Partially Delocalized Allylic Lithium Compounds:  Dynamics of Inversion, 1,3 Li Shift, and C−Li Bond, Exchange Influence of the Stereochemistry of Solvation

Abstract

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.

Restricted Stereochemistry of Solvation of Allylic Lithium Compounds: Structural and Dynamic Consequences

“…Recently, a long anticipated, third general structure of allylic lithium compounds was reported that lies between the two reference compounds, 1 and 2 . , In experiments to slow down exchange of ions among ion pairs, to facilitate structure determination, it was proposed to encapsulate the ions by means of internal coordination to lithium using a pendant ligand. However, instead of producing such an ion pair, metalation of 4 produced a species that, from its NMR spectra, is best described as 5 , internally coordinated, partially delocalized with small detectable C, Li covalence.…”

Complexes with Sterically Bulky Allyl Ligands: Insights into Structure and Bonding

“…[35][36][37] Other trimethylsilyl-substituted propenes with donor functionalities in the 2-position have been described; [38,39] these have been used in studies of internal solvation on lithium coordination modes.…”

“…[9][10][11]34,[36][37][38][39] The tmeda adduct of Li[AЈ] is a monomer in the solid state, with slightly asymmetrical π-bonding (e.g., the C-C distances in the allyl ligand differ by 0.041 Å); [9] in solution, however, the molecule appears symmetrical. Lappert has described the structure of the related complex Li{1,3-[Si(tBu)Me 2 ] 2 C 3 H 3 }(tmeda); it is more symmetrical than Li[AЈ] (e.g., ∆C-C = 0.007 Å), despite the greater bulk of the silyl substituents.…”

A tetrameric allyl complex of sodium, and computational modeling of the 23Na–allyl chemical shift

“…Silylated allyls have provided the majority of examples of the effects of such sterically enhanced ligands, and their complexes have been the subject of recent reviews [1,2]. Fraenkel reported the synthesis of lithiated derivatives of [(SiMe 3 ) n C 3 H 5Àn ] À in the early 1990s [3][4][5][6], and donor functionalized allyl ligands that provide both steric bulk and internal solvation to metal complexes are known [7][8][9][10][11][12][13]. More complex bisand tris(allyl) ligands have also been prepared [14][15][16][17][18].…”

“…c This work; the experimental value is for Na(1,3-(SiH 3 ) 2 C 3 H 3 )(thf).dominate the shift calculation, and the previously observed effects of silyl substitution and THF addition on the monomeric 6 are muted. There is essentially no change ($1 ppm downfield) with {Na(C 3 H 5 )(thf)} 4(11), or with the further addition of silyl groups to form {Na(1,3-(SiH 3 ) 2 C 3 H 3 )(thf)} 4…”