Syntheses and reactions of a stable 1,2-dichloro-1,2-diborolane and aromatic tetraboranes

Article abstract of DOI:10.1002/ejic.200400078

The 1,2-dichloro-1,2-diborolane 1b was isolated in good yield after the reaction of the 1,2-bis(dimethylamino)-1,2-diborolane 2c with BCl₃. We prepared 2c from 1,2-bis(dimethylamino)-1,2-dichlorodiborane(4) (3) and TMEDA-free [1,3-bis(trimethylsilyl)allyl]lithium (5a·Li), which is accessible by deprotonation of 1,3-bis(trimethylsilyl)propene (4a) with n-butyllithium in diethyl ether. Upon its reaction with lithium in THF, the 1-allyl-2-chlorodiborane(4) 6a obtained in the first step undergoes an unprecedented ring closure with 1,2-migration of a trimethylsilyl substituent to yield the boron-stabilized carbanion 7a. Reaction of 7a with either chlorotrimethylsilane or HCl gives 2c or 2d, respectively. The latter compound cannot be transformed into an isolable 1,2-dichloro-1,2-diborolane; this observation demonstrates that a trimethylsilyl substituent next to each boron atom is essential for the stability of 1,2-dichloro-1,2-diborolanes. Compound 1b is a versatile starting material, inter alia for the syntheses of the aromatic tetraboranes(6) cis-11 and trans-13 and the aromatic tetraborane(4) cis-12. The isomerization cis-11 → trans-13 can be explained on the basis of reactions occurring under the conservation of two-electron aromaticity. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400078

FULL PAPER
Syntheses and Reactions of a Stable 1,2-Dichloro-1,2-diborolane and Aromatic
Tetraboranes
Carsten Präsang,[a] Yüksel Sahin,[a] Matthias Hofmann,[b] Gertraud Geiseler,[a]
Werner Massa,[a] and Armin Berndt*[a]
Keywords: Aromatics / Boranes / Carbanions / Cyclizations / Heterocycles / Rearrangements
The 1,2-dichloro-1,2-diborolane 1b was isolated in good
yield after the reaction of the 1,2-bis(dimethylamino)-1,2-di-
borolane 2c with BCl3. We prepared 2c from 1,2-bis(dimeth-
compound cannot be transformed into an isolable 1,2-
dichloro-1,2-diborolane; this observation demonstrates that a
trimethylsilyl substituent next to each boron atom is essential
ylamino)-1,2-dichlorodiborane(4) (3) and TMEDA-free [1,3- for the stability of 1,2-dichloro-1,2-diborolanes. Compound
bis(trimethylsilyl)allyl]lithium (5a·Li), which is accessible by
deprotonation of 1,3-bis(trimethylsilyl)propene (4a) with n-
butyllithium in diethyl ether. Upon its reaction with lithium
in THF, the 1-allyl-2-chlorodiborane(4) 6a obtained in the
first step undergoes an unprecedented ring closure with 1,2-
migration of a trimethylsilyl substituent to yield the boron-
stabilized carbanion 7a. Reaction of 7a with either chlorotri-
methylsilane or HCl gives 2c or 2d, respectively. The latter
1b is a versatile starting material, inter alia for the syntheses
of the aromatic tetraboranes(6) cis-11 and trans-13 and the
aromatic tetraborane(4) cis-12. The isomerization cis-11 Ǟ
trans-13 can be explained on the basis of reactions occurring
under the conservation of two-electron aromaticity.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
In this paper we describe the synthesis of the stable 1,2-
dichloro-1,2-diborolane 1b (Scheme 3) via the new 1,2-di-
amino-1,2-diborolane 2c (Scheme 2). Compound 1b turned
out to be a versatile starting material, inter alia for the
syntheses of the aromatic tetraboranes cis-11,[5] cis-12,[6]
and trans-13[5] as well as for triboracyclopropanates[7] and
a pentaborane(7).[6]
1,2-Dichloro-1,2-diborolanes appear to be versatile start-
ing materials for the syntheses of 1,2-diborolanes that lack
strong donor substituents at the boron centers, but only 1,2-
diborolanes possessing such donors, i.e., 1,2-diamino-1,2-
diborolanes, are known experimentally.[1Ϫ3] Computations
have revealed a strong tendency for the unsubstituted 1,2-
diborolane to isomerize to nonclassical forms that have
two-electron homoaromatic structures.[4] Attempts to pre-
pare 1,2-dichloro-1,2-diborolane (1a) by reaction of the 1,2-
Syntheses
The TMEDA-free allyllithium compounds 5a·Li and
diamino-1,2-diborolane 2a with BCl3 were unsuccessful,[2] 5b·Li were prepared by deprotonation of 1,3-bis(silyl)prop-
as were efforts to obtain 2b by reaction of 1,2-dichloro-1,2- enes 4a[8,9] and 4b[10] with n-butyllithium in diethyl ether at
bis(dimethylamino)diborane(4) (3) with 1,3-dilithio-2-meth- room temperature. Compound 4b was synthesized from 3-
ylenepropane (Scheme 1).[3]
[dimethyl(phenyl)silyl]propene,[11] which was deprotonated
Scheme 1
in the same way and then treated with chlorodimethyl-
(phenyl)silane.
[a]
Fachbereich Chemie der Philipps-Universität Marburg,
Hans-Meerwein-Strasse, 35032 Marburg
Fax: (internat.) ϩ 49-(0)6421-2828917
E-mail: berndt@chemie.uni-marburg.de
The bis(trimethylsilyl)propene 4a is deprotonated usually
in the presence of N,N,N,N-tetramethylethylenediamine
(TMEDA), THF, or HMPA.[8,12Ϫ14] The deprotonation
[b]
Anorganisch-Chemisches Institut, Universität Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
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Scheme 2
Scheme 3
with n-butyllithium in diethyl ether requires a reaction 6b even when stoichiometric amounts of lithium naph-
time of 8 d, but the workup of the products obtained thalenide were used. The constitution of 7a is in agreement
after the reaction of lithiopropene 5a·Li with 1,2-dichloro- with all the NMR spectroscopic data and was confirmed
1,2-bis(dimethylamino)diborane(4) (3) is considerably eas- definitely by an X-ray crystal structure analysis (see below).
ier and yields of allylchlorodiboranes(4) 6a,b are much While the methyl groups of the dimethylamino substituent
higher than they are when using the lithium salts of 5a,b of the uncharged boron atom of 7a show two signals in the
containing TMEDA. The 1-allyl-2-chlorodiboranes(4) 6a,b 1H and 13C NMR spectra (300 K, 300/75 MHz), those of
thus obtained are mixtures of (E) and (Z) stereoisomers, the charged boron atom are averaged, which indicates a less
which could not be separated by distillation. The consti- hindered rotation.
1
tutions of (E/Z)-6a,b were deduced from their H and 13C
During the formation of 7a from (E/Z)-6a, a CϪB bond,
NMR spectra and confirmed by the X-ray crystal structure as well as a boron-stabilized carbanion, is formed, and a
analysis of the product obtained by reaction of the former trimethylsilyl group must migrate from a carbon atom to
with lithium in THF (see below). Nearly identical coupling the neighboring carbon atom, which, for 6a, does not pos-
constants J(1H,1H) of the alkene moieties of the isomers sess a silyl substituent. A mechanism for this unprecedented
prevented assignments of configurations by means of 1H ring closure is proposed below.
NMR spectroscopy. Probably, the main products (6a: 85%;
6b: 90%) are the (E) isomers because of their lower degree silane or HCl gave 1,2-diborolanes 2c and 2d, respectively.
of steric hindrance when compared to the (Z) isomers. Their constitutions and stereochemistries are in agreement
The reaction of the anion 7a with either chlorotrimethyl-
1
Reaction of (E/Z)-6a with lithium powder in THF at with all of their NMR spectroscopic data. The H NMR
room temperature yielded 1,2-diborolan-3-ide 7a as the sole spectrum of 2c exhibits two signals for the silyl groups (2:1),
product. This ring-closing reaction was not observed with the aminomethyl groups (1:1), and the ring protons (2:1; d
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Stable 1,2-Dichloro-1,2-diborolane and Aromatic Tetraboranes
FULL PAPER
and t, respectively). The crude product of 2c was used for
One or both chlorine atoms in 1b can be exchanged with
subsequent reactions without further purification because it amino groups by adding stoichiometric amounts of silylam-
contained very few impurities after removal of the volatile ines. In this way, we synthesized the monoaminodiborol-
components and filtration.
anes 10a,b and the diaminodiborolanes 2c,e,f. Reaction of
Reaction of 7a with 1,1,1-trifluoro-2-iodoethane did not 10a with lithium powder in diethyl ether did not yield a
yield the expected 3-iodo-1,2-diborolane after warming to tetraborane(6) by dimerization,[5] but gave the 1,2-diboro-
room temperature, but, instead, the elimination product di- lan-3-ide 7b, as proven by its X-ray structural analysis. This
boracyclopentene 8. The constitution of compound 8 was transformation, like that of 6a to 7a, requires the 1,2-mi-
derived from its one- and two-dimensional 1H, 11B, and 13
NMR spectra.
C
gration of a trimethylsilyl substituent (here to a neighboring
boron atom) and the generation of a boron-stabilized car-
After the addition of ca. 4 equiv. of neat boron trichlo- banion. The NMR spectroscopic data of 7b are similar to
ride to a solution of 2c in n-pentane at Ϫ78 °C, the solution those of 7a, but some of the chemical shifts differ distinctly.
was warmed to ambient temperature during which time The stronger acceptor in 7b [B(SiMe3)], when compared to
both amino groups were replaced cleanly by chlorine atoms. B(NMe2) in 7a, causes a downfield shift of the signals of
While small-scale experiments yielded nearly pure 1b, the the sp2-hybridized carbon atom and its proton relative to
1
crude products of experiments starting with Ͼ 10 g of 2c those signals for 7a (13C: δ ϭ 149.2 vs. 109.0 ppm; H: δ ϭ
had to be crystallized from n-pentane at Ϫ30 °C to obtain 6.57 vs. 4.58 ppm). The boron atom bearing the trimethyl-
satisfactory yields (ca. 68%) of pure 1b. Diborolane 2d, silyl group, however, is more shielded (δ ϭ 33 vs. 38 ppm),
which lacks one trimethylsilyl group when compared to 2c, even though it lacks a second π-donating group. Attempts
could not be chlorinated in a similar manner. Reaction with to let 7b react with electrophiles (MeI, Me3SiCl, Me3SnCl)
BCl3 resulted in the recovery of 2d or led to an inseparable to yield a B-trimethylsilyldiborolane were unsuccessful. The
mixture of products containing varying amounts of start- mixtures of products obtained obviously contained no com-
ing material.
pounds featuring a BϪB(SiMe3)C moiety, which would re-
The diborolane 1b reacts with bis(trimethylsilyl)ethyne at sult in a 11B NMR signal for a strongly deshielded boron
room temperature to form the 3,7-diboracycloheptene 9. Its center.[17]
constitution was confirmed by NMR spectroscopy experi-
Reactions of 1b with K2.8Na in n-pentane at room tem-
ments. Compound 9 displays three signals for the trimethyl- perature yielded the nonclassical dichlorotetraborane(6) cis-
silyl groups (ratio: 2:1:2) and two for the methylene groups 11 (Scheme 4).[5,18,19] As seen from its classical isomer 11*,
(2:1; d and t). Formation of the isomeric 1-methylene-2,6- there are only six electrons to connect the four centers of
diboracyclohexane, i.e., the product of the often observed the B4 ring of cis-11. Four of them are used for two three-
1,1-diborylation of bis(trimethylsilyl)ethyne,[15,16] was ruled center two-electron (3c2e) σ-bonds and the remaining two
out by 13C{11B} NMR spectroscopy experiments. The sig- for a 4c2e π-bond.[5] Thus, cis-11 is a 2e-aromatic system
nals of the boron-bound carbon atoms at δ ϭ 181.2 and having a nonclassical σ-skeleton. Computations for model
34.8 ppm sharpened considerably, but no sharp signal ap- molecules show the energetic advantage of 2e-aromatic
pears for a tricoordinated carbon atom that is not bound compounds of type 11 over orthogonal classical isomers,
to boron atoms (i.e., for an Si2CϭCB2 moiety). Similar 1,2- which benefit only from twofold BϪB hyperconjugation
diborylations of ethynes are known to occur from 1,2-diam- (see below). The aromatic tetraborane(4) cis-12 was ob-
ino-1,2-diborolanes, which, however, require BF3·OEt2 as tained by reaction of cis-11 with 2 equiv. of lithium naph-
catalyst and elevated temperatures.[1]
thalenide at low temperatures.[6] Reduction of cis-12 to its
Scheme 4
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FULL PAPER
dianion and a subsequent reaction with either HCl or
methyl iodide leads to the tetraborane(6) cis-13 or cis-14,
respectively.[20] Surprisingly, the stereoisomer trans-13 was
obtained by exchange of the Cl atoms of cis-11 for hydrogen
by use of sodium triethylhydroborate.[5] A reasonable
mechanism for this stereoisomerization is presented below.
Crystal Structures
The lithium cations of all the derivatives of allyllithium
analyzed by crystal structure determination so far are coor-
dinated by ligands like TMEDA or diethyl ether.[14,21Ϫ24]
The structure of 5b·Li is the first one of a solvent-free allyl-
lithium derivative. Attempts to obtain single crystals of
5a·Li were unsuccessful: it is insoluble, even in boiling para-
xylene, and it decomposed in boiling mesitylene.
Figure 2. Perspective view along the c axis of the unit cell of 5b·Li
having alternating clockwise (Li1) and anticlockwise (Li2) spiral
chains
The 5b·Li complex exhibits a polymeric structure in the
solid state with two independent spiral LiϪallylϪLi chains
parallel to the c axis (Figure 1). According to the space
group I41, one spiral chain with Li1 turns around clockwise
(41 screw axis at 1/4, 1/4, z, 90° rotation of subsequent allyl
groups); the second one with Li2 turns anticlockwise (43
screw axis at 3/4, 1/4, z). By the twofold axis in 0.5,0.5,z, a
second pair of equivalent chains is generated in 3/4,1/4,z
(Li2) and 3/4,3/4,z (Li1) leading to a tetragonal chain pack-
ing of alternating 41 and 43 spirals (Figure 2). Figure 1 dis-
plays a section of the Li1 chain that indicates the η3-con-
nection of the allyl moieties; the C3 planes are located at an
angle of 60.4(3)° [61.7(3)° for the Li2 chain] with respect to
While the differences in the C3 moieties are not worth
mentioning, the absence of an additional ligand coordinat-
ing the cation in 5b·Li has a distinct effect, unsurprisingly,
on the CϪLi distances. In (1,3-diphenylallyl)lithium·OEt2,
the cation is ca. 10 to 20 pm more remote from the carbon
atoms [CϪLi: 230(3), 232(3), 248(3), 240(3), 250(3), 252(3)
pm].
Compound 7a·Li[THF]2 forms a contact ion pair; the
cation is coordinated by N1, B1ϪC1, C1ϪH1, and two
molecules of THF, one of which shows disorder. The bond
B1ϪC1 [146.6(3) pm] is significantly shorter than the
B2ϪC3 bond [160.8(3) pm] because of its double bond
character. This situation results in a long N1ϪB1 bond
[153.1(2) pm] while the N2ϪB2 distance [140.8(2) pm] is in
the typical range of NϪB bonds having a strong π-interac-
tion.
the chain axis. In contrast, the structure of (1,3-
[22]
diphenylallyl)lithium·OEt2
possesses chains having 21-
symmetry (180° rotation of subsequent allyl groups); the
inclination of the allyl planes to the chain axis is similar
(60°).
Like 7a·Li[THF]2, compound 7b·Li[Et2O]2 forms a con-
tact ion pair: the cation is coordinated by C1ϪB1 and two
molecules of diethyl ether in this case. The unit cell contains
two independent molecules of 7b·Li[Et2O]2 with only
slightly different bond lengths and bond angles (the second
set of data is given in brackets in the following discussion).
Complexes 7a·Li[THF]2 and 7b·Li[Et2O]2 are similar with
regard to their B2C3 rings (see Figures 3 and 4); while the
Li cation of the latter is placed approximately above the
CϪB double bond {B1ϪLi1: 238.9(5) pm [238.3(4)];
C1ϪLi1: 239.7(5) pm [243.1(4)]; B2ϪB1ϪC1ϪLi1:
101.9(2)° [101.0(2)°]}, the cation in 7a·Li[THF]2 is located
nearer to the ring plain because of the coordination to the
amino group [Li1ϪC1ϪB1ϪB2 146.5 (2)].
In the structure of 1b (Figure 5), the BϪB [169.2(4) pm]
and BϪCl [175.9(3), 176.0(3) pm] bond lengths are quite
similar to those found in 1,2-dichloro-1,2-dimesityldibor-
ane(4)[25] [168.0(6), 177.0(5) and 177.4(4) pm, respectively].
Compound 1b deviates from Cs symmetry in the solid state
mainly because the orientations of the SiMe3 groups at C1
and C3 break the symmetry of a mirror plane, but also
because of slight nonplanarity around the BϪB moiety
[Cl1ϪB1ϪB2ϪCl2: 5.8(3)°] and the inequivalent BϪC and
Figure 1. Section of the spiral chain with Li1 in the crystal structure
of 5b·Li; selected bond lengths [pm] and angles [°], the correspond-
ing values for the second spiral chain with Li2 are provided in
brackets (same e.s.d.s): C1ϪC2 140.6(4) [140.2], C2ϪC3 140.0(4)
[139.9], C1ϪLi1 231.4(6) [233.9], C2ϪLi1 215.8(6) [215.6], C3ϪLi1
231.8(6) [228.7], C1ЈϪLi1 228.8(6) [229.7], C2ЈϪLi1 219.3(6)
[216.2], C3ЈϪLi1 235.3(6) [229.7]; C1ϪC2ϪC3 129.9(3) [128.9],
C2ϪC1ϪSi1 123.2(2) [124.4], C2ϪC3ϪSi2 123.8(2) [123.5],
Si1ϪC1ϪC2ϪC3 Ϫ176.1(2) [177.9]
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Stable 1,2-Dichloro-1,2-diborolane and Aromatic Tetraboranes
FULL PAPER
CϪC bond lengths (see caption to Figure 5). The short
BϪC [B2ϪC1: 150.1(4) pm; B1ϪC3: 151.6(3) pm; usually
158Ϫ160 pm] and long CϪSi bonds [C1ϪSi1 193.0(2) pm;
C3ϪSi3 192.5(2) pm; usually 186Ϫ187 pm] are mainly the
result of CϪSi hyperconjugation of the boron atoms with
the CϪSi bonds.
Figure 3. Structure of 7a·Li[THF]2 in the crystal (most H atoms
have been omitted for clarity); selected bond lengths [pm] and
angles [°]: B1ϪB2 169.1(3), B1ϪC1 146.6(3), B2ϪC3 160.8(3),
C1ϪC2 152.4(2), C2ϪC3 156.8(3), N1ϪLi1 203.3(4), B1ϪLi1
230.2(4), C1ϪLi1 223.6(4); C1ϪB1ϪB2 103.1(2), C3ϪB2ϪB1
105.1(2), C2ϪC1ϪB1 115.4(2), C2ϪC3ϪB2 104.9(2), sum of
angles (without Li1) C1 359.6, N1 344.7, N2 359.8
Figure 6. Structure of cis-11 in the crystal; selected bond lengths
[pm] and angles [°]: B1ϪB2 182.6(2), B2ϪB3 186.3(2), B3ϪB4
182.2(2), B4ϪB1 186.0(2), B2ϪB4 150.2(2), B1ϪCl1 179.8(2),
B3ϪCl2 179.8(2), B1ϪC1 157.1(2), C1ϪC2 155.6(2), C2ϪC3
157.1(2), C3ϪB2 156.4(2); Cl1ϪB1ϪC1 112.1(1), Cl2ϪB3ϪC4
112.0(1),
C3ϪB2ϪB4
169.4(1),
C6ϪB4ϪB2
170.1(1),
B1ϪB2ϪB4ϪB3 135.2(1)
Compound cis-11 (Figure 6) possesses
a
folded
[135.2(1)°] diamond-like structure that has a remarkably
short diagonal of only 150.2(2) pm. The edges of this dia-
mond are considerably longer [182.2(2) to 186.3(2) pm]
than the BϪB bond in 1b [169.2(4) pm], which indicates
multicenter bonding. The boron centers that carry chloro
substituents show an approximately planar surrounding by
their four neighbors [Cl1ϪB1ϪB4ϪB2: 174.1(1)°;
Cl2ϪB3ϪB2ϪB4: 176.7(1)°]. All trimethylsilyl substituents
at carbon atoms bound to boron atoms lie on the same side
of the B4 ring. We refer to this stereochemistry as ‘‘cis’’ in
this text.
Figure 4. Structure of 7b·Li[Et2O]2 in the crystal; selected bond
lengths [pm] and angles [°], the corresponding values for the second
independent ion pair are provided in brackets: B1ϪB2 171.3(3)
[170.0(4)], B1ϪC1 146.1(3), B2ϪC3 160.1(3), C1ϪC2 151.9(3)
[152.3(3)], C2ϪC3 157.4(3) [156.2(3)], B1ϪSi1 200.9(2) [201.2(2)],
B1ϪLi1 238.9(5) [238.3(4)], C1ϪLi1 239.7(5) [243.1(4)];
C1ϪB1ϪB2 101.4(2) [101.8(2)], C3ϪB2ϪB1 107.2(2) [106.9(2)],
C2ϪC1ϪB1 117.2(2) [116.9(2)], C2ϪC3ϪB2 105.1(2) [104.9(2)];
sum of angles (without Li1) C1 359.6 [359.2], N1 359.1 [359.3]
Figure 7. Structure of cis-14 in the crystal; selected bond lengths
[pm] and angles [°]: B1ϪB2 179.9(3) B2ϪB3 183.0(3), B3ϪB4
180.2(3), B4ϪB1 183.5(3), B2ϪB4 150.1(3), B1ϪC01 164.8(3),
B3ϪC02 160.9(3), B1ϪC1 159.6(2), C1ϪC2 156.1(2), C2ϪC3
157.0(2), C3ϪB2 158.0(2); C01ϪB1ϪC1 112.0(2), C02ϪB3ϪC4
113.0(2),
C3ϪB2ϪB4
169.4(2),
C6ϪB4ϪB2
170.1(2),
Figure 5. Structure of 1b in the crystal; selected bond lengths [pm]
and angles [°]: B1ϪB2 169.2(4), B2ϪC1 150.1(4), C1ϪC2 156.0(3),
C2ϪC3 157.7(3), C3ϪB1 151.6(3), B1ϪCl1 176.0(3), B2ϪCl2
175.9(3), C1ϪSi1 193.0(2), C2ϪSi2 189.9(2), C3ϪSi3 192.5(2);
B1ϪB2ϪC1 107.2(2), B1ϪB2ϪCl2 128.3(2), B2ϪB1ϪC3 106.4(2),
B2ϪB1ϪCl1 128.3(2), B1ϪC3ϪC2 107.4(2), C3ϪC2ϪC1 109.8(2),
C2ϪC1ϪB2 107.5(2), C1ϪB2ϪB1ϪC3 4.5(2)
B1ϪB2ϪB4ϪB3 140.2(1)
Compound cis-14 (Figure 7) is isostructural to cis-11 and
has a similar folding angle of 140.2(1)° for the B4 diamond.
Its diagonal is short [150.1(3) pm], and the edges of the
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diamond are a bit shorter [179.9(3) to 183.5(3) pm] than this substituent next to one of the boron centers were un-
those in cis-11. The boron centers that carry methyl sub- successful. This result explains the failure of earlier efforts
stituents are almost planar-tetracoordinate (max. deviations to prepare 1a lacking any substituents.[2]
from best planes: B1,B2,B4,C01,C1, 9.9(2) pm;
B2,B3,B4,C02,C4, 5.3(2) pm].
So far, reaction of metals with 1,2-dichloro-1,2-dibor-
anes(4) having appropriate substituents, like sterically de-
manding amino or tert-butyl substituents, have led to the
elimination of both chlorine atoms and generation of folded
or tetrahedral tetraboranes(4).[28,29] The reductive dimeriz-
ation of 1b to cis-11 is the first example where only one
chlorine atom of each 1,2-dichloro-1,2-diborane(4) is elim-
inated. The dichlorotetraborane(6) cis-11 is a 2e-aromatic
tetraborane(6) that has a nonclassical σ-skeleton. Compu-
tations (B3LYP/6-311ϩG**//B3LYP/6-31G*ϩZPE)[30] of
model molecules lacking silyl substituents show that the
aromatic folded diamond 11u is preferred over the classical
planar 11u*p by 24.7 kcal·molϪ1. The orthogonal 11u*o
(Scheme 6), which benefits from twofold BϪB hyperconjug-
ation, is only 13.2 kcal·molϪ1 lower in energy than 11u*p.
While the cis configuration is retained during the trans-
formations of cis-11 to cis-13 and cis-14 via cis-12 and its
dianion,[20] the stereoisomer trans-13 is obtained upon reac-
tion of cis-11 with 2 equiv. of sodium triethylhydroborate.
All transformations leading to cis-13 occur with conser-
vation of 2e-aromaticity and retention of the stereochem-
istry in the four-membered rings. The inversion of the
stereochemistry of three adjacent centers during the forma-
tion of trans-13 requires opening of one ring and then re-
closure after inversion at these three centers. This process is
possible without loss of 2e-aromaticity by formation of
three-membered aromatic systems as intermediates. The in-
itially formed cis-17 can open to syn-18 with conservation
of 2e-aromaticity. Rotation about the BϪC bond marked
with a curved arrow in Scheme 7 in syn-18 leads to anti-18
in which three trimethylsilyl substituents pointing in op-
posite directions relative to those in syn-18. This stereo-
chemistry is locked by the reaction to form trans-13; thus,
the transformation of cis-11 into trans-13 occurs with con-
servation of 2e-aromaticity in all steps. Our mechanism is
supported by computations that show that the prototype
molecules 17u and 18u are low-energy minima.[31] In ad-
dition, attack of electrophiles at the 2c2e BϪB bond of tri-
boracyclopropanates 18, like that which occurs during the
transformation of anti-18 into trans-13, are well known ex-
perimentally.[7]
Discussion
The ring closure, accompanied by an 1,2-migration of a
trimethylsilyl group, which takes place upon reaction of 6a
with lithium metal and leads to the boron-stabilized car-
banion 7a, is unprecedented to the best of our knowledge.
It may be explained, however, by analogy to reactions of
carbon electrophiles with allylsilanes,[26] by invoking the
borylborenate 15a as an intermediate (Scheme 5). Since
borylborenates are isoelectronic with methyleneboranes[27]
and vinyl cations, their dicoordinate boron center can be
expected to react intramolecularly as a strong electrophile
with the allylsilane moiety to generate a carbocation in the
β-position relative to a silyl substituent. 1,2-Migration of
silyl substituents in cations of this type are well-known pro-
cesses in the chemistry of allylsilanes.[26]
Scheme 5
By analogy, the formation of the boron-stabilized car-
banion 7b upon reaction of 10b with lithium metal can be
rationalized by assuming that the borylborenate 15b is the
intermediate. In this case, the silyl substituent migrates to
the neighboring electrophilic dicoordinate boron center.
Obviously, the first 1,2-dichloro-1,2-diborolane, 1b, owes
its persistence to the trimethylsilyl substituents at all of the
Our assumption that the stereoisomerization does not
carbon atoms. All attempts to generate a derivative lacking take place during the substitution of the first chloride by
Scheme 6
3068
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Eur. J. Inorg. Chem. 2004, 3063Ϫ3073

Stable 1,2-Dichloro-1,2-diborolane and Aromatic Tetraboranes
FULL PAPER
Scheme 7
1,3-Bis[dimethyl(phenyl)silyl]allyllithium (5b·Li): In a similar reac-
tion to the preparation of 5a·Li, 4b (12.0 g, 38.6 mmol) was treated
with n-butyllithium solution (1.6  in hexanes, 26.3 mL 42.1 mmol)
for 4 d. After workup, 5b·Li (8.56 g, 70%) was obtained as a slightly
yellow, pyrophoric powder. 1H NMR (300 MHz, [D8]THF, 300 K):
δ ϭ 0.20 (s, 12 H, SiMe), 2.95 (d, 3JH,H ϭ 16 Hz, 2 H, CHSi), 6.77
hydride, but requires a hydrogen atom for the transform-
ation cis-17 Ǟ syn-18 to be easy, is supported by the follow-
ing observation. Substitution of one chloride in cis-11 for
CH2SiMe3, by reaction with LiCH2SiMe3, occurs with con-
servation of the cis configuration, as is evidenced by the X-
ray structure determination of the obtained monochloro-
tetraborane(6).[32]
3
(t, JH,H ϭ 16 Hz, 1 H, C2CH), 7.15Ϫ7.65 (m, 10 H, Ph) ppm.
Transformation of the versatile 1b into 1,2-diorganyl-1,2-
diborolanes and their spontaneous and thermally induced
reactions will be described in a forthcoming paper.
(E/Z)-1-[1,3-Bis(trimethylsilyl)allyl]-2-chloro-1,2-bis(dimethyl-
amino)diborane(4) (6a): 1,2-Dichloro-1,2-bis(dimethylamino)dibor-
ane(4) (3)[33] (17.2 g, 95.0 mmol) was added dropwise to a solution
of [1,3-bis(trimethylsilyl)allyl]lithium (5a·Li) (18.0 g, 93.6 mmol) in
diethyl ether (150 mL) at Ϫ78 °C. The mixture was warmed to
room temperature and all volatile compounds were evaporated in
vacuo. After filtration and washing the residue with n-pentane (2
ϫ 50 mL), the filtrate was distilled at 1 ϫ 10Ϫ1 mbar to yield
allyldiborane(4) (E/Z)-6a (29.4 g, 95% relative to 3; 85:15) as a col-
orless oil (b.p. 68Ϫ72 °C). (E)-6a: 1H NMR (300 MHz, CDCl3,
300 K): δ ϭ Ϫ0.01, 0.02 (each s, each 9 H, SiMe3), 2.00 (d, 3JH,H ϭ
11 Hz, 1 H, BCHSi), 2.68, 2.69, 2.77, 2.88 (each s, each 3 H, NMe),
Experimental Section
General: All reactions were carried out under argon, using standard
Schlenk techniques. Solvents were dried, distilled, and saturated
with argon. Glassware was dried using a heat gun under high vac-
uum. 1H, 11B, and 13C NMR: Bruker ARX 300 and AMX 500
spectrometers. NMR references: Me4Si, BF3·OEt2.
3
3
5.20 (d, JH,H ϭ 18 Hz, 1 H, CϭCHSi), 6.09 (dd, JH,H ϭ 18,
11 Hz, 1 H, CHϭCHSi) ppm. 11B NMR (96 MHz, CDCl3, 300 K):
δ ϭ 42, 46 ppm. 13C NMR (75 MHz, CDCl3, 300 K): δ ϭ Ϫ1.1,
(E)-1,3-Bis[dimethyl(phenyl)silyl]propene (4b): n-Butyllithium solu-
tion (1.6  in hexanes, 73.1 mL 117 mmol) was added to a solution
of 3-(dimethylphenylsilyl)propene (20.6 g, 116.9 mmol) in n-pen-
tane/diethyl ether (1:1, 100 mL) at 0 °C within 2 h. After the solu-
tion had been stirred at room temperature for 3 d, chlorodimethyl-
(phenyl)silane (20.5 g, 120 mmol) was added at Ϫ78 °C and the
mixture was warmed to room temperature. The organic layer was
washed with water (2 ϫ 200 mL), dried with MgSO4, and distilled
at 1 ϫ 10Ϫ3 mbar. Yield: 29.8 g (82%); colorless oil (b.p. 98 °C).
For spectroscopic data, see ref.[10]
1
Ϫ0.8 (each q, SiMe3), 41.2 (br. d, JC,H ϭ 94 Hz, BCHSi), 37.6,
1
39.3, 42.2, 44.6 (each q, NMe), 123.3 (d, JC,H ϭ 132 Hz, CHϭ
1
CHSi), 149.3 (d, JC,H ϭ 148 Hz, CHϭCHSi) ppm.
(E/Z)-1-{1,3-Bis[dimethyl(phenyl)silyl]allyl}-2-chloro-1,2-bis(di-
methylamino)diborane(4) (6b): (E/Z)-6b was synthesized by a simi-
lar procedure to that of (E/Z)-6a from 3 (4.42 g, 24.44 mmol)
and {1,3-bis[dimethyl(phenyl)silyl]allyl}lithium (5b·Li, 6.9 g,
22.2 mmol). Distillation at 1 ϫ 10Ϫ3 mbar yielded (E/Z)-6b (8.29 g,
82% relative to 3; 9:1) as a colorless oil (b.p. 105Ϫ110 °C). (E)-6b:
1H NMR (300 MHz, CDCl3, 300 K): δ ϭ 0.27, 0.29, 0.37 (each s,
total 12 H, SiMe), 2.28 (d, 3JH,H ϭ 11 Hz, 1 H, BCHSi), 2.35, 2.62,
[1,3-Bis(trimethylsilyl)allyl]lithium (5a·Li): n-Butyllithium solution
(1.6  in hexanes, 45 mL 72 mmol) was added at 0 °C to a solution
of 4a (11.9 g, 64.0 mmol) in n-pentane/diethyl ether (1:1, 100 mL).
After the solution had been stirred at room temperature for 8 d,
all volatile components were evaporated in vacuo and the residue
was suspended in n-pentane (100 mL) and stirred for 2 h. Insoluble
5a·Li was separated using a reverse glass frit and washed with n-
pentane (2 ϫ 30 mL). Yield: 9.6 g (78%); colorless, pyrophoric
3
2.71, 2.85 (each s, each 3 H, NMe), 5.29 (d, JH,H ϭ 18 Hz, 1 H,
3
CHϭCHSi), 6.24 (dd, JH,H ϭ 18, 11 Hz, 1 H, CHϭCHSi),
7.25Ϫ7.60 (m, 10 H, Ph) ppm. 11B NMR (96 MHz, CDCl3, 300 K):
δ ϭ 42 ppm. 13C NMR (75 MHz, CDCl3, 300 K): δ ϭ Ϫ3.4, Ϫ2.5,
1
1
powder. H NMR (200 MHz, [D8]THF, 300 K): δ ϭ Ϫ0.11 (s, 18
Ϫ2.2, Ϫ2.1 (each q, SiMe), 40.9 (br. d, JC,H ϭ 126 Hz, BCHSi),
3
3
1
H, SiMe3), 2.64 (d, JH,H ϭ 16 Hz, 2 H, CHSi), 6.57 (t, JH,H
ϭ
37.6, 39.0, 42.1, 44.5 (each q, NMe), 122.2 (d, JC,H ϭ 136 Hz,
16 Hz, 1 H, C2CH) ppm. 13C NMR (75 MHz, [D8]THF, 300 K): CHϭCHSi), 127.3, 127.6, 128.6, 128.6, 133.8, 134.0 (each d, o-, m-,
1
1
δ ϭ 2.1 (q, SiMe3), 67.3 (d, JC,H ϭ 128 Hz, CHSi), 154.3 (d, p-C), 139.6, 139.9 (each s, i-C), 150.9 (d, JC,H ϭ 150 Hz, CHϭ
1JC,H ϭ 137 Hz, C2CH) ppm.
CHSi) ppm.
Eur. J. Inorg. Chem. 2004, 3063Ϫ3073
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3069

C. Präsang, Y. Sahin, M. Hofmann, G. Geiseler, W. Massa, A. Berndt
FULL PAPER
1
1
Lithium 1,2-Bis(dimethylamino)-4,5-bis(trimethylsilyl)diborolan-3-
ide (7a·Li): Naphthalene (ca. 10 mg) was added to a mixture of
lithium powder (600 mg, 86.4 mmol) in THF (150 mL) and then
the suspension was stirred until it turned dark-green (5Ϫ30 min).
The flask was then placed in a big water bath and allyldiborane(4)
6a (12.0 g, 36.3 mmol) was added within 2 min. After the mixture
had been stirred overnight, all volatile compounds were evaporated
in vacuo and the residue was suspended in n-pentane (150 mL).
After filtration, a stable solution of 7a was obtained, which was
used for further syntheses. Crystallization from n-pentane at Ϫ30
°C yielded 7a·Li[THF]2 (8.76 g, 50%) as colorless crystals (m.p. 43
°C). 1H NMR (300 MHz, C6D6, 300 K): δ ϭ 0.31, 0.33 (each s,
each 9 H, SiMe3), 1.20 (s, 1 H, BCHSi), 1.41 (m, 8 H, THF), 2.08
BCH2), 20.9 (d, JC,H ϭ 120 Hz, C2CHSi), 27.0 (br. d, JC,H ϭ
109 Hz, BCHSi), 40.7, 40.9, 45.1, 45.6 (each q, NMe) ppm.
1,2-Dichloro-3,4,5-tris(trimethylsilyl)-1,2-diborolane (1b): A solu-
tion of 2c (6.8 g, 18.5 mmol) in n-pentane (150 mL) was mixed with
neat BCl3 (8.67 g, 74 mmol, 4.4 equiv.) at Ϫ78 °C and then warmed
to 0 °C within 3 h. All volatile components were evaporated at this
temperature and then the residue was warmed to room temperature
in vacuo and evacuation was continued for 2 h. The residue was
treated with n-pentane (50 mL) and the insoluble byproducts were
filtered off. Evaporation of the solvent yielded 1b (6.2 g, 96%) as a
highly viscous oil, which was essentially pure based on its 1H NMR
spectra. Crystallization from n-pentane (necessary for reactions
starting with more than 10 g of 2c) at Ϫ30 °C gave light-yellow
crystals [68%; m.p. 44 °C (decomposition)]. 1H NMR (300 MHz,
CDCl3, 300 K): δ ϭ Ϫ0.15, 0.16 (each s, total 27 H, SiMe3), 1.92
3
(d, JH,H ϭ 3 Hz, 1 H, C2CHSi), 2.78, 3.03, 3.25 (each s, total 12
H, NMe), 3.48 (m, 8 H, THF), 4.58 (d, JH,H ϭ 3 Hz, 1 H, Bϭ
3
CH) ppm. 11B NMR (96 MHz, C6D6, 300 K): δ ϭ 38, 52 ppm. 13
C
3
3
(t, JH,H ϭ 1 Hz, 1 H, C2CHSi), 2.13 (d, JH,H ϭ 2 Hz, 2 H,
BCHSi) ppm. 11B NMR (96 MHz, CDCl3, 300 K): δ ϭ 84 ppm.
13C NMR (75 MHz, CDCl3, 300 K): δ ϭ Ϫ3.4, 1.0 (each q, SiMe3),
NMR (75 MHz, C6D6, 300 K): δ ϭ Ϫ2.5, 0.8 (each q, SiMe3), 25.4
1
1
(t, THF), 30.4 (br. d, JC,H ϭ 109 Hz, BCHSi), 33.6 (d, JC,H
ϭ
115 Hz, C2CHSi), 42.5, 46.5, 46.8 (each q, NMe), 68.5 (t, THF),
1
1
28.4 (d, JC,H ϭ 124 Hz, C2CHSi), 53.2 (br. d, JC,H ϭ 117 Hz,
1
109.0 (br. d, JC,H ϭ 118 Hz, BϭCH) ppm.
BCHSi) ppm.
3,4-Bis(dimethylamino)-5-(trimethylsilyl)-3,4-diboracyclopentene
(8): 1,1,1-Trifluoro-2-iodoethane (8.19 g, 39.0 mmol) was added to
a solution of 7a (prepared from 12.9 g 6a) in diethyl ether (100 mL)
at Ϫ78 °C. The mixture was warmed to room temperature within
10 min by using a water bath and then distilled at 20 mbar to yield
3,7-Dichloro-1,2,4,5,6-pentakis(trimethylsilyl)-3,7-diboracyclo-
heptene (9): 1b (ca. 4.5 g, 12.8 mmol) dissolved in n-pentane
(50 mL) was treated with bis(trimethylsilyl)ethyne (3.27 g,
19.2 mmol, 1.5 equiv.) at 0 °C. After warming to room temperature,
all volatile components were evaporated in vacuo, to yield 9 (6.35 g,
95%) as a highly viscous oil that contained some minor impurities.
1H NMR (300 MHz, CDCl3, 300 K): δ ϭ 0.10, 0.18, 0.33 (each s,
1
8 (4.33 g) as colorless liquid (b.p. 60Ϫ62 °C). H NMR (300 MHz,
3
CDCl3, 300 K): δ ϭ Ϫ0.08 (s, 9 H, SiMe3), 1.45 (d, JH,H ϭ 3 Hz,
1 H, BCHSi), 2.78, 2.87, 2.95, 2.99 (each s, each 3 H, NMe), 6.13
3
total 45 H, SiMe3), 1.26 (t, JH,H ϭ 7 Hz, 1 H, C2CHSi), 1.43 (d,
3
3
(d, JH,H ϭ 9 Hz, 1 H, BCH), 7.01 (dd, JH,H ϭ 9, 3 Hz, 1 H,
C2CH) ppm. 11B NMR (96 MHz, CDCl3, 300 K): δ ϭ 46, 51 ppm.
13C NMR (75 MHz, CDCl3, 300 K): δ ϭ Ϫ1.3 (q, SiMe3), 34.4 (br.
3JH,H ϭ 7 Hz, 2 H, BCHSi) ppm. 11B NMR (96 MHz, CDCl3,
300 K): δ ϭ 67 ppm. 13C NMR (75 MHz, CDCl3, 300 K): δ ϭ 0.7,
1
1.6, 3.0 (each q, SiMe3), 18.4 (d, JC,H ϭ 127 Hz, C2CHSi), 34.8
1
d, JC,H ϭ 112 Hz, BCHSi), 41.2, 41.4, 44.9, 45.5 (each q, NMe),
137.1 (br. d, JC,H ϭ 149 Hz, BCH), 159.5 (d, JC,H ϭ 152 Hz,
C2CH) ppm.
1
(br. d, JC,H ϭ 112 Hz, BCHSi), 181.2 (br. s, BCSi) ppm.
1
1
1,2-Dipiperidino-3,4,5-tris(trimethylsilyl)-1,2-diborolane (2e): N-
(Trimethylsilyl)piperidine (ca. 4 equiv.) was added to a stirred solu-
tion of 2c (2.2 g, 6.27 mmol) in n-pentane (50 mL) at room tem-
perature. After 24 h, all volatile components were evaporated in
vacuo to yield 2e (2.64 g, 94%) as a slightly yellow solid. 1H NMR
(300 MHz, C6D6, 300 K): δ ϭ 0.26, 0.43 (each s, total 27 H, SiMe3),
1,2-Bis(dimethylamino)-3,4,5-tris(trimethylsilyl)-1,2-diborolane (2c):
A solution of 7a·Li [prepared from 6a (12.0 g, 36.3 mmol)] in n-
pentane/diethyl ether (2:1, 150 mL) was cooled to Ϫ78 °C and
mixed with 1.1 equiv. of chlorotrimethylsilane (4.34 g, 39.9 mmol).
After warming to room temperature, all volatile components were
evaporated in vacuo and the residue was suspended in n-pentane
(100 mL). Filtration and evaporation of the solvent yielded 2c
(12.43 g, 93% relative to 6a) as a highly viscous oil that crystallized
overnight (m.p. 42 °C). Usually, the n-pentane solutions were used
for the synthesis of 1b without further workup. 1H NMR
(300 MHz, CDCl3, 300 K): δ ϭ Ϫ0.15, 0.04 (each s, total 27 H,
SiMe3), 0.64 (d, 3JH,H ϭ 3 Hz, 2 H, BCHSi), 1.46 (t, 3JH,H ϭ 3 Hz,
1 H, C2CHSi), 2.82, 2.90 (each s, each 6 H, NMe) ppm. 11B NMR
(96 MHz, CDCl3, 300 K): δ ϭ 54 ppm. 13C NMR (75 MHz,
3
1.00 (d, JH,H ϭ 5 Hz, 2 H, BCHSi), 1.5Ϫ1.7 (m, 12 H, pip), 1.91
3
(t, JH,H ϭ 5 Hz, 1 H, C2CHSi), 3.3Ϫ3.5 (m, 8 H, pip) ppm. 11B
NMR (96 MHz, C6D6, 300 K): δ ϭ 53 ppm. 13C NMR (75 MHz,
C6D6, 300 K): δ ϭ Ϫ2.6, 2.0 (SiMe3), 22.7 (br., BCHSi), 24.3
(C2CHSi), 25.3, 27.8, 28.2, 51.9, 53.9 (pip) ppm.
1,2-Bis(diethylamino)-3,4,5-tris(trimethylsilyl)-1,2-diborolane (2f):
By a procedure analogous to that for the preparation of 2e, 2f was
synthesized from 2c (4.0 g, 11.39 mmol) and diethyl(trimethyl-
silyl)amine (4 equiv.) to yield 2f (4.59 g, 95%) as a slightly yellow
CDCl3, 300 K): δ ϭ Ϫ3.3, 1.5 (each s, SiMe3), 23.6 (br. d, 1JC,H
123 Hz, BCHSi; d, JC,H ϭ 123 Hz, C2CHSi), 42.7, 45.5 (each q,
ϭ
1
solid. H NMR (300 MHz, CDCl3, 300 K): δ ϭ Ϫ0.14, 0.03 (each
1
3
s, total 27 H, SiMe3), 0.65 (d, JH,H ϭ 3 Hz, 2 H, BCHSi), 1.05 (t,
3
NMe) ppm.
12 H, NCH2CH3), 1.47 (t, JH,H ϭ 3 Hz, 1 H, C2CHSi), 3.22 (q, 8
H, NCH2CH3) ppm. 11B NMR (96 MHz, CDCl3, 300 K): δ ϭ 54
ppm. 13C NMR (75 MHz, CDCl3, 300 K): δ ϭ Ϫ2.8, 1.5 (each q,
SiMe3), 14.6, 15.0 (each q, NCH2CH3), 22.8 (br. d, 1JC,H ϭ 110 Hz,
1,2-Bis(dimethylamino)-3,4-bis(trimethylsilyl)-1,2-diborolane (2d):
Starting from 6a (12.2 g), 2d was synthesized by a similar procedure
to that of 2c from 7a and 6  HCl (1 equiv., diethyl ether). Distil-
lation (10 mbar, 65Ϫ70 °C) yielded 2d (8.53 g, 78%) as a colorless
1
BCHSi), 23.8 (d, JC,H ϭ 122 Hz, C2CHSi), 43.2, 45.8 (each t,
NCH2CH3) ppm.
liquid. 1H NMR (300 MHz, CDCl3, 300 K): δ ϭ Ϫ0.15, 0.13 (each
2
s, each 9 H, SiMe3), 0.76 (s, 1 H, BCHSi), 0.77 (dd, JH,H ϭ 17, 1-Chloro-2-(dimethylamino)-3,4,5-tris(trimethylsilyl)-1,2-diborolane
3JH,H ϭ 4 Hz, 1 H, BCHHЈ), 1.05 (dd, 2JH,H ϭ 17, 3JH,H ϭ 10 Hz,
(10a): Dimethyl(trimethylsilyl)amine (530 mg, 4.52 mmol) was ad-
3
1 H, BCHHЈ), 1.17 (dd, JH,H ϭ 10, 4 Hz, 1 H, C2CHSi), 2.75, ded to a solution of 1b (1.6 g, 4.56 mmol) in n-pentane (50 mL) at
2.79, 2.89, 2.91 (each s, each 3 H, NMe) ppm. 11B NMR (96 MHz,
CDCl3, 300 K): δ ϭ 53 ppm. 13C NMR (75 MHz, CDCl3, 300 K): all the volatile components were evaporated in vacuo and the resi-
δ ϭ Ϫ3.3, Ϫ0.9 (each q, SiMe3), 20.5 (br. pseudo t, 1JC,H ϭ 117 Hz,
due was treated with n-pentane (25 mL) and filtered. Crystalliza-
room temperature. After the mixture had been stirred overnight,
3070
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Eur. J. Inorg. Chem. 2004, 3063Ϫ3073

Stable 1,2-Dichloro-1,2-diborolane and Aromatic Tetraboranes
FULL PAPER
tion from n-pentane (5 mL) at Ϫ30 °C yielded 10a (990 mg, 60%) (50 mL) and then the solution was separated from the solids using
as slightly yellow crystals [m.p. 52 °C (some decomposition)]. 1H a pipette. After washing the solids with additional n-pentane
NMR (300 MHz, C6D6, 300 K): δ ϭ Ϫ0.05, 0.25, 0.31 (each s, each (25 mL), the combined solutions were reduced to a volume of
3
9 H, SiMe3), 1.29 (s, 1 H, NBCHSi), 1.67 (d, JH,H ϭ 2 Hz, 1 H,
20 mL. Crystallization at Ϫ30 °C yielded trans-13 (1.02 g, 60%) in
3
ClBCHSi), 1.79 (d, JH,H ϭ 2 Hz, 1 H, C2CHSi), 2.62, 3.04 (each
three fractions. Spectroscopic data: see ref.[5]
s, each 3 H, NMe) ppm. 11B NMR (96 MHz, C6D6, 300 K): δ ϭ
48, 91 ppm. 13C NMR (75 MHz, C6D6, 300 K): δ ϭ Ϫ4.6, 0.1, 0.2
(SiMe3), 24.4 (C2CHSi), 27.9 (br., NBCHSi), 40.7, 41.4 (NMe),
46.8 (br., ClBCHSi) ppm.
Tetraborane(4) (cis-12): Lithium naphthalenide solution (1.0 ,
26.7 mL, 26.7 mmol) was added to a suspension of cis-11 (8.4 g,
13.3 mmol) in diethyl ether (150 mL) at Ϫ100 °C. After warming
to room temperature, all of the volatile components were evapo-
rated in vacuo and the naphthalene was sublimed (60 °C/1 ϫ 10Ϫ3
mbar). The residue was treated with n-pentane (120 mL) and the
1-Chloro-2-piperidino-3,4,5-tris(trimethylsilyl)-1,2-diborolane (10b):
N-(Trimethylsilyl)piperidine (2.62 g, 16.7 mmol) was added to a
solution of 1b (5.85 g, 16.7 mmol) in n-pentane (50 mL) at Ϫ78 °C. insoluble solids were filtered off. Crystallization at Ϫ30 °C yielded
After warming to room temperature, the mixture was stirred for an
additional 18 h and then all the volatile components were evapo-
rated in vacuo. Crystallization of the residue from n-pentane (15
mL) at Ϫ30 °C yielded 10b (4.86 g, 73%) as colorless crystals [m.p.
cis-12 (7.08 g, 95%) as a yellow solid. Spectroscopic data: see ref.[6]
Dimethyltetraborane(6) (cis-14): solution of cis-12 (1.9 g,
A
3.39 mmol) in THF (25 mL) was treated with lithium naphthalen-
ide (2 equiv.), followed by iodomethane (0.95 g, 6.79 mmol), at
Ϫ100 °C. After warming to room temperature, all of the volatile
compounds were evaporated in vacuo, the residue was treated with
n-pentane, and the insoluble material was filtered off. Crystalliza-
tion from n-pentane (10 mL) at Ϫ30 °C yielded cis-14 (1.60 g, 80%)
as yellow crystals [m.p. 125 °C (decomposition)]. Spectroscopic
data: see ref.[20]
72 °C (some decomposition)]. 1H NMR (300 MHz, CDCl3, 300 K):
3
δ ϭ Ϫ0.15, 0.01, 0.13 (each s, each 9 H, SiMe3), 1.21 (d, JH,H
ϭ
3
2 Hz, 1 H, BCHSi), 1.43 (d, JH,H ϭ 2 Hz, 1 H, BCHSi), 1.62 (dd,
3JH,H ϭ 3, 2 Hz, 1 H, C2CHSi), 1.32Ϫ1.75 [m, 6 H, N(CH2)5], 3.06,
3.29, 3.95 [each m, total 4 H, N(CH2)5] ppm. 11B NMR (96 MHz,
CDCl3, 300 K): δ ϭ 47, 92 ppm. 13C NMR (75 MHz, CDCl3,
300 K): δ ϭ Ϫ3.4, 1.1, 1.3 (each q, SiMe3), 25.1 (d, 1JC,H ϭ 123 Hz,
1
C2CHSi), 27.9 (br. d, JC,H ϭ 123 Hz, NBCHSi), 47.3 (br. d,
Tetraborane(6) (cis-13): A solution of cis-12 (1.1 g, 1.96 mmol) in
THF (25 mL) was treated with lithium naphthalenide (2 equiv.)
1JC,H ϭ 104 Hz, ClBCHSi), 25.1, 28.1, 28.6, 51.1, 51.7 [each d,
N(CH2)5] ppm.
followed by HCl solution (2
 in diethyl ether, 0.99 mL,
1.98 mmol). After warming to room temperature and evaporation
of the volatile compounds in vacuo, the naphthalene was separated
by sublimation (60 °C/1 ϫ 10Ϫ3 mbar). The residue was treated
with n-pentane and the insoluble compounds were filtered off. cis-
13 of only 95% parity was obtained by crystallization from n-
pentane at Ϫ30 °C. Yield (1H NMR spectroscopy): 90%. 1H NMR
(300 MHz, C6D6, 300 K): δ ϭ 0.07, 0.24, 0.28 (each s, each 18 H,
Lithium
1-Piperidino-2,4,5-tris(trimethylsilyl)-1,2-diborolan-3-ide
(7b·Li): A solution of 10b (2.42 g, 6.1 mmol) in diethyl ether
(25 mL) was added to a suspension of lithium powder (ca. 500 mg)
in diethyl ether (25 mL) at room temperature. After the mixture
had been stirred for 2 h, all the volatile components were evapo-
rated in vacuo. The residue was treated with n-pentane (50 mL)
and the excess lithium powder and LiCl were filtered off. Crystalli-
zation from n-pentane (20 mL) at Ϫ30 °C yielded 7b·Li[Et2O]2
(1.67 g, 62%) as colorless crystals (m.p. 93 °C). 1H NMR
(400 MHz, C6D6, 300 K): δ ϭ 0.15, 0.23, 0.45 (each s, each 9 H,
SiMe3), 0.86 (t, 6 H, Et2O), 1.26 (s, 1 H, BCHSi), 1.55Ϫ1.87 [m, 6
3
SiMe3), 1.74 (pseudo t, JH,H ϭ 4 Hz, 2 H, C2CHSi), 1.93 (d,
3JH,H ϭ 4 Hz, 2 H, B3BCHSi), 2.23 (m, 2 H, HBCHSi), 7.01 (very
br. s, 2 H, BH) ppm. 11B NMR (96 MHz, C6D6, 300 K): δ ϭ 27,
51 ppm. 13C NMR (75 MHz, C6D6, 300 K): δ ϭ Ϫ3.6, 0.4, 0.5
1
(SiMe3), 28.9 (d, JC,H ϭ 123 Hz, C2CHSi), 30 (br., BCHSi), 41.6
3
H, N(CH2)5], 2.33 (d, JH,H ϭ 3 Hz, 1 H, C2CHSi), 3.01 (q, 4 H,
1
(br. d, JC,H ϭ 110 Hz, BCHSi) ppm.
Et2O), 3.13, 3.42, 3.66, 4.21 [each m, each 1 H, N(CH2)5], 6.57 (d,
3JH,H ϭ 3 Hz, 1 H, BCH) ppm. 11B NMR (96 MHz, C6D6, 300 K):
δ ϭ 33, 58 ppm. 13C NMR (75 MHz, C6D6, 300 K): δ ϭ Ϫ2.5, 0.3,
3.2 (each q, SiMe3), 14.4 (q, Et2O), 27.5 (br., BCHSi), 26.3, 28.8,
Crystal Structure Determinations: All single-crystal diffraction
experiments were performed with an image plate area detector sys-
tem (IPDS, Stoe) using an Mo-Kα radiation source equipped with
a graphite monochromator. Crystals were selected from dry inert
paraffin oil, mounted on top of a quartz capillary, and measured
with ϕ scans. All crystal and experimental data of the six structure
determinations are listed in Table 1. The structures were all solved
by direct methods using the SHELXS program[34] and refined by
the full-matrix least-squares method against all F2 data.[35] ‘‘Inter-
esting’’ hydrogen atoms at the nuclei of the structures were lo-
calized from difference Fourier maps and refined using individual
isotropic displacement parameters; those of peripheral moieties,
like the SiMe3 groups, were treated as riding on idealized calculated
positions. Their displacement parameters were fixed to 1.2- or (for
methyl groups) 1.5-times the Ueq values of their bonding partners.
All heavier atoms were refined using anisotropic displacement par-
ameters. Images of the structure were drawn using DIAMOND.[36]
Crystals of 1b suitable for X-ray analysis were grown from an n-
pentane solution at Ϫ30 °C. Single crystals of 5b·Li were obtained
1
29.3, [each t, N(CH2)5], 42.3 (d, JC,H ϭ 119 Hz, C2CHSi), 51.8,
56.2 [each t, N(CH2)5], 66.1 (t, Et2O), 149.2 (br. d, 1JC,H ϭ 122 Hz,
BCH) ppm.
Dichlorotetraborane(6) (cis-11): K2.8Na (3 mL) was added to a solu-
tion of 1b (5.92 g, 16.9 mmol) in n-pentane (150 mL; water bath,
reflux condensor) at room temperature. After the reaction started
(reflux, precipitation of a deep-blue salt), the mixture was stirred
for an additional 30 min. The insoluble components were filtered
off and washed with n-pentane (3 ϫ 50 mL). Removal of the sol-
vent yielded cis-11 (5.11 g, 96%) as a yellow solid, essentially pure
based on its 1H NMR spectra. The use of larger quantities (Ͼ 10 g)
of 1b in this reaction caused the formation of some impurities. In
this case, crystallization from n-pentane at Ϫ30 °C yielded 1b (ca.
75%) as yellow crystals. Spectroscopic data: see ref.[5]
Tetraborane(6) (trans-13): NaBEt3H solution (1.0  in toluene,
6 mL, 6 mmol) was added to a solution of cis-11 (1.9 g, 3 mmol) by dissolving 5b·Li (ca. 2 g) in boiling toluene (80 mL) and then
in diethyl ether (50 mL) at Ϫ78 °C. The mixture was warmed slowly
(12 h) to room temperature and then all the volatile components
were evaporated in vacuo. The residue was treated with n-pentane
keeping the solution at 95 °C for 18 h. The diffraction pattern of
the very sensitive crystal showed weak powder rings, which prob-
ably caused the somewhat reduced quality of the results, especially
Eur. J. Inorg. Chem. 2004, 3063Ϫ3073
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3071

C. Präsang, Y. Sahin, M. Hofmann, G. Geiseler, W. Massa, A. Berndt
Table 1. Crystal data and experimental details of the crystal structure determinations; all crystals were measured at T ϭ 193(1) K
FULL PAPER
1b
5b·Li
7a·Li[THF]2
7b·Li[Et2O]2
cis-11
cis-14
Empirical formula C12H30B2Cl2Si3
C19H25Li1Si2
316.51
tetragonal
I41 (I43)
2447.8(1)
2447.8(1)
1289.8(1)
90
90
90
7728.1(7)
16
C21H49B2LiN2O2Si3 C25H60B2LiN2O2Si3 C24H60B4Cl2Si6
C26H66B4Si6
590.57
Formula mass
Crystal system
Space group
351.15
monoclinic
P21/n
446.36
519.57
631.40
triclinic
P1
triclinic
P1
triclinic
P1
triclinic
¯
¯
¯
¯
P1
Unit cell [pm, °]
a ϭ 909.6(1)
b ϭ 1762.4(1)
c ϭ 1325.3(1)
α ϭ 90
973.2(1)
1126.0(1)
1426.2(1)
94.46(1)
98.76(1)
104.04(1)
1487.7(2)
2
1011.5(1)
1768.5(2)
1986.4(2)
83.09(1)
90.00(1)
81.67(1)
3489.8(6)
4
1015.6(1)
1273.5(1)
1652.0(1)
103.10(1)
90.28(1)
109.30(1)
1956.7(3)
2
1014.5(1)
1280.6(1)
1643.9(1)
103.40(1)
90.05(1)
108.69(1)
1961.4(3)
2
β ϭ 90.650(10)
γ ϭ 90
2124.4(3)
4
3
˚
V [A ]
Z
Calculated
1.098
1.088
0.996
0.989
1.072
1.000
density [g·cmϪ3
]
Absorption
0.462
752
0.177
2720
0.136
492
0.155
1152
0.363
684
0.227
652
coefficent [mmϪ1
]
F(000)
Crystal size [mm]
Θmax. [°]
0.12 ϫ 0.10 ϫ 0.03 0.65 ϫ 0.30 ϫ 0.25 0.45 ϫ 0.30 ϫ 0.30 0.55 ϫ 0.45 ϫ 0.30 0.45 ϫ 0.40 ϫ 0.40 0.35 ϫ 0.30 ϫ 0.30
26.13
25.90
25.92
25.93
25.91
25.92
Index range
Ϫ11 Ͻ h Ͻ 11,
Ϫ21 Ͻ k Ͻ 21
Ϫ16 Ͻ l Ͻ 15
ϕ scans
Ϫ29 Ͻ h Ͻ 29
Ϫ29 Ͻ k Ͻ 30
Ϫ15 Ͻ l Ͻ 15
ϕ scans
Ϫ11 Ͻ h Ͻ 11
Ϫ13 Ͻ k Ͻ 13
Ϫ17 Ͻ l Ͻ 17
ϕ scans
Ϫ12 Ͻ h Ͻ 12
Ϫ21 Ͻ k Ͻ 21,
Ϫ24 Ͻ l Ͻ 24
ϕ scans
Ϫ12 Ͻ h Ͻ 12
Ϫ15 Ͻ k Ͻ 15,
Ϫ20 Ͻ l Ͻ 20
ϕ scans
Ϫ12 Ͻ h Ͻ 11
Ϫ15 Ͻ k Ͻ 15
Ϫ20 Ͻ l Ͻ 20
ϕ scans
Scan type
No. of reflections
16576
22589
16018
34646
17996
11838
Unique reflections 4133 (0.061)
(Rint
7427 (0.0511)
5411 (0.0490)
12717 (0.0718)
7099 (0.0289)
7067 (0.0306)
)
Observed reflections 2472
6331
3349
7402
5674
5125
[I Ͼ 2σ(I)]
Parameters,
193, 21
430, 17
303, 18
655, 19
367, 19
371, 19
data/param. ratio
Goodness-of-fit (F2) 0.818
1.023
0.885
0.827
0.966
0.947
R [I Ͼ 2σ(I)]
wR2 (all refl.)
Largest diff.
0.0327
0.0648
ϩ0.221/Ϫ0.261
0.0463
0.1236
ϩ0.690/Ϫ0.220
0.0427
0.1156
ϩ0.329/Ϫ0.238
0.0427
0.0974
ϩ0.419/Ϫ0.285
0.0298
0.0770
ϩ0.286/Ϫ0.225
0.0348
0.0878
ϩ0.340/Ϫ0.252
Ϫ3
˚
peak/hole [e·A
]
3
˚
the relatively high residual electron density of 0.69 e/A . The abso-
lute structure could not be determined clearly because of the ab-
sence of suitably strong anomalous scatters [Flack parameter x ϭ
0.21(11)]. Possible twinning by merohedry could be excluded. Crys-
tals of 7a·Li[THF]2 suitable for X-ray analysis were grown from an
n-pentane solution at Ϫ30 °C. In its structure, one of the THF
molecules coordinated to lithium shows disorder: C15 has two
alternative positions (1:1) above and below the plane of the remain-
ing atoms. A single crystal of 7b·Li[Et2O]2 was obtained from an
n-pentane/diethyl ether (1:1) solution at Ϫ30 °C. Crystals of the
isostructural compounds cis-11 and cis-14 were grown from an n-
pentane solution at Ϫ30 °C. CCDC-225331 (1b), -225332 (5b·Li),
-225333 (7a·Li[THF]2), -225334 (7a·Li[Et2O]2), -225335 (cis-11),
and -225336 (cis-14) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; Fax: (internat.) ϩ 44-1223-336-033; E-mail:
deposit@ccdc.cam.ac.uk].
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 3063Ϫ3073

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Received January 30, 2004
Early View Article
Published Online May 25, 2004
T. Mennekes, P. Paetzold, R. Boese, D. Bläser, Angew. Chem.
Eur. J. Inorg. Chem. 2004, 3063Ϫ3073
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3073