<TABLE>jo702239vn00001>journals/joceah/73/i04/figures/jo702239vn00001.gif" ALIGN="left" HSPACE=5> |
"Formal" and s
tandard Ru(II)-ca
talyzed [2 + 2 + 2] cycloaddition of 1,6-diynes
1 to alkenes gave bicyclic1,3-cyclohexadienes in relatively good yields. The neutral Ru(II) ca
talyst was formed in situ by mixingequimolecular amounts of [Cp*Ru(CH
3CN)
3]PF
6 and Et
4NCl. Two isomeric bicyclic 1,3-cyclohexadienes
3 and
8 were ob
tained depending on the cyclic or acyclic nature of the alkene partner. Mechanisticstudies on the Ru ca
talytic cycle revealed a clue for this difference: (a) when acyclic alkenes were used,linear coupling of 1,6-diynes with alkenes was observed giving 1,3,5-trienes
6 as the only initial reactionproducts, which after a thermal disro
tatory 6e
- electrocyclization led to the final 1,3-cyclohexadienes
3 as probed by NMR studies. This cascade process behaved as a formal Ru-ca
talyzed [2 + 2 + 2]cycloaddition. (b) With cyclic alkenes, the s
tandard Ru-ca
talyzed [2 + 2 + 2] cycloaddition occurred,giving the bicyclic 1,3-cyclohexadienes
8 as reaction products. A complete ca
talytic cycle for the formaland s
tandard Ru-ca
talyzed [2 + 2 + 2] cycloaddition of acetylene and cyclic and acyclic alkenes withthe Cp*RuCl fragment has been proposed and discussed based on DFT/B3LYP calculations. The mostlikely mechanism for these processes would involve the formation of ruthenacyclohep
tadiene intermediates
XXIII or
XXVII depending on the alkene nature. From these complexes, two alternatives could beenvisioned: (a) a reductive elimination in the case of cyclic alkenes
7 and (b) a
ta2.gif" BORDER=0 ALIGN="middle">-elimination followedby reductive elimination to give 1,3,5-hexatrienes
6 in the case of acyclic alkenes. Final 6e
-electrocyclization of
6 gave 1,3-cyclohexadienes
3.