Tetrakis(triphenylphosphine)palladium(0)

The oxidative addition of 4-substituted iodobenzenes on Pd(0) catalysts under CO atmosphere was investigated by means of density functional calculations employing the M06//B97-D3 level of theory. The 18-electron triphenylphosphine-tricarbonyl complex was found to be the global minimum. Several coordinatively unsaturated species are predicted to be present both in N,N-dimethylformamide and toluene solution. In terms of activating iodobenzene, bis(triphenylphosphine)palladium(0) was proved to be the most active. However, due to its lower thermodynamic stability, it is slightly inferior to the Pd-triphenylphosphine-carbonyl complex, which is predicted to react with a free energy of activation of 23.2 kcal mol-1 with respect to the initial resting state tetrakis(triphenylphosphine)palladium(0). The effect of 4-substituents of iodobenzene on reaction energetics is also discussed. The activity of the Pd(0) catalyst was found to be governed by the donor-acceptor strength of the ancillary ligands: the barrier decreases with increasing basicity and decreasing back-donating capability.

Tetrakis(triphenylphosphine)palladium(0) effects the regio- and stereo-selective alkylation of 2-acetoxy-5,6-dihydro-2H-pyrans and 1-S-acetyl-1-thiohex-2-enopyranosides. Use of stabilized carbanions resulted in the formation of alkylated dihydropyrans with net retention at the oxygen-bearing carbon atom. Examples include the preparation of 2-[acetamidobis(ethoxycarbonyl)methyl]-5,6-dihydro-2H-pyran, 2-[acetamidobis(methoxycarbonyl)methyl]-6-ethoxy-5,6-dihydro-2H-py ran, and 2-[acetamidobis(methoxycarbonyl)methyl]-6-methoxymethyl-5,6-dihydr o-2H- pyran. Use of nonstabilized carbanions, such as arylzinc chlorides, resulted in the formation of alkylated dihydropyrans and C-glycosyl compounds with net inversion at the oxygen-bearing carbon atom. Examples include the preparation of 2-[(6-ethoxy-5,6-dihydro-2H-pyranyl)methyl]-4,4-dimethyl-2-oxazoli ne, trans-methoxymethyl-2-phenyl-5,6-dihydro-2H-pyran, trans-methoxymethyl-2-vinyl-5,6-dihydro-2H-pyran, trans-2-[2,2-bis(ethoxy)ethyl]-6-methoxymethyl-5,6-dihydro-2H-pyran, (4,6-di-O-methyl-2,3-dideoxy-alpha-D-erythro-hex-2-enopyranosyl)be nzene, (2,3-dideoxy-4,6-di-O-methyl-beta-D-erythro-hex-2-enopyranosyl)ben zene, 1-(2,3-dideoxy-4,6-di-O-methyl-alpha and -beta-D-erythro-hex-2-enopyranosyl)naphthalene, 4-(2,3-dideoxy-4,6-di-O-methyl-alpha- and -beta-D-erythro-hex-2-enopyranosyl)toluene, and 1-(2,3-trideoxy-4-O-methyl-alpha-L-erythro-hex-2-enopyranosyl)naph thalene.

An elegant general synthesis route for the preparation of two coordinate palladium(0) and platinum(0) complexes was developed by reacting commercially available tetrakis(triphenylphosphine)palladium/platinum with π-accepting cyclic alkyl(amino) carbenes (cAACs). The complexes are characterized by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. The palladium complexes exhibit sharp color changes (crystallochromism) from dark maroon to bright green if the C-Pd-C bond angle is sharpened by approximately 6°, which is chemically feasible by elimination of one lattice THF solvent molecule. The analogous dark orange-colored platinum complexes are more rigid and thus do not show this phenomenon. Additionally, [(cAAC)2 Pd/Pt] complexes can be quasi-reversibly oxidized to their corresponding [(cAAC)2 Pd/Pt](+) cations, as evidenced by cyclic voltammetry measurements. The bonding and stability are studied by theoretical calculations.