Chelate restrained boron cations for intermolecular electrophilic arene borylation

Article abstract of DOI:10.1021/om900893g

Highly electrophilic boron species that borylate arenes are generated by halide abstraction from CatBX (Cat = catecholato, C₆H ₄O₂^2-, X = Cl or Br) by [Et₃Si] [CbBr₆] (CbBr₆ = [closo-1-H-CB₁₁H ₅Br₆]^-). A transient [CatB][CbBr₆] related species reacts as a synthetic equivalent of [CatB]⁺ in intermolecular electrophilic borylation, with reactions proceeding rapidly at 25 °C. The [CatB]⁺ moiety was shown to be strongly Lewis acidic on the basis of ¹H and ³¹P{¹H} NMR spectroscopy of the crotonaldehyde and triethylphosphine oxide adducts, respectively. Catalytic quantities of [Et₃Si][CbBr₆] and CatBX were effective for the high-yielding borylation of arenes by CatBH in a highly atom efficient cycle with H₂ the only byproduct. Successful catalysis was dependent on the robust [CbBr₆]^- anion and the use of electrophile-resistant borane sources

Full text of DOI:10.1021/om900893g

Organometallics 2010, 29, 241–249 241
DOI: 10.1021/om900893g
Chelate Restrained Boron Cations for Intermolecular Electrophilic Arene
Borylation
Alessandro Del Grosso, Robin G. Pritchard, Chris A. Muryn, and Michael J. Ingleson*
Department of Chemistry, University of Manchester, Manchester, M13 9PL U.K.
Received October 13, 2009
Highly electrophilic boron species that borylate arenes are generated by halide abstraction from
CatBX (Cat = catecholato, C6H4O22-, X = Cl or Br) by [Et3Si][CbBr6] (CbBr6 = [closo-1-H-
CB11H5Br6]-). A transient [CatB][CbBr6] related species reacts as a synthetic equivalent of
[CatB]þ in intermolecular electrophilic borylation, with reactions proceeding rapidly at 25 °C. The
1
[CatB]þ moiety was shown to be strongly Lewis acidic on the basis of H and 31P{1H} NMR
spectroscopy of the crotonaldehyde and triethylphosphine oxide adducts, respectively. Catalytic
quantities of [Et3Si][CbBr6] and CatBX were effective for the high-yielding borylation of arenes by
CatBH in a highly atom efficient cycle with H2 the only byproduct. Successful catalysis was
dependent on the robust [CbBr6]- anion and the use of electrophile-resistant borane sources.
The electrophilicity of boron Lewis acid reagents, such as
BBr3, while strong, is limited. This is exemplified by the lack
of reaction with unfunctionalized arenes by an electrophilic
aromatic substitution (EAS) mechanism.1-5 Increasing ca-
tionic character by replacement of halide for a weakly inte-
racting anion enhances electrophilic reactivity at boron.6-13
Gas phase studies, where anion coordination is precluded,
boron cations, termed borenium cations, were calculated to
be more electrophilic than Me3Cþ and remarkably appear to
activate H2 analogously to transition metal superelectro-
philes.19 Borenium species with weakly stabilizing substitu-
ents can be classed as superelectrophiles, combining a mono-
cationic charge with a formally unoccupied p orbital.17,20,21
An alternative class of boron cations with the potential to be
superelectrophiles are the two-coordinate boron cations,
termed borinium cations.6,7 Borinium cations are invariably
ligated by bulky, strongly π-donating substituents that effec-
tively shield the boron cation from solvent and anion.6,7
Monodentate systems predominate due to the stabilization
derived from a linear geometry at boron that maximizes
electron donation from orthogonal π-donors (Figure 1),
analogous to the isoelectronic allenes. While these factors
enabled the synthesis of borinium cations in the condensed
phase, they are undesirable from a reactivity perspective due
to reduced electrophilicity and effective steric shielding at
boron.
Bidentate ligation of boron conceptually generates a “che-
late restrained” borinium cation in which electrophilicity is
enhanced by the nonlinear geometry at boron, resulting in an
empty boron orbital that cannot be stabilized by ligand π-
donation (Figure 1). Chelation also ensures a sterically more
accessible boron cation, which is advantageous for substrate
accessibility. In the condensed phase bona fide chelate re-
strained borinium cations are not feasible. The boron Lewis
acid will readily interact with any Lewis base present in
solution, generating borenium cations or anion-coordinated
species. However, in a weakly nucleophilic environment
chelate restrained boron species will be generated with
electrophilicity approaching that of the superelectrophilic
þ
with BH2 confirmed the ability of an electrophilic boron
cation to bind and activate unreactive molecules, including
H2 and CH4.14,15 Recent elegant work by Vedejs et al. uti-
lized the BH2þ cation stabilized by R3N coordination for in-
tramolecular arene borylation.16-18 These three-coordinate
*Corresponding author. E-mail: michael.ingleson@manchester.ac.
uk.
(1) Yamamoto, H. Lewis Acids in Organic Synthesis; Wiley-VCH,
2000.
(2) Muetterties, E. L. J. Am. Chem. Soc. 1959, 81, 2597.
(3) Muetterties, E. L. J. Am. Chem. Soc. 1960, 82, 4163.
(4) Muetterties, E. L.; Tebbe, F. N. Inorg. Chem. 1968, 7, 2663.
(5) Erker, G. Dalton Trans. 2005, 1883.
€
(6) Koelle, P.; Noth, H. Chem. Rev 1985, 85, 399.
(7) Piers, W. E.; Bourke, S. C.; Conroy, K. D. Angew. Chem., Int. Ed.
2005, 44, 5016.
(8) Vidovic, D.; Findlater, M.; Cowley, A. H. J. Am. Chem. Soc. 2007,
129, 8436.
(9) Kato, T.; Tham, F. S.; Boyd, P. D. W.; Reed, C. A. Heteroat.
Chem. 2006, 17, 209.
(10) Wei, P.; Atwood, D. A. Inorg. Chem. 1998, 37, 4934.
(11) Imamoto, T.; Asakura, K.; Tsuruta, H.; Kishikawa, K.;
Yamaguchi, K. Tetrahedron Lett. 1996, 37, 503.
(12) Ott, H.; Matthes, C.; Ringe, A.; Magull, J.; Stalke, D.;
Klingebiel, U. Chem.;Eur. J. 2009, 15, 4602.
(13) Uddin, M. K.; Nagano, Y.; Fujiyama, R.; Kiyooka, S.; Fujio,
M.; Tsuno, Y. Tetrahedron Lett. 2005, 46, 627.
(14) DePuy, C. H.; Gareyev, R.; Hankin, J.; Davico, G. E.; Krempp,
M.; Damrauer, R. J. Am. Chem. Soc. 1998, 120, 5086.
€
(15) Schneider, W. F.; Narula, C. K.; Noth, H.; Bursten, B. E. Inorg.
Chem. 1991, 30, 3919.
(16) Vedejs, E.; Nguyen, T.; Powell, D. R.; Schrimpf, M. R. Chem.
Commun. 1996, 2721.
(17) Vries, T. S. D.; Prokofjevs, A.; Harvey, J. N.; Vedejs, E. J. Am.
Chem. Soc. 2009, 131, 14679.
(19) Kubas, G. J. Metal Dihydrogen and Sigma Bond Complexes;
Kluwer Academic/Plenum Publishers: New York, 2001.
(20) Olah, G. A.; Klump, D. A. Superelectrophiles and Their Chem-
istry; Wiley: New York, 2008.
(18) Vries, T. S. D.; Vedejs, E. Organometallics 2007, 26, 3079.
(21) Chiu, C.-W.; Gabbai, F. P. Organometallics 2008, 27, 1657.
r
2009 American Chemical Society
Published on Web 12/15/2009
pubs.acs.org/Organometallics

242 Organometallics, Vol. 29, No. 1, 2010
Del Grosso et al.
Figure 1. Linear and chelate restrained borinium cations de-
monstrating the different degrees of boron stabilization by π-
donation (potential anion/solvent coordination excluded for
simplicity).
BX2þ gas phase cations (X = H, Me, OMe).14,22,23 An initial
benchmark reaction of particular interest is the intermole-
cular electrophilic aromatic substitution of the C-H bonds
of unactivated arenes. This requires an electrophilicity great-
er than BBr3, with only directed intramolecular arene C-H
borylations proving successful using neutral borane Lewis
acids.17,24-30
Herein we report attempts to synthesize and isolate chelate
restrained boron cations based predominantly on the [CatB]þ
(cat = catecholato, C6H4O22-) moiety partnered with a range
of weakly coordinating anions. The phrase “chelate restrained
boron cation” is used to signify a [CatB]þ group stabilized by
interactions with weakly nucleophilic species and is repre-
sented throughout as [CatB][X] (where X is a weakly coordi-
nating anion). Subsequent stoichiometric and catalytic (in
superacid) reactivity studies are presented utilizing chelate
restrained boron cations to generate aryl-boronic esters (Ar-
B(OR)2). These findings are significant given the importance of
aryl boronic esters and acids in transition metal catalyzed
synthesis,31-33 combined with the scarcity of intermolecular
reactivity studies between electrophilic boron cations and
carbon nucleophiles.7
both 25 °C and at reflux in C6D6, with the reactants re-
maining unchanged. Use of a large excess (∼100 equiv)
ofTMSOTf yielded 1 as the major product along with
unreacted CatBCl. Subsequent removal of excess TMSOTf
in vacuo regenerated CatBCl, indicating that this system was
in an equilibrium favoring CatBCl.36 Access to [CatB][OTf]
was achieved through silver salt metathesis with AgOTf (eq
1). The 11B chemical shift of 1 at 21.7 ppm was consistent
with a three-coordinate B(OR)3 environment and close to
that reported for compound A (23.3 ppm).34 A benzene
solution of 1 was heated to 80 °C for 3 days; however there
was no reaction, with 1 recovered unchanged. The triflate
anion in 1 interacts strongly with the boron center, as
indicated by the 11B chemical shift, limiting the electrophi-
licity at boron and preventing arene borylation.37 The synth-
esis of a [CatB]þ species partnered with a more weakly
coordinating anion was sought to further enhance the Lewis
acidity at boron.
AlCl3 Reactivity. Halide abstraction is the prevalent syn-
thetic route to linear borinium cations from (R2N)2BCl and
strong MX3 Lewis acids (M = Al, Ga, B, X = halide).6,7,12 It
is also noteworthy that AlCl3 enabled arene borylation with
BCl3 (or BBr3) to yield aryldihaloboranes, presumably in-
volving an unobserved boron species related to
[BCl2][AlCl4].2,3,38 In contrast combination of stoichiometric
CatBCl and AlCl3 in arene solvents resulted in effectively no
reaction at 25 °C (by 11B and 27Al NMR spectroscopy). With
similar B-Cl bond strengths expected for BCl3 and CatBCl
Results and Discussion
€
[OTf]. Noth et al. previously reported a diazaborole
system that utilized the relatively coordinating triflate anion
(= [OTf]-) to stabilize the formally cationic chelate re-
strained boron species, A.34 The reduced degree of π-dona-
tion provided by chalcogens compared to pnicogens of the
same period will afford a modest enhancement of electro-
philicity at boron in the related compound [CatB][OTf], 1.35
Initial attempts to synthesize 1 from CatBCl using stoichio-
metric TMSOTf as the halide-abstracting reagent failed at
39,40
˚
(B-Cl bond lengths are 1.75 and 1.74 A, respectively),
the lack of reactivity with CatBCl is attributed to the
stabilization afforded by adopting a linear geometry at
boron in the resultant borinium cation.
Heating equimolar CatBCl/AlCl3 to 100 °C in toluene
resulted in a slow reaction that could be followed by
11B NMR (Figure 2). A new resonance observed at
46.4 ppm corresponds to BCl3, formed by a ligand redis-
tribution reaction between CatBCl and AlCl3. Prolonged
heating led to a further increase in BCl3 along with the
appearance of two other minor products, one attributed to
isomers of (C6H4CH3)BCl2 (55.0 ppm, generated by electro-
philic borylation of the solvent toluene on activation of BCl3
by AlCl3)2,41 and the other consistent with an aryl boronic
€
(22) Ranatunga, T. D.; Kennady, J. M.; Kenttamaa, H. I. J. Am.
Chem. Soc. 1997, 119, 5200.
(23) Ranatunga, T. D.; Kenttamaa, H. I. J. Am. Chem. Soc. 1992,
€
114, 8600.
(24) Allaoud, S.; Frange, B. Inorg. Chem. 1985, 24, 2520.
(25) Zhou, Q. J.; Worm, K.; Dolle, R. E. J. Org. Chem. 2004, 69, 5147.
(26) Davis, F. A.; Dewar, M. J. S. J. Am. Chem. Soc. 1968, 90, 3511.
(27) Laaziri, H.; Bromm, L. O.; Lhermitte, F.; Gschwind, R. M.;
Knochel, P. J. Am. Chem. Soc. 1999, 121, 6940.
(28) Arcus, V. L.; Main, L.; Nicholson, B. K. J. Organomet. Chem.
1993, 460, 139.
(29) Dewar, M. J. S.; Kubba, V. P.; Pettit, R. J. Chem. Soc. 1958,
3073.
(30) Lee, G. T.; Prasad, K.; Repic, O. Tetrahedron Lett. 2002, 43,
3255.
(36) See Supporting Information.
(37) Reed, C. A. Chem. Commun. 2005, 1669.
(38) Bujwid, Z. J.; Gerrard, W.; Lappert, M. F. Chem. Ind. 1959,
1091.
(31) Hall, D. Boronic Acids; Wiley-VCH: New York, 2005.
(32) Nicolau, K. C.; Bulger, P. C.; Sarlah, D. Angew. Chem., Int. Ed.
2005, 44, 4442.
(39) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;
Butterworth-Heinemann: Oxford, 1997.
(40) Coapes, R. B.; Souza, F. E. S.; Fox, M. A.; Batsanov, A. S.;
Goeta, A. E.; Yufit, D. S.; Leech, M. A.; Howard, J. A. K.; Scott, A. J.;
Clegg, W.; Marder, T. B. Dalton Trans. 2001, 1201.
(41) Haubold, W.; Herdtle, J.; Gollinger, W.; Einholz, W. J.
Organomet. Chem. 1986, 315, 1.
(33) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
€
(34) Narula, C. K.; Noth, H. Inorg. Chem. 1984, 23, 4147.
(35) Rissler, J.; Hartmann, M.; Marchand, C. M.; Grutzmacher, H.;
Frenking, G. Chem.;Eur. J. 2001, 7, 2834.

Article
Organometallics, Vol. 29, No. 1, 2010 243
supports the conclusions derived from NMR spectroscopy:
B/Al ligand redistribution and arene borylation. The absence
in the 11B NMR spectra of any significant boronic ester
products from EAS with a “[CatB][AlCl4]” species is attrib-
uted to the competing ligand redistribution reaction. In a
related attempt to achieve halide abstraction from CatBBr a
stronger neutral Lewis acid, BBr3,47 was added in stoichio-
metric quantities to a toluene solution of CatBBr. This
produced no arene borylation at both 25 and 100 °C after
prolonged periods, indicating that neutral MX3 species are
insufficiently Lewis acidic to abstract halide from CatBX.
This is due to the respective M-Cl/CatB-Cl bond strengths
combined with the lower stability of chelate restrained boron
cations compared to the linear borinium analogues.
Having observed the reaction of TMSOTf with CatBCl,
halide abstraction with triethylsilylium ([Et3Si]þ) salts of the
weakly coordinating anions [closo-1-H-CB11H5Br6] (termed
CbBr6) and [B(C6F5)4]- was investigated.48,49 A chelate
restrained boron cation partnered with these very weakly
coordinating anions can be expected to react with arene
solvents, thus driving the halide abstraction equilibrium to
completion.
[CbBr6]- Anion. [Et3Si][CbBr6], generated in situ from
[Ph3C][CbBr6] and Et3SiH,48 reacted rapidly with CatB-X
(X = Cl, Br) in benzene at 25 °C, with Et3Si-X observed as
the expected byproduct (by 29Si{1H} NMR). The CatB-
containing products were CatB-Ph and CatBH, with no
cationic boron species observed. CatB-Ph originated from
the intermolecular reaction of a highly electrophilic boron
species with the solvent benzene (Scheme 1), while CatBH
was formed by a boron electrophile abstracting a hydride
from Ph3CH, regenerating [Ph3C][CbBr6] (by 1H NMR),
consistent with the absence of reactivity between Ph3C-
[CbBr6] and CatBH at 25 °C. However at elevated tempera-
tures [Ph3C][CbBr6] and CatBH did react slowly in benzene
to produce CatB-Ph.36 Removal of the Ph3CH byproduct (to
eliminate the low-temperature side reaction between
[CatB][CbBr6] and Ph3CH) by isolation of [Et3Si][CbBr6]
was not possible in our hands due to the extreme sensitivity
of cationic silylium species.50
Numerous unsuccessful attempts were made to detect any
highly electrophilic boron intermediates. This included the
use of the less nucleophilic arenes, o-C6H4Cl2 and C6H5F,
that at 25 °C produced the respective arene borylation
products (C6H3Cl2(BCat) and C6H4F(BCat)) and CatBH
as the only new boron-containing species. Likewise the low-
temperature combination of reagents resulted in no detect-
able intermediates. At -10 °C in d8-toluene [Et3Si][CbBr6]
and CatBCl reacted to give tolyl-BCat; below this tempera-
ture CatBCl and [CbBr6]- were the only species observed by
11B NMR spectroscopy. Closely comparable results were
obtained when the low-temperature reaction was performed
in the more polar solvent C6H5F (with 10% C6D6) to improve
the solubility of ionic species at lower temperatures.36
Figure 2. (Top) 11B NMR spectra of a 1:1 combination of AlCl3
and CatBCl in toluene. Resonance at 28.1 ppm corresponds to
CatBCl. (Bottom) Molecular structure of 2 with thermal ellip-
˚
soids at 50% probability. Selected bond lengths (A): B1-C7 =
1.526(4), B1-O1 = 1.348(3), B1-O2 = 1.434(3), Al1-O2 =
1.866(2), Al1-O3 = 1.805(2), Al2-O3 = 1.913(2), Al2-O4 =
1.828(2), and Al3-O4 = 1.830(2).
ester (32.7 ppm). Heating beyond 48 h did not lead to the
complete consumption of all CatBCl, signifying a nonstoi-
chiometric reaction between CatBCl and AlCl3. This was
confirmed by the use of excess AlCl3 in refluxing toluene,
completely converting CatBCl to predominantly BCl3 and
(tolyl)BCl2. The 27Al NMR spectra of this reaction consisted
of two broad resonances, at 104 and 60 ppm, in the regions
expected for four- and five-coordinate aluminum, res-
pectively.42-44 Slow cooling of a toluene solution of CatBCl
and AlCl3 (1:3 equiv, heated at 100 °C for two days) led to the
deposition of a small quantity of yellow needles. X-ray
diffraction analysis revealed a product derived from arene
borylation and extensive ligand redistribution, comprised of
a borylated catechol coordinated to two AlCl2 and one AlCl3
(compound 2, Figure 2 bottom). The two Al-Cl distances of
the asymmetrically bridged Al-Cl-Al moiety (Al2-Cl5 =
˚
˚
2.681(1) A, Al3-Cl5 = 2.145(1) A) are elongated compared
to the terminal Al-Cl bonds,44-46 with the distinct bridging
Al-Cl bond lengths indicative of stronger Lewis acidity of
the lower coordinate aluminum center. The structure of 2
The mild conditions required for intermolecular electro-
philic borylation implied the presence of a highly reactive
[CatB][CbBr6] species, with previous arene borylations
(42) Atwood, D. A. Coord. Chem. Rev. 1998, 176, 407.
(43) Moravec, Z.; Sluka, R.; Necas, M.; Jancik, V.; Pinkas, J. Inorg.
Chem. 2009, 48, 8106.
(44) Sharma, V.; Simard, M.; Wuest, J. D. Inorg. Chem. 1990, 30, 579.
(45) Cowley, A. H.; Cushner, M. C.; Davis, R. E.; Riley, P. E. Inorg.
Chem. 1981, 20, 1179.
(47) Beckett, M. A.; Brassington, D. S.; Coles, S. J.; Hursthouse,
M. B. Inorg. Chem. Commun. 2000, 3, 530.
(48) Reed, C. A.; Xie, Z.; Bau, R.; Benesi, A. Science 1993, 262, 402.
(49) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science
1993, 260, 1917.
(46) Belanger-Gariepy, F.; Hoogsteen, K.; Sharma, V.; Wuest, J. D.
(50) Scott, V. J.; Celenligil-Cetin, R.; Ozerov, O. V. J. Am. Chem. Soc.
Inorg. Chem. 1991, 30, 4140.
2005, 127, 2852.

244 Organometallics, Vol. 29, No. 1, 2010
Scheme 1. Arene Borylation through Chelate Restrained Boron Cationsa
Del Grosso et al.
a For clarity the anion [CbBr6] is omitted. Throughout formal net charges correspond to the molecule as a whole with assignment of specific charge at
individual atoms avoided due to the complexity in locating the delocalized charge in such species.
Figure 3. (Inset) C-H insertion mechanism for the intramolecular arene borylation by a borenium cation.17 (Bottom) EAS and C-H
insertion mechanisms applied to intermolecular arene borylation proceeding through [CatB][CbBr6]. The subsequent cationic species
can be three- or four-coordinate at boron (by addition/loss of a weak nucleophile L).
requiring directing agents to precoordinate boron and/or
aggressive reaction conditions.2-4,17,24-30 The absence of
detectable [CatB]þ species, either anion coordinated or
ligated with a weak nucleophile from the reaction mixture,
also contrasts with the reactivity of closely related main
group cations. Examples of particular relevance are the
chelated borenium cation, compound B (Figure 3 inset),
and [Et2Al][CbBr6], which are both indefinitely stable in
arene solvents.17,51 Other highly electrophilic cationic spe-
cies, E[1-H-CB11Me5Br6] (E = Meþ, R3Siþ, Hþ), react with
arenes to give isolable arenium cations of the general formula
[C6R6E]þ (R = H or Me or a combination thereof).37,48,52-56
The transient nature of any [CatB][CbBr6] compound is
attributed to its highly reactive nature, which results in rapid
arene borylation. Arenium cations ([C6R6(BCat)]þ) required
for an EAS mechanism were also not detected. It is feasible
that the Lewis acidic nature of the three-coordinate boron
center in [C6R6(BCat)]þ destabilizes this cation, resulting in
rapid Hþ loss and rearomatization to generate C6R5-BCat.
However, an alternative mechanism can equally account for
the absence of [C6R6(BCat)]þ species, with arenium inter-
mediates not required for borylation reactions that proceed
by the insertion of a borenium electrophile into an aromatic
C-H bond (Figure 3 inset).17 This mechanism can be applied
to the intermolecular borylation by [CatB][CbBr6], where
highly electron deficient borenium cations are equally fea-
sible (C-H insertion mechanism, Figure 3). With no inter-
mediates experimentally observed both EAS and C-H
insertion mechanisms are viable, as are three-coordinate
[CatB(arene)]þ and four-coordinate [CatB(arene)L]þ transi-
tion states (L = a weak nucleophile).
An alternative route to cations that possess enhanced
electrophilicity at boron is through coordination of a catio-
nic Lewis acid (e.g., Et3Siþ) to an oxygen lone pair in CatBX.
This is feasible, particularly in weakly nucleophilic arene
environments, and is consistent with the reactivity observed
between CatBCl and AlCl3. However, boron electrophiles
generated by this route are unlikely to be the active borylat-
ing species due to the following observations: (i) the ability of
the significantly weaker Lewis acid [Ph3C][CbBr6] to enable
arene borylation with CatBH (the weakly basic oxygen
donor in CatBH does not coordinate to the [Ph3C]þ cation,
precluding enhancement of electrophilicity at boron); (ii) the
lack of arene borylation products when equimolar amounts
of [Et3Si][CbBr6] and CatBH were combined at 25 and 100 °C
in ortho-dichlorobenzene. Coordination of Et3Siþ to oxygen
in CatBH would produce a stronger boron electrophile than
the respective Et3Siþ adduct of CatBX, due to the stabilizing
effect of halide π-donation to boron in the latter. By this
rationale arene borylation via a C-H insertion mechanism
should also proceed using CatBH, with concomitant loss
of H2, analogous to the production of B. The divergent
(51) Kim, K.-C.; Reed, C. A.; Long, G. S.; Sen, A. J. Am. Chem. Soc.
2002, 124, 7662.
(52) Stasko, D.; Reed, C. A. J. Am. Chem. Soc. 2001, 124, 1148.
€
(53) Kato, T.; Stoyanov, E.; Geier, J.; Grutzmacher, H.; Reed, C. A.
J. Am. Chem. Soc. 2004, 126, 12451.
(54) Reed, C. A. Acc. Chem. Res. 1998, 31, 325.
(55) Reed, C. A.; Kim, K.-C.; Stoyanov, E. S.; Stasko, D.; Tham,
F. S.; Mueller, L. J.; Boyd, P. D. W. J. Am. Chem. Soc. 2003, 125, 1796.
(56) Hubig, S. M.; Kochi, J. K. J. Org. Chem. 2000, 65, 6807.

Article
Organometallics, Vol. 29, No. 1, 2010 245
reactivity actually seen for CatBX and CatBH is more con-
sistent with the inability of [Et3Si][CbBr6] to abstract hydride
from CatBH (due to the relative Si-H and B-H bond
strengths),57 preventing formation of [CatB][CbBr6]. The only
reaction observed between [Et3Si][CbBr6] and CatBH was
slow ligand exchange (at 100 °C) to give CatB-Et as the new
boron-containing product.36
Table 1. 31P{1H} Chemical Shifts of the Et3PdO Adducts of
a Range of Lewis Acids
Et3PO adduct
δ 31P{1H} (ppm)a
Lewis acid
B(C6F5)3
AlCl3
F2B(O3SCF3)
CatB(O3SCF3)
BBr3
Et3Si[CbBr6]
Me3Si(O3SCF3)
CatB[CbBr6]
76.6b
80.3b
84 6c
85.4d
903b
[B(C6F5)4]- Anion. It was desirable, particularly from a
cost perspective, to determine if arene borylation was possi-
ble with the perfluorinated tetraphenyl borate anion, [B-
(C6F5)4]-. Combination of CatBCl and [Et3Si][B(C6F5)4]49
in benzene at 25 °C produced PhBCat and [B(C6F5)4]- as the
major boron-containing products. No low-temperature in-
termediates were observed, with NMR spectra closely com-
parable to the [CbBr6]- system obtained. The only notable
difference between the [CbBr6]- and [B(C6F5)4]- borylation
reactions was the observation of anion decomposition in the
latter. In addition to PhBCat and [B(C6F5)4]- two minor
boron-containing products were detected and identified as
B(C6F5)3 and CatB-C6F5 (by 11B NMR and independent
synthesis).36 CatB-C6F5 was generated from the attack on a
B-C6F5 bond of [B(C6F5)4]- by a transient [CatB][CbBr6]
electrophile analogous to that previously observed for re-
lated [R2Al]þ species, the borenium cation B (Figure 3), and
protonated arenes.17,58,59
91.2d
92.8b
106.9d
a NMR spectra recorded in C6D6. b Ref 47. c Ref 65. d This work.
-Ph3CH
CatBH þ Ph3C½BðC6F5Þ4ꢀ
CatBðC F Þ
f
6
5
Δ
þ BðC6F5Þ3 ð2Þ
The anion decomposition products, CatB-C6F5 and
B(C6F5)3, were produced as the major products by the stoi-
chiometric hydride abstraction from CatBH with Ph3C[B-
Figure 4. (Left) Molecular structure of 3, thermal ellipsoids at
˚
P
50% probability, P1-O1 = 1.595(4) A, (O-B-O) = 359.6°.
The closest B Br contact (dashed line) is extremely long at
3 3 3
1
˚
3.41 A. (Right) Phosphonium and borenium resonance struc-
tures.
(C6F5)4] at 80 °C in benzene (eq 2, Ph3CH observed by H
NMR). There was no anion decomposition in the corre-
sponding reaction between CatBH and [Ph3C][CbBr6], con-
sistent with the extreme resistance to electrophilic attack that
halogenated carborane anions are well noted for.60,61 The
observation of [B(C6F5)4]- decomposition, while severely
limiting the usefulness of the anion in this system, does
provide further indirect evidence for the exceptional Lewis
acidity of [CatB]þ species in a weakly nucleophilic environ-
ment.
106.9 ppm is shifted considerably downfield compared to the
Et3PdO adducts of other strong Lewis acids (Table 1),
suggesting exceptional Lewis acidity for the {CatB}þ moiety.
In contrast the 31P{1H} chemical shift of the triflate ana-
logue, [CatB(OPEt3)][OTf], at 85.4 ppm (comparable to
BF2(OPEt3)(OTf)) is indicative of a significant B-OTf
interaction.65 This was corroborated by the 11B chemical
shift at 7.9 ppm, in the region expected for four-coordinate
boron centers and significantly upfield from the 11B reso-
nance of the [CbBr6] congener (21.9 ppm). The lack of arene
borylation with CatB(OTf) is a result of the lower Lewis
acidity at boron on strong anion coordination. For direct
comparison of the Lewis acidity of {CatB}þ and {R3Si}þ
moieties, [Et3Si(OPEt3)][CbBr6] was synthesized and found
to have a 31P{1H} chemical shift of 91.2 ppm. The similarity
to the 31P{1H} chemical shift of TMS(OPEt3)(OTf) (92.8
ppm) precludes any hypervalence by anion coordination.65
Thus [CatB]þ is more Lewis acidic than Et3Siþ toward
Et3PdO, attributable to the additional formally empty
orbital present in [CatB]þ.
Lewis Acidity. The Lewis acidity of [CatB]þ was directly
assessed using two complementary NMR spectroscopy
based methods; the first is based on the 31P{1H} chemical
shift of OdPEt3 on adduct formation,47,62 and the second on
the change in the 1H NMR chemical shift of the H3 proton of
crotonaldehyde on binding of a Lewis acid to the carbonyl
oxygen.63,64
[CatB(OdPEt3)][CbBr6], compound 3, was the major
product on reaction of Ag[CbBr6] with CatBCl in the pre-
sence of 0.9 equiv of OdPEt3 and was purified by recrystal-
lization from DCM/hexane. The 31P{1H} resonance of 3 at
The solid-state structure of 3 was determined to confirm
the absence of cation-anion interactions. The structure
reveals a planar three-coordinate boron center with the nea-
(57) Rablen, P. R.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 4648.
(58) Bochmann, M.; Sarsfield, M. J. Organometallics 1998, 17, 5908.
(59) Reed, C. A.; Fackler, N. L. P.; Kim, K.-C.; Stasko, D.; Evans,
D. R. J. Am. Chem. Soc. 1999, 121, 6314.
˚
rest anion boron interaction at 3.41 A (Figure 4), con-
3 3 3
(60) Reed, C. A. Acc. Chem. Res. 1998, 31, 133.
(61) Douvris, C.; Ozerov, O. V. Science 2008, 321, 1188.
(62) Gutmann, V. Coord. Chem. Rev. 1976, 18, 225.
(63) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J. Chem. 1982,
60, 801.
siderably longer than compounds possessing significant
(65) Myers, E. L.; Butts, C. P.; Aggarwal, V. K. Chem. Commun.
2006, 4434.
(64) Lazlo, P.; Teston, M. J. Am. Chem. Soc. 1990, 112, 8750.

246 Organometallics, Vol. 29, No. 1, 2010
Del Grosso et al.
Table 2. Magnitude of Downfield Shift of the H3 Proton of
Crotonaldehyde on Binding to a Lewis Acid (in CD2Cl2 unless
otherwise stated)
Δδ of the H3 proton
of crotonaldehyde
Lewis acid
Me3Si(O3SCF3)
B(C6F5)3
AICI3
CatB[CbBr6]
BF2(O3SCF3)
BBr3
0.7d
1.05e
1.23a
1.28c
1.46b
1.49a
Figure 5. Superelectrophile-catalyzed production of aryl boro-
nic esters from CatBH.
process involving undesirable halogenated aromatics and
stoichiometric organolithium or organomagnesium re-
agents.31,70 Recent advances have improved the synthesis
of ArB(OR)2, particularly with the development of direct
arene borylation using iridium catalysts.71-75 A catalytic (in
Lewis acid) electrophilic borylation route to ArB(OR)2 from
Ar-H is attractive, with the potential to provide complemen-
tary selectivity patterns to iridium catalysts. Stoichiometric
(in cationic Lewis acid) arene borylation produces a strongly
a Ref 47. b Ref 65. c This work. d Ref 65, the lower than expected value
was attributed to the presence of a hypervalent five-coordinate silicon
center bonded to OTf. e Ref 66. The 1H NMR of [CatB(crotonal-
dehyde)][CbBr6] was recorded in C6D6.
anion cation interactions (e.g., [CH3CHCH3][1-H-CB11-
Me5Br6], anion, and carbocation Cþ---Br distance =
3 3 3
53
2.104(3) A). The combination of planarity at boron with
˚
the long anion cation distance indicates a very weak
3 3 3
Bronsted acidic byproduct, and these have been previously
e
interaction at most. The NMR and structural data combined
are more consistent with the phosphonium cation resonance
structure dominating in 3.36,67 The P1-O1 distance (1.595(4)
demonstrated to react with hydridic B-H bonds in neutral
boranes, liberating H2 and generating a formally cationic boron
electrophile.76,77 A combination of these two steps using CatBH
and a cationic initiator will enable catalytic electrophilic arene
borylation (Figure 5). The anion [CbBr6]- is essential for this
cycle, having a nucleophilicity comparable to toluene37,60,61,78
˚
A) is elongated compared to free OdPEt3 and to the Lewis
˚
acid/base adduct Et3PdOfB(C6F5)3 (P-O = 1.4973(17) A,
47
˚
B-O = 1.533(3) A), whileþphosphonium P-O distances
t
(e.g., [Ph2(Me)P-O-CH2 Bu] , P-O = 1.57 A)68 are closely
˚
and a conjugate Bronsted superacidity,37,55 essential for pro-
e
comparable to the P1-O1 distance observed in 3. The short
bond distance for O1-B1 (1.380(7) A) is also consistent with a
phosphonium cation formalism for 3 and comparable to the
tonation of the strong Bδþ-Hδ- bond in catecholborane. This
cycle requires only catalytic quantities of the cationic Lewis acid,
in sharp contrast to conventional EAS, which requires stoichio-
metric or excess Lewis acid reagents.1,3,4,79 This distinct behavior
is enabled by the inherent polarity of the Bδþ-Hδ- bond, which
reacts with superacidic “Hþ” species, in an entropically driven
process irreversibly producing H2 and regenerating the active
boron electrophile.
˚
˚
two B-Ocatecholato distances (1.374(2) and 1.372(2) A).
Initial attempts to probe the Lewis acidity of [CatB]þ by
formation of a crotonaldehyde adduct in CH2Cl2 were
unsuccessful, with numerous intractable products formed.
This was consistent with a cationic electrophile reacting with
chlorinated solvent18,69 and distinct to neutral Lewis acid-
crotonaldehyde complexes that are stable at 25 °C in
CH2Cl2. [CatB(crotonaldehyde)][CbBr6] was synthesized in
arene solvents and had an 11B NMR resonance at 23.8 ppm,
Twenty equivalents of CatBH (with respect to starting
[Ph3C][CbBr6]) was added to the reaction mixture resulting
from the stoichiometric combination of [Et3Si][CbBr6] and
CatBX in benzene (Scheme 1). No reaction was observed at
25 °C, but heating to reflux resulted in the complete conver-
sion of all CatBH to CatB-Ph through the catalytic (in
superacid) intermolecular electrophilic borylation. The ob-
1
consistent with a three-coordinate boron species. The H
NMR spectrum (in C6D6) confirmed the deshielding of the
H3 resonance, which is shifted 1.28 ppm downfield relative
to free crotonaldehyde (in C6D6 at 25 °C). This shift is less
than reported for the Br3B-crotonaldehyde adduct, albeit
recorded in CD2Cl2 (Table 2). The discrepancy between the
two Lewis acidity scales can be attributed to a greater
hard-soft mismatch between the hard Lewis acidic boron
center in [CatB]þ and the predominantly covalent double-
bond character of the pπ-pπ double bond of the CdO group
in crotonaldehyde. The reversal of relative Lewis acidities in
the Et3PdO adducts is due to the more ionic OdP pπ-dπ
bond, as noted previously66 and consistent with calculations
on borenium cations.15
1
servation of H2 (and HD when performed in C6D6, by H
NMR) and the absence of catalysis when the hindered base
2,6-di-tert-butylpyridine is added to sequester Hþ is consis-
tent with the protonation of CatBH by a Bronsted acidic
e
species during the catalytic cycle.36 Catalytic electro-
philic borylation of alkyl-substituted arenes with CatBH
(70) Constable, D. J. C.; Dunn, P. J.; Hayler, J. D.; Humphrey, G. R.;
Leazer, J. L.; Linderman, R. J.; Lorenz, K.; Manley, J.; Pearlman, B. A.;
Wells, A.; Zaks, A.; Zhang, T. Y. Green Chem. 2007, 9, 411.
(71) Mkhalid, I. A. I.; Coventry, D. N.; Albesa-Jove, D.; Batsanov,
A. S.; Howard, J. A. K.; Perutz, R. N.; Marder, T. B. Angew. Chem., Int.
Ed. 2005, 45, 489.
(72) Murphy, J. M.; Lao, X.; Hartwig, J. F. J. Am. Chem. Soc. 2007,
129, 15434.
(73) Cho, J.-Y.; Tse, M. K.; Holmes, D.; Maleczka, R. E.; Smith,
Catalysis. Aryl boronic esters (ArB(OR)2) are important
synthetic intermediates widely utilized in metal-catalyzed
C-C bond-forming reactions.31-33 Their production from
arenes classically proceeds in an atom-inefficient, multistep
M. R., III Science 2002, 295, 305.
(74) Ishiyama, T.; Miyaura, N. J. Organomet. Chem. 2003, 680, 3.
(75) Miyaura, N. Bull. Chem. Soc. Jpn. 2008, 81, 1535.
(76) Olah, G. A.; Aniszfeld, R.; Prakash, G. K. S.; Williams, R. E.;
Lammertsma, K.; Guner, O. F. J. Am. Chem. Soc. 1988, 110, 7885.
(77) Shapland, P.; Vedejs, E. J. Org. Chem. 2004, 69, 4094.
(78) Moxham, G. L.; Douglas, T. M.; Brayshaw, S. K.; Kociok-
Kohn, G.; Lowe, J. P.; Weller, A. S. Dalton Trans. 2006, 5492.
(79) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; Wiley
Interscience: NJ, 2003.
(66) Britovsek, G. J. P.; Ugolotti, J.; White, A. J. P. Organometallics
2005, 24, 1685.
(67) Dureen, M. A.; Lough, A.; Gilbert, T. M.; Stephan, D. W. Chem.
Commun. 2008, 4303.
(68) Henrick, K.; Hudson, H. R.; Kow, A. Chem. Commun. 1980,
226.
(69) Gunnoe, T. B.; Caldarelli, J. L.; White, P. S.; Templeton, J. L.
Angew. Chem., Int. Ed. 1998, 37, 2093.

Article
Organometallics, Vol. 29, No. 1, 2010 247
Table 3. Superacid-Catalyzed Aromatic Borylationj
entry
substrate
C6H6
C6H5Me
m-C6H4Me2
m-C6H4Me2
p-C6H4Me2
C6H5Et
catalyst
temp. (°C)
time (h)
product
(C6H5)BCat
yield (%)a
1
2
3
4
5
6
7
8
A
A
Ac
A
A
A
A
A
A
A
80
100
100
140
140
100
140
25
100
100
80
40
80
15
15
45
10
7
96
10
48
5
99
(C6H4(Me))BCat
(C6H3(Me)2)BCat
(C6H3(Me)2)BCat
(C6H3(Me)2)BCat
(C6H4(Et))Bcat
1,2-Cl2,-4-Bcat-C6H3
(C6H5)BCat
99(93)b
97d
92d
57d, e
35d, f
99
o-C6H4Cl2
g
C6H5SiMe3g
C6H5SiMe3
C6H5Meh
C6H6
C6H6
C6H6
99
99
80
<5
<1
<1
9
(C6H5)BCat
10
11
12
41
5
(C6H4(Me))BCat
(C6H5)BCat
(C6H5)BCat
Et3Si[B(C6F5)4]i
TMSOTfi
AlCl3
18
18
18
i
(C6H5)BCat
a Yield of borylated aromatic based on initial CatBH by NMR assay combined with GC analysis. b Number in parentheses is isolated analytically pure
material, ortho:para ratio = 1.2:1. c 10 mol % catalyst loading. d For exact product isomer ratios see Supporting Information. e Other major borane
products are isomers of (C6H4Me)BCat at 33%. f Other major products are isomers of (C6H3Et2)BCat, 37%, and (C6H2Et3)BCat, 17%. g CatBBr used in
place of CatBH. h Run with 20 equiv of CatBBr and PhSiMe3 added, low yield due to 20% PhBCat produced; o:p ratio of tolylBCat = 1.1:1. i 5 mol %
loading of a 1:1 ratio of catalyst and CatBBr. j Unless otherwise stated the arene reagent is used as the solvent throughout. [Cat] A = Ph3C[CbBr6]/
Et3SiH/CatBBr at 5 mol % loading with respect to CatBH.
proceeded efficiently with the respective boronic esters
isolated (as isomeric mixtures) by extraction with hexane
(Table 3). All catecholborane was quantitatively converted
to the respective ArBCat species, in a highly atom-efficient
process, with H2 the only byproduct. Regiochemistry at
lower temperatures is dominated by arene electronic effects;
borylation of toluene gives ortho and para products only,
with no meta or benzylic activation observed (entry 2),
consistent with an electrophilic mechanism and precluding
radical reactivity.80 Superacid-catalyzed borylation there-
fore provides products with selectivities complementary to
those produced by transition metal catalysts, which operate
under steric control (e.g., meta and para substitution of
toluene).71-73,81 Increasing the reaction temperature led to
shorter reaction times (entries 4 and 5); however extensive
isomerization toward the thermodynamic products re-
sulted.79 Arenes more amenable to 1,2 carbocation shifts
(e.g., Etþ versus Meþ, entry 6) are more susceptible to
pervasive alkyl isomerization as expected.79 Deactivated
arenes were also borylated efficiently (entry 7), in contrast
to the equimolar BCl3/AlCl3 system, which does not borylate
haloarenes,3,4 demonstrating the greater relative electrophi-
licity of the [CatB][CbBr6] species. Both CatBOTf and a 1:1
CatBBr/AlCl3 combination gave no borylation (entries 12
and 13), while the [B(C6F5)4]- anion resulted in extremely
poor catalytic activity (entry 11), due to anion instability.36
This catalytic methodology is limited due to the raised
temperatures required combined with the high reactivity of
the chelate restrained boron cations. While fluorinated
arenes (C6H5F) were borylated at room temperature using
stoichiometric [Et3Si][CbBr6]/CatBX, catalytic borylation
of C6H5F at 80 °C was unsuccessful. The raised tempera-
tures led to sp2 C-F activation and anion decomposition
presumably by the strongly Lewis acidic and fluorophilic
boron species present in solution.36 Strongly coordinat-
ing functionalities and alkoxy-substituted arenes are also
incompatible; for example anisole reacts to give CatB-
OPh as the major boron-containing product. Attempts to
correlate relative rates of reaction to arene nucleophilicity
were complicated by the continued presence of a variable
quantity of insoluble material whose major component
1
was the poorly arene soluble [Ph3C][CbBr6] (by H NMR
on the isolated solid in CD2Cl2). Use of the more polar arene
o-C6H4Cl2 produced a homogeneous catalytic system, but
on periodic monitoring throughout the catalysis the only
boron-containing species observed by 11B NMR spectrosco-
py were CatBH, the borylated product CatB(C6H3Cl2), and
[CbBr6]-.
The absence of any catalytic turnover at 25 °C implies that
protonation of CatBH by the Bronsted acidic byproduct was
e
the rate-limiting step, requiring elevated temperatures. This
was indirectly confirmed by using ipso-directing82 PhSiMe3
as the arene reagent and CatBBr in place of CatBH. Catalytic
borylation proceeded at 25 °C (entry 8) with a Me3Siþ
leaving group (ligated by a weak nucleophile or anion
coordinated), completing the cycle by reaction with CatBBr.
In control reactions CatBBr did not react with PhSiMe3, in
contrast to the more Lewis acidic BBr3.83,84 PhSiMe3 can be
used as an additive to accelerate arene borylation; cata-
lysis with equimolar CatBBr and PhSiMe3 (at 5 mol %
[Et3Si][CbBr6] loading) in toluene resulted in slow, but
catalytic, arene borylation at 25 °C and relatively rapid
borylation at 100 °C (entry 10). The Bronsted acidic bypro-
e
duct from electrophilic borylation protodesilates PhSiMe3,85
presumably generating the reactive silylium cation [Me3Si]-
[CbBr6], which closes the cycle by reaction with CatBBr. This
circumvented the rate-limiting protonation of CatBH, but
led to mixed arene products due to the competing boryla-
tion of PhSiMe3 and toluene by the reactive [CatB][CbBr6]
species.
(82) Lambert, J. B.; Zhao, Y.; Emblidge, R. W.; Salvador, L. A.; Liu,
X.; So, J.-H.; Chelius, E. C. Acc. Chem. Res. 1999, 32, 183.
(83) Kaufmann, D. Chem. Ber. 1987, 120, 853.
(80) Fahr, A.; Stein, S. E. J. Phys. Chem. 1988, 92, 4951.
(81) Miyaura, N. Bull. Chem. Soc. Jpn. 2008, 81, 1535.
(84) Kaufmann, D. Chem. Ber. 1987, 120, 901.
(85) Szele, I. Helv. Chim. Acta 1981, 64, 2733.

248 Organometallics, Vol. 29, No. 1, 2010
Del Grosso et al.
Summary
Chelate restrained boron cations partnered with robust
and weakly coordinating anions are among the strongest
Lewis acids produced in the condensed phase. The potent
electrophilicity of transient “[CatB][CbBr6]” species enabled
the intermolecular borylation of arenes, which proceeded
rapidly even at low temperatures. Arene borylation was
made catalytic, with respect to the cationic initiator, by using
Figure 6. H/Br ligand redistribution catalyzed by Ph3C[CbBr6].
the strongly Bronsted acidic byproduct to protonate the
e
hydridic CatBH, producing H2 and the active [CatB]-
[CbBr6] species. This new catalytic route to synthetically
important aryl boronic esters proceeds in one step, from the
arene and catecholborane, atom efficiently with H2 as the
only byproduct. The low-temperature stoichiometric bory-
lation of arene C-H bonds highlights the potential of boron
cations to provide a complementary method to transition
metal catalysis for the synthesis of aryl boronic esters. A key
challenge to be overcome in the future will be the catalytic
synthesis of boron cations of sufficient electrophilicity to
enable low-temperature arene borylation yet tolerate a wider
substrate scope; this will require boron species significantly
less reactive than the [CatB][CbBr6] complexes studied here.
Figure 7. Electrophile-initiated ring-opening of PinBH, con-
trasted with electrophile-resistant CatBH. (Inset) Molecular
˚
structure of the cationic portion of 4: C3-S = 1.858(15) A,
P
˚
˚
C9-S=1.829(14) A, C10-S=1.752(14) A, (C-S-C)=312°.
Catalysis performed in the presence of excess Et3SiH
resulted in no arene borylation. Instead rapid H/X ligand
exchange took place at 25 °C, driven by formation of the
stronger Si-Br and B-H bonds (Figure 6). Under analo-
gous conditions no reaction is observed between CatB-X and
Et3SiH, although stronger boron Lewis acids (e.g., B(C6F5)3,
BCl3) undergo similar ligand redistributions without addi-
tional catalyst.86-88
Experimental Details
General Considerations. All manipulations were performed
using standard Schlenk techniques or in an argon-filled MBraun
glovebox (O2 levels below 0.5 ppm). Solvents were distilled from
Na/benzophenone, CaH2, or P2O5 and degassed prior to use.
Catecholborane was distilled under reduced pressure. Triethyl-
silane was dried with CaH2 and distilled under reduced pressure.
Et3PO was sublimed before use. [Ph3C][closo-1-H-CB11H5Br6]
and Ag[closo-1-H-CB11H5Br6] were prepared according to lit-
erature procedures.90,91 All other materials were purchased
from commercial vendors and used as received.
Catalytic electrophilic arene borylation with pinacolbor-
ane (PinBH pin = pinacol {Me2(O-)C}2), a borane widely
used in iridium-catalyzed arene borylation, was unsuccess-
ful. No Ar-BPin species were formed; instead oligomeric
materials were produced (by mass spectrometry) derived
from the cation-initiated ring-opening of the five-membered
ring of pinacolborane,36 analogous to the Lewis acid-in-
itiated ring-opening of THF.89 This mechanism was corro-
CatB(OTf). A Schlenk flask was charged with CatBCl (150
mg), 1 equiv of AgOTf (249 mg), and 10 mL of C6H5F. The
resulting suspension was stirred in the dark for 16 h and filtered,
and the volatiles were removed in vacuo to leave a white solid.
Redissolving in minimum hexane and cooling to -20 °C for 2
borated by the use of PinBH SMe2, which upon reaction
1
3
days yielded pure material. H NMR (C6D6, 25 °C): 6.87 (m,
with [Ph3C][CbBr6] led, albeit in low yield, to the formation
of the ring-opened species [Me2(H)CCMe2(SMe2)][CbBr6],
4. The carbocation produced on pinacolborane ring-opening
has been trapped by coordination of Me2S, producing a
sulfonium cation (Figure 7).36 The low isolated yield com-
bined with intractable byproduct precludes an in-depth
mechanistic discussion of the formation of compound 4.
The divergent reactivity for catechol- and pinacol-boranes
originates from the relative stability of the respective carbo-
cations produced on ring-opening by C-O cleavage. Pina-
colborane ring-opening would lead to a relatively stable
tertiary carbocation; in contrast, ring-opening of catechol-
borane would lead to a Phþ cation (Figure 7). It is the
formation of the high-energy Phþ intermediate that pre-
cludes ring-opening reactions when using catecholborane.
Catalytic arene borylation via chelate restrained boron ca-
tions therefore requires both anion and cation components
stable to extremely Lewis acidic species.
2H), 6.72 (m, 2H). 11B NMR (C6D6, 25 °C): 21.6 (br s). 19F{1H}
NMR (C6D6, 25 °C): -77.2.
CatB(C6F5). A 50 mg amount of B(C6F5)3 was loaded into a
Schlenk flask fitted with a J. Youngs tap, and 5 mL of toluene
and 2 equiv of HBcat (1.2 ꢁ 10-4mol, 20 μL) were added. The
Schlenk flask was then sealed and the solution refluxed for 5 h, in
which time all B(C6F5)3 and HBCat had been consumed. Drying
in vacuo produced CatB-C6F5 analytically pure as a white solid.
Isolated yield (based on CatBH) = 75% (26 mg, 9 ꢁ 10-5 mol).
1H NMR (C6D6, 25 °C): 7.04 (m, 2H), 6.80 (m, 2H). 11B NMR
(C6D6, 25 °C): 29.5 (br s, pwhh = 205 Hz). 19F{1H} NMR
(C6D6, 25 °C): -128.3 (m), -147.3 (m), -161.5 (m). 13C{1H}
1
NMR (C6D6, 25 °C): 150.7 (br d JC-F = 256 Hz), 148.3 (s),
144.3 (br d, 1JC-F = 265 Hz), 137.9 (br d, 1JC-F 257 Hz), 124.1
(s), 113.5 (s) (ipso carbon bound to B is not observed). Anal.
Calcd for C12H4O2B1F15: C = 50.35, H = 1.41. Found: C =
48.81, H = 1.25.
[CatB(OdPEt3)][CbBr6]. In a J. Youngs tube, 2-bromo-1,3,2-
benzodioxaborole (8 mg, 40 μmol) was added to a solution of
triethylphosphine oxide in CD2Cl2 (approximately 0.6 mL), and
the solution was shaken. Ag[closo-1-H-CB11H5Br6]- (30 mg,
(86) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1996, 26, 345.
(87) Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000,
65, 3090.
(88) Neilson, R. H. Inorg. Chem. 1980, 19, 755.
(89) Welch, G. C.; Masuda, J. D.; Stephan, D. W. Inorg. Chem. 2006,
45, 478.
(90) Liston, D. J.; Lee, Y. J.; Scheidt, W. R.; Reed, C. A. J. Am. Chem.
Soc. 1989, 111, 6643.
(91) Xie, Z.; Jelinek, T.; Bau, R.; Reed, C. A. J. Am. Chem. Soc. 1994,
116, 1907.

Article
Organometallics, Vol. 29, No. 1, 2010 249
41 μmol) was added as a solid, and this solution was shaken,
resulting in the rapid precipitation of an insoluble off-white
solid. Filtration and subsequent crystallization from the di-
chloromethane/hexane layer by slow diffusion yielded colorless
platelets. 1H NMR (CD2Cl2, 25 °C): 7.15-7.08 (m, 2H),
7.07-7.00 (m, 2H), 2.61 (dq, JHP =10.85 Hz, JHH =7.57 Hz,
6H), 1.26 (dt, JHP = 20.18 Hz, JHH= 7.57 Hz, 9H). 11B NMR
(CD2Cl2, 25 °C): 21.9 (br s), -1.8 (s, 1B), -9.9 (s, 5B), 20.3 (d,
tube was sealed and warmed while stirring, with aliquots removed
periodically for monitoring by 11B NMR. When the reaction was
complete (HBCat consumed) hexane was added, resulting in the
precipitation of an off-white solid. The solution was filtered and the
volatile components were removed in vacuo to leave a colorless
solid that was the pure boronic ester (albeit a mixture of isomers).
The reaction was monitored by 11B NMR, and the products were
identified and yields determined by GC and GC-MS.
J
BH= 162.0 Hz, 5B). 31P NMR (CD2Cl2): 106.89.
Catalytic Borylation of Arenes. In a Schlenk flask fitted with a
Acknowledgment. M.J.I. thanks the University of
Manchester and the Royal Society for the provision of a
University Research Fellowship.
J. Youngs tap, [Ph3C][closo-1-H-CB11H5Br6] (25 mg, 29 μmol)
was suspended/partially dissolved in arene solvent, Et3SiH (4.5 μL,
29 μmol) was added, and the mixture was stirred. 2-Bromo-1,3,2-
benzodioxaborole (6 mg, 29 μmol) was added to the suspension,
and the mixture was shaken for approximately 1 min, resulting in a
rapid color change from colorless to orange. Catecholborane
(20 equiv, 31 μL, 290 μmol) was then added, and the J. Youngs
Supporting Information Available: Full experimental infor-
mation, data, and crystallographic details are available free of
charge via the Internet at http://pubs.acs.org.