Isomerization of Olefins Triggered by Rhodium‐Catalyzed CH Bond Activation: Control of Endocyclic β‐Hydrogen Elimination

Abstract

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 ...

Direct Production of Aviation Fuel Range Hydrocarbons and Aromatics from Oleic Acid without an Added Hydrogen Donor

“…Besides, the double bond could shift. 30 In our experiment, 1-heptene, 2-heptene, and 3heptene could be detected in the decarboxylation of 3-octenoic acid. Therefore, the double bond could shift and 1-heptadecene could be chosen to support the mechanism of aromatization.…”

Catalytic Enantioselective Transformations Involving C–H Bond Cleavage by Transition-Metal Complexes

“…262 Both aryl and alkyl substitution was well-tolerated, and yields and enantioselectivities ranged from 73 to 96% and 91 to 99% ee, respectively. On the basis of related literature reports 264,265 and their own mechanistic studies, Dong and co-workers proposed that the reaction proceeds via an isomerization/hydroacylation sequence. Specifically, oxidative addition intermediate 402 undergoes an intramolecular migratory insertion of an alkene into the Rh− hydride bond, leading to the five-membered rhodacycle 403.…”

Rhodium‐Catalyzed Intramolecular C−H Silylation by Silacyclobutanes

“…[12] In general, endocyclic b-Helimination of related metallacycles (especially that of 3-to 6-membered rings) are conceived to be extremely challenging. [16] Herein we reported that SCB is an efficient reagent for CÀHb ond silylation [17] under Rh catalysis in the presence of ahindered ligand, possibly enabled by endocyclic b-H elimination of Rh silametallacycles ( Figure 1b). [14] Although DFT calculations suggested that endocyclic b-H elimination might be possible, [15] only af ew examples were reported very recently.…”