Theoretical studies on palladium mediated or catalyzed C-C coupling reactions

Theoretical studies on the chemical bonding in palladium complexes and mechanistic aspect on palladium mediated or catalyzed reactions are reported in this thesis. Part I: The insertion reactions of allenes with the palladium aryl complex [PdI(Ph)(PPh3)]2 have been systematically investigated with the aid of DFT calculations. The calculated results show that formation of allyl complex PdI(η3-MeCHC(Ph)CHSO2H)(PH3) from PdI(Ph)MeCH=C=CHSO2H)(PH3) is both thermodynamically and kinetically favored among the possible pathways, which is consistent with the experimental results. The regioselectivity observed experimentally has also been explained. Part II: The relative stability of the trans and cis isomers in the square planar palladium(II) complexes Pd(I)(PPh3)(η3-XCHC(Ph)CHR) (X = H, Me, CMe3, CO2Me, P(O)(OMe)2 and SO2H; R = H, Me) was investigated with the aid of DFT calculations at the B3LYP level. How the substituents X, with different electronic properties, of the η3-allyl ligands affect the relative stability of the trans and cis isomers has been examined. Through the investigation, the trans/cis relative stability derived from the experimentally measured trans/cis isomer ratios in the palladium(II) complexes has been explained. Part III: The multiple insertion of a silyl vinyl ether into the Pd-C bond in (α-diimine)PdMe+ has been thoroughly studied with the aid of DFT calculations. Through the study, it was found that the 1,2-insertion is preferred over 2,1-insertion for the chain growth. For the β-OSiH3 elimination, the overall barrier is comparable to the VE insertion barriers in the chain growth. The DFT calculations provide deep insights into these processes of the chain growth, the β-OSiH3 elimination and the chain walking in the polymerization reactions. Part IV: Decarboxylation processes in a series of PdL2X(OOCArR) complexes BOSR (L = DMSO; X = OOCCF3; R = H, OMe, NO2, Me and CN) with the substituent R in an ortho, meta or para position were investigated with the aid of DFT calculations. Through our study, it was found that the OOCCF3- ligand is not just a spectator ligand but assists the decarboxylation process. The results indicated that electron-donating substituents have greater promotion effect than electron-withdrawing substituents on the decarboxylation process. An ortho substituent in the substrate ligands OOCArR- is normally necessary for a successful decarboxylation. The reason behind this has been explained. Part V: Palladium-catalyzed processes for the formation of carbon-carbon bonds, i.e., coupling reactions of organic halides (R1X) and main-group organometallic compounds (R2M) are most frequently applied in synthesizing biologically and pharmaceutically important organic molecules. In chapter 6, we provide comments and discussion on current understanding of the palladium-catalyzed carbon-carbon cross-coupling reactions, and give an up-to-date summary of the current understanding of the cross-coupling reactions from a theoretical point of view. The successfully application by using modern computational techniques to understand organometallic chemistry has been demonstrated. Through the theoretical studies, the nature of boning in a number of palladium allyl complexes and reaction mechanisms of palladium catalyzed or mediated reactions are understood well. Furthermore, studies of reaction mechanisms are important in designing better and effective catalysts, and selecting appropriate substrates