Structure and Reactivity of an Allylpalladium N-Heterocyclic Carbene Enolate Complex

Abstract

A novel anionic N-heterocyclic carbene was prepared and ligated to [Pd(allyl)Cl] 2 to yield the complex (NHC-enolate)Pd(allyl). At elevated temperatures in acetonitrile, the allyl ligand undergoes a stereoselective η 3 -η 1 -η 3 isomerization in an associative process that is first order in CD 3 CN with ∆H q ) 13.5 ( 0.7 kcal mol -1 and ∆S q ) -17 ( 2 eu. This complex reacts with electrophiles (HCl, CH 3 I) at the enolate position.

Pyrazole‐Bridged NHC Ligands and Their Dimetallic (Allyl)palladium Complexes

“…Allyl resonances could be assigned from intra-allyl NOE correlations (gray peaks in Figure 3) and an NOE correlation between H cis,syn and the N-Me group; the assignment is in accordance with literature data. [27] This type of allyl rotation is known to proceed via an η 1 binding mode where the second coordination site may be temporarily occupied by a solvent molecule. [27] Due to the pronounced trans effect of the NHC donor, the CH 2 group trans to the carbene is weakened and more labile than the CH 2 group cis to the carbene (i.e., trans to the pyrazolate).…”

Recyclable Catalysts for Palladium‐Catalyzed CO Coupling Reactions, Buchwald–Hartwig Aminations, and Sonogashira Reactions

“…The central N,N'-biaryl-1H-imidazolium unit is synthesized either by multicomponent condensation of the corresponding aniline with a-dicarbonyl compounds (glyoxal or 2,3-butadione) and paraformaldehyde or by sequential twostep reactions. [18] Both strategies allow for the preparation of imidazolium salts with different substituents in the 1-, 3-, 4-, and 5-positions. After selective deprotonation at the 2-position, the resulting carbene was methylated in situ.…”

Optically Active (Peptido‐carbene)palladium Complexes: Towards True Solid‐Phase Combinatorial Libraries of Transition Metal Catalysts

“…During initial experiments it had been observed that no complex formation took place with commonly used bases such as DBU and tBuOK. [32,33] Only a few examples of anionic ligands derived from carbenes are known, [34] but recent results from Waymouth et al [35] demonstrated a preference for N-heterocyclic carbene enolate complexation when a carbonyl group is positioned β to the carbene donor, enabling the formation of very stable six-membered chelates. No decomposition of these complexes could be observed by MS (ESI) even after three months in a TFA buffer (0.1 % aq.…”