Acyl Radicals from Benzothiazolines: Synthons for Alkylation, Alkenylation, and Alkynylation Reactions

Article abstract of DOI:10.1021/acs.orglett.9b01717

We describe herein a fundamentally new visible light-driven homolytic C-C bond breaking mode for the generation of acyl radicals from C2-acyl-substituted benzothiazolines. The reactive species can be used as versatile synthons for formal radical alkylatio

Full text of DOI:10.1021/acs.orglett.9b01717

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Acyl Radicals from Benzothiazolines: Synthons for Alkylation,
Alkenylation, and Alkynylation Reactions
Lei Li, Shan Guo, Qi Wang, and Jin Zhu*
Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, State Key Laboratory of
Coordination Chemistry, Nanjing National Laboratory of Microstructures, Collaborative Innovation Center of Chemistry for Life
Sciences, Nanjing University, Nanjing 210023, China
S
* Supporting Information
ABSTRACT: We describe herein a fundamentally new visible light-
driven homolytic C−C bond breaking mode for the generation of acyl
radicals from C2-acyl-substituted benzothiazolines. The reactive species
can be used as versatile synthons for formal radical alkylation,
alkenylation, and alkynylation reactions.
central theme of organic synthesis is the invention of new
bond breaking modes and use of reactive species generated
generating discrete carbon-centered radical intermediates for
synthetic purpose. In particular, we demonstrate herein that C2-
acyl groups, when assisted with hypervalent iodine reagents
(HIRs),5 can be converted into acyl radicals under visible light
irradiation (Scheme 1b). The acyl radicals produced thereof can
be utilized as versatile synthons for formal radical alkylation,
alkenylation, and alkynylation reactions.
A
thereof for forging new chemical bonds. Compared to C−H
bonds, the scission of structurally closely related C−C bonds
poses a significantly more demanding challenge due to their
lower degree of polarization and lower affinity for transition
metals.1 However, the tempting prospect of discovering new
reaction pathways, new retro-synthetic schemes, and powerful
synthetic tools has prompted efforts for the identification of new
C−C bond breaking modes.2
Benzothiazolines are extremely useful organic synthesis
reagents and have been used, primarily, as a C2-hydrogen
donor for various reductive processes under Brønsted acid or
Lewis acid catalysis (Scheme 1a).3 Only a single report briefly
Acyl radicals are tremendously useful intermediates for
organic synthesis.6 Early thermal or photolytic acyl radical
generation strategies generally witness harsh reaction conditions
(e.g., strong oxidant, UV irradiation, high CO pressure, high
temperature), thus inevitably restricting reactivity scope.7
Visible light driven bond breakage has recently been adopted
for the achievement of mild reaction conditions and expansion
of reactivity profiles.8 Albeit effective, a high-valued precious
transition metal (Ir, Ru, etc.) or a toxic organic dye as
photosensitizer is typically involved, which can be a drawback
in certain synthetic contexts.9 To address this issue, several
efforts have been directed at transition metal- and organic dye-
free conditions.10 α-Ketoacids have been exploited in
alkynylation reaction and oxindole synthesis through activation
with HIRs.11 However, the lability of α-ketoacids12 raises serious
concerns over their routine use as synthetic reagents. Aldehydes
have been combined with N-hydroxyphthalimide13 or 2-tBu-
anthraquinone14 for denitrative alkenylation reaction or
coumarin synthesis, but only aromatic aldehydes can be
tolerated. Hantzsch esters15 have recently been added to the
repertoire, but (1) their preparation involves complicated
multiple reflux/high temperature steps, and (2) their utility is
demonstrated only in organocatalysis-coupled, Giese-type
addition reaction under prolonged irradiation. The acyl radical
system reported herein provides an efficient, versatile visible
light-driven platform for diverse C−C bond-forming reactions
from benzothiazolines, the preparation of which involves
minimum reagents and efforts.
Scheme 1. Benzothiazolines as Donors of Reactive Species for
Organic Synthesis
mentioned the possibility of thermally driven, concerted C2-
alkyl transfer (heterolytic C−C bod cleavage), in the form of
carbanion, to imines (with three examples) under high
temperature and prolonged reaction time (∼2 days) (Scheme
1a).4 With these precedents in mind, we have opted to pursue a
different reaction course, exploiting aromatization as the
enabling tool for homolytically cleaving C−C bonds and
Received: May 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01717
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We set out to evaluate the feasibility of generating acyl radical
from 2-benzoyl-2-phenylbenzothiazoline (1a) under visible
light irradiation and, with electron-deficient 1,1-dicyano-2-
phenylethylene (2a) as the acceptor, achieving formal radical
alkylation (or alternatively, alkene hydroacylation) reaction. By
using hydroxybenziodoxolone (BI−OH) (2.0 equiv) as the
hypothetical 1a N−H hydrogen (proton or hydrogen atom)
abstraction reagent, initial screening experiments were per-
formed against the photosensitizer (Table S1). The radical
alkylation reaction can proceed quite easily under overnight blue
light LED irradiation in DCM with the addition of a variety of
transition metal complexes (2.5 mol %) as the photosensitizer
and NaH2PO4 (2.0 equiv) as the base. However, the yield of
target product 3aa is generally not satisfactory (Ru(ppy)3(PF6)2,
68%; Ir(dtbbpy)(ppy)2PF6, 76%; Ir(ppy)3, 88%). The switching
to the organic photosensitizer rhodamine B witnesses a
reduction in the yield (44%). The adoption of 9-mesityl-10-
methylacridinium perchlorate (Acr+-Mes) provides a seemingly
significant boost to the reactivity. Depending on the solvent
(DCM, DCE, EtOAc, dioxane)/base (NaH2PO4, Na2HPO4,
Na3PO4, NaH2PO4, NaOAc, NaHCO3)/BI−OH derivative
combination, the yield can be largely maintained, with a few
exceptions, at the level of 80−92%. Further control experiments
indicate, however, not surprisingly in hindsight from mecha-
nistic standpoint, that photosensitizer can be completely
eliminated from the system without negatively impacting the
reaction. Compound 3aa can be efficiently obtained in 92% yield
even with the BI−OH quantity reduced to the 30 mol % catalytic
level. No reaction occurs in the dark under otherwise identical
conditions.
Scheme 2. Substrate Scope of Formal Radical Alkylation
Reaction
a b
,
With the visible light-driven, photosensitizer-free reaction
conditions established, we next proceeded to the assessment of
substrate scope (Scheme 2). The substrate scope for
benzothiazolines with respect to C2-acyl substituent was
examined by coupling with 2a. Both electron-donating (p-Me,
1b; p-OMe, 1c) and electron-withdrawing (p-Br, 1d) groups can
be tolerated on the phenyl ring, affording corresponding
products in 80−90% yields. The use of a heterocyclic substituent
such as 2-furyl (1e) or 2-thienyl (1f) only leads to a slight
reduction of the yield. Notably, an alkyl substituent (1g, 1h, 1i)
provides an effective reaction system (85% yield). The substrate
scope for trisubstituted alkenes with respect to C2-substituent
was explored by coupling with 1a. The yields stay at a high level
(80−95%) irrespective of the electronic character of the
substituent on the phenyl ring. The electron-donating groups
(p-Me, 2b; p-OMe, 2c) exert a slightly negative effect on the
product yields as compared to the electron-withdrawing groups
(p-F, 2d; p-Cl, 2e; p-Br, 2f; p-CF3, 2g; o-Cl, 2h; m-Cl, 2i). This is
understandable since electron deficiency is the prime parameter
governing alkene reactivity as acceptors for nucleophilic acyl
radicals. The product yields for alkenes bearing di- (2j) and
trisubstituted (2k) phenyl rings are comparable to those bearing
monosubstituted ones. The replacement of phenyl group with
the bulky naphthyl group (2l) poses no hurdle for the reaction.
However, a heterocyclic group (2-furyl, 2m; 2-thienyl, 2n)
significantly retards the reaction. Switching to the alkyl group
(2o, 2p) also shows compatibility with the reaction. The
replacement of one cyano group to an ester group (2q)
decreases the reactivity. Significantly, two fundamentally
different types of alkene substrates also exhibit the coupling
reactivity (2r, 2s).
a
Reaction conditions: 1 (0.12 mmol), 2 (0.1 mmol), DCM (2 mL).
Isolated yield.
b
reaction pathway. The reaction between 1a and 2a can be
completely inhibited by the addition of a radical scavenger of
either (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) or 1,2-
dinitrobenzene (eq 1). Together with the isolation of 2-
phenylbenzothiazole and 2-iodobenzoic acid, the following
reaction mechanism is proposed (Scheme 3): the reaction
between 1a and BI−OH generates intermediate I; the I−N bond
of I is cleaved under visible light irradiation, leading to the
formation of radical intermediate II and BI• radical III;
aromatization of II provides the driving force for C−C bond
cleavage and formation of acyl radical IV; nucleophilic attack of
IV on 2a generates intermediate V; abstraction of hydrogen
atom by V from 1a generates 3aa as the formal alkylation
product and reinitiates the formation of I for the next chain
reaction cycle; BI• radical III can also abstract the hydrogen
atom from 1a to initiate the radical reaction. Dicyano groups are
versatile reactive functionalities toward further elaboration (with
or without the incorporation of two cyano groups into target
products).16 As an illustration, 3aa can be conveniently
transformed into 3aa-t1 (eq 2) and 3aa-t2 (eq 3).
With the substrate scope surveyed, preliminary mechanistic
experiments were conducted and are consistent with a radical
B
DOI: 10.1021/acs.orglett.9b01717
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Organic Letters
Letter
reaction. The radical alkylation product 3aa can be converted to
4aa under the radical alkenylation condition employed herein
(eq 4). Monitoring with thin layer chromatography (TLC)
Scheme 3. Mechanistic Proposal for the Formal Radical
Alkylation Reaction
shows that the reaction between 1a and 2a features an initial
rapid accumulation of 3aa (within 2 h) and subsequent
formation of 4aa at the expense of 3aa. The transition metal
photosensitizer Ru(1,10-phen)3Cl2 can be changed to the
organic photosensitizer Acr+-Mes without affecting the trans-
formation (eqs 5 and 6). Alternatively, the formal alkenylation
product (4ia, 4ao) can be directly obtained from the
corresponding alkylation product (3ia, 3ao) under DDQ
oxidative conditions (Scheme 4).
With both radical alkylation and alkenylation reactions
demonstrated, we further pursued the possibility of achieving
radical alkynylation. To this end, alkynylbenziodoxolone (BI-
alkyne) is selected as the potential source of alkynyl group.
Initial screening was carried out using 1a and BI-phenyl-
acetylene (5a) as the coupling partners, Acr+-Mes (2.0 mol %) as
the photosensitizer, and NaH2PO4 (2.0 equiv) as the base
(Table S3). With the quantity of 5a first set at 2.0 equiv, a variety
of solvents (DCM, DCE, EtOH, EtOAc, THF, CH3CN) was
scrutinized. DCM proves to be the best reaction medium, with
the yield of product 6aa reaching 84% after 2 h of blue light LED
irradiation. The reaction can still proceed smoothly when the
quantity of 5a is decreased to either 1.5 or 1.2 equiv, albeit with a
slightly lower yield. The increase of 5a quantity to 3.0 equiv is
not beneficial. As in the case of benzothiazoline/BI−OH system,
the acyl radical generation from benzothiazoline/BI-alkyne
system does not require the participation of Acr+-Mes. Without
Acr+-Mes, 6aa can be obtained from 1a and 5a in 89% yield.
The substrate scope for benzothiazolines and BI-alkynes was
then investigated (Scheme 5). For benzothiazolines, the
electronic character of C2-acyl phenyl ring, whether electron-
rich (1b, 1c) or electron-poor (1d), does not significantly
influence the reactivity. The change of phenyl ring to a
heterocycle (1e) or an alkyl group (1g) provides a less effective
reaction system. The reactions using benzothiazolines con-
structed from unsymmetrical 1,2-diketones (1j, 1k, 1l)
demonstrate that an aromatic ring on the C2 position is not
required for achieving the high reactivity. For BI-alkynes, the
effect of phenylacetylene substitution was comprehensively
studied. For the para substitution, an electron-donating group
(p-Me, 5b) is generally not as ideal as an electron-withdrawing
group (p-F, 5c; p-Cl, 5d; p-C(O)CH3, 5e; p-CH(O), 5f; p-
CO2Me, 5g), but with a notable exception (p-CN, 5h). The
ortho substitution (o-Cl, 5i) provides a more reactive substrate
as compared to the para counterpart. The meta substitution (m-
Cl, 5j; m-OMe, 5k; m-OCF3, 5l) also enables the generation of
respective product in high yield. Extension of the single phenyl
ring to the 1,1′-biphenyl ring (5m) largely preserves the
Even though photosensitizer has proven to be an unnecessary
component for the radical alkylation reaction, during the course
of reaction condition screening, we have identified an interesting
formal radical alkenylation reaction (Table S2). The reaction
between 1a and 2a can proceed to the stage of 4aa in 80% yield,
with the restoration of BI−OH quantity to 1.2 equiv. and the
additional participation of Ru(1,10-phen)3Cl2 (2.0 mol %),17
under blue light LED irradiation. Preliminary substrate scope
inspection (Scheme 4) suggests the reaction compatibility of
phenyl substitution on both the C2-acyl side (1b) and alkene C2
side (2b, 2c, 2d, 2e, 2f, 2j). The product yields for radical
alkenylation are generally inferior to those for radical alkylation,
and again, electronic deficiency on the alkene C2 phenyl ring
dictates the reaction outcome. Careful control experiments
illustrate a stepwise mechanism for the radical alkenylation
Scheme 4. Substrate Scope of Formal Radical Alkenylation
a b
,
Reaction
a
Reaction conditions: 1 (0.12 mmol), 2 (0.1 mmol), DCM (2 mL).
b
c
d
Isolated yield. Compounds 3ia and 3ao as starting material. DDQ
(2.0 equiv), O2 balloon, CHCl3 (1 mL).
C
DOI: 10.1021/acs.orglett.9b01717
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Letter
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a b
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a
Reaction conditions: 1 (0.1 mmol), 5 (0.2 mmol), DCM (2 mL).
b
Isolated yield.
reactivity. Delightfully, an alkyl substitution (5n) or a silyl
substitution (5o) on the acetylene unit can also be tolerated.
In conclusion, we have developed herein an acyl radical
generation strategy from benzothiazolines under visible light
irradiation. This not only provides a mechanistically distinct C−
C bond breaking mode but also enables the efficient achieve-
ment of radical alkylation, alkenylation, and alkynylation
reactions. These findings promise the discovery of more
aromatization-derived synthetic systems.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b01717.
Experimental procedures and product characterization;
1
copies of the H and 13C NMR spectra of selected
products (PDF)
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A.-L.; Ollivier, C.; Ryu, I.; Fensterbank, L. Angew. Chem., Int. Ed. 2019,
58, 1789−1793. (b) Micic, N.; Polyzos, A. Org. Lett. 2018, 20, 4663−
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jinz@nju.edu.cn.
ORCID
Jin Zhu: 0000-0003-4681-7895
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
J.Z. gratefully acknowledges support from the National Natural
Science Foundation of China (21425415, 21774056), the
National Basic Research Program of China (2015CB856303),
and Science and Technology Department of Jiangsu Province
(BRA2017360, BK20181255).
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