Determination of the effect of cation-π interactions on the stability of α-oxy-organolithium compounds

Article abstract of DOI:10.1021/jo801176d

(Chemical Equation Presented) Sn-Li exchange equilibria have allowed the quantification of the stabilizing effect of cation-π interactions in organolithium chemistry. Stabilization energy data on the effect of Li-π complexation of an aromatic ring or a C=C double bond in organolithium compounds are presented. The amount of stabilization gained by complexation of the Li atom with a π system in α-oxy-organolithium compounds is quite comparable to the one observed in systems containing Li-N or Li-O interactions.

Full text of DOI:10.1021/jo801176d

cation-π interaction is fundamentally an electrostatic interaction
between a positively charged species (a cation) and the electrons
that make up one or more π bonds. Theoretical investigations8
and experimental evidence9 for cation-π interactions are well-
documented and place it among the strongest of noncovalent
binding forces. From a synthetic point of view, cation-π
interactions have been occasionally proposed to explain the
stereochemical outcome of several organic transformations.10
For instance, five-membered rings have been prepared in a very
stereoselective manner by anionic cyclizations of different
alkenyllithiums.11 The stereoselectivity of this reaction is a
consequence of an energetically favorable coordination of the
lithium atom with the remote π bond. More recent applications12
of cation-π interactions include their use as a conformation-
controlling tool13 in a variety of regio- and stereoselective
syntheses, as well as in the design of chiral metal catalysts for
enantioselective Diels-Alder and Mukaiyama-Michael reac-
tions.14
Determination of the Effect of Cation-π
Interactions on the Stability of
r-Oxy-Organolithium Compounds
Pablo Monje,† M. Rita Paleo,† Luis Garc´ıa-R´ıo,‡ and
F. Javier Sardina*,†
Departamento de Qu´ımica Orga´nica and Departamento de
Qu´ımica F´ısica, UniVersidad de Santiago de Compostela,
15782 Santiago de Compostela, Spain
jaVier.sardina@usc.es
ReceiVed May 30, 2008
We have recently reported15 the measurement of the relative
stabilities of R-heterosubstituted benzylic organolithium com-
pounds and secondary R-oxy-organolithium compounds in THF,
where the effects of alkyl substituents and Li-O and Li-N
chelation on carbanion stability were quantified. Our approach
is based on the establishment of a Sn-Li exchange between
the organolithium under study and a reference compound (as
shown in eq 1 in Figure 1), an equilibrium reaction which favors
the pairing of the most stable carbanion with the more
electropositive Li atom.16,17
Sn-Li exchange equilibria have allowed the quantification
of the stabilizing effect of cation-π interactions in organo-
lithium chemistry. Stabilization energy data on the effect of
Li-π complexation of an aromatic ring or a CdC double
bond in organolithium compounds are presented. The amount
of stabilization gained by complexation of the Li atom with
a π system in R-oxy-organolithium compounds is quite
comparable to the one observed in systems containing Li-N
or Li-O interactions.
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Noncovalent interactions involving unsaturated systems1 play
a dominant role in many forefront areas of modern chemistry2
and molecular biology.3 Together with other attractive interac-
tions, cation-π interactions4 are playing a key role in both
chemical and biological recognition,1,5 the structural and
functional properties of proteins,6 and enzymatic catalysis.7 The
† Departamento de Qu´ımica Orga´nica.
‡ Departamento de Qu´ımica F´ısica.
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7394 J. Org. Chem. 2008, 73, 7394–7397
10.1021/jo801176d CCC: $40.75 2008 American Chemical Society
Published on Web 08/07/2008

SCHEME 2. Relative Stability Data of
r-Oxy-Organolithium Compounds 9-12b
FIGURE 1. Sn-Li exchange equilibrium and reference compounds
used.
SCHEME 1. Effect of Cation-π Interaction on
r-Oxy-Organolithium Stability
forward and reverse reactions was obtained. The data obtained
from the tin-lithium equilibration with the reference compounds
are displayed in Scheme 1 (the reference level of 0.0 kcal mol-1
was assigned to 9-xanthenyllithium, the most stable reference
compound used in our previous studies).15 These data clearly
show the presence of sizable stabilizing effects when the phenyl
group is located two or three carbon atoms away from the Li,
when compared with compounds possessing the phenyl group
at a greater distance or an alkyl side chain15b (∆∆Geq ) 1.5-2.2
kcal mol-1, compare entries 1 and 2 with 4-6 of Scheme 1).
This stabilization can be explained by the formation of a pseudo-
four- (3b) or pseudo-five-membered ring chelate (4b) by
interaction of the Li cation with the aromatic ring. Apparently,
cation-π stabilizing interactions through the formation of
pseudo-six-membered and larger rings are not taking place since
these systems show the same relative stabilities as those
possessing a purely alkyl side chain (compound 8) (∆∆Geq
)
0-0.2 kcal mol-1, compare entries 4-6 with entry 7, Scheme
1).
Pb-Li exchange equilibrium15b was showed to be a useful
alternative to Sn-Li for the determination of relative stabilities
of organolithium compounds, especially for those more sterically
hindered. We decided to determine the relative stability of
compound 4 by using both Sn-Li and Pb-Li exchange
methodologies, and we were pleased to see that the same value
for its relative stability was obtained (see entries 2 and 3,
Scheme 1).
The values for the observed Keq are temperature and
concentration independent, as the same value was obtained when
the equilibrium constant for 4b was measured at different
temperatures (-35, -50, and -65 °C) and concentrations (0.02,
0.1, and 0.2 M), which suggest that these compounds have
definite aggregation states under the experimental conditions
used. We15b and others18 have established that O-carbamoyl-
organolithiums are monomers in THF solution at low temper-
ature based on NMR data.
To check the general validity of these results, we have also
prepared a similar set of stannanes and organolithium com-
pounds now containing an alkene substituent instead of a phenyl
ring (9-12) and studied their Sn-Li exchange equilibria with
reference compounds of known stability (1 and 2). The results
obtained are collected in Scheme 2 and indicate that these
carbanions are displaying a behavior similar to their phenyl-
substituted counterparts: when a pseudo-four- or pseudo-five-
membered ring chelate can be established by interaction between
the lithium cation and the CdC π-system, the corresponding
Since quantification of the thermodynamic effect of cation-π
interactions on simple organic systems would put the develop-
ment of new synthetic uses for them on firmer ground, we
decided to apply our methodology for determining the relative
stabilities of organolithium compounds to establish if cation-π
interactions can exert a significant stabilization on organolithi-
ums possessing an unsaturated system on the side chain. We
report herein the application of this Sn-Li exchange equilibrium
methodology to the quantification of the cation-π interaction
effects on the stability of phenyl- and vinyl-substituted
organolithiums.
We have prepared a series of R-oxy-organolithium com-
pounds with different phenyl-group-containing side chains from
the appropriate stannanes and measured their relative stabilities.
To set up the tin-lithium exchange equilibria, equimolar
amounts of stannanes 3-7a and organolithium reagents 1-2b
(obtained from the corresponding stannanes 1-2a and 100 mol
% of BuLi) were mixed and kept for the time and at the
appropriate temperature as indicated in Scheme 1. The equi-
librium concentrations were determined after low-temperature
quenching of the reactions with methanol and quantitative
analysis of the resulting mixtures of hydrocarbons 3-7c (arising
from the protonation of the corresponding organolithium species
present in the equilibria) and stannanes 3-7a by 1H NMR. The
reverse reaction (organolithium reagents 3-7b and stannanes
1-2a) was performed in each case to establish that the
equilibrium had been reached. The conditions for each particular
reaction were optimized until the same Keq value from the
(18) Boche, G.; Bosold, F.; Lohrenz, J. C. W.; Opel, A.; Zulauf, P. Chem.
Ber. 1993, 126, 1873–1885.
J. Org. Chem. Vol. 73, No. 18, 2008 7395

organolithium is 1.3-1.7 kcal mol-1 more stable than when
the side chain does not possess an unsaturated substituent or
when the CdC moiety is separated by more than two methylene
groups from the negatively charged carbon atom (compare
entries 1 and 2 and 3 and 4, Scheme 2). As was the case with
the phenyl-substituted compounds, the possibility of forming a
pseudo-six-membered ring chelate gave no appreciable stabi-
lization to the organolithium, as the stabilization observed in
this case is similar to that of 12b (∆∆Geq ) 0.2 kcal mol,-1
compare entries 3 and 4, Scheme 2) or 8b15b (∆∆Geq ) 0 kcal
mol,-1 compare entry 3, Scheme 2 with entry 7, Scheme 1).
To the best of our knowledge, these results can be considered
the first quantitative evidence of the importance of the cation-π
interactions in organolithium chemistry. It is interesting to note
that the amount of stabilization gained by complexation of the
Li atom with a π-system is quite comparable to the one shown
by systems containing Li-N or Li-O interactions.19,20
3-Phenyl-1-(trimethylstannanyl)propyl 2,2,4,4-tetramethylox-
azolidine-3-carboxylate (4a). Following the general procedure, 4c
(1.0 g, 3.43 mmol) afforded 4a as a pale yellow oil (1.52 g, 98%):
1H NMR (300 MHz, CDCl3, rotamers) δ 7.29 (m, 5H), 4.53 (m,
1H), 3.71 (s, 2H), 2.70 (m, 2H), 2.13 (m, 2H), 1.54 (s, 6H), 1.39
(s, 6H), 0.10 (s, J(117,119Sn-1H) ) 52.5, 9H); 13C NMR (75 MHz,
CDCl3, rotamers) δ 153.2/152.5, 141.6, 128.3, 128.2, 125.8, 95.6/
94.5, 76.1/76.0, 71.2 (J(117Sn-13C) ) 423.3, J(119Sn-13C) ) 442.9),
60.3/59.4, 35.9, 34.3 (J(117,119Sn-13C) ) 38.5), 26.6/26.5, 25.2, 25.3/
25.1, 24.1/24.0, -9.1 (J(117Sn-13C) ) 318.1, J(119Sn-13C) ) 333.0);
IR (NaCl) ν 1678 cm-1. Anal. Calcd for C20H33NO3Sn: C, 52.89;
H, 7.32; N, 3.08. Found: C, 53.29; H, 7.52; N, 3.07.
4-Phenyl-1-(trimethylstannanyl)butyl 2,2,4,4-tetramethyloxazo-
lidine-3-carboxylate (5a). Following the general procedure, 720 mg
1
of 5c (2.39 mmol) gave 5a as a colorless oil (950 mg, 85%): H
NMR (250 MHz, CDCl3, rotamers) δ 7.23 (m, 2H), 7.13 (m, 3H),
4.51 (m, 1H), 3.67 (s, 2H), 2.60 (m, 2H), 1.79 (m, 2H), 1.55 (s,
2H), 1.49 and 1.46 (2s, 6H), 1.35 and 1.30 (2s, 6H), 0.04 (s,
J(117,119Sn-1H) ) 52.4, 9H); 13C NMR (62.9 MHz, CDCl3, rotamers)
δ 153.1/152.4, 141.9, 128.1, 128.0, 125.5, 95.4/94.4, 76.0/75.8, 71.3
(J(117Sn-13C) ) 429.1, J(119Sn-13C) ) 448.9), 60.2/59.2, 35.4, 33.1,
29.5 (J(117,119Sn-13C) ) 35.8), 26.4/26.3, 25.2/25.1, 24.0/23.9, -9.3
(J(117Sn-13C) ) 316.8, J(119Sn-13C) ) 331.3); IR (NaCl) ν 1678
cm-1. Anal. Calcd for C21H35NO3Sn: C, 53.87; H, 7.53; N, 2.99.
Found: C, 53.84; H, 7.88; N, 2.96.
Experimental Section
Preparation of 2,2,4,4-Tetramethyloxazolidine-3-carboxylates
3-7c and 9-12c. General Procedure. A suspension of NaH (60%
in mineral oil, 0.3 g, 7.5 mmol) in THF (6 mL) was treated with
the corresponding alcohol (5 mmol) and stirred at room temperature
for 30 min. A solution of 2,2,4,4-tetramethyloxazolidine-3-carbonyl
chloride21 (1.1 g, 6.0 mmol) in THF (6 mL) was then added, and
the resulting mixture was stirred at room temperature for 5 days,
quenched by addition of pH 7.0 phosphate buffer (5 mL), and
partitioned between CH2Cl2 (10 mL) and phosphate buffer (10 mL).
The aqueous phase was extracted with CH2Cl2 (3 × 10 mL), and
the combined organic phase was washed with brine (15 mL), dried
with anhydrous Na2SO4, filtered, and concentrated. The residue was
purified by column chromatography (SiO2, EtOAc/hexane 1:6) to
give 3-7c and 9-12c as oils (87-99% yield).
General Procedure for the Synthesis of Organostannanes
3-7a and 9-12a. s-BuLi (1.25 mL, 1.50 mmol, 1.2 M in hexane)
was added to a precooled solution (-78 °C) of the corresponding
carbamate (1.00 mmol) and TMEDA (230 µL, 1.50 mmol) in Et2O
(3.0 mL). After stirring for 5 h at-78 °C, Me3SnCl (1.5 mL, 1.5
mmol, 1.0 M in THF) was added to the reaction mixture. The
resulting solution was stirred at the same temperature for 1 h,
quenched by addition of pH 7.0 phosphate buffer, and then
partitioned between Et2O (10 mL) and phosphate buffer (10 mL).
The aqueous phase was extracted with Et2O (10 mL), and the
combined organic phase was washed with brine (10 mL), dried
with anhydrous Na2SO4, filtered, and concentrated. The residue was
purified by column chromatography (grade III neutral Al2O3, hexane
to EtOAc/hexane 1:50).
2-Phenyl-1-(trimethylstannanyl)ethyl 2,2,4,4-tetramethyloxazo-
lidine-3-carboxylate (3a). Following the general procedure, 3c (670
mg, 2.42 mmol) afforded 3a as a pale yellow oil (969 mg, 91%):
1H NMR (300 MHz, CDCl3, rotamers) δ 7.24 (m, 5H), 4.78 (m,
1H), 3.65 (s, 2H), 3.23 (m, 1H), 3.06 (dd, J ) 13.9, 6.9 Hz, 1H),
1.54 and 1.52 (2 s, 3H), 1.40, 1.37, 1.25, 1.22, and 1.12 (5 s, 9H),
0.05 (s, J(117,119Sn-1H) ) 52.8, 9H); 13C NMR (75 MHz, CDCl3,
rotamers) δ 153.1/152.4, 139.5, 128.7, 128.2, 126.2, 95.5/94.6, 76.2/
75.9, 71.6 (J(117,119Sn-13C) ) 429.5), 60.3/59.4, 39.7, 26.1/26.0,
25.2/25.1, 24.9/24.8, 24.0, -9.2 (J(117Sn-13C) ) 320.4, J(119Sn-
13C) ) 335.4); IR (NaCl) ν 1681 cm-1. Anal. Calcd for
C19H31NO3Sn: C, 51.85; H, 7.10; N, 3.18. Found: C, 52.04; H, 7.28;
N, 3.22.
5-Phenyl-1-(trimethylstannanyl)pentyl 2,2,4,4-tetramethyloxazo-
lidine-3-carboxylate (6a). Following the general procedure, 780 mg
1
of 6c (2.44 mmol) gave 6a as a colorless oil (969 mg, 82%): H
NMR (250 MHz, CDCl3, rotamers) δ 7.26 (m, 2H), 7.16 (m, 3H),
4.51 (m, 1H), 3.71 (s, 2H), 2.62 (t, J ) 7.5 Hz, 2H), 1.74 (m, 4H),
1.56-1.26 (m, 14H), 0.07 (s, J(117Sn-1H) ) 53.3, J(119Sn-1H) )
51.4, 9H); 13C NMR (62.9 MHz, CDCl3, rotamers) δ 153.1/152.5,
142.1, 125.5, 95.5/94.4, 76.1/75.9, 71.6 (J(117Sn-13C) ) 430.9,
J(119Sn-13C) ) 450.9), 60.2/59.3, 35.6, 33.5, 30.9, 27.3 (J(117,119Sn-
13C) ) 35.0), 26.3, 25.2/25.1, 25.0, 24.0/23.9, -9.3 (J(117Sn-13C)
) 316.3, J(119Sn-13C) ) 331.0); IR (NaCl) ν 1678 cm-1. Anal.
Calcd for C22H37NO3Sn: C, 54.79; H, 7.73; N, 2.90. Found: C,
54.52; H, 8.10; N, 2.79.
8-Phenyl-1-(trimethylstannanyl)octyl 2,2,4,4-tetramethyloxazo-
lidine-3-carboxylate (7a). Following the general procedure, 7c (553
1
mg, 1.53 mmol) afforded 7a as a colorless oil (613 mg, 76%): H
NMR (250 MHz, CDCl3, rotamers) δ 7.23 (m, 5H), 4.53 (m, 1H),
3.72 (s, 2H), 2.62 (t, J ) 7.5 Hz, 2H), 1.87 (m, 2H), 1.58 (m, 8H),
1.40 (m, 14H), 0.14 (s, J(117,119Sn-1H) ) 53.0, 9H); 13C NMR (62.9
MHz, CDCl3, rotamers) δ 153.2/152.5, 142.5, 128.2, 128.0, 125.4,
95.5/94.5, 76.1/75.9, 71.7 (J(117Sn-13C) ) 432.6, J(119Sn-13C) )
452.7), 60.2/59.3, 35.8, 33.6, 31.3, 29.2, 29.1, 29.0, 27.7 (J(117,119Sn-
13C) ) 33.8), 26.4, 26.3, 25.2/25.1, 24.1/24.0, -9.2 (J(117Sn-13C)
) 315.7, J(119Sn-13C) ) 330.4); IR (NaCl) ν 1677 cm-1. Anal.
Calcd for C25H43NO3Sn: C, 57.27; H, 8.27; N, 2.67. Found: C,
57.49; H, 8.49; N, 2.72.
1-(Trimethylstannanyl)but-3-enyl 2,2,4,4-tetramethyloxazolidine-
3-carboxylate (9a). Following the general procedure, 9c (638 mg,
2.81 mmol) afforded 9a as a colorless oil (1.03 g, 94%): 1H NMR
(250 MHz, CDCl3, rotamers) δ 5.79 (m, 1H), 5.08 (d, J ) 24.7
Hz, 1H), 5.07 (s, 1H), 4.57 (m, 1H), 3.71 (s, 2H), 2.61 (m, 2H),
1.54 and 1.50 (2 s, 6H), 1.40 and 1.35 (2 s, 6H), 0.11 (s, J(117,119Sn-
1H) ) 52.8, 9H); 13C NMR (62.9 MHz, CDCl3, rotamers) δ 153.2/
152.5, 136.3 (J(117,119Sn-13C) ) 36.2), 116.7, 95.6/94.7, 76.3/76.0,
70.5 (J(117Sn-13C) ) 421.8, J(119Sn-13C) ) 441.4), 60.4/59.6, 38.1,
26.4, 25.2/25.1, 24.1/24.0, -9.0 (J(117Sn-13C) ) 319.2, J(119Sn-
13C) ) 334.1); IR (NaCl) ν 1679 cm-1. Anal. Calcd for
C15H29NO3Sn: C, 46.18; H, 7.49; N, 3.59. Found: C, 46.50; H, 7.56;
N, 3.58.
(19) ∆Geq (kcal/mol): Me2NCH2CHLiOCby ) 5.8; Me2N(CH2)2CHLiOCby
) 5.0; MeO(CH2)2CHLiOCby ) 5.4-5.5.
1-(Trimethylstannanyl)pent-4-enyl 2,2,4,4-tetramethyloxazoli-
dine-3-carboxylate (10a). Following the general procedure, 10c (758
(20) An approximate acidity scale can be derived from these data that should
be of value in the planning of syntheses involving carbanions (compound/pK):
3c/36.8-36.9, 4c/37.3, 5c/38.8, 6c/38.8, 7c/39.0, 8c/38.8, 9c/37.3, 10c/37.5, 11c/
38.8, 12c/39.0. See ref 15 for an explanation.
1
mg, 3.14 mmol) afforded 10a as a colorless oil (1.23 g, 97%): H
NMR (250 MHz, CDCl3, rotamers) δ 5.83 (m, 1H), 5.01 (d, J )
24.1 Hz, 1H), 5.00 (s, 1H), 4.52 (m, 1H), 3.72 (s, 2H), 2.01 (m,
(21) Hintze, F.; Hoppe, D. Synthesis 1992, 1216–1218.
7396 J. Org. Chem. Vol. 73, No. 18, 2008

4H), 1.54 and 1.53 (2 s, 6H), 1.40 and 1.37 (2 s, 6H), 0.10 (s,
J(117,119Sn-1H) ) 52.4, 9H); 13C NMR (62.9 MHz, CDCl3, rotamers)
δ 153.0/152.4, 137.5, 114.8, 95.4/94.4, 76.0/75.8, 71.0 (J(117Sn-
13C) ) 426.9, J(119Sn-13C) ) 447.0), 60.2/59.2, 33.0, 32.0 (J(117,119Sn-
13C) ) 37.5), 26.4/26.3, 25.1, 24.0/23.9, -9.3 (J(117Sn-13C) ) 318.2,
J(119Sn-13C) ) 333.0); IR (NaCl) ν 1678 cm-1. Anal. Calcd for
C16H31NO3Sn: C, 47.55; H, 7.73; N, 3.47. Found: C, 47.23; H, 7.59;
N, 3.49.
1-(Trimethylstannanyl)hex-5-enyl 2,2,4,4-tetramethyloxazolidine-
3-carboxylate (11a). Following the general procedure, 11c (656 mg,
2.57 mmol) afforded 11a as a colorless oil (967 mg, 90%): 1H NMR
(250 MHz, CDCl3, rotamers) δ 5.80 (m, 1H), 4.99 (m, 2H), 4.53
(m, 1H), 3.72 (s, 2H), 2.09 (q, J ) 7.1 Hz, 2H), 1.86 (m, 2H), 1.53
(m, 8H), 1.41, 1.40 and 1.36 (3s, 6H), 0.10 (s, J(117Sn-1H) ) 51.2,
J(119Sn-1H) ) 53.2, 9H); 13C NMR (62.9 MHz, CDCl3, rotamers)
δ 153.2/152.5, 138.3, 114.6, 95.6/94.5, 76.2/76.0, 71.5 (J(117Sn-
13C) ) 429.6, J(119Sn-13C) ) 449.8), 60.3/59.4, 33.3, 33.0, 27.0
(J(117,119Sn-13C) ) 35.5), 26.4/26.3, 25.2/25.1, 24.1/24.0, -9.3
(J(117Sn-13C) ) 316.5, J(119Sn-13C) ) 331.3); IR (NaCl) ν 1678
cm-1. Anal. Calcd for C17H33NO3Sn: C, 48.83; H, 7.95; N, 3.35.
Found: C, 49.15; H, 8.25; N, 3.30.
1-(Trimethylstannanyl)dec-9-enyl 2,2,4,4-tetramethyloxazolidine-
3-carboxylate (12a). Following the general procedure, 12c (419 mg,
1.35 mmol) afforded 12a as a colorless oil (544 mg, 85%): 1H NMR
(250 MHz, CDCl3, rotamers) δ 5.80 (m, 1H), 4.95 (m, 2H), 4.53
(m, 1H), 3.72 (s, 2H), 2.03 (q, J ) 6.7 Hz, 2H), 1.83 (m, 2H), 1.54
and 1.52 (2s, 6H), 1.36 (m, 16H), 0.09 (s, J(117Sn-1H) ) 51.2,
J(119Sn-1H) ) 53.4, 9H); 13C NMR (62.9 MHz, CDCl3, rotamers)
δ 153.3/152.7, 139.0, 114.1, 95.6/94.6, 76.3/76.1, 71.9 (J(117Sn-
13C) ) 432.3, J(119Sn-13C) ) 452.0), 60.4/59.5, 33.8, 29.3, 29.2,
29.0, 28.8, 27.7 (J(117,119Sn-13C) ) 33.6), 26.5/26.4, 25.3/25.2, 24.2/
24.1, -9.3 (J(117Sn-13C) ) 315.4, J(119Sn-13C) ) 329.9); IR (CsI)
ν 1679 cm-1. Anal. Calcd for C21H41NO3Sn: C, 53.18; H, 8.71; N,
2.95. Found: C, 53.20; H, 8.97; N, 2.93.
General Procedure for Sn-Li Exchange. A cold solution of
stannane 3-7a or 9-12a (0.1 mmol) in THF was treated with BuLi
(0.1 mmol) and stirred for 30 min. A solution of the reference
stannane (1-2a) in THF was then added, and the resulting solution
was allowed to equilibrate while stirring for the time and at the
appropriate temperature as indicated in Schemes 1 and 2. The
reaction mixture was treated with dry and deoxygenated MeOH
followed by pH 7.0 phosphate buffer, then partitioned between
CH2Cl2 (10 mL) and phosphate buffer (10 mL). The aqueous phase
was extracted with CH2Cl2 (2 × 10 mL), and the combined organic
phase was washed with brine (15 mL), dried with anhydrous
Na2SO4, filtered, and concentrated. Analysis of the resulting
mixtures of stannanes (3-7a or 9-12a) and protonated compounds
1
(3-7c or 9-12c) by H NMR spectroscopy allowed the determi-
nation of the equilibrium concentrations. To confirm that the
equilibrium had been reached, the reverse reaction was performed
starting from a THF solution of the reference organolithium 1-2b
which was then treated with a solution of stannane 3-7a or 9-12a.
Acknowledgment. Financial support from MEC (Spain,
Grant CTQ2006-07854/BQU and fellowship to P.M.) and the
Xunta de Galicia (Grant PGIDIT03PXIC20910PN) is gratefully
acknowledged.
Supporting Information Available: General experimental
methods, preparation and full characterization for compounds
1
3-7c and 9-12c, equilibria data, and copies of H and 13C
NMR spectra for all new compounds are included. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO801176D
J. Org. Chem. Vol. 73, No. 18, 2008 7397