Base-promoted, deborylative secondary alkylation of N-heteroaromatic: N -oxides with internal gem -bis[(pinacolato)boryl]alkanes: A facile derivatization of 2,2′-bipyridyl analogues

Article abstract of DOI:10.1039/c7cc03731g

A base-promoted, secondary alkylation of N-heteroaromatic N-oxides using internal gem-bis[(pinacolato)boryl]alkanes as alkylation reagents is reported. The reaction exhibits a broad scope, providing deoxygenated secondary alkylated N-heteroaromatic compounds with high efficiency. The usefulness of the developed protocol is evidenced by the sequential direct alkylation of 2,2′-bipyridine-N-oxide.

Full text of DOI:10.1039/c7cc03731g

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DOI: 10.1039/C7CC03731G
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Base-Promoted, Deborylative Secondary Alkylation of N-
Heteroaromatic N-Oxides with Internal gem-
Bis[(pinacolato)boryl]alkanes: A Facile Derivatization of 2,2′-
Bipyridyl Analogues
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Chiwon Hwang, Woohyun Jo, and Seung Hwan Cho*
www.rsc.org/
A base-promoted, secondary alkylation of N-heteroaromatic N-
oxides using internal gem-bis[(pinacolato)boryl]alkanes as
alkylation reagents is reported. The reaction exhibits broad scope,
providing deoxygenated secondary alkylated N-heteroaromatic
compounds with high efficiency. The usefulness of the developed
protocol is evidenced by sequential direct alkylation of 2,2′-
bipyridine-N-oxide.
Alkylated N-heteroaromatic compounds constitute important
core structures in numerous pharmaceuticals, natural products
and ligands.1 Over the last few decades, several direct
alkylations of electron-deficient N-heteroarenes such as
pyridines and quinolines have been extensively studied.2
However, most approaches focus primarily on the introduction
of primary alkyl groups,2,3 and limited examples of the
secondary alkylation of electron-deficient N-heteroarenes have
been studied.4 For instance, Cu-,4a-c Pd-,4d,e and lanthanide-
4f,gcatalyzed C2-selective secondary alkylation reactions of
pyridine N-oxides or pyridines via C-H activation strategies were
recently reported (Fig. 1a). The radical-mediated secondary
alkylation of pyridines or quinolines using stoichiometric
amounts of oxidants or photocatalysts has also been developed
(Fig. 1b).5 Despite these advances, the development of more
practical methods without the use of metal catalysts or
expensive oxidants is desirable. In this context, nucleophilic
aromatic substitutions of N-heteroarenes or N-oxides with
alkyllithium6 or Grignard reagents7 were reported, but the
reactions showed limited scope and were accompanied by a
two-step sequence: the addition of organometallics to N-
heteroaromatic N-oxides and subsequent re-aromatization in
the presence of oxidant. Herein, we report the base-promoted,
transition-metal-free, regioselective secondary alkylation
Fig.
1 Approaches for Direct Secondary Alkylation of N-Heteroaromatic
Compounds at C2 Position
reaction of N-heteroaromatic N-oxides with internal gem-
bis[(pinacolato)boryl]alkanes without employing an additional
oxidant (Fig. 1c). This straightforward method enables the
formation of an array of secondary alkylated N-heteroaromatic
compounds from easily accessible starting materials with high
efficiency. Moreover, a facile preparation method of 2,2ʹ-
bipyridyl analogues is also demonstrated.
Recently, our group reported transition-metal-free, primary
alkylation of N-heteroaromatic N-oxides using gem-
bis[(pinacolato)boryl]alkanes as alkylating reagents (R2 = alkyl
or H, R3 = H, Fig. 1c). The reaction provided the deoxygenated
C2- alkylated N-heteroaromatic compounds with excellent
regioselectivity.8 On the basis of several mechanistic studies, we
have shown that an in-situ generated a-boryl carbanion from a
reaction between gem-bis[(pinacolato)boryl]alkanes and an
alkoxide base9 could attack the electrophilic C2 position of N-
heteroaromatic N-oxide,2c,10 followed by elimination and re-
aromatization delivered the C2-alkylated N-heteroaromatic
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Table 1. Optimization of reaction conditions
Table 2. Scope of N-Heteroaromatic N-Oxides a,b
View Article Online
DOI: 10.1039/C7CC03731G
Entry
1
2
3
4
5
6
7
8
Base
NaOMe
NaOMe
NaOMe
LiOMe
KOMe
NaOtBu
LiOtBu
KOtBu
CsF
Solvent
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
1,4-dioxane
THF
Temp (oC)
80
Yield (%)b
<5
65
100
120
120
120
120
120
120
120
97(95)c
<1
7
62
9
62
<1
83
93
<1
9
10
11
12
NaOMe
NaOMe
-
120
120
120
toluene
a Reaction conditions: 1a (0.1 mmol), 2a (2 equiv), base (3 equiv) and solvent (1 mL)
b
at the indicated temperature for
3
h.
1H-NMR yield using 1,1,2,2-
a Reaction conditions: 1 (0.2 mmol), 2 (2 equiv), NaOMe (3 equiv) and toluene (1
mL) at 120 oC for 3 h. b Isolated yields are given. c 3.0 Equivalents of 2a was used.
tetrachloroethane as an internal standard. c Isolated yield is given in parenthesis.
pin = pinacolato.
selectivity at the less-bulky site (3l), probably owing to steric
hindrance between the phenyl and isopropyl groups, albeit with
a low yield. Qunoxaline N-oxide was proved to be an effective
substrate under the standard reaction conditions, giving the
desired product 3m in 32% yield. The reaction was also applied
to the late-stage modification of a complex N-heteroaromatic
N-oxide.12 When 9-O-methylquinine N-oxide and 2a were
subjected to the standard reaction conditions in the presence
of 3 equivalents of 2a, the corresponding secondary alkylated
products 3n were obtained with 79% yield, demonstrating the
synthetic usefulness of the developed protocol.
compounds. On the basis of these observations, we envisioned
that a secondary alkylation protocol for N-heteroaromatic N-
oxides could be developed by employing internal gem-
bis[(pinacolato)boryl]alkanes as alkylation sources.
Our investigation was initiated with benzo[h]quinoline-N-
oxide (1a) and 2,2ʹ-bis[(pinacolato)boryl]propane (2a) as model
substrates in the presence of 3 equiv. of NaOMe in toluene at
80 °C (Table 1, entry 1).11 Unfortunately, the reaction was not
effective, and only trace amount of the desired product (3a) was
formed. Further optimization studies revealed that the reaction
was sensitive to temperature. We found that elevating the
reaction temperature from 80 °C to 100 °C improved the yield
(65%, entry 2), and the reaction when performed at 120 °C
exhibited the highest yield (97%) of 3a (entry 3). Screening of
other bases showed moderate to poor reactivity (entries 4–9).
The use of ethereal solvents such as 1,4-dioxane and THF
afforded 3a with slightly lower efficiency compared with
toluene (entries 10 and 11). The desired product (3a) was not
formed in the absence of NaOMe (entry 12).
We next surveyed the scope of internal gem-
bis[(pinacolato)boryl]alkanes with a range of N-heteroaromatic
Table 3. Scope of Internal gem-Bis[(pinacolato)boryl]alkanes a,b
Under the optimized reaction conditions, we explored the
scope of N-heteroaromatic N-oxides by employing 2a as an
alkylating reagent (Table 2). The secondary alkylation occurred
smoothly with electron neutral (3b) and donating quinoline (3c
)
N-oxides with moderate-to-good yields. The reaction could be
extended to pyridine derivatives. We found that 2-arylpyridine
N-oxides with 2a proceeded smoothly, providing the
corresponding products 3d–3g with good-to-excellent yields.
The reactions of phenyl-containing pyridine N-oxide at the C4
position delivered the secondary alkylated product (3h) with 48%
yield. The reaction conditions were compatible with pyridine N-
oxides bearing amide, protected alcohol, and masked aldehyde
at the C4 or C2 positions, which generated the corresponding
a Reaction conditions: 1 (0.2 mmol), 2 (2 equiv), NaOMe (3 equiv) and toluene (1
mL) at 120 oC for 3 h. b Isolated yields are given. c 3.0 Equivalents of 2 was used.
products 3i–3k in moderate yields. Notably, 3-phenyl pyridine-
N- oxide underwent secondary alkylation with excellent site-
2 | J. Name., 2012, 00, 1-3
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Table 4. Scope of 2,2′-Bipyridine and 2,2′:6′,2″-Terpyridine N-oxidesa,b
View Article Online
DOI: 10.1039/C7CC03731G
Scheme 1. Sequential direct alkylation of 2,2'-bipyridine N-oxide.
To illustrate the synthetic utility of the developed process, we
tried sequential functionalization of 5a (Scheme 1). The reaction
of 5a with 2,2ʹ-bis[(pinacolato)boryl]phenylpropane or 2,2ʹ-
bis[(pinacolato)boryl]propane was easily run on a 5 mmol scale,
affording the products 6e and 6j with 92% yield and 71%,
respectively. The selective oxidation of 6e and 6j to N-oxide at
the non-alkylated pyridine ring with m-chloroperoxybenzoic
acid in CH2Cl2 gave the corresponding N-oxides 7a and 7b in
good yields. Subjection of 7a and 7b under the direct
methylation conditions with bis[(pinacolato)boryl]methane at
80 °C in toluene,8a the corresponding di-alkylated 2,2ʹ-
bipyridine 8a and 8b in 75% and 79% yield, respectively. These
results highlight the potential of the developed procedure to
derivatize 2,2ʹ-bipyridyl ligands.
aReaction conditions: 5 (0.2 mmol), 2 (2 equiv), NaOMe (3 equiv), and toluene (1
mL) at 120 oC for 3 h. bIsolated yields are given.
N-oxides under the standard conditions (Table 3).
Benzo[h]quinoline-N-oxide (1a) efficiently reacted with a range
of internal gem-bis[(pinacolato)boryl]alkanes to form the
corresponding
secondary
alkylated
benzo[h]quinoline
compounds 4a
–4e with excellent yields. The compound 6-
methoxy quinoline readily participated in the reaction with 2,2ʹ-
bis[(pinacolato)boryl]butane, giving 4f with 84% yield. The
reactions of 2-phenylpyridine N-oxide were highly
regioselective and afforded a good yield of 4g and 4h. The
simple pyridine N-oxide could also be used as a coupling partner,
furnishing a synthetically acceptable yield of 4i. Finally, 9-O-
methyl quinine N-oxide was converted to the product 4j in 86%
yield.
In summary, we have developed transition-metal-free,
regioselective, secondary alkylation of N-heteroaromatic N-
oxides using internal gem-bis[(pinacolato)boryl]alkanes as
alkylating reagents. Our process shows a broad scope for N-
heteroaromatic
N-oxides
and
internal
gem-
bis[(pinacolato)boryl]alkanes, providing an operationally simple
process to afford degoxygenated, secondary alkylated N-
heteroaromatic compounds. The power of this developed
method is demonstrated by a modification of 2,2ʹ-bipyridine
ligand through sequential direct alkylation procedures. Further
studies to expand the scope of N-heteroaromatic compounds
and to develop related transition-metal-free reactions are
ongoing in our laboratory.
Since 2,2ʹ-bipyridines have been widely used in the fields of
materials science,13 molecular recognition,14 and metal-
catalyzed organic transformations,15 their derivatizations are
highly desirable, but have rarely been reported.16 Classical
methods for the alkylation of 2,2ʹ-bipyridines involve the
reaction of lithiated 2,2ʹ-bipyridines with alkyl electrophiles,17
Pd-catalyzed cross-coupling of 6-halo-2,2ʹ-bipyridines with alkyl
nucleophiles18 or the addition of alkyllithium reagents to 2,2ʹ-
bipyridines.19
However, these methods required exotic
reagents and showed limited scope under harsh reaction
conditions. Therefore, we immediately investigated the
Acknowledgement
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Science, ICT & Future Planning
(NRF-2015R1C1A1A02036326).
secondary alkylation reaction of 2,2ʹ-bipyridine-N-oxide (5a
)
and their analogues under the standard reaction conditions
(Table 4). We found that 5a underwent the secondary alkylation
reaction with 2a in 63% yield. Moreover, 4,4ʹ-disubstituted-2,2ʹ-
bipyridine N-oxides (5b and 5c) and 2,2ʹ:6ʹ,2ʺ-terpyridine N-
oxide (5d) efficiently reacted with 2a, affording 6b–6d in good
yields. The reactions of 5a and 5d with various internal gem-
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