Efficient and Selective Cu/Nitroxyl-Catalyzed Methods for Aerobic Oxidative Lactonization of Diols

Article abstract of DOI:10.1021/jacs.5b01036

Cu/nitroxyl catalysts have been identified that promote highly efficient and selective aerobic oxidative lactonization of diols under mild reaction conditions using ambient air as the oxidant. The chemo- and regioselectivity of the reaction may be tuned by changing the identity of the nitroxyl cocatalyst. A Cu/ABNO catalyst system (ABNO = 9-azabicyclo[3.3.1]nonan-N-oxyl) shows excellent reactivity with symmetrical diols and hindered unsymmetrical diols, whereas a Cu/TEMPO catalyst system (TEMPO = 2,2,6,6-tetramethyl-1-piperidinyl-N-oxyl) displays excellent chemo- and regioselectivity for the oxidation of less hindered unsymmetrical diols. These catalyst systems are compatible with all classes of alcohols (benzylic, allylic, aliphatic), mediate efficient lactonization of 1,4-, 1,5-, and some 1,6-diols, and tolerate diverse functional groups, including alkenes, heterocycles, and other heteroatom-containing groups.

Full text of DOI:10.1021/jacs.5b01036

Communication
pubs.acs.org/JACS
Efficient and Selective Cu/Nitroxyl-Catalyzed Methods for Aerobic
Oxidative Lactonization of Diols
Xiaomin Xie† and Shannon S. Stahl*
Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States
S
* Supporting Information
systems exhibit broad scope and functional group compatibility,
ABSTRACT: Cu/nitroxyl catalysts have been identified
that promote highly efficient and selective aerobic
oxidative lactonization of diols under mild reaction
conditions using ambient air as the oxidant. The chemo-
and regioselectivity of the reaction may be tuned by
changing the identity of the nitroxyl cocatalyst. A Cu/
ABNO catalyst system (ABNO = 9-azabicyclo[3.3.1]-
nonan-N-oxyl) shows excellent reactivity with symmetrical
diols and hindered unsymmetrical diols, whereas a Cu/
TEMPO catalyst system (TEMPO = 2,2,6,6-tetramethyl-1-
piperidinyl-N-oxyl) displays excellent chemo- and regiose-
lectivity for the oxidation of less hindered unsymmetrical
diols. These catalyst systems are compatible with all classes
of alcohols (benzylic, allylic, aliphatic), mediate efficient
lactonization of 1,4-, 1,5-, and some 1,6-diols, and tolerate
diverse functional groups, including alkenes, heterocycles,
and other heteroatom-containing groups.
and they operate efficiently at room temperature with ambient
air as the source of oxidant. The identity of the nitroxyl can be
varied to alter the activity and selectivity of the catalyst. A Cu/
TEMPO catalyst, consisting of [Cu(CH3CN)4]OTf, 2,2′-
bipyridine (bpy), N-methyl imidazole (NMI), and 2,2,6,6-
tetramethylpiperidine-N-oxyl (TEMPO), is very sterically
sensitive and promotes highly selective oxidation of diverse
primary alcohols to aldehydes.6b Meanwhile, a Cu/ABNO
catalyst system, which employs the less sterically hindered
nitroxyl 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO),6c shows
excellent reactivity with both 1° and 2° alcohols. Here we show
that these catalyst systems exhibit high activity and selectivity in
the aerobic oxidative lactonization of diols.
Our initial catalyst development efforts focused on oxidative
lactonization of the symmetrical diol, diethylene glycol (1a).
The lactone product 1b is an important precursor to degradable
polyesters that have diverse biomedical applications.7 We tested
the previously reported Cu/TEMPO catalyst system6b with 1a
in acetonitrile at room temperature under ambient air. The
reaction reached moderate conversion (78%) of 1a after 4 h,
affording a mixture of lactone 1b and hemiacetal 1d in 34 and
44% yields, respectively. Elevated temperatures and higher
loading of TEMPO did not improve the outcome (see
Supporting Information, Scheme S1). A number of other
nitroxyl cocatalysts were then tested under similar reaction
conditions. Examples include the more oxidizing TEMPO
derivatives 4-MeO-TEMPO and 4-oxo-TEMPO and the
sterically less hindered ABNO, keto-ABNO, AZADO, and
Me-AZADO nitroxyls (Figure 1). The latter bi- and tricyclic
nitroxyls were quite effective, in each case affording near-
quantitative yield of lactone 1b within 1 h at room temperature.
ABNO exhibited a slightly higher conversion rate. Because of
its commercial availability and lower cost than the AZADO
derivatives, we proceeded with this nitroxyl. A quantitative yield
of 1b was obtained within 1 h, even when the ABNO loading
was decreased to 1 mol % (see Table S1). Catalysts with
electron-rich bipyridyl ligands, such 4,4′-dimethoxybipyridine,
show faster initial rates, but they deactivate prior to complete
conversion to the lactone (see Supporting Information for
additional catalyst optimization data).
actones are important structural motifs in natural products
L
and pharmaceuticals, building blocks for the production of
fine chemicals, and monomers for the preparation of
polyesters.1 The oxidative lactonization of diols, involving
sequential oxidation of a 1° alcohol and an intermediate
hemiacetal (lactol), is an appealing route to these molecules
(Scheme 1; steps i−iii). Numerous stoichiometric oxidants and
Scheme 1. Oxidative Lactonization of Diols
catalytic methods have been explored to achieve this goal.2,3
Aerobic oxidation methods offer a compelling alternative,4 but
existing catalysts face limitations associated with forcing
reaction conditions, restricted functional group tolerance,
and/or poor chemo/regioselectivity. Oxidation of the second
alcohol, which in many cases is a more reactive 2° alcohol
(Scheme 1, step iv or v), presents a key challenge for this
transformation. We anticipated that recently reported Cu/
nitroxyl catalyst systems for aerobic alcohol oxidation5,6 could
provide a unique solution to this challenge. These catalyst
Reaction profiles for the oxidative lactonization of 1a
catalyzed by Cu/TEMPO and Cu/ABNO were monitored by
1H NMR spectroscopy (Figure 2). The hemiacetal intermediate
1d is evident with both catalyst systems. The Cu/TEMPO
Received: January 30, 2015
Published: March 9, 2015
© 2015 American Chemical Society
3767
DOI: 10.1021/jacs.5b01036
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Journal of the American Chemical Society
Communication
Table 1. Scope of Cu/ABNO-Catalyzed Aerobic Oxidative
a
Lactonization of Symmetrical Diols
Figure 1. Results of Cu/nitroxyl-catalyzed aerobic oxidative
lactonization of diethylene glycol with different nitroxyls after 0.5
and 1 h (0.5 mmol scale).
a
Reaction conditions: diols (1.0 mmol), [Cu(CH3CN)4]OTf (0.05
mmol, 5%), bpy (0.05 mmol, 5%), ABNO (0.01 mmol, 1%), NMI
(0.10 mmol, 10%), CH3CN (5.0 mL), yield based on 1H NMR
analysis (int. std. = biphenyl), reactions run in an open round-bottom
flask (50 mL). Diol (5.0 g, 47 mmol). Isolated yield. ABNO (0.05
mmol, 5%).
Figure 2. Reaction profiles of the oxidative lactonization of diethylene
glycol 1a, catalyzed by (A) Cu/TEMPO and (B) Cu/ABNO (1 mmol
scale). Reaction conditions: 1a (1.0 mmol), [Cu(CH3CN)4]OTf (0.05
mmol, 5%), bpy (0.05 mmol, 5%), TEMPO or ABNO (0.05 mmol,
5%), NMI (0.1 mmol, 10%), CD3CN (5.0 mL).
b
c
d
the α,ω-dialdehyde (adipaldehyde, 74% yield). The reaction
conditions scale effectively; oxidative lactonization of diethylene
glycol (1a) afforded a 92% isolated yield of lactone 1b when
performed on a 5 g scale.
catalyst exhibits good initial reactivity but appears to deactivate
after 1.5 h, with negligible further conversion beyond this time
(Figure 2A). Build-up and decay of the hemiacetal is also
evident with the Cu/ABNO catalyst system, but complete
conversion to the lactone is achieved within 1.5 h (Figure 2B).8
The relative catalyst activity trends in this reaction may be
rationalized by steric effects. The Cu/TEMPO catalyst is very
sterically sensitive, for example, showing high selectivity in the
oxidation of 1° over 2° alcohols.6 On the other hand, the Cu/
ABNO catalyst mediates efficient oxidation of both 1° over 2°
alcohols.6c,9 These considerations account for the higher
reactivity of Cu/ABNO in the oxidation of the hemiacetal,
which is more sterically hindered than the initial 1° alcohol.
The scope of (bpy)CuI/ABNO-catalyzed lactonization was
probed initially with a series of symmetrical diols (Table 1).
The reactions led to efficient formation of a number of five- and
six-membered lactones, including the bioactive molecule
mevalonolactone (10b), at room temperature under ambient
air. Most reactions achieved complete substrate conversion
within 2 h and afforded isolated products in >90% yield.
Efficient reactivity was observed with benzylic, allylic, and
aliphatic substrates. The seven-membered-ring lactone 11b was
generated in 97% yield,10 while the less conformationally
constrained 1,6-hexanediol afforded caprolactone in only 22%
yield with 5 mol % of ABNO. The major product in this case is
Oxidative lactonization of unsymmetrical diols presents a
more stringent challenge. The catalyst must not only promote
oxidation of the hemiacetal intermediate but also discriminate
between two different alcohols, for example, between 1° and 2°
alcohols (cf. Scheme 1) or between sterically or electronically
differentiated 1° alcohols. The first classes of substrates
examined were aliphatic and benzylic diols that featured one
1° alcohol together with a 2° or 3° alcohol (Table 2). The Cu/
ABNO catalyst system showed good activity and selectivity in
the lactonization of these substrates; however, dicarbonyl
compounds (cf. f, Scheme 1) were obtained as byproducts in
some cases, reflecting the high activity of the Cu/ABNO
catalyst for oxidation of both 1° and 2° alcohols.6c For example,
the oxidation of 1,5-hexanediol 16a affords 5-hexanolide 16b in
88% yield, together with an 8% yield of 5-oxohexanal 16f.11
Substrates 13a, 14a, and 16a exhibited improved results when
ABNO was replaced with the more sterically hindered TEMPO
cocatalyst. For example, use of 5 mol % of TEMPO in the
lactonization of 16a led to a 97% yield of the lactone 16b. A
variation of the Cu/TEMPO catalyst conditions even enabled
selective conversion of heptane-1,6-diol 18a to the seven-
membered-ring product 6-methyl-ε-caprolactone 18b in 92%
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Table 2. Scope of Cu/Nitroxyl-Catalyzed Aerobic Oxidative
Lactonization of Unsymmetric Diols with Primary and
Secondary Hydroxyl Groups
Table 3. Scope of Cu/Nitroxyl-Catalyzed Aerobic Oxidative
Lactonization of Unsymmetrical Diols with Two Different
a
a
Primary Hydroxyl Groups
a
Reaction conditions: diols (1.0 mmol), [Cu(CH3CN)4]OTf (0.05
mmol, 5%), bpy (0.05 mmol, 5%), nitroxyl (0.01 mmol, 1%), NMI
(0.10 mmol, 10%), CH3CN (5.0 mL), yield based on 1H NMR
analysis (int. std. = biphenyl), reactions run in an open round-bottom
b
c
flask (50 mL). Isolated yield. With 5 mol % of nitroxyl (0.05 mmol).
d
With 10 mol % of TEMPO (0.10 mmol), 3 Å M.S. (300 mg), O2
balloon, rt.
yield.12 These observations are consistent with the known
ability of Cu/TEMPO to discriminate more effectively between
1° and 2° alcohols, relative to Cu/ABNO.6b,13
The most difficult class of unsymmetrical diols features two
different primary alcohols. The steric and/or electronic
differences between the two alcohols in these substrates can
be quite subtle, and successful examples of these trans-
formations are rare.14 Previously reported aerobic lactonization
methods exhibit low selectivity in these cases,4 but considerable
success was achieved with Cu/nitroxyl catalysts. The Cu/
TEMPO catalyst generally performed better than Cu/ABNO
(Table 3, blue vs red). The enhanced steric discrimination of
Cu/TEMPO relative to Cu/ABNO presumably accounts for
the selectivities observed here; however, electronic effects also
contribute (see below).
A ≥9:1 regioselectivity was observed with the 2-methyl- or
2,2-dimethyl-substituted diols 19a and 20a. Use of the Cu/
ABNO catalyst with these substrates led to reduced selectivity
(<2:1 and 4:1, respectively). In the naphthalene-derived diol
21a, a phenyl substituent provided adequate steric differ-
entiation of the alcohols to enable formation of lactone 21b in
high yield. Successful discrimination between the benzylic and
aliphatic alcohols was achieved in substrate 22a, consistent with
the significantly higher rates for benzylic versus aliphatic
alcohols observed previously with the Cu/TEMPO catalyst.6b
The 2-NHBoc-substituted pentanediol 23a was unreactive with
the Cu/TEMPO catalyst, but use of the Cu/ABNO catalyst
afforded lactones 23b/b′ in excellent yield with 2:1
regioselectivity, favoring alcohol oxidation proximal to the
NHBoc group. Preferential oxidation of the more hindered
alcohol could arise from chelation of the NHBoc group or from
an electronic effect, as elaborated below. The Cu/ABNO
a
Reaction conditions: diols (1.0 mmol), [Cu(CH3CN)4]OTf(0.05
mmol, 5%), bpy (0.05 mmol, 5%), nitroxyl (0.01 mmol, 1%), NMI
(0.10 mmol, 10%), CH3CN (5.0 mL), yield based on 1H NMR
analysis (int. std. = biphenyl), reactions run in an open round-bottom
b
c
flask (50 mL). Isolated yield. TEMPO or ABNO (0.05 mmol, 5%).
d
e
TEMPO (0.03 mmol, 3%). TEMPO (0.1 mmol, 10%).
catalyst showed similar reactivity with 2-aminobutanediol
derivatives 24a−26a. It afforded the five-membered lactones
in good yield, again favoring oxidation of the more hindered
alcohol. Diols 24a−26a underwent more selective lactonization
with the Cu/TEMPO catalyst, favoring oxidation of the less
hindered alcohol in a range from 4.5 to 11.5:1. Finally, the
pyridine-derived diols 27a and 28a underwent regioselective
lactonization with the Cu/TEMPO and Cu/ABNO catalysts,
respectively. The hydroxymethyl groups in the ortho and para
positions of diols 27a and 28a should be more acidic than those
in the meta position, and we have previously reported Hammett
studies of different benzyl alcohols, which show that more
acidic alcohols are oxidized more rapidly.8a The observed
selectivity is readily rationalized on the basis of the expected
alcohol acidity. These results show that electronic effects can be
as important as steric effects in controlling the reaction
selectivity.
In summary, the results described herein show that Cu/
nitroxyl catalyst systems exhibit excellent activity and selectivity
for the oxidative lactonization of diols. The selectivity patterns
observed with unsymmetrical diols match that expected from
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ASSOCIATED CONTENT
* Supporting Information
■
S
Full reaction development data, experimental procedures, and
product characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
(7) Numerous catalyst oxidative and dehydrogenative routes have
been explored for the preparation of this lactone. See refs 1c, 4b, 4h,
and the following: (a) Guest, H. R.; Kiff, B. W. U.S. Patent 2,900,395,
1959. (b) Forschner, T. C. U.S. Patent 5,310,945, 1994. (c) Kaneda,
K.; Mitsudome, T.; Mizugaki, T.; Jitsukawa, K. Molecules 2010, 15,
8988−9007. (d) Musa, S.; Shaposhnikov, I.; Cohen, S.; Gelman, D.
Angew. Chem., Int. Ed. 2011, 50, 3533−3537.
AUTHOR INFORMATION
Corresponding Author
stahl@chem.wisc.edu
■
Present Address
†School of Chemistry and Chemical Engineering, Shanghai Jiao
Tong University, 800 Dongchuan Road, Shanghai 200240,
China.
(8) Mechanistic studies have shown that the rate of these reactions
depends on the partial pressure of O2 and can be affected by mass
transfer into solution. The slightly longer reaction time in Figure 2
relative to that in Figure 1 reflects the larger reaction scale, which can
affect gas−liquid mixing. For details, see: (a) Hoover, J. M.; Ryland, B.
L.; Stahl, S. S. J. Am. Chem. Soc. 2013, 135, 2357−2367. (b) Hoover, J.
M.; Ryland, B. L.; Stahl, S. S. ACS Catal. 2013, 3, 2599−2605.
(c) Rogan, L.; Hughes, N. L.; Cao, Q.; Dornan, L. M.; Muldoon, M. J.
Catal. Sci. Technol. 2014, 4, 1720−1725.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the DOE for financial support of this work
(DE-FG02-05ER15690) and Sigma-Aldrich for generous
donation of samples of ABNO.
(9) For mechanistic studies that provide insights into the origin of
the different steric effects in the Cu/TEMPO and Cu/ABNO catalyst
systems, see ref 13.
́
(10) Fetizon, M.; Golfier, M.; Louis, J. M. J. Chem. Soc. D 1969,
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