Tridentate SCS Palladium(II) Complexes:  New, Highly Stable, Recyclable Catalysts for the Heck Reaction

Abstract

A new pincer-type SCS ligand containing Pd(II) is a simple, robust catalyst for Heck chemistry using a variety of alkene acceptors and aryl iodides. It is less active with aryl bromides. While certain palladium(II) species insert slowly into the aryl C−H bond of an unsubstituted version of this ligand, the introduction of activating groups into the 5 position of the aromatic ring readily allows quantitative metal insertion. These ligands were synthesized and attached to soluble polymers by simple modification of inexpensive starting materials. For example, both 5-oxy and 5-amido SCS ligands were successfully appended to 5000 M n poly(ethylene glycol) via ether or amide linkages, respectively. Both the 5-oxo and 5-amido complexes are active as Heck catalysts in DMF solution in air. The PEG-bound 5-amido-SCS−Pd complex was recycled via solvent precipitation three times with no observed catalyst deactivation. While the 5-amido-SCS−Pd complexes are very robust, their 5-oxo counterparts decompose slowly under certain conditions. These SCS catalysts are analogous to PCP-type catalysts previously reported in the literature but avoid the requirement of an air-sensitive phosphine synthesis.

Tiara-like Octanuclear Palladium(II) and Platinum(II) Thiolates and Their Inclusion Complexes with Dihalo- or Iodoalkanes

“…[44][45] When the same reaction was performed at room temperature, it produced an insoluble solid in nearly quantitative yield; the product appeared to be a monomer and/or oligomers of Pd1-C1 = 1.980(2), Pd1-S1 = 2.2967 (7), Pd1-S2 = 2.3151 (7), Pd1-Cl1 = 2.3961 (7), C1-Pd1-S1 = 83.11(6), C1-Pd1-S2 = 84.46(6), S1-Pd1-Cl1 = 96.27(3), S2-Pd1-Cl1 = 96.12(2), Pd1-S1-C2 = 99.24 (7), Pd1-S1-C4 = 105.61 (7), C2-S1-C4 = 104.64(10), Pd1-S2-C3 = 97.67 (7), Pd1-S2-C5 = 102.18 (7), C3-S2-C5 = 100.81(11), C1-Pd1-S1-C2 = 25.12 (10) by HCl followed by E1 elimination similar to that described above occurs again to give 4 and a mononuclear palladium thiolate, [Pd(SCH 2 CO 2 Me) 2 ], which then aggregates to construct a stable tiara-like octanuclear architecture. Although the above mechanism explains the formation of 1, the bidentate coordination of 2 to PdCl 2 without aromatic C(sp 2 )-H bond activation 45 and the subsequent stepwise elimination of the thiolate groups, similar to the above mechanism, can proceed concurrently to form 1. 39 In this reaction, we presume that the formation of the hexanuclear tiara ring with carboxylic acids as substituents occurred first, and ester condensation between the carboxylic acid moieties and MeOH followed.…”

Novel silica immobilized NCN-pincer palladium(II) and platinum(II) complexes: Application as Lewis acid catalysts

“…The physico-chemical properties of the pincer metal complexes allow for a 'hands-on' structural analysis via reversible colorization of pincer platinum materials by SO 2 binding and, in addition, offer various opportunities in catalysis as exemplified by the corresponding pincer palladium materials described in this study. Moreover, the NCN-pincer manifold can be replaced by PCP, SCS, or a variety of other pincer-type combinations and together with specific choices of the metal site this gives access to a variety of catalysts for notable conversions in organic synthesis [20,23,32,45,64].…”

“…Current efforts focus on the use of organometallic pincersiloxane reagents for surface modification and on the use of pincer-metal silica hybrid materials for other catalytic conversions (e.g., C-C and C-X bond formation) [45,[64][65][66][67][68][69]. The integrity and stability of the silica support will be another important aspect of these studies.…”

The Heck reaction in the production of fine chemicals

“…More variations on this theme are the use of bulky phosphite (18,41) or phosphoramidite ligands (42), carbene ligands (43,44), and pincer ligands (31,45). Another interesting, cheap, and very active class of catalysts are the palladacycles based on aromatic compounds with side chains containing N (41,46), O (47), or S (48,49). The more active catalysts also have allowed the use of chloroarenes as arylating agents, which is tremendously important because of the much wider commercial availability and the lower costs of these compounds (16)(17)(18)(19)(20).…”