Quaternizing diboranes(4): Highly divergent outcomes and an inorganic Wagner-Meerwein rearrangement

Article abstract of DOI:10.1021/ja403037k

Apart from a few compounds under heavy use in organic chemistry, diboranes are relatively exotic and poorly understood. Recently, interest in these molecules has intensified with the advent of so-called sp ²-sp³ diboranes which exhibit useful reactivity toward organic substrates. In our hands, addition of Lewis bases to dihalodiorganyl diboranes(4) has previously shown some very surprising reactivity, including a substituent exchange between the boron atoms, and diboranes in which halide atoms bridge the B-B bond. Herein we have expanded the range of diborane(4) and Lewis base reaction partners, in the process uncovering three new reactivity patterns as well as some cases where mixtures are obtained. Trends are established for the preferential formation of certain products which rationalize the results based on electronic and steric effects. The substituent exchange, clearly based on an inorganic version of the well-known Wagner-Meerwein rearrangement, was also found to be an equilibrium reaction with the halide-bridged Lewis adducts on the other side.

Full text of DOI:10.1021/ja403037k

Article
pubs.acs.org/JACS
Quaternizing Diboranes(4): Highly Divergent Outcomes and an
Inorganic Wagner−Meerwein Rearrangement
Holger Braunschweig,* Alexander Damme, Rian D. Dewhurst, Thomas Kramer, Thomas Kupfer,
Krzysztof Radacki, Eva Siedler, Alexandra Trumpp, Katharina Wagner, and Christine Werner
Institut fur Anorganische Chemie, Julius-Maximilians-Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: Apart from a few compounds under heavy use
in organic chemistry, diboranes are relatively exotic and poorly
understood. Recently, interest in these molecules has
intensified with the advent of so-called “sp2−sp3” diboranes
which exhibit useful reactivity toward organic substrates. In our
hands, addition of Lewis bases to dihalodiorganyl diboranes(4)
has previously shown some very surprising reactivity, including
a substituent exchange between the boron atoms, and
diboranes in which halide atoms bridge the B−B bond.
Herein we have expanded the range of diborane(4) and Lewis
base reaction partners, in the process uncovering three new reactivity patterns as well as some cases where mixtures are obtained.
Trends are established for the preferential formation of certain products which rationalize the results based on electronic and
steric effects. The substituent exchange, clearly based on an inorganic version of the well-known Wagner−Meerwein
rearrangement, was also found to be an equilibrium reaction with the halide-bridged Lewis adducts on the other side.
INTRODUCTION
■
The vastly disparate levels of interest in chains of carbon (i.e.,
organic chemistry) and hypovalent boron belies their proximity on
the periodic table. Synthetic and structural understanding of organic
compounds has reached a stunning level of sophistication,1 while in
contrast, even compounds as simple as diboranes(4) are still poorly
understood. Diboranes(4), despite their increasing popularity in
organic chemistry,2 still suffer from synthetic limitations and
unexplored reactivity. The many recent surprises in diborane(4)
chemistry, particularly with regard to their interaction with Lewis
bases, serve as regular reminders of how little we know these
compounds. In the 1990s, the groups of Marder and Norman3
explored the chemistry of single and double pyridine adducts of
diboranes(4). More recently, the groups of Santos,4 Hoveyda,5
́ ́
Gulyas, Bo and Fernan
dez,6 Hodgkinson, and Lin and Marder7
have noted changes in reactivity of diboranes when one of the
boron atoms is tetracoordinate, particularly in conjugate addition
and diboration reactions. Currently, these research groups are
changing the way we think about diboranes, and the field now
known as sp2−sp3 diboranes is very promising indeed.
Figure 1. Comparison of the Wagner−Meerwein rearrangement with
the proposed mechanism of the recently reported base-induced
diborane(4) rearrangement. L = Lewis base.
In 2011, we reported the 1,2-exchange of a mesityl group with a
chloride upon addition of an N-heterocyclic carbene (NHC) to a
diborane(4).8 Recently we also discovered a diborane in which
one bromide substituent bridged the two boron atoms upon
addition of a phosphine, followed by a halide-bridged diborane
resulting from a C−H activation of triethylamine.9 It seems
obvious that such a halide-bridged compound is an intermediate in
the mesityl/halide 1,2-exchange.
shift that occurs during the generation of a carbocation in strained
alkane systems, known as the Wagner−Meerwein rearrange-
ment.10 In both cases a substituent migrates from an sp3 atom to
an adjacent atom bearing an empty p orbital (Figure 1), although
The 1,2-halide/mesityl exchange reaction of diboranes(4) in
Received: March 26, 2013
the presence of a Lewis base is reminiscent of the 1,2-sigmatropic
© XXXX American Chemical Society
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Figure 2. Reaction of 1,2-dihalodiboranes(4) with Lewis bases leads to a range of different classes of compounds. Compound numbers in red denote
those that have been published previously. Percentages in parentheses are the relative amounts of each isomer measured by NMR.
in the diborane case this rearrangement must occur twice to
provide the products.
small Lewis base PMe3 to mesityl diboranes(4) 1a−c led
exclusively to the halide-bridged adducts 2a, 3a, and 4a (Figure 2).
No rearranged adducts were detected either by NMR
spectroscopy or X-ray crystallography. When the slightly larger
base PEt3 was added to the diboranes 1a−c, inseparable
mixtures of halide-bridged and rearranged adducts 5a/b, 6a/b
and 7a/b were formed, respectively. In each case, the halide-
bridged adduct (5a/6a/7a) predominated in C6D6 solution
(87−93% relative to total yield as judged by 1H NMR
spectroscopy). The relative preference for rearrangement
increases as the halide becomes heavier (Cl < Br < I). When
the more sterically hindered phosphine PMeCy2 was applied to
the diboranes(4) 1a−c, mixtures are again obtained, however
the isomeric preference is flipped. In these reactions the
rearranged adducts (8b/9b/10b) are preferred over the halide-
bridged adducts (8a/9a/10a). As in the compounds 5−7,
the relative preference for rearrangement increases as the halide
becomes heavier. Interestingly, for the compounds which were
observed as mixtures (5−10), the product ratios did not change
at lower temperatures as judged by variable-temperature NMR
experiments. When much larger NHCs IDip and SIMes are
This article contains an exploration of the reactivity of
dihalodiboranes(4) in the presence of a range of phosphine and
NHC Lewis bases. This reaction results in an astounding
diversity of products, which can be favored or disfavored by
altering the diborane(4) substituents or the Lewis base. We
investigate the scope and mechanism of the 1,2-exchange
reaction with reference to the Wagner−Meerwein rearrange-
ment and determine the preferences of each diborane substrate
for the halide-bridged or rearranged adducts. We also provide
experimental confirmation that in some cases an equilibrium
exists between these two isomers. Depending on the reagents,
adducts both with and without bridging halide groups can be
observed, while other combinations lead to halide displacement
and borenium formation, or C−H activation of a carbene side-
group.
RESULTS AND DISCUSSION
■
Synthetic Studies. In contrast to our previously reported
additions of Lewis bases to diboranes(4),8,9 addition of the
B
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added to the diboranes(4) 1a and 1b, the rearranged adducts
11b/12b/13b are the exclusive products. Overall, these results
show a very clear trend for an increasing preference for
rearrangement with increasing size of the Lewis base, from no
rearrangement with PMe3 to complete rearrangement with
large NHCs. A large base appears to be a requirement for the
rearrangement, perhaps as a way of “pushing” one halide closer
to the second boron atom. It should be noted here that the
solid material obtained from the reactions of 1a−c with
phosphines PEt3 or PMeCy2 showed slightly different isomeric
ratios when dissolved in CD2Cl2. This suggests that the relative
amounts of each isomer in the mixtures are not static and that
the two isomers may exist in an equilibrium, the position of
which is slightly different in a different solvent.
The tert-butyl-substituted diborane(4) 1d provides a distinct
contrast to the above reactivity. Treating 1d with PMe3, PEt3,
IMes or SIMes leads to the simple, unbridged adducts 14c/
15c/16c/17c, regardless of the size or nature of the Lewis base.
As a rationale for this behavior, we cannot ignore the fact that
the organyl substituents (mesityl) which migrate have sp2
carbon atoms bound to the boron centers, while those that
do not migrate (tert-butyl) have sp3 carbon atoms. The latter
are presumably much less likely to form three-membered CBB
transition states (dictating a pseudopentacoordinate carbon
atom) that were calculated to be intermediates in this mi-
gration.9 The lack of halide bridging is presumably due to the
chloride substituent’s lower propensity toward bridging
combined with the lower steric bulk of the tert-butyl group
(i.e., a lack of methyl groups that point toward the boron atoms
as in the mesityl-substituted compounds).
Although the addition of Lewis bases to the diborane(4) 1d
led in four cases to simple, unbridged adducts, attempting this
reaction with the NHC ItBu (1,3-bis-(tert-butyl)imidazol-2-
ylidene) provided an unexpected product. The white solid
obtained had one high-field 11B NMR signal, and the 1H NMR
spectrum indicated that only one B-tBu group was present per
NHC unit. Single crystal X-ray crystallography (see Supporting
Information) indicated that a C−H bond of one carbene tert-
butyl group, as well as the B−B bond, had been cleaved,
resulting in the bicyclic species 18d (Figure 2). Based on this
formula, a yield of 97% was calculated. We had earlier reported
the synthesis of a similar bicyclic species,8 however, this was
prepared using monoborane starting materials. Although it was
not detected, the presumed byproduct of this reaction is the
monoborane HBCl(tBu) or a dimer thereof.
The boron atoms of 1,2-diamino-1,2-dihalodiboranes(4)
were expected to be reluctant to accept Lewis base donation,
given the effective π donation from the amino groups. Thus the
reaction of 1e with the carbene IDip led to divergent products
from the displacement of the bromide by the carbene. These
products, 19e and 20e (Figure 2), are the first known base-
stabilized borenium cations based on a diborane backbone (i.e.,
diborenium cations).11
Table 1. Summary of Spectroscopic Data for the Products of
a
the Base-Addition Reactions
solvent
11B (BRX/BR2)
11B (L→B)
31P
Halide-Bridged Adducts, a Series
2a
3a
4a
5a
C6D6
CD2Cl2
C6D6
80.8
56.5
41.8
81.0
75.4
58.1
37.1
37.1
80.8
60.3
61.0
n.d.
−4.11
−2.31
1.61
−9.37
−10.4
−14.3
3.64
Spectroscopic Studies. Selected 11B and 31P NMR
spectroscopic data for the compounds/mixtures 2−18 can be
found in Table 1. The most apparent features of these data are
the high-field 11B signals (δ −20.8 to 4.70) for the base-stabilized,
quaternary boron atoms and the low-field signals (δ 37.1 to 95.5)
C6D6
−2.50
−2.77
−1.62
2.30
CD2Cl2
C6D6
3.54
6a7
7a
0.19
C6D6
−2.24
−0.14
−2.26
−1.45
−1.97
−4.28
CD2Cl2
C6D6
4.70
8a
9a7
−1.02
−0.43
1.30
C6D6
CD2Cl2
C6D6
10a
5b
n.d.
Rearranged Adducts, b Series
C6D6
CD2Cl2
C6D6
n.d.
n.d.
n.d.
2.11
2.16
94.6
6b7
7b
91.0
−7.87
n.d.
−0.25
−3.36
−3.04
−2.26
−4.76
−4.35
−8.93
n.a.
C6D6
90.0
CD2Cl2
C6D6
90.3
−20.4
−1.02
−6.74
−6.62
−20.8
−0.17
−5.03
0.60
8b
9b7
93.6
C6D6
95.5
CD2Cl2
C6D6
94.6
10b
11b6
12b
13b
88.6
CD2Cl2
CD2Cl2
CD2Cl2
87.4
79.7
n.a.
85.3
n.a.
Unbridged Adducts, c Series
14c
15c
16c
17c
C6D6
C6D6
C6D6
C6D6
95.2
95.0
93.9
91.4
−1.89
−1.2
−1.5
−1.3
−13.18
3.3
n.a.
n.a.
C−H Activation Product, d Series
n.a. 3.42
Figure 3. Molecular structures of 2a, 3a, 5a, and 7a. Thermal ellipsoids
represent 50% probability. Aryl and alkyl substituents have been
simplified for clarity. Selected bond lengths and angles can be found in
Table 2.
18d
C6D6
n.a.
a
n.d. is not determined; n.a. is not applicable; and chemical shifts are
expressed in ppm.
C
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for the nonbase-stabilized boron atoms. In most cases, these signals
are very broad as a result of 11B−11B coupling. The distance
between the two signals for each compound is also much larger for
the compounds without halide bridging (b and c series) than those
of the halide-bridged isomers (a series). Presumably the bridging
halide atoms exert a ″quaternizing″ effect on the secondary boron
atoms, resulting in higher-field 11B NMR signals. The tricoordinate
boron nuclei of the unbridged adducts 14c/15c/16c/17c show
11B NMR signals (δ 91−95) further downfield of those of the
precursor diborane(4) (δ 84.3),12 a strong indication that no
bridging halide is present in solution. The 31P NMR signals of the
phosphine adducts for the most part lie in a small range on either
side of zero (δ −14.3 to 3.64). No significant distinction in the 31P
NMR properties can be observed between the a, b, or c series. The
C−H activated product 18d exhibits an 11B NMR resonance in the
expected range for a quaternary boron nucleus (δ 3.42). Only single
11B NMR signals were found for the base-stabilized diborenium
salts 19e (δ 37.9) and 20e (δ 37.7). Averaging of the two boron
signals due to rapid swapping of the base between boron atoms was
ruled out by 1H and variable-temperature 11B NMR, thus we attri-
bute the single 11B NMR signals to either accidental super-
imposition of the signals or extreme broadness of the borenium
nuclei signal due to B−B coupling, making them invisible.
Selected structural parameters for these compounds can be found in
Table 2. The mesityl groups of the halide-bridged isomers are found,
in each case, on opposite sides of the B2 unit. Therefore, while
different diastereomers of these molecules are possible in theory,
only one is observed, it being a racemic mixture of the two cor-
responding enantiomers. The two series of compounds (a and b)
have obvious structural differences related to the presence of bridgi-
ng halide groups. For instance, short B2−X1 distances (2.44−
2.62 Å) and acute B2−B1−X1 angles (82.6−87.9°) are found in the
a series, while the B2−B1−X1 angles are all obtuse in the b series.
An unexpected difference between the two series is the significantly
shorter B1−B2 distances in the a series, perhaps also a result of the
bridging halide groups.
One possible quantifier of the extent of halide bridging in the
complexes of the a series is the B2−B1−X1 angle. This angle is
smaller when the base is larger (i.e., PEt3 vs PMe3), and when
the base is kept constant, is smaller when the halide is heavier.
A second method for measuring this effect is to compare the
distance of the weak B2−X1 bond with that of the conventional
B2−X2 bond. The parameter ΔBX denotes the d(B2−X1)/
d(B2−X2) ratio. As in the B2−B1−X1 angles, this parameter is
lower (i.e., stronger bridging) with larger phosphines and
heavier halides. However, these ΔBX values are significantly
higher than a previously reported diborane, which contained an
almost symmetrically bridging iodide group and had a ΔBX
value of 1.06.9b Both of these metrics point to a higher
propensity toward bridging with larger Lewis bases and heavier,
more polarizable halides. As expected, the B1−Ccarbene distances
of the NHC adducts 11b/12b/13b are much shorter than the
B1−P1 distances of comparable phosphine adducts 9b/10b/
11b, however, the nature of the Lewis base appears to have
little effect on the B1−B2 distance in these complexes.
Structural Studies. Molecular structures of the halide-
bridged isomers 2a, 3a, 5a, and 7a are shown in Figure 3, and
the rearranged adducts 8b, 10b, 12b, and 13b in Figure 4.
A crystal structure determination of diborane(4) base adduct
15c showed clearly the absence of a bridging chloride atom, with a
B2−B1−X1 angle corresponding to a tetrahedral base-stabilized
boron atom. C−H activated species 18d was found to be structurally
Figure 4. Molecular structures of rearranged adducts 8b, 10b, 12b,
and 13b. Thermal ellipsoids represent 50% probability. Aryl and alkyl
substituents have been simplified for clarity. Selected bond lengths and
angles can be found in Table 2.
Figure 5. Molecular structures of the base-stabilized borenium salts 19e
(with SIMes) and 20e (with IDip). Thermal ellipsoids represent 50% pro-
bability. Carbene substituents have been simplified and counterions omitted
for clarity. Selected bond lengths and angles can be found in Table 2.
D
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a
Table 2. Summary of Crystallographic Data for the Products of the Base-Addition Reactions
B1−B2 (Å)
B1−P1/Ccarbene (Å)
B1−X1 (Å)
B2−X1 (Å)
B2−X2 (Å)
ΔBX
B2−B1−X1 (°)
Halide-Bridged Adducts, a Series
2a
3a
1.721(3)
1.719(3)
1.715(3)
1.711(3)
1.711(2)
1.970(2)
1.9753(18)
1.9896(18)
1.990(2)
1.955(2)
2.1572(18)
1.9754(18)
2.178(2)
2.555(2)
2.6197(19)
2.4439(18)
2.437(2)
1.812(2)
1.9820(19)
1.8177(18)
2.004(2)
1.41
1.32
1.34
1.22
1.15
87.85(12)
84.22(10)
82.62(9)
76.5(1)
5a
6a7
7a
1.9904(18)
2.3989(17)
2.5867(17)
2.2496(17)
75.96(8)
Rearranged Adducts, b Series
8b
9b7
1.755(5)
1.749(3)
1.769(7)
1.774(3)
1.762(4)
1.758(2)
1.975(4)
1.988(2)
1.967(4)
1.653(2)
1.652(4)
1.639(2)
1.901(4)
2.067(3)
2.293(4)
1.906(2)
2.061(3)
1.9020(16)
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
1.886(4)
2.053(2)
2.283(4)
1.889(2)
2.083(3)
1.8994(17)
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
107.7(2)
103.8(1)
102.6(2)
101.2(1)
98.64(15)
112.24(10)
10b
11b6
12b
13b
Unbridged Adducts, c Series
1.945(3) n.a.
C−H Activation Product, d Series
1.9728(19) n.a.
Base-Stabilized Borenium Salt, e Series
15c
18d
1.724(4)
n.a.
2.018(3)
1.657(3)
1.816(3)
n.a.
n.a.
n.a.
109.9(2)
n.a.
19e
20e
1.710(3)
1.707(3)
1.612(3)
1.608(3)
n.a.
n.a.
n.a.
n.a.
1.996(3)
1.984(2)
n.a.
n.a.
n.a.
n.a.
a
For atom numbering see the individual molecular structures in Figures 3 and 4. X = Cl, Br, or I. ΔBX is the ratio of the B2-X1 and B2-X2 distances.
This is a way of gauging the extent of the halide bridging relative to an internal B-X standard (i.e. the conventional B2−X2 distance).
similar to the previously reported C−H activation prod-
uct from a different route.8 Details of the structure determination
of 18d can be found in the Supporting Information.
The molecular structures of base-stabilized borenium salts 19e
and 20e are shown in Figure 5, and some parameters are listed
in Table 2. The structures show significantly shorter B−Ccarbene
distances (1.612(3) and 1.608(3) Å) than the other carbene-
coordinated compounds in this study (11b: 1.653(2) Å; 12b:
1.652(4) Å; 13b: 1.639(2) Å), understandable given the trico-
ordinate nature of the boron atoms in the former.
Mechanistic Computational Study. In order to rule out
the spontaneous rearrangement of the diboranes(4) in the absence
of base, we investigated this possibility by DFT calculations on a
computational model of 1,2-dibromo-1,2-dimesityldiborane(4),
1bOPT (Figure 6). These calculations showed that the rearrangement
(ΔG = 79.37 kJ·mol−1)9, suggesting that the quaternization of one
boron (and hence the Lewis base) is integral to the rearrangement
process. Figures illustrating the relative energies of all of these
processes can be found in the Supporting Information.
CONCLUSION
■
Our work combining Lewis bases with diboranes(4) has
uncovered a truly fascinating unpredictability, as demonstrated
by the five distinct classes of compounds (classes a−e, Figure 2)
obtained herein. We have presented spectroscopic and structural
data of all five classes and drawn some clear conclusions about the
reactivity which should help predict the outcome of future
diborane/Lewis base reactions. We also hope our results will be of
assistance in the design of sp2−sp3 diboranes for use as reagents in
organic chemistry.
ASSOCIATED CONTENT
* Supporting Information
Experimental and crystallographic details. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
h.braunschweig@uni-wuerzburg.de
■
Notes
The authors declare no competing financial interest.
Figure 6. Calculated mechanism for a hypothetical base-free
diborane(4) rearrangement process, calculated using the PBE level
of theory (ΔG298 (1b′OPT) and ΔG‡298 (1bTS) energies are shown).
ACKNOWLEDGMENTS
This work was supported by the Deutsche Forschungsgemein-
schaft (DFG).
■
of 1bOPT to the unsymmetrical 1,1-dibromo-2,2-dimesityldibor-
ane(4) (1b′OPT) in the absence of a Lewis base is endothermic
by ΔG = 15.09 kJ·mol−1. The reaction, through transition state
1bTS, involves an activation barrier of ΔG = 115.22 kJ·mol−1.
This barrier is higher than those found for the Lewis-base-induced
rearragements 6a→6b(ΔG = 96.30 kJ·mol−1)9 and 9a→9b
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