Intramolecular and intermolecular reactivity of localized singlet diradicals: The exceedingly long-lived 2,2-diethoxy-1,3-diphenylcyclopentane-1,3-diyl

Article abstract of DOI:10.1021/ja992507j

The direct and benzophenone-sensitized photodenitrogenation of the diethoxy-substituted diazene 3 in nonprotic solvents, e.g. n-hexane, benzene, and acetonitrile, afforded exclusively the housane 4 through the intermediary 2,2-diethoxy-1,3-diphenylcyclope

Full text of DOI:10.1021/ja992507j

J. Am. Chem. Soc. 2000, 122, 2019-2026
2019
Intramolecular and Intermolecular Reactivity of Localized Singlet
Diradicals: The Exceedingly Long-Lived
2,2-Diethoxy-1,3-diphenylcyclopentane-1,3-diyl
Manabu Abe,*,‡ Waldemar Adam,† Thomas Heidenfelder,† Werner M. Nau,*,§ and
Xiangyang Zhang§
Contribution from the Department of Materials Chemistry, Faculty of Engineering, Osaka UniVersity,
Suita 565-0871, Osaka, Japan, the Institut fu¨r Organische Chemie der UniVersita¨t Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany, and the Insitut fu¨r Physikalische Chemie der UniVersita¨t Basel,
Klingelbergstrasse 80, CH-4056 Basel, Switzerland
ReceiVed July 15, 1999. ReVised Manuscript ReceiVed NoVember 9, 1999
Abstract: The direct and benzophenone-sensitized photodenitrogenation of the diethoxy-substituted diazene 3
in nonprotic solvents, e.g. n-hexane, benzene, and acetonitrile, afforded exclusively the housane 4 through the
intermediary 2,2-diethoxy-1,3-diphenylcyclopentane-1,3-diyl diradical 2b. Alternatively, in the presence of
methanol, the formation of the adduct 6 competed through trapping of the allylic cation 5. The intervention of
the intermediates was corroborated by laser-flash photolysis experiments of the diazene 3. The diradical
intermediate was characterized by its strong transient absorption at 545 nm, which decayed with a first-order
lifetime in the microsecond range. The diradical lifetimes increased in the order n-hexane (0.52 µs), benzene
(0.88 µs), acetonitrile (1.01 µs), and chloroform (3.73 µs). The singlet ground state of the 2,2-diethoxy-substituted
1,3-diradical 2b is suggested by the absence of EPR signals at low temperature, the lack of quenching by
molecular oxygen, and the calculated (UB3LYP/6-31G*) singlet preference of ca. 4 kcal mol-1 for
2,2-dihydroxy-1,3-diphenyl-1,3-cyclopentanediyl as a model. Trapping of the 1,3-diradical 2b by external
additives, e.g., olefins, dienes, and TEMPO, was not observed. In protic solvents, a decrease of the diradical
lifetimes with increasing acidity was noted, e.g., the lifetime in acetic acid dropped to 70 ns. Moreover, a
concomitant growth of an additional transient at 470 nm was found in acidic solvents, which was assigned to
the allylic cation 5. The factors governing the intramolecular and intermolecular chemical reactivity of the
localized singlet diradical 2b are discussed.
Introduction
chemistry to compete efficiently; moreover, its housane product
does not persist near ambient temperature such that its com-
prehensive characterization has been difficult.2 On the other
hand, the spiroepoxy case 1d, for which intermolecular trapping
by methanol has been demonstrated through product studies,4
lacks a chromophore for spectroscopic detection.
The theoretical and experimental examination of localized
singlet diradicals presents a topic of considerable current
interest.1-4 2,2-Difluoro substitution in 1,3-cyclopentanediyl
diradicals has been theoretically predicted1 to promote a singlet
ground state for the parent 1a and this has been experimentally
confirmed2 for the 1,3-diphenyl derivative 2a. Similarly, the
most recent theoretical and experimental results suggest that
2,2-disilyl3 or 2-spiroepoxy4 substitution facilitates also a singlet
ground state in such diradicals, e.g., derivatives 1c and 1d. The
hitherto known singlet diradicals suffer some drawbacks,
however, which have encumbered a detailed characterization
of their intermolecular and intramolecular chemistry. On one
hand, the difluoro-substituted diradical 2a, while spectroscopi-
cally detectable, possesses a still too short lifetime (80 ns in
n-pentane and 6 ns in acetonitrile)2 to allow intermolecular
Herein, we present a detailed spectroscopic characterization
and product studies for the most persistent localized singlet
diradical known to date, namely the 2,2-diethoxy-1,3-diphenyl-
1,3-cyclopentanediyl (2b). Despite the seemingly innocuous
substitution (as opposed to the previously employed fluoro1,2
and silyl3 substituents), the lifetimes of this diradical are orders
of magnitude longer (3.7 µs in chloroform!) than those of the
difluoro derivative 2a. This has enabled us to study the factors
that govern the intramolecular and intermolecular reactivity of
this singlet diradical in detail.
‡ Osaka University.
† Universita¨t Wu¨rzburg.
§ Universita¨t Basel.
(1) (a) Xu, J. D.; Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc. 1994,
116, 5425. (b) Borden, W. T. Chem. Commun. 1998, 1919.
(2) Adam, W.; Borden, W. T.; Burda, C.; Foster, H.; Heidenfelder, T.;
Heubes, M.; Hrovat, D. A.; Kita, F.; Lewis, S. B.; Scheutzow, D.; Wirz, J.
J. Am. Chem. Soc. 1998, 120, 593.
(3) Skancke, A.; Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc. 1998,
120, 7079. The calculation refers to the 2,2-disilyltrimethylene-1,3-diyl
species.
(4) Abe, M.; Adam, W.; Nau, W. M. J. Am. Chem. Soc. 1998, 120, 11304.
10.1021/ja992507j CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/17/2000

2020 J. Am. Chem. Soc., Vol. 122, No. 9, 2000
Abe et al.
Table 1. Product Distributions for Photodenitrogenation of
Scheme 1
Diazene 3a
entry
solvent
benzene
n-hexane
yields of 4b
ratios of 4/6c
1
2
3
4
97
98
95
88
72
90
73
CH3CN
MeOH
92/8
77/23
93/7
5
MeOH/CF3CH2OHd
MeOH
6e
7e
MeOH/CF3CH2OHd
75/25
a The photodenitrogenation of diazene 3 (27 mM) was performed at
25 °C with a high-pressure Hg lamp (300 W, 320-380 nm band-path
filter); diazene consumption was 100%. b Yields of isolated product
after chromatography on silica gel. c The ratios of 4/6 (normalized to
100%) were determined on the basis of 1H NMR (270 MHz) peak areas;
reproducibility was (0.9% for independent experiments. d A 1:1
mixture of MeOH/CF3CH2OH was used as solvent. e With benzophe-
none (270 mM) as triplet sensitizer.
and conjugated dienes, e.g., vinyl ether, acrylonitrile, furan, etc.7
However, only housane 4 was observed quantitatively in the
photolyzates. Similarly, trapping experiments of the diradical
(photolysis of diazene 3 or thermolysis of housane 4 at 65 °C
for 24 h) with the radical scavengers TEMPO8a (1.1 M in CHCl3
or benzene) and tributyltin hydride8b (neat) provided only
housane 4 in >90% yield.
Results
Photoproducts of Diazene 3. Diazene 3 was prepared from
4,4-diethoxy-3,5-diphenyl-4H-pyrazol5 through the Hu¨nig route6
(Scheme 1). On direct photolysis with a high-pressure Hg lamp
in benzene, n-hexane, or acetonitrile, diazene 3 (ꢀ ) 116
M-1cm-1 at λmax ) 364 nm) afforded quantitatively housane 4
(> 95%, entries 1-3 in Table 1, Scheme 1). The anti
configuration of 4 was established by NOE effects (cf. Scheme
1). For photodenitrogenations in the presence of methanol
(entries 4-7), the methanol adduct 6 was observed by NMR
spectroscopy along with housane 4. Since the latter persisted
in the presence of methanol and even trifluoroethanol, the
methanol adduct 6 is not formed by reaction with housane 4
(vide infra). Trapping by methanol was more efficient in CF3-
CH2OH (entries 5 and 7). The ratios of 4/6 in the direct and
triplet (benzophenone) sensitized photodenitrogenation were the
same within experimental error (compare entries 4,5 with 6,7).
The structure of the methanol trapping adduct 6 was
established on the basis of NMR spectra (1H and 13C DEPT). It
was formed as a single diastereomer and its exo configuration
Trapping experiments with molecular oxygen (3O2) were
pursued with particular interest, because oxygen is well-known
to be an excellent scavenger of virtually all carbon-centered
radicals and is sterically the least hindered trapping agent. High
oxygen concentrations were achieved in a 1:5 mixture of
n-hexane and perfluorodecalin (C10F18), in which the solubility
of dioxygen is much higher than that in n-hexane alone.9 An
elevated temperature was needed to achieve homogeneous
conditions10 and the oxygen concentration was found to be 81.5
( 2.0 mM at 4 atm and 65 °C.11 After irradiation of diazene 3
for 4 h under O2 (4 atm), the oxygenated products 8 (5%), 9
(6%), and 10 (10%) were isolated in addition to housane 4
(72%).
1
was readily determined by means of H NMR-NOE measure-
ments for the MeOH/CF3CH2OH photolysate (cf. NOE effects
in eq 1). The bridgehead and methoxy protons are well
(7) Intermolecular reactions of non-Kekule´ (delocalized) singlet diradi-
cals: (a) Berson, J. A. Acc. Chem. Res. 1997, 30, 238. (b) Stone, K. J.;
Greenberg, M. M.; Blackstock, S. C.; Berson, J. A. J. Am. Chem. Soc. 1989,
111, 3659. (c) Bush, L. C.; Heath, R. B.; Feng, X. W.; Wang, P. A.;
Maksimovic, L.; Song, A. I.; Chung, W.-S.; Berinstein, A. B.; Scaiano, J.
C.; Berson, J. A. J. Am. Chem. Soc. 1997, 119, 1406. (d) Heath, R. B.;
Bush, L. C.; Feng, Wu, X.; Berson, J. A.; Scaiano, J. C.; Berinstain, A. B.
J. Phys. Chem. 1993, 97, 13355. Note that orbital-symmetry considerations
suggest an allowed cycloaddition of conjugated dienes, e.g., furan, with
the singlet diradical, and a forbidden one for monoolefins.
(8) (a) The rate constant for reaction of benzyl radicals with TEMPO in
benzene has been reported to be 1.8 × 108 M-1 s-1, cf.: Beckwith, A. L.
J.; Bowry, V. W.; Ingold, K. U. J. Am. Chem. Soc. 1992, 114, 4983. (b)
The rate constant for reaction of benzyl-type radicals with tributyltin hydride
has been reported to be 3.6 × 104 M-1 s-1, cf.: Franz, J. A.; Suleman, N.
K.; Alnajjar, M. S. J. Org. Chem. 1986, 51, 19.
(9) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,
2nd ed.; Marcel Dekker: New York, 1993; p 289.
(10) Klement, I.; Lu¨tjens, H.; Knochel, P. Angew. Chem., Int. Ed. Engl.
1997, 36, 1454.
(11) The previously reported method was employed to determine the
dioxygen solubility: Adam, W.; Hannemann, K.; Wilson, R. M. J. Am.
Chem. Soc. 1986, 108, 929.
distinguished from the protons of housane 4 in d6-benzene. The
observed stereochemistry was expected from an exo attack of
methanol on the intermediary allylic cation 5 (cf. Discussion).
Since adduct 6 was too labile for isolation by silica gel or
alumina chromatography, additional structural evidence was
obtained by chemical transformation to the keto ester 7 (8%
yield from diazene 3) in the ozonolysis of the crude photolyzate
in MeOH/CF3CH2OH (eq 1); housane 4 persisted under the
ozonolysis conditions.
Diradical Trapping Experiments. Additional trapping ex-
periments were performed to obtain evidence for the involve-
ment of singlet diradical 2b as an intermediate in the photolysis
of diazene 3. For example, the photodenitrogenation was
performed in the presence of an excess (up to 0.5 M) of alkenes
(5) Gerninghaus, C.; Ku¨mmell, A.; Seitz, G. Chem. Ber. 1993, 126, 733.
(6) Beck, K.; Hu¨nig, S. Chem. Ber. 1987, 120, 477.

2,2-Diethoxy-1,3-diphenylcyclopentane-1,3-diyl
J. Am. Chem. Soc., Vol. 122, No. 9, 2000 2021
Scheme 2
Figure 1. Transient decay traces of the singlet diradical 2b (λobs
)
545 nm), generated by photolysis from the diazene 3 (λexc ) 351 nm)
in (a) chloroform, (b) methanol, (c) acetonitrile, and (d) n-hexane.
unstable due to ring strain,13 e.g., the thermally labile 1,2-
dioxolane 2,3-dioxabicyclo[2.2.1]heptane decomposes readily
even at 50 °C. This may explain the failure to detect the
presumed intermediary endoperoxide 11. The decomposition of
the latter may involve R cleavage at one of the oxyl radical
centers to form diradical 13, which cyclizes14 to the 1,5 diradical
14. The tricyclic ortho ester 10 results from radical coupling of
the 1,5 diradical 14 and the bicyclic product 9 is presumably
produced by ethyl migration. The cis-1,2-dibenzoylcyclopentane
(8) is probably formed from the elimination of diethoxycarbene
in the 1,5-dioxyl diradical 12 (double R cleavage).15 However,
trapping of this carbene by electron-deficient olefins, e.g.,
acrylonitrile and fumaronitrile, was not observed.
Transient Absorption Spectroscopy. The transient absorp-
tion spectra and decay traces of diazene 3 were measured in
various solvents by means of laser-flash photolysis (λexc ) 351
nm, 20 ns pulse). These experiments could not be performed
in the most polar (water) and least polar (perfluorohexane)
solvents due to insolubility. In solvents of higher acidity than
acetic acid, e.g., trifluoroacetic acid, rapid decomposition of the
diazene 3 occurred. In the other solvents, a strong transient
absorption was observed at around 545 nm, which decayed with
clean first-order kinetics (Figure 1) and displayed a linear
Arrhenius temperature dependence (Figure 2). The lifetimes
were unaffected by the presence of oxygen (1 atm of air or
oxygen atmosphere), even in chloroform and in the fluorocarbon
solvent mixture used for the preparative trapping experiments
(C10F18/n-hexane/C6H5CF3 ) 5/1/1). The transient lifetimes and
activation parameters are summarized in Table 2.
The stereochemical assignments of the acid-labile products
9 and 10 were made by NOE measurements in C6D6 (cf. NOE
effects above). In the oxygenated product 9, significant NOE
enhancements between the aromatic protons and the bridgehead
protons, and also between the protons of the carboethoxy and
ethoxy groups, were found, while for the ortho ester 10 strong
NOE effects between the aromatic protons and the bridgehead
ones, but no NOE enhancement between the bridgehead protons
and the ethoxy group, were observed. The cis configuration of
the 1,2-dibenzoylcyclopentane (8) was deduced by comparison
with the spectral data of its known trans isomer.12
A control experiment was performed to check the thermal
stability of housane 4. After 18 h at 65 °C, housane 4 was also
converted to the oxygenated products 8 (23%), 9 (23%), and
10 (43%). To perform the trapping experiments at ambient
temperature (25 °C) at which housane 4 persists, benzotrifluoride
(CF3C6H5) was added to a suspension of diazene 3 in n-hexane/
C10F18 to achieve homogeneous conditions also at ambient
temperature (C10F18/n-hexane/C6H5CF3 ) 5/1/1). The oxygen
concentration in this solvent system was found to be 58 ( 2
mM at 25 °C and 4 atm.11 After direct or benzophenone-
sensitized irradiation (320-380 nm) for 4 h (conversion 100%),
housane 4 was obtained almost quantitatively (93%).
The combined results suggest that housane 4 undergoes ring
opening to diradical 2b and subsequent oxygen trapping at 65
°C (Scheme 2). The observed oxygen trapping during the high-
temperature photolysis proceeds presumably also through the
intermediary housane since a control experiment at ambient
temperature did not afford any oxygen-trapping product. While
the experiments establish oxygen trapping of diradical 2b, it
cannot be decided whether the singlet or the triplet state of the
diradical is being trapped (Scheme 2).
With respect to the mechanism for formation of the oxygen-
ated products 8-10 (Scheme 2), we assume homolytic decom-
position of endoperoxide 11 at the elevated temperature (65 °C).
Related bicyclic 1,2-dioxolanes have been reported to be quite
The quantitative formation of the corresponding housane 4,
which in contrast to that derived from diradical 2a is persistent
near ambient temperature, allows the assignment of this transient
as the intermediary singlet diradical 2b. The singlet multiplicity
is further indicated by the following facts and analogies: (a)
the absorption maximum (λmax ) 545 ( 5 nm in all solvents)
is very similar to that of the difluoro-substituted diradical 2a
(λmax ) 530 nm);2 (b) the absorption obtained on photolysis in
(13) (a) Balci, M. Chem. ReV. 1981, 81, 91. (b) Coughlin, D. J.; Salomon
R. G. J. Am. Chem. Soc. 1979, 101, 2761. (c) Adam, W.; Duran, N. J. Am.
Chem. Soc. 1977, 99, 2729. (d) Clennan, E. L.; Foote, C. S. In Organic
Peroxides; Ando, W., Ed.; Wiley: New York, 1992; p 276.
(14) (a) Beckwith, A. L. J.; Hay, B. P. J. Am. Chem. Soc. 1989, 111,
230. (b) Beckwith, A. L. J.; Hay, B. P. J. Am. Chem. Soc. 1989, 111, 2674.
(15) For a related extrusion of silylene in the thermolysis of an
endoperoxide see: Satoh, T.; Moritani, I.; Matsuyama, M. Tetrahedron Lett.
1969, 5113. Nakadaira, Y.; Nomura, T.; Kanouchi, S.; Satoh, R.; Kabuto,
C.; Sakurai, H. Chem. Lett. 1983, 209.
(12) Fuson, R. C.; Fleming, C. L.; Warfield, P. F.; Wolf, D. E. J. Org.
Chem. 1945, 10, 121.

2022 J. Am. Chem. Soc., Vol. 122, No. 9, 2000
Abe et al.
Figure 3. Transient decay trace of the singlet diradical 2b (λobs
)
545 nm, lower trace, OD′ scale), generated by photolysis of the diazene
3 (λexc ) 351 nm) in trifluoroethanol and the concomitant growth of
absorbance of the allylic cation 5 (λobs ) 470 nm, OD scale).
Figure 2. Arrhenius plots for the decay rate constants (ln k) of the
singlet diradical 2b (λobs ) 545 nm), generated by photolysis of the
diazene 3 (λexc ) 351 nm) in a variety of solvents.
Table 2. Solvent Effect on the Lifetimes and Activation
Parameters for Decay of Singlet Diradical 2b
Table 3. Lifetimes and Relative Absorbances of Transients
(Singlet Diradical 2b at 545 nm and Allylic Cation 5 at 470 nm)
Formed in the Photolysis of Diazene 3 in Protic Solvents
Ea/(kcal
c
c
solvent
n-hexane
CCl4
ET(30)a
τ (2b)/µsb
0.52
0.82
0.88
0.91
0.72
3.73
1.55
1.01
1.29
1.54
1.19
mol-1
7.6
)
log(A/s-1
)
c
d
e
solvent
MeOH
MeOH/
τ(2b)/µsa
τ(5)b
23 µs
230 µs
ODrel
φ5
φ4
33.1
33.6
34.8
36.0
38.1
39.1
41.1
46.0
48.6
51.9
54d
12.0
1.89
2.37
0.33
2.0
0.04 [0.08] 0.96 [0.92]
[0.23]g
[0.77]g
benzene
CF3CH2OHf
1,4-dioxane
ethyl acetate
CHCl3/CDCl3
CH2Cl2
CH3CN
iPrOH
CF3CH2OH
CH3COOH
0.64
0.07
∼10 minh 6.0
0.69
0.92
0.31
0.08
71 µs
ca. 8
10.3
8.0
13.2
12.0
a From Table 2, the same lifetime was measured for the growth of
the allylic cation 5 at λobs ) 470 nm, λexc ) 351 nm. b Lifetime for the
decay of the allylic cation 5 determined by laser-flash photolysis (λobs
) 470 nm, λexc ) 351 nm, 20 ns pulse) at 20.0 ( 0.2 °C. c Ratio of
the absorbances at 470 and 545 nm (OD470/OD545), determined from
the initial maximum absorbance at 545 nm and the absorbance
maximum in the plateau region at 470 nm, cf. Figure 3, under identical
experimental conditions (same pulse energy, etc.). d Efficiency of allylic
cation 5 formation obtained from eq 2; the values in brackets were
acquired from product studies (trapping of the allylic cation with
methanol). e Taken as 1 - φ5. f 1:1 mixture by volume. g This ratio
(from product studies) was used as reference to determine the extinction
coefficients from eq 2, which were subsequently employed to calculate
the efficiencies in the other solvents, cf. text. h Measured with
conventional UV spectrophotometry.
EtOH
CH3CN (10% H2O)
CH3CN (20% H2O)
MeOH
55d
55.5
1.32
1.89 [≈2.0]e
9.0
12.5
MeOH/CF3CH2OHf [57.5]g 2.37 [≈3.1]e
CF3CH2OH
CH3COOH
59.5
51.7
0.64 [≈2.0]e
5.8h
10.5h
0.07 [≈1.0]e
a From ref 23. b Lifetime of the singlet diradical 2b determined by
laser flash photolysis (λobs ) 545 nm, λexc ) 351 nm, 20 ns pulse);
due to the strong temperature dependence, all values were obtained in
thermostated cells at 20.0 ( 0.2 °C; 5% error. c Determined from
Arrhenius plots of the lifetimes of the singlet diradical 2b at ca. 10
temperatures between 10 and 60 °C; statistical error in the data is ca.
0.1 kcal mol-1 for Ea and 5% for logA. d From: Balakrishnan, S.;
Easteal, A. J. Aust. J. Chem. 1981, 34, 943-947. e Values in brackets
are the extrapolated lifetimes used in Figure 4 (correction for the
interference from allylic cation formation) taken as 1/k4 according to
eq 3e with φ5 from Table 3. f 1:1 mixture by volume. g Estimated to
be the average of the values in the neat solvents. h Arrhenius plots for
the growth kinetics of the allylic cation 5 at 470 nm provided consistent
results.
In nonprotic solvents, the transient at 545 nm was the only
detectable species. In protic solvents, however, the decay of
the transient at 545 nm was accompanied by an isokinetic
growth at 470 nm (Figure 3). The tail absorption of the 545 nm
transient was very small at this wavelength. Moreover, the
activation parameters, determined independently from the grow-
ing and decaying absorption, were the same within error. This
provides evidence that the transient at 545 nm is the precursor
of the transient at 470 nm. The secondary transient species at
470 nm decayed also with first-order kinetics (Table 3). In the
case of trifluoroethanol, the transient persisted long enough to
allow, after laser excitation, measurement of its slow decay by
conventional UV spectrophotometry in the presence of air.
The transient at 470 nm was assigned to the allylic cation 5
on the basis of the following observations: (a) its lifetime was
not influenced by molecular oxygen;17a (b) the observed
absorption maximum is very similar to that of the allylic cation
derived from the difluoro-substituted diradical 2a (470 nm)2 and
to that of the related acyclic E,E-1,3-diphenylallyl cation (490
nm);18 (c) the lifetime in methanol was much shorter than that
a 2-methyltetrahydrofuran glass was thermally persistent at 77
K and resembled that of the transient in solution (λmax ) 566
nm), but it was EPR silent;16 (c) the lifetime of the transient
was insensitive to the presence of molecular oxygen; and (d)
the activation parameters are similar to those for the decay of
the difluoro-substituted diradical 2a,2 in particular, the high (ca.
1012-1013 s-1) preexponential Arrhenius factor (Table 2) is
indicative of a spin-allowed reaction, namely ring closure to
housane 4.
(16) In contrast to diradical 2a, which proved to be relatively photostable
in MTHF glass at 77 K, the diradical 2b underwent very efficient
photodecomposition upon broad-band UV-visible irradiation with a
medium-pressure mercury lamp. In fact, due to the high extinction coefficient
of 2b, it was only observable when special precautions were taken to exclude
visible light (cobalt glass filters, Schott UG 1) or, alternatively, when 351
nm laser excitation was employed for the generation from the diazene 3.
(17) (a) Miranda, M. A.; Pe´rez-Prieto, J.; Font-Sanchis, E.; Ko´nya, K.;
Scaiano, J. C. J. Phys. Chem. A 1998, 102, 5724. (b) Mayer, R.; Fabian, J.;
Viola, H.; Jakisch, L. Phosphorus Sulfur 1987, 31, 109.

2,2-Diethoxy-1,3-diphenylcyclopentane-1,3-diyl
J. Am. Chem. Soc., Vol. 122, No. 9, 2000 2023
in the less nucleophilic solvents trifluoroethanol and acetic acid
(Table 3); and (d) the methanol trapping product 6 is formed
(vide infra).
Scheme 3
The chemical trapping in the presence of methanol suggests
that the corresponding faster first-order decays are attributable
to pseudo-first-order reactions with methanol. The lifetimes in
neat methanol and acetic acid (Table 3) then suggest that the
apparent reaction rate constants (kr) of the allylic cation 5 with
methanol and the less nucleophilic acetic acid are ca. 1.8 and
0.8 × 103 M-1 s-1, taking kr to be the inverse of τ × [solvent].
For trifluoroethanol one obtains a lower limit of <0.6 M-1 h-1
,
based on the very long first-order lifetime. These rate constants
for nucleophilic trapping are orders of magnitude lower than
those recently measured or estimated for the E,E-1,3-dipheny-
lallyl cation, e.g., 1.3 × 105 M-1 s-1 with methanol and 8.0 ×
103 M-1 s-1 with trifluoroethanol,18 which indicates a dramatic
stabilization of the allylic cation 5 by the electron-donating
ethoxy and alkyl groups.
The extinction coefficients of the singlet diradical 2b at 545
nm and that of the allylic cation 5 at 470 nm were determined
in a 1:1 mixture of methanol/trifluoroethanol relative to that of
triplet benzophenone in benzene at 525 nm (7800 ( 800 M-1
cm-1) as actinometer.18 The quantum yield for the formation
of the diradical 2b was assumed to be unity and that for the
allylic cation 5 was taken as 23%, based on the relative yield
of methanol trapping product in the same solvent mixture (see
above). Extinction coefficients of 3350 M-1 cm-1 for 2b and
27000 M-1 cm-1 for 5 were obtained. These agree well with
the previously estimated value of 104 M-1 cm-1 for both 2a2
and the parent E,E-1,3-diphenylallyl cation.17b The efficiencies
for allylic cation formation in other protic solvents were obtained
from the experimental ratio of the absorbances of the diradical
and the allylic cation (Table 3). Correction for the known
extinction coefficients was made according to eq 2 and a
possible solvent dependence of the extinction coefficients was
neglected.
mol-1) and its 1,3-diphenyl derivative 15b (∆EST ) -3.96 kcal
mol-1) resemble the corresponding values for the 2,2-difluoro
derivatives 1a (∆EST ) -7.07 kcal mol-1)4 and 15a (∆EST
)
-4.72 kcal mol-1, this work), which indicate a singlet ground
state in all cases. The calculated singlet preference for the 2,2-
dihydroxy diradical 1b is larger than that for the 2-monoalkoxy
(spiroepoxy) case 1d (∆EST ) -1.00 kcal mol-1).4
Diradical 15b should be an excellent model system for the
experimentally examined system 2b, since the effect of the 4,5-
annelated cyclopentane ring is expected to have a minor effect.
Note that the 4,5-ethano bridge in 1,3-cyclopentanediyl diradi-
cals is a much weaker through-bond coupler than the 2-methano
bridge.20
φ5 ) [OD5× ꢀ2b]/[OD2b × ꢀ5]
(2)
For methanol as solvent, the efficiency for allylic cation
formation calculated from eq 2 (4%) agreed reasonably well
with that obtained from chemical trapping (8%). The efficiency
of allylic cation formation in neat trifluoroethanol (no trapping)
was estimated from transient absorption to be 69%. In the most
acidic solvent examined, acetic acid, the allylic cation 5 is nearly
quantitatively formed (92%). The efficiency for the formation
of housane 4 (φ4, Table 3) was taken to be 1 - φ5, in agreement
with the observed product distributions.
Calculation of the Diradical Ground State. Our previously
employed and tested4 density function theoretical procedure
(UB3LYP/6-31G*)19 was used to assess the effect of 2,2-
dihydroxy substitution on the singlet-triplet energy gap (∆EST)
of 1,3-cyclopentanediyl diradicals. The calculated data (not
corrected for the small4 zero-point vibrational energy differ-
ences) for the 2,2-dihydroxy diradical 1b (∆EST ) -7.38 kcal
Discussion
The photolysis of diazene 3 is proposed to proceed through
the pathways in Scheme 3. The involvement of the singlet
diradical 2b and the allylic cation 5 has been established by
their transient spectra and kinetics, as well as product studies
(cf. Results). For example, intervention of the allylic cation 5
has been confirmed by trapping with methanol. Diradical 2b
and the previously reported derivative 2a2 bear a number of
similarities, namely their strong transient absorption in the
visible region21 and their competitive follow-up reactions,
(20) (a) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1980, 102,
5409. (b) Dixon, D. A.; Dunning, T. H., Jr.; Eades, R. A.; Kleier, D. A. J.
Am. Chem. Soc. 1981, 103, 2878.
(18) Wintgens, V.; Johnston, L. J.; Scaiano, J. C. J. Am. Chem. Soc.
1988, 110, 511.
(19) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (b)
Becke, A. D. Phys. ReV. A 1988, 38, 3098. (c) Becke, A. D. J. Chem. Phys.
1993, 98, 5648. (d) The calculations were performed within Gaussian 94;
Frich, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B.
G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Peterson, G. A.;
Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V.
G.; Oritz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Wong,
M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Gordon,
M.; Gonzalez, C.; Pople, J. A. Gaussian 94; Gaussian, Inc.: Pittsburgh,
PA, 1995.
(21) A CF2-induced mixing of the parity-forbidden transition of benzyl
radicals at 460 nm with allowed transitions at higher energies has been
held responsible for the strong, bathochromically shifted absorption band
in the singlet diradical 2a (ref 2). An alternative interpretation, already under
discussion in the previous work, would involve a transition between the
symmetric and antisymmetric SOMO orbitals, which is allowed for singlet
diradicals. With respect to the latter possibility, the bathochromic shift for
the diethoxy-substituted diradical 2b versus the difluoro-substituted diradical
2a (by ca. 15 nm) could be due to a smaller singlet-triplet energy splitting
(cf. calculated values for diradicals 15a,b in Results).

2024 J. Am. Chem. Soc., Vol. 122, No. 9, 2000
Abe et al.
namely cyclization to the housane and heterolytic cleavage to
the allylic cation 5 (Scheme 3). However, the two singlet species
2a and 2b display three important differences: (i) in contrast
to the housane derived from diradical 2a, that of diradical 2b
persists at ambient temperature which facilitates reliable product
studies; (ii) the lifetimes of diradical 2b are up to 2 orders of
magnitude longer than those of diradical 2a (Table 2), which
dramatically increases the dynamic range for intermolecular
trapping; and (iii) heterolytic cleavage (ethoxide ion expulsion)
of diradical 2b to yield the allylic cation 5 competes only in
acidic protic solvents (Table 3). In contrast, diradical 2a
undergoes this competitive reaction even in nonpolar solvents.2
The exceptional properties of diradical 2b render this intermedi-
ate first choice to examine the intermolecular and intramolecular
reactivity of localized singlet diradicals, in regard to both product
studies and spectroscopic measurements.
Figure 4. Correlation of the rate constant for housane 4 formation
from the singlet diradical 2b (log k4) versus the ET(30) solvent polarity
parameter; the data are taken from Table 2 and corrected where
appropriate, as discussed in the text.
Ground-State Multiplicity and Persistence of Diradical 2b.
The singlet ground state of the localized diradical 2b has been
established by product studies and, in analogy to the difluoro
derivative 2a, by transient absorption spectroscopy (cf. Results).
The assignment of a singlet ground state is supported by density-
functional calculations (UB3LYP/6-31G*), which reveal a
singlet ground state for the 2,2-dihydroxy diradical 1b and also
the 2,2-dihydroxy-1,3-diphenyl derivative 15b. Interestingly, the
singlet spin-state preference resulting from 2,2-dihydroxy
substitution is calculated to be as large as for 2,2-difluoro
substitution (cf. Results).
however, to observe significant curvature of the Arrhenius plots
and thereby to discriminate between the activation parameters
for the two competitive processes. Moreover, there was little
variation in the absorbance ratios with temperature, which
precluded further analysis.
Intramolecular Reactivity: Ring Closure to Housane 4.
Significant solvent effects were observed on the lifetimes of
the singlet diradical 2b. The lifetimes and activation energies
decreased in the order chloroform > methanol > acetonitrile
> n-hexane. The rate of ring closure of diradical 2b to the
corresponding housane 4 (k4) decreased roughly with solvent
polarity, as confirmed by the moderate correlation with the ET-
(30) parameter23 in Figure 4 [log(k4) ) 6.5 ( 0.3 - (1.3 (
0.5)ET(30), r ) 0.55, n ) 16].
The values for the rate constants k4 were obtained as the
inverse of the diradical lifetime, i.e., 1/τ ) kobs ) k4, unless
formation of the allylic cation 5 decreased the housane 4 yield
below unity. In the latter case, the values for k4 and k5 may be
obtained by correction of the observed diradical lifetime for
the efficiencies of formation of the allylic cation 5 (φ5) and the
housane 4 (φ4, Table 3) according to the simple relations in
eqs 3a-e. The “apparent” lifetimes of the singlet diradical 2b
were taken as the inverse of k4, i.e., they are those expected if
the formation of the allylic cation 5 were not to occur in protic
solvents, cf. values in brackets in Table 2.
In view of the seemingly innocuous alkoxy substitution in
diradical 2b compared to the strongly electron-withdrawing
fluoro- or electron-donating silyl substituents (theory has
suggested that these serve best to promote singlet diradical
character),1,3 the exceptional persistence of diradical 2b comes
much as a surprise. This happenstance demands an adequate
theoretical rationalization. Comparison of the activation para-
meters in nonpolar solvents (n-hexane for 2b, Table 2, and
cyclohexane for 2a)2 suggests that the higher persistency of
diradical 2b versus 2a originates mainly from a decreased
preexponential factor [log(A/s-1) ) 12.0 versus 12.8] and not
from an increased activation barrier (EA ) 7.6 versus 7.8 kcal
mol-1). This implies that an entropic rather than an enthalpic
effect is responsible. In addition, the ethoxy substituents are
expectedly poorer leaving groups and render diradical 2b more
reluctant to undergo heterolytic cleavage to the allylic cation.
Intramolecular Reactivity: Heterolytic Cleavage of the
Singlet Diradical 2b to the Allylic Cation 5. The formation
of allylic cation 5 was corroborated by chemical trapping
experiments (cf. Results). Its ease of formation increased with
the acidity of the solvent,22 as evidenced by efficiencies, e.g.,
acetic acid > trifluoroethanol > methanol (Table 3). The
presence of a protic solvent alone, e.g., in acetonitrile-water
mixtures, did not promote formation of the allylic cation 5.
Evidently, the elimination of the ethoxide ion is assisted by
protons (formation of the better leaving group EtOH), as shown
in Scheme 3. As may be seen in Table 2, the lifetime of the
singlet diradical 2b is correspondingly shortened in acidic
solvents, most prominently in acetic acid (70 ns). Under acidic
conditions, it is expected that proton-catalyzed heterolytic
cleavage competes with housane 4 formation. This change in
mechanism manifests itself not only in a shortened transient
lifetime, but also in a reduced activation energy (Table 2). The
examined temperature range (10-60 °C) was too small,
kobs ) 1/τ ) k4 + k5
φ5 ) k5/(k4 + k5)
(3a)
(3b)
φ4 ) 1 - φ5 ) k4/(k4 + k5)
(3c)
k5 ) φ5/τ
(3d)
(3e)
k4 ) kobs - k5 ) kobs - φ5/τ
The strong solvent dependence of the lifetime of the singlet
diradical 2b is unprecedented and tentatively attributed to a
stabilization of its dipolar mesomeric structures II and III in
(22) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. In Organic SolVents;
Physical Properties and Methods of Purification; John Wiley & Sons: New
York, 1986.
(23) (a) Reichardt, C.; Harbusch-Go¨rnert, E. Lieb. Ann. Chem. 1983, 721.
(b) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry;
VCH: Weinheim, 1988.

2,2-Diethoxy-1,3-diphenylcyclopentane-1,3-diyl
J. Am. Chem. Soc., Vol. 122, No. 9, 2000 2025
polar solvents. In contrast, the lifetimes of closely related triplet
diradicals do not display a significant solvent dependence.24b
Unfortunately, the housane is oxidized at this temperature, such
that oxygen trapping may have also taken place through
equilibration with the energetically disfavored (by ca. 4 kcal
mol-1 according to calculations for 15b) triplet diradical T-2b
(Scheme 2). The latter is expected to be 3 orders of magnitude
(kr g 109 M-1 s-1 24
more reactive than the singlet diradical
)
2b. Presently we have no experimental criteria to exclude the
intervention of the triplet pathway.27
In Table 2 and Figure 4, the high persistence of the diradical
2b in chloroform (and to a lesser degree also in methylene
chloride) stands out: Although the polarity of chloroform lies
between that of dioxane and acetonitrile, the lifetime of diradical
2b in chloroform is four times longer! Presumably the weak
hydrogen-bonding (yet nonprotic) properties of the C-H bonds
in chloroform25 stabilize the singlet diradical 2b.26 Evidently,
in addition to the bulk polarity, specific interactions with the
solvent molecules may stabilize the localized singlet diradical
2b.
Intermolecular Reactivity. All our extensive efforts to trap
the localized singlet diradical 2b by external additives and, thus,
to explore the intermolecular reactivity of a localized singlet
diradical met with failure. Even at very high concentrations
(g0.5 M) of trapping agents (diylophiles, TEMPO, tributyltin
hydride) no new products except housane 4 were detected. Not
even in the time-resolved spectroscopic experiments, in which
the exceedingly long lifetime (e.g., in chloroform ca. 3.7 µs)
should facilitate the observation of subtle effects, did we obtain
clear-cut evidence for trapping by external additives. These
results suggest, if a lower limit of 0.60 µs is assumed for the
lifetime of the singlet diradical 2a (Table 2) and if a conservative
estimate of 10% for the detection limit of trapping products is
made, that the trapping rate constants are less than 4 × 105
M-1 s-1. Such low values contrast sharply with those for
delocalized (non-Kekule´) singlet diradicals, for which trapping
by diylophiles is efficient and occurs with rate constants up to
108 M-1 s-1, e.g., for acrylonitrile.7c,d
The estimated lower limits of the rate constants for intermo-
lecular reactions of the localized singlet diradical 2b, e.g., e1.4
× 106 M-1 s-1 for reaction with triplet molecular oxygen and
e4 × 105 M-1 s-1 for trapping by alkenes, dienes, TEMPO,
etc., characterize such singlet diradicals as exceptionally unre-
active. We take the sluggish intermolecular chemical reactivity
as experimental evidence that the electronic configuration of
the singlet 1,3-diphenyl-1,3-cyclopentanediyl structure I has
significant or even major contributions from the zwitterionic
forms II and III or the covalent mesomer IV. In fact, such
singlet diyls may well be viewed as planarized housanes with
a “central π bond” and an activation barrier of ca. 8 kcal mol-1
toward rehybridization to the bent housane.
Conclusions
The three important conclusions from the present work on
localized singlet 1,3-diphenyl-1,3-cyclopentanediyl diradicals are
the following: (1) simple 2,2-dialkoxy substitution is sufficient
to promote a singlet ground state and an extraordinary persis-
tence; (2) the intramolecular reactivity of localized singlet
diradicals may be affected by the medium, whereby nonpolar
solvents accelerate cyclization to the housane, while acidic
solvents accelerate cleavage to the allylic cation; and (3) the
localized singlet diradicals are reluctant to undergo intermo-
lecular trapping reactions, which may be indicative of significant
zwitterionic and covalent contributions. The experimental find-
ings, along with the observation of the strong long-wavelength
absorption of such localized singlet diradicals,21 challenge more
detailed theoretical investigations. The present state of the art
displays the lack of a comprehensive theoretical understanding
of localized singlet diradicals, which presents an obstacle in
the design of appropriate experimental test cases for further
investigation.
Strikingly, no dioxygen trapping products could be isolated
even when the photolysis of diazene 3 was carried out at ambient
temperature in a fluorocarbon solvent mixture to ensure a very
high molecular oxygen concentration of ca. 60 mM. By
employing the experimental lifetime of the singlet diradical in
C10F18/n-hexane/C6H5CF3 ) 5/1/1 (τ ) 0.74 µs) and assuming
a detection limit to 10%, this failure of diylophilic trapping
suggests an oxygen-scavenging rate constant below 3.0 × 106
M-1 s-1. The absence of trapping of the singlet diradical 2b in
chloroform under oxygen (1 atm) implies an even lower value
of 1.4 × 106 M-1 s-1, estimated on the basis that 5% trapping
would have been detectable in laser flash photolysis. While these
low values are reasonable in comparison to non-Kekule´ (delo-
calized) singlet diradicals, which display values of 104-107 M-1
Acknowledgment. We express our gratitude to the Deutsche
Forschungsgemeinschaft, the Volkswagen Foundation, the Fonds
der Chemischen Industrie, and the Swiss National Science
Foundation for generous financial support. This work was
supported in part by the grant-in-aid for Scientific Research on
Priority Areas “Molecular Physical Chemistry” from the Min-
istry of Education, Science, Sports, and Culture of Japan. M.A.
thanks the Alexander-von-Humboldt Foundation for a postdoc-
toral fellowship (1997-1998) and Prof. M. Nojima for his
encouragement.
s-1 7d
they are, nevertheless, surprising if one considers the
,
presence of benzylic radical sites in this intermediate.
It should be noted that dioxygen trapping was observed upon
photolysis of diazene 3 at elevated temperature (cf. Results).
Note Added in Proof: The generous gift (Dr. M. Tinkl, CIBA
SC) of a high-pressure cuvette (up to 10 atm) for transient
absorption measurements has enabled us to obtain a value of
8.4 × 105 M-1 s-1 for the oxygen trapping rate constant in
C10F18/n-hexane/C6H5CF3 ) 5/1/1 at ambient temperature. This
(24) (a) Adam, W.; Grabowski, S.; Wilson, R. M. Acc. Chem. Res. 1990,
23, 165. (b) Kita, F.; Adam, W.; Jordan, P.; Nau, W. M.; Wirz, J. J. Am.
Chem. Soc. 1999, 121, 9265.
(25) For solvent parameters on hydrogen-donating ability (R values) cf.
textbooks and: Daniel, D. C.; McHale, J. L. J. Phys. Chem. A 1997, 101,
3070.
(26) The supposition that the “lower concentration” of C-H bonds in
chloroform may be responsible (C-H and O-H bonds could serve to
promote this chemical reaction by electronic-to-vibrational energy transfer,
akin to the deactivation of singlet molecular oxygen, cf.: Gorman, A. A.;
Rodgers, M. A. J. In Handbook of Organic Photochemistry; Scaiano, J. C.,
Ed.; CRC Press: Boca Raton, FL, 1989; Vol. II, pp 229-247), is discarded
in view of the absence of a deuterium isotope effect and the “normal”
behavior (short lifetime) in carbon tetrachloride (Table 2).
(27) Oxygen trapping of related triplet diradicals has been previously
observed in the thermally induced ring opening of the parent 1,3-
diphenylbicyclo[2.1.0]pentane (Adam, W.; Platsch, H.; Wirz, J. J. Am.
Chem. Soc. 1989, 111, 6896); however, unlike for the latter housane, NMR
coalescence experiments could not be employed to evaluate the bond
dissociation of the housane 4 to its diradical 2b due to its unsymmetric
structure, i.e., anti and syn diastereomers.

2026 J. Am. Chem. Soc., Vol. 122, No. 9, 2000
Abe et al.
result is in line with our assumed lower limit. This finding has
once more encouraged the search for oxidation products in
preparative trapping experiments and, indeed, 2% of one
oxidation product, cis-1,2-dibenzoylcyclopentane (8), could be
isolated along with 91% housane 4 after photolysis of diazene
3 in CDCl3 under 4 atm of oxygen at ambient temperature.
Supporting Information Available: Experimental Section
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA992507J