CH 409/609, Spring 2011 Dr. Shaughnessy

Problem Set #4 Due: Friday, April 29th, 2011

Name:

1. Stereoerrors during propylene polymerization catalyzed by early transition metals may not be the result of insertion of the wrong olefin face. Rather they may result from epimerization of the Zr-alkyl species.

a. Propose a mechanism for the isomerization shown above. You can assume that the polymer chain does not dissociate from the metal center during this process. Feel free to simplify the metallocene structures in your mechanisms ((EBTHI)Zr), but be sure to show the appropriate stereochemistry for the chirality center.

b. One piece of data supporting epimerization as a source of stereoerrors, is that many metallocene systems give lower isotacticity at low propylene concentration. Explain how this observation supports epimerization rather than insertion errors as the mechanism for stereoerror formation.

Zr L P

HCH3

Zr L P

CH3H

2. Provide a catalytic cycle for the reactions below. Provide the oxidation state and total electron count for each intermediate species.

O

N

NPd

H

NCCH3

+

BF4

catalyticO

CH3a.

OTsN SiMe2

CH2RuCl

ClCy3P

PCy3TsN O SiMe2

Hb.

c. Cp2TaH3D2

DD

DD

D

D You only need to show replacement of one H with D

3. The compound Cp2Ti(η2-Me3SiCCSiMe3) (1) is a ready precursor of the ‘titanocene’ fragment. The C-C bond in the coordinated alkyne measures 1.283(6) Å and the C-C-Si angles are 146° and 148°. Compare these structural features those of a typical alkyne and explain how coordination to the metal center affects the alkyne structure.