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An Extraordinary Structure and Topomerization Mechanism for УDiboramethylenecyclopropaneФ.

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that binding is weak. The upfield shift of the methyl signal
on complexation is in marked contrast[’41to the downfield
shifts characteristic of most H bonded systems. However,
this shift may reflect slight changes induced in the relative
orientations of the phenyl and methyl groups by the approach of the crown ether to the guest cation. Under such
circumstances, intramolecular aromatic ring current effects
will probably influence the observed chemical shift of the
methyl group in the [Ph,PMe]@ cation.
Received: June 18, 1984 [Z 888 IE]
German version: Angew. Chem. 96 (1984) 806
An Extraordinary Structure and Topomerization
Mechanism for “Diboramethylenecyclopropane”**
By Peter H . M . Budzelaar, Paul von Rag& Schleyer*,
and Karsten Krogh-Jespersen*
Compounds containing a B-C double bond were unknown until the recent report by Berndt et al., claiming the
The
synthesis of a diboramethylenecyclopropane, 1
compound was found to undergo a rapid topomerization
at room temperature (activation energy 11.4 kcal/mol),
represented by the equilibrium 2 + 2‘.
-
CAS Registry number:
[(PR,PMe)2’ 18C61[PF6],, 92346-47-9
c1 c1
[I] Top. Curr. Chem. 109 (1983).
[2] As far as alkylammonium cations are concerned, the strengths of complexes in MeOH decrease (R. M. Izatt, N. E. Izatt, B. E. Rossiter, J. J.
Christensen, B. L. Haymore, Science 199 (1978) 994) in the order NKm,
RNH? > R2NHP> R3NH” to the extent that macrocycles of the 18C6
type do not bind %N” ions (B. Roland, J. Smid, J . Am. Chem. SOC.105
(1983) 5269). However, nitrogen ylides are not nearly as stable as their
phosphorus counterparts, and encouragingly, certain features of the ’HNMR spectra of alkylphosphonium salts (%Paxe where X=CI, Br, I)
have been interpreted (L. V. Nesterov, N. A. Aleksandrova, 1. D. Temyachev, A. A. Musina, R. G. Gainullina, Zh. Obshch. Khim. 47 (1977)
1259; Chem. Abstr. 87 (1977) 1172030 in terms of C-H.. . X hydrogen
bond formation.
[3] F. Vogtle, E. Weber in S. Patai: R e Chemistry of Functional Groups.Supplement E: The Chemistry of Ethers, Crown Ethers, Hydroxyl Groups, and
rheir Sulphur Analogues, Wiley, Chichester 1980, p. 59; F. Vogtle, H.
Sieger, W. M. Miiller, Top. Curr. Chem. 98 (1981) 107.
141 J. A. Bandy, M. R. Truter, F. Vogtle, Acta Crystallogr. 8 3 7 (1981)
1568.
[51 J. A. A. de Boer, D. N. Reinhoudt, S . Harkema, G. J. Hummel, F. de
Jong, J. Am. Chem. SOC.104 (1982) 4073.
161 A. Elbasyouny, H. J. Briigge, K. von Deuten, M. Dickel, A. Knochel, K.
U. Koch, J. Kopf, D. Melzer, G. Rudolph, J. Am. Chem. Soc. 705 (1983)
6568.
I71 Experimental: [(Ph,PMe),. 18C6][PF6]2: [Ph3PMe][PF.,] (64 mg, 0.151
mmol) and 18C6 (20 mg, 0.075 mmol) were dissolved in the minimum
volume of warm MeOH; long needle-shaped crystals, suitable for X-ray
investigation, were obtained by vapor diffusion using light petroleum (P.
G. Jones, Chem. Brit. 17 (1981) 222). After 16 h, the crystals were filtered
off, washed with Et20, and dried. Yield 62 mg (75%); m.p. 141--143”C;
‘H-NMR (CD2C12):6=2.77 (6H, d, J pH=12 Hz, Me), 3.59 (24H, s,
OCH2), 7.55-7.94 (30H, m, Ph).
181 The complex is rhombohedral, a = 11.592(3)
y=71.39(2)’,
U = 1357 space group Rj, Z =1, p,=1.36 g ern-,. Data were measured on a Nicolet R3m diffractometer with Cu,, radiation (graphite
monochromated) and using w-scans. The structure was solved by the
heavy atom method. The unique methyl hydrogen atom was clearly located in a AF map and refined isotropically; the remaining non-hydrogen atoms were refined anisotropically to give R=0.064 (R,=0.079)
for 1192 independent observed reflections [OS 58”. lFol > 3a(lF01)]. Further details of the crystal structure investigation can be obtained from
the Director of Cambridge Crystallographic Data Centre, University
Chemical Laboratory, Lensfield Road, Cambridge CB2 IEW. Any request should be accompanied by the full literature citation for this communication.
[9] The PFF ion lies on a crystallographic 3-fold axis and is involved in only
one short intermolecular contact (3.25 A) between one of the F atoms
and a phenyl ring C atom; the H . . .F distance is 2.65
[lo] D. J. Cram, K. N. Trueblood, Top. Curr. Chem. 98 (1981) 43.
1111 K. N. Trueblood, C. B. Knobler, D. S. Lawrence, R. V. Stevens, J. Am.
Chem. SOC.104 (1982) 1355.
1121 The double perching arrangement has been observed in the X-ray crystal structures of a number of 2 : 1 complexes between CH-acidic neutral
guests and 18C6: for example, Me2S02[4] and MeN02 151.
[I31 Apart from exhibiting a P-C(Ph) bond length (1.795
that is slightly
longer than the P-C(Me) bond length (1.796 A), the structure of the guest
cation is very similar to those reported (see, for example, F. J. Hollander, D. H. Templeton, A. Zalkin, Inorg. Chem. 12 (1973) 2262 and R. E.
Cramer, D. M. Ho, W. van Doorne, J. A. Ibers, T. Norton, M. Kashiwagi, ibid. 20 (1981) 2457) for the [Ph,PMe]’ cation.
[I41 Indeed, the Me doublet in [Ph3PMe]Br resonates at 6=3.18 in
CD2C12-C6D6 (1 : 1) indicating a strong C-H.. .Br hydrqgen bonding
interaction.
A,
A,
A.
A)
Angew. Chem. Int. Ed. Engl. 23 (1984) No. 10
Our further theoretical study of various C2B,H4 isome d 2 ] now leads us to propose a related but significantly
different structure (still containing a B=C bond) for 1, and
also to confirm a pathway for its topomerization suggested
by Berndt et al.l3].The C2B,H, species relevant to the present discussion are the isomers 2-6. Geometries were optimized at the Hartree-Fock (HF) level using the split-valence plus polarization 6-31G* basis set, and energies were
then obtained from single-point calculations at these optimized geometries using Mdler-Plesset electron correlation estimates to third order (MP3)I4].Table 1 gives the relative energies of 3 -6 .
While the “classical” diboramethylene-cyclopropane
structure 2 is indicated to be a local minimum with small
3
4
5
6
Table I. Relative energies of 3-6 and substituent effects [a].
Compound
Ere,,[b]
C2B2H4
Substituent Effects [c]
CHI at B
SiH, at C
3
0
0
4
5
14.3
26.2
24.4
- 5.6
6
- 7.3
1.9
0
1.4
-3.6
- 2.5
Erei Id1
(H3Si)2CC(BCH3)2
0
10.2
15.3
23.8
la] Energies in kcal/mol, relative to 3 E 0 kcal/mol. b ] MP3/6-31G*//631G*. [c] Single point HF/6-31G with HF/6-31G* optimized skeleton geometries and standard bond lengths and angles for the XH3 substituents. [d] Values estimated by assuming additivity of substituent effects (see text).
[*] Prof. Dr. P. von R. Schleyer, Dr. P. H. M. Budzelaar
Institut fur Organische Chemie der Universitit Erlangen-Niirnberg
Henkestrasse 42, D-8520 Erlangen (FRG)
Prof. Dr. K. Krogh-Jespersen
Department of Chemistry, Rutgers,
The State University of New Jersey
New Brunswick, NJ 08903 (USA)
[**I This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen lndustrie. P. H. M. B. expresses his gratitude
for a Fellowship sponsored by the Netherlands Organization for the Advancement of Pure Research (Z.W.O.). We thank Dr. T. Clark for preliminary calculations on 2 and 6 and general assistance.
0 Verlag Chemie GmbH, 0-6940 Weinheim, 1984
0570-0833/84/1010-0825 $ 02.50/0
825
basis sets (e.g., 3-21G), this is not so at the HF/6-31G* level: optimization (starting with the 3-21G geometry for 2)
gives structure 3 directly. The carbene 4[2",33
(CZvsymmetry) corresponded to a local minimum at all theoretical levels employed. This means that a transition state (roughly
resembling 2) must separate 3 and 4.This was located ( 5 )
at 6-31C*; some geometrical details are provided (Fig. 1).
Finally, structure 6 , another possible species on the rearrangement pathway was considered (Table l)['I. Since 3
is by far the most stable of the CZBZH,isomers examined
here, we suggest that Berndt's "diboramethylenecyclopropane" l['I may have a similar structure.-The spectroscopic data obtained for 1 may also be reconciled with a
structure related to 3.
The curious species 3 has normal B1 -C 1 and C1 -C2 Gbonds and a three-center n-bond (C2-B2-B1) (see Fig. 1).
Only four electrons are involved in bonding the o-framework of the BlC2B2 ring (Fig. I). The bonding Walsh-type
orbital o2is occupied, but there is no antibonding counterpart to oz. This results in weak B1-C2 and B1-B2 bonds
(represented here by dotted lines) and a strong C2-B2 double bondr5].
by assuming additivity of the substituent effects. We conclude that path 1 can account for the observed topomerization barrier. The intermediacy of 4 in the proposed mechanism can also explain some of the observed reactions of
1 [3l.
Received: May 15, 1984;
revised: July 9, 1984 ( Z 834 IE]
German version: Angew. Chem. 96 (1984) 809
[ l ] H . Klusik, A. Berndt, Angew. ('hem. 95 (1983) 895; Angew. Chem. lnr. Ed.
Engl. 22 (1983) 877.
[2] a) K. Krogh-Jespersen, D. Cremer, D. Poppinger, J. A. Pople, P. von R.
Schleyer, J. Chandrasekhar, J. Am. Chem. SOC.101 (1979) 4843; b) K.
Krogh-Jespersen, D. Cremer, J. I>. Dill, J. A. Pople, P. von R. Scbleyer,
ibid. 103 (1981) 2589; c) P. von R. Schleyer, P. H. M. Budzelaar, D. Cremer, E. Kraka, Angew. Chem. 96 (1984) 374; Angew. Chem. Int. Ed. Engl. 23
(1984) 374: d) P. H. M. Budzelaar, P. von R. Schleyer, T. Clark, K.
Krogh-Jespersen, unpublished.
[3] H. Klusik, Dissertation, Universitat Marburg 1983; R. Wehrmann, H.
Klusik, A. Berndt, Angew. Chrm. 96 (1984) 810; Angew. Chem. Int. Ed.
Engl. 23 (1984) 826.
141 The GAUSSIAN 82 series of programs were employed (J. S . Binkley, M.
Frisch, K. Raghavachari, D. J. DeFrees, H. B. Schlegel, R. A. Whiteside,
E. Fluder, R. Seeger and J. A. l'ople, GAUSSIAN 82, release A), with the
standard basis sets and correlation treatments included in these programs.
[5] Recent calculations on 3 are in agreement with our calculations (G.
Frenking, H. F. Schaeffer 111, ('hem. Phys. Lerf., in press.
1,3-Diboretanes**
Fig. I . Calculated bond lengths [A] in 3 and 5 (top) and a schematic representation of the orbitals responsible for the bonding in the BIC2B2-ring of 3
(bottom).
What is the mechanism of the topomerization of 1
(modeled in our study by 3)? The two obvious possibilities
involving only planar species are:
+
4 i [ 5 ]i
3
161
(path 1)
(path 1)
We have also searched for possible non-planar pathways,
but have not found any likely candidates. Thus, the barrier
for the topomerization of 3 is either 26.2 kcal/mol (path 1)
or 23.7 kcal/mol (path 2). Before comparing this with the
experimental value of 11.4 kcal/mol"l for Berndt's compound 1, however, we have to take into account the electronic and steric effects of the substituents. The electronic
effects have been estimated by modeling %Me3 and tBu in
1 by SiH3 and CH3 groups, respectively. Relative to 3,
these groups lower the energy of 4 and 5, but hardly affect
that of 6 (Table 1, last column). Thus, the energy of path 1
is lowered while that of path 2 remains virtually unchanged. Hence, for the particular choice of substituents in
1, path 2 can be ruled out, since it is ca. 10 kcal/mol
higher in energy. Steric effects will be important for 3,
where the larger tBu substituents o n boron are forced close
together. This should destabilize 3 by a few kcal/mol relative to 4 and 5,and reduce the topomerization barrier further below the estimated'value of 15.3 kcal/mol obtained
826
0 Verlag Chemie GmbH. 0-6940 Weinheim. 1984
By Rolf Wehrmann, Hartmut Klusik, and Armin Berndt*
Dedicated to Professor Roland Koster on the occasion
of his 60th birthday
1,3-Diboretane has so far merely been the subject of theoretical studies"]. Herein we report the synthesis and characterization of the derivatives 3 - 5 .
The ethenylidene-1,3-diboretane 3 is formed as sole
product on heating a mixture of 2-tert-butylborandiylborirane 1 [2.31 and bis(trimethylsilyl)acetylene in pentane under reflux for ten days. In this reaction, 1 behaves as if it is
initially converted into the carbene
This behavior is
also observed in the reaction of 1 with triphenylarsane and
the well known carbene trapping reagentr6]triphenylphosphane, which leads at 20°C to the ylides 4a and 4b;the
reaction with PPh3 leads to formation of equal amounts of
4b and the 1,3-dihydro-1,3-diborete617],which is also obtained on heating 4b to 180°C. The ylides 4a and 4b can
be protonated with trifluoromethanesulfonic acid in chloroform to give the 1,3-diboretanes 5a and 5b,respectively.
Reduction of 3 with K/Na alloy in tetrahydrofuranldimethoxyethane furnishes the radical anion 3 ''(a("B) = 0.58
mT, ~ ( ~ ' s =
i )3.43 mT, g = 2.0024).
The structure of the compounds 3- 5 follows from their
'H-, I3C-, "B-, 3'P-NMR-, and mass spectra'']; in its I R
spectrum, 3 shows an allene band at 1870 cm-I. The
shielding of the boron atoms in 4a, b (6("B)=61 and 59,
respectively) compared to that in 3 (6=78) reflects the
delocalization of the negative charge of the ylidic C-atoms
(6(I3C)=83.1 and 81.5, respectively). In the meantime,
similar "B-NMR shifts (6 = 51 -58) have been described
for dialkylboryltriphenylphosphonium ylides['I. The ylide
structure of 4b is also supported by the 31P-13Ccoupling
2[4351.
[*] Prof. Dr. A. Berndt, R. Wehrmann, Dr. H. Klusik
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, D-3550 Marburg (FRG)
[**I
This work was supported by the Fonds der Chemischen lndustrie
0570-0833/84/1010-0826 $ 02.50/0
Angew. Chem. Int. Ed. Engl. 23 (1984) No. 10
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