This contribution reports a kinetic/mechanistic/stereochemical/moiecular mechanics study of olefin insertion into the actinide-hydrogen bonds of Cp′2An(H)(OR) complexes (Cp′ = η5-(CH3)5C5; An = Th, U; R = achiral or chiral alkyl group). For the reaction Cp′2An(H)(O-t-Bu) + cyclohexene (An = Th), the rate law is first order in organoactinide and first order in olefin, with kTh-H/kTh-D = 1.4 (1), kTHF/ktoluene = 0.59 (5), ΔH⋆ = 9.0 (5) kcal/mol, and ΔS⋆ =-47.2 (1) eu; kU/kTh ≈ 1.5. For Cp′2Th(H)[OCH(t-Bu)2] + 1-hexene, kTh-H/kTh-D = 1.3 (2). Alkoxide effects on insertion rates can be large, and for Cp′2Th(H)(OR) + cyclohexene, ko-t-Bu/kOCH(t-Bu)2 ≳ 103. For Cp′2Th(H) [OCH(t-Bu)2], relative insertion rates follow the ordering ethylene > 1-hexene > 4-methoxystyrene > styrene ≫ cyclohexene; k4-methoxystyrene/kStyrene = 2.2 (5). For cis-2-butene, insertion reactions yield sec-butyl derivatives that slowly rearrange to the n-butyl isomers. Insertion experiments with Cp2′An(H)(OR⋆) complexes having chiral alkoxide ligands (OR⋆ = (1R,2S,5R)-menthoxide; (R)-2-butoxide; [(1S)-endo]-bornoxide; (1S,2S,5R)-neomenthoxide) and prochiral olefins (norbornene, cis-2-butene) or ketones (acetophenone, butyrophenone) demonstrate that maximum diastereoselection occurs for sterically encumbered reagents at low temperatures (de = 36% for norbornene at-45 °C). A molecular mechanics/molecular graphics analysis suggests that the sterically most favorable direction of olefin approach toward the actinide center is between the U-H and U-O bonds rather than from the side. These results provide additional insight into ancillary ligand effects on the kinetics of organo-f-element-catalyzed olefin hydrogenation.
ASJC Scopus subject areas
- Colloid and Surface Chemistry