Synthesis and Ring‐Opening Metathesis Polymerization (ROMP) Reactivity of endo‐and exo‐Norbornenylazlactone Using Ruthenium Catalysts

Abstract

AbstractSummary: Norbornenylazlactone (NBAz) was prepared by a Diels‐Alder cycloaddition between cyclopentadiene and 2‐vinyl‐4,4‐dimethyl‐5‐oxazolone. The stereoselectivity of this pericyclic reaction was in favour of endo isomers. The ring‐opening metathesis polymerization (ROMP) of NBAz was carried out in presence of ruthenium catalysts [(PCy3)2(Cl)2RuCHPh 1; (SIMes)(PCy3)(Cl)2RuCHPh 2 (SIMes: 1,3‐dimesityl‐4,5‐dihydroimidazol‐2‐ylidene); (PiPr3)2(Cl)2RuCHSPh 3; (PiPr3)(Cl)2RuCH(CH2)2C, N‐2‐C5H4N 4]. It was found that the exo isomer is much more reactive than the corresponding endo isomer. Some hypotheses of mechanism are suggested to justify the low polymerization rate of endo isomer using the results of NBAz derivatives as the hydrolyzed NBAz, 5, and the coupling product between NBAz and glycine methyl ester hydrochloride, 6. For exo‐NBAz, the propagating rate constant, kp, was determined with the four Ru‐initiators and their activity decreases in the order 2 > 1 > 3 > 4. Complementary kinetic measurements with 1 and 2 were given as kp/ki ratio. The ROMP of exo isomer was faster with 2 than 1 in spite of a worse control of the chains size. The azlactone ring survived intact during the polymerization and may be used for further reactions with nucleophiles.ROMP of NBAz by ruthenium initiators.magnified imageROMP of NBAz by ruthenium initiators.