Chiral Palladium Bis(acyclic diaminocarbene) Complexes as Enantioselective Catalysts for the Aza-Claisen Rearrangement

Abstract

Palladium complexes of chiral bis(acyclic diaminocarbene) ligands with seven-membered chelate rings catalyze the aza-Claisen rearrangement of an allylic benzimidate to an allylic amide with moderate yields and enantioselectivities of 30–59% ee. The promotion of electrophilic catalysis by complexes of these ligands highlights their distinct stereoelectronic properties relative to those of common bis(N-heterocyclic carbenes).

Palladium complexes of bis(acyclic diaminocarbene) ligands with chiral N-substituents and 8-membered chelate rings

“…In addition to providing the first chiral ADC ligands, this one-step synthetic route provides a strategy for systematically varying the chiral structure and chelate ring size. The 7-membered chelate rings of C and D were found to engender non-ideal C-Pd-C bite angles of 82-87°, which we viewed as a likely cause of the lower effective donor abilities of these ligands relative to common bis(NHC) or bis(phosphine) ligands [6]. This allowed the use of C and D as electrophilic catalysts for aza-Claisen rearrangement reactions [6], an unusual departure from the prevailing use of NHCs and ADCs to stabilize electron-rich metal centers.…”

“…The 7-membered chelate rings of C and D were found to engender non-ideal C-Pd-C bite angles of 82-87°, which we viewed as a likely cause of the lower effective donor abilities of these ligands relative to common bis(NHC) or bis(phosphine) ligands [6]. This allowed the use of C and D as electrophilic catalysts for aza-Claisen rearrangement reactions [6], an unusual departure from the prevailing use of NHCs and ADCs to stabilize electron-rich metal centers. In addition, the 7-membered chelate structure constrains the bis(ADCs) of C and D to C 1 -symmetric conformations with the chiral elements relatively remote from the metal center.…”

“…The ability of NHCs to boost the activities of organometallic catalysts relative to the corresponding phosphine-ligated systems [3] has been frequently attributed to the higher r-donating abilities of the NHCs [4], although there is growing recognition that NHCs may not act as stronger donors than phosphines under all circumstances [5,6]. Whereas a large and growing literature documents the organometallic chemistry of the imidazolium-derived imidazolin-2-ylidenes and their backbone-saturated analogues [7], which represent the most popular classes of NHCs, the related acyclic diaminocarbenes (ADCs) [8] have been largely neglected as ancillary ligands.…”

“…Subsequently, we extended this synthetic approach to chiral bis(ADC) palladium complexes C and D (Fig. 1) by reaction of arylisocyanide complexes with chiral secondary diamines [6,29]. In addition to providing the first chiral ADC ligands, this one-step synthetic route provides a strategy for systematically varying the chiral structure and chelate ring size.…”

“…In addition, the 7-membered chelate structure constrains the bis(ADCs) of C and D to C 1 -symmetric conformations with the chiral elements relatively remote from the metal center. While the influence of the chiral backbone on the metal coordination sphere was sufficient to provide modest enantiomeric excesses of 30-59% ee in aza-Claisen rearrangement reactions [6], the need for highly enantioselective catalysts led us to pursue other ligand architectures that might provide enhanced chiral environments about the metal. The development of structurally diverse chiral bis(ADC) ligands is particularly important given that reported examples of chiral bis(NHC) ligands are still relatively scarce [30].…”