Metal carbenes appended with two electron-donating groups, known as "donor/donor" carbenes, undergo diastereo- and enantioselective rhodium-catalyzed C-H insertion reactions with ether substrates to form benzodihydrofurans. Unlike the reactions of metal carbenes with electron-withdrawing groups attached, the attenuated electrophilicity enables these reactions to be conducted in Lewis basic solvents (e.g., acetonitrile) and in the presence of water. The diazo precursors for these species are prepared in situ from hydrazone using a mild and chemoselective oxidant (MnO ). Although this sequence often can be performed in one-pot, control experiments have elucidated why a "two-pot" process is often more efficient. A thorough screening of achiral catalysts demonstrated that sterically encumbered catalysts are optimal for diastereoselective reactions. Although efficient insertion into allylic and propargylic C-H bonds is observed, competing dipolar cycloaddition processes are noted for some substrates. The full substrate scope of this useful method of benzodihydrofuran synthesis, mechanisms of side reactions, and computational support for the origins of stereoselectivity are described.
Here we report that chiral Mn(I) complexes are capable of H–P bond activation. This activation mode enables a general method for the hydrophosphination of internal and terminal α,β-unsaturated nitriles. Metal−ligand cooperation, a strategy previously not considered for catalytic H–P bond activation, is at the base of the mechanistic action of the Mn(I)-based catalyst. Our computational studies support a stepwise mechanism for the hydrophosphination and provide insight into the origin of the enantioselectivity.
We propose a novel mechanism for the deoxydehydration (DODH) reaction of glycols catalyzed by a [Bu 4 N][VO 2 (dipic)] complex (dipic= pyridine-2,6-dicarboxylate) using triphenylphosphine as a reducing agent. Using density functional theory (DFT) we have confirmed that the preferred sequence of reaction steps involves reduction of the V(V) complex by phosphine, followed by condensation of the glycol into a [VO(dipic)(- (6), which then evolves to the alkene product and the recovery of the catalyst. In contrast to the usually invoked closed-shell mechanism for the latter steps, where 6 suffers a [3+2] retrocycloaddition, we have found that the homolytic cleavage of one of the C-O bonds in 6 is preferred by 12 kcal/mol. The resulting diradical intermediate then collapses to a metallacycle which evolves to the product through an aromatic [2+2] retrocycloaddition. We use this key change in the mechanism to propose ways to design better catalysts for this transformation. The analysis of the mechanisms in both singlet and triplet potential energy surfaces, together with the location of the MECPs between them, showcase this reaction as an interesting example of two-state reactivity.2
Oxygen atom transfer reactions are receiving increasing attention because they bring about paramount transformations in the current biomass processing industry. Significant efforts have therefore been made lately in the development of efficient and scalable methods to deoxygenate organic compounds. One recent alternative involves the modification of the Cadogan reaction in which a Mo(VI) core catalyzes the reduction of o -nitrostyrene derivatives to indoles in the presence of PPh 3 . We have used density functional theory calculations to perform a comprehensive mechanistic study on this transformation, in which we find two clearly defined stages: an associative path from the nitro to the nitroso compound, characterized by the reduction of the catalyst in the first step, and a peculiar mechanism involving oxazaphosphiridine and nitrene intermediates leading to an indole product, where the metal catalyst does not participate.
Dearomative functionalization of heteroaromatics, a readily available chemical feedstock, is one of the most straightforward approaches for the synthesis of three-dimensional, chiral heterocyclic systems, important synthetic building blocks for both synthetic chemistry and drug discovery. Despite significant efforts, direct nucleophilic additions to heteroaromatics have remained challenging because of the low reactivity of aromatic substrates associated with the loss of aromaticity, as well the regio- and stereoselectivities of the reaction. Here we present a catalytic system that leads to unprecedented, high-yielding dearomative C-4 functionalization of quinolines with organometallics with nearly absolute regio- and stereoselectivities and with a catalyst turnover number (TON) as high as 1000. The synergistic action of the chiral copper catalyst, Lewis acid, and Grignard reagents allows us to overcome the energetic barrier of the dearomatization process and leads to chiral products with selectivities reaching 99% in most cases. Molecular modeling provides important insights into the speciation and the origin of the regio- and enantioselectivity of the catalytic process. The results reveal that the role of the Lewis acid is not only to activate the substrate toward a potential nucleophilic addition but also to subtly control the regiochemistry by preventing the C-2 addition from happening.
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