A Career in Catalysis: Maurice Brookhart

Stable group 8 metal porphyrin mono- and bis(dialkylcarbene) complexes: synthesis, characterization, and catalytic activity

Alkyl-substituted carbene (CHR or CR2, R = alkyl) complexes have been extensively studied for alkylcarbene (CHR) ligands coordinated with high-valent early transition metal ions (a.k.a. Schrock carbenes or alkylidenes), yet dialkylcarbene (CR2) complexes remain less developed with bis(dialkylcarbene) species being little (if at all) explored. Herein, several group 8 metal porphyrin dialkylcarbene complexes, including Fe- and Ru-mono(dialkylcarbene) complexes [M(Por)(Ad)] (1a,b, M = Fe, Por = porphyrinato dianion, Ad = 2-adamantylidene; 2a,b, M = Ru) and Os-bis(dialkylcarbene) complexes [Os(Por)(Ad)2] (3a-c), are synthesized and crystallographically characterized. Detailed investigations into their electronic structures reveal that these complexes are formally low-valent M(ii)-carbene in nature. These complexes display remarkable thermal stability and chemical inertness, which are rationalized by a synergistic effect of strong metal-carbene covalency, hyperconjugation, and a rigid diamondoid carbene skeleton. Various spectroscopic techniques and DFT calculations suggest that the dialkylcarbene Ad ligand is unique compared to other common carbene ligands as it acts as both a potent σ-donor and π-acceptor; its unique electronic and structural features, together with the steric effect of the porphyrin macrocycle, make its Fe porphyrin complex 1a an active and robust catalyst for intermolecular diarylcarbene transfer reactions including cyclopropanation (up to 90% yield) and X-H (X = S, N, O, C) insertion (up to 99% yield) reactions.

Gold Difluorocarbenoid Complexes: Spectroscopic and Chemical Profiling

Gold carbenes of the general type [LAu=CR₂ ]⁺ are sufficiently long-lived for spectroscopic inspection only if the substituents compensate for the largely missing stabilization of the carbene center by the [LAu]⁺ fragment. π-Donation by two fluorine substituents (R=F) is insufficient; rather, difluorocarbene complexes are so deprived in electron density that they sequester even "weakly coordinating" anions such as triflate or triflimide. This particular bonding situation translates into unmistakable carbenium ion chemistry upon reaction with stilbene as a model substrate.

General, Mild, and Selective Method for Desaturation of Aliphatic Amines

A novel method for desaturation of aliphatic amines into enamines as well as allylic and homoallylic amines has been developed. This general protocol operates via putative aryl hybrid Pd-radical intermediates, which combine the signature features of radical chemistry, a hydrogen atom transfer (HAT) process, and transition metal chemistry, a selective β-hydride elimination step, to achieve efficient and selective desaturation of amines. These hybrid Pd-radical intermediates are efficiently generated under mild photoinduced conditions and are capable of a 1,n-HAT (n = 5-7) event at C(sp3)-H sites. The selectivity of HAT is tunable by varying different auxiliaries, which highlight the generality of this method. Remarkably, this desaturation method, which operates under mild conditions and does not require employment of exogenous photosensitizers or oxidants, can be performed in a practical scalable fashion from simple amines.

Glyoxalbis(2-methylmercaptoanil) complexes of nickel and ruthenium: radical versus non-radical states

The molecular and electronic structures of nickel(ii) and ruthenium(ii) complexes of glyoxalbis(2-methylmercaptoanil) and their reduced and oxidized analogues are reported.