Mh. Cheng et al., NUCLEOPHILIC-ADDITION TO TUNGSTEN ETA(4)-2-(METHOXYCARBONYL)-1,3-PENTADIENE CATIONS - CONTROL OF NUCLEOPHILIC REGIOCHEMISTRY BY THE DIENE CONFORMATION, Organometallics, 13(10), 1994, pp. 4082-4091
Treatment of CpW(CO)3(eta1-vinylpropargyl) (1) with CF3SO3H in 1.2 equ
imolar proportion in Et2O at -78-degrees-C, followed by addition of CH
3OH, produced (CO)2(eta3-2-(methoxycarbonyl)-2,4-pentadien-1-yl) (2) i
n good yield. Further treatment of 2 with CF3SO3H in cold diethyl ethe
r (-78-degrees-C) generated an orange precipitate of a4-s-trans-2-(met
hoxycarbonyl)-1,3-pentadiene)]BF4 (3a), which was characterized by H-1
and C-13 NMR spectra. Above -10-degrees-C, this cation underwent an i
rreversible isomerization to the more stable s-cis-eta4-diene conforme
r 3b. The sites of nucleophilic addition to these two conformers are d
istinct for most nucleophiles. For 3a, hard nucleophiles such as water
, alcohols, thiols, and amines added exclusively at the =C(delta)HMe c
arbon, whereas organocopper reagents preferred the C(alpha)H2 carbon t
o give Michael reaction products. For 3b, nucleophiles regardless of t
heir natures all add unambiguously at the C(alpha)H2 carbon. The disti
nct nucleophilic regiochemistries of these two eta4-diene conformers a
re discussed. To demonstrate the synthetic utility of these nucleophil
ic additions, we performed demetalation of several allyl products deri
ved from 3a and 3b.