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A mild, oxidant-free, and selective Cp*CoIII -catalyzed amidation of thioamides with robust dioxazolone amidating agents via C(sp3 )-H bond activation to generate the desired amidated products is reported. The method is efficient and allows for the C-H amidation of a wide range of functionalized thioamides with aryl-, heteroaryl-, and alkyl-substituted dioxazolones under the Cp*CoIII -catalyzed conditions. The observed regioselectivity towards primary C(sp3 )-H activation is supported by computational studies and the cyclometalation is proposed to proceed by means of an external carboxylate-assisted concerted metalation/deprotonation mechanism. The reported method is a rare example of the use of a directing group other than the commonly used pyridine and quinolone classes for Cp*CoIII -catalyzed C(sp3 )-H functionalization and the first to exploit thioamides.

An in situ generated Pd-Cy*Phine catalyst has been successfully applied to the N-arylation of primary and secondary amines, and it exhibited high performance across multiple substrate classes. The performance induced by the meta-terarylphosphine motif of the Cy*Phine ligand for C-N cross-coupling displayed only subtle differences to that of its biarylphosphine congener XPhos. DFT studies demonstrated comparable reaction energetics in the catalytic cycle steps for both Pd-Cy*Phine and Pd-XPhos, which was consistent with previous findings. The computational investigation also indicated that a putative rate-determining step occurred after amine binding, which was likely to have annulled the expected benefits of having a meta-terarylphosphine ligand architecture.

Computations (density functional theory) of the post-transmetallation stages (carborhodation, 1,4-shift consisting of C–H oxidative addition/C–H reductive elimination, and hydrolysis) in the catalytic cycle for the arylation of 2-butyne with phenylboronic acid mediated by [(dppe)RhI] catalyst [dppe = 1,2-bis(diphenylphosphanyl)ethane] shows that (1) carborhodation is facile (ΔG‡ ≈ 10 kcal mol–1); (2) the barriers of the 1,4-shift and hydrolysis are approximately equal (ΔG‡ ≈ 19–24 kcal mol–1); (3) the 1,4-Rh shift product is ca. 5 kcal mol–1 more stable than the non-rearranged product, in good agreement with the experimental results. Analogous computations for (Z)-2-butene as a model substrate show that (1) carborhodation is much slower (ΔG‡ ≈ 19 kcal mol–1); (2) the sp3 → sp2 1,4-shift is faster than the sp2 → sp2 1,4-shift; (3) the hydrolysis of Rh–C σ bonds depends more on the steric environment than the hybridization of the C atom; and (4) β-hydride elimination is the most likely reaction after carborhodation.

The C-H activation in the tandem, "merry-go-round", [(dppp)Rh]-catalyzed (dppp=1,3-bis(diphenylphosphino)propane), four-fold addition of norborene to PhB(OH)2 has been postulated to occur by a C(alkyl)H oxidative addition to square-pyramidal Rh(III) -H species, which in turn undergoes a C(aryl)-H reductive elimination. Our DFT calculations confirm the Rh(I) /Rh(III) mechanism. At the IEFPCM(toluene, 373.15 K)/PBE0/DGDZVP level of theory, the oxidative addition barrier was calculated to be 12.9 kcal mol(-1) , and that of reductive elimination was 5.0 kcal mol(-1) . The observed selectivity of the reaction correlates well with the relative energy barriers of the cycle steps. The higher barrier (20.9 kcal mol(-1) ) for norbornyl-Rh protonation ensures that the reaction is steered towards the 1,4-shift (total barrier of 16.3 kcal mol(-1) ), acting as an equilibration shuttle. The carborhodation (13.2 kcal mol(-1) ) proceeds through a lower barrier than the protonation (16.7 kcal mol(-1) ) of the rearranged aryl-Rh species in the absence of o- or m-substituents, ensuring multiple carborhodations take place. However, for 2,5-dimethylphenyl, which was used as a model substrate, the barrier for carborhodation is increased to 19.4 kcal mol(-1) , explaining the observed termination of the reaction at 1,2,3,4-tetra(exo-norborn-2-yl)benzene. Finally, calculations with (Z)-2-butene gave a carborhodation barrier of 20.2 kcal mol(-1) , suggesting that carborhodation of non-strained, open-chain substrates would be disfavored relative to protonation.

Attaching 2-thienyl residues to the Pechmann dye core chromophore (5,5-exo-dilactone situated around a C-C double bond) results in a novel magenta-coloured compound (UV/Vis spectroscopy λ(max) =570 nm in CHCl(3)), which can be rearranged to a yellow 6,6-endo-dilactone (λ(max) =462 nm in CHCl(3)). Single and double amidation results in pronounced redshift in the 5,5-exo series (violet, λ(max) =570 nm and blue, λ(max) =606 nm in CHCl(3), respectively) but pronounced blueshift in the 6,6-endo series (yellow, λ(max) =424 nm and pale yellow bordering on colourless, λ(max) =395 nm in CHCl(3), respectively). Incorporation of a 3-alkyl substituent on the thiophene ring allows for sharp increase of solubility in organic solvents concomitant with fine-tuning of the colour: a redshift in 5,5-exo-dilactones but a blueshift in 5,5-exo-dilactams. DFT computations demonstrate that both lactone classes are planar regardless of the presence of a 3-alkyl group. The lactam derivatives are non-planar: the thiophene-core chromophore dihedral angles increase on going from 5,5-exo to 6,6-endo and from thiophene to 3-alkyl thiophene. Depending on the core heteroatom (O vs. N-alkyl), ring junction (5,5-exo vs. 6,6-endo) and 3-thiophene substituent (H vs. alkyl), two, three, four or six conformers are possible. All of these conformers were characterised by DFT and were found to be very close in energy at both IEFPCM/B3LYP/DGDZVP and SMD/M06/DGDZVP levels of theory. Within each conformer set, the HOMO and LUMO energies were within 0.05 eV and the predicted λ(max) values (TD-DFT) within 10 nm, and this implies low sensitivity of the optical and electronic properties to conformation. Cyclic voltammetry measurements of selected compounds demonstrated good matching to the HOMO and LUMO energies from IEFPCM/B3LYP/DGDZVP computations. M06-2X was the best DFT functional for TD-DFT, giving predicted λ(max) values within about 20 nm.

The electronic structures of the triplet ground states and first three excited singlet states for phenylnitrene, 14 meta-, and 17 para-substituted congeners have been characterized using density functional theory and multireference second-order perturbation theory (CASPT2). Ring expansion pathways to form didehydroazepines have activation enthalpies of about 9 kcal⋅mol−1 and are fairly insensitive to substitution—in the case of the strongest para donor, MeNH–, this barrier increases to about 13 kcal⋅mol−1. The trends in state energies as a function of substitution are rationalized using a (2,2) configuration interaction theory and qualitative molecular orbital theory. Analysis of spin-orbit coupling in the nitrenes using the same model in conjunction with explicit calculation of spin-orbit coupling matrix elements rationalizes why electron donating substituents increase rates of intersystem crossing. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001