Bulky Substituent Effect on Reactivity of Localized Singlet Cyclopentane-1,3-diyls with π-Single Bonding (C-π-C) Character

Article abstract of DOI:10.1002/asia.201901253

Thorough investigation of key intermediates, such as long-lived singlet diradicals, is essential to understand the homolytic bond cleavage reactions. In this study, we evaluate the effect of bulky substituents at the meta-position of the phenyl ring on the bond formation process in singlet 2,2-diethoxy-1,3-diarylcyclopentane-1,3-diyls. The bulky groups have significant influence on the diradical lifetime, as such, when the triisopropylphenyl group was used, the lifetime was 45 times longer than that of the parent diradical in benzene at 293 K.

Full text of DOI:10.1002/asia.201901253

DOI: 10.1002/asia.201901253
Full Paper
Bulky Substituent Effect on Reactivity of Localized Singlet
Cyclopentane-1,3-diyls with p-Single Bonding (C-p-C) Character
Rikuo Akisaka[a] and Manabu Abe*[a, b, c]
This paper is dedicated to Professor Jun-ichi Yoshida who passed away on September 14th 2019.
Abstract: Thorough investigation of key intermediates, such
as long-lived singlet diradicals, is essential to understand the
homolytic bond cleavage reactions. In this study, we evalu-
ate the effect of bulky substituents at the meta-position of
the phenyl ring on the bond formation process in singlet
2,2-diethoxy-1,3-diarylcyclopentane-1,3-diyls.
The
bulky
groups have significant influence on the diradical lifetime, as
such, when the triisopropylphenyl group was used, the life-
time was 45 times longer than that of the parent diradical in
benzene at 293 K.
Introduction
Selectivity and kinetics of chemical reactions, such as Diels–
Alder reaction, are largely affected by the electronic and steric
features of substituents near the reaction center.[1–4] The steric
effect has been known to influence stereoselectivity in asym-
metric syntheses.[5–8] Kinetic stabilization induced by steric hin-
drance has also played a crucial role in stabilizing and isolating
low-valent compounds.[9–11] It is essential for understanding the
steric effect on fundamental reactions.
Radical-radical coupling reaction[12–14] is an important funda-
mental reaction, involved in processes such as termination of
radical polymerization[15–17] and deactivation of open-shell spe-
cies.[18–20] Therefore, it is important to understand how the cou-
pling reaction is influenced by steric effects in order to develop
new organic materials with open-shell characters.
In the past two decades we have studied the electronic,[21–23]
stretch,[24,25] and steric effects[26,27] on the reactivity of 2,2-dia-
lkoxy-1,3-diarylcyclopentane-1,3-diyls (S-DR1) generated in the
photodenitrogenation of the corresponding azoalkanes AZ1
(Scheme 1). These studies have shown that the alkoxy group
greatly influences the lifetime of S-DR1 (Table 1). Thus, the flex-
Scheme 1. Generation of singlet diradical S-DR1 and the formation of ring-
closed product.
Table 1. Effect of alkoxy and aryl groups on the lifetime (t293) of S-DR1 in
benzene at 293 K, and activation parameters (DH° and DS°) for the radi-
cal-radical coupling reaction
S-DR1
OR
Ar
t293 [ns] DH° [kJmolÀ1
]
DS° [JmolÀ1 TÀ1
]
S-DR1a
S-DR1b
S-DR1c
S-DR1d
OCH3
OC10H21
OCH3
Ph
Ph
Ar’ 580
299
2146
32.7Æ0.2
36.1Æ0.4
33.0Æ0.2
36.5Æ0.5
À8.1Æ1.2
À13.1Æ1.1
À12.9Æ1.0
À19.4Æ1.0
OCH2Ar’ Ar’ 5394
Ar’=3.5-dimethoxybenzene.
ible and large substituents (OR=OC10H21, OCH2Ar’) increase
the activation enthalpy and decrease the activation entropy of
the radical-radical coupling (ring-closing) reaction to increase
the lifetime from ꢀ300 ns (OR=OCH3) to ꢀ5.4 ms in benzene
at 293 K (Table 1).
[a] R. Akisaka, Prof. M. Abe
Department of Chemistry
Graduate School of Scienece
The results prompted us to investigate the effect of more
rigid and bulkier substituents (R’), such as mesityl (Mes) and
triisopropylphenyl (Tip) groups, at the meta position of the
phenyl ring, on the reactivity of singlet diradicals S-DR2-4
(Figure 1), which have p-single bonding character between the
radical centers.[20] The bulky substituents are well known to ki-
Hiroshima University
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526 (Japan)
E-mail: mabe@hiroshima-u.ac.jp
[b] Prof. M. Abe
Hiroshima University
Research Center for Photo-Drug-Delivery System (HiU-P-DDS)
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526 (Japan)
[c] Prof. M. Abe
JST-CREST
K’s Gobancho 6F, 7, Gobancho, Chiyoda-ku, Tokyo 102-0075 (Japan)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/asia.201901253.
This manuscript is part of a special collection celebrating the 100th Annual
Meeting of the Chemical Society of Japan (CSJ). Click here to see the Table
of Contents of the special collection.
Figure 1. Localized singlet diradicals S-DR2-4 with bulky substituents.
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netically stabilize the low-valent species[9] and open-shell mole-
cules.[28–31] The effect of R’ groups on the reactivity of the sin-
glet diradicals was assessed by theoretical calculations, transi-
ent absorption spectroscopic analysis, and product analysis of
S-DR2-4.
Results and Discussion
Theoretical calculations were conducted at the UB3LYP-6-
31G(d)[32–34] with broken-symmetry (BS) methods[35] to evaluate
the effect of bulky substituents R’ had on the ring-closing reac-
tion of S-DR2 (Figure 2). The previous study has demonstrated
Scheme 2. (a) Syntheses of AZ3a–c and AZ4a–c (b) Syntheses of AZ3d,e
and AZ4d,e.
For all the azo compounds AZ3a–e,4a–e, the product analy-
ses of the photochemical denitrogenation reactions were con-
ducted at room temperature in C6D6 using a light-emitting
diode (LED) lamp (365 nm) in nitrogen atmosphere (Figure 3
for AZ4e). The individual products were detected using NMR
analysis. The NOESY NMR spectroscopic analysis clearly indicat-
ed that all the products possess the structure of ret-CP3,4 (see
SI).
Figure 2. Electronic energy difference including zero-point energy correc-
tions DE (and the Gibbs energy difference DG298 at 298 K in the parenthesis)
between CP2/TS2 and S-DR2.
that the level of theory reproduced well the experimental ob-
servation.[26] The substituent effect on the ring-closing reac-
tions were examined for R’=bromine (Br), methoxy (MeO),
Mes, and Tip group as bulky substituents. The most thermody-
namically stable isomer was found to be ret-CP2, although the
activation energy of the ring-closing reaction from S-DR2 to
inv-CP2 was smaller than that for ret-CP2. As found in the pre-
vious study,[26] the repulsive interaction between the aryl
group and the cyclopentane ring in ret-TS is larger than that in
inv-TS. In the ring-closing reaction for inv-CP2, the introduction
of the bulky group resulted in increased activation energy and
destabilized the ring closure product. Interestingly, the singlet
diradical S-DR2e (R’=Tip) proved to be more stable than inv-
CP2e.[36] In contrast to the large substituent effects on the
ring-closing reaction to inv-CP2, the relatively small substituent
effects on the ring-closing reaction to ret-CP2 were demon-
strated by the quantum chemical calculations.
Figure 3. 1H NMR spectrum (C6D6, 400 MHz) bottom: before photochemical
irradiation of AZ4e. middle: 1-min irradiation, top: 6-min irradiation using
365-nm LED under nitrogen.
Singlet diradicals S-DR4a–e were generated in the photoly-
sis of AZ4a–e with a 500-W Xenon lamp through a monochro-
mator (lirr =360Æ10 nm) in a glassy matrix of 2-methyltetrahy-
drofurane (MTHF) at 110 K. Absorption bands were observed in
the visible region (lmax =570–600 nm) for all azo compounds
AZ4 (Table 1, for AZ4e in Figure 4a, for other AZ4 in Fig-
ure S1). The absorption bands were similar to those of the pre-
viously observed singlet diradicals S-DR3a, which have an ab-
sorption maximum at 570 nm,[38] indicating that the bulky sub-
stituents at the meta position have no strong effect on the
electronic nature of the singlet diradicals. The 570-nm band
Azoalkanes AZ3a–c,4a–c, which are the precursors of S-
DR3a-c,4a–c, were synthesized from the arylnitriles
(Scheme 2).[38] The Mes-substituted AZ3d,4d and Tip-substitut-
ed AZ3e,4e were synthesized from AZ3b,4b by Suzuki–
Miyaura coupling (Scheme 2b). The proton NMR analysis of
azo compounds AZ3 and AZ4 displayed clear NOE correlations
between the methine proton and the ethoxy methylene pro-
tons to determine their stereochemistries (Figures S2–S9 in
Supporting information).
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Table 2. Aryl groups effect on the lifetime (t293) of S-DR3,4 in benzene at
293 K, and activation parameters (Ea, logA, DH°, and DS°) for the radical-cou-
pling reaction.
S-DR
3a/4a
3b/4b
3c/4c
3d/4d
3e/4e
lmax [nm][a]
t293 [ms][b]
n.d./575
0.94Æ0.3
n.d./595
1.38Æ0.4
n.d./587
2.21Æ0.2
n.d./598
6.24Æ0.4 19.6Æ0.8
n.d./598
/0.53Æ0.2 /0.88Æ0.3 /1.30Æ0.3 /5.68Æ0.4 /23.8Æ1.4
Ea [kJmolÀ1
]
37.2Æ0.6
37.7Æ0.5
40.8Æ0.2
42.6Æ0.1 48.5Æ0.1
[c]
/35.1Æ0.5 /35.8Æ0.6 /35.3Æ0.2 /43.1Æ0.2 /47.5Æ0.1
Log(A [sÀ1])[c] 12.7Æ0.1
12.6Æ0.1
13.0Æ0.1
12.8Æ0.1 13.3Æ0.1
/12.4Æ0.1 /12.4Æ0.1 /12.1Æ0.1 /12.9Æ0.1 /13.1Æ0.1
DH°
34.6Æ0.6
35.1Æ0.5
38.2Æ0.2
40.1Æ0.1 45.9Æ0.1
[d]
[kJmolÀ1
DS°
]
/32.6Æ0.5 /33.2Æ0.6 /32.7Æ0.4 /40.7Æ0.2 /45.0Æ0.4
À11.0Æ2.0 À12.5Æ1.6 À5.9Æ0.6 À8.2Æ0.4 2.1Æ0.3
/À13.2Æ1.7 /À15.4Æ2.0 /À20.3Æ1.3 /À5.7Æ0.5 /À2.5Æ0.5
[JmolÀ1 TÀ1
]
[d]
[a] Measured in MTHF at 110 K. Not determined (n. d.) for S-DR3. [b] In ben-
zene at 293 K; calculated from the Arrhenius parameter, Ea and LogA. The
error was obtained from the mean error of activation energy Ea. [c] Deter-
mined from the Arrhenius plot of the lifetimes of the singlet diradicals at
5 temperatures between 285 and 333 K. The mean error obtained from re-
gression analysis is shown as errors. [d] Determined from the Eyring plot of
the lifetimes of the singlet diradicals at 5 temperatures between 285 and
333 K. The mean error obtained from regression analysis is shown as errors.
Figure 4. (a) Absorption spectrum in the photolysis (lirr =360Æ10 nm) of
AZ4e (2 mm) in MTHF matrix at 110 K (blue line); (b) Transient absorption
spectrum in benzene after laser flash photolysis (355 nm, 7 mJ, 4–6-ns pulse)
of AZ4e (5 mm) under nitrogen at 295 K (orange point). (c,d) Time profile of
S-DR4e (lobs =600 nm) generated by the laser flash photolysis of AZ4e
(lexc =355 nm) in benzene at 295 K under nitrogen (c, black point) and
under air (d, red point).
has been assigned to the p–p* electronic transition of the p-
single bonding (C-p-C) system.[39]
The laser flash photolysis (LFP) of 5 mm benzene solution of
AZ3a–e and AZ4a–e was conducted using a 355-nm laser
(7 mJ, 4–6-ns pulse) in nitrogen atmosphere to directly observe
the transient species in the photolysis. The strong absorption
band at around 580 nm (Figure 4b) was attributed to the pho-
tolysis of AZ4e, which decayed with first-order kinetics having
a lifetime of about 20000 ns (Table 2, Figures 5 for the photol-
ysis of other AZ3,AZ4). The absorption band was consistent
with that observed for low-temperature photolysis of AZ4e
(Figure 4a). The lifetimes of the transient species were nearly
the same with those obtained in air (21000 ns, Figure 4c,d).
From the experimental results, the transient species detected
at around 580 nm was assigned to the singlet diradicals S-
DR3,4, which decay to the ring-closing compounds CP3,4
(Figure 3). Interestingly, the lifetime of S-DR4e is longer than
that of S-DR3e despite S-DR3 having a longer lifetime than S-
DR4 for other substituents (Table 2).
The lifetimes of singlet diradicals S-DR3,4 were largely de-
pendent on the substituent R’ (Table 2, Figure 5). The activa-
tion parameters, Ea, logA, DH°, and DS°, were determined
from the Arrhenius and Eyring plots, which were obtained by
measuring the temperature dependency of the lifetime at
285–333 K (Table 2, see also Figure S25–S28). As shown in
Table 2, the lifetime of S-DR3,4 lengthens with an increasing
size of the substituent. The bulkier group increased the activa-
tion energy (Ea) and activation enthalpy (DH°). To obtain infor-
mation on the mechanistic insights and substituent effect on
the decay of S-DR3,4, the activation entropy (DS°) was plotted
Figure 5. Time profile of S-DR3a–e (a,c) and S-DR4a–e (b,d) generated by
laser flash photolysis of AZ3,4a–e (lexc =355 nm) in benzene at 295 K.
against DH° (Figure 6). The isokinetic relationship[40,41] was
roughly observed for S-DR3a–e and S-DR4a–e. The correlation
displays that the introduction of the bulky groups increases
both the activation enthalpy and activation entropy (Enthalpy
and Entropy Compensation).
By comparison of the experimentally observed activation pa-
rameters (Table 2) with the computationally predicted reaction
pathways (Figure 2), the fate of S-DR3,4 was to produce the
corresponding inv-CP3,4, because the predicted activation en-
ergies from S-DR2 to inv-CP2 fit with the observed energy bar-
riers.[26,42] However, the observed product was ret-CP3,4 in the
photodenitrogenation of AZ3,4 at room temperature
(Figure 3). To understand the contradiction, AZ4e was irradiat-
ed in deuterium toluene (d8-tol) at 200 K in the NMR cavity
using a quartz rod with a 355-nm YAG laser (Figure 7).[42] After
12-min irradiation, two products were observed in a ratio of
75/25 by in situ NMR analysis (Figure 7b). By comparing the
NMR signals of the authentic sample of ret-CP4e, one of the
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Figure 6. Enthalpy-entropy compensation plot for the ring closing process
of single diradicals S-DR3 and S-DR4.
Figure 8. (a) Absorption spectra of S-DR4e at 122 K in MTHF. (blue: just after
generation, orange: after 10 min, gray: after 30 min, yellow after 1 h), (b) the
decay trace of S-DR4e at 597 nm under 122 K in M-THF.
At 205 K, inv-CP4e fully converted to ret-CP4e after 11 h
(Figure 9), from which the lifetime of inv-CP4e was determined
to be 1.5ꢁ104 s (Figure 9e). The rate constant for the forma-
tion of ret-CP4e was consistent with the decay rate constant
of inv-CP4e. The activation parameters of isomerization were
determined to be Ea =63.8Æ0.9 kJmolÀ1, log (AsÀ1À1)=12.0Æ
0.2, DH° =62.0Æ0.9 kJmolÀ1, and DS° =À20.3Æ4.4 JmolÀ1 KÀ1
using the Arrhenius plot (Figure 10) and the Eyring plot (Fig-
ure S30) in the temperature range from 200 K to 223 K (Fig-
ure S29). In comparison with inv-CP1a to ret-CP1a (Ea =60.4Æ
0.9 kJmolÀ1, log (AsÀ1À1)=12.0Æ0.1),[42] a small increase in acti-
vation energy was found. The singlet diradical S-DR4e must
exist as an intermediate in the isomerization process from inv-
CP4e to ret-CP4e (Figure 2). The determined activation energy
would correspond to the activation barrier from inv-CP4e to S-
DR4e, or that from S-DR4e to ret-CP4e. The theoretical calcu-
lations suggest that introducing bulky substituents slightly in-
creases the activation barrier of the formation ret-CP2 from S-
DR2. Thus, the obtained activation energy is proposed to be
the latter process, that is, from S-DR4e to ret-CP4e.
The two energy barriers from S-DR3,4 were experimentally
determined from the LFP measurements and NMR analysis. In-
terestingly, the process to the inversion product inv-CP3,4 was
profoundly affected by introducing the bulky substituents. In
contrast, the energy barrier to ret-CP3,4 was not affected as
much by the introduction of the bulky substituents. Thus, the
energy barriers of the conversion from S-DR3e/4e to inv-
CP3e/4e was much higher than those from S-DR3a/4a to inv-
CP3a/4a (Table 2), although the difference in energy barriers
between S-DR1a to ret-CP1a and S-DR4e to ret-CP1e was
small.
Figure 7. 1H-NMR spectrum at 200 K (a)AZ4e, (b) after irradiation of AZ4e,
(c) after irradiation of AZ4e with benzophenone, (d) after 440 min under
dark condition of (c), (e) after warming to 234 K then re-cooling to 200 K,
(f) after irradiation of AZ4e at room temperature then cooling to 200 K.
products was assigned to ret-CP4e (Figure 7b vs. 7 f). Another
pair of signals at 5.44 and 5.67 ppm was attributed to the sin-
glet diradical S-DR4e or inv-CP4e (Figure 7b), because the
energy difference between S-DR2e and inv-CP2e was comput-
ed to be very small (Figure 2).
To examine the stability of S-DR4e at low temperature, S-
DR4e was produced in MTHF at 122 K in the photochemical
denitrogenation of AZ4e (Figure 8a). The lifetime of S-DR4e
was found to be ꢀ15 min using UV-vis absorption spectro-
scopic analysis (Figure 8b). Thus, another species having 5.44
and 5.67 ppm NMR signals at 200 K was not S-DR4e, but inv-
CP4e because the NMR signals were observed for at least for
1 h at 200 K, suggesting that the DFT computations overesti-
mate the stability of S-DR2e, or underestimate the stability of
inv-CP2e.
To confirm the formation of inv-CP4e (Figure 7b), the triplet
sensitized denitrogenation of AZ4e was conducted in the
presence of benzophenone at 200 K (Figure 7c). As found pre-
viously for the photoreaction of AZ1a,[42] the selective forma-
tion of inv-CP4e at 5.44 and 5.67 ppm was observed by in situ
NMR analysis. The compound thermally isomerized to ret-CP4e
(Figures 7c–e), indicating that the decay process observed in
the LFP experiments produced the kinetically favored inv-
CP4e.
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To obtain more insights into the substituent effect, the tran-
sition structures for the two processes were compared to each
other (Figure 11). In the transition state producing inv-CP2e,
the distance between two methyl carbons was calculated to
be 4.29 ꢂ. In ret-TS2e, the distance was found to be 5.88 ꢂ,
longer than that of inv-TS2e, indicating that inv-TS4e has a
more crowded structure than ret-TS4e.
Figure 11. Optimized structures of ret-TS2e (left) and inv-TS2e (right) at the
BS-UB3LYP/6-31G(d) level of theory. Hydrogen atoms were omitted for clari-
ty.
Conclusions
The substituent effect of bromine (Br), methoxy (MeO), mesityl
(Mes), and triisopropylphenyl (Tip) group at the meta-position
was examined in the ring-closing reaction, radical-radical cou-
pling, of the singlet diradicals S-DR3,4. The bulkier substitu-
ents, such as Mes and Tip groups, increased the lifetime of the
singlet diradicaloid species. Thus, when the Tip group was
used, the lifetime of S-DR4e (24 ms) was 45 times longer than
that of parent diradical S-DR4a (530 ns) at 293 K in benzene.
The entropy of activation for the Tip-substituted diradical was
positive. Using low-temperature NMR analysis, the singlet dir-
adical S-DR4e was found to produce inv-CP4e, followed by
thermal isomerization to give ret-CP4e via S-DR4e.
Figure 9. 1H-NMR spectra at 205 K in [D8]toluene: (a) after irradiation of
AZ4e in the presence of benzophenone (* marks were correspond to AZ4e
and + marks were corresponded to inv-CP4e), (b) after 127 min under dark
conditions, (c) after 673 min under dark conditions (* marks were correspond
C
to AZ4e and marks were corresponded to ret-CP4e), (d) ret-CP4e; (e) Time
profiles of integration ratio of NMR signals between inv-CP4e (red,
5.45 ppm) or ret-CP4e (blue, 5.78 ppm) and AZ4e(5.35 ppm)at 205 K.
Experimental Section
Full experimental methods including detailed synthetic procedures,
characterization data, (NMR, MS), Arrhenius plot, Eyring plot for
LFP and low temperature NMR analysis, and DFT calculations are
available in the Supporting Information.
Conflict of interest
The authors declare no conflict of interest.
Keywords: enthalpy-entropy
compensation
·
Kinetic
stabilization · laser flash photolysis · singlet diradicals · steric
effect
Figure 10. Arrhenius plot of thermal isomerization process from inv-CP4e to
ret-CP4e.
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Version of record online: && &&, 0000
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Chem. Asian J. 2019, 00, 0 – 0
www.chemasianj.org
6
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ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
The effect of bulky substituents at the
meta-position of the phenyl ring on the
bond formation process in singlet dirad-
icals has been evaluated. When the trii-
sopropylphenyl group is introduced, the
lifetime is found to be 40 times longer
than that of the parent diradical in ben-
zene at 293 K.
Rikuo Akisaka, Manabu Abe*
&& – &&
Bulky Substituent Effect on Reactivity
of Localized Singlet Cyclopentane-1,3-
diyls with p-Single Bonding (C-p-C)
Character
Chem. Asian J. 2019, 00, 0 – 0
www.chemasianj.org
7
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ