Chemo-, regio- and stereoselective tricyclohexylphosphine-catalyzed [3+2] cycloaddition of enynes with [60]fullerene initiated by 1,4-michael addition: Synthesis of cyclopenteno[60]fullerenes and their electrochemical properties

Article abstract of DOI:10.1002/adsc.201300255

Herein we demonstrate a tricyclohexylphosphine-catalyzed cycloaddition of (E)- or (Z)-alkyl 5-substituted phenylpent-2-en-4-ynoates with [60]fullerene to give cyclopentenofullerenes in good to excellent yields, through initial chemo- and regioselective 1,4-addition of phosphines at the b-carbon of the enyne substrates. The nucleophilic addition pattern of P (cHx)3 is found to be different from that of Gilman or Grignard reagents toward the studied enynes. The resulting cyclopentenofullerenes, characterized with spectrometric methods and single crystal X-ray diffraction analysis, exhibit comparable or higher LUMO energy levels than a typical n-type material, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).

Full text of DOI:10.1002/adsc.201300255

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DOI: 10.1002/adsc.201300255
Chemo-, Regio- and Stereoselective Tricyclohexylphosphine-Cata-
lyzed [3+2]Cycloaddition of Enynes with [60]Fullerene Initiated
by 1,4-Michael Addition: Synthesis of Cyclopenteno[60]fullerenes
and their Electrochemical Properties
Po-Yen Tsenga and Shih-Ching Chuanga,*
a
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, R.O.C.
Fax : (+886)-3572-3764; phone: (+886)-3573-1858; e-mail: jscchuang@faculty.nctu.edu.tw
Received: March 29, 2013; Revised: June 20, 2013; Published online: August 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300255.
Abstract: Herein we demonstrate a tricyclohexyl-
phosphine-catalyzed cycloaddition of (E)- or (Z)-
alkyl 5-substituted phenylpent-2-en-4-ynoates with
[60]fullerene to give cyclopentenofullerenes in good
to excellent yields, through initial chemo- and regio-
selective 1,4-addition of phosphines at the b-carbon
of the enyne substrates. The nucleophilic addition
duce g-lactones having a-phosphorus ylides with alde-
hydes in one-pot.[10] Other alkylamines have been also
shown to react with A through the a(d’)-addition pat-
tern.[11] Our continuing interest in uncovering the nu-
cleophilic addition site of relevant enynes prompted
us to explore the regioselectivity of 1/2/3 with phos-
phines. It had been shown that enynes 1/2/3 can take
up nucleophiles such as Gilman or Grignard reagents
at the d-carbon through 1,6-conjugate addition to
yield allenes B after work-up [Eq. (2), Scheme 1].[12]
Herein, we wish to disclose a regioselective cycloaddi-
tion of enynes 1/2/3 with C60 catalyzed by tricyclohex-
pattern of PACHTUNGTRENNUNG(cHx)3 is found to be different from
that of Gilman or Grignard reagents toward the
studied enynes. The resulting cyclopentenofuller-
enes, characterized with spectrometric methods and
single crystal X-ray diffraction analysis, exhibit
comparable or higher LUMO energy levels than
a typical n-type material, [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM).
ylphosphine [PACTHNUTRGNEUNG(cHx)3] to give cyclopentenofullerenes
in one-pot; we also carried out a study of the reaction
scope and achieved an unambiguous characterization
with aid of X-ray diffraction analyses on a crystallized
product. The addition pattern is inferred through an
Keywords: cycloaddition; enynes; fullerenes; Mi-
chael addition; phosphanes
Organocatalysis has become one of the most powerful
synthetic methods for building up complex molecular
structures in one-pot with features of atom-economy
and efficiency.[1] Among the developed organocatalyt-
ic systems, phosphine-mediated reactions[2] with elec-
tron-deficient alkynoates/allenoates/alkenoates enable
the syntheses of heterocyclic compounds such as cou-
marins,[3] tetrahydropyridines,[4] 2,3-dihydrofurans,[5] 2-
pyrones,[6] spiro compounds[7] and other natural prod-
ucts as well as bioactive compounds efficiently.[8] This
methodology is also applicable to the syntheses of
fullerene derivatives with exohedrally functionalized
cyclopentene and cyclopropane moieties.[9] Recently,
we have reported an unusual enyne A [Eq. (1),
Scheme 1] that can react with nucleophilic phosphines
Scheme 1. Nucleophilic conjugate additions of phosphines
at the a(d’)-carbon to generate 1,3-dipole Ia and pro- and Gilman or Grignard reagents toward enyne substrates.
Adv. Synth. Catal. 2013, 355, 2165 – 2171
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Table 1. Optimization of the reaction conditions.[a]
[c]
Entry
1a (equiv.)
P(cHx)3 (equiv.)
Temperature [oC]
Time [h]
Yield[b] [%]
C60
1
2
3
4
5
6
7
8
1
1
1
1
1
1
2
3
4
0.5
0.4
0.3
0.2
0.1
0.1
0.05
100
120
120
130
120
120
120
120
120
120
120
120
120
120
6
6
6
3
6
6
6
6
6
6
6
6
24
6
41 (66)
56 (79)
67 (92)
56 (85)
59 (83)
53 (56)
49 (53)
58 (97)
60 (97)
50 (78)
45 (82)
8 (36)
38
29
27
34
29
6
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
7
40
38
36
45
78
37
88
9
10
11
12
13
14
53 (84)
trace
[a]
Reactions were carried out using C60 (36 mg, 0.050 mmol) in 10 mL of o-DCB.
Isolated yields after column chromatography; values in parentheses are based on converted C60.
Recovered C60 after column chromatography.
[b]
[c]
initial nucleophilic attack of phosphines at the b- a higher yield (entry 3, 67%). However, it provided
carbon of enynes 1/2/3 [Eq. (3), Scheme 1] to generate a lower yield when the reaction was carried out at
the 1,3-dipole Ic – which is inconsistent with the find- a higher temperature, 1308C (entry 4). We further
ings of previous studies using Gilman or Grignard re- noted that the best yield was achieved from the use of
agents. Furthermore, these isolated cyclopentenoful- 0.4 equiv. of PACTHNUGTRENUNG(cHx)3 (entry 9) after a survey of the
lerenes display unusually comparable or higher loadings of the catalysts (entries 8–11). Although the
LUMO energy levels than a typical n-type material, reaction provided an 8% yield with only 0.1 equiv. of
PCBM, through an electrochemical study.
We first prepared a number of enynes (Z)-1a–h and to 24 h gave a 53% yield (entry 13). Further reduction
(E)-2a–h,[13] and selected 1a as a model substrate for of the P
(cHx)3 loading to 5 mol% provided only trace
optimization of the reaction conditions. The choice of amounts of 4a (entry 14). In a compromise between
(cHx)3 was determined from preliminary reactivity the reaction time and the loading of P(cHx)3 for ob-
screenings by reacting 1a with trialkyl phosphites taining an optimal yield, the conditions of entry 9
[P(OR3), R=Me and n-Bu], trialkylphosphines [PR3, were selected for a study of the scope.
R=Me, n-Bu and cHx], and triarylphosphines PAr3 We next evaluated the scope of the reaction with
[Ar=phenyl, 4-methylphenyl, 4-methoxyphenyl] in o- (Z)-enynes 1b–h having various substituents on the
DCB – colour changes were observed with PMe3, phenyl ring (Table 2). P(cHx)3 reacted with these
(n-Bu)3 and P(cHx)3 at room temperature within enynes and C60 to produce cyclopentenofullerenes 4b–
PACHTNGUTRENNU(G cHx)3 (entry 12), an extension of the reaction time
AHCTUNGTRENNUNG
P
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
P
N
ACHTUNGTRENNUNG
a few hours and while phosphites and triarylphos- h in 40–63% isolated yields (entries 2–8; 62–84%
phines showed negative results at 24 h. To our delight, based on converted C60). Among these substrates, re-
our initial test with PACHTNUGRTENUNG(cHx)3 (1 equiv.) and enyne 1a actions with 4-fluorophenyl-substituted enyne 1f pro-
(1 equiv.) with C60 at 1008C for 6 h gave a 41% yield vided the lowest yield (40%, entry 6). In addition, we
(Table 1, entry 1) and that at 1208C provided a 56% noted that trans-configured enynes 2a–h were slightly
yield of 4a (entry 2). When we increased the amount less reactive than cis-configured enynes with P
ACHTUNGTRENNUNG(cHx)3.
of 1a (1.2 equiv.), compound 4a was isolated in They also produced the same fullerene derivatives
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Chemo-, Regio- and Stereoselective Tricyclohexylphosphine-Catalyzed [3+2]Cycloaddition
Table 2. Scope of the reaction to give 4.[a]
Entry
R
Product
Yield [%][b]
C60 [%][c]
1
2
3
4
5
6
7
8
1a; H
4a
4b
4c
4d
4e
4f
4g
4h
4a
4b
4c
4d
4e
4 f
4g
4h
60 (97)
49 (82)
56 (84)
45 (68)
56 (66)
40 (62)
57 (71)
63 (73)
37 (74)
45 (76)
48 (52)
57 (93)
59 (73)
47 (80)
57 (79)
57 (79)
38
40
33
34
15
35
20
14
50
41
8
39
19
41
28
28
1b; 4-OMe
1c; 4-t-Bu
1d; 4-Me
1e; 3-F
1f; 4-F
1g; 3-Cl
1h; 4-Cl
2a; H
2b; 4-OMe
2c; 4-t-Bu
2d; 4-Me
2e; 3-F
2f; 4-F
2g; 3-Cl
2h; 4-Cl
9
10
11
12
13
14
15
16
[a]
All reactions were performed using C60 (36 mg, 0.050 mmol), 1a–h or 2a–h (0.060 mmol), PACHTUNTRGNE(NUG cHx)3 (5.6 mg, 0.020 mmol) in
10 mL of o-DCB.
[b]
[c]
Isolated yields after column chromatography; values in parentheses are based on converted C60.
Recovered C60 after column chromatography.
4a-h in 37–59% isolated yields (entries 9–16; 52–93% moieties of 5a–h with the ethyl group while facing the
based on converted C60). These isolated fullerenes ex- plane of the functionalized five-membered ring as
hibit poor solubility in CHCl3 (ca. 2.6 mgmLÀ1 in compared to those of 4a–h with a methyl group –
CHCl3) and CH2Cl2. We dissolved these compounds such pseudo-symmetrization may cause a higher ten-
in carbon disulfide with C6D12 as an external standard dency for molecular packing in solvents leading to re-
and Cr
ACHTUNGTRENNUNG
troscopic measurements.
Our attempt at improving the solubility of the cy- en-4-ynoate (3i) with C60 showed the best solubility
clopentenofullerenes led us to prepare the ethyl ester (entry 9).[14]
enynes. We investigated only the more reactive (Z)-
The molecular structures of these cyclopentenoful-
enynes 3a–h because their isomeric (E)-enynes pro- lerenes were characterized by mass spectrometry
duced the same products. We prepared cyclopenteno- (MS), 1H and 13C NMR and infrared spectroscopic
fullerenes 5a–h under the optimal reaction conditions, methods (see the Supporting Information). All the
giving 47–64% yields (64–93% based on converted MS data corresponded to the expected formulae of
C60, Table 3). We noted that these ethyl ester enynes the isolated cyclopentenofullerenes 4–6. For example,
generally provided better yields than the methyl ester in the IR spectrum of 4a, C=O stretching bands ap-
counterparts. However, their solubilities are about peared at 1717 cmÀ1. In the 13C NMR spectra of 4a,
three times lower when compared to 4a–h. Among excluding the addend carbons, 30 peaks were ob-
these products, compound 5e with a 3-fluorophenyl served in the range of 128–156 ppm for the 58 sp2-car-
moiety exhibits the best solubility (ca. 1 mgmLÀ1 in bons of the C60 skeleton due to the existence of a sym-
CHCl3). The reduced solubility is likely due to rela- metrical plane, and another two peaks in the range of
tively more weighing-symmetrization of the two-wing 72–76 ppm corresponded to the two sp3-carbons on
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Table 3. Scope of the reaction to give 5 and 6.[a]
Entry
R1
R2
Product
Yield [%][b]
C60 [%][c]
1
2
3
4
5
6
7
8
9
Et
Et
Et
Et
Et
Et
Et
Et
3a; H
5a
5b
5c
5d
5e
5f
5g
5h
6
64 (83)
55 (93)
50 (64)
58 (76)
63 (84)
54 (77)
47 (89)
55 (69)
50 (58)
23
41
22
24
25
30
47
20
14
3b; 4-OMe
3c; 4-t-Bu
3d; 4-Me
3e; 3-F
3f; 4-F
3g; 3-Cl
3h; 4-Cl
3i; H
n-octyl
[a]
All reactions were performed using C60 (36 mg, 0.050 mmol), 3a–i (0.060 mmol), PChx3 (5.6 mg, 0.020 mmol) in 10 mL of
o-DCB.
[b]
[c]
Isolated yields after column chromatography; values in parentheses are based on converted C60.
Recovered C60 after column chromatography.
C60. One of the compounds, 4b, was crystallized from cleophilic addition to C60. The initial attack by P-
its CS2/o-DCB solution through slow evaporation of (cHx)3 could take place either at the b- or d-carbon
AHCTUNGTRENNUNG
carbon disulfide. Single crystal X-ray diffraction anal- of enynes 1–3 to generate 1,3-dipolar species Ic or Id
ysis shows that the original enyne a and b carbons, C- (see Scheme 1). However, the d-attack pattern was
3 and C-5, are covalently bonded to the carbons of C- excluded due to steric hindrance of the phenyl ring
15 and C-14 of the fullerene cage, respectively and the fact that P
ACHTUNGERTN(NUNG cHx)3 was found to be unreactive
(Figure 1).[15]
toward methyl 3-phenyl-2-propynoate. Upon forma-
The mechanism of formation of cyclopentenofuller- tion of reactive intermediate Ic, it undergoes nucleo-
enes 4–6 is proposed in Scheme 2. According to the philic addition to C60 to form II. This species then
X-ray diffraction results, the carbanion was generated does an intramolecular 5-exo-trig cyclization to give
either at the a or g-carbon of 1–3 for subsequent nu- III. After subsequent proton transfer and elimination
of PACTHNUTRGNE(NUG cHx)3, compounds 4–6 were formed. These prod-
ucts are formed stereoselectively because the substi-
tuted phenyl ring stays trans with respect to the C60
cage due to a steric effect. In short, PACTHNURTGNENG(U cHx)3 attacks
the enynes chemoselectively and regioselectively and
then the intermediates react with C60 to give 4–6 ste-
reoselectively. The mode of phosphine attack is al-
tered when the ester group on the alkynyl carbon of
A (Scheme 1) is replaced with an aryl moiety in this
study, likely due to the switch of electropositivity of
sp2-carbons.
We studied the electrochemical properties of 4–6
by cyclic voltammetry (CV) and noted that their elec-
trochemical properties are quite similar with first
half-wave reduction potentials (1E1/2) in the range of
À1.21 to À1.23 V (Table 4 and Table S2 in the Sup-
porting Information). Among these, derivatives 4e
Figure 1. Solid state structure of compound 4b with solvent
molecule (1,2-dichlorobenzene) omitted for clarity.
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Adv. Synth. Catal. 2013, 355, 2165 – 2171

Chemo-, Regio- and Stereoselective Tricyclohexylphosphine-Catalyzed [3+2]Cycloaddition
Scheme 2. Proposed catalytic cycle.
Table 4. Half-wave reduction potentials E1/2 (V) of com- pounds 4e and 4f possess the highest LUMO energy
pounds 4a–h and 6.[a]
levels and could provide the highest open-circuit volt-
age (Voc) upon use as n-type materials in organic pho-
Entry
Compound
1E1/2
2E1/2
3E1/2
tovoltaics (OPVs).[16] Examples with other substitu-
ents only exhibit minute changes of the E1/2 with less
than 10 mV differences. Similarly, compound 5f with
a 4-fluoro moiety remains to be the one exhibiting
a higher LUMO energy level among 5a–h with the
ethyl ester moiety. These new cyclopentenofullerenes
all showed comparable or higher LUMO energy
levels than that of PCBM (entry 10) according to
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4f
4g
4h
6
PCBM
À1.21
À1.21
À1.21
À1.22
À1.23
À1.23
À1.21
À1.22
À1.22
À1.21
À1.58
À1.58
À1.58
À1.59
À1.62
À1.63
À1.59
À1.60
À1.61
À1.58
À2.09
À2.08
À2.08
À2.09
À2.13
À2.14
À2.08
À2.10
À2.11
À2.08
1
their E1/2 values.
9
10[b]
In conclusion, we have developed a chemo-, regio-
and stereoselective PACTHNUTRGNE(UGN cHx)3-catalyzed [3+2]cycload-
[a]
Versus ferrocene/ferrocenium. Conditions: ca. 0.50 mM
4–6 and 0.050 mM Bu4NPF6 in anhydrous o-DCB; refer-
ence electrode: Ag/0.01M AgNO3 and 0.050 mM (n-
Bu)4NClO4 in anhydrous acetonitrile; working electrode:
glassy carbon; auxiliary electrode: Pt; scanning rate:
50 mV sÀ1. See Table S1 and Table S2 in the Supporting
Information for Osteryoung square wave voltammetry
(OSWV) and differential pulse voltammetry (DPV) data
of 4–6, and CV data for 5a–h.
dition of (E)- or (Z)-enynes with C60 to give cyclopen-
tenofullerenes in good to excellent yields. The ob-
tained cyclopentenofullerenes showed comparable or
higher LUMO energy levels than that of PCBM. The
photovoltaic study utilizing cyclopentenofullerenes as
n-type materials will be reported in due course.
[b]
See ref.[16a]
Experimental Section
General Procedure for the Synthesis of
Cyclopentenofullerenes 4a–h and 5a–h
and 4f with 3-fluoro and 4-fluoro substituents showed
1
the lowest E1/2 value of À1.23 V (entries 5 and 6),
likely due to an electron-donating behavior of the
To a reaction flask containing C60 (36 mg, 0.050 mmol) and
fluoro moiety through resonance effects. Thus, com- 1a–h/2a–h/3a–h (0.060 mmol) were added 10 mL of anhy-
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Po-Yen Tseng and Shih-Ching Chuang
COMMUNICATIONS
drous o-dichlorobenzene, and then the solution was bubbled
with argon for 20 min. Then, tricyclohexylphosphine
(5.6 mg, 0.020 mmol) was added into the solution. The mix-
ture was stirred at 1208C for 6 h. After the reaction was
completed, the solution was subjected to flash silica gel
column chromatography using toluene/hexanes (1/1.2) as an
eluent for recovery of the unreacted C60. Continuing elution
with toluene gave 4a–h or 5a–h. Compound 6 could not be
separated from C60 by flash chromatography, but required
further purification with a preparative Buckyprep column.
Betzer, A. Voituriez, A. Marinetti, ChemCatChem
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Acknowledgements
We
thank
the
National
Science
Council
(NSC101211M009008) and NCTU for financial support of
this research.
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