DENDRITIC BIS(OXAZOLINE)COPPER(II) CATALYSTS .2. SYNTHESIS, REACTIVITY, AND SUBSTRATE SELECTIVITY

A series of dendritic bis(oxazoline) Ligands 1-4 were synthesized to e valuate the effects of the degree of branching of a dendritic sector o n both the reactivity and selectivity of their corresponding copper(II ) complex-catalyzed Diels-Alder reaction between cyclopentadiene and a crotonyl imide. Kinetic studies unveiled a two-step mechanism of the Diels-Alder reaction, in which a reversible binding of the dienophile to the copper complex was followed by a rate-determining reaction betw een the resulting dienophile-catalyst complex with the diene. Furtherm ore, two interesting features emerged: first, the formation constant o f the dienophile-catalyst complex decreased gradually on going from th e lower to higher generations, and secondly, while the Diels-Alder rea ction rate constant remained essentially the same from the zeroth to s econd generation catalysts, it dropped abruptly for the third generati on one. These observations were rationalized as a consequence of a fol ding-back of the dendritic sectors toward the catalytic unit at the th ird generation, so that increase in steric size impeded both the react ivity and binding profiles of the catalytic system. This behavior was reminiscent of related phenomena observed by others from solvatomatic, photophysical, and viscosity studies. In line with this reasoning, a slight but noticeable substrate selectivity was observed for the third generation catalyst, which was absence for the lower ones, in competi tive kinetic studies involving two dienophiles of different steric siz es.