Triboracyclobutanides: Four-membered two-electron aromatic compounds with fluctuating skeletal bonds

Article abstract of DOI:10.1002/anie.200390184

A distorted triboratetrahedrane anion, 2, is proposed as the transition state of the fluctuation of the skeletal bonds of triboracyclobutanide 1, a two-electron aromatic compound, B3LYP/6-31G* computations support this suggestion (R = SiMe₃, Du

Full text of DOI:10.1002/anie.200390184

Angewandte
Chemie
Fluctuating Skeletal Bonds
herein the synthesis, crystal structure, and the ring inversion
barrier of the anion 3a (Scheme 2), and show, based on line
shape analyses of its temperature-dependent NMR spectra,
that its skeletal bonds fluctuate via a distorted triboratetra-
hedrane anion 6a (see Scheme 3) as transition state. B3LYP
computations[5] on model molecules support this proposal.
The unsymmetrically substituted triboracyclobutanide 3a
is obtained by reaction of lithium in diethyl ether with the
symmetrically substituted derivative 4, which is accessible
Triboracyclobutanides: Four-Membered Two-
Electron Aromatic Compounds with Fluctuating
Skeletal Bonds**
Y¸ksel Sahin, Carsten Pr‰sang, Peter Amseis,
Matthias Hofmann, Gertraud Geiseler, Werner Massa,
and Armin Berndt*
from
5[6]
and
dichloro(trimethylsilylmethyl)borane
(Scheme 2). Attempts to detect the symmetrically substituted
compound 3b by NMR spectroscopic monitoring of the
reaction of 4 with lithium naphthalenide in [D8]THFat ꢀ808C
proved unsuccessful. The constitutions of 3a and 4 are in
accord with their NMR data (Table 1) and are confirmed
unambiguously by their crystal structure determinations[7]
(Figure 1).
Among the series of isoelectronic four-membered, two-
electron aromatic compounds 1 3 (Scheme 1), to date
derivatives of the dicationic and uncharged skeletons 1[1]
4]
and 2,[2 respectively, have been realized experimentally.
The barriers of their ring inversions, which are a measure of
the stabilization of the folded over the planar rings, have not
been determined experimentally for any example. We report
Scheme 1. Frameworks of isoelectronic four-membered two-electron ar-
omaticcompounds 1 3. Circles denote two cyclic delocalized elec-
trons.
Scheme 2. Synthesis of the triboracyclobutanide 3a by ring expansion
of the diboracyclopropanide 5 via 4. The compound 3b, expected as
the primary product of the reaction of 4 with lithium, was not detected.
(R=SiMe3, Dur=2,3,5,6-tetramethylphenyl).
[*] Prof. Dr. A. Berndt, Dr. Y. Sahin, Dr. C. Pr‰sang, Dr. P. Amseis,
G. Geiseler, Prof. Dr. W. Massa
Fachbereich Chemie
Universit‰t Marburg
35032 Marburg (Germany)
Fax: (+49)6421-282-8917
E-mail: berndt@chemie.uni-marburg.de
Figure 1. Structures of the anion 3a (a) and of 4 (b) in the crystal. Hy-
drogen atoms were omitted for clarity, except those of the methylene
groups. Selected bond lengths [pm] and angles [8]. 3a: C1-B2 150.6(5),
B2-B1 162.7(5), B1-B3 162.8(5), B3-C1 152.2(5), C1-B1 199.8(4), B2-B3
214.0(5), C1-Si1 182.7(3); C1,B2,B3/B3,B2,B1 59.3(3)8. 4: C1-B1
142.8(8), C1-B2 145.4(7), B2-B1 169.5(9), B1-B3 192.6(8), B2-B3
189.0(9), C1-Si1 183.7(5), B1-C10 156.9(8), B2-C20 156.8(8), B3-C2
153.9(8), B3-Cl1 179.0(6); C1,B1,B2/B3,B2,B1 0.8(6)8.
Dr. M. Hofmann
Anorganisch-Chemisches Institut
Universit‰t Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
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Table 1: Selected physical and spectroscopic properties of 3a¥Li(DME)2
and 4.[a]
The anion 3a forms a solvent-separated ion pair with a
lithium ion, which is coordinated by three molecules of
diethyl ether. The four-membered ring of the triboracyclo-
butanide 3a is strongly (by 598) folded, as expected for an
3a¥Li(DME)2: pale yellow solid, m.p. 73 758C (without decomp), yield
90%. 1H NMR (500 MHz, [D8]THF, ꢀ238C): d=6.36, 6.20 (each s, each
1H, p-H, Tcoal =408C); 3.41, 3.26 (each DME); 2.04, 2.02 (each s, each
6H, o- or m-CH3, Tcoal =178C); 1.98, 1.94 (each s, each 6H, o- or m-CH3,
4]
isoster of the 1,3-dihydro-1,3-diborete[2 2 and the cyclo-
butadienyl dication[1] 1. The substitution pattern of the boron
atoms of 3a does not correspond to the symmetric substitu-
tion pattern of its precursor 4. Evidently, the initially formed,
symmetrically substituted triboracyclobutanide 3b rapidly
isomerizes to 3a. Line width phenomena in the NMR spectra
of 3a provide a hint for the mechanism of this isomerization.
At low temperature there are 1H and 13C NMR signals for two
different Duryl substituents, at higher temperatures their
broadenings and coalescences are observed. The 11B NMR
spectrum of 3a in [D8]THF at 278C shows three signals at d =
16, 38, and 42ppm, of which that at d = 42ppm remains
unchanged at higher temperatures, whereas the other two
signals broaden. This shows that not only the Duryl sub-
stituents but also the boron atoms to which they are attached
participate in the exchange. The barrier for the exchange of
the environments of the Duryl substituents, that is
the topomerization in 3a, was determined to be
19.9 kcalmolꢀ1 based on the line shape analysis of
the signals of the o-Me, m-Me, and p-H atoms.[8] For
the diastereotopic methylene protons of the Me3-
SiCH2 substituents, which reveal the planar chirality
of 3a, a more rapid exchange than for the Duryl
substituents is determined from the line shape
analysis of their signals. This is plausible, if one
considers, that two pathways are available for the
enantiomerization of 3a: one by ring inversion via
the planar transition state 7a (Scheme 3b) and a
second by rearrangement of the framework via a
distorted triboratetrahedrane anion 6a,[9,10] which
also leads to an enantiomerization, however, with-
out ring inversion (see Scheme 3a). The barrier of
the enantiomerization by ring inversion is calcu-
T
coal =278C); 0.92, 0.43 (each d, each 1H, BCH2, Tcoal =378C); ꢀ0.05,
ꢀ0.08 ppm (each s, each 9H, Me3Si); 13C NMR (75 MHz, [D8]THF,
278C): d=156.4, 155.6 (each br. s, each 1C, i-C); 133.4, 131.9, 130.4,
130.0 (each s, each 2C, o- and m-C); 129.0 (br. s, 1C, CB2); 126.6, 124.5
(each d, each 1C, p-C); 20.7, 20.0 (each q, each 4C, o- and m-CH3); 9.7
(br. t, 1C, BCH2), 2.4, 1.8 ppm (each q, each 3C, Me3Si); 11B NMR
(160 MHz, [D8]THF, 278C): d=42, 38, 16 ppm
1
4: colorless solid, m.p. 133 1348C (decomp), yield 55%. H NMR
(500 MHz, C6D6 , 278C): d=7.04 (s, 2H, p-H); 2.54, 2.19 (each s, each
12H, o- and m-CH3); 1.49 (s, 2H, BCH2); 0.32, ꢀ0.21 ppm (each s, 9H,
Me3Si); 13C NMR (125 MHz, C6D6, 278C): d=137.2, 134.6 (each s, each
4C, o- and m-C); 134.0 (br. s, 2C, i-C); 132.9 (d, 2C, p-C); 22.0 (br. t, 1C,
BCH2); 22.2, 19.7 (each q, each 4C, o- and m-CH3); 0.6, 0.3 ppm (each q,
each 3C, Me3Si); 11B NMR (160 MHz, C6D6, 278C): d=71 (1B), 38 ppm
(2B)
[a] DME=1,2-dimethoxyethane.
lated to be 13.2kcalmol ꢀ1 [8]
.
B3LYP/6-311 + G** computations[5] reveal that
the transition state 6c (Scheme 4) lies
17.2kcalmol ꢀ1 above 3c, and planar 7c is
9.6 kcalmolꢀ1 higher in energy than 3c. The dis-
torted tetrahedral transition state of the isomer-
ization of the 1,2-dihydro-1,2-diborete to the un-
substituted 1,3-dihydro-1,3-diborete 2u, which is
isoelectronic to the framework of 6, lies
22.9 kcalmolꢀ1 above 2u according to computations
by McKee; planarization of 2u requires only
Scheme 3. a) Distorted triboratetrahedrane anions of the type 6a[9] as transition
states of the isomerization 3b!3a and of the enantiomerization of 3a without
ring inversion but with topomerization, that is exchange of the boron atoms with
the Duryl substituents. There are only ten electrons available for the six skeletal
bonds of the tetrahedron. b) Enantiomerization of 3a with ring inversion the planar
transition state 7a.
Scheme 4. Computed[5] energy differences be-
tween 3c and distorted tetrahedral 6c and planar
7c, respectively.
670
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Angewandte
Chemie
16.9 kcalmolꢀ1.[4] For the dianion of B4Me4 the distorted
[10]
Cambridge CB21EZ, UK; Fax: (+ 44)1223-336033; or deposit
@ccdc.cam.ac.uk).
tetrahedral form according to our computations lies only
4.9 kcalmolꢀ1 above the folded two-electron aromtic com-
pound; its planarization requires 7.3 kcalmolꢀ1. Thus, an
increasing number of boron atoms in four-membered two-
electron aromatic compounds facilitates the planarization as
well as the fluctuation of the skeletal bonds.
[8] The line shape analysis was carried out with WIN-DYNA 32,
Bruker Analytik GmbH, Version 1.01; the changes in the
chemical shifts in the investigated temperature interval were
considered to be linear and the line widths of the trimethylsilyl
groups were used as the reference line widths. The rate constants
of the ring inversion kRInv were computed from the difference in
rate constants of the enantiomerization without ring inversion
ktet (coalescence of the methyl groups and protons at the
aromatic group) and the enantiomerization with ring inversion
Received: August 6, 2002 [Z19899]
ktotal (coalescence of the methylene protons) according to kRInv
=
ktotalꢀktet
.
[1] M. Bremer, P. von R. Schleyer, U. Fleischer, J. Am. Chem. Soc.
1989, 111, 1147, and references therein.
[2] M. Hildenbrand, H. Pritzkow, W. Siebert, Angew. Chem. 1985,
97, 769; Angew. Chem. Int. Ed. Engl. 1985, 24, 759, and
references therein.
[9] Tetrahedra of CB3 anions are, like those of the isoelectronic B4
dianions, distorted.[10] There are three forms for distorted
tetrahedra of the type 6a: two enantiomers with short C BDur
edges (the transition state between 3b and 3a), and one with a
ꢀ
ꢀ
short C BCH2R edge (the transition state of the degenerate
[3] P. H. M. Budzelaar, K. Krogh-Jespersen, T. Clark, P. von R.
Schleyer, J. Am. Chem. Soc. 1985, 107, 2723.
rearrangement of 3a).
[10] A. Neu, T. Mennekes, U. Englert, P. Paetzold, M. Hofmann, P.
von R. Schleyer, Angew. Chem. 1997, 109, 2211; Angew. Chem.
Int. Ed. Engl. 1997, 36, 2117.
[4] M. McKee, Inorg. Chem. 2000, 39, 4206, and references therein.
[5] All geometries were optimized by employing the B3LYP hybrid
functional together with the 6-31G(d) basis set, and the nature of
the stationary points was characterized by analytical frequency
calculations. Relative energies are based on energy calculations
with 6-311 + G(d,p) and are corrected for zero-point energies.
a) Gaussian 98, Revision A.7, M. J. Frisch, G. W. Trucks, H. B.
Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G.
Zakrzewski, J. A. Montgomery, Jr., R. E. Stratmann, J. C. Bur-
ant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C.
Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.
Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A.
Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski,
J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P.
Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T.
Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C.
Gonzalez, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S.
Replogle, und J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1998;
b) A. D. Becke, J. Chem. Phys. 1993, 98, 1372; A. D. Becke, J.
Chem. Phys. 1993, 98, 5648; c) C. Lee, W. Yang, R. G. Parr, Phys.
Rev. B 1988, 37, 785.
Boron-Containing Rings
A Diboracyclopropane with a Planar-
Tetracoordinate Carbon Atom and a
Triborabicyclobutane**
Y¸ksel Sahin, Carsten Pr‰sang, Matthias Hofmann,
Govindan Subramanian, Gertraud Geiseler,
Werner Massa, and Armin Berndt*
[6] The synthesis of 5 will be described elsewhere.
[7] Crystal structure analyses: 3a¥Li(Et2O)3: A colorless crystal
(0.45 î 0.35 î 0.25 mm3) was measured at 193 K on an IPDS area
detector system (Stoe) with MoKa radiation. C40H76B3LiO3Si2,
monoclinic, space group P21/c, Z = 4, a = 1183.4(1), b =
2277.6(2), c = 1756.7(1) pm, b = 98.32(1)8, V= 4685.0(6)
Dedicated to Professor Paul von Raguÿ Schleyer
Known molecules with a planar-tetracoordinate carbon
atom[1] contain metal centers.[2] According to computations[3]
the prototype of such molecules without metal centers is the
diboracyclopropane 1u (Scheme 1). Derivatives of the lower
energy isomer 2u with planar-tetracoordinate boron atoms[4]
î10ꢀ30 m3, 1calcd = 0.993 Mgmꢀ3 50384 reflections up to q =
,
25.848, 8557 independent (Rint = 0.1160), 3910 with I > 2s(I).
The structure was solved with direct methods and refined against
F2 with full matrix. Hydrogen atoms were considered as riding at
calculated positions, wR2 = 0.1527 for all reflections, R = 0.0691
for observed reflections. Limited accuracy due to the disorder of
the diethyl ether in the Li(Et2O)3 cation. 4: Pale yellow crystal
(0.30 î 0.15 î 0.05 mm3) C28H46B3ClSi2, monoclinic, space group
C2/c, Z = 8, a = 3031.1(2), b = 988.1(1), c = 2217.6(1) pm, b =
[*] Prof. Dr. A. Berndt, Dr. Y. Sahin, Dr. C. Pr‰sang, Dr. G. Subramanian,
G. Geiseler, Prof. Dr. W. Massa
Fachbereich Chemie
Universit‰t Marburg
35032 Marburg (Germany)
Fax: (+49)6421-282-8917
E-mail: berndt@chemie.uni-marburg.de
111.06(1)8, V= 6198.1(8)î10ꢀ30 m3, 1ber = 1.086 Mgmꢀ3 meas-
,
urement as for 3a¥Li(Et2O)3, 12475 reflections up to q = 24.08,
4627 independent (Rint = 0.1414), 1857 with I > 2s(I) (only very
thin platelets available). The structure was solved analogously to
that of 3a, resulting in wR2 = 0.1144 for all reflections, and R =
0.0539 for the observed reflections. CCDC-187178 (3a¥Li-
(Et2O)3) and CCDC-187179 (4) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.can.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Centre, 12Union Road,
Dr. M. Hofmann
Anorganisch-Chemisches Institut
Universit‰t Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
Angew. Chem. Int. Ed. 2003, 42, No. 6
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671