Stabilization of γ-lactam and lactone ring-fused norcaradienes by protonation: DFT calculations of norcaradiene and the corresponding cycloheptatriene structures

Article abstract of DOI:10.1016/j.molstruc.2009.10.049

Stabilization of norcaradiene structures of γ-lactone and lactam ring-fused 7-vinylnorcaradienes 1a-1e in their norcaradiene - cycloheptatriene valence isomerization was achieved by protonation. Induced ¹³C NMR chemical shifts caused by protona

Full text of DOI:10.1016/j.molstruc.2009.10.049

Journal of Molecular Structure 964 (2010) 47–51
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Stabilization of
c-lactam and lactone ring-fused norcaradienes by protonation: DFT
calculations of norcaradiene and the corresponding cycloheptatriene structures
*
Shigeo Kohmoto , Tatsuya Motomura, Masahiro Takahashi, Keiki Kishikawa
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 August 2009
Received in revised form 30 October 2009
Accepted 30 October 2009
Available online 6 November 2009
Stabilization of norcaradiene structures of c-lactone and lactam ring-fused 7-vinylnorcaradienes 1a–1e
in their norcaradiene – cycloheptatriene valence isomerization was achieved by protonation. Induced
13C NMR chemical shifts caused by protonation were monitored for the cyclopropane ring carbons (C1,
C6 and C7) of the norcaradiene structures as indices of their stabilities. DFT calculations (B3LYP/6-
311+G(d) level) of norcaradienes, the corresponding cycloheptatrienes, and their protonated structures
supported the experimental results.
Keywords:
Ó 2009 Elsevier B.V. All rights reserved.
Norcaradiene
Tautomerization
13C NMR
B3LYP calculations
1. Introduction
lactone moiety as a proton acceptor. Since the c-lactone ring-fused
system could provide norcaradienes of moderate stabilities
Norcaradines are usually unstable valence isomers and the cor-
responding cycloheptatrines exist predominantly [1–3]. They are
proposed as intermediates in some rearrangements [4–6]. Certain
structural devices are required to prepare stable norcaradienes.
In the past, great efforts have been devoted to prepare stable nor-
caradienes either by structural restrictions (an incorporation of
fused ring system [7–9], steric hindrance [10,11], or resonance sta-
depending on their substituents, they are an appropriate candidate
for stability study. Therefore, we chose the 1,7-
c-lactone ring-
fused norcaradiene together with -lactam ring-fused one as a
c
model system. Herein, we report on our approach to the control
of the norcaradiene – cycloheptatriene valence isomerization by
protonation which increases the
p-acceptor character of the sub-
stituents at the 7-position. This kind of approach to stabilize norca-
radiene structures by heteroatom-stabilized carbocations as
substituents at the 7-position has been reported by Daub et al.
[26,27].
bilization [12–14]) or electronic effects (an introduction of
p-
acceptors at 7-position [15–17] or -donors at 3- and 4-positions
p
[18,19]). However, almost no effort has been made to control the
stability of norcaradienes not by these factors but by changing
some external conditions using the same molecules. We are inter-
ested in this stability tunable norcaradienes. Since an introduction
of an electron withdrawing group at the 7-position is known to sta-
bilize the norcaradiene structure, we have planned to furnish a
proton acceptor as a substituent at this position. Depending on
the degree of protonation, an electron withdrawing character of
the substituent varies. Thus the stability of norcardiene can be
tuned externally by the degree of protonation of the proton accep-
tor at the 7-position.
2. Results and discussion
The stability of norcaradienes could be roughly estimated from
their 13C NMR chemical shifts of cyclopropane carbons, C1, C6, and
C7 [28]. If the nocaradiene structure exists predominantly in the
valence isomerization, these carbons possess an sp3 character.
However, C1 and C6 should be sp2 carbons if the cycloheptatriene
structure predominates completely in the valence isomerization
(Scheme 1). In equilibrium, chemical shifts of these carbons appear
as their weight average depending on their ratios. Norcaradienes
examined, 1a, 1b [21], 1c [24], 1d [20], and 1e, were prepared
according to our reported method (Scheme 2). Thermolysis of
cyclopropenes generated vinylcarbenes whose intramolecular
cycloaddition to a benzene or a naphthalene ring afforded the cor-
responding 7-vinylnorcaradienes. Table 1 shows the chemical
shifts of cyclopropane carbons (C1, C6, and C7) of 1a–1e and their
In our previous study, we reported the synthesis of unusually
stable 7-vinylnorcaradienes fused with a
c-lactone ring at the
1,7-position [20–22] and their thermal [23], photochemical [24],
and acid-catalyzed rearrangements [25]. We have chosen this c-
* Corresponding author. Tel.: +81 43 290 3420; fax: +81 43 290 3422.
E-mail address: kohmoto@faculty.chiba-u.jp (S. Kohmoto).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.10.049

48
S. Kohmoto et al. / Journal of Molecular Structure 964 (2010) 47–51
induced chemical shifts (Dd) data after the addition of TFA at ambi-
ent temperature (ca. 300 K). Judging from these data, 1a–1c exist
as an equilibrium mixture with their cycloheptatriene structures
2a–2c. The c-lactam ring-fused norcaradiene 1a shows the lowest
shift values in CDCl3 for C1(d 72.1), C6(d 67.9), and C7(d 31.9)
among them. In contrast, the stable norcaradienes 1d and 1e
showed reasonable chemical shift values for C1, C6, and C7 as sp3
carbons. In comparison to them, C1 and C6 of 1b and 1c appeared
at considerably lower field than those of 1d and 1e. This indicates
that these compounds exist as equilibrium mixtures and their C1
and C6 bears some sp2 character. Therefore, 1a is the least stable
norcaradiene among them. Norcaradiene 1d was reported to be
stable from its X-ray analysis [19]. Due to the resonance stabiliza-
tion, it is hard to take the cycloheptatriene structure in 1e.
Fig. 1a and b shows the induced 13C NMR chemical shifts of C1,
C6, C7, C8, C10, C11, and C12 for 1a and 1d with increasing amount
of TFA, respectively. Induced 13C NMR chemical shifts (
D
d are ob-
d = dTFA À d0, where dTFA and d0 are the chemical shifts
of 1a and 1d with and without TFA, respectively. Negative and po-
sitive signs of d indicate upfield and downfield induced shifts,
tained as
D
D
respectively. Large upfield shifts, À21.5 and À19.2 ppm after addi-
tion of 8 eq. of TFA, were observed for C1 and C6 of 1a, respec-
tively. An upfield shift of À5.3 ppm was observed for C7
(Fig. 1a). A relatively small shift value for C7 is reasonable since
it remains as an sp3 carbon in both norcaradiene and cyclohepta-
triene structures. On the contrary, C8 and C10 showed downfield
shifts due to the protonation of the carbonyl group. These results
indicated that the norcaradiene structure of 1a was stabilized sig-
nificantly by the addition of TFA in the valence isomerization. The
acid-catalyzed thermal acetylene–allene rearrangement of 2-ethy-
nylcycloheptatriene has been reported in which protonation of an
acetylene moiety initiated the rearrangement of the corresponding
norcaradiene structure [29,30]. In contrast to this, no rearrange-
ment had occurred for 1a under the reaction conditions. Similar
tendency of the induced shifts by the addition of TFA was observed
Scheme 1. Norcaradience – cycloheptatriene valence isomerization.
for the
ever, the upfield shift values for the C1 and C6 were smaller com-
pared to those of 1a. The -lactone ring fusion stabilizes the
norcaradiene structure more than by the ring fusion with the
c-lactone ring-fused 7-vinylnorcaradienes 1b and 1c. How-
c
c-lactam.
In contrast to the upfield shifts observed for 1a, 1b, and 1c, the
stable norcaradienes 1d and 1e showed slight downfield shifts for
their C1, C6 and C7 carbons after the addition of TFA. Fig. 1b shows
the induced chemical shifts for 1d. These downfield shifts are the
normal tendency caused by protonation in a static system if no va-
lence isomerization exists. Protonation strengthen the electron
withdrawing ability of the lactone moiety resulting in the down-
field shifts of the contiguous carbons. The degree of downfield
shifts for C8 and C10 is nearly the same for all norcaradienes,
which indicates the protonation takes place in the similar manner.
The results proved the fact that 1d and 1e exist almost completely
as the norcaradiene structures.
In order to evaluate the experimental results, DFT calculations
were carried out. DFT calculations of some norcaradienes were re-
ported previously [31]. Norcardienes, 1a and 1b, the correspond-
ing cycloheptatrienes, 2a and 2b, and their protonated structures
were calculated using Gaussian 03 program package [32] and
B3LYP [33,34] method in conjunction with 6-311+G(d) basis set.
Depending on direction of the vinyl substituents, two conforma-
tions are possible to be taken for norcaradiene, 1a and 1b, and
cycloheptatriene, 2a and 2b. Four possible conformations could
be existed for their protonated structures because of the direction
of protonation. Geometry optimizations were carried out for all
these possible conformations to find out their most stable confor-
mations. Fig. 2 shows the most stable conformations of 1a, 1b, 2a,
2b, and their protonated forms together with 1f and 2f. Stabilities
Scheme 2. Synthesis of 7-vinylnorcaradienes.
Table 1
Chemical shifts (d) for C1, C6 and C7 of 1a–1e and their induced chemical shifts (Dd)
caused by the addition of TFAa.
Carbon
C1
1ab
1bb
1cb
1dc
1eb
d
72.1
À21.5
67.9
À19.2
31.9
À5.3
52.4
À5.8
47.9
À5.3
23.0
À0.3
56.2
À8.7
52.5
À7.9
26.1
À1.2
41.4
+1.3
39.4
+2.0
D
dd
dd
dd
C6
C7
d
39.6
+1.6
37.8
+1.5
D
d
21.6
+1.2
24.3
+1.5
D
a
Measurements were carried out on ca. 0.5 M solution of 1a–1e in CDCl3 at
ambient temperature (ca. 300 K).
b
With 8 eq. amount of TFA.
With 10 eq. amount of TFA.
Plus and minus signs designate downfield and upfield shifts, respectively.
c
d

S. Kohmoto et al. / Journal of Molecular Structure 964 (2010) 47–51
49
Fig. 1. Induced 13C NMR chemical shifts of 1a (a) and 1d (b) with addition of TFA.
Fig. 2. Stable conformations of norcaradienes and cycloheptatrienes of 1a, 1b, and 1f, and the corresponding protonated structures calculated by B3LYP/6-311+G(d). Stability
of cycloheptatriene structures relative to the corresponding norcaradine structures are presented in kcal/mol in parentheses.
of cycloheptatrienes relative to the most stable corresponding
norcaradiene conformers are presented in kcal/mol in parenthe-
ses. Relative energies of all possible conformers can be found in
Supplementary data. Comparison between the norcaradiene and
the corresponding cycloheptatrines shows that the lactam ring-
fused norcaradiene 1a is less stable than that of the lactone

50
S. Kohmoto et al. / Journal of Molecular Structure 964 (2010) 47–51
ring-fused one 1b. This agrees with experimental results in which
the chemical shifts of C1 and C6 of 1a appeared in lower fields
than those of 1b. Calculations show that cycloheptatriene struc-
tures are more stable than the corresponding norcaradiene struc-
tures in 2.35 and 1.30 kcal/mol for 1a and 1b, respectively.
However, they seem to have a similar stability judging from their
observed chemical shifts. Protonation resulted in the stabilization
of norcaradine structures in both cases as observed in upfield
shifts of cyclopropane carbons C1 and C6. Calculations also pre-
dicted that protonation stabilized the norcaradiene structure.
The results indicate that the protonated forms of norcaradienes
3a and 3b are more stable than those of the corresponding pro-
tonated cycloheptatrienes 4a and 4b in 4.56 and 6.08 kcal/mol,
respectively. Protonation causes an increase of electron with-
drawing ability of substituents at the C7 position, which results
in the stabilization of the norcaradiene structures. We reported
that 1d was the very stable norcaradiene in our previous study
because of the electron withdrawing nature of an ester substitu-
ent [20]. In order to evaluate the stability of 1d, the calculation
was carried out on the simplified structure, the corresponding
methyl ester 1f. In contrast to 1a and 1b, it was calculated that
the norcaradiene structure of 1f is more stable than the corre-
sponding cycloheptatriene structure 2f in 1.86 kcal/mol. An intro-
duction of an ester group to the vinyl moiety stabilizes the
norcaradiene structure.
diazomethane in THF at À78 °C until the orange color of dimethyl-
diazomethane disappeared. After the removal of solvent by
evaporation, the residue was purified by flash column chromatog-
raphy on silica gel with hexane/ethyl acetate as an eluent to give
0.680 g (68%) of 6a as white crystals. Mp. 115.5–116.5 °C; 1H
NMR (CDCl3) d 8.15 (br. s, 1H), 7.62 (s, 1H), 7.33 (s, 5H), 4.68 (d,
J = 11.2 Hz, 2H), 1.45 (s, 6H); 13C NMR (CDCl3) d 159.6 (s), 150.4
(d), 149.1 (s), 137.6 (s), 128.2 (d), 127.5 (d), 127.1 (d), 95.0 (s),
42.9 (t), 19.5 (q); HRMS (FAB) calcd for C13H16ON3 (MH+)
230.1293, found 230.1296; Anal. Calcd for C13H15N3O: C, 68.10;
H, 6.59; N, 18.33. Found: C, 67.81; H, 6.37; N, 18.52.
4.2.1.2. Naphthalen-2-ylmethyl 3,3-dimethyl-3H-pyrazole-5-carbox-
ylate 8. In a similar manner as for the synthesis of 6a, 8 was pre-
pared from naphthalene-2-ylmethyl propiolate 7 in 75% yield as
white crystals. Mp. 99.5–101.0 °C; 1H NMR (CDCl3) d 8.0–7.7 (m,
4H), 7.65 (s, 1H), 7.6–7.4 (m, 3H), 5.54 (s, 2H), 1.45 (s, 6H); 13C
NMR (CDCl3) d 160.6 (s), 154.2 (d), 146.5 (s), 133.0 (s), 132.9 (s),
132.4 (s), 128.3 (d), 127.8 (d), 127.7 (d), 127. 5 (d), 126.2 (d),
126.1 (d), 125.9 (d), 94.9 (s), 67.1 (t), 19.5 (q); MS (EI) 280 (M+,
10), 142 (13), 141 (100), 115 (15), 83 (26); Anal. Calcd for
C17H16N2O2: C, 72.84; H, 5.75; N, 9.99. Found: C, 72.64; H, 5.58;
N, 9.77.
4.2.2. Synthesis of norcaradienes
4.2.2.1. Norcaradiene 1a. A benzene solution (20 mL) of 6a (0.213 g,
0.929 mmol) was irradiated for 40 min with a high pressure mer-
cury lamp (USHIO UM452) at 0 °C. After removal of benzene by
evaporation, the residue was dissolved in toluene (40 mL) and then
refluxed for 15 h. After evaporation of solvent, the residue was
chromatographed on silica gel to give norcaradiene 1a (0.099 g,
53%) as white crystals. Mp. 188–189 °C; 1H NMR (CDCl3); 6.28 (t,
J = 7.8 Hz, 1H), 6.23 (br. s, 1H), 6.18 (t, J = 7.8 Hz, 1H), 6.10 (dd,
J = 7.8, 6.3 Hz, 1H), 5.97 (d, J = 7.8 Hz, 1H), 4.52 (sept, J = 1.3 Hz,
1H), 3.91 (d, J = 12.0 Hz, 1H), 3.66 (d, J = 12.0 Hz, 1H), 3.48 (d,
J = 6.3 Hz, 1H), 1.63 (d, J = 1.3 Hz, 3H), 1.50 (d, J = 1.3 Hz, 3H); 13C
NMR (CDCl3) 180.1 (s), 140.4 (s), 126.4 (d), 124.9 (d), 123.6 (d),
121.8 (d), 116.7 (d), 71.7 (s), 67.5 (d), 47.0 (t), 31.8 (s), 25.5 (q),
18.7 (q). HRMS (FAB): calcd for C13H16NO (MH+) 202.1232, found
202.1225. Anal. Calcd for C13H15NO: C, 77.58; H, 7.51; N, 6.96.
Found: C, 77.50; H, 7.31; N, 6.77.
3. Conclusion
As we demonstrated, stability tunable norcaradienes could be
attained by protonation of
c-lactam and lactone ring-fused 7-
vinylnorcaradienes. Protonation increases electron withdrawing
nature of substituents which stabilize the norcaradiene structure
significantly in the norcaradiene – cycloheptatriene valence isom-
erization. The degree of stability was monitored by 13C NMR spec-
troscopy. Geometry optimizations (B3LYP/6-311+G(d) level) on all
possible conformers of norcaradienes, cycloheptatrienes, and the
corresponding protonated forms were carried out to support these
experimental results.
4. Experimental
4.1. General
4.2.2.2. Norcaradiene 1e. In a similar manner, norcaradiene 1e is
prepared from 8 in 74% as colorless oil, 1H NMR (CDCl3) d 7.49–
7.18 (m, 4H), 6.63 (d, J = 10.0 Hz, 1H), 6.11 (dd, J = 10.0, 5.0 Hz,
1H), 5.12 (d, J = 9.6 Hz, 1H), 4.50 (qq, J = 1.4, 1.3 Hz, 1H), 4.38 (d,
J = 9.6 Hz, 1H), 2.67 (d, J = 5.0 Hz, 1H), 1.51 (d, J = 1.3 Hz, 3H),
1.41 (d, J = 1.4 Hz, 3H); 13C NMR (CDCl3) d 177.6 (s), 144.1 (s),
131.7 (s), 129.4 (d), 128.5 (d), 127.8 (d), 127.3 (d), 127.1 (d),
125.2 (d), 120.8 (d), 110.6 (d), 68.1 (t), 39.0 (s), 35.1 (d), 24.7 (q),
24.3 (s), 19.3 (q); HRMS (FAB): calcd for C17H17O2 (MH+)
253.1231, found 253.1233.
Mps were determined on a Yanaco MP-S3 apparatus and are
uncorrected. 1H and 13C NMR spectra were recorded on JEOL FX-
270 and EX-90 spectrometers in CDCl3 with Me4Si as an internal
standard; J values are given in Hz. Mass spectra were measured
with a Hitachi RMU-7M mass spectrometer. Elemental analyses
were performed on a Perkin-Elmer 240 analyser. Reaction mixtures
were concentrated on a rotary evaporator at 10–15 mm Hg. Chro-
matographic separations were accomplished by flash column chro-
matography on silica gel (Fuji gel BW 200; 150-350 mesh). All
solvents were distilled and stored over 4 Å molecular sieves.
Appendix A. Supplementary data
4.2. Synthesis
The results of full geometry optimization (B3LYP/6-311+G(d)
level) of 1a, 2a, 1b, 2b, and their protonated forms, 1f and 2f
(PDF, 63 pages) is available. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.molstruc.2009.10.049.
Previously reported norcaradienes 1b, 1c, and 1d were pre-
pared according to the reported method by us. Norcaradies 1a
and 1e were synthesized in a similar manner from 3H-pyrazoles
as described below.
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