Synthesis of Unsymmetrical Diboron(5) Compounds and Their Conversion to Diboron(5) Cations

Article abstract of DOI:10.1021/acs.organomet.8b00288

Reaction of the bis-catecholatodiboron-NHC adducts B₂Cat₂(NHC) (NHC = IMe (tetramethylimidazol-2-ylidene), IMes (1,3-dimesitylimidazol-2-ylidene), IDIPP (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)) with BCl₃ results in the replacement of the catecholato group bound to the four-coordinate boron atom with two chlorides to yield diboron(5) Lewis acid-base adducts of the formula CatBBCl₂(NHC). These compounds are precursors to diboron(5) monocations, accessed by adding AlCl₃ or K[B(C₆F₅)₄] as halide abstraction agents in the presence of a Lewis base. The substitution of the chlorides of CatBBCl₂(NHC) for hydrides was achieved using Bu₃SnH and a halide abstracting agent to form 1,1-dihydrodiboron(5) compounds, CatBBH₂(NHC). Attempts to generate diboron(4) monocations of formula [CatBB(Y)(NHC)]⁺ (Y = Cl, H) led to the rapid formation of CatBY.

Full text of DOI:10.1021/acs.organomet.8b00288

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis of Unsymmetrical Diboron(5) Compounds and Their
Conversion to Diboron(5) Cations
†
‡,§
†
†
,‡,§
Jessica Cid, Alexander Hermann, James E. Radcliffe, Liam D. Curless, Holger Braunschweig,*
,
†
and Michael J. Ingleson*
†
School of Chemistry, The University of Manchester, Manchester M13 9PL, United Kingdom
‡
Institute for Inorganic Chemistry, Julius-Maximilians-Universitat
̈
Wu
Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universitat
rzburg, Germany
S Supporting Information
̈
rzburg, Am Hubland, 97074 Wu
̈
rzburg, Germany
§
̈
Wurzburg, Am Hubland, 97074
̈
Wu
̈
*
ABSTRACT: Reaction of the bis-catecholatodiboron-NHC
adducts B Cat (NHC) (NHC = IMe (tetramethylimidazol-2-
2
2
ylidene), IMes (1,3-dimesitylimidazol-2-ylidene), IDIPP (1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene)) with BCl re-
3
sults in the replacement of the catecholato group bound to the
four-coordinate boron atom with two chlorides to yield diboron(5) Lewis acid−base adducts of the formula CatBBCl (NHC).
2
These compounds are precursors to diboron(5) monocations, accessed by adding AlCl or K[B(C F ) ] as halide abstraction
3
6 5 4
agents in the presence of a Lewis base. The substitution of the chlorides of CatBBCl (NHC) for hydrides was achieved using
2
Bu SnH and a halide abstracting agent to form 1,1-dihydrodiboron(5) compounds, CatBBH (NHC). Attempts to generate
3
2
+
diboron(4) monocations of formula [CatBB(Y)(NHC)] (Y = Cl, H) led to the rapid formation of CatBY.
INTRODUCTION
et al., who during studies on the reactivity of Lewis bases with
■
R(X)B-B(X)R (X = halide, R = mesityl (Mes), tBu, NMe2)
synthesized an array of diboron(4)-based borenium cations (A;
Organoboron compounds are ubiquitous reagents in synthesis
due to their utility in many functional group transformations.
Many modern routes to organoboranes use diboron(4)
compounds, e.g. bis-pinacolatodiboron and bis-catecholatodi-
boron, B Pin and B Cat , respectively; thus, these reagents
7
Figure 1). More recently, Prokofjevs proposed the formation
2
2
2
2
have received considerable interest, as have diboron(5)
derivatives that possess two distinct boron moieties with one
1
being four-coordinate. These unsymmetrical diboron(5)
species can be easily accessed by adding a neutral or anionic
Lewis base to a (RO) B-B(OR) precursor, quaternizing one
2
2
boron center. Quaternization leads to polarization of the B−B σ
2
3
bond, and the resulting mixed sp /sp diborons then react as
2
sources of nucleophilic boron. This enables the reaction with
an array of carbon electrophiles even in the absence of
precious-metal catalysts, thus providing a powerful route for
forming C−B bonds.
Figure 1. Structures of select previously reported unsymmetric
electrophilic diboron(4) and diboron(5) compounds and (inset)
those in this work.
Until recently, the reactivity of diboron species as strong
electrophiles had been mainly limited to highly reactive species
such as B Cl . While B Cl reacts with H , diborylates alkynes,
2
3
of a cationic sp -sp diboron(5) cation formed from [(L-
(μ-H)]+ (L = Me
BH CCH NMe ), with calculations
indicating that the diboron(5) cation [L(H) B-BHL] is key
2
8
)
2
4
2
4
2
2
2
3
2
2
+
alkenes, and aromatics, it decomposes above 0 °C and its use3
historically has also been limited by a complex synthesis.
However, Braunschweig et al. have recently reported much
in the intramolecular activation of aliphatic C−H bonds. Other
2
3
notable work in this area includes the formation of an sp −sp
9
4
boryl-substituted boronium salt (B) (Figure 1) and the
simpler routes to B X species. Other symmetrical diboron(4)
2
4
unsymmetrical diboron(4) compound PinBBMes (C), which
compounds that have appreciable electrophilicity have also
2
10
reacts with CO, isocyanides, and pyridine. More recently,
Wang and co-workers synthesized an N-heterocyclic carbene
been recently reported (e.g., B (o-tolyl) ) and shown to react
2
4
5
with H and other small molecules. In contrast, reports on the
2
development of unsymmetrical diboron(4) and diboron(5)
compounds that have significant electrophilicity are less
common. Notable work in this area comes from Braunschweig
Received: May 4, 2018
6
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00288
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Organometallics
NHC) coordinated borylborenium cation (D) that on
Article
(
treatment with H resulted in addition of H to the borenium
2
2
11
center along with the formation of Mes-BPin. In addition to
small-molecule activation, Himmel and co-workers described
the reactivity of diboron(5) monocations, which included
12
dimerization to generate an unusual B dication.
4
These studies demonstrate that unsymmetrical diboron
compounds are able to activate a range of molecules in novel
ways. However, the number of unsymmetrical and significantly
electrophilic diboron compounds is still limited. Herein, we
report the synthesis of readily available unsymmetrical
diboron(5) compounds that can be converted to cationic
diboron(5) species.
RESULTS AND DISCUSSION
■
Our previous work on the interaction of Lewis bases with
diboron(4) compounds described the synthesis of the mono-
NHC adduct B Cat (IMe) (IMe = tetramethylimidazol-2-
2
2
1
3
ylidene). The reaction of B Cat (IMe) (1-IMe) with 1
2
2
equiv of BCl in toluene at room temperature results in the
3
Figure 2. (top) Solid-state structure of 2-IMe. Ellipsoids are shown at
the 50% probability level. Selected bond lengths (Å) and angles (deg)
(for one of the four metrically similar molecules in the asu): B1−B2
1.687(11), B1−C1 1.627(10), Cl1−B1−Cl2 108.1(3), Cl1−B1−C1
2
3
formation of the new sp −sp NHC adduct 2-IMe, which was
isolated in 72% yield, with chlorocatecholborane observed as
1
1
the side product (Scheme 1). The B NMR spectrum of 2-IMe
1
07.9(5), Cl2−B1−C1 105.5(5). (bottom) NBO charges calculated at
the M06-2X/6-311G(d,p) level. Legend: (a) {BCat} refers to the
Scheme 1. Reaction of 1-NHC with BCl To Give 2-NHC
3
2
overall charge on the BCat unit containing the sp -B center.
substituent exchange, but no B−B cleavage. To determine a
feasible mechanism for the formation of 2-NHC, we performed
a DFT study on the initial steps of the reaction. Calculations
were carried out at the M06-2X/6-311G(d,p) level with a DCM
polarizable continuum medium (PCM) solvation model.
Despite extensive searching, we were not able to locate any
2
reveals two resonances, one at 36.4 ppm (broad) for the sp -B
3
center and one at −6.2 ppm (sharp) for the sp -B center. The
same methodology was applied to synthesize the IMes and
IDipp derivatives (2-IMes and 2-IDipp, respectively; IMes =
2
pathway involving the CatB-sp unit reacting as a nucleophile to
−
+
BCl to form [CatBBCl ] and [CatB(NHC)] . Analysis of the
3 3
1
,3-dimesitylimidazol-2-ylidene, IDipp = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene) in good yields (77 and
1%), with the 11B NMR spectra of both compounds being
very similar to that observed for 2-IMe. It should be noted that
charge distribution by NBO calculations (Figure 2, right)
2
showed that in 1-IMe the sp -B atom remains positively
3
8
charged; meanwhile the sp -B atom gains electron density
(relative to that in B Cat ), and overall this is similar to the
2
2
14
16
analogous chemistry was observed with BBr in place of BCl ;
however, attempts using Lewis acids such as BPh , B(C F ) ,
charge distribution calculated previously for B pin (IMes).
Notably, the overall charge on the entire sp -BCat unit is
3
3
2 2
2
3
6 5 3
and L-BH3 either resulted in no reaction or produced
intractable products with no analogues of 2-NHC isolable.
Single crystals of 2-IMe suitable for X-ray crystallography
were obtained by dissolution in o-dichlorobenzene (o-DCB)
and layering with pentane. This confirmed the formulation,
with the structure containing one four-coordinate boron and
one three-coordinate trigonal-planar (angles sum to 359.7°)
boron center (Figure 2). The B−B bond length in 2-IMe is in
the range 1.682(10)−1.701(10) Å (four inequivalent molecules
of 2-IMe are present in the asymmetric unit), which is slightly
shorter than in compound 1-IMe (1.729(3) Å) and B pin -
effectively zero in 1-IMe, which is distinct from that found for
alkoxide-activated diboron(4) compounds (where the sp
2
B(OR) unit is overall partially negatively charged). Thus, as
2
16
previously observed, NHC activation of diboron compounds
results in the formation of a less nucleophilic sp -B(OR) group
2
2
in comparison to that formed by activation with alkoxide. For
the second possible mechanism, Scheme 2 shows the calculated
relative electronic and total energies of the different calculated
intermediates for the first O/Cl exchange. Starting from
compound 1-IMe, coordination of BCl to an oxygen atom
3
of the two different catechol moieties gives two different
intermediates, 1a and 1b. The difference in electronic energy
2
2
NHC adducts reported previously (B pin -NHC adduct:
2
2
1
5
−1
1
1
.743(2) and 1.730(3) Å). The B−C distance (range
.602(9)−1.627(10) Å) does not differ significantly from that
between them is 19.1 kcal mol , with the more stable
intermediate being that with BCl coordinated to the catechol
3
3
of its 1-IMe precursor (1.647(2) Å).
bound to the sp -B center (1a), as expected. From this
Compound 2 may be formed by a process involving either
intermediate, a transfer of chloride from the coordinated BCl3
(
{
i) a B−B cleavage step, where transfer of a nucleophilic
to the boron center occurs through TS
energy barrier of 4.6 kcal mol . This results in the formation of
with a relatively low
1
a‑1c
BCat}− moiety from 1-NHC to BCl occurs followed by
−1
3
−
+
chloride transfer from [CatBBCl ] to a [CatB(NHC)] cation
intermediate 1c, containing a ROBCl moiety, which is slightly
3
2
and subsequent NHC transfer, or (ii) interaction of BCl with
the oxygen atom of a catechol group, leading to B−O/B−Cl
less stable than 1a. From here, a second oxygen/chloride
transfer would give compound 2-IMe and CatBCl.
3
B
DOI: 10.1021/acs.organomet.8b00288
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Article
Scheme 2. Calculated Energy Profile for the Possible Initial
a
Steps of the B−O Cleavage Mechanism
a
Figure 3. (top and middle) HOMO and LUMO representations for 2-
Electronic and Gibbs free energies (in parentheses) are given in kcal
+
−
1
IMe and [3-IMe] , respectively, at an isosurface value of 0.04.
mol .
+
(
bottom) Relative (to AlCl ) chloride ion affinity of [3-IMe] .
3
Before investigation of the synthesis of diboron(4)
achieved using other halide abstracting agents, e.g. K[B(C F ) ]
6 5 4
Cl
Cl
−
monocations derived from 2-NHC, calculations were per-
formed to probe the charge distribution (using the NBO
model), frontier orbital energy, and Lewis acidity. The change
of catecholato for two chlorides decreases the positive charge
on the B(IMe) center in 2-IMe relative to 1-IMe, consistent
with a lower degree of B−Cl σ bond polarization (charge on Cl
and NaBAr (BAr = [B(3,5-Cl -C H ) ] ), with CatBCl
2 6 3 4
observed to be formed on reaction of 2-IMe with each of these
salts. The formation of CatBCl from 2-IMe/AlCl also occurs
3
in chlorobenzene, indicating that the halide in CatBCl is
unlikely to derive from the solvent. The results of the reaction
of 2-IMes and 2-IDIPP with AlCl and K[B(C F ) ] did not
3
6 5 4
−
0.3) relative to B−O (charge on O −0.7). The polarization of
lead to any identifiable or isolable species in our hands.
+
the B−B bond is effectively unchanged on OR for Cl exchange,
Attempts to trap the putative cation [3-IMe] by performing
as indicated by the overall charge on the entire {BCat} unit
halide abstraction in the presence of a range of small molecules
+
remaining effectively zero. In [3-IMe] the removal of chloride
(e.g., H , CO, alkynes) still produced CatBCl along with other
2
to generate the positive charge does not significantly alter the
overall charge on the {BCat} unit. Instead, the positive charge
is localized on the B-IMe moiety, despite the effectively
coplanar arrangement of the C−B−Cl and O−B−O moieties.
In the B-IMe unit the boron center carries the largest degree of
positive charge, as would be expected on the basis of its more+
electropositive nature. The HOMOs for 2-IMe and [3-IMe]
intractable products. Therefore, attempts to trap the putative
cation by adding better nucleophiles, specifically phosphine and
amine Lewis bases, were explored. It should be noted that 2-
IMe shows no reaction with the phosphines and amines used in
this study in the absence of exogenous Lewis acid (by
multinuclear NMR spectroscopy), consistent with the low
Lewis acidity of 2-IMe indicated by calculations. The reaction
of 2-IMe with AlCl in the presence of 1 equiv of PPh results
(Figure 3) were found to be very similar in character, while the
3
3
LUMOs are slightly more distinct, with more boron character
present in the LUMO of [3-IMe] . As expected, the relative
in the formation of the Lewis base stabilized diboron(5)
+
11
monocation 4 (Figure 4). The 2 B NMR spectrum of 4 shows
LUMO energies indicate that compound 2-IMe is a weak Lewis
two resonances, one for the sp -B atom at 37.7 ppm and one
+
3
31
1
acid while the diboron(4) monocation [3-IMe] is a stronger
for the sp -B atom at −14.8 ppm. The P{ H} NMR spectrum
of 4 shows one broad signal at 1.44 ppm. The structure of 4
was confirmed by X-ray diffraction (Figure 4b), which
Lewis acid. This is supported by calculation of the chloride ion
affinity (CIA) of [3-IMe]+ relative to AlCl , which was
3
3
2
estimated using an approach based on the energy change for
confirmed the presence of an sp -B and an sp -B center. The
B−B bond length (of one of the two metrically similar
molecules in the asymmetric unit) in 4 was 1.722(8) Å, which
is longer than that in 2-IMe but shorter than in Wang’s
diboron(5) (1.754(11) Å) monocation (PinBB(NHC)(4-
DMAP)Mes, the 4-DMAP adduct of compound D). The
P−B distance in 4 (1.972(6) Å) is consistent with that found in
other phosphine-containing boronium cations. The addition
−
the transfer of chloride from [AlCl ] to the borenium cation
4
(
Figure 3, bottom). This value gives insight into the
accessibility of the borenium cation by halide abstraction.
The calculated CIA for [3-IMe] is 12.3 kcal mol (at the B−
Cl(IMe) center) relative to AlCl , which is comparable with
+
−1
11
3
+
17
[
(amine)BCl ] borenium cations synthesized previously and
2
12b
thus indicates the feasibility of halide abstraction from 2-IMe
using Lewis acids such as AlCl3.
of a second equivalent of AlCl to 4 did not abstract the second
3
Compound 2-IMe was reacted with AlCl using a range of
chloride, with resonances for the cationic portion of 4 being
unchanged.
3
conditions (varying temperature and solvent), and this led to
−
the complete consumption of 2-IMe and formation of [AlCl ]
When 2-IMe was treated with 1 equiv of 2-DMAP and 1
4
1
11
(
by multinuclear NMR spectroscopy). However, in the 1
B
equiv of K[B(C F ) ] in o-DCB, the B NMR spectrum
6
5 4
NMR spectrum, we were only able to identify BCl (minor)
and CatBCl (major) as products of this reaction. No
intermediates were observed, and similar outcomes were
indicated conversion to a new boron-containing product
3
2
3
11
containing both sp and sp boron centers: δ 29.9 and
B
−3.7 ppm, respectively. Crystals were isolated from the reaction
C
DOI: 10.1021/acs.organomet.8b00288
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
dihydrodiboron derivative CatBB(H) IMe (6-IMe) within
2
minutes. The 11B NMR spectrum of the reaction mixture
containing 6-IMe shows a broad signal at 42.9 ppm
2
corresponding to the sp -B and a broad singlet at −40.1 ppm
corresponding to the dihydroborane (Figure 5), which are
Figure 5. (a) Isodesmic reaction used to calculate HIA. (b) Borenium
cation E that possesses a similar HIA. (c) LUMO representation of [7-
IMe] (isosurface value of 0.04).
Figure 4. (a) Synthesis of diboron monocations 4 and 5. (b) X-ray
crystal structures of 4 and 5. Selected bond lengths (Å): 4, B1−B2
+
1
.722(8), B1−C1 1.618(7), B1−P1 1.972(6); 5, B1−B2 1.710(5),
B1−C1 1.630(4), B1−N3 1.600(4).
comparable to the signals of similar compounds reported
19
previously. Presumably, halide abstraction to form the [3-
mixture after layering with pentane. The structure of 5 confirms
the formation of a diboron monocation of formula [CatBB-
+
IMe] cation facilitates hydride transfer from Bu SnH, as in the
3
Cl
absence of NaBAr the reaction is extremely slow and yields
(
1
IMe)Cl(2-DMAP)][B(C F ) ]. While the B−B distance of
6
5 4
multiple products. Bu SnCl is observed as the expected tin
3
.710(5) Å is very similar to that in 4, the presence of the
11
byproduct from this reaction (and the B resonance for
[BAr ] persists). In the reaction mixture, compound 6-IMe
proved to be thermally sensitive, with the B NMR spectrum
pendant NMe group has a noticeable structural effect. The
Cl
2
NMe group of 2-DMAP is orientated toward the boron atom
11
2
of the BCat moiety with a B−N distance of 2.056 Å, suggesting
after times >10 min at room temperature showing increasing
amounts of decomposition products, as indicated by the growth
2
a weak interaction between the NMe group and the sp -B
2
center. Consistent with a weak interaction between N and B is
of a quartet at −36.6 ppm (J = 84.1 Hz) that corresponds to
the small deviation from planarity of the BCat moiety (sum of
20
BH -IMe. The other product(s) that must form alongside
3
2
11
B
angles 354.3°). The distinct δ chemical shift of the sp -B
IMe-BH proved intractable and were not able to be identified
3
center in 5 relative to 4 (29.9 and 37.7 ppm, respectively)
suggests that this interaction persists in solution.
in our hands. The purification of compound 6-IMe from
Cl
Na[BAr ], Bu SnCl, and the decomposition products was also
3
Attempts to abstract the remaining chloride from 5 upon
addition of K[B(C F ) ] or by reaction of 2-IMe with 2-DMAP
not successful in our hands.
The accessibility of the cation derived from 6-IMe initially
6
5 4
and excess AlCl failed (with 5 persisting by multinuclear NMR
3
was explored by calculation of the relative (to BEt ) hydride ion
3
spectroscopy). The relative (to AlCl ) chloride ion affinity of
+
−1
3
affinity of [7-IMe] , which was found to be −52.7 kcal mol
Figure 5, top). This suggests that the putative [7-IMe] is
the putative diboron(4) dication derived from chloride
17
+
(
abstraction from 5 (and before binding of NMe to form a
+
2
less Lewis acidic toward hydride in comparison to Ph C but
3
21
+
boronium cation as observed in [BCl (2-DMAP)] ; Scheme
+
2
more so than B(C F ) . The hydride ion affinity of [7-IMe]
6 5 3
18
−1
3)
was calculated and found to be −1.2 kcal mol , consistent
is in fact comparable to that of other borenium cations (e.g. E,
−
1
16
−
47.8 kcal mol ). Inspection of the frontier molecular
+
Scheme 3. CIA of the Dication Derived from 5
orbitals revealed that the LUMO of [7-IMe] was found to be
lower in energy than its precursor (6-IMe, −0.39 eV; [7-IMe] ,
2.63 eV) and was mainly located on the B-IMe group. In
contrast, the LUMO of 6-IMe was located mainly on the Bcat
moiety. The LUMOs of the two cations [7-IMe] and [3-
IMe] are similar in both character and energy.
Attempts were made to form [7-IMe] directly from the
+
−
+
+
+
Cl
with the lack of chloride abstraction using AlCl . The LUMO
crude reaction mixture derived from 2-IMe/Na[BAr ]/
3
energy of compound 5 (−1.93 eV) is much higher than for [3-
Bu SnH. The addition of trityl salts to this reaction mixture
3
+
IMe] , suggesting 5 is a weaker Lewis acid; consistent with this,
in haloarene solvents resulted in formation of Ph CH; however,
3
compound 5 did not react with alkynes such as 3-hexyne.
Due to the instability of putative [3-IMe] , our next target
the only identifiable new boron-containing species was CatBH.
CatBH is not derived from solvent activation, as CatBH is also
+
was exchanging a chloride group for an alternative substituent
that is also amenable to abstraction using common reagents,
specifically the 1,1-dihydrodiboron congener of 2-IMe. While
reaction of 2-IMe with silanes such as HSiEt or HSiPh gave
formed when hydride abstraction is repeated in d -bromoben-
5
zene; therefore the decomposition pathway of putative [7-
+
IMe] , which forms CatBH (and unidentified byproducts),
+
appears to be comparable to that for [3-IMe] , which forms
3
3
complex mixtures of products that were intractable in our
CatBCl and unidentified byproducts. Seeking to trap the
Cl
+
hands, the reaction of 2-IMe with 10 mol % of Na[BAr ] and 2
putative [7-IMe] using competent nucleophiles, we reacted a
equiv of Bu SnH resulted in the rapid formation of the 1,1-
mixture containing 6-IMe (<2 min after its formation) with a
3
D
DOI: 10.1021/acs.organomet.8b00288
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Organometallics
Article
+
number of phosphine/B(C F ) frustrated Lewis pairs. The
NHC] and Wang’s closely related compound D is noteworthy
and is presumably due to the superior π-donor properties of
pinacol versus catechol and the greater steric bulk of the Mes
group (relative to H or Cl). However, it should be noted that,
with compound D, related B−B bond cleavage reactions occur,
leading to the formation of MesBPin (Scheme 4, bottom).
6
5 3
t
addition of P Bu and B(C F ) gave the best result, producing
3
6 5 3
1
1
a single major new boron compound with its B NMR
spectrum showing a resonance at 39.2 ppm attributed to the B
of the BCat moiety, one at −37.2 ppm attributed to the BH,
and one doublet at −25.8 ppm which is consistent with
3
1
1
HB(C F ) . However, the P{ H} NMR spectrum revealed
6
5 3
several products and we were not able to isolate any of the
desired unsymmetrical diboron(5) cations derived from 6-IMe
despite multiple attempts.
Scheme 4. (a) Possible Reaction Pathway for the
+
+
Decomposition of [3-NHC] and [7-NHC] and (b)
Reaction of Wang’s Borylborenium (Compound D) with H2
or D2
In order to increase the stability of the B−H diboron species,
IMe was replaced with IMes and IDipp. Using the same
Cl
procedure (addition of 10 mol % of Na[BAr ] and 2 equiv of
Bu SnH), we were able to synthesize the corresponding 1,1-
3
dihydrodiboron(5) compound for both NHCs, termed 6-IMes
and 6-IDipp. However, the isolation of 6-IMes and 6-IDipp
Cl
was not possible to achieve since Na[BAr ] cocrystallized with
both species and could not be separated in our hands.
1
1
Nevertheless, the B NMR spectrum of 6-IMes shows
resonances similar to those observed for 6-IMe, including a
broad signal at 42.4 ppm and a broad signal at −40.0 ppm,
which supports the formation of 6-IMes. The solid-state
structure of compound 6-IMes, crystallized by slow evaporation
of a saturated solution in DCM, further confirmed the
formulation of this series of compounds (Figure 6). The B−
CONCLUSIONS
■
In this work we have reported the synthesis of NHC-
coordinated diboron compounds 2-NHC. When the IMe
congener of these diboron(5) compounds was treated with a
Lewis base and AlCl or K[B(C F ) ], formation of Lewis base
3
6 5 4
stabilized diboron(5) monocations occurred. However, in the
absence of competent nucleophiles, the putative diboron(4)
+
monocation [CatBB(NHC)Cl] reacted to form CatBCl and
other unidentified boron-containing species. The reaction of 2-
Cl
NHC with 10 mol % of Na[BAr ] and 2 equiv of Bu SnH
3
provides access to a series of 1,1-dihydrodiboron(5) com-
Figure 6. (a) Synthesis of 1,1-dihydrodiboron(5) compounds. (b)
Solid-state structure of 6-IMes, with ellipsoids at 50% probability.
Selected bond lengths (Å): B1−B2 1.686(4), B1−C1 1.605(3).
pounds. However, it was not possible to achieve the synthesis
and isolation of the putative [7-IMe] cation derived from
hydride abstraction due to its instability, with CatBH observed
to form rapidly. Thus, there appears to be a minimum steric
and electronic stabilization requirement to afford access to
diboron(4) monocations.
+
B bond distance in 6-IMes at 1.686(4) Å is slightly shorter than
those reported by Braunschweig et al. (1.702(6) Å) for a
19
related dihydrodiboron(5) but comparable to the B−B
distance in 2-IMe. In all of these compounds, the B−H
coupling is not clearly resolved, which was also the case for
EXPERIMENTAL SECTION
All appropriate manipulations were performed using standard Schlenk
techniques or in an argon-filled MBraun glovebox (O /H O levels
■
18
other 1,1-dihydroboranes.
2
2
below 0.5 ppm). Glassware was dried in a hot oven overnight and
heated under vacuum before use. Solvents and amines were distilled
Compounds 6-IMes and 6-IDIPP proved to be more
thermally stable in solution, and formation of the hydrodiboron
from NaK, CaH , or K and degassed prior to use. Unless otherwise
+
2
monocations [7-NHC] was explored. The reaction of 6-IMes
stated, all compounds were purchased from commercial sources and
used as received. NMR spectra were recorded on Bruker AvanceIII-
400 or Bruker Ascend-400 spectrometers. Chemical shifts are reported
as dimensionless δ values and are frequency referenced relative to
Cl
or 6-IDIPP (containing Na[BAr ] impurity) with [Ph C][B-
3
(
C F ) ] again led to decomposition, with CatBH again
6 5 4
1
observed at 27.8 ppm ( J = 210 Hz) as one of the products
BH
in the 11B NMR spectrum. Attempts to form a Lewis base
stabilized hydrodiboron(4) monocation with these compounds
using phosphine/B(C F ) FLPs gave complex mixtures that
residual protio impurities in the NMR solvents for H and C{ H},
1
13
1
respectively, while 11B, F, P, and Al shifts are referenced relative
19
31
27
to external BF ·Et O, hexafluorobenzene, 85% phosphoric acid, and
3
2
6
5 3
3+
Al(NO ) in D O (Al(D O) ), respectively. Coupling constants J are
3
3
2
2
6
proved intractable in our hands. Attempts to assess the
+
given in hertz (Hz) as positive values regardless of their real individual
signs. The multiplicities of the signals are indicated as s, d, t, q, pent,
sept, or m for singlet, doublet, triplet, quartet, pentet, septet, or
multiplet, respectively. Unless otherwise stated, all NMR spectra were
recorded at 293 K. Carbon atoms directly bonded to boron are not
always observed in the 13C{ H} NMR spectra due to quadrupolar
relaxation leading to signal broadening. All calculations were
reactivity of the putative cations [7-NHC] by performing
hydride abstraction from 6-NHC in the presence of alkynes
also led to complex mixtures.
The decomposition of the putative diboron(4) cations to
form CatBCl and CatBH appears to be a common reaction
1
+
pathway. The disparity between the stabilities of [3-NHC] /[7-
E
DOI: 10.1021/acs.organomet.8b00288
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
conducted at the M06-2X/6-311G(d,p) level with a solvation model
CD Cl ): δ 148.1, 134.8 (d, J = 8.7 Hz), 133.9 (d, J = 2.7 Hz), 130.3
2
2
22
31
1
(
PCM, CH Cl ) using the Gaussian software package. All geometry
(d, J = 10.8 Hz), 129.7, 123.8, 123.6, 122.9, 113.3, 34.7, 9.9. P{ H}
2
2
27
optimizations were full, with no restrictions. All stationary points
located in the potential energy hypersurface were characterized as
minima or transition states by vibrational analysis. Transition states
had one and only one imaginary frequency, whose normal mode
NMR (162 MHz, CD Cl ): δ 1.44. Al NMR (104 MHz, CD Cl ): δ
2
2
2
2
103.8.
[CatBBCl(IMe)2-DMAP][B(C F ) ] (5). 2-DMAP (0.15 mmol, 19
6
5 4
μL) was added to a stirred solution of CatBBCl IMe (2-IMe; 0.15
2
13
23
corresponded to the expected motion. 1-IMe, 1-IMes, and sodium
mmol, 49 mg) in 1 mL of o-dcb. After the solution was stirred for 30
Cl
tetrakis(3,5-dichlorophenyl)borate (Na[BAr ]) were synthesized in
min, K[B(C F ) ] (0.15 mmol, 164 mg) was added and the reaction
6
5 4
24
accordance with the literature. Purity was indicated by multinuclear
NMR spectroscopy in organic solvents (in which the sample fully
dissolved) and supported by elemental microanalysis.
mixture was stirred overnight. Afterward, the reaction mixture was
filtered and all volatiles were removed in vacuo. The residue was
washed with pentane and benzene to yield a yellow solid (120 mg,
B Cat IDipp (1-IDipp). IDipp (1.80 mmol, 700 mg) was added to
73%). Anal. Calcd for C44
Found: C, 48.17; H, 2.41; N, 5.10. H NMR (400 MHz, CD
8.75 (d, J = 6.1, 1H), 8.39 (dd, J = 7.6, 7.5 Hz, 1H), 7.74 (m, 2H), 7.09
H
26
B
3
ClF201
N
4
O
2
: C, 48.46; H, 2.40; N, 5.14.
2
2
a stirred solution of B cat (1.80 mmol, 428 mg) in 25 mL of toluene.
2
Cl ): δ
2
2
2
After the solution was stirred overnight, all volatiles were removed in
vacuo. The residue was washed three times with pentane to yield a
white solid (995 mg, 88%). Anal. Calcd for C H B N O : C, 74.78;
1
1
(m, 2H), 6.98 (m, 2H), 3.44 (s, 6H), 2.60 (s, 6H), 2.18 (s, 6H).
NMR (128 MHz, CD Cl ): δ 29.9 (br s), −3.7 (br s), −16.7 (sharp,
). C{ H} NMR (101 MHz, CD Cl ): δ 163.2, 149.5, 148.7
B
3
9
44
2
2
4
2
2
1
13
1
H, 7.08; N, 4.47. Found: C, 74.95; H, 7.17; N, 4.58. H NMR (400
B(C
6
F
5
)
4
2
2
MHz, CD Cl ): δ 7.16 (m, 4H), 7.08 (m, 4H), 6.92 (m, 2H), 6.88 (m,
(d, J = 240 Hz) 147.4, 147.3, 138.8 (d, J = 245 Hz), 136.9 (d, J = 245
2
2
Hz), 128.4, 125.7, 122.4, 121.7, 112.1, 46.2, 34.4, 9.4. 19F NMR (376
2
H), 6.18 (m, 2H), 6.06 (m, 2H), 2.74 (sept, J = 6.9 Hz, 4H), 1.33 (d,
11
J = 6.8 Hz, 12H), 1.11 (d, J = 6.8 Hz, 12H). B NMR (128 MHz,
MHz, CD
−167.1 to −169.6 (m).
CatBBH (IMes) (6-IMes). Bu
added to a stirred solution of CatBBCl
2
2 2
Cl ): δ −132.1 to −134.6 (m), −163.5 (t, J = 20.3 Hz),
13
1
CD Cl ): δ 36.8 (br s), 6.9 (br s). C{ H} NMR (101 MHz,
2
2
CD Cl ): δ 153.7, 148.9, 145.9, 133.5, 130.6, 124.8, 124.0, 121.8,
2
3
SnH (0.80 mmol, 0.215 mL) was
IMes (2-IMes; 0.40 mmol, 200
2
2
1
17.5, 112.3, 109.2, 29.6, 26.3, 22.3.
Cl
CatBBCl IMe (2-IMe). BCl 1 M in hexanes (2.42 mmol, 2.42 mL)
mg) and Na[BAr ] (0.04 mmol, 25 mg) in 10 mL of DCM. After the
solution was stirred for 1 h, all volatiles were removed in vacuo. The
residue was washed three times with pentane to yield a white solid
(132 mg, 76%). An analytically pure sample of 6-IMes could not be
2
3
was added to a stirred solution of B Cat IMe (1-IMe; 2.42 mmol, 876
2
2
mg) in 20 mL of toluene at −77 °C. After the addition, the reaction
mixture was warmed to room temperature and was stirred for 2 h.
Afterward, all volatiles were removed in vacuo. The residue was
washed three times with pentane to yield a white solid (786 mg, 72%).
Anal. Calcd for C H B Cl N O : C, 48.07; H, 4.97; N, 8.62. Found:
Cl
obtained, as during purification Na[BAr ] was cocrystallized with 6-
1
IMes and could not be separated. H NMR (400 MHz, CD
Cl
2
): δ
2
7.03 (s, 2H), 6.95 (m, 2H), 6.93 (s, 4H), 6.88 (m, 2H), 2.27 (s, 6H),
1
3
16
2
2
2
2
1
1
11
C, 48.23; H, 4.82; N, 8.51. H NMR (400 MHz, CD Cl ): δ 7.27 (m,
H), 7.08 (m, 2H), 3.73 (s, 6H), 2.16 (s, 6H). B NMR (128 MHz,
CD Cl ): δ 36.4 (br s), −6.2 (br s). C{ H} NMR (101 MHz,
CD Cl ): δ 149.0, 126.5, 122.8, 112.9, 34.3, 9.3.
2.10 (s, 12H). H{ B} NMR (400 MHz, CD
Cl ): δ 7.03 (s, 2H),
2 2
2
2
11
2
6.95 (m, 2H), 6.93 (s, 4H), 6.88 (m, 2H), 2.27 (s, 6H), 2.10 (s, 12H),
13
1
11
0.68 (s, 2H). B NMR (128 MHz, CD
Cl
Cl ): δ 149.5, 139.7, 135.7, 135.0,
2 2
): δ 42.4 (br s), −40.0 (br
2
2
2
2
s). 13C{ H} NMR (101 MHz, CD
129.5, 121.5, 121.3, 111.5, 21.4, 18.0.
CatBBH IDipp (6-IDIPP). Bu SnH (0.68 mmol, 0.183 mL) was
added to a stirred solution of CatBBCl IDipp (2-IDIPP; 0.34 mmol,
00 mg) and NaBArCl (0.034 mmol, 21 mg) in 10 mL of DCM. After
1
2
2
CatBBCl IMes (2-IMes). BCl 1 M in hexanes (0.92 mmol, 0.92
2
3
mL) was added to a stirred solution of B Cat IMes (1-IMes; 0.92
2
3
2
2
mmol, 500 mg) in 20 mL of hexane and 5 mL of toluene at −77 °C.
After the addition, the reaction mixture was warmed to room
temperature and stirred for 2 h. Afterward, all volatiles were removed
in vacuo. The residue was washed three times with pentane to yield a
white solid (356 mg, 77%). Anal. Calcd for C H B Cl N O : C,
2
2
the solution was stirred for 1 h, all volatiles were removed in vacuo.
The residue was washed three times with pentane to yield a white solid
(
106 mg, 60%). An analytically pure sample of 6-IMes could not be
2
7
28
2
2
2
2
Cl
1
obtained, as during purification Na[BAr ] was cocrystallized with 6-
IMes and could not be separated. H NMR (400 MHz, CD Cl ): δ
6
(
(
4.21; H, 5.59; N, 5.55. Found: C, 64.02; H, 5.59; N, 5.73. H NMR
1
400 MHz, CD Cl ): δ 7.10 (s, 2H), 7.00 (m, 2H), 6.94 (m, 2H), 6.79
2
2
2
2
11
7
.43 (t, J = 7.7 Hz, 2H), 7.26 (d, J = 7.8 Hz, 4H), 6.93 (m, 2H), 6.86
m, 4H), 2.19 (s, 12H), 2.09 (m, 6H). B NMR (128 MHz, CD Cl ):
δ 36.3 (br s), − 6.3 (br s). C{ H} NMR (101 MHz, CD Cl ): δ
49.2, 141.0, 136.1, 133.3, 129.5, 128.9, 124.2, 122.2, 112.2, 21.3, 18.5.
2
2
13
1
(
m, 2H), 2.73 (sept, J = 6.8 Hz, 4H), 1.18 (d, J = 6.9 Hz, 12 H), 1.15
2
2
1
1
(d, J = 6.9 Hz, 12 H). B NMR (128 MHz, CD Cl ): δ 42.5 (br s),
1
2 2
1
−
39.3 (br s). 13C{ H} NMR (101 MHz, CD Cl ): δ 149.62, 146.34,
CatBBCl IDipp (2-IDIPP). BCl 1 M in hexanes (0.80 mmol, 0.80
2 2
2
3
1
34.97, 130.55, 124.51, 122.87, 121.35, 111.66, 29.21, 25.75, 23.01.
mL) was added to a stirred solution of B Cat IDipp (1-IDIPP; 0.80
2
2
mmol, 500 mg) in 20 mL of hexane and 5 mL of toluene at −77 °C.
After the addition, the reaction mixture was warmed to room
temperature and stirred for 2 h. Afterward, all volatiles were removed
in vacuo. The residue was washed three times with pentane to yield a
white solid (384 mg, 81%). Anal. Calcd for C H B Cl N O : C,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00288.
■
S
33
40
2
2
2
2
1
6
7.27; H, 6.84; N, 4.75. Found: C, 67.01; H, 6.87; N, 5.04. H NMR
(
400 MHz, CD Cl ): δ 7.16 (m, 8H), 6.90 (s, 4H), 2.76 (sept, J = 7.0
2
2
11
Hz, 4H), 1.45 (d, J = 6.7 Hz, 12H), 1.11 (d, J = 6.8 Hz, 12H).
B
Coordinates for all calculations (XYZ)
Additional experimental details and NMR spectra (PDF)
13
1
NMR (128 MHz, CD Cl ): δ 36.6 (br s), −6.3 (br s). C{ H} NMR
2
2
(
101 MHz, CD Cl ): δ 149.1, 146.6, 133.4, 131.3, 125.2, 124.2, 121.8,
2 2
Accession Codes
1
12.3, 29.8, 26.3, 22.3.
CatBBClIMe(PPh )][AlCl ] (4). PPh (0.15 mmol, 39 mg) was
[
CCDC 1840161−1840163 and 1840193 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
3
4
3
added to a stirred solution of CatBBCl IMe (2-IMe; 0.15 mmol, 49
2
mg) in 1 mL of o-dcb. After the solution was stirred for 30 min, AlCl3
(
0.15 mmol, 20 mg) was added and the reaction mixture was stirred
for 3 h. Afterward, all volatiles were removed in vacuo. The residue
was washed three times with pentane to yield a white solid (96 mg,
8
3
9%). Anal. Calcd for C H AlB Cl N O P: C, 51.68; H, 4.34; N,
.89. Found: C, 51.33; H, 4.45; N, 4.01. H NMR (400 MHz,
31 31 2 5 2 2
1
AUTHOR INFORMATION
Corresponding Authors
*E-mail for H.B.: h.braunschweig@uni-wuerzburg.de.
CD Cl ): δ 7.73−7.63 (m, 10H), 7.60−7.55 (m, 5H), 7.22 (m, 2H),
■
2
2
1
1
7
.13 (m, 2H), 2.93 (s, 6H), 2.17 (s, 6H). B NMR (128 MHz,
13
1
CD Cl ): δ 37.7 (br s), −14.8 (br s). C{ H} NMR (101 MHz,
2
2
F
DOI: 10.1021/acs.organomet.8b00288
Organometallics XXXX, XXX, XXX−XXX

Organometallics
E-mail for M.J.I.: Michael.ingleson@manchester.ac.uk.
Article
*
2016, 35, 2563−2566. (d) Katsuma, Y.; Asakawa, H.; Yamashita, M.
Chem. Sci. 2018, 9, 1301−1310.
11) Zheng, J.; Li, Z. H.; Wang, H. Chem. Sci. 2018, 9, 1433−1438.
(12) (a) Litters, S.; Kaifer, E.; Enders, M.; Himmel, H.−J. Nat. Chem.
2013, 5, 1029−1034. (b) Litters, S.; Ganschow, M.; Kaifer, E.;
Himmel, H.−J. Eur. J. Inorg. Chem. 2015, 2015, 5188−5195. (c) Litters,
S.; Kaifer, E.; Himmel, H. − J. Angew. Chem., Int. Ed. 2016, 55, 4345−
347. (d) Litters, S.; Kaifer, E.; Himmel, H.−J. Eur. J. Inorg. Chem.
016, 2016, 4090−4098. (e) Widera, A.; Kaifer, E.; Wadepohl, H.;
ORCID
(
Holger Braunschweig: 0000-0001-9264-1726
Michael J. Ingleson: 0000-0001-9975-8302
Notes
The authors declare no competing financial interest.
4
2
Himmel, H.-J. Chem. - Eur. J. 2018, 24, 1209−1216.
(
ACKNOWLEDGMENTS
■
13) (a) Pietsch, S.; Paul, U.; Cade, I. A.; Ingleson, M. J.; Radius, U.;
The authors gratefully acknowledge the award of a Marie Cure
Fellowship to J.C. (703227-DIBOR), the University of
Manchester, and the EPSRC (Grant No. EP/K03099X/1 and
EP/R001499/1). A.H. thanks the Fonds der Chemischen
Industrie for a Ph.D. fellowship. H.B. thanks the European
Research Council for an Advanced Grant under the European
Union Horizon 2020 Research and Innovation Program (grant
agreement no. 669054). Additional research data supporting
this publication are available as Supporting Information
accompanying this publication.
Marder, T. B. Chem. - Eur. J. 2015, 21, 9018−9021. (b) Cade, I. A.;
Chau, W. Y.; Vitorica-Yrezabal, I.; Ingleson, M. J. Dalton Trans. 2015,
44, 7506−7511.
(14) The synthesis of the CatB-BBr
reported in a separate paper along with their reduction chemistry.
15) (a) Kleeberg, C.; Crawford, A. G.; Batsanov, A. S.; Hodgkinson,
P.; Apperley, D. C.; Cheung, M. S.; Lin, Z.; Marder, T. B. J. Org. Chem.
rtemberger-Pietsch, S.; Eichhorn,
(NHC) derivatives will be
2
(
2
012, 77, 785−789. (b) Eck, M.; Wu
̈
A.; Berthel, J. H. J.; Bertermann, R.; Paul, U. S. D.; Schneider, H.;
Friedrich, A.; Kleeberg, C.; Radius, U.; Marder, T. B. Dalton Trans.
2
(
1
(
1
(
017, 46, 3661−3680.
16) Cid, J.; Carbo,
2794−12802.
17) Clark, E.; Del Grosso, A.; Ingleson, M. J. Chem. - Eur. J. 2013,
9, 2462−2466.
18) Lawson, J. R.; Clark, E. R.; Cade, I. A.; Solomon, S. A.; Ingleson,
M. J. Angew. Chem., Int. Ed. 2013, 52, 7518−7522.
19) Arrowsmith, M.; Braunschweig, H.; Radacki, K.; Thiess, T.;
Turkin, A. Chem. - Eur. J. 2017, 23, 2179−2184.
20) Brahmi, M. M.; Monot, J.; Murr, M. D. E.; Curran, D. P.;
Fensterbank, L.; Lacote, E.; Malacria, M. J. Org. Chem. 2010, 75,
983−6985.
21) Clark, E.; Ingleson, M. J. Angew. Chem., Int. Ed. 2014, 53,
1306−11309.
22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
́ ́
J. J.; Fernandez, E. Chem. - Eur. J. 2012, 18,
REFERENCES
■
(
1) (a) Dewhurst, R. D.; Neeve, E. C.; Braunschweig, H.; Marder, T.
B. Chem. Commun. 2015, 51, 9594−9607. (b) Neeve, E. C.; Geier, S.
J.; Mkhalid, I. A. I.; Westcott, S. A.; Marder, T. B. Chem. Rev. 2016,
1
(
2
16, 9091−9161.
2) (a) Lee, K.; Zhugralin, A. R.; Hoveyda, A. H. J. Am. Chem. Soc.
009, 131, 7253−7255. (b) Bonet, A.; Gulyas, H.; Fernandez, E.
Angew. Chem., Int. Ed. 2010, 49, 5130−5134. (c) Bonet, A.; Pubill-
Ulldemolins, C.; Bo, C.; Gulyas, H.; Fernandez, E. Angew. Chem., Int.
(
́
́
(
̂
́
́
6
(
1
(
Ed. 2011, 50, 7158−7161. (d) Wu, H.; Radomkit, S.; O’Brien, J. M.;
Hoveyda, A. H. J. Am. Chem. Soc. 2012, 134, 8277−8285. (e) Pubill-
́ ́
Ulldemolins, C.; Bonet, A.; Bo, C.; Gulyas, H.; Fernandez, E. Chem. -
Eur. J. 2012, 18, 1121−1126. (f) Pietsch, S.; Neeve, E. C.; Apperley, D.
C.; Bertermann, R.; Mo, F.; Qiu, D.; Cheung, M. S.; Dang, L.; Wang,
J.; Radius, U.; Lin, Z.; Kleeberg, C.; Marder, T. B. Chem. - Eur. J. 2015,
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision C1; Gaussian, Inc., Wallingford, CT, 2009.
23) Perras, F. A.; Bryce, D. L. J. Am. Chem. Soc. 2013, 135, 12596−
2599.
24) Chaplin, A. B.; Weller, A. S. Eur. J. Inorg. Chem. 2010, 2010,
124−5128.
2
́
1, 7082−7099. (g) Cuenca, A. S.; Shishido, R.; Ito, H.; Fernandez, E.
Chem. Soc. Rev. 2017, 46, 415−430. (h) Pecharman, A.-F.; Hill, M. S.;
McMullin, C. L.; Mahon, M. F. Angew. Chem., Int. Ed. 2017, 56,
1
(
6363−16366.
3) (a) Urry, G.; Kartik, T.; Moore, R. E.; Schlesinger, H. I. J. Am.
Chem. Soc. 1954, 76, 5293−5298. (b) Timms, P. L.; Ittel, S. D.; Kuck,
M. Inorg. Synth. 2007, 19, 74−78. (c) Morrison, J. A. Chem. Rev. 1991,
9
1, 35−48.
4) Arrowsmith, M.; Bo
Dewhurst, R. D.; Ewing, W. C.; Ho
Commun. 2017, 53, 8265−8267.
5) (a) Tsukahara, N.; Asakawa, H.; Lee, K.-H.; Lin, Z.; Yamashita,
(
̈
hnke, J.; Braunschweig, H.; Deißenberger, A.;
rl, C.; Mies, J.; Muessig, J. H. Chem.
̈
̈
(
M. J. Am. Chem. Soc. 2017, 139, 2593−2596. (b) Katsuma, Y.;
Tsukahara, N.; Wu, L.; Lin, Z.; Yamashita, M. Angew. Chem., Int. Ed.
(
1
(
5
2
(
018, 57, 6109−6114.
6) For unsymmetrical diboron(4) compounds with relatively low
electrophilicity see the following (and references therein): (a) Os-
chmann, W.; Borner, C.; Kleeberg, C. Dalton Trans. 2018, 47, 5318−
5
327.
(
7) (a) Braunschweig, H.; Damme, A.; Dewhurst, R. D.; Kramer, T.;
Radacki, K.; Siedler, E.; Trumpp, A.; Wagner, K.; Werner, C. J. Am.
Chem. Soc. 2013, 135, 8702−8707. (b) Arnold, A.; Braunschweig, H.;
Dewhurst, R. D.; Hupp, F.; Radacki, K.; Trumpp, A. Chem. - Eur. J.
2
(
(
016, 22, 13927−13934.
8) Prokofjevs, A. Angew. Chem., Int. Ed. 2015, 54, 13401−13405.
9) Rixin Wang, S.; Arrowsmith, M.; Braunschweig, H.; Dewhurst, R.
D.; Paprocki, V.; Winner, L. Chem. Commun. 2017, 53, 11945−11947.
(
10) (a) Asakawa, H.; Lee, K. H.; Lin, Z.; Yamashita, M. Nat.
Commun. 2014, 5, 4245. (b) Kojima, C.; Lee, K.-H.; Lin, Z.;
Yamashita, M. J. Am. Chem. Soc. 2016, 138, 6662−6669. (c) Katsuma,
Y.; Asakawa, H.; Lee, K.-H.; Lin, Z.; Yamashita, M. Organometallics
G
DOI: 10.1021/acs.organomet.8b00288
Organometallics XXXX, XXX, XXX−XXX