Novel organocycloborates via Grignard reagents

Article abstract of DOI:10.1039/b404864d

An unprecedented cyclisation reaction of α,ω-borylbromoalkanes is seen upon treatment with magnesium turnings, forming organocycloborates in high yield. The novel boratacyclopentanes and -hexanes have been characterised by X-ray crystallography.

Full text of DOI:10.1039/b404864d

Novel organocycloborates via Grignard reagents†
Holger Braunschweig,*a Giovanni D’Andola,ab Tom Weltonb and Andrew J. P. White‡b
a Institute of Inorganic Chemistry, Am Hubland, 97074 Würzburg, Germany.
E-mail: h.braunschweig@mail.uni-wuerzburg.de; Fax: +499318884623; Tel: +499318885260
b Department of Chemistry, Imperial College London, Exhibition Road, South Kensington, London, UK SW7
2AZ
Received (in Cambridge, UK) 1st April 2004, Accepted 27th May 2004
First published as an Advance Article on the web 25th June 2004
An unprecedented cyclisation reaction of a,w-borylbromoalk-
anes is seen upon treatment with magnesium turnings, forming
organocycloborates in high yield. The novel boratacyclo-
pentanes and -hexanes have been characterised by X-ray
crystallography.
The cyclisation reaction, which yields the thermodynamically
favoured 5- and 6-membered boratacycles, occurs instantaneously
upon treatment of 1 and 2 with magnesium. So far, it has proven
difficult to isolate the proposed intermediate. However, in the case
of related compounds with shorter alkanediyl spacers, and hence, a
less pronounced tendency for cyclisation, we were able to
spectroscopically characterise Grignard species of the type
R2B(CH2)3MgBr.14
The X-ray structure§ determination of crystals of 3 showed it to
contain the expected tetraalkylborate anion (Fig. 1). The five-
membered BC4 ring adopts a typical twisted geometry with C(2)
and C(3) lying ca. 0.36 and 0.30 Å “above” and “below” the
{B,C(1),C(4)} plane respectively. Except for the restricted bite
The chemistry of tetraorganoborates has received considerable
attention,1 due to their importance for the preparation of organic
and organometallic compounds and as analytical reagents.2 Tetra-
aryl- or mixed alkylarylborates have been widely used as non-
coordinating anions for organometallic compounds, and as sources
for nucleophilic organyl groups.3,4 In contrast, relatively little is
known about non-cyclic tetraalkylborates, which can be obtained in
several ways,5 e.g. from reaction of alkaline metal organyls with
BR3 (R = alkyl)6,7 or from reaction of Na[HBR3] with alkenes.8 In
the case of sterically demanding trialkylboranes, however, there is
a strong tendency to form trialkylborohydrides M[HBR3].9 The
chemistry of cyclic tetraalkylborates is even less well developed,
and none of the aforementioned methods can be applied to their
synthesis.
To the best of our knowledge, only one very specific formation
of a single spirocyclic tetraalkylborate at elevated temperature has
been reported, by Koster in 1969.10 In addition, only a few 9-BBN
(where 9-BBN = 9-borabicyclo[3.3.1]nonane) derivatives of the
t
type C8H14BRRA9 (R = CH3, RA = Bu) or C8H14BArPh11 (Ar =
C6H4-o-CH2NMe2) are known.
A boron-centred cyclisation reaction appeared to us to be a
preferable method for the synthesis of tetraalkylcycloborates. This
approach has been, so far, restricted to cyclic boranes with three
coordinate boron centres, obtained via hydroboration of dienes,12 or
dialkylation of R2NBCl2.13
As summarized in Scheme 1, 4-bromo-1-butene and 5-bromo-
1-pentene were treated with suitable diboranes(6) {(R2BH)2} to
give the w-bromoalkyl(diorgano)boranes 1 and 2 in high yield as
colourless oils. Subsequent reaction of 1 with activated Mg-
turnings and then 1,4-dioxane furnished the boratacyclopentane 3
with elimination of MgBr2. Similarly, the 9-BBN derivative 2 was
treated with magnesium to give the boratacyclohexane 4. In
contrast to the former reaction, no 1,4-dioxane was employed here,
and hence, 4 was isolated as a MgBr2 salt.
Scheme 1 Synthetic pathway for the synthesis of tetraalkylcycloborates. (a)
4-bromo-1-butene, THF; (b) 5-bromo-1-pentene, THF; (c) Et2O, Mg
turnings; (d) 1,4-dioxane, 2MgBr2·(dioxane)n; (e) THF, Mg turnings.
The two cyclic tetraalkylborates were isolated as colourless,
modestly air- and moisture-sensitive crystals in yields above 85%.
Their structure in solution was derived from multinuclear NMR
spectroscopic data. The 11B NMR spectra exhibit expected signals
at d = 212.5 (3) and 220.5 (4), which are high field shifted by
approximately 95 ppm with respect to the triorganyl boranes 1 and
2, indicating boron in coordination number four. The formation of
the borates 3 and 4 must proceed via Grignard species of the type
R2B(CH2)nMgBr (n = 4, 5) with intramolecular attack of the
nucleophilic carbon atom at the Lewis-acidic boron centre.
† Electronic supplementary information (ESI) available: experimental
details and 1H, 13C and 11B NMR data for 1, 2, 3and 4, single crystal X-ray
data and additional figures for 3and 4. See http://www.rsc.org/suppdata/cc/
b4/b404864d/
‡ This paper is dedicated to Prof. Dr. Wolfdieter Schenk on the occasion of
his 60th birthday.
Fig. 1 The structure of the anion (B-B-dicyclohexylboratacyclopentane) (3),
determined by X-ray crystallography.
1738
C h e m . C o m m u n . , 2 0 0 4 , 1 7 3 8 – 1 7 3 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Ka) = 1.075 mm21, T = 203 K, colourless blocks; 7040 independent
measured reflections, F2 refinement, R1 = 0.046, wR2 = 0.095, 4555
independent observed absorption corrected reflections [¡Fo¡ > 4s(¡Fo¡),
2qmax = 50°], 513 parameters. CCDC 236835. See http://www.rsc.org/
suppdata/cc/b4/b404864d/ for crystallographic data in .cif or other elec-
tronic format.
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14 H. Braunschweig and G. DAAndola, unpublished results. For example, in
the case of R2B(CH2)3MgBr with R2 = 9-BBN or Cy2, the 1H NMR
spectra show a highfield shifted triplet for the a-CH2 methylene protons
adjacent to the boron centre, and deshielded signals in the 11B NMR
spectra at approximately 80 ppm, thus proving the presence of three
coordinate boron.
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Wrackmeyer, B. Schwarze and W. Milius, J. Organomet. Chem., 1997,
545, 297; M. Sigl, A. Schier and H. Schmidbaur, Chem. Ber., 1997, 130,
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Fig. 2 The structure of the anion 1,5-pentadiyl-9-borabicyclo[3.3.1]nonane
(4), determined by X-ray crystallography.
angle within the BC4 ring [C(1)–B–C(4) 100.9(3)°] the angles at the
boron centre are close to ideal tetrahedral, being in the range
109.9(3) to 112.0(3)°. Within statistical significance, all four B–C
bond lengths are the same [1.655(5) to 1.668(5) Å].
The solid state structure§ of
4
was shown to be
[(thf)5MgBr][C13H24B]·thf, confirming the presence of a magne-
sium bromide cation and the anion shown in Fig. 2. The geometry
at the boron is distorted tetrahedral with angles in the range
102.7(3)–113.2(3)°, the greater flexibility of the six-membered ring
causing less strain at the boron centre than is seen in 3, the C(1)–B–
C(5) angle being 104.8(3)° for the intra-ring boron angle in 4 cf.
100.9(3)° in 3. The six-membered (CH2)5B ring adopts a chair
conformation, as do both of the C5B portions of the 9-BBN ring.
The B–C bond lengths are in the narrow range 1.643(5)–1.649(5)
Å, at the high end of those seen for trialkyl boron centres with either
a (CH2)5B or 9-BBN ring system (ca. 1.60–1.64).15
Though a few examples of BC4H8 and BC5H10 rings have been
previously reported, we believe these to be the first examples of
such rings at a tetraalkyl boron centre.16 The most closely related
analogues are trialkylborohydride moieties {(H)(PhCH2)BC4H8}
and {(H)(PhCH2)BC5H10} in Cp2Zr{(m-H)B(X)CH2Ph}{(m-
H)2BX} (X = C5H10, C4H8), which are linked to the zirconium
centre via a B–H–Zr bridge.17 All other examples involving a
(H)2BC4H8 or (H)2BC5H10 unit are linked to a metal centre by two
B–H–M bridges.
In conclusion, this unprecedented ring closure reaction at the
boron centre represents a facile and versatile synthesis for
tetraalkylborates, which will be further exploited.
Notes and references
§
Crystal data for 3: [(C4H8O)6Mg](C16H30B)2·4C4H8O, M = 1211.77,
monoclinic, P21/n (no. 14), a = 13.8628(19), b = 17.177(3), c = 15.580(2)
Å, b = 96.390(11)° V = 3686.9(9) Å3, Z = 2 (Ci symmetry), Dc = 1.092
g cm23, m(Cu–Ka) = 0.613 mm21, T = 203 K, colourless plates; 4964
independent measured reflections, F2 refinement, R1 = 0.073, wR2 =
0.204, 3417 independent observed reflections [¡Fo¡ > 4s(¡Fo¡), 2qmax =
120°], 426 parameters. CCDC 236834.
Crystal data for 4: [(C4H8O)5MgBr](C13H24B)·C4H8O, M = 727.98,
monoclinic, P21/c (no. 14), a = 10.953(2), b = 14.4213(16), c = 25.584(3)
Å, b = 93.597(15)° V = 4033.4(11) Å3, Z = 4, Dc = 1.199 g cm23, m(Mo–
16 A search of the Jan-04 update of the Cambridge Crystallographic
Database (version 5.25) revealed no examples of a tetraalkylborate
centre with BC4H8 or BC5H10 rings.
17 F.-C. Liu, J. Liu, E. A. Meyers and S. G. Shore, Inorg. Chem., 1999, 38,
2169.
C h e m . C o m m u n . , 2 0 0 4 , 1 7 3 8 – 1 7 3 9
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