Tin(II) Hydrides as Intermediates in Rearrangements of Tin(II) Alkyl Derivatives

Reactions of the Sn­(II) hydrides [ArSn­(μ-H)]₂ (<b>1</b>) (Ar = Ar^iPr4 (<b>1a</b>), Ar^iPr6 (<b>1b</b>); Ar^iP4 = C₆H₃-2,6-(C₆H₃-2,6-^<i>i</i>Pr₂)₂, Ar^iPr6 = C₆H₃-2,6-(C₆H₂-2,4,6-^<i>i</i>Pr₃)₂) with norbornene (NB) or norbornadiene (NBD) readily generate the bicyclic alkyl-/alkenyl-substituted stannylenes, ArSn­(norbornyl) (<b>2a</b> or <b>2b</b>) and ArSn­(norbornenyl) (<b>3a</b> or <b>3b</b>), respectively. Heating a toluene solution of <b>3a</b> or <b>3b</b> at reflux afforded the rearranged species ArSn­(3-tricyclo­[2.2.1.0^2,6]­heptane) (<b>4a</b> or <b>4b</b>), in which the norbornenyl ligand is transformed into a nortricyclyl group. ¹H NMR studies of the reactions of <b>4a</b> or <b>4b</b> with <i>tert</i>-butylethylene indicated the existence of an apparently unique reversible β-hydride elimination from the bicyclic substituted aryl/alkyl stannylenes <b>2a</b> or <b>2b</b> and <b>3a</b> or <b>3b</b>. Mechanistic studies indicated that the transformation of <b>3a</b> or <b>3b</b> into <b>4a</b> or <b>4b</b> occurs via a β-hydride elimination of <b>1a</b> or <b>1b</b> to regenerate NBD. Kinetic studies showed that the conversion of <b>3a</b> or <b>3b</b> to <b>4a</b> or <b>4b</b> is first order. The rate constant <i>k</i> for the conversion of <b>3a</b> into <b>3b</b> was determined to be 3.33 × 10^–5 min^–1, with an activation energy <i>E</i>_a of 16.4 ± 0.7 kcal mol<sup>–1