Bishomoaromatic 1,2,4-triboracyclopentane dianions: Strong three-center, two-electron bonds between three boron atoms

Article abstract of DOI:10.1002/(SICI)1521-3773(20000403)39:7<1272::AID-ANIE1272>3.0.CO;2-Z

Much greater than the bishomoaromaticity for the archetype 1 of all bishomoarenes is that of 1,2,4-triboracyclopentane dianions 2, although the skeletons of 2 are isoelectronic with that of the experimentally unknown monocyclic prototype 3. Donor substitu

Full text of DOI:10.1002/(SICI)1521-3773(20000403)39:7<1272::AID-ANIE1272>3.0.CO;2-Z

COMMUNICATIONS
Diederich, Chem. Eur. J. 1998, 4, 1351 ± 1361; H.-F. Chow, T. K.-K.
Mong, M. F. Nongrum, C.-W. Wan, Tetrahedron 1998, 54, 8543 ± 8660;
O. A. Matthews, A. N. Shipway, J. F. Stoddart, Prog. Polym. Sci. 1998,
23, 1 ± 56; M. Fischer, F. Vögtle, Angew. Chem. 1999, 111, 934 ± 955;
Angew. Chem. Int. Ed. 1999, 38, 884 ± 905.
Bishomoaromatic 1,2,4-Triboracyclopentane
Dianions: Strong Three-Center, Two-Electron
Bonds between Three Boron Atoms**
David Scheschkewitz, Abolfazl Ghaffari, Peter Amseis,
Markus Unverzagt, Govindan Subramanian,
[2] C. Heim, A. Affeld, M. Nieger, F. Vögtle, Helv. Chim. Acta 1999, 82,
746 ± 759; it was found that the deslipping reaction is kinetically first
order.
[3] For slipping syntheses, see a) M. Händel, M. Plevoets, S. Gestermann,
F. Vögtle, Angew. Chem. 1997, 109, 1248 ± 1250; Angew. Chem. Int. Ed.
Engl. 1997, 36, 1199 ± 1201; b) D. B. Amabilino, M. Asakawa, P. R.
Ashton, R. Ballardini, V. Balzani, M. Belohradsky, A. Credi, M.
Higuchi, F. M. Raymo, T. Shimizu, J. F. Stoddart, M. Venturi, K. Yase,
New J. Chem. 1998, 959 ± 972, and references therein.
[4] Reviews: G. Schill, Catenanes, Rotaxanes, and Knots, Academic Press,
New York, 1971; D. B. Amabilino, J. F. Stoddart, Chem. Rev. 1995, 95,
2725 ± 2825; H. W. Gibson in Large Ring Molecules (Ed.: J. A.
Semlyen), Wiley, Chichester, 1996, pp. 191 ± 262; Comprehensive
Supramolecular Chemistry, Vol. 9 (Eds.: J. L. Atwood, J. E. D. Davies,
D. D. MacNicol, F. Vögtle, J.-P. Sauvage, M. W. Hosseini), Pergamon,
Oxford, 1996; R. Jäger, F. Vögtle, Angew. Chem. 1997, 109, 966 ± 980;
Angew. Chem. Int. Ed. Engl. 1997, 36, 930 ± 944; Molecular Catenanes,
Rotaxanes and Knots (Eds.: J.-P. Sauvage, C. Dietrich-Buchecker),
WILEY-VCH, Weinheim, 1999.
Â
Matthias Hofmann, Paul von Rague Schleyer,
Henry F. SchaeferIII, Gertraud Geiseler,
Werner Massa, and Armin Berndt*
The three-center, two-electron (3c,2e) bonding of bisho-
moaromatic cations of type 1[1±4] are known to be weakened by
[2b, 3a]
R ˆ C6H5
and displaced by R ˆ p-MeOC6H4 and R ˆ
OH.[4] Evidently, cations 1 are stabilized by the 3c,2e bonds,
whereas cations 2 are stabilized by donor substituents R.
According to computational estimates, 1u (R ˆ H) is more
1 [2a]
stable than 2u (R ˆ H) by 15 kcalmol .
The energy
difference between the prototypes 3u and 4u, neither of
which is known experimentally, is only 10.1 kcalmol .
1 [5]
R
C
R
Â
[5] C. J. Hawker, J. M. J. Frechet, J. Chem. Soc. Chem. Commun. 1990,
1010 ± 1013.
C
[6] For the first studies of the size complementarity of trityl stoppers and
macrocyclic wheels, see I. T. Harrison, J. Chem. Soc. Chem. Commun.
1972, 231 ± 232; I. T. Harrison, J. Chem. Soc. Perkin Trans. 1 1974,
301 ± 304; G. Schill, W. Beckmann, N. Schweikert, H. Fritz, Chem. Ber.
1986, 119, 2647 ± 2655.
[7] For the influence of the wheel and stopper size on the synthesis of
polyester rotaxanes, see H. W. Gibson, S. Liu, P. Lecavalier, C. Wu,
Y. X. Shen, J. Am. Chem. Soc. 1995, 117, 852 ± 874.
[8] For the molecular modeling of the deslipping of rotaxanes, see F. M.
Raymo, K. N. Houk, J. F. Stoddart, J. Am. Chem. Soc. 1998, 120,
9318 ± 9322.
[9] For the synthesis of stable rotaxanes with dendritic stoppers, see D. B.
Amabilino, P. R. Ashton, V. Balzani, C. L. Brown, A. Credi, J. M. J.
H
H
C
H
C
C
C
C
1
2
H
H
C
H
10.1
kcal mol–1
C
C
H
C
H
C
H
MP2/6-31G*
H
3u
4u
R
B
B
Â
B
R
Frechet, J. W. Leon, F. M. Raymo, N. Spencer, J. F. Stoddart, M.
B
B
Venturi, J. Am. Chem. Soc. 1996, 118, 12012 ± 12020.
[10] a) G. M. Hübner, J. Gläser, C. Seel, F. Vögtle, Angew. Chem. 1999,
111, 395 ± 398; Angew. Chem. Int. Ed. 1999, 38, 383 ± 386; b) C. Reuter,
W. Wienand, G. M. Hübner, C. Seel, F. Vögtle, Chem. Eur. J. 1999, 5,
2692 ± 2697; c) G. M. Hübner, C. Reuter, C. Seel, F. Vögtle, Synthesis
2000, 5, 103 ± 108; d) C. Seel, F. Vögtle, Chem. Eur. J. 2000, 6, 21 ± 24.
[11] F. Vögtle, M. Händel, S. Meier, S. Ottens-Hildebrandt, F. Ott, T.
Schmidt, Liebigs Ann. 1995, 739 ± 743; see also C. A. Hunter, J. Chem.
Soc. Chem. Commun. 1991, 749 ± 751.
B
5
6
We now report on bishomoaromatic dianions 5 of 1,2,4-
triboracyclopentanes that have skeletons isoelectronic to that
of 3u.[6, 7] The 3c,2e bonds of dianions of type 5 remain intact
even in the presence of strong donor substituents like R ˆ
NR2. Hence, the energy difference between 5 and 6 must be
[12] H. W. Gibson, S.-H. Lee, P. T. Engen, P. Lecavalier, J. Sze, Y. X. Shen,
M. Bheda, J. Org. Chem. 1993, 58, 3748 ± 3756.
1
larger than the 26 kcalmol stabilization afforded to a
[13] 5-Acetoxyresorcinol was synthesized by standard procedures from
phloroglucinol, acetylchloride, and triethylamine in THF and purified
by column chromatography on silica gel with CH2Cl2/MeOH 20/1
(Rf ˆ 0.54); G. M. Hübner, F. Vögtle, unpublished results.
tricoordinate boron atom by a dialkylamino substituent.[8]
Calculations at the MP2/6-31‡G* level[9] confirm this con-
clusion.
[14] For phenolic dendrimers, see H.-F. Chow, I. Y.-K. Chan, C. C. Mak,
M.-K. Ng, Tetrahedron 1996, 52, 4277 ± 4290.
The dianions 5a ± c (Durˆ 2,3,5,6-tetramethylphenyl) are
obtained by reduction of 7a ± c with lithium in diethyl ether.
[15] The steric influences of the dendrons are also mirrored in the long
reaction times: The mixtures had to be stirred for up to four weeks
before all of the starting materials had reacted while usually the
reaction is complete after a few days. Furthermore, between 77% (in
case of 9b) and 94% (in case of 10b) of the free axles were formed
here.
[16] To make sure that under these severe conditions the spectral changes
were caused by mechanical deslipping and not by chemical decom-
position, we exposed the free components of the rotaxanes, that is the
tetralactam 1 and the axles, to the same conditions. Indeed, there was a
slow color change of the solutions and slight changes in the spectra
were observed, however, these processes were significantly slower
than the changes observed in the spectra of the rotaxanes.
[17] N. Brinkmann, D. Giebel, G. Lohmer, M. T. Reetz, U. Kragl, J. Catal.
1999, 183, 163 ± 168.
[*] Prof. Dr. A. Berndt, Dr. D. Scheschkewitz, A. Ghaffari, P. Amseis,
Dr. M. Unverzagt, G. Geiseler, Prof. Dr. W. Massa
Fachbereich Chemie der Universität Marburg
35032 Marburg (Germany)
Fax : (‡ 49)6421-282-8917
E-mail: berndt@ chemie.uni-marburg.de
Dr. G. Subramanian, Dr. M. Hofmann, Prof. Dr. P. von R. Schleyer,
Prof. Dr. H. F. SchaeferIII
Center for Computational Chemistry, University of Georgia, 1004
Cedar Street, Athens, GA, 30602-2525
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(Schwerpunktprogramm ªPolyederº), the Fonds der Chemischen
Industrie, and by the U.S. National Science Foundation.
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Angew. Chem. Int. Ed. 2000, 39, No. 7

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R
B
The reaction of 8 with phenyllithium and methyllithium yields
5a and 5d, respectively. Compounds 7a ± c and 8 were
obtained from 7d, which can be synthesized from 11[10] via
10, 9, and 7e. The structures of the new compounds are
R
B
SiMe3
H
H
SiMe3
H
H
2 Li
C
C
C
C
Me3Si
Me3Si
Dur
B
B
B
B
a: R = C6H5
b: R = OEt
c: R = NEt2
Dur
Dur
Dur
5a-d
7a-c
1
deduced from their 11B, H, and 13C NMR spectra (Tables 1
d: R = Me
Me3Si-R
R
R
and 2). Similarities of the NMR data with those of the
known[11] isomers with cis-trimethylsilyl substituents allows
identification of 7d, 7e, and 8. The crystal structures[12] of 5a ´
2Li ´ 2Et2O and 5b ´ 2Li ´ 3Et2O are shown in Figure 1, their
relevant spectroscopic and physical data and that of 5c ´ 2Li ´
2Et2O are included in Table 2.
Cl
SiMe3
H
H
H
SiMe3
B
2 Li
C
B
C
C
C
Me3Si
Me3Si
Dur
H
B
B
B
B
Dur
Dur
Dur
7d
8
BCl3
OMe
Dianions 5a and 5b form contact triple ions with two
lithium cations, which, in addition, are coordinated to diethyl
ether molecules. Li1 is h2-coordinated to the B2 B3 s bond
and to the ipso- and ortho-C atoms of the duryl substituents.[13]
Li2 in 5a exibits a remarkably short distance to B1 (211 pm)
as well as contacts to C1 and C2 (230 pm).
The five-membered rings of 5a ± c feature distortions
characteristic of two-electron bishomoaromatics:[3] short
transannular distances (e.g. between B1 and B2,3) as well as
H H
H
SiMe3
B
C
SiMe3
2 Li
Dur
Me3Si
C
C
C
Me3Si
H
+ 3 B(OMe)3
B
B
B
B
Dur
Dur
Dur
- 2 LiB(OMe)4
7e
9
2 LiNMe2
Me3Si CH2 H2C SiMe3
Cl
Cl
2 LiCH2SiMe3
B
B
B
B
Dur
Dur
Dur
Dur
11
10
Table 1. Selected physical and spectroscopic properties of 5a ± c, 7a ± e, and 8 ± 10.
5a: yellow solid, yield 98%; 1H NMR (500 MHz, C6D6, 258C): d ˆ 7.92 (d,
2H, Ph-o-H), 7.20 (m, 2H, Ph-m-H), 7.04 (t, 1H, Ph-p-H), 6.71, 6.68 (each
s, each 1H, Dur-H), 2.93, 2.82, 2.45, 2.22, 2.18, 2.12, 2.08 (each s, total of
24H, Dur-CH3), 2.64 (br., 8H, Et2O), 0.59 (br., 12H, Et2O), 0.46, 0.15 (each
s, each 9H, SiMe3), 0.02, 0.43 (each s, each 1H, BCH); 13C{11B} NMR
(125 MHz, C6D6, 258C): d ˆ 156.2 (s, Ph-i-C), 152.2, 151.7 (each s, Dur-i-C),
139.5, 138.6, 138.2, 137.1, 133.8, 133.7, 133.5, 133.2 (each s, Dur-o- and m-C),
136.0 (d, Ph-o-C), 129.62, 129.57 (each d, Dur-p-C), 126.8 (d, Ph-m-C),
123.7 (d, Ph-p-C), 65.1 (t, Et2O), 22.4, 22.1, 21.3, 21.0, 20.9, 20.8 (each q,
Dur-CH3), 18.0 (d, 1J(C,H) ˆ 111 Hz, BCH), 14.0 (q, Et2O), 4.3, 3.8 (each q,
SiMe3); 11B NMR (96 MHz, C6D6, 258C): see Table 2
5b: orange-red solid, yield 97%; 1H NMR (500 MHz, C6D6, 258C): d ˆ
6.70, 6.65 (each s, each 1H, Dur-H), 3.95 (q, 2H, OEt), 2.79, 2.68, 2.64, 2.21,
2.15 (each s, total of 24H, Dur-CH3), 2.70 (br., 8H, Et2O), 1.22 (t, 3H,
OEt), 0.64 (br., 12H, Et2O), 0.45, 0.14 (each s, each 9H, SiMe3), 0.07,
0.98 (each s, each 1H, BCH); 13C{11B} NMR (125 MHz, C6D6, 258C): d ˆ
155.1, 152.6 (each s, i-C), 139.2, 136.9, 136.2, 133.5, 132.8, 132.4 (each s, o-
and m-C), 128.9, 128.8 (each d, p-C), 65.5 (t, Et2O), 61.4 (t, OEt), 24.0, 21.3,
21.1, 20.9, 20.5 (each q, Dur-CH3), 18.7 (q, OEt), 18.0 (d, 1J(C,H) ˆ 107 Hz,
BCH), 14.2 (q, Et2O), 12.1 (d, 1J(C,H) ˆ 113 Hz, BCH), 3.8, 3.7 (each q,
SiMe3); 11B NMR (160 MHz, C6D6, 258C): see Table 2
19.7, 19.0 (each br. q, Dur-CH3), 17.6 (q, OEt), 2.7 (q, SiMe3); 11B NMR
(96 MHz, CDCl3, 258C): d ˆ 96, 53 (2:1).
7c: yellow solid, m.p. 1288C, yield 98%; 1H NMR (300 MHz, CDCl3,
258C): d ˆ 6.91 (s, 2H, Dur-H), 3.51, 3.01 (each q, each 2H, NEt2), 3.01 (s,
2H, BCH), ca. 2.2 (br. s, 24H, Dur-CH3), 1.19 (t, 6H, NEt2), 0.02 (s, 18H,
SiMe3); 13C NMR (75 MHz, CDCl3, 258C): d ˆ 149.8 (br. s, i-C), 133.3,
133.1 (each s, o- and m-C), 130.9 (d, p-C), 52.5 (d, 1J(C,H) ˆ 107 Hz, BCH),
42.9 (t, NEt2), 21.6, 19.8 (each q, Dur-CH3), 15.8 (q, NEt2), 3.5 (q, SiMe3);
11B NMR (96 MHz, CDCl3, 258C): d ˆ 97, 49 (2:1)
7d: yellow solid, m.p. 1338C, yield 75%; 1H NMR (300 MHz, CDCl3,
258C): d ˆ 6.85 (s, 2H, Dur-H), 3.37 (s, 2H, BCH), 2.12 (s, 12H, Dur-CH3),
ca. 1.9 (br. s, 12H, Dur-CH3), 0.13 (s, 18H, SiMe3); 13C NMR (75 MHz,
CDCl3, 258C): d ˆ 148.4 (br.s, i-C), 133.1, 131.8 (each s, o- and m-C), 130.8
(d, p-C), 63.8 (d, 1J(C,H) ˆ 111 Hz, BCH), 20.6, 19.4 (each q, Dur-CH3), 2.6
(q, SiMe3); 11B NMR (96 MHz, CDCl3, 258C): d ˆ 97, 74 (2:1)
1
7e: yellow oil, yield 99%; H NMR (300 MHz, CDCl3, 258C): d ˆ 6.82 (s,
2H, Dur-H), 3.89 (s, 3H, OMe), 2.82 (s, 2H, BCH), 2.16 to 1.58 (br.s, total
of 24H, Dur-CH3), 0.01 (s, 18H, SiMe3); 13C NMR (75 MHz, CDCl3, 258C):
d ˆ 149.5 (br. s, i-C), 132.8, 131.3 (each s, o- and m-C), 130.2 (d, p-C), 54.7
1
(q, OMe), 53.2 (d, J(C,H) ˆ 104 Hz, BCH), 20.3, 19.2 (each q, Dur-CH3),
2.6 (q, SiMe3); 11B NMR (96 MHz, CDCl3, 258C): d ˆ 97, 53 (2:1)
1
5c: orange-red solid, yield 95%; H NMR (500 MHz, [D10]Et2O, 308C):
1
8: yellowish solid, yield 59%; H NMR (500 MHz, [D8]THF, 808C): d ˆ
d ˆ 6.65 (s, 2H, Dur-H), 3.38 (q, Et2O), 2.68, 2.54, 2.50, 2.49, 2.12, 2.11, 2.09
(each s, total of 24H, Dur-CH3), 1.13 (t, Et2O), 0.14, 0.31 (each s, each
9H, SiMe3), BCH and NEt2 not observed; 13C{11B} NMR (125 MHz, [D10]-
Et2O, 308C): d ˆ 153.0, 152.2 (each s, i-C), 140.5, 137.3, 136.8, 134.8, 133.6,
133.5, 133.1 (each s, o- and m-C), 129.14, 129.10 (each d, p-C), 66.2 (t,
Et2O), 51.5, 49.0 (t, NEt2), 23.1, 22.4, 21.3, 21.2, 21.1, 20.9, 20.7, 20.0 (each q,
Dur-CH3), 18.2, 17.2 (q, NEt2), 15.6 (q, Et2O), 4.3, 3.7 (each q, SiMe3), BCH
not observed; 11B NMR (160 MHz, C6D6, 258C): see Table 2
6.34 (s, 2H, Dur-H), 3.35 (Et2O), 2.10, 1.80, 1.63 (each s, total of 24H,Dur-
CH3), 1.11 (Et2O), 0.05 (s, 2H, BCH), 0.28 (s, 18H, SiMe3); 13C NMR
(125 MHz, [D8]THF, 808C): d ˆ 153.6 (br.s, i-C), 132.8, 135.0, 134.5,
130.6, 130.1 (each s, o- and m-C), 126.9 (d, p-C), 66.8 (Et2O), 21.4, 20.2
(each q, Dur-CH3), 2.9 (d, 1J(C,H) ˆ 129 Hz, BCH), 2.2 (q, SiMe3); 11B
NMR (96 MHz, Et2O, 258C): d ˆ 24, 20 (2:1)
9: colorless solid, m.p. 1188C, yield 85%; 1H NMR (300 MHz, C6D6, 258C):
d ˆ 6.67 (s, 2H, Dur-H), 3.25 (q, 8H, Et2O), 3.24 (s, 2H, BCH), 2.23, 2.16
(each s, each 12H, Dur-CH3), 1.01 (t, 12H, Et2O), 0.08 (s, 18H, SiMe3);
13C NMR (75 MHz, C6D6, 258C): d ˆ 155.7 (br.s, i-C), 132.2, 131.4 (each s,
o- and m-C), 128.0 (d, p-C), 83.7 (d, 1J(C,H) ˆ 90 Hz, BCH), 66.1 (t, Et2O),
22.2, 20.6 (each q, Dur-CH3), 3.2 (q, SiMe3); 11B NMR (96 MHz, C6D6,
258C): d ˆ 51
7a: yellow solid, m.p. 988C, yield 99%; 1H NMR (300 MHz, CDCl3, 258C):
d ˆ 7.76 (m, 2H, Ph-H), 7.53 (m, 3H, Ph-H), 6.98 (s, 2H, Dur-H), 3.98 (s,
2H, BCH), 2.25 (s, 12H, Dur-CH3), 2.1 (br. s, 12H, Dur-CH3), 0.08 (s,
18H, SiMe3); 13C NMR (75 MHz, CDCl3, 258C): d ˆ 149.5 (br. s, Dur-i-C),
146.2 (br. s, Ph-i-C), 133.0, 132.5 (each s, Dur-o- and m-C), 131.7 (d, Ph-m-
C), 130.7 (d, Dur-p-C), 129.9 (d, Ph-p-C), 127.5 (d, Ph-o-C), 65.0 (d,
1J(C,H) ˆ 108 Hz, BCH), 20.9, 19.6 (each q, Dur-CH3), 3.0 (q, SiMe3); 11B
NMR (96 MHz, CDCl3, 258C): d ˆ 101, 82 (2:1)
10: greenish, fluorescent solid, m.p. 1348C, yield 98%; 1H NMR (300 MHz,
CDCl3, 258C): d ˆ 6.80 (s, 2H, Dur-H), 2.16, 1.90 (each s, each 12H, Dur-
CH3), 1.79 (s, 4H, BCH2), 0.06 (s, 18H, SiMe3); 13C NMR (75 MHz, CDCl3,
258C): d ˆ 149.6 (br. s, i-C), 133.0, 129.5 (each s, o- and m-C), 129.3 (d, p-C),
28.6 (t, 1J(C,H) ˆ 113 Hz, BCH2), 20.3, 19.3 (each q, Dur-CH3), 1.7 (q,
SiMe3); 11B NMR (96 MHz, CDCl3, 258C): d ˆ 97
7b: yellow oil, yield 99%; 1H NMR (300 MHz, CDCl3, 258C): d ˆ 6.86 (s,
2H, Dur-H), 4.20 (m, 2H, OEt), 2.80 (s, 2H, BCH), 2.25, 2.20 (each s, each
12H, Dur-CH3), 1.43 (t, 3H, OEt), 0.12 (s, 18H, SiMe3); 13C NMR
(75 MHz, CDCl3, 258C): d ˆ 149.5 (br. s, i-C), 132.8, 131.1 (each br. s, o- and
1
m-C), 130.2 (d, p-C), 62.9 (t, OEt), 53.3 (d, J(C,H) ˆ 101 Hz, BCH), 20.4,
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Table 2. Selected distances [pm] and interplanar angles [8] of 5a ´ 2Li ´ 2Et2O, 5b ´ 2Li ´ 3Et2O, and 5c ´ 2Li ´ 2Et2O (crystal structures) as well as of model
compounds 5u-R (R ˆ H, Me, Ph, OH, NH2) (//MP2/6-31 ‡ G*); 11B NMR chemical shifts [ppm] are also given.
B1 ´´´ B2
B1 ´´´ B3
B2-B3
C1-B2-B3-C2/
C1-B1-C2[a]
d(B1)
d(B2), d(B3)
5a ´ 2Li ´ 2Et2O
5u-Ph[b]
185.7(6)
185.8
186.5(4)
184.7
192.7(6)
183.6
184.1(6)
185.8
187.8(4)
184.7
190.8(5)
183.6
163.7(6)
166.4
163.0(4)
165.3
165.0(6)
165.5
80.0(4)
82.4
82.7(3)
81.8
85.1(4)
80.2
20
35.6
2
7.1
4
17.2
21
15, 18
20.8
13, 15
20.5
7, 21
21.6, 19.7
13, 16
20.0
5b ´ 2Li ´ 3Et2O
5u-OH[b]
5c ´ 2Li ´ 2Et2O
[b]
5u-NH2
5d
5u-Me[b]
5u-H[b]
184.6
184.8
184.6
184.8
165.4
165.5
81.4
81.7
28.5
47.8
9.2
[a] Interplanar angle. [b] NMR chemical shifts were computed at GIAO-SCF/6-311‡G**//MP2(fc)/6-31‡G*.
the bishomoaromatic cations of type 1, even the relatively
weak phenyl donor substituent at C7 leads to a considerable
elongation of the transannular C C distance from 172.1 pm[14]
in 1u (R ˆ H) to 190.1 pm in 1a (R ˆ C6H5). The interplanar
angle also is increased (from 81.3 to 93.68, //MP2(fc)/6-
31G*).[2b] Similar geometries are found in the X-ray structures
of 1'a[3b] and 1a-Me2.[3a]
C6H5
C
Me
C
Me
Me
Me
C
C
Me
C
C
Me
Cl
Me
Me
1a-Me2
1´a
The monocyclic cation 3u-Me (with a transannular distance
1
of 182.1 pm) is only 2.1 kcalmol higher in energy than the
classical 4-methyl-4-cyclopentenyl cation, 4u-Me, and no
local minima of the 3u type could be localized when effective
p- or n-donors like phenyl or OH and NH2 were present.
These structures must be much less stable than the corre-
sponding classical cations 4u-Ph, 4u-OH, and 4u-NH2.[15]
Likewise, strong donors influence the NMR chemical shifts
of the pentacoordinate boron atoms of dianions 5 (Table 2) to
a lesser extent than the shifts of the pentacoordinate C atoms
of 1. Replacing R ˆ Me in the nonclassical 1 by OH gives the
classical 2b (R ˆ OH), as indicated by the deshielding of C7
by 153 ppm (d13C ˆ 72 !225). An analogous exchange of
substituents in the dianions 5 (5d !5b) leads to a 11B
deshielding of only 19 ppm.[16, 17]
Figure 1. Structures of 5a ´ 2Li ´ 2Et2O (top) and 5b ´ 2Li2 ´ 3Et2O (bot-
tom) in the crystal, the methyl substituents at Si1 and Si2 as well as most of
the H atoms are omitted for clarity. Selected bond lengths (in addition to
those in Table 2) [pm] and bond angles [8]. 5a: B1-C1 165.1(5), B1-C2
163.6(5), C1-B3 162.3(5), C2-B2 163.3(5); C1-B1-C2 110.1(3), B1-C2-B2
69.2(2), C2-B2-B3 108.9(3), B2-B3-C1 109.0(3), B3-C1-B1 68.4(2); 5b: B1-
C1 161.4(4), B1-C2 162.7(4), C1-B3 164.1(4), C2-B2 164.4(4); C1-B1-C2
113.7(2), B1-C2-B2 69.5(2), C2-B2-B3 109.3(2), B2-B3-C1 109.1(2), B3-C1-
B1 70.5(2).
The ineffectiveness of the donor substituents is expressed
most impressively in the geometries characterizing the 3c,2e
bonds of the bishomoaromatics 5a ± c: these parameters, like
those in 5u-Ph, 5u-OH, and 5u-NH2, are almost identical
(Table 2).
Hence, the 3c,2e bonding in bishomoaromatic dianions 5 is
much stronger than those of the isoelectronic cations of type 1.
A quantitative measure for the 3c,2e bond strengths is given
by the computed energy differences between bishomoaro-
matic 5u-R and classic 6u-R. At the MP2/6-31‡G* ‡ 0.89
ZPE(HF/6-31G*) level 5u-H is more stable than 6u-H by
small interplanar angles (here between the C1-B3-B2-C2 and
C1-B1-C2 planes). The experimental data are compared to
values computed for the model compounds 5u-R (R ˆ H, Me,
Ph, OH, NH2) in Table 2.
1
39.0 kcalmol 1; this is about 30 kcalmol greater than the
The strong donors OEt, OH, NEt2, and NH2 distort
bishomoaromatics of type 5 to a small extent (Table 2). For
difference between the isoelectronic 3u and 4u . The differ-
ence between nonclassical and classical forms is only about
1274
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Angew. Chem. Int. Ed. 2000, 39, No. 7

COMMUNICATIONS
[10] A. Höfner, B. Ziegler, W. Massa, A. Berndt, Angew. Chem. 1989, 101,
188; Angew. Chem. Int. Ed. Engl. 1989, 28, 186.
[11] M. Unverzagt, G. Subramanian, M. Hofmann, P. von R. Schleyer, S.
Berger, K. Harms, W. Massa, A. Berndt, Angew. Chem. 1997, 109,
1567; Angew. Chem. Int. Ed. Engl. 1997, 36, 1469.
[12] Crystal structure determination: 5a ´ 2Li ´ 2Et2O: A yellow platelet-
like crystal (0.4 Â 0.35 Â 0.05 mm3) was measured on a Stoe IPDS
diffractometer at 808C using MoKa radiation. C42H70B3Li2O2Si2,
monoclinic, space group P21/n, Z ˆ 4, a ˆ 1022.0(1), b ˆ 1970.0(2), c ˆ
2267.2(3) pm, b ˆ 100.02(2)8, Vˆ 4495.0(9)  10 30 m3, 1calcd
ˆ
1.048 gcm 3; a total of 30705 reflections were recorded in the range
1.828 < q < 24.918, resulting in 7636 independent reflections from
which 3397 with F0 > 4s(F0) were observed. All 7636 independent
reflections were used for subsequent calculations; no absorption
correction was applied (m ˆ 1.1 cm 1). The structure was solved with
direct methods and refined against Fo2 with full matrix. Non-hydrogen
atoms were refined using anisotropic displacement factors. The H
atoms at C1 and C2 were refined; the remaining H atoms were kept at
calculated positions, and 1.2 or 1.5 times (CH3) the equivalent
isotropic U value of the corresponding C atom was used as displace-
ment factors. The refinement converged at wR2 ˆ 0.1534 for all
reflections, corresponding to a conventional R ˆ 0.0632 for 3397
reflections with Fo > 4s(Fo). 5b ´ 2Li ´ 3Et2O: A yellow plateletlike
crystal (0.6 Â 0.2 Â 0.06 mm3) was measured on a Stoe IPDS diffrac-
tometer at 808C using MoKa radiation. C42H80B3Li2O4Si2, mono-
clinic, space group P21/c, Z ˆ 4, a ˆ 2355.0(2), b ˆ 1107.2(1), c ˆ
1
1
10 kcalmol smaller (29.6 kcalmol ) for the monoanionic
prototypes which result from protonating the B B bond in
dianions of type 5.[18]
In conclusion, both the statement, based on results for
homoaromatic cations and neutral compounds, that ªthe very
existence of homoaromaticity is
a matter of a few
kcalmol º[19] as well as the one that anionic homoaromaticity
is ªeither quite insignificant or nonexistent altogetherº[20] are
not valid any longer.
1
1899.6(1) pm,
b ˆ 96.30(1)8,
Vˆ 4923.2(7)  10 30 m3,
1calcd
ˆ
1.014 gcm 3; a total of 38176 reflections was observed in the range
1.748 < q < 24.118, resulting in 7734 independent reflections from
which 3283 with Fo > 4s(Fo) were observed. All 7734 reflections were
used for the subsequent calculations; no absorption correction (m ˆ
1.06 cm 1) was applied. The structure was solved by direct methods
and refined against Fo2 with full matrix as described above. The
refinement converged at wR2 ˆ 0.0785 for all reflections, correspond-
ing to a conventional R ˆ 0.0406 for 3283 reflections with Fo > 4s(Fo).
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC-133560 (5a ´ 2Li ´ 2Et2O) and CCDC-133561 (5b ´ 2Li ´ 3E-
t2O). Copies of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
(‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
Received: November 26, 1999 [Z14319]
[1] S. Winstein, M. Shatavsky, C. Norton, R. B. Woodward, J. Am. Chem.
Soc. 1955, 77, 4183; W. G. Woods, R. A. Carboni, J. D. Roberts, J. Am.
Chem. Soc. 1956, 78, 5653.
[2] a) M. Bremer, K. Schötz, P. von R. Schleyer, U. Fleischer, M.
Schindler, W. Kutzelnigg, W. Koch, P. Pulay, Angew. Chem. 1989,
101, 1063, Angew. Chem. Int. Ed. Engl. 1989, 28, 1042; b) P.
von R. Schleyer, C. Maerker, Pure Appl. Chem. 1995, 67, 755.
[3] a) T. Laube, J. Am. Chem. Soc. 1989, 111, 9224; b) T. Laube, C. Lohse,
J. Am. Chem. Soc. 1994, 116, 9001, and references therein; T. Laube,
Acc. Chem. Res. 1995, 28, 399; W. J. Evans, K. J. Forrestal, J. W. Ziller,
J. Am. Chem. Soc. 1995, 117, 12635.
[13] Similar lithium cation coordinations were found recently for 1,2-
diboratacyclopentadienides: D. Scheschkewitz, M. Hofmann,
P. von R. Schleyer, G. Geiseler, W. Massa, A. Berndt, Angew. Chem.
1999, 111, 3116; Angew. Chem. Int. Ed. 1999, 38, 2936.
[14] Quantum Chemistry Archive, Computer Chemie Centrum, Universi-
tät Erlangen-Nürnberg.
[4] G. A. Olah, G. Liang, J. Am. Chem. Soc. 1975, 97, 6803; G. A. Olah,
A. L. Berrier, M. Arvanaghi, G. K. Surya Prakash, J. Am. Chem. Soc.
1981, 103, 1122.
[5] P. von R. Schleyer, T. W. Bentley, W. Koch, A. J. Kos, H. Schwarz, J.
Am. Chem. Soc. 1987, 109, 6953; K. J. Szabo, E. Kraka, D. Cremer, J.
Org. Chem. 1996, 61, 2783.
[15] Without symmetry constraints the geometry optimizations of 3u-OH,
3u-NH2, and 3u-Ph converge to the planar five-membered ring
structures 4u-OH, 4u-NH2, and 4u-Ph. However, the transition states
3u-Ph and 3u-OH as well as a second-order stationary point for 3u-
NH2 (with planar amino group) can be obtained by choosing
conformations that exclude effective conjugation between substituent
R and the cationic center in 3u-R. The energies of 3u-Ph, 3u-OH, and
[6] The following communication[7] describes a trishomoaromatic 1,3,5-
triboracyclohexane dianion, a system isoelectronic with the trisho-
mocyclopropenylium cation.
[7] W. Löûlein, H. Pritzkow, P. von R. Schleyer, L. Schmitz, W. Siebert,
Angew. Chem. 2000, 112, 1336, Angew. Chem. Int. Ed. 2000, 39, 1297.
[8] T. Totani, K. Tori, J. Murakami, H. Watanabe, Org. Magn. Reson.
1971, 3, 627.
1
3u-NH2 are 14.9, 24.7, and 47.3 kcalmol (MP2(fc)/6-31G* ‡ 0.89
ZPE(HF/6-31G*)) relative to the corresponding classical structures
4u-R.
[9] The geometries of the dianions were optimized at the MP2(fc)/6-
31‡G* level, while MP2(fc)/6-31G* was applied for the cations.
Relative energies were corrected for scaled (0.89) zero-point vibra-
tional energies from HF/6-31G* frequency calculations. Chemical
shifts were computed at the GIAO-SCF/6-311‡G** level employing
the optimized geometries. The Gaussian 94 program was used for all
computations: M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill,
B. G. Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A.
Petersson, J. A. Montgomery, K. Raghavachari, M. A. Al-Laham,
V. G. Zakrzewski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B.
Stefanov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala,
W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts,
R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P.
Stewart, M. Head-Gordon, C. Gonzalez, J. A. Pople, Gaussian 94
Revision C.3, Gaussian, Inc., Pittsburgh PA, 1995.
[16] Application of the Nöth ± Wrackmeyer relation,[17] which correlates
13C and 11B chemical shifts of isoelectronic carbocations and boron
compounds, gives d11B ˆ 43.6 for a boron compound corresponding to
2b; however, d11B ˆ 2 is observed experimentally for 5b.
[17] B. Wrackmeyer, Z. Naturforsch. B 1980, 35, 439, and references
therein.
[18] M. Hofmann, D. Scheschkewitz, A. Ghaffari, G. Geiseler, W. Massa,
H. F. Schaefer, A. Berndt, J. Mol. Model. 2000, 6, in press.
[19] R. C. Haddon, J. Org. Chem. 1979, 44, 3608; R. C. Haddon, Acc.
Chem. Res. 1988, 21, 243.
[20] V. I. Minkin, M. N. Glukhovtsev, B. Y. Simkin, Aromaticity and
Antiaromaticity, Wiley, New York, 1994, p. 243.
Angew. Chem. Int. Ed. 2000, 39, No. 7
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1275