Origins of Protons in C–H Bond Insertion Products of Phenols: Proton-Self-Sufficient Function via Water Molecules

Is Enol Always the Culprit? The Curious Case of High Enantioselectivity in a Chiral Rh(II) Complex Catalyzed Carbene Insertion Reaction

The mechanism of Rh2 (S-NTTL)4 catalyzed carbene insertion into C(3)-H of indole is investigated using DFT methods. Since the commonly accepted enol mechanism cannot account for enantioinduction, a concerted oxocarbenium pathway was proposed in an earlier work using a model catalyst. However, after considering the full catalytic system, this study finds that akin to other reactions, here, too, the enol pathway is of lower energy, which now naturally raises a conundrum regarding the mode of chiral induction. Herein, a new water promoted mechanistic pathway involving a metal-associated enol intermediate hydrogen bonding and stereochemical model are proposed to solve this puzzle. It is shown how the catalyst bowl-shaped structure along with substrate-catalyst binding is crucial for achieving high levels of enantioselectivity. A stereodetermining water-assisted proton transfer is proposed and confirmed through deuterium-labeling experiments. The water molecules are held together by H-bonding interactions with the carboxylate ligands that is reminiscent of enzyme catalysis. Although several previous studies have aimed at understanding the mechanism of metal catalyzed carbene insertion reactions, the origin of high stereoinduction especially with chiral metal complexes remains unclear, and till date there is no transition state model that can explain the high enantioselectivity with such chiral Rh complexes. The metal-associated enol pathway is currently underrepresented in catalytic cycles and may play a crucial role in catalyst design. Since the enol pathway is commonly adopted in other metal-catalyzed X-H insertion reactions involving a diazoester, the presented results are not specific to the current reaction. Therefore, this study could provide the direction for achieving high levels of enantioselectivity which is otherwise difficult to achieve with a single metal catalyst.

Insights into the regioselectivity and diastereoselectivity of the Nazarov cyclization of 3-alkenyl-2-indolylmethanol with tryptophol

Density functional theory calculations of Nazarov cyclizations showed that 3-alkenyl-2-indolylmethanol and tryptophol formed binary and ternary complexes, respectively, in the presence and absence of H2O to produce C–N and C–C products.

Computation-Guided Development of the "Click" ortho-Quinone Methide Cycloaddition with Improved Kinetics

We report here a deep mechanistic study of the "click" ortho-quinone methide (oQM) cycloaddition between ortho-quinolinone quinone methide (oQQM) and thio-vinyl ether (TV), named as TQ-ligation. DFT calculations revealed the unexpected fact that dehydration of oQQM precursors is the rate-determining step of this transformation, and two highly reactive oQQM precursors were predicted. Guided by the calculations, a new "click" oQM cycloaddition which shows significantly improved kinetics and remarkable efficiency on protein labeling was developed.

How CuCl and CuCl2 Insert into C-N Bonds of Diazo Compounds: An Electronic Structure and Mechanistic Study

The transition-metal Cu catalysts CuCl and CuCl₂ have been widely employed to catalyze a series of chemical reactions with diazo compounds because of their high efficiency and selectivity. However, how to yield the active Cu carbene species from the Cu catalysts and diazo compounds still remains unclear. In this work, we performed a comprehensive theoretical investigation on the electronic structures of CuCl and CuCl₂ in solution. The results indicate that the most stable structures for CuCl and CuCl₂ are dimer and monomer, respectively. The C-N bond insertion of aryldiazoacetate by CuCl yields a stable bimetallic carbene species, which differs from the monometallic carbene generated from CuCl₂.

Selective functionalisation of aromatic alcohols with supramolecularly regulated gold(i) catalysts

Phosphite-based Au(i) catalytic systems containing an array of structurally diverse polyether-based regulation sites were designed and synthesised.