Palladium Carbene Complexes for Selective Alkene Di- and Oligomerization

Abstract

Publication informationOrganometallics, 31 (3) In one set, the pyridine and the carbene are linked by a flexible CH 2 group (a), in the other two sets, the two ligand units are directly linked and feature a shielding mesityl substituent on the carbene and either an unsubstituted pyridine (b) or a xylyl-substituted pyridine unit (c).Investigation of the reactivity of cationic complexes [Pd(C^N)Me(NCMe)] + , 6, analogues to Brookhart's α-diimine system, towards alkenes showed a strong correlation between the catalytic activity and selectivity and the ligand setting. While 6a was inactive in ethylene conversion, 6b afforded low-molecular weight olefins (oligomerization), and 6c produced exclusively butene (dimerization). With styrene as substrate, exclusive dimerization occurred with all three complexes. Steric and electronic factors were identified that govern the disparate activity and selectivity, and that allow for efficient tailoring of the catalytic performance.2

Solid-state structure, solution-state behaviour and catalytic activity of electronically divergent C,N-chelating palladium–N-heterocyclic carbene complexes

“…28 Pd complexes in which a pyridyl donor is tethered to the NHC ligand have previously been reported [30][31][32][33][34] and, in some cases, the steric effects of the ligand have been examined in catalysis. 35 Following our recent findings regarding the electronic effects of hemilabile NHC-appended pyridyl donors on Cu, 36 it was decided to prepare a family of related complexes with Pd. Altering the electronic properties of the pyridyl donor, in addition to the ligand rigidity/flexibility, was found to have a notable effect on the structural chemistry of the complexes and their catalytic activities in Suzuki-Miyaura cross-coupling reactions.…”

“…Pd-NHC complexes, in the presence of ancillary nitrogen donor ligands, have shown remarkable activities in numerous Pdcatalysed cross-coupling reactions. 28,29,35,[42][43][44][45][46] To evaluate catalytic activity and examine the effect of tethered pyridyl donor basicity and flexibility on catalysis, a benchmark Suzuki Miyaura cross-coupling reaction between phenylboronic acid and bromobenzene was examined (Table 1) Examination of entries 2-4 shows a subtle increase in conversion as an electron-withdrawing substituent on the hemilabile pyridyl ring is replaced by an electron-donating substituent. This implies that electron-rich pyridyl donor wingtips may enhance catalytic turnover.…”

σ-Alkenyl endo-palladacycle formation via regiospecific functionalisation of an unreactive NHC-tethered C(sp2)–H bond

“…18,19 Two equivalents of pyridylappended imidazolium ion 1a were stirred in refluxing MeCN with Pd(OAc) 2 to deliver the corresponding bis-NHC coordinated Pd II complex 2a (Scheme 1). Single crystals of 2a suitable for X-ray diffraction analysis were grown via vapour diffusion of Et 2 O into an MeCN solution ( Figure 2).…”

Synthesis of a sterically modulated pyridine–NHC palladium complex and its reactivity towards ethylene

“…A new cationic pyridine-carbene palladium complex has been prepared that features a C,N-bidentate coordinating ligand with a shielded pyridine and a sterically less protected carbene moiety; evaluation of this complex in ethylene polymerization revealed competitive reductive elimination 10 processes and provides guidelines for further catalyst design.…”

“…10 Direct interlinking of the 40 pyridine and the NHC provides a planar ligand in which the steric shielding is independently adjustable on the NHC and on the pyridine side of the ligand (C, Fig. 1).…”