Generation and intermolecular trapping of 1,2-diaza-4silacyclopentane-3,5- diyls in the denitrogenation of 2,3,5,6-tetraaza-7silabicyclo[2.2.1]hept-2-ene: An experimental and computational study

Article abstract of DOI:10.1021/jo902714c

Chemical Equation Presented In our previous computational study, we found that silicon and nitrogen atoms have a notable effect on the reactivity of l,2-diaza-4-silacyclopentane-3,5-diyls. Thus, the singlet state of the diradical was calculated to be much more stable than the corresponding ring-closing product, i.e., 2,3-diaza-5silabicyclo[2.1.0]pentane, and the triplet state of the diradical. In the present study, derivatives of the diradical were generated experimentally in the denitrogenation of precursor azoalkanes, i.e., 2,3,5,6tetraaza-7-silabicyclo[2.2.1]hept-2-enes, which can be prepared by cycloaddition of a diazasilole with 4-phenyl-1,2,4-triazole-3,5- dione (PTAD) or 4-methyl-1,2,4-triazole-3,5-dione (MTAD). The diradicals were trapped intermolecularly to afford polycyclic compounds. The computational studies (UB3LYP/ 6-31G) of the denitrogenation of a model azoalkane suggested that stepwise denitrogenation with an activation energy of ca. 22 kcal/mol is the thermodynamically favored pathway for generation of the singlet diradical 1,2-diaza-4-silacyclopentane-3,5-diyl derivative via a 1,4-diazenyldiradical intermediate. The low activation energy of the denitrogenation reaction was consistent with the experimental observation that the azoalkane was labile under the preparation conditions used in this study.

Full text of DOI:10.1021/jo902714c

pubs.acs.org/joc
Generation and Intermolecular Trapping of 1,2-Diaza-4-
silacyclopentane-3,5-diyls in the Denitrogenation of 2,3,5,6-Tetraaza-7-
silabicyclo[2.2.1]hept-2-ene: An Experimental and Computational Study
Takeshi Nakamura,† Akinobu Takegami,‡ and Manabu Abe*,†
†Department of Chemistry, Graduate School of Science, Hiroshima University (HIRODAI),
Higashi-Hiroshima, Hiroshima 739-8526, Japan, and ‡Department of Materials Chemistry,
Graduate School of Engineering, Osaka University, Suita 565-0871, Japan
mabe@hiroshima-u.ac.jp
Received December 24, 2009
In our previous computational study, we found that silicon and nitrogen atoms have a notable effect
on the reactivity of 1,2-diaza-4-silacyclopentane-3,5-diyls. Thus, the singlet state of the diradical
was calculated to be much more stable than the corresponding ring-closing product, i.e., 2,3-diaza-5-
silabicyclo[2.1.0]pentane, and the triplet state of the diradical. In the present study, derivatives of the
diradical were generated experimentally in the denitrogenation of precursor azoalkanes, i.e., 2,3,5,6-
tetraaza-7-silabicyclo[2.2.1]hept-2-enes, which can be prepared by cycloaddition of a diazasilole with
4-phenyl-1,2,4-triazole-3,5-dione (PTAD) or 4-methyl-1,2,4-triazole-3,5-dione (MTAD). The diradicals
were trapped intermolecularly to afford polycyclic compounds. The computational studies (UB3LYP/
6-31G*) of the denitrogenation of a model azoalkane suggested that stepwise denitrogenation with an
activation energy of ca. 22 kcal/mol is the thermodynamically favored pathway for generation of the
singlet diradical 1,2-diaza-4-silacyclopentane-3,5-diyl derivative via a 1,4-diazenyldiradical intermediate.
The low activation energy of the denitrogenation reaction was consistent with the experimental
observation that the azoalkane was labile under the preparation conditions used in this study.
Introduction
states are significantly different from those of the triplet
states.2 In localized diradicals (Scheme 1),3 for example, the
singlet states are intermediates in processes that involve
homolytic bond cleavage and formation. Thus, localized
singlet diradicals are intramoleculary reactive and very
short-lived.4 By contrast, the triplet states are connected to
the singlet states via the spin-forbidden intersystem-crossing
Diradicals are defined as molecules in which two electrons
occupy two degenerate or nearly degenerate molecular orbi-
tals.1 Thus, there are two spin-multiplicities—singlet vV and
triplet vv states. The reactivity and properties of the singlet
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Res. 1997, 30, 238–244. (c) Matsuda, K.; Iwamura, H. J. Chem. Soc., Perkin
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1956 J. Org. Chem. 2010, 75, 1956–1960
Published on Web 02/12/2010
DOI: 10.1021/jo902714c
r
2010 American Chemical Society

Nakamura et al.
JOCArticle
SCHEME 1. Chemistry of Localized Singlet and Triplet
Diradicals
be isolable. In a very recent computational study, we found a
notable cooperative effect of nitrogen and silicon atoms in
the energetic stabilization of the singlet state of a 1,2-diaza-4-
silacyclopentane-3,5-diyl S-K, which was calculated to be
more stable than the ring-closing molecule CP-K and the
triplet state T-K.13 These calculations suggest that the singlet
diradical S-K is long-lived enough to be intercepted in
bimolecular reactions. In the present study, the generation
and intermolecular trapping reaction of the 1,2-diaza-4-
silacyclopentane-3,5-diyl derivative 1 was experimentally
investigated in the denitrogenation reaction of the corre-
sponding azoalkane. Indeed, the diradical species was found
to be trapped intermolecularly by 4-phenyl-1,2,4-triazole-
3,5-dione (PTAD) and 4-methyl-1,2,4-triazole-3,5-dione
(MTAD) to give polycyclic compounds. The mechanism of
the thermal denitrogenation process was also investigated at
the (U)B3LYP/6-31G(d) level of theory.
process (ISC). Thus, the lifetime of localized triplet diradicals
is long enough for conformational change and intermole-
cular reactivity.5 The structure and reactivity of triplet
diradicals have been thoroughly investigated because these
species can be detected using conventional spectroscopic
analyses, including ESR measurements.2,6-8
Experimental studies of localized singlet 1,3-diradicals
recently became possible after it was discovered that appro-
priate substituents at the C(2) position make the singlet
energetically more stable than the triplet.9 For example,
singlet 1,3-diradicals A-C in five-membered ring systems
were found to have lifetimes of up to 10 ms.10 By taking
advantage of the kinetic stability of four-membered ring-
systems and the unique properties of heteroatoms, singlet
diradicals D-H were isolated, and their structures were
determined by X-ray crystallography.11 Computational cal-
culations predicted that the singlet diradicals I and J12 would
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Results and Discussion
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The denitrogenation of cyclic azoalkanes may be the best
way to cleanly generate diradical species. Our reaction
scheme for the synthesis of the promising azo compound
AZ, which was the precursor of the diradical 1, is shown in
Scheme 2. Thus, the synthesis of the 1,2,4-diazasilole 2 was
begun with the denitrogenation reaction of the commercially
available tetrazine 3 with silylene, which was generated by
thermolysis of the silirane 4. Thus, an equimolar mixture of
the tetrazine 3 and silirane 4 was heated for 2 days at 125 °C
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SCHEME 2. Generation of 1,2-Diaza-4-silacyclopentane-3,5-
diyls 1 from Azoalkane AZ
SCHEME 4. Computational Study of the Denitrogenation of a
Model Azoalkane 7
in dry benzene in a sealed tube. After the products of the
thermolysate were purified, the diazasilole 2 was obtained
as a yellow powder in 27% yield. The freshly prepared
diazasilole (1 equiv) was mixed with PTAD (3 equiv) in
benzene-d6 and sealed. The reaction was monitored using 1H
NMR spectroscopy. At a reaction temperature of approxi-
mately 60 °C, the signals corresponding to the diazasilole 2
decreased and new signals were observed. After 4 h at 60 °C,
compound 2 was completely consumed with concomitant
product formation. After removal of the solvent under
reduced pressure, the products were subjected to silica gel
column chromatography. The polycyclic compound 5a was
isolated in 53% yield (Scheme 3).
diradical 1 is trapped by PTAD to afford 5. It is well-known
that triplet diradicals are trapped by molecular oxygen at the
diffusion rate constant, and the singlet of compound 8 has
been calculated to be much more stable than the triplet state
with a large singlet-triplet energy gap of ΔEST = ES - ET =
-9.3 kcal/mol (Scheme 4).13 To understand the experimental
observations and determine the mechanism of the denitro-
genation reaction, we decided to compute the denitrogena-
tion of the model azoalkane 7 (Scheme 4), in which the
phenyl groups were replaced by hydrogen atoms and a
methyl group was used in place of the tert-butyl group on
the silicon atom.
SCHEME 3. Formation of Polycyclic Adducts 5 in the
Denitrogenation of Azoalkanes AZ
The ring-closed compound 6a was not detected, even in
trace amounts. As a result of the reaction of the diazasilole
with MTAD (3 equiv), the corresponding adduct 5b was
obtained in 90% yield. The structures of 5a,b were deter-
mined unequivocally by spectroscopic analyses, including
1H, 13C NMR, HRMS, and elemental analysis (see the
Experimental Section). The formation of the compounds
5a,b strongly suggests that 1,2-diaza-4-silacyclopentane-3,5-
diyls 1a,b were generated from the azoalkanes AZa,b, which
formed during the cycloaddition reaction of the diazasilole 2
with either PTAD or MTAD. Trace amounts of the azoal-
kanes AZ were not detected during the Diels-Alder (DA)
cycloaddition reaction of 2 with PTAD or MTAD. These
results suggest that the denitrogenation of AZ is faster than
the DA reaction of 2. When the DA reaction with MTAD
was performed in the presence of 10 equiv of vinylene
carbonate or cyclopentadiene, the compound 5b was ob-
tained in 86% yield. We could not detect trapping products
other than 5b. When the DA reaction was conducted under
oxygen atmosphere (O2), the PTAD adduct 5a was obtained
in 58% yield. Thus, we propose that the singlet state of the
FIGURE 1. Potential energy surface for the C(1)-N(2) and C(4)-
N(3) bond-breaking during the denitrogenation of azoalkane 7.
In general, denitrogenation of azoalkanes proceeds via
one of two possible mechanisms14—stepwise versus con-
certed (Scheme 4). First, to obtain information on the
mechanism, a three-dimensional (3-D) potential energy sur-
face (PES) for the two C-N bonds that are broken, i.e.,
C(1)-N(2) and C(4)-N(3), was calculated at the UB3LYP/
6-31G(d) theoretical level (Figure 1). Apparently, the shape
of the 3-D plot is consistent with the energetically preferred,
stepwise denitrogenation mechanism that proceeds via the
diazenyl diradical DZ. Thus, the stepwise pathway via TS1
(ΔE = ∼22 kcal/mol) was calculated to be the energetically
(14) (a) Engel, P. S. Chem. Rev. 1980, 80, 99–150. (b) Simpson, C. J. S. M.;
Wilson, G. J.; Adam, W. J. Am. Chem. Soc. 1991, 113, 4728–4732. (c) Reyes,
M. B.; Carpenter, B. K. J. Am. Chem. Soc. 2000, 122, 10163–10176.
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2004, 17, 643–655.
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FIGURE 2. Substituent effects on the electronic configuration of
the lowest singlet state of cyclopentane-1,3-diyls DR and the deni-
trogenation mechanism of the azoalkanes, diazabicyclo[2.2.1]hept-
2-enes.
FIGURE 3. Orbital interaction diagram of the high-lying 2p AOs
with the low-lying Si-C σ*, depicting the electronic configura-
tion of the lowest singlet state of the diradical 8 and the relative
energy level of the symmetric NBMO (ψS) and the antisymmetric
NBMO (ψA).
favored process by ca. 6 kcal/mol compared with the con-
certed process, which proceeds via the transition state TS2
(ΔE = ∼28 kcal/mol). Actually, the transition state TS1,
ΔE = 21.7 kcal/mol including zero-point-energy correction,
was optimized with an imaginary frequency of 302 cm-1 at
the same level of theory. The bond lengths of the two C-N
bonds were calculated to be 242 and 154 pm, respectively.
The concerted transition state TS2 was not located at the
same level of theory. Thus, optimization of TS2 ultimately
produced the structure of the transition state TS1. The
diazenyl diradical DZ was also labile at the same level of
theory to afford molecular nitrogen and the singlet diradical
8. This result means that the activation energy of the second
C-N bond cleavage is a process without barriers. The
question then becomes why the stepwise denitrogenation
process is energetically favored.
We recently reported that the mechanism for the denitro-
genation of bicyclic azoalkanes, e.g., diazabicyclo[2.2.1]-
hept-2-enes, is largely dependent upon the electronic config-
uration of the lowest singlet state of the diradicals DR in the
denitrogenation reaction, i.e., SS-DR versus SA-DR, where
electronic configuration is determined by the substituent X
(Figure 2).15 When the two electrons selectively occupy the
symmetric, nonbonded molecular orbital (NBMO, ψS, X =
F, OR), concerted denitrogenation is a symmetry-forbidden
process, since the phases of the diyl-HOMO (ψS) and the N2-
LUMO (π*) do not match. Thus, the stepwise denitrogena-
tion reaction was the energetically favored process. By
contrast, concerted denitrogenation is the energetically
favored mechanism for azoalkanes (X=H, SiR3), since the
two electrons selectively occupy the antisymmetric NBMO
(ψA) of the resulting diradical SA-DR. Thus, the phases of the
diyl-HOMO (ψA) and the N2-LUMO (π*) match. To under-
stand the energetic preference of the stepwise denitrogena-
tion of the azoalkane AZ (Scheme 4 and Figure 1), the
electronic configuration of the lowest singlet state of the
diradical 8 was calculated using the complete active space
multiconfiguration SCF (CASSCF) method. As shown in
Figure 3, the symmetric NBMO (ψS), i.e., diyl-HOMO, was
at a lower energy than the antisymmetric NBMO (ψA), i.e.,
diyl-LUMO. At the CASSCF(2,2)/6-31G(d) level of theory,
74% of the two electrons are calculated to selectively occupy
the bonding ψS orbital. This result indicates that the diradi-
cal is weakly π-single bonded between the two radical sites.
The electronic configuration of the lowest singlet state of the
diradical 8 can be explained by examining the orbital inter-
action between the high-lying, symmetric NBMO (ψS) and
the low-lying Si-C σ* orbital,16 which energetically stabi-
lizes the ψS (Figure 3). These computational results clearly
suggest that the concerted denitrogenation reaction is a
symmetry-forbidden process. Thus, stepwise denitrogena-
tion was calculated to be the energetically favored process
(Scheme 4 and Figure 1).
As clearly indicated in Figure 1, the model azoalkane 7
loses molecular nitrogen in a stepwise manner with an
activation energy of 21.7 kcal/mol to give the 1,2-diaza-4-
silacyclopentane-3,5-diyl 8 (Scheme 4). The computational
results strongly suggest that the singlet diradicals 1a,b are
generated in a stepwise manner during the denitrogenation
of AZ. The activation energy of the denitrogenation of AZ
should be significantly lower than that of the compound 7,
since the radical moiety is well stabilized by the phenyl group.
Thus, it would be reasonable that we could not detect the
azoalkane AZ during thermolysis of the diazasilole with
either PTAD or MTAD at 60 °C (Scheme 2).
As mentioned in the trapping experiments of the diradical
1 with vinylene carbonate and cyclopentadiene, we could not
obtain trace amounts of the trapping adduct. The cycloaddi-
tion reaction of 1 with vinylene carbonate is a symmetry-
forbidden process, since the diyl-HOMO (LUMO) does not
match the alkene-LUMO (HOMO). Thus, it would be
understandable that the diradical 1 cannot be trapped by
vinylene carbonate. Alternatively, the cycloaddition reaction
with cyclopentadiene is a symmetry-allowed process; thus, it
would be possible to trap 1 by cyclopentadiene. However,
trace amounts of the adduct were not detected. The experi-
mental results clearly indicate that the trapping process is
faster by MTAD than by cyclopentadiene.
Conclusions
As predicted in our previous computational study, the 1,2-
diaza-4-silacyclopentane-3,5-diyl species was intramole-
cularly unreactive, but reacted intermolecularly to give the
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adduct 5. The mechanism of denitrogenation of the precur-
sor azoalkane was also investigated by computations at the
UB3LYP/6-31G(d) theoretical level. Stepwise denitro-
genation via the diazenyl diradical was the energetically
favored process compared with concerted denitrogenation.
We found that the electronic configuration of the lowest
singlet state of the 1,2-diaza-4-silacyclopentane-3,5-diyl
plays an important role in controlling the mechanism of
the denitrogenation reaction. Thus, the two spins of the
singlet diradicals 1 and 8 selectively occupy the symmetric
NBMO, as shown by the CASSCF calculations. The results
of the present study indicate that singlet diradicals, which
were previously considered intermediates only of intramole-
cular reactions, can be applied for intermolecular reactions.
1476, 1472, 1459, 1435, 1367, 763, 694 cm-1; HRMS (EI) calcd
for C22H28N2Si 348.2023, found 348.2024. Anal. Calcd for
C22H28N2Si: C, 75.81; H, 8.10; N, 8.04. Found: C, 75.58; H,
7.99; N, 8.11.
Diels-Alder Reaction of 2 with PTAD. A degassed suspension
of 2 (5 mg, 0.014 mmol) and the commercially available PTAD
(7.5 mg, 0.042 mmol) was heated in dry benzene-d6 at 60 °C for
7 h; the reaction was monitored by 1H NMR. After the reaction
mixture was cooled to room temperature, the products were
subjected to silica gel column chromatography (EtOAc/n-hex-
ane = 10/90) to separate the products. The PTAD-adduct 5a
was obtained as a white powder (5 mg, 7.46 Â 10-3 mmol) in
53% yield: 1H NMR (CDCl3) δ7.97 (d, J = 7.29 Hz, 4 H),
7.5-7.2 (m, 16 H), 1.26 (s, 18 H); 13C NMR (CDCl3) δ152.5,
130.7, 129.9, 128.8, 128.5, 128.2, 127.9, 126.4, 125.9, 82.3, 30.6,
23.1; IR (KBr) ν 1793, 1745, 1502, 1396, 1151, 1023, 786, 758,
690, 569, cm-1; HRMS (EI) calcd for C38H38N6O4Si 670.2726,
found 670.2731. Anal. Calcd for C38H38N6O4Si: C, 68.04; H,
5.71; N, 12.53. Found: C, 67.87; H, 5.42; N, 12.46.
Diels-Alder Reaction of 2 with MTAD. A degassed suspen-
sion of 2 (34.4 mg, 0.098 mmol) and commercially available
MTAD (30.5 mg, 0.269 mmol) was heated in dry benzene-d6 at
50 °C for 24 h; the reaction was monitored by 1H NMR. After
being cooled to room temperature, the products were subjected
to silica gel column chromatography (EtOAc/n-hexane = 20/
80) for separation. The MTAD adduct 5b was obtained as a
white powder (48 mg, 0.088 mmol) in 90% yield: 1H NMR
(CDCl3) δ 7.86-7.85 (d, J = 7.56 Hz, 4 H), 7.43-7.40 (t, J =
7.56 Hz, 4 H), 7.35-7.32(t, J = 7.39 Hz, 2 H), 3.01 (s, 6 H), 1.17
(s, 18 H); 13C NMR (CDCl3) δ 153.5 (s), 129.9 (s), 128.1 (d),
127.8 (d), 126.2 (d), 81.2 (s), 30.5 (q), 25.6 (q), 22.9 (s); IR (KBr) ν
1796, 1740, 1498, 1448, 1391, 1281, 1149, 1037, 935, 889, 815,
768, 697, 681, 634, 590, 508, 499 cm-1; HRMS (EI) calcd for
C28H34N6O4Si 546.2411, found 546.2396. Anal. Calcd for
C28H34N6O4Si: C, 61.52; H, 6.27; N, 15.37. Found: C, 61.54;
H, 6.32; N, 15.31.
Computational Methods
Structures were optimized at the B3LYP/6-31G(d)17,18 level
of theory. The computations for the singlet state of diradicals
were optimized using the UB3LYP/6-31G(d) with a broken-
symmetry method (initial guess <S2> = 1.00). The azoalkane
was analyzed at the restricted B3LYP/6-31G(d) theoretical
level. The geometries of stationary points and transition states
were all located, and vibrational analyses were performed with
the Gaussian 03 suite of programs.19 The optimized geometries
and their electronic energies are available in the Supporting
Information. The occupation number in the symmetric NBMO
(ψS) of the singlet diradical 8 was calculated at the CASSCF20-
(2,2)/6-31G(d)//UB3LYP/6-31G(d) theoretical levels.
Experimental Section
Synthesis of 4,4-Di-tert-butyl-3,5-diphenyl-4H-1,2,4-diazasi-
lole (2). 2,3-cis-Dimethyl-1,1-di-tert-butylsilacyclopropane (4)21
(991 mg, 5.0 mmol) and commercially available 3,6-diphenyl-
1,2,4,5-tetradine (3) (1.0 g, 4.25 mmol) were dissolved in dry
benzene (15 mL) under nitrogen atmosphere. After the mixture
was degassed, the reaction tube was sealed and heated to 125 °C .
After 2 days at 125 °C, the mixture was cooled to room
temperature, and the thermolysate was subjected to silica gel
column chromatography (EtOAc/n-hexane = 5/95). The dia-
zasilole 2 was isolated as a yellow solid in 27% yield (403 mg,
1.15 mmol): 1H NMR (CDCl3) δ7.99 (m, 4 H), 7.45 (m, 6 H), 1.20
(s, 18 H); 13C NMR (CDCl3) δ177.6 (s), 137.0 (s), 130.5 (d), 129.6
(d), 128.6(d), 29.0 (q), 19.5(s); IR(KBr) ν 2969, 2953, 2937, 2859,
Acknowledgment. This work was supported by a Grant-
in-Aid for Science Research on Priority Areas (No.
20030006, Synergy of Elements), (No. 21108516, Emergence
of Highly Elaborated π-Space and Its Function), and (B)
(No. 19350021) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan, and by the Toku-
yama Science Foundation.
Note Added after ASAP Publication. This paper was pub-
lished on the Friday, February 12, 2010, with errors in
the abstract graphic. The corrected version was reposted on
February 18, 2010.
(17) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213–222.
(18) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C.;
Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (c) Foresman, J. B.;
Head-Gordon, M.; Pople, J. A.; Frisch, M. J. J. Phys. Chem. 1992, 96, 135–
149.
(19) Frisch, M. J. et al. Gaussian 03, Revision C.02, Gaussian Inc.,
Wallingford, CT, 2004.
(20) Eade, R. H. E.; Robb, M. A. Chem. Phys. Lett. 1981, 83, 362–368.
(21) Boudjouk, P.; Samaraweera, U.; Sooriyakumaran, R.; Chrusciel, J.;
Anderson, K. R. Angew. Chem., Int. Ed. Engl. 1988, 27, 1355–1356.
Supporting Information Available: Complete ref 19, com-
1
putational details, and copies of H and 13C NMR spectra of
compounds 2 and 5a,b. This material is available free of charge
via the Internet at http://pubs.acs.org.
1960 J. Org. Chem. Vol. 75, No. 6, 2010