Correlating the Reactivity of Molybdenum Imido Alkylidene <i>N</i> -Heterocyclic Carbene Complexes in Ene-Yne Metathesis and Cyclopolymerization

The neutral and cationic initiators Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OC(CH3)3)2 (Mo1), Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OCH(CH3)2)2.quinuclidine (Mo2), Mo(N-2,6-iPr2–C6H3)(CHCMe2Ph)(OCH(CH3)2)2 (Mo3), [Mo(N-2-tBuC6H4)(IMes)(CHCMe2Ph)(OTf)][B(ArF)4] (Mo4) and [Mo(N-tBu)(IMes)(CHCMe2Ph)(MeCN)2(OTf)][B(ArF)4] (Mo5, IMes = 1,3-dimesitylimidazol-2-ylidene, OTf = CF3SO3, B(ArF)4– = tetrakis(bis(3,5-trifluoromethyl)phenyl)borate) have been investigated for their regioselectivity in the cyclopolymerization of eight different diynes (4-methoxymethyl-1,6-heptadiyne (D1), 4-methoxycarbonyl-1,6-heptadiyne (D2), 4-ethoxycarbonyl-1,6-heptadiyne (D3), 4,4-bis(methoxymethyl)-1,6-heptadiyne (D4), 4,4′-bis(methoxycarbonyl)-1,6-heptadiyne (D5), 4,4′-bis(ethoxycarbonyl)-1,6-heptadiyne (D6), N,N-dipropargyl-p-toluenesulfonamide (D7), 4-(p-toluylsolfonylmethyl)-1,6-heptadiyne (D8)). Complementary, the structurally analogous ene-ynes 4-methoxymethylhept-6-ene-1-yne (E1), 4-methoxycarbonylhept-6-ene-1-yne (E2), 4-ethoxycarbonylhept-6-ene-1-yne (E3), 4,4-bis(methoxymethyl)hept-6-ene-1-yne (E4), 4,4-bis(methoxycarbonyl)hept-6-ene-1-yne (E5), 4,4-bis(ethoxycarbonyl)hept-6-ene-1-yne (E6), N-allyl-N-propargyl-p-toluenesulfonamide (E7), 4-(p-toluylsulfonylmethyl)hept-6-ene-1-yne (E8) were used as substrates in ring-closing ene-yne metathesis (RCEYM). While Mo1 – Mo3 were inactive in RCEYM, the use of the cationic complexes Mo4 and Mo5 allowed for the first exo-selective RCEYM. A strong correlation between α-selectivity in cyclopolymerization and exo-selectivity in ene-yne metathesis was found. Density functional theory (DFT) calculations confirm that exo-selectivity with these catalysts is also based on an α-selective, “yne-first” pathway.