NMR detection of living intermediates prepared from activated [NON]ZrMe2 ([NON]2- = [(t-Bu-d6-N-o-C6H4)2O]2-) and olefins

The C-13-NMR spectrum of {[NON]Zr((CH3)-C-13)(S)}(+) (S = bromobenzene-d(5)) after addition of one equivalent of 1-hexene reveals resonances at 30.8 ppm for the terminal (CH3)-C-13 group in the first insertion product, at 24.0 ppm for the terminal (CH3)-C-13 group in the second insertion product and near 20 ppm for the terminal (CH3)-C-13 group in higher insertion products. The latter are consistent with 'insertion' of the 1-hexene into the Zr-CH3 bond in a 1,2 manner. Addition of ten equivalents of 1-nonene to {[NON]Zr(CH3)(S)}(+) followed by one equivalent of (CH3)-C-13=CHC7H15 led to a C-13-NMR spectrum consistent with formation of {[NON]Zr[(CH3CH)-C-13(C7H15)(Polymer)](S)}(+), which confirms that 1-nonene 'inserts' into the Zr-C bond primarily in a 1,2 fashion. A discussion as to why beta elimination is relatively slow in {[NON]Zr(R)(S)}(+) systems that have been examined so far focuses on reversible addition of a terminal olefin only to the CNN face of the pseudo-tetrahedral cation, {[NON]Zr(R)}(+), to yield a trigonal bipyramidal transition state. After the equatorial alkyl group migrates to the substituted carbon of the incoming olefin, the new bulky alkyl in {[NON]Zr(CH2CHPR')}(+) cannot 'back up' toward the two t-butyl groups in preparation for beta elimination relative to the rate at which {[NON]Zr(CH2CHPR')}(+) reads with either base or more olefin. (C) 1998 Elsevier Science S.A. All rights reserved.