Promoting the Hydrosilylation of Benzaldehyde by Using a Dicationic Antimony-Based Lewis Acid: Evidence for the Double Electrophilic Activation of the Carbonyl Substrate

Article abstract of DOI:10.1002/chem.201600971

The concomitant activation of carbonyl substrates by two Lewis acids has been investigated by using [1,2-(Ph₂MeSb)₂C₆H₄]²⁺ ([1]²⁺), an antimony-based bidentate Lewis acid obtained by methyl

Full text of DOI:10.1002/chem.201600971

DOI: 10.1002/chem.201600971
Communication
&
Homogeneous Catalysis
Promoting the Hydrosilylation of Benzaldehyde by Using
a Dicationic Antimony-Based Lewis Acid: Evidence for the Double
Electrophilic Activation of the Carbonyl Substrate
[
a]
Masato Hirai, Junsang Cho, and FranÅois P. Gabba ¨ı *
Abstract: The concomitant activation of carbonyl sub-
strates by two Lewis acids has been investigated by using
2
+
2+
[
1,2-(Ph MeSb) C H ] ([1] ), an antimony-based biden-
2 2 6 4
tate Lewis acid obtained by methylation of the corre-
sponding distibine. Unlike the simple stibonium cation
+
2+
[
Ph MeSb] , dication [1] efficiently catalyzes the hydro-
3
silylation of benzaldehyde under mild conditions. The cat-
alytic activity of this dication is correlated to its ability to
doubly activate the carbonyl functionality of the organic
substrate. This view is supported by the isolation of [1-m2-
to activate strong element-fluorine bonds. As part of our con-
tribution to the chemistry of these new Lewis acids, we have
also synthesized bidentate distiboranes, such as E, and found
evidence of strong cooperativity between the two Lewis acidic
DMF][OTf] , an adduct, in which the DMF oxygen atom
2
bridges the two antimony centers.
[
8]
centers in the binding of fluoride anions. Encouraged by
these ongoing developments, we have now decided to test
whether bidentate antimony derivatives could also be used as
catalysts for the double electrophilic activation of organic car-
Electrophilic phosphonium cations are attracting an increasing
interest as Lewis acids for the complexation of small anions or
[
9]
bonyls, as illustrated in F. Herein, we present a series of re-
sults, which support this possibility.
[1]
for the activation of various organic reactions. The unique
Lewis acidic properties displayed by these saturated derivatives
arise from the ability of phosphorus to form hypervalent com-
pounds, a phenomenon facilitated by the introduction of elec-
[1a,b,2]
tron-withdrawing ligands.
Another methods that has been
explored as a means to achieve greater Lewis acidity is based
on the incorporation of two electrophilic moieties positioned
to cooperatively interact with an incoming nucleophile. This is,
for example, the case with the phosphonium borane derivative
+
[3]
A , which acts as a bidentate Lewis acid toward fluoride. The
Stephan group has recently investigated the Lewis acidic prop-
2
+
erties of the bis-fluorophosphonium species (B ) and found
that the proximity of the two Group 15 cations leads to en-
hanced catalytic activity in a range of reactions, including Frie-
del Crafts, hydrosilylation, and hydrodefluorination reac-
[
2e,4]
tions.
Organoantimony(V) derivatives are another class of Lewis
To initiate our studies, we decided to target a bifunctional
antimony Lewis acid with a binding pocket that is readily sub-
strate accessible. This consideration led us to target the ortho-
[5]
acidic derivatives drawing attention. Such derivatives, includ-
2
+[6]
+ [7]
ing C
and D , are emerging as air-stable Lewis acids,
2
+
which can be used to promote CÀC bond-forming reactions or
phenylene derivative [1] , which features two Lewis acidic an-
timony sites predisposed to interact with incoming nucleo-
philes. Distibonium salts [1][OTf] and [1][BF ] could be con-
[
a] M. Hirai, J. Cho, Prof. Dr. F. P. Gabba¨ı
Department of Chemistry, Texas A&M University
College Station, Texas 77843 (USA)
Fax: (+1)979-845-4719
E-mail: francois@tamu.edu
Homepage: http://www.chem.tamu.edu/rgroup/gabbai/
2
4 2
veniently generated by treatment of o-phenylene-bis(diphenyl-
[
10]
stibine)
with methyl trifluoromethylsulfonate (MeOTf) and
trimethyloxonium tetrafluoroborate ([Me O][BF ]), respectively
3
4
(
Scheme 1). Both [1][OTf] and [1][BF ] have been fully charac-
2 4 2
terized, and their compositions have been verified by elemen-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600971.
1
tal analyses. The H NMR spectrum of [1][OTf] and [1][BF ] in
2
4 2
Chem. Eur. J. 2016, 22, 6537 – 6541
6537
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Scheme 1. Synthesis of [1][OTf]
2
and [1][BF
4 2
] : i) 4 equiv MeOTf, toluene,
9
08C; ii) 2.05 equiv [Me O][BF ], 1,2-C
3
4
2
H
4
2
Cl /toluene 1:2, 908C.
CD Cl showed a diagnostic methyl resonance at d=2.18 and
2
2
2
.17 ppm, respectively, indicative of the formation of the meth-
ylstibonium moiety. Both [1][OTf] and [1][BF ] are very soluble
2
4 2
in CH Cl , THF, and CH CN and sparingly soluble in CHCl . Salt
2
2
3
3
[
1][BF ] is stable over prolonged periods of time and showed
4 2
no tendency towards decomposition by fluoride transfer from
À
the BF4 anion to the Lewis acidic antimony center. For com-
parison, we also prepared the monofunctional model com-
[11]
[12]
pound [Ph MeSb][OTf]
and [Ph MeSb][BF ],
which have
3
3
4
both been previously described.
With these compounds in hand, we first decided to quanti-
tatively examine their Lewis acidity by applying the Gutmann–
3
1
Beckett method, which relies on the P NMR chemical-shift
change observed upon coordination of Et PO to a Lewis
3
[
13]
acid.
In the case of the monofunctional Lewis acids
[
(
Ph MeSb][OTf] and [Ph MeSb][BF ], CH Cl solutions of Et PO
3
3
4
2
2
3
À2
7.5”10 m) containing an eightfold excess of the stibonium
31
salt feature a broad P NMR signal at d=57.0 ppm, downfield
shifted from the free Et PO (d=51.0 ppm) by 6.0 ppm. This
3
suggests that these two salts display similar Lewis acidity de-
spite the differing counteranions. When the same measure-
Figure 1. Solid-state structure of [1][OTf]
Thermal ellipsoids are drawn at the 50% probability level. The hydrogen
atoms are omitted for clarity, except for the water molecule in [1-OH ][BF ] .
2 4 2
Pertinent metric parameters can be found in the text.
2 2 4 2
(top) and [1-OH ][BF ] (bottom).
ment was repeated with the distibonium salts [1][OTf] and [1]
2
À2
[
BF ] by using CH Cl solutions of Et PO (7.5”10 m) contain-
4
2
2
2
3
3
1
ing a fourfold excess of the distibonium, the P NMR chemical
shift of the phosphine oxide was observed at d=61.4 and
6
2.2 ppm, respectively. These resonances are significantly more
downfield than those observed with the simple stibonium salts
Ph MeSb][OTf] and [Ph MeSb][BF ] indicating that the distibo-
cannot be overlooked and likely diminishes the Lewis acidity
of the antimony centers. Next, we moved to the crystallization
[
3
3
4
[
14]
nium salts [1][OTf] and [1][BF ] are more Lewis acidic and
of [1][BF4]2. In all attempts that involved a variety of solvents
or solvent mixtures, this salt only precipitated in a powder
form. In a few cases, we observed that precipitation of [1][BF4]2
was accompanied by formation of a small number of single
crystals. Analysis of these crystals indicate that they correspond
to the hydrate [1-OH ][BF ] , which probably results from the
2
4 2
more effectively polarize the P=O bond of Et PO. This suggests
3
that this greater Lewis acidity arises from the preorganization
of the two stibonium moieties and their ability to simultane-
ously interact with the oxygen atom of the phosphine oxide.
Last, we note a small influence of the counteranions for the bi-
2
4 2
À
functional derivatives, with the BF4 salt displaying a slightly
presence of adventitious water in the solvent (Figure 2). The
water molecule interacts with one of the antimony centers
(Sb2), as indicated by a Sb2ÀO1 distance of 2.938(3) Š. The
other antimony atom interacts with a tetrafluoroborate anion,
as indicated by the Sb1ÀF4 contact of 3.066(6) Š.
higher Lewis acidity than its triflate counterpart.
Although we failed to crystallize the above-mentioned Et PO
3
adducts, single crystals of the distibonium salt [1][OTf] were
2
obtained as colorless blocks by diffusion of Et O into a CH Cl
2
2
2
[
14]
(
Figure 1). In the crystal, one of the triflate anions is well sep-
Encouraged by these results, we next investigated the cata-
lytic properties of these stibonium compounds in the hydrosi-
arated from the distibonium complex. In contrast, the other tri-
flate anion bridges the two antimony centers resulting in Sb1À lylation of benzaldehyde by using triethylsilane in CDCl3
O1 and Sb2ÀO2 separations of 2.8541(12) and 2.9838(13) Š, re-
spectively. These SbÀO distances are shorter than the SbÀO
separation of 3.1518(16) Š found in the monofunctional analog
(Scheme 2). Although [Ph MeSb][OTf] and [Ph MeSb][BF ]
3 3 4
(3 mol%) did not promote the reaction at room temperature,
we observed some moderate catalytic activity in the case of
[
Ph MeSb][OTf], the structure of which was also determined
3
[1][OTf] (1.5 mol%), with 11 % conversion after 8 h. A surpris-
2
for the purpose of this study (see the Supporting Informa-
ingly contrasting behavior was observed in the case of [1][BF4]2
(1.5 mol%), which proves to be much more active leading to
[
14]
tion). In turn, coordination of the triflate anion in [1][OTf]2
Chem. Eur. J. 2016, 22, 6537 – 6541
www.chemeurj.org
6538
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
to distorted trigonal bipyramidal geometries at each antimony
[
5a]
center. The solid-state IR spectrum of single crystals of [1-m2-
DMF][OTf] displayed a weakening of the CÀO bond, when the
2
À1
stretching frequency was lowered to 1634 cm
from
À1
1
675 cm in neat DMF (see the Supporting Information). A
natural bond orbital analysis carried out by using the crystal
2
+
geometry of [1-m -DMF] supported the concomitant interac-
2
tion of the DMF oxygen atom with each antimony center, as il-
lustrated by the presence of multiple O“Sb interactions involv-
ing filled oxygen p orbitals as donor orbitals and vacant SbÀ
C
Ph
s* orbitals as acceptor orbitals (Figure 2). The energy of
these O”Sb interactions was estimated to be approximately
À1
[17]
1
2 kcalmol by using the NBO deletion protocol.
The four stibonium salts investigated in this study have also
been evaluated for the hydrosilylation of 4-nitro-, 4-trifluoro-
methyl-, 4-methoxy-, and 4-dimethylaminobenzaldehyde. Hy-
drosilylation reaction was not observed for these substrates.
We propose that this lack of activation arises from the relative-
ly weak Lewis acidity of the stibonium cations and their inabili-
ty to activate weakly basic substrates, such as 4-nitro- and 4-
trifluorobenzaldehyde or overcome the stability of electron-
rich substrates, such as 4-methoxy- and 4-dimethylaminoben-
zaldehyde. To support this proposal, we have also tested the
reactivity of 4-fluorobenzaldehyde and found that it undergoes
clean hydrosilylation with [1][BF ] and Et SiH as silane. Howev-
2 2
Figure 2. Top: Solid-state structure of [1-m -DMF][OTf] . Thermal ellipsoids
are drawn at the 50% probability level. The triflate anions and the hydrogen
atoms are omitted for clarity. Pertinent metric parameters can be found in
the text. Bottom: NBO plot (isovalue 0.05) showing two representative
lp(O)!s*(SbÀCPh) donor–acceptor interactions.
4
2
3
er, the fluorine atom appears to play a slight deactivating role
with the reaction only proceeding when heated to 608C. We
have also tested a few other tertiary silanes and found that
iPr SiH, Ph MeSiH, and Ph SiH are not reactive toward benzal-
3
2
3
dehyde in the presence of [1][BF ] . We assign this lack of reac-
tivity to the bulk of these silanes. Finally, the H NMR spectrum
4
2
Scheme 2. Hydrosilylation of benzaldehyde.
1
of Et SiH remained unchanged upon mixing with [1][BF ] . This
3
4 2
observation suggests that a mechanism involving SiÀH bond
activation as with catalysts such B(C F ) or [(C F ) FP] is
[
18]
+[4]
complete conversion after 8 h. This reaction is unaffected by
6
5 3
6 5 3
[
19]
addition of 3 mol% of Mes P as a Brønsted acid scavenger in-
unlikely; instead, it suggests that the catalyst may be directly
activating the carbonyl substrate, as was observed for other
3
dicating that protons are not responsible for the observed cat-
[
15]
[20]
alytic activity.
We also note that Et SiH reacts with acids
main-group catalysts. Collectively, these results can be rec-
3
making the involvement of protons an even more remote pos-
sibility. These results showed that: 1) the distibonium catalysts
are more active that their monofunctional analogs; and 2) the
tetrafluoroborate salt of the distibonium is significantly more
active than the triflate salt. We propose that: 1) the higher ac-
tivity of the distibonium catalysts arises from their ability to
doubly activate the carbonyl functionality of the aldehyde; and
onciled by invoking the double electrophilic activation of ben-
2
+
zaldehyde by [1]
followed by
silane reduction as depicted in
Figure 3.
In summary, we described the
synthesis and structure of a distibo-
nium dication, which promotes the
hydrosilylation of benzaldehyde
under mild conditions. The unusual
catalytic properties of this dication
2
) the higher activity of [1][OTf] versus [1][BF ] results from
2 4 2
À
the more weakly coordinating nature of the BF4 anion. To
support the concept of double electrophilic activation of the
Figure 3. Double electrophilic
activation of benzaldehyde by
2
+
2+
carbonyl substrate by [1] in these reactions, we failed to iso-
are proposed to result from its [1]
ability to doubly activate the car-
bonyl functionality of the sub-
.
late the benzaldehyde adduct. An adduct was obtained with
[
14]
the more basic carbonyl substrate DMF and [1][OTf]2. Eluci-
dation of the structure of this adduct revealed a DMF molecule
bridging the two antimony centers in an unsymmetrical fash-
ion (Figure 2). The resulting Sb1ÀO1 (2.555(2) Š) and Sb2ÀO1
strate. This proposal is supported by the fact that simple stibo-
nium monocations failed to promote this reaction, as well as
by the isolation of the DMF adduct [1-m -DMF][OTf] , in which
2
2
(
2.992(2) Š) bonds are well within the sum of the van der
the DMF oxygen atom is engaged with the two antimony cen-
ters.
[16]
Waals radii of the two elements (SbÀO 3.75 Š).
The DMF
oxygen atom is positioned directly trans from a phenyl ligand
(
](O1-Sb1-C7) 175.44(10)8, ](O1-Sb2-C19) 175.49(11)8) leading
6539
Chem. Eur. J. 2016, 22, 6537 – 6541
www.chemeurj.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Synthesis and isolation of (benzyloxy)triethylsilane: Triethylsilane
Experimental Section
À3
À5
(
0.319 mL, 2.0”10 mol), hexamethylbenzene (9.0 mg, 5.6”10
À5
À3
mol), and [1][BF ] (12.5 mg, 1.5”10 mol; 1.5 mol%) were mixed
Synthesis of [1][OTf] : MeOTf (0.21 mL, 1.9”10 mol) was added
4 2
2
in 4 mL of dry CHCl , and the reaction mixture was stirred at ambi-
to a solution of o-phenylene-bis(diphenylstibine) (302 mg, 4.8”
3
À4
ent temperature. After 12 h, the reaction mixture was directly
transferred to a short silica plug and chromatographed by using
1
0
mol) in toluene (3 mL). The mixture was sealed under N at-
2
mosphere in a 25 mL Schlenk tube and heated for 908C for 12 h,
after which a white precipitate was formed. The solid was filtered,
washed with Et O (3”5 mL), and dried in vacuo to give [1][OTf] in
hexanes/Et N (99:1) mixture as an eluent. The solvent was removed
3
in vacuo to give the pure product as a colorless oil in 88% isolated
2
2
À4
1
À4
yield (195.7 mg, 8.8”10 mol). The H NMR spectrum of the prod-
6
2% yield (285 mg, 3.0”10 mol). Single crystals of [1][OTf] were
2
[19b]
1
uct is in agreement with that previously reported.
H NMR
obtained as colorless blocks by diffusing Et O into a CH Cl solu-
2
2
2
1
(399.508 MHz, CDCl ): d=7.41–7.32 (m; 4H; o- and m-Ph), 7.28–
tion. H NMR (399.508 MHz, CD CN, 258C, TMS): d=7.88–7.84 (m;
3
3
3
3
7.25 (m; 1H; p-Ph), 4.70 (s; 2H; CH ), 0.97 (t; J (H,H)=8.0 Hz, 9H;
CH CH Si,), 0.68 ppm (q; J (H,H)=8.0 Hz, 6H, CH CH Si).
4
H; C H ), 7.71 (pseudo t; J (H,H)=6.0 Hz, 4H; p-Ph), 7.56 (pseudo
2
6
4
3
3
3
t; J (H,H)=6.4 Hz, 8H; o-Ph), 7.49 (pseudo d; J (H,H)=6.4 Hz, 8H;
3
2
3
2
1
3
1
m-Ph), 2.14 ppm (s; 6H; Sb-CH3); C{ H NMR (125.60 MHz, CD CN,
3
2
1
1
58C, TMS): d=141.38 (o-phenylene), 136.61 (o-Ph), 134.91 (p-Ph),
34.91 (quat. Ph), 134.81 (o-phenylene), 134.05 (quat. o-phenylene), Acknowledgements
À
31.93 (o-Ph), 124.46 (o-phenylene), 120.8 (q; CF SO ), 6.43 ppm
3
3
(
9
SbÀCH ); elemental analysis calcd (%) for C H F O S Sb : (M
3
34 30
6
6
2
2
w
Financial support from the Welch Foundation (A-1423), the Na-
tional Science Foundation (CHE-1300371), Texas A&M Universi-
ty (A.E. Martell Chair), and the Laboratory for Molecular Simula-
tion at Texas A&M University (software and computation re-
sources) is gratefully acknowledged.
56.24): C 42.71, H 3.16; found: C 42.85, H 3.20.
À4
Synthesis of [1][BF ] : [Me O][BF ] (49 mg, 3.3”10 mol) was
4
2
3
4
À4
added to a solution of 1 (101 mg, 1.6”10 mol) in a mixture of
,2-dichloroethane (1 mL) and toluene (2 mL). The mixture was
1
sealed in a 25 mL Schlenk tube under N atmosphere and heated
2
for 908C for 12 h, after which a white precipitate was formed. The
Keywords: antimony · homogeneous catalysis · cations ·
hydrosilylation · Lewis acids
solid was filtered, washed with Et O (3”5 mL), and dried in vacuo
2
À5
to give [1][BF ] in 48% yield (64 mg, 7.7”10 mol). Single crystals
4
2
of [1-OH ][BF ] were obtained in low yield as colorless blocks by
2
4 2
layering pentane on a saturated CH Cl solution of [1][BF4]2.
[1] a) M. Perez, C. B. Caputo, R. Dobrovetsky, D. W. Stephan, Proc. Natl.
Acad. Sci. USA 2014, 111, 10917–10921; b) M. PØrez, Z.-W. Qu, C. B.
Caputo, V. Podgorny, L. J. Hounjet, A. Hansen, R. Dobrovetsky, S.
Grimme, D. W. Stephan, Chem-Eur J 2015, 21, 6491–6500; c) W. Zhao,
P. K. Yan, A. T. Radosevich, J. Am. Chem. Soc. 2015, 137, 616–619;
d) K. D. Reichl, A. T. Radosevich, Chem. Commun. 2014, 50, 9302–9305;
e) T. Mukaiyama, S. Matsui, K. Kashiwagi, Chem. Lett. 1989, 18, 993–996;
f) T. Mukaiyama, K. Kashiwagi, S. Matsui, Chem. Lett. 1989, 18, 1397–
2
2
1
H NMR (399.508 MHz, CD Cl , 258C, TMS): d=7.74 (broad s; 4H),
2
2
3
7
.66 (m; 4H), 7.55 (pseudo t; J (H,H)=6.0 Hz, 8H; o-Ph), 7.47
3
(
pseudo d; J (H,H)=6.0 Hz, 8H; m-Ph), 2.16 ppm (s; 6H; SbÀCH );
3
1
3
1
C{ H} NMR (125.60 MHz, CD Cl , 258C, TMS): d 139.55 (o-phenyl-
2
2
ene), 134.89 (o-Ph), 133.21 (o-phenylene), 132.86 (p-Ph), 130.56 (m-
Ph), 30.60 ppm (SbÀCH ). The Sb-bound quaternary carbon could
3
not be detected. Elemental analysis calcd (%) for C H B F Sb (M
w
3
2
30
2
8
2
1400.
8
31.72): C 42.21, H 3.64; found: C 42.44, H 3.58. This elemental
[
2] a) C. B. Caputo, L. J. Hounjet, R. Dobrovetsky, D. W. Stephan, Science
2013, 341, 1374–1377; b) L. J. Hounjet, C. B. Caputo, D. W. Stephan,
Dalton Trans. 2013, 42, 2629–2635; c) M. PØrez, L. J. Hounjet, C. B.
Caputo, R. Dobrovetsky, D. W. Stephan, J. Am. Chem. Soc. 2013, 135,
18308–18310; d) M. H. Holthausen, M. Mehta, D. W. Stephan, Angew.
Chem. Int. Ed. 2014, 53, 6538–6541; Angew. Chem. 2014, 126, 6656–
6659; e) M. H. Holthausen, R. R. Hiranandani, D. W. Stephan, Chem. Sci.
2015, 6, 2016–2021; f) M. Mehta, M. H. Holthausen, I. Mallov, M. PØrez,
Z.-W. Qu, S. Grimme, D. W. Stephan, Angew. Chem. Int. Ed. 2015, 54,
8250–8254; Angew. Chem. 2015, 127, 8368–8372.
analysis was performed on the bulk product; it points to the ab-
sence of water in bulk [1][BF4]2.
Synthesis of [1-m -DMF][OTf] : A sample of [1][OTf] (32 mg; 3.3”
2
2
2
À5
1
0
mol) was placed in a vial and dissolved in 0.5 mL of DMF. Et O
2
was slowly diffused into this mixture leading to the crystallization
À5
of [1-m -DMF][OTf] in 64% yield (22 mg, 2.1”10 mol). The
2
2
1
H NMR data showed that the adduct is fully dissociated in solu-
tion. H NMR (399.508 MHz, CD CN, 258C, TMS): d=7.89 (broad;
H; C(O)H), 7.88–7.84 (m; 4H; C H ), 7.71 (pseudo t; J (H,H)=
.0 Hz, 4H; p-Ph), 7.56 (pseudo t; J (H,H)=6.4 Hz, 8H; o-Ph), 7.49
pseudo d; J (H,H)=6.4 Hz, 8H; m-Ph), 2.88 (s; 3H; DMF-CH ), 2.76
3
s; 3H; DMF-CH ), 2.14 ppm (s; 6H; Sb-CH ); elemental analysis
1
3
3
[3] T. W. Hudnall, Y.-M. Kim, M. W. P. Bebbington, D. Bourissou, F. P. Gabba ¨ı ,
1
6
(
(
6
3
4
J. Am. Chem. Soc. 2008, 130, 10890–10891.
[4] M. H. Holthausen, J. M. Bayne, I. Mallov, R. Dobrovetsky, D. W. Stephan,
3
J. Am. Chem. Soc. 2015, 137, 7298–7301.
3
3
[5] a) A. P. M. Robertson, S. S. Chitnis, H. A. Jenkins, R. McDonald, M. J. Fer-
guson, N. Burford, Chem. Eur. J. 2015, 21, 7902–7913; b) A. P. M. Robert-
son, N. Burford, R. McDonald, M. J. Ferguson, Angew. Chem. Int. Ed.
calcd (%) for C H F NO S Sb (Mw 1029.33): C 43.17, H 3.62, N
3
5
37
6
7
2
2
1
.36; found: C 43.22, H 3.55, N 1.38.
2
014, 53, 3480–3483; Angew. Chem. 2014, 126, 3548–3551; c) I.-S. Ke,
Hydrosilylation reactions: In a glovebox, an NMR tube was
M. Myahkostupov, F. N. Castellano, F. P. Gabba ¨ı , J. Am. Chem. Soc. 2012,
134, 15309–15311; d) C. R. Wade, F. P. Gabba ¨ı , Organometallics 2011, 30,
4479–4481; e) M. Jean, Anal. Chim. Acta 1971, 57, 438–439; f) L. H.
Bowen, R. T. Rood, J. Inorg. Nucl. Chem. 1966, 28, 1985–1990.
À4
charged with benzaldehyde (0.023 mL, 2.0”10 mol), triethylsilane
À4
(
1
0.064 mL, 4.0”10 mol), hexamethylbenzene (1.8 mg, 1.1”
0
À5
mol), and the corresponding stibonium salts (1.5 mol% [1]
[
6] N. Li, R. Qiu, X. Zhang, Y. Chen, S.-F. Yin, X. Xu, Tetrahedron 2015, 71,
[
[
OTf] , 1.5 mol% [1][BF ] , 3.0 mol% [Ph MeSb][OTf], 3.0 mol%
2 4 2 3
4275–4281.
Ph MeSb][BF ] with all concentrations based on benzaldehyde) in
3
4
1
[7] B. Pan, F. P. Gabba ¨ı , J. Am. Chem. Soc. 2014, 136, 9564–9567.
8] M. Hirai, F. P. Gabba ¨ı , Angew. Chem. Int. Ed. 2015, 54, 1205–1209;
Angew. Chem. 2015, 127, 1221–1225.
9] a) J. Vaugeois, J. D. Wuest, J. Am. Chem. Soc. 1998, 120, 13016–13022;
b) M. Tschinkl, A. Schier, J. Riede, F. P. Gabba ¨ı , Organometallics 1999, 18,
1747–1753; c) I. D. Kostas, G.-J. M. Gruter, O. S. Akkerman, F. Bickel-
haupt, H. Kooijman, W. J. J. Smeets, A. L. Spek, Organometallics 1996,
dry CDCl (1 mL). After recording an initial H NMR spectrum, the
3
[
[
NMR samples were kept at room temperature and monitored peri-
odically. For 4-fluorobenzaldehyde (21 mL, 0.2 mmol) with [1][BF4]2
(
1.5 mol%) as a catalyst, no reaction was observed at room tem-
perature. Placing the NMR tube in an oil bath heated to 608C gave
a conversion of 33% after 8 h and >95% after 22 h.
Chem. Eur. J. 2016, 22, 6537 – 6541
www.chemeurj.org
6540
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
1
5, 4450–4458; d) J. D. Wuest, Acc. Chem. Res. 1999, 32, 81–89; e) J. B.
[16] S. S. Batsanov, Inorg. Mater. 2001, 37, 871–885.
King, F. P. Gabba ¨ı , Organometallics 2003, 22, 1275–1280; f) T. Ooi, M. Ta-
kahashi, M. Yamada, E. Tayama, K. Omoto, K. Maruoka, J. Am. Chem. Soc.
[17] A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88, 899–926.
[18] a) D. J. Parks, W. E. Piers, J. Am. Chem. Soc. 1996, 118, 9440–9441;
b) D. J. Parks, J. M. Blackwell, W. E. Piers, J. Org. Chem. 2000, 65, 3090–
3098.
[19] a) S. E. Denmark, Y. Ueki, Organometallics 2013, 32, 6631–6634; b) J.
Koller, R. G. Bergman, Organometallics 2012, 31, 2530–2533.
[20] a) A. L. Liberman-Martin, R. G. Bergman, T. D. Tilley, J. Am. Chem. Soc.
2015, 137, 5328–5331; b) R. Calas, J. Organomet. Chem. 1980, 200, 11–
36; c) J. L. Fry, M. Orfanopoulos, M. G. Adlington, W. P. Dittman, S. B. Sil-
verman, J. Org. Chem. 1978, 43, 374–375; d) M. P. Doyle, C. T. West, S. J.
Donnelly, C. C. McOsker, J. Organomet. Chem. 1976, 117, 129–140.
2
004, 126, 1150–1160.
10] W. Levason, C. A. McAuliffe, S. G. Murray, J. Organomet. Chem. 1975, 88,
71–174.
11] B. A. McCortney, B. M. Jacobson, M. Vreeke, E. S. Lewis, J. Am. Chem. Soc.
990, 112, 3554–3559.
[
[
1
1
[
[
12] M. C. Henry, G. Wittig, J. Am. Chem. Soc. 1960, 82, 563–564.
13] M. A. Beckett, D. S. Brassington, S. J. Coles, M. B. Hursthouse, Inorg.
Chem. Commun. 2000, 3, 530–533.
[
14] CCDC 1448773 ([1][OTf]
2 4 2 2
), 1448774 ([1][BF ] ), 1448775 ([1-m -DMF]
[
OTf] ), 1448776 ([Ph MeSb][OTf]) contain the supplementary crystallo-
2
3
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data request/cif.
[
15] R. K. Schmidt, K. Müther, C. Mück-Lichtenfeld, S. Grimme, M. Oestreich,
J. Am. Chem. Soc. 2012, 134, 4421–4428.
Received: March 1, 2016
Published online on March 16, 2016
Chem. Eur. J. 2016, 22, 6537 – 6541
www.chemeurj.org
6541
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim