No abstract

Exposure of heptane solutions of Mo(NAr)(CHCMe2Ph)(Me2Pyr)(OAr) (1a; Ar = 2,6-diisopropylphenyl), Mo(NAr)(CHCMe3)(Me2Pyr)[OCMe(CF3)2] (1b), and Mo(NAr)(CHCMe2Ph)(Me2Pyr)(OSiPh3) (1c) to one atmosphere of ethylene for 12 h yields the ethylene complexes, Mo(NAr)(CH2CH2)(Me2Pyr)(OAr) (2a), Mo(NAr)(CH2CH2)(Me2Pyr)[OCMe(CF3)2] (2b), and Mo(NAr)(CH2CH2)(Me2Pyr)(OSiPh3) (2c). Addition of 1 equiv of triphenylsilanol to a solution of 2c gives Mo(NAr)(CH2CH2)(OSiPh3)2 (3) readily. Mo(NAr)(CHCMe2Ph)(OTf)2(dme) reacts slowly with ethylene (60 psi) in toluene at 80 °C to give cis and trans isomers of Mo(NAr)(CH2CH2)(OTf)2(dme) (4a) in the ratio of ∼2(cis):1. Addition of lithium 2,5-dimethylpyrrolide to 4a under 1 atm of ethylene produces Mo(NAr)(CH2CH2)(η1-Me2Pyr)(η5-Me2Pyr) (5a). Mo(NAr)(CHCMe2Ph)(η1-MesPyr)2 (MesPyr = 2-mesitylpyrrolide) reacts cleanly with ethylene in benzene at 60 °C over a period of four days to give exclusively Mo(NAr)(CH2CH2)(MesPyr)2 (5b). Treatment of 5b with 2 equiv of (CF3)2CHOH in ether yields Mo(NAr)(CH2CH2)[OCH(CF3)2]2(Et2O) (6). Neat styrene reacts with 2c and 3 to generate the styrene complexes, Mo(NAr)(CH2CHPh)(Me2Pyr)(OSiPh3) (7) and Mo(NAr)(CH2CHPh)(OSiPh3)2 (8), respectively. Similarly, the trans-3-hexene complex, Mo(NAr)(trans-3-hexene)(OSiPh3)2 (9a), can be prepared from 3 and neat trans-3-hexene. When 3 is exposed to 1 atm of ethylene, the molybdacyclopentane species, Mo(NAr)(C4H8)(OSiPh3)2 (10), is generated. X-ray structural studies were carried out on 2c, 5a, 6, 8, 9a, and 10. All evidence suggests that alkene exchange at the Mo(IV) center is facile, followed by cis,trans isomerization and isomerization via double bond migration. In addition, trace amounts of alkylidene complexes are formed that result in slow metathesis reactions of free olefins to give (e.g.) a distribution of all possible linear olefins from an initial olefin and its double bond isomers.

An X-ray structural study of Mo(NAd)(CHCMe2Ph)(2,5-Me2NC4H2)2 (1; Ad = 1-adamantyl) reveals it to contain one η1-2,5-Me2NC4H2 ring and one η5-2,5-Me2NC4H2 ring. The structures of Mo(NAr)(CHCMe2Ph)(pyrrolide)2 (Ar = 2,6-i-Pr2C6H3) complexes that contain 2,3,4,5-tetramethylpyrrolides, 2,5-diisopropylpyrrolides, or 2,5-diphenylpyrrolides are analogous to that of 1. In contrast, Mo(NAr)(CH2CMe2Ph)(indolide)2 (6) was shown to contain two η1-bound indolides. Monohexafluoro-t-butoxide pyrrolide (MAP) species can be prepared, either through addition of one equiv of Me(CF3)2COH to a bispyrrolide or through reactions between the lithium pyrrolide and the bishexafluoro-t-butoxide. A trimethylphosphine adduct of a bispyrrolide, Mo(NAd)(CHCMe2Ph)(η1-NC4H4)2(PMe3), has been prepared and structurally characterized, while a PMe3 adduct of MAP hexafluoro-t-butoxide species was found to have PMe3 bound approximately trans to the pyrrolide. This adduct serves as a model for the structure of the initial olefin adduct in olefin metathesis.

Grafting [Mo(⋮NAr)(CHCMe2R)(NR2)2] on SiO2-(700) gives [(⋮SiO)Mo(⋮NAr)(CHCMe2R1)(NR2)] as a major surface species along with [(⋮SiO)Mo(NAr)(CH2CMe2R1)(NR2)2] according to mass balance analysis, IR and NMR spectroscopies. The replacement of one amido group by a siloxy group from the surface of silica dramatically increased the activity of the Mo complex in olefin metathesis, and tuning the imido ligands allows the reactivity to be increased by at least one order of magnitude so that an overall turnover of 231 000 can be achieved in the metathesis of propene. Additionally, ethyl oleate can also be converted into a thermodynamic equilibrated mixture in 8 h with only 0.02% of catalyst.