A DENSITY-FUNCTIONAL STUDY ON OLEFIN INSERTION AND HYDROGEN-TRANSFER IN THE REACTION BETWEEN CL2TI-ETHYL AND ETHYLENE - POSSIBLE IMPLICATIONS FOR THE STEREOCHEMISTRY AND CHAIN TERMINATION IN OLEFIN POLYMERIZATION()
Ly. Fan et al., A DENSITY-FUNCTIONAL STUDY ON OLEFIN INSERTION AND HYDROGEN-TRANSFER IN THE REACTION BETWEEN CL2TI-ETHYL AND ETHYLENE - POSSIBLE IMPLICATIONS FOR THE STEREOCHEMISTRY AND CHAIN TERMINATION IN OLEFIN POLYMERIZATION(), Canadian journal of chemistry, 73(7), 1995, pp. 989-998
Calculations based on density functional theory have been carried out
on the reaction between Cl2Ti+-ethyl (1) and ethylene. In this study 1
was taken as a model for the cationic metallocenes of group-4 element
s, which have been developed by Kaminsky and Brintzinger as efficient
catalysts for the polymerization of olefins. The ground state structur
e of 1 has a beta-agostic conformation in which a single C-beta-H bond
is directed towards the metal center. It was assumed that this confor
mation also serves as a model for the resting state of the growing cha
in attached to the cationic group-4 metallocenes between insertions. T
wo paths were considered for the reaction between 1 and ethylene. The
first (2) has ethylene approaching the agostic C-beta-H bond, whereas
ethylene in the second approach (3) attacks the Ti-C-alpha link from t
he side opposite to the C-beta-H bond. The front-side attack (2) resul
ts in a transfer of hydrogen from the beta-carbon of ethyl to ethylene
and represents a chain-terminating step with an activation energy of
5.3 kcal/mol. It was not possible to locate a path leading to olefin i
nsertion from the front-side attack (2). The back-side attack (3) resu
lted readily in insertion with an activation energy of 3.9 kcal/mol. T
he study made use of full transition state optimization as well as a t
racing of the reaction paths by the intrinsic reaction coordinate (IRC
) method of Fukui. Previous investigations have all assumed that olefi
n insertion takes place via a front-side approach (2) based on the kno
wn stereochemistry of alpha-olefin polymerization. The present study s
uggests that insertion takes place by a back-side approach (3), and th
is suggestion is discussed in connection with the known stereochemistr
y of olefin polymerization.