12806
J. Am. Chem. Soc. 2000, 122, 12806-12812
Alkyl-Substituted Allylic Lithium Compounds: Structure and
Dynamic Behavior†
Gideon Fraenkel* and Fayang Qiu
Contribution from the Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210-1185
ReceiVed June 22, 2000. ReVised Manuscript ReceiVed October 10, 2000
Abstract: Several methyl-substituted allylic lithium compounds have been prepared by CH3Li cleavage of
their corresponding bis(methyl)bis(allylic)stannanes. Low-temperature 13C and proton NMR studies of 1:1
complexes of these allylic lithium compounds with TMEDA establish their structures. NMR line shape changes
with temperature provide barriers to rotation. Results are listed in order as follows (allyl substituents, compound
number, barrier to rotation in kcal‚mol-1, and bonds undergoing rotation): 1,1-dimethyl, 26, 18, 2-3; endo-
1-methyl, 27, 19, 2-3; endo-1-exo-3-dimethyl, 28, 21, 1-2 and 2-3. These observations together with the
allylic 13C NMR chemical shifts indicate that in the case of unsymmetrical alkyl substitution at the termini the
allyl C-C bond to the more substituted terminus is of higher bond order than that to the less substituted
terminus. Unsymmetrical substitution is proposed to reduce the degree of delocalization compared to the
symmetrically substituted allylic lithium compounds. A mechanism is proposed for the rotation process which
is consistent with the Eyring activation parameters.
Allylic lithium compounds,1 which are the simplest of the
conjugated carbanionic substances, have been extensively
investigated, via X-ray crystallogaphy,2 spectroscopy,3 and
calculations.4 These studies show that most solvated allylic
lithium compounds assume the delocalized contact ion-paired
structure 1, within which coordinated lithium lies normal to the
allylic plane.2,3 In contrast unsolvated alkane-soluble allylic
lithium compounds such as 2 exhibit 13C NMR shifts which
† This article is dedicated to Professor Chengxue Zhao of Shanghai
Jiaotong University on the occasion of his 60th birthday.
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are consistent with a localized structure.5 These unsolvated
species are most likely aggregated.5 One might have expected
to find examples of allylic lithium compounds, such as 3, in
which the degree of π-delocalization lies between that of 1 and
2. We recently reported that certain internally coordinated allylic
lithium compounds, 4, exhibit such intermediate degrees of
π-delocalization.6 We proposed that the short tether of the
pendant ligand restricts the stereochemistry of coordination of
lithium.6 This places lithium off the vertical above the allyl plane
above C1 thus favoring unusual partial C-Li covalency.
Low-temperature NMR studies of selected solvated allylic
and benzylic lithium compounds showed that contrary to
prevailing opinion these ion-paired species adopt favored
structures. Furthermore, below 160 K the rate of reorganization
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10.1021/ja002248u CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/05/2000