Allyllithiums have been variously described, omitting solvation and aggregation, as contact ion-pairs, 1, including delocalized anions, a localized species, 2, or something in between 3.
While exo-exo-[1,3-bis(trimethylsilyl)allyl]lithium (15) and [1-(trimethylsilyl)allyl]lithium (16) were previously shown to be contact ion pairs containing delocalized carbanions, the corresponding species with a pendant ligand at the 2-position, [2-[[bis(2-methoxyethyl)amino]methyl]-1,3-bis(trimethylsilyl)allyl]lithium (14) and [2-[[bis(2-methoxyethyl)amino]methyl]-1-(trimethylsilyl)allyl]lithium (12), respectively, appear from their 13C NMR shifts and the first observation of 13C lithium spin coupling in an allylic lithium to be partially delocalized with detectable C−Li covalence. In proposed structures 12 and 14, lithium is tridentately complexed. N and Li lie within the allyl carbon plane with the two oxygens normal to it on opposite sides. NMR line shape analysis and 13C1 of signal averaging of the 13C−6Li coupling of 12 provides dynamics of intermolecular C−Li bond exchange with ΔH e ⧧ and ΔS ⧧ of 11.6 kcal/mol and −11.5 eu, respectively. Inversion at the lithium-bound carbon of 12 averages nonequivalent ligand shifts. Line shape analysis gives ΔH i ⧧ and ΔS i ⧧ of 8 kcal/mol and −10 eu, respectively. Line shape changes observed for the methylsilyl (13C and 1H) resonances as well as of the terminal 13C's of 14 due to a 1,3 Li sigmatropic shift yield activation parameters ΔH s ⧧ and ΔS s ⧧ of 18 kcal/mol and +15 eu. These results show that electronic structure of nominally conjugated organolithium compounds can be significantly altered by changing the stereochemistry of solvation, by use of pendant ligands, producing structures previously described in other systems as transition states for allylic rotation.
Several methyl-substituted allylic lithium compounds have been prepared by CH 3 Li cleavage of their corresponding bis(methyl)bis(allylic)stannanes. Low-temperature 13 C 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 13 C 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 13 C NMR shifts which 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 C 1 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 † This article is dedicated to Professor Chengxue Zhao of Shanghai Jiaotong University on the occasion of his 60th birthday.(1) (a) Wardell, J. L. Etzrodt, H.; Marsh, M.; Massa, W.; Baum, G.; Dietrich, H.; Mahdi, W. Angew. Chem. 1986, 98, 84. (e) Boche, G.; Fraenkel, G.; Cabral, J.; Harms, K.; Eikema-Hommes, N. J. P. Van.; Lohrenz, J.; Marsch, M.; Schleyer, P. v. R.
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