Classical 1,2,4-triboracyclopentanes and their rearrangement into nonclassical 2-boryl-1,3-diboracyclobutanes: Intramolecular C-H bond activation by a B-B moiety

Article abstract of DOI:10.1002/ejic.200500279

1,2,4-Triboracyclopentanes 1a-c in solution spontaneously transform into their nonclassical isomers 2a-c. These have been characterised by NMR spectroscopy and X-ray structural analyses. Compounds 2a-c are strongly distorted when compared with the five-membered ring of 1a, the structure of which was determined for comparison. In addition, the pattern of the substituents at the boron centres in 2a,b is different from that in 1a,b. This can be rationalised on the basis of a rapid topomerisation in 2c as determined by analysis of its temperature dependent ¹³C NMR spectra. The environment of the boron centres B1 and B3 is exchanged by transforming the two 3c-2e bonds which both involve B1 into a B2-H and a B1-C 2c-2e bond. This leads to the 2-boryl-1,3-diboracyclobutane 4c and formation of corresponding new 3c-2e bonds employing B3 gives the topomer of 2c. The low barrier of this topomerisation reveals that 2a-c may also be regarded as nonclassical isomers of the 2-boryl-1,3-diboracyclobutanes 4a-c. Ab initio computations of models at the MP4SDTQ/6-311+G* *// MP2/6-311+G* * levels support this interpretation. In addition, they show that the transformation of 1u to 2u occurs by an intramolecular C-H bond activation process initiated by the B-B moiety. Derivatives of the type 1 and 2 described here represent the first examples of 1,2-diboron heterocycles which can be isolated as classical as well as nonclassical isomers. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200500279

FULL PAPER
Classical 1,2,4-Triboracyclopentanes and Their Rearrangement into
Nonclassical 2-Boryl-1,3-diboracyclobutanes: Intramolecular C–H Bond
Activation by a B–B Moiety
David Scheschkewitz,[a] Peter Amseis,[a] Gertraud Geiseler,[a] Werner Massa,[a]
Matthias Hofmann,[b] and Armin Berndt*[a]
Keywords: Ab initio computations / Boron heterocycles / Three-centre, two-electron bonds
1,2,4-Triboracyclopentanes 1a–c in solution spontaneously
transform into their nonclassical isomers 2a–c. These have
been characterised by NMR spectroscopy and X-ray struc-
tural analyses. Compounds 2a–c are strongly distorted when
compared with the five-membered ring of 1a, the structure
of which was determined for comparison. In addition, the
pattern of the substituents at the boron centres in 2a,b is dif-
ferent from that in 1a,b. This can be rationalised on the basis
of a rapid topomerisation in 2c as determined by analysis of
its temperature dependent 13C NMR spectra. The environ-
ment of the boron centres B1 and B3 is exchanged by trans-
forming the two 3c–2e bonds which both involve B1 into a
B2–H and a B1–C 2c–2e bond. This leads to the 2-boryl-1,3-
diboracyclobutane 4c and formation of corresponding new
3c–2e bonds employing B3 gives the topomer of 2c. The low
barrier of this topomerisation reveals that 2a–c may also be
regarded as nonclassical isomers of the 2-boryl-1,3-diboracy-
clobutanes 4a–c. Ab initio computations of models at the
MP4SDTQ/6-311+G**// MP2/6-311+G** levels support this
interpretation. In addition, they show that the transformation
of 1u to 2u occurs by an intramolecular C–H bond activation
process initiated by the B–B moiety. Derivatives of the type
1 and 2 described here represent the first examples of 1,2-
diboron heterocycles which can be isolated as classical as
well as nonclassical isomers.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
In this paper we show that 1,2,4-triboracyclopentanes
1a–c (Scheme 2) isomerise into nonclassical isomers 2a–c.
Ab initio computations at the MP2/6-311+G** level[7] for
the prototypes 1u and 2u (Scheme 3) reveal that these trans-
formations are intramolecular C–H bond activations by B–
B moieties. However, 2a,b are isomers of the products to
be expected from these C–H bond activations in 1a,b. This
discrepancy can be rationalised by an additional rearrange-
ment via 4a,b (Scheme 4) which shows up in 2c as a topo-
merisation via 4c. Therefore, compounds of type 2 may be
regarded equally well as nonclassical 1,2,4-triboracyclopen-
tanes and as nonclassical 2-boryl-1,3-diboracyclobutanes.
Introduction
Classical forms of unsubstituted 1,2-diboron heterocycles
of type I–III (Scheme 1) have been computed to be con-
siderably higher in energy than their nonclassical forms IV–
VI.[1,2] This is due to the two-electron aromaticity of IV and
homoaromaticity of V and VI, respectively. The electron
deficiency at the boron atoms in I–III is reduced in IV–VI
by formation of two three-centre, two-electron (3c–2e)
bonds. Experimentally, derivatives are known for nonclassi-
cal IV[3] and V[2] as well as for classical II[4] and III.[5] While
the boron atoms of all nonclassical species bear only weakly
donating substituents, those of the experimentally known
classical ones carry amino substituents which strongly re-
duce the electron deficiency at these boron atoms. As a con-
sequence, isomerisation of the classical compounds II or III
into nonclassical compounds of type V or VI has not been
observed so far. Recently, 1,2,4-triboracyclopentanes with
only weakly donating substituents at the boron atoms be-
came accessible in their classical form 1.[6]
Results and Discussion
Syntheses and NMR Characterisation
Details of the preparation of 1,2,4-triboracyclopentanes
1a,b[6] and 1d[6] are described in the experimental section.
Compound 1d does not react with duryllithium to yield 1c
which, finally, was synthesised by oxidation of its dianion
1c2–. This can be obtained by addition of duryllithium to
the borenate 3.[6] In CDCl3 solution, 1a,b transform into
their nonclassical isomers 2a,b within four and nine days,
respectively. Isomerisation of 1c into 2c requires only 2 h in
diethyl ether. Interestingly, CDCl3 solutions of the stereo-
[a] Fachbereich Chemie der Universität Marburg,
Hans-Meerwein-Strasse, 35032 Marburg
Fax: +49-6421-2828917
E-mail: berndt@chemie.uni-marburg.de
[b] Anorganisch-Chemisches Institut der Universität Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg
4078
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200500279
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Rearrangement of 1,2,4-Triboracyclopentanes into 2-Boryl-1,3-diboracyclobutanes
FULL PAPER
Scheme 1.
Scheme 2.
isomers of 1a,b having the trimethylsilyl substituents in cis
These chemical shifts are characteristic for homoaro-
positions[8] do not show any isomerisation over a period of matic species containing a 3c–2e bond between one carbon
three months.
and two boron atoms. Compounds 2a–c are rare examples
1
Compounds 2a–c were characterised by H, 13C and 11B of homoaromatics having two-membered homobridges. The
NMR spectroscopy. The chemical shifts of the skeleton boron centres B1 of 2a,b, which are not part of the 3c–2e
atoms of 2a,b are in good agreement with those computed bond, are considerably shielded (48, 42 ppm) compared
for 2u at the IGLO/II/MP2/6-31+G* level and resemble with those of 1a,b (88, 82 ppm). This can be explained
those of Va (see Table 1).
mainly by the interactions of the formally empty p orbital
at B1 with the 3c–2e bond between B2, B3 and C1 (see
NBO analysis of 2u in section “Ab initio computations”).
The substitution patterns at the boron atoms of 2a,b are
different from those of the precursors 1a,b. While the boron
centres between two carbon atoms carry a methyl or phenyl
group in 1a,b, it is a duryl group in 2a,b. The methyl and
phenyl substituents are found at a boron atom adjacent to
a boron atom in 2a,b. This was demonstated for 2a,b by
the X-ray structure analyses discussed below and for 2a in
solution by a selectively 11B-decoupled 13C NMR spectrum.
Table 1. Selected NMR chemical shifts [ppm] of 2a–c, Va (exp.)
and 2u (calcd. at IGLO/II/MP2/6-31+G*).
2a
2b
2c
2u
Va
3[c]
δ11B B3[a]
δ11B B2[a]
δ11B B1[a]
δ13C C1[a]
δ13C C2[a]
0
48
56
74.6
27.6
–2
42
57
70.7
26.8
ca. 30[b] -–6.0
ca. 47[b] 46.9
ca. 47[b] 48.5
105
24.8
48[d]
72.6
16.6
68.2[e]
32.1[f]
[a] Numbering as in Figures 1, 2, 3 and 4. [b] Broad signal. [c] B1.
[d] B2. [e] C3. [f] C4.
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FULL PAPER
Scheme 3.
Scheme 4.
Decoupling using the frequency corresponding to the chem- peratures and show coalescence between –70 and –60 °C,
ical shift δ11B = –2 ppm strongly sharpens the 13C NMR respectively. From the chemical shift differences δν of corre-
signal of the boron bound methyl group at 4.9 ppm. This sponding signals and their coalescence temperatures Tc, the
indicates this methyl group to be bound to the pentacoordi- following ∆G# values can be calculated using Equation (1):
nate boron atom. Computations for 2u (see below) clearly 9.2, 10.0, 9.7 and 10.0 kcalmol–1.
support the assignment of the most shielded boron atom to
that of the highest coordination number.
∆G# = 4.57·Tc·[9.97 + log (Tc/δν)]
(1)
The chemical shifts of the skeleton atoms of 2c are dis-
On average, a barrier of exchange of 9.7 kcalmol–1 can
tinctly different from those of 2a,b. There is no sharp 11B be obtained. We have assigned this barrier to the enantio-
signal at about δ = 0 for a pentacoordinate boron atom but merisation of 2cЈ to 2c which arises through opening of the
only a broad shoulder at about 30 ppm of a very broad 3c–2e bonds in 2cЈ to form the 2-boryl-1,3-diboracyclobut-
signal at about 47 ppm. The 13C NMR spectrum of 2c at ane 4c and the corresponding new 3c–2e bonds employing
ambient temperature shows signals for only two different the C1–B3 and B2–H bond. These transformations lead to
kinds of duryl substituents and not for three as would be 2c, the enantiomer of planar-chiral 2cЈ. The enantiomeri-
expected from its asymmetric structure which has been un- sation of 2c can be observed by NMR spectroscopy because
ambiguously deduced from the X-ray analysis described be- it includes a topomerisation of two duryl substituents.
low. However, at –90 °C signals for three different duryl
On the basis of the topomerisation of 2cЈ, the unexpected
substituents can be observed which broaden at higher tem- pattern of substituents in 2a,b can easily be explained by
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Rearrangement of 1,2,4-Triboracyclopentanes into 2-Boryl-1,3-diboracyclobutanes
FULL PAPER
the corresponding process leading to isomerisation instead
of topomerisation. Opening of both 3c–2e bonds in the pri-
marily formed 2e,f generates the classical 2-boryl-1,3-dibor-
etanes 4a,b. If the ring boron centre carrying the sterically
less demanding substituent is employed in reforming the
corresponding 3c–2e bonds, 2a,b are obtained which are
isomers of 2e,f.
Crystal Structures
Figures 1, 2, 3 and 4 show the structures of 1a and 2a–c
in the crystal. Characteristic distances and angles of 2a–c
and 1a are compared with those computed for 2u (Table 2).
Figure 3. Structure of 2b in the crystal. Most of the hydrogen atoms
have been omitted for clarity. Selected bond lengths [pm] and
angles [°] (completing Table 2): C1–B1 156.9(3), B1–C2 155.8(3),
C2–B3 160.8(3), C1–Si1 189.3(2), B1–C10 157.9(3), C2–Si2
185.8(2), B3–C30 158.1(3), B2–C20 155.3(3); C1–B1–C2 107.9(2),
B1–C2–B3 76.8(1).
Figure 1. Structure of 1a in the crystal. Most of the hydrogen atoms
have been omitted for clarity. Selected bond lengths [pm] and
angles [°] (completing Table 2): C1–B3 156.6(3), B3–C2 157.5(3),
C2–B1 156.0(2), C1–Si1 189.8(2), B1–C10 158.0(2), C2–Si2
191.6(2), B2–C20 157.6(2); B1–C2–B3 107.6(1).
Figure 4. Structure of one of the two independent molecules 2c in
the crystal structure of 2c·pentane. Most of the hydrogen atoms
have been omitted for clarity. Selected bond lengths [pm] and
angles [°] (completing Table 2): C1–B1 151.8(8), B1–C2 160.5(8),
C2–B3 153.3(8), C1–Si1 189.2(6), B1–C10 159.3(7), C2–Si2
186.9(6), B3–C30 156.6(8), B2–C20 155.5(7); C1–B1–C2 109.7(4),
B1–C2–B3 92.7(4).
261.6(3) pm]. The bonding distances and angles of the
C1B2B3 moiety of 2a,b resemble those of the correspond-
ing part of homoaromatic Va[3] (Table 2). Elongation of
C1···B3 in 2a,b relative to Va can be explained by weaken-
ing of the 3c–2e bond due to interaction with the formally
Figure 2. Structure of 2a in the crystal. Most of the hydrogen atoms
have been omitted for clarity. Selected bond lengths [pm] and
angles [°] (completing Table 2): C1–B1 156.7(5), B1–C2 156.2(5), empty orbital at B1 (see computations). For 2c, the transan-
C2–B3 160.8(5), C1–Si1 188.3(3), B1–C10 159.0(5), C2–Si2
nular B···C distance (219.9 pm) is about midway between
185.8(4), B3–C3 158.1(5), B2–C20 155.6(5); C1–B1–C2 106.6(3),
those of 2a,b and 1a. In 2a the C1,B1,B3 plane is folded
B1–C2–B3 78.2(3).
towards the C1,B2,B3 and B1,C2,B3 planes by angles of 48
The five-membered rings of 2a–c are strongly distorted and 25°, respectively. In 2c the corresponding angles are
in comparison with that in 1a. This can be demonstrated, only 39 and 6°. Clearly, the sterically demanding duryl sub-
inter alia, by the much shorter transannular C1···B3 dis- stituent at B3 in 2c prevents the strong folding present in
tances in 2a–c [178.0(5), 179.6(3) and 219.9(7) pm, respec- 2a,b and 2u, all of which lack a large substituent at the
tively] compared with the shortest in 1a [C2···B2 = boron centre B3.
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D. Scheschkewitz, P. Amseis, G. Geiseler, W. Massa, M. Hofmann, A. Berndt
Table 2. Comparison of characteristic distances [pm] and angles [°] of 2a–c, Va, 1a (exp.) and 2u (calcd. at MP2/6-311+G**).
FULL PAPER
2u
2a
2b
2c⋅pentane
Va
1a
B2–B3
C1–B2
C1···B3
B1···B3
C1–B2–B3
B2–C1–B3
C1–B3–B2
B2–B3–C1–B1
180.3
147.1
180.5
193.1
66.0
65.9
48.1
186.9(6)
149.5(5)
178.0(5)
200.0(5)
62.7(2)
69.0(2)
48.3(2)
187.2(3)
147.8(3)
179.6(3)
196.8(3)
63.6(1)
69.0(1)
47.5(1)
192.4(9)
146.5(7)
219.9(9)
227.3(8)
79.7(4)
59.4(4)
41.0(3)
179.9(3)[a]
147.2(3)[c]
172.3(3)[d]
–
175.9(2)[b]
156.0(2)
263.2(2)[e]
252.9(3)[f]
62.7(1)[g]
68.0(1)[i]
49.3(1)[j]
133.7(2)[l]
104.8(1)[h]
106.6(1)
111.9(1)[k]
158.6(2)[m] 163.5(2)[n]
–145.3
–131.9(3)
–133.6(2)
–140.7(5)
[a] B2–B1. [b] B1–B2. [c] C3–B2. [d] C3···B1. [e] C1···B1. [f] B3···B2. [g] B1–B2–C3. [h] B1–B2–C1. [i] B2–C3–B1. [j] C3–B1–B2. [k] C1–
B3–C2. [l] B2–B1–C3–C4, positive sign since the corresponding enantiomer was used. [m] B2–B1–C1–B3. [n] B1–B2–C2–B3.
Ab Initio Computations
duced by the substituents in 4a–c and, as a result, the bar-
rier from 2e,f,cЈ to 2a,b,c can be expected to be larger.
Energies and geometries of model compounds 1u and 2u
were computed at the MP2/6-311+G** level.[7] Bond
lengths and angles obtained for 2u are in good agreement
(Table 2) with the corresponding data from the X-ray crys-
tal structure determinations of 2a,b. Chemical shifts com-
puted at the IGLO/II/MP2/6-31+G* level for 2u agree
(Table 1) with the experimental data of 2a,b but not with
those of 2c. This indicates the structure of 2c to be different
from those of 2a,b in solution as well.
NBO analyses for 2u reveal a 3c–2e bond between the
neighbouring boron atoms and the formally tricoordinate
carbon atom occupied by 1.84 electrons. The 0.26 electrons
in the p orbital of the boron centre between carbon atoms
indicate its interaction with the CB2 3c–2e bond and ex-
plain the NMR shielding of the boron atom of the two-
membered homo bridge. This interaction is essential for the
stabilisation of 2u relative to 1u by 7.4 kcalmol–1: the re-
lated nonclassical VIu with CH2 instead of BH in the two-
membered homo bridge is only 0.5 kcalmol–1 lower in en-
ergy than the classical Cs symmetric IIIu.[2,9]
Conclusions
Classical and nonclassical 1,2,4-triboracyclopentanes 1a–
c and 2a–c are the first representatives of 1,2-diboron het-
erocycles which can be isolated as classical as well as non-
classical isomers. Isomerisation of 1a,b can be easily ob-
served and computations for models reveal an intramolecu-
lar C–H bond activation by a B–B moiety as the mechanism
for this process. Nonclassical 1,2,4-triboracyclopentanes
2a–c are only slightly lower in energy than the correspond-
ing classical 2-boryl-1,3-diboretanes 4a–c which are con-
siderably stabilised by C–B hyperconjugation, the general
alternative to stabilisation by formation of 3c–2e bonds.
Compounds of type 2 may therefore be regarded as non-
classical 2-boryl-1,3-diboracyclobutanes as well as nonclas-
sical 1,2,4-triboracyclopentanes.
The transition state TS1/2u for the transformation of 1u
into 2u was computed to be 29.3 kcalmol–1 higher in energy
than 1u. TS1/2u has a short C–B distance (151.8 pm), a
very long C–H (159.9 pm) distance as well as short
(130.9 pm) and long (158.1 pm) B–H distances. According
to an NBO analysis it contains a 3c–2e C–H–B bridge occu-
pied by 1.79 electrons (29% at C, 24% at B and 37% at H).
Only 4 and 7% of the two electrons are found in the p
orbitals of the adjacent boron centres. This transition state
thus describes the activation of a C–H bond by a neigh-
bouring B–B moiety. The five-membered ring of the transi-
Experimental Section
General: Reactions were carried out under dry argon or nitrogen
using standard Schlenk techniques. Solvents were dried, distilled
and saturated with nitrogen. Glassware was dried with a heat gun
under high vacuum. 1H, 13C NMR spectroscopy: Bruker DRX 200
and Bruker AC 500 instruments, 11B NMR: Bruker DRX 200 in-
strument. The NMR references used were Me4Si and BF3·Et2O.
Melting points (uncorrected) were measured under argon.
4-Methyl-1,2-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-trans-3,5-bis(trimeth-
ylsilyl)-1,2,4-triboracyclopentane (1a): MeLi solution (Et2O, 1.6 ,
4.20 mL) was added by syringe to a solution of 1d (3.45 g,
tion state is still considerably less distorted than that of the 6.8 mmol) in Et2O (50 mL) at –78 °C. The solution was then al-
lowed to reach –10 °C over 1 h with stirring. Stirring was then sus-
pended to allow the salts to settle and the solution was decanted
through a precooled reversed frit (–15 °C, about 4 cm diameter).
All solvents were removed at –10 °C. The yellow residue consisted
of spectroscopically pure 1a which was used for subsequent reac-
tions immediately after preparation. Yield: 3.31 g (99%) of deeply
yellow 1a, m.p. 105–106 °C (dec.) 1H NMR (300 MHz, CDCl3,
25 °C): δ = 6.88 (s, 2 H, Dur-H), 3.31 (s, 2 H, BCH), 2.12–1.98
(br., 24 H in total), 1.10 (s, 3 H, BCH3), 0.03 (s, 18 H, SiMe3) ppm.
product 2u.
Computations also revealed that classical 4u is only
0.6 kcalmol–1 higher in energy than nonclassical 2u and
that the barrier for the 2u Ǟ 4u rearrangement via transi-
tion state TS2/4u is only 3.2 kcalmol–1.
Compound 4u is considerably stabilised by C–B hyper-
conjugation as revealed by an NBO analysis which showed
the formally empty orbitals of the boron atoms to be occu-
pied by 0.145 and 0.163 electrons at the boryl and ring bo-
13C{11B}NMR (125 MHz, CDCl3, –10 °C): δ = 149.2 (s, i-C),
1
ron centres, respectively. This stabilisation is certainly re- 132.8, 131.5 (both s, o- and m-C), 130.0 (d, p-C), 66.4 [d, J(C,H)
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Rearrangement of 1,2,4-Triboracyclopentanes into 2-Boryl-1,3-diboracyclobutanes
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= 107 Hz, BCH], 20.5, 19.5 (both q, Dur-CH3), 13.9 [br. q, 1J(C,H)
= 120 Hz, BCH3], 2.8 (q, SiMe3) ppm. 11B NMR (96 MHz, CDCl3,
25 °C): δ = 104 (2B), 88 (1B) ppm.
constitution. Yield: 1.97 g (35 %) of white 2b, m.p. 128 °C. 1H
NMR (500 MHz, CDCl3, 25 °C): δ = 7.5 (br., BHB, located by 1H/
1H COSY to d at 1.43), 7.5, 7.4 (br.s, 6 H in total, Ph–H and B–
H–B), 7.17, 6.91 (both s, 1 H each, Dur-H), 2.61, 2.32, 2.28, 2.20,
4-Phenyl-1,2-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-trans-3,5-bis(trimeth-
ylsilyl)-1,2,4-triboracyclopentane (1b): PhLi solution (cyclohexane/
Et2O, 1.85 , 2.05 mL) was added to a solution of 1d (1.91 g,
3.8 mmol) in Et2O (50 mL) at –78 °C. The mixture was allowed to
reach room temperature over 2 h and stirring was continued for 1 h
at this temperature. The solvents were removed in vacuo and the
residue was digested with pentane (50 mL). The salts were sepa-
rated from the deep yellow solution by filtration with a reversed
frit and washed with pentane (10 mL). The filtrates were reduced
to dryness in vacuo yielding spectroscopically pure 1b. Yield: 2.04 g
(99%) of deep yellow 1b, m.p. 99 °C (dec.). For spectroscopic data
see ref.[6]
3
2.2 (br.), 1.7 (br) (all s, 24 H in total, Dur-Me), 1.43 [d, J(H,H) =
4.6 Hz, 1 H, CHSi, identified by a cross peak to δ13C = 26.8 in a
C,H correlated spectrum], 0.09, –0.12 (both s, 9 H each,
SiMe3) ppm. 1H NMR (500 MHz, CDCl3, –40 °C) 7.7–7.6 (br., 6
H in total, Ph–H and B–H–B), 7.24, 6.99 (both s, 1 H each, Dur-
H), 2.65, 2.36, 2.31, 2.29, 2.28, 2.18, 2.02, 1.67 (all s, 3 H each,
Dur-Me), 1.51 [d, 3J(H,H) = 4.6 Hz, 1 H, CHSi], 0.09, –0.12 (both
s, 9 H each, SiMe3) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ =
146.8 (br. s, i-C), 140.8, 139.8, 134.0, 133.5, 132.9, 132.5 (all s, Dur-
o- and m-C), 136.7, 127.3 (both d, Ph-o- or m-C), 135.5, 130.0
(both d, Dur-p-C), 128.3 (d, Ph-p-C), 71.9 (br. s, B2C), 26.8 [d,
1J(C,H) = 113 Hz, B2CH], 21.1, 20.3, 20.2, 20.0, 19.55, 19.50 (all
q, Dur-CH3), 1.6, 0.8 ppm (both q, SiMe3), Ph-i-C and one Dur-i-
C not observed. 13C NMR (125 MHz, CDCl3, –40 °C): δ = 146.5,
139.2 (both br., Dur-i-CЈs), 141.0, 139.7, 133.9, 133.4, 133.0,. 132.9,
132.6, 132.3 (Dur-o- and m-CЈs), 136.6, 127.2 (br., Ph-o- or m-CЈs),
135.5, 129.7 (Dur-p-CЈs), 133.1 (br., Ph-i-C), 128.2 (Ph-p-C), 70.7
(B2C), 26.4 (B2CH), 21.0, 20.6, 20.4, 20.3, 20.0, 19.9, 19.6, 19.5
(Dur-CH3), 1.4, 0.6 (SiMe3) ppm. 11B NMR (160 MHz, CDCl3,
25 °C): δ = 57, 42, –2 ppm (each 1B).
4-Chloro-1,2-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-trans-3,5-bis(trimeth-
ylsilyl)-1,2,4-triboracyclopentane (1d): B(OMe)3 (9.15 g, 88.0 mmol,
10 mL) was added slowly by syringe to a suspension of dilithium-
2,3-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-1,4-bis(trimethylsilyl)-2,3-di-
boratabuta-1,3-diene[6] (13.4 g, 21.5 mmol) in pentane (200 mL)
at –78 °C. The mixture was allowed to reach room temperature
during which time it gradually became yellow. Stirring was contin-
ued overnight and the solvents were then removed in vacuo. The
viscous residue was thoroughly dried in vacuo for 2 h and was then
digested with pentane (200 mL) and cooled again to –78 °C. After
addition of BCl3 (30 g, 256 mmol) with a syringe, the mixture was
warmed to room temperature over 2 h and stirring was continued
for 30 min at this temperature. All volatile materials were then re-
moved in vacuo and the solid residue digested with pentane
(200 mL). The salts were filtered through a reversed frit and washed
with pentane (2×20 mL). The combined filtrates were reduced to
dryness in vacuo and thoroughly dried thereafter. The deep yellow
residue was of almost spectroscopic purity. Crystallisation from
pentane (50 mL) at –30 °C yielded 1d as yellow crystals. Yield:
8.17 g (75%) of bright-yellow 1d, m.p. 134 °C (dec.). For spectro-
scopic data see ref.[6]
1,2,4-Tris(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-3,5-bis(trimethylsilyl)-
µ(1,2)-hydro-3-dehydro-1,2,4-triboracyclopentane (2c): A mixture of
1d (5.5 g, 10.85 mmol) and lithium powder (0.25 g) in diethyl ether
(70 mL) was stirred at room temperature and monitored frequently
by 11B NMR spectroscopy until the signals of 1d were replaced by
those of 3 at δ = 24 and 21 ppm (about 1 h). After cooling to
–78 °C, duryllithium (1.53 g, 10.85 mmol) was added with stirring.
Formation of the dianion of 1c during warming to room tempera-
ture was indicated by the presence of a 11B NMR signal at δ =
–16 ppm. After disappearance of the signals of 3 and cooling to
–78 °C, 2,3-dibromo-2,3-dimethylbutane (2.6 g, 10.85 mmol) was
added and the mixture allowed to warm to room temperature with
stirring for an additional 1 h. 11B NMR signals at δ = 97 and
80 ppm unambiguously indicated the formation of 1c. All volatile
materials were removed in vacuo and the residue was dissolved in
pentane (50 mL). The salts were separated by filtration with a re-
versed frit and washed further with pentane (10 mL). The filtrate
was reduced to dryness in vacuo yielding yellow 2c which could be
crystallised at –30 °C from pentane. Yield: 6.2 g (84%) of yellow
2c, m.p. 134 °C. 1H NMR (500 MHz, CD2Cl2, 25 °C): δ = 7.87 (br.,
1 H, BHB), 7.02, 6.98 (both s, 1H and 2 H, respectively, p-H), 2.30,
2.26, 2.20, 2.10, 1.95 (all s, 36 H in total, Dur-Me), 1.17 (d, 1J =
1-Methyl-2,4-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-3,5-bis(trimethylsi-
lyl)-µ(1,2)-hydro-3-dehydro-1,2,4-triboracyclopentane (2a): A solu-
tion of freshly prepared 1a (3.00 g, 6.17 mmol) in CDCl3 (20 mL)
was stored for 4 d at room temperature and the solution was moni-
tored frequently by 1H NMR spectroscopy. After completion of
the reaction all volatile materials were removed in vacuo. The white
residue consisted of essentially pure 2a which could be crystallised
at –30° from toluene. Yield: 2.97 g (99%) of white 2a, m.p. 113 °C.
1H NMR (500 MHz, CDCl3, –40 °C): δ = 7.21, 6.99 (both s, 1 H
each, Dur-H), 6.72 (br. s, 1 H, B–H–B), 2.49, 2.43, 2.37, 2.31, 2.28,
2.19 (all s, 24 H in total, Dur-CH3), 1.10 (s, 1 H, CHSi), 0.66 (s, 3
H, BCH3), 0.01, –0.26 (both s, 9 H each, SiMe3) ppm. 13C NMR
(125 MHz, CDCl3, –40 °C): δ = 146.7, 133.9 (each br. s, i-C), 140.0,
138.4, 134.0, 133.1, 133.05, 132.9, 132.8, 131.8 (each s, o- and m-
C), 134.9, 129.6 (both d, p-C), 74.6 (s, B2C), 27.6 [d, 1J(C,H) =
116 Hz, B2CH], 20.6, 20.5, 20.3, 20.1, 19.6, 19.3 (all q, Dur-CH3),
4.9 [br. q, 1J(C,H) = 117 Hz, BCH3], 0.9, 0.2 (both q, SiMe3) ppm.
11B NMR (96 MHz, CDCl3, 25 °C): δ = 56, 48, 0 (each 1B) ppm.
6.2 Hz, 1 H, CHSi), 0.06, –0.15 (both s, 9 H each, SiMe3) ppm. 13
C
NMR (125 MHz, CD2Cl2, 25 °C): δ = 143.4 (br. s, i-C), 136.9,
133.9, 133.6, 133.4 (all s, Dur-o- and m-C), 132.6, 131.0 (all d, 1C
and 2C, p-C), 102.3 (br. s, B2C), 25.0 (br. B2CH), 21.1, 19.9, 19.7,
19.6 (all q, Dur-CH3), 1.6, 0.9 (both q, SiMe3) ppm. 13C NMR
(125 MHz, CD2Cl2, –90 °C): δ = 149.0, 146.0, 142.3, 137.5, 136.0,
135.6, 135.0, 134.5, 133.3,. 132.5, 132.1, 131.6 (o- m- and i-CЈs),
131.6, 130.1, 129.1 (p-C), 89.1 (B2C), 21.6 (B2CH), 22.2, 21.6, 21,4,
20.3, 20.2, 20.1, 19.1, 18.7, 18.1 (Dur-CH3), –0.01, –0.21
(SiMe3) ppm. 11B NMR (96 MHz, CDCl3, 25 °C): δ = ca. 47 (2B),
ca. 30 ppm (1B).
1-Phenyl-2,4-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-3,5-bis(trimethylsi-
lyl)-µ(1,2)-hydro-3-dehydro-1,2,4-triboracyclopentane (2b): A solu-
tion of 1b (5.67 g, 10.3 mmol) in CDCl3 (25 mL) was stored for 9 d X-ray Crystal Structure Analyses of 1a, 2a, 2b and 2c·pentane: Sin-
at room temperature and was monitored frequently by 1H NMR
spectroscopy. All volatile materials were then removed in vacuo and
the residue was dissolved in a 1:1-mixture of Et2O and pentane
(30 mL). After four days at –30 °C 2b precipitated as a white solid
which was contaminated with approx. 5% of a product of unknown
gle crystals were grown from toluene/Et2O at 0 °C for 2a and pen-
tane at –30 °C for 2b and 2c. Those of 1a were obtained from di-
ethyl ether at –30 °C. The crystals were investigated on imaging
plate systems (IPDS Stoe) using graphite monochromated Mo-Kα
radiation at 193 K. The space groups were determined from the
Eur. J. Inorg. Chem. 2005, 4078–4085
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4083

D. Scheschkewitz, P. Amseis, G. Geiseler, W. Massa, M. Hofmann, A. Berndt
Table 3. Crystal and experimental data for the structure determinations of 1a, 2a, 2b and 2c·pentane.
FULL PAPER
1a
2a
2b
2c·pentane
Empirical formula
Formula mass
Measuring temperature
Crystal system
C29H49B3Si2
486.29
193 K
monoclinic
P21/c
C29H49B3Si2
486.29
193 K
C34H51B3Si2
548.36
193 K
monoclinic
P21/c
C43H71B3Si2
676.61
193 K
triclinic
triclinic
¯
¯
Space group
P1
P1
Unit cell
a
b
c
α
β
γ
[pm]
[pm]
[pm]
[°]
[°]
[°]
1187.1(1)
1427.7(1)
1905.1(1)
90
104.45(1)
90
3126.7(4)
4
1.033
877.8(1)
1131.1(2)
1725.1(3)
96.37(1)
97.65(1)
109.46(1)
1578.3(4)
2
2036.4(1)
1121.6(1)
1596.7(1)
90
110.49(1)
90
3416.2(4)
4
1.066
15.985(2)
16.758(2)
17.223(2)
77.21(1)
79.31(2)
89.87(2)
4417.5(9)
4
Volume [Å3]
Z
Calculated density [gcm–3]
Absorption coefficient [mm–1]
F(000)
1.023
0.127
532
1.017
0.107
1488
0.128
1064
0.124
1192
crystal size [mm]
Θmax [°]
Index range
Scan type
No. of reflections
0.55×0.40×0.10
25.02
–14/14, –16/16, –22/22
ω-scans
23617
0.30×0.30×0.05
24.97
–10/10, –13/13, –20/20
ω-scans
0.30×0.25×0.15
25.98
–25/24, –13/13, –19/19
φ-scans
23778
0.45×0.30×0.05
25.96
–19/19, –20/20, –21/21
φ-scans
14411
42952
Unique refl., (Rint
)
5467 (0.0399)
4529
331, 16.5
1.071
5408 (0.0905)
2878
330, 16.4
0.904
6647 (0.0624)
3669
374, 17.8
0.814
16051 (0.1418)
4713
916, 17.5
0.852
Observed refl. [I Ͼ 2σ(I)]
Parameters, data/param. ratio
Goodness-of-fit (F2)
R [I Ͼ 2σ(I)]
0.0375
0.0588
0.0403
0.0771
wR2 (all refl.)
0.1140
+0.173/–0.214
0.1572
+0.303/–0.234
0.0879
+0.259/–0.212
0.1829
+0.462/–0.259
Largest diff. peak/hole [eÅ–3]
systematic absences and intensity statistics, no absorption correc-
tions were applied. The structures were solved by direct methods
and refined against all F2 data using full-matrix least-squares and
difference Fourier techniques [SHELX programmes[10,11]]. Most hy-
drogen atoms were kept riding in calculated positions with isotropic
displacement factors taken as 1.2 times (1.5 times for CH3) the Ueq
value of the corresponding C atom. The hydrogen atoms at the
central ring were located and refined with individual isotropic dis-
placement factors. For all heavier atoms, anisotropic displacement
parameters were used. The structure of 2c·pentane showed two in-
dependent molecules and, in addition, two strongly vibrating or
disordered pentane molecules in the asymmetric unit. The solvent
molecules were refined with restraints in geometry and in the an-
isotropy of the displacement factors. Due to this problem and the
weak data set [29% of reflections Ͼ 4σ(F) only] the precision of
the structural data is reduced. Details of the experimental and crys-
tal data are summarised in Table 3. Crystallographic data (exclud-
ing structure factors) for the structures reported in this paper have
been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication no. CCDC-245202 (for 1a), -245204
(for 2a), -245203 (for 2b) and -245205 (for 2c). Copies of the data
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [Fax: +44-1223-336-033; E-mail:
deposit@ccdc.cam.ac.uk]
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[7]
All structures were optimized at the MP2(fc)/6-311+G** level
and relative energies were determined at the MP4SDTQ/6-
311+G**//MP2(fc)/6-311+G** + 0.93 ZPE[MP2(fc)/6-
311+G**] level using Gaussian 98: M. J. Frisch, G. W. Trucks,
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Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft
(SPP Polyeder) and the Fonds der Chemischen Industrie.
4084
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2005, 4078–4085

Rearrangement of 1,2,4-Triboracyclopentanes into 2-Boryl-1,3-diboracyclobutanes
FULL PAPER
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-Gor-
don, E. S. Replogle, J. A. Pople, Gaussian 98, Revision A.6,
Gaussian, Inc., Pittsburgh PA, 1998.
[9] At the MP2/6-31G* level nonclassical VIu is more stable than
classical III by 2.5 kcalmol–1: ref.[2]
.
[10] G. M. Sheldrick, SHELXS-97, Program for the Solution of
Crystal Structures, University of Göttingen, 1997.
[11] G. M. Sheldrick, SHELXL-97, Program for the Refinement of
Crystal Structures, University of Göttingen, 1997.
Received: March 31, 2005
[8] M. Unverzagt, G. Subramanian, M. Hofmann, P. von Ragué
Schleyer, S. Berger, K. Harms, G. Geiseler, W. Massa, A.
Berndt, Angew. Chem. 1997, 109, 1567–1569; Angew. Chem.
Int. Ed. Engl. Ed. 1997, 36, 1469–1472.
Published Online: August 17, 2005
Eur. J. Inorg. Chem. 2005, 4078–4085
www.eurjic.org
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4085