Helical chirality palladium complexes

It was presumed from molecular models that the 3,6-substituents of quinox-aline units may play an important role in the stabilization of optically active helical structures. 1,2-Diisocyanobenzene 86 (4 equiv) bearing p-tolyl groups at the 3- and 6-positions was subjected to oligomerization in the presence of the chiral palladium complex 84 (Scheme 54) [96]. The resulting mixture of oligomers was separated by preparative GPC, leading to the isola-... 

Metallomesogens have been shown to form helical supramolecular organisations in their mesophases [95]. Chiral oxazoline complexes with various metal ions and six alkyl chains did not show LC behaviour, but when mixed with trinitrofluorenone form achiral smectic A phases [96]. Furthermore, when a branch was included in the structure of the ligands (Fig. 12) the corresponding complexes with copper(II) and palladium(II) form columnar mesophases which have a helical organisation [97]. The presence of the stereogenic centre near the central metal ion in these complexes (Fig. 12) is enough to cause the parallel molecules to stack in a tilted manner with... 

Enantiomerically pure 5-palladatricyclo[4.1.0.0 " ]heptanes have been prepared and been converted into enantio-merically pure complexes with helical chirality at palladium. The reaction of an excess of dimethyl 3,3-dimethyl-cyclopropene-l,2-dicarboxylate with [Pd2(dba)3]-CHCl3 gave diastereoselectively as side-product a new palladacycle 122 308 Purthermore, a highly diastereoselective synthesis of palladepanes 120 and 121 has been achieved. ... 

The reaction of [2+2+2] cycloaddition of acetylenes to form benzene has been known since the mid-nineteenth century. The first transition metal (nickel) complex used as an intermediate in the [2+2+2] cycloaddition reaction of alkynes was published by Reppe [1]. Pioneering work by Yamazaki considered the use of cobalt complexes to initiate the trimer-ization of diphenylacetylene to produce hexasubstituted benzenes [54]. Vollhardt used cobalt complexes to catalyze the reactions of [2+2+2] cycloaddition for obtaining natural products [55]. Since then, a variety of transition complexes of 8-10 elements like rhodium, nickel, and palladium have been found to be efficient catalysts for this reaction. However, enantioselective cycloaddition is restricted to a few examples. Mori has published data on the use of a chiral nickel catalyst for the intermolecular reaction of triynes with acetylene leading to the generation of an asymmetric carbon atom [56]. Star has published data on a chiral cobalt complex catalyzing the intramolecular cycloaddition of triynes to generate a product with helical chirality [57]. 

The first use of chiral helical polymers bearing no chiral side chains for chiral reaction induction was realized by Reggelin et at. in 2002 [69]. Two poly(methyl methacrylate)-based chiral polymers (40) was prepared by hehx-sense selective anionic polymerization of sterically congested methacrylates with a chiral base mixture as initiator. The pyridine moieties in helical polymers allowed various metal coordinations [70] or formation of ionic pairs [71]. Their complexes with palladium precursor were found to be active catalysts for the allyHc substitution reaction of l,3-diphenylprop-2-enyl acetate (Figure 4.36). Although the ee values were only moderate (<33%), this research opened up a new area for asymmetric catalysis with unnatural helical chiral polymers. 

The helical structure of (oUgoquinoxalinyl)palladium(II) complexes was confirmed by optical resolution of a diastereomeric mixture of pentameric and hexameric 5,8-di-p-tolylquinoxaline oligomers 20, which were produced by oligomerization of 3,6-di-p-tolyl-l,2-diisocyanobenzene 17 with methylpalladium(II)bromide complex with chiral bis((5)-2-methylbutyl)phenylphosphine (Scheme The right-handed and left-... 

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