Inhibition, propene insertion

Inhibition of propene insertion by ether or monomer 7A is mild but kinetically important at the concentrations used for NMR experiments ( 0.05 M), slowing the rates by a factor of 20-30. Inhibition is very pronounced with the stronger ligands Al(0113)3 and THF. This kinetic inhibition is due to the thermodynamic stability of the adducts (see Figure 4). In fact, complexation of 7L by both Al (0112)3 slows the propene... 

Under the standard conditions, alternating copolymerization is initiated by the insertion of propene to the Pd-carbon bond of Pd(COOMe) species. Addition of a large amount of an oxidant results in shortening the copolymer chain, that is, inhibiting propagation. Thus, with this protocol, dimers were prepared to estimate the enantiopurity of the chiral centers in the polymer main chain [131). For example, a head-to-tail and anti-dimer with >98% ee was obtained in the reaction catalyzed by a cationic Pd-[MeO-BICHEP] complex with 85% selectivity over other diastereomers (Scheme 7.13). 

For the (C5Hj)2Zr-catalyzed coupling of propene and a-picoline, the turnover-limiting steps are the insertion (34— 31), and the hydrogenolysis steps (31— 32), and only 34 and 31 are observed in active solutions by H NMR. Catalysis is inhibited by added picoline, owing to inhibition of picoline dissociation from 34, but is promoted by added propene and Hj. Thus, highest activities are observed at low picoline concentrations and higher propene and H2 pressures. Catalysis can be initiated by either 34 or... 

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