Abstract
Treatment of CpW(CO) 3 (η 1 -vinylpropargyl) (1) with CF 3 SO 3 H in 1.2 equimolar proportion in Et 2 O at -78°C, followed by addition of CH 3 OH, produced CpW(CO) 2 (η 3 -2-(methoxycarbonyl)-2,4-pentadien-1-yl) (2) in good yield. Further treatment of 2 with CF 3 SO 3 H in cold diethyl ether (-78°C) generated an orange precipitate of [CpW(CO) 2 (η 4 -s-trans-2-(methoxycarbonyl)-1,3-pentadiene)]BF 4 (3a), which was characterized by 1 H and 13 C NMR spectra. Above -10°C, this cation underwent an irreversible isomerization to the more stable s-cis-η 4 -diene conformer 3b. The sites of nucleophilic addition to these two conformers are distinct for most nucleophiles. For 3a, hard nucleophiles such as water, alcohols, thiols, and amines added exclusively at the =C δ HMe carbon, whereas organocopper reagents preferred the C α H 2 carbon to give Michael reaction products. For 3b, nucleophiles regardless of their natures all add unambiguously at the C α H 2 carbon. The distinct nucleophilic regiochemistries of these two η 4 -diene conformers are discussed. To demonstrate the synthetic utility of these nucleophilic additions, we performed demetalation of several allyl products derived from 3a and 3b. © 1994 American Chemical Society.