Palladium: Organometallic Chemistry
Published Online: 15 MAR 2006
Copyright © 2006 John Wiley & Sons, Ltd
Encyclopedia of Inorganic Chemistry
How to Cite
William Suggs, J. 2006. Palladium: Organometallic Chemistry. Encyclopedia of Inorganic Chemistry. .
- Published Online: 15 MAR 2006
Four oxidation states of palladium are encountered in organometallic chemistry. In order of importance, they are PdII, Pd0, PdIV, and PdI. With the reduction of palladium from PdII to Pd0, the metal changes its reactivity from electrophile to nucleophile. Ligands π-bound to PdII, such as π-allyl, alkene, and alkyne, show enhanced reactivity toward nucleophiles, particularly less basic nucleophiles such as malonates. Pd0 species, such as (PR3)nPd, are powerful nucleophiles. They undergo oxidative addition to alkyl, aryl, and vinyl halides to produce LnPdII-R species. If the R group is an alkyl group with β-hydrogens, a beta-elimination of H usually takes place, giving a palladium hydride and an alkene. If the R group is aryl or vinyl, the species does not undergo beta-elimination. Instead, if a second R′ group is bonded to the palladium center, reductive elimination can take place, forming R–R′. Alternatively, the PdR bond can add across an unsaturated group, such as an alkene, forming a new carbon–carbon bond. New carbon–palladium bonds can also form through transmetalation. Organotin, organoboron, organosilicon (and other main group and transition metal organometallic reagents) will react with PdII halides to give new PdII-R bonds and R3SnX (or analogous) species. Because PdII-C bonds are neither very strong nor very weak (on the order of 160 kJ mol−1), palladium can catalyze many organic reactions. For example, in the Suzuki reaction, an aryl boronic acid (ArB(OH)2) and an aryl halide (Ar′X) are coupled to form a biaryl, Ar–Ar′. The mechanism involves a Pd0 species undergoing oxidative addition to the Ar′X, forming LnPdIIAr′(X), then transmetalation giving a LnPdIIAr(Ar′) and, finally, reductive elimination forming Ar–Ar′ and regenerating LnPd0. Palladium-catalyzed reactions are increasingly being used in the synthesis of high value-added chemicals such as pharmaceuticals. With basic, hindered phosphine ligands, such as P(t-Bu)3, Pd0 reagents undergo oxidative addition even with aryl chlorides, and new reactions, such as the arylation of amines (ArX + HNR2 giving ArNR2) are possible.
- Heck reaction;
- Wacker process;
- Stille reaction;
- Suzuki reaction;
- Buchwald–Hartwig reaction