Catalyst-free and additive-free reactions enabling C-C bond formation: a journey towards a sustainable future

This review focuses on the catalyst- and additive-free C-C bond forming reactions reported mostly from the year 2005 to date. C-C bond forming reactions are highly important as large and complex organic molecules can be derived from simpler ones via these reactions. On the other hand, catalyst- and additive-free reactions are economical, environmentally friendly and less sensitive to air/moisture, allow easy separation of products and are operationally simple. Hence, a large number of research articles have been published in this area. Though a few reviews are available on the catalyst-free organic reactions, most of them were published a few years ago. The current review excludes catalysts as well as additives and is specific to only C-C bond formation. Besides many organic name reactions, catalyst/additive-free C-H functionalizations, coupling reactions and UV-visible-light-promoted reactions are also discussed. Undoubtedly, the contents of this review will motivate readers to do more novel work in this area which will accelerate the journey towards a sustainable future.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Acetaldehyde Silyl Enol Ethers in Enantioselective Mukaiyama Aldol Reactions: Enzyme-Like Organocatalysis in Action

Touched for the very first time! It is herein highlighted how acetaldehyde silyl enol ethers undergo enantioselective Mukaiyama aldol reaction with aliphatic and aromatic aldehydes. The chemistry relies on the use of the highly efficient and substrate-selective imidodiphosphorimidate catalyst, which displays some of the features of enzymatic catalysis.

The Role of Water in the Catalyst-Free Aldol Reaction of Water-Insoluble N-Methyl-2,4-thiazolidinedione with N-Methylisatin from QM/MM Monte Carlo Simulations

The role of water in the uncatalyzed aldol reaction of N-methyl-2,4-thiazolidinedione with N-methylisatin is investigated through Monte Carlo statistical mechanics simulations that utilize free energy perturbation theory and the mixed quantum mechanics and molecular mechanics (QM/MM) model with PDDG/PM3 for the QM method in "on-water" and DMSO environments. There are several conceivable orientations between thiazolidinedione and isatin in the process of C-C bond formation. However, the formation of the C-C bond takes place between the re face of isatin and the si face of (E)-enol of the thiazolidinedione to form the preferred anti-type product, which results from enhanced hydrogen-bonding interactions between water molecules and the oxygen atoms undergoing bond breakage and bond formation during the reaction. Novel insights into the effect of water on the aldol reaction are presented herein.

A versatile access to vicinal diamine motifs by highly anti-selective asymmetric vinylogous Mannich reactions: an efficient total synthesis of (+)-absouline

An anti-stereoselective vinylogous Mannich reaction between 2-TBSO-pyrrole and N-tert-butanesulfinyl imines and its application to the efficient synthesis of (+)-absouline were reported.

Studies on the asymmetric synthesis of pandamarilactonines: an unexpected syn-selective vinylogous Mannich reaction of N-tert-butanesulfinimines

Through an unanticipatedsyn-selective VMR, a highly enantioselective total synthesis of (−)-pandamarilactonine-A has been achieved.

Beyond classical reactivity patterns: shifting from 1,4- to 1,6-additions in regio- and enantioselective organocatalyzed vinylogous reactions of olefinic lactones with enals and 2,4-dienals

Organocatalysis is shown to expand the classical reactivity pattern for conjugate addition reactions. It is demonstrated that the site selectivity can be extended from 1,4- to 1,6-additions for the enantioselective vinylogous additions of methyl-substituted vinylogous lactones to enals and 2,4-dienals. This novel reactivity is demonstrated for methyl-substituted olefinic azlactones and butyrolactones. Their synthetic potential is first highlighted by the development of the organocatalytic regioselective vinylogous 1,4-addition to enals which proceeds with a very high level of double-bond geometry control and excellent enantioselectivity. The concept is developed further for the unprecedented intermolecular enantioselective organocatalyzed vinylogous 1,6-addition to linear 2,4-dienals, by which the site selectivity of the process is extended from the β-position to the remote δ-position of the 2,4-dienal. The organocatalyst controls the newly generated stereocenter six bonds away from the stereocenter of the catalyst with a high level of enantiocontrol, and the products are obtained with full control of double-bonds configuration. The scope of these new reaction concepts is demonstrated for a series of aliphatic and aryl-substituted enals and 2,4-dienals undergoing enantioselective vinylogous reactions with methyl-substituted olefinic azlactones and butyrolactones. Furthermore, mechanistic considerations are presented which can account for the change from 1,4- to 1,6-selectivity. Finally, a number of different transformations of the optically active 1,4- and 1,6-addition products are demonstrated.