Synthesis of Multisubstituted Cyclopentadiene Derivatives from 3,3-Disubstituted Cyclopropenes and Internal Alkynes Catalyzed by Low-Valent Niobium Complexes

A low-valent niobium species generated from NbCl₅ and 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (<i>Si</i>-Me-CHD) in combination with PPh₃ catalyzed a [2+2+1]-cycloaddition reaction of 3,3-disubstituted cyclopropenes and 2 equiv of diaryl/dialkylalkynes, leading to isomeric mixtures of multisubstituted cyclopentadienes <b>3</b>-<b>5</b>. The initial catalyst activation process was a one-electron reduction of NbCl₅ with <i>Si</i>-Me-CHD to provide [NbCl₃(μ-Cl) (L)]₂ (L = PMe₂Ph (<b>6</b>), L = PPh₃ (<b>7</b>)) in the presence of phosphine ligands. An NMR spectroscopic time course experiment using complex <b>7</b> as the catalyst revealed an induction period for the product formation, corresponding to an additional one-electron reduction of <b>7</b> by the substrates to give catalytically active η²-alkyne complexes of NbCl₃. A combined computational and experimental study clarified the mechanism of this unprecedented [2+2+1]-cyclopentadiene synthesis; a rate-determining 1,2-insertion of cyclopropene into η²-alkyne niobium species to form cyclopropane-fused metallacyclopentene followed by ring-opening β-C elimination provides a dienylalkylidene intermediate prior to incorporation of the second alkyne through carbene/alkyne metathesis. We also demonstrated the synthetic utility of the multisubstituted cyclopentadienes as the cyclopentadienyl ligands by derivatizing to the corresponding lithium cyclopentadienide, which is applicable for the synthesis of ferrocene <b>10</b>.