Cryptocatalytic 1,2‐Alkene Migration in a σ‐Alkyl Palladium Diene Complex

[(RCN)2PdCl2]‐Catalyzed E/Z Isomerization of Alkenes: A Non‐Hydride Binuclear Addition–Elimination Pathway

“…Unpublished observations5 made during mechanistic studies6 of Pd‐catalyzed 1,6‐diene cycloisomerization7 led us to suspect that there is a mechanistic dichotomy in [(RCN) 2 PdCl 2 ]‐catalyzed alkene isomerization,1 and thus to investigate selective decoupling of migration from E / Z isomerization using [Pd‐H] traps 6e,f. 8 We began with the isomerization of deuterium labeled methyl allyl malonate ( Z )‐1‐[ 2 H]‐ 1 (Figure 1).…”

“…However, detailed analysis of the kinetics was precluded by extensive H/D exchange 9. Repeating the isomerization in the presence of 1 to 5 mol % 1,5‐diene 3 ,6e,f an efficient [Pd‐H] trap, induced substantial suppression of both the alkene migration and the isotopic scrambling. Simple pseudo first‐order equilibrium kinetics were then observed for E / Z interconversion, see solid line in Figure 1 b 10…”

“…In summary, based on prior observations in diene cycloisomerization,6 we have reinvestigated the isomerization of alkenes catalyzed by [(RCN) 2 PdCl 2 ] 1. 2 Using [Pd‐H] traps6e,f, 8 to inhibit migration, we reveal a background interconversion of E / Z isomers that does not involve [Pd‐H] species,1a,i or allylic CH insertion,1b,d–h, 2a or incipient β‐carbocations 1f. 4, 19 Kinetic simulations, isotopic labeling and DFT studies support E / Z interconversion proceeding through reversible syn and anti binuclear chloropalladation of the alkene (Scheme and Figure 3).…”

“…The geometries of the proposed intermediates should also facilitate the design of selective isomerization catalysts based on variation of the spectator alkenes (L) in complexes 8 – 11 . The selective inhibition of Pd‐catalyzed alkene migration, using [Pd‐H] traps such as 1,5‐diene 3 6e,f or TEMPO,8 may also find application in other catalytic processes, for example in Mizoroki–Heck reactions 24. It is also evident from Figure 3, that 9 → 10 → 11 → 9 can effect intramolecular enantiofacial interconversion of a coordinated prochiral alkene, that is without dissociation from the metal or involvement of an external ligand; a feature that may be of importance in asymmetric catalysis.…”