https://pubs.rsc.org/en/content/articlepdf/2019/NP/C8NP00092A

Marine natural products.

N-Heterocyclic Carbenes in Late Transition Metal Catalysis

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes.

Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

Cyclobutanes and cyclobutenes are important structural motifs found in numerous biologically significant molecules, and they are useful intermediates for chemical synthesis. Consequently, [2+2] cycloadditions to access cyclobutanes and cyclobutenes have been established to be particularly useful transformations. Within the last 10 years, an increase in the frequency of publications for catalytic enantioselective [2+2] cycloadditions has occurred. These reactions provide access to a wide array of enantiomerically enriched chemical diversity that was not previously attainable. Described in this review are the advances made in catalytic enantioselective [2+2] cycloadditions to access cyclobutanes and cyclobutenes.

Cyclobutane and cyclobutene synthesis: catalytic enantioselective [2+2] cycloadditions.

Copper-catalyzed Suzuki–Miyaura-type cross-coupling and carboboration processes are reported. The cross-couplings function well with a variety of substituted aryl iodides and aryl boronic esters and allows for orthogonal reactivity compared to palladium-catalyzed processes. The carboboration method includes both alkynes and allenes and provides access to highly substituted and stereodefined vinyl boronic esters. The alkyne carboboration method is highlighted in the simple one-pot synthesis of Tamoxifen.

Copper‐Catalyzed Cross‐Coupling of Boronic Esters with Aryl Iodides and Application to the Carboboration of Alkynes and Allenes

All‐Carbon Quaternary Stereogenic Centers by Enantioselective Cu‐Catalyzed Conjugate Additions Promoted by a Chiral N‐Heterocyclic Carbene

We present the first examples of Cu-catalyzed enantioselective conjugate additions of alkyl- and arylzinc reagents to unactivated cyclic beta-substituted enones. Transformations are promoted in the presence of 2.5-15 mol % of a readily available chiral NHC-based Cu complex, affording the desired products bearing all-carbon quaternary stereogenic centers in 67-->98% yield and in up to 97% ee. Catalytic enantioselective reactions can be carried out on a benchtop, with undistilled solvent and commercially available (not further purified) Cu salts. Mechanistic models, accounting for the observed levels and trends in enantioselectivity are provided.

A Practical Method for Enantioselective Synthesis of All-Carbon Quaternary Stereogenic Centers through NHC-Cu-Catalyzed Conjugate Additions of Alkyl- and Arylzinc Reagents to β-Substituted Cyclic Enones

An efficient method for catalytic asymmetric allylic alkylation (AAA) of allylic phosphates with vinylaluminum reagents is reported. The vinylmetal reagents are prepared by reaction of commercially available DIBAL-H and a terminal alkyne. The resulting vinylaluminum reagent can be used directly, without isolation or purification. AAA reactions are promoted in the presence of 0.5−2.5 mol % of a readily available chiral N-heterocyclic carbene (NHC) complex and 1−5 mol % commercially available and air stable Cu salt (CuCl2·2H2O). The desired products are typically obtained within 2−12 h in 74% to 95% isolated yield, 77% to >98% ee, and in >98% E selectivity; >98% SN2‘ selectivity is obtained in all but one instance (90%). The hydroalumination/catalytic AAA sequence can be performed in a single vessel, on gram scale.

M. Kevin Brown

Papers

Cyclobutane and cyclobutene synthesis: catalytic enantioselective [2+2] cycloadditions.

Copper‐Catalyzed Cross‐Coupling of Boronic Esters with Aryl Iodides and Application to the Carboboration of Alkynes and Allenes

All‐Carbon Quaternary Stereogenic Centers by Enantioselective Cu‐Catalyzed Conjugate Additions Promoted by a Chiral N‐Heterocyclic Carbene

A Practical Method for Enantioselective Synthesis of All-Carbon Quaternary Stereogenic Centers through NHC-Cu-Catalyzed Conjugate Additions of Alkyl- and Arylzinc Reagents to β-Substituted Cyclic Enones

Highly site- and enantioselective Cu-catalyzed allylic alkylation reactions with easily accessible vinylaluminum reagents.