Reactivity of Lewis acid activated diaza- and dithiaboroles in electrophilic arene borylation

Article abstract of DOI:10.1021/om201228e

Hydride abstraction from N,N′-bis(adamantyl)-1-hydrido-1,3,2- benzodiazaborole with catalytic [Ph₃C][closo-CB₁₁H ₆Br₆] resulted in a low yield of arene borylation and a major product derived from migration of both adamantyl groups to the arene backbone. In contrast, the related aryl-substituted diazaborole N,N′-(2,6-diisopropylphenyl)-1-bromo-1,3,2-diazaborole did not borylate benzene or toluene, being resistant to halide abstraction even with strong halide acceptors: e.g., [Et₃Si][closo-CB₁₁H ₆Br₆]. The reactivity disparity arises from greater steric shielding of the boron p_z orbital in the 2,6-diisopropylphenyl- substituted diazaboroles. Boron electrophiles derived from 1-chloro-1,3,2- benzodithiaborole ((CatS₂)BCl) are active for arene borylation, displaying reactivity between that of catecholato- and dichloro-boron electrophiles. [(CatS₂)B(NEt₃)][AlCl₄] is significantly less prone to nucleophile-induced transfer of halide from [AlCl₄] to boron compared to catecholato and dichloro borocations, enabling it to borylate arenes containing nucleophilic -NMe₂ moieties in high conversion (e.g., N,N,4-trimethylaniline and 1,8-bis(dimethylamino) naphthalene). Calculations indicate that the magnitude of positive charge at boron is a key factor in determining the propensity of chloride transfer from [AlCl₄] to boron on addition of a nucleophile.

Full text of DOI:10.1021/om201228e

Article
pubs.acs.org/Organometallics
Reactivity of Lewis Acid Activated Diaza- and Dithiaboroles in
Electrophilic Arene Borylation
S. A. Solomon, A. Del Grosso, E. R. Clark, V. Bagutski, J. J. W. McDouall, and M. J. Ingleson*
Department of Chemistry, University of Manchester, Manchester M13 9PL, U.K.
S
* Supporting Information
ABSTRACT: Hydride abstraction from N,N′-bis(adamantyl)-
1-hydrido-1,3,2-benzodiazaborole with catalytic [Ph3C][closo-
CB11H6Br6] resulted in a low yield of arene borylation and a
major product derived from migration of both adamantyl
groups to the arene backbone. In contrast, the related aryl-
substituted diazaborole N,N′-(2,6-diisopropylphenyl)-1-
bromo-1,3,2-diazaborole did not borylate benzene or toluene, being resistant to halide abstraction even with strong halide
acceptors: e.g., [Et3Si][closo-CB11H6Br6]. The reactivity disparity arises from greater steric shielding of the boron pz orbital in the
2,6-diisopropylphenyl-substituted diazaboroles. Boron electrophiles derived from 1-chloro-1,3,2-benzodithiaborole ((CatS2)BCl)
are active for arene borylation, displaying reactivity between that of catecholato- and dichloro-boron electrophiles.
[(CatS2)B(NEt3)][AlCl4] is significantly less prone to nucleophile-induced transfer of halide from [AlCl4]¯ to boron compared
to catecholato and dichloro borocations, enabling it to borylate arenes containing nucleophilic −NMe2 moieties in high
conversion (e.g., N,N,4-trimethylaniline and 1,8-bis(dimethylamino)naphthalene). Calculations indicate that the magnitude of
positive charge at boron is a key factor in determining the propensity of chloride transfer from [AlCl4]¯ to boron on addition of a
nucleophile.
INTRODUCTION
■
Intermolecular direct borylation is an highly efficient method
for the generation of synthetically ubiquitous aryl boronates,1
particularly using iridium catalysis.2,3 An alternative direct arene
borylation route which represents a boron analogue of the
Friedel−Crafts reaction has been recently reported using
molecular4−7 and polyhedral boron electrophiles.8 This
alternative boron electrophiles following approaches 1 and 2. It
proceeds under electronic control, complementing iridium-
was envisaged that diazaborole derivatives, (R2N)2BH, while
catalyzed direct borylation, which operates predominantly
less electrophilic (due to enhanced N→B π donation)15,16
under steric control. A key step in electrophilic borylation is
would undergo H2 loss and regeneration of the active
sequestering the Brønsted acidic byproduct from electrophilic
electrophile more readily due to the weaker B−H bond relative
to that in the catechol analogue. Furthermore, the Brønsted
acid byproduct from electrophilic borylation would more
strongly coordinate to aza functionalities (relative to the
oxygen moiety in CatBH),15,17 reducing the Brønsted acidity
and preventing alkyl migration. In contrast, 1,2-benzenedithiol-
ligated boron electrophiles will have enhanced electrophilicity
relative to the catechol analogues, increasing the arene
nucleophile substrate scope via approach 1.18 The Lewis
acidities of B(SR)3 species have been calculated to be
significantly greater (toward hydride) than that of BF3,19
suggesting that the arene substrate scope of 1,2-dithiol-ligated
boron electrophiles may be comparable to or even broader than
that reported for electrophiles derived from BCl3.6
aromatic substitution to prevent protodeboronation and
provide an additional energetic driving force for the
reaction.9−14 We have reported two distinct approaches to
ultimately sequester the protic byproduct from direct electro-
philic arene borylation with catecholboranes (CatBH and
CatBCl): (1) “H+” is trapped by a good Lewis base, and (2)
“H+” is combined with an hydrically polarized B−H to evolve
H2.4−6 Both of these approaches currently have limitations; the
former requires stoichiometric Lewis acid and base and is
restricted in substrate scope to activated arenes. In contrast, the
second approach is effective for deactivated arenes (e.g., 1,2-
dichlorobenzene) and is catalytic in Lewis acid activator.
However, it has poor turnover frequencies (due to the slow
regeneration of the active electrophile and poor solubility of
ionic species in low dielectric solvents) and alkylated arenes
undergo extensive alkyl migration during borylation.
Herein we report on (i) our studies into the factors
determining the stability of borocations derived from diaza- and
dithia-ligated boroles and (ii) the borylating ability of these
Received: December 10, 2011
Published: February 7, 2012
© 2012 American Chemical Society
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28 ppm). The major boron-containing species, 2, had low
symmetry and contained B−H and N−H resonances.
Increasing the loading of [Ph3C][closo-CB11H6Br6] to 33 mol
% relative to 1 increased the amount of (C6H4{NAd}2)BPh
formed compared to the resonance of 2 (δ 23.8 ppm).
Repeated recrystallization attempts from saturated arene
solutions of 2 afforded only small single crystals of 2 which
diffracted weakly (maximum 2θ of 42°). Nevertheless, X-ray
diffraction analysis unambiguously revealed this species to be
derived from the formal exchange of aryl C−H protons with
the N-adamantyl groups (Figure 1). The angles at boron in 2
sum to 360°, while the short B−N distances (N1−B1 =
1.413(7) Å, N2−B1 = 1.401(7) Å) are indicative of N→B π
donation.
RESULTS AND DISCUSSION
■
(1). Diazaboroles. 1-Hydrido-1,3,2-benzodiazaborole,
(C6H4(NH)2)BH, was readily synthesized by reaction of 1,2-
diaminobenzene with H3B·SMe2.20 Attempts to access the base-
stabilized borenium cation (a three-coordinate borocation using
the Noth terminology)15 [(C6H4(NH)2)B(NEt3)]+ were not
̈
successful by sequential addition of a Lewis base adduct of
B(C6F5)3 to (C6H4(NH)2)BH (a route previously used to
synthesize [CatBPtBu3][HB(C6F5)3]);21 this instead formed
Et3NB(C6F5)3 as the only new product. In the absence of an
amine nucleophile, abstraction of a hydride from
(C6H4(NH)2)BH by [Ph3C][closo-CB11H6Br6]22 proceeded at
1
20 °C in benzene and toluene (Ph3CH observed by H NMR
spectroscopy). The arene borylation product, (C6H4(NH)2)-
BAr, was not observed; instead, a solid insoluble in common
organic solvents was formed. The insoluble material may be
due to the formation of oligomeric amine-bridged cationic
species, [{(C6H4(NH)2)B}n]n+, as previously proposed by
Parry et al.23 and as intermediates after protonation and H2
loss from ammonia−borane.24 N−H was exchanged for N−
adamantyl to enhance the steric bulk at nitrogen and preclude
both oligomerization after hydride abstraction and the
coordination of Lewis acids larger than a proton to nitrogen.
N,N′-Diadamantyl-1-hydrido-1,3,2-benzodiazaborole (1; Figure
1) was readily synthesized, and IR spectroscopy confirmed a
The repeated observation of Ph3CH and (C6H4{NAd}2)BPh
indicates that both hydride abstraction from 1 and borylation of
the solvent (e.g., benzene) take place. Arene borylation
generates a Brønsted acid byproduct that in weakly basic
media will protonate an additional molecule of 1 at nitrogen
(Figure 1). Previous work by Noth and Corey has
̈
demonstrated that strong Lewis acids (e.g., H+ and AlBr3)
will coordinate to the nitrogen of a diazaborole (or an
oxazaborole) in weakly basic media, generating an extremely
strong boron Lewis acid.15,25 Instead of the desired H2 loss
from this intermediate (Figure 1 center) to regenerate an active
arene borylating electrophile, cleavage of the N−C bond
occurs. N−C cleavage would initially generate an adamantyl
cation, [Ad]+, active for electrophilic aromatic substitution of
the benzodiazaborole arene backbone. Arene alkylation then
forms a new C−C bond and regenerates the Brønsted
superacid, enabling further [Ad]+ loss and ultimately complete
formal H/Ad exchange. This mechanism is supported by the
identification of phenyladamantane as a minor product (by GC-
MS) formed by reaction of the adamantyl cation with the
solvent benzene instead of a diazaborole. Protonated 1-hydrido-
1,3,2-benzodiazaboroles are isoelectronic with amidine dica-
tions that are stronger methylating agents than MeI and
Me2SO4 (eq 1).26 Therefore, the formal alkyl migration
converting 1 to 2 and the alkylation of benzene are both
consistent with the formation of a highly electrophilic
intermediate that evolves a carbocation. Amidine dications
and protonated diazaboroles are both gitonic superelectro-
philes, using Olah’s formalism, where related alkyl migration
and skeletal rearrangements by carbocation shifts are well
precedented.27 The susceptibility of borenium cations to N−C
cleavage is not restricted to 1; attempts to generate the
borenium cation [CatB(κ1-N(Me2)CH2N(Me2))][AlCl4] also
resulted in the formation of products from N−C cleavage,
namely CatBNMe2 and the iminium cation [H2CNMe2]-
[AlCl4] (by NMR spectroscopy; eq 2). In this case the
Figure 1. Formation of 2 proceeding via the proposed protonated 1-
hydrido-1,3,2-benzodiazaborole intermediates. Inset bottom left:
ORTEP representation of 2, with thermal ellipsoids at the 50%
probability level (N−H and B−H were located in the penultimate
Fourier difference map and freely refined).
weaker B−H bond relative to CatBH (ν(B−H): 1 at 2607
cm−1, CatBH at 2656 cm−1, both in toluene solution). The
addition of 10 mol % of [Ph3C][closo-CB11H6Br6] to 1 in C6D6
resulted in a slow reaction (incomplete after 7 days at 20 °C)
that could be accelerated by moderate heating. 1 is completely
consumed within 2 days (at 50 °C), with the 1H NMR
spectrum confirming the formation of the Ph3CH byproduct
from hydride abstraction. However, the expected arene
borylation product, (C6H4{NAd}2)BPh, was only a minor
component (observed as a broad 11B resonance centered at δ
developing positive charge on the methylene CH2 is stabilized
by N→C π donation.28 Thus, highly electrophilic borocations
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are not accessible when a relatively stable carbocation leaving
group (e.g., tertiary alkyl or heteroatom stabilized carbocations)
can be formed. The generation of a carbocation from a
borenium cation (also observed in the C−O cleavage on
reaction of pinacolborane with strong electrophiles)4 empha-
sizes the necessity for aryl substituents (or other poor cationic
leaving groups) on the pnictogen or chalcogen boron
substituents to successfully access strongly electrophilic boron
species.
To preclude N−C cleavage, N,N′-(2,6-diisopropylphenyl)-1-
bromo-1,3,2-diazaborole (3) was synthesized following the
procedure of Nozaki et al.29 The diisopropylphenyl (DIPP)
substituents and the alkenyl backbone of 3 were chosen to
disfavor analogous carbocation deactivation routes, while still
providing sufficient steric bulk at nitrogen to prevent
borocation oligomerization. Addition of the silicenium cation
[Et3Si][closo-CB11H6Br6] (made in situ by addition of Et3SiH
to [Ph3C][closo-CB11H6Br6]) to 3 led to a broadening of the
1H NMR resonances of 3, suggesting a fluxional process.
However, attempts to reach the slow-exchange regime failed to
−40 °C in 1/1 d8-toluene/C6H4Cl2 (to enhance the solubility
of the ionic species at low temperature). We attribute the
broadening of the 1H resonances to an interaction between the
silicenium cation and the bromine of 3 (eq 3) analogous to that
X, would involve a two-coordinate chelate restrained borinium
cation that would be extremely high in energy. The inability of
bases to coordinate to 3 is further demonstrated by the absence
of Py→3 adduct formation (by 11B NMR). The lack of adduct
formation precludes borenium cation formation by sequential
addition of pyridine and AlCl3 to 3, with the only observed
products being 3 and pyridine adducts of AlCl3. The
requirement in these chelated systems for base coordination
to boron prior to abstraction of X by a Lewis acid is also
consistent with the observed reactivity of 1 with [Ph3C][closo-
CB11H6Br6]. The axial steric bulk in 1 is significantly lower
relative to 3 (while we have been unable to obtain suitable
single crystals of 1, it will be structurally related to the
previously reported N-heterocyclic carbene, Figure 2 right),
enabling interaction of a Lewis base with the pz orbital during
abstraction of the hydride.
(2). Dithiaboroles. 1-Chloro-1,3,2-benzodithiaborole (4;
(CatS2)BCl)32 forms the 1/1 Lewis acid/base adduct (CatS2)-
BCl(NEt3) (5) with 1 equiv of triethylamine in dichloro-
methane. The borenium cation [(CatS2)B(NEt3)][AlCl4]
(6[AlCl4]) is then readily accessed by addition of stoichio-
metric aluminum trichloride. While attempts to obtain single
crystals failed, 6[AlCl4] could be readily obtained analytically
pure (by elemental microanalysis) simply by solvent removal in
vacuo. Multinuclear NMR spectroscopy on this isolated
material was fully consistent with an ionic formalism for
6[AlCl4]. Addition of 1 equiv of N-methylindole or N-TIPS-
indole to 6[AlCl4] resulted in rapid (complete within 3 and 4 h,
respectively) arene borylation. The reaction time for the
borylation of N-TIPS-indole with 6[AlCl4] was considerably
reduced compared to borylation of this substrate with
[CatB(NEt3)][AlCl4] (7[AlCl4]),5 indicating that 6[AlCl4] is
a more reactive electrophile (Table 1).18 Standard pinacol
transesterification conditions were applicable to the (1,3,2-
benzodithiaborol-2-yl)aryl products.5 For example, the 3-
indolyl boronic ester 1-methyl-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)indole could be isolated in 70% yield
(unoptimized; Table 1, entry 2).
observed in halide-bridged [(R3Si)2X]+ cations.30 On standing,
equimolar 3/[Et3Si][closo-CB11H6Br6] did not borylate ben-
zene or toluene even at raised temperatures. In contrast,
CatBBr/Et3Si[closo-CB11H6Br6] rapidly borylates these arenes
even at −10 °C.4 Furthermore, when the reaction is repeated in
the presence of excess Et3SiH, the CatBBr/Et3Si[closo-
CB11H6Br6] combination rapidly forms CatBH, while 3/
Et3Si[closo-CB11H6Br6] does not react (24 h at 80 °C). We
attribute this reactivity disparity and the contrasting behavior of
1 and 3 toward Lewis acids to the significant steric shielding of
the boron pz orbital by the DIPP groups in 3. The DIPP phenyl
and diazaborole rings are orientated orthogonally in DIPP-
substituted diazaboroles (e.g., Figure 2, left).29 This arrange-
ment results in significant axial steric bulk that prevents the
interaction of Lewis bases with the pz orbital of boron in 3.
Nucleophile coordination to the pz orbital of boron in Y2BX
systems (X = H, halide, Y = chelating bis-amido or bis-
alkoxide) appears to be vital to facilitate abstraction of X by a
Lewis acid. The alternative, proceeding via initial abstraction of
The borylation of the less activated heteroarene 2-
methylthiophene (N = 1.26 on the Mayr scale)33 with
6[AlCl4] was extremely slow, with no borylation observed
after 24 h at 20 °C. In an attempt to generate a more reactive
borylating agent, NEt3 was replaced with the less nucleophilic
amine N,N-4-trimethylaniline (Me2NTol). [(CatS2)B-
(Me2NTol)][AlCl4] (8[AlCl4], characterized by resonances at
δ 60.0 ppm, 11B; δ 104 ppm, 27Al; and downfield-shifted
1
aliphatic H resonances at δ 3.90 and 2.44 ppm) was only the
minor product (∼20%) from sequential addition of equimolar
Me2NTol and AlCl3 to 4. The major species present in solution
were 4 and Me2NTol−AlCl3 (the latter has a broad 27Al
resonance centered at δ 109 ppm). The amine-dependent
Figure 2. Steric comparison of a DIPP-substituted diazaborole (left) and an adamantyl-substituted N-heterocyclic carbene, R = N(Ad)C:31 (right),
both at 100% van der Waals radii. Views are perpendicular to the heterocyclic rings.
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only resulted in partial borylation of 2-methylthiophene, which
did not proceed beyond a maximum of 60% thiophene
borylation after 96 h (by H NMR). The comparatively slow
Table 1. Variation in Borylation Times for a Range of Boron
Electrophiles
1
borylation indicates that the reactivity of thiocatechol-ligated
boron electrophiles is intermediate between that of catechol-
and dichloro-ligated congeners. This we attribute to the lower
reactivity of electrophiles derived from 4 compared to BCl3
analogues, a conclusion indirectly supported by the competition
reactions between 4/(Me2NTol)BCl3 (no reaction) and BCl3/
(CatS2)BCl(Me2NTol) (amine transfer forming (Me2NTol)-
BCl3 and 4).
The reasons for incomplete thiophene borylation using the
equimolar 4/Me2NTol/AlCl3 mixture were found to be as
follows: (i) base-induced disproportionation of 4 and (ii)
competitive borylation of Me2NTol. These were both indicated
by the 1H and 11B NMR spectra, which after 96 h revealed that
all 8[AlCl4] is consumed yet 2-methylthiophene is still present
in a ratio of 2/3 to the expected product 2-methyl-5-B(S2Cat)-
thiophene. Compound 4, (CatS2)3B2 (from disproportionation
of 4), and minor resonances at δ +59 and +57 ppm were also
present in the 11B NMR spectrum. Additional resonances were
a
1
Time for full consumption of starting materials by H and 11B NMR
b
1
spectroscopy. In situ yield by H NMR versus an internal standard.
1
c
1
also observed in the H spectra attributable to [HMe2NTol]-
Maximum thiophene borylation of 60% (by H NMR spectroscopy)
reached after 96 h; no further borylation of thiophene is observed at
longer reaction times.
[AlCl4] and one other minor Me2NTol-containing product.
The minor “Me2NTol” product (1/3 ratio relative to
[HMe2NTol]+) contained a singlet of relative intensity 1 in
the aromatic region of the 1H NMR spectrum, which in
combination with the additional 11B resonances suggested
borylation of Me2NTol (Figure 3, bottom). As borylation of
Me2NTol had not been previously observed with catecholato-
and dichloro-ligated boron electrophiles, confirmation was
sought that this was viable with thiocatecholato−borenium
cations. The addition of 1 equiv of Me2NTol to 6[AlCl4] led to
the complete regioselective borylation of Me2NTol ortho to
NMe2 in 24 h, forming N,N,4-trimethyl-2-(1,3,2-benzodithio-
borolan-2-yl)aniline as the only boron-containing product
(Figure 3, top right). The relatively slow borylation of the
highly activated Me2NTol substrate can be attributed to steric
reactivity is due to the relative amine nucleophilicities
producing different equilibrium positions, with Me2NTol
favoring the free species 4 and Me2NTol over the adduct
(CatS2)BCl(Me2NTol). This disparity is exemplified by the 11
B
NMR spectra of equimolar mixtures of 4 and Et3N or Me2NTol
(δ 16.0 or 38.7 ppm, respectively). Equilibria between
borocations and neutral species are well documented,34−36
including for BCl3/AlCl3/Me2NTol, which is a highly active
combination, rapidly borylating 2-methylthiophene in excellent
yield despite the borocation being a minor component (Table
1, entry 9).6 In contrast, an equimolar 4/Me2NTol/AlCl3
mixture (where the borocation is also the minor component)
Figure 3. Scheme for the reaction of 6[AlCl4] and 8[AlCl4] with 2-methylthiophene and Me2NTol.
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crowding of the positions ortho to −NMe2. Subsequent pinacol
esterification produced N,N,4-trimethyl-2-(4,4,5,5-tetramethyl-
dioxaborolan-2-yl)aniline in a moderate unoptimized yield of
51%.
halide transfer process is the strength of the interaction
between the cationic boron center and the anion [AlCl4]¯. This
interaction will have a considerable electrostatic component,
which will be strongly affected by the charge on boron. To
determine the origin of the disparity to halide transfer observed
between catechol- and benzene-1,2-dithiol-ligated borenium
cations, the charge at boron and the nature of the LUMO were
probed computationally for the relevant borenium cations. The
results (Table 2) are at the DFT MPW1K/6-311++G(d,p)
level for comparison to those previously reported for
[CatBPMe3]+,21 and at this level the crystallographic geometry
of [Cl4CatBNEt3]+ (9) is in good agreement with that
determined computationally.5
The character of all three LUMOs is grossly similar, with a
considerably increased boron contribution to the LUMO in 6
(54% boron contribution in 6, 33% in 9, and 20% in 7).
Cations 7 and 9 have extremely similar LUMOs, the only
significant difference being their relative energies, with the
LUMO of 9 being 0.6 eV lower. Despite this difference, both
7[AlCl4] and 9[AlCl4] undergo Al−Cl cleavage on addition of a
range of amines, including Me2NTol. The LUMO of 6 is 0.87
eV lower in energy than that of 7, yet 6[AlCl4] does not
undergo halide transfer to boron on addition of Me2NTol. The
calculated natural bond order (NBO) charges at boron
correlate with the observed reactivity toward chloride transfer.
The similar degrees of NBO positive charge localized at boron
in 7 and 9 are consistent with their comparable reactivities. The
replacement of oxygen for the less electronegative sulfur in 6
results in a significant decrease in the overall charge at boron
Previous attempts to borylate Me2NTol by using stoichio-
metric [Cl2B(NR3)][AlCl4] or [CatB(NR3)][AlCl4] borenium
cations had resulted in Al−Cl bond cleavage and formation of
predominantly (NR3)AlCl3 and (Me2NTol)B(Cl)Y2 (Y =
catecholato, Cl). This contrasting reactivity suggested either a
lower chloride ion affinity for 6[AlCl4] (relative to dichloro and
catecholato analogues) or a more significant kinetic barrier to
halide transfer. The latter can be precluded, as addition of 1
equiv of Et3N to 6[AlCl4] did induce rapid Al−Cl cleavage and
formation of 5 and (Et3N)AlCl3 (Et3N is more bulky with a
cone angle of 150° compared to 140° for Me2NTol).37 The
dependence of Al−Cl cleavage on amine nucleophilicity
indicates the concerted nature of chloride transfer from
[AlCl4]¯ to boron, which we propose will proceed via a SN2
type mechanism via a five-coordinate aluminum, as observed in
bis-amine-AlCl3 adducts (eq 4).38 The other key variable in this
Table 2. Calculated (at the DFT MPW1K/6-311++G(d,p) level) LUMOs and NBO Charges for Cations 6 (LUMO, Top Left), 7
(LUMO, Middle Left) and 9 (LUMO, Bottom Left), All at the 0.04 Isosurface
a
Values taken from ref 21.
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Figure 4. Borylation of 1,8-bis(dimethylamino)naphthalene with 6[AlCl4] (top) and with 7[AlCl4] (bottom).
(Table 2). The lower magnitude of charge at boron in 6 entails
a relatively weaker electrostatic interaction with the anion,
preventing chloride transfer initiated by the addition of a poor
nucleophile (e.g., Me2NTol). While 6 has a significantly lower
LUMO energy and is more reactive in arene borylation than 7,
the degree of positive charge localized at boron is a key factor in
nucleophile-induced halide transfer.
with 7[AlCl4] indicates that catechol ligation and its associated
O→B π donation does little to attenuate the reactivity of
borenium cations toward fluoride anion sources.
For the borylation of arenes that contain coordinating
functionalities (without resorting to relatively expensive weakly
coordinating anions such as [B(C6F5)4] and closo-carboranes),
it is essential to decouple borylation reactivity from rapid
chloride transfer by minimizing the positive charge at boron
and thus enabling [AlCl4]¯ to be used as the anion. This is
demonstrated by the borylation of 1,8-bis(dimethylamino)-
naphthalene using 6[AlCl4], which was highly effective, leading
to the 4-borylated product as the only boron-containing
product within 1 h (as a mixture of protonated at nitrogen and
nonprotonated species, Figure 4 top). Combination of 1,8-
bis(dimethylamino)naphthalene with equimolar 7[AlCl4] also
resulted in an equally rapid reaction (all 7[AlCl4] is consumed
within 1 h); however, this produced the aryl boronate ester in
Comparison of amine- and phosphine-ligated catecholato
borocations reveals a related ligand electronegativity (χN = 3.0,
χP = 2.2) controlled variation in NBO charge at boron, with
[CatBPMe3]+ having a significantly lower positive charge than 7
(Table 2). For reactivity comparison purposes [CatBPtBu3]-
[AlCl4] (10[AlCl4]) was synthesized and recrystallized from
CH2Cl2/pentane (the structure of the cation is identical within
3σ with that previously reported for 10[HB(C6F5)3]).21,39
Dissolution of this recrystallized material showed only
resonances attributable to 10[AlCl4] with B−P coupling
observed in the 31P{1H) and 11B NMR spectra (at 20 °C in
CD2Cl2). The observation of B−P coupling precludes halide
transfer equilibria in the absence of additional base. Were halide
transfer to occur, this would form CatBCl(PtBu3) and result in
loss of B−P coupling due to rapid phosphine dissociation, as
previously reported.40 In contrast to the reactivity of 7[AlCl4],
the addition of 1 equiv of Me2NTol to 10[AlCl4] resulted in no
Al−Cl cleavage of [AlCl4] (27Al NMR spectrum showed only a
sharp resonance for [AlCl4]¯). Instead a mixture of 10[AlCl4]
and resonances attributable to the boronium cation [CatB-
(PtBu3)(Me2NTol)][AlCl4] were observed as the major
products. The absence of [AlCl4]¯ cleavage is consistent with
the lower magnitude of NBO positive charge at boron in
[CatBPMe3]+ compared to 7.
An obvious alternative that would also preclude halide
transfer would be to utilize weakly coordinating anions in place
of [AlCl4]¯. However, the vast majority of classic weakly
coordinating anions are either oxo based (e.g., triflate, [ClO4]
¯), which coordinate too strongly to boron for arene borylation
purposes,15 or are Lewis acid/anion adducts vulnerable to
analogous anion transfer.41 The latter is exemplified by
attempts to form 7[SbF6] from CatBCl(NEt3) and AgSbF6,
instead producing multiple B−F-containing products. There-
fore, the borenium cation 7 has a higher fluoride ion affinity
than SbF5, which is itself extremely fluorophilic.41 Furthermore,
7[AlCl4] activates C−F bonds in aryl-CF3 compounds,
precluding other common weakly coordinating anions. Vedejs
et al. previously reported the C−F activation of [B-
(C6H3(CF3)2)4]¯ with a less stabilized aryl hydrido ligated
borenium cation.42 The fact that C−F activation is observed
1
only ∼60% yield (by in situ H and 11B NMR spectroscopy).
The other major products were CatBCl(Et3N) and the 1,8-
bis(dimethylamino)naphthalene adduct of AlCl3 (Figure 4,
bottom). This indicates that arene borylation and chloride
transfer are kinetically competitive when borylating 1,8-
bis(dimethylamino)naphthalene with 7[AlCl4]. In contrast, in
the borylation of 1,8-bis(dimethylamino)naphthalene with
6[AlCl4] no products derived from halide transfer are observed,
due to the relatively lower charge localized at boron.
CONCLUSIONS
■
The generation of highly electrophilic borenium cations
requires a judicious choice of anion and neutral ligands to
prevent anion or cation decomposition. Borenium cations,
[(RY)2BL]+ (Y = O, S, RN; L = neutral two-electron-donor
ligand, e.g., NR3), can be designed to prevent decomposition by
C−Y bond cleavage by using R groups that can only generate
high-energy carbocations on C−Y cleavage (e.g., R = Me+,
Ph+). Only once all decomposition routes have been removed
can the electrophilicity of borenium cations be directed toward
external substrates. The order of reactivity of borenium cations
with constant L in direct electrophilic arene borylation is
[(halide)2BL]+ > [(CatS2)BL]+ > [CatBL]+. Our attempts to
place a number of chelate restrained diazaboroles ((R2N)2BX)
in this series have failed to date. Stable Lewis base/acid adducts
between (R2N)2BX (X = H, halide) and L are not observed,
due to the low Lewis acidity and significant steric bulk of the
(R2N)2BX species investigated herein. The inability to form
stable adducts prevents abstraction of X− and formation of the
borenium cation on addition of a Lewis acid. Finally, we have
1913
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Organometallics
Article
demonstrated that halide transfer from [AlCl4]− to boron in
borenium cations is dependent on the charge at boron. This
enables borenium cations that are highly electrophilic
(possessing a low-lying LUMO) but have a low degree of
positive charge localized at boron to borylate arenes containing
nucleophilic substrates (e.g., −NMe2) with no evidence for
[AlCl4]− decomposition by Al−Cl cleavage.
The mixture was inverted repeatedly to ensure reaction, and aluminum
trichloride was then added (25 mg, 0.19 mmol); dissolution of all
AlCl3 and the iminium chloride precipitate occurred concomitantly to
form CatBNMe2 and [Me2NCH2][AlCl4] as the major products.
aromatics), 2.80 (br s, 5.9 H, CatBNMe CH3); 11B NMR (CD2Cl2) δ
25.6 (s); 13C{1H} NMR (CD2Cl2) δ 149.51 (broad, weak singlet, Ar
O), 122.06 (s CatB Ar H), 111.74 (s, CatB Ar H), 36.58 (s,
CatBNMe2, CH3).
1
[CatBNMe2]: H NMR (CD2Cl2) δ 6.9−7.1 (br. m, 4 H, CatB
[Me2NCH2][AlCl4] (consistent with that previously reported by
EXPERIMENTAL SECTION
■
28
1
Mayr et al. ): H NMR (CD2Cl2) δ 7.99 (b s, 2H, iminium CH2),
3.87 (br s, 6H, iminium NMe2); 13C{1H} NMR (CD2Cl2) δ 168.27
(1:1:1 sharp triplet, JCN = 13.1 Hz, iminium CH2), 50.82 (1:1:1 triplet,
JCN = 5.0 Hz, iminium NMe2); 27Al NMR (CD2Cl2) δ 104.1 (sharp
singlet, AlCl4).
General Considerations. All reactions were performed using
standard glovebox or Schlenk line techniques, unless otherwise
specified. Solvents used were either purified by an Innovative
Technology PS-MD-5 solvent purification system or distilled from
appropriate drying agents and degassed. Deuterated solvents were
distilled from appropriate drying agents and degassed. [Ph3C][closo-
CB11H6Br6],22 1-hydrido-1,3,2-benzodiazaborole,20 N,N′-(adaman-
tyl)2-1,2-diaminobenzene,43 N,N′-(2,6-diisopropylphenyl)-1-bromo-
1,3,2-diazaborole (3),29 and 1-chloro-1,3,2-benzodithiaborole
((CatS2)BCl, 4)32 were synthesized according to published routes.
All other materials were purchased from commercial vendors and used
as received. NMR spectra were recorded with a Bruker AV 400
spectrometer (400 MHz; 1H, 100 MHz; 13C. 100 MHz; 11B, 128
MHz; 31P, 162 MHz; 19F, 376.5 MHz; 27Al, 104.3 MHz; 29Si, 79.5
MHz). 1H NMR chemical shifts are reported in ppm relative to protio
impurities in the deuterated solvents, and 13C NMR shifts are reported
using the center line of the CDCl3 (or CD2Cl2 as appropriate) triplet
as an internal standard. 11B NMR spectra were referenced to external
BF3·Et2O, 31P to H3PO4, 19F to Cl3CF, and 27Al to Al(NO3)2 in D2O
(Al(D2O)63+).Unless otherwise stated, all NMR spectra were recorded
at 293 K. Elemental analysis of air -sensitive compounds was
performed by London Metropolitan University. For borylation
conditions using borenium cations derived from CatBCl, see ref 5.
Compound 1. A Schlenk flask fitted with a J. Young valve was
charged with N,N′-(adamantyl)2-1,2-diaminobenzene (300 mg, 0.78
mmol) and dissolved in 5 mL of toluene. H3BSMe2 (1.2 equiv, 2 M
solution in toluene, 0.5 mL) was added by syringe, resulting in rapid
gas evolution. The flask was sealed under vacuum and heated to 100
°C for 1 h. Toluene was then removed in vacuo and the colorless solid
Borylation of 2-Methylthiophene with Equimolar BCl3/
Me2NTol/AlCl3. A Schlenk tube containing a stirring bar was flame-
dried under reduced pressure and charged in a glovebox with solid
Me2NTol·BCl3 adduct (265 mg, 1.05 mmol) and anhydrous AlCl3
(140 mg, 1.05 mmol). Then anhydrous CH2Cl2 (3.5 mL) was added
and the reaction mixture was stirred at ambient temperature for 1 h.
After the AlCl3 had completely dissolved, 2-methylthiophene (87 μL, 1
mmol) was added and stirring was continued at ambient temperature
until an 1H NMR spectrum of an aliquot of the reaction mixture
showed no resonances attributable to 2-methylthiophene in the
aromatic region (2 h). The borylated product was the major species
observed in situ (>90%), with a currently unidentified minor
byproduct, occurring at <10% of the major product’s intensity (by
1H NMR spectroscopy).
(CatS2)BCl(NEt3) (5). A J. Young NMR tube was charged with
triethylamine (0.21 mmol, 21 mg, 29 μL, 1.0 equiv) in anhydrous
CD2Cl2/CH2Cl2 (0.7 mL). This was then treated with 4 (0.21 mmol,
40 mg, 1.0 equiv) to give a colorless solution which was sealed and
rotated for 1 h. The solution was transferred via cannula to a Schlenk,
the NMR tube was washed with anhydrous CH2Cl2 (3 × 2 mL), and
the washings were transferred to the Schlenk. The CH2Cl2 was
removed under vacuum, and the product was dried under vacuum for
2 h, to give a cream/off-white solid (27 mg, 45%). 1H NMR (CDCl3):
δ 7.21 (m, 2H, ThioCatB), 6.91 (m, 2H, ThioCatB), 3.27 (q, 3J = 8.0
3
1
Hz, 6H, 3 × NCH2CH3), 1.25 (t, J = 8.0 Hz, 6H, 3 × NCH2CH3).
dried in vacuo overnight to give 255 mg of 1 (yield 84%). H NMR
13C NMR (CDCl3): δ 10.68, 51.39, 124.3, 124.5, 140.8. 11B NMR
(CDCl3): δ 15.95 (s, broad). Anal. Calcd for C12H19Cl4BNS2: C,
50.10; H, 6.66; N, 4.87. Found: C, 49.98; H, 6.75; N, 4.74.
(C6D6): δ 7.61 (2H, m Ar H), 7.12 (2H, M, Ar H), 2.27 (6H, CH2),
1
1
2.02 (3H, CH), 1.62 (6H, CH2). H{11B} NMR (C6D6, as H apart
from additional resonance): δ 4.99 (br s, 1H, B−H). 11B NMR
(C6D6): δ 22.8 (s, v br), 11B{1H} NMR (C6D6): δ 22.8 (s, v br). IR
(toluene solution): ν(B−H) 2607 cm−1. Anal. Calcd for C26H35BN2:
C, 80.82; H, 9.13; N, 7.25. Found C, 79.87; H, 9.12; N, 6.87.
[(CatS2)BNEt3][AlCl4] (6[AlCl4]). A J. Young NMR tube was
charged with triethylamine (0.21 mmol, 21 mg, 29 μL, 1.0 equiv) in
CD2Cl2/CH2Cl2 (0.7 mL). This was then treated with 4 (0.21 mmol,
40 mg 1.0 equiv) and AlCl3 (0.21 mmol, 28 mg, 1.0 equiv), resulting in
immediate (<10 min) formation of 6[AlCl4]. The solution was
transferred via cannula to a Schlenk, the NMR tube was washed with
anhydrous CH2Cl2 (3 × 2 mL), and the washings were transferred to
the Schlenk. The CH2Cl2 was removed under vacuum, and the
Compound 2. A J. Young NMR tube was charged with 1 (50 mg,
0.13 mmol) that was dissolved in C6D6 (0.6 mL). [Ph3C][closo-
CB11H6Br6] (10 mg, 0.012 mmol) was added as a solid and the
reaction mixture subsequently heated to 50 °C with periodic
monitoring by NMR spectroscopy. After 48 h all 1 had reacted and
on cooling gave 23 mg of 2 as small colorless crystals (46% yield). The
moderate yield is due to the partial solubility of 2 in benzene. 1H NMR
(CDCl3): δ 7.03 (1H, Ar H), 6.95 (1H, Ar H), 6.63 (2H, NH, br);
2.16 (9H, CH and CH2); 2.11 (3H, CH) 1.97 (6H, CH2), 1.86 (6H,
1
product was dried under vacuum to produce a colorless oil. H NMR
(400.13 MHz, CDCl3): δ 7.93 (m, 2H, ThioCatB), 7.55 (m, 2H,
ThioCatB), 3.73 (q, 3J = 8.0 Hz, 6H, 3 × NCH2CH3), 1.43 (t, 3J = 8.0
Hz, 6H, 3 × NCH2CH3). 13C NMR (100.61 MHz, CDCl3): δ 9.42,
56.18, 128.3, 128.4, 137.1. 11B NMR (128.38 MHz, CDCl3): δ 59.84
(s, broad). 27Al NMR (104.26 MHz, CDCl3): δ 103.98 (s, sharp).
Anal. Calcd for C12H19AlBCl4NS2: C, 34.23; H, 4.55; N, 3.33. Found:
C, 34.35; H, 4.51; N, 3.25.
N-Methyl-2-(4,4,5,5-tetramethyldioxaborolan-2-yl)indole. A
J. Young NMR tube was charged with triethylamine (0.21 mmol, 21
mg, 29 μL, 1.0 equiv) in CH2Cl2/CD2Cl2 (0.7 mL). This was then
treated with 4 (0.21 mmol, 40 mg, 1.0 equiv) and AlCl3 (0.21 mmol,
28 mg, 1.0 equiv). The reaction mixture was then treated with N-
methylindole (0.21 mmol, 28 mg, 26 μL, 1.0 equiv) and monitored by
NMR spectroscopy. The reaction was 97% complete within 3 h, with
1H NMR indicating the major product (>99%) was the borylated
arene. 1H NMR (CD2Cl2): δ 8.01 (m, 1H, Ar H), 7.81 (q, 3J = 8.0 Hz,
2H, ThioCatB), 7.71 (s, 1H, Ar H), 7.42 (m, 1H, Ar H), 7.33 (m, 4H,
Ar H), 3.83 (s, 3H, NCH3). 11B NMR (CD2Cl2): δ 53.64 (s, broad).
1
1
CH2), 1.79 (6H, CH2). H{11B} NMR (CDCl3, as H spectra apart
from additional resonance): δ 4.58 (br s, 1H, B−H). 13C{1H} NMR
(CDCl3): δ 142.67, 136.17, 132.55, 130.65, 112.96, 106.27, 43.88,
41.69, 37.09, 36.96, 36.82, 36.13, 28.18, 28.96. 11B NMR (CDCl3): δ
23.8 (s, v br), 11B{1H} NMR (CDCl3): δ 23.6 (s, v br). IR (toluene
solution): ν(B−H) 2593 cm−1. Anal. Calcd for C26H35BN2: C, 80.82;
H, 9.13; N, 7.25. Found: C, 79.53; H, 8.82; N, 6.67.
Reactivity of CatBCl with N,N,N′,N′-Tetramethyldiamino-
methane. Chlorocatecholborane (30 mg, 0.19 mmol) was added to a
J. Young NMR tube. After dissolution in CD2Cl2 (1 mL) N,N,N′,N′-
tetramethyldiaminomethane (26.5 μL, 0.19 mmol) was added, giving
immediate formation of a cloudy solution. 1H and 11B NMR
spectroscopy at this stage indicates the formation of N,N-
dimethylamidocatecholborane (CatBNMe2); the precipitate is attrib-
uted to autoionization and elimination of insoluble iminium chloride.
1914
dx.doi.org/10.1021/om201228e | Organometallics 2012, 31, 1908−1916

Organometallics
Article
Standard Workup Procedure. The solution was then treated
with triethylamine (3.15 mmol, 319 mg, 0.44 mL, 15 equiv) and
pinacol (0.42 mmol, 50 mg, 2.0 equiv) and rotated for 30 min to give
an orange suspension. The suspension was transferred to a round-
bottom flask and the NMR tube washed with dry CH2Cl2 (3 × 1 mL).
The CH2Cl2 was removed under vacuum, dried for 1 h, and replaced
with hexane (25 mL) to give an orange suspension. The suspension
was filtered to give a colorless solution, hexane was removed under
vacuum, and the product was dried overnight to give N-methyl-2-
(4,4,5,5-tetramethyldioxaborolan-2-yl)indole (52 mg, 70% identified
by comparison to our previous report).
and treated with CatBCl (0.32 mmol, 50 mg, 1.0 equiv), AlCl3
(0.35 mmol, 48 mg, 1.10 equiv), and 1,8-bis(dimethylamino)-
naphthalene (0.32 mmol, 68 mg, 1 equiv) and stirred to give a
yellow solution. The reaction reached a maximum 60%
conversion (by 11B NMR) after stirring for 18 h. Isolation
was as described above in Standard Workup Procedure
(triethylamine (4.8 mmol, 0.485 g, 0.67 mL, 15 equiv) and
pinacol (0.96 mmol, 114 mg, 3 equiv)) with further purification
by column chromatography with hexane, followed by DCM
and ethyl acetate. 4-(4,4,5,5-Tetramethyldioxaborolan-2-yl)-1,8-
(dimethylamino)naphthalene was isolated as an off-white solid
1
(62.6 mg, 57%). H NMR (CDCl3): δ 1.29 (s, 12H, 4 × Me,
N-TIPS-3-(1,3,2-benzodithioborolan-2-yl)indole. A J. Young
NMR tube was charged with triethylamine (0.21 mmol, 21 mg, 29 μL,
1.0 equiv) in CH2Cl2/CD2Cl2 (0.7 mL). This was then treated with 4
(0.21 mmol, 40 mg, 1.0 equiv) and AlCl3 (0.21 mmol, 28 mg, 1.0
equiv). The reaction mixture was treated with N-TIPS-indole (0.21
mmol, 88 mg, 1.0 equiv) and rotated with periodic monitoring by
pinacol), 2.67, 2.74 (2 × s, 12H, 2 × NMe2), 6.80 (m, 2H, Ar
H), 7.24 (m, 1H, Ar H), 7.84 (d, J = 8.0 Hz, 1H, Ar H), 8.26
3
3
(d, J = 8.0 Hz, 1H, Ar H). 13C NMR (CDCl3): δ 24.89 (4 ×
Me, pinacol), 43.66, 43.85 (2 × NMe2), 83.06, 110.76, 111.76,
121.43, 125.86, 136.23, 141.51, 150.68, 163.48. 11B NMR
(CDCl3): δ 32.14 (s, broad). Accurate mass ES+: calcd for MH+
(C20H29BN2O2) m/z 341.2400, found: m/z 341.2388.
1
NMR spectroscopy. The reaction was complete within 4 h, with H
NMR indicating >99% borylation. 1H NMR (CD2Cl2): δ 8.02 (d, 3J =
3
Method B. A J. Young tap NMR tube was charged with
triethylamine (0.113 mmol, 11.4 mg, 16 μL, 1.05 equiv) in CH2Cl2/
CD2Cl2 (0.8 mL) and treated with 4 (0.107 mmol, 20 mg, 1.0 equiv),
AlCl3 (0.118 mmol, 16 mg, 1.10 equiv), and then 1,8-bis-
(dimethylamino)naphthalene (0.107 mmol, 23 mg, 1 equiv) and
stirred to give a yellow solution. The reaction reached >99%
conversion (by 11B NMR spectroscopy) within 1 h at room
temperature. NMR data of 4-(1,3,2-benzodithioborolan-2-yl)-1,8-
8.0 Hz, 1H, Ar H), 7.90 (s, 1H, Ar H), 7.82 (q, J = 4.0 Hz, 2H,
3
3
CatS2B), 7.63 (d, J = 8.0 Hz, 1H, Ar H), 7.31 (q, J = 4.0 Hz, 2H,
CatS2B), 7.29 (m, 2H, Ar H), 1.78 (m, 3H, 3 × NCH(CH3)2), 1.18 (d,
3J = 8.0 Hz, 18H, 3 × NCH(CH3)2). 11B NMR (CD2Cl2): δ 53.56 (s,
broad).
N,N,4-Trimethyl-2-(4,4,5,5-tetramethyldioxaborolan-2-yl)-
aniline. A J. Young NMR tube was charged with triethylamine (0.21
mmol, 21 mg, 29 μL, 1.0 equiv) in CH2Cl2/CD2Cl2 (0.7 mL). This
was then treated with 4 (0.21 mmol, 40 mg, 1.0 equiv.) and AlCl3
(0.21 mmol, 28 mg, 1.0 equiv). The reaction mixture was treated with
Me2NTol (0.21 mmol, 28.4 mg, 30 μL, 1.0 equiv) and monitored
periodically by NMR spectroscopy. The reaction was complete within
1
bis(dimethylamino)naphthalene: H NMR (CD2Cl2) 2.78, 2.88 (2 ×
3
3
s, 2 × 6H, 2 × NMe2), 6.88 (d, J = 4.0 Hz, 1H, Ar H), 6.95 (d, J =
4.0 Hz, 1H, Ar H), 7.32 (m, 2H, ThioCatB), 7.79 (m, 2H, ThioCatB),
7.87 (d, 3J = 4.0 Hz, 1H, Ar H), 8.00 (dd, 3J = 8.0, 4.0 Hz, 2H, Ar H);
11B NMR (CD2Cl3) δ 58.3 (s, broad, peak width at half height 500
Hz).
1
1
24 h, with in situ H NMR indicating >99% borylation. H NMR
(CD2Cl2): δ 7.80 (q, 3J = 4.0 Hz, 2H, ThioCatB), 7.66 (s, 1H, Ar H),
7.30 (q, 3J = 4.0 Hz, 2H, ThioCatB), 7.28 (dd, 3J = 8.0, 4.0 Hz, 1H, Ar
H), 7.13 (d, 3J = 8.0 Hz, 1H, Ar H), 2.74 (s, 6H, 2 × N(CH3)2), 2.34
(s, 3H, CH3). 11B NMR (CD2Cl2): δ 58.3 (s, broad, peak width at half-
height =339 Hz).
ASSOCIATED CONTENT
* Supporting Information
■
S
Tables, figures, and CIF files giving full crystallographic and
computational details. This material is available free of charge
via the Internet at http://pubs.acs.org.
Esterification and isolation was as described in Standard Workup
Procedure (using triethylamine (3.15 mmol, 319 mg, 0.44 mL, 15
equiv) and pinacol (0.42 mmol, 50 mg, 2.0 equiv)) and afforded
N,N,4-trimethyl-2-(4,4,5,5-tetramethyldioxaborolan-2-yl)aniline (28
mg, 51%) as a colorless solid. A combination of one- and two-
dimensional NMR experiments unambiguously allowed the product to
be identified with the BPin group ortho to the amino group.
1H NMR (CDCl3): δ 1.37 (s, 12H, 4 × CH3), 2.27 (s, 3H, CH3),
2.83 (s, 6H, N(CH3)2), 6.79 (d, 3J = 8.0, 1H, Me2NC6H3-ortho), 7.13
(dd, 3J = 4.0, 2.0, 1H, PinB-C6H3-para), 7.46 (s, 1H, PinB-C6H3-
ortho). 13C NMR (CDCl3): δ 20.28 (CH3), 24.74 (4 × CH3 pinacol),
45.28 (2 × CH3 amino group), 83.41 (quaternary C × 2, pinacol),
115.1, 128.3, 131.9, 136.9, 155.9. 11B NMR (CDCl3): δ 31.57 (s,
broad). Accurate mass ES+: calcd for MH+ (C15N25BNO2) m/z
262.1978, found m/z 262.1965.
AUTHOR INFORMATION
Corresponding Author
*E-mail: michael.ingleson@manchester.ac.uk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the Royal Society (M.J.I. for a
University Research Fellowship), the Leverhulme Trust (E.W.),
and the University of Manchester for funding. Dr. Christopher
Muryn is thanked for assistance with crystallography.
[CatB(PtBu3)][AlCl4] (10[AlCl4]). In an oven-dried Schlenk CatBCl
(0.90 mmol, 140 mg, 1.0 equiv) was added to a stirred solution of tri-
tert-butylphosphine (0.90 mmol, 183 mg, 1 equiv) in dry DCM (1
mL). After 3 min AlCl3 (0.90 mmol, 120 mg, 1 equiv) was added to
the mixture, which was stirred for 1 h and then layered with pentane.
Slow diffusion of the layers yielded colorless crystals of [CatB-
(PtBu3)][AlCl4] (406 mg, 92%) suitable for single-crystal X-ray
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1
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1
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1
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1915
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