Study of the lithiated phenylacetonitrile monoanions and dianions formed according to the lithiated base used (LHMDS, LDA, or n-BuLi). 1. Evidence of heterodimer ( Quadac ) or dianion formation by vibrational spectroscopy

Article abstract of DOI:10.1021/jo020492t

It is evidenced through vibrational spectroscopy that a heterodimer or Quadac is formed when an excess of base (LHMDS, LDA, or n-BuLi) is added to PhCH₂CN in THF, THF-hexane, or THF-toluene solution. The amount of heterodimer increases with the pK_Ha of the lithiated base. A dianionic species may be formed through decomposition of this heterodimer if the pK_Ha of the base is sufficiently high, as in the case of n-BuLi. With LDA, only a very small amount of dianion is observed, and with LHMDS, no dianion is detected. The predominant dianionic species observed are the linear and bridged separated ion pairs of the dilithiated dianion. The presence of the amine in the medium is of paramount importance. The PhCHCNLi monomer-dimer equilibrium is entropy driven toward the dimer solvated by the amine.

Full text of DOI:10.1021/jo020492t

Study of the Lithiated Phenylacetonitrile Monoanions and
Dianions Formed According to the Lithiated Base Used (LHMDS,
LDA, or n-BuLi). 1. Evidence of Heterodimer (“Quadac”) or
Dianion Formation by Vibrational Spectroscopy
Jacques Corset,† Martine Castella`-Ventura,*,† Franc¸oise Froment,† Tekla Strzalko,‡ and
Lya Wartski‡
LADIR - CNRS (UMR 7075), 2 Rue Henri Dunant, BP 28, 94320 Thiais, France, and Laboratoire des
Carbocycles - CNRS (UMR 8615), Institut de Chimie Mole´culaire et des Mate´riaux d’Orsay, Baˆt 420,
Universite´ de Paris Sud, 91405 Orsay, France
martine.ventura@glvt-cnrs.fr
Received July 25, 2002
It is evidenced through vibrational spectroscopy that a heterodimer or “Quadac” is formed when
an excess of base (LHMDS, LDA, or n-BuLi) is added to PhCH2CN in THF, THF-hexane, or THF-
toluene solution. The amount of heterodimer increases with the pKH of the lithiated base. A
a
dianionic species may be formed through decomposition of this heterodimer if the pKH of the base
a
is sufficiently high, as in the case of n-BuLi. With LDA, only a very small amount of dianion is
observed, and with LHMDS, no dianion is detected. The predominant dianionic species observed
are the linear and bridged separated ion pairs of the dilithiated dianion. The presence of the amine
in the medium is of paramount importance. The PhCHCNLi monomer-dimer equilibrium is entropy
driven toward the dimer solvated by the amine.
Introduction
gem-Dilithiated reagents prepared by metalation of
compounds containing acidic hydrogens, as, for instance,
alkyl, allyl, benzyl, phenyl sulfones and phenyl1,2 or
trimethylsilyl acetonitriles,3 possess great synthetic po-
tential.
the structure of the heterodimer, in which LHMDS is the
6
base, was confirmed by Li/15N NMR solution study in
Kaiser et al.4 have postulated the formation of a
dianion (3) from PhCH2CN (1) with an excess of n-
butyllithium (n-BuLi), on the basis of deuteriation and
alkylation experiments. Later on, Crowley et al.5 inves-
tigated the reaction of PhCH2CN with 2 equiv of lithium
hexamethyldisilazide (LHMDS) by 13C NMR experiments
and have not observed dianion formation. According to
the base used, a sequential process involving a rapid
intra-aggregate lithiation through a species called a
heterodimer or quasi dianion complex “Quadac” (2a with
R′ ) n-Bu, or 2b with R ) i-Pr or Me3Si) rather than
direct dianion (3) formation was proposed.
TMEDA/toluene.7
We have been particularly interested in the elucidation
of the structures of the lithiated species derived from
PhCH2CN under a slight excess of base in different media
by IR, FT-Raman, and 13C NMR spectroscopies.8 We have
thus shown that the 1,2- versus 1,4-regioselectivity of
lithiated phenylacetonitrile toward R,â-unsaturated car-
bonyl compounds9,10 was related to the ion-pair monomer-
dimer equilibrium.
In this paper, we present the IR, FT-Raman, and 13C
NMR spectroscopic study of the species formed from
PhCH2CN with an excess (1-3 equiv) of different bases
(LHMDS, LDA, and n-BuLi) in THF, THF-hexane, and
THF-toluene (30/70 v/v) media. We have calculated the
structures and the spectra11 of most species by the B3-
The structure of the heterodimer, in which lithium
diisopropylamide (LDA) is the base, was established by
Zarges et al. by X-ray crystal analysis.6 More recently,
* To whom correspondence should be addressed. Fax: 33 (1) 49 78
11 18.
(6) Zarges, W.; Marsh, M.; Harms, K.; Boche, G. Angew. Chem., Int.
Ed. Engl. 1989, 28, 1392.
† LADIR - CNRS.
(7) Carlier, P. R.; Lucht, B. L.; Collum, D. B. J. Am. Chem. Soc.
1994, 116, 11602.
‡ Laboratoire des Carbocycles - CNRS.
(1) Marek, I.; Normant, J. F. Chem. Rev. 1996, 96, 3241.
(2) Langer, P.; Wuckelt, J.; Do¨ring, M.; Go¨rls, H. J. Org. Chem. 2000,
65, 3603.
(3) Boche, G. Angew. Chem., Int. Ed. Engl. 1989, 28, 277.
(4) Kaiser, E. M.; Solter, L. E.; Schwarz, R. A.; Beard, R. D.; Hauser,
C. R. J. Am. Chem. Soc. 1971, 93, 4237.
(5) Crowley, P. J.; Leach, M. R.; Meth-Cohn, O.; Wakefield, B. J.
Tetrahedron Lett. 1986, 27, 2909.
(8) Croisat, D.; Seyden-Penne, J.; Strzalko, T.; Wartski, L.; Corset,
J.; Froment, F. J. Org. Chem. 1992, 57, 6435.
(9) Strzalko, T.; Seyden-Penne, J.; Wartski, L.; Corset, J.; Castella`-
Ventura, M.; Froment, F. J. Org. Chem. 1998, 63, 3287.
(10) Strzalko, T.; Seyden-Penne, J.; Wartski, L.; Corset, J.; Castella`-
Ventura, M.; Froment, F. J. Org. Chem. 1998, 63, 3295.
(11) Corset, J.; Castella`-Ventura, M.; Froment, F.; Strzalko, T.;
Wartski, L. Spectrochim. Acta, Part A 2002, 58, 1971.
10.1021/jo020492t CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/12/2003
3902
J. Org. Chem. 2003, 68, 3902-3911

Study of Lithiated Phenylacetonitrile Monoanions and Dianions
FIGURE 1. Evolution of the IR spectra of a 0.25 M solution
of PhCH2CN in THF in the ν(CN) spectral region: (A) with
increasing amounts of LHMDS (time, 10 min); (B) with time
(LHMDS, 3 equiv).
FIGURE 2. IR spectra of a solution of PhCH2CN in THF-
toluene solvent mixture (30/70 v/v) at two concentrations [(a)
and (b) 0.25 M; (c) and (d) 0.12 M] and for two LHMDS
amounts [(a) and (c) 1.2 equiv; (b) and (d) 2.4 equiv] (time, 10
min). The bands marked by a star are related to (Me3Si)2NH
amine formed.
LYP method,12,13 based on density functional theory
(DFT). The calculated spectra have been compared to the
experimental spectra of the mono- and dianionic species
observed in the presence of n-BuLi.11 This comparative
study allows identification the species by their vibrational
spectra. The influence of the secondary amine (i-Pr)2NH
or (Me3Si)2NH on the monomer-dimer equilibrium will
be discussed. These results will be used in the second
part of this paper to explain the nature of the products
obtained by alkylation with benzyl chloride or by deute-
riolysis with DCl solution in D2O (4 N) of the anions
formed with different bases.
Besides the different monoanionic (monomers, dimers,
heterodimer) and dianionic species, we have observed a
THF cleavage reaction with n-BuLi leading to the forma-
tion of the acetaldehyde lithium enolate. The mechanism
of this reaction and the structure of the species have been
presented in another paper.14
This indicates the formation of a new species. Figure 1B
shows the evolution with time of the spectra of PhCH2-
CN in THF in the ν(CN) range for 3.0 equiv of LHMDS
added to the solution. The appearance of the new band
within 10 min indicates that the formation of the related
species is rather fast. To favor the formation of this new
species, we have used a less dissociating media such as
a THF-toluene solvent mixture (30/70 v/v). Figure 2
shows the IR spectra of PhCH2CN for two different
concentrations (0.25 and 0.12 M) in this solvent mixture.
With 1.2 equiv of LHMDS, only a small amount of
PhCHCNLi still remains, while (PhCHCNLi)2 is pre-
dominant, with the two coupled antisymmetric and
symmetric ν(CN) modes at 2056 and 2070 cm-1 (shoul-
der), respectively. With 2.4 equiv of LHMDS, the inten-
sity of the dimer band at 2056 cm-1 decreases, while a
band about 2070 cm-1 related to the new species appears.
No great change is observed in the spectral region of the
benzenic skeleton (Figure 2). We noticed the appearance
of a shoulder on the low wavenumber side of the (Me3-
Si)2NH amine at 1250 cm-1, this shoulder being assigned
to the excess of LHMDS (Figure 2b,d). As shown by the
X-ray structure of the heterodimer formed with LDA,6
or by the calculated structure of the (PhCHCNLi, CH3-
Li) heterodimer11 (Figure 3A), the structure of the anion
in this new entity is very close to that of (PhCHCNLi)2
or of the linear PhCHCNLi observed in [PhCHCNLi, 12-
crown-4].9,10,15 The only difference expected in the spec-
trum of the new species with respect to that of (PhCH-
CNLi)2 is the presence of only one ν(CN) mode. Thus, the
band appearing at 2072 cm-1 in THF solution (Figure 1)
is assigned to the ν(CN) vibration of the (PhCHCNLi,
Results and Discussion
(1) Vibrational Study of the Anionic Species
Formed in Solution According to the Nature and
the Concentration of the Base. (1.1) LHMDS. Figure
1A shows the evolution of the IR spectra of a 0.25 M
solution of PhCH2CN in THF in the ν(CN) spectral range
with increasing amounts of LHMDS added to the solu-
tion. With 1.2 equiv of LHMDS, the main IR bands
observed at 2090 and 2057 cm-1 are related to PhCH-
CNLi ion pair and to its dimer (PhCHCNLi)2, respec-
tively, as already described.8-10 When the base amount
increases up to 3.0 equiv, only small changes are ob-
served. The intensity of the bands at 2090 and 2057 cm-1
slightly decreases, and a new band appears at 2072 cm-1
.
(12) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(13) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter
1988, 37, 785.
(14) Corset, J.; Castella`-Ventura, M.; Froment, F.; Strzalko, T.;
Wartski, L. J. Raman Spectrosc. 2002, 33, 652.
(15) Langlotz, I. Ph.D. Thesis, Universita¨t Marburg, 1993.
J. Org. Chem, Vol. 68, No. 10, 2003 3903

Corset et al.
FIGURE 4. Evolution of the IR spectra of a 0.25 M solution
of PhCH2CN in THF in the ν(CN) spectral region with
increasing amounts of LDA (time, 2 h).
THF-toluene medium, but not in neat THF, due to the
solvation by the conjugate acid formed, as will be seen
further.
(1.2) LDA. Figure 4 shows the evolution of the IR
spectra of a 0.25 M solution of PhCH2CN in THF in the
ν(CN) spectral range with increasing amounts of LDA
added to the solution. With 1.1 equiv of LDA, PhCHCNLi
(at 2089 cm-1) predominates over (PhCHCNLi)2 (at 2057
cm-1). With an excess of LDA, the intensity of the bands
of PhCHCNLi and (PhCHCNLi)2 strongly decreases and
a band at 2070 cm-1 related to a (PhCHCNLi, LDA)
heterodimer appears (equilibrium [3], Scheme 1). Despite
the large decrease of PhCHCNLi and (PhCHCNLi)2
bands with LDA, the variation of the intensity ratio of
the two bands indicates that equilibrium [1] (Scheme 1)
is shifted to the right because of solvation of the species
by the amine, as will be seen later on. On the other hand,
with LDA, a small band around 1910 cm-1 appears. As
for LHMDS, the heterodimer formation is fast and the
equilibrium seems to be reached within 30 min. With 2.7
equiv of LDA, the heterodimer formation increases at the
expense of equilibrium [1] (Scheme 1) and a shoulder
around 1890 cm-1 appears on the low wavenumber side
of the band at 1910 cm-1. As will be seen below, these
bands are related to a small amount of dianionic species.
(1.3) n-BuLi. Figure 5 shows the evolution of the IR
spectra of a 0.25 M solution of PhCH2CN in THF-hexane
solvent mixtures with increasing amounts of n-BuLi
added to the solution. With 1.1 equiv of n-BuLi, PhCH-
CNLi and (PhCHCNLi)2 are formed. The equilibrium
between both species is shifted toward the dimer when
compared to the equilibrium observed with 1.1 equiv of
LHMDS (Figure 1A) or LDA (Figure 4) in THF. This is
due both to the lowering of the dielectric constant of the
THF-hexane medium (82/18 v/v) with respect to that of
THF and to the decrease of the THF content (solvation).
With an excess of n-BuLi (2.2-2.7 equiv), several new
bands appear. The band at 2071 cm-1 is related to the
(PhCHCNLi, n-BuLi) heterodimer (equilibrium [4], Scheme
FIGURE 3. Calculated structures of (PhCHCNLi, CH3Li)
heterodimer (A), (PhCCN)2- (B), linear (PhCCNLi)- (C),
bridged (PhCCNLi)- (D),19 and PhCCNLi2 (E) by the B3-LYP/
DFT method using the 6-31+G* basis set (see ref 11). The
hydrogen atoms are numbered as the carbon atoms to which
they are bound. νexp(CN) and νcalc(CN) are the experimental
and calculated wavenumbers of the CN bond stretching,
respectively.
LHMDS) heterodimer, taking into account the smaller
basicity of LHMDS compared to that of CH3Li. In the
THF-toluene solvent mixture, the intensity of the (Ph-
CHCNLi)2 band at 2056 cm-1 appreciably decreases and
that of the PhCHCNLi band at 2088 cm-1 slightly
increases on going from 1.2 to 2.4 equiv of LHMDS
(Figure 2).
Under the addition of an excess of LHMDS, equilibria
[2] and [3] (Scheme 1) appear. The formation of the
heterodimer (equilibrium [3], Scheme 1), evidenced by
the shoulder around 2072 cm-1, leads to the decrease of
the total PhCHCNLi and (PhCHCNLi)2 concentration.
This shifts equilibrium [1] (Scheme 1) to the left, which
decreases the dimer concentration at the benefit of the
monomer concentration. This variation is observed in
3904 J. Org. Chem., Vol. 68, No. 10, 2003

Study of Lithiated Phenylacetonitrile Monoanions and Dianions
SCHEME 1
1). But the most noticeable fact is the appearance in the
2068 cm-1 in this solvent mixture decreases at least
during 70 min, while those of the dianion bands at 1930
and 1890 cm-1 slightly increase. The intensity decrease
of the heterodimer band indicates that this species is in
the way of the dianion formation, as will be confirmed
further.
ν(CN) region of two strong bands at 1930 and 1890 cm-1
,
the intensity of which increases with the excess of base
added. Simultaneously, the intensity of the monoanionic
species bands decreases, thus showing that the formation
of the dianionic species is irreversible. In the benzenic
skeleton region, two groups of bands are observed. First,
bands appear in correlation with the disappearance of
bands related to the benzenic skeleton modes of PhCH-
CNLi, (PhCHCNLi, n-BuLi) heterodimer, and (PhCH-
CNLi)2. For instance, the bands appearing at 1576 and
1481 cm-1 correspond, respectively, to the bands at 1585
cm-1 (mode 8a) and 1488 cm-1 (mode 19a), the intensity
of which decreases. These benzenic skeleton bands,
slightly shifted, and the strong bands at 1930 and 1890
cm-1 correspond well to the vibrational modes expected
for a dianion (Figure 3), as previously shown by the
calculations.11 Second, three bands at 1608, 1314, and
1215 cm-1 (marked by an asterisk in Figure 5), the
intensities of which do not correlate with the disappear-
ance of the bands of the monoanionic species, are related
to the most intense bands of the acetaldehyde lithium
enolate formed by reaction of n-BuLi in excess with THF.
They correspond to the ν(CC), â(CH), and ν(CO) vibra-
tional modes of this enolate anion, as has been shown
recently.14
Figure 7 shows the evolution of the Raman spectra of
a 0.25 M solution of PhCH2CN in THF-hexane solvent
mixtures with increasing amounts of n-BuLi. Behaviors
similar to those of the IR spectra are expected for the
Raman spectra. Indeed, with 1.1 equiv of n-BuLi, the
ν(CN) band of PhCHCNLi at 2091 cm-1 and the sym-
metric ν(CN) band of (PhCHCNLi)2 at 2070 cm-1 are
observed. With an excess of n-BuLi, a broad ν(CN) band
at 1912 cm-1 appears, related to the dianionic species. It
is similar to the band observed at 1910 cm-1 in the IR
spectra with LDA in THF (Figure 4). Two shoulders
around 1932 and 1890 cm-1 (Figure 7) correspond to the
strong IR bands at 1930 and 1890 cm-1 (Figure 5). The
benzenic skeleton modes are shifted similarly as in the
IR spectra. The wavenumbers of the observed ν(CN) new
bands at 1912, 1890, and 1932 cm-1 are assigned to
dianionic species. Indeed, they appear in the spectral
range of the calculated ν(CN) wavenumbers of the four
dianionic entities: (PhCCN)2- free dianion, (PhCCNLi)-
monolithiated linear and bridged ion pairs, and PhC-
Figure 6 shows the evolution with time of the IR
spectra of a 0.25 M solution of PhCH2CN in THF-hexane
solvent mixture (65/35 v/v) with 2.2 equiv of n-BuLi. The
intensity of the ν(CN) bands of PhCHCNLi at 2089 cm-1
and (PhCHCNLi)2 at 2057 cm-1 no longer varies after
20 min. This indicates that the formation of the (PhCH-
CNLi, n-BuLi) heterodimer (equilibrium [4], Scheme 1)
is fast. The intensity of the heterodimer band located at
CNLi2 dilithiated species (1880-1950 cm-1 11
(Figure
)
3B-E), in good agreement with the IR absorption re-
ported for similar dianionic entities.4,16 As expected, the
structure of the CCN group in these dianionic species is
very close to that observed in ketene imine molecules
(16) Gornowicz, G. A.; West, R. J. Am. Chem. Soc. 1971, 93, 1714.
J. Org. Chem, Vol. 68, No. 10, 2003 3905

Corset et al.
FIGURE 7. Evolution of the Raman spectra of a 0.25 M
solution of PhCH2CN in THF-hexane solvent mixtures [(a)
82/18 v/v; (b) 65/35 v/v; (c) 62.5/37.5 v/v; (d) 57.5/42.5 v/v] with
increasing amounts of n-BuLi (time, 1 h). The bands marked
by a star are related to the acetaldehyde lithium enolate
formed.
FIGURE 5. Evolution of the IR spectra of a 0.25 M solution
of PhCH2CN in THF-hexane solvent mixtures [(a) 82/18 v/v;
(b) 65/35 v/v; (c) 62.5/37.5 v/v; (d) 57.5/42.5 v/v] with increasing
amounts of n-BuLi (time, 1 h). The bands marked by a star
are related to the acetaldehyde lithium enolate formed.
ity of these bands by a Fermi resonance. Calculations
indicate only one stable conformer for (PhCCN)2- free
dianion and PhCCNLi2 dilithiated species, while two
isomers are calculated for the (PhCCNLi)- dianionic
monolithiated species,19 as for the monoanionic PhCH-
CNLi. The IR band at 1910 cm-1 in THF with an excess
of LDA is related to traces of dianions, probably as
solvent-separated ion pairs (Figure 4). The wavenumbers
of the other IR and Raman bands assigned to benzenic
skeleton modes of the dianionic entities fit rather the
calculated values rather well.11 This is noticeable for the
8a and 19a ν(CC) modes, which appreciably shift on going
from the monoanionic to the dianionic species. Neverthe-
less, there is a discrepancy between the calculated ν(C2C3)
mode and the observed one, which has been discussed.11
The IR bands at 1930 and 1890 cm-1 in THF-hexane
solvent mixtures with an excess of n-BuLi correspond to
lithiated dianionic species (Figure 5). To assign these
bands, we have tried to vary the dianion concentration
on a range as large as possible. Figure 8 shows the
evolution of the IR spectra of a solution of PhCH2CN in
THF-hexane solvent mixtures (65/35 to 91/9 v/v) with
2.2 equiv of n-BuLi for decreasing concentrations of
PhCH2CN from 0.25 to 0.06 M after 1 h of reaction. As
expected, the dianion formation, proved by the intensity
of the bands at 1930 and 1890 cm-1 after 1 h, depends
on the initial concentration. For instance, it is more than
twice as high for a 0.12 M initial total concentration than
FIGURE 6. Evolution with time of the IR spectra of a 0.25
M solution of PhCH2CN in THF-hexane solvent mixture (65/
35 v/v) with 2.2 equiv of n-BuLi. The bands marked by a star
are related to the acetaldehyde lithium enolate formed. The
dotted spectrum corresponds to 1.1 equiv of n-BuLi.
such as N-p-tolyl-2,2-diphenylvinylideneamine.17 The
ν(CN) is observed around 2000 cm-1 in such ketene imine
molecules.18 Furthermore, it was impossible to find
overtones or harmonics that could explain the multiplic-
(18) Lambrecht, J.; Gambke, B.; von Seyerl, J.; Huttner, G.; Koll-
mannsberger-von Nell, G.; Herzberger, S.; Jochims, J. C. Chem. Ber.
1981, 114, 3751.
(19) We acknowledge one of the reviewers for suggesting that we
investigate more extensively the potential energy surface for the
dianionic species. With the help of more powerful computers, we have
found a bridged isomer for the (PhCCNLi)- monolithiated ion pair,
(17) Naqvi, R. R.; Wheatley, P. J. J. Chem. Soc. A 1970, 2053.
3906 J. Org. Chem., Vol. 68, No. 10, 2003
more stable than the linear isomer by ∼14 kcal mol-1
.

Study of Lithiated Phenylacetonitrile Monoanions and Dianions
FIGURE 8. Evolution of the IR spectra of a solution of PhCH2-
CN at different concentrations [(a) 0.06M; (b) 0.12 M; (c) 0.18
M; (d) 0.25 M] with 2.2 equiv of n-BuLi in THF-hexane solvent
mixture [(a) 91/9 v/v; (b) 82/18 v/v; (c) 75/25 v/v; (d) 65/35 v/v]
(time, 1 h).
FIGURE 9. Evolution with time of the IR spectra of a 0.25
M solution of PhCD2CN in THF-hexane solvent mixture (65/
35 v/v) with 2.2 equiv of n-BuLi. The dotted spectrum corre-
sponds to 1.1 equiv of n-BuLi. The bands marked by a star
are related to the acetaldehyde lithium enolate formed.
for a 0.06 M initial concentration. This indicates that the
reactive species is probably the heterodimer. Indeed, the
intensity of the monoanionic species bands at 2089, 2068,
and 2056 cm-1 shows that the equilibria of Scheme 1 are
strongly shifted toward the heterodimer formation as the
initial total concentration increases. Thus, after 1 h of
reaction, the predominant monoanionic species is the
monomer for 0.06 M, while it is the heterodimer at 0.25
M. For the dianionic species, both the decrease of the
THF proportion in the solvent mixture and the concen-
tration effect increase the association of the species, as
observed for the monoanionic species. The relative in-
tensity variation of the two bands at 1930 and 1890 cm-1
is small and is consequently in favor of their assignment
to an equilibrium between the two isomers found for
(PhCCNLi)- (Figure 3C,D) or the corresponding solvent-
separated ion pairs (PhCCNLi)-//Li+. On going from 0.06
to 0.25 M, the small intensity increase of the 1890 cm-1
band relative to the 1930 cm-1 band is due to the lowering
of the THF content in the solvent mixture. The 1930 cm-1
component is thus assigned to the linear trisolvated ion
pair (Figure 3C), and the 1890 cm-1 component is
attributed to the bridged disolvated ion pair (Figure 3D),
in excellent agreement with their calculated wavenum-
bers in the absence of solvent. We may not exclude the
appearance of a small amount of higher aggregates such
as PhCCNLi2, the absorption of which being calculated
between these two bands at 1914 cm-1 (Figure 3E).
Figure 9 shows the evolution with time of the IR
spectra of a 0.25 M solution of PhCD2CN in THF-hexane
solvent mixture (65/35 v/v) with n-BuLi. As illustrated
in Figure 9, the second proton abstraction reaction shows
a strong isotopic kinetic effect. With 1.1 equiv of n-BuLi,
the species mainly formed is PhCDCNLi. On going from
PhCHCNLi to PhCDCNLi, the ν(C2C3) mode is shifted
from 1377 to 1350 cm-1, in good agreement with the
calculated shift from 1352 to 1335 cm-1 for the pyramidal
PhCHCNLi.11 With 2.2 equiv of n-BuLi, after 10 min, the
predominant formation of the heterodimer (PhCDCNLi,
n-BuLi) characterized by the strong 2071 cm-1 band is
observed. This confirms that with n-BuLi the equilibria
of Scheme 1 are strongly shifted toward the heterodimer,
which is much more stable than the dimer. The intensity
of this band decreases much more slowly with time than
that of the corresponding hydrogenated species (see
spectrum at 20 min in Figure 6). This indicates that the
dianion formation is much slower from the (PhCDCNLi,
n-BuLi) heterodimer, which is the reactive species. As
the kinetics is much slower, the reaction of n-BuLi
involved in this heterodimer with THF leads to a larger
amount of enolate, as indicated by the large intensity
increase of the ν(CC) band at 1607 cm-1 (Figure 9). Under
these conditions, only a part of the (PhCDCNLi, n-BuLi)
heterodimer contributes to the dianion formation. After
reaction of its n-BuLi with the solvent to form (PhCD-
CNLi, CH2CHOLi) heterodimer, this last one partly
dissociates to form again PhCDCNLi and (PhCDCNLi)2,
as shown by the increase of their ν(CN) bands at 2091
and 2061 cm-1, respectively, between 10 min and 4 h
(Figure 9).
(2) 13C NMR Spectroscopic Study of the Anionic
Species Formed from PhCH2CN in Different Solu-
tions according to the Nature and the Concentra-
tion of the Base. In Table 1, the 13C chemical shifts
observed according to the nature of the solvent (THF,
THF-toluene, or THF-hexane) and of the base used
(LHMDS, LDA, or n-BuLi) are reported. As seen by IR
J. Org. Chem, Vol. 68, No. 10, 2003 3907

Corset et al.
TABLE 1. 13C Chemical Shifts of Anionic Species Derived from PhCH2CN in Function of the Nature of the Solvent and
of the Base Used
chemical shifts δa,b
solvent (v/v)
THF
THF-toluene (30/70)
THF
THF-toluene (30/70)
THF-hexane (82/18)
THF-hexane (65/35)
base (equiv)
C1
C2
C3
C7,C5
127.8
C4,C8
C6
speciesc,d
LHMDS (1.2-3.0)
LHMDS (1.2-3.0)
LDA (1.1-3.0)
LDA (1.1-2.2)
n-BuLi (1.1)
144.4
150.0
143.9e
150.5
145.4
31.9
32.0
32.0
32.0
31.8
31.9
41.9f
41.9f
149.5
148.0
149.4
148.1
149.2
149.0
118.2
119.2
118.4
119.4
118.3
118.6
-
112.1
114.3
112.4
114.5
112.4
112.9
113.6
114.1
mono, M
mono, D
mono, M
mono, D
mono, M
mono, M
di
127.9
127.8
127.8
127.1;127.5
127.3;127.6
n-BuLi (2.2)
155.7
155.5
THF-hexane (58/42)
n-BuLi (2.7)
118.8
di
a δ are reported in ppm from TMS at 20 °C. The PhCH2CN concentration is 0.25 M. b With LHMDS and LDA, δ values not significantly
varying from 1.1 to 3.0 equiv, a mean value is observed. c mono, monoanionic species; di, dianionic species. d M (monomer) and D (dimer)
represent the predominant species in the medium. e Between 1.1 and 3.0 equiv of LDA, the C1 signal is not observed at 20 °C; the δ value
at 0 °C for 2.2 equiv of base is reported. f Partly masked by a peak of n-BuLi/hexane solution.
TABLE 2. Properties of the Lithiated Bases and Their Conjugate Acids
conjugate acid
lithiated base
a
∆Gacida (kcal mol-1
)
pKa
pKH b (THF)
pKH b (DMSO)
agg statec
a
a
n-BuH
407.1
382.8
349.0
686.2
645.2
588.2
n-BuLid
LDA
LHMDS
PhCHCNLi
T, D
D
D, M
D, M
(i-Pr)2NH
(Me3Si)2NH
PhCH2CN
35.7
25.8
18.6e
26
21.9f
a Gas-phase acidity of the R′H or R2NH conjugate acid or gas-phase basicity of the corresponding anionic base expressed as ∆Gacid or
pKa, respectively. b pKH of the lithiated base in THF or in DMSO. c Aggregation state in THF: M, monomer; D, dimer; T, tetramer. The
predominant species is ain italics. d pKH of n-BuLi in benzene is 44 (see ref 45). e See ref 46. f See ref 47.
a
vibrational study, very similar results are found for
LHMDS and LDA: with LHMDS, the dianion does not
form, and with LDA, only traces are observed. The
species in equilibrium are PhCHCNLi, (PhCHCNLi)2,
and (PhCHCNLi, NR2Li) heterodimers. Their relative
proportions depend on the solvent and on the quantity
of base used. We have previously shown that in THF the
major species is PhCHCNLi, while in THF-toluene (30/
70 v/v) (PhCHCNLi)2 is predominant.8 In 13C NMR
spectra, a mean δ value was observed for monoanionic
species. This allows us to estimate a monomer and dimer
chemical shift difference of 6 ppm for C1.8 The very small
chemical shift differences observed when the base con-
centration increases from 1.1 to 3.0 equiv whatever the
solvent (Table 1) are related to the fast Li cation exchange
between species. It is thus impossible to distinguish
PhCHCNLi or (PhCHCNLi)2 from the heterodimer in
THF. The largest chemical shift difference, previously
noticed,8 observed for C1 (∆δ ∼6 ppm) on going from THF
to THF-toluene solvent mixture is also related to the
predominance of either monomers or dimers in these
media, respectively.
The addition of 2.2 equiv of n-BuLi in THF leads to
the formation of dianionic species and a small amount
of monoanionic species (Figure 5), which give rise to two
different sets of signals. Only the dianionic entities are
observed with 2.7 equiv of n-BuLi. The dianionic species
are then characterized by a ∆δ C1 of 10.1 ppm compared
to PhCHCNLi with 1.1 equiv of n-BuLi and a strong ∆δ
C2 of 10.1 ppm. The observation of two signals for the
aromatic meta carbons C5 and C7, although predicted
almost equivalent by the calculations,11 seems to indicate
a higher rotation barrier of the phenyl group in the
dianionic species. These results may be compared to those
of Bestmann et al.,20 who have been able to deprotonate
the cyanomethylenetriphenylphosphorane into a dian-
ionic species containing a C2C1N moiety as in our case.
The chemical shift of the cyanomethylenetriphenylphos-
phorane C2 is observed at -2.7 ppm,21 while the C2 of
the dianionic species appears at 6.6 ppm.20 The ∆δ C2 of
9.3 ppm thus observed compares well with the ∆δ C2 of
10.1 ppm observed in our case.
In all the 13C NMR spectra of solutions containing 2.2-
2.7 equiv of n-BuLi, two signals at about 158.5 and 83.0
ppm due to the formation of the acetaldehyde lithium
enolate are observed.14
(3) Structure and Stability of the Species (Dimers,
Heterodimers) Formed in the Presence of the Vari-
ous Lithiated Bases. (3.1) Structure and Stability
of the Species Formed according to the pKH of the
a
Lithiated Bases. We have reported in Table 2 the
properties of the lithiated bases R′Li or R2NLi used and
those of the corresponding conjugate acids R′H or R2NH.
The gas-phase acidities (or anion proton affinities) of the
parent acids R′H or R2NH measured by their free
energies22 are in the inverse order of the R′ or R2N group
polarizability: n-Bu < (i-Pr)2N < (Me3Si)2N < PhCH2-
CN. The basicity of the lithiated ion pair R′Li or R2NLi
(pKH measured in THF or DMSO23) thus decreases with
a
the R′ or R2N group polarizability. This polarizability
favors the charge delocalization in the anion and hence
decreases the neat charge on the negative carbon center
bonded to the Li+ cation, leading to a less stable and less
polar ion pair. The stability of this ion pair naturally
follows that of its dimer in THF. The more the ion pair
is polar, the more the dimer formation through dipole-
(20) Bestmann, H. J.; Schmidt, M. Angew. Chem., Int. Ed. Engl.
1987, 26, 79.
(21) Kunze, U.; Merkel, R. J. Organomet. Chem. 1983, 248, 205.
(22) Grimm, D. T.; Bartmess, J. E. J. Am. Chem. Soc. 1992, 114,
1227.
(23) Fraser, R. R.; Mansour, T. S.; Savard, S. J. Org. Chem. 1985,
50, 3232.
3908 J. Org. Chem., Vol. 68, No. 10, 2003

Study of Lithiated Phenylacetonitrile Monoanions and Dianions
TABLE 3. Comparison of the X-ray Structures of the LDA and PhCHCNLi Dimers with That of the (PhCHCNLi, LDA)
Heterodimer
bond lengths (Å)
bond angle (deg)
LiNLi
dimer or heterodimer
LiN
C1N
C1C2
C2C3
a
[LDA, THF]2
2.00(5)
2.018
b
[PhCHCNLi, TMEDA]2, C6H6
1.171
1.372
1.442
80.9
[(PhCHCNLi, LDA), (TMEDA)2]c:
LDA
2.025d
2.092d
82.7
80.0
PhCHCNLi
1.173
1.383
1.430
a See ref 48. b See ref 41. c See ref 6. d Average value of the two LiN lengths.
TABLE 4. Influence of the Presence of the Conjugate
Acid of the Base Used on the Monomer-Dimer
Equilibrium of PhCHCNLi Solution in Neat THF or
THF-Hexane Mixture
dipole interaction is favored. It is indeed well-known that
n-BuLi exists in THF as a tetramer in equilibrium with
a dimer24-26 that is at the origin of the THF cleavage.14
It has been shown by NMR that for LDA the dimer is
the exclusive species in THF solution27-29 and that this
dimer may undergo successive solvation without de-
aggregation.30 Kimura and Brown31 have reported that
LHMDS forms a dimer-monomer ion pair equilibrium
in THF. This has been confirmed by solvation studies in
THF-HMPA media.32 Our IR studies have proved that
in THF the monomer-dimer equilibrium of lithiated
phenylacetonitrile is strongly shifted toward the mono-
meric ion pair. The stability of the mixed dimeric ag-
gregate formed by the association of PhCHCNLi with a
lithiated base R′Li (2a) or R2NLi (2b) follows the same
stability order as that of the lithiated base dimer. This
is in agreement with the relative order of stability found
by vibrational spectroscopy for the different heterodimers
observed: (PhCHCNLi, n-BuLi) > (PhCHCNLi, LDA) >
(PhCHCNLi, LHMDS).
PhCH2CN
entry concentration/M
base useda/ amine addeda,b
/
c
equiv
equiv
AD/AM
1
2
3
4
5
6
7
8
9
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.37
0.33
n-BuLid/1.1
LHMDSe/1.2
n-BuLif/1.1
n-BuLif/1.1
n-BuLif/1.1
n-BuLif/1.1
LDAe/1.1
0.27
0.38
0.50
0.63
0.73
0.93
0.31
0.40
0.55
(Me3Si)2NH/1
(Me3Si)2NH/2
(Me3Si)2NH/3
LDAe/1.1
LDAe/1.1
(i-Pr)2NH/2
a The amine or conjugate acid of the base is present at the same
concentration in the medium. b The amine was added 10 min after
formation of PhCHCNLi, and the anion concentration was kept
constant. c Ratio of the IR integrated intensities of the PhCHCNLi
band around 2090 cm-1 (AM) and of the (PhCHCNLi)2 band around
2057 cm-1 (AD), after decomposition of the spectra into three
components corresponding to the bands at 2090 and 2057 cm-1
,
and to the symmetric mode of the dimer around 2070 cm-1 d After
.
evaporation of the solvents (hexane and THF) and their replace-
ment by neat THF. e In neat THF. f In THF-hexane mixture (82/
18 v/v).
It is further interesting to notice that the heterodimer
structure is not a simple addition of the structures of the
two entities involved in the dimers (Table 3). The X-ray
structure shows indeed that the NLi distance increases,
mainly for the PhCHCNLi part. This is probably due to
the charge delocalization overall the anion into the
PhCHCNLi part, due to the larger charge on the LDA
nitrogen atom. This charge delocalization is confirmed
by the C2C3 shortening of 0.01 Å, which explains the LiN
lengthening of 0.07 Å in the mixed dimeric species, the
CN bond length remaining about the same.
(3.2) Influence of the Solvation by the Amine on
the Monomer-Dimer Equilibrium. In a previous
paper,10 we had attempted to calculate the apparent
equilibrium constant of the monomer-dimer equilibrium
[1] (Scheme 1) for the lithiated phenylacetonitrile formed
in THF with 1.2 equiv of LHMDS. We had observed a
strong increase of the apparent equilibrium constant with
the decrease of the total concentration. We had explained
this variation by the appearance of a new monomeric
species when the lithiated phenylacetonitrile concentra-
tion decreases. We had then neglected the solvation of
the anionic species formed by the (SiMe3)2NH amine
present in the medium, though it is of paramount
importance, as it will be seen below. To check the
influence of the (SiMe3)2NH amine, we have measured
the ratio AD/AM of the IR integrated intensity of the
(PhCHCNLi)2 band at 2057 cm-1 (AD) to that of the
PhCHCNLi band at 2090 cm-1 (AM) for a 0.25 M PhCH2-
CN solution in THF after addition of 1.1 equiv of n-BuLi
(1.6 M in hexane) and evaporation of the solvents and
their replacement by neat THF (entry1, Table 4). The
same result has been obtained when LHMDS is used
instead of n-BuLi, after elimination of (SiMe3)2NH. When
the amine formed in the medium is not eliminated (entry
2, Table 4), AD/AM strongly increases from 0.27 to 0.38,
indicating a shift of the equilibrium toward the dimer
formation. This is confirmed by the AD/AM increase from
0.50 to 0.93 when (Me3Si)2NH is added from 0 to 3 equiv
to an anion formed with n-BuLi in a THF-hexane solvent
(82/18 v/v) (entries 3-6, Table 4).
(24) McGarrity, J. F.; Ogle, C. A. J. Am. Chem. Soc. 1985, 107, 1805
and references therein.
(25) Heinzer, J.; Oth, J. F. M.; Seebach, D. Helv. Chim. Acta 1985,
68, 1848.
(26) Keresztes, I.; Williard, P. G. J. Am. Chem. Soc. 2000, 122,
10228.
(27) Galiano-Roth, A. S.; Collum, D. B. J. Am. Chem. Soc. 1989, 111,
6772.
(28) Gilchrist, J. H.; Collum, D. B. J. Am. Chem. Soc. 1992, 114,
794.
(29) Romesberg, F. E.; Collum, D. B. J. Am. Chem. Soc. 1992, 114,
2112.
(30) Romesberg, F. E.; Gilchrist, J. H.; Harrison, A. T.; Fuller, D.
J.; Collum, D. B. J. Am. Chem. Soc. 1991, 113, 5751.
(31) Kimura, B. Y.; Brown, T. L. J. Organomet. Chem. 1971, 26,
57.
Collum et al. have largely studied the solvation of
LHMDS33-35 and LDA36-38 monomers and dimers by
(32) Romesberg, F. E.; Bernstein, M. P.; Gilchrist, J. H.; Harrison,
A. T.; Fuller, D. J.; Collum, D. B. J. Am. Chem. Soc. 1993, 115, 3475.
(33) Lucht, B. L.; Collum, D. B. J. Am. Chem. Soc. 1995, 117, 9863.
J. Org. Chem, Vol. 68, No. 10, 2003 3909

Corset et al.
SCHEME 2
ethers and amines, using cosolvents such as pentane or
toluene. They have shown that the base dimers of such
hindered amines are usually disolvated, while the mono-
mers are trisolvated rather than disolvated by ethers
(mainly by THF34). They have shown that with polyden-
tate amines35,38 and even with TMEDA37,39 or DMEDA,40
only one nitrogen is bound to each lithium atom in the
dimer. On the contrary, with the less hindered lithiated
phenylacetonitrile base, the two nitrogen atoms of one
TMEDA are bound to each lithium in the dimer, as shown
by Boche.41 We thus think that in THF, each lithium
atom of the dimer is solvated by two THF molecules
(equilibrium [1], Scheme 1). This has been confirmed by
recent calculations, although a semiempirical method has
been used for the geometry optimization.42 The monomer-
dimer equilibrium in neat THF is thus entropy driven
toward the dimer as usually observed for equilibria,33 the
enthalpy of which does not much depend on the solvent.
When a hindered amine S′ as (Me3Si)2NH or (i-Pr)2NH
is added to the medium, equilibria [5]-[7] (Scheme 2)
take place. Two THF molecules are replaced by one S′
amine molecule in the dimer and equilibria [6] and [7]
are entropy driven toward the formation of the dimer
solvated by the amine. Nevertheless, we may not exclude
the formation of some amine triTHF tetrasolvated dimer
or of some amine disolvated dimer over the THF tetra-
solvated dimer. This replacement of two THF molecules
by one amine molecule explains the large shift toward
the dimer in the presence of (Me3Si)2NH (entries 1-2 and
3-6, Table 4) or the smaller one observed with (i-Pr)2NH
(entries 7-9, Table 4). Amine (i-Pr)2NH has a more basic
nitrogen atom than (Me3Si)2NH, but is a less hindering
solvating agent,43 and may partly lead to the formation
of an amine triTHF tetrasolvated dimer. To confirm its
solvating effect, we have added to a 0.37 M PhCH2CN
solution in the presence of 1.1 equiv of LDA (entry 8,
Table 4), 2 equiv of (i-Pr)2NH (entry 9, Table 4). Taking
into account the small dilution effect by the amine added,
the shift of AD/AM from 0.40 to 0.55 indicates a large shift
of equilibria [6] and [7] (Scheme 2) toward the dimer
solvated by the amine. To take into account the solvation
by the amine S′ conjointly with the dimerization,10 the
measured dimerization equilibrium constant (Kmeas) must
be written as
Kmeas ) {CD[S]2/CM2 }
where
CD ) [(PhCHCNLi)2,4S] + [(PhCHCNLi)2,2S,S′]
and
CM ) [PhCHCNLi,3S] + [PhCHCNLi,2S,S′]
It may be expressed as
Kmeas ) {K1(1 + K7[S′]/[S]2)}/{(1 + K5[S′]/[S])}2
(34) Lucht, B. L.; Collum, D. B. J. Am. Chem. Soc. 1996, 118, 2217.
(35) Lucht, B. L.; Bernstein, M. P.; Remenar, J. F.; Collum, D. B. J.
Am. Chem. Soc. 1996, 118, 10707.
where K1, K5, and K7 are the equilibrium constants of
[1], [5], and [7] equilibria, respectively.
As [S] . [S′], Kmeas becomes
(36) Rutherford, J. L.; Collum, D. B. J. Am. Chem. Soc. 2001, 123,
199.
(37) Remenar, J. F.; Lucht, B. L.; Collum, D. B. J. Am. Chem. Soc.
1997, 119, 5567.
K
meas = K1(1 - 2K5[S′]/[S])
(38) Remenar, J. F.; Collum, D. B. J. Am. Chem. Soc. 1998, 120,
4081.
(39) Bernstein, M. P.; Collum, D. B. J. Am. Chem. Soc. 1993, 115,
789.
This explains the increase of the measured dimeriza-
tion equilibrium constant,10 when the total PhCHCNLi
(40) Lucht, B. L.; Collum, D. B. J. Am. Chem. Soc. 1996, 118, 3529.
(41) Boche, G.; Marsch, M.; Harms, K. Angew. Chem., Int. Ed. Engl.
1986, 25, 373.
(43) Graton, J.; Berthelot, M.; Laurence, C. J. Chem. Soc., Perkin
Trans. 2 2001, 2130.
(42) Carlier, P. R.; Madura, J. D. J. Org. Chem. 2002, 67, 3832.
3910 J. Org. Chem., Vol. 68, No. 10, 2003

Study of Lithiated Phenylacetonitrile Monoanions and Dianions
The LHMDS and LDA solutions in THF were prepared from
the corresponding amines, freshly distilled over CaH2, and
n-BuLi (1.6 M in hexane). The hexane and THF were evapo-
rated under argon until a white powder was obtained. To 1.2-
3.0 equiv of dry LHMDS or LDA dissolved in 4 mL of THF or
THF-toluene solvent mixture (30/70 v/v) was added 10-3 mol
of PhCH2CN. In all cases, the solutions were stirred during
30 min for 13C NMR experiments and during various times
for IR and Raman studies.
NMR Spectra. The 13C NMR spectra of anionic solutions
were recorded on a Bruker AM 250 spectrometer operating at
62.9 MHz. A 10 mm tube filled with CD2Cl2 located outside
the 8 mm tube containing the solution was used for 2H external
lock.
IR Spectra. IR spectra were measured on a Perkin-Elmer
983 spectrometer. The spectral resolution was 3 cm-1. Cells
with CaF2 windows were used, and their thickness was 0.0040
or 0.0052 cm for diluted solutions. The filling of the cells was
performed under argon in a glovebag. Other windows such as
KBr or CsI cannot be used because they react with the
carbanions. The solvent bands were subtracted from the
spectra.
Raman Spectra. Raman spectra were carried out on a
Perkin-Elmer system 2000R NIR FT-Raman spectrometer
equipped with a stabilized Nd:YAG laser emitting at 1064 nm.
The spectral resolution was 4 cm-1 in the 3600-200 cm-1
range. To obtain a good signal-to-noise, about 50-100 scans
were accumulated for each spectrum. More detailed experi-
mental conditions have been described previously.9,10 The
solvent bands were subtracted from the spectra. Because of
the disorder in liquids and solutions, the broadening of the
foot of the Rayleigh line prevents from properly observing low
wavenumber modes with small diffusion cross sections, such
as LiN modes. Their low diffusion cross section is related to
their ionic character.
concentration decreases, since the amine S′ concentration
also decreases.10
Conclusion
The formation of lithiated phenylacetonitrile from
PhCH2CN with LHMDS or LDA leads to the presence of
the corresponding amine in the medium. The presence
of the amine is of paramount importance, since it induces
a shift of the monomer-dimer equilibrium of lithiated
phenylacetonitrile toward the dimer, through a solvation
entropic effect.
In the presence of an excess of base, a heterodimer is
formed with the lithiated phenylacetonitrile. The quan-
tity of this heterodimer increases with the pKH of the
a
lithiated base. This heterodimer is the reactive interme-
diate, which leads to the formation of a dianionic species.
With LHMDS, no dianion is observed, because the pKH
a
of the base is too low to lead to a second proton
abstraction inside the heterodimer. With LDA only a
small amount of solvent-separated dianion ion pair is
present. With n-BuLi the larger part of the heterodimer
leads to lithiated dianions. A strong isotopic effect is
observed for this second deprotonation, which is much
slower for the deuterium abstraction and competes with
the reaction between n-BuLi and the THF solvent. The
absence of an appreciable concentration effect on the
relative proportion of the two dianionic species observed
at 1930 and 1890 cm-1 and comparison with the calcula-
tions suggest that these species are the two isomers of
(PhCCNLi)-//Li+, linear and bridged, respectively.
Experimental Section
JO020492T
Preparation of Anionic Solutions.8,9 THF was distilled
under argon atmosphere from sodium/benzophenone immedi-
ately before use. Toluene was distilled over CaH2. PhCH2CN
(Aldrich) was distilled prior to use. n-BuLi (1.6 M in hexane)
was a fresh Aldrich commercial solution, checked against salt
content.44 The preparation of the solutions was performed
under argon at 20 °C. To 10-3 mol of PhCH2CN dissolved in
the required amount of THF was added 1.1-2.7 equiv of
n-BuLi.
(44) Corset, J.; Froment, F.; Lautie´, M.-F.; Ratovelomanana, N.;
Seyden-Penne, J.; Strzalko, T.; Roux-Schmitt, M.-C. J. Am. Chem. Soc.
1993, 115, 1684.
(45) Elschenbroich, Ch.; Salzer, A. Organometallics. A Concise
Introduction; 2nd revised ed.; VCH: New York, 1992.
(46) Kaufman, M. J.; Gronert, S.; Bors, D. A.; Streitwieser, A., Jr.
J. Am. Chem. Soc. 1987, 109, 602.
(47) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456.
(48) Williard, P. G.; Salvino, J. M. J. Org. Chem. 1993, 58, 1.
J. Org. Chem, Vol. 68, No. 10, 2003 3911