An observable model for the SN2 transition state [1a]. Hypervalent trigonal bipyramidal carbon (10‐C‐5) [1b]

Dicationic 10‐C‐5 species 14 , formally two‐electron oxidation products of 1,8‐bis(arylthio)‐9‐(2,6‐dimethoxyaryl)‐10‐phenylanthracenes, are prepared and characterized by NMR spectroscopy. Evidence is presented for a bis‐sulfonium structure containing a hypervalent trigonal bipyramidal (TBP) carbon atom. Dicationic species 14 , prepared by the reaction of the appropriate diol with strong acid in liquid sulfur dioxide or sulfurylchlorofluoride, are stable for indefinite periods at room temperature in these solutions. Structural features common to these compounds which stabilize the TBP geometry sufficiently to allow their direct observation are as follows: (a) electronegative apical ligands (sulfonium sulfur atoms), (b) five‐membered rings linking each apical position to an equatorial position, (c) geometric constraints built into the tridentate ligand in a pattern designed to stabilize TBP, relative to tetrahedral carbon, (d) three relatively electropositive (sigma‐donor) equatorial ligands, which decrease the effective electronegativity of the central carbon atom of the threecenter bond, (e) a strain‐free, bidentate π‐acceptor equatorial ligand with its π‐axis parallel with the electron‐rich three‐center four‐electron bond axis, and (f) possible residual aromaticity (bis‐ipso‐aromaticity) in the equatorial six‐membered ring incorporating the hypervalent carbon atom. A low‐temperature 19 F NMR study of 14b sets an upper limit for ΔG≠ −100°C of ca. 4 kcal/mol for a hypothetical process which would interconvert unsymmetrical species 30 and 30 ′ if such a tautomeric mixture were to represent the structure of 14 . Evidence against such a tautomeric equilibrium is adduced from the upfield shift of 0.65 ppm observed for the proton meta to the two methoxyl substituents of dication 14a relative to the comparable proton in 21a , the model dication lacking one of the two arylthio substituents of 14a . This is most easily explained by postulating a nucleophilic interaction of the second thioaryl group in 14 at the hypervalent, 10‐C‐5 carbon.