p-benzyne

Article abstract of DOI:10.1002/(SICI)1521-3773(19980420)37:7<955::AID-ANIE955>3.0.CO;2-T

A noble gas matrix at low temperature was used to investigate the photochemical behavior of diacetyl terephthaloyl diperoxide and dipropionyl terephthaloyl diperoxide as well as 1,4-diiodobenzene. All three photoreactions formed small quantities of a compound with IR absorption bands at 725 and 980 cm^-1, which disappeared when the matrix was annealed. These bands correspond to the most intense of the calculated bands (B3LYP) for 1,4-didehydrobenzene (p-benzyne) (1). That decomposition of the peroxides in fact leads to 1 is confirmed by vapor-phase pyrolysis experiments in which (Z)-2 was obtained in high yield.

Full text of DOI:10.1002/(SICI)1521-3773(19980420)37:7<955::AID-ANIE955>3.0.CO;2-T

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be trapped in the gas phase with O2 or NO.[7] Roth et al.
[7] H. Langhals, J. Gold, Liebigs Ann. 1997, 1151 ± 1153.
[8] See also A. Osuka, H. Shimidzu, Angew. Chem. 1997, 109, 93 ± 95;
Angew. Chem. Int. Ed. Engl. 1997, 36, 135 ± 137.
[9] H. Langhals, S. Demmig, T. Potrawa, J. Prakt. Chem. 1991, 333, 733 ±
748.
[10] H. Langhals, Nachr. Chem. Tech. Lab. 1980, 28, 716 ± 718 [Chem.
Abstr. 1981, 95, R9816q].
[11] M. Klessinger, J. Michl, Lichtabsorption und Photochemie organischer
determined the heat of formation of 1 from the kinetics of
1
these trapping reactions to be 138.0 Æ 1 kcalmol (which
1
means it is 8.5 kcalmol less stable than (Z)-2), and the
1 [7]
barrier toward ring opening to be 19.8 kcalmol .
Although several attempts at the synthesis of 1 from p-
substituted benzene derivatives are reported in the litera-
ture,[8, 9] the only derivative of 1 so far characterized spec-
troscopically, at low temperature, is 9,10-didehydroanthra-
cene (3).[10] Squires et al. determined thermochemical data for
1 through collision-induced dissociation of the p-chlorophenyl
anion in the gas phase in a quadrupole mass spectrome-
Moleküle, VCH, Weinheim, 1989.
7
[12] The concentrations of 5a, 6a, 6c, and 6e in CHCl3 were 4.12 Â 10
,
7
7
1
2.33 Â 10
,
2.57 Â 10
,
and 2.66 Â 10 7 molL
,
respectively. The
quantum yields F are quoted with respect to that of the reference
compound 1a (F ˆ 100%).
[13] W. E. Ford, J. Photochem. 1987, 37, 189 ± 204.
[14] T. Förster, Naturwissenschaften 1946, 166 ± 175.
[15] H. Zollinger, Color Chemistry, 2nd ed., VCH, Weinheim 1991.
ꢀ
1
ter.[11, 12] These data (DHf ˆ 137 Æ 3 kcalmol ) are in excel-
lent agreement with the values reported by Roth et al.[7] and
with theoretical data from ab initio calculations at the
CCSD(T)[13, 14] and CASSCF/CASPT2[15] levels of theory.
Thus, both experiment and theory predict 1 to lie in an
energy well sufficiently deep to permit, in principle, its
isolation at low temperature. Here we report a matrix
isolation and IR spectroscopic characterization of p-benzyne
(1).
p-Benzyne**
Acyl peroxides of type 4 were first used by Pacansky
et al.[16, 17] for the matrix isolation of radicals, and were
recently utilized for the generation of the phenyl radical[18]
and the synthesis of m-benzyne.[17] Upon irradiation, 4
decomposes to an aryl radical 5, CO2, and a second radical,
which in the case of acetyl peroxides is a methyl radical
(Scheme 1). As long as the temperature of the matrix is kept
Ralph Marquardt, Andreas Balster, Wolfram Sander,*
Elfi Kraka, Dieter Cremer,* and
J. George Radziszewski*
Dedicated to Professor Dr. Günther Maier
on the occasion of his 66th birthday
Since the discovery of the enediyne antibiotics there has
been great interest in p-benzyne (1,4-didehydrobenzene, 1)
and its derivatives.[1±3] Compound 1 is formed as a reactive
intermediate at very low stationary concentrations in the
rearrangement of (Z)-3-hexene-1,5-diyne [(Z)-2],[4±6] and can
[*] Prof. Dr. W. Sander, Dr. R. Marquardt, A. Balster
Lehrstuhl für Organische Chemie II der Universität
D-44780 Bochum (Germany)
Fax: (‡49)234-709-4353
E-mail: sander@neon.orch.ruhr-uni-bochum.de
Prof. Dr. D. Cremer, Prof. Dr. E. Kraka
Department of Theoretical Chemistry
University of Göteborg
Ã
Kemigarden 3, S-41296 Göteborg (Sweden)
Scheme 1. Photolysis of 4a, b in argon at 10 K.
Fax: (‡46)31-772-2933
E-mail: cremer@theoc.chalmers.se
Dr. J. G. Radziszewski
low enough to prevent the diffusion of small molecules, the
two radical fragments are separated by two CO2 molecules,
but at higher temperatures the radicals react rapidly to give 6
and 7.
National Renewable Energy Laboratory (NREL, 1613)
1617 Cole Boulevard, Golden, CO 80401 (USA)
Fax: (‡1)303-275-2905
E-mail: jradzisz@nrel.nrel.gov
Similarly, diacetylterephthaloyl diperoxide (8)[19] should
yield 1, CO2, and methyl radicals (Scheme 2). Vacuum flash
pyrolysis of 8 at temperatures between 300 and 6008C and
trapping of the products in argon at 10 K resulted in the
formation of Z enediyne 2 and CO2 as the main products,
together with a small amount of dimethyl terephthalate (6b).
[**] This work was supported financially by the Deutschen Forschungsge-
meinschaft, the Fonds der Chemischen Industrie, and the Swedish
Natural Science Research Council. Calculations were carried out on a
CRAY YMP/464 and a CRAY C90 of the Nationellt Superdatorcen-
trum (NSC), Linköping, Sweden, whose generous support we grate-
fully acknowledge. J. G. R. acknowledges support from the NREL
FIRST program.
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amounts of the p-tolyl radical (5a). Annealing of the matrix at
40 K reduced the number of these radicals. IR absorption
bands of medium to low intensity at 1445, 980, 814, 725, and
1
691 cm also disappeared when the matrix was warmed;
however, it was not possible to assign these to any of the
known compounds (Figure 1).
Scheme 2. Photolysis of 8 in argon at 10 K.
At higher pyrolysis temperatures (Z)-2 is partially isomerized
to (E)-2. No other products were observed, nor were IR
absorption bands that could be attributed to 1. This suggests
that 1 is formed during pyrolysis of 8 but is not stable toward
ring opening at the temperatures necessary for fragmentation
of the peroxide. The fact that the Bergman rearrangement[4] is
rapid under these conditions was readily demonstrated by
vacuum flash pyrolysis of (Z)-[1,6-D2]hex-3-en-1,5-diine at
300 ± 5008C, which resulted in an equilibrium mixture of the
[1,6-D2] and [3,4-D2] isotopomers. Again, the E isomer was a
secondary product formed only at temperatures above 8008C.
The radicals expected to be formed upon photolysis of 8 are
p-benzyne (1), the methyl radical, the p-tolyl radical (5a), and
the 4-(methoxycarbonyl)phenyl radical (5b, Scheme 2). To
obtain reference spectra of 5a, b the photochemistry of acyl
peroxides 4a, b[19] was first investigated under matrix conditions.
Photolysis (l ˆ 254 nm, Scheme 1) of matrix-isolated acetyl
4-methylbenzoyl peroxide (4a) yielded CO2, methyl p-toluate
(6a), methyl radical, and a new species with strong IR
Figure 1. Difference IR absorption spectra after photolysis (254 nm, 10 K)
of 8 (a) and [D6]8 (c) as well as spectra after subsequent annealing at 408C
(b and d). After irradiation, no bands remain for the starting material, only
those of the photolysis products are observed. Upon disappearance of the
bands for the radicals and 1, bands for unknown products appear.
Irradiation of matrix-isolated [D6]8 (with perdeuterated
acetyl groups) again resulted in formation of a set of IR
absorption bands for a thermolabile species. The bands at 980
and 725 cm 1 were the only ones that did not exhibit isotopic
shifts (Figure 1) and therefore cannot be associated with a
compound that contains methyl groups. It is tempting to
assign these vibrations to p-benzyne (1). To obtain a third
1
absorption bands at 1450, 1029, 767, and 452 cm . The two
latter species disappeared when the matrix was annealed at
30 K, and p-xylene (7a) was formed. This thermal reaction
and the similarity of the observed IR absorption bands to
those of the phenyl radical[17] strongly suggest that the new
species is the p-tolyl radical (5a).
Photolysis (l ˆ 248 nm) of acetyl 4-(methoxycarbonyl)ben-
zoyl peroxide (4b) produced CO2, dimethyl terephthalate
(6b), methyl radical, and the 4-(methoxycarbonyl)phenyl rad-
1
ical (5b) with strong bands at 1438, 1372, and 739 cm . Because
of the strong carbonyl absorption of ester 6b at 1740 cm 1 the
carbonyl absorption of 5b could not be clearly observed.
When the matrix was annealed at 40 K the methyl radicals
reacted with 5b to produce methyl p-toluate (7b ˆ 6a).
The main products of photolysis (l ˆ 254 nm) of 8 were
dimethyl terephthalate (6b) and CO2 (Scheme 2). Other
products were the methyl radical, radical 5b, and small
isotopomer of 8 the benzene ring was perdeuterated to give
[D4]8. Photolysis of [D4]8 under the same conditions did not
1
result in absorption bands at 980 and 725 cm . Instead, a very
1
weak absorption appeared at 767 cm , which was tentatively
assigned to [D4]1.
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Irradiation (l ˆ 254 nm) of matrix-isolated dipropionyl
terephthaloyl peroxide (9) produced CO2, diethyl terephtha-
late, ethyl radicals,[20] and small amounts of the 4-(ethoxycar-
bonyl)phenyl radical, identified by the similarity of its IR
spectrum to that of the methyl derivative 5b. When the matrix
was annealed, all the absorption bands assigned to the radicals
In Table 1, scaled B3LYP vibrational frequencies and
corresponding IR intensities calculated for the harmonic
approximation are compared with the observed IR bands.
Table 1. Calculated IR frequencies nÄ [cm 1] and intensities I [kmmol 1] for
1 and [D4]1 at the UB3LYP/6-311 ‡ G(3d1f,3p1d) level.[a]
1
decreased in intensity. Again, the bands at 980 and 725 cm
Entry Sym.
1
I
nÄexp.
[D4]1
nÄscaled
[b]
[b]
assigned to 1 became apparent during photolysis and dis-
appeared after the matrix was annealed; therefore, the
photochemistry of 9 is completely analogous to that of 8.
Iodobenzenes are independent precursors of radicals and
didehydrobenzenes. Indeed, irradiation of matrix-isolated 1,2-
diiodobenzene produces o-benzyne.[21] Since the C I bond
nÄ
nÄscaled for 10/8
nÄ
I
1
au
414
0
396 ± / ±
360
0
345
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
b3u
b3g
b2g
ag
b1g
b3u
b2g
b1u
au
445 20
427 435(w)/ ±
392 26 376
577
614
625
731
0
0
0
0
554
589
600
702
± / ±
± / ±
± / ±
± / ±
557
544
603
568
0
0
0
0
535
523
579
546
1
energy is about 65 kcalmol and aromatic iodides begin to
758 76
915
923 18
728 721(s)/725(s) 625 24 600
876 ± / ± 760 729
886 918(w)/ ± 806 20 774 (767)
absorb UV light at 270 nm, 248- or 254-nm irradiation
provides direct energetic access to radicals.[22] From our
earlier work with the phenyl radical we concluded on the basis
of photoorientation and polarization measurements that
partial in-plane rotation of the ring is necessary to permit
the formation from iodobenzene of radical pairs that do not
immediately recombine.
Neon matrices (at 6 K) provide unique viscosity conditions
that allow rupture of the first C I bond and subsequent partial
rotation of the ring, which in the photolysis of 1,4-diiodoben-
zene (10) leads to radical 11.[23] A small fraction of 11 is
0
0
947
1021
0
0
909
980
± / ±
± / ±
772
849
0
0
6
0
0
741
815
875
766
950
ag
b1u
b2u
ag
b2u
b3g
b2u
ag
b1u
b3g
b1u
b3g
b2u
ag
1047 17
1005 976(w)/980(s) 911
1050
1151
1231
1279
1391
1394
1442
1633
3173
3175
3189
3190
3
0
5
0
5
0
2
0
1
0
3
0
1008
1105
± / ±
± / ±
798
990
1182 1207(w)/ ±
1228 ± / ±
1335 1331(w)/ ±
1338 ± / ±
1384 1403(w)/ ±
1149
977
1 1103
938
0
1361 < 1 1306
1369
1320
1614
2340
2339
2357
2360
0 1315
1 1267
0 1549
1 2246
0 2245
2 2263
0 2266
1568
3046
3048
3061
3062
± / ±
± / ±
± / ±
± / ±
± / ±
[a] The experimentally determined values were obtained by irradiation of
10 (first value) and 8 (second value). Relative intensities are indicated in
parentheses, as is the only detected band for [D4]1. UB3LYP/6-311+G
(3d1f, 3p1d) geometry: R( ´ CC) ˆ 1.366, R(CC) ˆ 1.419, R( ´ C ´ C) ˆ 2.664,
R(CH) ˆ 1.082Š; a(C ´ CC) ˆ 125.8, a( ´ CCC) ˆ 117.2, a( ´ CCH) ˆ
123.48. [b] Scaling factor 0.96.
converted into 1 by the subsequent absorption of photons. The
observed set of IR absorption bands is exceedingly weak
3
(A ꢁ 10 ) but reproducible, and it corresponds well with
Calculations predict that the IR spectrum of 1 should be
characterized by four relatively strong bands: the b3u-sym-
metric ring torsion mode (boat conformation, out-of-plane) at
427 (entry 2), the b3u-symmetric CH wagging mode at 728
(entry 7), the b1u-symmetric ring deformation mode at 886
(entry 9), and the b1u-symmetric C ± C stretching and C-C-H
bending mode at 1005 cm 1 (entry 12). In addition, four rather
weak bands should appear at 1008 (entry 13, probably masked
results obtained from the photolysis of peroxides. These bands
disappear from the spectra after annealing at about 8 K, while
facile recombination of monoradical 11 with iodine atoms to
form 10 is observed at 9.5 K. It is impossible to decide whether
a small fraction of 10 originates from recombination of iodine
atoms with 1. The frequencies of the observed bands are very
probably affected by the proximity of heavy iodine atoms,
whereas the intensities may be distorted by nonlinearity in the
detector response to such extremely weak signals.
To provide further support for this assignment, the vibra-
tional spectrum of 1 was calculated at different levels of
theory ranging from MP2 through MP4 to GVB, CCSD,
CCSD(T), and density functional theory (DFT) with the
B3LYP functional. Most of these descriptions turned out to be
insufficient because of the multireference character of 1 and
the necessity of providing a reasonable description of both
static and dynamic electron correlation in 1. Useful descrip-
tions were obtained only at the CCSD(T)[24] and B3LYP[25, 26]
levels of theory. Basis sets ranging from 6-31G(d,p) to 6-311 ‡
G(3df,3pd)[27] were utilized; in the first case the size of the
basis set was limited by the available computer capacity. All
calculations were carried out with the programs ACES[28] and
GAUSSIAN 94.[29]
1
by entry 12), 1182 (entry 15), 1335 (entry 17), and 1384 cm
(entry 19). Of these bands, all but the weak bands 13 and 17
could be identified in the recorded IR spectra, with differ-
ences from the calculated frequencies of only 3 to at most
1
30 cm . The agreement for the intense bands 12 and 7 is
particularly satisfying.
Upon perdeuteration of 1 the IR spectrum changes
significantly (Table 1). Of the six bands calculated for [D4]1
1
in the region 700 to 1500 cm with weak (entries 15, 17, and
19), strong (entries 9 and 12), or very strong intensity
(entry 7), all but band 9 are considerably weaker than in the
non-deuterated case. According to our calculations, it should
be nearly impossible to observe modes 12, 15, 17 (also not
observed for 1), or 19. The band corresponding to the b3u-
symmetric motion (entry 7) should have a reduced intensity of
1
1
24 kmmol , but its frequency is shifted to 600 cm (into a
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[2] K. C. Nicolaou, Angew. Chem. 1993, 105, 1462 ± 1471; Angew. Chem.
Int. Ed. Engl. 1993, 32, 1377 ± 1385.
[3] P. Chen, Angew. Chem. 1996, 108, 1584 ± 1586; Angew. Chem. Int. Ed.
Engl. 1996, 35, 1478 ± 1480.
region in which the IR spectrum could not be recorded). The
1
only detectable IR band of [D4]1 in the region 700 ± 1500 cm
1
is predicted to fall at 774 cm and should belong to the b1u-
symmetric mode 9 (masked in the case of 1 by IR absorption
of other compounds, Table 1). This is in agreement with the
fact that the only IR band observed for [D4]1 in the region
[4] R. R. Jones, R. G. Bergman, J. Am. Chem. Soc. 1972, 94, 660 ± 661.
[5] R. G. Bergman, Acc. Chem. Res. 1973, 6, 25 ± 31.
[6] T. P. Lockhart, P. B. Comita, R. G. Bergman, J. Am. Chem. Soc. 1981,
103, 4082 ± 4090.
[7] W. R. Roth, H. Hopf, C. Horn, Chem. Ber. 1994, 127, 1765 ± 1779.
[8] R. S. Berry, J. Clardy, M. E. Schafer, Tetrahedron Lett. 1965, 15, 1003 ±
1010.
[9] I. P. Fisher, F. P. Lossing, J. Am. Chem. Soc. 1963, 85, 1018 ± 1019.
[10] O. L. Chapman, C. C. Chang, J. Kolc, J. Am. Chem. Soc. 1976, 98,
5703 ± 5705.
[11] P. G. Wenthold, J. A. Paulino, R. R. Squires, J. Am. Chem. Soc. 1991,
113, 7414 ± 7415.
[12] P. G. Wenthold, R. R. Squires, J. Am. Chem. Soc. 1994, 116, 6401 ±
6412.
[13] E. Kraka, D. Cremer, Chem. Phys. Lett. 1993, 216, 333 ± 340.
[14] E. Kraka, D. Cremer, J. Am. Chem. Soc. 1994, 116, 4929 ± 4936.
[15] J. Cramer, J. J. Nash, R. R. Squires, Chem. Phys. Lett., 1997, 277, 311 ±
320.
[16] a) J. Pacansky, J. Bargon, J. Am. Chem. Soc. 1975, 97, 6896 ± 6897;
b) J. Pacansky, W. Koch, M. D. Miller, ibid. 1991, 113, 317 ± 328.
[17] a) J. Pacansky, G. P. Gardini, J. Bargon, Ber. Bunsen-Ges. Phys. Chem.
1978, 82, 19 ± 20; b) J. Pacansky, J. S. Chang, D. W. Brown, Tetrahedron
1982, 38, 257 ± 260.
[18] J. G. Radziszewski, M. R. Nimlos, P. R. Winter, G. B. Ellison, J. Am.
Chem. Soc. 1996, 118, 7400 ± 7401.
[19] R. Marquardt, W. Sander, E. Kraka, Angew. Chem. 1996, 108, 825 ±
827; Angew. Chem. Int. Ed. Engl. 1996, 35, 746 ± 748.
[20] J. Pacansky, M. Dupuis, J. Am. Chem. Soc. 1982, 104, 415 ± 421.
[21] J. G. Radziszewski, unpublished results.
[22] ªVibrational and Electronic Energy Levels of Polyatomic Transient
Moleculesº: M. E. Jacox, J. Phys. Chem. Ref. Data Monogr. 1994, 3.
[23] Recorded ESR, UV/Vis, and IR absorption spectra are compatible
with expectations for the 4-iodophenyl radical. In addition, the
experimental IR spectrum is nicely reproduced by calculations at the
B3LYP level of theory. A full account of this work will be presented
elsewhere.
1
700 ± 1500 cm 1 is at 767 cm .
We were thus able to prepare in a matrix from two
precursors a thermolabile species with IR absorption at 435,
1
721/725, 976/980, 1207, and 1403 cm . These IR bands are in
agreement with calculated frequencies for p-benzyne (1). The
IR spectrum of [D4]1 is characterized by a single intense IR
1
1
absorption (767 cm ) between 700 and 1500 cm . The Berg-
man rearrangement is very rapid in the gas phase, and for this
reason 1 is converted completely into ring-opened enediyne
(Z)-2.
Experimental Section
General procedure for the synthesis of acyl peroxides: a solution of the acyl
chloride (5 mmol for the synthesis of 4, 10 mmol for 8) in CHCl3 (20 mL)
and pyridine (12 mmol in 20 mL of CHCl3) was added within one hour at
108C to a suspension of the corresponding peroxy- or diperoxyacid
(5 mmol) in CHCl3 (100 mL) and stirred at this temperature for 2 h. The
cold solution was filtered and washed with water (30 mL), 10% HCl
(30 mL), and a saturated solution of NaHCO3 (2 Â 30 mL). The CHCl3
solution was dried over Na2SO4, and solvent was removed in vacuo at 158C.
The peroxides were obtained in 40 ± 80% yield as crystalline material of
generally high purity and, if necessary, purified by preparative HPLC
(SiO2, CH2Cl2/CHCl3). Compounds 4a,[30] 8,[31] and 11[31] were identified by
comparison with literature data.
4b: M.p. 958C; IR (KBr): nĈ 3106 (vw), 2961 (vw), 1814 (w), 1814 (m),
1771 (vs), 1723 (vs), 1578 (w), 1503 (vw), 1440 (m), 1408 (m), 1368 (w), 1288
(vs), 1229 (s), 1154 (m), 1110 (s), 1034 (vs), 1010 (vs), 958 (vw), 894 (w), 870
(w), 836 (w), 823 (w), 787 (vw), 720 (vs), 668 (m), 643 (w), 584 cm (vw);
UV/Vis (EtOH): lmax (lge) ˆ 208 (3.45), 244 (4.12), 286 nm (3.16); 1H NMR
[24] K. Raghavachari, G. W. Trucks, J. A. Pople, M. Head-Gordon, Chem.
Phys. Lett. 1989, 157, 479 ± 483.
[25] A. Becke, J. Chem. Phys. 1993, 98, 5648 ± 5652.
[26] P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys.
Chem. 1993, 98, 11623 ± 11627.
(400 MHz, CDCl3): d ˆ 8.09 (dd, J ˆ 27.2, 8.4 Hz, 4 H), 3.93 (s, 3 H), 2.26 (s,
13
ˆ
ˆ
3 H); C NMR (400 MHz, CDCl3): d ˆ 166.0 (s, C O), 165.8 (s, C O),
ˆ
162.2 (s, C O), 135.1 (s), 129.9 (d), 129.7 (d), 129.3 (s), 52.6 (q), 16.6 (q); MS
(70 eV): m/z (%): 238 (3) [M ], 207 (4), 180 (13), 164 (10), 163 (100), 149
‡
[27] a) P. C. Hariharan, J. A. Pople, Chem. Phys. Lett. 1972, 16, 217 ± 219;
b) R. Krishnan, M. Frisch, J. A. Pople, J. Chem. Phys. 1980, 72, 4244 ±
4245.
(36), 135 (13), 121 (8), 104 (14), 103 (10), 77 (9), 76 (17), 75 (9), 65 (11), 50
(15), 45 (7), 44 (11), 43 (27), 39 (6), 28 (6); HR-MS: calcd for C11H10O6
238.0477, found 238.0472.
[28] J. F. Stanton, J. Gauss, J. D. Watts, W. J. Lauderdale, R. J. Bartlett,
ACES II, Quantum Theory Project, University of Florida, Gainesville,
FL (USA), 1992.
[29] M. J. Frisch, M. Head-Gordon, G. W. Trucks, J. B. Foresman, H. B.
Schlegel, K. Raghavachari, M. A. Robb, J. S. Binkley, C. Gonzales,
D. J. Defrees, D. J. Fox, R. A. Whiteside, R. Seeger, C. F. Melius, J.
Baker, R. L. Martin, L. R. Kahn, J. J. P. Stewart, S. Topiol, J. A. Pople,
Gaussian 94, Gaussian Inc., Pittsburgh PA (USA), 1994.
[30] K. Rübsamen, W. P. Neumann, R. Sommer, U. Frommer, Chem. Ber.
1969, 102, 1290 ± 1298.
1
8: H NMR (200 MHz, CDCl3): d ˆ 8.11 (s, 4 H), 2.27 (s, 6 H); 13C NMR
(200 MHz, CDCl3): d ˆ 164.9, 160.8, 129.6, 129.0, 15.6; MS (70 eV): m/z
‡
(%): 282 (2) [M ], 207 (87), 191 (5), 166 (15), 149 (100), 135 (9), 121 (16),
104 (53), 76 (31), 65 (20), 60 (17), 50 (32), 44 (60), 43 (95).
[D6]8: 1H NMR (200 MHz, CDCl3): d ˆ 8.11 (s); 13C NMR (200MHz,
CDCl3): d ˆ 164.9, 160.8, 129.6, 129.0, 15.6; MS (70 eV): m/z (%): 288 (1)
‡
[M ], 210 (61), 194 (16), 166 (15), 149 (100), 138 (7), 121 (12), 104 (39), 76
(22), 65 (17), 63 (12), 50 (24), 46 (85), 44 (57).
[D4]8: 1H NMR (200 MHz, CDCl3): d ˆ 2.27 (s); 13C NMR (200 MHz,
[31] Y. A. Ol'dekop, G. S. Belina, M. S. Matveentseva, Zh. Org. Khim.
1986, 4, 585 ± 587.
CDCl3): d ˆ 164.9, 160.8, 129.6, 129.0, 15.6; MS (70 eV) m/z (%): 286 (3)
‡
[M ], 211 (100), 170 (16), 153 (58), 139 (11), 125 (13), 108 (54), 80 (28), 69
(17), 60 (16), 52 (27), 44 (57), 43 (66).
Received: February 13, 1997
Revised version September 23, 1997 [Z10107IE]
German version: Angew. Chem. 1998, 110, 1001±1005
Keywords: ab initio calculations ´ density functional calcu-
lations ´ matrix isolation ´ photochemistry ´ radicals
[1] K. C. Nicolaou, W. M. Dai, Angew. Chem. 1991, 103, 1453 ± 1481;
Angew. Chem. Int. Ed. Engl. 1991, 30, 1387 ± 1416.
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