Sören Randoll | PubMed Health

PubMed2006

Stereoselective allyl transfer to chiral alpha-methoxycarbaldehydes: a model study related to the C-9/C-15 fragment of geldanamycin.

The enantiomerically pure alpha-methoxycarbaldehyde 3 was prepared from l-leucine in five steps and 31% overall yield. The aldehyde was subjected to a diastereoselective BF3-mediated crotylation with silane 4 and to various reagent-controlled addition reactions. The configuration of aldol addition products 19 and 20 was proven by single crystal X-ray crystallography. Based on these data spectral comparison allowed an unambiguous assignment of the desired anti,syn-crotylation product 2b and of the syn,syn-crotylation product 2a. Unexpectedly, product 2a of the BF3-mediated crotylation is formally the product of chelation-control. The anti,syn-crotylation product 2b, which is a model for the C-9/C-15 fragment of geldanamycin, was obtained by a reagent-controlled crotylation with the chiral (Z)-crotylborane 23.

Aldehydes↗

PubMed2005

Imidazolin-2-iminato titanium complexes: synthesis, structure and use in ethylene polymerization catalysis.

The Staudinger reaction of the imidazolin-2-ylidenes, 1,3-di-tert-butylimidazolin-2-ylidene (1a), 1,3-diisopropylimidazolin-2-ylidene (1b), 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (1c), 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (1d) and 1,3-bis(2,6-diisopropylphenylimidazolin-2-ylidene (1e), with trimethylsilyl azide furnishes the corresponding N-silylated 2-iminoimidazolines 2a-e, which react with [(eta-C5H5)TiCl3] to afford half-sandwich cyclopentadienyl titanium complexes of the type [CpTi(L)Cl2] (3) (L = imidazolin-2-iminato ligand). Similarly, the reactions of 1,3-di-tert-butyl-2-(trimethylsilylimino)imidazoline (2a) with [(eta-tBuC5H4)TiCl3] results in the formation of [(eta-tBuC5H4)Ti(L)Cl2] (4) (L = 1,3-di-tert-butylimidazolin-2-imide). Bis(1,3-di-tert-butylimidazolin-2-iminato)titanium dichloride (5) is obtained from the reaction of two eq. of 2a with TiCl4. Treatment of 5 with methyllithium results in the formation of the corresponding dimethyl complex [L2Ti(CH3)2] (6), whereas [CpTi(L)(CH3)2] (7) is similarly obtained from 3a. The molecular structures of 3a, 3b, 3c, 3e x C7H8, 4 and 7 are reported revealing linearly coordinated imidazolin-2-iminato ligands together with very short Ti-N bond distances. All dichloro complexes (3a-e, 4 and 5) can be activated with methylaluminoxane (MAO) to give active catalysts for ethylene homopolymerization. In most cases, moderate to high activities are observed together with the formation of high (HMWPE) or even ultra high molecular weight polyethylene (UHMWPE).

Catalysis↗

PubMed2004

Titanium complexes with imidazolin-2-iminato ligands.

The reaction of 1,3-di-tert-butylimidazolin-2-ylidene with trimethylsilyl azide furnishes the corresponding N-silylated 2-iminoimidazoline which reacts with titanium(IV) chlorides to yield imidazolin-2-iminato titanium complexes.

Journal Article↗