Visible-light-driven palladium-catalyzed Dowd-Beckwith ring expansion/C-C bond formation cascade

Article abstract of DOI:10.1039/d0sc04399k

A visible-light-induced palladium-catalyzed Dowd-Beckwith ring expansion/C-C bond formation cascade is described. A range of six to nine-membered β-alkenylated cyclic ketones possessing a quaternary carbon center were accessed under mild conditions. Besides styrenes, the electron-rich alkenes such as silyl enol ethers and enamides were also compatible, providing the desired β-alkylated cyclic ketones in moderate to good yields.

Full text of DOI:10.1039/d0sc04399k

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Visible-light-driven palladium-catalyzed Dowd–
Beckwith ring expansion/C–C bond formation
cascade†
Cite this: DOI: 10.1039/d0sc04399k
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
a
a
ab
Li Chen,a Li-Na Guo,
Shuai Liu,a Le Liu
and Xin-Hua Duan
*
A visible-light-induced palladium-catalyzed Dowd–Beckwith ring expansion/C–C bond formation cascade
is described. A range of six to nine-membered b-alkenylated cyclic ketones possessing a quaternary carbon
center were accessed under mild conditions. Besides styrenes, the electron-rich alkenes such as silyl enol
ethers and enamides were also compatible, providing the desired b-alkylated cyclic ketones in moderate to
good yields.
Received 11th August 2020
Accepted 26th November 2020
DOI: 10.1039/d0sc04399k
rsc.li/chemical-science
Since the seminal pioneering works of Dowd and Beckwith,3,6
a variety of radical initiating systems, especially photocatalytic
Introduction
systems have been established for this transformation (Scheme
1, eqn a).7 Although many progress have been made in the
reaction system, the ring expansion/H-atom abstraction is still
the mainstream transformation, which limited its application
in the radical cascade.8 Recently, the light-induced transition-
metal catalysis has emerged as a versatile platform to realize
transformations that are otherwise difficult to achieve. In this
eld, the groups of Gevorgyan, Fu, Yu and others recently
demonstrated a series of fantastic chemical transformations
through visible light-induced Pd-catalyzed alkyl couplings,
wherein the hybrid alkyl Pd(I)-radical species enabled the alkyl
couplings achievable.9 As a part of our interest in radical
chemistry and transition-metal catalysis,10 we hope to explore
an intermolecular Dowd–Beckwith ring-expansion/Heck-type
coupling cascade (Scheme 1, eqn b). Challenges for devel-
oping such cascade include the fast competitive H-atom
abstraction, the possible b-H elimination and other side reac-
tions due to several different reactive radical species involved.
Herein, we report the rst visible light induced, Pd-catalyzed
The medium-sized carbocycles constitute the core frameworks
of many natural products, valuable pharmaceutical compounds
and so on.1 Given their widely recognized importance, contin-
uous efforts from chemists have been devoted to construct the
related skeletons.1,2 In addition to the cyclization reactions, the
ring-expansion reactions represent complementary and useful
strategy to access these frameworks.3 Different methods
including fragmentation of the zero bridge in fused bicyclic
units, inter- and intramolecular carbon-insertion reactions and
others have been developed to achieve the ring-expansion.4 In
this eld, the ring expansion of cycloalkanones provided an
efficient strategy for the medium-sized cyclic ketones' con-
struction.3c–e The ring expansion of unstrained cyclic ketones,
particularly ve- and six-membered rings is more challenging
and attractive due to the relatively much weaker ring strain.
Recently, Dong's group presented an elegant Rh-catalyzed
strategy based on the installation of temporary directing
group to activate the C–C bond, followed by carbon insertion to
enlarge the cyclic ketones.3b,3f,5 Unfortunately, this strategy is
limited to cyclopentanones. The development of ring-expansion
reactions to form functionalized medium-sized carbonyl
compounds still remains highly demanding.
The Dowd–Beckwith reaction and variants represent an
important class of radical ring-expansion reaction, which offers
useful alternative to obtain the medium-sized cyclic ketones.
aSchool of Chemistry, Xi'an Key Laboratory of Sustainable Energy Material Chemistry,
MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed
Matter Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, China
bState Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou730000, P. R. China. E-mail: duanxh@xjtu.edu.cn
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization of new compounds are provided. See DOI:
10.1039/d0sc04399k
Scheme 1 The classical Dowd–Beckwith reactions and our design on
the Dowd–Beckwith ring-expansion/Heck-type coupling cascade.
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intermolecular Dowd–Beckwith ring-expansion/C–C bond MeOC6H4)3) were effective (entries 5 and 6). Both inorganic and
formation cascade. A range of medium-sized b-alkenylated organic bases were examined, and K2CO3 turned out to be the
cyclic ketones bearing a quaternary carbon centre were obtained optimal one (entries 7 and 8). Solvent screening reveals that
under mild conditions with good yields and excellent PhCF3, xylene and DCE were inferior to toluene, while polar
stereoselectivity.
solvents such as CH3CN and DMF were completely unsuitable
(entries 9 and 10). Lowering the loading of Pd/Xantphos to 5/
10 mol% resulted in a diminished yield of 3a (entry 1). Control
experiments indicated that catalyst, ligand and base were all
essential for this reaction (entry 11). Furthermore, visible light
irradiation was also required (entry 12). Without irradiation, the
Results and discussion
Inspired by the pioneering works on visible-light induced Pd-
catalyzed alkyl-Heck coupling reactions,11 we envisioned that
photoirradiation might facilitate the SET event between Pd(0)
complex and Dowd–Beckwith halides to initiate the ring
expansion process. The interaction of the tertiary radical
species and Pd(I) complex generated in situ would provide
opportunities for further transformation (Scheme 1, eqn b). To
check this hypothesis, we set to examine the reaction of a-bro-
momethyl b-keto ester 1a and styrene 2a in the presence of Pd(0)
complex under visible light irradiation. To our delight, treat-
ment of 1a and 2a with 10 mol% of Pd(OAc)2, 20 mol% of 4,5-
bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos) and
K2CO3 (2.0 equiv.) in toluene under blue LEDs irradiation
afforded the desired ring-expansion/alkenylation product 3a in
66% yield (Table 1, entry 1). Other palladium catalysts such as
Pd(TFA)2, PdCl2 and Pd(PPh3)4 also displayed some catalytic
activity, but gave lower yields than Pd(OAc)2 (entries 2 and 3).
When Ni(OAc)2 was used instead of Pd(OAc)2 as the catalyst, no
reaction occurred (entry 4). The ligand proved to be crucial for
this reaction. Except for Xantphos, other phosphine ligands
neither bidentate (DPEPhos, BINAP) nor monodentate (P(o-
ꢀ
reaction did not work even heating up to 80 C (entry 13). It is
noteworthy that some amount of the direct alkyl-Heck coupling
product 3a0 (without ring-expansion) and ring-expansion/b-H
elimination product 1a0 were also observed during the reaction.
Under the optimal conditions, the byproduct 3a0 was isolated in
10% yield and the 1a0 was obtained in 5% yield. Finally, to
demonstrate the utility of this procedure, the reaction of 1a and
2a was conducted on a 1.0 mmol scale, and 62% yield of 3a was
still obtained.
With the optimized reaction conditions in hand, the gener-
ality of this reaction with respect to alkenes was evaluated
(Table 2). A variety of para-, meta-, and ortho-substituted
styrenes reacted with 1a smoothly to afford the desired six-
membered products 3b–3p in 15–76% yields. In general,
styrenes with electron-donating groups gave better yields than
Table 2 Scope of the alkenesa
Table 1 Optimization of reaction conditionsa
Entry
Variants from standard conditions
Yield (%)
1
2
3
4
5
6
None
66 (55)b
14, 16
33
Pd(TFA)2, or PdCl2 as the catalyst
Pd(PPh3)4 as the catalyst
Ni(OAc)2 as the catalyst
DPEPhos or BINAP as the ligand
P(o-MeOC6H4)3
n.r.c
Trace, n.r.c
n.r.c
7
8
9
10
11
12
13
Cs2CO3, Li2CO3 or KOAc as the base
Et3N or DIPEA as the base
PhCF3, xylene or DCE as the solvent
CH3CN or DMF as the solvent
Without catalyst or ligand or base
In the dark
50, 16, 48
12, 28
39, 43, 20
n.r.c
n.r.c or trace
n.r.c
Without irradiation, heating to 80 ꢀ
C
n.r.c
a
a
Reaction conditions A: 1a (0.2 mmol, 1.0 equiv.), 2a (0.4 mmol, 2.0
Reaction conditions A: 1a (0.2 mmol, 1.0 equiv.), 2 (0.4 mmol, 2.0
equiv.), Pd(OAc)2 (10 mol%), Xantphos (20 mol%) and K2CO3 equiv.), Pd(OAc)2 (10 mol%), Xantphos (20 mol%) and K2CO3
(0.4 mmol, 2.0 equiv.) in toluene (2.0 mL) were irradiated with 30 W (0.4 mmol, 2.0 equiv.) in toluene (2.0 mL) were irradiated with 30 W
blue LEDs at room temperature for 24 h under N2. Yields of isolated blue LEDs at room temperature for 24 h under N2. Yields of isolated
b
product were given. 5 mol% of Pd(OAc)2 and 10 mol% of Xantphos product were given. E/Z and t/l ratios of products were determined by
c
were used. n.r. ¼ no reaction.
1H NMR analysis: t ¼ terminal and l ¼ linear.
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those bearing strong electron-withdrawing groups (3b–3g vs. 3k functionalization of complex molecules. Unfortunately,
and 3l), probably due to the polarity-matching of radicals. In aliphatic alkenes, allylbenzene and acrylate esters were inert
these cases, most of the substrate 1a was recovered. Remark- under the standard conditions.
ably, excellent stereoselectivity was observed for all these reac-
Subsequently, the scope and limitations of b-keto esters 1
tions, and only E-isomers were obtained as sole products. 2- were examined using 2a as the coupling partner (Table 3).
Vinylpyridine was also efficient for this reaction, providing the Besides the a-bromomethyl substrate 1a, the homologous
desired product 3r in 27% yield. 1,1-Disubstituted alkenes chloride and iodide gave the product 3a in 10% and 53% yield,
participated in this reaction to deliver the corresponding respectively. A range of a-bromomethyl substrates containing
products 3s–3z in moderate yields. Satisfactorily, the exocyclic ve-to eight-membered ring could react smoothly to afford the
alkenes derived from 1-tetralone and 1-indanones furnished the one-carbon extended products 5a–5g. For six-, seven-, eight-
products 3x–3z in 60–80% yields. It is worth noting that the membered or other substrates, PdCl2(PPh3)2 was found as
bromo groups in styrenes were retained in this palladium optimal catalyst (conditions B). The relatively strained ve-
catalytic system (3j, 3o, 3z), thereby providing opportunity for membered substrates displayed much better reaction effi-
further derivatization. Besides terminal alkenes, the 1H-indene ciency, giving the six-membered products 5a and 5b in good
containing an internal alkene also gave the product 4a, albeit yields. It is a pity that some of the yields were unsatised due to
with somewhat low yield due to poor conversion. Notably, the the low conversion even aer prolonged reaction time. Therein,
estrone-derived olens underwent this reaction efficiently to the Dowd–Beckwith bromides could be recovered. When the
give the products 4b and 4c in satised yields, which high- substrate 1k bearing a three-carbon side chain was subjected to
lighted the potential of this method in late-stage the reaction, the anticipated product 5j was formed in a low
yield, along with the by-product 5j’ in 31% isolated yield.
Substrates with two- or four-carbon side chain (1j and 1l) failed
to give any expected products, delivering 87% and 36% of the
Table 3 Scope of the b-keto estersa
direct coupling by-products, respectively.3a The 2-phenyl cyclo-
heptanone only afforded trace amount of the desired product
(not shown).
As we know, the Dowd–Beckwith ring-expansion led to
a tertiary alkyl radical species bearing an ester group, which is
quite electrophilic in nature. Thus, we thought the electron-rich
alkenes would be compatible for this tandem transformation.
Satisfactorily, the Dowd–Beckwith bromide 1a reacted smoothly
with a series of silyl enol ethers to provide the expected b-
alkylated cyclic ketones 7a–7d in moderate to good yields.
Substrate with a three-carbon side chain afforded the desired
Table 4 Scope of the silyl enol ethersa
a
Reaction conditions A: 1 (0.2 mmol, 1.0 equiv.), 2a (0.4 mmol, 2.0
equiv.), Pd(OAc)2 (10 mol%), Xantphos (20 mol%) and K2CO3
(0.4 mmol, 2.0 equiv.) in toluene (2.0 mL) were irradiated with 30 W
blue LEDs at room temperature for 24 h under N2. Yields of isolated
b
product were given. Reaction conditions B: 1 (0.2 mmol, 1.0 equiv.),
2a (0.4 mmol, 2.0 equiv.), PdCl2(PPh3)2 (5 mol%), PPh3 (12 mol%),
a
Reaction conditions C: 1 (0.2 mmol, 1.0 equiv.), 6 (0.4 mmol, 2.0
Xantphos (6 mol%) and KOAc (0.3 mmol, 1.5 equiv.) in 1,4-dioxane equiv.), Pd(PPh3)4 (5 mol%), Xantphos (6 mol%) and KOAc (0.3 mmol,
(2.0 mL) were irradiated with 30 W blue LEDs at room temperature 1.5 equiv.) in 1,4-dioxane (2.0 mL) were irradiated with 30 W blue
c
for 24 h under N2. Yields of isolated product were given. The LEDs at room temperature for 24 h under N2. Yields of isolated
reactions were irradiated for 48 h.
product were given.
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Table 5 Scope of the enamidesa
a
Reaction conditions B: 1a (0.2 mmol, 1.0 equiv.), 8 (0.4 mmol, 2.0
equiv.), PdCl2(PPh3)2 (5 mol%), PPh3 (12 mol%), Xantphos (6 mol%)
and KOAc (0.3 mmol, 1.5 equiv.) in 1,4-dioxane (2.0 mL) were
irradiated with 30 W blue LEDs at room temperature for 24 h under
N2. Yields of isolated product were given.
Scheme 2 Proposed reaction mechanism.
Further increase of the amount of TEMPO led to a higher yield
of 10. These results implied that a radical intermediate might be
involved in this reaction. In addition, the reaction of 1a with 1-
(1-cyclopropylvinyl)benzene 2d0, a radical clock substrate affor-
ded 3x as the sole product in 55% yield (eqn (3)), which provided
clear evidence for radical pathway. Moreover, light on-off
studies revealed that constant photoirradiation is essential for
this transformation (for details, see the ESI†). The UV-vis anal-
ysis indicated that the Pd0 complex formed in the catalytic
system is the photoabsorbing species (for details, see the ESI†).
Based on the above results and literature,11 a plausible
mechanism for this ring-expansion/alkenylation reaction is
proposed (Scheme 2). Firstly, the LnPd0 complex is photoexcited
to form an excited state LnPd0* species upon irradiation, which
promotes a single-electron transfer event with 1a to generate the
putative PdI species and a primary alkyl radical I. Subsequently,
intermediate I undergoes intramolecular cyclization of the
carbon radical on the carbonyl group, followed by alkoxyl
radical triggered C–C bond cleavage to yield the ring extended
radical intermediate III. Finally, radical addition of III to styrene
2a provides the benzyl radical IV, which produces the target
product 3a and regenerates the Pd0 catalyst through b-H elim-
ination. Due to the equilibrium between hybrid alkyl Pd(I)-
radical species and alkyl Pd(II) species,13 the intermediate III
could also deliver the product 1a0 and regenerate the Pd0 cata-
lyst through b-H elimination. In addition, the direct coupling
product 3a0 could also be formed during the reaction.
Fig. 1 Ring expansion/b-H elimination reaction.
product 7f in 18% yield, along with by-product 7f0 in 25% yield.
Substrate 1m with four-carbon side chain didn't undergo the
ring-expansion process, only delivering the direct alkylated
product 7g0 in 62% yield (Table 4). In addition, the enamides
also worked with 1a, yielding the b-alkylated cyclic ketones 9 in
moderate yields (Table 5).
Notably, the byproduct 1a0 formed through ring-expansion/
b-H elimination was always observed in above reactions, which
is a valuable synthetic intermediate but is hard to obtain in
organic synthesis.12 Thus we hope to amplify this useful trans-
formation (Fig. 1). Aer several trails, we were delighted to nd
that the substrate 1a delivered the desired product 1a0 in 80%
yield by using TMEDA as the base and PhH as the solvent.
While, the analogue 1d only afforded the product 1d0 in 26%
isolated yield due to incomplete conversion.
To shed light on the mechanism of this reaction, a series of
control experiments were performed (Fig. 2). The addition of
TEMPO and BTH, well-known radical scavengers, both inhibi-
ted the model reaction signicantly (eqn (1) and (2)). When 1.0
equiv. of TEMPO was added, the yield of 3a was reduced to 20%.
Meanwhile, the TEMPO-adduct 10 was isolated in 13% yield.
Conclusions
In conclusion, we have developed a Dowd–Beckwith ring
expansion/C–C bond formation cascade by light-induced
palladium catalysis. A range of ve to eight-membered Dowd–
Beckwith halides reacted smoothly with styrenes to afford the
enlarged b-alkenylated cyclic ketones in moderate to good yields
with excellent stereoselectivity. Besides styrenes, the electron-
rich alkenes such as silyl enol ethers and enamides were also
compatible, providing the desired b-alkylated cyclic ketones.
This research would arouse the application of the Dowd–Beck-
with reaction.
Fig. 2 Control experiments.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
We gratefully acknowledge the nancial support from the
National Natural Science Foundation of China (No. 21971201),
Natural Science Basic Research Plan in Shaanxi Province of
China (No. 2019JM-299) and Key Laboratory Construction
Program of Xi'an Municipal Bureau of Science and Technology
(No. 201805056ZD7CG40). We also thank Mr Chang, Miss Bai
and Miss Zhang at the Instrument Analysis Center of Xi'an
Jiaotong University for their assistance with NMR and HRMS
analysis.
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