Asymmetric Photocatalytic C(sp3)-H Bond Addition to α-Substituted Acrylates

Article abstract of DOI:10.1021/acs.orglett.1c00801

Asymmetric functionalization of inert C(sp3)-H bonds is a straightforward approach to realize versatile bond-forming events, allowing the precise assembly of molecular complexity with minimal functional manipulations. Here, we describe an asymmetric photocatalytic C(sp3)-H bond addition to α-substituted acrylates by using tetrabutylammonium decatungstate (TBADT) as a hydrogen atom transfer (HAT) photocatalyst and chiral phosphoric acid as a chiral proton-transfer shuttle. This protocol is supposed to occur via a radical/ionic relay process, including a TBADT-mediated HAT to cleave the inert C(sp3)-H bond, a 1,4-radical addition, a back hydrogen abstraction, and an enantioselective protonation. A variety of inert C-H bond patterns and α-substituted acrylates are well tolerated to enable the rapid synthesis of enantioenriched α-stereogenic esters from simple raw materials.

Full text of DOI:10.1021/acs.orglett.1c00801

pubs.acs.org/OrgLett
Letter
Asymmetric Photocatalytic C(sp3)−H Bond Addition to
α‑Substituted Acrylates
Zhen-Yao Dai, Zhong-Sheng Nong, Shun Song, and Pu-Sheng Wang*
Cite This: Org. Lett. 2021, 23, 3157−3161
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ABSTRACT: Asymmetric functionalization of inert C(sp3)−H
bonds is a straightforward approach to realize versatile bond-forming
events, allowing the precise assembly of molecular complexity with
minimal functional manipulations. Here, we describe an asymmetric
photocatalytic C(sp3)−H bond addition to α-substituted acrylates by
using tetrabutylammonium decatungstate (TBADT) as a hydrogen
atom transfer (HAT) photocatalyst and chiral phosphoric acid as a
chiral proton-transfer shuttle. This protocol is supposed to occur via
a radical/ionic relay process, including a TBADT-mediated HAT to
cleave the inert C(sp3)−H bond, a 1,4-radical addition, a back
hydrogen abstraction, and an enantioselective protonation. A variety
of inert C−H bond patterns and α-substituted acrylates are well tolerated to enable the rapid synthesis of enantioenriched α-
stereogenic esters from simple raw materials.
Scheme 1. Enantioselective C(sp3)−H Addition to Michael
Acceptors
he direct asymmetric functionalization of inert C(sp3)−H
T
bonds represents an entirely new perspective in
comparison with the conventional logic of organic synthesis,1
allowing for the rapid assembly of densely functionalized
molecules from simple molecules and with minimal functional
manipulations. Among the advent of modern methods of
C(sp3)−H bond activation, hydrogen atom transfer (HAT)
stands out as an attractive process because it can mediate inert
C(sp3)−H bond activation with mild conditions and thereby
generate highly reactive alkyl radicals that can be leveraged to
forge versatile chemical bonds.2 However, due to the high
reactivity of radical intermediates,3 successful catalytic
strategies for controlling the stereochemical outcome of
HAT-mediated C(sp3)−H functionalization are still under-
developed.4
The reaction of C(sp3)−H bond addition to Michael
acceptors is one of the most widely utilized transformations
in the light-mediated HAT literatures (Scheme 1A).5 In recent
years, the combination of HAT photocatalyst and small-
molecule chiral catalysts6 has proven to be efficient catalytic
systems for enantioselective C(sp3)−H bond addition to
electrophilic alkenes,7 enabling the direct asymmetric
alkylation of versatile inert C(sp3)−H bonds with 100%
atom economy and mild conditions. While the use of chiral
amine7a and Lewis acid7b as cocatalysts has provided
encouraging results in generating the β-stereocenter via a
stereocontrolled radical addition to prochiral alkene, the
generation of the α-stereocenter still lies in the lack of effective
strategy for stereoinduction.8,7c
Received: March 12, 2021
Published: March 29, 2021
To address this issue, our group has recently described
preliminary results concerning an asymmetric photocatalytic
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© 2021 American Chemical Society
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C(sp3)−H bond addition to versatile exocyclic enone7c with
the use of tetrabutylammonium decatungstate (TBADT)9 as a
HAT photocatalyst and chiral phosphoric acid as a chiral
proton-transfer shuttle (CPTS).10 This protocol is supposed to
occur via a radical/ionic relay process, and the key
enantiodetermining step is the enantioselective protonation
of in situ generated cyclic ketoenol with CPTS (Scheme 1A).
In the same way, a C(sp3)−H bond addition to α,β-
unsaturated ester may also be viable to generate a transient
ester enolate. However, enantioselective protonation of this
less stable intermediate may be more difficult than cyclic
ketoenol.11 In addition, for the hydrocarbon substrates bearing
relatively low dissociation energy of C(sp3)−H bonds, the
existence of a nonstereoselective HAT pathway12 to afford
racemic products makes it more challenging to achieve high
levels of enantioinduction (Scheme 1A).13 Given these
challenges, we imagine that acyclic α-substituted acrylates
might be superior alkylating agents for asymmetric function-
alization of a wider range of inert C(sp3)−H bonds (Scheme
1B) because of their structural diversity and the potential to
regulate steric hindrance,14 albeit with the risk of a side
reaction of radical polymerization.15 Therefore, we propose
that a TBADT-mediated HAT process is capable of cleaving
the inert C(sp3)−H bond to generate a carbon-centered
radical, which then undergoes a Giese radical addition to α-
substituted acrylate to give an α-acyl radical. Back hydrogen
abstraction (HA) from the reducing photocatalyst to an α-acyl
radical furnishes an enol species, and enantioenriched α-
stereogenic ester is yielded via enantioselective protonation
with CPTS.16
a
Table 1. Optimization of Reaction Conditions
b
c
entry
variation
yield (%)
er
1
2
3
4
5
6
7
8
9
none
89
46
52
76
55
54
78
<1
<1
95
95:5
A2 instead of A1
A3 instead of A1
A4 instead of A1
A5 instead of A1
no A1
no 5 Å MS
no TBADT
no light
gram-scale reaction
80:20
82:18
86:14
64:36
N.D.
95:5
N.D.
N.D.
94:6
d
10
a
Reaction conditions: 1 (0.1 mmol), 2 (4 mmol), TBADT (0.002
mmol), A1 (0.002 mmol), PhCN (0.5 mL), 5 Å MS (30 mg), 0 °C,
under nitrogen, 6 W 390 nm LEDs, 3 h. Determined by H NMR
analysis of the crude reaction mixture based on 1,3,5-triacetylbenzene
as an internal standard. Determined by chiral-phase HPLC analysis.
1 (3 mmol), 2 (120 mmol), TBADT (0.06 mmol), A1 (0.06 mmol),
b
1
c
d
Dehydroalanine esters, a useful kind of α-substituted
acrylates, are widely used as fundamental building blocks for
the synthesis of enantioenriched α-amino acid derivatives.17,18
Therefore, we initially targeted the development of an
enantioselective C(sp3)−H bond addition to N-acyl dehy-
droalanine benzyl ester 1 with the use of TBADT as a HAT
photocatalyst and chiral spiro phosphoric acid (S)-A1 as a
CPTS (Table 1), enabling a facile and highly atom-economic
approach to prepare chiral α-amino esters.19 To our delight,
the desired C(sp3)−H addition product 3 was smoothly
furnished in 89% NMR yield and with 95:5 er by exposing 1
and cyclohexane 2 to near-ultraviolet light in benzonitrile
(entry 1). The examination of a range of chiral spiro
phosphoric acids revealed that the substituents at the 6,6′-
positions of the SPINOL had obvious influence on
enantioinduction, and (S)-A1 still turned out to be a superior
CPTS (entries 2−5). The evaluation of other reaction
parameters was shown in the Supporting Information.
Interestingly, this reaction also proceeded well in the absence
of chiral phosphoric acid (S)-A1 (entry 6), implying the
existence of an alternative nonstereoselective pathway.
Considering the high levels of enantioselectivity in the
presence of chiral phosphoric acid (S)-A1, the reaction rate
of the enantioselective pathway was certain to be much faster
than that of the nonstereoselective pathway (entry 1 vs entry
6). In the absence of 5 Å MS, 3 was afforded in a slightly
eroded NMR yield but with maintained enantioselectivity
(entry 7), suggesting that 5 Å MS might not only serve as a
desiccant to lighten H2O-mediated nonstereoselective proto-
nation but also act as a solid acid20 to promote the Giese
radical addition. Finally, control experiments verified that both
HAT photocatalyst TBADT and near-ultraviolet light were
essential to the success of the reaction (entries 8 and 9). A
PhCN (15 mL), 5 Å MS (900 mg), 10 °C, under nitrogen, 40 W 390
nm LEDs, 6 h. PMP = p-methoxyphenyl.
gram-scale reaction of N-acyl dehydroalanine benzyl ester 1
and cyclohexane 2 conducted under slightly altered reaction
conditions furnished the desired product 3 in a slightly
enhanced yield and with maintained enantioselectivity (entry
10).
With the optimal reaction conditions in hand, the scope of
this asymmetric C(sp3)−H bond addition protocol was
examined (Scheme 2). The variations at either N-acyl groups
or the ester substituents of N-acyl dehydroalanines were nicely
tolerated to afford the corresponding C(sp3)−H addition
products (4−7) in moderate to good yields and with high
levels of enantioselectivity. With respect to the hydrocarbon
substrates, the reaction tolerated a wide range of cyclic
hydrocarbons and methylarenes. For example, 5-, 7-, and 8-
membered cycloalkanes were smoothly converted to the
corresponding alkylated products (8−10) in good to excellent
yields and with good enantioselectivities. Methylarenes bearing
either electron-donating or electron-withdrawing substituents
in the ortho-, meta-, and para-position of the aromatic moiety
were capable of delivering C(sp3)−H adducts (11−22) with
good levels of enantioselectivity. Notably, 3-methylthiophene
could also be directly alkylated at the benzylic site (23) in 43%
yield and with 93:7 er. Isopentyl benzoate was capable of
undergoing site-selective tertiary C(sp3)−H alkylation to
provide the corresponding C(sp3)−H adduct (24) in moderate
yield, albeit with a slightly decreased enantioselectivity. In
addition, 1,3-benzodioxole, phenyl ethers, and methyl tert-
butyl ether were amenable to afford the desired coupling
products (25−29) through enantioselective C(sp3)−H
alkylation occurring exclusively at the carbon atom adjacent
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a
Scheme 2. Substrate Scope
a
Unless indicated otherwise, reactions run with N-acyl dehydroalanine ester (0.1 mmol), cyclic alkane (4 mmol), TBADT (0.002 mmol), (S)-A1
(0.002 mmol), PhCN (0.5 mL), 5 Å MS (30 mg), 0 °C, under nitrogen, 6 W 390 nm LEDs, 3 h. The absolute configurations of 26 and 37 were
b
assigned based on X-ray crystallography, and the absolute configurations of all other compounds were assigned by analogy. With 20 equiv of
c
d
e
f
methylarene. With 5 mol % (S)-A1. With 20 equiv of isopentyl benzoate. With 10 equiv of benzodioxole, 0.5 mL MeCN. With 60 equiv of
g
h
i
ether, 2 mL of PhCN/CH2Cl2 (1:1). With 20 equiv of ether, 1 mL of MeCN. With 10 mol % (S)-A1. With 2 equiv of dimethylcarbamate, 0.25
mL of MeCN. With 3 mL of CH2Cl2. With 2 equiv of aldehyde, 0.5 mL CH2Cl2. With 0.5 mL of PhCN/CD2Cl2 (1:9), rt, 8 h. With 0.5 mL of
PhCN/CH2Cl2 (1:1), rt, 4 h.
j
k
l
m
to the oxygen. Aliphatic amide and dichloromethane were also
viable to afford the corresponding C(sp3)−H adducts (30−
31) with acceptable results. In particular, aliphatic aldehydes,
capable of serving as acyl radical precursors, were employed
successfully to provide chiral hydroacylation-type products
(32−33) in high yields and synthetically useful enantiose-
lectivities.
Notably, α-aryl acrylates were capable of serving as effective
substrates in asymmetric C(sp3)−H addition under slightly
altered conditions. Either electron-donating or electron-
withdrawing functional groups at the para-position were
suitable to deliver the corresponding adducts (34−38) in
49−80% yields and with 94:6 to 96:4 er. It should be noted
that this protocol was also applicable by using readily available
methacrylate, furnishing the C(sp3)−H adduct (39) with
synthetically acceptable results.
To gain some insight into the mechanism of this
photocatalytic C(sp3)−H bond addition protocol, a series of
deuterium labeling experiments were performed (Scheme 3).
In the presence and absence of chiral spiro phosphoric acid
(S)-A1, the incorporation of an excess amount of D2O into the
reaction of N-acyl dehydroalanine benzyl ester 1 and toluene
40 afforded the corresponding deuterated product 11-d1 with
82% and 93% deuteration, respectively (Scheme 3a). The
observed high levels of deuterium incorporation suggested that
the nonstereoselective pathway was significantly less likely to
be a direct hydrogen atom transfer from toluene to an α-acyl
radical intermediate but most likely to be a H2O-mediated
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product from transition state TS-1 with an R-configuration,24
and transition state TS-2 is disfavored by the steric repulsion
between the aryl group of the enol and the 2,4,6-
triisopropyphenyl group of chiral spiro phosphoric acid (S)-
A1. For the α-amidoester enolate, as suggested by Zhu and
Zhou,22 the presence of the N−H bond is also capable of
serving as a hydrogen-bond donor to facilitate seven-
membered-ring transition states TS-3 and TS-4. The sterically
demanding benzyl ester moiety is oriented to minimize
interaction with the bulky substituent of (S)-A1; thus, the
formation of R-configuration is favored (TS-3 vs TS-4).
In summary, we have developed an asymmetric photo-
catalytic C(sp3)−H bond addition to α-substituted acrylates by
using TBADT as a HAT catalyst and chiral spiro phosphoric
acid as a chiral proton-transfer shuttle. This protocol is
supposed to occur via a radical/ionic relay process, including a
TBADT-mediated HAT to cleave the inert C(sp3)−H bond, a
1,4-radical addition, a back hydrogen abstraction, and an
enantioselective protonation. A wide variety of inert C(sp3)−H
bond patterns and α-substituted acrylates are nicely tolerated
to enable rapid synthesis of enantioenriched α-stereogenic
esters from simple raw materials. We anticipate that this report
will facilitate the future development of asymmetric function-
alization of inert C(sp3)−H bonds through the combination of
photocatalysis and organocatalysis.
Scheme 3. Control Experiments and Kinetic Studies
protonation of the carbanion intermediate.21 In addition, a
significant decrease of enantioselectivity (Scheme 3a),
observed in the presence of an excess amount of D2O,
strongly indicated that enantioselective protonation of the
carbanion intermediate should constitute the main enantio-
differentiating step in this protocol, and chiral spiro phosphoric
acid was capable of showing a significant acceleration effect on
the proton shift step in comparison with H2O. Interestingly,
the block of the amide N−H moiety in N-acyl dehydroalanine
41 also resulted in the generation of the corresponding
C(sp3)−H adduct 42 but the enantioinduction completely
disappeared (Scheme 3b), implying that the amide N−H bond
might play a key role in the enantio-differentiating step
probably through a hydrogen-bonding interaction with chiral
spiro phosphoric acid (S)-A1.22 Furthermore, kinetic studies
toward clarifying the rate-determining step were also carried
out (Scheme 3c). A significant deuterium kinetic isotope effect
(KIE, kH/kD = 2.5) was observed for the reaction of 1 and
40(d), suggesting that the TBADT-mediated C(sp3)−H bond
cleavage was involved in the rate-determining step.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00801.
Complete experimental procedures and characterization
data for the prepared compounds (PDF)
Accession Codes
CCDC 2024891 and 2046103 contain 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
The absolute configuration of these C(sp3)−H adducts is
unambiguously established by X-ray crystallographic analysis of
product 26 and 37. Based on these mechanistic studies, two
plausible stereochemical pathways for protonation are
speculated to rationalize the absolute configuration (Scheme
4). With regard to the α-aryl ester enolate, a CPTS-assisted
[1,3]-proton shift23 is a plausible pathway to generate the final
AUTHOR INFORMATION
■
Corresponding Author
Pu-Sheng Wang − Department of Chemistry, University of
Science and Technology of China, Hefei 230026, China;
orcid.org/0000-0002-7229-3426; Email: pusher@
ustc.edu.cn
Scheme 4. Proposed Stereochemical Pathways for
Protonation
Authors
Zhen-Yao Dai − Department of Chemistry, University of
Science and Technology of China, Hefei 230026, China
Zhong-Sheng Nong − Department of Chemistry, University of
Science and Technology of China, Hefei 230026, China
Shun Song − Department of Chemistry, University of Science
and Technology of China, Hefei 230026, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00801
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful for financial support from University of Science
and Technology of China and Youth Innovation Promotion
Association CAS.
(14) Srikanth, G. S. C.; Castle, S. L. Advances in radical conjugate
additions. Tetrahedron 2005, 61, 10377.
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