Kinetic stabilization and reactivity of π single-bonded species: Effect of the alkoxy group on the lifetime of singlet 2,2-dialkoxy-1,3- diphenyloctahydropentalene-1,3-diyls

Article abstract of DOI:10.1002/chem.201300038

Kinetic stabilization and reactivity of π single-bonded species have been investigated in detail by generating a series of singlet 2,2-dialkoxy-1,3-diphenyloctahydropentalene-1,3-diyls (DRs). The lifetime at 293 K in benzene was found to increase when the carbon chain length of the alkoxy groups was increased; 292 ns (DRb; OR=OR′=OCH₃) <880 ns (DRc; OR=OR′=OC₂H₅) <1899 ns (DRd; OR=OR′=OC₃H₇) ≈2292 ns (DRe; OR=OR′=OC ₆H₁₃) ≈2146 ns (DRf; OR=OR′=OC₁₀H ₂₁). DRh (OR=OC₃H₇, OR′=OCH₃; 935 ns) with the mixed-acetal moiety is a longer-lived species than another diastereomer DRg (OR=OCH₃, OR′=OC₃H₇; 516 ns). Activation parameters determined for the first-order decay process reveal that the enthalpy factor plays a crucial role in determining the energy barrier of the ring-closing reaction, that is, from the π-bonding to the σ-bonding compounds. Computational studies using density functional theory provided more insight into the structures of the singlet species with π single-bonded character and the transition states for the ring-closing reaction, thereby clarifying the role of the alkoxy group on the lifetime and the stereoselectivity of the ring-closing reaction. Copyright

Full text of DOI:10.1002/chem.201300038

FULL PAPER
DOI: 10.1002/chem.201300038
Kinetic Stabilization and Reactivity of p Single-Bonded Species:
Effect of the Alkoxy Group on the Lifetime of
Singlet 2,2-Dialkoxy-1,3-diphenyloctahydropentalene-1,3-diyls
Tomoyuki Nakagaki,[a] Tomoko Sakai,[a] Tsutomu Mizuta,[a] Yoshihisa Fujiwara,[b] and
Manabu Abe*[a, c]
This paper is dedicated to Professor Waldemar Adam on the occasion of his 75th birthday and Professor Herbert Mayr on the
occasion of his 65th birthday
Abstract: Kinetic stabilization and re-
activity of single-bonded species
OR=OR’=OC10H21). DRh (OR=
OC3H7, OR’=OCH3; 935 ns) with the
mixed-acetal moiety is a longer-lived
species than another diastereomer
crucial role in determining the energy
barrier of the ring-closing reaction, that
is, from the p-bonding to the s-bonding
compounds. Computational studies
using density functional theory provid-
ed more insight into the structures of
p
have been investigated in detail by gen-
erating a series of singlet 2,2-dialkoxy-
1,3-diphenyloctahydropentalene-1,3-
diyls (DRs). The lifetime at 293 K in
benzene was found to increase when
the carbon chain length of the alkoxy
groups was increased; 292 ns (DRb;
OR=OR’=OCH3) <880 ns (DRc;
OR=OR’=OC2H5) <1899 ns (DRd;
OR=OR’=OC3H7) ꢀ2292 ns (DRe;
OR=OR’=OC6H13) ꢀ2146 ns (DRf;
DRg
(OR=OCH3,
OR’=OC3H7;
516 ns). Activation parameters deter-
mined for the first-order decay process
reveal that the enthalpy factor plays a
the singlet species with
p single-
bonded character and the transition
states for the ring-closing reaction,
thereby clarifying the role of the
alkoxy group on the lifetime and the
stereoselectivity of the ring-closing re-
action.
Keywords: density functional calcu-
lations · diradicals · kinetics · pi sin-
gle bonds · substituent effects
Introduction
The three-dimensional molecular structure and electronic
structure play important roles in molecular functions. The
structures are determined by chemical bonds, which are one
of the most important concepts in chemistry. There are two
types of carbon–carbon covalent bonds, that is, s bonds and
p bonds (Figure 1); p bonds are weaker than s bonds,
DEs>DEsp. Thus, the p-bonding system is believed to
exist only together with a strong s bond. The typical exam-
ple is a p bond in ethylene. Is it possible to make a p single
bond without a s bond? The p single-bonded compounds
possess planar four-coordinate carbon atoms[1] (Figure 1).
[a] T. Nakagaki, T. Sakai, T. Mizuta, Prof. M. Abe
Department of Chemistry, Graduate School of Science
Hiroshima University (HIRODAI), 1-3-1 Kagamiyama
Higashi-Hiroshima, Hiroshima 739-8526 (Japan)
Figure 1. Chemical bonds: s bonds and p bonds in ethylene, and p single
bond.
The generation and the molecular functions derived from
such an unusual bonding system are worth investigating.
The HOMO–LUMO energy gap (DEp) should be smaller
than the normal p-bonding system (DEsp) in multiple
bonds.
To make such a p single-bonding system, the singlet
ground state of localized diradicals is indispensable. Over
the last few decades, a significant development of the local-
[b] Y. Fujiwara
Department of Mathematical and Life Sciences
Graduate School of Science, Hiroshima University (HIRODAI)
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526 (Japan)
[c] Prof. M. Abe
Institute for Molecular Science (IMS), Myoudaiji
Okazaki, Aichi 444-8787 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300038.
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10395

ized singlet diradical chemistry has been achieved after the
pioneering studies by Borden et al.,[2] Niecke et al.,[3] Ber-
trand et al.,[4] and this laboratory[1g–j] on the ground-state
spin multiplicity of propane-1,3-diyls such as cyclopentane-
1,3-diyls DR and heterocyclic butane-1,3-diyls H-DR
(Scheme 1).[5]
states S-DR1 possess a p single-bonding character between
the two radical sites and the zwitterionic character. The
characters can be represented in the resonance structures P
and ZI1.[1g–j,7] In the Type-2 molecules, however, the anti-
symmetric MO yA is selectively occupied by the two elec-
trons. The Type-2 molecules S-DR2 possess the zwitterionic
character of ZI2. The reactivity of singlet diradicals is large-
ly dependent on the most stable electronic configuration,
Type-1 versus Type-2.[2,8] For example, a disrotatory ring clo-
sure is an energetically favored process for Type-1 molecules
to give s-bonded compounds. In contrast, the conrotatory
ring closure is required for Type-2 singlet state.
1,3-Diradicals H-DR in heterocycles are also classified as
Type-1 and Type-2. Diradicals H-DR1[4,9] are Type-1 mole-
cules because the low-lying s* orbital (pseudo-p orbital)
exists between the two p orbitals. Thus, 2-silapropane-1,3-
diyls and 1,2-diaazacyclopentane-3,5-dilys belong to Type-1
molecules. Very recently, Fischer and Frenking et al. report-
ed a p single bond in a Ge2Ga2 four-membered molecule.[10]
Diradicals H-DR2[3,6a,11,12] belong to Type-2 molecules be-
cause the n orbital of the lone-pair electrons of N, O, and P
interact with yS.
This laboratory has reported so far the generation and
detection
of
2,2-dialkoxyoctahydropentalene-1,3-diyls
DRa–c,[1g–j,13] which are categorized as Type-1 molecules, in
the photochemical denitrogenation of the corresponding
azoalkanes AZa–c (Scheme 2). Ring-closing compounds
Scheme 1. Localized singlet 1,3-diradicals of Type-1 and Type-2.
The localized singlet 1,3-diradicals can be categorized as
Type-1 and Type-2 on the basis of the most stable electronic
configuration of the singlet state.[1j,6] Diradicals DR1 with a
substituted electron-withdrawing group (W=F, OR, and so
forth) at C(2) are typically classified as Type-1 diradicals,
whereas diradicals DR2 with a substituted electron-donating
group (D=H, SiR3, and so on) belong to Type-2. In the
Type-1 molecules, two electrons preferentially occupy the
symmetric molecular orbital (MO) yS. Thus, the singlet
Scheme 2. Alkoxy-group effect on the lifetime of DRs.
CPa–c were quantitatively formed in the denitrogenation re-
actions. The singlet species with lmax ꢀ570 nm were persis-
tent at liquid-nitrogen temperature (ꢀ77 K) and detected
on the nanosecond (ns) timescale at room temperature. The
strong absorption band in the visible region is due to the
Abstract in Japanese:
HOMO(yS)–LUMO(yA) electronic excitation of the
p
single-bonding system between the two radical sites. The
lifetime at 293 K (t293) was found to be largely dependent
upon the alkoxy group (OR/OR’) as shown in Scheme 2.
Thus, the diethoxy-substituted compound DRc (OR/OR’=
OC2H5/OC2H5, t293 =880 ns in benzene and 1010 ns in aceto-
nitrile)[1h] was a longer-lived species than both the dime-
thoxy-substituted compound DRb (OR/OR’=OCH3/OCH3,
302 ns in benzene)[1i] and the ethylene–ketal-substituted
ꢁ
ꢁ
compound DRa (OR/OR’= OACHTNUGRTNEUNG(CH2)2O , approximately
2 ns in acetonitrile).[13]
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Single-Bonded Species
FULL PAPER
In the present study, azoalkanes AZd–f with a series of
alkyl chains (R/R’) of C3H7, C6H13, and C10H21 were synthe-
sized to clarify the effect of the alkoxy group (OR=OR’)
on the lifetime of the Type-1 molecules with p single bonds
(Scheme 3). Furthermore, compounds DRg,h with mixed-
ACHTUNGTRENNUNG
The maximum absorption of the azo chromophore (n,p*)
was found at the typical position of approximately 360 nm
with low molar absorption coefficients (eꢀ100mꢁ1 cmꢁ1).
The structural determination of AZd–h was performed on
the basis of the spectroscopic analyses, in particular by
NMR spectroscopic measurements. The nuclear Overhauser
1
effect (NOE) in conjunction with H NMR spectroscopic ex-
periments was used to determine the stereochemical config-
uration of the azoalkanes. The configurational assignment
for AZ’g (OR/OR’=OCH3/OC3H7) was unequivocally con-
firmed by X-ray crystallographic analysis (see Figure S1 in
the Supporting Information). The stereochemical determina-
tion was consistent with the structure determined by the
NOE measurements.
Scheme 3. Present study: generation and reactivity of DRd–h in the
denitrogenation of AZd–h.
Product analysis was performed in the photochemical de-
nitrogenation of azoalkanes AZd–h (0.2m in [D6]benzene)
by using a Xe lamp through a band-path filter (hn=320–
380 nm) for 12 h at room temperature (ꢀ295 K). The pho-
acetal moieties (OR¼OR’) were designed and generated
from AZg,h to gain more insight into the role of the alkoxy
group on the reactivity of S-DR. Quantum chemical calcula-
tions clarified the structure of the singlet diradical with p
single-bonded character, the role of the alkoxy group on the
lifetime, and the stereoselectivity of the conrotatory ring-
closing reaction. The combined experimental and computa-
tional study made a significant contribution to research on
the kinetic stabilization of p single-bonded molecules and
understanding their reactivity.
1
tolysates were directly analyzed by H and 13C NMR spec-
troscopic measurements. As shown in Scheme 4, trans-con-
figured ring-closed compounds CPd–h were quantitatively
obtained. The results clearly indicate that intermediates
DRd–h are generated in the photochemical reaction. The
formation of trans-configured ring-closing compounds was
determined by the NOE measurements, for example, 2.2%
NOE for trans-CPe.
Steady-state absorption spectroscopy at 80 K: Photochemi-
cal irradiation of the corresponding azoalkanes AZd–h was
examined to directly observe the singlet-state molecules
DRd–h in a 2-methyltetrahydrofuran (MTHF) glass matrix
at 80 K using an Xe lamp through a band-path filter (320–
380 nm; Figure 2 and Table 1). For comparison, 2,2-dime-
thoxycyclopentane-1,3-diyl DRb (OR/OR’=OCH3)[13b] was
also generated under the same irradiation conditions. As
shown in Figure 2, strong absorption spectra at lmax
ꢀ570 nm were detected under the photochemical irradiation
conditions. The species were persistent at 80 K at least for
one night. The persistent species were electron-spin-reso-
nance (ESR) silent under the similar photoirradiation condi-
tions at 79 K in an ESR cavity resonator. The strong absorp-
tion signals in the visible region are characteristic of the sin-
glet 2,2-dialkoxy-1,3-diphenylcyclopentane-1,3-diyls.[1h,i] The
absorption band was assigned to the HOMO(yS)–
LUMO(yA) electronic excitation of the p single-bonding
system (Figures 1 and 2, and Scheme 1). The assignment was
confirmed by time-dependent (TD) DFT calculations.[10] As
shown in Figure 2 and Table 1, the absorption maxima were
not significantly dependent on the alkoxy group. Thus, the
HOMO–LUMO energy spacing in DRb,d–h was not de-
pendent upon the chain length. The energy gap affects the
Results and Discussion
Synthesis of azoalkanes AZd–h and their denitrogenation
reaction: The synthesis of azoalkanes AZd–h is shown in
Scheme 4. Oxadiazole derivatives 1d–g were prepared ac-
cording to the method reported by Warkentin and co-work-
ers.[14] 4,4-Dialkoxypyrazoles 2d–g were obtained by the
[4+1] cycloaddition reaction of the thermally generated di-
Scheme 4. Synthesis of azoalkanes AZd–h and their photodenitrogena-
tion reaction.
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10397

M. Abe et al.
Figure 2. Electronic absorption spectra of singlet diradicals S-DRb,d-h in
MTHF glass at 80 K. To see the respective spectra for DRb,d–h clearly,
the original intensity of the signals were modified, that is: DRbꢁ1.0,
DRdꢁ1.8, DReꢁ2.5, DRfꢁ4.5, DRgꢁ6.5, and DRhꢁ8.2.
Figure 3. Transient decay trace of the singlet diradicals DRb,d–h (lobs
=
570 nm), generated by photolysis of AZb,d–h (lexc =355 nm) in benzene
at 293 K.
Table 1. Maximum absorption (lmax) and lifetime (t293) of singlet diradicals DRb,d–h; and activation parame-
ters (Ea, logA, DH&#8225;, DS&#8225;) for the first-order decay process of the singlet diradicals.
the denitrogenation of AZd–f
were determined by comparison
with that reported for a similar
azoalkane.[16] All of the transi-
ent species derived from
AZb,d–f were not quenched by
molecular oxygen. The results
clearly support the theory that
the transient species at approxi-
mately 570 nm are the singlet
species DRb,d–f because the
Entry
S-DR (OR/OR’)
lmax
t293
Ea
log(A [sꢁ1])[c]
DH°
DS°
[c]
[d]
[d]
[nm][a] [ns][b]
G
]
G
]
[Jmolꢁ1 Kꢁ1
]
1
2
3
4
5
6
S-DRb (OCH3/OCH3)
S-DRd (OC3H7/OC3H7)
S-DRe (OC6H13/OC6H13)
S-DRf (OC10H21/OC10H21)
S-DRg (OCH3/OC3H7)
S-DRh (OC3H7/OCH3)
574
571
572
572
568
568
292
1899
2292
2146
516
33.4
39.3
38.8
38.2
36.4
37.8
12.5
12.7
12.6
12.5
12.8
12.8
30.7
36.7
36.5
36.1
33.8
35.2
ꢁ15.0
ꢁ10.0
ꢁ14.6
ꢁ13.1
ꢁ9.0
935
ꢁ9.2
[a] The maximum absorption was determined by the absorption spectra of DRb,d–h measured in MTHF glass
at 80 K (Figure 2). [b] In benzene at 293 K; errors of approximately 5% are standard deviations of the mean
(five data points). [c] Determined from Arrhenius plots of the lifetimes of the singlet diradical at ten tempera-
tures between 20 and 608C. Error in the data is less than approximately 0.4 kJmolꢁ1 for Ea and approximately triplet diradicals, lmax ꢀ330 nm,
0.1 for logA. [d] Determined from Eyring plots of the lifetimes of the singlet diradical at five temperatures be-
were known to be quenched by
molecular oxygen with diffu-
tween 20 and 608C. Error in the data is less than approximately 0.4 kJmolꢁ1 for DH° and approximately 1.1
for DS° (Figure S2 in Supporting Information).
sion-controlled
stants.[17]
rate
con-
thermodynamic stability of the p single-bonded species, for
The lifetime was found to be largely dependent upon the
alkoxy groups (OR/OR’) of the acetal moiety, although the
absorption maximum was not dependent upon the alkoxy
group. Thus, the propoxy-substituted compound DRd (OR/
OR’=OC3H7/OC3H7, 1899 ns at 293 K in benzene; Table 1,
entry 2) was a longer-lived species by one order of magni-
tude than the methoxy-substituted compound DRb (OR/
OR’=OCH3/OCH3, 292 ns; Table 1, entry 1). The lifetime
of DRe (OR/OR’=OC6H13/OC6H13, 2292 ns; Table 1,
entry 3) was found to be only a little bit longer than that of
DRd. The lifetime of DRf (OR/OR’=OC10H21/OC10H21,
2146 ns; Table 1, entry 4) was nearly the same as that of
DRe. The activation parameters (Ea, logA, DH°, and DS°)
were determined for the ring-closing reaction by measuring
the temperature-dependent change of the decay rate con-
stant (k) of the singlet diradicals (Table 1 and Figure S2 in
the Supporting Information).
[1i,j]
ꢁ
example, the p-bonding strength of C(1) C(3).
Time-resolved absorption spectroscopy: The transient ab-
sorption spectra and decay traces in the photolysis of azoal-
kanes AZd–f were measured in degassed benzene by means
of laser-flash photolysis (lexc =355 nm, 5 ns pulse). The
measurements were expected to provide valuable informa-
tion about the alkoxy-group effect on the reactivity of the p
single-bonded species (Table 1). The spectroscopic data
were compared with those observed for AZb (OR/OR’=
OCH3/OCH3). As found under the low-temperature matrix
conditions, the strong transient absorption bands were ob-
served at around 570 nm, which decayed with clean first-
order kinetics (Figure 3) and displayed linear Eyring tem-
perature dependencies (see Figure S2 in the Supporting In-
formation). The first-order process corresponds to the ring-
closing reaction that produces the ring-closed compounds
CP, because the quantitative formation of CP was found
with high quantum yields (ꢀ0.90). The quantum yields for
The large preexponential Arrhenius factor (logAꢀ12–13)
for all compounds DRb,d–h clearly indicates that the first-
order decay process is spin-allowed, which is consistent with
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Single-Bonded Species
FULL PAPER
the intramolecular cyclization from DRd–h to CPd–h.[2b,18]
The negative activation entropy of DS° ꢀ10 Jmolꢁ1 Kꢁ1 is
reasonable for the intramolecular ring-closing reaction. The
values were not significantly different among DRb,d–h
(Table 1, entries 1–5). Thus, the alkoxy-group-dependent
change of the Gibbs energy barrier in the ring-closing reac-
tion was mainly controlled by the enthalpy factor (DH°).
The nonbonded repulsive interaction, that is, steric hin-
drance of the alkoxy group, plays an important role in deter-
mining the energy barrier of the ring-closing reaction.
The chain-length effect over C3 of the alkoxy group on
the lifetime of the singlet diradicals was found to be quite
small: DRd (OC3H7, 1899 ns)ꢀDRe (OC6H13, 2292 ns)ꢀ
DRf (OC10H21, 2146 ns). The small effect of the alkoxy
group over the C3 chain length suggests that the alkyl-chain
moiety over C3 locates far from the nonbonded interaction
site during the ring-closing reaction. The details will be dis-
cussed in the computational studies on the structures of the
singlet species and the transition states for the disrotatory
ring-closing reaction.
acetal moiety, but the in–in conformer of the acetal moiety
S-DRb-ii (Figure 4). Thus, the in–in conformer S-DRb-ii
(S2 =0.923) was calculated to be energetically more stable
than the in–gauche isomer S-DRb-ig (S2 =0.929), the
gauche–in isomer S-DRb-gi (S2 =0.938), and the gauche–
gauche conformer S-DRb-gg by 4.6, 12.5, and 27.2 kJmolꢁ1,
respectively. The singlet states were computed to be lower
in energy than those triplet states by DEST =ESꢁET =ꢁ16.4,
ꢁ15.6, ꢁ15.2, and ꢁ14.9 kJmolꢁ1, respectively.
The gauche–gauche conformer was expected to be the
most stable structure among the possible conformers be-
cause of the stereoelectronic stabilization in the acetal
moiety. The conformational preference of the in–in isomer
S-DRb-ii can be explained by the spiroconjugative stabiliza-
[13,23]
tion interaction of the lone pair of electrons nO with yA
and the energetic destabilization of the gauche–gauche
isomer due to the repulsive interaction between the me-
thoxy and phenyl group (Figure 4).
The p single-bonding character of S-DRb-ii was assessed
by calculating the occupation number of electrons in the
bonding orbital yS and the antibonding orbital yA at the
Computational studies on S-DRb: To obtain insight into the
effect of the alkoxy group on the lifetime of the singlet spe-
cies, structural analysis was first carried out for the dime-
thoxy-substituted singlet species S-DRb at the UB3LYP/6-
31G(d) level[19] with the broken-symmetry (BS)[20] method
(Figure 4). The DFT level of theory is known to reproduce
the activation parameters determined by the experiment in
CASSCFACTHNGUTER(NNUG 2/2)/6-31G(d) level of theory (Figure 5). The occu-
pation numbers in the yS and yA orbitals were found to be
1.37 and 0.63, respectively As shown in the molecular orbi-
tals, S-DRb-ii thus possesses a significant p single-bonding
character.
The transition states for the ring-closing reaction of
S-DRb were calculated at the same level of theory, that is,
BS-(U)B3LYP/6-31G(d) (Figure 5, entry 1 in Table 2). Sur-
prisingly, the transition state cis-TSb (DHrel =43.9 kJmolꢁ1,
ni =28.8i cmꢁ1, S2 =0.00) that produces the cis-configured
ring-closed compound cis-CPb was found to be energetically
a ring-opening reaction of 1,4-diphenylbicycloACHTUNTRGNEU[GN 2.1.0]pentane
to the singlet 1,3-diphenylcyclopentane-1,3-diyl.[21] The geo-
metries of stationary points and transition states were all lo-
cated, and vibrational analyses were performed with the
Gaussian 09[22] suite of programs.
more stable than the transition state trans-TSb (DHrel
=
Surprisingly, the most stable conformer was calculated to
be not the gauche–gauche conformer S-DRb-gg of the
65.4 kJmolꢁ1, ni =228.8i cmꢁ1, S2 =0.176) that produced the
trans-configured isomer trans-CPb by DDHrel (DDEzpe)=
21.5 (20.2) kJmolꢁ1. The imagi-
nary frequency (ni) of the tran-
sition state cis-TSb was found
to be very low. The intrinsic re-
action coordinate (IRC) calcu-
lations confirmed that cis-TSb
and
trans-TSb
connected
S-DRb with the ring-closing
compounds cis-CPb and trans-
CPb, respectively.
The energetic preference for
the formation of the cis isomer
is
reasonably
explained
(Figure 6). The phenyl ring in
trans-TSb has to go downward
to the cyclopentane ring during
the formation of the trans
isomer. On the other hand, the
phenyl ring goes upward in cis-
TSb during the formation of
the cis isomer. Thus, the repul-
sive interaction of “a” in trans-
Figure 4. Three equilibrated structures of the singlet diradicals S-DRb optimized at the BS-UB3LYP/6-31G(d)
level of theory. The value DHrel is the relative thermal enthalpy calculated at 298 K and 1 atm; DEzpe is the
electronic energy after zero-point energy corrections.
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M. Abe et al.
depictions of “a” to “d”) because the energetic destabiliza-
tion of the interaction “b” in trans-TSb and “c” in cis-TSb
should be similar each other. The repulsive interaction of
“d” in cis-TSb also influences the energy of the transition
state cis-TSb, but the effect was found to be smaller than
the interaction “a.” trans-CPb was calculated to be energeti-
cally more stable than cis-CPb by DDHrel (DDEzpe)=15.5
(15.9) kJmolꢁ1. Thus, cis-CPb is the kinetically controlled
product in the ring-closing reaction of S-DRb.
In our experimental studies of the denitrogenation reac-
tion of AZb,d–f at room temperature, the trans isomers of
the ring-closing compounds CP were quantitatively isolated.
At first glance, the computational results, which predict the
selective formation of the cis isomer, contradict the experi-
mental observation of the exclusive formation of the trans-
configured CP. The transition state trans-TSb energetically
locates above the ring-closing compound cis-CPb by just
DDHrel (DDEzpe)=78.7 (79.4) kJmolꢁ1, which is equal to 18.8
(19.0) kcalmolꢁ1 (Figure 6). Thus, the cis-configured ring-
closing compound cis-CPb is equilibrated with the trans
isomer trans-CPb at room temperature, which is energetical-
ly more stable than cis-CPb by approximately 15 kJmolꢁ1.
The DFT calculations in the gas phase overestimated the
experimentally determined transition-state energy of cis-
TSb by approximately 13 kJmolꢁ1 (entry 1 in Table 1). The
transition-state energy of trans-TSb is also supposed to be
overestimated in a similar way. Thus, the isomerization reac-
tion from the cis isomer to the trans isomer would be faster
than that predicted by the computations.
Figure 5. The calculated HOMO (yS) and LUMO (yA) orbitals of
S-DRb-ii and the occupation numbers of two electrons in their orbitals at
the CASSCFACHTUNGTRENNUNG(2/2)/6-31G(d) level of theory.
Computational studies on S-DRg–j: To clarify in more
detail the effect of the alkoxy group on the transition-state
energy of the ring-closing reactions, computations on the
singlet species S-DRg–j with the mixed-acetal moiety were
calculated at the same level of theory (Scheme 5, entries 2–5
in Table 2). The transition-state energy of cis-TSi (OR/
OR’=OCH3/OH) from S-DRi, DHrel (DEzpe)=42.0
(45.0) kJmolꢁ1 with ni =33.2i cmꢁ1 (Table 2, entry 2), was
calculated to be a little bit smaller than that of cis-TSb (OR/
OR’=OCH3/OCH3, DHrel (DEzpe)=43.9 (46.4) kJmolꢁ1,
Figure 5, entry 1 in Table 2). The results clearly indicate that
the energetic destabilization caused by the interaction “f”
(Scheme 5) is small but not negligible in the transition states
for the formation of the cis isomers. Indeed, the transition-
state energy of cis-TSg (OR/OR’=OCH3/OC3H7) from
S-DRg, DHrel (DEzpe)=44.7 (47.3) kJmolꢁ1 with ni =
Figure 6. Reaction profiles of S-DRb at the BS-UB3LYP/6-31G(d) level
of theory. The value DHrel is the relative thermal enthalpy calculated at
298 K and 1 atm; DEzpe is the electronic energy after zero-point energy
corrections.
TSb increases the energy of the transition state trans-TSb
that produces the trans isomer trans-CPb (see Figure 6 for
Table 2. Computational results of the ring-closing reaction of S-DRb,g–j at the BS-UB3LYP/6-31G(d) level of theory.
[a]
Entry
S-DR (OR/OR’)
DHrel (DEzpe) [kJmolꢁ1
]
cis-TS
trans-TS
cis-CP
trans-CP
1
2
3
4
5
S-DRb (OCH3/OCH3)
S-DRi (OCH3/OH)
S-DRj (OH/OCH3)
S-DRg (OCH3/OC3H7)
S-DRh (OC3H7/OCH3)
43.9 (46.4)
42.0 (45.0)
18.3 (20.2)
44.7 (47.3)
48.0 (46.9)
65.4 (66.6)
35.5 (37.2)
62.8 (64.3)
66.9 (68.6)
65.5 (67.1)
ꢁ13.3 (ꢁ12.8)
ꢁ14.5 (ꢁ13.3)
ꢁ18.9 (ꢁ17.9)
ꢁ13.0 (ꢁ12.4)
ꢁ13.6 (ꢁ12.6)
ꢁ28.8 (ꢁ28.7)
ꢁ36.3 (ꢁ35.4)
ꢁ27.9 (ꢁ26.6)
ꢁ40.0 (ꢁ41.0)
ꢁ40.7 (ꢁ39.8)
[a] At the BS-(U)B3LYP/6-31G(d) level of theory. The values of the calculated enthalpy (DHrel) are relative to those of the singlet diradicals S-DRb,g–j.
The values of the zero-point-energy-corrected electronic energies (DEzpe) are relative to those of the singlet diradicals S-DRb,g–j.
10400
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Chem. Eur. J. 2013, 19, 10395 – 10404

Single-Bonded Species
FULL PAPER
Scheme 5. Computations on the reactivity of S-DRb,g–i at the
BS-UB3LYP/6-31G(d) level of theory.
Figure 7. Optimized transition-state structure of cis-TSh at the
BS-UB3LYP/6-31G(d) level of theory.
29.2i cmꢁ1 (Table 2, entry 4), was found to be larger than
that of cis-TSb (Table 2, entry 1).
the two phenyl rings. Thus, the interaction “e” is expected to
be quite small, when the alkyl-chain length is greater than
C3 (Scheme 5). Indeed, as mentioned above, the chain-
length effect over C3 of the alkoxy group on the lifetime of
singlet diradicals was found to be quite small: DRd (OC3H7,
1899 ns)ꢀDRe (OC6H13, 2292 ns)ꢀDRf (OC10H21, 2146 ns)
(Table 1 and Figure 3).
DFT calculations clearly indicate that the kinetically fa-
vored result of the singlet diradicals is to produce the cis iso-
mers, except in the case of the reactivity of the diradical
S-DRi (OR/OR’=OCH3/OH).
The transition-state energy of cis-TSj (OR/OR’=OH/
OCH3), DHrel (DEzpe)=18.3 (20.2) kJmolꢁ1 with ni =
278.9i cmꢁ1 (Table 2, entry 3), was found to be significantly
smaller than that of cis-TSi (Table 2, entry 2). Thus, the
effect of the repulsive interaction “e” between the phenyl
ring and the alkoxy group was found to play a crucial role in
determining the energy barrier in the formation of the cis
isomers. Indeed, the transition-state energy of cis-TSh (OR/
OR’=OC3H7/OCH3) from S-DRh, DHrel (DEzpe)=48.0
(46.9) kJmolꢁ1 with ni =38.1i cmꢁ1 (Table 2, entry 5) was cal-
culated to be larger than those of cis-TSb (Table 2, entry 1).
The transition-state energy of trans-TSj (OR/OR’=OH/
OCH3), DHrel (DEzpe)=62.8 (64.3) kJmolꢁ1 with ni =
245.3i cmꢁ1 (Table 2, entry 3), was calculated to be a little
bit smaller than that of trans-TSb, DHrel (DEzpe)=65.4
(66.6) kJmolꢁ1 (Figure 6, entry 1 in Table 2). The results
clearly indicate that the energetic destabilization caused by
the interaction “h” (Scheme 5) is small in determining the
transition-state energy for the formation of the cis isomers.
Thus, interactions “g” and “a” determine the energy barrier
for the formation of the trans isomer. Although the similar
energy barriers for trans-TSg and trans-TSh were obtained
at the same level of theory (Table 2, entries 4 and 5), the
energy of trans-TSg was calculated to be a little bit larger
than that of trans-TSh because the interaction of the pro-
poxy group with the phenyl ring is larger than that between
the methoxy group and the phenyl ring (Scheme 5).
Low-temperature NMR spectroscopic measurements: An
attempt to detect the kinetically favored cis isomer cis-CPb
was examined in the photodenitrogenation of AZb in
[D8]toluene in a NMR spectroscopy tube at 213 K using an
Xe lamp. After 6 h irradiation at 213 K, the photolysate was
transferred to the precooled NMR spectrometer, and the
1H NMR spectrum was measured at 213 K as quickly as pos-
sible. However, only the trans isomer trans-CPb was detect-
ed in the photolysate. The conversion of the azoalkane was
12%. The experimental result suggests that the kinetically
favored cis-CPb is converted into the thermodynamically fa-
vored trans isomer trans-CPb during the transfer of the
sample to the NMR spectroscopic cavity, or that the lifetime
of cis-CPb is less than 6 h under the NMR spectroscopic
measurement conditions. In situ NMR spectroscopic meas-
urements[24] under irradiation conditions that used a high-
power light source (such as a laser) would be indispensable
in detecting such a short-lived species.
The
energy
of
trans-TSi,
DHrel
(DEzpe)=35.5
(37.2) kJmolꢁ1 with ni =155.5i cmꢁ1 (Table 2, entry 2), was
found to be significantly smaller than those of trans-TSb,j
(Table 2, entries 1,3). Thus, the energetic destabilization de-
rived from the interaction “g” of the phenyl ring with the
alkoxy group was also found to be important in determining
the transition-state energy for the formation of the trans iso-
mers.
Photochemical generation of S-DRg and S-DRh and their
reactivity: The singlet diradicals S-DRg (OR/OR’=OCH3/
OC3H7) and S-DRh (OR/OR’=OC3H7/OCH3) were experi-
mentally generated from the corresponding azoalkanes and
the lifetimes were compared with each other (Scheme 6).
The experiments were expected to provide experimental
evidence for the formation of the kinetically favored cis
isomer (entries 5 and 6 in Table 1, entries 4 and 5 in
The optimized transition-state structure of cis-TSh (OR/
OR’=OC3H7/OCH3) is shown in Figure 7. The methyl
group of the propoxy group was found located at the end of
Chem. Eur. J. 2013, 19, 10395 – 10404
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10401

M. Abe et al.
On the other hand, the lifetime of S-DRh (OR/OR’=
OC3H7/OCH3, 935 ns) was found to be shorter than S-DRd
(OR/OR’=OC3H7/OC3H7, 1899 ns). The result is reasonably
explained again by the repulsive interaction between the
alkoxy group and the cyclopentane ring. Interaction “d” of
the methoxy group with the cyclopentane ring should be
smaller than interaction “i” of the propoxy group with the
cyclopentane ring.
Conclusion
A series of singlet 2,2-dialkoxy-1,3-diphenyloctahydropenta-
lene-1,3-diyls was generated using the photochemical deni-
trogenation of the corresponding azoalkanes to clarify the
effect of the alkoxy group of C(OR)ACTHNUTRGNEUNG(OR’) at the C(2) posi-
tion on the lifetime of the singlet diradicals with p single-
bonding character DR. The combined experimental and
computational studies clarified the hitherto unknown role of
the alkoxy group on the reactivity. The fate of singlet diradi-
cals with p single-bonding character was not to produce the
trans-configured ring-closed products, but to produce the
cis-configured ring-closed compounds.[25] The kinetically
controlled cis-configured products were not directly ob-
served by NMR spectroscopic analyses owing to the short-
lived character of the cis isomers. The steric interaction be-
tween the alkoxy group and the phenyl ring, which deter-
mines the energy barrier of the ring-closing reaction, was
found to play a crucial role in determining the lifetime of
the singlet-state molecules. The present studies provide a
concrete strategy to obtain the long-lived p single-bonded
molecules.
Scheme 6. Generation of S-DRg,h and their reactivity.
Table 2). When the ring-closing mode, that is, the result of
the singlet diradicals, produces the cis isomer, the singlet dir-
adical S-DRh is predicted to be a longer-lived species than
S-DRg because the steric interaction “k” between the pro-
poxy group (OC3H7) and the phenyl ring (Ph) in cis-TSh
was calculated to be higher than interaction “c” between the
methoxy (OCH3) and Ph in cis-TSg (entries 4 and 5 in
Table 2). Actually, the singlet diradical S-DRh (935 ns) was
found to be a significantly longer-lived species than S-DRg
(516 ns). If the ring-closing mode produces the trans iso-
mers, the lifetime of S-DRh should be shorter than S-DRg
because the interaction “b” in trans-TSh is smaller than the
interaction “j” in trans-TSg (Scheme 6). All of the combined
experimental and computational results revealed that the
ring-closing mode, which determines the lifetime of the sin-
glet diradicals, produces the cis isomers.
The lifetime of S-DRg (OR/OR’=OCH3/OC3H7, 516 ns)
was longer than that of S-DRb (OR/OR’=OCH3/OCH3,
292 ns). The results suggest that the effect of interaction “i”
in cis-TSg is not negligible in determining the activation
energy of the ring-closing mode of the reactions. The ener-
getic destabilization caused by interaction “i” between the
propoxy group and the cyclopentane ring should be larger
than that derived from interaction “d” between the methoxy
group and the cyclopentane ring in cis-TSb (Scheme 6,
Figure 6). Actually, the transition-state energy of cis-TSg
(Table 2, entry 4) was calculated to be a little bit larger than
that in cis-TSb (Table 2, entry 1). Thus, the experimental re-
sults of the longer-lived species S-DRg (relative to S-DRb)
can be reasonably explained by the increase in the repulsive
interaction between the alkoxy group and the cyclopentane
ring.
Experimental Section
General procedure for the synthesis of endo-2,3-diazo-10,10-dialkoxy-1,4-
diphenyltricyclo[5.2.1.05,9]dec-2-ene (AZd–h): Trifluoroacetic acid
(0.5 equiv) was added dropwise to
a solution of cyclopentadiene
(20 equiv) and 4,4-dialkoxy-3,5-diphenyl-pyrazole[1i,13] in CH2Cl2 (1 mL)
at 0C8 under nitrogen. After stirring for 10 min, the reaction was
quenched with NaHCO3 (aq) until the pH of the solution reached 8.
After washing the solution with water and brine, and drying by MgSO4,
the organic phase was then filtered, concentrated, and purified by
column chromatography. The [4+2] cycloadduct AZ’d–h was dissolved in
ethyl acetate (2 mL), and palladium (5 mg) on a charcoal catalyst was
added. The mixture was stirred under a hydrogen gas atmosphere for
24 h at room temperature, the palladium catalyst was removed by filtra-
tion over Celite, and the ethyl acetate was evaporated under reduced
pressure. Cycloadducts AZd–AZh were purified by column chromatogra-
phy and recycling column chromatography. The endo configuration was
determined by NOE experiments.
endo-2,3-Diazo-10,10-dipropoxy-1,4-diphenyltricyclo[5.2.1.05,9]dec-2-ene
(AZd): Yield: 222 mg (43% from 2d) as
a
viscous oil. 1H NMR
(500 MHz, CDCl3): d=7.96–7.92 (m, 4H), 7.48–7.34 (m, 6H), 3.60–3.52
(m, 2H), 3.03 (t, J=6.70 Hz, 2H), 2.75 (t, J=6.70 Hz, 2H), 1.68–1.36 (m,
8H), 1.16 (m, 2H), 0.89 (t, J=7.39 Hz, 3H), 0.61 ppm (t, J=7.39 Hz,
3H); 13C NMR (68 MHz, CDCl3): d=136.8 (C), 128.3 (CH), 128.1 (CH),
127.6 (CH), 118.5 (C), 94.5 (C), 65.3 (CH2), 65.0 (CH2), 49.0 (CH), 27.8
(CH2), 25.7 (CH2), 23.0 (CH2), 22.7 (CH2), 10.8 (CH3), 10.4 ppm (CH3);
IR (neat): n˜ =2962, 2940, 2876, 1602, 1496, 1446, 1175, 1075 cmꢁ1; UV
10402
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Chem. Eur. J. 2013, 19, 10395 – 10404

Single-Bonded Species
FULL PAPER
(benzene): e365nm =99.7 Lmolꢁ1 cmꢁ1
;
HRMS (EI): m/z calcd for
3,3-Dihexoxy-2,4-diphenyltricyclo[3.3.1.02,4]octane (trans-CPe): Yield:
16 mg (100% from AZe) as a viscous oil. H NMR (500 MHz, C6D6): d=
1
C26H32N2O2: 404.2464; found: 404.2473.
endo-2,3-Diazo-10,10-dihexoxy-1,4-diphenyltricyclo[5.2.1.05,9]dec-2-ene
(AZe): Yield: 151 mg (41% from 2e) as
viscous oil. 1H NMR
7.45–7.41 (m, 4H), 7.24–7.06 (m, 6H), 3.95 (t, J=6.47 Hz, 2H), 3.25 (t,
J=6.76 Hz, 2H), 3.07–3.03 (m, 2H), 2.00–0.70 ppm (m, 28H); 13C NMR
(125 MHz, C6D6): d=135.2 (C), 130.0 (CH), 127.4 (CH), 125.8 (CH), 96.9
(C), 66.5 (CH2), 64.5 (CH2), 47.2 (C), 41.2 (CH), 31.7 (CH2), 31.2 (CH2),
29.9 (CH2), 29.3 (CH2), 27.9 (CH2), 26.1 (CH2), 25.5 (CH2), 24.8 (CH2),
22.5 (CH2), 22.3 (CH2), 13.7 (CH3), 13.6 ppm (CH3); ESI-MS: m/z calcd
for C32H44O2Na [M+Na]+: 483.3234; found: 483.3232.
a
(500 MHz, CDCl3): d=7.86–7.82 (m, 4H), 7.38–7.25 (m, 6H), 3.48–3.43
(m, 2H), 2.95 (t, J=6.40 Hz, 2H), 2.67 (t, J=6.40 Hz, 2H), 1.58–0.76 (m,
22H), 0.80 (t, J=7.23 Hz, 3H), 0.72 ppm (t, J=7.31 Hz, 3H); 13C NMR
(125 MHz, CDCl3): d=136.7 (C), 128.2 (CH), 128.0 (H), 127.4 (CH),
118.5 (C), 94.4 (C), 63.7 (CH2), 63.4 (CH2), 48.9 (CH), 31.3 (CH2), 31.2
(CH2), 29.6 (CH2), 29.2 (CH2), 28.7 (CH2), 28.7 (CH2), 25.6 (CH2), 25.5
(CH2), 22.3 (CH2), 22.2 (CH2), 13.8 (CH3), 13.7 ppm (CH3); IR (neat):
n˜ =3058, 3028, 2931, 2860, 1742, 1603, 1496, 1447, 1174, 1090, 1027, 748,
699 cmꢁ1; UV (MTHF): e365nm =83 Lmolꢁ1 cmꢁ1; ESI-MS: m/z calcd for
C32H45O2N2: 489.3476 [M+H]+; found: 489.3473.
endo-2,3-Diazo-10,10-didecyloxy-1,4-diphenyltricyclo[5.2.1.05,9]dec-2-ene
(AZf): Yield: 181 mg (39% from 2 f) as a viscous oil. Rf =0.63 (hexane/
EtOAc=7:3); 1H NMR (500 MHz, CDCl3): d=7.94 (d, J=7.60 Hz, 4H),
7.45–7.36 (m, 6H), 3.52–3.58 (m, 2H), 3.05 (t, J=6.47 Hz, 2H), 2.77 (t,
J=6.47 Hz, 2H), 1.67–1.00 (m, 42H), 0.89 (t, J=6.78 Hz, 3H), 0.88 ppm
(t, J=6.93 Hz, 3H); 13C NMR (125 MHz, CDCl3): d=136.9 (C), 128.4
(CH), 128.1 (CH), 127.6 (CH), 118.7 (C), 94.6 (C), 63.9 (CH2), 63.6
(CH2), 49.1 (CH), 31.9 (CH2), 29.9 (CH2), 29.5 (CH2), 29.5 (CH2), 29.3
(CH2), 29.3 (CH2), 29.3 (CH2), 29.2 (CH2), 27.8 (CH2), 26.2 (CH2), 25.8
(CH), 22.7 (CH2), 14.1 ppm (CH3); IR (neat): n˜ =2926, 2855, 1603, 1496,
1466, 1176, 747, 698 cmꢁ1; UV (benzene): e364nm =119.3 Lmolꢁ1 cmꢁ1; ele-
mental analysis (%) calcd for C40H60N2O2: C 79.95, H 10.06, N 4.66;
found: C 79.93, H 10.23, N, 4.64.
3,3-Didecyloxy-2,4-diphenyltricyclo[3.3.1.02,4]octane (trans-CPf): Yield:
1
18 mg (100% from AZf) as a viscous oil. H NMR (500 MHz, C6D6): d=
7.44–7.43 (m, 2H), 7.22–7.07 (m, 8H), 3.98 (t, J=6.4 Hz, 2H), 3.27 (t, J=
6.7 Hz, 2H), 3.02–3.08 (m, 2H), 1.93–1.81 (m, 4H), 1.58–1.03 (m, 34H),
0.91 (t, J=6.4 Hz, 3H), 0.90 ppm (t, J=6.7 Hz, 3H); 13C NMR
(125 MHz, C6D6): d=136.4 (C), 131.2 (CH), 128.6 (CH), 127.0 (CH), 98.1
(C), 67.8 (CH2), 65.8 (CH2), 48.4 (C), 42.4 (CH), 33.0 (CH2), 31.2 (CH2),
30.8 (CH2), 30.7 (CH2), 30.6 (CH2), 30.6 (CH2), 30.5 (CH2), 30.4 (CH2),
30.3 (CH2), 29.1 (CH2), 27.7 (CH2), 26.0 (CH2), 23.8 (CH2), 15.0 ppm
(CH3); EI-MS: m/z calcd for C40H60O2 [M]+: 572.4593; found: 572.4588.
3-Methoxy-3-propoxy-2,4-diphenyltricyclo[3.3.1.02,4]octane (trans-CPg):
Yield: 17 mg (100% from AZg) as a viscous oil. 1H NMR (500 MHz,
CDCl3): d=7.32–7.18 (m, 10H), 3.67 (s, 3H), 2.94 (t, J=6.77 Hz, 2H),
2.89–2.83 (m, 2H), 1.88–1.46 (m, 8H), 1.12 (sext, J=7.05 Hz, 3H),
0.55 ppm (t, J=7.39 Hz, 3H); 13C NMR (125 MHz, CDCl3): d=134.8
(C), 129.9 (CH), 127.3 (CH), 125.6 (CH), 97.6 (C), 66.0 (CH2), 54.0
(CH3), 47.1 (C), 40.9 (CH), 27.8 (CH2), 24.7 (CH2), 22.5 (CH2), 10.2 ppm
(CH3); IR (neat): n˜ =2922, 2852, 1738, 1603, 1495, 1447, 748, 698 cmꢁ1
APPI-MS: m/z calcd for C24H28O2: 348.2080; found: 348.2090.
;
endo-2,3-Diazo-10-methoxy-10-propoxy-1,4-diphenyltricyclo[5.2.1.05,9
ACHTUNGTRENNUNGdec-2-ene (AZg): Yield: 42 mg (23% from 2g) as a viscous oil. Rf =0.70
]
3-Methoxy-3-propoxy-2,4-diphenyltricyclo[3.3.1.02,4]octane (trans-CPh):
Yield: 19 mg (100% from AZh) as a viscous oil. 1H NMR (500 MHz,
C6D6): d=7.40–7.32 (m, 4H), 7.20–7.02 (m, 6H), 3.78–3.71 (t, J=6.40 Hz,
2H), 2.99–2.91 (m, 2H), 1.96–1.32 (m, 8H), 0.98 ppm (t, J=7.37 Hz,
3H); 13C NMR (125 MHz, CDCl3): d=134.8 (C), 129.8 (CH), 127.4
(CH), 125.7 (CH), 97.4 (C), 68.4 (CH2), 52.2 (CH3), 47.0 (C), 40.9 (CH),
27.8 (CH2), 24.7 (CH2), 22.9 (CH2), 10.9 ppm (CH3); IR (neat): n˜ =2922,
2852, 1738, 1603, 1495, 1447, 748, 698 cmꢁ1; IR (neat): n˜ =2929, 2893,
1606, 1496, 1367, 1330, 1184, 752, 711, 559 cmꢁ1; APPI-MS: m/z calcd for
C24H28O2: 348.2080; found: 348.2090.
(hexane/EtOAc=7:3); 1H NMR (500 MHz, CDCl3): d=7.95–7.91 (m,
4H), 7.48–7.35 (m, 6H), 3.60–3.52 (m, 2H), 2.94 (s, 3H), 2.73 (t, J=
6.47 Hz, 2H), 1.70–1.35 (m, 6H), 1.19 (m, 2H), 0.63 ppm (t, J=7.31 Hz,
3H); 13C NMR (125 MHz, CDCl3): d=136.5 (C), 128.2 (CH), 128.0
(CH), 127.5 (CH), 118.7 (C), 94.3 (C), 65.1 (CH2), 51.4 (CH), 48.6 (CH3),
27.6 (CH2), 25.6 (CH2), 22.5 (CH2), 10.2 ppm (CH3); IR (neat): n˜ =2975,
2928, 1601, 1498, 1445, 1382, 1258, 1112, 1044, 700 cmꢁ1; UV (benzene):
e355nm =110 Lmolꢁ1 cmꢁ1; ESI-MS: m/z calcd for C24H28O2N2Na: 399.2043
[M+Na]+; found: 399.2047.
endo-2,3-Diazo-10-methoxy-10-propoxy-1,4-diphenyltricyclo[5.2.1.05,9
]
ACHTUNGTRENNUNG
(hexane/EtOAc=7:3); 1H NMR (500 MHz, CDCl3): d=7.97–7.91 (m,
4H), 7.49–7.35 (m, 6H), 3.65–3.54 (m, 2H), 3.02 (t, J=6.36 Hz, 2H), 2.64
(s, 3H), 1.86–1.36 (m, 8H), 0.89 ppm (t, J=7.39 Hz, 3H); 13C NMR
(125 MHz, CDCl3): d=136.5 (C), 128.2 (CH), 128.1 (CH), 127.5 (CH),
118.7 (C), 94.4 (C), 64.9 (CH2), 51.8 (CH), 48.7 (CH3), 27.6 (CH2), 25.5
(CH2), 22.5 (CH2), 10.6 ppm (CH3); IR (neat): n˜ =2975, 2928, 1601, 1498,
Acknowledgements
NMR spectroscopic and MS measurements were performed at N-BARD,
Hiroshima University. This work was supported by a Grant-in-Aid for
Science Research on Innovative Areas “Stimuli-Responsive Chemical
Species” (no. 24109008), “pi-Space” (no. 21108516), and from the Minis-
try of Education, Culture, Sports, Science and Technology, Japan, and by
the Tokuyama Science Foundation (no. 19350021).
1445, 1382, 1257, 1112, 1044, 700 cmꢁ1
; UV (benzene): e355nm =
100 Lmolꢁ1 cmꢁ1
; ESI-MS: m/z calcd for C24H28O2N2Na: 399.2043
[M+Na]+; found: 399.2046.
Photodenitrogenation of AZd–AZh: A solution of endo-2,3-diazo-10,10-
dialkoxy-1,4-diphenyltricyclo[5.2.1.05,9]dec-2-ene in C6D6 was added into
an NMR spectroscopy tube. After bubbling N2 through it for 5 min, the
solution was exposed to light (lexc =355 nm) for 24 h at room tempera-
ture. The quantitative formation of the ring-closing products CPd–h was
observed by the NMR spectroscopic analysis of the photolysate. The
trans configuration was determined by NOE experiments.
3,3-Dipropoxy-2,4-diphenyltricyclo[3.3.1.02,4]octane (trans-CPd): Yield:
15 mg (100% from AZd) as a viscous oil. 1H NMR (500 MHz, CDCl3):
d=7.41–7.39 (m, 4H), 7.20–7.06 (m, 6H), 3.83 (t, J=6.42 Hz, 2H), 3.15
(t, J=6.53 Hz, 2H), 3.01 (d, J=6.53 Hz, 2H), 1.95–1.89 (m, 3H), 1.73
(sext, J=7.39 Hz, 2H), 1.56–1.36 (m, 3H), 1.14 (sext, J=7.39 Hz, 2H),
1.03 (t, J=7.33 Hz, 3H), 0.57 ppm (t, J=7.39 Hz, 3H); 13C NMR
(125 MHz, CDCl3): d=136.4 (C), 131.2 (CH), 128.6 (CH), 127.0 (CH),
97.9 (C), 69.2 (CH2), 66.9 (CH2), 48.3 (C), 42.3 (CH), 29.1 (CH2), 26.0
(CH2), 24.2 (CH2), 23.8 (CH2), 11.9 (CH3), 11.4 ppm (CH3); IR (neat):
n˜ =3084, 3056, 3024, 2957, 2875, 1945, 1602, 1498, 1261 cmꢁ1; HRMS
(EI): m/z calcd for C26H32O2: 376.2402; found: 376.2394.
[1] a) R. Hoffmann, R. W. Alder, C. F. Wilcox, J. Am. Chem. Soc. 1970,
92, 4992; b) O. J. Cooper, A. J. Wooles, J. McMaster, W. Lewis, A. J.
Blake, S. T. Liddle, Angew. Chem. 2010, 122, 5702; Angew. Chem.
Int. Ed. 2010, 49, 5570; c) J. B. Collins, J. D. Dill, E. D. Jemmis,
P. v. R. Schleyer, R. Seeger, J. A. Pople, J. Am. Chem. Soc. 1976, 98,
5419; d) U. D. Priyakumar, G. N. Sastry, Tetrahedron Lett. 2004, 45,
1515; e) U. Deva Priyakumar, A. S. Reddy, G. N. Sastry, Tetrahedron
Lett. 2004, 45, 2495; f) B. Sateesh, A. S. Reddy, G. N. Sastry, J.
Comput. Chem. 2007, 28, 335; g) M. Abe, W. Adam, W. M. Nau, J.
Am. Chem. Soc. 1998, 120, 11304–11310; h) M. Abe, W. Adam, T.
Heidenfelder, W. M. Nau, X. Zhang, J. Am. Chem. Soc. 2000, 122,
2019; i) M. Abe, W. Adam, M. Hara, M. Hattori, T. Majima, M.
Nojima, K. Tachibana, S. Tojo, J. Am. Chem. Soc. 2002, 124, 6540;
j) M. Abe, J. Ye, M. Mishima, Chem. Soc. Rev. 2012, 41, 3808.
[2] a) J. D. Xu, D. A. Hrovat, W. T. Borden, J. Am. Chem. Soc. 1994,
116, 5425; b) W. Adam, W. T. Borden, C. Burda, H. Foster, T. Hei-
denfelder, M. Jeubes, D. A. Hrovat, F. Kita, S. B. Lewis, D. Scheut-
Chem. Eur. J. 2013, 19, 10395 – 10404
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10403

M. Abe et al.
zow, J. Wirz, J. Am. Chem. Soc. 1998, 120, 593; c) W. T. Borden,
Chem. Commun. 1998, 1919.
Chem. Soc. 2004, 126, 6510; e) H. Cox, P. B. Hitchcock, M. Lappert,
L. J.-M. Peirssens, Angew. Chem. 2004, 116, 4600; Angew. Chem. Int.
Ed. 2004, 43, 4500; f) D. A. Dickie, P. T. K. Lee, O. A. Labeodan, G.
Schatte, N. Weinberg, A. R. Lewis, G. M. Bernard, R. E. Wasylishen,
J. A. C. Clyburne, Dalton Trans. 2007, 2862; g) X. Wang, Y. Peng,
M. M. Olmstead, J. C. Fettinger, P. P. Power, J. Am. Chem. Soc.
2009, 131, 14164; h) T. Beweries, R. Kuzora, U. Rosenthal, A.
Schulz, A. Villinger, Angew. Chem. 2011, 123, 9136; Angew. Chem.
Int. Ed. 2011, 50, 8974; i) K. Takeuchi, M. Ichinohe, A. Sekiguchi, J.
Am. Chem. Soc. 2011, 133, 12478; j) W. W. Schoeller, E. Niecke,
Phys. Chem. Chem. Phys. 2012, 14, 2015; k) S. Demeshko, C. Gode-
mann, R. Kuzora, A. Shultz, A. Villinger, Angew. Chem. 2013, 125,
2159; Angew. Chem. Int. Ed. 2013, 52, 2105.
[3] a) E. Niecke, A. Fuchs, F. Baumeister, M. Nieger, W. W. Schorller,
Angew. Chem. 1995, 107, 640; Angew. Chem. Int. Ed. Engl. 1995, 34,
555; b) M. Sebastian, M. Nieger, D. Szieberth, L. Nyulaszi, E.
Niecke, Angew. Chem. 2004, 116, 647; Angew. Chem. Int. Ed. 2004,
43, 637.
[4] D. Scheschkewitz, H. Amii, H. Gornitzka, W. W. Schoeller, D. Bour-
issou, G. Bertrand, Science 2002, 295, 1880.
[5] For delocalized diradicals, see: a) W. T. Borden, H. Iwamura, J. A.
Berson, Acc. Chem. Res. 1994, 27, 109; b) J. A. Berson, Acc. Chem.
Res. 1997, 30, 238; c) K. Matsuda, H. Iwamura, J. Chem. Soc. Perkin
Trans. 2 1998, 1023; d) T. Kubo, A. Shimizu, M. Sakamoto, M. Urui-
chi, K. Yakushi, M. Nakano, D. Shiomi, K. Sato, T. Takui, Y. Morita,
K. Nakasujii, Angew. Chem. 2005, 117, 6722; Angew. Chem. Int. Ed.
2005, 44, 6564; e) Z. Sun, Q. Ye, C. Chi, J. Wu, Chem. Soc. Rev.
2012, 41, 7857.
[6] a) W. W. Schoeller, A. Rozhenko, D. Bourissou, G. Bertrand, Chem.
Eur. J. 2003, 9, 3611; b) D. A. Dougherty, Acc. Chem. Res. 1991, 24,
88.
[7] a) D. Y. Zhang, D. A. Hrovat, M. Abe, W. T. Borden, J. Am. Chem.
Soc. 2003, 125, 12823; b) D. M. Lemal, J. Org. Chem. 2009, 74, 2413;
c) D. M. Lemal, J. Org. Chem. 2010, 75, 6411.
[13] M. Abe, W. Adam, W. T. Borden, M. Hattori, D. A. Hrovat, M.
Nojima, K. Nozak, J. Wirz, J. Am. Chem. Soc. 2004, 126, 574.
[14] X. Lu, D. Reid, J. Warkentin, Can. J. Chem. 2001, 79, 319.
[15] a) K. Beck, S. Huenig, Chem. Ber. 1987, 120, 477; b) W. Adam, T.
Heidenfelder, C. Sahin, Synthesis 1995, 1163.
[16] a) W. D. Clark, C. Steel, J. Am. Chem. Soc. 1971, 93, 6347; b) M.
Abe, S. Watanabe, H. Tamura, S. Boinapally, K. Kanahara, Y. Fuji-
wara, J. Org. Chem. in press, DOI: 10.1021/jo3019472.
[17] F. Kita, W. Adam, P. Jordan, W. M. Nau, J. Wirz, J. Am. Chem. Soc.
1999, 121, 9265.
[8] a) W. T. G. Johnson, D. A. Hrovat, A. Skancke, W. T. Borden, Theor.
Chem. Acc. 1999, 102, 207; b) M. Abe, S. Kawanami, C. Ishihara, M.
Nojima, J. Org. Chem. 2004, 69, 5622; c) M. Abe, C. Ishihara, S. Ka-
wanami, A. Masuyama, J. Am. Chem. Soc. 2005, 127, 10.
[18] S. L. Buchwalter, G. L. Closs, J. Am. Chem. Soc. 1979, 101, 4688–
4694.
[19] a) P. C. Hariharan, J. A. Pople, Theor. Chim. Acta 1973, 28, 213;
b) A. D. Becke, J. Chem. Phys. 1993, 98, 5648; c) C. Lee, W. Yang,
R. G. Parr, Phys. Rev. B 1988, 37, 785; d) P. C. Harihara, A. Pople,
Theor. Chim. Acta 1973, 28, 213.
[20] K. Yamaguchi, F. Jensen, A. Dorigo, K. N. Houk, Chem. Phys. Lett.
1988, 149, 537.
[21] M. Abe, H. Furunaga, D. Ma, L. Gagliardi, G. J. Bodwell, J. Org.
Chem. 2012, 77, 7612.
[22] M. J. Frisch, et al., Gaussian 09, Revision B.01, Gaussian Inc., Wall-
ingford, CT, 2009.
[23] H. E. Simmons, T. Fukunaga, J. Am. Chem. Soc. 1967, 89, 5208.
[24] G. E. Ball, Spectrosc. Prop. Inorg. Organomet. Compd. 2010, 41,
262.
[25] For the stereoselectivity during the thermal and photochemical deni-
trogenation reaction of azoalkanes, see: a) M. Abe, I. Iwakura, A.
Yabushita, S. Yagi, J. Liu, K. Okamura, T. Kobayashi, Chem. Phys.
Lett. 2012, 527, 79; b) S. Yagi, Y. Hiraga, R. Yakagi, J. Phys. Org.
Chem. 2011, 24, 894; c) E. L. Allred, R. L. Smith, J. Am. Chem. Soc.
1969, 91, 6766; d) M. B. Reyes, B. K. Carpenter, J. Am. Chem. Soc.
2000, 122, 10163; e) W. R. Roth, M. Martin, Liebigs Ann. Chem.
1967, 702, 1; f) W. Adam, M. Diedering, A. Trofimov, J. Phys. Org.
Chem. 2004, 17, 643; g) W. Adam, H. Garcia, M. Diedering, V.
Marti, M. Olivucci, E. Palomares, J. Am. Chem. Soc. 2002, 124,
12192; h) M. Hamaguchi, M. Nakaishi, T. Nagai, T. Nakamura, M.
Abe, J. Am. Chem. Soc. 2007, 129, 12981.
[9] a) D. Scheschkewitz, H. Amii, H. Gornitzka, W. W. Schoeller, D.
Bourissou, G. Bertrand, Angew. Chem. 2004, 116, 595; Angew.
Chem. Int. Ed. 2004, 43, 585; b) A. Rodriguez, R. A. Olsen, N. Gha-
deri, D. Scheschkewitz, F. Tham, L. J. Mueller, G. Bertrand, Angew.
Chem. 2004, 116, 4988; Angew. Chem. Int. Ed. 2004, 43, 4880; c) G.
Fuks, N. Saffon, L. Maron, G. Bertrand, D. Bourissou, J. Am. Chem.
Soc. 2009, 131, 13681; d) P. Henke, T. Pankewitz, W. Klopper, F.
Breher, H. Schnçckel, Angew. Chem. 2009, 121, 8285; Angew.
Chem. Int. Ed. 2009, 48, 8141; e) J. Ma, Y. Ding, K. Hattori, S. Ina-
gaki, J. Org. Chem. 2004, 69, 4245; f) M. Abe, C. Ishihara, A. Take-
gami, J. Org. Chem. 2004, 69, 7250; g) M. Abe, E. Kubo, K. Nozaki,
T. Matsuo, T. Hayashi, Angew. Chem. 2006, 118, 7992; Angew.
Chem. Int. Ed. 2006, 45, 7828; h) T. Nakamura, L. Gagliardi, M.
Abe, J. Phys. Org. Chem. 2010, 23, 300; i) T. Nakamura, A. Takega-
mi, M. Abe, J. Org. Chem. 2010, 75, 1956; j) K. C. Mondal, H. W.
Roesky, M. C. Schwarzer, G. Frenking, I. Tkach, H. Wolf, D. Krat-
zert, R. Herbst-Irmer, B. Niepçtter, D. Stalke, Angew. Chem. 2013,
125, 1845; Angew. Chem. Int. Ed. 2013, 52, 1801.
[10] A. Doddi, C. Gemel, M. Winter, R. A. Fischer, C. Goedecke, H. S.
Rzepa, G. Frenking, Angew. Chem. 2013, 125, 468; Angew. Chem.
Int. Ed. 2013, 52, 450.
[11] F. Breher, Coord. Chem. Rev. 2007, 251, 1007.
[12] a) H. Sugiyama, S. Ito, M. Yoshifuji, Angew. Chem. 2003, 115, 3932;
Angew. Chem. Int. Ed. 2003, 42, 3802; b) S. Ito, J. Miura, N. Morita,
M. Yoshifuji, A. J. Arduengo III, Angew. Chem. 2008, 120, 6518;
Angew. Chem. Int. Ed. 2008, 47, 6418; c) M. Yoshifuji, Y. Hirano, G.
Schnakenburg, R. Streubel, E. Niecke, S. Ito, Helv. Chim. Acta 2012,
95, 1723; d) C. Cui, M. Brynda, M. M. Olmstead, P. P. Power, J. Am.
Received: January 5, 2013
Revised: April 22, 2013
Published online: June 20, 2013
10404
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Chem. Eur. J. 2013, 19, 10395 – 10404