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.

Liquid-liquid biphasic systems that exhibit an increase in phase miscibility at elevated temperature together with soluble polymer-bound catalysts that have a strong phase preference at ambient temperature are described. In such systems, product isolation and catalyst recovery are effected by a liquid/liquid separation. This report describes the use of such thermomorphic catalyst recovery systems for palladium-catalyzed carboncarbon bond forming reactions. Poly(N-isopropylacrylamide) (PNIPAM)-bound phosphine ligands with a Pd(0) catalyst used previously in allylic substitution chemistry are efficient catalysts in Heck, Suzuki, and sp-sp 2 cross-coupling reactions. Air-stable tridentate SCS-Pd(II) catalysts bound to PNIPAM or poly(ethylene glycol) (PEG) are also described. A particular advantage of these SCS catalysts is that no precautions against adventitious catalyst oxidation need be taken with the polymer-bound SCS-PdCl catalysts, thus avoiding time-consuming solvent purification and degassing protocols.

Air-stable SCS palladacycles that can be used to promote C À C coupling chemistry were studied mechanistically. Using a small library of electronically varied SCS ligands, a collection of palladacycles was synthesized. Kinetic studies showed that these complexes all had induction periods, induction periods that were effected by concentration of substrates, products and trace impurities. Hammet correlations showed that electronically diverse palladacycles had identical 1 values, values that suggested that aryl halide electrophilic addition to a Pd species was not the rate-determining step. Phosphine addition experiments led to increased reactivity of the starting palladacycles, possibly by trapping an in situ generated Pd(0) species. Studies that examined reactivity in biphasic thermomorphic reactions showed residual activity in phases that do not contain polymer-bound palladacycle and provided convincing evidence that palladacycles are not the actual catalyst. Poisoning experiments using mercury metal to test for the presence of a Pd colloid were very effective with low molecular weight palladacycles, completely suppressing Heck chemistry. Similar studies with polymer-bound palladacycles showed mercury poisoning too. However, since so little decomposition of the palladacycle occurred, the polymer-bound palladacycle could still be recycled multiple times. However, mercury poisoned subsequent cycles of the experiment too. The conclusion is that SCS palladacycles are actually reservoirs of a catalytically active but ill-defined form of palladium(0).

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The extent of the phase-selective solubility of poly(N-alkylacrylamide)s was studied by UV-vis and fluorescence spectroscopy using poly(N-isopropylacrylamide) and poly(N-octadecylacrylamide) as representative polar and nonpolar poly(N-alkylacrylamide)s in a mixture of polar and nonpolar thermomorphic solvents. Phase-selective solubilities of greater than 10000:1 were seen with each labeled polymer in polar and nonpolar solvents such as heptane and DMF or heptane and 90% EtOH-H(2)O. Using a poly(N-acryloxysuccinimide) as a common precursor, a pool-split synthesis was devised to prepare a library of poly(N-alkylacrylamide)s whose members varied only in the size of their N-alkyl substituent. The solubilities of these library members were measured in both the polar and nonpolar phases of a thermomorphic heptane/90% EtOH-H(2)O mixture at 25 degrees C. Such solvent mixtures are miscible hot (70 degrees C) and biphasic cold (25 degrees C). The results show that poly(N-pentylacrylamide) is selectively soluble (>99.5%) in the polar EtOH-rich phase at rest. Poly(N-alkylacrylamide)s with larger N-alkyl groups are predominantly (C(6), 85%; C(7), 95%) or exclusively (>C(8), >99.5%) in the heptane-rich phase at rest.