Photocatalytic ?±-oxyamination of stable enolates, silyl enol ethers, and 2-oxoalkane phosphonic esters

Article abstract of DOI:10.1002/ejoc.201403433

Fast ?±-oxyamination of stable enolates, silyl enol ethers, and in situ deprotonated dialkyl 2-oxoalkane phosphonates and diphenyl-2-oxoalkyl phosphine oxides was performed in the presence of [Ru(bpy)₃]²⁺ (bpy = 2,2a?2-bipyridyl) as a photocatalyst, 2,2,6,6-tetramethylpiperidine nitroxide (TEMPO), and visible light. The key step was the light-induced one-electron oxidation of TEMPO into the 2,2,6,6-tetramethylpiperidine- 1-oxoammonium ion, which was nucleophilically attacked to yield ?±-functionalized carbonyl compounds. The reaction time was significantly reduced by the use of the microreactor flow technique.

Full text of DOI:10.1002/ejoc.201403433

Job/Unit: O43433
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Date: 04-12-14 16:38:06
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SHORT COMMUNICATION
DOI: 10.1002/ejoc.201403433
Photocatalytic α-Oxyamination of Stable Enolates, Silyl Enol Ethers,
and 2-Oxoalkane Phosphonic Esters
Peter Schroll[a] and Burkhard König*[a]
Keywords: Photooxidation / Photocatalysis / Radicals / Flow catalysis / Microreactors
Fast α-oxyamination of stable enolates, silyl enol ethers, and
in situ deprotonated dialkyl 2-oxoalkane phosphonates and
diphenyl-2-oxoalkyl phosphine oxides was performed in the
presence of [Ru(bpy)3]2+ (bpy = 2,2Ј-bipyridyl) as a photocat-
alyst, 2,2,6,6-tetramethylpiperidine nitroxide (TEMPO), and
visible light. The key step was the light-induced one-electron
oxidation of TEMPO into the 2,2,6,6-tetramethylpiperidine-
1-oxoammonium ion, which was nucleophilically attacked to
yield α-functionalized carbonyl compounds. The reaction
time was significantly reduced by the use of the microreactor
flow technique.
tetramethylpiperidine nitroxide (TEMPO).[16–18] All of
these reactions allow α-functionalization of carbonyls
through an oxidative pathway.
Introduction
Catalytic and stoichiometric one-electron oxidation reac-
tions of different classes of substrates are well estab-
lished.[1–4] For such oxidations, visible-light photocatalysis
has recently been widely applied to provide the necessary
redox energy. The photocatalytic oxidation of alkenes by
using 9-mesityl-10-methylacridinium perchlorate and subse-
quent intra- or intermolecular trapping of the resulting rad-
ical cation with nucleophiles, such as alcohols, amines, and
carboxylates, is an example that was described by Nicewicz
et al.[5–9] The generation of carbon-centered radicals R·
through one-electron oxidation of borates of the type
TEMPO catalyzes a variety of oxidation reactions.[19] An
important application as reagent is the oxidation of
alcohols to aldehydes or carboxylic acids.[20,21] The catalyti-
cally active species was found to be the 2,2,6,6-tetrameth-
ylpiperidine-1-oxoammonium ion (2Ј, TEMPO+), which is
generated in the presence of an oxidant. The oxoammon-
ium ion can react with enolates to give, for example, alk-
oxyamines.[22] More recently, TEMPO+ was prepared
photocatalytically with visible light by using dye-sensitized
titanium dioxide and transition-metal complexes based on
ruthenium and copper. Following this procedure, the
TEMPO-mediated aerobic photocatalytic oxidation of
alcohols to aldehydes was reported both heterogeneously
and homogeneously.[23–28]
–
R–BX3 K+ induced by photoredox catalysis followed by re-
action with permanent radicals, electron-deficient alkenes,
or alkynes was reported by different groups.[10–12]
Enolates and their equivalents are another class of im-
portant building blocks in synthetic organic transforma-
tions that can be oxidized. Dependent on the oxidant, they
yield either an α-carbonyl radical or an α-carbonyl carb-
enium ion intermediate. With a stoichiometric amount of
the weak one-electron oxidant tris-(p-methoxyphenyl)amin-
ium hexafluoroantimonate (E = +0.51 V vs. saturated calo-
mel electrode, SCE), α-carbonyl radicals are obtained from
stable enolates. Two equivalents of the stronger oxidizing
agent tris(1,10-phenanthroline)iron(III) hexafluorophos-
phate {[FeIII(phen)3(PF6)3], E = +1.08 V vs. SCE} afforded
α-carbonyl cations as a result of two subsequent one-elec-
tron oxidation steps.[13–15] The Jahn group reported the oxi-
dation of ester enolates with CuCl2 or ferrocenium ions and
subsequent trapping of the α-ester radicals with 2,2,6,6-
1,3-Dicarbonyl compounds have also been the target of
one-electron oxidation reactions[29] and have been used in
TEMPO-mediated oxidations with the use of visible
light.[30,31] The oxidation of the enol form occurs in the
presence of TEMPO (2 equiv.) to afford α-oxyaminated
products. However, reaction times of 24–36 h are necessary
and different reaction pathways have been proposed con-
sidering either a light-induced disproportionation reaction
of TEMPO to yield TEMPO+ and TEMPO– or the re-
duction of the enol form. We apply now stable enolates in
the reaction with photocatalytically generated TEMPO+ to
lead to significantly reduced reaction times. The proposed
reaction mechanism was supported by control experiments
by using different trapping reagents.
[a] Institut für Organische Chemie, Universität Regensburg,
Universitätsstraße 31, 93053 Regensburg, Germany
Results and Discussion
E-mail: Burkhard.Koenig@chemie.uni-regensburg.de
http://www-oc.chemie.uni-regensburg.de/koenig/index.php
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403433.
We started our investigations by treating a 1:1 mixture of
stable enolate 1a with TEMPO (2) in the presence of
Eur. J. Org. Chem. 0000, 0–0
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SHORT COMMUNICATION
[Ru(bpy)3]2+ (bpy = 2,2Ј-bipyridyl) as a photocatalyst and therefore shortened the reaction time from 3 h to 10 min
ammonium peroxodisulfate as an oxidant. Dry DMSO and increased the yield of the α-oxyaminated product to
turned out to be the solvent of choice to dissolve all of the 93%. Despite the fact that oxygen was an appropriate oxi-
ionic compounds. Irradiation with a blue high-power light- dant for the reaction, the sealed conditions in the microre-
emitting diode (LED, λmax = 455Ϯ15 nm, P = 3.0 W) at actor demanded ammonium peroxodisulfate as a nongas-
ambient temperature afforded desired α-oxyaminated prod- eous electron acceptor.
uct 3a in 87% yield (Scheme 1). Control reactions in the
The α-oxyaminated product may be obtained by two dif-
dark gave only trace amounts of product 3a. Without the ferent reaction pathways: (a) oxidation of the enolate to the
photocatalyst, a background reaction yielding 4% of α-oxy- α-carbonyl radical followed by radical recombination with
aminated product occurred. The results exclude efficient di- the TEMPO radical or (b) TEMPO oxidation and subse-
rect oxidation of TEMPO by ammonium peroxodisulfate.
quent nucleophilic attack of the enolate (Scheme 2).[22] The
structure of the product allows no distinction between the
two reaction pathways.
Scheme 1. Photocatalytic α-oxyamination of stable enolates.
Full conversion was achieved after 3 h of irradiation.
Hence, the reaction of the enolate was much faster than
that of the enol, which required at least 24 h for conversion.
By using ammonium peroxodisulfate as the terminal oxi-
dant, the amount of TEMPO could be reduced to 1 equiva-
lent. Without peroxodisulfate, a second equivalent of
TEMPO was necessary for full conversion of the enolate. A
catalyst loading of 2 mol-% was found to be optimal. Dif-
ferent photocatalysts were screened: Eosin Y gave the
α-oxyamination product in 37% yield under green light
(λmax = 520Ϯ15 nm) irradiation. 9-Mesityl-10-methyl-
acridinium perchlorate (“Fukuzumi’s dye”) as a strongly
oxidizing photocatalyst gave the product in 64% yield.
However, additional unidentifiable side products were ob-
tained. The product was obtained in 70% yield by using
2,4,6-triphenylpyrylium tetrafluoroborate as the photocata-
lyst. Both the acridinium and the pyrylium dyes were ex-
cited with purple light (λmax = 400Ϯ10 nm). The best re-
sults were obtained with [Ru(bpy)3]2+, which yielded α-oxy-
aminated product 3a in 87% yield in a clean reaction. Al-
though enolates are potential ligands for ruthenium ions,
the photocatalyst remained stable under these condi-
tions.[32] The reaction time was further reduced by applying
flow chemistry in a photomicroreactor. The benefits of flow
photochemistry have been discussed through many exam-
ples in the literature.[33–37] Compared to the batch reaction,
the reaction in a glass capillary offers a larger irradiation
surface. By using Equation (1) for the penetration depth l of
light, a layer of l = 1.0 mm depth contributes to the photo-
reaction for ε = 2.1ϫ103 m–1 cm–1 and c = 2.0ϫ10–3 m in
which ε is the molar extinction coefficient and c the concen-
Scheme 2. Generation of α-oxyaminated 1,3-dicarbonyls by (a) rad-
ical recombination or (b) nucleophilic attack of the enolate to the
oxoammonium ion.
The oxidation potentials of enolate 1a and TEMPO (2)
were determined by cyclic voltammetry to be +0.61 and
+0.68 V versus SCE in DMSO, respectively. Both cyclic vol-
tammograms show an irreversible oxidation wave (see the
Experimental Section). The potentials of the enolate and
TEMPO are identical within the error of the measurement
and both lie within the photocatalyst’s oxidizing ability.
Distinction as to which compound is photooxidized is
therefore not possible.
However, considering the reactivity of the postulated in-
termediates, the two mechanisms should be different. In the
case of enolate oxidation to the corresponding α-carbonyl
radical, other radical trapping reagents should be appli-
cable. Therefore, different π- and σ-electron donors were
added to the enolate, but no coupling product was observed
with styrene, furan, thiophene, N-methylpyrrole, or di-
methyl disulfide (5 equiv. each). Moreover, no dimerization
product resulting from radical recombination of two
α-carbonyl radicals was detected. Even if a catalytic amount
of TEMPO (10 mol-%) was added to initiate the oxidation,
no reaction was observed with the exception of a small
amount of α-oxyaminated product corresponding to the
amount of TEMPO added. The presence of TEMPO is es-
sential for the reaction. The absence of any addition prod-
uct of a postulated α-carbonyl radical and π- or σ-electron
donors or the formation of radical dimerization products
tration of the chromophore [Ru(bpy)3]2+ [38,39]
.
l = (ε·c·ln10)–1
(1)
All of the reaction mixture was exposed to light upon indicates the one-electron oxidation of TEMPO to the cor-
pumping the solution through the irradiated glass capillary responding TEMPO+ ion as the key step of the reaction.
of 0.5 mm radius (for further details, see the Supporting
On the basis of these results, the following mechanism is
Information). Switching from batch to a microreactor setup proposed: After excitation of the photocatalyst by blue
2
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Photocatalytic α-Oxyamination
light, ammonium peroxodisulfate acts as an oxidative the α-carbon atom: enolate 4 is more basic, which led to
quencher to form the strongly oxidizing species decomposition in DMSO solution before irradiation was
[Ru(bpy)3]3+ [40]
The oxidant is reduced to sulfate and a started. The same applied to enolates of simple aldehydes
.
sulfate radical anion that is capable of accepting an ad- and ketones, and this limits the scope of the method. Ma-
ditional electron.[41] Regeneration of the photocatalyst is lononitrile 5 did not afford the oxyaminated product. In
achieved by oxidizing TEMPO (2) to corresponding the presence of a strong base, self-condensation occurred
oxoammonium ion 2Ј. Finally, enolate 1a attacks the oxid- instead, which resulted in the formation of 2-amino-1,1,3-
ized species in a nucleophilic manner to produce α-oxy- tricyanopropene dimer 7 (Scheme 4).[43,44]
aminated product 3a (Scheme 3).
Scheme 4. Limitations in the oxyamination reaction.
According to the mechanistic proposal (Scheme 3), the
enolate reacts as a nucleophile. Oxyamination was therefore
expected to proceed also with other nucleophiles. To prove
the hypothesis, other classes of nucleophiles were examined
in the reaction with photocatalytically generated TEMPO+.
Scheme 3. Proposed mechanism for the α-oxyamination of stable
The nucleophiles were selected by using Mayr’s nucleophi-
enolates.
licity scale.[45–48]
The principle of α-oxyamination of carbon nucleophiles
was extended to β-keto phosphine oxides and β-keto alkyl
phosphonic esters. After preparing the enolate in situ by
treatment with sodium hydride, nucleophilic attack to the
photocatalytically generated oxoammonium ion occurred.
Product 9a could only be identified in trace amounts, but
compounds 8b and 8c gave desired products 9b and 9c in
yields of 68 and 57%, respectively. The reaction time was
30 min (Table 2).
Stable enolates are easily accessible from 1,3-diketones
by deprotonation with sodium hydride in diethyl ether at
room temperature. Under these conditions, the (Z)-enolate
is formed selectively.[42] A range of enolates were tested in
the α-oxyamination reaction (Table 1).
Table 1. Photooxidation reaction of stable enolates with TEMPO.[a]
Table 2. Oxyamination of organophosphorus compounds.[a]
Entry Reactant
R1
R2
Product
Yield [%]
1
2
3
4
5
1a
1b
1c
1d
1e
Ph
Ph
Me
Ph
Ph
Me
Me
OEt
OEt
3a
3b
3c
3d
3e
93
81
82
96
86
Entry Reactant
R1
R2
Product
Yield [%]
Me
1
2
3
8a
8b
8c
Ph
Ph
EtO
OEt
Ph
Ph
9a
9b
9c
trace
68
57
[a] Reaction conditions: Enolate (0.6 mmol), TEMPO (0.6 mmol),
(NH4)2S2O8 (0.3 mmol), Ru(bpy)3Cl2·6H2O (0.012 mmol), DMSO
(6.0 mL, absolute), photomicroreactor, irradiation with eight high-
power LEDs (λmax = 455Ϯ15 nm, P = 3.0 W). Yields of the iso-
lated products are given.
[a] Reaction
conditions:
Organophosphorus
compound
(0.6 mmol), NaH (0.72 mmol), TEMPO (0.6 mmol), (NH4)2S2O8
(0.3 mmol), Ru(bpy)3Cl2·6H2O (0.012 mmol), DMSO (6.0 mL, ab-
solute), photomicroreactor, irradiation with eight high-power
LEDs (λmax = 455Ϯ15 nm, P = 3.0 W). Yields of the isolated prod-
The α-oxyamination proceeded smoothly with 1,3-di-
keto- and β-keto ester enolates 1a–e. The scope of the reac-
tion was limited to β-keto enolates without substitution at ucts are given.
Eur. J. Org. Chem. 0000, 0–0
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P. Schroll, B. König
SHORT COMMUNICATION
Enamines have already been successfully used in oxy-
amination reactions. After hydrolysis, α-functionalized
carbonyls were obtained.[49,50] Therefore, related silyl enol
ethers were examined as nucleophilic carbonyl deriva-
tives.[51] These enolate equivalents were converted in 30 min
under photooxidation conditions into α-oxyaminated prod-
ucts 11a–c in good yields (Table 3).
Acknowledgments
Financial support by the Evonik Foundation (stipend to P. S.) and
the Deutsche Forschungsgemeinschaft (DFG) (GRK 1626, Chemi-
cal Photocatalysis) is acknowledged.
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Experimental Section
General Procedure for Photocatalytic α-Oxyamination Reactions of
Stable Nucleophiles: A 10 mL flask was equipped with Ru(bpy)3-
Cl2·6H2O (9.0 mg, 0.012 mmol, 0.02 equiv.), nucleophile
(0.6 mmol, 1.0 equiv.), TEMPO (93.8 mg, 0.6 mmol, 1.0 equiv.),
ammonium peroxodisulfate (68.5 mg, 0.3 mmol, 0.5 equiv.), and
dry DMSO (6.0 mL). The mixture was transferred to a glass micro-
reactor (115ϫ60ϫ6 mm, diameter of capillary: 1.0 mm) by a sy-
ringe and was irradiated with an array of eight blue high-power
LEDs (λmax = 455Ϯ15 nm, P = 3.0 W) for 10 min. The tempera-
ture in the microreactor was kept at 20 °C. After that, the mixture
was diluted with water (5 mL) and extracted with diethyl ether (3ϫ
5 mL). The combined organic layer was concentrated in vacuo. Pu-
rification of the crude product was achieved by flash column
chromatography (petroleum ether/ethyl acetate, 19:1). α-Oxyamin-
ation reactions of organophosphorus compounds differ by initial
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Published Online: ᭿
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P. Schroll, B. König
SHORT COMMUNICATION
Photocatalysis
P. Schroll, B. König* ......................... 1–6
Photocatalytic α-Oxyamination of Stable
Enolates, Silyl Enol Ethers, and 2-Oxoalk-
ane Phosphonic Esters
Keywords: Photooxidation
lysis / Radicals / Flow catalysis / Microreac-
tors
/ Photocata-
Fast and efficient visible-light-mediated α-
oxyamination of β-keto enolates, silyl enol
ethers, and deprotonated dialkyl 2-oxoalk-
anephosphonates occurs in a microreactor
in the presence of [Ru(bpy)3]2+ (bpy = 2,2Ј-
bipyridyl) as a photocatalyst, 2,2,6,6-tetra-
methylpiperidine nitroxide (TEMPO), and
blue light. The key step is the light-induced
one-electron oxidation of TEMPO.
6
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