Acetylenes and noble-metal compounds. Part 13. Formation of cyclobutadienepalladium complexes from σ,π-butadienyl complexes in the PdCl2-induced dimerisation of t-butyl(methyl)acetylene(4,4-dimethylpent-2-yne)
Abstract
4,4-Dimethylpent-2-yne [t-butyl(methyl)acetylene, bma] reacts with [Pd(NCPh)2Cl2](1) at 20 °C in CH2Cl2 to give the ionic cyclobutadiene complex [Pd2(C4But2Me2)2Cl3]2[Pd2Cl6](2), which on reaction with pyridine gives the cyclobutadiene complex [Pd2(C4But2Me2)2Cl4] and [Pd(py)2Cl2]. At –50 °C in CH2Cl2 the π-acetylene complex [Pd3(bma)2Cl6Ln] is formed, which rearranges (at 0 °C) to the σ,π-butadienyl complex [Pd3(CButCMeCMeCButCl)2Cl4](10A). Complex (10A) reacts with bipy to give the kinetically favoured adduct [Pd(CButCMeCMeCButCl)(bipy)Cl](11) which rearranges to the thermodynamically more stable isomer (12), the X-ray structure of which has been determined. 1H and 13C n.m.r. spectra have been used to deduce the structures of complexes (10A), (11), [{Pd(CButCMeCMeCButCl)Cl}2], and [Pd(CButCMeCMeCButCl)L](L = pentane-2,4-dionate or S2CNMe2). The rearrangement (11)→(12)(ΔG‡= 95 kJ mol–1) involves a rotation about the sterically hindered CMe–CMe bond of the ligand. The rearrangement of the σ,π-butadienyl complexes (10A) and (10B) to the cyclobutadiene complexes (2) and (3) respectively is shown to have ΔG‡= 95–97 kJ mol–1 and to involve an essentially non-polar transition state. The mechanism of the formation of the σ,π-butadienyl complex (10A) is discussed and reasons are proposed for the acceleration in rate caused by positive charge on the complex and the absence of a detectable σ-alkenylpalladium intermediate.