Ilan Marek

Selectivity in Metal‐Catalyzed CarbonCarbon Bond Cleavage of Alkylidenecyclopropanes

When more complex system leads to simpler reactivity profile; the ring-opening of strained three-membered rings such as methylene- and alkylidenecyclopropanes generally lead to several products. If one starts with more functionalized carbon skeletons, selective reactions are now observed and rationalization as well as synthetic applications are described in this concept article. This methodology could be used to the preparation of challenging structural motifs possessing quaternary carbon stereocenters in acyclic systems.

A unique approach to aldol products for the creation of all-carbon quaternary stereocentres

Among the challenges facing asymmetric synthesis the selective construction of quaternary stereogenic centres, particularly those bearing all-carbon substituents, stands out. The aldol reaction has the potential to create such stereocentres, but, despite the many advances in this field, a highly selective and efficient procedure has remained elusive. Here we show that, by choosing an alternative retrosynthetic approach, such products can be prepared in a highly selective fashion. Through the consecutive addition of several organometallic derivatives, an allylzinc reagent is prepared and, on addition to an aldehyde, an aldol-type product is created. The one-pot operation involves the construction of three new carbon-carbon bonds and the formation of an all-carbon quaternary stereocentre. The products of the reaction are amenable to downstream modification, and the procedure should find applications in the fields of natural product synthesis and drug discovery.

Selective carbon-carbon bond cleavage for the stereoselective synthesis of acyclic systems.

Most of the efforts of organic chemists have been directed to the development of creative strategies to build carbon-carbon and carbon-heteroatom bonds in a predictable and efficient manner. In this Review, we show an alternative approach where challenging molecular skeletons could be prepared through selective cleavage of carbon-carbon bonds. We demonstrate that it has the potential to be a general principle in organic synthesis for the regio-, diastereo-, and even enantioselective preparation of adducts despite the fact that C-C single bonds are among the least reactive functional groups. The development of such strategies may have an impact on synthesis design and can ultimately lead to new selective and efficient processes for the utilization of simple hydrocarbons.

All-Carbon Quaternary Stereogenic Centers in Acyclic Systems through the Creation of Several C–C Bonds per Chemical Step

In the past few decades, it has become clear that asymmetric catalysis is one of the most powerful methods for the construction of carbon-carbon as well as carbon-heteroatom bonds in a stereoselective manner. However, when structural complexity increases (i.e., all-carbon quaternary stereogenic center), the difficulty in reaching the desired adducts through asymmetric catalytic reactions leads to a single carbon-carbon bond-forming event per chemical step between two components. Issues of efficiency and convergence should therefore be addressed to avoid extraneous chemical steps. In this Perspective, we present approaches that tackle the stimulating problem of efficiency while answering interesting synthetic challenges. Ideally, if one could create all-carbon quaternary stereogenic centers via the creation of several new carbon-carbon bonds in an acyclic system and in a single-pot operation from simple precursors, it would certainly open new horizons toward solving the synthetic problems. Even more important for any further design, the presence of polyreactive intermediates in synthesis (bismetalated, carbenoid, and oxenoids species) becomes now an indispensable tool, as it creates consecutively the same number of carbon-carbon bonds as in a multi-step process, but in a single-pot operation.

Forming all-carbon quaternary stereogenic centres in acyclic systems from alkynes

The formation of all-carbon quaternary stereocentres in acyclic systems is one of the most difficult contemporary challenges in modern synthetic organic chemistry. Particularly challenging is the preparation of all-carbon quaternary stereocentres in aldol adducts; this difficulty is problematic because the aldol reaction represents one of the most valuable chemical transformations in organic synthesis. The main problem that limits the formation of these stereocentres is the absence of an efficient method of preparing stereodefined trisubstituted enolates in acyclic systems. Here we describe a different approach that involves the formation of two new stereogenic centres—including the all-carbon quaternary one—via a combined carbometalation–oxidation reaction of an organocuprate to give a stereodefined trisubstituted enolate. We use this method to generate a series of aldol and Mannich products from ynamides with excellent diastereomeric and enantiomeric ratios and moderate yields.

Fluoroform: an Efficient Precursor for the Trifluoromethylation of Aldehydes

Abstract Fluoroform is shown to be an efficient trifluoromethylating agent when deprotonated using standard reagents in DMF. The important role of DMF as a solvent but also as a reactant was demonstrated.

Merging allylic carbon–hydrogen and selective carbon–carbon bond activation

Since the nineteenth century, many synthetic organic chemists have focused on developing new strategies to regio-, diastereo- and enantioselectively build carbon–carbon and carbon–heteroatom bonds in a predictable and efficient manner. Ideal syntheses should use the least number of synthetic steps, with few or no functional group transformations and by-products, and maximum atom efficiency. One potentially attractive method for the synthesis of molecular skeletons that are difficult to prepare would be through the selective activation of C–H and C–C bonds, instead of the conventional construction of new C–C bonds. Here we present an approach that exploits the multifold reactivity of easily accessible substrates with a single organometallic species to furnish complex molecular scaffolds through the merging of otherwise difficult transformations: allylic C–H and selective C–C bond activations. The resulting bifunctional nucleophilic species, all of which have an all-carbon quaternary stereogenic centre, can then be selectively derivatized by the addition of two different electrophiles to obtain more complex molecular architecture from these easily available starting materials.

Enantioselective Carbometalation of Cinnamyl Derivatives: New Access to Chiral Disubstituted Cyclopropanes— Configurational Stability of Benzylic Organozinc Halides

Enantioselectiveadditionofan alkyllithiumcompound acrossa doublebond is achieved by the complexation of various primary or secondary organolithium compounds with (−)-sparteine [Eq. (a)]. This synthetic route is effective, simple, and may be useful for the synthesis of chiral disubstituted cyclopropanes.

FLUOROFORM : AN EFFICIENT PRECURSOR FOR THE TRIFLUOROMETHYLATION OF ALDEHYDES

Abstract Fluoroform is shown to be an efficient trifluoromethylating agent when deprotonated using standard reagents in DMF. The important role of DMF in that reaction was demonstrated.

Remote functionalization through alkene isomerization

Exploiting the reactivity of one functional group within a molecule to generate a reaction at a different position is an ongoing challenge in organic synthesis. Effective remote functionalization protocols have the potential to provide access to almost any derivatives but are difficult to achieve. The difficulty is more pronounced for acyclic systems where flexible alkyl chains are present between the initiating functional group and the desired reactive centres. In this Review, we discuss the concept of remote functionalization of alkenes using metal complexes, leading to a selective reaction at a position distal to the initial double bond. We aim to show the vast opportunity provided by this growing field through selected and representative examples. Our aim is to demonstrate that using a double bond as a chemical handle, metal-assisted long-distance activation could be used as a powerful synthetic strategy.