Transient imine as a directing group for the Pd-catalyzed anomeric C(sp3)-H arylation of 3-aminosugars.

The first example of Pd(II)-catalyzed anomeric arylation of 3-aminosugars is reported by using an L,X-type transient directing group (TDG) approach combined with an external 2-pyridone ligand. The released free amine was in situ transformed into an azide function, which was then exploited in a CuAAC to increase the molecular complexity and prepare a variety of complex substituted C3-triazolo C-glycosides in good yields.

Expedient Access to Polyaromatic Biaryls by Unconventional Ag-Catalyzed Cycloaromatization of Alkynylthiophenes and Au-Catalyzed Double C-H Activation.

An unconventional approach for the regioselective synthesis of polyaromatic biaryls via site-selective Ag-catalyzed twofold electrophilic cycloisomerization followed by Au-catalyzed double C-H activation is described. The developed process allows the synthesis of highly decorated biaryls with excellent regioselectivity. As revealed by DFT computations, the reaction represents a rare example of C1-C5 endo-exo and C1-C6 endo-endo cycloaromatization. The formation of the 6-membered ring is predicted to be the fruit of an uncommon SEAr on a vinyl carbocation.

Kinetic resolution of sulfur-stereogenic sulfoximines by Pd(ii)-MPAA catalyzed C-H arylation and olefination.

A direct Pd(ii)-catalyzed kinetic resolution of heteroaryl-enabled sulfoximines through an ortho-C-H alkenylation/arylation of arenes has been developed. The coordination of the sulfoximine pyridyl-motif and the chiral amino acid MPAA ligand to the Pd(ii)-catalyst controls the enantio-discriminating C(aryl)-H activation. This method provides access to a wide range of enantiomerically enriched unreacted aryl-pyridyl-sulfoximine precursors and C(aryl)-H alkenylation/arylation products in good yields with high enantioselectivity (up to >99% ee), and selectivity factor up to >200. The coordination preference of the directing group, ligand effect, geometry constraints, and the transient six-membered concerted-metalation-deprotonation species dictate the stereoselectivity; DFT studies validate this hypothesis.

Thermal decomposition of hexamethylenetetramine: mechanistic study and identification of reaction intermediates via a computational and NMR approach.

In a joint DFT and chemometrics study applied to NMR spectra, we disclose the structure of the main decomposition products of hexamethylenetetramine. The combination of these techniques enabled us to propose the structures of near-identical intermediates of the process and to unveil the structure of the main decomposition product of this priviliged structure.

Synthesis of axially chiral biaryl thioglycosides through thiosugar-directed Pd-catalyzed asymmetric C-H activation.

Herein we report for the first time that the thiosugar moiety can be used both as a directing group enabling the regioselective activation of a C-H bond of biaryl scaffolds and as a chiral source inducing axial chirality. Our approach enables the easy generation of complex thioglycoside atropoisomers, thus paving the way to new products of potential biological interest.

Looking at the big picture in activation strain model/energy decomposition analysis: the case of the ortho-para regioselectivity rule in Diels-Alder reactions.

The regioselectivity of the Diels-Alder reaction is predicted by the ortho-para rule which has been explained from FMO theory. Using DFT calculations, the activation-strain model and energy decomposition analysis we studied the reaction of methyl acrylate with four unsymmetrical dienes. We found that if the analysis is carried out considering the TS structures, the selectivity would not be explained by the interaction energy as expected considering the FMO arguments. However, a thorough analysis along the reaction path revealed that the interaction energy is responsible for the regioselectivity. A deeper analysis with the EDA model showed that the decisive term that accounts for the HOMO-LUMO interactions favors the ortho and para paths, as predicted by FMO arguments.

Combining the Power of J Coupling and DP4 Analysis on Stereochemical Assignments: The J-DP4 Methods.

A systematic study to include 3 JHH couplings into DP4 formalism ( J-DP4) led to the development of three alternative strategies. The d J-DP4 (direct) approach involves a new DP4-like equation including an additional probability term given by 3 JHH. The i J-DP4 (indirect) approach explores the original DP4 method with a restricted conformational search. Despite both strategies performing better than DP4, their combined use (i J/d J-DP4) provided the best results, with a 2.5-fold performance improvement at similar or lower computational cost.

Mechanistic insight into the acid-catalyzed isomerization of biomass-derived polysubstituted pyrrolidines: an experimental and DFT study.

The 1,3-dipolar cycloaddition reactions of azomethine ylides is one of the preferred methods for the synthesis of polysubstituted pyrrolidines. The use of chiral dipolarophiles derived from carbohydrates yields enantiomerically pure pyrrolidines, usually in good to excellent endo selectivities, along with other minor stereoisomers. Recently, we found an unusual isomerization event that allowed the isolation of useful pyrrolidines with relative stereochemistries difficult to obtain otherwise. Although a simple and efficient protocol to promote these transformations was developed, the mechanism was not fully unravelled. Herein, after a combination of experimental, spectroscopic and computational studies (using DFT methods) we propose that this isomerization event takes place through a retro-Mannich//Mannich cascade, via the formation of an iminium ion with E geometry.

Computational Chemistry Driven Solution to Rubriflordilactone B.

The structure of rubriflordilactone B (2) was determined by X-ray crystallography. However, the NMR data of the synthetic sample did not match those reported for 2. It was then suggested that the original sample contained an additional isomer of different solubility, pseudorubriflordilactone B (3), whose structure remained unknown. From theoretical calculations, reexamination of the NMR data, and biogenetic considerations, it is proposed that 3 should be the 16S,17R isomer of 2.

A theoretical study of the Duff reaction: insights into its selectivity.

The Duff reaction is one of the most employed methods for the ortho-formylation of phenols; however, not much is truly known about its mechanism. Using DFT calculations, we disclose the first theoretical study regarding the selectivity-determining step of the reaction. We have found that this stage is governed by a hydrogen bond, that gives rise to a cyclohexa-2,4-dienone intermediate and establishes the position where the formylation will take place. These findings were evaluated by analysis of the reaction outcome of several non-symmetrically substituted phenols.

Computational Chemistry to the Rescue: Modern Toolboxes for the Assignment of Complex Molecules by GIAO NMR Calculations.

The calculations of NMR properties of molecules using quantum chemical methods have deeply impacted several branches of organic chemistry. They are particularly important in structural or stereochemical assignments of organic compounds, with implications in total synthesis, stereoselective reactions, and natural products chemistry. In studying the evolution of the strategies developed to support (or reject) a structural proposal, it becomes clear that the most effective and accurate ones involve sophisticated procedures to correlate experimental and computational data. Owing to their relatively high mathematical complexity, such calculations (CP3, DP4, ANN-PRA) are often carried out using additional computational resources provided by the authors (such as applets or Excel files). This Minireview will cover the state-of-the-art of these toolboxes in the assignment of organic molecules, including mathematical definitions, updates, and discussion of relevant examples.

Beyond DP4: an Improved Probability for the Stereochemical Assignment of Isomeric Compounds using Quantum Chemical Calculations of NMR Shifts.

The DP4 probability is one of the most sophisticated and popular approaches for the stereochemical assignment of organic molecules using GIAO NMR chemical shift calculations when only one set of experimental data is available. In order to improve the performance of the method, we have developed a modified probability (DP4+), whose main differences from the original DP4 are the inclusion of unscaled data and the use of higher levels of theory for the NMR calculation procedure. With these modifications, a significant improvement in the overall performance was achieved, providing accurate and confident results in establishing the stereochemistry of 48 challenging isomeric compounds.

A hydrogen bond rationale for the enantioselective ß-alkenylboration of enones catalyzed by O-monoacyltartaric acids.

DFT calculations suggest that O-monoacyl L-tartaric acids catalyze the asymmetric conjugate alkenylboration of enones through transition structures that are stabilized by hydrogen-bonding interactions. Formation of a five-membered acyloxyborane is proposed. The hydrogen of the free carboxy group derived from the catalyst interacts with the carbonyl group of the cyclic acyloxyborane, stabilizing the transition structure and reducing the flexibility of the system. Additional stabilizing nonclassical CH···O hydrogen-bond interactions seem to determine the observed enantioselectivity.

Reactivity and selectivity of boron-substituted alkenes in the Diels-Alder reaction with cyclopentadiene. A study of the electron charge density and its Laplacian.

The effect of the nature of the boron moiety upon the reactivity and the selectivity of a variety of vinylboron dienophiles (1-12) in the Diels-Alder (DA) reaction was investigated using density functional theory and the quantum theory of atoms in molecules. The calculated reactivity of the dienophiles decreases in the order vinylborane (1) > dihalovinylboranes (2-4) > dialkylvinylboranes (5-7) ≈ vinyl boronic acid (8) > vinylboronates (9, 10) > vinyl MIDA boronate (11) ≈ vinyltrifluoroborate (12). The DA reactions of 1-7 were slightly endo-selective due to the stronger C6-B secondary orbital interaction in the endo transition structures (TSs) evaluated by the C6|B delocalization index. In the TSs of 5 and 7, a combination of electronic and steric factors reduce the endo selectivity. The moderate exo selectivity calculated for the DA reactions of boronates 8-11 was attributed mainly to the hydrogen bond between the oxygen atom of boronate moieties and one of the acidic hydrogens of the methylene of cyclopentadiene in the exo TSs, which also reduces the ability of the oxygen lone pairs to donate electron density into the vacant boron orbital. Interestingly, the cooperative effect between the two hydrogen bonds in the exo TS of the DA reaction of vinyltrifluoroborate (12) determines the almost exclusive exo selectivity predicted for this DA reaction. We propose that the relative reactivities of the dienophiles can be estimated by the charge density (ρr) and its Laplacian (∇(2)ρr) at the (3,+1) critical point in the topology of ∇(2)ρr, evaluated at the reactant molecules in the ground state. The profiles of the several topological parameters along the reaction are affected by the nature of the substituents attached to the boron atom and by the mode of addition (endo and exo) in the DA reactions.

Theoretical investigation of the Diels-Alder reactions of unsaturated boronates.

The Diels-Alder reactions of simple unsaturated boronates have been investigated using computational methods and the results were compared with those for the analogue dihalo- and dialkylboranes. Our results indicate that the activating effect of the boronate moiety is small. All the studied reactions are concerted normal electron-demand Diels-Alder reactions with asynchronous transition structures and weak [4 + 3] C-B secondary orbital interactions, which explains the low experimental reactivity. Both electronic and steric effects contribute to give the observed low stereo- and regioselectivities.