Chemo- and regioselective catalytic reduction of N-heterocycles by silane

Article abstract of DOI:10.1021/om400269q

The ruthenium complex [Cp(iPr₃P)Ru(NCCH₃) ₂]⁺ (1) catalyzes the regioselective hydrosilylation of pyridines to 1,4-dihydropyridines. Substitution in the 3- and 5-positions is tolerated, whereas pyridines with substituents in the 2-, 4-, and 6-positions are not reduced. Reduction of functionalized pyridines having keto and ester substituents results in a mixture of products. N-Silyl-1,4-dihydropyridine reacts with ketones and aldehydes to give products of N-Si addition across the C=O bond. Hydrosilylation of pyridine in acetone results quantitatively in the addition product PhMe₂SiO-CMe₂-NC₅H ₆, which decomposes in hexane to give the parent dihydropyridine HNC₅H₆. The phenanthroline complex [Cp(phen)Ru(NCCH ₃)₂]⁺ (10) catalyzes regioselective 1,4-reduction of phenanthroline by a 3-4-fold excess of silane/water or silane/alcohol mixtures. The Cp* analogue [Cp*(ph n)Ru(NCCH ₃)₂]⁺ (9) catalyzes 1,4-regioselective monohydrosilylation of phenanthroline, quinoline, acridine, and 1,3,5-triazine and the 1,2-reduction of isoquinoline. In contrast, 2-substituted phenanthroline, pyrazine, 2-ethylpyridine, 2,6-lutidine, 2,4-lutidine, and pyrimidine are not reduced under these conditions by either of the catalysts studied.

Full text of DOI:10.1021/om400269q

Article
pubs.acs.org/Organometallics
Chemo- and Regioselective Catalytic Reduction of N‑Heterocycles by
Silane
Sun-Hwa Lee, Dmitry V. Gutsulyak,† and Georgii I. Nikonov*
Chemistry Department, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario L2S 3A1, Canada
S
* Supporting Information
ABSTRACT: The ruthenium complex [Cp(iPr3P)Ru-
(NCCH3)2]+ (1) catalyzes the regioselective hydrosilylation of
pyridines to 1,4-dihydropyridines. Substitution in the 3- and 5-
positions is tolerated, whereas pyridines with substituents in the
2-, 4-, and 6-positions are not reduced. Reduction of
functionalized pyridines having keto and ester substituents
results in a mixture of products. N-Silyl-1,4-dihydropyridine
reacts with ketones and aldehydes to give products of N−Si
addition across the CO bond. Hydrosilylation of pyridine in
acetone results quantitatively in the addition product
PhMe2SiO−CMe2−NC5H6, which decomposes in hexane to
give the parent dihydropyridine HNC5H6. The phenanthroline
complex [Cp(phen)Ru(NCCH3)2]+ (10) catalyzes regioselec-
tive 1,4-reduction of phenanthroline by a 3−4-fold excess of silane/water or silane/alcohol mixtures. The Cp* analogue
[Cp*(phen)Ru(NCCH3)2]+ (9) catalyzes 1,4-regioselective monohydrosilylation of phenanthroline, quinoline, acridine, and
1,3,5-triazine and the 1,2-reduction of isoquinoline. In contrast, 2-substituted phenanthroline, pyrazine, 2-ethylpyridine, 2,6-
lutidine, 2,4-lutidine, and pyrimidine are not reduced under these conditions by either of the catalysts studied.
catalytically.23 Second, with the notable exception of Ir-
INTRODUCTION
■
catalyzed hydrogenation of quinolines reported by Crabtree
Heterocycles constitute about 80% of pharmaceutically active
et al.,24 hydrogenation of nitrogen heteroaromatics usually
ingredients,1 with nitrogen-containing cycles accounting for
requires high pressure (30−100 atm) and/or elevated temper-
atures. Third, most of these methods provide complete
reduction to piperidine derivatives in the case of pyridines
and to tetrahydroquinolines in the case of quinolines.
>92% of drug candidates.2 Reduced pyridine and quinoline
derivatives are a very important motif in natural products,3 of
which NADH, a coenzyme responsible for the reduction
processes in living matter, is probably the most notable
example.4 The reduction of pyridine rings is an attractive
strategy to prepare other six-membered N-heterocycles.5
However, this approach faces a serious chemoselectivity
problem, as the reduction can afford dihydropyridines,
tetrahydropyridines, and the fully reduced piperidines. It also
presents a significant regioselectivity problem, as the formation
of 1,4-dihydropyridines is usually accompanied by 1,2-
reduction.6
For nitrogen heterocycles several reduction methods have
been developed, including stoichiometric reactions with alkali
metals in alcohol,7,8 borohydrides,9 and reduced pyridines
(reduction by Hantsch’s ester10,11 or disproportionation of
dihydropyridines12) and catalytic methods such as transfer
hydrogenation13,14 and hydrogenations.15−18 Hydrogen is the
cheapest as well as the cleanest reductant. The catalytic
hydrogenation of six-membered N-heterocycles has three main
features. First, it has been mainly developed for ring-fused
substrates, such as quinolines,19,20 quinoxalines,19b,c,21 and
similar polycyclic substrates, which are more activated due to
the diminished aromatic stabilization.22 Pyridines, the most
aromatic and hence most robust substrates, are rarely reduced
The dihydropyridine ring is an important structural unit in
natural products and pharmaceuticals, as well as a useful
starting point for the construction of functionalized tetrahy-
dropyridines and piperidines.6,25 Therefore, the development of
chemo- and regioselective reduction of pyridines under mild
conditions is an important goal, which requires the application
of milder reducing reagents, such as boranes and silanes.
Suginome et al. reported Rh-catalyzed regioselective hydro-
boration to give N-boryl 1,2-dihydropyridines.26 Hill et al.
studied the hydroboration of pyridines catalyzed by a
magnesium NacNac complex.27 With pyridines as substrates,
mixtures of 1,2- and 1,4-dihydropyridines formed, whereas the
hydroboration of quinolines showed good regioselectivity
toward the 1,2-products.
Although heterogeneous catalytic hydrosilylation of N-
heterocycles has been known since the mid-1960s,28 its
homogeneous variant appeared relatively recently. Murata et
al. reported the first Rh-catalyzed hydrosilylation of quinox-
alines.21a Harrod et al. found that Ti-catalyzed hydrosilylation
© XXXX American Chemical Society
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a
Table 1. Hydrosilylation of Pyridines by Complex 1 (5%) in CH2Cl2
a
b
In the general procedure, 5 mol % of 1 was added to a solution of substrate and silane in CH2Cl2. Based on the 1H NMR data. NR = no reaction.
c
d
96% under solvent-free conditions. 70% under solvent-free conditions.
of pyridines results in mixtures of 1,2-dihydropyridines and
simple pyridines (Table 1). The silane HSiMe2Ph was chosen
as the reducing reagent because it behaved the best in previous
studies,31a,32 most likely due to the optimum Si−H bond
activation in the transient silane σ complex.34 To our delight,
pyridine was reduced in only 30 min under very mild
conditions chemoselectively to the N-silyl dihydropyridine.
Even more remarkably, this reduction was 1,4-regiospecific
(entry 1). Substrates having substitution in the 3- and 5-
positions are also reduced smoothly (entries 2 and 3), whereas
blocking the 4-position by a group as small as methyl stops the
reaction completely (entry 4). To our disappointment,
substitution in the 2- and 6-positions also hampers the reaction
(entries 5 and 6), and heating to 70 °C results only in
chlorination of the silane by the solvent. Although we did
observe 50% reduction of the 2-bromopyridine into the N-silyl
dihydropyridine (entry 7), this reaction most likely goes via
initial conversion of the substrate into pyridine followed by 1,4-
reduction. We, however, found chemoselective reduction of
quinoline (entry 8), although this reaction was compromised by
a moderate yield and loss of regioselectivity, as a mixture of 1,4-
and 1,2-reduction products was observed. As is typical for many
dihydropyridines,6 N-silyl dihydropyridines from Table 1 are air
and moisture sensitive.
more reduced tetrahydropyridines.29 Stoichiometric mechanis-
tic studies suggested the 1,2-insertion of pyridine across the
Ti−H bond as the first step. Crabtree et al. reported that Rh-
catalyzed hydrosilylation of quinolines also leads to mixtures of
dihydro- and tetrahydroquinolines.13a The best regioselectivity
was observed in the reduction of 2-methylquinoline by
phenylsilane,30 which gives the 97:3 preference of the 1,2-
dihydroquinoline over the tetrahydro product. To the best of
our knowledge, the 1,4-regioselectivity of reduction has been so
far achieved only in the case of hydrosilylation of 2-
phenylquinoline catalyzed by B(C6F5)3.18 Here we report Ru-
catalyzed chemoselective reduction of pyridines, quinolines,
and some related N-heterocycles to their respective dihydro
derivatives which is also 1,4-regiospecific.31
RESULTS AND DISCUSSION
■
Hydrosilylation of Pyridines. Our initial studies were
inspired by the recent discovery that the cationic complex
[Cp(iPr3P)Ru(NCCH3)2]+ (1) catalyzes chemoselective re-
duction of nitriles to N-silyl imines32 and the understanding
that this reaction should proceed by an ionic hydrosilylation
mechanism.33,34 Anticipating a similar reactivity pattern for
pyridines, we tested the catalyst 1 in the hydrosilylation of
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Scheme 1. Reactivity of the N-Silyl Dihydropyridine toward Ketones and Acid Chlorides
Unlike the hydrosilylation of nitriles by 1, which tolerates
such reactive functionalities as aldo, keto, and ester groups,
attempted hydrosilylation of functionalized pyridines afforded
mixtures of products.31a However, by carrying out competition
reactions with carbonyl compounds, we discovered that at least
in the case of aldo- and keto-substituted substrates this lack of
chemoselectivity can be due to secondary reactions of
dihydropyridines with carbonyls, not the loss of chemo-
selectivity of the reduction itself. Thus, we found that under
these catalytic conditions the N-silyl dihydropyridine adds
across CO bonds, so that coupling with ketones affords the
silyloxy amines 2 and reactions with acid chlorides35 lead
cleanly to the secondary amides 3 due to facile elimination of
chlorosilane (Scheme 1).36 In fact, the chemoselectivity of Ru-
catalyzed hydrosilylation of pyridine is so high that the reaction
can be carried in acetone as a solvent to give the coupling
product PhMe2SiO−CMe2−NC5H6 (4) in quantitative yield!
This fact is remarkable, because pyridines are usually
considered to be much more inert substrates in hydrosilylation
than carbonyls.37 However, the N-silyl dihydropyridine does
not react with esters and imines.
Another interesting observation is that dissolution of 4 in
hexane results in clean decomposition of 4 into the parent 1,4-
dihydropyridine and O-silyl enolate (eq 1). To the best of our
Figure 1. Ionic hydrosilylation mechanism for catalytic hydrosilylation
of pyridines.
conditions. Initial rate analysis was performed to minimize
the effect of possible catalyst deactivation. When the reaction
was carried out with an 8−12-fold excess of pyridine, first-order
kinetics in silane concentration was observed (Figure 2).
knowledge, alternative routes to dihydropyridine have not yet
been reported. Unfortunately, numerous attempts to isolate
pure dihydropyridine by distillation or chromatography failed.38
Mechanistic Considerations. Our preliminary mechanis-
tic studies suggested an ionic hydrosilylation mechanism,33,39
which starts with nitrile dissociation from 1 to give the reactive
intermediate 5 (Figure 1), followed by silane coordination to
give the σ complex 6. These two steps have been previously
studied for the related hydrosilylation of carbonyls.34 The
substrate then abstracts the silylium ion from 6. This
nucleophilic abstraction process has two major consequences:
(i) the substrate is activated for an electrophilic attack in the
form of N-silyl pyridinium salt 7 and (ii) a neutral Ru(II)
hydride complex 8 is formed which has increased hydridic
character. The hydride transfer from 8 to the sterically more
accessible 4-position of 7 leads to the product and regenerates
the catalyst 5. Further supporting this scheme were the
observations that (i) the reaction of the hydride Cp(iPr3P)Ru-
(py)H with the pyridinium cation [(Et3Si)(Py)]+ gives a 1,4-
dihydropyridine derivative, (ii) the deuterated silane DSiMe2Ph
in the 1-catalyzed hydrosilylation of pyridine delivers the
deuterium exclusively to the 4-position of the product, (iii) the
catalytic hydrosilylation is reversible, and (iv) a Lewis acid,
B(C6F5)3, abstracts the hydride from the 4-position of the N-
silyl dihydropyridine.31
Figure 2. Dependence of −ln([silane]/[silane]0) on time with a 12-
fold excess of pyridine.
Moreover, the dependence of the keff on the concentration of
pyridine shows saturation behavior (Figure 3). Assuming the
steady-state approximation for reactive species shown in Figure
1, the following kinetic law can be derived:40
k1k2[HSiMe2Ph][pyridine]
rate =
To shed more light on this mechanistic proposal, we
performed kinetic studies at 21 °C under the catalytic
k−1k−2 + k−1k3[pyridine]
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phosphine was found to be the best in the case of catalyst 1.
However, this phosphine fails in the case of a Cp* complex
(entry 4). Our rationalization of this fact is that increased steric
hindrance in the complex [Cp*(iPr3P)Ru(CH3CN)2]+ ham-
pers silane coordination and activation.
Hydrogenation and Hydrosilylation of Phenanthro-
line. Attempted hydrosilylation of phenanthroline by complex
1 failed because of catalyst deactivation by means of formation
of the stable chelate complex [Cp(iPr3P)Ru(phen)]+. Because
our mechanistic rationalization of this catalytic process requires
the availability of only one coordination site on the metal (vide
supra), we also studied catalysis by the phenanthroline complex
[Cp(phen)Ru(NCCH3)]+ (10). Attempted hydrosilylation of
phenanthroline by 10 and by [CpRu(NCCH3)3]+ resulted in a
very sluggish reaction.42 However, we found that phenanthro-
line can be reduced to a mixture of 1,2- and 1,4-
dihydrophenanthroline (in a 1:4.5 ratio) by using a slight
excess (4−5 equiv) of silane and water or alcohol as the proton
source. No N−C coupling products akin to 4 were observed
when the reaction was carried out in acetone. The reaction is
accompanied by intense hydrolysis (or alcoholysis in the case of
alcohol) of silane and evolution of hydrogen gas. However,
control experiments with hydrogen showed that in the absence
of silane the hydrogenation does not take place.
Our rationalization of this reduction process is that for steric
reasons the phenanthroline cannot abstract the silylium ion
[SiMe2Ph]+ from the proposed intermediate [Cp(phen)Ru(η1-
HSiMe2Ph)]+, whereas a smaller nucleophile, such as water or
alcohols, can. The deprotonation of the species [RO(H)−
SiMe2Ph] + by phenanthroline affords the activated form of the
substrate, the cation [phenH]+. The abstraction of silylium ion
in the first step also generates the neutral hydride complex
Cp(phen)RuH, which is able to reduce [phenH]+ in a way
similar to that shown in Figure 1.
With the bulkier catalyst 9 we found that the reduction can
proceed more regioselectively to furnish the 1,4-dihydrophe-
nanthroline in quantitative yield (by NMR). Even more
surprisingly, complex 9 catalyzes regioselective 1,4-hydro-
silylation of phenanthroline already at room temperature,
although a longer reaction time is required (Scheme 2). This
difference in reactivity between 9 and 10 may reflect the
difference in complexation of silane to ruthenium and hence the
different accessibility of the silylium ion.43 An interesting
feature of this process is that addition of the product, N-silyl
dihydrophenanthroline, to the solvent (acetone) does not take
place, presumably for steric reasons. Unfortunately, 2-
substituted phenanthrolines do not enter this hydrosilylation
reaction. Attempted reduction of 2-substituted phenanthrolines
by 3 equiv of the silane/alcohol mixture in the presence of a
catalytic amount of 9 (5%) also failed. There was also no
Figure 3. Dependence of keff on the amount of pyridine.
which agrees well with our kinetic studies.
The proposed ionic hydrosilylation mechanism leading to the
1,4-regioselectivity is different from the conventional inner-
sphere mechanism of Ir-catalyzed hydrogenation, which
involves 1,2-insertion of a N-coordinated substrate into the
Ir−H bond. Ionic hydrogenation, another example of the outer-
sphere mechanism,41 has been previously suggested for the
hydrogenation of quinolines.19a,24 In particular, calculations by
Eisenstein et al. showed that hydrogenation of quinoline gives
the 1,4-dihydroquinoline as the first kinetic product, but further
hydrogenation of this species to give the tetrahydroquinoline is
thermodynamically favorable and has reaction barriers com-
parable with the first hydrogenation step.24 For this reason, the
hydrogenation cannot stop at the dihydroquinoline product. In
the case of the reduction process shown in Figure 1, selective
monoreduction is achieved because the product, N-silyl
dihydropyridine, is a much worse nucleophile than the parent
pyridine. The transfer of a silylium ion to the CC bond of
dihydropyridine does not take place either, so that the over-
reduction does not happen.
Catalyst Optimization. We also briefly screened a series of
phosphine- and phenanthroline-supported half-sandwich com-
plexes of Ru in the catalytic hydrosilylation of pyridine. A
significantly decreased activity in comparison with that of the
catalyst 1 was observed (Table 2). Our previous studies on the
hydrosilylation of nitriles catalyzed by half-sandwich complexes
of Ru showed that the reaction is very sensitive to the nature of
both the silane and the supporting phosphine complex,32 so
that success in hydrosilylation is provided by an interplay of
steric and electronic factors. In the case of an unsubstituted Cp
ligand, as in complex 1, a basic and bulky phosphine ligand is
required, most likely because it (i) provides the η1 coordination
of silane enabling the silylium ion transfer39 and (ii) makes the
hydride intermediate CpL(CH3CN)RuH more “hydridic”,
which is a prerequisite for the reduction step. Triisopropyl-
a
Table 2. Hydrosilylation of Pyridine Catalyzed by Half-Sandwich Ru Complexes
a
b
Reaction condition: catalyst load 5 mol %, pyridine (0.06 mmol), CD2Cl2 (0.5 mL), room temperature. 70 °C
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Scheme 2. Hydrosilylation of Phenanthroline by Complex 9 (5%)
hydrogenation of 2,6- and 2,4-lutidines and 2-ethylpyridine by
the same reducing mixture.
newly formed nonconjugated benzene rings. The reaction with
1,3,5-triazine generated the silylated product in a relatively
short time (Table 3, entry 4). As for the catalyst 1, the
hydrosilylation of triazine is quite easy. The reduction can be
achieved under solvent-free conditions on the millimole scale,
but the product tends to decompose during the isolation. To
our surprise, the attempted hydrosilylation of pyrazine under
the same conditions failed. An NMR study revealed the
formation of the stable complex Cp*(phen)Ru(κ1-N-
(CHCH)2N) (11). We believe that this difference in reactivity
stems from the lack of a carbon atom in the 4-position and
more sluggish 1,2-reduction in comparison with the 1,4-
reduction. No reactivity was observed for other pyridine
derivatives, such as 2-ethylpyridine, 2,6-lutidine, 2,4-lutidine,
and pyrimidine. With these substrates the only reaction
observed is the slow formation of ClSiMe2Ph upon the
reaction of HSiMe2Ph with the solvent (CH2Cl2). Substituted
quinolines, such as 2-phenylquinoline, 7-chloro-2-methylquino-
line, 7-methyl-8-nitroquinoline, and 2-methylquinoxaline, do
not enter this reaction either.
Hydrosilylation of Quinoline and Related Hetero-
cycles. As mentioned above, the hydrosilylation of quinoline
by complex 1 is sluggish and results in the loss of
regioselectivity. Given the fact that complex 9 catalyzes the
hydrosilylation of such a challenging substrate as phenanthro-
line, we screened its activity toward other N-heterocycles of
biological importance. To our delight, the hydrosilylation of
quinoline was achieved under solvent-free conditions and with
reduced catalyst loading (Table 3, entry 1). Although a mixture
Table 3. Hydrosilylation of N-Heterocycles Catalyzed by
a
Complex 9
Reduction of N-Heterocycles by a Silane/Alcohol
Mixture. Similar to the 9-catalyzed reduction of 1,10-
phenanthroline with excess HSiMe2Ph/ethanol discussed
above, catalytic hydrogenation of acridine can be achieved in
quantitative yield under the same conditions. However, all
attempts to reduce quinoline led to a mixture of products. The
reduction of 1,3,5-triazine by the silane/alcohol mixture did not
lead to a hydrogenation product either.
CONCLUSION
■
The complex [Cp(iPr3P)Ru(CH3CN)2]+[B(C6F5)4]− catalyzes
regioselective hydrosilylation of pyridines to N-silyl 1,4-
dihydropyridines. Substitution in the 3- and 5-positions is
tolerated, whereas substitution in the 2-, 6-, and 4-positions
stops the reaction. Sensitive functionalities (cyano, aldo, keto,
and ester groups) are not tolerated. In the case of cyano-
substituted pyridines, the nitrile moiety is hydrosilylated
preferably. The N-silyl 1,4-dihydropyridine cleanly adds across
the CO bond of aldehydes and ketones under standard
catalytic conditions. The compound C6H6N−CMe2−OSi-
Me2Ph decomposes in hexane to give the parent 1,4-
dihydropyridine and the silylated enol, but separation of
these two products proved difficult. The complexes [CpRu-
(CH3CN)3]+[B(C6F5)4]− and [Cp(phen)Ru(CH3CN)]+[B-
(C6F5)4]− catalyze reduction of phenanthroline into a mixture
of 1,4- and 1,2-dihydrophenanthroline by excess silane/water
and silane/alcohol mixtures. The complex [Cp*(phen)Ru-
(CH3CN)]+[B(C6F5)4]− catalyzes the regioselective hydro-
silylation of ring-fused N-heteroaromatics, such as quinoline,
isoquinoline, acridine, etc. However, in the case of 2-substituted
quinolines the reaction does not occur.
a
Reaction conditions: Ru complex (5 mol %), substrate (0.05−0.1
b
c
mmol), CD2Cl2 (0.5 mL), 70 °C. Room temperature. 0.7 mol % of
Ru catalyst, 2 mmol of quinoline. 1 mmol of isoquinoline. Isolated
yield.
d
e
of 1,2- and 1,4-addition products was still generated, very high
regioselectivity toward the 1,4-hydrosilylation was observed
(3:97). Because the polarity of the media changes during the
reaction,33 the product can be easily isolated by removing the
cationic catalyst by simple filtration and evaporation of volatiles,
leading to the high isolated yield (98%). For comparison, the
hydrogenation of quinolines leads to tetrahydroquinolines.19
Isoquinoline can be hydrosilylated equally well with excellent
selectivity to give the N-silyl 1,2-dihydroisoquinoline (entry
2),44 although this reaction requires a much longer time. We
attribute the sluggishness of the latter process to the fact that
the more activated 8-position is blocked. Complex 9 can
hydrosilylate even sterically hindered substrates such as acridine
(entry 3). Full conversion to the corresponding hydrosilylated
product was achieved after 24 h at 70 °C in CD2Cl2. Obviously,
the reaction is driven by the increased aromaticity of the two
E
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7.3 Hz, 1, NCHCHCH), 7.62 (m, 2, SiMe2Ph(o)), 7.29 (m, 1,
NCHCHCH), 7.22 (m, 3, SiMe2Ph(m, p)), 7.10 (d, J(H-H) = 7.9 Hz,
1, CHCH), 6.93 (d, J(H-H) = 7.9 Hz, 1, CHCH), 6.55 (d, J(H-H) =
7.6 Hz, 1, Si-NCHCHCH2), 4.82 (m, 1, Si-NCHCHCH2), 3.66 (m, 2,
Si-NCHCHCH2), 0.54 (s, 6, SiMe2Ph).
Hydrogenation of 1,10-Phenanthroline by a Silane/ROH
Mixture. Method 1. To a solution of 1,10-phenanthroline (0.047 g,
0.26 mmol), HSiMe2Ph (0.4 mL, 2.6 mmol), and H2O (50 μL, 2.6
mmol) in acetone-d6 (0.6 mL) was added [CpRu(CH3CN)3]PF6
(0.006 g, 5% mol). The reaction mixture turned brown, and gas
evolution was observed. After 24 h at ambient temperature, 96% of
1,10-phenanthroline was converted into a mixture of 1,2- and 1,4-
dihydrophenanthrolines (1:4.5). All silane was converted into a
mixture of HOSiMe2Ph and O(SiMe2Ph)2.
Method 2. To a solution of 1,10-phenanthroline (0.050 g, 0.28
mmol), HSiMe2Ph (0.2 mL, 1.4 mmol), and EtOH (0.08 mL, 1.4
mmol) in acetone-d6 (0.6 mL) was added [CpRu(CH3CN)3]PF6
(0.006 g, 5% mol). The reaction mixture turned brown, and gas
evolution was observed. After 2 h at ambient temperature, 99% of
1,10-phenanthroline was converted into a mixture of 1,2- and 1,4-
dihydrophenanthrolines (1:4.5).
Method 3. To a solution of 1,10-phenanthroline (0.30 g, 1.66
mmol), HSiEt3 (2.67 mL, 16.6 mmol), and EtOH (0.96 mL, 16.6
mmol) in acetone (20 mL) was added [CpRu(CH3CN)3]PF6 (0.036
g, 5 mol %). The reaction mixture turned brown, and gas evolution
was observed. After 1 h at ambient temperature, 100% of 1,10-
phenanthroline was converted into a mixture of 1,2- and 1,4-
dihydrophenanthrolines (1:4.5). Hexane (80 mL) was then added,
and the volume of the solution was reduced to 30 mL by solvent
evaporation. The resulting mixture, a yellow solution and a brown
precipitate, was filtered, and the products were extracted with Et2O (3
× 20 mL). The mixture of 1,2- and 1,4-dihydrophenanthrolines was
obtained as a yellow solid after removal of volatiles. Yield: 0.27 g
(90%).
EXPERIMENTAL DETAILS
■
All manipulations were carried out using conventional high-vacuum or
nitrogen-line Schlenk techniques. NMR spectra were recorded on a
Bruker DPX-300 (1H, 300 MHz; 13C, 75.4 MHz) and/or Bruker DPX-
600 (1H, 600 MHz; 13C, 150.8 MHz) spectrometers at 295 K. All
chemicals were purchased from Sigma-Aldrich and Alfa Aesar, apart
from HSiMe2Ph, which was purchased from Gelest. These reagents
were used without further purification. Acetone-d6 and CD2Cl2 were
purchased from Cambridge Isotope Laboratories. CD2Cl2 was dried
́
over CaH2, and acetone-d6 was dried over molecular sieves (3 Å).
Other solvents were dried by distillation from appropriate drying
agents or using a Grubbs-type solvent purification system. The
complexes [CpRu(CH3CN)3]PF6,45 [Cp*Ru(CH3CN)3]PF6,46 [Cp-
(iPr3P)Ru(CH3CN)2]An (1; An = PF6 , B(C6F5)4 ),47 and [Cp(1,10-
−
−
32
phenanthroline)Ru(CH3CN)]PF6 were prepared according to
literature procedures.
Preparation of [Cp*(1,10-phenanthroline)Ru(CH3CN)]PF6 (9-
PF6). To a solution of [Cp*Ru(CH3CN)3]PF6 (0.30 g, 0.6 mmol) in
acetonitrile (60 mL) was added 1,10-phenanthroline (0.11 g, 0.6
mmol). The reaction mixture was stirred for 12 h at room
temperature. The solvent was evaporated and the residue was washed
with hexane (3 × 20 mL). The yellow solid was dried under vacuum.
1
Yield: 0.34 g (94%). H NMR (acetone-d6): δ 9.68 (m, 2, o-phen),
8.72 (m, 2, 1, p-phen), 8.23 (m, 2, phen), 8.13 (m, 2, m-phen), 2.13 (s,
3, CH3CN), 1.70 (s, 15, Cp*). 31P NMR (acetone-d6): δ −144.2
1
(septet, PF6). H−13C HSQC (acetone-d6): δ 209.0 (CH3CN), 152.5
(o-phen), 135.4 (p-phen), 127.2 (phen), 125.6 (m-phen), 8.7 (Cp),
2.3 (CH3CN). IR (neat): ν (CH3CN) 2339 cm−1. Mp: 217 °C dec.
Anal. Calcd for C24H26F6N3PRu: C, 47.84; H, 4.35; N, 6.97. Found: C,
47.20; H, 4.19; N, 6.72.
Preparation of [Cp*(1,10-phenanthroline)Ru(CH3CN)]BAF
(9-BAF). To a solution of 9-PF6 (0.15 g, 0.25 mmol) in CH2Cl2
was added LiBAF·Et2O (0.19 g, 0.25 mmol). The solution was stirred
for 2 h at room temperature. The solvent was evaporated under
vacuum, and the complex 9-BAF was extracted with ether (2 × 40
mL). The product was obtained as a bright yellow powder after
removal of ether under vacuum. Yield: 0.27 g (95%). 1H NMR
(acetone-d6): δ 9.68 (d, 2, 1, o-phen), 8.73 (d, 2, 1, p-phen), 8.24 (s, 2,
Method 4. To a solution of HSiMe2Ph (15.4 μL, 0.1 mmol), 1,10-
phenanthroline (18.0 mg, 0.1 mmol), and EtOH (4.6 μL, 0.3 mmol) in
acetone-d6 was added [Cp*(phen)Ru(CH3CN)]PF6 (3 mg, 0.005
mmol). The reaction was periodically monitored by NMR spectros-
copy. Full conversion was achieved after 20 min at room temperature.
1,4-Dihydrophenanthroline and CH3CH2OSiMe2Ph were obtained.
phen), 8.13 (m, 2, m-phen), 2.15 (s, 3, CH3CN), 1.70 (s, 15, Cp*), 11
B
NMR (acetone-d6): δ −16.62 (s, BAF). 1H−13C HSQC (acetone-d6):
δ 152.4 (o-phen), 135.5 (p-phen), 127.3 (phen), 125.4 (m-phen), 8.5
(Cp), 2.0 (CH3CN). IR (neat): ν (CH3CN) 2337 cm−1. Mp: 223 °C
dec. Anal. Calcd for C48H26BF20N3PRu: C, 50.72; H, 2.31; N, 3.70.
Found: C, 49.76; H, 2.34; N, 2.84.
1
1,4-Dihydrophenanthroline. H NMR (acetone-d6): δ 8.72 (dd,
J(H−H) = 4.1 and 1.6 Hz, 1, NCHCHCH), 8.15 (dd, J(H−H) = 8.1
and 1.6 Hz, 1, NCHCHCH), 7.40 (m, 1, NCHCHCH), 7.28 (d, J(H−
H) = 8.5 Hz, 1, CHCH), 7.13 (d, J(H−H) = 8.5 Hz, 1, CHCH), 6.42
(m, 1, NCHCHCH2), 4.52 (m, 1, NCHCHCH2), 3.75 (m, 2,
NCHCHCH2). 1H−13C HSQC (acetone-d6): δ 147.7 (NCHCHCH),
135.6 (NCHCHCH), 128.3 (CHCH), 127.2 (NCHCHCH2), 120.7
(NCHCHCH), 117.8 (CHCH), 95.3 (NCHCHCH2), 27.0
(NCHCHCH2). IR (Nujol): ν 3432 cm−1 (N−H).
Hydrosilylation of Pyridine. Method 1. To a solution of pyridine
(1.00 mL, 12.3 mmol) and HSiMe2Ph (2.00 mL, 13.0 mmol) in
CH2Cl2 (20 mL) was added 1-PF6 (0.20 g, 3% mol). The resulting
solution was stirred for 6 h at ambient temperature. All the volatiles
were removed under vacuum, and the resulting yellow oil was distilled
under reduced pressure. Yield: 1.97 g (74%).
Method 2 (Reaction without Solvent). To a mixture of pyridine
(1.00 mL, 12.3 mmol) and HSiMe2Ph (1.9 mL, 12.3 mmol) was added
1-BAF (0.03 g, 0.2 mol %). The resulting mixture turned slowly from
red to yellow. There was 96% conversion of pyridine after 30 min at
ambient temperature with a selective formation of N-silyl 1,4-
dihydropyridine.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text and figures giving further experimental details on catalytic
hydrosilylation. This material is available free of charge via the
Internet at http://pubs.acs.org.
N-(SiMe2Ph)-1,4-dihydropyridine. 1H NMR (CDCl3): δ 7.55 (m, 2,
Ph), 7.39 (m, 3, Ph), 5.89 (d, J(H−H) = 8.3 Hz, 2, NCHCH), 4.45
(dt, J(H−H) = 8.3 and 3.2 Hz, 2, NCHCHCH2), 2.95 (m, 2, CH2),
0.42 (s, 6, SiMe). 13C NMR (CDCl3): 136.5, 133.8, 129.8, 128.7,
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for G.I.N.: gnikonov@brocku.ca.
1
128.1, 100.1, 22.5, −2.4. H−29Si HSQC (CDCl3): δ 1.1.
Hydrosilylation of 1,10-Phenanthroline. To a solution of
HSiMe2Ph (15.4 μL, 0.1 mmol) and 1,10-phenanthroline (18.0 mg,
0.1 mmol) in acetone-d6 was added 9-PF6 (3 mg, 0.005 mmol). The
reaction was periodically monitored by NMR spectroscopy. After 20 h
at room temperature 70% of the starting 1,10-phenanthroline was
converted into N-silyl 1,4-dihydrophenanthroline (further conversion
was not observed due to reversibility of the reaction).
Present Address
†Chemistry Department, University of Calgary, 2500 University
Drive NW, Calgary, Alberta T2N 1N4, Canada.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. S.-H.L. and D.V.G. contributed equally.
1
N-Silyl-1,4-dihydrophenanthroline. H NMR (acetone-d6): δ 8.64
(dd, J(H-H) = 3.8 Hz and 1.1 Hz, 1, NCHCHCH), 8.12 (d, J(H-H) =
F
dx.doi.org/10.1021/om400269q | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(16) For an example of reduction of the benzene rings of acridine,
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the NSERC (DG grant to
G.I.N.) and OGS (graduate student fellowship to D.V.G.). We
thank the CFI/OIT for a generous equipment grant.
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dx.doi.org/10.1021/om400269q | Organometallics XXXX, XXX, XXX−XXX