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

• Published in: Journal of the American Chemical Society 2004-03, Vol.126 (12), p.3983-3995
• Main Authors: Fraenkel, Gideon, Chow, Albert, Fleischer, Roland, Liu, Hua
• Format: Article
• Language: English
• Subjects: Alkali and alkaline-earth metals derivatives; Chemistry; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Organic chemistry; Organic compounds; Organometalloidal and organometallic compounds; Physics; Preparations and properties; Structure of solids and liquids; crystallography; Structure of specific crystalline solids
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja030370y

Several allylic lithium compounds have been prepared with ligands tethered at C2. These are with (CH3OCH2CH2)2NCH2−, 6, 1-TMS 5, 1,3-bis(TMS) 8, and 1,1,3-tris(TMS) 9. Allylic lithiums with (CH3OCH2CH2)2NCH2C(CH3)2−, are 10, 1-TMS 11, and 1,3-bis(TMS), 12 compounds with −C(CH3)2CH2N-((S)-(2-methoxymethyl)-pyrrolidino) at C2 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 13C−7Li or 13C1−7Li spin coupling, for the former typically ca. 3 Hz and for the latter 6−8 Hz. In every structure, the C1−C2 allyl bond is longer than the C2−C3 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 13C NMR shifts. Lithium lies 40−70° off the axis perpendicular to the allyl plane at C1. These effects are variable, so the trend is that the differences between the C1−C2 and C2−C3 bond lengths, 13δ3−13δ1 values, and the 13C1−7Li or 13C−6Li coupling constants all increase with decreasing values of the torsional angle that C1−Li makes with respect to the allyl plane. In the solid state, all internal ligands are fully coordinated to lithium and none of the X-ray structures incorporate solvent. Also, in the solid-state monomers 8−10 and 12, Li lies near the center of one face of a trigonal pyramid with two methoxy oxygens and nitrogen at its corners. Nitrogen occupies the apex. The above results support a variable degree of delocalization in the allylic moiety due to the proximity of lithium to one of the terminal allyl carbons. The relative contribution of ionic and covalent interactions to the C−Li bond have yet to be determined. The ligand tether constrains the stereochemistry of lithium coordination, thus stabilizing these otherwise abberant structures. Dynamics of inversion at lithium-bound carbon obtained from changes in the 13C1 NMR line shapes due to the ligands gave ΔH ⧧ values (kcal/mol) of 6.5 ± 0.4 and 15 ± 1.3 for ligand tethers of one and two carbons, respectively. Averaging of the 13C1−7Li or 13C1−6Li coupling constants observed with increasing temperature, which is due to bimolecular C1−Li exchange, typically involves ΔH ⧧ values of 12 ± 0.5 kcal/mol. In the case of