Two general mechanisms (or various versions of them) for alkene metathesis were originally considered. In the first (pair wise) mechanism, shown in Fig. P18.81, the catalyst brings about cyclobutane formation between two alkenes. The second mechanism involves metallacycle formation, as shown in Eqs. 18.59a-d (pp. 854-855).
Fig. P18.81

Eqs. 18.59a-d

Transcribed Image Text:

Pairwise (cyclobutane) mechanism: H R R2 H cyclobutane intermediate CH2 CH2 CH2 CD2 CD2 CH, CD2 cyclohexene ethylene ethylene-1,1-d ethylene-d, 1,7-octadiene- 1,1,8,8-d4 1,7-octadiene NHC NHC NHC RCH CHR C,Ru CHPh a 14e complex + :P(Cy)s RCH CHR P(Cy) a l6e complex cycloaddition cycloreversion NHC NHC NHC NHC CHPh l CHR very minor by-product RCH RCH RCH CHR a metallacycle RCH CHR NHC NHC NHC RCH CHR first metathesis product RCH CHR NHC NHC NHC CI,Ru RCH CH second RCH CHR RCH CHR RCH metathesis product catalyst enters another cycle (Eq. 18.59c)