Electro-organocatalysis: Enantioselective α-alkylation of aldehydes

Article abstract of DOI:10.1002/ejoc.201000453

The asymmetric organocatalyzed a-alkylation of aldehydes via a cationic radical enamine intermediate was performed under environmentally benign electro-oxidation conditions without the use of chemical, oxidants. To promote the desired a-alkylation reaction of aldehydes, various aldehydes with xanthene or cycloheptatriene groups were exposed to elec-tro-organocatalytic conditions to afford optically active αsubstituted aldehydes (α-alkylated aldehydes) in good yield. A reaction mechanism involving the cationic radical enamine was proposed based on the cyclic voltammetry (CV) results, DFT calculations, and control experiments.

Full text of DOI:10.1002/ejoc.201000453

FULL PAPER
DOI: 10.1002/ejoc.201000453
Electro-Organocatalysis: Enantioselective α-Alkylation of Aldehydes
Xuan-Huong Ho,[a] Sun-il Mho,[a] Hyuk Kang,[b] and Hye-Young Jang*[a]
Keywords: Organocatalysis / Electrochemistry / Enantioselectivity / Alkylation / Radicals
The asymmetric organocatalyzed α-alkylation of aldehydes
via a cationic radical enamine intermediate was performed
under environmentally benign electro-oxidation conditions
without the use of chemical oxidants. To promote the desired
α-alkylation reaction of aldehydes, various aldehydes with
xanthene or cycloheptatriene groups were exposed to elec-
tro-organocatalytic conditions to afford optically active α-
substituted aldehydes (α-alkylated aldehydes) in good yield.
A reaction mechanism involving the cationic radical enamine
was proposed based on the cyclic voltammetry (CV) results,
DFT calculations, and control experiments.
free method, a stoichiometric amount of an organic oxi-
dant, 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ), was
used to oxidize the benzylic C–H bond that participates in
the reaction with the enamine.[3e]
Introduction
With the aim of broadening the scope and applications
of conventional enamine-mediated organocatalytic reac-
tions with electrophiles, a range of organocatalytic pro-
cesses involving enamine radicals, which are formed from
the 1-electron oxidation of the enamine, have been re-
ported.[1,2] For the oxidation of the enamine, either stoi-
chiometric amounts of transition-metal oxidants [ceric am-
monium nitrate (CAN) and Fe3+] or catalytic amounts of
metal complexes with inorganic oxidants (Cu2+/Na2S2O8)
are commonly used to generate a cationic radical enamine,
which undergoes asymmetric coupling reactions with non-
polar hydrocarbon substrates or radical species. This new
concept was proposed by MacMillan as a singly occupied
molecular orbital (SOMO) concept.[2a]
In parallel with the development of SOMO protocols,
including the in situ oxidation of enamine species to form
cationic enamine radicals, a variety of methods for selec-
tively oxidizing nonconventional coupling partners under
organocatalytic reaction conditions by using metal com-
plexes and organic oxidants have also been proposed.[3] In
a photoredox transition-metal-mediated protocol presented
by MacMillan, the direct alkylation of aldehydes occurs in
the presence of photoredox transition-metal complexes and
organocatalysts upon the irradiation of light. A catalytic
amount of photoredox metal complexes {[Ru(bpy)3]2+ or
[Ir(ppy)2(dtb-bpy)]+; bpy = bipyridine, ppy = 2-phenylpyr-
idine, dtb-bpy = 4,4Ј-di-tert-butyl-2,2Ј-dipyridyl} promotes
the redox process of the catalytic reaction without generat-
ing the cationic radical enamine.[3b,3c] As a transition-metal
Our research interest has been directed toward the devel-
opment of green catalytic processes by avoiding the use of
large amounts of toxic organic/inorganic chemicals. Ac-
cordingly, we have explored new catalytic reactions merging
electro-oxidation and organocatalysis to establish green or-
ganocatalytic reactions.[4] By using electro-oxidation, en-
amine radicals could be generated, performing radical cou-
pling reactions with 2,2,6,6-tetramethylpiperidine N-oxide
(TEMPO) under environmentally benign conditions. Al-
though there have been extensive studies on the application
of electro-oxidation to organic synthesis,[5] there are few re-
ports demonstrating electro-organocatalysis.[6] Therefore,
we have examined a wide range of asymmetric transforma-
tions by using our electro-organocatalysis method under
simple, aerobic and environmentally friendly conditions.
This paper reports the first asymmetric organocatalyzed α-
alkylation of aldehydes under galvanostatic conditions and
electrochemical studies as well as DFT calculation results
to provide mechanistic information. By using the electro-
organocatalysis method, the direct incorporation of a ben-
zylic carbon unit into synthetically versatile aldehydes was
accomplished in a stereoselective manner.
Results and Discussion
The optimization began with an achiral sec-amine cata-
lyst, pyrrolidine (A), which promoted the coupling of hy-
drocinnamaldehyde (1a) with xanthene (2a) under galvano-
static conditions to afford the desired product (3a) in 43%
yield (Table 1, entry 1). With this result in mind, a range of
proline derivatives and imidazolidinone-based catalysts
were evaluated for the electro-organocatalytic coupling re-
action, as shown in Table 1. Relative to compounds B and
[a] Division of Energy Systems Research, Ajou University,
Suwon 443-749, South Korea
Fax: +82-31-219-1615
E-mail: hyjang2@ajou.ac.kr
[b] Department of Chemistry, Ajou University,
Suwon 443-749, Korea
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000453.
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Electro-Organocatalysis
Table 1. Organocatalytic coupling reaction of 1a and 2a under galvanostatic conditions.[a]
Entry
Organocatalyst (mol-%)
t [h]
T
% Yield
ee [%]
1
2
3
4
5
6
7
8
A (50)
B (50)
C (50)
D (50)
E (50)
F (50)
F (50)
F (20)
4
3
3
4
3
4
6
4
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
4 °C
r.t.
43
81
25
14
18
74
80
68
–
3
17
68
65
70
70
68
[a] Experimental procedure: organocatalyzed coupling reactions of 1a and 2a were conducted in an undivided cell with a dichloromethane
solution [30 mL; tetrabutylammonium perchlorate (TBAP), 0.1 ] under galvanostatic conditions. The mixture of compound 1a (1 mmol),
2a (2 mmol), and catalyst was allowed to react at the indicated temperature until the starting material was completely consumed. Platinum
gauze (2.5 cmϫ2.5 cm) was used as the anode and cathode materials. A regulated power supply (Hanil Electric Co, maximum output
power;180 W) was used for the constant current electrolyses. With this power supply, a fine adjustment of small current was not possible
and around 50 mA of the current was applied. Over the course of the electrolysis, around 11–13 V of the potential was observed for each
reaction.
C, compounds D, E, and F catalyzed the reaction and af- enantioselectivity observed with 3b was dramatically lower
forded the product with high enantioselectivity at room than that of 3a. The possibility that the existence of a
temperature (Table 1, entries 2–6). Compound F was cho- phenyl ring at the aldehyde would affect the enantio-
sen as a catalyst for further optimization considering the selectivity was examined. Aldehydes 1c and 1d were tested
good enantioselectivity (70% ee; ee = enantiomeric excess) for the coupling reaction. The phenyl ring of 1c is closer to
and yield (74%) of the reaction (Table 1, entry 6). To im- the aldehyde than that of 1a and the phenyl ring of 1d is
prove the enantioselectivity, the reaction was run at a lower three carbon atoms away from the aldehyde. Based on the
temperature (4 °C) with a prolonged reaction time. How- results of Table 2, entries 1, 3, and 4, each enantioselectivity
ever, the yield and enantioselectivity did not increase dra- of products 3a, 3c, and 3d appears to be related to the dis-
matically (entry 7). With the reduced catalyst loading tance of the phenyl ring position from the aldehyde. With
(20 mol-%), a similar yield (68%) and enantiomeric excess p-methoxy-substituted hydrocinnamaldehyde 1e, product 3e
(68% ee) of the reaction were observed (Table 1, entry 8). was obtained in modest yield and enatioselectivity (Table 2,
In contrast to dichloromethane as a solvent, acetonitrile, entry 5). When a disubstituted aldehyde at the β-position
DMF, and acetone did not provide the desired product. (isovaleraldehyde) was applied, the reaction did not pro-
Therefore, the reaction conditions of entry 8 were used to ceed, which is not shown in Table 2. Other than xanthene
examine the substrate scope. The stereochemical outcome (2a), cycloheptatriene (2b) was also applied for the reaction
of this product was assigned by comparing its optical rota- with 1a, yielding a coupling product in 8% yield and with
tion with that of the previously reported compound 3a.[3e]
The α-alkylation of various aldehydes with xanthene (2a)
10% ee (Table 2, entry 6).
To account for the product formation under anodic oxi-
and cycloheptatriene (2b) was examined. The results are dation conditions, two possible routes (enamine radical
summarized in Table 2. With organocatalyst F in 30 mL of pathway and xanthene cation pathway) were considered
a dichloromethane solution containing TBAP, a mixture of (Scheme 1). The enamine radical pathway involves the 1-
one equivalent of compounds 1a–e and two equivalents of electron oxidation of both the enamine intermediate and
compounds 2a and 2b was exposed to the current con- xanthene, followed by the coupling of two radical species.
trolled electrolysis conditions in an undivided cell equipped On the other hand, the reaction may proceed though two-
with platinum gauze as the anode and cathode. The ali- electron oxidation and the C–H bond cleavage of xanth-
phatic aldehyde 1b and xanthene (2a) subjected to the reac- enes, generating xanthene cations during oxidation.[7]
tion conditions produced the desired coupling product 3b Since the benzylic C–H bond can be oxidized to form cat-
in 84% yield and with 24% ee (Table 2, entry 2). The ionic species V in the presence of chemical oxidants, the
Eur. J. Org. Chem. 2010, 4436–4441
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X.-H. Ho, S.-i. Mho, H. Kang, H.-Y. Jang
FULL PAPER
Table 2. Scope of the reaction.[a]
[a] Experimental procedure: under galvanostatic conditions, F-catalyzed reactions of the aldehyde (1 mmol) and 2a or 2b (2 mmol) were
conducted in an undivided cell with a dichloromethane solution (30 mL; TBAP, 0.1 ). Platinum gauze (2.5 cmϫ2.5 cm) was used as the
anode and cathode materials. A regulated power supply (Hanil Electric Co, maximum output power, 180 W) was used for the constant
current electrolyses. With this power supply, a fine adjustment of small current was not possible, and around 50 mA of the current was
applied. Over the course of the electrolysis, around 11–13 V of the potential was observed for each reaction. The reaction times of each
reaction are provided in the Supporting Information. [b] Catalyst F (20 mol-%). [c] Catalyst F (50 mol-%).
Scheme 1. Plausible catalytic cycles.
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Electro-Organocatalysis
xanthene cation pathway was also considered as a possible the presence of 50 mol-% of BHT with respect to 1a, which
mechanism.[3e,8]
indicated the existence of radical intermediates as catalyti-
To determine the mechanism for the organocatalyzed al- cally competent species. Next, compound 1f containing a
dehdyes and xanthene coupling reactions under anodic oxi- cyclopropyl group was exposed to these reaction conditions,
dation conditions, the electrochemical properties of each as indicated in Scheme 2. The formation of the cationic rad-
component and the reaction mixtures of the substrates and ical enamine species during the reaction conditions is ex-
catalyst were evaluated by cyclic voltammetry (CV) on a pected to provide the radical character at the α-position of
platinum electrode in a dichloromethane solution contain- the aldehyde, rendering the ring opening of the cyclopropyl
ing 0.1  TBAP as the electrolyte (Figure 1). During the ring on 1f. Although no coupling product was obtained
anodic scan over a potential range 0.0 to 2.5 V (or 3.0 V), from the reaction of 1f and 2a, the disappearance of the
1a (0.01 ), pyrrolidine (0.01 ), and 2a (0.01 ) showed an cyclopropyl ring from the aldehyde was observed under the
oxidation peak at 2.7, 1.4, and 2.0 V, respectively versus a reaction conditions shown in Scheme 2.
silver-wire pseudoreference electrode at a scan rate of
0.1 Vs–1 (Figure 1, a).[4,9] An oxidation peak for a mixture
of compound 1a (0.01 ) and pyrrolidine (0.005 ) was ob-
served at 0.8 V, which was confirmed to be the oxidation
peak potential corresponding to the enamine formed from
1a and pyrrolidine (Figure 1, b).[4] In the cyclic voltamme-
try of a mixture of 1a (0.01 ), pyrrolidine (0.005 ), and
2a (0.02 ), three oxidation waves appeared at Ep,a = 0.8,
2.3, and 2.9 V, respectively, which corresponded to the oxi-
dation of in situ formed enamines 1b and 1a. No reduction
wave was observed during the reverse scan. In accordance
Scheme 2. Reaction of 1f and 2a.
with the analysis of the cyclic voltammograms shown in
Figure 1, the 1-electron oxidation of enamine intermediates
Assuming the catalytically competent intermediate of
was presumed to be facile, affording catalytically competent this reaction is the enamine radical, the ground-state-opti-
cationic radical species. Subsequently, the 1-electron oxi- mized geometries and relative energies of the enamine radi-
dation of xanthene[8] and the immediate coupling of oxid- cal derived from compound 1a and F were calculated at
ized enamine and xanthene radicals are believed to occur B3LYP/6-31G(d) level of theory with the Gaussian 03 pro-
(enamine radical pathway).
gram package. The structures of the three lowest-energy
The effect of the radical inhibitor (2,6-di-tert-butyl-4- conformers (geometries 1, 2, and 3) devoid of solvents are
methylphenol, BHT) was examined to gain additional sup- given in Figure 2. Other geometries that lie much higher in
port for the proposed enamine radical pathway. During the energy (Ͼ7 kcal/mol) are not considered. Relative energies
organocatalytic coupling of 1a and 2a under anodic oxi- and abundances at 25 °C of the three conformers are given
dation conditions, the desired product was not formed in in Table 3. Relative abundances were calculated by as-
Figure 1. (a) Cyclic voltammograms of 1a (0.01 ), pyrrolidine (0.01 ), 1b (0.01 ), a mixture of 1a (0.01 ) and pyrrolidine (0.005 ),
and a mixture of 1a (0.01 ), pyrrolidine (0.005M), and 2a (0.02 ) in a 0.1  TBAClO4 (TBA = tetrabutylammonium) dichloromethane
solution at a scan rate of 100 mV/s. (b) A cyclic voltammogram of a mixture of 1a (0.01 ) and pyrrolidine (0.005 ) with a different
current scale bar (30 µA).
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X.-H. Ho, S.-i. Mho, H. Kang, H.-Y. Jang
FULL PAPER
Figure 2. Ground-state-optimized geometries and relative energies of the enamine radical intermediate derived from compounds 1a
and F.
suming a Boltzmann distribution. As illustrated in Figure 2 reaction and identify the reaction mechanism are currently
and Table 3, the enamine radical mostly has the geometry underway.
3 in vacuo, with a minor contribution from geometries 1
and 2. The geometry 3 has internal π hydrogen bonding, in
Experimental Section
which a hydrogen atom of one aromatic ring points to the
other aromatic ring, making the geometry 3 stabilized. In
geometry 3, the benzene ring from compound F efficiently
blocks the lower side of the reaction center. As a result,
xanthene is added to the top side of the enamine radical,
providing the coupling product with the indicated stereo-
chemistry of 3a.
1
General: Dichloromethane was distilled from calcium hydride. H
NMR spectra were recorded with Varian Mercury plus
a
(400 MHz) spectrometer. Chemical shifts are reported as delta (δ)
values (ppm) downfield from trimethylsilane. Coupling constants
are reported in Hertz (Hz). 13C NMR spectra were recorded with
a Varian Mercury plus (100 MHz) spectrometer. Chemical shifts
are reported as delta (δ) values, part per million (ppm) relative to
the center of the triplet at δ = 77.00 ppm for deuteriochloroform.
Electrochemical data were recorded on a potentiostat/galvanostat
[PARC (Princeton Applied Research), model 263] with the Electro-
chemistry Power Suite Module and Cyclic Voltammetry software.
The three-electrode cell consisting of a Pt disc (working electrode),
a Ag wire (pseudo reference electrode), and a Pt wire (auxiliary
electrode) was employed. Each compound was dissolved in a
dichloromethane solution containing 0.1  TBAClO4. The re-
sulting solutions were subject to cyclic voltammetry experiments at
a scan rate of 100 mV/s. The enantiomeric excess (ee) of the prod-
ucts was determined by chiral stationary-phase HPLC. Optical ro-
tations were measured on a DIP370 (JASCO) polarimeter. Product
3a and 3b exhibited spectral properties consistent with previous
literature reports.[3e,10]
Table 3. Relative energies and abundances of the conformers (geo-
metries 1, 2, and 3) of the enamine radical intermediate derived
from compounds 1a and F.
Geometry Relative energy [kcal/mol]
Abundance [%]
1
2
3
1.847
1.842
0
4.06
4.10
91.84
Conclusions
The organocatalytic reaction protocol was extended to
the enantioselective coupling of aldehydes and xanthene by
utilizing anodic oxidation, affording the α-alkylation prod-
ucts in good yields and enantioselectivities. This study is the
first to show that anodic oxidation can be applied to benz-
ylic C–H functionalization for coupling with the enamine
in the absence of large amounts of chemical oxidants. In
addition, DFT calculations were conducted to understand
the origin of the reaction selectivity. A pathway involving
an enamine radical intermediate is believed to be suitable
for the present reaction based on electrochemical studies
and control experiments. However, the xanthene cation
pathway cannot be ruled out completely because the homo-
coupling product of the radical intermediates supporting
the enamine radical mechanism was not isolated from the
Representative Experimental Procedure of the α-Alkylation of Alde-
hydes Under Anodic Oxidation Conditions: Organocatalyzed cou-
plings of aldehydes and xanthene or cycloheptatriene were con-
ducted in an undivided cell in a dichloromethane solution (30 mL;
TBAP, 0.1 ). The mixture of the aldehyde (1 mmol), 2a or 2b
(2 mmol), and catalyst was exposed to constant current electrolysis
conditions until the aldehyde was consumed completely. Platinum
gauze (2.5ϫ2.5 cm) was used as the anode and cathode materials.
A regulated power supply (Hanil Electric Co, maximum output
power, 180 W) was used for the constant current electrolyses. With
this power supply, a fine adjustment of small current was not pos-
sible; ca. 50 mA of electric current was applied.
Supporting Information (see also the footnote on the first page of
this article): Spectral data for compounds 3a–3f and Table 2 with
reaction mixture. Further studies to extend the scope of this reaction times.
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Electro-Organocatalysis
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Received: April 5, 2010
Published Online: July 2, 2010
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