Reduction of acetylated tetraphenylethylenes: Electrochemical behavior and stability of the related reduced anions

Article abstract of DOI:10.1021/jo951027h

The synthesis and characterization of 1-(4-acetylphenyl)-1,2,2-triphenylethylene (2), 1,1-bis(4-acetylphenyl)-2,2-diphenylethylene (3a), trans-1,2-bis(4-acetylphenyl)-1,2-diphenylethylene (3b), cis-1,2-bis(4-acetylphenyl)-1,2-diphenylethylene (3c), 1,1,2-tris(4-acetylphenyl)-2-phenylethylene (4), and 1,1,2,2-tetrakis(4-acetylphenyl)ethylene (5) are described. The molecular orbital energies, charge densities, and electronic spectra of the neutral and reduced species of 2 and 3a-3c were calculated using INDO/1 parameters for structures that had been optimized geometrically with AM1 parameters. The calculated electronic spectra for the neutral compounds compare favorably with the experimentally observed absorption spectra. Cyclic voltammetry of these compounds in deoxygenated THF containing 0.3 M [n-Bu₄N]PF₆ showed that 2 and 3a have irreversible one-electron first reduction waves, whereas 3b, 3c, 4, and 5 display quasi-reversible two-electron first reduction waves. Chemical reduction of 2-4 with Na/Hg in THF results in highly colored solutions, the absorbance spectra of which have been compared to the calculated electronic spectra for the radical anions and dianions of these compounds. Reduction of 2 results in dimerization to a pinacol product, whereas those of 3b and 4 result only in formation of the tetraarylethane product. 3a gives both pinacol and tetraarylethane upon reduction.

Full text of DOI:10.1021/jo951027h

J . Org. Chem. 1996, 61, 287-294
287
Red u ction of Acetyla ted Tetr a p h en yleth ylen es: Electr och em ica l
Beh a vior a n d Sta bility of th e Rela ted Red u ced An ion s
Michael O. Wolf,† Harold H. Fox, and Marye Anne Fox*
Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712
Received J une 6, 1995 (Revised Manuscript Received November 1, 1995X)
The synthesis and characterization of 1-(4-acetylphenyl)-1,2,2-triphenylethylene (2), 1,1-bis(4-
acetylphenyl)-2,2-diphenylethylene (3a ), trans-1,2-bis(4-acetylphenyl)-1,2-diphenylethylene (3b), cis-
1,2-bis(4-acetylphenyl)-1,2-diphenylethylene (3c), 1,1,2-tris(4-acetylphenyl)-2-phenylethylene (4),
and 1,1,2,2-tetrakis(4-acetylphenyl)ethylene (5) are described. The molecular orbital energies,
charge densities, and electronic spectra of the neutral and reduced species of 2 and 3a -3c were
calculated using INDO/1 parameters for structures that had been optimized geometrically with
AM1 parameters. The calculated electronic spectra for the neutral compounds compare favorably
with the experimentally observed absorption spectra. Cyclic voltammetry of these compounds in
deoxygenated THF containing 0.3 M [n-Bu4N]PF6 showed that 2 and 3a have irreversible one-
electron first reduction waves, whereas 3b, 3c, 4, and 5 display quasi-reversible two-electron first
reduction waves. Chemical reduction of 2-4 with Na/Hg in THF results in highly colored solutions,
the absorbance spectra of which have been compared to the calculated electronic spectra for the
radical anions and dianions of these compounds. Reduction of 2 results in dimerization to a pinacol
product, whereas those of 3b and 4 result only in formation of the tetraarylethane product. 3a
gives both pinacol and tetraarylethane upon reduction.
In tr od u ction
transfer can be initiated photochemically.5,7,14 In this
vein, our group has prepared substituted tetraphenyl-
ethylenes with varying redox potentials in order to
explore the effect of substitution on their observed
electrochemical behaviors.7 The electrochemistry of these
compounds was more complex than that observed for 1.
Although reduction waves were sometimes broad and ill-
defined, it was clear that Kdisp for most of these tetra-
substituted compounds is large.
Tetraphenylethylene (TPE) (1) has been of interest for
some time because of its unusual electrochemical behav-
ior and excited state properties.1-15 Electrochemical
reduction of 1 has been shown to yield the radical anion,
which rapidly disproportionates according to eq 1.12
kf
2TPE•- y
\
k z TPE2- + TPE Kdisp ) kf/kr
(1)
r
In an effort to expand our understanding of the
geometric changes induced by excitation and/or reduction,
we have prepared a series of substituted tetraphenyl-
ethylenes 2-5 in which the effect of increasing substitu-
tion by a single functional group on the electrochemical
behavior could be examined. We chose electron-with-
drawing acetyl groups for this study, since successive
substitutions by Friedel-Crafts acylation proved to be a
facile route to multiply substituted tetraphenylethylenes.
The disproportionation equilibrium constant for this
reaction, Kdisp, is large in relatively polar solvents such
as THF or CH3CN, since solvation and ion pairing
stabilize the highly charged dianion over the radical
anion in these solvents.1 The cyclic voltammogram of 1
thus contains a single two-electron “reversible” wave,
which is actually two closely spaced one-electron waves.
We have proposed that tetraphenylethylene derivatives
could be used as two-electron transfer catalysts since they
can undergo rapid two-electron reduction, and electron
† Current address: Department of Chemistry, University of British
Columbia, Vancouver, BC, V6T 1Z1.
X Abstract published in Advance ACS Abstracts, December 15, 1995.
(1) Troll, T.; Baizer, M. M. Electrochim. Acta 1974, 19, 951-953.
(2) Sun, Y.-P.; Bunker, C. E. J . Am. Chem. Soc. 1994, 116, 2430-
2433.
(3) Sun, Y.-P.; Fox, M. A. J . Am. Chem. Soc. 1993, 115, 747-750.
(4) Stuart, J . D.; Ohnesorge, W. E. J . Am. Chem. Soc. 1971, 93,
4531-4536.
(5) Schultz, D. A.; Fox, M. A. J . Org. Chem. 1990, 55, 1047-1055.
(6) Schuddeboom, W.; J onker, S. A.; Warman, J . M.; de Haas, M.
P.; Vermeulen, M. J . W.; J ager, W. F.; de Lange, B.; Feringa, B. L.;
Fessenden, R. W. J . Am. Chem. Soc. 1993, 115, 3286-3290.
(7) Muzyka, J . L.; Fox, M. A. J . Org. Chem. 1991, 56, 4549-4552.
(8) Ma, J .; Dutt, G. B.; Waldeck, D. H.; Zimmt, M. B. J . Am. Chem.
Soc. 1994, 116, 10619-29.
(9) Leigh, W. J .; Arnold, D. R. Can. J . Chem. 1981, 59, 3061-75.
(10) Leigh, W. J .; Arnold, D. R. Can. J . Chem. 1981, 59, 609-612.
(11) Kumar, R.; Singh, P. R. Indian J . Chem. 1972, 951-952.
(12) Grzeszczuk, M.; Smith, D. E. J . Electroanal. Chem. 1984, 162,
189-206.
(13) Go¨rner, H. J . Phys. Chem. 1982, 86, 2028-35.
(14) Fox, M. A.; Schultz, D. J . Org. Chem. 1988, 53, 4386-4390.
(15) Farnia, G.; Maran, F.; Sandona J . Chem. Soc., Faraday Trans.
1 1986, 82, 1885-1892.
The acetyl group does not behave solely as an inert
substituent in this system, since it can contribute both
to the reduction behavior of the compounds and to the
reactivity of the reduced species. The structure of the
singly and doubly reduced anions in this series is
therefore critical in determining the result of electro-
chemical or chemical reduction of 2-5. In this paper,
we describe the observed and calculated absorption
spectra of neutral 2-5 and the mono- and dianions
derived from them. We then correlate this data with the
results of the electrochemical and chemical reductions
of these compounds.
0022-3263/96/1961-0287$12.00/0 © 1996 American Chemical Society

288 J . Org. Chem., Vol. 61, No. 1, 1996
Wolf et al.
solution was heated at reflux for 12 h and was then quenched
by being poured into water. The crude product was extracted
into CH2Cl2, washed with H2O and brine, and dried over anhyd
MgSO4. The solvent was removed in vacuo, yielding an oily
solid which was purified by chromatography (silica, ethyl
acetate/CH2Cl2, 1:1), yielding pure 5 (0.05 g, 3%): mp ) 241
°C. 1H NMR: 7.71 (d, 8 H, J ) 8.4 Hz), 7.07 (d, 8 H, J ) 8.4
Hz), 2.52 (s, 12 H). 13C NMR: 197.39, 146.91, 141.62, 135.80,
131.26, 128.13, 25.51. HRMS (CI): m/ z calcd for C34H29O4
(M + H)+, 501.2066; found, 501.2056.
Exp er im en ta l Section
Gen er a l. All reactions were carried out under nitrogen,
and reaction workups, after quenching, were done in air.
Solvents were reagent grade and were used as received. CDCl3
was used as a solvent for 1H and 13C NMR spectra. Reagents
were purchased from Aldrich and were used as received.
1-(4-Acetylp h en yl)-1,2,2-tr ip h en yleth ylen e (2). A solu-
tion of acetyl chloride (0.21 mL, 3.0 mmol) and AlCl3 (0.40 g,
3.0 mmol) in CH2Cl2 (50 mL) was added via cannula to a
solution of tetraphenylethylene (1.0 g, 3.0 mmol) in CH2Cl2
(50 mL) at room temperature. The solution was stirred for 2
h before being quenched by being poured into water. The
crude product was extracted into CH2Cl2. The extract was
washed with H2O and brine and was dried over anhyd MgSO4.
The solvent was removed in vacuo, leaving an oily solid which
was purified by chromatography (silica, CH2Cl2), yielding pure
2 (0.68 g, 61%): mp ) 111 °C. 1H NMR: 7.67 (d, 2 H, J ) 8.4
Hz), 7.15-7.05 (m, 11 H), 7.05-6.95 (m, 6 H), 2.51 (s, 3 H).
13C NMR: 197.6, 149.0, 143.1, 143.1, 143.0, 142.6, 139.8, 134.9,
131.4, 131.2, 131.2, 131.2, 127.8, 127.8, 127.7, 127.7, 126.9,
126.7, 126.7, 26.5. HRMS (CI): m/ z calcd for C28H22O (M)+,
374.1671; found, 374.1672.
1,1-Bis(4-a cetylp h en yl)-2,2-d ip h en yleth ylen e (3a ) a n d
tr a n s-1,2-Bis(4-a cetylp h en yl)-1,2-d ip h en yleth ylen e (3b).
A solution of acetyl chloride (1.29 mL, 18.1 mmol) and AlCl3
(2.46 g, 18.4 mmol) in CH2Cl2 (50 mL) was added via cannula
to a solution of tetraphenylethylene (3.0 g, 9.0 mmol) in CH2-
Cl2 (25 mL) at rt. The solution was stirred for 12 h before
being quenched by being poured into water. The crude product
was extracted into CH2Cl2. The extract was washed with H2O
and brine and was dried over anhyd MgSO4. The solvent was
removed in vacuo, leaving an oily solid which was purified by
chromatography (silica, CH2Cl2, followed by ethyl acetate/CH2-
Cl2, 1:100) yielding pure 3a (0.44 g, 12%) and 3b (0.34 g, 9%).
3a : mp ) 151 °C. 1H NMR: 7.69 (d, 4 H, J ) 7.5 Hz), 7.2-
7.0 (m, 10 H), 7.0-6.9 (m, 4 H), 2.51 (s, 6 H). 13C NMR: 197.5,
148.2, 144.1, 142.5, 138.6, 135.1, 131.4, 131.1, 127.9, 127.9,
127.2, 26.5. HRMS (CI): m/ z calcd for C30H24O2 (M)+,
416.1776; found, 416.1769. 3b: mp ) 212 °C. 1H NMR: 7.68
(d, 4 H, J ) 8.4 Hz), 7.1-7.0 (m, 10 H), 7.0-6.9 (m, 4 H), 2.51
(s, 6 H). 13C NMR: 197.7, 148.4, 142.5, 141.4, 135.2, 131.4,
131.2, 128.1, 127.8, 127.2, 26.5. HRMS (CI): m/ z calcd for
C30H24O2 (M)+, 416.1776; found, 416.1777.
Electr och em istr y. [n-Bu4N]PF6 (Aldrich) was recrystal-
lized three times from absolute ethanol. THF was distilled
from Na/benzophenone immediately before use and degassed
by three freeze-pump-thaw cycles. Experiments were done
in
a three-electrode cell with a platinum mesh counter
electrode, a silver wire quasi-reference electrode, and
a
platinum working electrode. Potentials were referenced in-
ternally to added ferrocene and are reported against SCE. Bulk
electrolysis was conducted in the same cell at constant
potential (-2.00 V vs Ag) under a N2 atmosphere.
Ch em ica l Red u ction s. The compound to be reduced was
dissolved in dry THF and stirred with an excess (10-20 equiv)
of 5 wt % Na in Hg. After the reaction was complete, the
mixture was poured into water, and the product was extracted
into CH2Cl2 and dried over anhyd MgSO4. The solvents were
then removed in vacuo.
Molecu la r Or bita l Ca lcu la tion s. Structural minimiza-
tions for all neutral compounds were conducted with MOPAC
6.01 and AM1 parameters.16 Geometry optimization was
achieved when the energy change was less than 0.0002 kcal
mol-1; the resulting structures were then used in all subse-
quent calculations. Semiempirical electronic structure calcu-
lations for 1-3 were carried out using the ZINDO package
included in the CAChe 3.7 software package. Compounds 4
and 5 were too large to be handled by this software. The
INDO/1 parameters used in these calculations were taken from
the literature.17 Singlet RHF and doublet ROHF calculations
were performed on the neutral and monoanions, respectively.
The dianions of 1, 2, 3a , and 3b were treated as singlets (RHF).
The spectral calculations were performed following the pro-
cedure (CI) included in the ZINDO software. The solvent
parameters for CH2Cl2 (F ) 1.4218, ꢀ ) 9.08; used for the
neutral compounds) or THF (F ) 1.407, ꢀ ) 7.58; used for the
reduced compounds) were included as a dielectric continuum
in the model. Molecular radii were determined directly from
the minimized structures.
cis-1,2-Bis(4-a cetylp h en yl)-1,2-d ip h en yleth ylen e (3c).
A solution of 3b (0.10 g, 0.24 mmol) in benzene (1 mL) was
prepared, degassed by thorough purging with nitrogen, and
irradiated with a medium pressure Hg lamp in a Pyrex tube
for 1 h. The solvent was removed in vacuo, and the crude
product was purified by chromatography (silica, CH2Cl2),
yielding pure 3c (10 mg, 10%). 1H NMR: 7.69 (d, 4 H, J )
8.4 Hz), 7.2-7.1 (m, 10H), 7.1-7.0 (m, 4 H), 2.51 (s, 6 H). 13C
NMR: 197.65, 148.37, 142.43, 141.39, 135.20, 131.39, 131.16,
128.06, 127.80, 127.22, 26.51. HRMS (CI): m/ z calcd for
C30H24O2 (M)+, 416.1776; found, 416.1774.
Resu lts
Syn th esis. We prepared acetylated tetraphenyleth-
ylenes 2-5 by Friedel-Crafts acylation of 1 in CH2Cl2
using AlCl3, as shown for the preparation of 2 in eq 2.
1,1,2-Tr is(4-a cetylp h en yl)-2-p h en yleth ylen e (4). A so-
lution of acetyl chloride (1.7 mL, 24 mmol) and AlCl3 (3.2 g,
24 mmol) in CH2Cl2 (50 mL) was prepared. This was added
via cannula to a solution of tetraphenylethylene (1.0 g, 3.0
mmol) in CH2Cl2 (50 mL). The solution was stirred for 2 h,
and the reaction was quenched by being poured into water.
The crude product was extracted into CH2Cl2. The extract was
washed with H2O and brine and was dried over anhyd MgSO4.
The solvent was removed in vacuo, leaving an oily solid which
was purified by chromatography (silica, ethyl acetate/CH2Cl2,
1:10), yielding pure 4 (0.44 g, 32%): mp ) 165 °C. 1H NMR:
7.7 (m, 6 H), 7.15-7.05 (m, 9 H), 6.96 (m, 2 H), 2.52 (s, 9 H).
13C NMR: 197.4, 197.3, 147.6, 147.5, 147.4, 142.8, 141.9, 140.1,
135.5, 135.4, 131.3, 131.3, 131.3, 131.0, 128.0, 128.0, 127.9,
127.9, 127.5, 26.4 (one resonance obscured in the aryl region).
HRMS (CI): m/ z calcd for C28H22O (M)+, 459.1960; found,
459.1957.
Compounds 2-4 were obtained by controlling the
molar ratio of acetyl chloride and AlCl3 to TPE at room
temperature. In order to obtain 5, it was necessary to
heat the reaction mixture at reflux. However, by this
method the compound could be isolated only in very low
yield (∼3%). All of the acetylated compounds were
purified by column chromatography on silica and were
obtained as crystalline solids.
1,1,2,2-Tetr a k is(4-a cetylp h en yl)eth ylen e (5). A solution
of acetyl chloride (1.7 mL, 24 mmol) and AlCl3 (3.2 g, 24 mmol)
in CH2Cl2 (50 mL) was added via cannula to a solution of
tetraphenylethylene (1.0 g, 3.0 mmol) in CH2Cl2 (50 mL). The
(16) Dewar, M. J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J . P.
J . Am. Chem. Soc. 1985, 107, 3902-3909.
(17) Pople, J . A.; Beveridge, D. L.; Dobosh, P. A. J . Chem. Phys.
1967, 47, 2026.

Reduction of Acetylated Tetraphenylethylenes
J . Org. Chem., Vol. 61, No. 1, 1996 289
Ta ble 1. INDO/1 Ca lcu la tion s of F or m a l Ch a r ge on Olefin ic a n d Ca r bon yl Atom s in Acetop h en on e a n d 1-3 a s Neu tr a l
a n d Sin gly a n d Dou bly Red u ced Ion s
formal chargea
compound
C1 olefin
C2 olefin
C carbonyl 1
O carbonyl 1
C carbonyl 2
O carbonyl 2
1
-0.001
-0.134
-0.154
-0.011
0.010
-0.041
-0.010
0.011
0.013
0.001
0.021
-0.076
0.001
0.026
-0.002
-0.135
-0.151
0.010
-0.030
-0.110
0.010
-0.039
-0.082
0.000
-0.049
-0.081
-0.003
-0.034
1•-
12-
2
0.401
0.256
0.272
0.403
0.290
0.294
0.403
0.297
0.240
0.404
0.267
0.388
0.185
-0.620
-0.779
-0.801
-0.617
-0.756
-0.776
-0.616
-0.751
-0.677
-0.615
-0.771
-0.625
-0.820
2•-
22-
3a
0.403
0.396
0.308
0.403
0.396
0.240
0.403
0.399
-0.618
-0.639
-0.755
-0.617
-0.638
-0.675
-0.616
-0.633
3a•-
3a 2-
3b
3b•-
3b2-
3c
3c•-
acetophenone
acetophenone•-
a
C1 olefin and C2 olefin are defined as the olefinic carbon atoms in 1-3. C carbonyl 1 and O carbonyl 1 are defined as the carbon and
oxygen atoms of the carbonyl group located closest to C olefin 1. C carbonyl 2 and O carbonyl 2 are defined as the carbon and oxygen
atoms of the second carbonyl group in 3a or those located closest to C olefin 2 in 3b and 3c.
Ta ble 2. INDO/1 Ca lcu la tion s of HOMOa a n d LUMO
Molecu la r Or bita l Ca lcu la tion s. The AM1-opti-
mized geometries of the acetyl-substituted tetraphenyl-
En er gies of 1-3 a n d th e Rela ted An ion s
ethylenes 1-3 have been investigated with the goal of
supporting the experimental results reported herein. All
of the minimized structures are propeller-shaped, similar
to the known structure of 1,18 with the acetyl groups in
2 and 3 lying in the plane of the attached phenyl rings.
The minimized structures of 1-3 were used in the
calculation of molecular orbital energies, charge densities,
and absorption spectra of the neutral and reduced species
using INDO/1 parameters. Compounds 4 and 5 could not
be studied directly because of software limitations. The
structures of the reduced species were not minimized
further in order to facilitate comparison of the electronic
structure of the neutral, radical anions, and dianions,
although it is known that the structure of the dianion of
1 is slightly twisted (∼30°) about the olefin.14
compound
HOMOa (eV)
LUMO (eV)
1
-7.385
-1.017a
3.158
-7.609
-1.238a
2.741
-7.850
-1.397a
2.286
-7.804
-1.387a
2.077
0.280
3.680
6.486
-0.368
2.639
5.591
-0.610
1.766
4.883
-0.662
1.537
4.072
-0.535
1.804
1•-
12-
2
2•-
22-
3a
3a •-
3a 2-
3b
3b•-
3b2-
3c
-7.820
3c•-
-1.335a
a
The HOMO of a radical anion might appropriately be called a
SOMO, but in order to facilitate direct comparison with the related
orbitals in the closed shell neutral and dianion, the more general
term (HOMO) is employed here.
The formal charges of the olefinic carbon atoms and
carbonyl carbon and oxygen atoms for acetophenone and
1-3 and their related mono- and dianions are listed in
Table 1. As 1 is reduced, there is an increased negative
charge (decreased formal charge) on the olefinic carbon
atoms, consistent with reduction of the C-C π-bond, as
is observed in the literature.15 In 2, the carbonyl of the
acetyl group is the site of significant reduction; the formal
charge on the carbonyl C and O became significantly
more negative upon addition of one electron, whereas the
olefinic carbons become only slightly more negative.
When the dianion is produced, a greater fraction of
negative charge resides on the olefin carbons as well.
Compound 3a behaves in a similar fashion to 2 when
reduced by one electron, but both carbonyl groups and
the olefin bear more negative charge in the dianion of
3a . A similar response is observed for the dianion of 3b,
except that charge is more evenly distributed across the
molecule. Thus, these calculations indicate that reduc-
tion of acetyl-substituted tetraphenylethylenes affects
both the carbonyl and olefinic moieties.
The calculated energies of the HOMO and LUMO of
1-3 and the corresponding reduced anions are listed in
Table 2. The energies of both the HOMO and LUMO
increase upon reduction, as expected. In addition, the
HOMO-LUMO gaps decrease substantially in the
monoanions.
F igu r e 1. Absorption spectra of 1 (‚ ‚ ‚), 2 (-‚-), 4 (- - -), and
5 (s) in CH2Cl2.
Absor p tion Sp ectr a . The absorption spectra of 1-5
in CH2Cl2 shown in Figures 1 and 3 can be compared with
the calculated (INDO/1) electronic spectra of 1-3 shown
in Figures 2 and 4. The calculated and observed spectra
(18) Hoekstra, A.; Vos, A. Acta Crystallogr. 1975, B31, 1716-1721.

290 J . Org. Chem., Vol. 61, No. 1, 1996
Wolf et al.
F igu r e 5. Cyclic voltammogram of 2 (∼10 mM) at Pt in
deaerated 0.3 M [n-Bu4N]PF6 in THF at room temperature.
Scan rate ) 100 mV/s.
F igu r e 2. Calculated (INDO/1) absorption spectra of 1 (s)
and 2 (- - -).
Ta ble 3. Cyclic Volta m m etr y Da ta of 1-5 a n d
Acetop h en on e
b
E1/2 vs SCE
(V ( 0.05 V) electrons
no. of
Epc - Epa
(mV)
a
compound
ipc/ipa
1
2
3a
3b
3c
-3.01
-2.83
-2.69
-2.44
-2.44
-2.32
-2.18
-2.93
2
1
1
2
irreversible
irreversible
0.98
1.00
0.96
210
183
220
140
4
5
2
2
1
0.96
irreversible
acetophenone
a
b
ipc ) peak cathodic current; ipa ) peak anodic current. Scan
rate ) 100 mV/s.
Since acetophenone has strong absorptions at 240 and
275 nm,19 the new band in the spectrum of 2 can likely
be assigned to the carbonyl group. The three diacetyl
isomers 3a -c have slightly different spectra, although
the two principal bands appear at approximately the
same energy, Figure 3. In the spectrum of the cis isomer
3c, the low-energy band is split, exhibiting two maxima
at 310 and 340 nm, whereas in both 3a and the trans
isomer 3b the low-energy band exists as a single peak
at ∼325 nm. Compounds 4 and 5 have similar absorption
spectra, Figure 1, in which the strongest absorption is
observed in the low-energy band (at 300 nm) and in which
a low energy shoulder is present.
F igu r e 3. Absorption spectra of 3a (s), 3b (- - -), and 3c (-‚-)
in CH2Cl2.
Cyclic Volta m m etr y. Cyclic voltammograms of 2-5
were measured in deaerated THF containing 0.3 M
[n-Bu4N]PF6 as the supporting electrolyte. The cyclic
voltammogram of 2 shows an irreversible reduction wave
and a broad reoxidation wave at -0.5 V vs Ag which
appears only after scanning through the reduction wave,
Figure 5. The cyclic voltammogram of 3a is similar (from
0 to -2.0 V vs Ag) to that of 2, except that the reduction
wave appears at a slightly less negative potential, Table
3. The peak position of the reduction wave (Ep,c) of 2
shifts negatively with increasing scan rate, while the
maximum cathodic current (ip,c) increases, Figure 6.
Interestingly, at fast scan rates (>1 V/s), an oxidation
wave directly associated with the reduction wave ap-
pears. This anodic wave also increases in intensity with
increasing scan rate, suggesting that at fast scan rates
some of the initial reduced species is oxidized back to the
neutral species before it could have been converted to the
secondary product. Since the oxidation wave at ∼-0.5
V is less intense than the cathodic wave, it is likely that
F igu r e 4. Calculated (INDO/1) absorption spectra of 3a (s),
3b (- - -), and 3c (‚ ‚ ‚).
are similar in shape, although the peaks in the observed
spectra are less well-defined. Each of the observed
spectra contains at least two distinct bands, and, in some
cases, shoulders. Both the higher and lower energy
bands are red-shifted with increasing acetylation, with
the intensities of both bands increasing with substitution.
The absorption spectrum of 2 is significantly red-shifted
from that of 1, and a third band appears at ∼275 nm.
(19) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry; Marcel Dekker, Inc.: New York, 1993.

Reduction of Acetylated Tetraphenylethylenes
J . Org. Chem., Vol. 61, No. 1, 1996 291
F igu r e 6. Scan rate dependence of the cyclic voltammogram
of a deaerated ∼10 mM solution of 2 in 0.3 M [n-Bu4N]PF6 in
THF at Pt at room temperature.
F igu r e 8. Cyclic voltammogram of (a) ∼10 mM 3a and (b)
∼10 mM 3b in deaerated 0.3 M [n-Bu4N]PF6 in THF at Pt at
room temperature containing 1 equiv of 1. Scan rate ) 100
mV/s.
tetrakis(4-nitrophenyl)ethylene in CH3CN and DMF,
where two successive two-electron waves have been
reported.21
The cyclic voltammograms of 3b and 3c are identical.
Since these compounds may interconvert by rotation
about the ethylenic bond, we suggest that reduction
produces a common species by rapid rotation about this
bond. This process is too rapid to be observed directly,
and so the cyclic voltammograms appear identical. Bulk
electrolysis of 3b yielded some 3c after workup, as would
be expected if rotation about the ethylenic bond occurs
in the intermediate reduced species. Our results are
consistent with literature measurements for cis- and
trans-1,2-bis(4-cyanophenyl)-1,2-bis(4-methoxyphenyl)-
ethylene, in which both isomers exhibit identical reduc-
tion potentials.9
In order to determine the number of electrons involved
in each reduction of 2-5, we examined the cyclic volta-
mmograms of these compounds in the presence of an
equimolar quantity of 1. We chose 1 as the internal
standard for these experiments because its diffusion
characteristics are expected to be similar to those of the
acetylated derivatives. The cyclic voltammograms for 3a
and 3b in the presence of 1 are shown in Figure 8. Those
compounds with an irreversible reduction wave (2 and
3a ) display single-electron reductions by cyclic voltam-
metry, whereas 1, 3b, 3c, 4, and 5 all display two electron
quasi-reversible reduction waves, Table 3.
Ch em ica l Red u ction s. Compounds 2-5 can be
chemically reduced upon reaction with Na/Hg amalgam.
Reduction of a solution of 2 gives a red solution, the
absorption spectrum of which is shown in Figure 9. The
addition of water to the reduction mixture causes the red
color to disappear rapidly. Workup of the resulting
mixture yields both diastereomers of pinacol 6,22 in
addition to recovered 2. The presence of residual 2 after
workup, even when excess Na/Hg was used, is likely the
result of equilibration between 62- and 2•-, analogous to
that reported for other pinacols.23 The observed cyclic
voltammetry of 2 (vide supra) suggests that dimerization
is rapid on the operative electrochemical time scale. The
calculated absorbance spectrum for 2 as a radical anion,
F igu r e 7. Cyclic voltammogram of ∼10 mM 5 in deaerated
0.3 M [n-Bu4N]PF6 in THF at Pt at room temperature over
the potential range (a) 0 to -1.75 V and (b) 0 to -2.5 V. Scan
rate ) 100 mV/s.
the chemically produced secondary product either is
unstable or is not formed quantitatively. We attribute
this behavior to an ECE process in which the initial
reduction product reacts rapidly, and the resulting
product is subsequently oxidized at ∼-0.5 V vs Ag.
Cyclic voltammetry of acetophenone under the same
conditions shows similar features. The radical anion of
acetophenone undergoes a rapid coupling reaction to yield
a pinacol (1,2-diphenyl-1,2-ethanediol).20 The oxidation
waves at ∼-0.5 V in both acetophenone and 2 and 3a
are assigned to oxidation of the pinacol product. We
conclude that the electrochemical reduction of 2 and 3a
in THF generates carbonyl-based radical anions which
dimerize rapidly.
The cyclic voltammograms obtained at a scan rate of
100 mV/s of 3b, 3c, 4, and 5 all have quasi-reversible
first reduction waves like that shown for 5, Figure 7a.
The quasi-reversibility of this wave indicates the produc-
tion of a reduced species that dimerizes less rapidly than
the radical anion of 2 or 3a . The reduction waves for
3b, 3c, 4, and 5 remain quasi-reversible even at the
slowest scan rates (10 mV/s). The electrochemical be-
havior of 3a and 3b is particularly interesting, as these
geometric isomers differ markedly, Figure 8. The cyclic
voltammogram of 5 also contains a second two-electron
cathodic wave, Figure 7b. The asymmetry of this wave
implies two closely spaced reductions involving the same
number of electrons as in first wave. This electrochemi-
cal behavior is very similar to that observed for 1,1,2,2-
(21) Ammar, F.; Save´ant, J . M. J . Electroanal. Chem. Interfacial
Electrochem. 1973, 47, 115-125.
(22) Mattielo, L.; Rampazzo, L. J . Chem. Soc., Perkin Trans. 2 1993,
2243-2247.
(20) Wawzonek, S.; Gunderson, A. J . Electrochem. Soc. 1960, 107,
537-540.
(23) Russell, G. A.; J anzen, E. G.; Strom, E. T. J . Am. Chem. Soc.
1962, 84, 4155-4157.

292 J . Org. Chem., Vol. 61, No. 1, 1996
Wolf et al.
F igu r e 11. Absorption spectra of 3a (- - -) and 3b (s) after
treatment with Na/Hg in THF.
F igu r e 9. Absorption spectra of 2 (s), 4 (‚ ‚ ‚), and 5 (- - -)
after treatment with Na/Hg in THF.
F igu r e 10. Calculated absorption spectra of 1•- (s) and 2•-
(- - -).
2•-, Figure 10, displays a broad absorbance around 710
nm similar to that expected for a ketyl radical anion. This
contrasts with the observed maximum of 450 nm, sug-
gesting that the observed species may be the pinacol of
6 which has been further reduced by excess Na/Hg
amalgam.
F igu r e 12. Calculated absorption spectra of (a) 3a •- (‚ ‚ ‚),
3b•- (s), and 3c•- (- - -) and (b) 12- (s) and 3b2- (- - -).
The reductions of 3a and 3b with Na/Hg give dark red
solutions, the absorbance spectra of which are shown in
Figure 11. The calculated spectra of 3a •- and 3b•-
,
Figure 12a, are somewhat similar to the observed spectra,
although shifted from the observed maximum absor-
bances. In addition there is an absorbance near 450 nm
that is similar to that observed for reduced 2 in Figure
9. Quenching the reduced 3a solution with water,
followed by workup, yields a product mixture containing
the pinacols of 3a as well as 7a . Dimerization appears
to be slower in 3a than in 2, leading to concomitant
spectroscopic observation of bands attributable to 3a •-
at wavelengths longer than 600 nm and to the pinacol
The product mixture showed no evidence for the
formation of semidione.24 Semidione formation is un-
likely under our experimental conditions which were
more mildly reducing, in a less polar solvent, and in the
absence of air, all of which differentiate our experiment
from those described earlier as conducive of semidione
formation.
(24) Russell, G. A.; Strom, E. T.; Talaty, E. R.; Weiner, S. A. J . Am.
Chem. Soc. 1966, 88, 1998.

Reduction of Acetylated Tetraphenylethylenes
J . Org. Chem., Vol. 61, No. 1, 1996 293
F igu r e 13. Energy and charge density changes in the HOMO and LUMO of 3b upon reduction to the radical anion and dianion.
Sch em e 1. Rea ction P a th w a ys of Red u ced 1-5
dianion at 450 nm.
The reduced olefin 7c was obtained from the reduction
of 4 with Na/Hg in THF. The absorption spectra of the
freshly reduced solutions of 4 and 5 are shown in Figure
9. The solution of reduced 4 is blue and that of reduced
5 is purple: these colors are also observed close to the
working electrode during bulk electrochemical reduction
of these compounds.
The major product obtained upon chemical reduction
of 3b is assigned from the H spectrum to the symmetric
Discu ssion
1
The differences observed in the reactivity of reduced
2-5 are caused by differences in the stabilities of the
corresponding radical anions. We propose Scheme 1 to
account for the reactivity of this series of compounds.
The radical anion formed upon electrochemical or
chemical reduction may react in several ways, although
the chemical reducing agent may induce over-reduction
in some cases. In 2 and 3a , compounds which lack acetyl
substituents on the phenyl at C1 and C2 of the central
ethane 7b resulting from reduction of the double bond.
There is no evidence for the formation of any products
arising from reduction of the carbonyl group. The
calculated positions of the long wavelength bonds for
singly and doubly reduced 3b (3b•- and 3b2-), Figure 12,
are essentially identical albeit with different extinction
coefficients. Thus, the absorbing species in the observed
spectrum, Figure 11, may be either the mono- or dianion
of 3b.

294 J . Org. Chem., Vol. 61, No. 1, 1996
Wolf et al.
double bond, rapid dimerization to a pinacol occurs. This
coupling appears to be more rapid for 2 than for 3a , since
with 2 no other product is observed, but with 3a other
products are also formed. The electrochemical studies
suggest that these reductions are one-electron processes,
and the coupled product 6 is structurally analogous to
that obtained in the one-electron reduction of acetophe-
and 5. We are unable to distinguish experimentally
between a reaction pathway involving disproportionation,
followed by protonation by solvent, and one proceeding
by protonation of the radical anion by solvent, followed
by reduction and a second protonation step. We can
conclude, however, that one-electron reductions of acety-
lated tetraphenylethylenes lead to coupled products
(pinacols) and two-electron reductions lead to reduction
of the olefin.
none. The formal charge distributions in 2•- and 3a •-
,
Table 2, suggest that these reactions can be regarded as
carbonyl group reductions in which the radical anion is
likely to combine with a second radical anion to afford
the expected pinacol via interaction of the singly occupied
molecular orbital of each anion. Further reduction to the
dianions of 3a and 3b localizes more negative charge on
the olefinic carbons, producing 7a and 7b, respectively.
Thus, upon reduction, 2 dimerizes rapidly to afford only
the pinacol, but when 3a is reduced, both the tetraaryl-
ethane product 7 and the dimerization product 6 are
formed, indicating that either the rate of dimerization
has decreased or over-reduction is easier in this com-
pound than in 2.
For 3b, 3c, 4, and 5 (and 3a , to a minor extent),
disproportionation followed by protonation produces the
related tetraarylethane, in parallel to reactivity previ-
ously reported for electrochemically15 or chemically11
reduced 1. Figure 13 shows the evolution of the molec-
ular orbitals as 3b is reduced by one and two electrons.
In the radical anion, the symmetry of the HOMO and
LUMO is broken but is reestablished upon further
reduction. Extensive delocalization in the HOMO of 3b2-
is expected to contribute to the stability of the dianion,
enabling facile double reduction. Once a dianion is
formed, however, the driving force for coupling decreases
since this would require the interaction of molecular
orbitals that are now completely filled. Protonation to
the reduced products 7b and 7c takes place with 3b and
4, although in the absence of a proton source, the
dianions of 3b, 3c, 4, and 5 appear to be quite stable.
The stabilizing effect of delocalization is consistent with
the two-electron reduction processes that are indicated
by the quasi-reversible electrochemistry of 1, 3b, 3c, 4,
Con clu sion s
The successive substitution of tetraphenylethylene by
acetyl groups results in significant changes in the
electronic structure of the compounds. These changes
are manifested in both the electronic spectra and the
electrochemistry of 2-5. The dramatic differences in
both the electrochemical behaviors and the reduction
product distributions observed for 3a and 3b, both
diacetylated derivatives of the parent compound, show
that the reactivity of the reduced compounds is very
sensitive to substitution. Ultimately, our goal is the
preparation of two-electron transfer catalysts with vary-
ing redox potentials and reactivity. This work demon-
strates that by using substituents such as the acetyl
group, these attributes can easily be tuned.
Ack n ow led gm en t. This work was supported by the
National Science Foundation and the Robert A. Welch
Foundation. M.O.W. gratefully acknowledges a post-
doctoral fellowship from the Natural Sciences and
Engineering Research Council of Canada. We thank
Scott Reese for assistance with the bulk electrolysis
experiment.
Su p p or tin g In for m a tion Ava ila ble: Copies of 13C NMR
spectra (6 pages). This material is contained in libraries on
microfiche, immediately follows this article in the microfilm
version of the journal, and can be ordered from the ACS; see
any current masthead page for ordering information.
J O951027H