Reaccions d'acoblament creuat: eines simples per a la construcció de molècules complexes

El premi Nobel de química de l'any 2010 ha reconegut el treball de tres químics que durant els anys 70 i 80 van obrir el camí a la realització de certes síntesis orgàniques que ara resulten fonamentals per a l'elaboració de substàncies d'interès en medicina, microelectrònica..

Halide abstraction competes with oxidative addition in the reactions of aryl halides with [Ni(PMenPh(3-n))4]

Density functional theory (DFT) calculations have been used to study the oxidative addition of aryl halides to complexes of the type [Ni(PMenPh(3-n))4], revealing the crucial role of an open shell singlet transition state for halide abstraction. The formation of NiI versus NiII has been rationalised through the study of three different pathways: (i) halide abstraction by [Ni(PMenPh(3-n))3], via an open shell singlet transition state; (ii) SN2-type oxidative addition to [Ni(PMenPh(3-n))3], followed by phosphine dissociation; and (iii) oxidative addition to [Ni(PMenPh(3-n))2]. For the case of [Ni(PMe3)4], a microkinetic model was used to show that these data are consistent with the experimentally-observed ratios of NiI and NiII. Importantly, [Ni(PMenPh(3-n))2] complexes often have little if any role in the oxidative addition reaction because they are relatively high in energy. The behaviour of [Ni(PR3)4] complexes in catalysis is therefore likely to differ considerably from those based on diphosphine ligands in which two coordinate Ni0 complexes are the key species undergoing oxidative addition

Steric effects determine the mechanisms of reactions between bis(N-heterocyclic carbene)-nickel(0) complexes and aryl halides

Computational investigations of the reactions of [Ni(NHC)2] complexes with aryl halides show that the outcomes are governed by the steric impact of the ligand (NHC = N-heterocyclic carbene). Small ligands permit facile oxidative addition, leading to NiII complexes, while larger NHC ligands prevent coordination of the aryl halide and favour the halide abstraction to form NiI

Mechanistic investigation of iridium-catalyzed C-H borylation of methyl benzoate: Ligand effects in regioselectivity and activity

The Ir-catalyzed C-H borylation of methyl benzoate has been studied with DFT methodology in order to understand the experimentally observed ligand-induced regioselectivity and activity when different [(ligand)Ir(Bpin)3] catalysts are employed. While bidentate ligands such as 4,4′-di-tBu-2,2′-bipyridine (dtbpy) completely inhibit ortho-borylation, the use of selected triphenylphosphine derivatives enables the reaction on that position, avoiding the meta- and para-regioisomers. The analysis of the catalytic cycles for the borylation reactions with dtbpy, PPh3, P(p-CF3C6H4)3, and P(m,m-(CF3)2C6H3)3 allows the interpretation of the observed ligand effects. The different reactivity observed for the different monodentate phosphine ligands can also be rationalized in terms of catalyst stability

Mechanistic study on the asymmetric synthesis of the Wieland-Miescher ketone analogs

The organocatalyzed synthesis of the Wieland‐Miescher ketone (WMK) via N‐sulfonyl‐binamprolinamide catalysis was investigated using experimental and computational tools. A mechanistic proposal is presented describing the origin of the high enantioselectivity, which rivals that of aldolases. The computational study reveals that the role of the prolinamide catalyst is to lower the reaction barrier and determine the stereoselectivity of the product achieved, while the role of the carboxylic acid is to facilitate proton transfer steps. The effect of the acid co‐catalyst was confirmed by experiments. The role of the structure of the BINAM backbone and the effect of the sulfonamide group are uncovered experimentally and computationally. Calculations show that a rigid highly defined catalytic pocket due to covalent and steric interactions induces conformational changes in the triketone substrate to maximize interactions

Oxidative Dehydrogenation of an Amine Group of a Macrocyclic Ligand in the Coordination Sphere of a Cu\u3csup\u3eII\u3c/sup\u3e Complex

The spontaneous oxidation of an amine group to an imine has been observed experimentally in an octa-aza macrocyclic dinucleating ligand LH4 coordinated to CuII. The reaction is bimolecular and spontaneous in which amine groups of one macrocycle are oxidised and the CuII centres of a second macrocyclic complex are reduced. No additional oxidating or external base agents are required. DFT calculations are carried out to compare the reaction with that recently reported for a ligand coordinated to an FeIII centre, but which requires an external base as proton acceptor. The computational results show that the copper and iron catalysed amine to imine reactions proceed via different mechanisms

Unexpected Nickel Complex Speciation Unlocks Alternative Pathways for the Reactions of Alkyl Halides with dppf-Nickel(0)

The mechanism of the reactions between dppf-Ni0 complexes and alkyl halides has been investigated using kinetic and mechanistic experiments and DFT calculations. The active species is [Ni(κ2-dppf)(κ1-dppf)], which undergoes a halide abstraction reaction with alkyl halides and rapidly captures the alkyl radical that is formed. The rates of the reactions of [Ni(COD)(dppf)] with alkyl halides and the yields of prototypical nickel-catalyzed Kumada cross-coupling reactions of alkyl halides are shown to be significantly improved by the addition of free dppf ligand

A Quantitative Model for Alkane Nucleophilicity Based on C−H Bond Structural/Topological Descriptors

A first quantitative model for calculating the nucleophilicity of alkanes is described. A statistical treatment was applied to the analysis of the reactivity of 29 different alkane C−H bonds towards in situ generated metal carbene electrophiles. The correlation of the recently reported experimental reactivity with two different sets of descriptors comprising a total of 86 parameters was studied, resulting in the quantitative descriptor‐based alkane nucleophilicity (QDEAN) model. This model consists of an equation with only six structural/topological descriptors, and reproduces the relative reactivity of the alkane C−H bonds. This reactivity can be calculated from parameters emerging from the schematic drawing of the alkane and a simple set of sums.We thank MINECO for support with Grants CTQ2017-82893-C2- 1-R, CTQ2015-73693-JIN, CTQ2017-87792-R and Red Intecat (CTQ2016-81923-REDC). Junta de Andalucía is also acknowledged for Grant P12-FQM-1765. R. G. and A. O. thank MINECO for FPI and Juan de la Cierva fellowships, respectively

Accelerated Ru-Cu Trinuclear Cooperative C−H Bond Functionalization of Carbazoles : A Kinetic and Computational Investigation

The mechanism of a trinuclear cooperative dehydrogenative C−N bond-forming reaction is investigated in this work, which avoids the use of chelate-assisting directing groups. Two new highly efficient Ru/Cu co-catalyzed systems were identified, allowing orders of magnitude greater TOFs than the previous state of the art. In-depth kinetic studies were performed in combination with advanced DFT calculations, which reveal a decisive rate-determining trinuclear Ru-Cu cooperative reductive elimination step (CRE)