A cooperative hydrogen-bond-promoted organophotoredox catalysis strategy for highly diastereoselective, reductive enone cyclization

Article abstract of DOI:10.1002/chem.201204573

Metal-free cooperation: The cooperative combination of EosinY as a photoredox catalyst with organocatalytic thiourea allows for the highly diastereoselective construction of trans-1,2-cycloalkanes and heterocycles. This new efficient, cooperative organoph

Full text of DOI:10.1002/chem.201204573

DOI: 10.1002/chem.201204573
A Cooperative Hydrogen-Bond-Promoted Organophotoredox Catalysis
Strategy for Highly Diastereoselective, Reductive Enone Cyclization
Matthias Neumann[a] and Kirsten Zeitler*[a, b]
À
Mild and efficient methods for catalytic C C bond forma-
tions are of great importance to both academic and industri-
al research, and green and sustainable processes pose an ad-
ditional major challenge for synthetic organic chemists.
Over the last decade, organocatalysis has successfully started
to seize the mantle and especially multicatalysis concepts—
combining robust, metal-free catalysts—allow access to com-
plex (asymmetric) structures and many formerly elusive
transformations.[1] Only recently has the combination of or-
ganocatalytic activation modes with other sustainable meth-
ods begun to evolve.[2] In this context, in particular the
merger with visible-light photoredox chemistry[3] has
emerged as a powerful approach, as was evidenced by pio-
neering examples by the groups of MacMillan,[4] Rueping,[5]
and Rovis.[6] Secondary amine and NHC catalysis were suc-
cessfully combined in synergistic processes with
[RuACHTUNGTRENNUNG
(bpy)3]2+-mediated photoredox catalysis. Although
metal-complex-promoted photocatalysis, which is often
based on expensive and scarce Ru and Ir complexes,[7] is still
mandatory for some applications, we have recently devel-
oped a photoredox protocol demonstrating the applicability
Scheme 1. Multicatalytic photoredox approaches.
bond catalysis,[9] mimicking enzymatic general acid catalysis,
is well-known to be a powerful tool for mild LUMO-lower-
ing carbonyl activation, we sought to transfer this attractive
alternative to Lewis acids to cooperative photoredox cataly-
sis to activate carbonyl groups towards electron accept-
ance.[10]
We anticipated that this noncovalent, weak interaction
would be favorable in terms of catalyst loading to avoid the
need of (super)stoichiometric amounts of Lewis acid activa-
tors.
Herein, we describe a highly diastereoselective hydrogen-
bond-promoted reductive cyclization of bisenones by coop-
erative organic photoredox catalysis. These reactions pro-
ceeded efficiently and fast at room temperature by using
simple, commercially available, inexpensive material as cata-
lysts.
of Eosin Y as
a
potent metal-free surrogate for
[8]
[RuACHTUNGTRENNUNG .
(bpy)3]2+
In this context, we could also show that simple organic
photoredox catalysts can participate in highly enantioselec-
tive synergistic transformations with two catalytic cycles
working perfectly in concert.[8a] Continuing this theme, we
questioned if the photoredox multicatalysis strategy could
be extended to other organocatalytic activation modes.
Apart from the above-mentioned successful examples to
combine photoredox catalysis with amine and NHC cataly-
sis, respectively (Scheme 1), a merger with metal-free car-
bonyl activation has not yet been described. As hydrogen-
[a] Dr. M. Neumann, Prof. Dr. K. Zeitler
Institut fꢀr Organische Chemie
Universitꢁt Regensburg
As outlined in Scheme 2, we considered the versatile class
of cyclization reactions of bisenones, promoted by single-
electron transfer (SET), as a suitable model system to prove
our concept. These cyclizations are not only interesting due
to the formal umpolung[11] triggered by electron donation
and hence the possible access to 1,6-or 1,4-difunctionalized
products 3 and 4, but also have already been studied exten-
sively for a number of catalytic systems and initiators. Early
examples use stannyl radical-chain reactions,[12] whereas al-
ternative methods range from metal catalysis,[13] arene
Universitꢁtsstrasse 31, 93053 Regensburg (Germany)
E-mail: kirsten.zeitler@ur.de
[b] Prof. Dr. K. Zeitler
New address:
Institut fꢀr Organische Chemie
Universitꢁt Leipzig
Johannisallee 29, 04103 Leipzig (Germany)
Fax : (+49)341-97-36599
E-mail: kzeitler@uni-leipzig.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201204573.
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Chem. Eur. J. 2013, 19, 6950 – 6955

COMMUNICATION
Scheme 2. Cyclizations of bisenones triggered by SET.[12–17]
anions,[14] and electrochemical activation[13b,15] to photocatal-
ysis.[16] Besides the common need of high loadings of addi-
tives and/or catalysts, product- and diastereoselectivity
entail possible issues.
alized by using the organophotoredox catalyst in excellent
yield and diastereoselectivity. However, unlike to former re-
sults, we could only detect the (thermodynamically more
stable)[14] trans-products 6 and 9 (Scheme 3).[18]
The fate of the initially formed distonic radical is known
to be strongly dependent on the reaction conditions. While
the presence of protic solvents or acids as activators causes
protonation or hydrogen transfer of the primarily cyclized
intermediate, generating the monocyclic products 3 in a re-
ductive cyclization (Scheme 2, left), nonprotic conditions
favor the formation of the bicyclic, formal [2+2] cycloaddi-
tion products 4 in a netto redox neutral reaction.[15b,16c] Com-
bining isopropanol solvent mixtures with dicyanoanthracene
(DCA) as photoredox catalyst, Pandey and Hajra described
Having demonstrated that Eosin could serve as a power-
ful organic photoredox catalyst to access ketyl radical
anions, we next focused on combining Eosin in a coopera-
tive manner with hydrogen-bond donor catalysts for carbon-
yl activation. We speculated that this catalytic combination
with its additional capability to mediate proton transfer
would allow for selective reductive bisenone cyclizations. As
common and easily accessible hydrogen-bond organocata-
lysts performing as Lewis acid type surrogates for carbonyl
activation, we chose TADDOL (a,a,a’,a’-tetraphenyl-1,3-di-
oxolan-4,5-dimethanol) 11[19] and thiourea 12 (“Schreinerꢃs
catalyst”).[20] As shown in Table 1, our design rendered reac-
tions with both catalysts successful, providing the expected,
the formation of monocyclic
3
under irradiation (l=
405 nm).[16] In the presence of [Ru(bpy)3]2+ as photocatalyst,
AHCTUNGTERNNUNG
Yoon and co-workers[17] could either access bicyclic products
4-cis by using an excess of
LiBF4 as templating Lewis acid
actiACHTUNGTRENNUNG
vator,[17a,b] or reductively cy-
clize the tethered bisenones to
give trans-configurated products
3,
if
superstoichiometric
amounts of HCOOH were ap-
plied.[17c]
Our investigation began with
an examination of the general
feasibility
to
substitute
[Ru
ACHTUNGTRENNUNG
presence of Lewis acids. Both
the intramolecular, as well as
the
formal [2+2] cycloaddition of
acyclic enones,[17a,b] could be re- lyst.
crossed
intermolecular
Scheme 3. Intra- and intermolecular formal [2+2] cycloadditions of enones with Eosin Y as photoredox cata-
Chem. Eur. J. 2013, 19, 6950 – 6955
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K. Zeitler and M. Neumann
Table 1. Optimization of reaction conditions.[a]
ditive (entry 7). For irradiation, an array of two
green 1 W LEDs that ideally match the absorption
maximum of Eosin Y was used (emission maximum
l=530 nm).[23] Changing the light source to a stand-
ard 23 W household fluorescent bulb resulted in
elongated reaction time, but still gave a comparable
yield (Table 1, entry 8). Control experiments with-
out irradiation left the starting material unchanged;
neither could we observe reductive cyclization, if
the organocatalyst or Eosin Y as photocatalyst were
left out (entries 9 and 10), hence verifying the indis-
pensability of all employed reaction components.
With these best conditions in hand, we next in-
vestigated the scope of the reductive cyclization of
symmetrical bisenones (Table 2). All tested aryl bis-
enones both with electron-withdrawing (e.g.,
entry 3) and electron-donating substitution pattern
(e.g., entry 2) proved to be suitable substrates selec-
tively providing the desired trans-cyclopentanes in
excellent yield and short reaction times (Table 2,
entries 1–3). As was expected, the reductive power
of the Eosin Y radical anion (E0 = À1.06 V vs.
SCE) proved to be insufficient for aliphatic enones
due to their more negative potential,[24] because
only trace amounts of product could be observed
after 24 h reaction time.
Entry Organo
G
Solvent
t
Yield of 10
[h] [%][a,b,c]
1
2
3
4
5
6
7
8
11
11
11
12
12
12
12
12
–
MeCN 48
THF
THF
MeCN 48
MeCN 12
CH2Cl2 3.5
traces
8
48
48
62
78
84
93
CH2Cl2
2
92
CH2Cl2 5.5
CH2Cl2 48
CH2Cl2 48
89[d]
9
0
0
10[e]
12
[a] Typical procedure: all components were dissolved in a Schlenk tube and degassed
by ꢀ2 freeze/pump/thaw cycles followed by irradiation by using two green LEDs
(530 nm, 1 W each) for the time indicated. [b] Only the trans-isomer was detected.
[c] Yield of isolated product. [d] A 23 W fluorescent bulb was used instead of LEDs.
[e] Without Eosin Y.
Table 2. Substrate scope of symmetrical bisenones.[a]
more stable trans-configurated racemic cyclopentane 10 with
excellent diastereoselectivity, but initially with only poor-to-
moderate yield, requiring long reaction times of up to 48 h
for full conversion (Table 1, entries 1–4).
Entry Substrate
Product
t
Yield
[h] [%][b]
Due to solubility problems of TADDOL catalyst 11 in the
presence of N,N-diisopropylethylamine (DIPEA; 13), good
yields could only be obtained if DIPEA hydrochloride was
used as reductive quencher instead (Table 1, entry 3). Never-
theless, thiourea 12 proved to be superior regarding solubili-
ty and activity (Table 1, entry 4). Because quenching of the
a-carbonyl radical resulting from the reductive cyclization
by direct hydrogen transfer (or by oxidation-followed hy-
dride transfer; also, see Scheme 4 for a mechanistic propos-
al) might be critical for successful catalysis,[21] further opti-
mization efforts were pursued with alternative reductive
quenchers (Table 1, entries 5–7). As DIPEA (13) is a rather
poor hydride or hydrogen donor, respectively, therefore, we
rationalized that exchanging DIPEA with a more powerful
donor, such as the commonly used Hantzsch ester 14 that
also can perform as reductive quencher,[22] should be benefi-
cial. In fact, the reaction was found to be significantly accel-
erated in the presence of biomimetic 14 (Table 1, entry 5).
During a survey of solvents, the reaction time shortened
to 3.5 h, when dichloromethane was employed (Table 1,
entry 6). Finally, optimal efficiency was achieved by using a
combination of Hantzsch ester and DIPEA (1 equiv) as ad-
1
2
3
R=Ph
2
92
R=PMP (p-methoxyphenyl)
R=p-Cl-C6H4
R=CH2CH2Ph
R=Me
1.5 95
2.5 96
2.5 91
0.5 93
4[c]
5[c]
6
7
8
X=O
X=CH2CH2
X=CH2CH2CH2
8
95
4.5 91
24
0
9[d]
48
71
[a] Conditions: thiourea 12 (20 mol%), Eosin Y (2.5 mol%), Hantzsch
ester (14; 1 equiv), DIPEA 13 (1 equiv), CH2Cl2 (cenone =0.2 molLÀ1).
[b] Yield of isolated product. [c] [IrCAHTUNGTREN(NNUG dtbbpy)ACHUTNTRGEG(NNNU ppy)2]ACHTNUGTREN[GUNN PF6] (1 mol%) was
used instead of Eosin Y. [d] No Hantzsch ester, DIPEA (13; 2 equiv)
used instead.
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Cooperative Hydrogen-Bond-Promoted Catalysis Strategy
Table 3. Substrate scope of unsymmetrical enones.[a]
COMMUNICATION
The yields were excellent in all cases; however, diastereo-
control was observed to be moderate for acrylonitrile
(Table 3, entry 5) relative to the other olefinic acceptor sub-
strates. Tethered enones with electron-deficient alkynes as
acceptor moiety were also competent in this transformation
(Table 3, entry 6) giving the product in high yield. In accord-
ance with results of Yoon and co-workers,[17c] aryl enones
could not be cyclized with styrenes (Table 3, entry 7); how-
ever, fast, productive cyclization for the corresponding ali-
phatic enone (entry 8) was observed, albeit missing the dia-
stereoselectivity of the other acceptors. Substrates with un-
substituted, terminal alkenes, lacking suitable stabilization
of the intermediary radical, failed to react (Table 3, entries 9
and 10).
Entry Substrate
Product
t
Yield
[h] [%][b]
1
2
3
4
R=OMe
R=SEt
R=Me
R=tBu
6
85
5.5 86
5
4
87
95
As shown in Scheme 4, our proposed mechanism starts
with the reductive quenching of the excited state of Eosin Y
either by DIPEA or Hantzsch ester to generate the corre-
sponding Eosin Y radical anion as a strong reductant. Upon
electron transfer to a simultaneously hydrogen-bond activat-
ed unsaturated (aryl) ketone, the photoredox catalyst re-
turns to its ground state. The resulting 1,4-distonic radical
anion undergoes an 5-exo-trig cyclization to the respective
Michael acceptor generating a stabilized a-carbonyl radical,
which subsequently can be trapped by direct hydrogen
transfer from the radical cation of the reductive quencher to
release the cyclopentane product; alternatively, oxidation of
the intermediary radical followed by hydride transfer could
be assumed.
5
6
7.5 85
8
82
7[c]
8[c]
R2 =Ph
12
0
R2 =CH2CH2Ph
2.5 72[d]
9[c]
R3 =Ph
12
12
0
0
To validate our preliminary mechanistic scheme, addition-
al experiments were performed. When employing
[D2]Hantzsch ester [D2]-14 as hydrogen donor, up to 60%
deuteration of the expected a-carbonyl position was ob-
10[c]
R3 =CH2CH2Ph
[a] Conditions: thiourea 12 (20 mol%), Eosin Y (2.5 mol%), Hantzsch
ester (14; 1 equiv), DIPEA (13; 1 equiv), CH2Cl2 (cenone =0.2 molLÀ1).
[b] Yield of isolated product. [c] [IrCAHTUNGTREN(NNUG dtbbpy)ACHUTNTRGEG(NNNU ppy)2]ACHTNUGTREN[GUNN PF6] (1 mol%) was
used instead of Eosin Y. [d] Diastereomeric mixture (1:1).
Replacing Eosin with the more potent ([IrACHTNUTRGNE(UNG dtbbpy)-
A
N
product in excellent yield and short reaction times (Table 2,
entries 4 and 5). Heterocycles (Table 2, entry 6) and cyclo-
hexanes (entry 7) could also be obtained in high yield,
whereas cycloheptanes were not accessible (entry 8). A dif-
ferent reactivity was observed upon shortening the alkyl
chain to four carbon atoms. According to Baldwinꢃs rules,[26]
the expected 4-exo-trig cyclization should result in the for-
mation of a cyclobutane; however, after prolonged reaction
time, only the rearrangement to a cyclopentene in good
yield was observed (Table 2, entry 9). Because 5-endo-trig
cyclizations are strongly disfavored and the formation via a
Rauhut–Currier pathway[27] can be ruled out,[28] the product
might stem from a base-promoted vinylogous deprotona-
tion/enolate addition/isomerization sequence.[15a]
We next examined the scope of this cooperative catalytic
approach by screening a variety of unsymmetrical enones
with respect to functional-group tolerance. As shown in
Table 3, substrates containing esters, thioesters (entries 1
and 2), as well as aliphatic ketones or nitrile Michael sys-
tems (entries 3–5) smoothly underwent reductive cyclization.
Scheme 4. Proposed mechanism of cooperative reductive cyclization (cat-
ionic pathway omitted for clarity).
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6953

K. Zeitler and M. Neumann
Acknowledgements
Generous support from the DFG (“Chemical Photocatalysis” GRK
1626) is gratefully acknowledged.
Keywords: organocatalysis
·
photocatalysis
·
redox
chemistry · reductive cyclization · thiourea
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À
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Cooperative Hydrogen-Bond-Promoted Catalysis Strategy
COMMUNICATION
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Received: December 23, 2012
Published online: April 23, 2013
Chem. Eur. J. 2013, 19, 6950 – 6955
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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