hydro-
silylation is reversible. In the presence of catalyst 1, com-
pound 2 transferred the silane moiety to benzonitrile to give
Scheme 1. Proposed mechanism for the hydrosilylation of pyridines.
mediates in these reactions.[15] To underpin the feasibility of
the hydride-transfer step, we prepared a neutral pyridine-
stabilized hydride complex 5b by the treatment of 1 with
tBuOK in iPrOH in the presence of excess pyridine. Complex
5b was characterized by NMR spectroscopy. As expected, the
reaction of 5b with the readily available pyridinium salt
[(Et3Si)(Py)][B(C6F5)4]À
(Py = pyridine)
yielded
the
1,4-hydrosilylated pyridine. The catalytic hydrosilylation of
pyridine with the deuterated silane DSiMe2Ph provided
further evidence for the suggested mechanism. The deuterium
atoms were found exclusively in the 4-position of the product.
In contrast, Harrod and co-workers postulated that in the
titanium-catalyzed hydrosilylation of pyridines, the products
of 1,4-addition stem from the kinetic products of
1,2-addition.[11]
Although we believe that the hydride is predominantly
transferred to the pyridinium cation in a single step (a two-
electron process), we cannot exclude the possibility of a
sequence of one-electron processes. For the closely related
+
À
system RO2C NC5H5 /[Cp(dppe)RuH] (dppe = 1,2-bis(di-
phenylphosphanyl)ethane), Norton and co-workers found
that both pathways are feasible.[17] In fact, the mechanism of
single-electron transfer (SET) can operate preferentially for
substrates with substituents in the 4-position. For example,
the SET mechanism can account for the formation of the
pinacol product in the hydrosilylation of 4-acetylpyridine
(Table 1, entry 11). We indeed observed the EPR spectrum of
a ruthenium-centered paramagnetic species in a frozen
reaction mixture of pyridine, HSiMe2Ph, and complex 1
(20%).[18]
=
pyridine and the imine PhHC NSiMe2Ph. With 3,5-lutidine,
an equilibrium mixture of 1,4-hydrosilylated pyridine deriv-
atives was formed. The treatment of compound 2 with
DSiMe2Ph in the presence of 1 (5 mol%) resulted in
deuteration exclusively at the 4-position. To the best of our
knowledge, these reactions are the first examples of a
reversible hydrosilylation.[16]
Compound 2 also underwent dehydrosilylation in ben-
zene upon the addition of B(C6F5)3, which abstracted hydride
from the 4-position with the precipitation of the pyridinium
salt [C5H5N·SiMe2Ph]HB(C6F5)3. This salt decomposed slowly
(in 24 h) in dichloromethane with the formation of C5H5N·B-
(C6F5)3 and HSiMe2Ph.
To check further the compatibility of pyridine hydro-
silylation with the presence of a carbonyl functionality, we
studied silane addition to a 1:1 mixture of pyridine and
acetone. Unexpectedly, we observed a mixture of 2 and the
À
=
product 3 of its formal N Si addition across the C O bond.
The attempted hydrosilylation of pyridine in acetone as the
solvent afforded 3 as the only product [Eq. (1)]. However,
extraction of the product with hexane resulted in a formal
For the addition of silanes to carbonyl compounds and
nitriles under the catalysis of 1, we previously suggested an
ionic hydrosilylation mechanism based on the formation of
cationic silane s complexes 4 and nucleophilic abstraction of a
silylium ion by the substrate, followed by hydride transfer
from the ruthenium hydride 5 (Scheme 1).[5c,15] We proved
previously the formation of silane s-complexes 4 as inter-
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Communications
dehydroamination reaction with the formation of 1,4-dihy-
dropyridine and a silylated enol.
The postulated catalytic cycle of phenanthroline reduc-
tion starts with the displacement of a labile ligand L in [Cp(k2-
phen)RuL]+ (L = NCCH3, phenH2) by the silane to give the
silane s complex 11 (Scheme 3). This complex is probably too
sterically loaded to react with phenanthroline but can react
with water to furnish the hydride 10 and the protonated
The reaction of 2 with acid chlorides afforded the
corresponding amide and ClSiMe2Ph. In contrast, 2 did not
react with ethyl acetate even at elevated temperatures (up to
1008C) and in the presence of catalyst 1 (5 mol%). Com-
pound 2 did not react with aldimines, even when complex 1
was used as an activator. However, we found that 2 slowly
(50% conversion after 2 days, as determined by NMR
=
spectroscopy) hydrosilylated the aldimine PhN CHPh in
ether in the presence of ZnCl2 (1 equiv). No reaction between
=
2 and PhN CHPh or between 2 and ZnCl2 was observed; nor
=
did HSiMe2Ph react with PhN CHPh in the presence of
ZnCl2 under these conditions. We believe that ZnCl2 effec-
tively plays the same role in this reaction as a phosphoric acid
in the reduction of imines by 1,4-dihydropyridines;[19] that is, it
promotes the formation of
a cyclic transition state
(Scheme 2). The requirement of very high temperatures
(150–2008C) for previously reported ZnCl2-mediated hydro-
silylation reactions further supports this hypothesis.[4a,20]
À
Scheme 3. Catalytic hydrogenation of phenanthroline. An=PF6
.
silanol [PhMe2SiOH2]+, which is then deprotonated by
phenanthroline to give [H-phen]+. Hydride transfer from 10
to the 4-position of [H-phen]+ affords the two-hydrogen-
atom-reduced phenanthroline, which coordinates to ruthe-
nium to give a latent form of the catalyst, the observed
complex 8.
In summary, we have discovered unprecedented catalytic
activity and 1,4-selectivity in the hydrosilylation of pyridines
by complex 1 and the catalytic reduction of phenanthroline by
HSiMe2Ph in the presence of the complex [Cp(H3CCN)Ru-
(k2-phen)]+ (7).
=
Scheme 2. Synchronized reaction of 2, PhN CHPh, and ZnCl2.
Although quinoline underwent hydrosilylation (Table 1,
entry 8), its chelating analogue, phenanthroline, poisoned the
catalyst by forming the very stable complex [Cp(iPr3P)Ru(k2-
phen)]+ (6). To free up a reaction site in the catalyst, we
attempted the hydrosilylation of phenanthroline in the
presence of [CpRu(NCCH3)3]+, which contains three poten-
tially labile ligands. However, only very low conversion was
observed. Accidentally, we discovered that adventitious water
triggers a different catalytic process: in the presence of water,
the compound [Cp(NCCH3)Ru(k2-phen)]+ (7) formed in situ
catalyzed the reduction of phenanthroline by excess HSi-
Me2Ph to 1,4-dihydrophenanthroline. An analogous process
occurred in the presence of an alcohol as the proton source.
The reaction was accompanied by intensive evolution of
dihydrogen as a result of concomitant silane hydrolysis
(alcoholysis). However, independent experiments showed
that no hydrogenation of phenanthroline by gaseous hydro-
gen occurred under these conditions in the absence of the
silane.
Monitoring of the course of phenanthroline reduction by
NMR spectroscopy revealed that the complex [Cp(k2-phen)-
Ru(k1-phenH2)]+ (8) with a partially reduced phenanthroline
ligand was the predominant ruthenium species in the reaction
mixture. At the end of reduction, 8 was slowly converted into
the hydride-bridged dimer [{Cp(k2-phen)Ru}2(m-H)]+ (9).
Although complex 9 itself is not an active catalyst, its
formation provides some evidence for the intermediacy of a
neutral ruthenium hydride, [Cp(phen)RuH] (10). Unfortu-
nately, numerous attempts to prepare compound 10 from the
precursor 7 have been unsuccessful so far. In all cases, the
complex 9 was observed as the major product.
Experimental Section
For general reaction conditions, see the Supporting Information.
Synthesis of 2: [CpRu(PiPr3)(CH3CN)2]PF6 (0.20 g, 3 mol%) was
added to a solution of pyridine (1.00 mL, 12.3 mmol) and HSiMe2Ph
(2.00 mL, 13.0 mmol) in CH2Cl2 (20 mL), and the resulting mixture
was stirred for 6 h at ambient temperature. The removal of all
volatiles under vacuum and distillation of the resulting oil under
reduced pressure then gave 2 (1.97 g, 74%) as a yellow oil. 1H NMR
(300 MHz, CDCl3): d = 7.58–7.52 (m, 2H, Ph), 7.42–7.32 (m, 3H, Ph),
=
5.89 (d, JH,H = 8.3 Hz, 2H, NCH CH), 4.45 (dt, JH,H = 8.3, 3.2 Hz, 2H,
=
NCH CHCH2), 2.95 (m, 2H, CH2), 0.42 ppm (s, 6H, SiMe);
13C NMR (75.5 MHz, CDCl3): d = 136.5, 133.8, 129.8, 128.7, 128.1,
1
100.1, 22.5, À2.4 ppm; H–29Si HSQC (CDCl3): d = 1.1 ppm.
Dehydrosilylation of
2
with benzonitrile: [CpRu(PiPr3)-
(CH3CN)2]PF6 (0.002 g, 2 mol%) was added to a solution of 2
(30 mL, 0.14 mmol) and PhCN (14 mL, 0.14 mmol) in [D6]acetone
(0.6 mL), and the resulting mixture was stirred at ambient temper-
=
ature. After 24 h, 40% conversion of 2 into PhCH NSiMe2Ph and
pyridine was observed. Conversion was complete after 18 days at
ambient temperature.
Exchange reaction of
2 with DSiMe2Ph: [CpRu(PiPr3)-
(CH3CN)2]BAF (0.005 g, 5 mol%) was added to a solution of 2
(20 mL, 0.09 mmol) and DSiMe2Ph (14.3 mL, 0.09 mmol) in CH2Cl2
(0.6 mL), and the resulting mixture was stirred at ambient temper-
ature. After 24 h, 25% deuteration of the 4-position of 2 was
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2
observed. According to H NMR spectroscopy, no deuterium incor-
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Received: September 30, 2010
Published online: January 5, 2011
Keywords: chemoselectivity · heterocycles · hydrosilylation ·
.
ruthenium · zinc
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