Journal of the American Chemical Society, Vol.126, No.12, 3983-3995, 2004
Perturbation of conjugation in allylic lithium compounds due to stereochemical control of internal lithium coordination: Crystallography, NMR, and calculational studies
Several allylic lithium compounds have been prepared with ligands tethered at C-2. These are with (CH3OCH2CH2)(2)NCH2-, 6, 1-TMS 5,1,3-bis(TMS) 8, and 1,1,3-tris(TMS) 9. Allylic lithiums with (CH3OCH2CH2)(2)NCH2C(CH3)(2)-, are 10, 1-TMS 11, and 1,3-bis(TMS), 12 compounds with -C(CH3)(2)CH2N((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 C-13-Li-7 or C-13(1)-Li-7 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 C-13 NMR shifts. Lithium lies 40700 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 C-13(1)-Li-7 or (13) C-Li-6 coupling constants all increase with decreasing values of the torsional angle that C-1-Li makes with respect to the allyl plane.