Activation of diboron reagents with bronsted bases and alcohols: An experimental and theoretical perspective of the organocatalytic boron conjugate addition reaction

Article abstract of DOI:10.1002/chem.201102209

Bases play an important role in organocatalytic boron conjugate addition reactions. The sole use of MeOH and a base can efficiently transform acyclic and cyclic activated olefins into the corresponding β-borated products in the presence of diboron reagents. Inorganic and organic bases deprotonate MeOH in the presence of diboron reagents. It is concluded, on the basis of theoretical calculations, NMR spectroscopic data, and ESI-MS experiments, that the methoxide anion forms a Lewis acid-base adduct with the diboron reagent. The sp ² B atom of the methoxide-diboron adduct gains a strongly nucleophilic character, and attacks the electron-deficient olefin. The methanol protonates the intermediate, generating the product and another methoxide anion. This appears to be the simplest method to activate diboron reagents and make them suitable for incorporation into target organic molecules. The simplest method to activate diboron reagents and make them suitable for incorporation in targeted organic molecules seems to be deprotonation of MeOH by base (e.g., Verkade's base) to generate the methoxide anion, which interacts with the diboron reagent to give a nucleophilic Lewis acid-base adduct that promotes the β-boration of α,β-unsaturated carbonyl substrates. Methanol protonates the intermediate to generate the product and another methoxide anion (see scheme). Copyright

Full text of DOI:10.1002/chem.201102209

FULL PAPER
DOI: 10.1002/chem.201102209
Activation of Diboron Reagents with Brønsted Bases and Alcohols: An
Experimental and Theoretical Perspective of the Organocatalytic Boron
Conjugate Addition Reaction
Cristina Pubill-Ulldemolins,[a, b] Amadeu Bonet,[a] Carles Bo,*[b] Henrik Gulyꢀs,*[a] and
Elena Fernꢀndez*[a]
Abstract: Bases play an important role
in organocatalytic boron conjugate ad-
dition reactions. The sole use of MeOH
and a base can efficiently transform
acyclic and cyclic activated olefins into
the corresponding b-borated products
in the presence of diboron reagents. In-
organic and organic bases deprotonate
MeOH in the presence of diboron re-
agents. It is concluded, on the basis of
theoretical calculations, NMR spectro-
scopic data, and ESI-MS experiments,
methoxide–diboron adduct gains
a
strongly nucleophilic character, and at-
tacks the electron-deficient olefin. The
methanol protonates the intermediate,
generating the product and another
methoxide anion. This appears to be
the simplest method to activate dibor-
on reagents and make them suitable
for incorporation into target organic
molecules.
that the methoxide anion forms
a
Lewis acid–base adduct with the dibor-
on reagent. The sp2 B atom of the
Keywords: boron · DFT · organo-
catalysis · reaction mechanisms ·
synthetic methods
Introduction
The presence of a base in the catalytic b-boration of activat-
ed olefins is essential,[1] but its role has not yet been eluci-
dated completely. In metal-mediated b-boration of a,b-unsa-
turated carbonyl compounds, the base has been used for in
situ generation of metal alkoxide complexes, to facilitate
transmetalation with diboron reagents[2] (Scheme 1, path a).
This is consistent with the fact that isolated transition metal
alkoxides, such as [(NHC)CuOR] (NHC=N-heterocyclic
carbene; OR=OMe, OtBu), can catalyze the reaction in the
absence of base.[3] However, other exceptions to the re-
quired use of base in boron conjugate addition reactions[4]
may allow other important roles to be attributed to the
base. For instance, Santos and co-workers have shown that
the pinacolboryl unit of a mixed sp2–sp3 diboron reagent can
be selectively added to the b-carbon atom of a,b-unsaturat-
ed carbonyl compounds and allenoates, by using phosphine-
free CuI complexes as catalysts, in the absence of base
Scheme 1. The role of the base in the formation of active metal species
for metal-mediated b-boration: path a describes the mechanism of Yun
and co-workers for Cu-mediated b-boration; path b illustrates the intra-
molecular diboron activation suggested by Santos and co-workers.
(Scheme 1, path b).[5] This “preactivated” diboron reagent
was developed from an original idea of Miyaura and co-
workers,[2b] who suggested that AcOÀ activated bis(pinacola-
to)diboron (B2pin2) by a Lewis acid–base interaction prior
to boryl transfer to the copper center (Scheme 2). For the
sake of completeness, we note that in early examples, in
which Pt0 complexes mediate the b-boration reaction,[6] base
is not required, probably because activation of the diboron
reagent occurs via homolytic oxidative addition to the tran-
sition metal. In recently developed transition-metal-free ap-
proaches to the catalytic b-boration of electron-deficient
[a] C. Pubill-Ulldemolins, A. Bonet, Dr. H. Gulyꢀs, Dr. E. Fernꢀndez
Department of Quꢁmica Fꢁsica i Iorgꢂnica
Universitat Rovira i Virgili
C/Marcel.lꢁ Domingo s/n (Spain)
Fax : (+34)977559563
E-mail: mariaelena.fernandez@urv.cat
henrik.gulyas@urv.cat
[b] C. Pubill-Ulldemolins, Dr. C. Bo
Institute of Chemical Research of Catalonia (ICIQ)
Avda. Paꢃsos Catalans, 17. 43007 Tarragona, (Spain)
E-mail: cbo@iciq.es
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102209.
Chem. Eur. J. 2012, 18, 1121 – 1126
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1121

Results and Discussion
To learn more about the catalytic potential of bases in the
b-boration of activated olefins, we conducted a systematic
study to convert ethyl crotonate to the b-borated product in
the presence of a series of bases. Alkoxides and inorganic
bases allowed low to moderate conversion at 708C over 6 h
with MeOH as additive (Table 1, entries 1–7). The benefit of
Scheme 2. First example of diboron activation with a base by Miyaura
and co-workers.
olefins, the presence of base is also required. In NHC-cata-
lyzed reactions, the base is used for in situ generation of the
nucleophilic carbene by deprotonation of imidazolium salts,
and the carbene subsequently activates the diboron reagent
through Lewis acid–base interaction.[7] By using a combina-
tion of 10 mol% imidazolium salt and 10 mol% NaOtBu,
cyclohexenone could be quantitatively converted into the
corresponding b-borated product at room temperature
(Figure 1, bar 1), whereas the base alone gives practically no
conversion under the same reaction conditions (Figure 1,
bar 2).[7] We decided to complement these literature experi-
ments, in order to generate more information on the role of
the base. Upon increasing the reaction temperature to 708C,
the conversion of the substrate increased to 9%, over the
same reaction time (Figure 1, bar 3). A first hypothesis was
that, considering the concentration of the base (10 mol%),
this conversion could correspond to one catalytic cycle.
Indeed, when we added MeOH (5 equiv with regard to sub-
strate) as a proton source for conjugate boron addition, the
conversion increased to 33% (Figure 1, bar 4). We made a
further advance in the organocatalytic approach by the in-
troduction of phosphines as catalytic additives. With phos-
phine/base (PPh3/Cs2CO3) as catalyst system and MeOH as
additive, quantitative formation of the b-borated product
could be observed at 708C (Figure 1, bar 5).[8] The most re-
markable fact is that under these reaction conditions, the
sole presence of base (Cs2CO3) allows the substrate to be b-
borated up to 30% conversion[9] (Figure 1, bar 6). Conceptu-
ally, the latter experiments suggest a novel and fundamental
role for the base: could it be the catalyst?
Table 1. Base-mediated catalytic b-boration of ethyl crotonate with
B2pin2.[a]
Entry
Base[b]
Solvent
Additive[b]
Conv [%][c]
1
2
NaOMe
KOMe
THF
–
MeOH
–
27
24
3
4
5
6
LiOMe
NaOtBu
Cs2CO3
K2CO3
–
–
–
–
–
–
–
–
23
40
45
28
7
CsF
–
–
29
8
9
10
11
12
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Verkadeꢅs base
toluene
THF
–
–
–
–
33
8
13
41
tBuOH
iPrOH
BuOH
MeOH
69 (90)[d]
[a] Standard conditions: substrate (0.5 mmol), diboron reagent (B2pin2;
1.1 equiv), base (15 mol%), alcohol (2.5 mmol), solvent (2 mL), 708C,
6 h. [b] pKaÀ (in water): MeOH 15.2, BuOH 16.0, iPrOH 17.1, tBuOH
18.0, HCO3 10.4, H+-Verkadeꢅs base 26.2. [c] Conversion calculated by
GC analysis and confirmed by ꢆH NMR spectroscopy. [d] 16 h.
using THF versus toluene as solvent is in accordance with
the higher solubility of the bases in the former (Table 1, en-
tries 5 and 8). The structure of the alcohol additive also had
a considerable influence on the catalytic activity. Primary al-
cohols (MeOH and BuOH)
provided significantly higher
conversions (Table 1, entries 5
and 11) than secondary
(iPrOH) and tertiary (tBuOH)
alcohols (Table 1, entries 9 and
10). The role of the alcohol ad-
ditive could be divided into
two parts: protonation of the
b-borated enolate intermediate
and the source of the corre-
sponding alkoxide ion, which
interacts with the diboron re-
agent. Since the protonation
step is guaranteed with all of
the alcohols tested under the
reaction conditions, the plausi-
ble Lewis acid–base interaction
Figure 1. Influence of base on the metal-free b-boration of cyclohexenone.
of the alkoxide ion with the
1122
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Chem. Eur. J. 2012, 18, 1121 – 1126

Activation of Diboron Reagents with Brønsted Bases and Alcohols
FULL PAPER
Table 2. Verkadeꢅs base mediated catalytic b-boration of ethyl crotonate
with B2pin2.
boron reagent may be more crucial and is, therefore, the
subject of discussion throughout the mechanistic proposal
(see below). Selecting MeOH as a suitable additive, we also
tested several organic bases in the b-boration reaction of
ethyl crotonate, but only the Verkade base conducted the
reaction to conversions up to 90% over 16 h (Table 1,
entry 12). The ability of this base to act as a catalyst has
been widely explored in a broad range of base-catalyzed re-
actions.[10] Since the nature of the diboron reagent has a
direct impact on the outcome of the reaction, we explored
different boron sources for the b-boration of ethyl crotonate
under the optimized reaction conditions. Over 24 h almost
quantitative conversion was observed for bis(pinacolato)di-
boron (B2pin2), bis(cathecolato)diboron (B2cat2), and bis(h-
exyleneglycolato)diboron (B2hex2), but only moderate con-
version was achieved with bis(neopentylglycolato)diboron
(B2neop2; Scheme 3). B2pin2 was selected for further studies
[a]
Entry
Substrate
Product
Conv[b] [%]
(yield[c] [%])
AHCTUNGTRENNUNG
1
2
3
99
99 (88)
94 (85)
4
40
5
6
65
90 (81)
7
8
99 (82)
99
9
99
97
10
[a] Standard conditions: substrate (0.5 mmol), B2pin2 (1.1 equiv), Ver-
kadeꢅs base (15 mol%), MeOH (2.5 mmol), THF (2 mL), 708C, 24 h.
[b] Conversion calculated by GC analysis and confirmed by 1H NMR
spectroscopy. [c] Yield of isolated product.
Scheme 3. Verkadeꢅs base mediates b-boration of ethyl crotonate with
different diboron reagents.
pletely protonated (31P{1H} NMR: d=À16.5 ppm, for more
information see the Supporting Information), and the gener-
ated MeOÀ forms a Lewis acid–base adduct with B2pin2.
The suggested mechanism for the base/alcohol-mediated b-
boration of activated olefin is depicted in Scheme 4, which
shows the elementary steps of the reaction between methyl
acrylate and B2pin2. The methoxide ion, generated from
MeOH in the presence of base, forms a Lewis acid–base
adduct with the diboron reagent, which we chose as the
origin of energies. In the next step, a transition state (TS)
was fully characterized, and it corresponds to a nucleophilic
attack of the sp2 boron moiety at the unsubstituted carbon
atom C1 of the alkene. This interaction was identified as the
À
due to its accessibility and chemical stability. To survey the
general scope of the Verkade base mediated b-boration re-
action, we carefully selected a series of activated olefins
with diverse structural features. a,b-Unsaturated esters were
quantitatively b-borated, regardless of the steric bulk of the
ester moiety (Table 2, entries 1–3). Nucleophilic attack at
the b position of the substrate is less favored in the case of
b,b-dimethyl-substituted a,b-unsaturated ester or a,b-di-
methyl substituted substrate (Table 2, entries 4, 5), probably
due to steric factors. Open-chain and cyclic a,b-unsaturated
ketones were quantitatively converted into the correspond-
ing organoboranes (Table 2, entries 6–8). The analogous a,b-
unsaturated amide and phosphonate were also efficiently
converted into the desired b-borated products (Table 2, en-
tries 9 and 10). To obtain deeper insight into the reaction
and postulate a plausible mechanism, the structures and sta-
bilities of possible adducts formed within the alkene/dibor-
on/alcohol/base system were investigated both computation-
ally and experimentally. The reaction between Verkadeꢅs
base and MeOH was studied in the absence and presence of
B2pin2. With an excess of MeOH Verkadeꢅs base is com-
overlap between the strongly polarized B B
(HOMO) of the activated diboron reagent and the anti-
s bond
bonding p* orbital (LUMO) of the alkene. The structural
À
features of the TS reflect the cleavage of the B B bond
(from 1.701 ꢇ in B2pin2 to 1.754 ꢇ in B2pin2·MeOÀ to
À
1.956 ꢇ in the TS) and the formation of the new B C bond
(2.208 ꢇ). The TS structure releases the “
A
product and directly leads to formation of anionic inter-
mediate I, which will be further protonated in the presence
of the excess of MeOH to provide the b-boration product
Chem. Eur. J. 2012, 18, 1121 – 1126
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1123

C. Bo, H. Gulyꢀs, E. Fernꢀndez et al.
Scheme 4. Proposed reaction pathway. Electronic energy and Gibbs free
energy (in parentheses) computed at the BP86 level relative to
B2pin2·MeOÀ adduct plus methyl acrylate. Methyl groups of the pinacol
Figure 3. Energy profile for several monosubstituted alkenes. Energy bar-
rier and reaction energies are given as electronic energy and Gibbs free
energy (in parentheses) computed at the BP86 level relative to
moieties are omitted for clarity. All values in kcalmolÀ1
.
B2pin2·MeOÀ adduct plus the respective alkene. All values in kcalmolÀ1
.
and generate another MeOÀ ion. Regarding I, we obtained
two types of structures (see Figure 2), which clearly reflect
the role of the carbonyl group in stabilization of the nega-
tive charge developed on C2. Indeed, substrates with ester,
showed in both experimental and theoretical studies.[11] We
also evaluated the effect of different substituents on the
energy barrier to explain the reactivities summarized in
Table 2. Considering esters as model substrates, we observed
that the energy barrier significantly increases when there
are methyl groups in the b position (Table 3, entries A, C,
and F), in agreement with the lower conversion observed in
the corresponding experimental results. The influence of a
methyl group in a position is not as pronounced as in the b
position. Only a slight increase of about 3 kcalmolÀ1 in the
activation energy was found (Table 3, entries A, B and C,
Figure 2. Molecular structures for a model transition state and the two
different intermediates.
Table 3. Relative electronic energy [kcalmolÀ1] and Gibbs energy [kcal
molÀ1] for the activation of different a,b-unsaturated esters.[a]
Entry Entry in
Table 2
Substrate
DE°
DEr
DG°
DGr
ketone, and aldehyde groups, which allow charge delocaliza-
tion, form more stable intermediates (IB) than unactivated
olefins such a propylene, in which the charge delocalization
is not so evident (IA). In the case of the structure IA the
negative charge is stabilized by an interaction between C2
and the Lewis acidic pinacolboryl unit. Note that in the case
of styrene the distribution of the negative charge density be-
tween C2 and the phenyl group is not as effective as in the
case of a carbonyl group, and the IA-type structure is
formed. Importantly, the thermodynamic stability of inter-
mediate I is also reflected in the reactivity of the corre-
sponding substrate; thus, a dramatic decrease of the energy
barrier is observed when electronic effects stabilize the TS
(see Figure 3). The largest difference of 22 kcalmolÀ1 was
found for the activation of propylene with respect to acrylal-
dehyde. Finally, it is also of great importance that the unac-
tivated olefins can also form intermediates which lead to or-
ganocatalytic diboration of the substrate, as we recently
6.9
(5.9)
À28.2
21.5
À28.4
A
B
C
D
1
2
(À33.2)
(20.5) (À31.4)
10.2
(6.3)
À25.6
26.0
À25.7
(À28.9)
(22.1) (À29.1)
14.0
À24.4
29.3 -24.2
(25.0) (À27.4)
(10.0) (À27.6)
16.3
À21.3
32.2
À20.7
(10.7) (À25.8)
(26.6) (À25.2)
17.1
À21.3
33.1
À19.5
E
F
5
(10.8) (À26.0)
(26.8) (À24.2)
23.0 À21.3
39.6 À20.2
(17.1) (À27.0)
(33.7) (À25.1)
[a] Molecular geometries for all the species were optimized with BP86
functional. Single-point energies at the metahybrid M06 level are in pa-
rentheses.
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Activation of Diboron Reagents with Brønsted Bases and Alcohols
FULL PAPER
D). The replacement of a methyl by an ethyl group in the
ester moiety (Table 3, entries D and E) results only in 1 kcal
molÀ1 difference, in accordance with the similar reactivities
of different esters of crotonic acid observed experimentally
(Table 2, entries 2, 3). The next step was to study the influ-
ence of the nature of the alcohol additive (Table 1, entries 5,
9–11). The dramatic decrease in activity when tBuOH is
used instead of MeOH can be explained by three facts:
1) tBuOH is less acidic than MeOH, and thus generation of
the alkoxide by the base would be less favored (initial step
in the suggested reaction pathway), 2) the alkoxide–diboron
adduct is thermodynamically more stable for MeOÀ
(À0.5 kcalmolÀ1 with respect to the energy of the B2pin2 and
MeOÀ before complexation) than for the tBuOÀ (3.4 kcal
molÀ1 above the energy of the free reagents), 3) in the case
of acrylaldehyde as model substrate, the relative energy for
the TS with B2pin2·tBuOÀ as nucleophile is 4 kcalmolÀ1
higher than with the B2pin2·MeOÀ. Finally, we were able to
elucidate the interaction of the diboron reagent with the
methoxide anion by means of NMR spectroscopy (Table 4)
theoretical methods. A plausible mechanism has been pro-
posed in which the role of the base is crucial to provide the
activated diboron reagent. It was concluded that, upon inter-
action with alcohols, it generates alkoxides, which interact
with the diboron reagent to form a nucleophilic Lewis acid–
base adduct. The activated diboron reagent is prone to
attack activated olefins. The nucleophilic attack forms the
borylated carbanionic intermediate I, which will be proton-
ated in the protic medium, forming the b-borated product
and generating another methoxide anion. The rate of forma-
tion of the carbanionic intermediate, as well as its thermody-
namic stability, is mainly controlled by the functional group
of the activated olefin. We believe that the sole use of base/
alcohol as catalytic system is an important achievement in
the emerging area of organocatalytic boron conjugate addi-
tion reactions. We are currently developing the asymmetric
version of this methodology, and also studying, in depth, the
related chiral phosphine/base/alcohol catalyst system, which
provides enhanced activity and asymmetric induction.
Experimental Section
Table 4. Experimental and computed 11B NMR chemical shifts in solu-
tion (THF) for different boron species.[a]
General procedure for b-boration of a,b-unsaturated carbonyl com-
pounds and other activated olefins: The base 2,8,9-trimethyl-2,5,8,9-
Species
Exptl 11B NMR
Calcd 11B NMR
B2pin2
30.5
30.5
tetraaza-1-phosphabicyclo
ACHTUNGERTN(NUNG pinacolato)diboron (140 mg, 0.55 mmol) were transferred to an oven-
ACHTUNGTRENUN[NG 3.3.3]undecane (8.4 mg, 0.076 mmol) and bis-
B2pin2·NHC[7,11]
1.8
(sp3)
36.2
(sp2)
À0.6
(sp3)
31.8
(sp2)
G
R
G
ACHTUNGTRENNUNG
dried Schlenk tube under argon. THF (2 mL) was added. The mixture
was stirred for 10 min at RT to dissolve the diboron reagent completely.
The substrate (0.5 mmol) and MeOH (100 mL, 2.5 mmol) were added,
and the reaction mixture was stirred for 24 h at 708C in an oil bath. The
reaction mixture was cooled to RT. An aliquot of 0.2 mL was taken from
the solution and diluted with CH2Cl2 (1 mL), then analyzed by using GC/
GC-MS to determine conversion and confirm selectivity. After the GC
analysis, the same aliquot was gently concentrated on a rotary evaporator
at RT and analyzed by using 1H NMR spectroscopy to confirm the con-
version and chemoselectivity previously observed by gas chromatography.
In each case, both methods indicated that a single product was formed
from the substrate. The differences in conversions obtained with the two
methods are within 5% in most cases.
(pin)BOtBu[12]
21.5
18.7
[B2pin2·MeO][HV]
5.9
35.0
(sp2)
5.4
35.9
(sp2)
28.3
(sp2)
A
E
A
ACHTUNGTRENNUNG
B2pin2·MeOÀ
À17.9
R
ACHTUNGTRENNUNG
[a] 11B NMR shifts are given in ppm. Boron trifluoride etherate was
taken as a reference.
and ESI-MS studies, as well as by calculations (see Support-
ing Information for details). Importantly, the calculated
values for 11B chemical shifts perfectly reproduce the mea-
sured values for B2pin2, for the B2pin2–N-heterocyclic car-
bene adduct reported by Hoped et al.,[12] and for the
Calculations: Molecular structures for all species were optimized without
constraints by using DFT-based methods as implemented in the Amster-
dam Density Functional (ADF v2009.01) package. A triple-z plus polari-
zation Slater basis set was used on all atoms. Relativistic corrections
were introduced by scalar-relativistic zero-order regular approximation
(ZORA).[15] For geometry optimizations we used the local VWN correla-
tion potential[16] together with the Becke exchange[17] and the Perdew cor-
relation[18,19] (BP86) generalized gradient corrections. Stationary points in
the potential energy hypersurface were characterized either as minima or
transition states by means of harmonic vibrational frequency calculations.
Standard corrections to Gibbs free energy at 298 K were evaluated, too.
Solvent effects were introduced by using the continuous solvent model
COSMO.[20] NMR chemical shifts were computed at the same level of
theory by doing single-point calculations with all-electron basis sets.
Single-point energy at the hybrid B3LYP21 and metahybrid M06[22] levels
were performed self-consistently (see the Supporting Information).
ACHTUNGTRENNUNG
(pin)BOtBu species reported by Marder et al.[13] The
B2pin2·MeOÀ adduct with the protonated Verkade base
(HV) as a cation ([HV][B2pin2·MeO]) is observed as a
broad signal at 5.9 ppm, and a very broad signal at around
35.0 ppm, which precisely coincides with the computed
values for the sp3 boron atom (5.4 ppm) and sp2 boron
(35.9 ppm). The broadening of the signals are probably due
to the fast inter- and intramolecular exchanges.[14] Further-
more, the molecular peak of the B2pin2·MeOÀ adduct, m/z=
285.1, was detected in an ESI-MS experiment.
Conclusion
Acknowledgements
The role of the base in the organocatalytic boron conjugate
addition reaction has been determined by experimental and
We thank MICINN (CTQ2010-16226/BQU, CTQ2008-06549-CO2-02/
BQU, and Consolider-Ingenio 2010 CSD2006-0003) and AGAUR
Chem. Eur. J. 2012, 18, 1121 – 1126
ꢄ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1125

C. Bo, H. Gulyꢀs, E. Fernꢀndez et al.
(2009SGR-00462 and 2009SGR-00116) for funding. A.B. thanks General-
itat Catalunya for an FI grant and C.P. thanks ICIQ Foundation for a
grant. We thank Prof. T. B. Marder and Dr. Ch. Kleeberg for useful dis-
cussions on 11B NMR experiments and Dr. N. Cabello for the ESI-MS ex-
periments.
[8] A. Bonet, H. Gulyꢀs, E. Fernꢀndez, Angew. Chem. 2010, 122, 5256;
Angew. Chem. Int. Ed. 2010, 49, 5130.
[9] For an early observation on the b-boration of ethyl crotonate with
Cs2CO3, see Table 1, entry 12 in A. Bonet, C. Sole, H. Gulyꢀs, E.
Fernꢀndez, Asian J. Chem. 2011, 6, 1011.
[10] a) P. B. Kisanga, B. DꢅSa, J. G. Verkade, J. Org. Chem. 1998, 63,
10057; b) B. A. DꢅSa, J. G. Verkade, J. Am. Chem. Soc. 1996, 118,
12832; c) J.-S. Tang, J. G. Verkade, Angew. Chem. 1993, 105, 934;
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Received: July 19, 2011
Published online: December 14, 2011
1126
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