Supporting InformationMaterials and Methods. All manipulations of air-sensitive materials were carried out with rigorous exclusion of oxygen and moisture in flame-or oven-dried Schlenk-type glassware on a dual-manifold Schlenk line, or interfaced to a high-vacuum line (10 -6 Torr), or in a nitrogenfilled Vacuum Atmospheres glovebox with a high capacity recirculator (< 1 ppm of O 2 ). Argon (Matheson, prepurified) was purified by passage through a MnO oxygen-removal column and a Davison 4A molecular sieve column. Pentane and toluene were dried using activated alumina columns according to the method described by Grubbs, 1 and were additionally vacuumtransferred from Na/K alloy immediately before use if employed for catalyst synthesis or catalytic reactions. Ether and THF were distilled before use from appropriate drying agents (sodium benzophenone ketyl, Na/K alloy) under nitrogen. Chloroform-d was purchased from Cambridge Isototope Laboratories. Benzene-d 6 , toluene-d 8 , and cyclohexane-d 12 (Cambridge Laboratories; all 99+ atom % D) used for NMR reactions and kinetic measurements were stored in vacuo over Na/K alloy in resealable bulbs, and were vacuum transferred immediately prior to use. All organic starting materials were purchased from Aldrich Chemical Co. or Lancaster Synthesis Inc., and were used without further purification unless otherwise stated. Substrates 3, 5, 7, 9, 11 were prepared as reported previously 2 , dried a minimum of two times as solutions in benzene-d 6 (substrates 3, 5, 11) or toluene-d 8 (substrates 7, 9) over freshly activated Davison 4A molecular sieves, and degassed by freeze -pump -thaw methods. They were then stored in vacuum-tight storage flasks. Solutions of 9 were protected from light to minimize non-catalytic endocyclization reactions. The organolanthanide precatalysts Ln[CH(SiMe 3 ) 2 ] 3 (Ln = La, Nd, Sm, Y, Lu), 3 and Ln[N(SiMe 3 ) 2 ] 3 4 (Ln = La, Nd, Sm, Y) were prepared by published procedures.
