Generation of iodobenzoborirene, a boraaromatic cyclopropabenzene derivative

Article abstract of DOI:10.1039/b419415b

Iodobenzoborirene is obtained photochemically (λ = 308 nm) from diiodophenylborane by elimination of HI in solid argon at 10 K. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b419415b

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Generation of iodobenzoborirene, a boraaromatic cyclopropabenzene
derivative{
Holger F. Bettinger*
Received (in Cambridge, UK) 5th January 2005, Accepted 9th March 2005
First published as an Advance Article on the web 13th April 2005
DOI: 10.1039/b419415b
from diiodophenylborane (5). The latter is readily available,19 and
Iodobenzoborirene is obtained photochemically (l 5 308 nm)
its reported instability20 towards irradiation encouraged us to
from diiodophenylborane by elimination of HI in solid argon
investigate its photochemical reactivity. The IR spectrum of
diiodophenylborane isolated in an argon matrix at 10 K is in very
good agreement with the one previously reported for a neat sample
at room temperature.20 Upon irradiation with 308 nm light of a
XeCl laser (1 Hz, y150 mJ/pulse) a decrease of the signals of 5 is
accompanied by the growing of IR bands (Fig. 1). Prolonged
308 nm irradiation does not result in complete disappearance of 5,
but the intensity of the signals of the photoproducts can be
increased by increasing the amount of matrix isolated 5, indicative
of a photostationary equilibrium.
at 10 K.
Two fundamental principles of chemistry, aromaticity and strain,
are closely intertwined in the cyclopropabenzenes 1.1 Derivatives
of 1 with a heteroatom replacing the methylene bridge in the three-
membered ring have been postulated (BD,2 NH,3) or isolated
(SiR2,4 GeR2,5 S,6 and Se6). The boron derivative, benzoborirene
2, is of particular interest: the BH unit allows delocalization of the
six p electrons over the entire molecule while the small boron atom
should be well suited to bridge the formal benzyne triple bond. It
has received some theoretical interest as a model system for
studying the reversed Mills–Nixon effect.7 Benzoborirene is
isoelectronic to the cycloproparenyl cation 3a, first observed as
the 2,5-diphenyl-7-chloro substituted compound 3b by Halton
et al.,8 without carrying its positive charge, thereby extending the
idea of isoelectronic heteroaromatics (CH+ vs. BH) to bicyclic
systems.9
By comparison of the relative intensities from a number of
experiments, one set of signals, with a strong absorption at
1450 cm21, a group of signals between 1080 and 1120 cm21 with a
band at 1080 cm21 the strongest, and a number of weaker bands,
is assigned to the major photoproduct. By comparison with
B3LYP computations, this photoproduct is identified as iodoben-
zoborirene 2b. The parent borirene 4 and the borirene radical,
cyclo-(CH)2B, have intensive ring stretching modes at around
1175 cm21 17
and benzannelation is red-shifting this most intense
,
mode of the system to about 1080 cm21. An additional band at
2159 cm21 grows at the same rate as absorptions assigned to 2b,
which upon careful annealing to 20–30 K shifts by 27 cm21 to
2186 cm21. These absorptions are assigned to HI, which is known
[D5]-Benzoborirene [D5]-2a has only been detected recently for
the first time by time-of-flight MS as the product of the boron
atom-[D6]-benzene molecule crossed-beams reaction.3
The monocyclic borirenes 4 are highly reactive species: early
experiments based on haloboration of alkynes and dehalogenation
with active metals resulted in polymeric materials.10 Photochemical
trapping studies by Eisch et al. produced evidence for the
intermediate existence of triphenylborirene 4b in 1982.11 Only a
few years later several isolations of borirenes with bulky
substituents had been described,12–15 culminating in the report
by Eisch et al. of the X-ray structure of trimesitylborirene.16 More
recently, the parent borirene 4a was observed by matrix isolation17
and by gas-phase crossed-beams experiments18 as the product of
reaction of boron atom with ethylene.
We here report on the first direct IR spectroscopic characteriza-
tion of a benzoborirene derivative 2b (R 5 I). In view of its
expected reactivity, we generated 2b in a solid argon matrix at 10 K
Fig. 1 (A) IR spectrum of diiodophenylborane (5) isolated in solid argon
at 10 K; (B) difference spectrum obtained after 308-nm-photolysis of 5
(bands pointing downwards decrease, bands pointing upwards
appear upon irradiation); (C) difference spectrum obtained after
subsequent 254-nm irradiation; (D) IR spectrum computed at the
B3LYP/6-311 + G** level of theory for iodobenzoborirene (2b) with
natural boron composition.
{ Electronic supplementary information (ESI) available: experimental and
computational details, computed structures of 2a, 2b, and IR spectral data
of 2b. See http://www.rsc.org/suppdata/cc/b4/b419415b/
*Holger.Bettinger@rub.de
2756 | Chem. Commun., 2005, 2756–2757
This journal is ß The Royal Society of Chemistry 2005

View Article Online
to interact with p systems resulting in a pronounced red-shift of the
n(HI) vibration from the 2227 cm21 gas-phase value.21
matrix isolation equipment in Bochum, and Professor Halton for
helpful comments. This paper is dedicated to Professor P. v. R.
Shleyer on the occasion of his 75th birthday.
Annealing also changes the relative intensities of the group of
absorptions observed in the 1080–1100 cm21 range possibly arising
from disturbances due to HI molecules lying nearby. The second
most intense signal of 2b, a C–C stretching vibration involving the
three-membered ring, is computed at 1673 cm21, but only a broad
band (centered at 1632 cm21) can be observed. No reliable
integration is possible, because of overlap with the water monomer
Holger F. Bettinger*
Lehrstuhl fu¨r Organische Chemie 2, Universita¨tsstr. 150, 44780,
Bochum, Germany. E-mail: Holger.Bettinger@rub.de;
Fax: +49 234 321 4353; Tel: +49 234 322-4353
Notes and references
at 1624 cm21
.
Monochromatic 254-nm-irradiation of the matrix after 308-nm-
irradiation completely bleaches the newly formed signals within
several minutes and results in formation of 5. It is well known that
HI can be cleaved into the atoms by 254 nm irradiation.22 We
conclude that the observed photochemistry is due to the
equilibrium:
1 (a) B. Halton, Chem. Rev., 2003, 103, 1327; (b) A. T. Balaban,
D. C. Oniciu and A. R. Katritzky, Chem. Rev., 2004, 104, 2777.
2 (a) R. I. Kaiser and H. F. Bettinger, Angew. Chem., 2002, 114, 2456,
Angew. Chem., Int. Ed., 2002, 41, 2350; (b) H. F. Bettinger and
R. I. Kaiser, J. Phys. Chem. A, 2004, 108, 4576.
3 (a) C. The´taz and C. Wentrup, J. Am. Chem. Soc., 1976, 98, 1258; (b)
G. I. Yranzo, J. E. Elguero, R. Flammang and C. Wentrup, Eur. J. Org.
Chem., 2001, 2209.
4 (a) K. Hatano, N. Tokitoh, N. Takagi and S. Nagase, J. Am. Chem.
Soc., 2000, 122, 4829; (b) T. Tajima, K. Hatano, T. Sasaki, T. Sasamori,
N. Takeda, N. Tokitoh, N. Takagi and S. Nagase, J. Organomet.
Chem., 2003, 686, 118.
5 N. Tokitoh, K. Hatano, T. Sasaki, T. Sasamori, N. Takeda and
N. Takagi, Organometallics, 2002, 21, 4309.
6 R. Schulz and A. Schweig, Tetrahedron Lett., 1984, 25, 2337.
7 (a) Z. B. Maksic, M. Eckert-Maksic and K.-H. Pfeifer, J. Mol. Struct.,
1993, 300, 445; (b) M. Eckert-Maksic, Z. Glasovac, Z. B. Maksic and
I. Zrinski, Theochem, 1996, 366, 173; (c) A. Stanger, J. Am. Chem. Soc.,
1998, 120, 12034.
Reversible photochemical reactions can also be observed by UV
spectroscopy. The UV/vis spectrum of 5 is characterized by three
major absorptions at 301, 274, 222 nm in good agreement with
computation (304, 287, and 219 nm at B3LYP/cc-pVTZ). These
absorptions decrease upon 308-nm-irradiation and subsequently
increase in intensity on short wavelength (l 5 254 nm) irradiation.
No new absorptions due to 2b can be detected. According to the
DFT computations, the strongest absorption of 2b (265 nm)
overlaps with the 274-nm-band of 5, but only has about half of its
oscillator strength.
8 B. Halton, A. D. Woolhouse, H. M. Hugel and D. P. Kelly, J. Chem.
Soc., Chem. Commun., 1974, 247.
9 (a) M. E. Volpin, Y. D. Koreshkov, V. G. Dulova and D. N. Kursanov,
Tetrahedron, 1962, 18, 107; (b) K. Krogh-Jespersen, D. Cremer,
J. D. Dill, J. A. Pople and P. v. R. Schleyer, J. Am. Chem. Soc.,
1981, 103, 2589; (c) P. H. M. Budzelaar, A. J. Kos, T. Clark and P. v.
R. Schleyer, Organometallics, 1985, 4, 429.
10 (a) J. J. Eisch and L. J. Gonsior, J. Organomet. Chem., 1967, 8, 53; (b)
J. J. Eisch and H. P. Becker, J. Organomet. Chem., 1979, 171, 141; (c)
R. Schlo¨gl and B. Wrackmeyer, Polyhedron, 1985, 4, 885.
11 J. J. Eisch, F. Shen and K. Tamao, Heterocycles, 1982, 18, 245.
12 S. M. van der Kerk, P. H. M. Budzelaar, A. van der Kerk, G. J. M. van
der Kerk and P. v. R. Schleyer, Angew. Chem., 1983, 95, 61, Angew.
Chem., Int. Ed. Engl., 1983, 22, 48.
The iodo substituent in 2b is found to destabilize the
benzoborirene ring system by 2.3 kcal mol21 [CCSD(T)/cc-
pVTZ-(PP) energies, see ESI{] compared to 2a according to
equation (1).9c Nonetheless, very similar geometries are obtained
˚
for 2a and 2b, with the largest deviations being less than 0.01 A.
13 B. Pachaly and R. West, Angew. Chem., 1984, 96, 444, Angew. Chem.,
Int. Ed. Engl., 1984, 23, 454.
14 C. Pues and A. Berndt, Angew. Chem., 1984, 96, 306, Angew. Chem., Int.
Ed. Engl., 1984, 23, 313.
2b + BH3 A 2a + BH2I (1)
Borirene 4a is highly strained, 69 kcal mol21, according to the
computational analysis of Budzelaar et al., but it enjoys a large
15 C. Habben and A. Meller, Chem. Ber., 1984, 117, 2531.
16 (a) J. J. Eisch, B. Shafii and A. L. Rheingold, J. Am. Chem. Soc., 1987,
109, 2526; (b) J. J. Eisch, B. Shafii, J. D. Odom and A. L. Rheingold,
J. Am. Chem. Soc., 1990, 112, 1847.
resonance energy of 48 kcal mol21 9c
.
Equation (2) measures
17 (a) D. V. Lanzisera, P. Hassanzadeh, Y. Hannachi and L. Andrews,
J. Am. Chem. Soc., 1997, 119, 12402; (b) L. Andrews, D. V. Lanzisera,
P. Hassanzadeh and Y. Hannachi, J. Phys. Chem. A, 1998, 102, 3259.
18 N. Balucani, O. Asvany, Y. T. Lee, R. I. Kaiser, N. Galland and
Y. Hannachi, J. Am. Chem. Soc., 2000, 122, 11234.
the strain that results from the fusion of 4a and benzene to be
14.3 kcal mol21
.
2a + H2CLCH2 A benzene + 4a
(2)
19 M. Schmidt, W. Siebert and F. Rittig, Chem. Ber., 1968, 101, 281.
20 F. C. Nahm, E. F. Rothergy and K. Niedenzu, J. Organomet. Chem.,
1972, 35, 9.
21 (a) A. J. Barnes, J. B. Davies, H. E. Hallam and J. D. R. Howells,
J. Chem. Soc., Faraday Trans. 2, 1973, 69, 246; (b) J. Weidlein, U. Mu¨ller
and K. Dehnicke, Schwingungsfrequenzen I, Georg Thieme Verlag,
Stuttgart, 1981.
22 M. Pettersson and J. Nieminen, Chem. Phys. Lett., 1998, 283, 1.
23 W. E. Billups, W. Y. Chow, K. H. Leavell, E. S. Lewis, J. L. Margrave,
R. L. Sass, J. J. Shieh, P. G. Werness and J. L. Wood, J. Am. Chem.
Soc., 1973, 95, 7878.
Similar values, 15 (experiment)23 and 16–18 (theory)24
kcal mol21, have been derived for the strain in 1 due to the
fusion of benzene and cyclopropane.
In summary, we could show that the benzoborirene system
forms upon 308-nm-irradiation of 5 in an argon matrix at 10 K.
The strain of 2a is similar to that of cyclopropabenzene 1, while the
iodo substituent causes some additional destabilization.
This work was supported by the Fonds der Chemischen
Industrie and DFG. I thank Professor Sander for access to the
24 Y. Apeloig and D. Arad, J. Am. Chem. Soc., 1986, 108, 3241.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2756–2757 | 2757