Catalytic Oxidation of Alcohols Using a 2,2,6,6-Tetramethylpiperidine-N-hydroxyammonium Cation

Article abstract of DOI:10.1002/ejoc.201801718

The oxidation of alcohols to aldehydes, ketones, and carboxylic acids is reported using 2,2,6,6-tetramethylpiperidine-4-acetamido-hydroxyammonium tetrafluoroborate as a catalyst in conjunction with sodium hypochlorite pentahydrate as a terminal oxidant. The reaction is generally complete within 30–120 min using an acetonitrile/water mix as the solvent, and no additives are required. Product yields are good to excellent and of particular note is that the methodology can be used to access aryl α-trifluoromethyl ketones.

Full text of DOI:10.1002/ejoc.201801718

A Journal of
Accepted Article
Title: Catalytic Oxidation of Alcohols Using a 2,2,6,6-
Tetramethylpiperidine-N-hydroxyammonium Cation
Authors: Shelli Miller, Kathryn Bisset, Nicholas Leadbeater, and
Nicholas Eddy
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201801718
Link to VoR: http://dx.doi.org/10.1002/ejoc.201801718
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10.1002/ejoc.201801718
European Journal of Organic Chemistry
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Catalytic Oxidation of Alcohols Using a 2,2,6,6-
Tetramethylpiperidine-N-hydroxyammonium Cation
Shelli A. Miller,a Kathryn A. Bisset,a Nicholas E. Leadbeatera,* and Nicholas A. Eddya,*
Abstract: The oxidation of alcohols to aldehydes, ketones, and
salts, avoiding the volatility and capricious reactivity of TEMPO;
carboxylic acids is reported using 2,2,6,6-tetramethylpiperidine-4-
they are soluble in water and insoluble in common organic
solvents, facilitating their removal and reuse; and they allow for
control of counterions, the variation of which has been postulated
to have some effect on reactivity.[10] Our attention has focused
acetamido-hydroxyammonium tetrafluoroborate as
a catalyst in
conjunction with sodium hypochlorite pentahydrate as a terminal
oxidant. The reaction is generally complete within 30-120 min using
an acetonitrile / water mix as the solvent, and no additives are required. particularly on the hydroxyammonium tetrafluoroborate salt of 3,
Product yields are good to excellent and of particular note is that the
and we present here a methodology for alcohol oxidation using
this species as a catalyst. A range of alcohols, including stubborn
-trifluoromethyl alcohols, can be oxidized rapidly and effectively
using sodium hypochlorite as a terminal oxidant.
methodology can be used to access aryl α-trifluoromethyl ketones.
Introduction
Oxidation is a net two-electron process that is often broken down
into a series of one-electron events and is one of the most
fundamental transformations in organic chemistry. From a
preparative chemistry perspective, oxidation reactions are widely
used for installing valuable functional handles or fragmenting
complex structures and, given its importance, significant research
effort has been focused on developing catalytic oxidation
methodologies.[1] Of the known catalytic oxidants, 2,2,6,6-
tetramethylpiperidine 1-oxyl (commonly known as TEMPO) is
among the most widely employed, particularly on industrial
scales.[2] TEMPO and its derivatives (notably 4-acetamido-
TEMPO, 1, also known as ACT,[3] Figure 1) facilitate an array of
oxidative transformations, but are best known for the conversion
of alcohols to the corresponding aldehyde, ketone, or carboxylic
acid. In such reactions, the nitroxide species are pro-oxidants, a
secondary oxidant generating the active oxidizing species, an
oxoammonium cation (2),[4] in situ via a single-electron transfer
(SET) step (Figure 1). Subsequent alcohol oxidation is a two-
electron process via a net hydride transfer[5] resulting in formation
of hydroxyammonium species such as 3. A series of two SET
steps are required to regenerate the oxoammonium cation, 2. The
initial one-electron oxidation of 1 to generate 2 has been
accomplished by the use, for example, of strong acid,[6]
Cu/oxygen,[7] nitrite[8] and photocatalysis.[9] The presence of these
or other additives complicate the reaction. To our knowledge,
there has not been development of catalytic strategies directly
utilizing hydroxyammonium salts. We believe that there are some
benefits to exploring this class of reagent: they are shelf stable
Figure 1. The interplay between nitroxide, oxoammonium, and
hydroxyammonium species.
Results and Discussion
Our first objective was to seek out reaction conditions that would
allow for re-oxidation of 3 to 2. Using 3-phenylpropan-1-ol (4) as
a test substrate and water as the solvent, we discovered that
some degree of oxidation occurred when employing 10 mol% 3
and 1 equiv. of commercially-available bleach solution as a
terminal oxidant. A 7% conversion to hydrocinnamaldehyde (5a)
together with 9% of the corresponding hydrate was obtained after
stirring at room temperature for 45 min (Table 1, entry 1). In an
attempt to increase conversion, we turned to a phosphate buffer
at pH 3.5 as the solvent, the objective being to maintain an acidic
reaction medium and hence disproportionate any nitroxide (1)
formed into 2 and 3.[11] When using 2 equiv. bleach, titrated to
confirm the concentration of sodium hypochlorite, the conversion
to 5a increased even when using 7.2 mol% of 3 as the catalyst,
but with this we also observed formation of dimeric ester 6a (entry
2). Titrating bleach solution to assure accurate stoichiometric
control is a laborious process. This, together with a desire to limit
byproduct formation led us to screen solid sodium hypochlorite
pentahydrate as a terminal oxidant,[12] and a binary solvent
system of acetonitrile and phosphate buffer as the solvent. The
[a]
S.A. Miller, K. A. Bisset, Dr. N. E. Leadbeater, Dr. N. A. Eddy
Department of Chemistry
University of Connecticut
55 North Eagleville Road
Storrs, Connecticut 06269
USA
nicholas.leadbeater@uconn.edu, nicholas.eddy@uconn.edu
chemistry.uconn.edu/person/nicholas-leadbeater/
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201801718
European Journal of Organic Chemistry
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former obviates the need for titration and we posited that the latter
would reduce byproduct formation. Our assertions were borne out
by the fact that when using 3.6 mol% of 3 as the catalyst, 1 equiv.
NaOCl  5H2O as the terminal oxidant, and 99:1 acetonitrile / 2 M
phosphate buffer as the solvent, a 78% conversion to 5a was
obtained with no evidence of concomitant dimeric ester formation
(entry 3). Probing the role of the buffer, we found that it could
simply be replaced by water and the product conversion improved
to 84% (entry 4). However, when the reaction was performed in
pure acetonitrile (i.e. without the addition of water), product
conversion dropped significantly (entry 5). A range of alternative
terminal oxidants were screened. No oxidation was observed
when using oxone, potassium persulfate, ceric ammonium nitrate,
silver carbonate, N-methyl morpholine-N-oxide, or sodium
perborate. A 67% conversion to 5a was obtained in the case of
(diacetoxyiodo)benzene, together with 13% of hydrocinnamic
acid (7a) (entry 6). Returning to sodium hypochlorite as the
terminal oxidant, increasing the loading from 1 equiv. to 1.3 equiv.
allowed us to consume all the alcohol starting material. Using
these conditions, a conversion to 5a of 81% was obtained,
together with 10% of 7a (entry 7). Upon increasing the quantity of
sodium hypochlorite to 2.5 equiv. we observed an 82%
conversion to 7a, this allowing us to increase the scope of the
methodology to the direct oxidation of primary alcohols to the
corresponding carboxylic acids (entry 8). Returning to the
targeted oxidation to the aldehyde, since separation of 5a from 4
is more challenging than that of 5a from 7a, we decided to use the
slightly increased sodium hypochlorite loading of 1.3 equiv.
moving forward. While reducing the quantity of 3 from 3.6 mol%
to 1 mol% was possible, a significantly increased reaction time
was required for full consumption of 4 (entry 9). Returning to a
catalyst loading of 3.6 mol%, we explored whether 1 or 2 could be
employed as the entry point for the catalytic cycle and found that
this was indeed possible without an effect on outcome (entries 10
and 11). Finally, to confirm that the reaction was not being
affected solely by the terminal oxidant, we performed a control
reaction in the absence of 3 and found that no product was
obtained (entry 12). Having completed our screen, as optimized
conditions we selected to move forward using 3 (3.6 mol%) as
the catalyst and NaOCl  5H2O (1.3 equiv.) as the terminal oxidant,
employing a 99:1 acetonitrile / water mix as the solvent, and
stirring the reaction mixture at room temperature
Table 1: Optimization of reaction conditions.a
Entry
Oxidant (equiv.)
Solvent
5a (%)
6a (%)
7a (%)
3 (mol %)
1
10
bleach (1)
water
7
-
-
2
7.2
3.6
3.6
3.6
3.6
3.6
3.6
1
bleach (2)
bufferb
39
78
82
62
69
81
14
85
80
85
0
21
-
-
3
99:1 acetonitrile / bufferb
99:1 acetonitrile / water
acetonitrile
-
NaOCl  5H2O (1)
NaOCl  5H2O (1)
NaOCl  5H2O (1)
PhI(OAc)2 (1)
4
-
-
5c
6
-
-
99:1 acetonitrile / water
99:1 acetonitrile / water
99:1 acetonitrile / water
99:1 acetonitrile / water
99:1 acetonitrile / water
99:1 acetonitrile / water
99:1 acetonitrile / water
-
17
16
82
7
7
-
NaOCl  5H2O (1.3)
NaOCl  5H2O (2.5)
NaOCl  5H2O (1.3)
NaOCl  5H2O (1.3)
NaOCl  5H2O (1.3)
NaOCl  5H2O (1.3)
8
2
-
9
10
11
12
3.6d
3.6e
-
2
-
18
13
0
0
a
Reactions performed using 1 mmol 4a (0.3 M in solvent) and the oxidant added as a single portion. Reaction run for 45 min at room temperature. Product
conversion determined by integration of signals in the 1H-NMR spectrum of the crude product mixture. b Phosphate buffer (pH 3.5). c Contained 35% 4a. d 1 used
as catalyst. e 2 used as catalyst.
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We moved next to a substrate screen using our optimized
reaction conditions (Table 2). The methodology can be applied to
variety of scaffolds, with isolated yields ranging from 65% to 98%.
Both primary and secondary alcohols are well tolerated. The
progress of primary alcohol oxidation was monitored closely to
ensure maximum conversion to the desired aldehyde product and
minimal formation of the corresponding carboxylic acid. Benzyl
alcohols are converted to the corresponding benzaldehydes (5c-
5g) and aryl carbinols to acetophenones (5h-l). Electron donating
groups were found to slow the reaction progress and required
increasing the sodium hypochlorite loading from 1.3 equiv. to 2.5
equiv. and employing a 95:5 acetonitrile / water mix as the solvent
in order to achieve complete oxidation. Both linear and cyclic
aliphatic alcohols were tested and proved successful (5m-5o),
including bulky substrates such as adamantanol (5p), although
photoredox catalysis.[18] We wanted to see if our current
methodology could be employed for this challenging oxidation,
and found that indeed it is (Table 3). A range of aromatic and
heteroaromatic α-trifluoromethyl alcohols could be oxidized to
TFMKs (5t-5y) but aliphatic examples proved unreactive under
our conditions (5z).
Table 2: Substrate scope for the oxidation of alcohols to aldehydes and
ketones.a
norbornene derivative 5q was unreactive. Extending to
a
heterocyclic example, we were able to oxidize 2-pyridinemethanol
in just 15 min (5r). In the absence of a base, stoichiometric
oxoammonium salt oxidation of substrates bearing a β-oxygen
substituent occurs so slowly as to be unreactive and in the
presence of a base, formation of the dimeric ester product is
observed.[13] To investigate this class of substrate using our
methodology, we screened 1-phenoxy-2-propanol. A 65% yield of
the desired ketone product (5s) was obtained when using 2.5
equiv. of sodium hypochlorite and performing the reaction in a
95:5 acetonitrile / water mix as the solvent. Interestingly, in this
case alongside 5s we observed a number of byproducts resulting
from chlorination of the aromatic ring, presumably arising from off-
target reactions of either 4s or 5s with the sodium hypochlorite
terminal oxidant.
An interesting reaction occurred when subjecting furfuryl
alcohol to our oxidation conditions. Instead of obtaining furfural,
we observed 8, the product of an Achmatowicz rearrangement
(Scheme 1). The identity of the six-membered ring heterocycle
was proven by comparing NMR data with that in the literature[14]
but proved challenging to isolate in pure form from the product
mixture. However, this proves that the oxidation conditions can
facilitate transformations other than simple oxidation.
Scheme 1. Formation of 8 from furfuryl alcohol through an Achmatowicz
a Unless noted otherwise, reaction performed using alcohol (2.5 mmol, 1 equiv.),
3 (0.09 mmol, 3.6 mol%), NaOCl · 5H2O (3.25 mmol, 1.3 equiv.), 0.3 M alcohol
in 99:1 MeCN / H2O, room temperature; all yields are isolated yields after
purification, unless otherwise noted. b 1 mmol scale. c Using NaOCl · 5H2O (6.25
mmol, 2.5 equiv.), 0.3 M alcohol in 95:5 MeCN / H2O. d Yield determined by
quantitative 1H-NMR spectroscopy.
rearrangement.
Oxidation of α-trifluoromethyl alcohols to the corresponding
trifluoromethyl ketones (TFMKs) offers a practical route to this
valuable class of compounds. However, classical methods for
alcohol oxidation are typically insufficient due to the presence of
the strongly electron-withdrawing trifluoromethyl group.[15,16] In
our group we have developed
a methodology using a
In a final substrate screen, we probed the conversion of
primary alcohols to the corresponding carboxylic acids, thereby
expanding the scope of the methodology. By using 2.5 equiv. of
sodium hypochlorite and performing the reaction in a 95:5
superstoichiometric quantity of 1 in conjunction with a pyridine
base,[17] and more recently reported an approach merging
catalytic oxoammonium cation oxidation with visible-light
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10.1002/ejoc.201801718
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acetonitrile / water mix as the solvent, four representative alcohols
were exhaustively oxidized to the carboxylic acids, including an
aliphatic substrate (7m) which, while difficult to isolate, gave an
86% yield as calculated by quantitative 1H-NMR spectroscopy
(Table 4).
as a model system, prior to addition of sodium hypochlorite we
see no evidence of the paramagnetic nitroxide 1. Immediately
after the addition of sodium hypochlorite to the reaction mixture,
a maximum of 0.25 mol% of 3 (or 7% of the total amount of 3 in
the reaction mixture) converts to radical 1 by means of
comproportionation. After 1 hour of reaction (the half-way point)
the radical is no longer observable. This evidence adds further
credence to our argument that one two-electron process is at play
rather than two one-electron steps.
Table 3: Scope of the methodology for the oxidation of various α-trifluoromethyl
alcohols.a
Conclusions
In summary, hydroxyammonium salt 3 has been employed as a
catalyst for the oxidation of alcohols in conjunction with sodium
hypochlorite pentahydrate as a terminal oxidant. The reaction is
generally complete within 30-120 min using an acetonitrile / water
mix as the solvent, and no additives are required. The
methodology has been employed for the generation of a range of
aldehyde, ketone, and carboxylic acid products. Of particular note
is that aryl trifluoromethyl carbinols can be converted to the
corresponding trifluoromethyl ketones in excellent yield, this class
of alcohol being notoriously hard to oxidize.
Experimental Section
a Reaction performed using alcohol (2.5 mmol, 1 equiv.), 3 (0.09 mmol, 3.6
mol%), NaOCl · 5H2O (3.25 mmol, 1.3 equiv.), 0.3 M alcohol in 99 / 1 MeCN :
H2O, room temperature; all yields are isolated yields after purification. b Contains
12% hydrate.
General Method for the oxidation of alcohols to aldehydes and ketones: A
vial, equipped with a stir bar, was charged with alcohol 4 (2.5 mmol, 1
equiv) and 3 (27.2 mg, 0.09 mmol, 0.036 equiv), and then a solution of
99:1 acetonitrile / water (8.33 mL, 0.3 M in 4) added. With stirring, solid
NaOCl · 5 H2O (0.535 g, 3.25 mmol, 1.3 eq) was added in portions over
10 minutes, and the reaction monitored by 1H-NMR spectroscopy through
the removal of aliquots. When the reaction was deemed complete, the
product mixture was diluted with Et2O (5 mL) and transferred to a
separatory funnel. The layers were separated, the aqueous layer was
extracted with Et2O (3 x 25 mL), and the combined organic layer washed
with 1 M HCl (25 mL), saturated NaHCO3 (25 mL), and brine (25 mL), then
dried over anhydrous Na2SO4 and concentrated in vacuo to afford the pure
aldehyde or ketone 5.
Table 4: Representative oxidation of alcohols to carboxylic acids.a
General Method for the oxidation of alcohols to carboxylic acids: A vial
equipped with a stir bar was charged with alcohol 4 (2.5 mmol, 1 equiv)
and 3 (27.2 mg, 0.09 mmol, 0.036 equiv), and then a solution of 95:5
acetonitrile / water (8.31 mL, 0.3 M in 4) added. With stirring, solid NaOCl
· 5 H2O (1.028 mg, 6.25 mmol, 2.5 equiv) was added as a single portion.
The reaction was monitored by 1H-NMR spectroscopy through the removal
of aliquots. When the reaction was deemed complete, the product mixture
was diluted with Et2O (10 mL) and transferred to a separatory funnel and
shaken for 30 s with a solution of sodium bisulfite (10 mL). The resulting
solution was extracted with deionized water (20 mL), separated, and the
aqueous layer extracted with Et2O (3 x 25 mL). The combined organics
were washed with 1 M HCl (25 mL) and water (3 x 25 mL), then dried over
sodium sulfate and the solvent removed in vacuo to afford the pure
carboxylic acid 7.
a Reaction performed using alcohol (2.5 mmol, 1 equiv.), 3 (0.09 mmol, 3.6
mol%), NaOCl · 5H2O (6.25 mmol, 2.5 equiv.), 0.3 M alcohol in 95:5 MeCN /
H2O, room temperature; yields are isolated yields after purification, unless noted
otherwise. b Yield determined by quantitative 1H-NMR spectroscopy.
We posit that the mechanism by which 1 is regenerated from
3 is by a two-electron oxidation as opposed to two one-electron
oxidation steps. This is supported by the observation that when
using ceric ammonium nitrate and silver nitrate, traditional one-
electron oxidants, in place of sodium hypochlorite, we see no Acknowledgments
product formed. To probe the reaction further, we performed an
EPR spectroscopy study. Using the oxidation of 1-octanol to 7m
We thank the University of Connecticut for funding.
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Keywords: Oxidation • Hydroxyammonium salt • Trifluoromethyl
ketone • Sodium hypochlorite • Alcohols
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Entry for the Table of Contents
FULL PAPER
Easy Oxidation
Shelli A. Miller, Kathryn A. Bisset,
Nicholas E. Leadbeater* and Nicholas A.
Eddy*
Page No. – Page No.
Catalytic Oxidation of Alcohols Using
a 2,2,6,6-Tetramethylpiperidine-N-
hydroxyammonium Cation
The fast, easy oxidation of alcohols to aldehydes, ketones, and carboxylic acids is
reported using a hydroxyammonium cation as a catalyst.
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