Reactions of an Osmium(IV) Complex with Allenedienes: Coordination and Intramolecular Cycloadditions

The reactions of OsH₂Cl₂(PⁱPr₃)₂ (1) with dimethyl (E)-2-(buta-2,3-dien-1-yl)-2-(penta-2,4-dien-1-yl)­malonate (I), dimethyl 2-((2E,4E)-hexa-2,4-dien-1-yl)-2-(buta-2,3-dien-1-yl)­malonate (II), and dimethyl (E)-2-(penta-2,3-dien-1-yl)-2-(penta-2,4-dien-1-yl)­malonate (III) have been studied. The reaction with I affords an OsCl₂(PⁱPr₃)₂(allenediene) complex with the organic ligand coordinated by the terminal double bond of the allene moiety. In toluene at 353 K, this compound evolves into an (η⁶-arene)­OsCl₂(PⁱPr₃)₂ species. The arene ligand results from an intramolecular (4 + 2) cycloaddition of the coordinated allenediene molecule and subsequent aromatization, with hydrogen loss, of the resulting tetrahydroindene. The reaction of 1 with II gives an allenediene complex similar to that of I. However, in this case, the allenediene species evolves into a hydride carbyne derivative. Complex 1 promotes both (2 + 2) and (4 + 2) cycloadditions of III to give respectively a bicyclo[3.2.0]­heptane skeleton featuring a cyclobutane ring and an Os arene derivative similar to that formed with I.