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Compounds with Three-Membered Rings Containing Boron.

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HIGHLIGHTS
Compounds with Three-Membered Rings Containing Boron
By Hansjorg Griitzrnacher*
Apart from the well-established cyclopropanes, a series of
homologous cyclopropanes of the general formula X,
(X = CR,, SIR,, GeR,, SnR,) has also been prepared in the
last few years. Several derivatives of these compounds, obtained by replacing a ring building block X by a different one
(Y, e.g., CR,, SIR,, NR, 0, S, Se, Te), have also been structurally characterized.[’ - 6 1
Common features of the molecular structures determined
can be summarized as follows: if a ring building block X in
an X, three-membered ring is replaced by a heteroatom or a
group Y of greater electronegativity, the basal X-X bond is
shortened. Within a series X,Y this reduction in the bond
length increases with increasing electronegativity of the ring
building block Y At the same time the angle a, a measure of
the planarization at the basal atoms, increases (Scheme 1).
w
Scheme 1
Kraka have translated this “molecular orbital model” into
an “electron density model” for three-membered rings.“] In
qualitative agreement with the 7c complex model a deformation of the o electron density is found, which is reminiscent
of that of a R bond and which is delocalized in the plane of
the three-membered ring (surface delocalization). The path
of the maximum (valence) electron density (MED), however,
does not agree with the nucleus-nucleus axes Y-X and X-X,
but takes a bent course.
It is possible to distinguish between cases with outwardly
(convex) and inwardly (concave) bent bonds. Cyclopropane
1 is an example for the first case and protonated oxirane 2 an
example for the second.[’”] Therefore, a short distance between the nuclei in the X-X unit is not necessarily a result of
particularly high R character in this bond, but of a strong
bend. In actual fact, the calculated length of the bent C-C
and Si-Si bond in the series of heterocyclopropanes agrees
well with those of the corresponding single bonds.[’] In both
models it is possible to establish simple rules, which allow the
prediction of the geometries of heterocyclopropanes as well
as substituent effects.[8b,
According to calculations by Allen et al. the parent compound of the oxadiboriranes (HB),O (3) has a remarkable
geometry.[”’ The calculated E B bond length in 3 is only
The unusual molecu.-r properties of three-membered
rings have stimulated numerous quantum chemical studies.
In accordance with the Dewar-Chatt-Duncanson model developed for transition metal-olefin complexes, the bonding
in heterocyclopropanes can be described in a simplified way
as a 7c complex of the X=X unit and the ring building block
Y (Scheme 2).17]The observed shortening of the basal X-X
1.594 A and the H-B-B-H grouping is almost linear
(a= 173.9”). Paetzold et al. have now been able to synthesize
the first alkyl-substituted oxadiborane 5 in a rather a simple
way [Eq. (a)].[121They started from a chlorine-substituted
diboryloxide 4, which was coupled reductively with sodium/
potassium alloy to give 5. Previously, Noth et al. and also
Paetzold et al. sucessfully prepared azadiboranes 6;[131however, these either had amino groups (isopropylamino;
XQx
Dewar-Chatf-Duncanson
u-bridged
7r-MO [I11
(Me,Si),C
Scheme 2.
C(SiMeS),
\
/
CI
Cl
bond with increasing electronegativity of Y is therefore considered to result from reduced backbonding from an occupied p(Y) orbital into the 7c* orbital of the X=X unit. Alternatively, the molecular structures of three-membered rings
can be explained by a high contribution of the “o-bridged 7c
orbital” in the bonding molecular orbitals.“ Cremer and
[‘I
Priv.-Doz. Dr. H. Griitzmacher
Anorganisch-chemisches Institut der Universitat
I m Neuenheimer Feld 270, D-W-6900 Heidelberg (FRG)
Angen. Chon. Int. Ed Engl. 1992, 31, No. 10
0 VCH
4
5
2,2,6,6-tetramethylpiperidino) at the boron atoms which
interact strongly with the electronically unsaturated boron
atoms, or the crystals were not suitable for an accurate structural investigation. Also in 5 the oxygen atom is disordered
with respect to the crystallographic C , axis; nevertheless, the
ring geometry can be determined accurately enough. In good
agreement with the calculations a very short B-B bond
Verlagsgesellschafi mbH, W-6940 Wernheim, 1992
0570-0833/92/fOfO-i329
$3.50+ .25/O
1329
(1.60 A) and an almost linear C-B-B skeleton (177.7') are
found. The B-0 distances are not that well reproduced (obsd
B-0 1.55, 3.52 A; calcd B-0 1.409 A). The packing of the
B-0-B three-membered ring in the sheath of the very bulky,
lipophilic (Me,Si),C ("trisyl") substituents, however, transforms it into a chemical "rock": in contrast to the remarkably reactive azadiborirane 6, R = R' = tBu, 5 does not
react with O,, H,O, CO, azides, BH,, phosphanes, or alkynes. The reaction of 6, R = R' = tBu, with CO leads to
the tricyclic product 7, whose structure with two B-N-C
Me
Ph
I
B
I
B
13
11
Following the synthesis and complete characterization of
the three-membered ring compounds containing boron, 14,
15, and 16 (Scheme 3),[", 'I Berndt et al. have now success-
R
Ph
\I
I
1.450(10)
/B
k.464(11)
14
1.597(8)
Ph
/B -
\1.578(8)
Ph
\ $'
Ph
B -
1.61(36
15
\
1.61(3)
16
Scheme 3. The C,B ring compounds 14-16 with the relevant experimentally
determined B-C bond lengths [A] given. R = 2,4.6-trimethylphenyI.
R'
I
I
Bu
0
three-membered rings is confirmed by an X-ray structural
analysis.['41 With azides the azadiborirane ring can be expanded to give diazadiboretidine 8. Only reactions of 5 with
formic or acetic acid to give 9 (R2 = H and Me, respectively)
were observed.
9 R
=
fully prepared the first boriranes 18 a-c from the borandiylboriranes 17a,b (Scheme 4). The molecular structures of 18a
and 18c have been confirmed by X-ray crystal structure
analysis.[*']
The new three-membered rings 18 a-c supplement the
family of the heterocyclopropanes with an example in which
a significantly more electropositive ring building block
Y = BR replaces a more electronegative X = CR,. Furthermore, an R,B group as strong 7t acceptor is connected to a
ring carbon atom. The structure determined experimentally
with a highly extended basal C-C bond (1.636 A) is in agreement with the theoretical results. Thermally (60 "C, 46 h),
I
R=Dur PhLi.
DurLi
H, Me
R
On the basis of the isolobal analogy B ts-f C', boriranes
are related to cyclopropyl cations. The latter generally isomerize exothermically (AG = - 38 kcalmol-') to give ally1
cations. An exception is the most recently prepared cyclo-
-
0
X
N(SiMe,),
CI
Me
I
R
20
Scheme 4. Dur
= 2,3,5,6-tetramethylphenyl.
10
propyl cation 10, which, because it is stabilized by a ferrocenyl group, can be studied by NMR spectroscopy in sohtion at -60"C.['51 Methylborirane (11) is in contrast
27.5 kcalmol-' more stable than its isomer 12.['61Denmark
et al. recently reported indications for 1,2,3-triphenylborirane (13)with a thermal stability (> 150 0C).L171
1330
R
a tBu
b Dur
c Dur
VCH firlugsgrselischufi mbH, M/-6940 Weinheim, i992
18 a rearranges into a C-borylaminomethyleneborane, and
at higher temperature (120"C, 4h) the borirane I8c rearranges into a 1,2,3,4-tetrahydro-1,3-diboranaphthaline.
The authors explain these results by the cleavage of a ring
bond (basal C-C bond, or C-B bond) opposing the R,B
substituents, but in both cases the proposed rearrangements
leave enough room for speculation.
o570-0833;92jlofO-1330
S 3.50f.2510
Angen>.Chem. Int. Ed. Engl. 1992, 31, N o . 10
Finally, Berndt et al. have reported the first unequivocal
proof of a negatively charged homoaromatic system 20,
which they obtained from the thermolysis of 19, the addition
product of aryllithium compounds to the borandiylborirane
17b (Scheme 4).[211
The fact that this compound actually
possesses the cyclic delocalized T[. bonding system of a homoaromatic compound with 27c electrons is revealed by
NMR spectroscopy. In comparison with 19 the signal due to
the tricoordinate carbon atom in 20 is shifted to lower field
by about A6 = 60 (20: 6 = 162.9; 19: 6 = 106.0/110.6). The
crystal structure was determined by X-ray structural analysis. The C,B, ring is folded about the B-B axis by roughly
33'; the transanular B-B distance (1.792 A) is in the range of
bonding B-B interactions. The energy barrier AG* (176K)
for ring inversion was estimated by dynamic NMR spectroscopy at approximately 7.9 kcalmol- '. The experimental
results were supported by a b initio calculations [fold angle
3 4 , B-B distance 1.859 A ; A E (ring, folded)-AE (ring, planar) = -7.4 kcalmol-', 6(I3C) = I 6 9 (IGLO)]. The somewhat lower calculated bond order of the 1,3 interaction relative to that in the homocyclopropenium ion (0.23 vs 0.40)
can be attributed to the lower electronegativity of the boron
atom in comparjson with the carbon atom.
Molecular chemistry is alive! Together with the authors of
the papers discussed we look forward to and eagerly await
new boron-containing three-membered rings. The synthesis
of a diborirane 21, in particular, now seems to be almost
within reach. For this compound the calculations predict a
carbon atom with a planar coordination geometry.[221
German version: Anyew. Chem. 1992, 104, 1358
Angeiv. Chrm. (tit. Ed. Eng/. 1992, 31, N u . 10
(C
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cited therein.
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therein.
[3] R. Peng-Koon Tan, G. R. Gillette, D. R. Powell, R. West, Orgunometallics
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Tsumuraya, Y Kahe, W Ando, J. Chem. Suc. Chem. Cummun. 1990,1159.
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[6] H. Grutzmacher, H. Pntzkow, Angew. Chem. 1991, 103, 976; Angew.
Chem. Int. Ed. Engl. 1991, 30, 1017.
[7] A. Greenberg, J. F. Liebman, StruinedUrgunic Molecules, Academic Press,
New York, 1978, S. 280-282, and references cited therein.
181 a) D. Cremer, E. Kraka, J. Am. Chem. SUC.1985,107,3800;ibid. 1985,107,
381 1. The advantage of this concept attributed to Bader et al. [9] is that a
random assignment of the electron density onto the bonding and nonbonding electron pairs IS avoided. The K character of a bond is described
by the anisotropy E of the charge concentration p(rb)at special points r,, of
the bond (bond critical points).
(91 R. F. W. Bader. P. J. MacDougall, C. D. H. Lau, J. Am. Chrm. Sur. 1984,
106. 1594, and references cited therein.
[lo] T. Clark, G. W. Spitznagel, R. Klose, P. vonR. Schleyer. J. Am. Chem. Suc.
1984, 106, 4412.
[ l l ] C. Liang, L. C. Allen, J. Am. Chem. SOC.1991, 113. 1878.
[12] P. Paetzold. L. Gkret-Baumgarten, R. Boese, Angew. Chem. 1992, 104.
1071; Angew. Chem. I n f . Ed. Engl. 1992,31, 1040.
[13] a) F. Dirschl, E. Hanecker, H. Noth, W Rattay, W. Wagner. Z. Nuturfursch. B 1986, 41, 32; h) R. Boese, B. Krockert, P. Paetzold, Chem. Ber.
1987, 120, 1913; c) K.-H. van Bonn, P. Schreyer, P. Paetzold. R. Boese,
ihid. 1988, 121, 1045.
[I41 P. Paetzold, B. Redenz-Stormanns, R. Boese, Angew Chem. 1990, 102,
910; Angew. Chem. I n t . Ed. Engl. 1990, 29, 900.
[IS] G. K. Surya Prakash, H. Buchholz, V. Prakash Reddy, A. de Meijere.
G. A. Olah, J. Am. Chem. SOL.1992, 114, 1097.
[16] M. Buhl, P. von R. Schleyer. M. A. Ibrahim, T. Clark, J. Am. Chem. Suc.
1991, 13, 2466, and references cited therein.
117) S. E. Denmark, K. Nishide, A.-M. Faucher, J. Am. Chem. SUC.1991, 113,
6675.
[I 81 J. J. Eisch, 8.Shafii. J. D. Ordon, A. L. Rheingold, J. Am. Chem. SOL..1990,
112, 1847.
[I91 M. A. Kropp, M. Baillargeon, K. M. Park, K. Bhamidapaty, G. B. Schuster, J. Am. Ch1.m. SUC.1991, 113, 2155.
[20] P. Willershausen, G . Schmidt-Lukasch, C. Kybart, J. Allwohn, W. Massa.
M. L. McKee, P. von R. Schleyer, A. Berndt, Angew. Chem. 1992. 104,
1417; Angen. Chem. In!. Ed. Engl. 1992, 31.1384
[21] P. Willershausen, C. Kybart, N. Stamatis, W. Massa, M. Buhl. P. von R.
Schleyer, A. Berndt, Angew. Chem., 1992, fU4, 1278; Angew. Chem. Int.
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[22] K. Krogh-Jespersen, D. Cremer, D. Poppinger, J. A. Pople, P. yon R.
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VCH Verlugsgesellschufi mbH, W-6940 Weinheim, 1992
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.25/0
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