| Abstract
| - A series of dendritic bis(oxazoline) ligands1−4 were synthesized to evaluate the effects of thedegreeof branching of a dendritic sector on both the reactivity andselectivity of their correspondingcopper(II) complex-catalyzed Diels−Alder reaction betweencyclopentadiene and a crotonyl imide.Kinetic studies unveiled a two-step mechanism of the Diels−Alderreaction, in which a reversiblebinding of the dienophile to the copper complex was followed by arate-determining reaction betweenthe resulting dienophile−catalyst complex with the diene.Furthermore, two interesting featuresemerged: first, the formation constant of the dienophile−catalystcomplex decreased gradually ongoing from the lower to higher generations, and secondly, while theDiels−Alder reaction rateconstant remained essentially the same from the zeroth to secondgeneration catalysts, it droppedabruptly for the third generation one. These observations wererationalized as a consequence of afolding-back of the dendritic sectors toward the catalytic unit at thethird generation, so that increasein steric size impeded both the reactivity and binding profiles of thecatalytic system. This behaviorwas reminiscent of related phenomena observed by others fromsolvatomatic, photophysical, andviscosity studies. In line with this reasoning, a slight butnoticeable substrate selectivity wasobserved for the third generation catalyst, which was absence for thelower ones, in competitivekinetic studies involving two dienophiles of different stericsizes.
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