Cover Picture: Measuring the Relative Reactivity of the Carbon–Hydrogen Bonds of Alkanes as Nucleophiles (Angew. Chem. Int. Ed. 42/2018)

Catalytic Functionalization of Methane and Light Alkanes in Supercritical Carbon Dioxide

International audience; The development of catalytic methods for the effective functionalization of methane yet remains a challenge. The best system known to date is the so-called Catalytica Process based on the use of platinum catalysts to convert methane into methyl bisulfate with a TOF rate of 10−3 s. In this contribution, we report a series of silver complexes containing perfluorinated tris(indazolyl)borate ligands that catalyze the functionalization of methane into ethyl propionate upon reaction with ethyl diazoacetate (EDA) by using supercritical carbon dioxide (scCO2) as the reaction medium. The employment of this reaction medium has also allowed the functionalization of ethane, prop…

Supercritical Carbon Dioxide: A Promoter of Carbon-Halogen Bond Heterolysis

as the leaving groupdeparts from the carbon atom prior to the entrance of thenucleophile. Polar protic solvents with high dielectric con-stants promote polar bond heterolysis by providing effectiveH-bonding and electron-pair donation interactions to theleaving group and the incipient carbocation, respectively.Then, the solvent captures the carbocation intermediate togive the corresponding S

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 th…

Titelbild: Measuring the Relative Reactivity of the Carbon–Hydrogen Bonds of Alkanes as Nucleophiles (Angew. Chem. 42/2018)

Favoring alkane primary carbon-hydrogen bond functionalization in supercritical carbon dioxide as reaction medium

The selectivity of a catalytic alkane functionalization process can be modified just changing the reaction medium from neat alkane to supercritical carbon dioxide (scCO2). A silica supported copper complex bearing an Nheterocyclic carbene ligand promotes the functionalization of carbon-hydrogen bonds of alkanes by transferring the CHCO2Et group from N2=CHCO2Et (ethyl diazoacetate, EDA). In neat hexane only 3% of the primary C-H bonds (ethyl heptanoate being the product) are functionalized in that manner, whereas the same reaction carried out in scCO2 provides a 30% yield in this linear ester. Such effect seems to be induced by an electronic density flux from the NHC ligand to the surroundin…

Functionalization of CnH2n+2Alkanes: Supercritical Carbon Dioxide Enhances the Reactivity towards Primary Carbon-Hydrogen Bonds

The functionalization of the primary sites of alkanes is one of the more challenging areas in catalysis. In this context, a novel effect has been discovered that is responsible for an enhancement in the reactivity of the primary C-H bonds of alkanes in a catalytic system. The copper complex Cu(NCMe) (=hydrotris{[3,5-bis(trifluoromethyl)-4-bromo]-pyrazol-1-yl}borate) catalyzes the functionalization of CnH2n+2 with ethyl diazoacetate upon inserting the CHCO2Et unit into C-H bonds. In addition, the selectivity of the reaction toward the primary sites significantly increased relative to that obtained in neat alkane upon using supercritical carbon dioxide as the reaction medium. This was attribu…

Discovering Copper for Methane C–H Bond Functionalization

The copper complex Tp(CF3)2,BrCu(NCMe) (1, Tp(CF3)2,Br = hydrotris((3,5-bis(trifluoromethyl)-4-bromo)-pyrazol-1-yl)borate) catalyzes the insertion of the CHCO2Et group (from ethyl diazoacetate N2CHCO2Et, EDA) into the C–H bonds of methane, in a homogeneous process that uses supercritical carbon dioxide (scCO2) as the reaction medium. Other light alkanes such as ethane, propane, and butane have been also functionalized with this copper-based catalyst, in the first example of the derivatization of the series of C1–C4 alkanes with this metal and a soluble catalyst.

Silver-Catalyzed C-C Bond Formation between Methane and Ethyl Diazoacetate in Supercritical CO2

Even in the context of hydrocarbons’ general resistance to selective functionalization, methane’s volatility and strong bonds pose a particular challenge. We report here that silver complexes bearing perfluorinated indazolylborate ligands catalyze the reaction of methane (CH4) with ethyl diazoacetate (N2CHCO2Et) to yield ethyl propionate (CH3CH2CO2Et). The use of supercritical carbon dioxide (scCO2) as the solvent is key to the reaction’s success. Although the catalyst is only sparingly soluble in CH4/CO2 mixtures, optimized conditions presently result in a 19% yield of ethyl propionate (based on starting quantity of the diazoester) at 40°C over 14 hours.

Measuring the Relative Reactivity of the Carbon-Hydrogen Bonds of Alkanes as Nucleophiles

We report quantitative measurements of the relative reactivities of a series of C-H bonds of gaseous or liquid CnH2n+2 alkanes (n = 1-8, 29 different C-H bonds) towards insitu generated electrophiles (copper, silver, and rhodium carbenes), with methane as the reference. This strategy surpasses the drawback of previous model reactions of alkanes with strong electrophiles suffering from C-C cleavage processes, which precluded direct comparison of the relative reactivities of alkane C-H bonds.

CCDC 1037192: Experimental Crystal Structure Determination

Related Article: Riccardo Gava, Andrea Olmos, Bárbara Noverges, Teresa Varea, Eleuterio Álvarez, Tomás R. Belderrain, Ana Caballero, Gregorio Asensio, and Pedro J. Pérez|2015|ACS Catalysis|5|3726|doi:10.1021/acscatal.5b00718

CCDC 1037194: Experimental Crystal Structure Determination

Related Article: Riccardo Gava, Andrea Olmos, Bárbara Noverges, Teresa Varea, Eleuterio Álvarez, Tomás R. Belderrain, Ana Caballero, Gregorio Asensio, and Pedro J. Pérez|2015|ACS Catalysis|5|3726|doi:10.1021/acscatal.5b00718

CCDC 1037193: Experimental Crystal Structure Determination

Related Article: Riccardo Gava, Andrea Olmos, Bárbara Noverges, Teresa Varea, Eleuterio Álvarez, Tomás R. Belderrain, Ana Caballero, Gregorio Asensio, and Pedro J. Pérez|2015|ACS Catalysis|5|3726|doi:10.1021/acscatal.5b00718