Phosphine adducts of 1,2-dibromo-1,2-dimesityldiborane(4): Between bridging halides and rearrangement processes

Article abstract of DOI:10.1002/anie.201201673

To bridge or not to bridge depends on the size of the Lewis base in mixed sp²-sp³ phosphine adducts of Br₂B ₂Mes₂. Whereas reaction with bulky PMeCy₂ induced a 1,2-rearrangement to afford the 1,1-dimesityl adduct as the major component, the normal 1,2-dimesityl adduct was favored for the smaller PEt₃ molecule. The latter contains an uncommon B-Br-B bridge and a rare dative B-Hal bonding interaction to the sp² boron center. Copyright

Full text of DOI:10.1002/anie.201201673

Angewandte
Chemie
DOI: 10.1002/anie.201201673
sp2–sp3 Diboranes
Phosphine Adducts of 1,2-Dibromo-1,2-dimesityldiborane(4): Between
Bridging Halides and Rearrangement Processes**
Holger Braunschweig,* Alexander Damme, Jose O. C. Jimenez-Halla, Thomas Kupfer, and
Krzysztof Radacki
Modern organic chemistry and the production of high-value
chemicals strongly rely on the possibility to incorporate
a manifold of different functionalities into the organic
framework. In this regard, the boryl group has been
established a long time ago, combining versatile accessibility
with a unique chemical diversity. Valuable secondary prod-
ucts include for example alkenes, alcohols, and ketones. Of
equal importance is their outstanding significance as reagents
in Suzuki–Miyaura-type coupling reactions.
Various synthetic approaches, such as salt elimination,
hydroboration, or diboration reactions, have been developed
to enable a convenient introduction of the boryl group. In
Scheme 1. Known sp2–sp3 diboron compounds (Cy=cyclohexyl,
Mes=mesityl).
particular, the diboration has been of great interest in
facilitating the simultaneous generation of two reactive sites
from commercially available diborane(4) reagents, that is,
B2cat2 (cat = catecholato) and B2pin2 (pin = pinacolato).[1]
One major drawback of this approach is the requirement of
transition-metal catalysis in almost all cases, except for highly
reactive diborane(4) derivatives.[2] The first example of such
diboration reactivity was published back in 1954 for B2Cl4,
which readily adds to the double bond of ethylene in the
absence of any additive.[2a] However, catalytic processes are
by far predominant, for which reason current research focuses
on alternative metal-free systems.
Hoveyda et al. recently reported the efficient metal-free
b-boration of a,b-unsaturated ketones promoted by an N-
heterocyclic carbene (NHC).[3] A neutral 1:1 NHC adduct of
B2pin2 (1; Scheme 1) was proposed as the catalytically active
species, even though its exact structure remained unknown at
that time. Nevertheless, these findings stimulated research in
this area and other metal-free systems for the borylation of
organic substrates have been developed based on both neutral
and anionic sp2–sp3 diboron compounds.[4] As a result, more
detailed information on the actual composition of the organic
catalyst is now available, which clearly emphasizes the high
relevance of sp2–sp3 diboranes as intermediates in both
transition-metal[5] and organocatalyzed[4] borylation reac-
tions. Elegant spectroscopic and theoretical studies by
Marder et al. eventually verified the existence of the neutral
NHC adduct 1 both in solution and in the solid state.[6]
Furthermore, Kleeberg et al. recently succeeded in the
isolation of related anionic species B2pin2·XÀ (2; XÀ =
OMeÀ, OtBuÀ, 4-tBu-C6H4OÀ, FÀ; Scheme 1), which
showed a high potential in the metal-free borylation of
organic electrophiles.[7]
With these fundamental developments in mind, it appears
rather surprising that simple 1:1 adducts of diboranes(4) with
Lewis bases have been neglected for a long time. Accordingly,
the number of fully characterized sp2–sp3 diboron compounds
is still comparatively small. Initially, Marder and Norman
studied the reactivity of B2cat2 and B2(1,2-S2C6H4) towards
nitrogen and phosphorus donors to afford the corresponding
mono- and bis-adducts B2(1,2-E2C6H4)·L (3; E = O, S ; L = 4-
picoline, PMe2Ph, PEt3; Scheme 1) and B2(1,2-E2C6H4)·L2
(4).[8] Later on, intramolecular Lewis base coordination was
established for pinacolato(diisopropanolaminato)diboron (5;
PDIPA-diboron; Scheme 1), which was shown to be a highly
useful sp2–sp3 diboron reagent in the copper-catalyzed b-
boration of a,b-unsaturated substrates.[9]
Together with the more recently published derivatives
1 and 2, it becomes evident that the few known sp2–sp3
diboranes are exclusively derived from oxo- and sulfur-
based diborane(4) precursors. We became interested in the
coordination chemistry of more reactive halide-substituted
diboranes(4), that is, X2B2Mes2 (X = Cl, Br), as part of our
ongoing efforts to generate unusual low-coordinate boron
species. Coordination of the NHC SIMes (1,3-dimesityl-
[*] Prof. Dr. H. Braunschweig, Dipl.-Chem. A. Damme,
Dr. J. O. C. Jimenez-Halla, Dr. T. Kupfer, Dr. K. Radacki
Institut fꢀr Anorganische Chemie
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: h.braunschweig@mail.uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/
Braunschweig/
imidazolin-2-ylidene) to Cl2B2Mes2 entailed
a carbene-
induced rearrangement process to afford the asymmetric
sp2–sp3 diborane 6.[10] As such 1,2-aryl migrations are rare in
diborane chemistry (limited to an early report by Berndt et al.
[**] This work was supported by the Deutsche Forschungsgemeinschaft.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201673.
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from 1991),[11] we began to elucidate whether this rather
uncommon transformation depends on the nature of the
Lewis base. Thus, the coordination of simple phosphines to
Lewis-acidic diborane(4) Br2B2Mes2 results in the formation
of two constitutional isomers of Br2B2Mes2·(PR2R’) (7: R,
R’ = Et; 8: R = Cy, R’ = Me), the ratio of which is related to
the size of the Lewis base. A non-rearranged structure
X-ray diffraction served to validate the non-rearranged
structure of the sp2–sp3 diborane 7a in the solid state
(Figure 1).[19] The most exciting structural feature of 7a is
the coordination mode of the bromide ligand Br2, which
À
À
featuring a rare B Br B bridge is found for PEt3, while the
increased steric bulk of PMeCy2 obviously favors the
rearrangement event. Herein, we present the results of
these studies along with X-ray diffraction data on the two
major isomers 7a and 8b.
Reaction of Br2B2Mes2 with one equivalent of PEt3 in
C6D6 was fast and occurred spontaneously at room temper-
ature within seconds.[12] Examination of the reaction mixture
by 31P NMR spectroscopy indicated quantitative conversion
of the starting materials to afford the phosphine adduct
Br2B2Mes2·(PEt3) (7), which was eventually obtained as
a colorless solid in 76% yield (Scheme 2). Initially, we were
Figure 1. Molecular structure of 7a in the solid state. Ellipsoids are set
at 50% probability; hydrogen atoms are omitted for clarity.
adopts a bridging position between the two boron center B1
and B2. As a result of this interaction, the two faces of B1
(sp2) become diasterotopic, making the formation of 7a
a highly stereoselective process. Only diastereoisomer 7a with
both mesityl ligands in “trans” position is formed, while the
corresponding “cis” diastereoisomer has not been observed at
any time, which is presumably due to steric congestion. Even
Scheme 2. Synthesis of sp2–sp3 diboron compounds 7 and 8.
À
though the B1 Br2 distance (2.437(2) ꢀ) in 7a is consider-
ably elongated by 21.8% and 11.8% with respect to the
À
À
puzzled by the appearance of four well-separated signals in
“normal” B1 Br1 (2.004(2) ꢀ) and B2 Br2 bonds
(2.178(2) ꢀ), respectively, the presence of a dative bonding
interaction is clearly indicated by an extremely small value for
the room-temperature 11B NMR spectrum of 7 (d = 91.0, 58.1,
1
À1.6, À7.9) and a rather complex H NMR spectrum, which
À
was not consistent with the observation of one broad
31P NMR resonance (d = 0.19) and the formation of a single
species. However, the 31P NMR signal readily splits into two
distinct resonances below À358C in CD2Cl2 (d = À0.69 (7a),
0.15 (7b); at room temperature: d = 0.21 (7)), which even-
tually verified the presence of two constitutional isomers 7a
and 7b in solution. With this in mind, all of the 1H NMR and
11B NMR signals could clearly be assigned to the different
isomers, while integration of the former revealed a ratio 7a/
7b of 85:15. It should also be noted that the ratio does not
change with temperature (À808C to 708C), which precludes
any interconversion between 7a and 7b and suggests that this
reactivity is driven by thermodynamics. Evidence for the
structural nature of the isomers comes from 11B NMR
spectroscopy. Thus, adduct formation is substantiated for
both species by upfield-shifted 11B NMR signals at d = À1.6
(7a) and d = À7.9 (7b), which appear in the typical region for
four-coordinate boron centers. Together with the pronounced
downfield shift of the second 11B NMR signal of the minor
isomer 7b (d = 91.0), it becomes obvious that the major
isomer 7a is a normal 1:1 adduct between Br2B2Mes2 and
PEt3, while 7b features a rearranged structure resulting from
a phosphine-induced 1,2-migration of one mesityl and one
bromide ligand. Also, the 11B NMR signal of the main product
7a (d = 58.1) is shifted upfield by 28 ppm with respect to
Br2B2Mes2 (d = 86), which suggests some kind of additional
electron donation to the sp2 boron center in solution.
the B1-B2-Br2 angle (76.55(12)8). All of the B Br bonds are
longer than those of the diborane(4) precursor Br2B2Mes2
(1.928(4) ꢀ, 1.932(4) ꢀ),[13] which is most likely due to the
increased electron density at B2 after adduct formation.
Consequently, electron density is also enhanced at Br2, which
now facilitates a dative bonding interaction to the highly
electrophilic B(Mes)(Br) fragment. With this interaction in
mind, the unusual upfield shift of the 11B BMR signal of B1
observed in solution is also readily rationalized.
The presence of a bridging bromide strongly affects the
geometries at B1 and B2, which deviate significantly from
ideal trigonal-planar (sp2-B1) and tetrahedral (sp3-B2),
respectively. By contrast, the effect of adduct formation on
À
the B B bond lengths is rather marginal, and an elongation of
only 4 pm is observed with respect to Br2B2Mes2
(1.673(6) ꢀ),[13] which is thus much smaller than in the related
NHC sp2-sp3 diboranes 1 (1.743(2) ꢀ)[6] and 6 (1.774(3) ꢀ).[10]
The observation of a dative bonding interaction between
a halide and an sp2 boron center in a neutral diboron species is
of exceeding significance with respect to the chemistry of
simple boron trihalides. It is well-known that halide redis-
tribution reactions readily occur between most mixtures of
boron trihalides to afford rapidly established equilibria.[14]
Even though the kinetics of these processes have been studied
theoretically and by different spectroscopic methods, mech-
anistic details are still rare.[15] Halide redistribution most
likely proceeds by an associative mechanism involving
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Scheme 3. Halide redistribution equilibria in mixtures of boron halides
À
(top) and the dative B I bonding interaction in 9 (bottom).
Figure 2. Calculated parameters for the interconversion of constitu-
tional isomers 7acal and 7bcal via transition state 7TS.
a
halide-bridged, four-center transition state dimer
(Scheme 3).
Conclusive experimental evidence for the existence of
such dimeric species is however limited to few matrix
isolation experiments with BF3 in krypton and argon matrices
structure 8bcal is 17 kJmolÀ1 lower in energy than 8acal. We
were able to locate the transition states for the interconver-
sion of the two isomeric species 7acal/7bcal (Figure 2) and 8acal/
8bcal. Accordingly, activation barriers for the interconversion
of the two constitutional isomers are high (DE = 84.24–
101.34 kJmolÀ1), which is in agreement with our experimental
findings that such a process is not accessible under ambient
conditions. Furthermore, the transition-state structures 7TS
and 8TS suggest that the rearrangement event proceeds via
an initial 1,2-shift of one mesityl ligand.
at 20 K.[16] In fact, we are only aware of one single example in
[17]
À
which a related dative B Hal interaction could be verified:
À
A rather short B I separation distance (2.383(4) ꢀ) between
the two boryl fragments in cis-3,4-bis(diiodoboryl)hex-3-ene
À
(9) clearly substantiated the presence of a dative B I bond
À
(compare with other B I bonds: 2.107(4)–2.231(4) ꢀ). Here,
the cis configuration and conjugation of the p system with the
empty pz orbitals at boron already create the geometrical pre-
À
conditions for an effective B I dative bonding interaction,
Determination of the molecular structure of 8b eventually
confirmed the phosphine-induced rearrangement upon addi-
tion of the bulky PMeCy2 to Br2B2Mes2. Again, isolation and
structural characterization was only successful in the case of
the major isomer 8b (Figure 3).[19] Most of the geometrical
parameters are not affected by the 1,2-shift of the mesityl and
which is clearly not the case in 7a, making its bonding
situation somewhat unique. All efforts to further substantiate
the nature of the minor reaction product 7b failed, as we were
not able to separate 7b from the reaction mixture.
The reaction of Br2B2Mes2 with PMeCy2 was used to
elucidate the steric influence of the Lewis base on the
observed reactivity patterns and the ratio of the resulting two
isomeric products. Again, adduct formation was fast and
proceeded quantitatively within seconds to afford sp2–sp3
diborane Br2B2Mes2·(PMeCy2) (8) as a colorless substance
in 88% yields.[12] Examination of the NMR spectroscopic
parameters in solution indicated the presence of two constitu-
tional isomers 8a and 8b in a ratio of 1:3, as evidenced by
their characteristic 11B NMR data (C6D6: 8a d = À0.4, 60.3;
8b d = À6.4, 95.5 ppm) and 31P NMR chemical shifts (C6D6:
8a d = 1.45; 8b d = À4.76; CD2Cl2: 8a d = À1.97; 8b d =
À4.35 ppm). However, coordination of the more bulky
PMeCy2 obviously favors the formation of the 1,2-shifted
product 8b, which is now the major component of the reaction
mixture. These findings clearly imply that the size of the
phosphine ligand exerts a strong influence on the strength of
the dative B Br bonding interaction. Thus, the larger PMeCy2
most likely prevents any effective B Br interaction stabilizing
a “normal” adduct structure (8a), for which reason the
phosphine-induced rearrangement pathway becomes more
favorable (8b).
This scenario is further supported by DFT calculations on
7 and 8 at the PBE level of theory.[18] To this end, geometry
optimizations were performed on both isomers 7a/7b and 8a/
8b without symmetry restraints. In any case, adduct formation
is an exothermic process regardless of the product nature
(DE = À34.83 to À63.14 kJmolÀ1). Consistent with our exper-
imental results, reaction with PEt3 energetically favors the
“normal” adduct structure 7acal over 7bcal by approximately
6 kJmolÀ1 (Figure 2), while for PMeCy2 the rearranged
Figure 3. Molecular structure of 8b in the solid state. Ellipsoids are set
at 50% probability; hydrogen atoms are omitted for clarity.
À
À
À
À
bromine ligands. Thus, the B B (1.749(3) ꢀ), B P
À
(1.988(2) ꢀ), and B C bond lengths (1.595(3) ꢀ,
1.602(3) ꢀ) lie in a similar region to those of 7a featuring
a “normal” adduct structure. Notable structural differences to
À
7a, such as the B Br distances (2.067(3) ꢀ, 2.053(2) ꢀ;
À
À
compare with 7a: B2 Br2 2.178(2) ꢀ; B2 Br1 2.437(2) ꢀ),
clearly result from the absence of a bridging halide in
rearranged 8b, which is also highlighted by a regular trigo-
nal-planar geometry at B2 (ꢀB2 = 360.08). Deviations from
the tetrahedral coordination sphere at B1 are caused by the
larger size of the PMeCy2 ligand. These findings imply that
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the steric requirements of the phosphine ligand play a critical
role in determining the nature of the product.
and subsequent Lewis base coordination appears very
unlikely.
Similar conclusions come from the following displacement
experiment. Reaction of a benzene solution of 8 as its
thermodynamic mixture (8a/8b = 1:3) with the smaller and
more Lewis-basic PEt3 results in the spontaneous exchange of
the phosphine donor, as readily evidenced by the appearance
of characteristic signals for 7 and free PMeCy2 in the 31P NMR
spectrum of the reaction mixture (Scheme 4). However, the
In conclusion, we have demonstrated that coordination of
simple phosphines (PEt3, PMeCy2) to electrophilic Br2B2Mes2
results in mixtures of two different types of sp2–sp3 diboron
species, the relative ratio of which strongly depends on the
size of the Lewis base. Thus, reaction with the more bulky
PMeCy2 afforded 8b as the major component formed by
phosphine-induced 1,2-rearrangement of mesityl and bro-
mine ligands. By contrast, the “normal” adduct structure 7a
was strongly favored when the smaller PEt3 was chosen as
reagent. Examination of the molecular structure of 7a by X-
À
À
ray diffraction revealed a B Br B bridge and the presence of
2
À
a dative B Hal bonding interaction to the sp boron center.
Such interactions have been suggested to be involved in
halide rearrangement processes observed for mixtures of
boron trihalides, while conclusive evidence for their existence
has remained exceedingly rare and limited to a few studies to
date. Furthermore, theoretical calculations showed that
adduct formation is an exothermic process in all cases and
that any interconversion of the constitutional isomers has to
be considered unlikely owing to large activation barriers,
results that are consistent with experimental findings.
Scheme 4. Ligand-exchange reaction and rearrangement of 8 by the
reaction with PEt3.
Received: March 1, 2012
Published online: May 10, 2012
initial isomer ratio of 1:3 for 8a/8b is not retained at all.
Instead, the above-mentioned 85:15 ratio for 7a and 7b
demanded by thermodynamics is established immediately.
We next tried to remove the phosphine ligand in 8b,
aiming at the generation and isolation of the unsymmetrical
Keywords: boron · bridging ligands · halogens · phosphines ·
.
rearrangements
À
diborane(4) Mes2B BBr2. When a reaction mixture contain-
ing 8 was treated with B(C6F5)3, the new phosphine–borane
À
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À
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[12] Experimental details are provided in the Supporting Informa-
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[18] Details on the theoretical calculations are provided in the
Supporting Information.
[19] CCDC 869358 (7a) and CCDC 869359 (8b) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[13] H. Hommer, J. Knizek, H. Nçth, W. Ponikwar, H. Schwenk-
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