Flash vacuum pyrolysis and photolysis of 3,3,5,5-tetramethylpyrazolin-4- one - A matrix isolation study

Article abstract of DOI:10.1002/(sici)1099-0690(200001)2000:1<91::aid-ejoc91>3.0.co;2-c

The flash vacuum pyrolysis of 3,3,5,5-tetramethylpyrazolin-4-one (4) with subsequent trapping of the products in argon at 10 K results in the formation of a complex product mixture with tetramethylcyclopropanone (5) as one of the minor constituents. The 193 nm photochemistry of matrix-isolated 4 is much cleaner, yielding cyclopropanone 5 and allene oxide 9 as the major products. The reaction of tetramethylallene 13 with oxygen atoms yields the same product mixture of 5 and 9 and thus provides an independent route to these species. The experimental IR spectra of 5 and 9 are in good agreement with the results from DFT calculations.

Full text of DOI:10.1002/(sici)1099-0690(200001)2000:1<91::aid-ejoc91>3.0.co;2-c

FULL PAPER
Flash Vacuum Pyrolysis and Photolysis of 3,3,5,5-Tetramethylpyrazolin-4-one –
A Matrix Isolation Study
Wolfram Sander,*[a] Roman Wrobel,[a] Peter Komnick,[a] Paul Rademacher,[b]
Heidi M. Muchall,[b] and Helmut Quast[c]
Keywords: Cyclopropanone / Allene oxide / Matrix isolation / Photochemistry / Flash vacuum thermolysis
The flash vacuum pyrolysis of 3,3,5,5-tetramethylpyrazolin-
4-one (4) with subsequent trapping of the products in argon
at 10 K results in the formation of a complex product mixture
oxide
9
as the major products. The reaction of
tetramethylallene 13 with oxygen atoms yields the same
product mixture of 5 and 9 and thus provides an independent
with tetramethylcyclopropanone (5) as one of the minor route to these species. The experimental IR spectra of 5 and
constituents. The 193 nm photochemistry of matrix-isolated
9 are in good agreement with the results from DFT
4 is much cleaner, yielding cyclopropanone 5 and allene
calculations.
Introduction
The elimination of nitrogen from pyrazolinones 1 leads
to highly reactive species such as oxyallyl diradicals 2 and
cyclopropanones 3. The photochemistry and thermolysis of
3,3,5,5-tetramethylpyrazolinone 4 in solution was investi-
gated by Quast and co-workers.[1]
Scheme 1
The main product of irradiation at λ > 300 nm of 4 was
2,3-dimethyl-2-butene 6 (up to 80%), presumably formed by
loss of CO from tetramethylcyclopropanone (5). The cyclo-
propanone 5 was not directly observed, but could be
trapped with methanol.[1a] A second product of the pho-
tolysis was acetone azine 7, produced by elimination of CO
from 4 (Scheme 1). Short-wavelength (185 nm) UV ir-
radiation yields 2,4-dimethylpent-1-en-3-one (8) as the main
product along with alkene 6 and azine 7 as minor constitu-
ents.[1b] This was rationalized by the assumption that cyclo-
propanone 5 was the main product at both 300 nm and
185 nm irradiation. Secondary photolysis of 5 results in de-
carbonylation to 6 or in rearrangement to ketone 8.
The flash vacuum pyrolysis (FVP) of pyrazolinone 4 was
investigated under a variety of conditions and product
identification carried out by gas chromatography or photo-
electron spectroscopy.[1b][2] The major products were again
enone 8 and alkene 6, while ketazine 7 was not formed dur-
ing the pyrolyses. This suggests that in the thermal reaction
the elimination of nitrogen is more efficient than during
the photolyses.
Matrix isolation spectroscopy is ideally suited to identify
labile reaction intermediates, which can be produced by
FVP with subsequent trapping in low-temperature matrices
or by direct irradiation of matrix-isolated precursors. Here
we describe a detailed mechanistic study of the photolysis
and pyrolysis of pyrazolinone 4 using the matrix isolation
technique.[3]
[a]
Lehrstuhl für Organische Chemie II der Ruhr-Universität,
D-44780 Bochum, Germany
Fax: (internat.) ϩ 49-234/709-4353
E-mail: sander@xenon.orch.ruhr-uni-bochum.de
[b]
Institut für Organische Chemie der Universität G H Essen,
D-45117 Essen, Germany
Results
Fax: (internat.) ϩ 49-201/183-4252
E-mail: paul.rademacher@uni-essen.de
[c]
Matrix Photolysis of Pyrazolinone 4
Institut für Organische Chemie der Universität Würzburg,
D-97074 Würzburg, Germany
Fax: (internat.) ϩ 49-931/888-4606
The IR spectrum of pyrazolinone 4, matrix-isolated in
Supporting information for this article is available on the WWW
under http://www.wiley-vch.de/home/eurjoc or from the author. argon at 10 K, exhibits a very strong absorption at 1771
Eur. J. Org. Chem. 2000, 91Ϫ98
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/0101Ϫ91 $ 17.50ϩ.50/0
91

W. Sander, R. Wrobel, P. Komnick, P. Rademacher, H. M. Muchall, H. Quast
FULL PAPER
cmϪ1 (CϭO stretching vibration) and a strong absorption vel of theory (Figure 1, Table 2). The most intense absorp-
at 1524 cmϪ1 (NϭN stretching vibration). In agreement tion in the IR spectrum of 9 is the CϪO stretching mode at
with literature data, pyrazolinone 4 is photochemically 1147 cmϪ1 (B3LYP: 1140 cmϪ1). Since the CϭC stretching
stable to irradiation at λ > 350 nm.[4] However, 60 min ir- vibration of 9 is located in the same area as the strong ab-
radiation at λ ϭ 248 nm (KrF Excimer Laser) results in a sorption of 5 at 1823 cmϪ1, this vibration is not observed
decrease of 4 and formation of small amounts of 2,3-di- in our experiments (Figure 1, Table 2). Otherwise, the ex-
methyl-2-butene (6), tetramethylcyclopropanone (5), and perimental and calculated IR data are in good agreement.
CO, all readily identified by comparison of the IR spectra In the parent methyleneoxirane the CϭC stretching vi-
with literature data.[5][6]
Irradiation of 4 at λ ϭ 193 nm (120 pulses, 60 mJ/pulse, bration at 1109 cmϪ1 [7Ϫ9]
ArF Excimer Laser) produces high yields of cylopropanone Further irradiation (420 pulses) leads to rearrangement
bration is found at 1824 cmϪ1 and the CϪO stretching vi-
.
5 and allene oxide 9, but only traces of ketazine 7 (1655 of 9 to 5 and its subsequent decarbonylation affording 6
and 1365 cmϪ1, Figure 1). Cyclopropanone 5 was identified (1452, 1369 and 1170 cmϪ1, identified by comparison with
by its characteristic band at 1844 cmϪ1 (carbonyl absorp- an authentic matrix-isolated sample). After 1200 pulses,
tion) and absorptions at 1823 and 967 cmϪ1 which are in a propene (10) (908 and 1448 cmϪ1) is formed as a further
good agreement with the IR data reported by Haller and fragmentation product (Scheme 2).
co-workers[5] and Sorensen and co-workers.[6] (the absorp-
Quantum chemical calculations predict that 5 is slightly
tion at 1823 cmϪ1 is a Fermi resonance of the vibrations more stable than 9 [B3LYP/6Ϫ311ϩϩG(d,p) ϩ ZPE: 0.42
at 967 and 1844 cmϪ1). A DFT calculation at the B3LYP/ kcal/mol, MP2/6Ϫ31G(d): 2.80 kcal/mol]. Oxirane 9 is only
6Ϫ311Gϩϩ(d,p) level of theory leads to an excellent repro- observed after the first pulses at λ ϭ 193 nm, prolonged
duction of the experimental spectrum (Table 1).
irradiation leads to cyclopropanone 5 and its decompo-
The second major product is the allene oxide 9, which sition products. The related rearrangement of di-tert-butyl-
was identified by comparison of the experimental IR spec- allene oxide to di-tert-cyclopropanone has been reported by
tra with data calculated at the B3LYP/6Ϫ311Gϩϩ(d,p) le- Chan and co-workers.[10]
Figure 1. IR difference spectrum showing the photolysis of 4 in argon at 10 K after irradiation of a matrix of 4 with λ ϭ 193 nm; bottom
part: bands of 4 disappearing; top part: bands of 5 and 9 appearing on irradiation.
92
Eur. J. Org. Chem. 2000, 91Ϫ98

Flash Vacuum Pyrolysis and Photolysis of 3,3,5,5-Tetramethylpyrazolin-4-one
FULL PAPER
Table 1. Comparison of IR spectroscopic data of tetramethylcyclopropanone 5, matrix-isolated in argon at 10 K, with calculations at the
B3LYP/6Ϫ311ϩϩG(d, p) (scaled by 0.97) and at the MP2/6Ϫ31G(d) (scaled by 0.95) level of theory
Nr.[a] Sym
5 (16O)
5 (18O)
[νi/ν]
5 (16O)
5 (18O)
[νi/ν]
5 (16O)
MP2/
5 (18O)
MP2/
[νi/ν]
Assignment[c]
argon,10 K
argon,10 K
B3LYP/
6Ϫ311ϩϩG
(d, p)
B3LYP/
6Ϫ311ϩϩG
(d, p)
ν˜, [cmϪ1
]
ν˜, [cmϪ1
]
6Ϫ31G(d)
6Ϫ31G(d)
b
[b]
(Irel
)
(Irel
)
ν˜, [cmϪ1
]
ν˜, [cmϪ1
]
ν˜, [cmϪ1
]
ν˜, [cmϪ1
]
(Irel
)
(Irel)
[b]
[b]
[b]
[b]
(Irel
)
(Irel)
13
14
17
19
20
21
24
25
28
31
32
36
37
39
40
41
b2
a1
b2
b2
b1
a1
b2
a1
b2
b1
b2
b1
b2
b1
a1
a1
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
1.000
1.000
Ϫ
Ϫ
Ϫ
Ϫ
0.999
Ϫ
Ϫ
Ϫ
Ϫ
1.000
Ϫ
1.000
1.004
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
0.998
1.000
Ϫ
Ϫ
1.000
Ϫ
1.000
1.000
Ϫ
573 (1)
612 (1)
780 (12)
958 (41)
959 (1)
986 (1)
1113 (5)
1123 (1)
1252 (15)
1386 (3)
1391 (1)
1472 (1)
1475 (8)
1486 (7)
1491 (1)
1501 (6)
Ϫ
571 (1)
612 (1)
778 (12)
958 (42)
959 (1)
984 (1)
1113 (5)
1123 (1)
1252 (15)
1386 (3)
1391 (1)
1472 (1)
1475 (8)
1486 (8)
1491 (1)
1500 (6)
Ϫ
0.996
1.000
0.998
1.000
1.000
0.998
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
Ϫ
794 (24)
967 (62)
Ϫ
794 (37)
967 (33)
Ϫ
Ϫ
Ϫ
Ϫ
1244 (12)
Ϫ
Ϫ
Ϫ
773 (6)
942 (21)
Ϫ
771 (6)
942 (22)
Ϫ
CϪC def.
Ϫ
Ϫ
Ϫ
1105 (8)
Ϫ
1245 (16)
1384 (4)
Ϫ
1095 (4)
Ϫ
1223 (6)
1366 (1)
Ϫ
1095 (4)
Ϫ
1223 (6)
1366 (1)
Ϫ
CϪC str.
CϪC str.
combination
Ϫ
Ϫ
Ϫ
Ϫ
1457 (11)
1461 (9)
Ϫ
1477 (7)
1823 (53)
Ϫ
1447 (4)
1456 (3)
Ϫ
1472 (2)
Ϫ
1447 (4)
1456 (4)
Ϫ
1472 (2)
Ϫ
1.000
1.000
Ϫ
1.000
Ϫ
CϪH def.
1461 (15)
Ϫ
1477 (8)
1829 (30)
Fermi-
Resonance
CϭO str.
42
44
45
46
49
50
52
53
54
a1
b2
b1
a1
b1
a1
b2
b1
a1
1844 (100)
1808 (100)
0.980
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
1836 (100)
2926 (16)
2928 (9)
2933 (11)
2982 (9)
2984 (14)
3013 (8)
3029 (11)
3029 (3)
1805 (100)
2926 (16)
2928 (10)
2933 (11)
2982 (9)
2984 (14)
3013 (8)
0.983
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1801 (100)
2939 (21)
2942 (25)
2945 (22)
3017 (10)
3019 (20)
3042 (14)
3057 (15)
3057 (5)
1774 (100)
2939 (21)
2942 (15)
2945 (12)
3017 (10)
3019 (21)
3042 (14)
3057 (15)
3057 (5)
0.985
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
CϪH str.
3029 (11)
3029 (3)
[a] Mode-number from the ab initio calculation. IR bands with relative intensities lower than 0.5 are not tabulated. Ϫ [b] Relative intensity
based on the strongest absorption (100%). Ϫ [c] The assignment is based on the comparison of the calculated and experimental spectrum
and is only tentative.
cyclopropanone (11) and methyleneoxirane (12) were
formed and in addition CO, formaldehyde, acrolein, and
ketene were produced in the ozone oxidation (Scheme 3).
This suggested to us that oxygen transfer to tetramethylal-
lene (13) should provide an alternative source of 5 and 9.
In addition, the advantageous use of 18O3 as the source of
the oxygen atoms should readily lead to incorporation of
an 18O isotopic label into 5 and 9.
Scheme 3
Irradiation of ozone at λ ϭ 420 nm rapidly produces
O(3P) and O2, while 13 is completely stable under these con-
Scheme 2
ditions. If an argon matrix, doped with 13 and an excess
(0.7% in argon) of O3, is irradiated with 420 nm light, both
13 and O3 disappear within several minutes, and a complex
mixture of oxidation products is formed (Figure 2). Most
The transfer of oxygen atoms to allene in cryogenic ma- of the products were identified by comparison of their IR
trices was investigated by Singmaster and Pimentel[7][8] and spectra with those of authentic matrix-isolated compounds
by Nakata and Frei[9] with ozone and N2O, respectively, Ϫ if available Ϫ or with spectra calculated at the B3LYP/
as photolytical sources of the oxygen atoms. In both cases, 6Ϫ311ϩϩG(d,p) level of theory. In addition, the oxidation
Oxidation of Tetramethylallene (13) with Oxygen
Atoms
Eur. J. Org. Chem. 2000, 91Ϫ98
93

W. Sander, R. Wrobel, P. Komnick, P. Rademacher, H. M. Muchall, H. Quast
FULL PAPER
Table 2. Comparison of IR spectroscopic data of tetramethylallenoxide 9, matrix-isolated in argon at 10 K, with calculations at the
B3LYP/6Ϫ311ϩϩG(d, p) (scaled by 0.97) and at the MP2/6Ϫ31G(d) (scaled by 0.95) level of theory
Nr.[a] Sym
9 (16O)
9 (18O)
[νi/ν]
9 (16O)
B3LYP/
6Ϫ311ϩϩG 6Ϫ311ϩϩG
9 (18O)
B3LYP/
[νi/ν]
9 (16O)
MP2/
9 (18O)
MP2/
[νi/ν]
Assignment[c]
argon,10 K
argon,10 K
ν˜, [cmϪ1
]
ν˜, [cmϪ1
]
6Ϫ31G(d)
6Ϫ31G(d)
[b]
[b]
(Irel
)
(Irel
)
(d, p)
(d, p)
ν˜, [cmϪ1
]
ν˜, [cmϪ1
]
ν˜, [cmϪ1
]
ν˜, [cmϪ1
]
(Irel
)
(Irel)
[b]
[b]
[b]
[b]
(Irel
)
(Irel)
12
13
15
16
17
19
20
22
25
26
27
29
30
31
32
33
36
37
38
39
40
42
43
44
45
46
47
48
50
51
52
53
54
AЈЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈЈ
AЈ
AЈЈ
AЈ
AЈ
AЈ
AЈЈ
AЈЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈ
AЈЈ
AЈ
AЈЈ
AЈЈ
AЈ
AЈ
AЈ
AЈЈ
AЈ
Ϫ
Ϫ
Ϫ
418 (7)
549 (8)
586 (22)
665 (15)
849 (10)
940 (5)
964 (8)
993 (25)
1089 (28)
1140 (100)
1200 (14)
1314 (11)
1363 (3)
409 (8)
0.978
0.987
0.984
0.988
0.998
0.999
1.000
0.986
1.000
0.993
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
0.999
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
412 (10)
537 (12)
590 (20)
667 (17)
866 (7)
954 (6)
979 (7)
1000 (27)
1107 (46)
1156 (100)
1229 (14)
1337 (11)
1385 (23)
1388 (6)
402 (11)
529 (16)
583 (19)
659 (16)
864 (7)
952 (11)
978 (4)
0.976
0.985
0.988
0.988
0.998
0.997
0.999
0.985
1.000
0.993
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
0.999
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
1.000
558 (15)
595 (5)
677 (28)
872 (20)
957 (14)
981 (13)
1011 (30)
1075 (13)
1147 (100)
1227 (26)
1333 (17)
1376 (80)
Ϫ
1396 (vw)
Ϫ
Ϫ
551 (13)
585 (4)
669 (34)
870 (18)
954 ( 16)
Ϫ
999 (28)
Ϫ
1141 (100)
1227 (23)
1332 (16)
1376 (78)
Ϫ
1396 (vw)
Ϫ
Ϫ
Ϫ
0.987
0.983
0.988
0.998
0.997
Ϫ
542 (12)
577 (21)
657 (14)
847 (10)
939 (9)
964 (10)
979 (27)
1089 (29)
1132 (100)
1200 (15)
1314 (12)
1363 (4)
1364 (18)
1374 (1)
1378 (2)
1436 (2)
1443 (5)
1445 (14)
1448 (2)
1454 (11)
1826 (51)
2909 (28)
2913 (75)
2933 (8)
2938 (30)
2947 (29)
2950 (21)
2993 (34)
3000 (24)
3012 (13)
3028 (9)
CϪC def.
0.988
Ϫ
985 (31)
ring- def.
1107 (51)
1148 (100)
1229 (15)
1336 (13)
1385 (26)
1388 (7)
1394 (2)
1401 (2)
1466 (3)
1472 (18)
1475 (7)
combination
combination
CϪC str.
0.995
1.000
0.999
1.000
Ϫ
1.000
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
1.000
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
CϪC str.
1364 (16)
1374 (1)
1378 (2)
1436 (2)
1443 (4)
1394 (2)
1401 (2)
1466 (3)
1472 (16)
1475 (6)
CϪH def.
CϭC str.
1447 (12)
Ϫ
Ϫ
Ϫ
Ϫ
1445 (13)
1448 (2)
1454 (10)
1827 (48)
2909 (26)
2913 (68)
2933 (8)
2938 (27)
2947 (26)
2950 (19)
2993 (31)
3000 (22)
3012 (12)
3028 (9)
1477 (3)
1477 (3)
1451 (15)
1451 (15)
1480 (15)
1829 (42)
2928 (32)
2931 (48)
2946 (8)
2949 (23)
2995 (29)
2998 (20)
3027 (30)
3028 (26)
3036 (149
3056 (7)
1480 (16)
1828 (45)
2928 (36)
2931 (54)
2946 (9)
2949 (26)
2995 (32)
2998 (23)
3027 (33)
3028 (29)
3036 (16)
3056 (7)
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
CϪH str.
Ϫ
Ϫ
Ϫ
3029 (14)
3029 (16)
3057 (16)
3057 (18)
[a] Mode-number from the ab initio calculation. IR bands with relative intensities lower than 0.5 are not tabulated. Ϫ [b] Relative intensity
based on the strongest absorption (100%). Ϫ [c] The assignment is based on the comparison of the calculated and experimental spectrum
and is only tentative.
was performed using 18O3 to obtain the 18O-labelled isoto- bration of the parent methyleneoxirane 12 is found at 1109
pomers (Tables 1 and 2).
cmϪ1 with an 18O isotopic shift of 8 cmϪ1 [7][8]
. Due to the
The co-deposition of 13 with ozone at 30 K in an argon overlap with the strong Fermi-resonance absorption of 5 at
matrix leads to a new, broad absorption in the UV/Vis spec- 1823 cmϪ1, the CϭC stretching vibration of 9 is not ob-
trum with a maximum at 427 nm, which disappears after served (Table 2). On 248 nm irradiation, 5 is decarbonylated
irradiation with λ > 420 nm. This absorption was assigned to give 2,3-dimethyl-2-butene (6) (Scheme 4). Thus, both
to the charge-transfer (CT) complex of allene 13 and ozone.
A similar complex of 2,3-dimethyl-2-butene and ozone has
been reported by Singmaster et al.[11][12] However, IR ab-
sorptions of the CT complex of 13 and O3 were not ob-
served in our experiments.
The IR spectra of the products show that minor constitu-
ents of the matrix are CO (IR: 2138 cmϪ1), CO2 (2347
cmϪ1), dimethylketene (14) (2130 cmϪ1) and acetone (15)
(1722 cmϪ1). The most intense absorption in the IR spec-
trum of the photolysis mixture is the carbonyl absorption
of cyclopropanone 5 at 1844 cmϪ1 with an 18O isotopic
shift of 36 cmϪ1 (Table 1). Another oxidation product of 13
is allene oxide 9 with the most intense absorption at 1147
cmϪ1 (6 cmϪ1 18O isotopic shift). The CϪO stretching vi- Scheme 4
94
Eur. J. Org. Chem. 2000, 91Ϫ98

Flash Vacuum Pyrolysis and Photolysis of 3,3,5,5-Tetramethylpyrazolin-4-one
FULL PAPER
Figure 2. a) IR difference spectrum showing the oxidation of tetramethylallene 13 with oxygen atoms produced by 420 nm irradiation of
0.7% O3 in argon at 10 K; bottom part: bands of 13 and O3 disappearing; top part: bands of 5 and 9 appearing on irradiation; bands
marked X correspond to a photolabile product of unknown constitution; b) Calculated IR spectrum of 9; c) Calculated IR spectrum of
5; the IR spectra were calculated at the B3LYP/6Ϫ311ϩϩG(d, p) level of theory and scaled by 0.97; the assignment of experimental and
calculated IR bands is based on band positions and is only tentative, especially in areas of overlapping bands
the 193 nm photolysis of pyrazolinone 4 and the reaction were subsequently trapped with a large access of argon at
of 13 with oxygen atoms result in the formation of a mix- 8Ϫ10 K. Above 650°C pyrolysis temperature, CO is
ture of 5 and 9.
formed, indicating the decomposition of 4 (Figure 3). At
1000°C, the pyrolysis is nearly complete, and a complex
product mixture is formed with alkene 6, ketone 8, enol 16
as major products and cyclopropanone 5 (1844 and 1823
cmϪ1) as one of the minor constituents.
Flash Vacuum Pyrolysis of Pyrazolinone 4
Pyrazolinone 4 was pyrolyzed in a quartz tube at 10Ϫ5
mbar and temperatures up to 1000°C, and the products
IR absorptions of alkene 6 (1452, 1369 and 1170 cmϪ1
and ketone 8 (1682 and 1053 cmϪ1) were identified by com-
)
Figure 3. IR spectrum showing the pyrolysis products of 4 (alkene 6, ketone 8, enol 16, and traces of cyclopropanone 5) trapped in argon
at 10 K; the pyrolyses were performed at 650, 800, 1000 and 1200°C
Eur. J. Org. Chem. 2000, 91Ϫ98
95

W. Sander, R. Wrobel, P. Komnick, P. Rademacher, H. M. Muchall, H. Quast
FULL PAPER
Table 3. Comparison of the IR spectroscopic data of enol 16, ma- and the products were co-deposited with a large excess of
trix-isolated in argon at 10 K, with calculations at the B3LYP/
6Ϫ31G(d, p) (scaled by 0.97) level of theory
argon onto a cold CsI window at 8Ϫ10 K. Again, a com-
plex mixture of products is obtained. In addition to alkene
6, ketone 8, and enol 16 as the main products, traces of
[b]
[b]
Nr.[a] 16 (16O)
(Irel
)
16 (16O)
(Irel
)
Assign-
ment[c]
argon,10 K
B3LYP/
cyclopropanone 5, propene (10) (908 and 1448 cmϪ1), al-
lene (18) (1955 cmϪ1), propyne (19) (629, 1034 and 1377
cmϪ1), and 2,3-dimethylbuta-1,3-diene (17) (893 and 1603
cmϪ1) were identified by comparison with authentic matrix-
isolated compounds (Scheme 6).
ν˜, [cmϪ1
]
6Ϫ311ϩϩG(d,p)
ν˜, [cmϪ1
]
13
15
16
18
19
25
522
Ϫ
Ϫ
Ϫ
905
1062
17
Ϫ
Ϫ
Ϫ
80
42
514
624
657
863
911
1069
7
2
CϪC def.
3
3
26
14
CϪH o.o.p.
CϪC def.ϩ
CϪH def.
CϪO str.
27
28
29
30
35
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
1152
Ϫ
1255
1315
Ϫ
Ϫ
Ϫ
1453
Ϫ
1641
1667
Ϫ
2865
Ϫ
Ϫ
2934
Ϫ
100 1132
100
5
Ϫ
19
42
Ϫ
Ϫ
Ϫ
23
Ϫ
7
1218
1240
1295
1443
1451
1462
1464
1467
1659
1696
2925
2931
2949
2965
2971
3003
3037
3054
3058
3065
3145
3688
23
38
3
CϪH def.
4
3
6
3
6
CϭC str.
CϭC str.
6
5
Ϫ
25
Ϫ
Ϫ
20
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
49
12
52
12
11
24
10
12
8
Scheme 6
CϪH str.
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
3630
6
6
9
Photolysis and Pyrolysis of Tetramethylcyclobutane-
1,3-dione (20)
20
OϪH str.
[a] Mode-number from the ab initio calculation. IR bands with rela-
tive intensities lower than 0.5 are not tabulated. Ϫ [b] Relative inten-
sity based on the strongest absorption (100%). Ϫ [c] The assignment
is based on the comparison of the calculated and experimental
spectrum and is only tentative.
An alternative entry to cyclopropanone 5 and its isomers
is the photochemical or thermal cleavage of dione 20. The
matrix photochemistry (λ > 250 nm, nitrogen at 4 K) of 20
has been investigated by Haller and Srinivasan.[5] Loss of
CO afforded cyclopropanone 5 and alkene 6 as the main
products. Another major pathway is the fragmentation of
20 to give two molecules of dimethylketene 14 (Scheme 7).
Scheme 5
parison with authentic matrix-isolated material. Enol 16
with the OϪH stretching vibration at 3631 cmϪ1 and the
CϭC stretching vibration at 1673 cmϪ1 was identified by
comparison of the IR spectrum with B3LYP/6Ϫ31G(d,p)
calculations (Table 3). Irradiation at λ ϭ 248 nm results in
the photolysis of cyclopropanone 5 (see above) and enol 16,
which rearranges to enone 8 (Scheme 5).
Scheme 7
Using a monochromatic light source (λ ϭ 248 or 193 nm)
and argon at 10 K, we obtained basically the same prod-
ucts. In addition, traces of ketone 8 were detected. FVP and
pulsed pyrolysis of 20 also lead to the same product mixture
as the UV photolysis. The only further product identified
Pulse Pyrolysis of Pyrazolinone 4
Pyrazolinone 4 was thermolyzed under the conditions of on FVP at 1000°C was enol 16 (trace amounts), which re-
pulse pyrolysis (1200°C, 2 bar, 10 ms pulse duration), [13Ϫ15] arranges to ketone 8 upon irradiation at λ ϭ 248 nm.
96
Eur. J. Org. Chem. 2000, 91Ϫ98

Flash Vacuum Pyrolysis and Photolysis of 3,3,5,5-Tetramethylpyrazolin-4-one
FULL PAPER
is only observed as a product of the thermolysis but not of
the photolysis of 4.
Discussion
Pyrazolinone 4 represents a bichromophoric substrate,
i.e., it contains both azo and carbonyl groups for photolytic
excitation. Conventional long wavelength (λ > 250 nm)[1a]
and short wavelength UV (185 nm)[1b] photolyses lead pref-
erentially to denitrogenation, although azine 7 is also
formed in 15Ϫ37% yield. Irradiation with λ ϭ 193 nm pro-
duces mainly cyclopropanone 5 and small amounts of al-
lene oxide 9 with only trace amounts of the azine 7. A
plausible precursor of 5 and 9 may be an oxyallyl, which
can either form a CϪC bond as in 5 or a CϪO bond as in
9 (Scheme 8). Such oxyallyls have been postulated several
times as intermediates in the allene oxideϪcyclopropanone
isomerization process,[16] but as yet have never been directly
observed. The IR spectrum of the planar diradical 21 was
calculated by Borden et al.,[17] however, in our experiments,
no IR absorptions could be assigned to 21.
In this study, we were able to generate and characterize
allene oxide 9 and enol 16 for the first time by the photoly-
sis and thermolysis, respectively, of pyrazolinone 4. Al-
though tetramethyloxyallyl probably is a key intermediate
in the transformations leading to 5 and 9, this diradical
could not be detected in our experiments. Obviously, the
ring-closure to cyclopropanone 5 is rapid under the exper-
imental conditions. Nitrogen loss of 4 results in 23 with the
methyl groups at the radical centers rotated out of plane.
To avoid the ring closure, the more stable planar 21 has to
be directly generated from a suitable precursor.
Scheme 8
The electrophilic addition of oxygen atoms [O(3P)] to al-
lene (18) has been investigated by Hammond et al. using ab
initio theory.[18] Two reaction pathways are calculated. In
the first, O-atom attack at the central carbon atom of allene
is followed by ring closure of the initially produced parent
orthogonal oxyallyl diradical to cyclopropanone 5. The en-
ergy barrier for this reaction was found to be 1.8 kcal/mol
lower than that of the reaction initiated by attack at the
terminal carbon atom. In the second pathway, the inter-
mediate is the parent allenoxy diradical which subsequently
cyclizes to allene oxide (12). However, the formation of
both cyclopropanone and allene oxide as products of the
oxygen transfer to allene in cryogenic matrices suggests that
both pathways are important.[7Ϫ9] This is in agreement with
our observation of the oxygen transfer to 13, which pro-
duces both 5 and 9.
Experimental Section
Matrix isolation experiments were performed by standard tech-
niques with an APD CSW-202 Displex closed-cycle helium cryo-
stat. Matrices were produced by deposition of argon (Messer-
Griesheim, 99.9999%) on top of a CsI window at a rate of approxi-
mately 0.15 mmol/min. Pyrazolinone 4 was matrix-isolated by sub-
limation at Ϫ20°C and deposition with a large excess of argon on
top of a cold window (30 K). Infrared spectra were recorded with
a Bruker IFS66 FTIR spectrometer with a standard resolution of
0.5 cmϪ1 in the range of 400Ϫ4000 cmϪ1
.
Irradiations were carried out with a KrF (λ ϭ 248 nm) or ArF
Excimer Laser (λ ϭ 193 nm; Lambda Physik COMPex 100) or Os-
ram HBO 500 W/2 mercury high-pressure arc lamps in Oriel hous-
ings equipped with quartz optics. IR irradiation from the arc lamps
was absorbed by a 10 cm path of water. Schott cut-off filters were
used (50% transmission at the wavelength specified) in combination
with dichroic mirrors.
Pyrolyses were carried out in a quartz tube placed 3 cm in front of
the cold window. In order to obtain short contact times, a heating
zone of only 3 cm was used. A large excess of argon and the precur-
sor were allowed to pass through the pyrolysis zone and the prod-
ucts were directly trapped on top of the cold window at 10 K.
Ketone 8 is presumably formed by rearrangement of
cyclopropanone 5. The first step is a [1,5]-H migration in 5
which results in the ring-opening and formation of enol 16.
The pulsed deposition unit was constructed similar to ChenЈs[13]
Since enol 16 is labile towards UV irradiation (248 nm), it and MaierЈs[14][15] description. We used a valve supplied by GEN-
Eur. J. Org. Chem. 2000, 91Ϫ98
97

W. Sander, R. Wrobel, P. Komnick, P. Rademacher, H. M. Muchall, H. Quast
FULL PAPER
[3]
The matrix photochemistry of pyrazolinone 4 has been inde-
pendently investigated by G. Maier and M. Cibulka, private
communication.
ERAL VALVE Corporation, with an orifice of 0.5 mm diameter.
The valve was connected to an alumina tube of 2 mm inner diam-
eter, which was mounted on a second alumina tube with 1 mm
inner diameter, each tube having a length of 30 mm. The latter tube
can be heated to 1400°C by means of a tantalum wire. The distance
between the orifice and the spectroscopic window was 30 mm. For
the use of pressures higher than 1 bar, the gas mixtures were pre-
pared in a stainless-steel gas mixing unit.
[4]
[5]
[6]
P. S. Engel, Chem. Rev. 1980, 80, 99.
I. Haller, R. Srinivasan, Can. J. Chem. 1965, 43, 3165.
C. Black, P. Lario, A. P. Masters, T. S. Sorensen, F. Sun, Can.
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K. A. Singmaster, G. C. Pimentel, J. Mol. Struct. 1989, 194,
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[8]
[9]
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[10]
Pyrazolinone 4 was prepared by the procedure described by R. J.
Crawford et al.[20]
[11]
[12]
[13]
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Calculations were performed with the Gaussian 94 program pack-
age.[21] Geometries and vibrations were calculated at the MP2/
6Ϫ31G, B3LYP/6Ϫ31G(df) or the B3LYP/6Ϫ311G(3df,3pd) level
of theory.[22] The calculated geometries of 5, 9, and 16 are included
in the Supporting Information.
S. Jonnalagadda, S. Chan, J. Garrido, J. Bond, K. A. Singmas-
ter, J. Am. Chem. Soc. 1995, 117, 562.
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[14]
[15]
G. Maier, T. Preiss, H. P. Reisenauer, Chem. Ber. 1994, 127, 779.
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W. T. Borden, private communication
[16]
[17]
[18]
Acknowledgments
B. L. Hammond, S. Y. Huang, W.A. Lester, Jr., M.Dupuis, J.
Phys. Chem. 1990, 94, 7969.
H. M. Muchall, K. Kowski, P. Rademacher, A. Fuss, R. Neub-
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1965.
[19]
This work was financially supported by the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie. We thank
Prof. G. Maier for helpful discussions and for communicating his
preliminary results on the photochemistry of pyrazolinone 4.
[20]
[21]
R. J. Crawford, H. Tokunaga, Can. J. Chem. 1974, 52, 4033.
M. J. Frisch, M. Head-Gordon, G. W. Trucks, J. B. Foresman,
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Gonzalez, D. J. Defrees, D. J. Fox, R. A. Whiteside, R. Seeger,
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[1] [1a]
H. Quast, A. Fuss, Angew. Chem. 1981, 93, 293; Angew.
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P. Mazenod, H. Quast, J. Am. Chem. Soc. 1981, 103, 998.
[22]
[2]
M. K. Kindermann, R. Poppek, P. Rademacher, A. Fuss, H.
Received June 25, 1999
Quast, Chem. Ber. 1990, 123, 1161.
[O99377]
98
Eur. J. Org. Chem. 2000, 91Ϫ98