Five-membered metallacycles are typically reluctant to undergo endocyclic β-hydrogen elimination. The rhodium-catalyzed isomerization of 4-pentenals into 3-pentenals occurs through this elementary step and cleavage of two C–H bonds, as supported by deuterium-labeling studies. The reaction proceeds without decarbonylation, leads to trans olefins exclusively, and tolerates other olefins normally prone to isomerization. Endocyclic β-hydrogen elimination can also be controlled in an enantiodivergent reaction on a racemic mixture.

Five-membered metallacycles are typically reluctant to undergo endocyclic b-hydrogen elimination. The rhodiumcatalyzedisomerization of 4-pentenals into 3-pentenals occurs through this elementary step and cleavage of two CÀHbonds, as supported by deuterium-labeling studies.T he reaction proceeds without decarbonylation, leads to trans olefins exclusively,a nd tolerates other olefins normally prone to isomerization. Endocyclic b-hydrogen elimination can also be controlled in an enantiodivergent reaction on ar acemic mixture.Five-membered metallacycles are important intermediates of numerous catalytic processes,b oth in academic laboratories and in large-scale industrial chemistry.[1] As shown in experimental [2] and theoretical [3] studies,g eometric constraints make these intermediates reluctant to undergo endocyclic b-hydrogen (b-H) elimination, especially in the case of square-planar complexes.H owever,a nd although thorough experimental studies are still lacking, theoretical studies suggest that five-membered metallacycles that are not square-planar could undergo b-H elimination more easily. [5] Importantly,s ubstrate decarbonylation is anotorious problem during the hydroacylation of 4-pentenals, especially in the case of a,a-disubstituted aldehydes (R ¼ 6 H). [6][7][8][9] In contrast, we have found that the rhodium-catalyzed isomerization of 4-pentenal 1 (R = Ph) into 3-pentenal 2 occurs without decarbonylation in 86 %yield and in ahighly stereoselective fashion. Them ost efficient catalyst was prepared with ligand L1, [10] whereas those prepared with L2-L5 led to incomplete conversion and decarbonylation. Hence,weassumed this isomerization to be triggered by C À H bond activation (a), and the catalytic cycle would be completed by migratory insertion of the terminal olefin into the rhodium-hydrogen bond thus engendered (b), followed by endocyclic b-H elimination of A (R = Ph) (c), and final reductive elimination (d).Herein, we report at horough study of the reaction depicted in Figure 1, including deuterium-labeling experiments that support the postulated mechanism and the endocyclic b-H elimination of rhodacyclopentanone A. Moreover,w ea lso show that the isomerization is chemoselective for olefins that enable the formation of A,a nd that olefins located elsewhere on the substrate remain intact under the reaction conditions,e ven in the challenging case of sensitive olefins normally prone to facile isomerization in the presence of transition-metal catalysts, [11] including rhodium catalysts.[12] Finally,w ed escribe how the endocyclic b-H elimination of rhodacyclopentanones can be prevented, whereby each enantiomer of the racemic 4-pentenal undergoes ad istinct and enantioselective rearrangement when treated with an enantiopure catalyst.We found that the rhodium-catalyzed isomerization of deuterated substrates 3 and 4 into compounds 5 and 6, respectively,occurred smoothly with complete transfer of the deuterium atom at the positions indicated in Scheme 1. Tr ansient intermediate 3-int was observed ...

Abstract4‐Pentenals isomerize to 3‐pentenals by rhodium catalysis.