Selectivity in carbon–carbon coupling reactions at palladium(IV) and platinum(IV)

Abstract

The isomerization and reductive elimination reactions from octahedral organometallic complexes of palladium(IV) and platinum(IV) usually occur through five-coordinate intermediates that cannot be directly detected. This paper reports a computational study of five-coordinate complexes of formulae [PtMe3(bipy)]+, [PtMe2Ph(bipy)]+, and [PtMe(CH2CMe2C6H4)(bipy)]+ (M = Pd or Pt, bipy = 2,2′-bipyridine), particularly with respect to reactivity and selectivity in reductive elimination. All of the complexes are predicted to have square pyramidal structures with the bipy and two R groups in the equatorial positions and one R group in the axial position, and axial–equatorial exchange occurs by a pairwise mechanism, with the transition state having a pinched trigonal bipyramidal (PTBP) stereochemistry, with one nitrogen and two R groups in the trigonal plane. The activation energy for isomerization is lower than that for reductive elimination in all cases. For the complexes [MMe2Ph(bipy)]+, the activation energies for reductive elimination with Me–Me or Me–Ph coupling are similar. For the complexes [MMe(CH2CMe2C6H4)(bipy)]+, the reductive elimination with Me–C6H4 bond formation from the isomer with the methyl group in the axial position is predicted and is attributed to it having the best conformation of the Me and C6H4 groups for C–C bond formation. In all cases, the selectivity for reductive elimination is similar for M = Pd or Pt, but reactivity is higher for M = Pd. The relevance of this work to selectivity in catalysis is discussed.

Geometric Control of Carbon–Carbon Reductive Elimination from a Platinum(IV) Pincer Complex

“…In a 2008 report, Goldman, Krogh-Jespersen, and co-workers highlighted a second key controlling factor in the selectivity of reductive elimination reactions in addition to hybridization: the importance of aryl orientation in reductive elimination. This important consideration has received more attention in recent years, and several researchers have taken aryl orientation into account in their analyses of C–C bond formation preferences. ,, Both the orientation of the aryl ring and the conformational flexibility of the ligand appear to play a crucial role in C–C reductive elimination selectivity.…”

Supramolecular chemistry of organoplatinum(IV) complexes: A syndiotactic polymer with uracil substituents

“…In related complexes [PtXMe 2 R(NN)], X = halogen, NN = diamine ligand, it is generally found that the bulkier alkyl group R prefers the axial coordination site trans to X, so complex 4 behaves in the expected way with 4b preferred over 4a when hydrogen bonding is not present. 19,20,30 The DFT calculations do not consider intermolecular interactions, so the quantitative aspects should be considered with caution, but the predictions do support the experimental observations in a qualitative way.…”

Cross-Coupling versus Homo-Coupling at a Pt(IV) Center: Computational and Experimental Approaches

“…Reductive elimination (RE) reactions are part of important catalytic processes. − RE with concomitant C–C cross-coupling plays a key role in the synthesis of organic molecules, including natural products and drugs. − RE from organometallic d 6 -configured octahedral Pt­(IV) and Pd­(IV) complexes with the concomitant formation of alkanes by Csp 3 –Csp 3 cross-coupling has been extensively studied, − while investigations on homo-coupling reactions are scarce. − …”