Donor–Acceptor Complex Enables Alkoxyl Radical Generation for Metal-Free C(sp3)–C(sp3) Cleavage and Allylation/Alkenylation

Article abstract of DOI:10.1002/anie.201707171

The alkoxyl radical is an essential and prevalent reactive intermediate for chemical and biological studies. Here we report the first donor–acceptor complex-enabled alkoxyl radical generation under metal-free reaction conditions induced by visible light. Hantzsch ester forms the key donor–acceptor complex with N-alkoxyl derivatives, which is elucidated by a series of spectrometry and mechanistic experiments. Selective C(sp³)-C(sp³) bond cleavage and allylation/alkenylation is demonstrated for the first time using this photocatalyst-free approach with linear primary, secondary, and tertiary alkoxyl radicals.

Full text of DOI:10.1002/anie.201707171

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Donor-Acceptor Complex Enables Alkoxyl Radical Generation for
Metal-Free C(sp3)-C(sp3) Cleavage and Allylation/Alkenylation
Authors: Jing Zhang, Yang Li, Ruoyu Xu, and Yiyun Chen
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707171
Angew. Chem. 10.1002/ange.201707171
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10.1002/anie.201707171
Angewandte Chemie International Edition
DOI: 10.1002/anie.201((will be filled in by the editorial staff))
Alkoxyl Radical / Donor-Acceptor Complex
Donor-Acceptor Complex Enables Alkoxyl Radical Generation for
Metal-Free C(sp3)-C(sp3) Cleavage and Allylation/Alkenylation**
Jing Zhang, Yang Li, Ruoyu Xu and Yiyun Chen†*
Abstract: The alkoxyl radical is an essential and prevalent reactive
intermediate for chemical and biological studies. Here we report the
first donor-acceptor complex-enabled alkoxyl radical generation
under metal-free reaction conditions induced by visible light.
Hantzsch ester forms the key donor-acceptor complex with N-
alkoxyl derivatives, which is elucidated by a series of spectrometry
and mechanistic experiments. Selective C(sp3)-C(sp3) bond cleavage
and allylation/alkenylation is demonstrated for the first time using
this photocatalyst-free approach with linear primary, secondary,
and tertiary alkoxyl radicals.
The alkoxyl radical is a prevalent reactive intermediate for
chemical and biological studies, however its generation under mild
reaction conditions has been challenging.[1] Traditionally, alkoxyl
radicals are generated with relatively harsh reaction conditions such
as heating or UV light irradiation, which limit their synthetic scope
and functional group compatibilities (Scheme 1a).[2] Recently, the
development of photoredox catalysis methods enables alkoxyl
radical generation under mild reaction conditions, however these
photoredox catalysis methods require expensive heavy metal
photocatalysts.[3,4] We envision a new alkoxyl radical generation
method under mild and metal-free conditions will be beneficial for
Scheme 1. Donor-acceptor complex enables alkoxyl radical generation for
metal-free C(sp3)-C(sp3) bond cleavage and functionalization
the synthetic, material, and biological applications of alkoxyl
radicals.[5]
The cleavage of unactivated C(sp3)-C(sp3) bond to engage in new
C-C bond formation is a desirable organic transformation, however
it is challenging to achieve the excellent regioselectivity and
chemoselectivity.[12] The alkoxyl radical is known for -
fragmentation reactivity to selectively cleave the adjacent C-C bond,
however its synthetic application for intermolecular C-C bond
formation is limited to alkoxyl radicals derived from strained
cycloalkanols.[13] For unstrained linear alkoxyl radicals, the resulting
C(sp3) radical from -fragmentation is usually trapped by reductive
hydrogenation or oxidation, and the intermolecular trapping of the
C(sp3) radical for C-C bond formation is challenging. [4b,14] Herein
we report the first donor-acceptor complex formation between
Hantzsch ester and N-alkoxyl derivatives to generate alkoxyl
radicals upon visible light irradiation (Scheme 1c). Selective C(sp3)-
C(sp3) bond cleavage and intermolecular allylation/alkenylation are
enabled via linear primary, secondary, and tertiary alkoxyl radicals.
Donor-acceptor complexes are molecular complexes resulting
from the association of an electron donor molecule (D) with an
electron acceptor molecule (A), which generally demonstrate partial
or total electron transfer from D to A upon irradiation (Scheme
1b).[6] However, its application in organic synthesis is limited and
the donor-acceptor complex for alkoxyl radical generation is
unknown.[7,8] The NADH (the reduced form of nicotinamide adenine
dinucleotide) is known to form the donor-acceptor complex when
bound to the enzyme,[9] however its synthetic analogue Hantzsch
ester (HE) has not been reported for donor-acceptor complex
formation.[10,11] We hypothesize that the donor-acceptor complex
formation between alkoxyl radical precursors and HE will enable
the alkoxyl radical generation without photocatalysts (Scheme 1c).
[]
Jing Zhang, Yang Li, Ruoyu Xu, Prof. Dr. Yiyun Chen
State Key Laboratory of Bioorganic and Natural Products Chemistry,
Shanghai Institute of Organic Chemistry, University of Chinese
Academy of Sciences, Chinese Academy of Sciences, 345 Lingling
Road, Shanghai 200032 China
We started our investigation with the unstrained linear alkoxyl
radical precursor N-alkoxyphthalimides, which was bench-stable
and could be easily prepared from readily-available alcohols.[4a,4d,15]
While the N-alkoxyphthalimide was unknown for donor-acceptor
complex formation, we hypothesized that its conjugated  system
and electron-deficient character would enable the donor-acceptor
complex formation with suitable electron donors. We then
synthesized a panel of N-alkoxyphthalimide analogues 1-4 with
different substituents, as well as N-alkoxysuccinimide 5 and N-
alkoxylnaphthalimide 6 (Scheme 2a). When mixed with HE 7, the
UV/vis absorption spectrometry showed no signal change from N-
E-mail: yiyunchen@sioc.ac.cn
[]
Financial support was provided by National Natural Science
Foundation of China 21472230, 21622207, National Basic Research
Program of China 2014CB910304, and Strategic Priority Research
Program of the Chinese Academy of Sciences XDB20020200.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201xxxxxx
1
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10.1002/anie.201707171
Angewandte Chemie International Edition
We next subjected N-alkoxyphthalimide 1 and HE 7 to the radical
acceptor allyl sulfone 11.[18] Under blue LED irradiation in
dichloromethane, we gratifyingly observed 12 in 31% yield, which
indicated that the alkoxyl radical was generated without
photocatalysts and the resulting alkyl radical successfully underwent
allyl addition (entry 1 in Table 1). The solvent screening indicated
that 1,4-dioxane was optimal to give 12 in 72% yield (67% isolated
yield, entries 2-3).[19] The use of N-alkoxyl derivatives 2, 3 and 6
gave 12 in similar 64-77% yields (entries 4, 5 and 8). The
alkoxysuccinimide 5, however there were clear red-shifts from N-
alkoxyl derivatives 1-4 and 6. The N-alkoxyphthalimides 3 and 4
bearing electron-withdrawing chlorides showed stronger red-shifts
compared to N-alkoxyphthalimides 2 bearing electron-rich methoxy
groups, which was consistent with the N-alkoxyphthalimides as the
electron acceptor. In particular, a charge-transfer orange-yellowish
color appeared when tetrachloro-substituted N-alkoxyphthalimides 4
was mixed with HE 7 (Scheme 2a). We also prepared substituted
HE 8-10 (methyl and isopropyl ester HE as well as N-methyl
substituted HE) and observed red-shifts when mixed with N-
alkoxyphthalimide 1 (see supporting information Figure S2).
tetrachloro-substituted N-alkoxyphthalimides
4
gave slow
conversion due to the product inhibition from tetrachloro-
phthalimides (entry 6).[20] The formation of donor-acceptor complex
was required that the N-alkoxysuccinimide 5 did not lead to any
conversion (entry 7). Among the substituted HE 8-10, the methyl
HE 8 gave similar 67% yield, the isopropyl HE 9 gave lower 21%
yield due to the steric hindrance, and no conversion was observed
with N-methyl HE 10 (entries 9-11).
We further investigated if the HE was directly photoexcited by
the shorter wavelength irradiation without donor-acceptor complex
formation.[11] Using the irradiation with band-pass at 450 nm, in
which HE has minimal absorption, the reaction went
uncompromised with 70% yield (entries 12-13). Using band-pass at
475 nm without emission at below 460 nm (see supporting
information Figure S15-16), 37% yield of 12 could be obtained
(entry 14). Taken together, the donor-acceptor complex formation is
essential for the reaction at longer wavelength.[21] The light
irradiation and HE were found both critical for the reaction (see
supporting information Table S3).
Table 1 Reactions between N-alkoxyl and Hantzsch ester derivatives under
visible light irradiation
Entry
Conditionsa
Conversionb
Yieldb
1
1 + HE 7, CH2Cl2
1 + HE 7, MeOH
49%
>95%
>95%
70%
31%
2
64%(60%)
72%(67%)
64%
3
1 + HE 7, dioxane
2 + HE 7, dioxane
3 + HE 7, dioxane
4 + HE 7, dioxane
5 + HE 7, dioxane
6 + HE 7, dioxane
1 + CO2Me-HE 8, dioxane
4
5
>95%
44%
66%
6
37%
7
<5%
<5%
8
>95%
79%
77%(73%)
67%
9
Scheme 2. Interactions between N-alkoxyl and Hantzsch ester derivatives
i
10
11
12
13
14
1 + CO2 Pr-HE 9, dioxane
38%
21%
1 + N-Me-HE 10, dioxane
6 + HE 7, MeOH, 12 h
<5%
<5%
The donor-acceptor complex between N-alkoxyphthalimides and
Hantzsch esters was further investigated by the Job’s plot with
UV/vis absorption experiments.[16] The maximal absorption at 50%
molar fraction of HE 7 suggested a 1:1 ratio between 4 and 7
(Scheme 2b). The dissociation constant KEDA between 4 and 7 was
calculated to be 8.4 M-1 in dichloromethane with Benesi-Hildebrand
methodology.[17] We also did NMR titration experiments and found
>95%
>95%
44%
70%(65%)
70%
entry 12, band-pass 450 nm
entry 12, band-pass 475 nm
37%
aReaction conditions: N-alkoxyl derivatives (0.10 mmol, 1.0 equiv.),
11 (0.30 mmol, 3.0 equiv.), and Hantzsch ester derivatives (0.50
mmol, 5.0 equiv.) in 1.0 mL solvent under nitrogen with 8 W blue LED
irradiation at ambient temperature for 36 h, unless otherwise noted.
Conversion and yields were determined by 1H NMR analysis and
isolated yields were in parentheses.
1
the H NMR signal of isopropyl HE 9 shifted downfield with the
addition of tetrachloro-substituted N-alkoxyphthalimide 4 (Scheme
2c, see supporting information Figures S6-S12 for the UV/vis
absorption spectrometry, KEDA, and NMR titration experiments of
other derivatives). Taken together, the donor-acceptor complex is
formed between N-alkoxyphthalimides and Hantzsch esters.[8b,8c]
Based on mechanistic investigations above, we propose that the
reaction is initiated by the formation of the donor-acceptor complex
between Hantzsch esters and N-alkoxyphthalimides (Scheme 3).
2
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10.1002/anie.201707171
Angewandte Chemie International Edition
the -fragmentation of alkoxyl radicals that 28a-30a reacted
smoothly to give excellent 94% yields.
With the diol-derived N-alkoxyphthalimide 31a, the alkoxyl
radical was selectively generated without interfering the free
hydroxyl group to yield homoallylic alcohol 31 in 64% yield. The
reaction was not limited to electron-withdrawing allyl sulfones that
styrene 16 could be obtained in 91% yield. In addition, the vinyl
sulfones acted as radical acceptors effectively to give the C(sp3)-
C(sp3) bond cleavage and alkenylation adducts 33-36 in 65-76%
yields.[23]
After visible-light-induced electron transfer,[22]
the N-
alkoxyphthalimide radical anion is formed and eliminates
phthalimides to generate the alkoxyl radical. The resulting alkoxyl
radical undergoes -fragmentation to yield the alkyl radical and is
trapped by allyl sulfones for radical allylation reaction.
Scheme 3. Mechanistic proposal of the reaction
With this metal-free donor-acceptor complex approach, we then
tested various linear secondary and tertiary N-alkoxyphthalimides
for C(sp3)-C(sp3) bond cleavage and allylation reaction (Scheme 4,
conditions in entry 3 of Table 1). Both acetaldehydes and
benzaldehydes could be eliminated smoothly from N-
alkoxyphthalimides 13a-16a, and the allylation went smoothly to
yield 12, 15 and 16 in 57-89% yields. The cyclopentanol- and
cyclobutanol-derived N-alkoxyphthalimides underwent C(sp3)-
C(sp3) bond cleavage and allylation to give alkenyl aldehydes 17
and 18 in 47-61% yields. The linear tertiary alkoxyl radical was
generated
with
N-alkoxyphthalimide
19a
and
N-
alkoxylnaphthalimide 20a, which both afforded 12 in 65-68% yields
after acetone elimination.
Scheme 5. C(sp3)-C(sp3) bond cleavage/functionalization with linear primary
alkoxyl radicals
In conclusion, we have developed the first donor-acceptor
complex method for alkoxyl radical generation under mild and
metal-free conditions. Selective C(sp3)-C(sp3) bond cleavage and
allylation/alkenylation is achieved via linear primary, secondary and
tertiary alkoxyl radicals. We envision this donor-acceptor complex
method will be instrumental for new visible-light-induced reaction
design and the further investigation is undergoing in our laboratory.
Scheme 4. C(sp3)-C(sp3) bond cleavage/functionalization with linear secondary
Received: ((will be filled in by the editorial staff))
and tertiary alkoxyl radicals
Published online on ((will be filled in by the editorial staff))
Finally, we challenged the reactivity of primary alkoxyl radicals,
which has not been realized by previous methods for C(sp3)-C(sp3)
bond cleavage/functionalization due to the facile side reactions
(Scheme 5).[4b-e] With linear N-alkoxyphthalimides 21a-23a, the
primary alkoxyl radicals were generated and yielded allylation
adducts 21-23 in 71-86% yields. The heterocycles furan and
thiophene were well tolerated to give 24 and 25 in 53-79% yields.
The alkoxyl radical fragmentation also afforded tertiary alkyl
radicals to yield 26-27 in 60-62% yields. The heteroatoms facilitated
Keywords: alkoxyl radical • donor-acceptor complex • Hantzsch ester
• C(sp3)-C(sp3) bond cleavage/functionalization
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4
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Alkoxyl Radical / Donor-Acceptor
Complex
J. Zhang, Y. Li, R. Xu, Y. Chen*
__________ Page – Page
Donor-Acceptor Complex Enables Alkoxyl
Radical Generation for Metal-Free C(sp3)-
C(sp3) Cleavage and Allylation/Alkenylation
Here we report the first donor-acceptor complex-enabled alkoxyl radical
generation under metal-free reaction conditions induced by visible light.
Selective C(sp3)-C(sp3) bond cleavage and allylation/alkenylation is
demonstrated using this photocatalyst-free approach via linear primary,
secondary, and tertiary alkoxyl radicals.
5
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