Metal complexes as catalysts for stereoselective synthesis
Metal complexes have been used as catalysts for stereoselective syntheses in CC bond formation reactions, namely, alkyl- and allyl- additions to aldehydes. The enantiomeric alkylations involved reactions between Et2Zn and arylaldehydes in the presence of chiral templates, e.g. zinc-, titanium- and boron-complexes of (S)-(-)-2-amino-3-(p-hydroxyphenyl)-1,1-diphenylpropanol. The reaction has been proved to be catalytic, using 3-5 mol% of the chiral ligand in the addition of Et2Zn to p-tolualdehyde. Study of the substituent effects in arylaldehydes showed that p-tolualdehyde leads to the highest optical yield (83%ee) when a chiral Schiff base-zinc complex is formed. The in-situ produced chiral templates for the alkylations were studied by two different methods. Firstly, the alkyl metal itself; i.e. Et2Zn, acts as the metal key atom. Secondly, other metal compounds; i.e. BH3.THF and Ti(OPri)4 formed chiral auxiliary complexes for the additions of Et2Zn to arylaldehyde. The highest optical yield for 1-p-tolylpropanol [ca. 80%ee (S)] was obtained using the zinc- and boron-complexes. The chiral amino alcohol was also deposited onto silica gel to act as a heterogeneous catalyst. The asymmetric ethylations of p-tolualdehyde using the silica gel supported ligand have been tested with zinc-, titanium- and boron-complexes. Diethylzinc alone provides the best selectivity (53%ee), the values for the boron and the titanium systems are 16 and < 1%ee, respectively. On subsequent use of the solid catalyst, the optical yields fall off. Allylic additions of Bu3SnCH2CH=CH2 to arylaldehydes have been examined in the presence of the chiral amino alcohol and titanium compounds. The optimum result was found to occur when TiCl4 was reacted with the amino alcohol prior to the simultaneous addition of the reacting materials. However, the chiral allylic alcohol was obtained in low optical yield (31%ee).