treffpunkt-naturwissenschaft.com

Folgende sigmatrope Umlagerung verstehe ich nicht:

"By clicking on this diagram two additional examples of thermal [1,5] hydrogen shifts will be displayed. These reactions are particularly informative in that [1,3] hydrogen shifts are not observed. The reactant in the first equation is a deuterium labeled 1,3,5-cyclooctatriene. On heating, this compound equilibrates with its 1,3,6-triene isomer, and the two deuterium atoms are scrambled among the four locations noted. If [1,3] or [1,7] hydrogen shifts were taking place, the deuterium atoms would be distributed equally among all eight carbon atoms. On prolonged heating, or at higher temperatures these cyclooctatrienes undergo electrocyclic ring opening to 1,3,5,7-octatetraene and reclosure to vinyl-1,3-cyclohexadienes."

(leider gibt es kein Diagramm, entnommen aus:https://www2.chemistry.msu.edu/faculty/r…ml/pericycl.htm)

Wieso ist der [1,5]-H-Shift bevorzugt im Gegensatz zum [1,3]-H-Shift?