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

Abstract

Several 1-sila allylic lithium compounds have been prepared with potential ligands for Li substituted at the 2-position. They are [2-[[cis-2,5-bis(methoxymethyl)-1-pyrrolidinyl]methyl]-1-(trimethylsilyl)allyl]lithium (22), [2-[[cis-2,5-bis(methoxymethyl)-1-pyrrolidinyl]methyl]-1-(dimethylethylsilyl)allyl]lithium (23), [2-[[bis(2-methoxyethyl)amino]methyl]-1-(dimethylethylsilyl)allyl]lithium (24), [2-[[bis(2-methoxyethyl)amino]methyl]-1-(tert-butyldimethylsilyl)allyl]lithium (25), and [2-[2-[bis(2-methoxyethyl)amino]-1,1-dimethylethyl](ethyldimethylsilyl)]allyllithium (26). Using diethyl ether or THF solutions all these compounds exhibited one bond 13C, 7Li spin coupling of ∼8 Hz, a 1:1:1:1 13C NMR pattern indicating monomeric structures; all show ligand resonances to be magnetically nonequivalent and reveal C1, C3 13C NMR shifts of about 40 and 75 δ which lie between those for model delocalized 1 and localized species 2. These compounds are concluded to be examples of the heretofore missing folded, internally tridentately coordinated partially delocalized structures with small detectable C, Li covalence. The exception is 25 which, in diethyl ether, consists of a rapidly interconverting equilibrium mixture of localized and delocalized more solvated forms, the former prevailing at 300 K and progressively converting mainly to the latter by 180 K. NMR line shape analysis of the diastereotopic gem methylsilyl 13C resonances as well as that due to the ligand carbons shows that all these line shape changes are due to the dynamics of inversion at lithium-bound carbon and that other ligand reorientation processes are slower than carbanide inversion; for inversion, ΔH ⧧ is found to be 6−9 kcal·mol-1, respectively. Averaging with increasing temperature of the 13C, 7Li spin coupling in 24 provides the dynamics of bimolecular carbon lithium bond exchange with ΔH ⧧ of 12 kcal·mol-1. Mechanisms are proposed on the basis of the data. We ascribe restricted stereochemistry of ligand lithium coordination to be responsible for these unusual internally coordinated structures.

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

“…It minimized to a structure with D 2d symmetry, and has a shift that is 14 ppm upfield of the experimental value, evidently reflecting the presence of two negatively charged allyls in the environment of the sodium. These now (12). The fact that the calculated chemical shifts of different models of the tetramer are all closely similar and are well over 10 ppm from that of the experimental shift of the substituted sodium allyl is strong evidence that there is a real difference between these values, greater than the reasonable error in the calculations.…”

“…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].…”

Alkali Metal Complexes of Silyl‐Substituted ansa‐(Tris)allyl Ligands: Metal‐, Co‐Ligand‐ and Substituent‐Dependent Stereochemistry

“…[40] The asymmetric allyl coordination in 8 is likely to arise from steric clashes between the three pmdeta ligands, and may indicate partial delocalization of the formal negative charges within the allyl anions as has previously been observed in intramolecularly solvated alkalimetal allyl compounds. [24][25][26][27]30,41] Indeed, it is notable that the allylic carbon-carbon bonds in 8 are unequal in length, with C(2)-C(3) and C(3)-C(4) being 1.382(7) and 1.426(7) Å, C(17)-C(18) and C(18)-C(19) being 1.367(7) and 1.424 (7), and C(32)-C(33) and C(33)-C(34) being 1.379 (7) 3 form, revealing that these two configurations are very close in energy. Consequently, it is possible that environment effects may play a role in determining the preference of one stereochemistry over the other.…”

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.…”