DIASTEREOSELECTIVE CONDENSATION-REACTIONS WITH AN EASILY ACCESSIBLE INTRAMOLECULARLY COORDINATED ZINC ENAMINE - MOLECULAR-STRUCTURE OF A SELF-ASSEMBLED POLYMER OF DIETHYLZINC AND ZINC ALDOLATES

The zinc-enamine EtZn(t-BuNCH=CHN(Et)-t-Bu) 1 is easily accessible via the reaction of Et2Zn with the alpha-diimine t-BuN=CHCH=N-t-Bu (t-BuD AB) and is reactive in C-C coupling reactions with aldehydes to form d imeric zinc-aldolates. The reactions of 1 with aryl aldehydes, like be nzaldehyde and 2-, 3-, and 4-pyridinecarboxaldehyde, afford exclusivel y the anti zinc-aldolates, whereas the reactions with alkyl aldehydes like propionaldehyde and 2-methylpropionaldehyde afford a mixture of s yn and anti zinc-aldolates. Quenching of either the anti zinc-aldolate or the of syn and anti zinc-aldolates with trimethylsilyl chloride af fords the anti diastereoisomers, which can be isolated in high yields. The condensation reaction of 1 with pyridine-2-carboxaldehyde results in the formation of a thermally unstable zinc-aldolate that subsequen tly rearranges to a 2,3-diaminoindolizine. With the enones, benzyliden eacetone, and chalcone, the corresponding Michael addition products ar e obtained in good yield. The dimeric zinc-aldolate that results from the C-C coupling reaction of 1 with pyridine-4-carboxaldehyde forms a coordination polymer upon addition of 0.5 equiv of Et2Zn. The X-ray st ructure of the polymer, wherein the 4-pyridine nitrogens act as comple xating agents to bridging Et2Zn entities, shows the monomeric zinc-ald olate units to have a distorted boat conformation resulting from the s teric hindrance of the N(Et)-t-Bu substituents. The polymeric structur e is a nice example of self-assembly of different molecules, (R)- and (S)-zinc-aldolates, and Et2Zn units, with the right match of stereoche mical and electronic information.