Perturbation of Conjugation in Allylic Lithium Compounds Due to Stereochemical Control of Internal Lithium Coordination:  Crystallography, NMR, and Calculational Studies

Abstract

Several allylic lithium compounds have been prepared with ligands tethered at C(2). These are with (CH(3)OCH(2)CH(2))(2)NCH(2)-, 6, 1-TMS 5, 1,3-bis(TMS) 8, and 1,1,3-tris(TMS) 9. Allylic lithiums with (CH(3)OCH(2)CH(2))(2)NCH(2)C(CH(3))(2)-, are 10, 1-TMS 11, and 1,3-bis(TMS), 12 compounds with -C(CH(3))(2)CH(2)N-((S)-(2-methoxymethyl)-pyrrolidino) at C(2) 13, 1-TMS 14, and 1,3-bis(TMS) 15. In the solid state, 8-10 and 12 are monomers, 6 and 13 are Li-bridged dimers, and 5 and 7 are polymers. In solution (NMR data), 5, 7-12, 14, and 15 are monmeric, and 6 is a dimer. All samples show lithium to be closest to one of the terminal allyl carbons in the crystal structures and to exhibit one-bond (13)C-(7)Li or (13)C(1)-(7)Li spin coupling, for the former typically ca. 3 Hz and for the latter 6-8 Hz. In every structure, the C(1)-C(2) allyl bond is longer than the C(2)-C(3) bond, and both lie between those for solvated delocalized and unsolvated localized allylic lithium compounds, respectively, as is also the case for the terminal allyl (13)C NMR shifts. Lithium lies 40-70 degrees off the axis perpendicular to the allyl plane at C(1). These effects are variable, so the trend is that the differences between the C(1)-C(2) and C(2)-C(3) bond lengths, (13)delta(3)-(13)delta(1) values, and the (13)C(1)-(7)Li or (13)C-(6)Li coupling constants all increase with decreasing values of the torsional angle that C(1)-Li makes with respect to the allyl plane.

Structural Studies on (–)‐Sparteine‐Coordinated Lithiosilanes

“…Moreover, due to the quadrupole moment of both 6 Li and 7 Li (spin quantum number Ͼ 1/2, 6 Li = 1, 7 Li = 3/2), NMR signals are often broadened and not fully resolved. [16,17] In general, desired couplings to adjacent atoms are often more visible at lower temperatures. Thereby, especially resolved coupling patterns are of great interest for the understanding of the structural situation in solution, as the coupling between two atoms is a hint to the actual structure.…”

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.…”

“…[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.…”