A new reaction between E-cinnamaldehyde and phenyllithium. Mechanistic studies

Article abstract of DOI:10.1039/a904635f

The reaction of phenyllithium (in excess) with E-cinnamaldehyde gives a surprising tandem addition β-alkylation, which can be successfully used for the synthesis of substituted dihydrochalcones. The mechanism by which these unexpected reactions could occur is unknown and experimental and theoretical studies were carried out as a contribution to its understanding. The observed concentration effects, the surprising changes in the product distribution upon changes in the reaction conditions, the haptomeric structure of the calculated intermediates and the calculated activation energies are consistent with a reaction pathway in which dimeric phenyllithium attacks the E-cinnamaldehyde without prior deaggregation. Solvated structures were also calculated using a "dielectric continuum" model for solvent effects.

Full text of DOI:10.1039/a904635f

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A new reaction between E-cinnamaldehyde and phenyllithium.
Mechanistic studies
Norma S. Nudelman* and Hernan G. Schulz
Departamento de Química Orgánica, Facultad de Ciencias Exactas y Naturales,
Universidad de Buenos Aires Pab. II, P.3, Ciudad Universitaria, 1428, Buenos Aires, Argentina
Received (in Cambridge, UK) 10th June 1999, Accepted 23rd September 1999
The reaction of phenyllithium (in excess) with E-cinnamaldehyde gives a surprising tandem addition β-alkylation,
which can be successfully used for the synthesis of substituted dihydrochalcones. The mechanism by which these
unexpected reactions could occur is unknown and experimental and theoretical studies were carried out as a
contribution to its understanding.
The observed concentration effects, the surprising changes in the product distribution upon changes in the reaction
conditions, the haptomeric structure of the calculated intermediates and the calculated activation energies are
consistent with a reaction pathway in which dimeric phenyllithium attacks the E-cinnamaldehyde without prior
deaggregation. Solvated structures were also calculated using a “dielectric continuum” model for solvent effects.
(RHF) level with the MNDO18 method in the MOPAC 9719
Introduction
program, by using the lithium parameters developed by Thiel.20
Organolithium compounds show a strong tendency to form
MNDO, in spite of its well-known shortcomings,21 has proven
oligomeric aggregates in solution, which influences their
to be reliable for studying large structures of organolithium
reactivity1,2 and, in many cases, also the regio- and stereo-
compounds,22 their predictions being, in many cases, in close
chemistry of their reactions.3,4 In solution, average aggregation
agreement with experimental data resulting from X-ray and/or
numbers of several alkyl- and aryllithium compounds have
spectroscopic studies.23 MNDO tends to overestimate C–Li
been obtained from freezing-point depression measurements in
bond strengths;24 but it has been shown that the current lithium
tetrahydrofuran (THF),5 and it has been found that phenyl-
lithium has an aggregation grade of 1.6 in THF. In addition
parameters are capable of embracing aggregation and the
particular structures of π-organolithium compounds.16
NMR studies have shown dimers and monomers of phenyl-
No geometrical constraints were imposed except for the
lithium in THF, while in ether tetramers and dimers can
cinnamyl derivative, for which the π system was kept planar
coexist.6,7 In the solid state, (PhLi–Et2O)4 has been isolated,8
throughout. Due to the MNDO overestimation of steric repul-
but several attempts to isolate Lewis-base free phenyllithium
sions, the corresponding non-planar structures were found to
have been unsuccessful; the full details of the solid-state struc-
be somewhat lower in energy. Stationary points were located by
use of the EF gradient optimization procedure and the energies
ture of PhLi remain unavailable.9
The special aggregation features of several organolithium
were obtained at the more rigorous criteria of the keyword
compounds can be conveniently used to direct their reactions
PRECISE, by minimization of the gradient norm at least below
towards the desired synthetic goal, in cases where different
0.05 kcal ÅϪ1 degϪ1
.
products would otherwise be obtained.2 We have recently
observed that the carbonylation of lithium amides10 and the
reaction of organolithiums with α,β-unsaturated carbonyl
compounds11 are good examples of this particular behavior.
The reaction of phenyllithium with E-cinnamaldehyde, 1, gives,
as expected, E-1,3-diphenylprop-2-en-1-ol in excellent yield
when it is carried out with equimolar amounts of PhLi and 1.12
However, we have recently found that when [PhLi]:[1] is ≥2
another reaction takes place that can be successfully used for
the synthesis of β-substituted dihydrochalcones.11
Atomic charges were obtained using the Mulliken scheme.
Each reported heat of formation is the result of a search for the
global minimum starting from several different initial geom-
etries. All the stationary points were characterized as minima or
saddle points by calculating and diagonalizing the Hessian
matrix and by checking the number of negative eigenvalues.25
Results and discussion
Experimental studies
In trying to find out appropriate explanations for the
unexpected products, we decided to carry out mechanistic
studies on the reaction, from experimental and theoretical
viewpoints. Initial complexation between the organolithium
and the carbonyl compound has been observed in previous
work,13 and has been proposed as the first step in additions to
ketones and aldehydes.14 The present work concentrates on the
further reaction steps; the MNDO semiempirical SCF–MO
method15 was chosen as the most appropriate to carry out the
study with hapticity,16 a considerably important structural
property of these types of π-organolithium compounds.17
The reaction of PhLi with E-cinnamaldehyde, 1, in THF at 0 ЊC
and protected from light, gives the addition product, 2, almost
exclusively; but, when the reaction conditions are slightly
changed, other compounds 3–5 are detected in minor amounts
(eqn. (1)).
We designed a few experiments to clarify the mechanistic
details. The influence of light (at 0 ЊC) and of the temperature is
shown in Table 1. It was observed that using equimolar
amounts of reactants in relatively low concentrations, 5 was not
detected at all and the distribution of products 2–4 was strongly
dependent on the reaction conditions.
Higher yields of side products 3 and 4 were observed when
the reaction was carried out unprotected from light. Under
these conditions, the relative yield of 2 decreases considerably.
Method of calculation
Calculations were performed at the restricted Hartree–Fock
J. Chem. Soc., Perkin Trans. 2, 1999, 2761–2767
This journal is © The Royal Society of Chemistry 1999
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Table 1 Addition of PhLi to E-cinnamaldehyde, 1,a in THF at several
The sensitivity to light suggests that radical processes might be
temperatures
involved, especially in those pathways leading to by-products 3
and 4. According to these results, alternative reaction mechan-
isms can be proposed (see Scheme 1):
Yields (%)b
a) Compound 2 can be formed by the classical polar path-
way; and/or by an initial electron transfer from PhLi to 1 giving
the radical anion–radical cation pair intermediate (I), which
by reaction within the cage would produce II, the anion pre-
cursor of 2.
b) A SET mechanism between II and 1 would also explain the
production of 3 and 4. According to this type of Meerwein–
Pondorf–Verley mechanism,26 adduct II could react with 1 giv-
ing the radical anion III and the radical IV. H transfer from IV
to III would yield 3 and 4. Light stimulates electron transfer,
and when the reaction is carried out unprotected from light the
yields of 3 and 4 increase (see Table 1).
T/ЊC
Ϫ78
2
3
4
79
88
94
70
95
73
10
9
0
16
0
8
11
3
6
14
5
19
Ϫ20
0
0c
20
20c
a [PhLi]0 = [1]0 = 0.07 M. Reaction time 3 h. b The yields represent per-
cent conversion. c These reactions were carried out unprotected from
light.
Alternatively, a polar way for the formation of 4 from II
(pathway c); and a solvent H abstraction by the radical anion I
for the formation of 3 (pathway d) could be proposed. Since
pathway b requires an excess of 1, and equimolar yields of 3
and 4, pathways c–d seem more probable for the formation of 4
and 3, respectively.
To test the proposed mechanisms, the effect of adding radical
traps at the beginning of the reaction was examined. Hydro-
quinone and quinhydrone have previously been found to be
effective radical traps in the reaction of naphthyllithium with
CO,27 and results given in Table 2 show that in both cases, only 2
was found.28
An additional experiment was carried out with the reaction
mixture unprotected from light at Ϫ20 ЊC, followed by quench-
ing with D2O. In the recovered product 3, the α carbon was not
deuterated, only the OD group attached to C(2). This could
indicate that the reaction involves a radical intermediate that
abstracts H from the solvent before quenching. Next we
Scheme 1 Alternative pathways in the reactions of PhLi with E-cinnamaldehyde, 1.
2762
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Table 2 Effect of the addition of radical traps in the reaction of PhLi
with E-cinnamaldehyde, 1,a in THF at 0 ЊCb
Yields (%)c
Recovered 1
(%)
Additive
2
3
4
5
None
Hydroquinone
Quinohydrone
88
53
26
0
0
0
11
0
0
1
0
0
0
47
63
a [PhLi]0 = [1]0 = 0.2 M. Reaction time 3 h. b The reactions were carried
out unprotected from light. c Yields represent percent conversion.
Table 3 Addition of PhLi to E-cinnamaldehyde, 1,a in several solvents
at 0 ЊCb
Scheme 2 Reaction of PhLi with E-cinnamaldehyde, 1, in [PhLi]:
[1] = 2.
Yields (%)c
Solvent
2
3
4
5
mediate, VI, which would be the precursor of compound 5.
Since the production of 5 increases with longer reaction times
and room temperature, this could indicate that the precursor to
5 is formed consecutively to the first intermediate leading to 2.
THF
Toluene
Ether
94
78
59
0
0
6
0
8
2
14
17
22
a [PhLi]0 = [1]0 = 0.07 M. Reaction times 3 h. b All the reactions were
carried out protected from light. c The yields represent percent
conversion.
Structure of the intermediates. The optimized structure of
dimer PhLi was obtained from MNDO calculations giving
a
planar arrangement, with two planar tetracoordinate
RRCLi2 ipso carbons as previously described.35 The calculated
stabilization energy due to dimerization is high, Ϫ72.8 kcal
molϪ1. Selected geometrical parameters for the calculated
intermediates and the transition state are shown in Table 4.
Intermediate V. The first reaction intermediate has a C1
symmetry. The optimized geometry is depicted in Fig. 1. Both
lithium atoms are bonded to the carbonyl oxygen; they are also
bonded to the C(8) (C ipso). Interestingly Li(7) is η2 interacting
with the C(3)–C(4) π bond, resulting in a tetracoordinate array.
The bond distance between C(3) and C(4), and the fact that the
hydrogens bonded to both carbon atoms remain almost in the
same plane, indicates the sp2 nature of these carbons. The dis-
tance between C(2) and C(3) corresponds to a single bond
length (see Table 4).
attempted to shed light on the mechanism of formation of
compound 5, which seems to be the more interesting feature in
the present study.
The mechanism proposed in Scheme 1 for the 1,2-addition
and for the formation of by-products 3 and 4 can be thought of
as occurring with PhLi as monomer or dimer; it is likely that the
dimer reacts also without prior deaggregation, as has been
observed in other cases.29 The propensity of organolithium
compounds for association in solution and in the solid state is
very well known;4,30 PhLi exists as a dimer–monomer equi-
librium in THF solution.31 When the reaction was carried out in
other solvents in which the aggregation state of PhLi is higher,
such as ether and toluene, the yields of 3 and 4 increased, and 5
was also obtained (see Table 3).
The influence of the [PhLi]:[1] ratio was then investigated.
When the reaction was carried out in a ratio [PhLi]:[1] = 2 at
20 ЊC and 7 h reaction time, a brilliant deep violet solution was
formed, 2 was no longer obtained, and 5 was formed in 97%
yield. The reaction pathway seems to change drastically, 3 was
not detected and the yield of 4 largely decreased ([2] = [3] = 0%,
[4] = 3%, [5] = 97%). The fact that 5 could also be detected when
the reaction was carried out in 1:1 ratio in more concentrated
solutions (Table 2) seems to confirm that the formation of 5
occurs without deaggregation of PhLi.
C(2) has sp3 character and is the most positive carbon in this
intermediate. Its positive charge even increases during the reac-
tion, especially when the keto form is reached in intermediate
VI. C(4) has only a poor carbanion character, but surprisingly
its negative charge is strongly increased while the lithium atom
Li(7) moves towards it, to form the intermediate VI; at the same
time, the light negative charge of C(3) becomes positive, when
the π interaction with Li(7) is lost (see Table 5).
Transition state. The transition state between V and VI was
located and optimized and its structure is shown in Fig. 2. By
frequency analysis the stationary state was characterized with
only one negative vibrational frequency whose magnitude is
Ϫ1556 cmϪ1, which coincides with the reaction coordinate.
H(10), at first bonded to C(2), increases its negative charge,
having a hydride character in the transition state, suggesting
that a 1,2-hydride transfer might occur.
Semiempirical studies
We carried out a theoretical study of the above-mentioned
unusual reaction between PhLi and E-cinnamaldehyde, 1, in
2:1 ratio. The most relevant part of this plan includes the
MNDO study32 of the probable intermediates in the form-
ation of 5 “in vacuo” and also in a “dielectric continuum”
(COSMO)33 to model solvated structures. We calculated differ-
ent pathways that could lead to the formation of 5 from 1 and
PhLi; the reaction pathway briefly depicted in Scheme 2 was
found to be the most energetically favorable.34
Concomitantly, C(3) and C(4) deviate from planarity; Li(7),
which is closer to C(8) than Li(9) (see Table 4), is interacting
with C(4) and with C(5) and C(6) (the ipso and ortho carbons of
the phenyl group);36 and Li(9) is also interacting with C(4), even
though it is attached to the oxygen. In this structure, the C(2)–
C(3) bond length is shorter than in the intermediates.
The reaction is formulated as occurring with dimeric
PhLi, taking into account the concentration effect and the
known structure of phenyllithium in tetrahydrofuran solution.5
Coordination to 1 through the lithium atom without deaggre-
gation is proposed, followed by the addition of a phenyl moiety,
probably through a SET pathway. Coordination of two lithium
atoms to the carbonyl oxygen gives the intermediate V, which
then could undergo a [1,2]-hydride shift giving a new inter-
Even though the 1,2-hydride shift is orbital symmetry
forbidden, the presence of a second phenyllithium is apparently
sufficient to catalyze it.37 Other examples of 1,2-hydride trans-
fer have been previously reported.38
Intermediate VI. In this intermediate (see Fig. 3), the bond
distance between C(3) and C(4) is close to a single bond, as
suggested from the transition state, in which the bond is elon-
gated with regard to species V (see Table 4). C(2) and O(1)
J. Chem. Soc., Perkin Trans. 2, 1999, 2761–2767
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Fig. 1 Optimized structure for the intermediate V, as obtained from
MNDO calculations. Atoms numbered 7 and 9 are lithiums and atom 1
is oxygen.
Fig. 2 Optimized structure for the transition state (TS), as obtained
from MNDO calculations.
have recovered the carbonyl character (see Tables 4 and 5). C(4)
carries a high negative charge, becoming the most negative
atom in the molecule; with both Li strongly bonded to it, Li(9)
remains interacting with the carbonyl oxygen. The carbanion
character of C(4) is very useful for synthetic purposes; we have
recently shown the convenience of this intermediate for the syn-
thesis of substituted dihydrochalcones.11 Li(7) is no longer
interacting with O(1), but it is bonded to the aryl carbons C(5)
and C(6), in a η3 haptomeric structure (strongest interaction
with C(4), see Fig. 3). The charges of both lithium atoms differ
considerably, Li(7) is the most positive making the Li(7)–C(4)
bond quite ionic. Finally, it can be observed that VI is found
preferably in a ketone form.
Scheme 3 gives the reaction energies (kcal molϪ1) for the
transformations. Strong stabilization of VI with respect to V
can be observed, due probably to the greater coordination of
lithium atoms. Even though the detailed reaction energy for
the conversion of V to VI is favorable, since the activation
energy for this transformation is rather high (34.5 kcal
molϪ1), we decided to make calculations including solvent
effects.
Solvation of the intermediates. To examine the solvent effect
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Table 5 MNDO net atomic charges (q) for selected atoms from dimer PhLi and reaction intermediates
Species
O(1)
C(2)
C(3)
C(4)
C(5)
C(6)
Li(7)
C(8)
Li(9)
H(10)
(PhLi)2
V
—
—
—
Ϫ0.06
0.15
—
—
Ϫ0.05
0.03
—
0.35
0.30
0.30
0.29
Ϫ0.27
Ϫ0.27
Ϫ0.28
Ϫ0.26
0.35
0.41
0.34
0.27
—
Ϫ0.56
Ϫ0.47
Ϫ0.26
0.31
0.33
0.32
Ϫ0.07
Ϫ0.37
Ϫ0.33
Ϫ0.04
Ϫ0.11
Ϫ0.12
Ϫ0.03
Ϫ0.14
0.00
TS
VI
0.03
0.03
mediate: these results are fully consistent with the carbanionic
character of C(4) in VI.
Further experimental results
Efforts were made to determine the possible existence of such
intermediates (see Scheme 4). When the reaction mixture is
Scheme 4 Quenching of the reaction mixture ([PhLi]:[1] = 2) with
different electrophiles.
Fig. 3 Optimized structure for the intermediate VI, as obtained from
MNDO calculations.
quenched with D2O, the β-carbon of the resulting dihydrochal-
cone is deuterated, which is consistent with the mechanism
depicted in Scheme 2 and the precursor VI proposed. Sim-
ilarly, when an alkyl halide RX is added to the reaction flask
before hydrolysis, high yields of β-substituted dihydrochalcones
are obtained, thus affording a very convenient method for the
synthesis of these compounds.11 On the other hand, independ-
ent treatment of 2 with 2 equivalents of PhLi gives 5 as the
main product.42 By treatment of the reaction mixture with 3
equivalents of trimethylchlorosilane, only silylation at the β-
carbon was observed, consistent with the less anionic character
of the oxygen atom in VI.
Scheme 3 MNDO energy change (kcal molϪ1) for the formation of
dihydrochalcone, 5, from the reaction intermediate V. Darkened square
shows the solvated energy change (COSMO).
The above mentioned results are fully consistent with the
proposed pathway for the conversion of 1 into 5; in particular,
they are convincing evidence for the existence of a lithium atom
bonded to the β-carbon. The rearrangement produced in the
reaction of 2 with PhLi, similar to that observed in a reaction
with [PhLi]:[1] = 2, indicates the likely structure of the
intermediate VI. Recent works describe the reactions of some
cinnamyl derivatives with alkyllithiums in different solvents and
in the presence of (Ϫ)-sparteine, but they constitute different
reactions since under those conditions only carbolithiation
takes place and α-substituted compounds are obtained.43
several calculations using discrete solvation39 and dielectric
continuum solvation were carried out. Discrete solvation using
dimethyl ether molecules bonded to the lithium atoms as a
working model did not account for the expected stabilization
of the intermediates, since the structures remained almost
unvaried.
Several continuum solvation models have been used suc-
cessfully for calculations in which solvation effects play a
fundamental role.40 For the present case, the COSMO32
model accounted reasonably well for the solvation effects
observed. This model was successfully used by Saá and co-
workers in related calculations of lithium compounds.17c The
results of minimizations using COSMO for intermediates V,
VI and the transition state are depicted in Table 6. The rela-
tive permittivity of THF (ε = 7.52) was used in this approxim-
ation.41
It can be seen that the relative energies of the intermediates
have changed drastically. The reaction energy has a value of
Ϫ29.7 kcal molϪ1 (see Scheme 3), and the activation enthalpy
decreases its magnitude in almost 11.5 kcal with respect to the
unsolvated form. The structures of the intermediates change
notably. The interaction of lithium atoms with the carbons
becomes weaker, and the hapticity in the intermediates is lower,
particularly in V. The bond distances Li(7)–C(3) and Li(7)–
C(4) are longer and Li(7) is located closer to O(1) in this inter-
Conclusions
The experimental and theoretical studies carried out on the
reaction of excess of PhLi with E-cinnamaldehyde, 1, under
conditions that allowed complete conversion of 1 into dihydro-
chalcone, 5, suggest a reaction pathway in which dimeric PhLi
adds to the carbonyl group, giving an intermediate, V, which
undergoes lithium attack on the β-carbon and a [1,2]-hydride
shift producing the precursor of 5, VI. The structures of the
likely intermediates involved in the transformation of 1 to 5
were located and minimized. The transition state for the con-
version of intermediate V to VI was also located and supports
the proposal.
This work sheds light on the mechanism of a very interesting
J. Chem. Soc., Perkin Trans. 2, 1999, 2761–2767
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reaction which has wide synthetic utility for the production of
substituted dihydrochalcones in a one step–one pot process.
MNDO calculations prove to be useful for the prediction of
relative trends of this type of organolithium reaction, when the
size of the molecular system precludes an ab initio calculation.
General reaction procedure
1 mL of a 1 M solution of PhLi in the required dry solvent,
contained in a septum-capped round-bottomed reaction flask
under nitrogen atmosphere, was cooled to the desired temper-
ature. Then 14 mL of the same solvent and 132 mg (1 mmol) of
(E)-cinnamaldehyde were added to the stirred solution all at
once; by this procedure, both reagents have a final concen-
tration of 0.07 M. Reactions protected or unprotected from
light were carried out. The reaction mixtures were worked up by
treatment with 0.5 mL of NH4Cl saturated solution.
Reaction in an excess of PhLi. 3 mL of 1 M PhLi in
anhydrous THF were placed in a septum-capped round-
bottomed reaction flask, at 20 ЊC under nitrogen atmosphere.
12 mL of THF and 132 mg (1 mmol) of E-cinnamaldehyde
were added at once to the stirred solution. The quenching was
carried out after 7 h, 1 mmol of the electrophile (RX, D2O) was
added; 3 equivalents were used in the case of TMSCl. The solu-
tion was allowed to stir until decoloration, and treated with 0.5
mL of NH4Cl saturated solution.
Reactions in the presence of radical inhibitors. The reaction
was carried out similarly to the general procedure already
described, in a reaction flask containing an equimolar amount
of the radical inhibitor.
Acknowledgements
H. S. is a grateful recipient of a fellowship from the Univer-
sidad de Buenos Aires (Argentina). Financial support from the
Universidad de Buenos Aires, the Agency for the Promotion of
Science and Technology and the National Research Council
(CONICET) of Argentina, and from the European Community
is gratefully acknowledged.
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24 R. Glaser and A. Streitwieser, J. Mol. Struct. (THEOCHEM),
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25 J. W. McIver and A. Komornicki, J. Am. Chem. Soc., 1972, 94, 2625.
26 (a) E. C. Ashby, Tetrahedron Lett., 1982, 23, 2273; (b) A.-C.
Malmvik, U. Obenius and U. Henriksson, J. Chem. Soc., Perkin
Trans. 2, 1986, 1905.
27 (a) N. S. Nudelman, F. Doctorovich, G. Garcia Liñares, H. Schulz
and S. Mendiara, Gazz. Chim. Ital., 1996, 126, 19; (b) N. S.
Nudelman and F. Doctorovich, J. Chem. Soc., Perkin Trans. 2, 1994,
1233; (c) F. Doctorovich and N. S. Nudelman, Magn. Reson. Chem.,
1990, 28, 576.
28 The reactions were carried out in more concentrated solutions,
[PhLi] = [1] = 0.2 M, and under these conditions the existence of 5
can be appreciated.
29 J. F. McGarrity and C. A. Ogle, J. Am. Chem. Soc., 1984, 107, 1805.
30 (a) T. Koizumi, K. Morihashi and O. Kikuchi, Bull. Chem. Soc. Jpn.,
1996, 69, 305; (b) T. Koizumi and O. Kikuchi, Organometallics,
1995, 14, 987; (c) D. Seebach, Angew. Chem., Int. Ed. Engl., 1990, 29,
1320; (d) E. Kauffman and P. v. R. Schleyer, J. Am. Chem. Soc.,
1985, 107, 5560.
31 W. Bauer, W. R. Winchester and P. v. R. Schleyer, Organometallics,
1987, 6, 2371.
32 We carried out the semiempirical studies with MNDO and PM3.
Both methods gave us similar geometrical results but dissimilar
relative energies. The previous successful use of MNDO for
calculations of very closely related structures (cinnamyl and aryl
derivatives, see references 16(b) and (c)) led us to take the decision to
present only the MNDO results.
33 COSMO (conductor-like screening model) evaluates the solvent
screening energy for a cavity based on the solvent-accessible surfaces
and for a charge distribution derived from a distributed multipole
analysis: A. Klamt and G. Schüürmann, J. Chem. Soc., Perkin Trans.
2, 1993, 799.
34 The calculated alternative pathways were: (a) attack of monomeric
PhLi and similar rearrangement with a [1,2]-hydride shift; (b)
previous deaggregation of dimer PhLi and attack followed by the
[1,2]-hydride rearrangement; (c) addition of PhLi followed by attack
of a second lithium with [1,2]-hydride shift and (d) attack of dimer
PhLi with H shift from the carbonyl C to O. Options (a) and (b)
seem attractive but the energetic changes in these options were
unfavorable.
35 E. Kauffmann, J. Gose and P. v. R. Schleyer, Organometallics, 1989,
8, 2577.
36 This η3 arrangement, with the lithium atom coordinately bonded to
the benzylic C, C ipso and C ortho, is also observed in the X-ray
structure of benzyllithium, in S. P. Patterman, I. L. Karle and
G. D. Stucky, J. Am. Chem. Soc., 1970, 92, 1150.
37 We thank one of the referees for this comment.
38 J. March, Advanced Organic Chemistry, Reactions, Mechanisms and
Structure, 4th edn., Wiley-Interscience, New York, 1992, ch. 18.
39 For a number of previous calculations in which discrete solvation
was used see: (a) J. F. Remenar, B. L. Lucht and D. B. Collum, J.
Am. Chem. Soc., 1997, 119, 5567; (b) A. Abbotto, A. Streitwieser
and P. v. R. Schleyer, J. Am. Chem. Soc., 1997, 119, 11255; (c) F. E.
Romesberg and D. B. Collum, J. Am. Chem. Soc., 1992, 114, 2112;
(d) refs. 17(b), 21(d).
40 Other “dielectric continuum” models are: (a) G. P. Ford and
B. Wang, J. Am. Chem. Soc., 1992, 114, 10563; (b) C. J. Cramer and
D. G. Truhlar, J. Am. Chem. Soc., 1991, 113, 8305; (c) H. S. Rzepa,
M. M. Yi, M. M. Karelson and M. C. Zerner, J. Chem. Soc., Perkin
Trans. 2, 1991, 635; (d) A. R. Katritzky and M. J. Karelson, J. Am.
Chem. Soc., 1991, 113, 1561.
41 CRC Handbook of Chemistry and Physics, ed. David R. Lide, CRC
Boca Raton, New York, 78th edn., 1997.
42 The reaction of 2 equivalents of PhLi with 2, at 20 ЊC in THF gives
only 79% of 5 and 21% of 4.
43 (a) S. Klein, I. Marek, J. F. Poisson and J. F. Normant, J. Am. Chem.
Soc., 1995, 117, 8853; (b) S. Norsikian, I. Marek, J. F. Poisson and
J. F. Normant, J. Org. Chem., 1997, 62, 4898.
Paper 9/04635F
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