Anti-Markovnikov Hydroazidation of Alkenes by Visible-Light Photoredox Catalysis

Article abstract of DOI:10.1002/chem.201806371

The anti-Markovnikov hydroazidation of alkenes has been accomplished under visible-light irradiation by using [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆ as the photocatalyst and trimethylsilyl azide as the azidating agent. The reactions were greatly facilitated by water, the beneficial effect of which can be attributed to its participation in the reaction as the hydrogen donor, as indicated by deuterium isotope experiments. The reactions proceed under solvent free conditions in the presence of water. 4-Dimethylaminopyridine also exhibited a beneficial effect on the reactions. The present method enabled hydroazidation of several types of unactivated alkenes with good yields and high regioselectivity.

Full text of DOI:10.1002/chem.201806371

A Journal of
Accepted Article
Title: Anti-Markovnikov Hydroazidation of Alkenes by Visible Light
Photoredox Catalysis
Authors: Juan-juan Wang and Wei Yu
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To be cited as: Chem. Eur. J. 10.1002/chem.201806371
Link to VoR: http://dx.doi.org/10.1002/chem.201806371
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Anti-Markovnikov Hydroazidation of Alkenes by Visible Light
Photoredox Catalysis
Juan-juan Wang and Wei Yu*
Dedication ((optional))
Renaud et al. reported a one-pot procedure towards this goal:
the alkene is first hydroborated to give an organoborane which is
then azidated with benzenesulfonyl azide via radical-mediated
azidyl transfer.[18] Recently, Chiba and Gagosz developed
Abstract: The anti-Markovnikov hydroazidation of alkenes has been
accomplished under visible light irradiation by using
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6 as photocatalyst and trimethylsilyl azide
as azidating agent. The reactions were greatly facilitated by water,
whose beneficial effect can be attributed to its participation in the
reaction as the hydrogen donor, as indicated by deuterium isotope
experiments. The reactions can be effected under solvent free
conditions in the presence of water. 4-Dimethylaminopyridine also
exhibited a beneficial effect on the reactions. The present method
enabled hydroazidation of several types of unactivated alkenes with
good yield and high regionselectivity.
another
radical
procedure
for
the
anti-Markovnikov
hydroazidation of functionalized alkenes.[19] This method
involves the addition of azidyl radical onto the alkene moiety
followed by intramolecular hydrogen atom transfer from an
internal benzyl protecting group. Although these studies provide
effective means for the hydroazidation of alkenes, simple
straightforward anti-Markovnikov hydroazidation protocols are
still lacking, and more approaches are needed to meet various
synthetic demands. In this concern, herein we report a visible
light-induced anti-Markovnikov hydroazidation of unactivated
alkenes (Scheme 1, (b)). Our method features the use of
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6 as photocatalyst and TMSN3 as
azidating agent. Moreover, water and 4-dimethylaminopyridine
were employed to facilitate the reaction. This system allowed us
to prepare some functionalized primary azides with good yield
and high selectivity.
Hydroamination of readily available alkenes constitutes a direct
and atom-economical means to gain access to amine
derivatives.[1] Despite its apparent simplicity, implement of this
type of reactions has proven to be a nontrivial task as
demonstrated by the enormous efforts made towards this
goal.[1,2] Particularly challenging in this line is to realize these
transformations in an anti-Markovnikov manner. While transition
metal catalysis provides viable solutions to this problem,[3-7]
radical-mediated addition reactions have also been applied to
effect anti-Markovnikov hydroamination.[8] Recent investigations
by Nicewicz et al.[9,10] and Knowles et al.[11] show that visible light
photoredox catalysis constitutes a powerful means for the anti-
Markovnikov hydroamination of both activated and unactivated
alkenes under mild catalytic conditions, and these excellent
studies well illustrate the usefulness of the photochemical
protocols in the synthesis of chain amines and sulfonamides.
Olefin azidation reactions are highly effective for the synthesis
of organic azides.[12] However, compared with olefin
difunctionalization azidation reactions, hydroazidation of
alkenes,[13,14] a variant type of hydroamination, has much less
been explored. There have only been several studies
addressing this issue during the last two decades.[15-19] In 2005,
Waser and Carreira developed
a
Markovnikov olefin
hydroazidation strategy featuring a cobalt-catalyzed radical
hydrogen addition and azidyl transfer process.[16] Later on,
Boger et al. realized similar transformations by using iron as
catalyst.[17] Besides their applications in Markovnikov
hydroazidation, radical reactions have also been employed for
the anti-Markovnikov hydroazidation (Scheme 1, (a)). In 2011,
Scheme 1. Anti-Markovnikov hydorazidation.
At the initial stage of this research, we envisioned that alkenes
might be hydroazidated with an azide anion in an anti-
Markovnikov manner under the conditions of visible light
photoredox catalysis. In this view, we began our investigation by
searching for a suitable catalytic system to realize this goal.
After extensive exploration of reaction conditions with several
olefins as substrates and TMSN3 as the azidating agent, we
found that when [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 was used as the
catalyst, compound N-phenylpent-4-enamide (1a) could be
converted to the desired hydroazidation product 2a in low yield
in acetonitrile in the presence of 4-dimethylaminopyridine
J-j. Wang, Professor W. Yu
State Key Laboratory of Applied Organic Chemistry, College of
Chemistry and Chemical Engineering
Lanzhou University
Lanzhou 730000 (China)
E-mail: yuwei@lzu.edu.cn
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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(DMAP). On the basis of this result, the reaction conditions were
further explored with 1a as the model compound and
CH3OH. It is noteworthy that water exhibited conspicuous
beneficial effect on the reaction. The reaction was also facilitated
by DMAP, but it was not indispensable (entry 15). The reaction
did not take place when both water and DMAP were absent
(entry 16). Although DMAP helped to improve the yield, it also
prolonged the reaction time to some extent (see Table 1 in
Supporting Information (SI)). The best result was obtained in
DBE (0.4 M for 1a) under the conditions of 5.0 equiv. of TMSN3,
1.0 equiv. of DMAP and 24 equiv. of water: not only the yield of
2a was raised to 87%, the reaction also proceeded quite fast (3
h,entry 12). Besides TMSN3, tetrabutyl ammonium azide and
sodium azide were also tested, but they were literally ineffective
under in the present circumstances (For detailed information
about optimization of reaction conditions, see Tables 1 and 2 in
Supporting Information (SI)).
The optimized conditions (Table 1, entry 12) were then
applied to variously unsaturated N-aryl amides to test the scope
of this hydroazidation protocol, and the result is presented in
Schemes 2-3 and Table 2. For ,δ-unsaturated anilides bearing
a substituent at the phenyl ring, the reaction took place smoothly
in most cases, giving rise to the anti-Markovnikov products in
good to excellent yields (Scheme 2). The only exceptions were
substrates bearing a p-OH or p-NO2 group, in which case the no
desired products 2o and 2p were obtained.
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6
as
photocatalyst.
The
representative results are summarized in Table 1.
Table 1. Screening of the conditions for the reaction of 1a[a]
Entry
TMSN3
(mmol)
DMAP (mmol)
H2O
(mmol)
Solv.
Yield of 2a
(%)
1
0.4
0.4
0.4
0.4
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.6
0.8
1.0
1.0
1.0
1.0
1.0
0.04
--
--
CH3CN
PhCF3
PhCF3
PhCF3
PhCF3
toluene
CH2Cl2
CH2Cl2
CH2Cl2
DCE[b]
10
2
--
0
3
0.04
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.2
--
--
23
4
--
26
5
4.8
4.8
4.8
0.6
56
53
6
23
7
56
8
46
9
36
10
11
12
13
14
15
16
17
18
19
20[d]
21[e]
22[f]
23
24
4.8
4.8
4.8
4.8
4.8
4.8
--
44
[b]
CHCl3
DBE[b]
DBE[b]
DBE
78
87
76
62
DBE[b]
DBE[b]
DBE[b]
DBE[b]
DBE[b]
DBE[b]
DBE[b]
DBE[b]
CH3OH
MeCN
48
.
--
0[c]
78
Scheme 2. Effect of substituent at the N-phenyl group.
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
4.8
4.8
4.8
4.8
4.8
4.8
4.8
4.8
48
Subsequent experiments revealed that the distance between
the double bond and the amidyl group has a crucial effect on the
reaction. As such, while compound 4c and 4d were obtained in
good yield, the yield of 4b dropped to 27%. Compound 4a, on
the other hand, cannot be formed from the corresponding
precursor under the current conditions (Scheme 3).
70
69
N. R.
N. R.
N. R.
Trace[g]
[a] The reactions were performed on a 0.2 mmol scale in 2 mL solvent under
an argon atmosphere with 2 mol% of photocatalyst unless otherwise specified.
The yields given are isolated yields. [b] The reaction was performed in 0.5 mL
solvent. [c] 1a decomposed. [d] 1 mol% of [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 was
used. [e] Control experiment in the dark. [f] Control experiment in the absence
of [Ir(dF(CF3)ppy)2(dtbbpy)]PF6. [g] Most of 1a recovered. DCE
= 1,2-
dichloroethane; DBE = 1,2-dibromoethane.
It can be seen from Table 1 that the desired reaction took
place in solvents such as PhCF3, toluene, CH2Cl2, CHCl3, 1,2-
dichloroethane (DCE) and 1,2-dibromoethane (DBE), but not in
Scheme 3. Effect of chain length in the unsaturated anilides.
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Variation of the substituent at other positions also influences
the reaction consequence. As shown in Table 2, while this
protocol is applicable to the preparation of compounds 5-9 and
11-16, it is ineffective when used to prepare compounds 10. For
generate the azidyl radical, which then adds onto the carbon-
carbon double bond from the less substituted side.[11,19] The
other is via the intermediacy of alkene radical cations generated
by single-electron oxidation of the alkenes.[9,10] Both mechanism
can explain the observed anti-Markovnikov selectivity. To see
which mechanism operates under the current conditions,
luminescence quenching experiments[20,21] were conducted with
both TMSN3 and 1a as the quenchers (Figure 1 in SI). It was
found that luminescence of the excited Ir[III] catalyst can be
quenched by TMSN3, While 1a has no such effect. Subsequent
cyclic voltammetry experiment shows that 1a has a high
oxidation potential (Ep = 1.65 V, vs SCE, see SI), and thus its
oxidation by the photoexcited [Ir(dF(CF3)ppy)2(dtbbpy)]PF6
(E1/2red[*Ir(III)/Ir(II) = +1.21 V vs SCE in MeCN][20,21] should be
endergonic. TMSN3, on the other hand, has an oxidation
potential of 0.66 V (vs. SCE),[22] and can be oxidized by
photoexcited [Ir(dF(CF3)ppy)2(dtbbpy)]PF6. Thus, we conclude
that the present reactions probably proceeded following a
mechanism where the azidation step involves addition of azidyl
radical onto the alkene double bond to give a carbon radical.
Evidence supporting the formation of this carbon radical can be
found in the reaction of compound 35, where cyclized product 36
was produced in good yield as a mixture of diastereoisomers
(Scheme 4). Luminescence quenching experiments indicate that
DMAP can also quench the excited state of the photocatalyst,
and that explains why the reaction became slower in the
presence of DMAP.
substrates bearing
a terminal double bond, the reaction
proceeded exactly in the anti-Markovnikov pattern. For
substrates bearing an internal 1,2-disubsituted double bond, the
azidyl was introduced to both positions in roughly the same
efficacy (11-1 and 11-2). It is noteworthy that when an allyl
group was tethered to the nitrogen group or to the ortho-position
of the phenyl group, the carbon-carbon double bond was
hydroazidated as well (14-16). N-phenyl-2-vinylbenzamide, on
the other hand, cannot be converted under the same conditions.
Table 2. Scope of alkenes-1[a]
[a] The reactions were conducted on a 0.2 mmol scale unless otherwise
specified. The yields given are isolated yields. [b] A complex mixture was
generated. [c] The Reaction was performed on 0.5 mmol scale.
Table 3. Scope of alkenes-2[a]
Scheme 4. Evidence supporting the involvement of a carbon radical.
[a] The reactions were conducted on 0.2 mmol scale unless otherwise
specified. The yields given are isolated yields. [b] Substrate decomposed. [c]
The reaction was performed on 0.5 mmol scale. [d] A complex mixture was
generated. [e] No reaction took place.
To further evaluate the scope of this method, more
functionalized alkenes were subjected to this protocol, and the
result is presented in Table 3. The method is applicable to the
preparation of 18, 19, 21, 22, 25-29 and 33, but failed in the
cases of 23, 24, 30-32 and 34. Styrene, allylbenzene, N-
vinylbenzenesulfonamide, ethoxyethene, and limonene also
cannot be hydroazidated in this way, for which either a complex
mixture was obtained or substrates decomposed (See SI).
Scheme 5. Experiment with deuterated reagents.
To determine where the added hydrogen atom in the products
came from, experiments were conducted by using deuterated
reagents. As shown in Scheme 5, the reaction was unaffected
by the use of CDCl3 and D6-DMAP, whereas using D2O in place
of H2O brought forth a big change: not only a deuterium atom
was introduced into the product, the yield also decreased
significantly. This result confirms that water is a source of the
There are two possible mechanisms that might work for the
present hydroazidation reactions. One involves the initial
oxidation of TMSN3 by the photoexcited iridium catalyst to
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hydrogen atom that appears in the product, probably brought in
via protonation. Organic acids such as acetic acid and benzoic
As shown in Table 1, the reactions are quite sensitive to the
reaction media, with DBE and CHCl3 being superior to other
acid are also effective proton donors for the reaction (Scheme 6), solvents. Initially we speculated that the beneficial effect of these
but they were detrimental to the reaction when used in large
amount.
two solvents might be caused by their participation in the
reactions as electron acceptor. However, subsequent
experiments show that the reactions can take place as well
under solvent free conditions as long as water is used (Scheme
8). Thus, we conclude that this beneficial effect is mainly related
to their solvating capacity.
Scheme 6. Reaction of 1a in the presence of an acid.
On the basis of these primary mechanistic investigations, a
plausible mechanism is proposed to rationalize the observed
results. As illustrated in Scheme 7 with 1a taken as an example,
the initiation step is believed to be the oxidative generation of
azidyl radical from TMSN3 by the action of photoexcited
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6. As TMSN3 can react readily with
H2O to form HN3 and TMSOH, it is also possible that the azidyl
radical derives from HN3 via single electron oxidation (path (b)).
The azidyl radical then adds onto 1a to give carbon radical A.
Subsequent single electron transfer (SET) between A and the
reduced iridium (II) catalyst affords carbon anion intermediate B,
from which product 2a is formed by capturing a proton from HN3.
Meanwhile the photocatalyst is recovered and enters into the
next catalytic cycle.[23] As the yield of 2a decreased considerably
when D2O was used in place of H2O (Scheme 5), there must
exist some other pathways to compete with the last protonation
step. The protonation needs to be fast to guarantee a clean
transformation and high yield; using D2O would slow down
protonation of B, thus resulting in decrease of the yield. This
protonation step, on the other hand, might be facilitated by
DMAP, which can act as proton carrier (DMAPH+) to transfer it to
intermediate B in the organic phase.
Scheme 8. Reactions under solvent-free conditions.
In summary, we have developed a photochemical protocol for
the anti-Markovnikov hydroazidation of unactivated alkenes. The
reactions were implemented by means of visible light
photoredox catalysis with [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 as
photocatalyst and trimethylsilyl azide as azidating agent. Water
was found to exhibit a crucial beneficial effect on the reaction,
which can be attributed to its role as hydrogen donor. This
method provides a simple and straightforward means to convert
terminal alkenes to structurally important primary azides.
Notably, the present reactions can take place as well under
solvent-free conditions, which is highly preferable from the point
of view of green chemistry.[24] Further work is being done in our
laboratory to improve on the reactions and expand the scope of
this method.
Acknowledgements
The authors thank the National Natural Science Foundation of
China (Nos. 21772077 and 21372108) for financial support.
Keywords: hydroazidation • photoredox catalysis • radical
reactions • trimethylsilyl azide • water
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10.1002/chem.201806371
Chemistry - A European Journal
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Juan-juan Wang, Wei Yu*
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Anti-Markovnikov Hydroazidation of
Alkenes by Visible Light Photoredox
Catalysis
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