Triple ion formation in localized organolithium reagents [11]

Full text of DOI:10.1021/ja9741382

J. Am. Chem. Soc. 1998, 120, 4035-4036
4035
Triple Ion Formation in Localized Organolithium
Reagents
Hans J. Reich,* William H. Sikorski,
Birgir O¨ . Gudmundsson, and Robert R. Dykstra
Department of Chemistry, UniVersity of Wisconsin
Madison, Wisconsin 53706
ReceiVed December 8, 1997
A variety of aggregation motifs dominate the structural
chemistry of organolithium compounds. At least four distinct
modes of dimerization exist: the common 4-center dimers,1
6-center dimers as in 2-lithio-2-methyldithiane,2 solvent-bridged
dimers as in (LiBr)2‚(HMPA)3,3,4a and triple ions (lithium ate
complexes) of type R-Li-R-//Li+.5,6a,7 The reactivity of these
various dimers compared to monomers or higher aggregates is
usually unknown. Triple ions possess an intriguing combination
of features of both a dimer and a solvent-separated ion pair.
Several lithium amides,8,9a â-keto10 and â-imino9b enolates, and
a few other lithium reagents7b have been rigorously shown through
crystal structures or NMR data to form triple ions. Delocalized,
lithiocene-type sandwich structures are also known;6a,7a however,
there is only a single report of a localized carbanion triple ion:
a crystal structure was obtained of the triple ion of tris-
(trimethylsilyl)methyllithium.5 Here, we present NMR evidence
that triple ions form in THF/HMPA solution4b for a variety of
6
Figure 1. 6Li NMR (52.98 MHz) spectra15 of Li isotopically enriched
aryllithiums (0.16 M in 4:1 THF/ether at -125 °C with 2 equiv of
HMPA): (A) 1a; (B and C) 2:1 and 1:2 ratio of 1a and 2a; (D) 2a; (E)
phenyllithium with 5 equiv of HMPA.
localized lithiated carbanions, such as aryllithiums and sulfur-
and silicon-substituted alkyllithiums.
(1) Thoennes, D.; Weiss, E. Chem. Ber. 1978, 111, 3157.
(2) Amstutz, R.; Seebach, D.; Seiler, P.; Schweizer, W. B.; Dunitz, J. D.
Angew. Chem., Int. Ed. Engl. 1980, 19, 53. Eight-centered dimers are also
known: Boche, G.; Marsch, M.; Harms, K.; Sheldrick, G. M. Angew. Chem.
1985, 97, 577.
(3) Barr, D.; Doyle, M. J.; Mulvey, R. E.; Raithby, P. R.; Reed, D.; Snaith,
R.; Wright, D. S. J. Chem. Soc., Chem. Commun. 1989, 318.
(4) (a) Reich, H. J.; Borst, J. P.; Dykstra, R. R. Organometallics 1994, 13,
1. (b) Reich, H. J.; Borst, J. P.; Dykstra, R. R.; Green, D. P. J. Am. Chem.
+
2,6-Diisopropylphenyllithium (1a) is monomeric: the 13C NMR
spectrum of 6Li-enriched 1a at -125 °C in 4:1 THF/ether shows
a 1:1:1 triplet (JC-Li ) 14.6 Hz) at 193.3 ppm for the ipso carbon,
demonstrating connectivity to a single 6Li (spin ) 1) nucleus.6b,11
Soc. 1993, 115, 8728. (c) The free Li(HMPA)4 signal (but not the phenyl-
coordinated lithium) of 1c was detected in an earlier study of PhLi: Reich,
H. J.; Green, D. P.; Phillips, N. H. J. Am. Chem. Soc. 1989, 111, 3444. (d)
Reich, H. J.; Green, D. P.; Phillips, N. H. J. Am. Chem. Soc. 1991, 113, 1414,
footnote 16. (e) Reich, H. J.; Kulicke, K. J. J. Am. Chem. Soc. 1995, 117,
6621. (f) For detection of a P-Li-P coupling in a 4-centered dimer, see:
Reich, H. J.; Dykstra, R. R. Organometallics 1994, 13, 4578.
(5) Eaborn, C.; Hitchcock, P. B.; Smith, J. D.; Sullivan, A. C. J. Chem.
Soc., Chem. Commun. 1983, 827. The triple ion structure of (Me3Si)3CLi is
at least partially maintained in solution: Avent, A. G.; Eaborn, C.; Hitchcock,
P. B.; Lawless, G. A.; Lickiss, P. D.; Mallien, M.; Smith, J. D.; Webb, A. D.;
Wrackmeyer, B. J. Chem. Soc., Dalton Trans. 1993, 3259. Buttrus, N. H.;
Eaborn, C.; Hitchcock, P. B.; Smith, J. D.; Stamper, J. G.; Sullivan, A. C. J.
Chem. Soc., Chem. Commun. 1986, 969.
6
Figure 1A shows the Li spectrum of 1a in the presence of 2
equiv of HMPA. A singlet at 1.8 ppm is seen for the monomer,12
but there are two additional signals present in a 1:1 ratio,
consistent with a triple ion of structure 2a: a singlet at 4.6 ppm
for the internal lithium and a quintet at -0.4 ppm for the external
lithium (coupled to four coordinated HMPA molecules).4b The
carbon NMR shows discrete 1:1:1 triplets for the carbanionic
carbon of the putative triple ion (δ 197.7, J ) 13.7 Hz).
Compound 1b is also monomeric,13 and the addition of HMPA
has a similar effect. Figure 1-D shows that with 2 equiv of
HMPA, 73% of the monomer was converted to triple ion (internal
lithium at δ 4.3).14
Support of our assignment comes from NMR studies of
mixtures of these two aryllithiums. Thus, 1a and 1b were mixed
in a 2:1 and a 1:2 ratio in the presence of 2 equiv of HMPA
(Figure 1B,C). The mixed experiments show three distinct signals
for the internal lithium, the middle one belonging to the mixed
triple ion 3. The mirror-like appearance of the two spectra is
(6) (a) Fraenkel, G.; Hallden-Abberton, M. P. J. Am. Chem. Soc. 1981,
103, 5657. (b) Fraenkel, G.; Henrichs, M.; Hewitt, J. M.; Su, B. M.; Geckle
M. J. J. Am. Chem. Soc. 1980, 102, 3345.
(7) (a) Eiermann, M.; Hafner, K. J. Am. Chem. Soc. 1992, 114, 135. Harder,
S.; Prosenc, M. H. Angew. Chem., Int. Ed. Engl. 1994, 33, 1744. Zaegel, F.;
Gallucci, J. C.; Meunier, P.; Gautheron, B.; Sivik, M. R.; Paquette, L. A. J.
Am. Chem. Soc. 1994, 116, 6466. Hoic, D. A.; Davis, W. M.; Fu, G. C. J.
Am. Chem. Soc. 1995, 117, 8480. (b) Veith, M.; Ruloff, C.; Huch, V.; To¨llner,
F. Angew. Chem., Int. Ed. Engl. 1988, 27, 1381. Pauer, F.; Rocha, J.; Stalke,
D. J. Chem. Soc., Chem. Commun. 1991, 1477. Gornitzka, H.; Stalke, D.
Angew. Chem., Int. Ed. Engl. 1994, 33, 693.
(8) Jackman, L. M.; Scarmoutzos, L. M.; Smith, B. D.; Williard, P. G. J.
Am. Chem. Soc. 1988, 110, 6058. Jackman, L. M.; Scarmoutzos, L. M.; Porter,
W. J. Am. Chem. Soc. 1987, 109, 6524.
(9) (a) 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. Romesberg,
F. E.; Gilchrist, J. H.; Harrison, A. T.; Fuller, D. J.; Collum, D. B. J. Am.
Chem. Soc. 1991, 113, 5751. Romesberg, F. E.; Collum, D. B. J. Am. Chem.
Soc. 1995, 117, 2166. (b) Galiano-Roth, A. S.; Collum, D. B. J. Am. Chem.
Soc. 1988, 110, 3546.
(12) The coupling to the phosphorus nuclei of two HMPA molecules is
not resolved because of rapid exchange with free HMPA.
(13) A study which revealed it to be dimeric was based on a much more
concentrated (3.0 M) solution: Wehman, E.; Jastrzebski, J. T. B. H.; Ernsting,
J.-M.; Grove, D. M.; van Koten, G. J. Organomet. Chem. 1988, 353, 133.
(14) Quenching a sample of 1b with 3 equiv of HMPA (80% conversion
to 2b) with Me2S2 produced 87% of 2,6-dimethylthioanisole; therefore, the
signal assigned to the triple ion is a structural form of the aryllithium and not
a decomposition product. Similarly, 1a with 2 equiv of HMPA (75% of 2a)
gave 75% of the sulfide.
(10) Raban, M.; Noe, E. A.; Yamamoto, G. J. Am. Chem. Soc. 1977, 99,
6527. Raban, M.; Haritos, D. P. J. Am. Chem. Soc. 1979, 101, 5178. Cambillau,
C.; Bram, G.; Corset, J.; Riche, C. NouV. J. Chim. 1979, 3, 9. Cambillau, C.;
Ourevitch, M. J. Chem. Soc., Chem. Commun. 1981, 996. Teixidor, F.; Llobet,
A.; Casabo´, J.; Solans, X.; Font-Altaba, M.; Aguilo´, M. Inorg. Chem. 1985,
24, 2315.
(11) Bauer, W.; Winchester, W. R.; Schleyer, P. v. R. Organometallics
1987, 6, 2371.
(15) Lithium NMR spectra were referenced to external 0.3 M LiCl in
methanol.
S0002-7863(97)04138-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/09/1998

4036 J. Am. Chem. Soc., Vol. 120, No. 16, 1998
Communications to the Editor
Figure 2. 7Li NMR (139.96 MHz) spectra15 of natural abundance and
13C-enriched 4a (0.16 M in 3:2 THF/ether at -125 °C) with 1.5 equiv of
HMPA.
6
Figure 3. 6Li NMR (52.98 MHz) spectra15 of Li isotopically enriched
strong evidence that triple ions are responsible for the spectral
data observed.
1:1 mixtures of bis(2-pyridylthio)methyllithium (6) and phenylthiometh-
yllithium (7), 0.16 M in total lithium reagent in 3:2 THF/ether at -125
°C, with and without 13C labeling at the carbanion carbons. The signal
for 6‚HMPA at 4.8 ppm splits into a doublet (JLi-P) at lower temperatures.
Figure 1E shows the parent phenyllithium 1c in the presence
of 5 equiv of HMPA. A small signal is observed at δ 3.2 ppm,
which is equal in intensity to the small signal at -0.4 ppm, and
we assign these signals to a triple ion, by analogy to the above
results.4c Wittig, when considering possible structures of a PhLi
dimer, proposed a triple ion structure.16 The species that he was
referring to is known today to be a 4-centered dimer, but we now
have evidence that a small amount of the triple ion he proposed
does form in THF/HMPA solutions of PhLi.
In addition, we have observed that small amounts of triple ion
also form in THF/HMPA solutions of 1e, 1g, and 2-lithio-5-
methylthiophene, while significant amounts form with 1f and 1d.
The relative amount of triple ion is sensitive to o-substitution on
the aryllithium: monomeric 1a and 1b form 65 to 80% triple ion
in the presence of 1-3 equiv of HMPA, whereas 1c and 1e (which
are a mixture of monomer and dimer in THF4d) formed less than
20% at 5 equiv of HMPA.
An electronic effect was observed by comparing 4a-e. The
better stabilized carbanions formed triple ions with less HMPA
and to a greater extent, consistent with the greater ease of ion
pair separation.
To probe the significance of chelation in promoting triple ion
formation, bis(2-pyridylthio)methyllithium (6) was investigated,
and it was found to form mixed triple ions with a variety of
nonchelated organolithium reagents. For instance, phenylthio-
methyllithium (7) formed a mixed triple ion 8 almost quantita-
tively when 2 equiv of HMPA (relative to total lithium reagent)
were added. The connectivity was proved by 13C labeling of the
carbanionic carbon in both components (Figure 3). The coupling
across the lithium (2JC-Li-C) in the doubly 13C-labeled material
was not observed. The coupling to HMPA of the monomer signal
at δ 4.7 ppm could be resolved at -145 °C (2JLi-P ) 9.2 Hz).4f
Lithium halide salts are often present in organolithium reac-
tions. A significant result is that LiCl also quantitatively formed
triple ions with 6 and, therefore, could affect its chemistry if the
triple ion is a reactive species. Lithium phenolate, intended as a
model for enolates and alkoxides which are also present in many
organolithium reactions, showed no evidence of mixed aggrega-
tion with 6.
Pyridylthio-substituted carbanions 4 show a propensity to form
7
triple ions in THF/HMPA solutions. Figure 2 compares the Li
spectra of unlabeled and 13C-labeled 4a in the presence of 1.5
equiv of HMPA. Singlets at 5.0 and 5.2 ppm have been assigned
to diastereomeric triple ions 5 since each becomes a triplet (JC-Li
) 16.9 and 16.7 Hz) in the 13C-labeled compound. The chemical
Triple ion formation is not unique to the chelated and aryl
systems. Several nonchelated sulfur-, selenium-, and silicon-
stabilized alkyllithiums also form significant fractions of triple
ions in ether/THF/HMPA solution. This was shown by 13C
labeling of the carbanion carbons of phenylselenomethyllithium
(9) and bis(trimethylsilyl)methyllithium (10), which converted the
internal lithium signals into triplets, whereas the labeled carbons
showed coupling to only one Li nucleus.
It is noteworthy that the addition of HMPA, which normally
deaggregates lithium reagents,17 converts a monomeric lithium
reagent into a dimeric triple ion. The presence of triple ions in
this range of systems suggests that even in cases where they are
not detectable ground-state structures, they should be considered
as possible reactive intermediates for all organolithium reactions
run in relatively polar media. Triple ions might be expected to
have reactivities competitive with contact ion pair monomers. For
instance, the downfield shift of the carbanion carbon of 1a from
the monomer (193.3 ppm) to the triple ion (197.7 ppm) is
indicative of increased electron density at the carbanion carbon.
It remains to be shown whether triple ions are reactive species,
but for lithium reagents such as 1, 7, 9, and 10 that do not form
classical separated ion pairs even with large excesses of HMPA,
triple ion involvement in their chemistry seems especially likely.
shifts of the triple ions indicate that they are bis-chelated, as each
successive pyridyl coordination causes a 2-2.5 ppm downfield
shift (ascribed to lithium’s location coordinated to the edge of an
aromatic ring, as amine coordination does not cause such large
chemical shift changes4e). The chemical shifts of the HMPA-
coordinated monomers demonstrate that chelation is intact for
them as well.
Two diastereomeric triple ions are observed due to the
stereogenic carbon centers. In principle, there should be a third
diastereomer due to the stereochemical arrangement about lithium.
The fact that only two are observed indicates one of the
following: (1) isomerization about lithium is fast on the NMR
time scale (even down to -150 °C), (2) the structure is not
chelated, (3) two of the signals are coincident, or (4) one isomer
does not form. We doubt the third explanation, since in 4c-e,
which have appreciably different chemical shifts for their triple
ion signals, we still see only two (4b, lacking a stereocenter, forms
only one). A â-ketohydrazone anion possessing stereogenic
centers also showed only two triple ion diastereomers.9b
Acknowledgment. We thank the National Science Foundation for
financial support of this research. The spectrometers are funded by the
NSF (CHF-8306121) and the National Institute of Health (NIH 1 S10
RR02388).
(16) Wittig, G. Angew. Chem. 1958, 70, 65.
(17) Exceptions are known: Jackman, L. M.; Chen, X. J. Am. Chem. Soc.
1992, 114, 403.
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