ROTATIONAL ISOMERS OF MO(VI) ALKYLIDENE COMPLEXES AND CIS-TRANS POLYMER STRUCTURE - INVESTIGATIONS IN RING-OPENING METATHESIS POLYMERIZATION

Low-temperature (-85-degrees-C) photolysis of complexes of the type sy n-M(NR)(CHR')(OR'')2 (where M = Mo (usually) or W; R = 2,6-i-Pr2C6H3, 4-Br-2,6-i-Pr2C6H2, 2,6-Me2C6H3, 2-t-BuC6H4, 2-i-PrC6H4, 1-adamantyl; R' = CMe3, CMe2Ph; OR'' = OCMe3, OCMe2(CF3), OCMe(CF3)2, OC(CF3)3, OC( CF3)2(CF2CF2CF3), O-2,6-i-Pr2C6H3) in thf-d8, toluene-d8, or methylcyc lohexane-d14 generated significant quantities of the anti rotamer. Ant i to syn isomerization rate constants (k(a/s)) were determined by NMR methods and correlated with the nature of R, R', R'', and solvent. Act ivation parameters were calculated in toluene-d8 and thf-d8 for the se ries Mo(N-2,6-i-Pr2C6H3)(CHCMe2-Rh)(OR'')2 (where OR'' = OCMe2(CF3), O CMe(CF3)2, OC(CF3)3, or OC(CF3)2(CF2CF2CF3)). An estimate of k(a/s) fo r Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OCMe3)2 at 25-degrees-C was obtained u sing activation parameters. Values for k(a/s) were found to vary by up to 7 orders of magnitude (at 298 K), the smallest values for k./s bei ng found in complexes that contain the most electron-withdrawing alkox ides in thf as a solvent. Equilibrium constants (K(eq) = k(a/s)/k(s/a) ) at 25-degrees-C were obtained from 500 MHz H-1 NMR spectra and found to vary by up to approximately 2 orders of magnitude. Values for k(s/ a) at 298 K were calculated and found to vary by up to 6 orders of mag nitude in the same direction as k(a/s). 2,3-Bis(trifluoromethyl)norbor nadiene (NBDF6) was added to photolyzed initiators of the type Mo(NR)( CHR')-(OR'')2 (where R = 2,6-i-Pr2C6H3, 2-t-BuC6H4, 1-adamantyl; R' = CMe3, CMe2Ph; OR'' = OCMe2(CF3), OCMe(CF3)2, OC(CF3)2(CF2CF2CF3)) in t oluene-d8 and thf-d8 at temperatures where the rate of isomerization o f anti rotamers to syn rotamers was negligible. The anti rotamer of Mo (N-2,6-i-Pr2C6H3)(CHCMe2Ph)[OCMe(CF3)2]2 was found to be at least 2 or ders of magnitude more reactive than the syn rotamer in thf-d8 and in toluene-d8. Results were similar for other complexes containing an ary limido ligand. The anti rotamer of every complex containing an arylimi do ligand reacted with NBDF6 to yield a syn first insertion product th at contained a trans C=C (anti --> syn,trans), and the syn rotamer pro duced a syn first insertion product that contained a cis C=C bond (syn --> syn,cis). For Mo(N-1-adamantyl)(CHCMe2Ph)[OCMe(CF3)2]2 both anti and syn isomers reacted at approximately the same rate with NBDF6; ant i yielded a syn,trans first insertion product (anti --> syn,trans); bu t syn yielded an anti,trans first insertion product (syn --> anti,tran s). The result of bulk polymerizations (40-100 equiv of NBDF6) at 25-d egrees-C suggests a relationship between alkylidene rotamer isomerizat ion rates and polymer cis/trans content when the initiator contains an arylimido ligand; high-cis polymers are formed when rotamer isomeriza tion rates are negligible on the time scale of polymerization, while h igh-trans polymers are obtained when rotamer isomerization rates are f ast on the time scale of polymerization. A mechanism is proposed for f ormation of cis polymers that consists of olefin attack (through the e xo face) on the CNO face of the syn rotamer of the catalyst with C-7 o f the monomer extending over the arylimido ring. Two plausible mechani sms for forming trans polymers are (i) a similar monomer approach to t he CNO face of anti rotamers or (ii) monomer approach to the COO face of the catalyst in which the alkylidene ligand has rotated by up to 90 -degrees. Bulk polymerizations with the 1-adamantylimido initiator con taining OCMe(CF3)2 ligands do not yield trans or cis polymers selectiv ely.