Substituent effect on the energy barrier for σ-bond formation from π-single-bonded species, singlet 2,2-dialkoxycyclopentane-1,3-diyls

Article abstract of DOI:10.3762/bjoc.9.106

Background: Localized singlet diradicals are in general quite short-lived intermediates in processes involving homolytic bondcleavage and formation reactions. In the past decade, long-lived singlet diradicals have been reported in cyclic systems such as cyclobutane-1,3-diyls and cyclopentane-1,3-diyls. Experimental investigation of the chemistry of singlet diradicals has become possible. The present study explores the substituents and the effect of their substitution pattern at the C(1)-C(3) positions on the lifetime of singlet octahydropentalene-1,3-diyls to understand the role of the substituents on the reactivity of the localized singlet diradicals. Results: A series of singlet 2,2-dialkoxy-1,3-diaryloctahydropentalene-1,3-diyls DR were generated in the photochemical denitrogenation of the corresponding azoalkanes AZ. The ring-closed products CP, i.e., 3,3-dialkoxy-2,4-diphenyltricyclo[ 3.3.0.0 _2,4]octanes, were quantitatively obtained in the denitrogenation reaction. The first-order decay process (k = 1/τ) was observed for the fate of the singlet diradicals DR (λ_max ≈ 580-590 nm). The activation parameters, ΔH^? and ΔS ^?, for the ringclosing reaction (σ-bond formation process) were determined by the temperature-dependent change of the lifetime. The energy barrier was found to be largely dependent upon the substituents Ar and Ar'. The singlet diradical DRf (Ar = 3,5-dimethoxyphenyl, OCH₂Ar' = OCH₂(3,5-dimethoxyphenyl)) was the longest-lived, τ₂₉₃ = 5394 ± 59 ns, among the diradicals studied here. The lifetime of the parent diradical DR (Ar = Ph, OCH₂Ar' = OCH ₃) was 299 ± 2 ns at 293 K. Conclusion: The lifetimes of the singlet 1,3-diyls are found to be largely dependent on the substituent pattern of Ar and Ar' at the C(1)-C(3) positions. Both the enthalpy and entropy effect were found to play crucial roles in increasing the lifetime.

Full text of DOI:10.3762/bjoc.9.106

Substituent effect on the energy barrier for σ-bond
formation from π-single-bonded species,
singlet 2,2-dialkoxycyclopentane-1,3-diyls
Jianhuai Ye1, Yoshihisa Fujiwara2 and Manabu Abe*1,3
Full Research Paper
Open Access
Address:
Department of Chemistry, Graduate School of Science, Hiroshima
Beilstein J. Org. Chem. 2013, 9, 925–933.
1
doi:10.3762/bjoc.9.106
University, Japan, 2Department of Mathematical and Life Sciences,
Graduate School of Science, Hiroshima University, Japan, and
Received: 14 March 2013
Accepted: 22 April 2013
Published: 14 May 2013
3Institute for Molecular Science (IMS), Okazaki, Aichi 444-8787,
Japan
Email:
This article is part of the Thematic Series "New reactive intermediates in
organic chemistry".
Manabu Abe* - mabe@hiroshima-u.ac.jp
*
Corresponding author
Guest Editor: G. Bucher
Keywords:
© 2013 Ye et al; licensee Beilstein-Institut.
License and terms: see end of document.
laser flash photolysis; lifetime; singlet diradicals (biradicals);
substituent effect; π-single bond
Abstract
Background: Localized singlet diradicals are in general quite short-lived intermediates in processes involving homolytic bond-
cleavage and formation reactions. In the past decade, long-lived singlet diradicals have been reported in cyclic systems such as
cyclobutane-1,3-diyls and cyclopentane-1,3-diyls. Experimental investigation of the chemistry of singlet diradicals has become
possible. The present study explores the substituents and the effect of their substitution pattern at the C(1)–C(3) positions on the
lifetime of singlet octahydropentalene-1,3-diyls to understand the role of the substituents on the reactivity of the localized singlet
diradicals.
Results: A series of singlet 2,2-dialkoxy-1,3-diaryloctahydropentalene-1,3-diyls DR were generated in the photochemical denitro-
genation of the corresponding azoalkanes AZ. The ring-closed products CP, i.e., 3,3-dialkoxy-2,4-diphenyltri-
cyclo[3.3.0.02,4]octanes, were quantitatively obtained in the denitrogenation reaction. The first-order decay process (k = 1/τ) was
observed for the fate of the singlet diradicals DR (λmax ≈ 580–590 nm). The activation parameters, ΔH‡ and ΔS‡, for the ring-
closing reaction (σ-bond formation process) were determined by the temperature-dependent change of the lifetime. The energy
barrier was found to be largely dependent upon the substituents Ar and Ar’. The singlet diradical DRf (Ar = 3,5-dimethoxyphenyl,
OCH2Ar’ = OCH2(3,5-dimethoxyphenyl)) was the longest-lived, τ293 = 5394 ± 59 ns, among the diradicals studied here. The life-
time of the parent diradical DR (Ar = Ph, OCH2Ar’ = OCH3) was 299 ± 2 ns at 293 K.
Conclusion: The lifetimes of the singlet 1,3-diyls are found to be largely dependent on the substituent pattern of Ar and Ar’ at the
C(1)–C(3) positions. Both the enthalpy and entropy effect were found to play crucial roles in increasing the lifetime.
925

Beilstein J. Org. Chem. 2013, 9, 925–933.
Introduction
Localized singlet diradicals are key intermediates in processes τ293 = 292 ns (DRa: OR = OCH3, λmax = 574 nm) and 2146 ns
involving the homolytic bond-cleavage and formation reactions (DRb: OR = OC10H21, λmax = 572 nm) [26]. The study
(
Figure 1) [1,2]. The singlet diradicals are, in general, quite prompted us to further investigate the kinetic stabilization of the
short-lived species due to the very fast radical–radical coupling singlet diradical species.
reaction [3]. However, in the past decade, the singlet diradicals
have been observed or even isolated in cyclic systems such as In the present study, the effect of the bulky 3,5-dimethoxy-
cyclobutane-1,3-diyls [4-20] and cyclopentane-1,3-diyls [17,21- phenyl group substituent was investigated on the lifetime of the
2
6]. Detailed experimental study of singlet diradical chemistry localized singlet diradicals. Thus, the aryl substituent was intro-
is thus now possible using the long-lived localized singlet dirad- duced at C(1), C(2), or/and C(3) positions of the diradicals
icals.
DRd–g, and the substituent effects on the lifetime of the singlet
diradicals were compared with the lifetime of a phenyl-group-
substituted diradical DRc and the parent diradical DRa. The
laser flash photolysis technique was used for the generation of
DRc–g from the corresponding azoalkanes AZc–g (Scheme 2).
Results and Discussion
Synthesis of AZc–g and their steady-state photolyses. The
Figure 1: Localized singlet diradicals.
precursor azoalkanes AZc–g were prepared in an analogous
So far, we have studied singlet diradical chemistry using long- method to the synthesis of AZa,b [28] (Scheme 3). Pyrazoles
lived 2,2-dialkoxy-1,3-diphenyloctahydropentalene-1,3-diyls 3c–f were synthesized in the reaction of tetrazines 1 (Ar = Ph or
DR with a singlet ground state, which can be cleanly generated 3,5-dimethoxyphenyl) with 2,2-dialkoxy-5,5-dimethyl-Δ3-
by the photochemical denitrogenation of the corresponding 1,3,4-oxadiazolines 2 [29], which are the precursor of the
azoalkanes AZ (Scheme 1). The 2,2-electron-withdrawing- dialkoxycarbene (Scheme 3a). Azoalkanes AZc–f (λmax ≈
group-substituted singlet 1,3-diradicals are categorized as 360 nm with ε ≈ 100) were obtained by a cycloaddition reac-
Type-1 diradicals [1,27], which possess a π-single-bonding tion with cyclopentadienes, and followed by a hydrogenation
character (–π–, closed-shell character) between the two radical reaction [30,31]. The synthesis of AZg (Ar = 3,5-dimethoxy-
sites. The role of the alkoxy group (OR) on the lifetime (k = 1/τ) phenyl, Ar’ = H) was performed from the corresponding 1,3-
was investigated by combined studies of experiments and diketone 4 (Scheme 3b). 2,2-Dimethoxy-1,3-diarylpropane-1,3-
quantum chemical calculations [26,28]. The steric repulsion dione 5g was prepared from 1,3-dione 4 (R = 3,5-dimethoxy-
between the alkoxy group and the phenyl ring, which is indi- benzene) according to the method of Tiecco [32]. Pyrazole 3g
cated in the transition-state structure for the ring-closing reac- was then synthesized by the reaction with hydrazine hydrate.
tion (Scheme 1), was found to play an important role in deter- AZg was obtained by the Diels–Alder [4 + 2]-cycloaddition
mining the energy barrier of the ring-closing process, with cyclopentadiene and hydrogenation using PtO2 as a cata-
Scheme 1: Alkoxy group effect on the lifetime of π-single-bonded species DR.
926

Beilstein J. Org. Chem. 2013, 9, 925–933.
Scheme 2: Generation of singlet diradicals DRc–g and their reactivity in the photochemical denitrogenation of AZc–g.
Scheme 3: Synthesis of azoalkanes AZc–f and AZg.
lyst. The endo-configured structure of azoalkanes AZc–g was Detection of singlet diradicals DRc–g. The detection of singlet
determined by X-ray crystallographic analysis as well as by diradicals DRc–g was examined by the photochemical denitro-
NOE measurements.
genation of azoalkanes AZc–g in a glassy matrix of 2-methyl-
tetrahydrofurane (MTHF) at 80 K, [AZ] ≈ 4 × 10−3 mol/L, and
The steady-state photolyses of AZc–g in benzene solution were by the laser flash photolysis experiments of AZc–g at room
performed with a Xenon lamp (500 W) through a Pyrex filter temperature in benzene solution. First of all, the MTHF matrix
(
hν > 300 nm). The ring-closed compounds CPc–g were solution of AZ was irradiated with a 500 W Xenon lamp
quantitatively obtained in the denitrogenation reaction through a monochromator (λirr = 360 ± 10 nm). A strong
Scheme 2). The quantum yields of the denitrogenation of absorption band, which corresponds to DRc–g, was observed in
(
AZc–g were determined to be ≈0.90 by comparison with those the visible region at 80 K (570–590 nm, Table 1), as exempli-
reported for similar azoalkanes [33]. The quantitative forma- fied for the photoirradiation of AZe in Figure 2a. The strong
tion of CPc–g and the high quantum yield of the denitrogena- absorption bands are quite similar to those of singlet diradicals
tion process suggest the clean generation of DRc–g in the DRa,b with λmax = 574 nm and 572 nm [1,28], respectively.
photoirradiation reaction of AZc–g.
The assignment of the strong band to the singlet diradical is
927

Beilstein J. Org. Chem. 2013, 9, 925–933.
further supported by the following facts: (a) The absorptions 580 nm, e.g., Figure 2c for DRe. As shown in Table 1, the life-
obtained on photolysis in a MTHF glass were thermally persis- time of the singlet diradical was largely dependent on the
tent at 80 K and resembled that of the transient absorption substituents Ar and Ar’. The activation parameters, ΔH‡, ΔS‡,
spectra in solution (for example, DRe, λmax = 590 nm, Ea, logA, were determined from the Eyring plots and Arrhenius
Figure 2b); (b) the species were ESR-silent in the MTHF-matrix plots, which were obtained from the temperature-dependent
at 80 K; (c) the lifetime of the transient was insensitive to the change of the lifetime (Table 1). For comparison, the lifetime of
presence of molecular oxygen (decay trace at 580 nm, diradical DRa (Table 1, entry 1) was also measured under
Figure 2c); and (d) the activation parameters (Table 1) are similar conditions, and determined to be 299 ns at 293 K. The
similar to those for the decay process of DRa, in particular, the obtained lifetime was nearly the same as that obtained previ-
high (ca. 1012 s−1) pre-exponential Arrhenius factors (logA) are ously by us (292 ns) [28].
indicative of a spin-allowed reaction to the ring-closed prod-
ucts CPc–g [34].
The lifetime of DRc (Ar = Ar’ = Ph) was found to be 1305 ns at
93 K (Table 1, entry 2), which was ca. 4.5 times longer than
2
the parent DRa. On introduction of a 3,5-dimethoxyphenyl ring
at C(2) position of the 1,3-diradical, i.e., DRd (Ar = Ph, Ar’ =
3
,5-dimethoxyphenyl), a further increase of the lifetime at 293
K was observed to be 1933 ns (Table 1, entry 3). The result
clearly indicates that the steric bulkiness plays an important role
in increasing the energy barrier for the ring-closing reaction.
Indeed, the activation enthalpy (ΔH‡ = 36.6 kJ mol−1, Table 1,
entry 3) for DRd was found to be higher than that for DRa
(
ΔH‡ = 32.7 kJ mol−1, Table 1, entry 1). Interestingly, the effect
of an aryl group substituent at C(1) and C(3) positions on the
lifetime was found to be larger than that at C(2); compare the
lifetime of DRe (4001 ns, Ar = 3,5-dimethoxyphenyl, Ar’ = Ph,
Table 1, entry 4) with that of DRd (1933 ns, Table 1, entry 3).
When the 3,5-dimethoxyphenyl group was introduced at all of
the C(1), C(2), and C(3) positions, the lifetime of the diradical
DRf (ΔG‡ = 42.2 kJ mol−1, Table 1, entry 5) was dramatically
Figure 2: (a) Absorption spectrum of the singlet diradical DRe in a
MTHF matrix at 80 K; (b) transient absorption spectrum of AZe
measured immediately after the laser pulse (λexc = 355 nm); (c) tran-
sient decay trace at 580 nm and 20 °C.
Lifetime of singlet diradicals DRc–g and activation parame- increased to 5394 ns at 293 K. The activation entropy (ΔS‡ =
ters for the ring-closing reaction. The decay traces of the −27.8 and −19.4 J mol−1, Table 1, entries 4 and 5) also plays an
intermediary singlet diradicals DRc–g at 293–333 K were important role in increasing the lifetime of the singlet species. A
measured in a benzene solution by the laser flash photolysis much shorter lifetime was found for the diradical DRg (Ar =
technique (λexc = 355 nm). The lifetime (τ = 1/k) was deter- 3,5-dimethoxyphenyl, Ar’ = H). Thus, the introduction of the
mined by the first-order decay rate constants (k) of DRc–g at bulky substituents is needed at all positions C(1)–C(3) of the
Table 1: Lifetimes and activation parameters of singlet diradicals DR.
λmax
nmb (at 80 K)
ΔG‡293Kc
/kJ mol−1
ΔH‡c
/kJ mol−1
ΔS‡c
/J mol−1 K−1
Eac
/kJ mol−1
entry
DR
τ293K /nsa
log Ac
/
1
2
3
4
5
6
DRa
DRc
DRd
DRe
DRf
299
1305
1933
4001
5394
580
573
583
584
592
593
583
35.1 ± 0.7
39.1 ± 0.9
39.6 ± 0.6
40.9 ± 0.8
42.2 ± 0.7
36.7 ± 0.4
32.7 ± 0.2
33.5 ± 0.6
36.6 ± 0.1
33.3 ± 0.4
36.5 ± 0.3
33.0 ± 0.2
–8.1 ± 1.2
–17.9 ± 1.7
–10.1 ± 1.1
–27.8 ± 1.3
–19.4 ± 1.0
–12.9 ± 1.0
35.3 ± 0.2
36.2 ± 0.6
39.2 ± 0.1
35.9 ± 0.4
39.1 ± 0.3
35.6 ± 0.2
12.8 ± 0.1
12.3 ± 0.1
12.7 ± 0.1
11.8 ± 0.1
12.2 ± 0.1
12.2 ± 0.1
DRg
aExperimental errors are ca. 5%.
bIn MTHF at 80 K.
cActivation parameters were determined by measurements of the lifetime of the singlet diradicals at five different temperatures in a temperature range
from 293 to 333 K.
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Beilstein J. Org. Chem. 2013, 9, 925–933.
1
,3-diradicals to increase the lifetime. The repulsive steric inter- washing with hot methanol (3 × 10 mL) gave the tetrazine as a
actions of the Ar group with the Ar’ group are suggested to play red solid (3.07 g, 81%). mp: 248–250 °C; IR (neat, cm−1):
important roles in increasing the energy barrier of the reaction 3022, 2981, 2947, 1611, 1462, 1393, 1221, 1067, 944, 843,
from the diradicals to the ring-closed compounds CP. The 683; 1H NMR (400 MHz, CDCl3) δ 3.88 (s, 6H), 6.60 (t, J =
results clearly indicate that the substituent effect using the steri- 2.23 Hz, 1H), 7.17 (d, J = 2.23 Hz, 2H); 13C NMR (125 MHz,
cally bulky group is effective to prolong the lifetime of the CDCl3) δ 55.68 (q, OCH3), 105.48 (d, CH), 105.90 (d, CH),
singlet diradicals.
133.44 (s, C), 161.54 (s, COCH3); ESIMS (m/z): [M + Na]+
calcd for C18H20O4N4, 379.13768; found, 379.13776.
Conclusion
We have succeeded in generating long-lived singlet diradical 5,5-Dimethyl-2,2-bis(3,5-dimethoxybenzyl)-Δ3-1,3,4-oxa-
species DRc–g, τ293 = 580–5394 ns, which were much longer- diazoline (2f). A solution of (3,5-dimethoxybenzyloxy-carbon-
lived species than DRa (τ293 = 299 ns). It was found that the yl)hydrazone of acetone (1.30 g, 4.87 mmol) in CH2Cl2 (5 mL)
lifetimes are largely dependent on the substituent pattern of Ar was added dropwise to a stirred solution of Pb(OAc)4 (2.59 g,
and Ar’ at the C(1)–C(3) positions of the 1,3-diyls. Thus, both 5.84 mmol) under nitrogen. The reaction mixture was stirred in
the enthalpy and entropy effect were found to play crucial roles an ice bath for 2 h, and then at room temperature for 24 h. After
in increasing the lifetime.
the stirring, the solid was filtered over Celite and the organic
layer was washed with 10% aq NaHCO3. The mixture was
filtered again until no precipitate was deposited. The organic
Experimental
All reagents were purchased from commercial sources and were phase was concentrated under reduced pressure. The corres-
used without additional purification, unless otherwise ponding 3,5-dimethoxybenzyl alcohol (2.46 g, 14.6 mmol) and
mentioned. Azoalkanes AZc–g were prepared according to the TFA (0.04 mL, 0.49 mmol) were then added to the organic mix-
methods described previously (Scheme 3) and were isolated by ture. The solution was heated to 40 °C and stirred for 24 h
silica gel column chromatography and GPC column chromatog- before KOH pellets were added, and stirring was continued for
raphy. 1H and 13C NMR spectra were reported in parts per another 3 h. After extracting with CH2Cl2, washing with brine,
million (δ) by using CDCl3 or C6D6 as internal standards. and drying by MgSO4, the organic layer was concentrated under
Assignments of 13C NMR were carried out by DEPT measure- reduced pressure. The product was purified by column chroma-
ments. IR spectra were recorded with a FTIR spectrometer. tography (eluent: EtOAc/hexane = 30/70, Rf = 0.17) to give the
UV–vis spectra were taken by a JASCO V-630 spectropho- product as a yellow liquid (0.48 g, 23%). IR (neat, cm−1): 3006,
tometer. Mass-spectrometric data were measured by a Mass 2959, 2843, 1750, 1622, 1480, 1386, 1236, 1076, 925, 846,
Spectrometric Thermo Fisher Scientific LTQ Orbitrap XL, 687; 1H NMR (500 MHz, CDCl3) δ 1.49 (s, 6H), 3.67 (s, 12H),
performed by the Natural Science Center for Basic Research 4.66 (q, J = 11.8 Hz, 36.78 Hz, 4H), 6.29 (t, J = 2.21 Hz, 2H),
and Development (NBARD), Hiroshima University.
6.41 (d, J = 2.21 Hz, 4H); 13C NMR (125 MHz, CDCl3) δ 24.12
q, CH3), 55.29 (q, OCH3), 66.72 (t, CH2), 99.94 (d, CH, Ar
ring), 105.50 (d, CH, Ar ring), 119.63 (s, C(CH3)2), 136.71 (s,
(
Preparation of diazenes AZc–g
3
,6-Diaryl-1,2,4,5-tetrazine 1. 3,6-Diphenyl-1,2,4,5-tetrazine C), 138.97 (s, C(OCH2Ar)2), 160.83 (s, COCH3); ESIMS (m/z):
was purchased and directly used. The preparation of 3,6-(3,5-di- [M + Na]+ calcd for C22H28O7N2Na, 455.17887; found,
methoxyphenyl)-1,2,4,5-tetrazine (Ar = 3,5-dimethoxyphenyl) 455.17899.
is as follows: In a 50 mL round-bottom flask, benzonitrile
(
3.7 g, 22.7 mmol) was dissolved in 10 mL of absolute ethanol. 1,3-Bis(3,5-dimethoxyphenyl)propane-1,3-dione (4). 3,5-
Hydrazine (3.6 mL, 90 mmol) and sulfur (0.43 g, 13.5 mmol) dimethoxyacetophenone (2.1 g, 11.7 mmol), 3,5-dimethoxy-
were quickly added, and the solution was stirred at room benzoic acid (2.75 g, 14.04 mmol), and NaH (0.94 g,
temperature for 1 h and then heated under reflux for 3 h. The 23.4 mmol) was dissolved in THF (20 mL) under N2 atmos-
remaining orange cake was solidified further in an ice bath. The phere in a 50 mL flask. The mixture was heated under reflux
solid was vacuum filtered, and washed with cold ethanol (75 °C) for 14 h under a N2 atmosphere, then cooled down to
(
3 × 10 mL) giving the crude dihydrotetrazine. The crude room temperature. The mixture was slowly added to cold HCl.
orange solid was then placed in a 50 mL beaker and dissolved The organic layer was extracted with ether, washed with brine,
in 20% acetic acid (15 mL) and 10 mL ether at room tempera- and dried with anhydrous MgSO4. The solvent was removed by
ture with stirring. An aqueous solution of 10% NaNO2 (20 mL) vacuum evaporator. The dry solid was then recrystallized in
was added to the solution in an ice bath. The immediate purple methanol to give the compound as a yellow crystal (2.77 g,
cloudiness signifies the completion of the reaction, as well as 69%). mp: 132 °C; IR (neat, cm−1): 3140, 3000, 2943, 1562,
the evolution of brown nitric oxide gas. Vacuum filtration and 1466, 1351, 1298, 1158, 1053, 842, 668; 1H NMR (500 MHz,
929

Beilstein J. Org. Chem. 2013, 9, 925–933.
CDCl3) δ 3.78 (s, 12H), 6.56 (t, J = 2.28 Hz, 2H), 6.65 (d, J = (125 MHz, C6D6) δ 55.76 (q, OCH3), 67.11 (t, CH2), 100.99 (d,
2
9
1
.28 Hz, 4H); 13C NMR (125 MHz, CDCl3) δ 55.56 (q, OCH3), CH, aryl), 106.15 (d, CH, aryl), 116.69 (s, C), 128.35 (d, CH,
3.56 (t, CH2), 104.62 (d, CH), 105.05 (d, CH), 137.52 (s, C), phenyl), 128.77 (s, C), 129.24 (d, CH, phenyl), 132.30 (d, CH,
60.92 (s, COCH3), 185.47 (s, OC=O); ESIMS (m/z): [M + phenyl), 138.40 (s, C), 161.30 (s, C), 167.29 (s, C); ESIMS
Na]+ calcd for C19H20O6Na, 367.11521; found, 367.11456.
(m/z): [M + Na]+ calcd for C33H32O6N2Na, 575.21526; found,
75.21387; Rf = 0.27 (EtOAc/hexane = 30/70).
5
1
,3-Bis(3,5-dimethoxyphenyl)-2,2-dimethoxypropane-1,3-
dione (5). 1,3-Dione 4 (2.76 g, 8 mmol) and diphenyl dise- 4,4-Dibenzyloxy-3,5-bis(3,5-dimethoxyphenyl)pyrazole (3e).
lenide (1.25 g, 4 mmol) were dissolved in methanol (50 mL), Yellow powder (from MeOH), mp: 183–184 °C; IR (neat,
and ammonium persulfate (3.65 g, 16 mmol) was added to the cm−1): 3025, 2952, 2847, 1605, 1551, 1456, 1426, 1371, 1160,
mixture. The solution was heated under reflux for 4 h with stir- 1117, 849, 670; 1H NMR (500 MHz, C6D6) δ 3.33 (s, 12H),
ring under nitrogen. Then the mixture was cooled to room 4.31 (s, 2H), 6.78 (t, J = 2.28 Hz, 2H), 6.93–7.01 (m, 10H),
temperature, and slowly added to ice water. The organic com- 8.02 (t, J = 2.28 Hz, 4H); 13C NMR (125 MHz, C6D6) δ 55.06
pound was extracted by chloroform and purified by silica-gel (q, OCH3), 67.02 (t, CH2), 105.54 (d, CH, aryl), 106.52 (d, CH,
column chromatography (eluent: EtOAc/hexane = 30/70, aryl), 116.22 (s, C), 128.59 (d, CH, phenyl), 128.45 (d, CH,
Rf = 0.30) to give the product as a yellow liquid (2.9 g, 90%). phenyl), 128.19 (d, CH, phenyl), 130.47 (s, C, aryl), 136.24 (s,
1
H NMR (500 MHz, CDCl3) δ 3.41 (s, 6H), 3.77 (s, 12H), 6.59 C, phenyl), 161.81 (s, COCH3), 167.53 (s, C); ESIMS (m/z):
t, J = 2.44 Hz, 2H), 7.29 (d, J = 2.44 Hz, 4H); 13C NMR (125 [M + H]+ calcd for C33H33N6O2, 553.23331; found, 553.23267;
MHz, CDCl3) δ 50.97 (q, C(OCH3)2), 55.53 (q, OCH3), 103.89 Rf = 0.13 (EtOAc/hexane = 20/80).
s, C), 107.22 (d, CH), 107.34 (d, CH), 135.50 (s, C), 160.66 (s,
COCH3), 192.47 (s, C=O); IR (neat, cm−1): 3027, 2965, 2845, 4,4-Bis(3,5-dimethoxybenzyloxy)-3,5-bis(3,5-dimethoxy-
691, 1605, 1466, 1429, 1321, 1162, 1070, 1036, 863, 671; phenyl)pyrazole (3f). IR (neat, cm−1): 3010, 2942, 1605, 1552,
ESIMS (m/z): [M + Na]+ calcd for C21H24O8Na, 427.13634; 1441, 1371, 1140, 1120, 849; 1H NMR (500 MHz, CDCl3) δ
found, 427.13602. 3.24 (s, 12H), 3.34 (s, 12H), 4.36 (s, 6H), 6.38 (d, J = 2.36 Hz,
H), 6.43 (t, J = 2.36 Hz, 2H), 6.70 (t, J = 2.28 Hz, 2H), 8.04
(d, J = 2.28 Hz, 4H); 13C NMR (125 MHz, C6D6) δ 54.74 (q,
(
(
1
4
4
,4-diaryloxy-3,5-diarylpyrazole (3c–f)
General Procedure. Oxadiazoline (1 mmol) was dissolved in OCH3), 55.03 (q, OCH3), 67.19 (t, CH2), 101.16 (d, CH),
benzene (2 mL) in a sealed tube. The mixture was stirred with 105.71 (d, CH), 106.08 (d, CH), 106.28 (d, CH), 116.36 (s, C),
tetrazine (1.10 mmol) in a sealed tube for 24 h at 120 °C under 130.46 (s, C), 138.47 (s, C), 161.33 (s, COCH3), 161.77 (s,
nitrogen. After filtration, the crude was purified by column COCH3), 167.58 (s, C); ESIMS (m/z): [M + Na]+ calcd for
chromatograph (in ca. 40% yield).
C37H40O10N2Na, 695.25752; found, 695.25775; Rf = 0.17
EtOAc/hexane = 30/70).
(
4
,4-Dibenzyloxy-3,5-diphenylpyrazole (3c). Yellow powder
(
from MeOH), mp: 150–151 °C; IR (neat, cm−1): 3070, 3066, 4,4-Dimethoxy-3,5-bis(3,5-dimethoxyphenyl)pyrazole (3g).
2
9
7
8
948, 2881, 1585, 1557, 1498, 1447, 1391, 1382, 1111, 970, To a solution of 1,3-bis(3,5-dimethoxyphenyl)-2,2-
16, 856, 694; 1H NMR (500 MHz, CDCl3) δ 4.21 (s, 4H), dimethoxypropane-1,3-dione (2.8 g, 6.92 mmol) in chloroform
.04–7.06 (m, 4H), 7.19–7.20 (m, 6H), 7.50–7.57 (m, 6H), (10 mL) was added dropwise NH2NH2·H2O (0.40 mL,
.38–8.39 (m, 4 H); 13C NMR (125 MHz, CDCl3) δ 66.73 (t, 8.30 mmol). The mixture was heated under reflux and kept
CH2), 115.96 (s, C), 127.74 (d, CH, benzyloxy), 127.95 (d, CH, under stirring for 6 h. The reaction was quenched with HCl. A
benzyloxy), 128.13 (d, CH, phenyl), 128.18 (d, CH, benzyloxy), solution of 10% NaHCO3 was added to the mixture. After
1
28.26 (d, CH, phenyl), 129.03 (d, CH, phenyl), 132.47 (s, C, extraction with chloroform, the organic phase was washed with
phenyl), 135.45 (s, C, benzyloxy), 167.09 (s, C); HRMS–EI brine, dried with Na2SO4, concentrated and then purified by
calcd for C29H24N2O2, 432.1838; found, 432.1842; Rf = 0.40 column chromatography to give 3g in 89.6% yield. mp:
(
EtOAc/hexane = 30/70).
179–180 °C; 1H NMR (500 MHz, CDCl3) δ 3.04 (s, 6H), 3.83
s, 12H), 6.62 (t, J = 2.28 Hz, 2H), 7.40 (d, J = 2.28 Hz, 4H);
,4-Bis(3,5-dimethoxybenzyloxy)-3,5-diphenylpyrazole (3d). 13C NMR (125 MHz, CDCl3) δ 51.93 (q, CH3), 55.52 (q,
(
4
Yellow powder (from MeOH), mp: 140–141 °C; IR (neat, OCH3), 105.21 (d, CH,), 105.27 (d, CH), 116.91 (s, C), 129.23
cm−1): 3024, 2951, 2845, 1603, 1475, 1456, 1388, 1160, 1118, (s, C), 161.00 (s, COCH3), 166.84 (s, C); IR (neat, cm−1): 3012,
1
4
102, 851; 1H NMR (500 MHz, C6D6) δ 3.19 (s, 12H), 4.21 (s, 2951, 1597, 1548, 1427, 1375, 1158, 1125, 1062, 980, 844;
H), 6.28 (d, J = 2.36 Hz, 4H), 6.42 (t, J = 2.36 Hz, 2H), 7.07 ESIMS (m/z): calcd for C21H25N6O2, 401.17071; found,
(
d, J = 8.15 Hz, 6H, phenyl), 8.68 (m, 4H, phenyl); 13C NMR 401.17041; Rf = 0.10 (EtOAc/hexane = 20/80).
930

Beilstein J. Org. Chem. 2013, 9, 925–933.
endo-2,3-Diazo-10,10-diaryloxy-1,4-
cm−1): 3014, 2972, 1600, 1461, 1450, 1357, 1157, 1069, 973,
856; UV (MTHF) λmax 365 (ε 184.2); 1H NMR (500 MHz,
diaryltricyclo[5.2.1.05,9]dec-2-ene (AZc–g)
General procedure. To a solution of cyclopentadiene (1 mL) C6D6) δ 1.20–1.83 (m, 6H), 3.41 (s, 12H), 3.57 (t, J = 4.47 Hz,
and pyrazole (2 mmol) in CH2Cl2 (2 mL) was added dropwise 2H), 4.31 (s, 2H), 4.46 (s, 2H), 6.68 (t, J = 2.28 Hz, 2H), 6.95
trifluoroacetic acid (1 mmol) in an ice bath under nitrogen. The (d, J = 2.28 Hz, 4H), 7.02–7.57 (m, 10H, overlapping with
reaction was traced by TLC analysis. After stirring for about C6D6); 13C NMR (125 MHz, C6D6) δ 26.13 (t, CH2, cyclopen-
1
5 min, the reaction was quenched with 10% aq NaHCO3 until tane), 28.19 (t, CH2, cyclopentane), 49.50 (d, CH, cyclopen-
the pH of the solution reached 8. After washing with water and tane), 54.91 (q, OCH3), 66.04 (t, COCH2), 66.32 (t, COCH2),
brine, the organic phase was dried with MgSO4, then filtered 94.96 (s, C), 101.03 (d, CH), 107.11 (d, CH), 119.74 (s, C),
and concentrated. The [4 + 2] cycloadduct was dissolved in 127.00 (d, CH), 127.16 (d, CH), 127.38 (d, CH), 127.72 (d,
benzene (2 mL), and 5 mg of PtO2 was added as a catalyst. The CH), 128.54 (d, CH), 128.60 (d, CH), 138.19 (s, C), 138.24 (s,
mixture was stirred under a hydrogen atmosphere for 24 h at C), 139.58 (s, C), 161.62 (s, C); ESIMS (m/z): [M + Na]+ calcd
room temperature. After stirring, the catalyst was removed by for C38H40O6N2Na, 643.27786; found, 643.27802.
filtration over Celite, and the solvent was evaporated under
reduced pressure. The product was purified by column chro- endo-2,3-Diazo-1,4-bis(3,5-dimethoxybenzyloxy)-10,10-
matograph to give the product as colorless liquid (ca. 60%). The bis(3,5-dimethoxyphenoxy)tricyclo[5.2.1.05,9]dec-2-ene
endo configuration was determined by NOE measurements.
(AZf). IR (neat, cm−1): 3009, 2965, 2842, 1606, 1467, 1430,
352, 1160, 1070, 1057, 943, 840; UV (MTHF) λmax 365 (ε
1
endo-2,3-Diazo-10,10-dibenzyloxy-1,4-diphenyltri- 249.6); 1H NMR (500 MHz, C6D6) δ 1.20–1.80 (m, 6H), 3.23
cyclo[5.2.1.05,9]dec-2-ene (AZc). IR (neat, cm−1): 3037, 2968, (s, 6H), 3.32 (s, 6H), 3.44 (s, 12H), 3.60 (t, J = 5.37 Hz, 2H),
2
886, 1739, 1607, 1498, 1456, 1387, 1139, 1085, 1029, 702; 4.38 (s, 2H), 4.56 (s, 2H), 6.33 (d, J = 2.29 Hz, 2H), 6.40 (t, J =
UV (MTHF) λmax 365 (ε 106.7); 1H NMR (500 MHz, C6D6) δ 2.29 Hz, 1H), 6.46 (t, J = 2.29 Hz, 1H) 6.54 (d, J = 2.29 Hz,
.20–1.75 (m, 6H), 3.69 (t, J = 5.13 Hz, 2H), 4.15 (s, 2H), 4.29 2H), 6.64 (t, J = 2.28 Hz, 2H), 7.62 (d, J = 2.28 Hz, 4H); 13C
1
(
(
s, 2H), 6.90–8.19 (m, 20H, overlapping with C6H6); 13C NMR NMR (125 MHz, C6D6) δ 26.11 (t, CH2, cyclopentane), 28.16
125 MHz, C6D6) δ 25.96 (t, CH2, cyclopentane), 28.16 (t, (t, CH2, cyclopentane), 49.52 (d, CH, cyclopentane), 54.69 (q,
CH2, cyclopentane), 49.27 (d, CH, cyclopentane), 66.16 (t, OCH3), 54.81 (q, OCH3), 54.91 (2×q, OCH3), 65.93 (t,
COCH2), 66.34 (t, OCH2), 94.83 (s, C), 119.57 (s, C), 126.66 COCH2), 66.32 (t, COCH2), 95.11 (s, C), 100.08 (d, CH),
(
d, CH), 126.87 (d, CH), 127.33 (d, CH), 128.61 (d, CH), 100.32 (d, CH), 100.92 (2×d, CH), 104.62 (s, C), 104.70 (s, C),
1
1
5
28.71 (d, CH), 129.01 (d, CH), 137.22 (s, C), 138.13 (s, C), 107.23 (2×s, C), 119.91 (s, C), 139.46 (2×s, C), 140.66 (s, C),
38.19 (s, C); HRMS–EI (m/z): calcd for C34H32O2N2, 140.77 (s, C), 161.33 (2×s, C), 161.58 (s, C), 161.62 (s, C);
00.6301; found, 500.2462. Rf = 0.57 (EtOAc/hexane = 20/80). ESIMS (m/z): [M + Na]+ calcd for C42H48O10N2Na,
7
63.32012; found,763.32043.
endo-2,3-Diazo-10,10-bis(3,5-dimethoxybenzyloxy)-1,4-
diphenyltricyclo[5.2.1.05,9]dec-2-ene (AZd). IR (neat, cm−1): endo-2,3-Diazo-1,4-bis(3,5-dimethoxybenzyloxy)-10,10-
3
8
022, 2966, 2844, 1751, 1603, 1473, 1326, 1162, 1072, 930, dimethoxytricyclo[5.2.1.05,9]dec-2-ene (AZg). IR (neat,
44, 703; UV (MTHF) λmax 364 (ε 175.1); 1H NMR (500 MHz, cm−1): 2973, 2846, 1602, 1464, 1359, 1158, 1088, 1022, 942,
CDCl3) δ 1.25–1.66 (m, 6H), 3.68 (s, 6H), 3.70 (t, J = 5.33 Hz, 848; UV (MTHF) λmax 364 (ε 169.9); 1H NMR (500 MHz,
H), 3.75 (s, 6H), 3.89 (s, 2H), 4.12 (s, 2H), 6.07 (d, J = 2.36 C6D6) δ 0.9–1.75 (m, 6H), 2.69 (s, 3H), 2.80 (s, 3H), 3.36 (t, J
Hz, 2H), 6.24 (t, J = 2.36 Hz, 1H), 6.30 (d, J = 2.36 Hz, 2H), = 5.49 Hz, 2H), 3.42 (s, 12H), 6.64 (t, J = 2.28 Hz, 2H), 7.48 (d,
.36 (t, J = 2.36 Hz, 1H), 7.40–8.03 (m, 10H); 13C NMR (125 J = 2.28 Hz, 4H); 13C NMR (125 MHz, C6D6) δ 26.08 (t, CH2,
2
6
MHz, C6D6) δ 26.21 (t, CH2, cyclopentane), 28.34 (t, CH2, cyclopentane), 28.16 (t, CH2, cyclopentane), 49.29 (d, CH,
cyclopentane), 49.42 (d, CH, cyclopentane), 55.69 (q, OCH3), cyclopentane), 51.46 (q, OCH3), 51.75 (q, OCH3), 51.92 (2×q,
5
5.74 (q, OCH3), 66.17 (t, COCH2), 95.15 (s, C), 99.80 (d, OCH3), 94.65 (s, C), 100.30 (d, CH), 107.33 (d, CH), 119.76 (s,
CH), 100.00 (d, CH), 104.26 (d, CH), 105.04 (d, CH), 119.54 C), 139.86 (s, C), 161.52 (s, C); ESIMS (m/z): [M + Na]+ calcd
(
(
s, C), 128.55 (d, CH), 128.92 (d, CH), 128.96 (d, CH), 136.67 for C26H32O6N2Na, 491.21526; found, 491.21466.
s, C), 140.41 (s, C), 140.67 (s, C), 161.00 (s, C) , 161.28 (s, C);
ESIMS (m/z): [M + H]+ calcd for C38H41O6N2, 621.29591; General procedure for photolysis. A sample (30.0 mg) of the
found, 621.29449; Rf = 0.27 (EtOAc/hexane = 20/80).
diazenes AZ was dissolved in 1.0 mL of C6D6. The photolysis
was performed with a 500 W Xenon-lamp through a Pyrex filter
endo-2,3-diazo-1,4-bis(3’,5’-dimethoxybenzyloxy)-10,10- (hν > 300 nm) at room temperature (ca. 20 °C). The photolysate
dibenzyloxytricyclo[5.2.1.05,9]dec-2-ene (AZe). IR (neat, was directly analyzed by NMR spectroscopy (1H: 500 MHz,
931

Beilstein J. Org. Chem. 2013, 9, 925–933.
1
3C: 125 MHz), which indicated the quantitative formation of 6H), 3.35 (s, 12H), 3.38 (s, 6H), 4.51 (s, 2H), 5.01 (s, 2H), 6.34
the housanes CP. The housanes CPc–g were isolated by using (d, J = 2.36 Hz, 2H), 6.41 (t, J = 2.36 Hz, 1H), 6.47 (t, J = 2.28
silica-gel column chromatography. The spectroscopic data are Hz, 2H), 6.54 (t, J = 2.36 Hz, 1H), 6.80 (d, J = 2.36 Hz, 2H),
as follows:
6.87 (d, J = 2.28 Hz, 4H); 13C NMR (125 MHz, C6D6) δ 25.46
t, CH2, cyclopentane), 28.44 (t, CH2, cyclopentane), 41.80 (d,
,3-Dibenzyloxy-2,4-diphenyltricyclo[3.3.0.02,4]octane CH, cyclopentane), 48.64 (s, C), 54.67 (q, OCH3), 54.78 (q,
(
3
(
CPc). 1H NMR (500 MHz, C6D6) δ 1.41–1.93 (m, 6H), 3.19 OCH3), 54.89 (2×q, OCH3), 67.29 (t, COCH2), 69.76 (t,
d, J = 6.34 Hz, 2H), 4.31 (s, 2H), 4.92 (s, 2H), 6.96–7.45 (m, COCH2), 98.41 (s, C), 99.18 (2×d, CH), 100.12 (d, CH), 100.32
(
2
2
4
6
1
0H, overlapping with C6D6); 13C NMR (125 MHz, C6D6) δ (d, CH), 104.93 (d, CH), 105.96 (d, CH), 108.89 (2×d, CH),
5.28 (t, CH2, cyclopentane), 28.38 (t, CH2, cyclopentane), 137.30 (2×s, C), 140.88 (s, C), 141.26 (s, C), 160.96 (2×s,
1.73 (d, CH, cyclopentane), 48.05 (s, C), 67.16 (t, COCH2), COCH3), 161.187 (s, COCH3), 161.59 (s, COCH3); ESIMS
9.66 (t, OCH2), 98.43 (s, C), 126.57 (d, CH), 127.26 (d, CH), (m/z): [M + Na]+ calcd for C42H48O10Na, 735.31397;
27.40 (d, CH), 127.92 (d, CH), 128.12 (d, CH), 128.35 (d, found,735.31415.
CH), 128.46 (d, CH), 128.68 (d, CH), 130.54 (d, CH), 135.25
s, C, phenyl), 138.59 (s, C, benzyloxy), 138.90 (s, C, benzyl- 3,3-Dimethoxy-2,4-bis(3’,5’-dimethoxyphenyl)tri-
oxy); HRMS–EI (m/z): calcd for C34H32O2, 472.2402; found, cyclo[3.3.0.02,4]octane (CPg). 1H NMR (500 MHz, C6D6) δ
(
4
72.2424.
1.43–2.03 (m, 6H), 2.94 (s, 3H), 2.96 (d, J = 6.47 Hz, 2H), 3.37
s, 12H), 3.48 (s, 3H), 6.51 (t, J = 2.28 Hz, 2H), 6.79 (d, J =
(
3
0
,3-Bis(3,5-dimethoxybenzyloxy)-2,4-diphenyltricyclo[3.3.0. 2.28 Hz, 4H); 13C NMR (125 MHz, C6D6) δ 25.49 (t, CH2,
2,4]octane (CPd). 1H NMR (500 MHz, CDCl3) δ 1.40–1.92 cyclopentane), 28.45 (t, CH2, cyclopentane), 41.83 (d, CH,
(
m, 6H), 3.12 (d, J = 6.29 Hz, 2H), 3.22 (s, 6H), 3.37 (s, 6H), cyclopentane), 48.13 (s, C), 52.37 (q, OCH3), 54.01 (q, OCH3),
4
2
2
.35 (s, 2H), 4.99 (s, 2H), 6.31 (d, J = 2.36 Hz, 2H), 6.41 (t, J = 54.85 (2×q, OCH3), 98.61 (s, C), 99.05 (d, CH), 108.88 (d,
.36 Hz, 1H), 6.56 (d, J = 2.36 Hz, 1H), 6.81 (d, J = 2.36 Hz, CH), 137.59 (s, COCH3), 161.07 (s, C); ESIMS (m/z): [M +
H), 7.02–7.46 (m, 10H, overlapping with C6D6); 13C NMR Na]+ calcd for C26H32O6Na, 463.20911; found, 463.20844.
(
125 MHz, C6D6) δ 25.22 (t, CH2, cyclopentane), 28.34 (t,
CH2, cyclopentane), 41.68 (d, CH, cyclopentane), 48.18 (s, C),
4.68 (q, OCH3), 54.89 (q, OCH3), 67.07 (t, COCH2), 69.78 (t,
COCH2), 98.46 (s, C), 100.04 (d, CH), 100.06 (d, CH), 104.91
Supporting Information
5
Supporting Information File 1
(
(
d, CH), 105.94 (d, CH), 126.48 (2×d, CH, phenyl), 128.04
2×d, CH, phenyl), 130.51 (2×d, CH, phenyl), 135.22 (2×s, C,
NMR spectra of compounds 1–5, AZc–g, and CPc–g.
[http://www.beilstein-journals.org/bjoc/content/
phenyl), 140.90 (s, C), 141.30 (s, C), 161.19 (s, COCH3),
61.64 (s, COCH); ESIMS (m/z): [M + Na]+ calcd for
C38H41O6Na, 615.27171; found, 615.27130.
supplementary/1860-5397-9-106-S1.pdf]
1
Acknowledgements
3
,3-Bisbenzyloxy-2,4-bis(3,5-dimethoxyphenyl)tri- NMR and MS measurements were performed at N-BARD,
cyclo[3.3.0.02,4]octane (CPe). 1H NMR (500 MHz, C6D6) δ Hiroshima University. This work was supported by a Grant-in-
1
4
2
.40–2.02 (m, 6H), 3.05 (d, J = 6.40 Hz, 2H), 3.29 (s, 12H), Aid for Science Research on Innovative Areas “Stimuli-respon-
.41 (s, 2H), 4.90 (s, 2H), 6.43 (t, J = 2.28 Hz, 2H), 6.80 (d, J = sive Chemical Species” (No. 24109008), “pi-Space” (No.
.28 Hz, 4H), 6.86–7.34 (m, 10H, overlapping with C6D6); 13C 21108516), and No. 19350021 from the Ministry of Education,
NMR (125 MHz, C6D6) δ 25.44 (t, CH2, cyclopentane), 28.43 Culture, Sports, Science and Technology, Japan, and by the
t, CH2, cyclopentane), 41.82 (d, CH, cyclopentane), 48.47 (s, Tokuyama Science Foundation.
C), 54.78 (q, OCH3), 67.44 (t, COCH2), 69.51 (t, COCH2),
(
9
1
8.39 (s, C), 99.22 (d, CH), 108.88 (d, CH), 127.19 (d, CH), References
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2
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