Insights in the rhodium-catalyzed tandem isomerization-hydroformylation of 10-undecenitrile: Evidence for a fast isomerization regime

Abstract

The tandem isomerization-hydroformylation of 10-undecenitrile (1) into the corresponding linear aldehyde (2) with a Rh-biphephos system was studied and the formation of internal olefin isomers (1-int-x) was monitored over time. The existence of an “isomerization phenomenon” was evidenced, where fast isomerization of 1 into up to 70% of 1-int-x followed by fast back-isomerization of 1-int-x into 1 and, in turn, into 2 occurs. This fast dynamic isomerization regime is favored at high syngas pressure (40 bar) and low biphephos-to-Rh ratio (5–10), and it is best observed at relatively high catalyst loadings ([1]0/[Rh] ≤ 3000). The latter regime is indeed evanescent, and gives place to a second stage in which isomerization of internal olefins (and eventual conversion into 2) proceeds much more slowly. The results are tentatively rationalized by the formation of an unstable species that promotes dynamic isomerization and which slowly vanishes or collapses into a Rh-biphephos species which is the one responsible for hydroformylation.