Bimacrocyclic NHC transition metal complexes

Synthesis, structure and catalytic activity of a bimacrocylic NHC palladium allyl complex

“…The 13 C resonance for the carbene carbon atom of 4 was observed at 211.4 ppm which is significantly more downfield than in palladium complex 3 (183.5 ppm) [8]. A comparison of related palladium complexes with the SIPr NHC ligand shows a very similar trend: the carbene carbon atom resonates at 215.4 ppm when SIPr is incorporated into an allyl palladium complex [7e], but in the 3-chloro-pyridine containing complex this resonance is observed at 184.9 ppm [11].…”

“…Selected bond lengths are given in Table 1. The structure of the NHC ligand in palladium complex 4 is similar to that in complex 3 [8], with the two phenyl substituents oriented almost perpendicular to the heterocycle, and the torsion angles C1-N1-C4-C5 and C1-N2-C23-C22 are 81.42(1)°and 70.89(1)°, respectively. The g 3 coordination of the allyl moiety to the palladium center is comparable to that reported for the respective complex using the SIPr NHC ligand (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene) [7e].…”

“…In an analogous fashion, catalytically active NHC palladium complexes can be formed in situ from a palladium source and an azolium salt as the NHC precursor [6], but recently also a number of well-defined, air-and moisture-stable NHC-bearing palladium complexes with excellent catalytic activities have been reported [7]. In our continuing investigation of the catalytic properties of bimacrocyclic NHCs [8], accessible from the respective azolium precursors [9], we explored the accessibility of bimacrocyclic NHC palladium complexes and their catalytic activity in cross-coupling reactions. We have recently reported [8] the synthesis of a number of NHC metal complexes derived from the concave bimacrocyclic imidazolinium salt 1.…”

“…In our continuing investigation of the catalytic properties of bimacrocyclic NHCs [8], accessible from the respective azolium precursors [9], we explored the accessibility of bimacrocyclic NHC palladium complexes and their catalytic activity in cross-coupling reactions. We have recently reported [8] the synthesis of a number of NHC metal complexes derived from the concave bimacrocyclic imidazolinium salt 1. In contrast to other transition metal complexes, palladium complex 3 was only obtained in poor yield via the free carbene, and 3 was not accessible by transmetalation from silver complex 2 [8].…”

“…We have recently reported [8] the synthesis of a number of NHC metal complexes derived from the concave bimacrocyclic imidazolinium salt 1. In contrast to other transition metal complexes, palladium complex 3 was only obtained in poor yield via the free carbene, and 3 was not accessible by transmetalation from silver complex 2 [8]. The use of NHC silver complexes as carbene transfer reagents is a popular method for the synthesis of NHC metal complexes by transmetalation, avoiding the handling of the sensitive free carbenes [10].…”