Stable CAAC-Triazenes: A New Nitrogen Ligand System With Donor and Conformational Flexibility, and With Application in Olefin Activation Catalysis.

N-heterocyclic imines such as pyridylidene amines impart high catalytic activity when coordinated to a transition metal, largely imposed by their electronic flexibility. Here, this donor flexibility has been applied for the first time to CAAC-based systems through the synthesis of CAAC-triazenes. These new ligands offer a larger π-conjugation that extends from the N-heterocyclic carbene through three nitrogens rather than just one, as observed in N-heterocyclic imines. We demonstrate the straightforward synthesis of three new CAAC-triazenes containing different substituents on the terminal triazene nitrogen. These compounds are remarkably stable up to 120 °C without loss of N2 as typically observed with similar triazenes. E-to-Z isomerization within the triazene is instigated by UV light and is partially reversible dependent on the triazene substituent. The quinoline-substituted CAAC-triazene 1-Q has been applied as an L,L'-type ligand in the synthesis of [PdCl2(1-Q)], [PdCl(Me)(1-Q)] and [Pd(Me)(H2O(1-Q)]+. E-to-Z ligand isomerization also occurs when coordinated to PdCl2, providing access to on-metal manipulation. The cationic complex [PdMe(H2O)(1-Q)]+ is a precatalyst for oligomerization of ethylene to form initially 2-butene and subsequently linear and branched C8-C12 products from butene activation. Moreover, isomerization of 1-hexene takes place efficiently with exceptionally low catalyst loading (10 ppm) and up to 74,000 turnover numbers.

Substituent Effects on Torsional Strain in Cyclopentene Derivatives: A Computational Study.

Density functional theory calculations were used to create a library of ring strain energies (RSEs) for 73 cyclopentene derivatives with potential use as monomers for ring-opening metathesis polymerization (ROMP). An overarching goal was to probe how substituent choice may influence torsional strain, which is the driving force for ROMP and one of the most understudied types of RSEs. Potential trends investigated include substituent location, size, electronegativity, hybridization, and steric bulk. Using traditional and recently developed homodesmotic equations, our results show that the size and substitution (bulk) of the atom directly bonded to the ring have the greatest influence on torsional RSE. A complex interplay between bond length, bond angle, and dihedral angle dictates the relative eclipsed conformations between the substituent and its neighboring hydrogens and was found to be responsible for the notable differences in RSEs. Furthermore, substituents placed on the homoallylic position resulted in higher RSEs than the same substituent placed on the allylic position due to increased eclipsing interactions. Different levels of theory were also assessed, and it was determined that consideration of electron correlation in calculations increased RSEs by ∼2-5 kcal mol-1. Further increasing the level of theory did not significantly change RSEs, indicating that the increased computational cost and time may not be necessary for improved accuracy.

Tailoring C-H amination activity via modification of the triazole-derived carbene ligand.

Two new C,O-bidentate chelating triazolylidene-phenolate ligands were synthesized that feature a diisopropylphenyl (dipp) and an adamantyl (Ad) substituent respectively on the triazole scaffold. Subsequent metalation afforded iron(II) complexes [Fe(C^O)2] that are active catalysts for the intramolecular C-H amination of organic azides. When compared to the parent complex containing a triazolylidene with a mesityl substituent (Mes) the increased steric bulk led to slightly lower activity (TOFmax = 23 h-1vs. 30 h-1), however selectivity towards pyrrolidine formation increases from 92% up to >99%. Kinetic studies indicate that the mechanism is similar in all three complexes and includes a half-order dependence in [Fe(C^O)2], congruent with the involvement of a dimetallic catalyst resting state within this catalyst class. Structural analysis suggests that enhanced bulkiness disfavors N2 loss and nitrene formation, yet shields the nitrene from intermolecular processes and thus favors intramolecular nitrene insertion into the C-H bond. This model rationalizes the high selectivity and the lower reaction rate observed with dipp and with Ad substituents on the ligand.