Metal-Free Oxidative Decarboxylative Acylation/Ring Expansion of Vinylcyclobutanols with α-Keto Acids by Visible Light Photoredox Catalysis

Article abstract of DOI:10.1002/cjoc.201600729

A metal-free visible-light-mediated decarboxylative acylation/ring expansion of vinylcyclobutanols with α-keto acids has been developed. Hypervalent iodine reagent was found to be important for promoting decarboxylation of α-keto acids in this tandem radical process. This protocol has been successfully applied to synthesis of a variety of substituted 1,4-dicarbonyl compounds and tolerated some functional groups well.

Full text of DOI:10.1002/cjoc.201600729

COMMUNICATION
DOI: 10.1002/cjoc.201600729
Metal-Free Oxidative Decarboxylative Acylation/Ring
Expansion of Vinylcyclobutanols with α-Keto Acids by
Visible Light Photoredox Catalysis
Jin-Jiang Zhang, Yuan-Bo Cheng, and Xin-Hua Duan*
Department of Chemistry, School of Science and MOE Key Laboratory for Nonequilibrium Synthesis and
Modulation of Condensed Matter, Xi'an Jiaotong University, Xi'an, Shaannxi 710049, China
A metal-free visible-light-mediated decarboxylative acylation/ring expansion of vinylcyclobutanols with α-keto
acids has been developed. Hypervalent iodine reagent was found to be important for promoting decarboxylation of
α-keto acids in this tandem radical process. This protocol has been successfully applied to synthesis of a variety of
substituted 1,4-dicarbonyl compounds and tolerated some functional groups well.
Keywords visible light, oxidative decarboxylation, radical, α-keto acids
Introduction
cross-couplings of carboxylic acids and their derivatives
have emerged as an efficient and straightforward meth-
od for the construction of C－C and C-heteroatom
bonds.[3,8] In 2014, Lei and co-workers[9] first reported
the decarboxylative amidation of α-keto acids with
amines promoted by visible-light photoredox catalysis.
Very recently, MacMillan, Fu and Shang et al. reported
the decarboxylative cross-coupling reactions between
α-keto acids and aryl halides by merging photoredox
and transition-metal catalysts.[10] Meanwhile, the de-
carboxylative alkynylation of α-keto acids with different
alkynylating reagents through photoredox catalysis has
also been well established.[11] Furthermore, Wang, Fu,
and Gu have also described a series of useful decarbox-
ylative transformations of α-keto acids by visible-light-
mediated photoredox catalysis.[12] On the other hand,
Toste and Glorius have reported visible-light photo-
redox-catalyzed arylation and trifluoromethylation of
vinylcyclobutanols via radical ring expansion pro-
cess.[13a,13b] Subsequently, Wallentin described a visible-
light mediated semipinacol rearrangement of vinylcy-
clobutanols using carboxylic anhydride as the acylating
agent.[13g] Inspired by these studies combined with our
ongoing interests in the oxidative decarboxylative cou-
pling of α-keto acids,[2e,3,5,7] we envisioned that this ele-
gant photoredox catalysis strategy might be applicable
to the acylation of vinylcyclobutanols, which would
provide an approach to 1,4-dicarbonyl compounds via
semipinacol-type rearrangement (Scheme 1).[13] Herein,
we wish to report a transition-metal free oxidative de-
carboxylative acylation/ring expansion of vinylcyclo-
butanols with α-keto acids through visible-light photo
Carboxylic acids are an important class of feedstock
chemicals which are widely used in organic synthesis
because of their easy availability, low toxicity as well as
high stability.[1] Among them, α-keto acids are particu-
larly relevant, which usually serve as acyl sources in
transition-metal catalyzed decarboxylative (dehydro-
genative) cross-coupling.[2] Furthermore, they are also
efficient acylating reagent in radical-mediated cross-
coupling reactions.[3] For instance, in the early 1990s,
the group of Minisci described first silver-catalyzed
homolytic acylation of pyridines and pyrazines with
α-keto acids via a decarboxylative radical process.[4]
Recently, our group have developed a series of radi-
cal-mediated tandem decarboxylative difunctionaliza-
tion of activated alkenes with α-keto acids for the syn-
thesis of acylated oxindoles, dihydroquinolinones, cou-
marins and so on.[5] During this period, Lei and others
have demonstrated a series of oxidative decarboxyla-
tion-cyclization of α-keto acids with various substrates
for the construction of structurally diverse heterocy-
cles.[6] Besides heterocycles, we have also explored the
oxidative decarboxylative coupling reactions of α-keto
acids for the synthesis of -fluorinated 3-aryl ketones
and functionalized ynones, respectively.[7] As part of our
continued interest in radical decarboxylation reactions,
we sought to further expand the scope and synthetic
applicability of decarboxylation of α-keto acids by de-
veloping mild and general strategies for generating and
trapping this kind of carbon-centered radicals.
Recently, visible-light-mediated decarboxylative
*
E-mail: duanxh@xjtu.edu.cn
Received October 28, 2016; accepted December 1, 2016; published online XXXX, 2017.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201600729 or from the author.
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Zhang, Cheng & Duan
Scheme 1 Visible-light-mediated decarboxylative acylation/ring expansion of vinylcyclobutanols with α-keto acids
COMMUNICATION
Table 1 Optimization of the reaction conditionsa
Entry
1
Photocatyalyst (mol%)
Ru(PPh3)Cl2 (2%)
Ru(bpy)3Cl2 (2%)
Solvent
Yieldb/%
n.r.c
15
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
DCE/H2O (1∶1)
Tol/H2O (1∶1)
2
3
Ir(dF(CF3)ppy)2(dtbbpy) (2%)
Rhodamine B (2%)
Vitamin B2 (2%)
58
4
37
5
Trace
Trace
62
6
3,4,5,6-Tetrachlorofluorescein (2%)
Rhodamine B (5%)
Rhodamine B (8%)
Rhodamine B (5%)
Rhodamine B (5%)
Rhodamine B (5%)
Rhodamine B (5%)
Rhodamine B (5%)
Rhodamine B (5%)
Rhodamine B (5%)
Rhodamine B (5%)
Rhodamine B (5%)
Rhodamine B (5%)
－
7
8
43
9
45
10
11
12
13
14
15
16
17
18
19
20
21
46
PhCl/H2O (1∶1)
EtOAc/H2O (1∶1)
DME/H2O (1∶1)
DMF/H2O (1∶1)
H2O
40
34
51
32
53
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
CH2Cl2/H2O (1∶1)
51d
28e
24f
0
0g
0h
Rhodamine B (5%)
Rhodamine B (5%)
a Reaction conditions: 1a (0.45 mmol, 1.5 equiv.), 2a (0.3 mmol, 1.0 equiv.), BIOH (0.6 mmol, 2.0 equiv.), and photocatalyst (2－8 mol%)
in 3.0 mL of solvent at room temperature were irradiated by a 23 W compact fluorescent light (CFL) bulb for 24 h under N2. b Yield of
c
e
f
isolated product. n.r.＝no reaction. d BIOAc was used as an oxidant. PhI(OAc)2 was used as an oxidant. IBX was used as an oxidant.
g No oxidant. h In the dark.
redox catalysis.
Results and Discussion
We commenced our study by investigating the de-
carboxylative reaction of α-keto acid 1a with vinylcy-
clobutanol 2a under photoredox conditions. Initially, the
reaction system containing 2 mol% of commercial
available Ru(PPh3)Cl2 as photocatalyst, and 2.0 equiv.
of BIOH as oxidant in 3.0 mL of CH2Cl2/H2O (1∶1) at
room temperature under irradiation of a 23 W CFL bulb
was applied to the reaction of 1a with 2a, but no desired
1,4-dicarbonyl compound 3a was observed (Table 1,
Entry 1). To our delight, replacement of photocatalyst
with Ru(bpy)3Cl2 led to the desired product 3a in 15%
yield (Entry 2). Further investigation revealed that
Ir(dF(CF3)ppy)2(dtbbpy) gave better yield. Considering
the high cost of the Ir catalyst, we then turn our atten-
2
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Chin. J. Chem. 2017, XX, 1—5

Metal-Free Oxidative Decarboxylative Acylation/Ring Expansion of Vinylcyclobutanols
Table 2 Scope of α-keto acids and vinylcyclobutanols
tion to the examination of some low cost, less toxic and
easy to handle organic dyes as photoredox catalyst (En-
tries 4－6). It was found that rhodamine B could give
the cyclopentanone 3a in 37% yield, while vitamin B2
and 3,4,5,6-tetrachlorofluorescein did not show any cat-
alytic activity for this reaction (Entries 5 and 6). In-
creasing the amount of rhodamine B to 5% resulted in a
satisfied yield (62%), while further increasing the
amount of photocatalyst led to a decreasing yield (En-
tries 7 and 8). Furthermore, using 1.0 and 0.5 equiv. of
BIOH as oxidant resulted in a decreased yield of 3a in
32% and 25%, respectively (not shown). In screening
different solvents, other two-phase systems were inferi-
or to CH2Cl2/H2O (Entries 9－12). When homogeneous
phase solvents, such as DME/H2O and DMF/H2O, were
used, the product 3a was obtained in 51% and 32%
yield, respectively (Entries 13 and 14). Notably, 3a was
still obtained in 53% yield even employing water as the
sole solvent (Entry 15). Using other oxidants such as
BIOAc led to slightly lower yield, while PhI(OAc)2 and
IBX were less effective for this reaction (Entries 16－
18). Finally, when the reaction was conducted in the
absence of photocatalyst, oxidant or light, no desired
product 3a was observed (Entries 19－21).
Ar2
O
Rhodamine B (5%)
BIOH (2.0 equiv.)
O
Ar1
OH
OH
Ar1
+
CH2Cl2/H2O (1:1), r.t.
Visible Light
Ar2
O
1
3
2
O
O
O
O
O
O
F
Me
3a, 62%
3b, 58%
3c, 56%
O
O
O
O
O
O
F3C
Cl
Br
3d, 60%
3e, 70%
3f, 60%
Cl
O
O
O
O
O
O
Cl
Br
3g, 63%
3h, 61%
3i, 63%
Cl
Me
O
O
O
O
O
O
Cl
Me
With the optimized reaction conditions in hand, we
then explored the scope of this decarboxylative ring
expansion reaction. As shown in Table 2, a variety of
α-keto acids reacted smoothly with 1-(1-phenylvinyl)-
cyclobutanol (2a) to give the desired products 3a－3k
in moderate to good yields. α-Keto acids having elec-
tron-donating or -withdrawing groups at the para-posi-
tion of the aromatic ring, provided good yields of the
products (3b－3f). Notably, some functional groups
such as chloride, bromo and trifluoromethyl on the aryl
ring were tolerated well (3d－3f). α-Keto acid bearing
an ortho substituent at the phenyl ring also afforded the
desired product 3g in 63% yield. When m-substituted
phenylglyoxylic acids 1h and 1i were subjected to the
reaction with 2a, the corresponding substituted cyclo-
pentanones 3h and 3i were isolated in 61% and 63%
yields, respectively. Satisfactorily, 2,4-disubstituted
phenylglyoxylic acids 1j and 1k also worked well with
2a, leading to the desired products 3j and 3k in moder-
ate to good yields. In addition to the substituted phenyl-
glyoxylic acids, β-naphthyloxoacetic acid 1l was also a
suitable reaction partner, generating the ring expansion
product 3l in good yield. Unfortunately, aliphatic α-keto
acids and oxamic acids, such as pyruvic acid and
N-phenyloxamic acid failed to give the desired products
under standard conditions (not shown). We then ex-
panded the scope to various substituted vinylcyclobuta-
nols which successfully afforded the desired products
3m－3o in moderate yields. However, treatment of 1a
with vinylcyclopentanol (4a) gave a complex mixture
under the present conditions (Eq. 1).
3j, 52%
3k, 73%
3l, 67%
O
O
O
O
O
O
Me
t-
Bu
3m, 52%
Cl
3n, 66%
3o, 64%
reagent TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)
was added to the reaction of 1a with 2a under the
standard conditions, only a trace amount of 3a was ob-
served, along with 18% isolated yield of TEMPO-COPh
6 (Eq. 2). Furthermore, the reaction of TEMPO and
α-keto acid 1a also directly furnished the adduct 6 in
60% yield (Eq. 3). These results implied that nucleo-
philic acyl radical was involved in this reaction, which
is consistent with the previous reports.[3] Additionally,
treatment of hypervalent iodine reagent 7a with 2a also
produced the desired product 3a in 65% yield. The re-
sult suggested that hypervalent iodine reagent 7a should
be an important intermediate for this transformation (Eq.
4).[11a,11b] Although the exact mechanism of the reaction
remains unclear, we proposed the following reaction
pathways based on the control experiments (Scheme 2).
Firstly, the hypervalent iodine reagent 7a generated in
situ is oxidized by photoexcited state PC* to produce
In order to understand the mechanism of this decar-
boxylative ring expansion reaction, some control ex-
periments were conducted. When the radical-trapping
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3

Zhang, Cheng & Duan
COMMUNICATION
Conclusions
O
O
standard
conditions
O
Ph
OH
In summary, we have developed a mild visible
light-mediated decarboxylative acylation/ring expansion
of vinylcyclobutanols with α-keto acids. This metal-free
method allows the formation of a variety of substituted
1,4-dicarbonyl compounds with a quaternary carbon
center in moderate to good yields. This facile approach
takes advantage of organic photoredox catalysis and
merges it with hypervalent iodine reagent.
+
2a
Ph
TEMPO (1.0 equiv.)
O
Ph
1a
3a, trace
Ph
O
N
(2)
O
6, 18%
O
standard
conditions
Ph
O
Acknowledgement
OH
N
Ph
+
(3)
TEMPO
O
O
1a
Financial supports from Natural Science Basic Re-
search Plan in Shaanxi Province of China (No.
2016JZ002) and the Fundamental Research Funds of the
Central Universities (Nos. 2015qngz17, xjj2016056 and
cxtd2015003) are greatly appreciated.
6, 60%
O
O
Ph
O
I
O
O
Rhodamine B (5%)
2a
+
CH2Cl2/H2O (1:1), r.t.
Visible Light
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