Electrochemical-Induced Ring Transformation of Cyclic α-(ortho-Iodophenyl)-β-oxoesters

Article abstract of DOI:10.1002/chem.201905570

Cyclic α-(ortho-iodophenyl)-β-oxoesters were converted in a ring-expanding transformation to furnish benzannulated cycloalkanone carboxylic esters. The reaction sequence started by electrochemical reduction of the iodoarene moiety. In a mechanistic rationale, the resulting carbanionic species was adding to the carbonyl group under formation of a strained, tricyclic benzocyclobutene intermediate, which underwent carbon–carbon bond cleavage and rearrangement of the carbon skeleton by retro-aldol reaction. The scope of the reaction sequence was investigated by converting cyclic oxoesters with different ring sizes yielding benzocycloheptanone, -nonanone and -decanone derivatives in moderate to good yields. Furthermore, acyclic starting materials and cyclic compounds carrying additional substituents on the iodophenyl ring were submitted to this reaction sequence. The starting materials for this transformation are straightforwardly obtained by conversion of β-oxoesters with phenyliodobis(trifluoroacetate).

Full text of DOI:10.1002/chem.201905570

A Journal of
Accepted Article
Title: Electrochemical Induced Ring Transformation of Cyclic α-(ortho-
Iodophenyl)-β-oxoesters
Authors: Jens Christoffers, Julia Strehl, Christoph Kahrs, Gerhard Hilt,
and Thomas Müller
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To be cited as: Chem. Eur. J. 10.1002/chem.201905570
Link to VoR: http://dx.doi.org/10.1002/chem.201905570
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Electrochemical Induced Ring Transformation of Cyclic
α-(ortho-Iodophenyl)-β-oxoesters
Julia Strehl, Christoph Kahrs, Thomas Müller,* Gerhard Hilt,* Jens Christoffers*
Abstract: Cyclic α-(ortho-iodophenyl)-β-oxoesters were converted in
a
ring-expanding transformation to furnish benzannulated
Stoltz et al., 2004
TMS
TfO
O
cycloalkanone carboxylic esters. The reaction sequence started by
electrochemical reduction of the iodoarene moiety. In a mechanistic
rationale, the resulting carbanionic species was adding to the
O
CO Et
2
2
.5 eq. CsF
carbonyl group under formation of
a
strained, tricyclic
MeCN, 80°C
CO Et
2
benzocyclobutene intermediate, which underwent carbon-carbon
bond cleavage and rearrangement of the carbon skeleton by retro-
aldol reaction. The scope of the reaction sequence was investigated
by converting cyclic oxoesters with different ring sizes yielding
benzocycloheptanone, -nonanone and -decanone derivatives in
moderate to good yields. Furthermore, acyclic starting materials as
well as cyclic compounds carrying additional substituents on the
iodophenyl ring were submitted to this reaction sequence. The
starting materials for this transformation are straightforwardly
1
a
2a, 50%
O
O
CO Et
2
CO Et
2
3
4
O
this work
obtained
by
conversion
of
β-oxoesters
with
I
O
O
electrochemical
phenyliodobis(trifluoroacetate).
reduction
CO2Et
5a
Br
CO Et
CO Et
2
Due to inherent entropic and enthalpic factors, the construction
2a, 72%
[
1]
[2]
Shono et al., 1990
of seven-membered and larger (medium) sized rings defines
a challenging task in synthetic organic chemistry. A common
strategy towards such synthetic targets is the ring-expansion of
more readily available compounds with five or six-membered
O
electrochemical
2
reduction
CO Et
2
[3]
[4]
6
7, 76%
rings. As an illustrative example, Stoltz and coworkers
introduced the ring-expansion of β-oxoester 1a with an aryne
furnishing benzocycloheptanone derivative 2a (Scheme 1). After
addition of the enolate of oxoester 1a to the aryne, the reaction
proceeds via intermediates 3 and 4. The aryne was generated in
situ from trimethylsilylphenyltriflate and excess CsF. As
alternative to this protocol we envisioned to generate
Scheme 1. Previously reported ring-transformation of oxoester 1a with an in
situ generated aryne via intermediates 3 and 4; entry into the same reaction
sequence leading to product 2a by electrochemical reduction of iodoarene 5a
(this work); and electrochemical ring-enlargement of oxoester 6 by cathodic
[4]
4
reduction; conditions: 3 equiv. TMSCl, nEt NOTos, DMF, 23 °C, 0.2 A, 4 F
–1
[8]
mol , Pb cathode, carbon anode.
carbanionic species
3
from iodo-compound 5a by
[5]
electrochemical reduction. Electrosynthesis is an attractive
concept with steadily growing importance, because it can
potentially reduce the amount of waste on one hand by avoiding
spent redox reagents. On the other hand, renewable energy can
be used to contribute to more sustainable conversions. The
preparation of the starting materials of this new synthetic
concept, α-(ortho-iodophenyl)-β-oxoesters 5, was recently
We started the screening program for optimal reaction
conditions based on a literature report by Shono et al. (Scheme
1), who have developed a cathodic ring-enlargement reaction of
α-(bromomethyl)-β-oxoesters like compound 6 by application of
DMF as solvent and 3 equiv. of TMSCl as additive to activate
the carbonyl group of the ketone to facilitate the nucleophilic
[
6]
[
8]
addition. As cathode material we have chosen leaded bronze,
which was recently recommended as innovative material for
[7]
introduced by Shafir and coworkers by conversion of β-
oxoesters 1 with PhI(O CCF (PIFA).
2
3 2
)
[
9]
4 4
reductive dehalogenations. Initially, we have used nBu NClO
as conducting salt and a current of 10 mA and achieved 54%
yield (Table 1, entry 1). Change of the conducting salt revealed
that the anion had almost no influence on the yield. Other
cations, however, lowered the yield. Therefore, we have chosen
[*]
M.Sc. J. Strehl, Dr. C. Kahrs, Prof. T. Müller, Prof. G. Hilt, Prof. J.
Christoffers, Institut für Chemie, Carl von Ossietzky Universität
Oldenburg, 26111 Oldenburg (Germany)
E-mails: jens.christoffers@uol.de, gerhard.hilt@uol.de,
thomas.mueller@uol.de
4
nBu NBr in all further experiments, since with this supporting
electrolyte the byproduct profile was least complicated (entry 2).
Variations of the cathode (graphite, glassy carbon, Cu, Pt, Pb)
and anode (glassy carbon, Pt) materials did not improve the
yields. Investigation of the Lewis acidic additive [TMSOTf,
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
Ti(OEt)
4 4 3 3 3 3 2 2 3 2
, TiCl , CeCl , LaCl , AlCl , InBr , ZnCl , ZnI , BF ∙ OEt ]
gave also no improvements, however, without or with only 1
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equiv. TMSCl, no conversion was achieved, with 5 equiv. TMSCl
the yield was lower. Survey of other solvents confirmed DMF to
be optimal (MeCN, DMPU, NMP, and DMA gave lower yields,
with 1,4-dioxane and THF no conductivity was achieved). The
first and only significant improvement was achieved, when the
electric current was lowered (entries 3 and 4). With 8 mA, 6.5
mA or 5 mA the yield raised to 71–72%. Upon further lowering
product dibenzocyclooctanone derivative 5f could be isolated in
low yield (10%). The acyclic starting materials 5g and 5h could
be converted to acetophenone derivative 2g (54%) and
benzophenone derivative 2h (18%). Finally, cyclopentanone
derivatives 5i–5m carrying additional substituents at the α-
(ortho-iodophenyl)-moiety were prepared and converted to
respective benzocycloheptanone derivatives 2i–2m. The yields
range from 44% to 59% almost independently from the
electronic nature and position of the additional residue.
Interestingly, a fluoro substituent in compound 5j is tolerated
adjacent to the proposed carbanionic center of the intermediate,
thus, competing aryne formation seems not to take place, albeit
product 2j was formed in 55% yield. While all these conversions
were performed on a 0.25 mmol scale, we have performed the
reaction of compound 5a also on a scale of 1 mmol and
obtained product 2a with a slightly lowered yield of 56%, which
might be due to the doubled concentration of the substrate 5a
(
2.5 mA, entry 5) it dropped to 43%. In favor of a high reaction
rate it was kept at 8 mA for all further optimizations. Variation of
the reaction temperature (entries 6 and 7) as well as lowering
the concentration of the conducting salt (entry 8) lowered the
yield. For more details on the screening program see the
Supporting Information.
Table 1. Selected examples of variation to the reaction conditions.
O
I
O
conditions[a]
–
1
(
36 vs. 71 mmol L ).
CO Et
2
CO Et
2
5
a
2a
Table 2. Scope of the transformation with various α-(ortho-iodophenyl)-β-
oxoesters 5; yields refer to isolated and purified products.
[
b]
[c]
Entry
Change from initial conditions
no change
Yield of 2a
3
.0 equiv. TMSCl
O
R3
1
R3
0.3 mol L nBu4NBr
1
2
3
4
5
6
7
8
54%
55%
72%
71%
43%
36%
41%
60%
I
DMF, 23 °C
R1
R2
O
–
1
[d]
nBu
4
NBr (c = 0.3 mol L ) as conducting salt
8 mA, 2 F mol1
R1
R2 CO2R
CuSn17Pb cathode
graphite anode
[
e]
CO R
I = 8 mA
2
5
2
I = 6.5 mA or 5 mA
I = 2.5 mA
O
O
O
O
T = 0 °C
T = 40 °C
CO Et
CO Et
CO Me
CO Me
2
2
2
2
a, 72%[
5
a]
2b, 0%[c]
2c, 64%
2d, 45%
2
–
1
4
c(nBu NBr) = 0.2 mol L
6%[b]
O
O
O
O
[
a] Reactions were performed in a divided cell on a 0.25 mmol scale with a
–1
Me
Me
Ph
substrate concentration of 36 mmol L . [b] Initial conditions: 3.0 equiv.
TMSCl, 0.3 mol L nBu
–1
–1
4 4
NClO , DMF, 23 °C, 10 mA, 2.0 F mol , CuSn17Pb
(
leaded bronze) cathode, graphite anode. [c] Yield determined by GLC of the
CO Me
e, 0%[d]
CO Et
CO Et
CO Et
2
2
2
2
unpurified reaction mixture with mesitylene as internal standard. [d] This
change was kept for entries 3–8. [e] This change was kept for entries 4–8.
2
2f, 10%
2g, 54%
2h, 18%
R3
O
O
With optimized reaction conditions in hand, we applied various
substrates of type 5 to investigate the scope and limitations of
the electrochemical induced rearrangement reaction. First, we
have investigated the ring size of the starting materials (Table 2).
Whereas the yield of the benzocycloheptanone 2a was good, it
was somewhat lower for the benzannulated products with nine-
O
Ph
R3
CO Et
2
CO Et
CO Et
2
3
2
3
2i, R = Me, 44% 2k, R = Br, 57%
j, R = F, 55%
8
3
3
2
2l, R = CF , 55%
3
2
m, R3 = CO Me, 59%
2
(
compound 2c, 64%) and ten-membered ring (2d, 45%).
Attempts of converting the cyclohexanone derivative 5b to
product 2b were never successful; instead, varying amounts of
the protodeiodinated compound 8 could be detected in the
reaction mixture. Interestingly, this type of starting material was
also investigated by Stoltz in the aryne induced ring-expansion
and the corresponding rearrangement was not observed as well
in their investigation. The reason for this failure remains
however unclear. Moreover, the conversion of indanone and
tetralone derivatives 5e and 5f gave also rather complex
reaction mixtures. However, in the latter case the desired
[
a] The reaction was performed on a 0.25 mmol scale. [b] The reaction was
performed on a 1 mmol scale. [c] Protodeiodination product 8 was formed
and isolated in 7% yield. [d] Unspecified decomposition.
The starting materials 5a–5g of this study were accessed from
the β-oxoesters 1a–g
chiometric amount of PIFA (9a, R = H) and TFAA (trifluoroacetic
anhydride) in a mixture of MeCN and TFA (trifluoroacetic acid;
Scheme 2; see Supporting Information). An exception was
[
10]
[7]
following the original report with stoi-
3
[
4]
compound 5h, which was prepared by TiCl
condensation from ethyl (2-iodophenylacetate) and benzoyl
4
-mediated Claisen-
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Chemistry - A European Journal
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COMMUNICATION
[
11]
chloride. Compounds 5i–5m were prepared accordingly from
substituted PIFA-derivatives 9i–9m. The latter were obtained by
oxidation of the corresponding iodobenzene derivatives with
aldol reaction, thus the ketene-silyl acetal 13 would be formed.
298
Although this process is slightly exergonic [G (12→13) = –6
–1
kJ mol ], it is kinetically highly unfavorable. It should proceed
via a transition state with a pentacoordinated silicon atom in a
trigonal bipyramidal coordination environment where the two
oxygen atoms adopt the axial positions. This arrangement
[
12]
oxone by following a literature protocol.
As also observed by
[
13]
3
others, donor substituted PIFA reagents (e.g. with R = OMe)
cannot be obtained by this procedure.
[
16]
cannot be realized within the strained tricyclic framework.
Therefore we suggest, that the ring transformation is finalized
after hydrolytic workup, which could be supported by
R3
O
I(O2CCF3)2
R3
I
1
.5 equiv. TFAA
O
CO R
R1
2
+
calculations:
The
concerted
ring-opening/proton-transfer
MeCNTFA 1:1
R1
R2
reaction that yields the (Z)-isomer of enol 15 is essentially
23 °C, 16 h
R2 CO R
2
298
–1
thermoneutral [G (1415) = +1 kJ mol ] and the subsequent
1
a1g
9a, 9i9m
1.3 equiv.)
5a5m
exergonic tautomerization step drives the reaction to completion
(
2
98
–1
[
G (152a’) = –82 kJ mol ]. The proton transfer to the ester-
Scheme 2. Preparation of starting materials 5a–5m from oxoesters 1a–1g and
PIFA 9a and PIFA derivatives 9i–9m.
carbonyl oxygen atom is assisted by one water molecule, which
allows the proton transfer to proceed via two hydrogen bridges
298 ǂ
–1 [17]
with a relative small barrier [G
(1415) = +61 kJ mol ].
Regarding the mechanism of the overall transformation, it could
be either proposed, that compound 5a is reduced to the
corresponding phenyl radical, which then enters the reaction
sequence by attack to the carbonyl group. More likely, however,
is formation of a phenyl anion derivative (cf. species 3 in
Scheme 1), which then proceeds to formation of the tricylic
intermediate 4. As evidence for the latter process we have
prepared the Grignard reagent from compound 5a by using a
TMS
O
I
+ 2 e
O
+
TMSCl
CO Me
 I,  Cl
CO2Me
5a'
2
11
2
98
= 8 kJ mol1
G
2
98
1
G
= 268 kJ mol
G298
O
=
[
14]
219 kJ mol1
protocol introduced by Knochel et al. (Scheme 3) and indeed
obtained the product 2a in 17% yield. Furthermore, when
converting compound 5a to the respective phenyl radical by
TMSO

G298
6 kJ mol
=
OTMS
CO2Me
workup
1

MeO
13
3
using nBu SnH and AIBN, only the protodeiodinated product 10
1
2
could be detected by NMR. There is actually literature
precedence on the electrochemical reduction of aryliodides. The
initial electron transfer to a radical intermediate was reported to
be relatively slow, while the second electron transfer to the anion
O
H O
G298
=
1 kJ mol
2
1
6
[
15]
HO
was much faster.

G298
+1 kJ mol
=
CO Me
2
1
OMe
H O HO
2
O
1
4
15
I
O
O

G298
82 kJ mol
=
+
1

CO Et
CO Et
2
2
CO Et
O
2
5
a
2a
iPrMgCl, LiCl, THF, 40 °C, 18 h: 17%
nBu SnH, cat. AIBN, toluene, 110 °C, 6 h: 0%
10
0%
50%
3
OMe
Scheme 3. Control experiments for the ionic or radical transformation of
strating material 5a.
OH2 O
2a'
Scheme 4. Mechanistic proposal for the transformation of compound 5a' by
two-electron reduction in the presence of TMSCl to product 2a'. DFT
calculations were performed at M06-2X/Def2-TZVP with the methyl instead of
the ethyl esters; species 14, 15, and 2a' were calculated as hydrogen-bonded
monohydrates.
Crucial in the overall process seems the role of TMSCl. We
propose it to activate the endocyclic carbonyl group
(
intermediate 11 in Scheme 4). Nucleophilic addition of the
phenyl anion to the activated carbonyl group gives intermediate
2 with CO Et and OTMS groups in relative cis-configuration.
The results of DFT-calculations at the M06-2X/Def2-TZVP level
see Supporting Information) revealed that this step is exergonic
1
2
In summary, a novel reaction pathway was realized for the
generation of the oxo-esters of type 2 from iodobenzene
derivatives 5 in the presence of TMSCl by a formal 1,3-acyl-shift.
The reaction sequence starts by an electrochemical reduction of
the iodobenzene derivative 5 and proceeds by nucleophilic
addition of the carbanion to the carbonyl group followed by C–C
(
2
98
–1
[
G (1112) = –268 kJ mol ] and the reaction proceeds
2 –1
98 ǂ
almost without barrier [G
(1112) = +8 kJ mol ]. Initially,
we had assumed that the TMS-group would then be transferred
to the ester-oxygen while the C–C-bond is cleaved in the retro-
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COMMUNICATION
bond cleavage by retro-aldol reaction. From cyclic oxoesters as
starting materials, a ring enlargement by two carbon atoms is
achieved, thus elegant access to medium-sized ring compounds
is attained.
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acknowledge
support
from
the
Deutsche
1
Forschungsgemeinschaft (GRK2226). We are furthermore
grateful to Evonik, Marl, Germany for a donation of MTBE. The
computations were performed at the HPC Cluster CARL (DFG,
INST 184/108-1 FUGG)
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2
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Electrosynthesis: Benzannulated cycloalkanone carboxylic esters are obtained by
ring-expansion of cyclic α-(ortho-iodophenyl)-β-oxoesters. The reaction sequence
starts with the electrochemical reduction of the iodoarene moiety. After nucleophilic
addition to the carbonyl group the transformation proceeds via a tricyclic
benzocyclobutene intermediate, which finally undergoes C–C bond cleavage by
retro-aldol reaction.
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