Structural and dynamic properties of propane coordinated to TpRh(CNR) from a confrontation between theory and experiment

Probing the Carbon-Hydrogen Activation of Alkanes Following Photolysis of Tp'Rh(CNR)(carbodiimide): A Computational and Time-Resolved Infrared Spectroscopic Study

Carbon-hydrogen bond activation of alkanes by Tp'Rh(CNR) (Tp' = Tp = trispyrazolylborate or Tp* = tris(3,5-dimethylpyrazolyl)borate) were followed by time-resolved infrared spectroscopy (TRIR) in the υ(CNR) and υ(B-H) spectral regions on Tp*Rh(CNCH₂CMe₃), and their reaction mechanisms were modeled by density functional theory (DFT) on TpRh(CNMe). The major intermediate species were: κ³-η¹-alkane complex (1); κ²-η²-alkane complex (2); and κ³-alkyl hydride (3). Calculations predict that the barrier between 1 and 2 arises from a triplet-singlet crossing and intermediate 2 proceeds over the rate-determining C-H activation barrier to give the final product 3. The activation lifetimes measured for the Tp*Rh(CNR) and Tp*Rh(CO) fragments with n-heptane and four cycloalkanes (C₅H₁₀, C₆H₁₂, C₇H₁₄, and C₈H₁₆) increase with alkanes size and show a dramatic increase between C₆H₁₂ and C₇H₁₄. A similar step-like behavior was observed previously with CpRh(CO) and Cp*Rh(CO) fragments and is attributed to the wider difference in C-H bonds that appear at C₇H₁₄. However, Tp'Rh(CNR) and Tp'Rh(CO) fragments have much longer absolute lifetimes compared to those of CpRh(CO) and Cp*Rh(CO) fragments, because the reduced electron density in dechelated κ²-η²-alkane Tp' complexes stabilizes the d⁸ Rh(I) in a square-planar geometry and weakens the metal's ability for oxidative addition of the C-H bond. Further, the Tp'Rh(CNR) fragment has significantly slower rates of C-H activation in comparison to the Tp'Rh(CO) fragment for the larger cycloalkanes, because the steric bulk of the neopentyl isocyanide ligand hinders the rechelation in κ²-Tp'Rh(CNR)(cycloalkane) species and results in the C-H activation without the assistance of the rechelation.

Mechanistic Aspects of Aryl-Halide Oxidative Addition, Coordination Chemistry, and Ring-Walking by Palladium

This contribution describes the reactivity of a zero-valent palladium phosphine complex with substrates that contain both an aryl halide moiety and an unsaturated carbon-carbon bond. Although η(2) -coordination of the metal center to a C=C or C≡C unit is kinetically favored, aryl halide bond activation is favored thermodynamically. These quantitative transformations proceed under mild reaction conditions in solution or in the solid state. Kinetic measurements indicate that formation of η(2) -coordination complexes are not nonproductive side-equilibria, but observable (and in several cases even isolated) intermediates en route to aryl halide bond cleavage. At the same time, DFT calculations show that the reaction with palladium may proceed through a dissociation-oxidative addition mechanism rather than through a haptotropic intramolecular process (i.e., ring walking). Furthermore, the transition state involves coordination of a third phosphine to the palladium center, which is lost during the oxidative addition as the C-halide bond is being broken. Interestingly, selective activation of aryl halides has been demonstrated by adding reactive aryl halides to the η(2) -coordination complexes. The product distribution can be controlled by the concentration of the reactants and/or the presence of excess phosphine.

Palladium-Catalyzed C8-Selective C-H Arylation of Quinoline N-Oxides: Insights into the Electronic, Steric and Solvation Effects on the Site-Selectivity by Mechanistic and DFT Computational Studies

We report herein a palladium-catalyzed C–H arylation of quinoline N-oxides that proceeds with high selectivity in favor of the C8 isomer. This site selectivity is unusual for palladium, since all of the hitherto described methods of palladium-catalyzed C–H functionalization of quinoline N-oxides are highly C2 selective. The reaction exhibits a broad synthetic scope with respect to quinoline N-oxides and iodoarenes and can be significantly accelerated to subhour reaction times under microwave irradiation. The C8-arylation method can be carried out on a gram scale and has excellent functional group tolerance. Mechanistic and density functional theory (DFT) computational studies provide evidence for the cyclopalladation pathway and describe key parameters influencing the site selectivity.

Understanding the factors affecting the activation of alkane by Cp'Rh(CO)2 (Cp' = Cp or Cp*)

Fast time-resolved infrared spectroscopic measurements have allowed precise determination of the rates of activation of alkanes by Cp'Rh(CO) (Cp(') = η(5)-C(5)H(5) or η(5)-C(5)Me(5)). We have monitored the kinetics of C─H activation in solution at room temperature and determined how the change in rate of oxidative cleavage varies from methane to decane. The lifetime of CpRh(CO)(alkane) shows a nearly linear behavior with respect to the length of the alkane chain, whereas the related Cp*Rh(CO)(alkane) has clear oscillatory behavior upon changing the alkane. Coupled cluster and density functional theory calculations on these complexes, transition states, and intermediates provide the insight into the mechanism and barriers in order to develop a kinetic simulation of the experimental results. The observed behavior is a subtle interplay between the rates of activation and migration. Unexpectedly, the calculations predict that the most rapid process in these Cp'Rh(CO)(alkane) systems is the 1,3-migration along the alkane chain. The linear behavior in the observed lifetime of CpRh(CO)(alkane) results from a mechanism in which the next most rapid process is the activation of primary C─H bonds (─CH(3) groups), while the third key step in this system is 1,2-migration with a slightly slower rate. The oscillatory behavior in the lifetime of Cp*Rh(CO)(alkane) with respect to the alkane's chain length follows from subtle interplay between more rapid migrations and less rapid primary C─H activation, with respect to CpRh(CO)(alkane), especially when the CH(3) group is near a gauche turn. This interplay results in the activation being controlled by the percentage of alkane conformers.