Visible light photoredox-catalysed intermolecular radical addition of α-halo amides to olefins

Article abstract of DOI:10.1039/c4cc00753k

We present α-chloro amides as a new class of α-acetyl radical precursors, which undergo a tin-free, photoredox-catalysed intermolecular α-alkylation with various olefins exclusively in an anti-Markovnikov fashion. The reaction represents a reductive atom

Full text of DOI:10.1039/c4cc00753k

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Visible light photoredox-catalysed intermolecular
radical addition of a-halo amides to olefins†
Cite this: Chem. Commun., 2014,
50, 3619
Masaki Nakajima, Quentin Lefebvre and Magnus Rueping*
Received 28th January 2014,
Accepted 10th February 2014
DOI: 10.1039/c4cc00753k
www.rsc.org/chemcomm
We present a-chloro amides as a new class of a-acetyl radical precur- bonds. Among others, highly sterically hindered bonds can be
sors, which undergo a tin-free, photoredox-catalysed intermolecular formed or the regioselectivity can be controlled. The atom
a-alkylation with various olefins exclusively in an anti-Markovnikov transfer radical addition (ATRA) features a popular synthetic
fashion. The reaction represents a reductive atom transfer radical tool for the simultaneous formation of a C–C and C–X bond
addition (ATRA) and provides a series of alkylated amides in good yields. across an alkene or alkyne.3a–c,11 Stephenson et al. reported a
photoredox-catalysed ATRA reaction10 employing a-bromo
The use of visible light in conjunction with photoredox catalysis esters.10b,c The same group also reported a reductive intra-
is of increasing interest.1 Photoredox catalysis has been shown molecular radical addition protocol utilizing an Ir-catalyst.8o,q
to facilitate a wide range of non-traditional transformations,
As part of our continuing efforts to employ photoredox
whereby operating with three different types of reactive key catalysis as an alternative and beneficial tool for carbon–carbon
intermediates: ions, radical ions or radicals.1a–j,2 The genera- and carbon–heteroatom bond formation,12 we became inter-
tion of the latter species can for instance be realized through ested in the development of the rather challenging intermole-
homolytic cleavage of C–X bonds or through deprotonation of cular radical addition reaction employing halo carbonyl
amine radical cations, generated via catalyst-initiated single compounds (Scheme 1).
electron transfer (SET) processes. Classical radical chemistry
The initial studies started with the reaction of a-bromo
methods are mainly driven by UV-mediated photochemistry or acetophenone (2) and 1,1-diphenylethylene (3a) in the presence
radical initiators (alkyl boranes or AIBN)3,4 in combination with of Ir(ppy)2(dtb-bpy)PF6 (1c, 1.0 mol%) and tributylamine in
hydrogen atom donors (alkyl stannanes, boranes, N-heterocyclic MeCN (Scheme 2). The reaction was clean and high yielding,
carbene–borane complexes, N-ethylpiperidine hypophosphite).5 but provided the dehalogenated acetophenone (4) instead of
These compounds are occupying a prominent position in radical the desired product, most probably by hydrogen abstraction
chemistry, even though they may have drawbacks including from the tributylammonium radical cation.13
toxicity,6 their explosive or pyrophoric nature7 or the potential
With these discouraging results in hand we questioned
hazard for scale-up. Harsh reaction conditions can also lead to whether different halo carbonyl derivatives or catalysts would
low functional group tolerance and lack of chemoselectivity. lead to formation of the desired product (Scheme 3).
Therefore, alternatives involving environmentally friendly setups
would be desirable.
Recently, visible-light photoredox catalysis with the use of
metal and metal-free homogeneous and heterogeneous cata-
lysts has shown to be an interesting alternative in free radical
chemistry.1j,8–10
Scheme 1 Proposed photoredox-catalysed radical addition reactions of
a-halo carbonyl derivatives.
In any event, radical chemistry offers an alternative way to
heterolytic reactions for the construction of C–C or/and C–X
Institute of Organic Chemistry, RWTH Aachen, Landoltweg 1, 52074 Aachen,
Germany. E-mail: Magnus.Rueping@rwth-aachen.de; Fax: +49-241-8092665;
Tel: +49-241-8094686
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc00753k
Scheme 2 Photoredox-catalysed dehalogenation of a-halo ketones.
Chem. Commun., 2014, 50, 3619--3622 | 3619
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Table 1 Scope of olefins in the photoredox-catalysed addition reaction
Scheme 3 Preliminary studies of the visible light mediated addition of
a-halo amides to olefins.
Upon switching to 2-bromo-N,N-diphenylpropanamide (5a) a more
electrophilic a-carbonyl radical was generated and to our delight, 39%
of product 7aa could be obtained along with the formation of side
products (for details see ESI†). Interestingly, modifying the substrate
to 6a had a strong influence on the reaction outcome, and the desired
coupling product was obtained in 79% yield.
Next, various Ru- and Ir-photocatalysts were tested for their
efficiency. Besides catalyst 1a, we found that the electron-rich Ir-
catalyst Ir(tbppy)2(dtb-bpy)PF6 (1b) was more effective in the trans-
formation of primary a-chloro amides. Further reaction parameters
were examined including solvent, amines and stoichiometry (for
details see ESI†). Among the amines, triethylamine, butylamine
and DIPEA gave comparable results. From the solvents tested,
acetonitrile was confirmed to be the solvent of choice. The reaction
could also be carried out in non-dried acetonitrile without detrimen-
tal effects. Under the optimal reaction conditions, we next turned our
attention to assess the generality and limitations of the method.
We were pleased to find that the optimised conditions proved to
be applicable to a broad range of styrene derivatives (Table 1), a
usually challenging class of substrates that often undergo uncontrol-
lable polymerizations under radical conditions. Electron poor olefins
such as 3g–i were compatible as well as their electron-neutral (3c–f)
and electron rich (3k) analogues. Substitution in the ortho, meta and
para positions in 3c–e along with heteroaryl groups (3l and 3m) was
also tolerated. However, the latter worked less efficiently. Interest-
ingly, a-acetoxy styrene derivatives 3n–p performed well, featuring an
efficient synthetic way to access g-hydroxyl amides.
Entrya Amide/olefin R1 R2/R3/R4 b
Product Yieldb (%)
1c
2
3
4
5
6a/3a
6b/3a
6b/3b
6b/3c
6b/3d
6b/3e
6a/3e
6b/3f
6b/3g
6b/3h
6b/3i
6a/3i
6b/3k
6b/3l
6b/3m
6b/3n
6b/3o
6b/3p
Me Ph/Ph/H
7aa
7ba
7bb
7bc
7bd
7be
7ae
7bf
79
90
28
82
74
85
73 f
72
67
68
65
61
86
73
48
70
86
73
H
H
H
H
H
Ph/Ph/H
H/Ph/Ph
Ph/2-Tol/H
Ph/3-Tol/H
Ph/4-Tol/H
6
7c
8e
9d
10
11
12c
13e
14
15
16
17
18
Me Ph/4-Tol/H
H
H
H
H
Ph/biphenyl/H
Ph/4-F-Ph/H
Ph/4-Cl-Ph/H
7bg
7bh
4-Cl-Ph/4-Cl-Ph/H 7bi
Me 4-Cl-Ph/4-Cl-Ph/H 7ai
H
H
H
H
H
H
4-MeO-Ph/Ph/H
Ph/2-pyridyl/H
Ph/2-thiophenyl/H 7bm
OAc/Ph/H
OAc/4-MeO-Ph/H
7bk
7bl
7bn
7bo
OAc/2-naphthyl/H 7bp
a
Unless otherwise specified, all reactions were performed with 6
(0.13 mmol), 3 (0.39 mmol), NBu3 (0.26 mmol) and 2 mol% of 1b in
b
MeCN (1.0 mL) under irradiation of blue LED at RT. Yield after
c
d
isolation. 2 mol% of 1a has been employed. 0.65 mmol of 3g has
e
been used. 1 mL of solvent mixture MeCN : DCM (3 : 1) has been used.
f
d.r. 1 : 1.
Next, we turned our attention to the amide moiety in order to
evaluate the a-substitution effects as well as the N-substitution
pattern. The unsubstituted a-chloro amide 6b proved to be efficient
in the alkylation reaction. Likewise, substrates 6c–d with an alkyl
substituent in the a-position underwent this transformation smoothly
(Table 2, entries 1 and 2). The malonic ester derived amide 6e could
also be transformed into the corresponding product 7ea (Table 2,
entry 3). Besides N,N-diphenyl substituted amides, the reaction
conditions also tolerated mono-substituted N-aryl amides 6g–i
(Table 2, entries 5–7). To our delight, the coupling between the
Weinreb amide 6f and 3a worked well to form the highly derivatisable
product 7fa in 73% yield (Table 2, entry 4).
Table 2 Scope of the amides
Entrya
Amide
R1
R2/R3
Product
Yieldb (%)
1
2
3
4c
5
6
7
6c
6d
6e
6f
6g
6h
6i
Et
Bn
CO2Me
H
Me
Me
Me
Ph/Ph
Ph/Ph
Ph/Ph
Me/OMe
H/Ph
H/4-Ac-Ph
H/4-CF3-Ph
7ca
7da
7ea
7fa
7ga
7ha
7ia
51
72
58
73
77
83
79
Finally, a variety of control experiments were carried out in order
to gain some insight into the reaction mechanism (Table 3). Impor-
tantly, no product formation could be observed in the absence of
base, photocatalyst or light source (Table 3, entries 4–6). Additionally,
the product formation was completely suppressed in the presence of
TEMPO, implying the involvement of a radical species in the reaction
mechanism (Table 3, entry 7).
a
Unless otherwise specified, all reactions were performed with 6
(0.13 mmol), 3a (0.39 mmol), NBu3 (0.26 mmol) and 2 mol% of 1a in
b
degassed MeCN (1.0 mL) under irradiation of a blue LED at RT. Yield
c
after isolation. 2 mol% of 1b has been employed.
excited state Ir(III)*, which is reduced by NBu3 (8), to give tributyl-
Based on the above observations, a putative reaction mechanism ammonium radical cation 80 and the reactive Ir(II) complex. This Ir(II)
is proposed (Scheme 4). Visible light irradiation of Ir(III) leads to the complex initiates a reductive cleavage of the C–Cl bond, generating an
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Table 3 Control experiments
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M. Oelgemoller and F. Ghetti, CRC Press, Boca Raton, FL, 2012;
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X
Deviation from the standard condition
Yieldb (%)
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M. Fagnoni and A. Albini, Chem. Soc. Rev., 2013, 42, 97;
1
2
3
4
5
6
7
Cl
Br
Cl
Cl
Cl
Cl
Cl
—
—
81
39
30
0
0
0
¨
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TEMPO as additivec
0
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