Azide/oxygen photocatalysis with homogeneous and heterogeneous photocatalysts for 1,2-aminohydroxylation of acyclic/cyclic alkenes and Michael acceptors

Article abstract of DOI:10.1007/s11164-012-0629-3

Homogeneous as well as heterogeneous photocatalysts that are able to oxidize the azide anion with low competitive singlet oxygen quantum yields are used to generate azidyl radicals. These radicals add to electron-rich as well as electron-poor (Michael acc

Full text of DOI:10.1007/s11164-012-0629-3

Res Chem Intermed (2013) 39:33–42
DOI 10.1007/s11164-012-0629-3
Azide/oxygen photocatalysis with homogeneous
and heterogeneous photocatalysts
for 1,2-aminohydroxylation of acyclic/cyclic
alkenes and Michael acceptors
•
Axel G. Griesbeck Jorg Steinwascher
•
¨
•
Melissa Reckenthaler Johannes Uhlig
¨
Received: 22 August 2011 / Accepted: 1 September 2011 / Published online: 12 June 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Homogeneous as well as heterogeneous photocatalysts that are able to
oxidize the azide anion with low competitive singlet oxygen quantum yields are used
to generate azidyl radicals. These radicals add to electron-rich as well as electron-poor
(Michael acceptors) alkenes, and carbon radicals are formed regioselectively. Trap-
ping with triplet oxygen (type I photooxygenation) is diffusion controlled, and the
initially formed peroxy radicals are reduced with regeneration of the photocatalyst.
Fluorescence quenching studies reveal rapid photoinduced electron transfer in the first
catalysis step. The lack of rearrangement products in the bicyclic terpene series
(pinenes, limonene) accounts for rapid subsequent oxygen trapping and back electron
transfer steps. The 1,2-azidohydroperoxidation enables synthesis of 1,2-azidoalcohols
and 1,2-aminoalcohols by different reduction protocols. Substrate modification and
combination of type II photooxygenation with electron transfer photocatalysis allows
the synthesis of 1-amino-2,3-diols and 2-amino-1,3-diols.
Keywords Photochemistry Á Photooxygenation Á Azidyl radicals Á Photoinduced
electron transfer
Introduction
Oxygen transfer reactions are exceedingly important synthetic processes in organic
synthesis and industrial chemistry of organic basic compounds [1]. With respect to
atom economy [2], direct use of molecular oxygen appears as the most sustainable
Dedicated to Professor Kazuhiko Mizuno on the occasion of his retirement from Osaka Prefecture
University.
¨
A. G. Griesbeck (&) Á J. Steinwascher Á M. Reckenthaler Á J. Uhlig
Department of Chemistry, University of Cologne, Greinstr. 4, 50937 Cologne, Germany
e-mail: griesbeck@uni-koeln.de
123

34
A. G. Griesbeck et al.
approach, encompassing activation of oxygen by specific oxygen transfer reagents
or by transition-metal catalysts that activate oxygen transfer reagents. Triplet
oxygen is however preferentially used in radical-type processes and not in polar
reactions. Photochemical activation of triplet oxygen can occur by two initial
processes: energy transfer and electron transfer, respectively, leading to singlet
oxygen (1Dg–1O2) or to the superoxide radical anion [3]. Reactions of singlet
oxygen with organic substrates that lead to oxygenated products (‘‘chemical
quenching’’) are termed type II reactions. These processes can be sensitized by
numerous energy transfer photocatalysts, mostly triplet sensitizers with high
intersystem crossing quantum yields and low-lying triplet states (1–2 eV). In these
cases, the catalyst functions in a one-step process, initiating the spin inversion of
oxygen from its triplet ground state with simultaneous formation of the catalyst in
its electronic ground state. In the alternative electron transfer (ET) processes that are
initiated by ET from or to a sensitizer molecule, a second electron transfer must
occur in order to regenerate the ground state of the photocatalyst. If products are
formed by concomitant combination, these processes are termed type A and type B
for semiconductor photocatalysis [4]. In type A photocatalysis, from the light-
generated electron–hole pair, two different products are formed, one resulting from
substrate electron transfer to the semiconductor hole, and the other from substrate
reduction by the semiconductor electron; in type B photocatalysis, combination
products are formed.
Recently we described photocatalytic 1,2-aminohydroxylations via intermediate
1,2-azidohydroperoxides that make use of the azidyl radical as amino group
precursor and molecular oxygen [5, 6]. This photochemical process is efficiently
catalyzed by several photoinduced electron transfer (PET) sensitizers. Beside
traditional fluorescent dyes, also nanosized semiconductor particles can be used [7].
It is crucial that these sensitizers do not (or hardly, e.g., UD for fluorescein = 0.03)
produce singlet oxygen by energy transfer from their excited triplet states, because
most alkenes tend to react quickly under type II (singlet oxygen) conditions [8].
Thus, sensitizers with low triplet quantum yields and high singlet excited-state
oxidation potential are most convenient. We have established rhodamine B and
rhodamine 6G as efficient catalysts that are quenched by azide anions with rate
constants of 1.6 9 109 and 3.3 9 109 M-1 s-1, respectively [5]. The well-studied
PET catalyst 9,10-dicyanoanthracence (DCA) is quenched by azide in methanol/
water with bimolecular rate constant of 6.4 9 109 M-1 s-1 [9]. The initial products
of the complex reaction cascade are 1,2-azido-hydroperoxides that can be converted
into azidoalcohols by mild reductants and into 1,2-aminoalcohols under stronger
reduction conditions (Scheme 1). Even more appealing are processes that combine
two photochemical steps, i.e., the type II photooxygenation that shifts the C=C
double bond with concomitant allylic oxygenation and a subsequent type B
photocatalysis that results in amino-1,2-diol structures.
The choice of sensitizer and solvent depends on the redox properties of the
excited sensitizer and the solubility of the azide salt. Sodium and lithium azide can
be used in water or in aqueous methanol, and trimethylsilyl azide is also applicable
in acetonitrile in the presence of acetic acid [10]. The oxidation potential of azide
anion in water has been reported as 1.32 V [11]. Thus, the limiting excited-state
123

Azide/oxygen photocatalysis
35
3O2
Typ II-
OOH
OOH
Photooxygenation
Sens./hν
N3
N3 , 3O2
-
Typ B-
Photocatalysis
Sens./hν
N3 , 3O2
-
3O2
OOH
Combined
II/B-process
N3
Sens./hν Sens./hν
OOH
O
OOH
OOH
OH
OH
OH
OH
or
N3
N3
NH2
or
Scheme 1 Reaction modes for the oxyfunctionalization and N,O-activation of alkenes
reduction potential is about 1.2 V, and most of the typical fluorescent dyes (e.g.,
fluoresceine, uranine, rhodamines, methylene blue) are applicable [12]. Rose Bengal
with an excited-state reduction potential of 0.99 V [13] cannot oxidize azide
exergonically and is used as a type II sensitizer for the generation of singlet oxygen.
Experimental
Photoreaction of substrates under homogeneous catalysis with sodium azide
A solution of 1 mmol of the alkene and 2.5 mmol (162 mg) sodium azide in 25 ml
stock solution of methanol/water (95:5 vol %) containing 2 9 10-4 M sensitizer
was irradiated under air for 24 h at 15 °C by means of a 70-W metal vapor lamp
(Osram Vialux Super 44). The crude reaction solution was filtered and extracted
with 3 9 15 ml methylene chloride. The organic phase was washed with 15 ml
brine and dried over MgSO4. After filtration, the crude reaction product was purified
by column chromatography.
For 10 mmol reactions, 100 ml of solvent mixture and 5 9 10-3 M sensitizer
concentration were applied.
Photoreaction of substrates under homogeneous catalysis with trimethylsilyl
azide
A solution of 1 mmol mesitylol 12 (100 mg), 5 mmol (576 mg) trimethylsilyl azide,
and 5 mmol (300 mg) acetic acid in 25 ml stock solution of methylene chloride
123

36
A. G. Griesbeck et al.
containing 2 9 10-4 M rhodamine B (or 5 mg CdS) was irradiated under air for
12 h at 15 °C by means of a 70-W metal vapor lamp. Workup was as described
above.
Photoreaction of substrates under heterogeneous semiconductor nanoparticle
catalysis
A solution of 1 mmol of the alkene and 2.5 mmol (162 mg) sodium azide in 25 ml
stock solution of methanol/water (95:5 vol %) containing 5 mg CdS was irradiated
under air for 12 h at 15 °C by means of a 70-W metal vapor lamp (Osram Vialux
Super 44) under vigorous stirring. Workup was as described above.
Results and discussion
Substrates for the azidohydroperoxidation reaction
The rate constants of azidyl radical addition to alkenes were determined by
Workentin and coworkers [11] and range from 106 M-1 s-1 (for cyclohexene) to
1.3 9 108 M-1 s-1 (for tetramethylethylene). No results for Michael acceptors are
known. In Scheme 2, an overview on the azidohydroperoxidation of cyclic electron-
rich alkenes is given. The pinenes 1, 3 can be prior to the azidyl radical addition be
converted to the corresponding allylic hydroperoxides by means of singlet oxygen
reaction and eventually give the amino diols 2, 4 in moderate overall yields [7].
Methylcyclohexene (5) and limonene (7) show moderate to high diastereoselectiv-
ity. In the case of limonene, the primary azidyl radical addition is trans selective
with respect to the isobutenyl group and with high regioselectivity concerning the
addition primarily to the higher-substituted double bond in Markovnikov fashion.
The secondary oxygen addition is stereodirected in the case of limonene with much
higher d.r. in comparison with the case of methylcyclohexene, possibly due to a
steric combination effect of both isobutenyl and azide groups [5].
In the kinetic studies by Workentin et al. [11], styrene and a-methylstyrene are
reported as azidyl acceptors with rate constants of 7 9 106 M-1 s-1 and
N3, 3O2
N3
1) 1O2
2) N3, 3O2
H
NH2
OH
OH
H
OOH
5
6
d. r. = 70 : 30
N3, 3O2
1
3
3) Red.
2
1) 1O2
N3
2) N3, 3O2
3) Red.
OH
OH
NH2
4
OOH
8
7
d. r. > 95
Scheme 2 Cycloalkenes as substrates under azidohydroperoxidation conditions
123

Azide/oxygen photocatalysis
37
3 9 108 M-1 s-1, respectively. We tested these substrates that are easily polymer-
izable under radical conditions and observed the corresponding azidohydroperox-
ides in moderate yields [5]. In all cases, the corresponding alcohols are formed as
side products, and only for the styrene case as the major product.
As new substrates, which we currently use in singlet oxygen reactions as starting
materials towards antimalarial peroxides [14], allylic alcohols were tested, among
them prenol (9) and mesitylol (12). Allylic alcohol 9 could be reacted in methanol/
water mixtures to give a mixture of azidohydroperoxide 10 and the azido 1,3-diol 11
with 36 % conversion and 30–40 % recovered substrate. In the case of the chiral
substrate 12, the solubility in protic media was low and only the heterogeneous
CdS-catalyzed version resulted in products. A 2:1 mixture of syn and anti products,
13a and b, respectively, was isolated. The relative configuration was determined by
3
the characteristic JHH-coupling constants. The major syn diastereoisomer has a
3JHH of 1 Hz whereas the minor anti diastereoisomer has a 3JHH of 8.2 Hz, which is
in agreement with a preferred conformation with near-anti periplanar arrangement
of the tertiary C–H bonds in the anti isomer. The direction of the stereodiscrim-
ination effect parallels the result with singlet oxygen that also delivers preferentially
the syn diastereoisomer [15]. This effect is slightly increased when changing the
solvent to methylene chloride. Trimethylsilyl azide was used as azide ion source,
and rhodamine B as photocatalyst (Scheme 3). This approach allows the synthesis
of 2-amino-1,3-diols and enlarges the synthetic potential of the reaction.
A third class of substrates that was investigated for this communication were
Michael acceptors. We used three a,b-unsaturated esters, the acrylate system 14 and
two trisubstituted compounds, the methacrylate 17 and the tiglate 20. In all cases,
the substrate conversions were high but large amounts of polymeric materials were
formed, especially from methylacrylate 14. The major product was the b-azido
propionate 15 beside the solvent incorporation product 16. Whereas we have no
convincing explanation for the Michael addition product 16 (which is not produced
under thermal conditions), the azide 15 is the first example of a C-radical reduction
N3
N3
NaN3, 3O2, hν
OH
76 : 24
OH
OH
OH
+
MeOH/H2O
rhodamine B
OOH
OH
9
10
11
36%
N3
N3
NaN3, 3O2, hν
OH
OH
+
MeOH/H2O
OOH
OOH
CdS
63 : 37
12
13b
13a
19% (isolated yield)
67 : 31
TMSN3 / CH2Cl2
rhodamine B
13b
13a
17% (isolated yield)
Scheme 3 Allylic alcohols 9 and 12 as substrates under azidohydroperoxidation conditions
123

38
A. G. Griesbeck et al.
by the sensitizer radical anion in the two-step electron transfer photocatalysis. This
reaction is less preferred for a-methylated substrates, and thus was not observed for
the compounds 17 and 20 (Scheme 4). Among all substrates investigated by us, the
methacrylate 17 is the most efficient starting material, delivering a 1:1 mixture of
azidohydroperoxide 18 and the corresponding alcohol 19 in good yields and with
high substrate conversion. The difference in chemoselectivity (i.e., formation of an
azide addition product 15 versus the regular trapping product 18) appears due to the
high tendency of C-radical A for back electron transfer by the sensitizer radical
anion, whereas the C-radical B can be trapped by molecular (triplet) oxygen.
Sensitizers for the azidohydroperoxidation reaction
As already described in the ‘‘Introduction,’’ the majority of the traditional
fluorescent dyes are capable of exergonic electron transfer from azide anions to
their electronically excited singlet states. The electron transfer rate can be
determined by fluorescence quenching with a high degree of precision because
accurate singlet lifetimes are available for all photocatalysts. The results for
fluorescence quenching of four dyes that can be used in aqueous media or in
acetonitrile (9,10-DCA) are summarized in Table 1. Typical fluorescence quench-
ing and Stern–Volmer plots are shown in Fig. 1 for rhodamine 6G and
coumain 151.
The efficiency of sensitizers as electron transfer photocatalysts were also studied
by product analysis. In the azidohydroperoxidation of a-pinene (3), a highly reactive
substrate with high regio- and stereocontrol, with total yields of 55–70 %, three
products were formed, a 9:1 mixture of the azidohydroperoxide 22 and the
azidoalcohol 23, and an unknown side product, possibly a cyclobutane ring-opening
derivative (Scheme 5). From the viewpoint of product chemoselectivity, uranine
and coumarine 151 are the most effective photocatalysts, whereas the amount of
azidoalcohol increases for rhodamine B and especially for CdS. This might be due
NaN3, 3O2, hν
H3CO
OMe
OMe
N3
OMe
+
MeOH/H2O
rhodamine B
O
O
O
O
14
17
N3
OMe
OMe
67 : 33
15
16
15%
O
A
OOH
OH
NaN3, 3O2, hν
OMe
N3
OMe
N3
OMe
+
N3
MeOH/H2O
rhodamine B
O
O
O
B
53 : 47
18
19
42%
OH
NaN3, 3O2, hν
OMe
N3
OMe
MeOH/H2O
O
O
rhodamine B
d.r. = 52 : 48
20
21
23%
Scheme 4 Michael acceptors as substrates under azidohydroperoxidation conditions
123

Azide/oxygen photocatalysis
39
Table 1 Fluorescence
Sensitizer
sS [ns]
kq [109 l mol-1 s-1
]
quenching in MeOH/H2O (1:1)
of dyes in the presence of azide
anions: sS and kq
9,10-DCA
14.8 [16]
6.2 [17]
3.7 [17]
6.7 [18]
6.4
1.6
3.3
2.3
Rhodamine B
Rhodamine 6G
Coumarin 151
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
2.2
2.0
1.8
30
25
20
15
10
5
1.6
A
B
1.4
25
1.2
1.0
20
0.8
-0.02
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
-0.02 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
conc. NaN [mol/L]
3
15
10
5
conc. NaN [mol/L]
3
0
0
500 520 540 560 580 600 620 640 660
wavelength [nm]
350
400
450
500
550
600
wavelength [nm]
Fig. 1 Fluorescence quenching by azide anions (aqueous media) for rhodamine 6G (a) and
coumarin 151 (b) in H2O/MeOH (1:1); insets: Stern–Volmer plots
to less efficient electron back transfer from the photocatalyst radical anions (from
uranine, coumarines, or 9,10-DCA) or radicals (from the cationic dyes rhodamine B
and 6G). In competition with this electron back transfer to the rapidly formed
peroxy radical, reductive pathways might exist that reduce the peroxy radical to the
hydroxyl group with irreversible oxygenation of the catalyst. In the case of
heterogeneous photocatalysis with semiconductor nanoparticles, this reaction leads
to the formation of oxidized catalyst that precipitates out of the solution;
rhodamine 6G shows rapid bleaching (Table 2).
Mechanism of the azidohydroperoxidation reaction
Describing a multistep catalytic process by kinetic data is obviously a challenging
task that can only be achieved if the individual steps of the process can be
investigated independently from the general process. Furthermore, molecular probes
that report the feasibility of the postulated reaction steps in the global process are
necessary. In the photocatalytic azidohydroperoxidation that we have developed,
NaN3, 3O2, hν
N3
H
H
H
+
+
N3
MeOH/H2O
N3
O
O
OH
catalyst
HO
HO
3
23
22
Scheme 5 a-Pinene under azidohydroperoxidation conditions
123

40
A. G. Griesbeck et al.
Table 2 a-Pinene (3) under azidohydroperoxidation conditions: product pattern [rel. ratios by 1H nuclear
magnetic resonance (NMR)]
Sensitizer
Hydroperoxide (%) 22
Alcohol (%) 23
Byproduct (%)
Rhodamine B
Rhodamine 6G
9,10-Dicyanoanthracene
Uranine
79
82
72
87
88
62
20
7
1
11
14
11
8
14
2
Coumarine 151
CdS
4
25
10
hν
bET
ET
Sens.*
Sens.-.
Sens.
OOH
Sens.
3O2
1O2
-
N3
N3
OO
-
eT
3O2
OO
H+
kAdd
N3
ene reaction and
physical quenching
N3
N3
N3
Scheme 6 1,2-Azidohydroperoxidation: the overall mechanism
four reaction steps can be identified: primary electron transfer with formation of the
azidyl radical, alkene addition of the azidyl radical, oxygen trapping of the carbon
radical, and secondary electron transfer with formation of the peroxy anion and
reformation of the photocatalyst.
In competition with step 1 (ET), triplet–triplet energy transfer (eT) can occur
with formation of singlet oxygen. This process was evaluated by typical singlet
oxygen acceptors and, under normal cases, cannot compete with ET in the case of
highly fluorescent dyes with appropriate excited-state reduction potentials. The rate
of azidyl radical addition to alkenes (kadd = 107–108 M-1 s-1) was evaluated in the
absence of oxygen [11], the formation of azidyl radicals (kq = 2–6 9 109 M-1 s-1
)
was evaluated by fluorescence quenching methods, and the rates of triplet oxygen
addition to carbon-centered radicals are well known (kadd = 2–5 9 109 M-1 s-1
for benzylic radicals [19]). This is in excellent agreement with the low amounts of
radical-induced ring-opening reactions that we have observed in the pinene series
(vide supra) (Scheme 6).
Conclusions
Organic azides are highly useful substrates for numerous reactions in organic
synthesis [20], including the fabulous ‘‘click’’ reaction [21]. Using the process
developed by us, the azide group can be introduced into unsaturated substrates with
123

Azide/oxygen photocatalysis
41
high regio- and diastereoselectivity. The usefulness of the photocatalytic process is
still limited concerning the azide source, the solvents, and especially the best
possible photocatalyst. Promising photocatalysts are nanosized semiconductor
particles such as CdS or TiO2 [7] that are currently being studied.
Acknowledgments We gratefully acknowledge financial support by the Deutsche Forschungsgeme-
inschaft (DFG) and the Volkswagen-Stiftung.
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