Perturbation of conjugation in internally solvated allylic lithium compounds: Variation of ligand structure. NMR and X-ray crystallography

Article abstract of DOI:10.1021/ja060272n

Several allyic lithium compounds were prepared with different potential ligands tethered at C₂. These are with CH₃OCH ₂CH₂NCH₃CH₂-, 5 and 1-TMS 6, with (CH₃)₂NCH₂CH₂NCH₃CH ₂-, 1-TMS 7, and with ((CH₃)₂NCH ₂CH₂)₂NCH₂-, 8 and 1-TMS 9. In all these compounds Li is fully coordinated to the pendant ligand and is sited off the axis perpendicular to the allyl plane at one of the allyl termini as indicated by a combination of X-ray crystallography and NMR spectra. Compounds 5 and 8 are Li-bridged dimers as shown by X-ray crystallography and also dimeric in benzene solution as determined from freezing point determinations. Compounds 6, 7, and 9 are monomeric in THF-d₈ or diethyl ether-d₁₀ solution and exhibit one bond ¹³C₁,⁶Li scalar coupling at low temperature. Taken together the crystallographic and NMR data indicate that all of these compounds incorporate partially delocalized allylic moieties. Compounds 5 and 8 undergo fast 1,3-Li-sigmatropic shifts that are proposed to take place within low concentrations of monomers in fast equilibrium with prevalent dimers. Averaging with increasing temperature of the one-bond ¹³C,⁶Li coupling constant in 6, 7, and 13 provided the dynamics of bimolecular C-Li exchange with ΔH? values of 6, 7, 12, and 13 kcal-mol^-1, respectively. Averaging of the diastereotopic N(CH₃)₂ ¹³C resonances of 7 is indicative of fast transfer of coordinated ligand between faces of the allyl plane ΔH? = 5.3 kcal-mol^-1 combined with slower inversion at nitrogen. Compound 8 exhibits similar effects. It is concluded that variation of the ligand structure changes dynamic behavior of the compounds but has little influence of their degrees of delocalization.

Full text of DOI:10.1021/ja060272n

Published on Web 06/06/2006
Perturbation of Conjugation in Internally Solvated Allylic
Lithium Compounds: Variation of Ligand Structure. NMR and
X-ray Crystallography
Gideon Fraenkel,* Judith Gallucci, and Hua Liu
Contribution from the Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210
Received January 13, 2006; E-mail: fraenkel@mps.ohio-state.edu
Abstract: Several allyic lithium compounds were prepared with different potential ligands tethered at C2.
These are with CH3OCH2CH2NCH3CH2-, 5 and 1-TMS 6, with (CH3)2NCH2CH2NCH3CH2-, 1-TMS 7, and
with ((CH3)2NCH2CH2)2NCH2-, 8 and 1-TMS 9. In all these compounds Li is fully coordinated to the pendant
ligand and is sited off the axis perpendicular to the allyl plane at one of the allyl termini as indicated by a
combination of X-ray crystallography and NMR spectra. Compounds 5 and 8 are Li-bridged dimers as
shown by X-ray crystallography and also dimeric in benzene solution as determined from freezing point
determinations. Compounds 6, 7, and 9 are monomeric in THF-d8 or diethyl ether-d10 solution and exhibit
one bond 13C1,6Li scalar coupling at low temperature. Taken together the crystallographic and NMR data
indicate that all of these compounds incorporate partially delocalized allylic moieties. Compounds 5 and 8
undergo fast 1,3-Li-sigmatropic shifts that are proposed to take place within low concentrations of monomers
in fast equilibrium with prevalent dimers. Averaging with increasing temperature of the one-bond 13C,6Li
coupling constant in 6, 7, and 13 provided the dynamics of bimolecular C-Li exchange with ∆Hq values of
6.7, 12, and 13 kcal‚mol-1, respectively. Averaging of the diastereotopic N(CH3)2 13C resonances of 7 is
indicative of fast transfer of coordinated ligand between faces of the allyl plane ∆Hq ) 5.3 kcal‚mol-1
combined with slower inversion at nitrogen. Compound 8 exhibits similar effects. It is concluded that variation
of the ligand structure changes dynamic behavior of the compounds but has little influence of their degrees
of delocalization.
Extensive chemical,1 X-ray crystallographic,2 NMR,3 and
calculational studies4 establish the delocalized character of
solvated allylithium and how coordinated lithium is sited normal
to the center of the allyl plane, 1.
line shape analysis.5 In related studies on ion-pairing we reported
how 13C NMR at 150 K revealed the unsymmetrical character
of 2. With increasing temperature above 150 K, NMR line shape
analysis of signal averaging effects in the 13C NMR of 2
distinguished the dynamics of two different first-order reorga-
nization processes. These are rotation of coordinated ligand on
one side of the allyl plane and transfer of coordinated ligand
between faces of the allyl plane.6
Barriers to rotation have been determined around the C1-C2
bond of different allylic lithium compounds by use of NMR
(1) (a) Wardell, J. L. In ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone, F. G. H., Abel, E. W., Eds.; Pergammon Press: Oxford, 1982;
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S.; Bywater, S.; Worsfold, D. J. J. Organomet. Chem. 1980, 199, 1. (g)
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In experiments to slow these molecular reorganization
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Organometallics 1983, 2, 1344. (f) Bushby, R. J.; Tytho, M. P. J.
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Chem. 1995, 16, 1070. (h) Eikemma-Hommes, N. J. R. Van; Bu¨hl, M.;
Schleyer, P. v. R. J. Organomet. Chem. 1991, 409, 307-320.
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(b) Cabral, J. A.; Fraenkel, G. J. Am. Chem. Soc. 1992, 114, 9067. (c)
Fraenkel, G.; Qiu, F. J. Am. Chem. Soc. 2000, 122, 1280. (d) Fraenkel, G.;
Chow, A.; Winchester, W. R. 1990, 112, 2582. (e) West, P.; Purmort, J. I.;
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9
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J. AM. CHEM. SOC. 2006, 128, 8211-8216
8211

A R T I C L E S
Fraenkel et al.
Table 1. Structures of Internally Solvated Allylic Lithium
different NOE-based procedures we prepared several internally
solvated allylic lithium compounds;7 see, for example, 3 shown
with 13C shifts. As revealed by our NMR and X-ray crystal-
Compounds Shown with 13C and (Proton) Chemical Shifts, δ Scale
lographic7,8 studies these structures do not incorporate the
expected delocalized allylic anion; rather, the anion is partly
localized as shown in 3.8 The terminal 13C NMR shifts of 3 lie
between those of a typical externally solvated delocalized allylic
lithium compound at 55 δ, 1 and values reported for an
unsolvated localized system, see the cis/trans equilibrium 4a
with 4b in eq 1, shown with terminal allyl 13C shifts of ca. 22
δ and 100 δ.9 We proposed that the ligand tethers in the
internally solvated compounds such as 3 are too short to place
coordinated Li+ normal to the center of the allyl plane as in 1
but instead places it off normal to the allyl plane at one of the
allyl termini.7,8 As a result, the Li+ polarizes the allyl moiety
to become partly localized. Thus, it appears that the site of Li+
relative to allyl determines the degree of delocalization of the
allyl carbanionic moiety.
Scheme 1
In principle the degree of allylic delocalization should be
influenced by the length of the ligand tether, the nature of
substitution on the allyl unit, and the structure of the tethered
ligand. Thus far in our studies of internal solvation effects we
have mainly used the bis(2-methoxyethyl)amino ligand.7,8 This
article is addressed to the effect of changing the structure of
the pendant ligand.
Results and Discussion
For the convenience of readers, Table 1 lists the structures
of the internally solvated organolithium compounds 5, 6, 7, 8,
and 9, which are reported in this article, together with their 1H
and 13C chemical shifts. Preparation of these compounds is
summarized in reactions 2-11 (see Scheme 1). In common,
compounds 5, 6, 7, 8, and 9 were prepared by metalating,
respectively, compounds, 10, 11, 13, 16, and 17 with n-
butyllithium in a mixture of hydrocarbon with diethyl ether or
THF. Note that treatment of the mixture of 13 and 14 with
n-butyllithium yielded only 7. Compound 14 remained un-
changed.
(6) (a) Fraenkel, G.; Cabral, J. A.; Lanter, C.; Wang. J. J. Org. Chem. 1990,
64, 1302. (b) Fraenkel, G.; Chow, A.; Winchester, W. R. J. Am. Chem.
Soc. 1990, 112, 1382.
(7) (a) Fraenkel, G.; Qiu, F. J. Am. Chem. Soc. 1996, 118, 5828. (b) Fraenkel,
G.; Qiu, F. J. Am. Chem. Soc. 1997, 119, 3571. (c) Fraenkel, G.; Duncan,
J. H.; Wang, J. J. Am. Chem. Soc. 1999, 121, 432. (d) Fraenkel, G.;
Fleischer, R.; Liu, H. ARKIVOC 2002, 13, 42.
(8) Fraenkel, G.; Chow, A.; Fleischer, R.; Liu, H. J. Am. Chem. Soc. 2004,
126, 3983.
(9) (a) Fraenkel, G.; Halasa, A. F.; Mochel, V.; Stumpe, R.; Tate, D. J. Org.
Chem. 1985, 50, 4563-4565. (b) Glaze, W. H.; Jones. P. C. J. Chem. Soc.
1969, 1434. (c) Glaze, W. H.; Hanicac, J. E.; Moore, M. L.; Chandpuri, J.
J. Organomet. Chem. 1972, 44, 39. (d) Glaze, W. H.; Hanicac, J. E.;
Chandpuri, J.; Moore, M. L.; Duncan, D. P. J. Organomet. Chem. 1973,
51, 13.
Dimeric Internally Solvated Allylic Lithium Compounds.
Compound 5, formed by metalation of 10 with n-butylithium
9
8212 J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006

Delocalized Character of Solvated Allylithium
A R T I C L E S
Table 2. Internally Solvated Allylic Lithium Compounds, Solid: Allylic Bond Lengths Å, and Aggregation, Agg. Solution: 13C Allyl Shifts δ,
Agg. and 1J (13C,6Li or 13C7Li); Numbering
solid
C2
soln
cmpd
C1−C2 Å
−C3 Å
agg.
C1
δ
C3
δ
agg.
1J(13C,Li) Hz
ref
3
5
1.397 (4)
1.442 (2)
1.361 (4)
1.347 (2)
poly.
dim
?
?
dim
?
dim
mon
mon
mon
41.10
38.70
40.29
39.28
52.24c
41.80
58.55c
42.81
54.12
51.62c
76.30
81.36
74.73
77.20
52.24c
74.92
58.55c
72.95
78.10
51.62c
mon
dim
mon
mon
dim
mon
dim
mon
mon
mon
3.0 6Li
a
d
d
d
d
d
a
a
a
a
c
6b
7b
8
-
-
-
-
4.05 6Li
3.7 6Li
c
1.4355 (12)
1.3585 (12)
9b
18
21
22
23
-
-
3.5 6Li
c
1.436 (4)
1.349 (1)
1.415 (7)
1.431 (3)
1.426 (2)
1.351 (8)
1.351 (3)
1.366 (2)
7.0 6Li
6.1 7Li
c
a Reference 8. b Did not crystallize. c Averaged. d This work.
was crystallized from diethyl ether. X-ray crystallography
reveals 5 to be an internally solvated lithium-bridged dimer.
The ORTEP diagram is shown in Figure 1. Selected structural
parameters are listed in Table 2.
1,3-lithium sigmatropic shift that accompanies a rapid dimer-
monomer interconversion with the latter in low concentration.
Compound 8 is also a dimer according to X-ray crystal-
lography, see Figure 2, in fashion similar to 5 and 18 and with
allyl bond lengths similar to those of the latter two, Table 2.
Clearly all three compounds display similar degrees of localiza-
tion associated with their allylic moieties.
Figure 1. ORTEP diagram of compound 5 showing 50% ellipsoids, and
selected atoms have been labeled. The hydrogen atoms have been drawn
with an artificial radius.
The structure of 5 has features in common with that of 18
that was reported previously.8 The allyl carbon-carbon bond
lengths of both compounds lie between those of the reference-
limiting localized 4a and 4b and delocalized 1 allylic lithium
compounds, respectively.
Figure 2. ORTEP diagram of compound 8 showing 50% ellipsoids, and
selected atoms have been labeled. The hydrogen atoms that are included
have been drawn with an artificial radius.
The arrangement around lithium in 8 is best described as a
distorted trigonal bipyramid within which lithium is closely
centered in the plane of C1, N2, and N3 and nearly collinear
with the span N1‚‚‚C1′, see Figure 3 and Table 3. By contrast
in 5, within the distorted tetrahedron N1, O1, C1, and C1′
The dimeric state of 5 also prevails in benzene solution as
determined by a freezing point determination. In principle, dimer
5 should consist of a mixture of meso and DL forms. This is
not detected among the NMR data most likely due to a fast
Figure 3. Stereochemistry of lithium in (a) 8 and (b) 5.
9
J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006 8213

A R T I C L E S
Fraenkel et al.
Table 3. Stereochemistry of Lithium in Compounds 5 and 8 (for
Numbering See Figures 1 and 2)
of a broad doublet, separation 5.8 ppm (400 Hz at 75.47 MHz)
centered at 56 δ. With increasing temperature above 130 K the
latter doublet progressively averages to a single line by 180 K.
In similar fashion, the doublet, centered at 52 δ, due to the N
(CH2CH2N(CH3)2)2 13C resonance, separation 1.060 ppm (81
Hz at 75.47 MHz), averages to a single line by 160 K. The
latter effect is most likely due to transfer of the coordinated
ligand between faces of the allyl plane within monomer 20. By
contrast, two processes are most likely responsible for changes
of the N(CH3)2 resonance. These would be face transfer,
described above, combined with fast reversible N-Li dissocia-
tion/recombination. During the dissociated phase, fast inversion
at nitrogen accompanied by rotation around the (CH3)2N-CH2
bond would average the shift between the geminal N-methyls.
Monomeric Internally Solvated Allylic Lithium Com-
pounds. Compounds 6, 7, and 9 are monomers as indicated by
the equal triplet multiplicity of the 13C resonance due to one-
bond 13C,6Li scalar coupling. The NMR data are consistent with
coordination of lithium to the pendant ligand. Terminal allyl
13C shifts which lie between those for reference delocalized 1
and localized, 4a a 4b, allylic lithiums respectively, show 6,
7, and 9 to be partially localized, see Table 2.
Monomers 6, 7, and 9 also display evidence of the dynamics
of interesting fast equilibrium reorganization behavior. The
13C,6Li coupling constants of all three compounds average with
increasing temperature as a result of bimolecular C,Li exchange.
Line shape analysis provides the associated activation parameters
listed in Table 3.
Averaging of the shift between the N(CH3)2 methyls of 7
with increasing temperature is indicative of a fast face transfer
of coordinated lithium combined with some contribution from
inversion at dimethylamino nitrogen. The activation parameters
of ∆Hq ) 5.3 ( 0.4 kcal‚mol-1 and ∆Sq ) -19 ( 5 eu are
very similar to values obtained for related internally solvated
allylic lithium compounds undergoing the face transfer reorga-
nization process alone.7,8 As noted above, similar effects were
ascribed as responsible for averaging of the diastereotopic 13C
shift between the geminal N(CH3)2 methyls of 8. Previous
results indicate that the nitrogen inversion process for both 8
and 7 would be significantly slower than that of face transfer
of coordinated lithium.
C1
C1
C1′
C1
C1′
N1
Li1
Li1
Li1
Li1
Li1
Li1
C1′
N1
N1
O1
O1
O1
113.87 (8)
87.55 (7)
123.81 (9)
119.07 (9)
120.05 (9)
85.14 (7)
C1
C1
C1
C1
C1′
C1′
Cl′
N1
N1
N2
Li1
Li1
Li1
Li1
Li1
Li1
Li1
Li1
Li1
Li1
C1′
N1
N2
N3
N1
N2
N3
N2
N3
N3
102.83 (7)
83.90 (6)
123.99 (8)
117.60 (7)
171.60 (8)
91.80 (6)
100.59 (6)
80.18 (6)
80.33 (6)
111.98 (8)
lithium is near coplanar with O1, C1, and C1′, see Figure 3b
and Table 3.
Carbon-13 NMR of 5 in THF does not exhibit one-bond
13C,6Li spin coupling. The allyl termini give rise to a single,
sharp 13C resonance at room temperature. This line progressively
broadens with decreasing temperature and resolves into two
broad lines at 38.78 δ (C1) and 81.36 δ (C3) by 140 K, Figure
4. These values are similar to those for terminal allyl shifts in
other internally solvated allylic lithium compounds reported
previously.7,8
NMR line shape analyis of the terminal allyl 13C resonance
of 5 gives rates for a dynamic process with ∆Hq ) 4.8 ( 0.5
kcal‚mol-1 and ∆Sq ) -10 ( 2 eu, Table 4. We propose that
these results are most consistent with a fast 1,3-lithium
sigmatropic shift (eq 12) which takes place within a low
undetectable concentration of monomer, with the latter in fast
equilibrium with dimer 5 (eq 13). Then fast recombination of
monomers from different dimers would be responsible for
averaging out the 13C,6Li coupling constant.
General Remarks
The X-ray crystallographic allylic carbon-carbon bond
distances of 5 and 8 are so similar to those reported previously
for 18, 21, 22, and 23 that one can conclude that the allylic
moieties in all the above compounds are partially localized to
the same extent.8 This conclusion is supported by close
similarities among the terminal allyl 13C shifts of 6, 7, and 9 to
those, respectively, of 3, 18, 21, and 22, see Table 2. As yet
The NMR behavior of 8 parallels that of 5. Compound 8 does
not exhibit 13C,6Li spin coupling down to 140 K. The terminal
allyl 13C NMR which is a single sharp line at room temperature,
broadens with decreasing temperature and disappears into the
baseline by 150 K and is not detected down to 140 K. Below
this temperature compound 8 precipitates from solution. A fast
interconversion of dimer 8 with monomer 20, in low concentra-
tion concurrent with a fast 1,3-lithium sigmatropic shift in
monomer would be consistent with the above behavior.
Two other temperature-dependent changes of the 13C NMR
line shapes of 8 provide qualitative insight into the nature of
dynamic effects. At 130 K the N(CH3)2 13C resonance consists
unavailable among these data are the bond distances for 6, 7,
and 9 and the terminal allyl 13C shifts of 8 and 23. The latter
9
8214 J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006

Delocalized Character of Solvated Allylithium
A R T I C L E S
Figure 4. 13C NMR compound 5, THF-d8 solution, 140 K. Resonances for terminal allyl carbons are circled.
Table 4. Dynamic Behavior Internally Solvated Allylic Lithium
for systems with ligand tethers of one to three carbons and
changes in the structure of the pendant ligand as presented herein
and previously.7,8
Compounds ∆Hq kcal‚mol-1 (∆Sq eu)
13C
−
6Li
1,3-Li
N(CH3)2
exchange
sigmatropic shift
inversion
While the compounds in Table 1 have common structural
features, they are differentiated by their dynamic behavior. For
example in cases where 13C,6Li or 13C7Li spin coupling has been
observed, it is always averaged with increasing temperature due
to fast intermolecular C,Li bond exchange. The corresponding
∆Hq values for 7 and 9 of 12.5 ( 0.5 kcal‚mol-1 are close to
all such values obtained in previous studies of several internally
solvated allylic lithium compounds.7,8 It was suggested that the
slow step for C,Li exchange involved complete decoordination
of Li to the pendant ligand. The smaller ∆Hq for C-Li exchange
between monomers of 6 is similar to ∆Hq values for face transfer
of coordinated lithium. Most likely in the case of 6 both
processes are due to a common mechanism. By contrast to the
dynamics of C-Li exchange the dynamics of the 1,3-Li
sigmatropic shift takes place at widely different rates. This
process has only been detected in the case of symmetrical
resonance used
13C1
-
C1, C3
4.8 ( 0.5
(-10 ( 2)
N(13CH3)2
5a
6b
7a
9a
6.7 ( 0.5
(-2.6)
12 ( 0.8
(-15 ( 4)
13 ( 0.8
(-14.6 ( 4)
5.3 ( 0.4
(-19 ( 5)
a THF-d8. b Diethyl ether-d10.
shifts remain averaged, even at low temperature due to fast 1,3-
lithium sigmatropic shifts. Also note that the allylic carbon-
carbon bond lengths of 3 are similar because 3 is a polymer in
the solid state via a fifth coordination of Li to the terminal allyl
CH2 carbon of a neighbor 3 unit.8
However, the totality of the data reflect a common partial
degree of localization for all the compounds in Table 1 and
most reported previously7,8 which prevails despite variations in
degrees of aggregation, substitution, lengths of the ligand tethers,
and structures of the ligands themselves.
(10) Kaplan, J. A.; Fraenkel, G. NMR of Chemically Exchanging Systems;
Academic Press: New York, 1980; Chapters 5 and 6.
(11) (a) Jantzi, K. L.; Puckett, C. L.; Guzei, I.; Reich, H. J. J. Org. Chem. 2005,
70, 7520. (b) Goldenberg, W. S.; Sanders, A. W.; Jantzi, K. L.; Tzschucke,
C. C. J. Am. Chem. Soc. 2003, 125, 3509. (c) Reich, H. J.; Goldenberg,
W. S.; Sanders, A. W.; Tzschucke, C. C. Org. Lett. 2001, 3, 33. (d) Reich,
H. J.; Sikorski, W. H.; Sanders, A. W.; Gudmundsson, B. J. Am. Chem.
Soc. 1998, 120, 4035. (e) Reich, H. J.; Kulicke, K. J. J. Am. Chem. Soc.
1996, 32, 1469. (f) Reich, H. J.; Kulicke, K. J. J. Am. Chem. Soc. 1996,
118, 273. (g) Klump, G. W. Recl. TraV. Chim. Pays-Bas 1986, 105, 1. (h)
Jastrzebski, J. T. B. H.; van Koten, G.; Koniju, M.; Stam, C. H. J. Am.
Chem. Soc. 1991, 113, 8546.
Within the context of results from previous studies of
internally coordinated organolithium compounds11 only the
allylic lithium compounds display partial localization of the
carbanionic entity. The degrees of partial localization are similar
9
J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006 8215

A R T I C L E S
Fraenkel et al.
point depression of 0.42 °C. The expected depression value for dimer
is 0.46 °C. Hence, 5 is best described as a dimer.
internally solvated allylic lithium compounds which are either
1,3-unsubstituted or 1,3-di-substituted. Thus at 240 K the first-
order rate constant in s-1 for 5, 188 (unsubstituted), and 228
((1,2-bis(TMS)) are, respectively, 1.45 × 106, 8.8 × 106, and
1.0. This effect is most likely the result of steric interactions.
Acknowledgment. This research was generously supported
by the National Science Foundation Grant No. 0315989 and
by the M. S. Newman Chair. We thank Dr. Charles Cottrell,
Central Campus Instrumentation Center, for untiring technical
assistance.
Experimental Section
2-[(N-(2-Methoxyethyl)-N-methyl)aminomethyl]allyllithium 5. Un-
der an atmosphere of argon 2-[N-(2-methoxyethyl)-N-methyl)amino-
methyl]-1-propene (10) (0.156 g, 1.1 mmol) in 4 mL of diethyl ether
was syringed into a Schlenk tube. After cooling to -78 °C n-
butyllithium (1.26 mL, 0.8 M, 1 mmol) was carefully added by syringe,
and the mixture was stirred for 3 h. Solvent was removed under vacuum.
The residue was crystallized in 5 mL of diethyl ether. Crystals were
washed with cold pentane (3 × 3 mL) and vacuum dried. For NMR
studies a solution was prepared of 30 mg of the title compound in 0.5
mL of THF-d8.
Note Added after ASAP Publication. After this paper was
published ASAP on June 6, 2006, the captions of Figures 1
and 2 were modified to better explain the drawings. The
corrected version was published on June 8, 2006.
Supporting Information Available: Synthetic procedures,
NMR spectra, Eyring plots, and crystallographic data (CIF
format). This material is available free of charge via the Internet
at http://pubs.acs.org.
Freezing Point Depression of 5 in Benzene. Compound 5 (250
mg) was dissolved in 8.8 g of benzene. This solution gave a freezing
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8216 J. AM. CHEM. SOC. VOL. 128, NO. 25, 2006