Copper-Catalyzed Aminoxylation of Different Types of Hydrocarbons with TEMPO: A Concise Route to N-Alkoxyamine Derivatives

Article abstract of DOI:10.1002/adsc.201500544

An efficient copper(II)/tert-butyl hydroperoxide catalyst system [(Bpy)Cu(II)/TBHP] for the aminoxylation of different types of hydrocarbons under mild and ambient air conditions has been developed to furnish N-alkoxyamine derivatives in good to high yields. Ketones, esters, nitriles, toluene, ethylbenzene, heterocycles, cyclohexene, and cyclohexanes are well compatible in this system and the catalyst loading could be lowered to 0.5 mol%.

Full text of DOI:10.1002/adsc.201500544

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DOI: 10.1002/adsc.201500544
Copper-Catalyzed Aminoxylation of Different Types of Hydrocar-
bons with TEMPO: A Concise Route to N-Alkoxyamine Deriva-
tives
Linyi Li,a Zhengwei Yu,a and Zengming Shena,*
a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240,
Peopleꢀs Republic of China
Fax: (+86)-21-5474-8325; e-mail: shenzengming@sjtu.edu.cn
Received: June 5, 2015; Revised: August 12, 2015; Published online: November 17, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500544.
Abstract: An efficient copper(II)/tert-butyl hydro-
peroxide catalyst system [(Bpy)Cu(II)/TBHP] for
the aminoxylation of different types of hydrocar-
bons under mild and ambient air conditions has
been developed to furnish N-alkoxyamine deriva-
tives in good to high yields. Ketones, esters, nitriles,
toluene, ethylbenzene, heterocycles, cyclohexene,
and cyclohexanes are well compatible in this system
and the catalyst loading could be lowered to
0.5 mol%.
Keywords: N-alkoxyamines; tert-butyl hydroperox-
ide (TBHP); copper catalysis; hydrocarbons;
2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO)
N-Alkoxyamines are widely recognized as effective
initiators in controlled radical polymerization.[1] They
provide powerful methods for controlling the polydis-
persity and molecular weight of growing polymer
chains. In addition, N-alkoxyamines are also applied
as chiral building blocks, fireproofing agents, rheology
modifiers,[2] as well as polyfunctional precursors espe-
cially in biologically active natural products and phar-
Scheme 1. Methods towards the preparation of N-alkoxy-
amine derivatives.
maceutical agents.[3]
Therefore, the development of approaches for pre-
paring N-alkoxyamines has witnessed substantial
progress in the past decades.[4–6] Several classical ap- bromides were discovered under Cu or photocatalyst
proaches were notable for the synthesis of N-alkoxy- systems [Scheme 1, Eq. (2)].[4e,5g] An improved ap-
amines.[4–6] The most classical method was reported by proach was reported by Frey and co-workers,[5n]
Matyjaszewski and co-workers,[5m] in which N-alkoxy- wherein the way to generate hydrocarbyl radicals was
amines were formed by treatment of an alkyl bromide entirely different via hydrogen atom abstraction of
with TEMPO (2,2,6,6-tetramethylpiperidine N-oxyl) hydrocarbon compounds catalyzed by copper halide
catalyzed by CuBr. Notably, this alkyl radical inter- and onium iodide in the presence of TBHP. However,
mediate was generated from the reaction of alkyl bro- a narrow substrate scope limited its practicability.
mide and CuBr [Scheme 1, Eq. (1)]. Analogous ap- Only three substrates were investigated. Moreover,
proaches using alkyl trifluoroborates instead of alkyl Tan[4d] and Koike[4f] respectivly reported photocatalyst
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Table 1. Optimization of the reaction conditions.[a]
catalyzed a-aminoxylation of 1,3-dicarbonyl com-
pounds with TEMPO in the presence of photoredox
catalysts such as organic dyes or iridium complexes.
In 2013, Li and co-workers[5o] developed a reusable
Cu/Fe catalyst for a-aminoxylation reactions between
ketones with TEMPO. More recently, Jiao and co-
workers[6f] first demonstrated a CAN-catalyzed a-ami-
noxylation of 1,3-dicarbonyl compounds with
TEMPO [Scheme 1, Eq. (3)]. Other examples were
found for the synthesis of the N-alkoxyamine deriva-
tives.[4–6] However, in most of the aforementioned
cases, there has been evidently limitation in substrate
scope. Therefore, a more simple and compatible pro-
tocol remains in demand.
Here we disclose an efficient (Bpy)Cu(II)/TBHP
catalyst system for the aminoxylation of different
types of hydrocarbons under mild and ambient air
conditions [Scheme 1, Eq. (4)]. This catalyst system
shows excellent reactivity and general substrate
scope, and the catalyst loading can be lowered to
0.5 mol%.
In the initial studies, we chose ethylbenzene (1a) as
a model substrate to test the a-aminoxylation reaction
with TEMPO. The optimization results are summar-
ized in Table 1. To our surprise, when 1a was treated
with 2 equivalents of TBHP (aqueous 65%) without
adding any metal catalysts under air at 608C, an 81%
yield of the desired a-aminoxylated product 2a was
observed after 21 h (Table 1, entry 1). With this prom-
ising result in hand, further screening of catalysts re-
vealed that the combination of Cu(OAc)2/bpy (2,2’-bi-
pyridine) (20 mol%) could dramatically accelerate the
reaction to provide 2a in 96% yield within 50 min. To
our delight, it was found that a decrease of catalyst
loading did not impact on the efficiency of this reac-
tion (Table 1, entries 2–4). When using as little as
0.5 mol% of Cu(OAc)2/bpy catalyst, a pleasing 97%
yield of product 2a was obtained within 50 min
(Table 1, entry 4). To evaluate the reactivity of other
copper salts for this transformation, various Cu(II)
and Cu(I) species were then examined, showing that
these copper salts could promote the aminoxylation
reaction to form the desired product 2a in 43–92%
yields (Table 1, entries 5–11). These results indicated
that cupric or cuprous complexes are able to serve as
an efficient catalyst. By varying the ligands, we found
that 2,2’-bipyridine is still the best ligand for this reac-
tion (Table 1, entries 12–15). In addition, the yield of
the reaction was not significantly affected in the ab-
sence of ligand, but the reaction time was prolonged
to 3 h (Table 1, entry 16). Next, a series of oxidants
was explored. Dramatically, no desired product was
obtained when employing Oxone, mCPBA, H2O2,
BPO, K2S2O8, DTBP, and CH3COOOH as oxidants
(Table 1, entries 17–19, 21–24). By running the reac-
tion at 1008C with H2O2, a trace amount of desired
product 2a was observed (Table 1, entry 20). CHP
[a]
Conditions: TEMPO (0.3 mmol), 1a (10 equiv.),
Cu(OAc)2 (0.5 mol%), bpy (0.5 mol%), oxidant
(2 equiv.), air, 608C, 50 min.
Isolated yield.
21 h.
3 h.
1008C, 48 h.
1008C.
Room temperature.
under O2, 2 h.
Under N2.
1508C, 4 h.
1a (1 equiv.), 4 h. TBHP=tert-butyl hydroperoxide,
mCPBA=meta-chloroperoxybenzoic acid, BPO=benzo-
yl peroxide, DTBP=di-tert-butyl peroxide, 1,10-phen=
1,10-phenanthroline, TMEDA =tetramethylethylenedia-
mine, Bpym=2,2’-bipyrimidine, CHP=cumyl hydroper-
oxide, n.r.=no reaction.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
[j]
[k]
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Copper-Catalyzed Aminoxylation of Different Types of Hydrocarbons
Table 2. Scope of the copper-catalyzed aminoxylation reac-
(cumyl hydroperoxide), which has a similar structure
to TBHP, was used as an oxidant to give the desired
product 2a with 75% yield (Table 1, entry 25). These
results demonstrated that TBHP is critical for the
generation of free radicals in this transformation.
Upon heating to 1008C with TBHP, an obviously de-
creased yield was obtained (57%, Table 1, entry 26),
implying that a higher temperature might lead to the
decomposition of product 2a. Two control experi-
ments were run under N2 or O2 atmosphere, both
giving an excellent yield of 2a, indicating that oxygen
did not play a role in this transformation (Table 1, en-
tries 28 and 29). Consequently, the optimal conditions
are 0.5 mol% Cu(OAc)2/bpy and TBHP (2 equiv.)
under an air atmosphere at 608C within 50 min
(Table 1, entry 4).
tion with TEMPO.[a]
With the optimal reaction conditions in hand, next,
we explored the scope and the utility of the reaction
with various kinds of hydrocarbons. To our surprise,
this catalyst system shows excellent reactivity and
broad scope of substrates, including ketones, esters,
nitriles, toluene, ethylbenzene, heterocycles, cyclohex-
ene and cyclohexanes. Under the standard conditions,
they generally furnished the desired N-alkoxyamine
products in good to high yields (Table 2). For exam-
ple, the treatment of ketone 1b (2-phenoxy-1-phenyl-
ethanone) with TEMPO afforded the desired N-alk-
oxyamine product 2b in 77% yield within 4 min. Grat-
ifyingly, when using 2-phenylacetonitrile 1c as a sub-
strate, an excellent result (98% yield) was obtained in
30 min. Furthermore, diphenylmethane (1d) and di-
methyl malonate (1e) reacted well under the standard
conditions, giving the corresponding products in 87
and 70% yields, respectively. Notably, cyclohexene
(1f) and cyclohexanone (1g) were found to be suita-
ble substrates for this aminoxylation reaction, leading
to the desired products 2f and 2g in 85% and 70%
yields. It is worth noting that heterocycles, such as
THF (1h) and 1,4-dioxane (1i), could be converted
into their corresponding products in 84 and 88% iso-
lated yields, respectively. Toluene 1j was tolerated
and gave the desired 2j in 54% yield. Owing to the
difficulty of hydrogen atom abstraction from cyclo-
hexane (1k), the reaction of 1k with TEMPO resulted
in a decreased yield (46% for 2k) despite using 4
equivalents of TBHP. In particular, when methylcy-
[a]
Conditions: TEMPO (0.3 mmol), 1a (10 equiv.),
Cu(OAc)2 (0.5 mol%), bpy (0.5 mol%), TBHP ( aqueous
65%, 2 equiv.), air, 608C, 4 min–42 h, isolated yield.
TBHP (aqueous 65%, 4 equiv.).
1008C.
808C.
[b]
[c]
[d]
However, when using morpholine (1m) as a sub-
clohexane (1l) was subjected to the standard condi- strate, it afforded the unexpected product trans-2m in
tions, a tertiary carbon free radical was selectively 83% yield. In order to determine the structure of
formed and subsequently reacted with TEMPO to trans-2m, the derivation of the unexpected product
give the desired N-alkoxyamine 2l in 53% yield. Sev- trans-2m was conducted by treatment of trans-2m
eral by-products from the coupling of secondary car- with TsCl to provide compound trans-3m which was
bons in 1l with TEMPO were found, but the tertiary confirmed by single crystal X-ray analysis
carbon displayed highly reactivity. Consequently, the (Scheme 2).[7] For pyrrolidine, the reaction gave an
wide range of substrate scope demonstrated the prac- unknown product, the structure of which was not de-
ticability of this protocol for preparing various kinds fined well. Therefore, N-containing heterocyclic sub-
of N-alkoxyamine derivatives.
strates gave abnormal products and the mechanism is
unclear at present.
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Scheme 3. Transformation and application of N-alkoxyamine
derivative.
lents of ethylbenzene (1a) under the standard condi-
tions, a 75% yield of compound 2a (1.87 g) was ob-
tained [Eq. (7)].
For the reaction mechanism, we proposed a possible
reaction mechanism for copper-catalyzed aminoxyla-
tion of hydrocarbons with TEMPO. As outlined in
Scheme 4, initially, the Cu catalyst is hypothesized to
Scheme 2. The derivation of trans-2m and X-ray structure of
trans-3m.
N-Alkoxyamines are versatile building blocks from promote the decomposition of TBHP to generate t-
the view of organic synthetic chemistry because they BuOC radical A and t-BuOOC radical B.[10] Conse-
can be transformed into different compounds such as quently, both cupric and cuprous catalysts could be
ketones by oxidation and alcohols by reduction, and responsible for this transformation (See Table 1, en-
can be used as a controller of weight in polymer tries 4–11). When RH reacts with t-BuOOC or t-BuOC
chemistry. The representative transformation of N- radical, the hydrocarbyl free radical C is smoothly
alkoxyamines is illustrated in Scheme 2. For example, provided by hydrogen atom abstraction. Subsequently,
compound 2b was treated with mCPBA in DCM to it is trapped by TEMPO to afford the corresponding
give phenyl 2-oxo-2-phenylacetate 3a in 88% yield N-alkoxyamine products.
[Scheme 3, Eq. (5)], which is a common motif in bio-
logically active natural products, for example, 3-
deoxy-2-ulosonic acids and their derivatives.[8] In addi-
tion, the preparation of allyl alcohols is an important
investigation area in organic chemistry. To our de-
light, compound 2f was smoothly converted to cyclo-
hex-2-enol 3f in 63% yield by treatment with excess
zinc powder in acetic acid at 508C for 1 h [Scheme 3,
Eq. (6).[3c] Furthermore, the N-alkoxyamines could be
easily transformed into carbocycles and lactones as
reported by the Studer group.[9]
To display the usefulness and practicability of this
method, we tested this reaction in a large scale. Ethyl-
benzene 1a was chosen as a representative example.
When 1.5 g of TEMPO were treated with 3 equiva- Scheme 4. Proposed reaction pathway.
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Copper-Catalyzed Aminoxylation of Different Types of Hydrocarbons
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In conclusion, we have developed a general and
practical protocol for the synthesis of N-alkoxyamines
using the TEMPO, (Bpy)Cu(II) (0.5 mol%)/TBHP
system. This system displays highly efficient reactivity
for this transformation, giving the desired N-alkoxy-
amines in high to excellent yields under mild and air
conditions within a short time. The investigation of
the reaction’s generality reveals a broad substrate
scope. The different types of substrates, such as ke-
tones, esters, nitriles, toluene, ethylbenzene, heterocy-
cles, cyclohexene, and cyclohexanes, are weel compat-
ible in this system. Moreover, the reaction is easily
handled because the color of TEMPO provides
a useful visual indication for the progress of reaction
and purification. Further studies on the reaction
mechanism, the scope of the reaction and the poten-
tial application of this reaction are ongoing in our lab-
oratory.
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Experimental Section
General Remarks
All experiments were carried out under air. Reactions were
monitored using thin-layer chromatography (TLC). H and
1
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13C NMR spectra were obtained on a Bruker DMX-400 at
400 and 100 MHz, respectively. High resolution mass spectra
were obtained with ACQUITYTM UPLC & Q-TOF MS
Premier Spectrometer. Mass spectra were determined on an
HPLC-MS LCQ Advantage Thermo Finningan instrument.
GC-MS analysis was performed on LECO Pegasus 4D GC”
GC-TOFMS. Infrared (IR) spectra were recorded on an
AVATAR 370 Spectrometer.
Synthesis of 2,2,6,6-Tetramethyl-1-(1-phenylethoxy)-
piperidine (2a) as an Example
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Under air, TEMPO (46.8 mg, 0.3 mmol), ethylbenzene 1a
(0.37 mL, 3 mmol), Cu(OAc)2 (0.27 mg, 0.5 mol%), bpy
(0.23 mg, 0.5 mol%), TBHP (aqueous 65%, 92.8 mL,
0.6 mmol) were added into a Schlenk tube. The reaction was
stirred at 608C for 50 min. Upon completion, the mixture
was purified by column chromatography (hexane/ethyl ace-
tate) to give the colorless oil 2a; yield: 97%.
Acknowledgements
This work was supported by NSFC (21272001), Shanghai
Education Committee (13ZZ014) and Shanghai Jiao Tong
University.
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