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Ethynol Photochemical Generation in an Argon Matrix IR Spectrum and Photoisomerization to Ketene.

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Fig. I. Molecular structure of dimeric l a (small circles = H atoms). Bond
lengths [pm]: B-0 158.0(1) (endocyclic), 137.711) (exocyclic), B-C 156.3159.5(1): bond angles ["I: in the ring: 0 - B - 0 86.1(1), B-0-B 93.9(1); external:
B-0-B 133.2(1), 0-B-C 119.8(1).
shows not only the centrosymmetry but also the two mutually perpendicular major planes of the molecule, whose
aesthetically appealing form more than compensates for
the nonappearance of a linear monomer.
Despite being dimeric, solid l a still represents an interesting example of isosterism since allene also forms a
dimer 2, the molecular geometry of which, as determined
by STO-3G calculations,[sJcorresponds in all details with
that of dimeric l a (C-C distances 131 and 154 pm, angles
in ring 87 and 93 "). The same is true for dimeric alkylideneiminoboranes 3, of which [Me2BNCHPh,]J91 has structural parameters which are almost identical with those of
l a (B-N 159 pm, N-C 127 pm, angles in ring 87 and
93 9.
/ \
The fact that l a is monomeric in the liquid and gas
phases indicates that the enthalpy of formation for the
dimer is low. The low temperature 'H NMR spectrum of
l a in solution shows a splitting of the signals, which was
originally attributed to the bent form of the molecule.['01
This result must now be reinterpreted as being due to
dimer formation at low temperature in solution. As shown
in Figure 1, the monomer conformation,['a1as determined
by electron diffraction, remains almost unchanged upon
formation of the dimer;"'] only one B-0 bond is lengthened to a considerable extent.
The greater tendency of the diboroxane to dimerize, as
compared to other R2B-0 compounds could perhaps be
explained by a synergic effect. The formation of the B-0-B
linkage means, firstly, that the electronic saturation of
the two boron atoms through pn-px interaction with the
one oxygen atom is insufficient, and secondly that more
negative charge is localized on the oxygen atom due to the
inductive effect of the two boron atoms. The diboroxane is
therefore both a stronger Lewis base and a stronger Lewis
acid than a simple alkoxyborane. If the substituents are
small enough, as in the title compound, an energetically
favorable intermolecular saturation of the Lewis basicity
and acidity is possible.
Received: August 31, 1988 [ Z 2950 IE]
German version: Angew. Chem. I01 (1989) 182
Angew Chem. Int. Ed. Engl. 28 (1989) No. 2
[I] a) G. Gundersen, H. Vahrenkamp, J. Mol. Sfrucf.33 (1976) 97: b) L.
Cynkier, N. Furmanova, Crysf. Sfrucf. Commun. 9 (1980) 307: c) C. J.
Cardin, H. E. Parge, J. W. Wilson, J. Chem. Res. Synop. 1983, 93; J .
Chem. Res. Miniprint 1983. 0801.
[2] a) J. D. Odom, A. J. Zozulin, S. A. Johnston, J. Durig, S. Riethmiller, E.
J. Stampf, J . Organomef. Chem. 201 (1980) 351; b) J. R. Dung, M. J.
Flanagan, E. J. Stampf, .I.D. Odom, J. Mol. Sfruct. 42 (1977) 13.
131 Monoclinic, P2,/c, Z = 4 , a=740.8(2), b=819.9(2), c = 1105.6(3) pm,
8=96.70(2)"; pcalcd=0.79 g cm-3 at the measurement temperature
- 160f2"C: (w/28)-scan in the range 3 " < 2 8 < 5 6 " ; 1624 symmetry-independent reflections, 1501 with F > 30(F); empirical absorption and
extinction corrections. Calculations with SHELXTL-PLUS from G. M.
Sheldrick, Gottingen 1987. R = 0.039 (R'=0.043 for all reflections),
R,=0.060 (0.061); goodness of f i t 2.474. The final Fourier difference
map showed no features greater than 0.4 e A-3. Further details of the
crystal structure investigation can be obtained from the Fachinformationszentrum Energie, Physik, Mathematik GmbH, D-75 14 EggensteinLeopoldshafen 2 (FRG), on quoting the depository number CSD-53 277,
the names of the authors, and the journal citation.
[4] A. Simon, H. J. Deiseroth, E. Westerbeck, B. Hillenkotter, Z . Anorg.
ANg. Chem. 423 (1976) 203.
[Sl The compounds (R0)2BX, ROBR2 and ROBC12are monomeric; ROBF2
and ROBH2 have been reported to be trimeric or polymeric: H. Steinberg: Organoboron Chemistry. Wiley, New York 1964.
[6] a) S. J. Rettig, J. Trotter, Can. J. Chem. 55 (1977) 958; ibid. 56 (1978)
1676; b) R. Boese, R. Koster, M. Yalpani, Chem. Ber. 118 (1985) 670.
171 J. B. Farmer, Adu. Inorg. Chem. Radiochem. 25 (1982) 187.
IS] P. Hemmersbach, M. Klessinger, P. Bruckmann, J. Am. Chem. SOC.ZOO
(1978) 6344.
191 J. R. Jennings, R. Snaith, M. M. Mahmoud, S. C. Wallwork, S. J. Bryan,
J. Halfpenny, E. A. Petch, K. Wade, J . Orgonomet. Chem. 249 (1983)
c 1.
[lo] G. F. Lanthier, W. A. G. Graham, Chem. Commun.1968, 715.
I l l ] The B-0-B angle decreases from 144 to 133" and the C-B-C angle from
122 to 112'; the angle between the two BC2 planes increases from 72 to
Ethynol: Photochemical Generation in an
Argon Matrix, IR Spectrum, and
Photoisomerization to Ketene**
By Remo Hochstrasser and Jakob Wirz*
Ethynol (hydroxyacetylene) has been considered as a
possible constituent of flames, planetary atmospheres, and
interstellar clouds.""] Several quantum mechanical calculations have led to the prediction that the isolated molecule
is kinetically stable with respect to the highly exothermic
tautomerization to ketene."' Recently, Schwarz and coworkers provided the first experimental proof for the existence of ethynolL2]and related molecules131
by tandem mass
spectrometry. Further evidence for ynols is limited to
rather tentative assignments of some IR bands observed
after cryogenic trapping of products formed by flash vacuum pyrolysis.[41Several unsuccessful attempts to generate matrix-isolated ethynol have been reported recently.['I
Acetylene diolate salts are well-known as products of oxocarbon chemistry[6a1and have been characterized recently
by matrix IR spectroscopy.[6b1Ynolate ions are gaining interest as reactive species for synthesis.[']
In the course of our investigations of keto-enol tautomerism,['] we were searching for photochemical methods to
generate ethynol under conditions suitable for its spectroscopic identification as well as for the study of its reaction
kinetics in solution. We found that irradiation of 3-hydroxycyclobutene-1,2-dione (semisquaric acid) 1 in an argon
[*] Prof. Dr. J. Wirz, DipLMath. R. Hochstrasser
lnstitut fur Physikalische Chemie der Universitat
Klingelbergstrasse 80, CH-4056 Basel (Switzerland)
[**I This work was supported by the Swiss National Science Foundation
(part of project No. 2.034-0.86), Ciba-Geigy SA, Sandoz SA, F. Hoffmann-La Roche & Cie SA and the Ciba-Stiftung.
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matrix cleanly yields ethynol 4 in three photochemical
Semisquaric acid 1 was synthesized by the procedure of
Serratosa et al.l9] Deuterated samples were obtained by exchange in D20. Selective isotopic enrichment was achieved
by exploiting the different exchange rates; exchange at
carbon required several days. A sample of 1 was sublimed
from a small quartz container at ca. 100°C, while a steady
stream of argon was condensed on a CsI sample window
at a temperature of 23 K. The resulting matrix was cooled
to 12 K and irradiated with pulses from a KrF excimer
laser (248 nm, 20 ns, ca. 10 mJ cm-2).['01The irradiation
was interrupted repeatedly in order to monitor the progress of the reaction by IR spectroscopy. Usually, 15 to 25
spectra were recorded in the range from 3630 to 630 cm-'
for each sample. With an instrumental resolution of 0.2
cm-', the peaks were generally well separated and sufficiently sharp for a base-line correction by graphical interpolation to be carried out with negligible ambiguity to obtain the absolute absorbances of the bands.
In the initial stages of the irradiation, complex product
mixtures formed, which were analyzed as follows. Peaks
belonging to the same compound were identified on the
basis that their absorbance ratio remained constant
throughout the sequence of spectra. This could best be determined graphically by a painvise comparison of peak intensities. Curved lines resulted, if the selected bands were
due to different compounds. Smaller, but still quite pronounced deviations from linearity were observed when a
pair of bands belonged to the same compound, but to different sites in the matrix; it is well-known that IR-bands of
matrix-isolated molecules often exhibit site splitting. Finally, if the chosen pair of peaks corresponded to the same
compound in the same site, the points accurately followed
a straight line through the origin. Other criteria, such as
the band splitting patterns and changes of site population
observed upon slight warming of the matrix, were used to
distinguish between independent components and different matrix sites for a single component. In this way, five
independent reaction components were identified. Figure
1 shows the change in intensity of one characteristic peak
of each of the five components with increasing irradiation
time. The curves represent concentration profiles with arbitrary units, since the extinction coefficients are unknown. The different induction periods and the corresponding displacements of the concentration maxima of
the intermediate products indicates that the components
are formed sequentially.
The first photolysis product of 1, 2-hydroxy-1,3-butadiene-1,4-dione (hydroxybiketenyl) 2, exhibits two very
strong, relatively broad carbonyl bands at 2112 and 2135
cm- '. The strongest bands of the next product, 2-hydroxy182
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Fig. 1. Concentration profiles of the components 1 to 5, plotted as the absorbance (A) of one characteristic band each: 1,1351.8; 2,2112.2; 3, 1857.2; 4,
3501.3;5, 2147.1 cm-'. The abscissa orders the spectra chronologically, but
approximately represents a logarithmic scale of the irradiation dose, as the
irradiation times were increased considerably towards the end of the reaction. The full abscissa range corresponds to a dose of ca. lo4 laser pulses
(I00 J cm-I).
cyclopropenone 3, are the C=O stretch at 1857 cm-' and
the C=C stretch at 1650 cm-'. Ethynol 4 and its deuterium-substituted isotopomers each exhibit five bands above
630 cm-' (Table 1). Work to determine the bending vibrations v, to v, in the spectral range below 630 cm-' is in
progress: the values 599, 523, 383, and 346 cm-' may be
quoted as preliminary results for HCCOH. The IR spectra
of matrix-isolated ketene 5 and its mono- and dideutero
derivatives are well-known.["]
The photochemical valence isomerization and decarbonylation of cyclobutenediones['21 to cyclopr~penones['~~
and of the latter to yield acetylenes['41are well-known reactions. We have obtained an authentic precedent for a reaction sequence of the type 1 - + 2 - + 3 + 4 under our conditions of matrix isolation by photolyzing 3,4-dimethylcyclob u t e n e d i ~ n e . ~Analysis
of the sequential spectra gave results analogous to those obtained with 1. In this case two
of the three expected intermediates, dimethylcyclopropenone['51 and 2-butyne, are known compounds which we
identified unambiguously by independent synthesis and
matrix isolation.
The observed IR bands leave no doubt that compound 4
retains both the OH/D and the C H I D groups of 1 ; moreover, the band at 2198 cm-' is characteristic for an unsymmetrically substituted alkyne. The stretching bands of 4,
apart from the OH-stretch, lie at much the same frequencies as those of flu~roacetylene.["~
A heuristic approach
would be to consider the OH-stretching vibration as localized and to treat 4 as a quasilinear isotopomer of fluoroacetylene (OH k "F, OD p "F). This very simple model
works amazingly well. The wave numbers of the HID-C,
C=C, and C - 0 stretching bands (Table I) obey both the
Redlich-Teller product rule['8a1and the Decius-Wilson
sum rule['8b1quite well and are reproduced within f 5
cm-' by the following force field Cf/Nm-'): 580 (HI
D-C), 1400 (C=C), 920 (C-0), and -40 for the cross
terms H/D-C=C and C=C-0. The agreement with the
frequencies and intensities predicted for ethynol by recent
ab initio ~ a l c u l a t i o n s [ ' "is~ ~as
~ good as can be expected.["]
The spectrum of the matrix after the disappearance of
components 1 to 3 is shown in Figure 2. Apart from bands
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Angew. Chem. Int. Ed. Engl. 28 (1989) No. 2
Table I. Wavenumbers (S/cm - ') and relative intensities (Yo) of the fundamental bands of 4 and its mono- and dideuterated derivatives in the range
3630 to 630 cm - I.
3501.3 (60)
3339.6 ( 2 5 )
2198.3 (100)
1232.1 (25)
1072.1 ( 2 5 )
2587.5 (55)
3339.6 (25)
2196.2 (100)
946.8 (10)
1069.2 (IS)
3501.7 (75)
2620.8 (100)
2037.4 (55)
1230.9 (25)
1050.5 (50)
2586.8 (100)
2620.4 (90)
2035.1 (50)
944.5 (20)
1046.9 (15)
vl(a'), OH/D stretching
v2(a'),CH/D stretching
%(a'), CC stretching
v4(a'), COH/D bending
vs(a?, CO stretching
t 0.20-
1000 cm-1
[4] a) M.-C. Lasne, J.-L. Ripoll, Bull. SOC.Chim. Fr. 1986, 766; b) Z. Jabry,
M.-C. Lasne, J.-L. Ripoll, J . Chem. Res. Synop. 1986, 188; c) C. Wentrup, P. LorenEak, J . Am. Chem. SOC.110 (1988) 1880.
[5] J. Dommen, M. Rodler, T.-K. Ha, Chem. Pbys. 117 (1987) 65.
[6] a) W. Biichner, Helu. Chim. Acta 49 (1966) 907; b) 0. Ayed, L. Manceron, B. Silvi, J . Phys. Chem. 92 (1988) 37.
[7] P. J. Stang, K. A. Roberts, J. Am. Chem. SOC.108 (1986) 7125; C. J.
Kowalski, G. S. Lal, M. S. Haque, h i d . 108 (1986) 7127.
[8] M. Capponi, 1. Gut, J. Win, Angew. Chem. 98 (1986) 358; Angew. Chem.
Int. Ed. Engl. 25 (1986) 344; Y. Chiang, M. Hojatti, J. R. Keeffe, A. J.
Kresge, N. P. Schepp, J. Wirz, J. Am. Chem. SOC.109 (1987) 4000 and
references cited therein.
191 A. Bou, M. A. Pericas, F. Serratosa, Tetrahedron Lett. 23 (1982) 361.
[lo] Similar results were obtained upon irradiation with a low-pressure mercury lamp through a cut-off filter (A> 220 nm). Without the filter ethynol
was not accumulated in detectable quantity.
[ I l l C. B. Moore, G. C. Pimentel, J . Chem. Phys. 38 (1963) 2816: J. L. Duncan, B. Munro, J. Mol. Srrucf. 161 (1987) 311.
1121 D. BelluS, B. Ernst, Angew. Chem. lOO(1988) 820; Angew. Chem. Int. Ed.
Engl. 27 (1988) 797.
[I31 E. V. Dehmlow, Tetrahedron Lett. 13 (1972) 1271; D. Eggerding, R.
West, J. Am. Chem. SOC.98 (1976) 3641; E. V. Dehmlow, R. Neuhaus,
H. G. Schell, Chem. Ber. 121 (1988) 569.
[I41 W. Sander, 0. L. Chapman, Angew. Chem. 100 (1988) 402; Angew.
Chem. Int. Ed. Engl. 27 (1988) 398.
[IS] A. Treibs, K. Jacob, R. Tribollet, Justus Liebigs Ann. Chem. 741 (1970)
[I61 R. Breslow, L. J. Altman, J . Am. Chem. SOC.88 (1966) 504.
[17] G. R. Hunt, M. K. Wilson, J. Chem. Phys. 34 (1961) 1301.
[IS] E. B. Wilson, J. C. Decius, P. C . Cross; Molecular Vibrations, McGrawHill, New York 1955, a) p. 183, b) p. 186.
[I91 B. A. Hess, L. J. Schaad, P. Carsky, R. Zahradnik, Chem. Reu. 86 (1986)
Fig. 2. Base-line corrected IR absorption spectrum of a mixture of ethynol 4
and ketene 5 . Other components: CO 6 , C 0 2 7, H20 8.
due to CO, COz, and HzO, all significant absorptions can
be assigned to either the intermediate ethynol 4 or the final product ketene 5 . The final reaction is very slow; presumably ethynol exhibits very little absorption at the irradiation wavelength of 248 nm.['*] Nevertheless, we maintain
that the conversion to ketene is a photo-induced reaction
of isolated ethynol. The rate of the isomerization 4 +5 was
independent of the water content of the matrix, which varied greatly from sample to sample. Furthermore, in deuterated samples the isotopic purity was retained through to
the end product 5 . This excludes the possibility that the
isomerization 4 4 5 is caused by a reaction with adventitious water in the matrix.
This work has provided conclusive evidence for the
identification of ethynol and suggests that hydroxycyclopropenones could be suitable precursors for the photochemical generation of hydroxyacetylenes in solution. The
two weak IR bands at 2230 and 1155 cm-', which were
tentatively assigned to ethynol by Lasne and RipoN,[4a1
not agree with our data, but we cannot exclude the possibility that the discrepancy is due to the different media.
Received: August 30, 1988;
revised: October 28, 1988 [Z 2948 IE]
German version: Angew. Chem. I01 (1989) 183
[I] a) D. J. DeFrees, A. D. McLean, J . Phys. Chem. 86 (1982) 2835 and
references cited therein; b) K. Tanaka, M. Yoshimine, J. Am. Chem. SOC.
102 (1980) 7655 and references cited therein; c) W. J. Bouma, R. H.
Nobes, L. Radom, C . E. Woodward, J . Org. Chem. 47(1982) 1869; d) H.
Huber, J. Vogt, J . Chem. Phys. 64 (1982) 399; e) S. Schroder, W. Thiel, J .
Am. Chem. SOC.108 (1986) 7985; f) B. L. M. van Baar, N. Heinrich, W.
Koch, R. Postma, J. K. Terlouw, H. Schwarz, Angew. Chem. 99 (1987)
153; Angew. Chem. Int. Ed. Engl. 26 (1987) 140.
[2] B. van Baar, T. Weiske, J. K. Terlouw, H. Schwarz, Angew. Chem. 98
(1986) 275; Angew. Chem. Int. Ed. Engl. 25 (1986) 282.
131 a) J. K. Terlouw, P. C. Burgers, B. L. M. van Baar, T. Weiske, H.
Schwarz, Chimia 40 (1986) 357; b) J. K. Terlouw, H. Schwarz, Angew.
Chem. 99 (1987) 829; Angew. Chem. In!. Ed. Engl. 26 (1987) 805.
Angew. Chem. Inr. Ed. Engl. 28 (1989) No. 2
Well-Resolved ESR Spectra of
Arenecarboxyl Radicals in Solution**
By Hans-Gert Korth, * Wolfgang Miiller, Janusz Lusztyk,
and Keith U.Ingold
Dibenzoyl peroxide and related bis(arenecarbony1) peroxides 1 are the most widely used initiators of free-radical processes. Their thermal and photochemical decomposition yields primarily arenecarboxyl radicals (arenecarbonyloxyl radicals) 2.") The molecular and electronic strucArylC(O)OOC(O)Aryl
& 2 ArylC(0)OO
a, Awl= C6Hs
tures of these radicals are currently under active investigation['] as are the kinetics of their reactions.[31Elucidation of
the structures of the radicals 2 would be greatly aided if
high resolution ESR spectra could be obtained. Benzoyloxyl 2a has been observed by ESR in the solid state at very
low temperature^[^.'^ and this radical along with two parasubstituted derivatives have been detected in solution at
room temperature using time-resolved ESR spectroscopy.16' In all these cases, the spectra were too poorly resolved to reveal any hyperfine splittings (HFS) by ring substituents. Earlier attempts to detect benzoyloxyl radicals by
conventional ESR spectroscopy using continuous UV photolysis of dibenzoyl peroxide in suitable solvents have met
Dr. H.-G. Korth ['I, Dipl.-Chem. W. Miiller
Institut fur Organische Chemie der Universitat-GHS
Universitatsstr. 5, D-4300 Essen 1 (FRG)
Dr. J. Lusztyk, Dr. K. U. Ingold
Division of Chemistry, National Research Council of Canada
Ottawa, Ontario KIA OR6 (Canada)
NRCC/Summit Postdoctoral Fellow 1987- 1988.
[**I Issued as NRCC No. 29827.
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matrix, spectrum, generation, argon, ketene, ethynol, photoisomerization, photochemical
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