A Kinetic, Mechanistic, and Molecular Mechanics Investigation of Olefin Insertion into Organoactinide-Hydride Bonds. Metal, Olefin, Ancillary Ligand, and Diastereoselection Effects

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 k_Th-H/k_Th-D = 1.4 (1), k_THF/k_toluene = 0.59 (5), ΔH^⋆ = 9.0 (5) kcal/mol, and ΔS^⋆ =-47.2 (1) eu; k_U/k_Th ≈ 1.5. For Cp′₂Th(H)[OCH(t-Bu)₂] + 1-hexene, k_Th-H/k_Th-D = 1.3 (2). Alkoxide effects on insertion rates can be large, and for Cp′₂Th(H)(OR) + cyclohexene, k_o-t-Bu/k_OCH(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; k_{4-methoxystyrene}/k_Styrene = 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.