Transition metal chemistry of cyclopropenes and cyclopropanes.

Transition-metal-catalyzed decarboxylative coupling presents a new and important direction in synthetic chemistry. Mechanistic studies on decarboxylative coupling not only improve the understanding of the newly discovered transformations, but also may have valuable implications for the development of more effective catalyst systems. In this work, a comprehensive theoretical study was conducted on the mechanism of Myers’ Pd-catalyzed decarboxylative Heck reaction. The catalytic cycle was found to comprise four steps: decarboxylation, olefin insertion, β-hydride elimination, and catalyst regeneration. Decarboxylation was the rate-limiting step, and it proceeded through a dissociative pathway in which Pd(II) mediated the extrusion of CO2 from an aromatic carboxylic acid to form a Pd(II)−aryl intermediate. Further analysis was conducted on the factors that might control the efficiency of Myers’ decarboxylative Heck reaction. These factors included Pd salts, ligands, acid substrates, and metals. (1) Regarding ...

Theoretical Analysis of Factors Controlling Pd-Catalyzed Decarboxylative Coupling of Carboxylic Acids with Olefins

Highly enantioselective cyclopropenation of internal alkynes with aryldiazoacetates was achieved using the binuclear gold catalyst (S)-xylylBINAP(AuCl)2, activated by silver hexafluoroantimonate.

Gold(I)-catalyzed asymmetric cyclopropenation of internal alkynes.

In addition to the appealing synthetic transformations that cyclopropanes present, they are also part of larger molecular structures that possess a wide range of biological properties. Therefore, the preparation of enantiomerically enriched cyclopropanes has consistently been a very interesting research topic in organic synthesis. In this Focus Review, we are presenting new methods for the synthesis of these target compounds through catalytic and asymmetric direct functionalization of simple achiral three-membered carbocycle precursors. These convergent and very flexible approaches allow the preparation of a large variety of polysubstituted alkyl-, vinyl-, alkynyl-, and arylcyclopropanes but also cyclopropanols and cyclopropylamines in very high diastereo- and enantiomeric ratios from a single precursor, underlining the power of this synthetic route.

Asymmetric Preparation of Polysubstituted Cyclopropanes Based on Direct Functionalization of Achiral Three-Membered Carbocycles

Nonconjugated hydrocarbons, like bicyclo[1.1.1]pentane, bicyclo[2.2.2]octane, triptycene, and cubane are a unique class of rigid linkers. Due to their similarity in size and shape they are useful mimics of classic benzene moieties in drugs, so-called bioisosteres. Moreover, they also fulfill an important role in material sciences as linear linkers, in order to arrange various functionalities in a defined spatial manner. In this Review article, recent developments and usages of these special, rectilinear systems are discussed. Furthermore, we focus on covalently linked, nonconjugated linear arrangements and discuss the physical and chemical properties and differences of individual linkers, as well as their application in material and medicinal sciences.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201804225

Nonconjugated Hydrocarbons as Rigid‐Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry

The gas-phase acidity of 3,3-dimethylcyclopropene (1) has been measured by bracketing and equilibrium techniques. Consistent with simple hybridization arguments, our value (deltaH degrees (acid) = 382.7 +/- 1.3 kcal mol(-)(1)) is indistinguishable from that for methylacetylene (i.e., deltadeltaH degrees (acid)(1 - CH(3)Ctbd1;CH) = 1.6 +/- 2.5 kcal mol(-)(1)). The electron affinity of 3,3-dimethylcyclopropenyl radical (1r) was also determined (EA = 37.6 +/- 3.5 kcal mol(-)(1)), and these quantities were combined in a thermodynamic cycle to afford the homolytic C-H bond dissociation energy. To our surprise, the latter quantity (107 +/- 4 kcal mol(-)(1)) is the same as that for methane, which cannot be explained in terms of the s-character in the C-H bonds. An orbital explanation (delocalization) is proposed to account for the extra stability of 1r. All of the results are supplemented with G3 and B3LYP computations, and both approaches are in good accord with the experimental values. We also note that for simple hydrocarbons which give localized carbanions upon deprotonation there is an apparent linear correlation between any two of the following three quantities: deltaH degrees (acid), BDE, and EA. This observation could be of considerable value in many diverse areas of chemistry.

Ralph E. McCarthy

Papers

Why does cyclopropene have the acidity of an acetylene but the bond energy of methane