Synthesis of a bicyclic diborane by selective boron carbon bond formation

Article abstract of DOI:10.1039/c3cc38789e

The bicyclic diboron species 1 was generated with high selectivity by a combination of boron carbon bond formation and hydrogen iodide elimination events. The nature of the boracycle 1 was evaluated using X-ray diffraction, NMR spectroscopy and density functional theory calculations. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc38789e

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Synthesis of a bicyclic diborane by selective boron
carbon bond formation†
Cite this: Chem. Commun., 2013,
49, 2774
Holger Braunschweig,* Alexander Damme and Thomas Kupfer
Received 7th December 2012,
Accepted 14th February 2013
DOI: 10.1039/c3cc38789e
www.rsc.org/chemcomm
The bicyclic diboron species 1 was generated with high selectivity
Based on these findings, we set out to elucidate the reactivity
by a combination of boron carbon bond formation and hydrogen of the more reactive halide-substituted diboranes(4) X2B2Mes2
iodide elimination events. The nature of the boracycle 1 was (X = Cl, Br) towards various Lewis bases. It turned out that the
evaluated using X-ray diffraction, NMR spectroscopy and density coordination of simple phosphines such as PEt3 and PMeCy2
functional theory calculations.
Br2B2Mes2 results in the formation of two constitutional
isomers Ia/Ib and IIa/IIb, whose relative ratios depend on the size
The development of metal-free alternatives of catalytic boryla- of the Lewis base.6 While a non-rearranged structure featuring a
tion and diboration processes has considerably advanced rare B–Br–B bridge is found for PEt3 (IIa), the sterical demand
during the last few years due to their crucial significance in of PMeCy2 evidently favors the 1,2-migration of one mesityl
preparative organic chemistry and of course due to economic and one bromide ligand to afford structure Ib as the major
reasons.1 Until very recently, the introduction of boryl groups component. A similar rearrangement process was observed
into organic substrates strongly relied on the classical hydro- upon coordination of NHC to Cl2B2Mes2 (III).7
boration protocol2 or the presence of catalytically active transi-
tion metal species.3 Only highly reactive boron reagents such as
B2Cl4, which is capable of functionalizing the double bond of
ethylene,4a were found to react in the absence of any additive.4
Recent results have emphasized the high value of neutral (A, B)
and anionic sp2–sp3 (C) diboranes in transition metal5 and
organo-catalyzed1 borylation processes. The catalytically active
species is formed either by (i) intermolecular coordination of
Lewis bases such as phosphines or N-heterocyclic carbenes
(NHC; A),1 (ii) intramolecular Lewis base coordination of the
nitrogen donor in pinacolato(diisopropanolaminato)diboron
Consequently, we now present our results regarding the
(B)1g or (iii) reaction of diboranes(4) with anionic nucleophiles interaction of amine donors with electrophilic I2B2Mes2.8
(C; XÀ = OMeÀ, OtBuÀ, 4-tBu–C6H4OÀ, FÀ).1f
Surprisingly, reaction with excess NEt3 goes along with formal
HI elimination and B–C bond formation to selectively afford the
bicyclic diboron enantiomers R,R-1 and S,S-1 featuring a B–I–B
bridge (Scheme 1). Such a B–C bond formation process is
without precedence in the literature providing an alternative
approach to the well-known salt-metathesis reactions of
lithium organyls and B–Hal bonds.9
B–C bond formation is most likely facilitated by the precipitation
of [Et3NH][I]10 from the pentane reaction medium, which provides
an extra driving force for the elimination of HI. Accordingly,
reactions proceed much slower and become increasingly unselective
in solvents of higher polarity such as benzene or CH2Cl2, for
which reason pentane is considered the solvent of choice for this
transformation. In addition to colorless [Et3NH][I], 1 precipitates
from solution as yellow crystals, which can be separated by
¨
¨
¨
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat, Wurzburg, Am
¨
Hubland, D-97074 Wurzburg, Germany. E-mail: h.braunschweig@uni-wuerzburg.de;
Fax: +49 931 31 84623; Tel: +49 931 31 85260
† Electronic supplementary information (ESI) available: Experimental details and
information on theoretical and crystallographic studies. CCDC 912614. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc38789e
c
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Scheme 1 Synthesis of racemic boracycles R,R-1 and S,S-1.
gradual sedimentation to afford 1 in 54% isolated yield.
Even though mechanistic details are not available so far, it
appears plausible that the reaction involves initial formation
of an adduct I2B2Mes2ÁNEt3. However, all attempts to obtain
conclusive evidence for the existence of a simple adduct A or
rearranged diborane species I–III consistently failed so far,
regardless of the applied reaction conditions (stoichiometry,
temperature and solvent). Thus, no reaction is observed below
temperatures of 0 1C (VT NMR; C7D8), and only 1 and I2B2Mes2
(d 89)8 can be identified with all certainty in the 11B NMR
spectra of the reaction mixture, even if only one or less
equivalents of NEt3 are used.
Fig. 1 Molecular structure of R,R-1 in the solid state. Thermal ellipsoids are
displayed at the 50% probability level. For clarity, hydrogen atoms and thermal
ellipsoids of the carbon atoms have been omitted. The racemic isomer S,S-1 is not
shown. Selected bond lengths [Å] and bond angels [1]: B1–B2 1.710(4), B1–N1 1.666(3),
B1–C1 1.608(4), B1–I1 2.501(3), B2–I1 2.645(3), B2–C2 1.611(4), B2–C3 1.612(4),
B1–I1–B2 38.70(9), B1–B2–I1 66.10(13), B2–B1–I1 75.20(14), N1–B1–I1 102.31(15),
C3–B2–I1 99.72(16), C1–B1–B2 135.8(2), N1–B1–B2–C3 À4.3(2).
Characterization of 1 posed no difficulties and the results of
NMR spectroscopy (1H, 11B, 13C), elemental analysis, and X-ray B2–I2 2.158(7) Å),8 and rather small values are found for all
diffraction clearly substantiate HI abstraction and formation of relevant bond angles (B1–B2–I1 66.10(13)1; B2–B1–I1 75.20(14)1;
a boracycle. The unsymmetrical environment of the two stereo- B1–I1–B2: 38.70(9)1). These data also indicate that the iodine
genic centers in R,R-1 and S,S-1 is clearly evident in all solution bridge is somewhat asymmetric with a stronger bonding inter-
NMR spectra of 1 (CD2Cl2). Thus, each hydrogen atom of the action between B1 and I1. Consequently, the geometry of B1 is
CH2 groups entails its own multiplet in the 1H NMR spectrum, rather distorted tetrahedral, while B2 is close to trigonal planar
while the resonances of the boron-bound CH2 group appear at with the empty pz orbital pointing towards the iodine lone
considerably higher field (d 2.14, 1.70; cf. d 3.83, 3.60, 3.50, 3.34, pairs. It should also be noted that the B1–B2 (1.710(4) Å) and
3.10, 3.00). In addition, 1H NMR signals of the mesityl protons B–C bond lengths (B1–C1 1.608(4) Å; B2–C2 1.611(4) Å) are larger
are now separated for both the CH (d 6.89, 6.75, 6.72) and CH3 than those in the precursor molecule I2B2Mes2 (B1–B2 1.664(9) Å;
groups (d 2.55, 2.23, 2.20, 2.09, 1.98). Similar results are found B1–C1 1.558(8) Å; B2–C2 1.548(8) Å).8
in the 13C NMR spectrum. The 11B NMR spectrum of 1 features
The uncommon bonding situation within the bridging B2I
two well-separated resonances at d = 30.7 (B1) and d = 41.5 (B2) fragment can further be evaluated by comparison with the
for the N- and CH2-bound boron nuclei, respectively. The B–Br–B bridge present in IIa.6 Thus, highly acute bond angles
chemical shift of both resonances is rather uncommon are found for the bridging halides in the molecular structures
for tetra-coordinated boron centers, which nicely reflects the of 1 (38.70(9)1) and IIa (43.1(9)1).6 Also, the B–I bonds in 1 are
bridging nature of the iodine ligand. However, the relative elongated by 15% (B1–I1) and 22% (B2–I1) with respect to
positions of the 11B NMR signals are as expected, that is the I2B2Mes2, which is of similar magnitude to the B–Br bond
nitrogen-bound B1 (d 30.7) appears at higher field than that of elongation observed in IIa (B1–Br1: 12%; B2Á Á ÁBr1: 26%). The
the CH2-substituted B2 (d 41.5), while both signals are con- differences highlight the stronger dative boron halide inter-
siderably shifted to higher field with respect to the diborane(4) action in 1 (Table 1). It should be emphasized that such dative
precursor I2B2Mes2 (d 89; DdB1 = 57 ppm, DdB2 = 46 ppm).8
boron halide interactions are still rare in neutral borane
The presence of a bicyclic diboron species was eventually species, and we are only aware of one other structurally
verified using X-ray diffraction of single crystals of 1 (Fig. 1). In characterized example reported by Siebert et al. Here, a dative
the solid state, the molecular structure of 1 features a central B–I bonding interaction (2.383(4) Å) was revealed in cis-3,4-
planar five-membered heterocycle (N1–B1–B2–C3 À4.3(2)1). bis(diiodoboryl)hex-3-ene, in which however, the geometrical
The bridging iodine atom is located almost perpendicularly preconditions of the cis configuration and the conjugated
above the B–B bonding axes (N1–B1–I1 102.31(15)1; C3–B2–I1 p-system promote an effective B–I interaction.11
99.72(16)1) (Fig. 1), thus completing the bicyclic structure. Since
DFT calculations on the B3LYP level of theory served to
(i) each boron atom is a stereogenic center and (ii) 1 crystallizes further assess the bonding situation within the B2I core of 1. To this
%
in the triclinic space group P1, a racemic mixture consisting of end, geometry optimizations were performed on R,R-1 without
enantiomers R,R-1 and S,S-1 is present. The bridging nature of symmetry restraints. Thus, the HOMO À 1 of R,R-1OPT is clearly
the iodine atom is nicely illustrated by the structural character- associated with B1–B2 s-bonding, while the LUMO + 1 represents
istics of the tri-membered B2I moiety. Thus, the B–I the empty p*-orbital (Fig. 2). The most significant orbital for
bond distances (B1–I1 2.501(3) Å; B2–I1 2.645(3) Å) are signifi- B–I bonding interactions is the HOMO À 8, which involves
cantly elongated with respect to I2B2Mes2 (B1–I1 2.165(6) Å; both boron centers. In addition, the calculated 11B NMR
c
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Table 1 Parameters relevant to dative boron–halide bonding interactions in 1
and IIa
´
´
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B1–X1a,b
B2–X2a,b
B2Á Á ÁX1a,b
I2B2Mes2
1
Br2B2Mes2
IIa
2.165(6)
2.501(3)
1.928(4)
2.178(2)
2.158(7)
—
1.932(4)
2.004(2)
—
2.645(3)
—
´
´
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2.437(2)
a
b
Distances given in Å. X = Br, I.
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Fig. 2 Calculated orbitals of R,R-1OPT used to describe the bonding situation in
the B2I core.
spectroscopic parameters of R,R-1OPT (dB1 31.0; dB2 52.4) are in
fairly good agreement with experimental values (dB1 30.7; dB2 41.5),
which suggests a reasonable description of the bonding situation
by the theoretical studies.
In summary, we unveiled a novel B–C bond formation
pathway, which was shown to be effective during reaction of
I2B2Mes2 with the Lewis base NEt3. In contrast to the reactivity
of diboranes(4) towards phosphines and NHCs, no simple
adduct is formed, but a sequence of B–C bond formation and
HI elimination processes is observed to selectively afford the
5-membered boracycle 1 as a racemic mixture of R,R-1 and
S,S-1. The iodine ligand in 1 adopts a rare B–I–B bridging
position, which is still uncommon in boron chemistry.
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c
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2776 Chem. Commun., 2013, 49, 2774--2776