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Spectroscopic Characterization, Reactivity, and Reactions of (Arene)Cr(CO)3-Stabilized γ-Propargyl−Allenyl Cations

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journal contribution
posted on 12.08.1999, 00:00 by Thomas J. J. Müller, Markus Ansorge, Kurt Polborn
(Arene)Cr(CO)3-stabilized γ-propargyl−allenyl cations Cr(CO)36-R1C6H4)C⋮CC+R2R3 (2) are reactive intermediates in the synthesis of arene-complex-substituted allenes or propargyl derivatives using mild in situ ionizations of the corresponding propargyl derivatives Cr(CO)36-R1C6H4)C⋮CCXR2R3 (X = OH, OAc). The regioselectivity of the nucleophilic attack is strongly dependent on the substitution pattern on the propargyl side chain, i.e., steric and electronic factors. In a representative case of a stable cation (2a; R1 = H, R2 = R3 = Ph) the structure investigation of this ambident electrophile by NMR spectroscopy and UV/vis spectroscopy reveals that a considerable contribution of the allenylium resonance structure accounts for a strong participation of the chromium carbonyl fragment in the efficient stabilization. The reactivity of 2a was studied by measuring the UV/vis kinetics for the nucleophilic trapping reactions revealing that the complexed cation 2a is by 2 orders of magnitude less reactive than the corresponding free ligand.