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Knorr, Rudolf; Lattke, Ernst; Ruhdorfer, Jakob; Roman, Ulrich von; Firl, Joachim; Böhrer, Petra (2016): What can ¹³C and ¹H NMR lithiation shifts tell us about the charge distribution in alpha-arylvinyllithium compounds? In: Journal of Organometallic Chemistry, Vol. 824: pp. 61-72
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The energetically stabilizing delocalization of negative electric charge from the Li-C(alpha) bond into the aryl pi system of alpha-arylvinyllithiums, Ar-C(Li)=CH2, is most efficient with an orthogonal relationship of the aryl ring plane and the C=C doublebond plane. This aryl conformation remains favored with at least one ortho-substituent in the alpha-aryl group. The lithiation NMR shifts, Delta delta = delta(R-Li) - delta(R-H), of the remote, para-positioned C-13 and H-1 nuclei appear to be dominated by the delocalized it charge (quasi-benzyllithium), as judged through comparison of the C-para/para-H lithiation shift quotients with that of benzyllithium. The magnitudes vertical bar Delta delta(C-para)vertical bar observed with two alpha-(2-alkylphenyl)vinyllithiums are highest for the trisolvated monomers in THE and decrease with decreasing solvation and increasing aggregation. In 3-lithio-1,1-dimethylindene, on the other hand, the alpha-aryl group is conformationally fixed in a coplanar relationship with the C=C double bond;this prevents a direct pi charge delocalization from the Li-C(alpha) bond into the adjacent aryl ring with the expected result of Delta delta(C-para) approximate to 0 in the absence of electron donor ligands. However, the disolvated dimer of this conformationally flattened alpha-arylvinyllithium exhibited a Delta delta(C-para) value whose magnitude was reduced by only 51% from that of the disolvated dimer of the above alpha-(2-methylphenyl)vinyllithium with its orthogonal orientation. (C) 2016 Elsevier B.V. All rights reserved.