The Synthesis of ABNO+BF4? and its Application to Alcohol Oxidation

Full text of DOI:10.1002/bkcs.11458

Communication
DOI: 10.1002/bkcs.11458
BULLETIN OF THE
M. J. Kim and J. Kim
KOREAN CHEMICAL SOCIETY
The Synthesis of ABNO+BF4 and its Application to Alcohol Oxidation
−
*
Myeong Jin Kim and Jinho Kim
Department of Chemistry, and Research Institute of Basic Sciences, Incheon National University, Incheon
22012, Republic of Korea. *E-mail: jinho@inu.ac.kr
Received March 5, 2018, Accepted March 27, 2018
Keywords: ABNO, Bicyclic nitroxyl radical, Oxoammonium salt, Alcohol oxidation, Organic synthesis
Nitroxyl radicals such as 2,2,6,6,-tetramethylpiperidine-N-
oxyl (TEMPO) are widely used in organic synthesis.1 Espe-
cially, they have played a crucial role in alcohol oxidation.2
A variety of alcohol oxidations using catalytic amounts of
nitroxyl radicals have been developed by the virtue of sec-
ondary oxidants such as NaOCl, Br2, and PhI(OAc)2.3 In
these catalytic alcohol oxidations, oxoammonium salts,
which were afforded by one-electron oxidation of nitroxyl
radicals, are active oxidants, and the alcohols undergo two-
electron oxidation during two-electron reduction of oxoam-
monium salts to hydroxylamine species. The regeneration
of oxoammonium salts through the oxidation of the hydrox-
ylamines species by the oxidants completed the catalytic
cycle. Oxoammonium salt-mediated alcohol oxidations
using stoichiometric amounts of oxoammonium salts were
also developed to avoid the use of secondary oxidants.4
Recently, Iwabuchi group disclosed that bicyclic nitroxyl
radicals such as 9-azabicyclo[3,3,1]nonane N-oxyl (ABNO)
and 2-azaadamantane N-oxyl (AZADO) exhibit superior cat-
alytic activity to TEMPO and its derivatives due to less steric
encumbrance (Figure 1).5 In addition, they reported the prep-
aration of various bicyclic oxoammonium salts such as AZA-
DO+X− (X = BF4, PF6, ClO4, SbF6, Cl, and NO3) and
utilized these salts in the oxidative conversion of silyl enol
ether.6 To the best of our knowledge, however, no reports
described the synthesis and characterization of ABNO salt.
In this paper, we describe the preparation and characteriza-
caused precipitation of yellow solid which is presumed to be
ABNO+BF4 salt (Scheme 1). Pleasingly, simple filtration
−
allowed isolation of this putative ABNO+BF4− salt.
To characterize the ABNO+BF4 , a variety of analyses
−
were carried out. The measurement of 1H NMR in
CF3COOD revealed that ABNO+BF4 is a diamagnetic
−
compound with 14 protons.8 While the λmax value of ABNO
was 395 nm in UV–Visible spectroscopy, the synthesized
ABNO+BF4 showed a maximum absorbance at 437 nm
−
(Figure 2).9 It was observed that ABNO+BF4 has a lower
−
melting point (111–112ꢀC) than ABNO (129–130ꢀC).5b In
IR spectrum, a strong N O double bond absorption was
observed at 1624 cm−1, and the HRMS (FAB) result for the
ABNO+ ion was 140.1076. These results indicated that
ABNO+BF4− was successfully synthesized.
Next, we tried to utilize the synthesized ABNO+BF4 in
−
the secondary alcohol oxidation with 1-(4-methylphenyl)
ethanol 1a as a model substrate. The use of basic conditions
showed better reactivity than neutral or acidic conditions in
the ABNO+BF4− mediated oxidation of 1a (entries 1–3).
Among solvents screened, CH3CN gave superior results
to others (entries 3–5). In light of comproportionation of
ABNO+BF4 in basic conditions,10 the increase of
−
ABNO+BF4 amount (2.0 equiv) gave 1-(4-methylphenyl)
−
ethanone 2a in a high yield (entry 6). Interestingly, the use
of KHCO3 gave a full conversion of 1a with an excellent
yield compare to other inorganic bases (entries 7–10).
tion of ABNO+BF4 . The application of the prepared
−
ABNO+BF4− to alcohol oxidation is also disclosed.
In 1980, Nelson et al. tried to synthesize ABNO+BF4− and
ABNO+PF6 through the addition of NO+BF4 or NO+PF6
−
−
−
rt, 0.5h
to ABNO in dichloromethane solution, but, they could not
succeed in purifying oxoammonium salts.7 To overcome the
isolation issue, we tried to apply Iwabuchi’s process for
Scheme 1. The synthesis of ABNO+BF4− from ABNO.
AZADO+BF4 to the synthesis of ABNO+BF4 . The addi-
tion of HBF4, NaOCl, and NaBF4 in water solution to ABNO
−
− 6
(a)
(b)
1.0
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
max =437nm
max =395nm
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 1. Structures of TEMPO, ABNO, AZADO, and AZDO+X−.
Figure 2. UV–visible spectrum of ABNO (a) and ABNO+BF4− (b).
Bull. Korean Chem. Soc. 2018
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BULLETIN OF THE
KOREAN CHEMICAL SOCIETY
The optimized conditions (Table 1, entry 10) were then
tested with several secondary benzyl alcohols (Table 2). It
was observed that the oxidation of 1b and 1c produced the
corresponding ketones in excellent yields. However, benzyl
alcohol 1d with strongly electron withdrawing nitro group
gave only moderate product yield. The synthesis of
1-indanone 2e was achieved through the present oxidation
Table 1. Optimization of ABNO+BF4− mediated secondary
with 1-indanol 1e. The developed ABNO+BF4 mediated
−
alcohol oxidation.a
protocol readily oxidizes secondary aliphatic alcohol 1f as
well as primary benzyl alcohol 1g, but not primary aliphatic
alcohol 1h [sluggish reaction with poor conversion (15%)
and yield (13%)].
In summary, we reported that oxoammonium salt
ABNO+BF4 could be synthesized by the addition of
−
HBF4, NaOCl, and NaBF4 in water solution to ABNO. The
synthesized ABNO+BF4 was characterized by a variety of
−
Entry
Base
Solvent
Yield(%)b
1
analyses such as H, 13C, 19F NMR, UV–Visible spectros-
1
2
None
None
CH3CN
AcOH
20
20
43
30
28
73
96
76
77
99
copy, melting point, IR, and HRMS. It was observed that
the synthesized ABNO+BF4 efficiently oxidized several
−
3
4
5
6c
7c
8c
9c
10c
a
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
Cs2CO3
NaHCO3
KHCO3
CH3CN
DCE
secondary alcohols.
1,4-dioxane
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Acknowledgments. This research was supported by the
Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Minis-
try of Science, ICT
2015R1C1A1A02037185).
&
Future Planning (NRF-
Reaction conditions: 1a (0.3 mmol), ABNO+BF4− (1.2 equiv), base
(1.0 equiv), and solvent (0.6 mL) under N2 (balloon) at 25ꢀC.
Yields determined by 1H NMR.
Supporting Information. Additional supporting informa-
tion is available in the online version of this article.
b
c
ABNO+BF4− (2.0 equiv) was used.
References
Table 2. Substrate scope of ABNO+BF4−mediated alcohol
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a
Reaction conditions: 1 (0.3 mmol), ABNO+BF4− (2.0 equiv), KHCO3
(1.0 equiv), and CH3CN (0.6 mL) under N2 (balloon) at 25ꢀC.
b
Isolated yields.
Bull. Korean Chem. Soc. 2018
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