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(C₄Bu^t₂Me₂)₂Cl₃]₂[Pd₂Cl₆](2), which on reaction with pyridine gives the cyclobutadiene complex [Pd₂(C₄Bu^t₂Me₂)₂Cl₄] and [Pd(py)₂Cl₂]. At –50 °C in CH₂Cl₂ the π-acetylene complex [Pd₃(bma)₂Cl₆L_n] is formed, which rearranges (at 0 °C) to the σ,π-butadienyl complex [Pd₃(CBu^tCMeCMeCBu^tCl)₂Cl₄](10A). Complex (10A) reacts with bipy to give the kinetically favoured adduct [Pd(CBu^tCMeCMeCBu^tCl)(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(CBu^tCMeCMeCBu^tCl)Cl}₂], and [Pd(CBu^tCMeCMeCBu^tCl)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.