PICOLINE AND TOLUENE - A COMPARISON OF STRUCTURAL-PROPERTIES AND RELATIVE STABILITIES OF THEIR MONOLITHIATED DERIVATIVES

Energy differences between five isomeric monolithiated 4-picolines 6a (eta3 at Calpha/C4/C3), 6b (eta5 at Nl/C2/C3/C6/C5), 6c (eta2 at Calph a/C4), 6da (N1-lithiated), and 6e (eta3 at C3/C4/C5) were calculated b y ab initio (MP2/6-31+G//6-31G*) and semiempirical (MNDO) methods. Wi th this basis set and inclusion of the ZPE correction, 6a-c and 6e tur n out to be quite close in energy (within 0.13 kcal mol-1). 6c and 6e are transition structures. The N-lithiated structure 6da is least stab le (about 5.5 kcal mol-1, compared with the above compounds). MNDO ove remphasizes the instability of 6da by about 11 to 16 kcal mol-1. Howev er, the ab initio calculated structural parameters of the anionic part of Gda compare quite well with those of a model of a ''dimeric'' stru cture 8, in which the 4-picolyl and the NH2 anion are arranged in the characteristic lithium-bridged bis-amide fashion as found, e.g., for [ LDA(TMEDA)]2 complexes. Furthermore, the heterocyclic moiety of the mo del compounds 8 and 6da reveal significant similarities to X-ray struc tural data of closely related [{4-(CH3CRCH3)-C5H4N}Li(TMEDA or (THF)2) ]2 complexes (R = H, CH3) 3,4a, and 4b. TMEDA or THF coligands almost do not influence the structural parameters of the anionic part. Theref ore, 6da and the Calpha-substituted models [4-(CH3CRC5H4N)Li (6db: R = H, 6dc: R = CH3), which are accessible to reliable ab initio calculat ions, represent the most simplified structures to describe this part o f such complexes adequately. Similar aspects for the interpretation of a variety of known benzyllithium X-ray structures (which differ in th e nature of the coligands involved) are found by calculating the four analogous ''pi-eta'' isomers la,b. In this context, MNDO should be use d with caution, as relative stabilities and activation barriers disagr ee significantly with high level ab initio calculations. In contrast t o MP2/6-31+G//6-31G* calculations, MNDO predicts ortho-lithiated tolu ene 7 to be more stable than la,b.