In the course of comparing the effectiveness of an HOAtderived coupling reagent (HAPyU) and a phosphonic acidbased condensation agent (T3P) by cyclization of model sequences, we found that T3P was a superior reagent with regard to conversion of the linear peptide into the stereochemically intact cyclic monomer when sterically hindered amino acids are found at the cyclization site.
3-Methoxyestra-1,3,5(lO)-trien-16-one (6) was converted to the corresponding fulvene (7). Methyllithium was added and the resulting (16a-methylestratrieny1)cyclopentadiene 8 separated by chromatography. Deprotonation gave the (16a-methylestratrienyl-C5H4)Li reagent 9 that was added to 0.5 molar equivalents of ZrC14(THF)2 to yield bis[q5-(3-methoxy16a-methylestra-1,3,5( 10)-trien-16~-yl)cyclopentadienyl]-zirconium dichloride (10,45%). Complex 10 was activated by treatment with a n excess of methylalumoxane. The resulting homogeneous Ziegler-type catalyst was employed for stereoselective propene polymerization. In the temperature range of -50 to -10°C partly isotactic polypropylene was obtained. A detailed analysis has revealed that stereocontrol is effected by a combination of enantiomorphic site a n d chain-end control. Compounds 7 and 10 were characterized by X-ray diffraction.In a series of papers we have recently described the synthesis and structural properties of a variety of group-4 metallocene complexes bearing steroid-and terpene-derived substituents at the cyclopentadienyl, indenyl, or tetrahydroindenyl ring systems. It was shown that these substituents, which were employed optically active from chiral pool sources, may very effectively control the conformational equilibrium of the bis(indeny1)-and bis(tetrahydroindeny1)zirconium backbone. In several cases this characteristic was utilized to generate efficient homogeneous Ziegler-type catalysts from such complexes (by activation with an excess of methylalumoxane) for the selective formation of highly isotactic polypropylene [']. The influence of the secondary and tertiary terpenyl and steroidal substituents at simple cyclopentadienyl ligands appears to be much less effective. We have recently shown that cholestan-3-yl-derived substituents [which very effectively controlled isotactic polypropylene formation at the substituted bis(indeny1)-and bis(tetrahydr0in-denyl)ZrCl,-derived catalyst systems] were quite ineffective in doing so in the corresponding (R*-C5H4)2ZrC12/MA0 catalyst systems (MAO = methylalumoxane). In the specific cases tested, no transfer of the metallocene chirality was obtained. The small degree of isotacticity observed was almost entirely due to stereochemical chain-end control taking place during the course of the catalytic polymerization reaction [']. We have checked a few additional steroid substitution patterns of single chiral substituents attached to the cyclopentadienyl rings in such systems and found that the degree of chirality transfer may vary with the specific steroid substituent used. In addition the present study has also shown that an aromatic ethereal functional group attached to the far end of the steroid framework does neither adversely interfere with the generation of the homogeneous Ziegler catalyst system nor with the rapidly proceeding repetitive carbon-carbon coupling process taking place at the activated early transition metal center. A typical example is described in this paper.
Results and Discussion
Synthesis a...
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