Synthesis of Multisubstituted\nCyclopentadiene Derivatives\nfrom 3,3-Disubstituted Cyclopropenes and Internal Alkynes Catalyzed\nby Low-Valent Niobium Complexes

A low-valent niobium species generated\nfrom NbCl<sub>5</sub> and\n1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (<i>Si</i>-Me-CHD) in combination with PPh<sub>3</sub> catalyzed a [2+2+1]-cycloaddition\nreaction of 3,3-disubstituted cyclopropenes and 2 equiv of diaryl/dialkylalkynes,\nleading to isomeric mixtures of multisubstituted cyclopentadienes <b>3</b>–<b>5</b>. The initial catalyst activation process\nwas a one-electron reduction of NbCl<sub>5</sub> with <i>Si</i>-Me-CHD to provide [NbCl<sub>3</sub>(μ-Cl) (L)]<sub>2</sub> (L = PMe<sub>2</sub>Ph (<b>6</b>), L = PPh<sub>3</sub> (<b>7</b>)) in the presence of phosphine ligands. An NMR spectroscopic\ntime course experiment using complex <b>7</b> as the catalyst\nrevealed an induction period for the product formation, corresponding\nto an additional one-electron reduction of <b>7</b> by the substrates\nto give catalytically active η<sup>2</sup>-alkyne complexes\nof NbCl<sub>3</sub>. A combined computational and experimental study\nclarified the mechanism of this unprecedented [2+2+1]-cyclopentadiene\nsynthesis; a rate-determining 1,2-insertion of cyclopropene into η<sup>2</sup>-alkyne niobium species to form cyclopropane-fused metallacyclopentene\nfollowed by ring-opening β-C elimination provides a dienylalkylidene\nintermediate prior to incorporation of the second alkyne through carbene/alkyne\nmetathesis. We also demonstrated the synthetic utility of the multisubstituted\ncyclopentadienes as the cyclopentadienyl ligands by derivatizing to\nthe corresponding lithium cyclopentadienide, which is applicable for\nthe synthesis of ferrocene <b>10</b>.