Thermal rearrangements of 3,3-spiroalkylated pyrazoles: Ring expansion and novel cases of sequential 1,5-shifts

Article abstract of DOI:10.3987/COM-01-9298

A 3,3-spiro-(cyclopentyl)pyrazole containing electron withdrawing ester groups undergoes readily ring expansion in the form of the van Alphen-Huettel rearrangement. Subsequent post van Alphen-Huettel rearrangement involved a sequence of 1,5-shifts different from that suggested earlier. Reactive intermediates have been isolated and identified. The corresponding phenyl analog does not exhibit post van Alphen-Huettel rearrangement. A rationale for the different behavior is offered. X-Ray crystallography has been applied to differentiate between structurally similar product.

Full text of DOI:10.3987/COM-01-9298

HETEROCYCLES, Vol. 55, No.10, 2001, pp. 1859 - 1872, Received, 13th July, 2001
THERMAL REARRANGEMENTS OF 3,3-SPIROALKYLATED
PYRAZOLES: RING EXPANSION AND NOVEL CASES OF
SEQUENTIAL 1,5-SHIFTS
Yao-pin Yen,a* Shih-Feng Chen, Zan-Cheng Heng, Jen-Chieh Huang, Li-Chun
Kao, Ching-Cheng Lai, and R. S. H. Liub
aDepartment of Applied Chemisty, Providence University, Shalu, Taichung
Hsien, Taiwan. E-mail: ypyen@pu.edu.tw
bDepartment of Chemistry, University of Hawaii, Honolulu, HI 96822, U. S. A.
Abstract - A 3,3-spiro-(cyclopentyl)pyrazole containing electron withdrawing
ester groups undergoes readily ring expansion in the form of the van Alphen-
Huettel rearrangement. Subsequent post van Alphen-Huettel rearrangement
involved a sequence of 1,5-shifts different from that suggested earlier. Reactive
intermediates have been isolated and identified. The corresponding phenyl
analog does not exhibit post van Alphen-Huettel rearrangement. A rationale for
the different behavior is offered. X-Ray crystallography has been applied to
differentiate between structurally similar product.
INTRODUCTION
1,5-Sigmatropic rearrangement of cyclopentadiene is a well known reaction that occupies an
important chapter in symmetry correlation reactions.1 Pyrazoles are hetero-cyclopentadienes; hence,
their propensity in undergoing 1,5-shift is also well known and continued to be of an interest.2 Such
reactions, first studied by van Alphen, later were investigated in more detail by Huettel and coworkers.3
Henceforth, the common alkyl rearrangements of 3-hydrogen or 3-alkylated pyrazoles (1) to the adjacent
C or N centers giving respectively the 4H-pyrazoles (2) or the 1H-pyrazoles (3) are known as the van
Alphen-Huettel rearrangements.4

R3
4
ϕ
R3
R3
4
R1 R3
ϕ
R2
R4
R2
R1
R2
R4
4
R4
R4
5
3
5
3
+
3
3
5
5
N N
ϕ
N N
R1
N N
N N
2
1
2
1
2
1
ϕ
4
3
1
2
Recent works,4 particularly those of Warkentin and coworkers,4 have shown that relative importance of
the two1,5-rearrangements and in some cases of possible formation of further rearrangement products is
a sensitive balance between the structure and electronic properties of the substituents. Hence, it was
shown that for 3-acyl-3-alkylpyrazoles, the acyl migration to the adjacent N-center is the exclusive
process taking place below room temperature5 while corresponding migration of an alkyl group usually
happens at >150℃. The 1,5-shift was shown to take place preferentially around the pyrazole ring
rather than the cyclopentadiene ring in the 3,3-spiro-pyrazole (4).6 And thermal rearrangement of 5
gave an unexpected product (7) in addition to the expected alkylated product (6) (corresponding to 3).2k
Since 7 was formed at the apparent expense of the C-alkyated product (8) (corresponding to 2), it was
suggested without proof that formation of 7 was achieved via two subsequent 1,5-shifts originating from
8 (a post van Alphen-Huettel rearrangement ). The involvement of ion-pair intermediates for cases
capable of forming stable carbocation added a new dimension to the rearrangement.7
R1
CO2Me
CO2Me
CO2Me
R2
R1
R2
R1
R2
2C
CO2Me
CO2Me
+
N N
N N
N N
O
Me
7
5
6
R1 CO2Me
R2
CO2Me
N N
8
In this paper, we would like to describe a study of thermal rearrangement and ring-expansion of
3,3-spiro-(cyclopentyl)pyrazoles. Because of our ability to isolate and identify the intermediates
involved in secondary rearrangements in one system, we ascertained a new reaction pathway involved in
the post van Alphen-Huettel rearrangement. These results are described below.
RESULTS AND DISCUSSION
Spiro-3H-pyrazoles (9a-c) were prepared following the procedure described in the literature.8 Each of

the three pyrazoles in benzene solution (2.40 × 10-2M) was degassed and sealed in Pyrex tubes and
heated in an oil bath. Product formation was monitored using analytical HPLC. The kinetic data
(Table 1, EXPERIMENTAL) followed unimolecular processes rigorously. For structural
characterization, products were isolated by column chromatography from mixtures obtained from scaled
up runs and identified by comparison of GC-MS, IR , 1H NMR and 13C NMR with those of authentic or
similar, known compounds, supplemented by X-Ray crystallography (see Figures 1-5). Compound (9a)
was studied in most detail. Its results were discussed first.
The diester spiral analog (9a). Compound (9a) was found to rearrange cleanly within the temperature
range of 64-92℃ with an activation energy of 26.3 ± 0.1 kcal/mol and a log A value of 12.8 ± 0.1. In a
preparative run, after heating a sample at 90℃ for 40 min, the reaction was found to be complete.
Only one major product (10a) was isolated in 70 % yield. Its structure was characterized by
comparison of spectral data with other van Alphen-Huettel rearranged products and by its X-Ray crystal
structure (Figure 1). A trace amount of a minor product (0.4 %) was also detected which was
subsequently characterized to be 11a (see below). These products are those expected from the van
Alphen-Huettel rearrangement involving ring expansion of the spiro-cyclopentyl ring. At 190℃ (60
min), the rearranged product mixture was found to contain, again, primarily a single, but different
product (12a, 69 % isolated yield) at the complete expense of 10a while the amount of 11a increased to
8 % (Scheme 1 and Table 2).
Scheme 1
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
N
+
+
CO2Me
N
N N
N N
N N
CO2Me
MeO2C
11a
12a
10a
9a
Table 2. Experimental Reaction Conditions and Product Yields
of the Thermal Reactions of Spiro-3H-Pyrazole (9a)
__________________________________________________
Yield (%)
Condition
10a
70
11a
0.4
12a
90 ℃
40 min
60 min
-
190 ℃
-
8
69
___________________________________________________

We were able to crystallize both of the isolated 11a and 12a. Their X-Ray crystal structures (Figures
2-3) removed any ambiguity in differentiating between them. Furthermore, the relative bond lengths
clearly reflect the delocalized nature of the pyrazole rings for these two compounds, expectedly different
from the localized structure of the isomeric (10a). Thus, that 11a and 12a are at the energy minima of
the potential surface for 1,5-shifts can be readily rationalized by aromatic resonance stabilization.
Figure 1. ORTEP Plots of Dimethyl 4,5,6,7-tetrahydro-3aH-
indazole-3,3a-dicarboxylate (10a)
Figure 2. ORTEP Plots of Dimethyl 4,5,6,7-tetrahydropyrazolo-
[1,5-a]pyridine-2,3-dicarboxylate (11a)

Figure 3. ORTEP Plots of Dimethyl 4,5,6,7- tetrahydro-
indazole-1,3-dicarboxylate (12a)
To test whether 12a originated from 10a or not, we heated a sample of 10a at a lower temperature (160
℃ for 5 h). While we readily confirmed that 12a (isolated yield 66 %) indeed originated from 10a,
much to our surprise, another isomer of pyrazole was also formed (13a, 19 %) (Scheme 2). But 13a
was an oil, thus its structure could not be elucidated by X-Ray crystallography. Fortunately, its spectral
data were most informative. Its 13C NMR spectrum exhibits three vinyl signals at 124.4, 132.5 and
149.6 ppm and two carbonyl ester carbons at 153.9 and 161.1 ppm, linked respectively to N and C-atom.
These data are consistent with 13a and not with the alternative structure (14a). The latter should have
one C-signal at <100 ppm as in the structurally similar 5,6,7,7a-tetrahydro-3-methyl-7a-phenyl-4H-
indazole (14b). 9 Finally with an isolated sample of 13a, we found that upon heating (>160℃ for 5 h),
it was cleanly converted to 12a (89% yield).
Scheme 2
160℃
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
160℃
+
N N
N N
N N
MeO2C
Y
13a
10a
12a
a: X = Y =CO2Me
b: X = C6H5; Y = Me
X
N N
14

The sequence of reactions shown below summarizes the thermal reactions of 9a. Indeed similar to that
proposed previously,2k the unexpected product (12a) originated from the van Alphen-Huettel product
(10a). However, the course of this post van Alphen-Huettel rearrangement is apparently two
consecutive 1,5-shifts to 13a10 followed by another N,N-1,5-shift to 12a. This is different from the
proposed mechanism2k involving two 1,5-shifts with compound (14a) being the reactive intermediate.
90℃
9a
10a
+
11a
12a
13a
160℃
+
160℃
12a
It is interesting that 10a prefers the longer pathway of three 1,5-shifts to reach the final product. We
suspect that the preference for the three consecutive “clockwise” 1,5-shifts rather than the two
consecutive “counterclockwise” 1,5-shifts for the 10a to 12a is not controlled by thermal stability (or the
lack of it) of the intermediate (14a) of the latter process. Rather, it is controlled by the conformation of
the six-membered ring which makes 1,5-shifts across the ring-junction (the first step of the
“counterclockwise” sequence) more difficult than the alternative “clockwise” 1,5-shift. However, it
should be emphasized that the isolation of intermediate (13a) does not exclude a parallel route involving
the unisolable 14a, especially in other analogous systems.
Other spiro-3H-pyrazoles (9b and 9c). For the hydrogen analog (9b), upon heating at 140℃ for 30
min, the compound was converted completely to the final post van Alphen-Huettel product (12b) (95%).
On the other hand, for the diphenyl analog (9c), a much higher temperature ( 230℃, 2 min) was required
for the van Alphen-Huettel rearrangement (activation energy = 32.1 ± 0.9 kcal/mol and log A = 13.8
± 0.6) giving products (10c) and (11c) (Scheme 3 and Table 3). Their structures were also confirmed
by their X-Ray crystal structures (Figures 4-5). The observed difference in rates (R = H > R = CO2Me
> R = C6H5) are clearly not a reflection of relative migratory aptitude (however, see below for additional
discussion) because all these cases involved the migration of the same alkyl group. Instead, it appears
to be controlled by product stability. In the phenyl case, the first step of 1,5-shift yielded a product
with the loss of resonance stabilization of a stilbene unit; the diester case, the loss of conjugation of one

of the two ester functional groups; and in unsubstituted 9b, no loss in resonance stabilization. These
conclusions have since been confirmed by calculations.11
Scheme 3
R
R
R
R
R
N
R
+
+
N
N N
N N
N N
R
R
10
11
12
9
Table 3. Experimental Reaction Conditions and Product Yields
of the Thermal Reactions of Spiro-3H-Pyrazoles (9b-c)
_____________________________________________________
Yield (%)
9
b
R
H
Condition
140℃ 30 min
10
11
12
95
-
-
c
Ph 230℃
2 min 74
25
-
_____________________________________________________
Furthermore, we found that prolonged heating of an isolated sample of the C-rearranged product (10c) at
230℃ only resulted in its complete conversion to the N-rearranged product (11c). Interestingly, at the
mid-point of the this rearrangement (51 % conversion ), we detected the presence of a small amount
(0.11 %) of the spiro-reactant (9c). Apparently, the rearrangement of 10c to 11c involved reversion to
the starting spiro structure.
Figure 4. ORTEP plot of 3,3a-Diphenyl 4,5,6,7-tetrahydro-3aH-indazole (10c)

Figure 5. ORTEP plot of 2,3-Diphenyl 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine (11c)
A few interesting points may be concluded from these observations. First, in this case no post van
Alphen-Huettel rearrangement product was detected. We believe this is due to the fact that additional
rearrangement would result in a further loss of resonance stabilization to give first, in the case of 10c,
compound (15). It is apparently too high in energy. Second, the reaction pathway for 10c to 11c has
been clarified. Conversion of 10c to 11c (and also 10a to 11a, previous section) involves a formal
1,3-sigmatropic reaction, a symmetry forbidden process. The sequence of consecutive symmetry
allowed 1,5-shifts with intermediacy of 9c is therefore a logical alternative. The secondary
rearrangement of 10c, and related compounds, provides a good measure of the migratory aptitude of the
two substituents at the quaternary carbon. In the case of 10c, the result clearly shows the lower
migratory aptitude of the phenyl group. Thus, it made reversion to the starting spiro-cyclopentyl
system competitively possible. In the equivalent structure (10a), the ester group clearly migrates much
more readily. This large difference in migratory aptitude for these two substituents is consistent with
analogous examples in cyclopentadienes12 and other pyrazoles,5 favoring systems with electron deficient
substituents.
9c
C6H5
C6H5
N N
10c
11c
15

In summary, because of our ability to isolate reaction intermediates in a van Alphen-Huettel
rearrangement and another case of a post van Alphen-Huettel rearrangement, we were able to
characterize the specific reaction pathways involved in different stages of the thermal rearrangements of
the pyrazoles. A better understanding of relative product stability and migratory aptitude allowed us to
rationalize the seemingly confusing and divergent pathways available to these compounds.
EXPERIMENTAL
General information. Melting point were determined on Yamaco micro melting points apparatus and
are uncorrected. IR spectra were obtained on a Perkin Elmer 2000 spectrophotometer in KBr. 1H
NMR spectra were determined on a Bruker AC-250 spectrometer in CDCl3 with TMS as an internal
standard. High performance liquid chromatography (HPLC) was performed on a Dynamax
Chromatograph equipped with Dynamax SD-200 pump and controller and a Water 486 detector.
Materials. The dimethyl 3,3-cyclopentyl-3H-pyrazole-4,5-dicarboxylate (9a), and 3,3-cyclopentyl-
3H-pyrazole (9b) were prepared according to published procedures.8 3,3-Cyclopentyl-4,5-diphenyl-
3H-pyrazole (9c) was prepared in sequence of reactions similar to those of 3,3-dimethyl-4,5-diphenyl-
3H-pyrazole.13 Hence, only characterization data are listed below.
3,3-Cyclopentyl-4,5-diphenyl-3H-pyrazole (9c)
Product (9c) was prepared following the literature13 procedure and recrystallized from n-hexane; yield
26 %; mp 134-135℃; 1H NMR (CDCl3) δ: 1.86-2.02 (m, 6H), 2.28-2.35 (m, 2H), 7.13-7.43 (m, 8H),
7.70-7.74 (m, 2H); IR (KBr): 3056, 2969, 2870, 1454, 771, 710 cm-1; MS m/z (%): 274 (M+, 8), 246
(68), 218 (45), 217 (100), 203 (27), 202 (34), 178 (36); Anal. Calcd for C19H18N2: C, 83.21; H, 6.57; N,
10.22. Found: C, 83.33; H, 6.49; N, 10.19.
General Procedure for Thermal Rearrangement of 3H-pyrazoles (9a-c)
For preparataive purpose, thermal rearrangement was carried out by heating a solution (1.5 mL, 2.40 ×
10-2 M) of the appropriate 3H-pyrazole in benzene in a sealed pyrex tube. After the rearrangement of
reactant was complete (>95 %), the products were isolated by flash column chromatography on silica gel
using the solvent mixtures: n-hexane/EA = 3/1 or n-hexane/EA = 5/1 for 10a and 11a.

Characterization data of the products are listed below.
Dimethyl 4,5,6,7-tetrahydro-3aH-indazole-3,3a-dicarboxylate (10a)
Yield 70 %; mp 94.5-95.5℃ (ethanol); 1H NMR (CDCl3) δ: 1.16-1.50 (m, 2H), 1.77-1.97 (m, 2H),
2.23-2.50 (m, 2H), 3.08-3.13 (m, 2H), 3.68 (s, 3H), 3.96 (s, 3H); 13C NMR (CDCl3) δ: 21.1, 27.5, 28.7,
35.3, 52.8, 53.1, 70.3, 159.8, 164.4, 167.6, 180.3; IR (KBr): 2955, 2860, 1754, 1720, 1562, 1443, 1367,
1219, 1091 cm-1; MS m/z (%): 238 (M+, 7), 206 (53), 179 (100), 163 (26), 147 (55), 135 (25), 119 (3),
91 (45); Anal. Calcd for C11H14N2O 4: C, 55.46; H, 5.92; N, 11.76. Found: C, 55.61; H, 5.87; N, 11.55.
Dimethyl 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-2,3-dicarboxylate (11a)
Yield 0.4 %; mp 65-66℃ (n-hexane); 1H NMR (CDCl3) δ: 1.87-1.98 (m, 2H), 2.04-2.15 (m, 2H), 3.04
(t, 2H, J= 6.3 Hz), 3.83 (s, 3H), 3.94 (s, 3H), 4.19 (t, 2H, J= 6.3 Hz); 13C NMR (CDCl3) δ: 19.3, 22.6,
23.1, 48.6, 51.6, 52.5, 110.5, 143.1, 144.8, 162.7, 163.0; IR (KBr) 2961, 1736, 1717, 1303, 1077 cm-1;
MS m/z (%): 238 (M+, 22), 207 (94), 206 (100), 177 (22), 163 (16), 135 (10), 57 (29); Anal. Calcd for
C11H14N2O 4: C, 55.46; H, 5.92; N, 11.76. Found: C, 55.41; H, 5.84; N, 11.94
General Procedure for Thermal Rearrangement of 10a
Dimethyl 4,5,6,7-tetrahydro-3aH-indazole-3,3a-dicarboxylate (10a) (1.5 mL × 2.50 × 10-2 M) in
benzene was degassed, sealed in pyrex tube and heated in an oil bath at 160℃ for 4 h. The reaction
mixture was concentrated in vacuo and the products were isolated by flash column chromatography on
silica gel using the solvent mixtures : n-hexane/EA = 9/1 for 12a and 13a. Characterization data of 12a
and 13a are listed below.
Dimethyl 4,5,6,7-tetrahydroindazole-1,3-dicarboxylate (12a)
Yield 66 %; mp 152-154℃ (ethanol); 1H NMR (CDCl3) δ: 1.69-1.90 (m, 4H), 2.74 (t, 2H, J = 6.3 Hz),
2.98 (t, 2H, J = 6.1 Hz), 3.93 (s, 3H), 4.06 (s, 3H); 13C NMR (CDCl3) δ: 21.2, 21.9, 22.0, 24.1, 52.0,
54.7, 122.3, 143.8, 144.8, 150.3, 162.5; IR (KBr) 2955, 1767, 1730, 1442, 1312, 1257, 1205, 1139, 806,
765 cm-1; MS m/z (%): 238 (M+, 76), 206 (73), 179 (100), 147 (25), 135 (39), 59 (25); Anal. Calcd for
C11H14N2O4: C, 55.46; H, 5.92; N, 11.76. Found: C, 55.44; H, 6.03; N, 11.69.
Dimethyl 4,5,6,7-tetrahydroindazole-2,3-dicarboxylate (13a)
Yield 19 %; bp 65℃/0.25 torr; 1H NMR (CDCl3) δ: 1.77-1.85 (m, 4H), 2.63 (t, 2H, J = 6.0 Hz), 2.71 (t,

2H, J = 6.3 Hz), 3.91 (s, 3H), 4.02 (s, 3H); 13C NMR (CDCl3) δ: 20.6, 22.2, 22.4, 23.4, 52.7, 55.0, 124.4,
132.4, 149.6, 153.9, 161.1; IR (neat) 2926, 2854, 1764, 1733, 1439, 1358, 1299, 1059, 804 cm-1; HRMS
calcd for C11H14N2O 4: 238.09536. Found: 238.09545.
4,5,6,7-Tetrahydroindazole (12b)
Yield 95%; mp 78-79℃ (petroleum ether); 1H NMR (CDCl3) δ: 1.69-1.80 (m, 4H), 2.52 (t, 2H, J = 5.8
Hz), 2.65 (t, 2H, J = 5.7 Hz), 7.27 (s, 1H), 11.06 (br s, 1H); 13C NMR (CDCl3) δ: 20.3, 21.9, 23.0, 23.4,
114.7, 131.5, 143.0; IR (KBr) 3160, 2931, 2852, 1443, 1342, 1087, 964, 854, 796 cm-1; MS m/z (%):
122 (M+, 35), 94 (100), 81 (4), 67 (7); HRMS calcd for C7H10N2: 122.0843. Found: 122.0841.
3,3a-Diphenyl-4,5,6,7-tetrahydro-3aH-indazole (10c)
Yield 75 %; mp 151-153℃ (ether); 1H NMR (CDCl3) δ: 1.47-1.64 (m, 2H), 1.80-1.91 (m, 2H),
2.13-2.26 (m, 2H), 2.91-2.99 (m, 1H), 3.17-3.24 (m, 1H), 7.09-7.38 (m, 8H), 7.70-7.74 (m, 2H); 13C
NMR (CDCl3) δ: 21.4, 26.6, 29.5, 34.6, 67.9, 125.3, 126.5, 127.1, 127.6, 127.8, 128.4, 129.5, 129.9,
130.5, 130.8, 132.5, 179.0, 183.2; IR (KBr) 3055, 2949, 2860, 1582, 1518, 1496, 1448, 1337, 772, 693
cm-1; MS m/z (%): 274 (M+, 32), 246 (81), 217 (54), 202 (20), 170 (38), 143 (87), 129 (100), 115 (29),
103 (36), 77 (29); Anal. Calcd for C19H18N2: C, 83.18; H, 6.61; N, 10.21; Found: C, 82.90, H; 6.70; N,
10.35.
2,3-Diphenyl-4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine (11c)
Yield 11 %; mp 142-144℃ (ether/n-hexane); 1H NMR (CDCl3) δ: 1.82-1.96 (m, 2H), 2.07-2.82 (m, 2H),
2.80 (t, 2H, J = 6.3 Hz), 4.28 (t, 2H, J = 6.1 Hz), 7.18-7.35 (m, 8H), 7.43-7.47 (m, 2H); 13C NMR
(CDCl3) δ: 20.2, 22.4, 23.2, 47.9, 116.3, 124.8, 126.1, 126.7, 127.1, 127.9, 128.2, 129.6, 130.9, 133.6,
137.9, 148.2; IR (KBr) 3048, 2950, 2863, 1600, 1542, 1496, 1431, 1347, 760 cm-1; MS m/z (%): 274
(M+, 100), 273 (42), 245 (10), 115 (8); Anal. Calcd for C19H18N2: C, 83.18; H, 6.61; N, 10.21. Found: C,
82.90, H; 6.75; N, 10.09; HRMS calcd for C19H18N2: 274.1470. Found: 274.1467.
For kinetic runs, benzene solutions of an appropriate pyrazole sealed in pyrex tubes were heated in a
constant temperature bath. Aliquots were taken for periodic analysis by HPLC. Conditions for
analysis were: hplc, Partisil-10 column, 25 cm × 4.6 mm, solvent hexane: ether = 3 : 1. Selected rate
constants are listed in Table 1.

Table 1. Rates of Rearrangement of 9a and 9c
______________________________________________
Compound
T (℃)
104 × k (s-1)
_______________________________________________
9a
9c
64
71
0.52 ± 0.01
1.31 ± 0.14
2.42 ± 0.05
3.81 ± 0.01
11.09 ± 0.52
2.03 ± 0.11
3.50 ± 0.29
5.34 ± 0.32
7.92 ± 0.08
14.44 ± 0.30
76
82
92
130
136
140
144
151
_______________________________________________
ACKNOWLEDGMENT
The authors gratefully acknowledge the National Science Council of the Republic of China for financial
support and Dr. Y. J. Lee for helpful discussions.
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10. Conceptually a single 1,3-shift could have achieved the same results. But we believe the long
accepted suprafacial 1,5-shifts for cyclopentadienes should apply here.
11. Molecular simulation using Cerius program. For example, relation energies of 9c and 10c were
found to be 88.7 and 96.0 kcal/mol.
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