Dendritic bis(oxazoline)copper(II) catalysts .2. Synthesis, reactivity, and substrate selectivity

A series of dendritic bis(oxazoline) Ligands 1-4 were synthesized to evaluate the effects of the degree of branching of a dendritic sector on 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 between the resulting dienophile-catalyst complex with the diene. Furthermore, two interesting features emerged: first, the formation constant of the dienophile-catalyst complex decreased gradually on going from the lower to higher generations, and secondly, while the Diels-Alder reaction rate constant remained essentially the same from the zeroth to second generation catalysts, it dropped abruptly for the third generation one. These observations were rationalized as a consequence of a folding-back of the dendritic sectors toward the catalytic unit at the third generation, so that increase in steric size impeded both the reactivity 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 competitive kinetic studies involving two dienophiles of different steric sizes.