Photoredox-Catalyzed Decarboxylative Alkylation of Silyl Enol Ethers to Synthesize Functionalized Aryl Alkyl Ketones

Article abstract of DOI:10.1021/acs.orglett.7b03587

Photoredox-catalyzed decarboxylative alkylation of silyl enol ethers has been developed. Diverse functionalized aryl alkyl ketones were afforded in modest to good yields using N-(acyloxy)phthalimide as an easy access alkyl radical source under mild and operationally simple conditions. The excellent performance of drug molecules such as fenbufen and indomethacin and naturally occurring carboxylic acids such as stearic acid and dehydrocholic acid further demonstrated the practicability of the reaction.

Full text of DOI:10.1021/acs.orglett.7b03587

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Photoredox-Catalyzed Decarboxylative Alkylation of Silyl Enol Ethers
To Synthesize Functionalized Aryl Alkyl Ketones
Weiguang Kong, Changjiang Yu, Hejun An, and Qiuling Song*
Institute of Next Generation Matter Transformation, College of Chemical Engineering & College of Material Sciences Engineering at
Huaqiao University, 668 Jimei Boulevard, Xiamen, Fujian 361021, P. R. China
S
* Supporting Information
ABSTRACT: Photoredox-catalyzed decarboxylative alkylation of
silyl enol ethers has been developed. Diverse functionalized aryl
alkyl ketones were afforded in modest to good yields using N-
(acyloxy)phthalimide as an easy access alkyl radical source under
mild and operationally simple conditions. The excellent perfor-
mance of drug molecules such as fenbufen and indomethacin
and naturally occurring carboxylic acids such as stearic acid and
dehydrocholic acid further demonstrated the practicability of the
reaction.
etones are among the most important and valuable func-
tional groups in organic molecules; they not only are
decarboxylative halogenation of aliphatic carboxylic acids in
the presence of stoichiometric silver salts.8 The Barton decar-
boxylation is another important application of this strategy
that has been widely used in chemical synthesis.9 Moreover,
decarboxylative functionalization catalyzed by transition metals
has also been developed as an efficient option for the synthetic
community.10 In addition to the reactions above, photo-
redox-catalyzed radical decarboxylative functionalization has
been raised recently as a powerful tool to access diverse com-
pounds.11 In consideration of the mechanism, the C-centered
radical involved in the reactions can be generated via two paths
from carboxylic acids and their derivatives under photoredox
conditions: (1) carboxylic acids deliver one electron to an oxida-
tive photocatalyst to generate a C-centered radical after CO2
extrusion, which is termed an reductive quenching process;12
(2) carboxylic acid derivatives such as N-(acyloxy)phthalimide
receive one electron from a reductive photocatalyst to form a
C-centered radical after CO2 and phthalimide anion extrusion,
which is termed an oxidative quenching process (Scheme 1a).13
These two processes complement each other to make photo-
redox-catalyzed radical decarboxylative functionalization a novel
and efficient method to create C−C and C−X bonds. On the
other hand, silyl enol ethers, which are frequently used platforms
to construct functionalized ketones and their derivatives, are
electron-deficient alkenes, and they can react rapidly with the
electrophilic carbon-centered radicals.14 On the basis of this
property, MacMillan and co-workers disclosed the photoredox-
catalyzed trifluoromethylation of silyl enol ethers and highly
valuable α-CF3 carbonyl compounds were synthesized in a gentle
manner (Scheme 1b).15 Herein, we investigated if N-(acyloxy)-
phthalimide could act as a source of alkyl radical to react with silyl
K
widespread in natural and man-made biologically active
molecules but also serve as basic building blocks for the synthesis
of other complex structures.1 Because of their versatility, the
development of diverse synthetic methods to prepare these
compounds has been the subject of intense research. Generally,
the oxidation of secondary alcohols could be employed to
synthesize the lower members of them, but this method was not
applicable to complicated structures because of the difficulty in
procuring the necessary alcohols and the sensitivity of some
substituent groups to oxidative conditions.2 Nucleophilic addi-
tion of organometallic reagents onto carboxylic acids or deriv-
atives constitutes another important ketone synthesis protocol;
the major challenges associated with these protocols are the
limited scope of organometallics and sometimes overaddition of
the reactive nucleophilic reagents to ketone products, which
results in the formation of tertiary alcohols and other side
reactions.3 Fortunately, the development of ketone synthesis
based on Weinreb amide overcomes these drawbacks to some
extent.4 Recent advances in cross-coupling reactions of activated
carboxylic acid derivatives with various transmetalating reagents
to forge new carbon−acyl bonds broaden the way of ketone
construction; however, the requirement of noble metal catalysts
and harsh reaction conditions makes this method far from
acceptable.5 Thus, the development of a mild, operationally
simple approach to the synthesis of functionalized ketones is
highly desired.
Carboxylic acids are basic synthetic materials because they are
stable, inexpensive, and easily available. Their conversion into
valuable fine chemicals such as medicinally relevant compounds
represents an important goal in organic chemistry.6 Among the
abundant transformations of carboxylic acids, decarboxylative
functionalization has been established as a powerful strategy
to construct all kinds of chemical bonds.7 In particular, the
Hunsdiecker reaction dates back to the 1930s and involves the
Received: November 18, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03587
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Letter
we next studied the influence of water on our reactions and found
that better results can be achieved with 25 equiv of water added in
dry CH3CN (entries 12−14, Table 1). However, other acids
including Brønsted acids and Lewis acids did not improve the
yield (entries 15−19, Table 1). An 88% yield was obtained when
trimethyl((1-phenylvinyl)oxy)silane (1a) was used on 0.2 mmol
(entry 20, Table 1). Control experiments found that light and
photoredox catalyst were essential to the reactions. The reactions
did not proceed without irradiation or in the absence of the
photocatalyst (entries 21 and 22, Table 1).
Scheme 1. (a) Mechanism of C-Centered Radical Generation
via Photoredox-Catalyzed Radical Decarboxylation; (b) and
(c) Photoredox-Catalyzed Functionalization of Silyl Enol
Ethers
With the optimized conditions in hand, we set out to explore
the substrate scope of the reaction with a range of different qN-
(acyloxy)phthalimides (Scheme 2). It was demonstrated that this
Scheme 2. Scope of the N-(Acyloxy)phthalimide
a
Compounds
enol ethers so that highly functionalized alkyl ketones would be
formed under mild photoredox-neutral conditions (Scheme 1c).
To evaluate the feasibility of our hypothesis on photoredox-
catalyzed decarboxylative alkylation of silyl enol ethers as a mild,
operationally simple approach to synthesize functionalized
ketones, trimethyl((1-phenylvinyl)oxy)silane (1a) and 1,3-
dioxoisoindolin-2-yl cyclohexanecarboxylate (2a) were selected
as model substrates to conduct the optimization of reaction
conditions (Table 1). Initially, screening of common photo-
Table 1. Optimization of Photoredox-catalyzed
Decarboxylative Alkylation of Silyl Enol Ethers
a
Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol), [Ir(ppy)2(dtbbpy)]-
(PF6) (2.5 mol %), H2O (25 equiv), CH3CN (2 mL), 5 W blue LED, N2,
b
rt, 12 h. The reaction was scaled up to 7.5-fold.
reaction showed good functional group compatibility. Simple
alkyl-substituted N-(acyloxy)phthalimides worked smoothly in
this transformation, and all of the corresponding products were
obtained in moderate yields (3ab−ae). Substituents such as Br,
alkynyl, ester, and carbonyl groups were compatible in the
reaction. The desired ketones that are useful compounds for
further transformation were obtained in moderate to good yields
(3af−ai). It is worth mentioning that when the reaction was
scaled up to 1.5 mmol scale, 44% (165 mg) of 3ai was obtained;
the drop in yield might be due to the formation of a decar-
boxylative protonation side product of N-(acyloxy)phthalimide.
N-(Acyloxy)phthalimide derived from 2-(4-methoxyphenyl)-
acetic acid could also participate in the reaction but with a
relatively low yield (3aj). This was probably due to the relatively
strong stability of the benzyl radical that would lead to the
1-methoxy-4-methylbenzene side product. In addition to
primary alkyl groups, secondary alkyl groups could also be
successfully introduced into the silyl enol ethers through this
photoredox-catalyzed decarboxylative alkylation reaction and
a
b
c
d
GC yield. Used directly from commercial source. Dry solvent. 1a
e
f
(0.2 mmol) and 2a (0.3 mmol) were used. Isolated yield. No light.
catalysts found that fac-Ir(ppy)3 and [Ir(ppy)2(dtbbpy)](PF6)
were efficient candidates to promote the reaction with the
desired 2-cyclohexyl-1-phenylethan-1-one (3aa) obtained in
60% and 67% yields, respectively (entries 1−5, Table 1). Further
investigation of solvents revealed that CH3CN was essential to
the reaction. Other solvents such as DMF, DMSO, THF,
toluene, DCE, and acetone led to terrible results (entries 3,
6−11, Table 1). Inspired by Glorius’ work that water can activate
N-(acyloxy)phthalimide through hydrogen-bond interactions,16
B
DOI: 10.1021/acs.orglett.7b03587
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
achieved good results (3aa, 3ak−an). As a special case, the
product of N-(acyloxy)phthalimide derived from trans-2-phenyl-
1-cyclopropanecarboxylic acid was afforded in 57% yield with
excellent diastereoselectivity (3an). Moreover, N-(acyloxy)-
phthalimides derived from α-amino acids were efficiently decar-
boxylated under the reaction conditions, and valuable β-amino
ketones were generated in moderate yields (3ao, 3ap). We
know that tertiary alkyl radicals are more stable and sterically
demanding than primary and secondary alkyl radicals. The
reflection of this characteristic was 3aq, which was obtained in a
relatively lower yield. To our surprise, drug molecules such as
fenbufen and indomethacin and naturally occurring carboxylic
acids such as stearic acid and dehydrocholic acid could be readily
applied to this photoredox-catalyzed ketone synthesis reaction.
All of the desired products were obtained in excellent yields
(3ar−au). The success of these remarkable cases further
demonstrated the practicability of the reaction.
Scheme 4. Experimental Probes on the Reaction Mechanism
Scheme 5. Proposed Mechanism
Next, we examined the generality of this transformation with
respect to different silyl enol ethers. N-(Acyloxy)phthalimide
derived from 4-oxo-4-phenylbutanoic acid (2i) was selected as
another reactant to participate in the reaction as the obtained 1,5-
dicarbonyl compounds are highly valuable building blocks for the
synthesis of other complex structures. As shown in Scheme 3,
a
Scheme 3. Scope of the Silyl Enol Ethers
interactions, single-electron transfer from visible light excited
photocatalyst to the N-(acyloxy)phthalimide 2 generates a
radical anion A, which then undergoes homolytic cleavage of the
N−O bond, leading to carboxyl radical B and phthalimide. Next
decarboxylation of B generates alkyl radical C. Radical addition of
alkyl radical C to silyl enol ether 1 generates the α-O radical D,
which is subsequently oxidized by the oxidative photocatalyst to
form the intermediate E. Finally, hydrolysis of the intermediate E
affords the desired ketone product.
In summary, photoredox-catalyzed decarboxylative alkylation of
silyl enol ethers as a mild and operational simple approach to
synthesize functionalizedarylalkylketoneshas been developed. N-
(Acyloxy)phthalimides were utilized as an easy access alkyl radical
source to afford the desired products in moderate to good yields.
The excellent performance of drug molecules such as fenbufen and
indomethacin and naturally occurring carboxylic acids such as
stearic acid and dehydrocholic acid makes this transformation
valuable and practical in organic synthesis.
a
Reaction conditions: 1 (0.2 mmol), 2i (0.3 mmol), [Ir-
(ppy)2(dtbbpy)](PF6) (2.5 mol %), H2O (25 equiv), CH3CN (2 mL),
5 W blue LED, N2, rt, 12 h.
α-aryl silyl enol ethers with functional groups such as −Me,
−OMe, −F, −Cl, and −Br proved to be well adapted to our
conditions without the influence of the position of functional
groups (3bi−mi). Furthermore, trimethyl((1-(naphthalen-2-
yl)vinyl)oxy)silane also exhibited robust reactivity in reaction
with the desired product obtained in good yield (3ni). How-
ever, heteroaromatics such as pyridine and thiofuran inhibited
the reaction strongly (3oi−pi). In addition, an alkyl-derived silyl
enol ether was also proven to be inappropriate in this protocol
(3qi).
To gain insight into the mechanism of the reaction, a radical
cascade experiment and radical-trapping experiments were
conducted (Scheme 4). When N-(acyloxy)phthalimide derived
from 6-heptenoic acid (2v) reacted with 1a under standard
conditions, radical cyclization product 3av was obtained in 50%
yield (eq 1). The reaction was totally suppressed with the radical-
trapping reagent (BHT or TEMPO or 1,1-diphenylethylene)
added, and radical-trapping products were all detected by
GC−MS (eqs 2−4). These results support the involvement of an
alkyl radical and its reaction with silyl enol ether.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03587.
Experimental details and spectral data for all products
(PDF)
According to the results above and other previous investi-
gations, a plausible mechanism for the photoredox-catalyzed
decarboxylative alkylation of silyl enol ethers is proposed in
Scheme 5. First, with the help of water through hydrogen-bond
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: qsong@hqu.edu.cn.
C
DOI: 10.1021/acs.orglett.7b03587
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
ORCID
Letter
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Qiuling Song: 0000-0002-9836-8860
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Natural Science Foundation of Fujian
Province (2016J01064), the Recruitment Program of Global
Experts (1000 Talents Plan), Program of Innovative Research
Team of Huaqiao University (Z14 × 0047), and the Graduate
Innovation Fund of Huaqiao University (to W. K.) is
acknowledged. We also thank the Instrumental Analysis Center
of Huaqiao University for analysis support.
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