A single transition state may lead to multiple intermediates or products if there is a post-transition-state reaction pathway bifurcation. These bifurcations arise when there are sequential transition states with no intervening energy minimum. For such systems, the shape of the potential energy surface and dynamic effects, rather than transition-state energetics, control selectivity. This Minireview covers recent investigations of organic reactions exhibiting reaction pathway bifurcations. Such phenomena are surprisingly general and affect experimental observables such as kinetic isotope effects and product distributions.

As a complement to Pd(0)-catalyzed cyclizations, seven Pd(II)-catalyzed cyclization strategies are reported. α,ω-Diynes are selectively hydroborated to bis(boronate esters), which cyclize under Pd(II)-catalysis producing a diverse array of small, medium, and macrocyclic polyenes with controlled E,E, Z,Z, or E,Z stereochemistry. Various functional groups are tolerated including aryl bromides, and applications are illustrated.

Palladium(II)-catalyzed macrocyclizations of bis(vinylboronate ester) compounds are demonstrated to provide a strategically efficient approach to transannular Diels-Alder reaction substrates. In several systems reported, the macrocycle is preorganized such that cycloaddition at room temperature occurs concomitantly with cyclization. Numerous advantages over palladium(0)-catalyzed cross-coupling approaches are demonstrated.

A new strategy to access macrocyclic enynes was developed. To block undesired ene−yne cyclization pathways, alkynes were protected via bromination and the resultant acyclic vic-(E)-dibromotrienes participated in selective ene−ene ring closing metathesis reactions. Zinc-promoted deprotection of (E)-dibromodienes provided macrocyclic enynes in high yields.

Ein einziger Übergangszustand kann zu mehreren Zwischenstufen oder Produkten führen, wenn sich der Reaktionsweg nach dem Übergangszustand gabelt. Solche Gabelungen (Bifurkationen) entstehen beim Vorliegen sequenzieller Übergangszustände ohne dazwischenliegendem Energieminimum. Die Selektivität dieser Reaktionen wird nicht von der Energetik des Übergangszustands, sondern von der Form der Potentialenergiehyperfläche und von dynamischen Effekten gesteuert. In diesem Kurzaufsatz werden jüngste Untersuchungen von organischen Reaktionen mit Reaktionswegbifurkationen vorgestellt. Diese Bifurkationen sind überraschend häufig und wirken sich auf experimentell bestimmbare Größen wie kinetische Isotopeneffekte und Produktverteilungen aus.