Primary, Secondary, and Tertiary γ-C(sp3)-H Vinylation of Amides via Organic Photoredox-Catalyzed Hydrogen Atom Transfer

Article abstract of DOI:10.1021/acs.orglett.8b02737

An efficient strategy for primary, secondary and tertiary aliphatic γ-C(sp³)-H vinylation of amides with alkenylboronic acids is reported. These reactions are catalyzed by visible-light organic photoredox agents. Regioselective γ-C(sp³)-H vinylation of amides is controlled by a 1,5-hydrogen atom transfer of an amidyl radical generated in situ.

Full text of DOI:10.1021/acs.orglett.8b02737

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Primary, Secondary, and Tertiary γ‑C(sp3)−H Vinylation of Amides
via Organic Photoredox-Catalyzed Hydrogen Atom Transfer
Hui Chen, Liangliang Guo, and Shouyun Yu*
State Key Laboratory of Analytical Chemistry for Life Science, Jiangsu Key Laboratory of Advanced Organic Materials, School of
Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China
S
* Supporting Information
ABSTRACT: An efficient strategy for primary, secondary and tertiary aliphatic γ-C(sp3)−H vinylation of amides with
alkenylboronic acids is reported. These reactions are catalyzed by visible-light organic photoredox agents. Regioselective γ-
C(sp3)−H vinylation of amides is controlled by a 1,5-hydrogen atom transfer of an amidyl radical generated in situ.
he development of selective strategies to convert inert
to date. Herein, we report our transition-metal-free solution to
this synthetic challenge (Figure 1b).
T
C(sp3)−H bonds to C−C bonds, which enables direct
access to molecules that would otherwise be inaccessible in a
single step, remains a formidable challenge in organic
chemistry.1 In a variety of functional groups capable of
directing C−H activation, carboxycarbonyls are attractive
because carboxylic acids and their derivatives are among the
most common organic molecules found in nature and
represent ideal starting materials with which to generate new
connections.2 Direct functionalizations of carboxylic acid
derivatives at the α- or β-positions, through enolate chemistry3
or transition-metal-catalyzed directing C−H activation,4
respectively, are well established, but derivatization at the γ-
position has been much less frequently reported. Transition-
metal-catalyzed arylation, olefination, and chalcogenation
reactions of the γ-C(sp3)−H of amides have been recently
developed by Chen,5 Yu,6 and Maiti7 (Figure 1a, top). These
reactions proceed through a sterically controlled and less
favorable six-membered metallacycle, and only primary γ-
C(sp3)−H moieties are amenable to these reactions.6b,7
Another elegant strategy, reported by Yu,8 Rovis,9 and
Leonori10 employs an intramolecular 1,5-hydrogen atom
transfer (1,5-HAT) process11 by amidyl radicals (Figure 1a,
bottom). Functionalization of tertiary and secondary γ-
C(sp3)−H units has been achieved using this method, but
unactivated primary C(sp3)−H bonds are not suited to this
reaction. There is a lack of an efficient and unified strategy for
the intermolecular functionalization of primary, secondary, and
tertiary inert γ-C(sp3)−H bonds of carboxylic acid derivatives
With their weak N−O bond, hydroxylamine derivatives have
recently been exploited as nitrogen-centered radical (N-
radical) precursors in visible-light photocatalysis.12 Their
synthetic potential can be controlled by tuning substituents
on the nitrogen and oxygen atoms. Based on this knowledge,
we believe that hydroxamic acid derivatives (1, Figure 1b)
could serve as the precursors to N-radicals and subsequently as
platforms for the remote C(sp3)−H functionalization under
photoredox catalysis. Mechanistically, the hydroxamic acid
derivative (1), upon single-electron transfer (SET) reduction
by the excited photocatalyst eosin Y*,13 forms an amidyl
−
radical (A) and a carboxylate anion (ArCO2 ). A 1,5-HAT
from the γ-C−H bond to the amidyl radical (A) provides a
carbon-centered radical, the C-radical (B), which is inter-
molecularly trapped by a vinylboronic acid (2), giving the
corresponding C-radical (C). This radical (C) is then oxidized
by eosin Y+•, delivering a cationic intermediate (D) and
regenerating the photocatalyst, eosin Y. After deboronation,14
the remote C(sp3)−H vinylation product (3) is ultimately
produced (Figure 1c).
Since the substituents on the nitrogen atom can affect the
stability and reactivity of the N-radical, a series of hydroxamic
acid derivatives (1a−f) with different functional groups on the
Received: August 27, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02737
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Letter
Figure 2. Investigation of the protecting groups on the nitrogen atom.
Reaction conditions: 1 (0.2 mmol, 1.0 equiv), 2a (0.6 mmol, 3.0
equiv), Na2CO3 (0.4 mmol, 2.0 equiv), eosin Y (2 mol %), DMSO
(2.0 mL), rt, 90 W white LEDs. Yields are for the isolated products.
N. D. = not detected.
a
Table 1. Variation of Reaction Parameters
b
entry
variation of reaction conditions
none
without light or eosin Y or Na2CO3
air instead of N2
Ir(ppy)3 instead of eosin Y
Ru(bpy)3Cl2 instead of eosin Y
DIPEA instead of Na2CO3
Na2HPO4 instead of Na2CO3
CH3CN instead of DMSO
MeOH instead of DMSO
yield (%)
c
1
2
3
4
5
6
7
8
9
86 (85 )
trace
33
11
22
21
44
19
21
a
Figure 1. γ-C(sp3)−H functionalization of amides.
Reaction conditions: 1f (0.1 mmol, 1.0 equiv), 2a (0.3 mmol, 3.0
equiv), base (0.2 mmol, 2.0 equiv), photocatalyst (2 mol %), solvent
(1.0 mL), rt, 90 W white LEDs. The yield was determined by GC in
the presence of tetradecane as an internal standard. Isolated yield on
b
c
nitrogen atom were prepared. Adjustments of the substituents
on the nitrogen atom provide an opportunity to control the
reactivity of the N-radicals and subsequently address remote
functionalization of secondary and even primary C(sp3)−H
bonds, which is still synthetically challenging. Hydroxamic acid
derivatives (1) were coupled with (E)-styrenylboronic acid
(2a) in the presence of eosin Y under visible-light irradiation
(Figure 2). Hydroxamic acids with a free nitrogen or an N-
phenyl group failed to give any of the desired products. It is
well documented that the higher electrophilic character of the
amidyl radicals the more likely they are to undergo hydrogen
atom abstraction.9,15 Polar effects of radicals also play a
significant role in the HAT process.16 Accordingly, electron
deficient group substituted hydroxamic acids were investigated.
The desired products (3ca and 3da) could be isolated, but
with unsatisfactory yields of 37% and 27%, respectively.
Finally, it was found that N-alkyl groups gave better results.
With tert-butyl (t-Bu) as the substituent,17 3fa was obtained in
85% isolated yield as the sole E-isomer (E/Z > 20/1).
a 0.2 mmol scale.
irradiated with 90 W white LEDs at room temperature in the
presence of 2 mol % of eosin Y as the photocatalyst and
Na2CO3 as the base. The desired γ-C(sp3)−H vinylation
product 3fa was obtained in 86% yield based on GC analysis
(85% isolated yield) (entry 1). Control experiments confirmed
that the photocatalyst, base, light, and nitrogen atomosphere
are all essential for this reaction (entries 2 and 3). Eosin Y,
Na2CO3, and DMSO were established as the optimal
combination among the photocatalysts (entries 4 and 5),
bases (entries 6 and 7), and solvents (entries 8 and 9),
respectively.
With the optimized conditions in hand, we evaluated the
generality of this photoredox-catalyzed γ-C(sp3)−H vinylation
of amides. Amides with different types of inert C(sp3)−H
bonds were first examined (Figure 3a−c). As functionalization
of unactivated secondary and primary C(sp3)−H bonds
through a radical pathway remains a synthetic challenge, we
decided to investigate vinylation of secondary inert C(sp3)−H
bonds and even more challenging primary inert C(sp3)−H
bonds. Consistent with our preliminary results, which were
obtained with product 3fa, other secondary C(sp3)−H bonds
reacted smoothly with boronic acid (2a) under our reaction
The reaction parameters were established using a
hydroxamic acid derivative (1f) and (E)-styrylboronic acid
(2a) as the reactants, and the results highlighted in Table 1
were obtained. For the comprehensive optimization of the
reaction conditions, see the Supporting Information (SI). A
mixture of 1f (1 equiv) and 2a (3 equiv) in DMSO was
B
DOI: 10.1021/acs.orglett.8b02737
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Figure 3. Scope of the remote vinylation of amides. Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), Na2CO3 (0.4 mmol), eosin Y (2 mol %),
DMSO (2.0 mL), rt, 90 W white LEDs. Yields for the isolated products. Ratios of stereoisomers were determined by 1H NMR analysis. (a) 6 mmol
scale. (b) 5 mmol scale.
conditions to give the desired products 3fa−oa in 53−92%
yields (Figure 3a). Functional groups, such as ether (3ga and
3ja), thioether (3oa), amide (3ha and 3ka), and ester (3na),
were all tolerated. With hydroxamic acid derivatives containing
two or more potential hydrogen abstraction sites, only γ-
vinylation products were observed in all cases (3ia−na). This
remarkable regioselectivity indicates that 1,5-HAT has a strong
preference over other HAT pathways in this reaction. The
trans-configuration of C4 and C5 in 3ia was established by X-
ray crystallography. Remarkably, this method can also be
applied to the vinylation of primary inert C(sp3)−H bonds, as
exemplified by the high-yield synthesis of products 3pa−ta
(52−84%), albeit with slightly lower E/Z stereoselectivity
(Figure 3b). Tertiary inert C−H bonds, such as in substrates
1u−x, were converted to 3ua−xa in good yields with excellent
E/Z stereoselectivities (Figure 3c). Amino acid derived
hydroxamic acids also undergo this remote vinylation, giving
3oa, 3sa, and 3za in good yields. Notably, the reaction can
perform at gram scale as demonstrated by the synthesis of 3fa
and 3ua, which were isolated with 83% and 78% yields,
respectively.
Next, we examined the vinylboronic acid partners.
Styrylboronic acids with electron-donating groups such as
Me, t-Bu, and OMe or electron-withdrawing groups such as F,
CF3, and CO2Me, at the para position, gave good yields of the
respective products (Figure 3d). Substituents at different
positions on the phenyl ring were well suited to this
transformation, affording satisfactory yields of the correspond-
ing products (3fi, 3fl, and 3uj−uk). Notably, all of the
products were isolated with a greater than 20/1 E/Z ratio.
The N-tert-butyl amide (3fa) could be hydrolyzed in hot
aqueous HCl solution. The acid (4) was obtained in 73% yield
and can support further transformations (eq 1).
C
DOI: 10.1021/acs.orglett.8b02737
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For a comprehensive understanding of this protocol, we
further studied the selectivity of 1,5-HAT when simultaneously
presented with two inert C(sp3)−H bonds. With the
hydroxamic acid derivative 1a′, the HAT occurred at the
tertiary C(sp3)−H bond (eq 2). When substrate 1b′ was
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In summary, we have developed an efficient approach for
primary, secondary, and tertiary γ-C(sp3)−H vinylation of
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02737.
Experimental details, NMR spectra, and details of
experiments (PDF)
Accession Codes
CCDC 1860263 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yushouyun@nju.edu.cn.
ORCID
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14897.
Shouyun Yu: 0000-0003-4292-4714
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (21732003), the National Key Research
and Development Program of China (2018YFC0310900), and
the Fundamental Research Funds for the Central Universities
(020514380131 and 020814380092) is acknowledged.
D
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