Synthesis and structure of novel organocycloborates

Article abstract of DOI:10.1002/chem.200500760

A series of α,ω-boryl-(bromo)alkanes of the general formula R₂B-(CH₂)_n-Br (n = 3, 4, 5, 6) was obtained in high yield following a standard hydroboration protocol. Upon treatment with Mg turnings and formation of the respective Grignard species, all alkanes with n = 4 to 6 underwent an unprecedented boron-centered cyclisation reaction with formation of boratacyclopentanes, - hexanes, and -heptanes, respectively. All new compounds were isolated in high yields as colourless, crystalline materials and characterised in solution by multinuclear NMR spectroscopy. Four representative examples were chosen for X-ray diffraction studies, thus providing the first structurally characterised ring systems of that size at a tetraalkyl borate centre.

Full text of DOI:10.1002/chem.200500760

DOI: 10.1002/chem.200500760
Synthesis and Structure of Novel Organocycloborates
Holger Braunschweig,*[a] Giovanni D’Andola,[b] Tom Welton,[b] and Andrew J. P. White[b]
Abstract:
A
series of a,w-boryl-
lisation reaction with formation of bor-
atacyclopentanes, -hexanes, and -hep-
tanes, respectively. All new compounds
were isolated in high yields as colour-
less, crystalline materials and character-
ised in solution by multinuclear NMR
spectroscopy. Four representative ex-
amples were chosen for X-ray diffrac-
tion studies, thus providing the first
structurally characterised ring systems
(bromo)alkanes of the general formula
ꢀ
ꢀ
R2B (CH2)n Br (n=3, 4, 5, 6) was ob-
tained in high yield following a stan-
dard hydroboration protocol. Upon
treatment with Mg turnings and forma-
tion of the respective Grignard species,
all alkanes with n=4 to 6 underwent
an unprecedented boron-centered cyc-
Keywords:
cycloborates
magnesium
borates
·
boron
·
·
of that size at
centre.
a tetraalkyl borate
·
hydroboration
Introduction
of the aforementioned methods can be applied to their syn-
thesis. As early as 1969 Kosterꢀs group reported the high
yield synthesis of the spirocyclic tetraalkyl borate 1 from
Na[BEt4] and a cyclic triorganyl borane precursor under
thermally harsh conditions at 1408C; however, they did not
obtain much spectroscopic or structural evidence.[7] Like-
wise, Murahashi and Kondo proposed the formation of the
spirocyclic compound 2 in 1979 from the reaction of trialkyl
boranes with BrMg(CH2)5MgBr, again without spectroscopic
support.[8] Both syntheses are very specific and their applica-
tion is restricted to the two examples described, the consti-
tution of which remains far from being definitively ascer-
tained. In addition, only very few cyclic tetraalkyl borates
are known, which like compound 3, were all obtained from
9-borabicyclononane (9-BBN) derivatives.[6]
To address this deficiency and to provide a facile and
preferably general access to cyclic tetraalkyl borates, we
started to investigate the potential of boron-centered cycli-
sation reactions very recently. This approach has been occa-
sionally applied to the synthesis of three-coordinate boranes,
such as 4 and 5, which have been obtained by hydroboration
of dienes[9] and alkylation of R2NBCl2,[10] respectively, but
should possess a much broader applicability. Indeed, our
preliminary studies did reveal the propensity of certain a,w-
boryl(bromo)alkanes to form novel cyclic borates upon
treatment with Mg turnings. Recently, we communicated the
first dialkylboratacyclopentanes 6 and -hexanes 7, respec-
tively, to exemplify this new synthetic approach.[11] Herein
we report full synthetic, spectroscopic, and structural details
of a representative range of novel five-, six-, and seven-
membered cyclic and spirocyclic borates thus obtained.
Tetraorganoborates of the general formula [R4B]ꢀ are a well
established class of compounds, which is important for the
preparation of organic and organometallic species and as an-
alytical reagents.[1] In particular, tetraaryl- and mixed alkyl-
(aryl) borates have attracted much attention due to their
widespread use as non-coordinating anions in organometal-
lic synthesis and as facile sources for nucleophilic organyl
groups.[2] In stark contrast to aryl borates, the chemistry and
application of tetraalkyl borates is less developed. Such
compounds can be obtained in various ways,[3] for example,
from reactions of alkaline metal organyls with BR3 (R=
alkyl),[4] or from the hydroboration of alkenes with Na-
[HBR3].[5] It should be mentioned that the synthesis of
bulky tetraalkylborates with four sterically demanding alkyl
substitutents is often hampered by the formation of trialkyl
borates by dehydroboration.[6] Surprisingly, the correspond-
ing cyclic tetralkyl borates are virtually unknown, and none
[a] Prof. Dr. H. Braunschweig
Institut für Anorganische Chemie, Bayerische Julius-Maximilians-
Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
Fax: ( +49)931-888-4623
E-mail: h.braunschweig@mail.uni-wuerzburg.de
[b] G. D’Andola, Prof. Dr. T. Welton, Dr. A. J. P. White
Department of Chemistry, Imperial College London
Exhibition Road, South Kensington, London SW7 2 AZ (UK)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
600
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Chem. Eur. J. 2006, 12, 600 – 606

FULL PAPER
plete after 16 h. Only in the
case of the sterically most en-
cumbered disiamyl derivative
14 were more forcing condi-
tions, that is, prolonged reflux
in THF solution for three days,
required to complete the reac-
tion. After workup the borata-
cyclopentanes 15, 19, the bora-
tacyclohexanes 7, 17, and the
boratacycloheptanes 16, 18
were isolated as their corre-
sponding MgBr salts. All of
these cyclic borates were
formed as colourless crystals in
very good yields up to 90%,
proved to be stable in THF sol-
utions under inert atmosphere,
and are only moderately sensi-
tive toward air and moisture.
Results and Discussion
9-BBN, dicyclohexylborane, and disiamylborane were
chosen as readily available starting materials for the synthe-
sis of a range of a,w-boryl(bromo)alkanes. The latter were
obtained according to Equation (1) by hydroboration of 4-
bromobutene(1), 5-bromopentene(1), and 6-bromohex-
ene(1), respectively. A standard protocol was applied[12] and
the new compounds 8–14 were isolated as viscous, colourless
oils in very high yields. As was demonstrated by 11B NMR
spectroscopy, the conversion into the products was quantita-
tive and the crude materials that were obtained after evapo-
ration of the reaction mixture proved to be of sufficient
purity for subsequent syntheses. Hence, further purification
procedures of these intractable oils were not pursued.
As a slight variation of the previously described synthesis,
the 5-bromobutylborane 11 was treated with Mg turnings in
THF for 16 h, but subsequently, an ethereal solution of 1,4-
dioxane was added, thus furnishing the precipitation of
.
MgBr2 C4H8O2 and the formation of the Mg salt of the bor-
atacyclopentane 6 according to Equation (3). Mg[(cyclo-
C6H11)2BC4H8]2 (6) was isolated in 86% yield as a colourless,
crystalline solid with properties similar to those of the relat-
ed MgBr salts 7, 15–19.
The most characteristic spectroscopic feature of the cyclic
borates is their 11B NMR resonances, which range from d=
ꢀ12.5 to ꢀ20.5 ppm. These signals are shielded by almost
The a,w-boryl(bromo)alkanes 8–10 and 12–14 were treat-
ed with Mg turnings in THF at ambient temperature accord-
ing to Equation (2). 11B NMR spectra of the reaction mix-
tures revealed that all conversions except one were com-
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601

H. Braunschweig et al.
Information) with C(2) about
0.36 Š “below” and C(3) about
0.31 Š “above” the B-C(1)-
C(4) plane. The related trialkyl
species COQTAD has a twisted
geometry for this ring, with the
central two carbon atoms of the
C4 chain deviating by about
ꢀ0.41 and +0.23 Š out of the
BC2 plane, with an intra-ring C-
B-C angle of 103.5(3)8.[14] In 7
100 ppm with respect to those of their borane precursors,
thus proving the increase of the coordination number of the
boron centre from 3 to 4. As to be expected, 1H and
13C NMR data are less diagnostic, as all signals appear in
the aliphatic region. Electrospray-MS data, however, also
confirmed the presence of the proposed borate anions.
The X-ray crystal structures of the boratacyclopentyl (15),
boratacyclohexyl (7)[11] and boratacycloheptyl (16) species,
which all derive from 9-BBN, and also the related bis(cyclo-
hexyl)boratacyclopentyl compound (6),[11] have been deter-
mined (Figures 1–4, respectively), and represent, we believe,
the only known structurally characterised examples of these
ring systems at a tetraalkyl boron centre.[13] The closest ana-
logue for the boratacyclopentyl ring containing compounds
6 and 15 is a trialkyl borohydride species (H)(PhCH2)BC4H8
linked to a zirconium centre through a B-H-Zr bridge
[CCDC refcode COQTAD].[14] For the boratacyclohexyl
moiety (7) there are two trialkyl analogues reported: the
first is a ring-enlarged version of the closest analogue for
the boratacyclopentyl compounds 6 and 15 [i.e., a (H)-
(PhCH2)BC5H10 species, COQSUW],[14] whilst the second
has a CH2CH2CH2N(Me)2 moiety occupying the other two
sites on the boron centre [WOPBEI].[15] Though the quality
of the final results from the diffraction experiments on crys-
tals of 16 is rather low,[16] the fact that no structural studies
on species including the seven-membered boratacycloheptyl
ring have previously been reported in the literature mean
that this determination represents the best structural evi-
dence to date for this ring system.
the boratacyclohexane ring adopts a chair conformation (see
Figure S3 in the Supporting Information), a geometry seen
for every example of this ring so far reported in the litera-
Figure 1. The molecular structure of the tetraalkylcycloborate anion pres-
ent in the structure of 15 with selected bond lengths [Š] and angles [8]:
ꢀ
ꢀ
ꢀ
ꢀ
B C(1) 1.671(4), B C(4) 1.656(4), B C(5) 1.634(4), B C(9) 1.638(4);
C(1)-B-C(4) 100.3(2), C(1)-B-C(5) 114.7(2), C(1)-B-C(9) 112.8(2), C(4)-
B-C(5) 113.6(2), C(4)-B-C(9) 112.6(2), C(5)-B-C(9) 103.3(2).
For the structures of 7, 15 and 16 the restricted bite of the
BBN macrocycle is clearly evident, the C-B-C angles being
102.7(2), 103.3(2) and 100.4(16)8, respectively. In the struc-
ture of 6 (Figure 4), which has two discrete cyclohexyl li-
gands in place of the BBN moiety in the other three struc-
tures, the release of strain at the boron centre is revealed by
the C-B-C angle between these two ligands (109.9(3)8) ap-
proaching ideal tetrahedral. Interestingly, this has no effect
on the C-B-C angle of the boratacyclopentane rings in 15
and 6, the respective angles being 100.3(2) and 100.9(3)8.
However, the conformation of the boratacyclopentane ring
is markedly different in each case. In 15 the C4B ring adopts
an envelope conformation (see Figure S2 in the Supporting
Information) with the C(3) centre lying about 0.64 Š out of
the B-C(1)-C(2)-C(4) plane (which itself is coplanar to
better than 0.01 Š), whereas in 6 the boratacyclopentane
ring has a twisted geometry (see Figure S6 in the Supporting
Figure 2. The molecular structure of the tetraalkyl cycloborate anion
present in the structure of 7 with selected bond lengths [Š] and angles
ꢀ
ꢀ
ꢀ
ꢀ
[8]: B C(1) 1.649(5), B C(5) 1.647(5), B C(6) 1.643(5), B C(10)
1.644(5); C(1)-B-C(5) 104.8(3), C(1)-B-C(6) 113.2(3), C(1)-B-C(10)
110.9(3), C(5)-B-C(6) 112.6(3), C(5)-B-C(10) 112.9(3), C(6)-B-C(10)
102.7(3).
602
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Chem. Eur. J. 2006, 12, 600 – 606

Organocycloborates
FULL PAPER
ꢀ
The B C distances within the five-membered boratacyclo-
pentyl rings in 15 and 6 range between 1.656(4) and
1.671(4) Š, compared to 1.625(5) and 1.633(5) Š in the tri-
alkyl borohydride species COQTAD.[14] The lengthening is
probably a consequence of the greater steric requirements
of the fourth alkyl substituent in 15 and 6 as compared to
hydride in the trialkyl borohydride compound. For the bora-
ꢀ
tacyclohexyl species 7, all four B C separations range be-
tween 1.643(5) and 1.649(5) Š, and are noticeably longer
than those in COQSUW (1.618(5) and 1.624(8) Š)[14] and
WOPBEI (in the range 1.613(4)–1.637(4) Š).[15] Here
though, the steric argument is not so simple since, whereas
COQSUW again shows a hydride in the fourth coordination
site on the boron centre, WOPBEI has the nitrogen atom of
a CH2CH2CH2N(Me)2 chain. However, the bond to this ni-
trogen is significantly longer at 1.682(4) Š (1.684(4) Š) than
those to either of the carbons in WOPBEI or in 7. The esti-
mated standard deviations in the structure of the boratacy-
cloheptyl species 16 are too high to allow any meaningful
comparisons to be made,[16] even if there were any closely
related analogues for this seven-membered C6B ring.
Figure 3. The molecular structure of the tetraalkyl cycloborate anion
present in the structure of 16 with selected bond lengths [Š] and angles
ꢀ
ꢀ
ꢀ
ꢀ
[8]: B C(1) 1.67(3), B C(6) 1.63(3), B C(7) 1.65(3), B C(11) 1.69(3);
C(1)-B-C(6) 111.3(17), C(1)-B-C(7) 111.5(16), C(1)-B-C(11) 109.5(15),
C(6)-B-C(7) 112.5(19), C(6)-B-C(11) 111.1(17), C(7)-B-C(11) 100.4(16).
Obviously, the formation of the cyclic borates proceeds
via an unprecedented boron-centered cyclisation reaction of
ꢀ
ꢀ
the intermediate Grignard species R2B (CH2)n MgBr with
intramolecular attack of the nucleophilic w-carbon atom at
the highly Lewis acidic boron centre. The cyclisation ap-
pears to occur instantaneously, once the reaction with Mg is
ꢀ
ꢀ
initiated. Only the bulky (C5H11)2B (CH2)4 Br (14) requires
further heating to give the borate formation (vide supra). In
that case, the spectroscopic data, particularly the deshielded
11B NMR resonance at d=85.0 ppm, which were obtained
once the exothermic Grignard reaction has subsided, indi-
ꢀ
cate the exclusive presence of the species (C5H11)2B
ꢀ
(CH2)4 MgBr (14a).
In addition, similar experiments targeted corresponding
boratacyclobutanes, thus probing for the lower limit of the
cyclisation reaction in terms of ring size and strain. To this
end potentially useful 3-bromopropylboranes were synthe-
sized by hydroboration of allyl bromide according to Equa-
tion (4). The bromides 20 and 21 were converted into the
corresponding Grignard species 20a and 21a and subse-
quently, into the magnesium dialkyls 20b and 21b by previ-
ously described protocols (vide supra). Attempts to achieve
a cyclisation reaction of any of these species under similar
conditions as in the case of 19, however, met with no suc-
cess.
Figure 4. The molecular structure of the tetraalkyl cycloborate anion
present in the structure of 6 with selected bond lengths [Š] and angles
ꢀ
ꢀ
ꢀ
ꢀ
[8]: B C(1) 1.663(5), B C(4) 1.668(5), B C(5) 1.655(5), B C(11)
1.659(5); C(1)-B-C(4) 100.9(3), C(1)-B-C(5) 112.0(3), C(1)-B-C(11)
110.4(3), C(4)-B-C(5) 111.4(3), C(4)-B-C(11) 112.0(3), C(5)-B-C(11)
109.9(3).
ture.[17] The C-B-C angle of 104.8(3)8 is slightly enlarged
compared to the values seen in the boratacyclopentane spe-
cies 15 and 6, but is still smaller than the angles of
111.5(5)[14] and 108.9(2)8 (109.0(2)8)[15] seen in the trialkyl
analogues (the value in square parentheses refers to the
second independent molecule present in WOPBEI). The
major occupancy orientation for the boratacycloheptane
ring in 16 (see Figure S5 in the Supporting Information) has
a ring-inverted chair conformation[16] with a C-B-C angle of
111.3(17)8. The intra-ring angles thus show the expected pat-
tern, with boratacyclopentane < boratacyclohexane < bor-
atacycloheptane.
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H. Braunschweig et al.
6.81 Hz, 2H, Cy2BCH2CH2CH2CH2Br); 13C NMR: d=23.64, 24.73, 28.14,
28.63, 34.21, 36.94, 37.44 ppm; 11B NMR: d=82.4 ppm. The crude materi-
al was used without further purification for the synthesis of 6.[11]
Conclusion
ꢀ
The boron-centered cyclisation of Grignard species R2B
(CH2)n MgBr (n=4, 5, 6) was established as a very conven-
12: According to the procedure described for 11 cyclohexene (6.0 mL,
ꢀ
59.2 mmol) in THF (20 mL) was treated with
a 1.0m solution of
ient and facile method for the high yield synthesis of novel
tetraalkyl cycloborates. A wide variety of unprecedented
boratacyclopentanes, -hexanes and -heptanes was thus pre-
pared, proving the general applicability of the new method
for the synthesis of medium-sized cycloborates. The com-
pounds can be obtained as colourless, crystalline materials
of the type [MgBr][R4B] or Mg[R4B]2, and the structures of
four representative examples in the solid state were eluci-
dated by single-crystal X-ray diffraction methods.
BH3·THF (30 mL, 30.0 mmol) in THF and subsequently treated with 5-
Br-1-pentene (4.59 g, 30.8 mmol) in THF (20 mL) to give 12 as a colour-
less oil in 90% yield, which was used without further purification for the
3
synthesis of 17. 1H NMR: d=1.20–1.90 (m, 30H), 3.40 ppm (t, JHH
=
6.92 Hz, 2H; R2BCH2)4CH2Br); 13C NMR: d=24.30, 25.72, 28.17, 28.65,
32.96, 33.96, 34.27, 36.95 ppm; 11B NMR: d=82.5 ppm.
13: According to the procedure described for 11 cyclohexene (4.0 mL,
39.5 mmol) in THF (20 mL) was treated with
a 1.0m solution of
BH3·THF (20 mL, 20.0 mmol) in THF and subsequently treated with 6-
Br-1-hexene (3.26 g, 20.0 mmol) in THF (20 mL) to give 13 as a colour-
less oil in 93% yield, which was used without further purification for the
3
synthesis of 18. 1H NMR: d=1.15–1.86 (m, 32H), 3.41 ppm (t, JH,H
=
6.80 Hz, 2H, R2B(CH2)5CH2Br); 13C NMR: d=24.91, 28.22, 28.67, 29.17,
33.71, 34.05, 34.40, 37.06 ppm; 11B NMR: d=82.5 ppm.
Experimental Section
14: A solution of 2-methyl-2-butene (8.0 mL, 75.5 mmol) in THF (20 mL)
was added dropwise to a stirred 1.0m solution of BH3·THF (38.0 mL,
38.0 mmol) at ꢀ158C. Subsequently, a solution of 4-Br-1-butene (3.8 mL,
37.4 mmol) in THF (20 mL) was added dropwise to the stirred suspen-
sion of disiamylborane at 08C. The resulting cloudy solution was allowed
to reach room temperature and stirred overnight. The solution was dried
in vacuo to afford 14 as a colourless oil in 85% yield, which was used
without further purification for the synthesis of 19. 1H NMR: d=0.40–
1.50 (m, 28H), 3.24 ppm (br, 2H; R2BCH2CH2CH2CH2Br); 13C NMR:
d=1.53, 13.30 (m), 21.70, 23.31 (br), 30.52 (br), 31.16, 31.41, 32.95,
38.50 ppm; 11B NMR: d=84.9 ppm.
General remarks: All manipulations of air-sensitive materials were per-
formed under an inert atmosphere of dry nitrogen using standard
Schlenk techniques. Solvents were purified and dried according to stan-
dard methods and stored over molecular sieve under an inert atmosphere
of nitrogen. All starting materials are commercially available and were
used as received without further purification. Mg turnings were activated
by using small amounts of 1,2-dibromoethane. NMR spectroscopy:
JEOL-EX 270 at 269.72 (1H, standard TMS internal), 86.54 (11B, standard
BF3·OEt2 in C6D6 external), 67.93 MHz (13C{1H}, APT, standard TMS in-
ternal), all NMR spectra were recorded at 258C in C4D8O as solvent
unless otherwise stated. Mass spectra were recorded on Micromass “Q-
TOF” (70 eV) and elemental analyses (C, H) were obtained from a Carlo
Erba EA1108 Elemental Analyser.
15: All of the previously obtained crude borane 8 was dissolved in THF
(20 mL) and added dropwise to a suspension of Mg turnings in THF
(50 mL) at ambient temperature. When the addition was complete, the
system was allowed to stir at ambient temperature for three days. The re-
sulting grey solution was filtered to give a clear solution, which was con-
centrated to a small volume in vacuo and cooled to 08C to give 15 as col-
ourless crystals in 74% yield. 1H NMR: d=0.04 (m, br, 6H), 1.20–
8: A solution of 9-BBN (dimer) (2.61 g, 10.70 mmol) in THF (50 mL)
was added dropwise to a solution of 4-Br-1-butene (2.89 g, 21.41 mmol)
in THF (20 mL) at 08C. When the addition was complete, the solution
was allowed to reach ambient temperature and stirred overnight. The re-
1
2.00 ppm (m, 16H); 13C NMR: d=26.81 (q, JC,B =33.67 Hz), 28.41, 32.02
sulting colourless solution was dried in vacuo to yield 8 as a colourless oil
(q, 1JC,B =38.73 Hz), 33.87, 35.42 ppm; 11B NMR: d=ꢀ13.9; MS (ESI):
3
in 92% yield. 1H NMR: d=1.16–1.92 (m, br, 20H), 3.43 ppm (t, JH,H
=
m/z (%): 177 (100) [C12H22B].
6.67 Hz, 2H; R2BCH2(CH2)2CH2Br); 13C NMR: d=23.68, 23.88, 25.73,
30.14, 33.32, 33.80, 36.58 ppm; 11B NMR: d=70.8 ppm. The crude materi-
al was used without further purification for the synthesis of 15.
7:[11] According to procedure described for 15 the crude borane 9 was
treated in THF (70 mL) with Mg turnings to yield colourless crystals of 7
1
in 89% yield. H NMR: d=ꢀ0.33 (br), 0.80–1.44 ppm (m, br); 13C NMR:
9: According to the procedure described for 8, 9-BBN (dimer) (2.68 g,
11.00 mmol) in THF (50 mL) was treated with 5-Br-1-pentene (2.6 mL,
21.95 mmol) in THF (20 mL) to give 9 as a colourless oil in 93% yield,
which was used without further purification for the synthesis of 7.[11]
1H NMR: d=1.20–1.94 (m, br, 22H), 3.42 ppm (t, 3JH,H =6.80 Hz, 2H;
R2BCH2(CH2)3CH2Br); 13C NMR: d=24.28, 24.72, 27.46, 31.02, 32.45,
33.82, 34.28 ppm; 11B NMR: d=75.8 ppm.
d=27.04, 28.23, 28.88 (br), 30.38, 34.07, 34.51 ppm; 11B NMR: d=
ꢀ20.5 ppm; MS (ESI): m/z (%): 191 (100) [C13H24B].
16: According to procedure described for 15 the crude borane 10 was re-
acted in THF (70 mL) with Mg turnings to yield colourless crystals of 7
in 65% yield. 1H NMR: d=0.14 (br), 0.29 (br), 1.20–2.00 ppm (br, m);
13C NMR: d=27.33, 28.43, 29.05 (br), 30.55, 34.07, 35.01 ppm; 11B NMR:
d=ꢀ 19.7 ppm; MS (ESI): m/z (%): 205 (100) [C14H26B].
17: According to procedure described for 15 the crude borane 12 was
treated in THF (35 mL) with Mg turnings to yield colourless crystals of
17 in 76% yield. 1H NMR: d=ꢀ0.17 (br), 0.08 (br), 0.72–1.57 ppm (br,
m); 13C NMR: d=23.48 (q, 1JC,B =39.49 Hz; R2B(CH2)2(CH2)3), 27.31,
28.73, 30.07, 32.09, 33.04, 36.82 ppm (q, 1JC,B =41.97 Hz; [(CH2)5CH]2B-
(CH2)5); 11B NMR: d=ꢀ18.6 ppm; MS (ESI): m/z (%): 247 (100)
[C17H32B].
10: According to the procedure described for 8, 9-BBN (dimer) (2.87 g,
11.77 mmol) in THF (50 mL) was treated with 6-Br-1-hexene (3.80 g,
23.30 mmol) in THF (20 mL) to give 10 as a colourless oil in 92% yield,
which was used without further purification for the synthesis of 16.
1H NMR: d=1.18–1.90 (m, br, 24H), 3.42 ppm (t, 3JH,H =6.78 Hz, 2H;
R2BCH2(CH2)4CH2Br); 13C NMR: d=24.43, 27.57, 29.14, 31.16, 34.26,
33.31, 33.91, 33.98, 34.40 ppm; 11B NMR: d=73.6 ppm.
11: A solution of cyclohexene (6.0 mL, 59.2 mmol) in THF (20 mL) was
18: According to procedure described for 15 the crude borane 13 was
treated in THF (35 mL) with Mg turnings to form 18 as a white solid in
added dropwise to
a stirred 1.0m solution of BH3·THF (30 mL,
30.0 mmol) in THF at ꢀ158C. The system was stirred for 1 h, during
which time white crystalline dicyclohexylborane precipitated out. Subse-
quently, a solution of 4-Br-1-butene (3.0 mL, 29.6 mmol) in THF (20 mL)
was added dropwise to the suspension of dicyclohexylborane with stirring
at 08C. The resulting cloudy solution was allowed to reach ambient tem-
perature and stirred overnight. The cloudy solution was centrifuged to
separate the off-white solid and the colourless solution was dried in
1
70% yield. H NMR: d=0.06 (br), 0.85–1.82 ppm (m, br); 13C NMR: d=
1
27.95, 28.05, 28.48 30.22, 30.62, 32.16, 32.87, 36.67, 39.92 ppm (q, JC,B
=
41.33 Hz; [(CH2)3(CH2)2CH]2B(CH2)6); 11B NMR: d=ꢀ16.6 ppm; MS
(ESI): m/z (%): 261 (100) [C18H34B].
14a and 19: All of the previously obtained crude borane 14 was dissolved
in THF (20 mL) and the resulting solution was added dropwise to a sus-
pension of Mg turnings in THF (20 mL) at ambient temperature. When
the reaction was complete, the system was allowed to reach ambient tem-
vacuo to give 11 as a colourless oil in 92% yield. 1H NMR: d=1.11–1.74
3
(m, 26H), 1.84 (m, 2H, R2BCH2CH2CH2CH2Br), 3.44 ppm (t, JH,H
=
604
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ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 600 – 606

Organocycloborates
FULL PAPER
perature and a white precipitate started to form. The mixture was stirred
overnight, diluted with THF (50 mL), and filtered hot to give a clear so-
lution. A small amount of this solution was dried in vacuo and analysed
by NMR spectroscopy.
to yield 21a; 1H NMR: d=1.19–1.72 ppm (m, br); 13C NMR: d=15.71,
28.00, 28.45, 29.95 (br), 36.80 ppm (br); 11B NMR: d=82.2 ppm.
21b: The previously obtained Grignard reagent 21a was redissolved in
Et2O and a solution of 1,4-dioxane (2.3 mL, 27.0 mmol) in Et2O (10 mL)
was added dropwise. The mixture was stirred overnight and the white
precipitate of MgBr2·dioxane was separated from the solution by centri-
fugation. The resulting clear solution was dried in vacuo to give 21b as
NMR data for 14a: 1H NMR: d=0.40–1.60 (m); 13C NMR: d=13.41,
22.50, 23.90 (br), 31.12 (br), 31.25, 32.11, 32.72, 33.50; 11B NMR: d=85.0.
The solution of 14a in THF was refluxed at 668C for three days. The re-
sulting clear solution was concentrated to a small volume in vacuo and
cooled to 08C to give colourless crystals of 19 in 76% yield. 1H NMR:
d=0.10 (br), 0.70–1.00 (br, m), 1.20–1.50 (br, m), 1.76 ppm (br);
13C NMR: d=12.19, 12.65, 14.40, 21.67, 31.17, 39.52 ppm (br); 11B NMR:
d=ꢀ 20.5 ppm; MS (ESI): m/z (%): 209 (100) [C14H30B].
6: A solution of the crude borane 11 in Et2O (20 mL) was added drop-
wise to a suspension of Mg turnings in Et2O (15 mL) at ambient tempera-
ture. When the reaction was complete, the system was allowed to reach
ambient temperature and a white precipitate started to form. The mix-
ture was stirred overnight, diluted with THF (50 mL), and filtered hot to
give a clear solution. 1,4-Dioxane (2.3 mL, 27.0 mmol) in Et2O (10 mL)
was added dropwise and the resulting solution was stirred overnight. The
white precipitate of MgBr2·dioxane was separated from the solution by
centrifugation, and the clear solution was concentrated to a small volume
in vacuo and cooled to 08C to give colourless crystals of 6 in 86% yield.
1H NMR: d=ꢀ0.17 (br), 0.08 (br), 0.72–1.57 ppm (m, br); 13C NMR: d=
30.10, 31.68, 32.66, 32.74 ppm; 11B NMR: d=ꢀ12.5 ppm; MS (ESI): m/z
(%): 233 (100) [C16H30B].
1
an off- white oil in 85% yield. H NMR: d=1.18–1.72 (m, br, 26H), 1.96
(m, 2H; R2BCH2CH2CH2Br), 3.42 ppm (t, 3JH,H =6.79 Hz, 2H;
R2BCH2CH2CH2Br); 13C NMR: d=19.39, 27.95, 28.42, 29.92, 36.74 ppm
(br); 11B NMR: d=82.5 ppm; elemental analysis calcd (%) for
C30H56B2Mg (462.7): C 77.87, H 12.20; found: C 77.82, H 11.98.
Crystal data for 15: [C20H40O5BrMg](C12H22B)·C4H8O, Mr =713.95, tri-
clinic, (no. 2), a=10.6422(5), b=13.8529(5), c=13.9367(6) Š, a=
71.397(4), b=89.688(4), g=87.648(4)8, V=1945.57(14) Š3, Z=2, 1calcd
=
1.219 gcmꢀ3, m(MoKa)=1.113 mmꢀ1, T=173 K, colourless prisms, Oxford
Diffraction Xcalibur 3 diffractometer; 11919 independent measured re-
flections, F2 refinement, R1 =0.075, wR2 =0.126, 11384 independent ob-
served absorption-corrected reflections [jFo j>4s(jFoj), 2qmax =648], 441
parameters. CCDC-273774 contains the supplementary crystallographic
data for this compound. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Crystal data for 16: [C20H40O5BrMg](C14H26B)·C4H8O, Mr =742.00, mon-
oclinic, P21/n (no. 14), a=10.7897(9), b=25.8464(19), c=14.5063(10) Š,
b=93.220(6)8, V=4039.1(5) Š3, Z=4, 1calcd =1.220 gcmꢀ3
, m(MoKa)=
1.074 mmꢀ1, T=173 K, colourless blocks, Oxford Diffraction Xcalibur 3
diffractometer; 5234 independent measured reflections, F2 refinement,
R1 =0.184, wR2 =0.448, 5166 independent observed absorption-corrected
reflections [jFo j>4s(jFoj), 2qmax =458], 429 parameters. CCDC-273775
contains the supplementary crystallographic data for this compound.
These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
20: A solution of 9-BBN (dimer) (6.7 g, 27.3 mmol) in THF (50 mL) was
added dropwise to
a colourless solution of allyl bromide (4.8 mL,
55.5 mmol) in CH2Cl2 (10 mL) at ꢀ308C. When the addition was com-
plete, the solution was allowed to reach ambient temperature and stirred
overnight. The resulting colourless solution was dried in vacuo to give 20
as a pale yellow oil in 94% yield, which was used without further purifi-
cation for the synthesis of 20a. 1H NMR: d=1.15 (t, 3JH,H =8.17 Hz, 2H;
R2BCH2CH2CH2Br), 1.34 (m, 4H; [(CH2)2(CH2)4(CH)2]B(CH2)3Br)),
1.71–1.80 (m, br, 10H; [(CH2)2(CH2)4(CH)2]B(CH2)3Br)), 1.96 (m, 2H;
R2BCH2CH2CH2Br),
3.40 ppm
(t,
3JH,H =7.05 Hz,
2H;
R2BCH2CH2CH2Br); 13C NMR: d=24.56, 29.59 (br), 30.10, 33.67,
38.04 ppm; 11B NMR: d=86.6 ppm.
Acknowledgements
20a: The previously obtained 20 was redissolved in Et2O (50 mL) was
added dropwise to Mg turnings in Et2O at ambient temperature. The
system was allowed to reach ambient temperature and stirred overnight.
The resulting grey solution was filtered and dried in vacuo to give 20a as
a white powder. 1H NMR: d=1.22–1.85 ppm (m, br); 13C NMR: d=
20.43, 24.29, 31.30 (br), 33.86 ppm; 11B NMR: d=78.2 ppm.
We thank EPSRC for financial support.
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20b: The previously obtained Grignard reagent 20a was redissolved in
Et2O and a solution of 1,4-dioxane (0.57 g, 6.47 mmol) was added drop-
wise. Immediately, a white precipitate of MgBr2·dioxane formed. After
the addition was complete the system was allowed to stir for 2 h. The
white precipitate was filtered and washed twice with Et2O (2”10 mL).
The mother liquor was dried in vacuo and 20b was isolated as a white
solid in 85% yield. 1H NMR: d=1.21–1.91 ppm (m, br); 13C NMR: d=
20.42, 24.31, 31.10 (br), 33.86 ppm; 11B NMR: d=77.8 ppm; elemental
analysis calcd (%) for C22H40B2Mg (350.5): C 75.39, H 11.50; found: C
75.61, H 11.36.
21: According to the procedure described for 20 a solution of cyclohex-
ene (6 mL, 59.2 mmol) in THF (20 mL) was treated with a 1.0m solution
of BH3·THF (30 mL, 30.0 mmol), and subsequently treated with a solu-
tion of allyl bromide (2.6 mL, 30.0 mmol) in THF (20 mL) to give 21 as a
colourless oil in 87% yield, which was used without further purification
for the synthesis of 20a. 1H NMR: d=1.16–1.73 (m, br, 24H), 1.96 (m,
2H;
R2BCH2CH2CH2Br),
3.42 ppm
(t,
3JH,H =6.79 Hz,
2H;
R2BCH2CH2CH2Br); 13C NMR: d=27.90, 28.06, 28.42, 29.16, 36.60 (br),
37.58 ppm; 11B NMR: d=81.4 ppm.
21a: The previously obtained 21 was dissolved in Et2O (20 mL) and
added dropwise to a suspension of Mg turnings in Et2O at ambient tem-
perature. The system was allowed to reach ambient temperature and stir-
red overnight. The resulting grey solution was filtered and dried in vacuo
Chem. Eur. J. 2006, 12, 600 – 606
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
605

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[12] H. C. Brown, Boranes in Organic Chemistry, Cornell University
Press, 1972.
[13] A search of the May-05 update of the Cambridge Crystallographic
Database (version 5.26) revealed no examples of a tetraalkylborate
centre with either a BC4H8, BC5H10, or a BC6H12 ring.
ray diffraction data from the available crystals, but somewhat inevi-
tably the final results are not satisfying (see Experimental Section).
The data reported in this paper for the structure of 16 represent the
best that could be obtained, and though we are restricted in what
conclusions can be drawn from the structure they unambiguously
prove the presence of the boratacycloheptane ring. We were also
still able to observe disorder in the boratacycloheptane ring, with
C(2) occupying two different positions (see Figure S5 in the Sup-
porting Information), though in each case the C6B ring adopts a
ring-inverted chair conformation.
[17] Though the search of the May-05 update of the Cambridge Crystal-
lographic Database (version 5.26) revealed no examples of a tetraal-
kylborate centre with a BC5H10 ring, a number of structures (11)
containing the H2BC5H10 moiety were found.
[14] [COQTAD] & [COQSUW] F.-C. Liu, J. Liu, E. A. Meyers, S. G.
Shore, Inorg. Chem. 1999, 38, 2169–2173.
[15] [WOPBEI] H. Schumann, B. C. Wassermann, S. Schutte, B. Heymer,
S. Nickel, T. D. Seuss, S. Wernik, J. Demtschuk, F. Girgsdies, R. Wei-
mann, Z. Anorg. Allg. Chem. 2000, 626, 2081–2095.
[16] Despite considerable effort it was not possible to grow crystals of
better quality of 16. Numerous attempts were made to get usable X-
Received: July 1, 2005
Published online: November 7, 2005
606
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ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 600 – 606