Anions of 1,3-dicarbonyl compounds are some of the most common ligands in transition-metal chemistry. [1, 2] They typically bind in an h 2 -O,O fashion, [3] have delocalized charge, and donate electron density more weakly to a metal center than alkyls or alkoxides. They are usually supporting ligands that are ancillary to the site of reaction. Anions of 1,3-dicarbonyl compounds are also common nucleophiles in metal-catalyzed allylic substitution, [4±6] but the facility of this chemistry relies on external attack of the anion without coordination to the metal center. If metal fragments could induce reactivity from coordinated versions of these anions, then complexes of these common ligands could serve as intermediates in catalytic processes.
Complexes of malonate anions are likely intermediates in recently developed palladium-catalyzed arylations of malonates. [7±11] Although palladium complexes of malonate anions have been isolated previously, [12±16] their reactivity has been limited. [17±20] We report here reductive elimination of arylmalonate and acetylarylacetone from isolated aryl palladium complexes of malonate and acetylacetone anions, respectively. Our results suggest that the propensity of these complexes to undergo reductive elimination depends critically on the steric properties of the ancillary phosphane ligand.
Our synthesis of aryl palladium malonates is summarized in Scheme 1. Addition of dimethyl malonate or diethyl phenylmalonate to the basic PPh 3 -ligated palladium hydroxide dimers 1 a [21, 22] and 1 b generated the O,O'-bound palladium dimethyl malonate complexes 2 a±c. Complexes 2 a and 2 c were characterized by X-ray diffraction (Figure ). No unusual angles at the palladium center were found in 2 a or ZUSCHRIFTEN