The effective use of ring strain has been applied to considerable advantage for the construction of complex systems. The focus here is directed towards cyclopropanes as building blocks for organic synthesis. Although thermodynamics should take the side of synthetic chemists, only a specific substitution pattern at the cyclopropane ring allows for particularly mild, efficient, and selective transformations. The required decrease in the activation barrier is achieved by the combined effects of vicinal electron-donating and electron-accepting moieties. This Review highlights the appropriate tools for successfully employing donor–acceptor cyclopropanes in ring-opening reactions, cycloadditions, and rearrangements.

A new golden age for donor-acceptor cyclopropanes.

Cyclization reactions of donor–acceptor (D–A) cyclopropanes are recognized as versatile methods for construction of carbocyclic and heterocyclic scaffolds. In the literature, many examples of these polarized cyclopropanes' reactivity with nucleophiles, electrophiles, and radicals are prevalent. Although intermolecular reactivity of donor–acceptor cyclopropanes is widely reported, reviews that center on their intramolecular chemistry are rare. Thereupon, this tutorial review focalizes on new intramolecular transformations of donor–acceptor cyclopropanes for cycloisomerizations, formal cycloadditions, umpolung reactions, rearrangements and ring-opening lactonizations/lactamizations from 2009 to 2013. Furthermore, the role of D–A acceptor cyclopropanes as reactive subunits in natural product synthesis is underscored.

Intramolecular donor–acceptor cyclopropane ring-opening cyclizations

Cyclization and annulation reactions of nitrogen-substituted cyclopropanes and cyclobutanes.

The carbon-carbon (C-C) bond cleavage of cyclopropanols is a wide area of research with much current activity. This review highlights new developments in this area over the past two decades. A summary is made of the three main reactivity modes, namely, homoenolate chemistry, β-keto radical chemistry, and acid-catalyzed ring-opening, as well as all other methods for the C-C bond cleavage and functionalization of cyclopropanols, including base-mediated ring-opening, metal-catalyzed C-C insertions and eliminations, oxidative fragmentation using hypervalent iodine reagents, reactions of donor-acceptor cyclopropanols, and pericylic reactions. Emphasis is placed on the synthetic utility of cyclopropanols and related derivatives, which have emerged as unique three-carbon synthons.

Selective Carbon–Carbon Bond Cleavage of Cyclopropanols

Until recently, computational tools were mainly used to explain chemical reactions after experimental results were obtained. With the rapid development of software and hardware technologies to make computational modeling tools more reliable, they can now provide valuable insights and even become predictive. In this review, we highlighted several studies involving computational predictions of unexpected reactivities or providing mechanistic insights for organic and organometallic reactions that led to improved experimental results. Key to these successful applications is an integration between theory and experiment that allows for incorporation of empirical knowledge with precise computed values. Computer modeling of chemical reactions is already a standard tool that is being embraced by an ever increasing group of researchers, and it is clear that its utility in predictive reaction design will increase further in the near future.

Design and Optimization of Catalysts Based on Mechanistic Insights Derived from Quantum Chemical Reaction Modeling.

Build a bridge: The first catalytic intramolecular [3+2] cycloaddition of monodonor-monoacceptor cyclopropanes (see scheme) provides a general and efficient strategy for construction of structurally diverse acetal[n.2.1] and 1,4-dioxygen-substituted cyclic skeletons, which are widely distributed in biologically important natural products.

Lewis Acid Catalyzed Intramolecular [3+2] Cross‐Cycloaddition of Donor–Acceptor Cyclopropanes with Carbonyls: A General Strategy for the Construction of Acetal[n.2.1] Skeletons

A highly enantioselective [4+2] annulation with respect to allenoates and 3-olefinic oxindoles catalyzed by Lewis base was reported, which proved to be an efficient way to synthesize chiral dihydropyran-fused indoles. The cycloaddition products were generally obtained in high yields (up to 98%) with very good enantioselectivities (up to 94% enantiomeric excess).

Enantioselective Synthesis of Dihydropyran-Fused Indoles through [4+2] Cycloaddition between Allenoates and 3-Olefinic Oxindoles

Equilibrium acidities of proline derived organocatalysts in DMSO.

Design and Applications of N-tert-Butyl Sulfinyl Squaramide Catalysts

Fluorine incorporation into organic molecules is expected to lower the pKa of neighboring functionality via its strong electron-withdrawing effect, and this strategy has been widely exploited in diverse disciplines. Herein, we report a striking anomalous α-fluorine substitution effect on the α-Csp3 -H acidity. We have experimentally measured the pKa values of a series of popular nucleophilic monofluoromethylating reagents α-fluoro(phenylsulfonyl)methane derivatives as well as their C-H analogues by Bordwell's overlapping indicator method in dimethyl sulfoxide solution. Contrary to expectations, we found that α-fluorine substituent does not generally enhance but rather weaken the α-Csp3 -H acidity of most (phenylsulfonyl)methane derivatives. DFT computations reproduce and provide insight into the anomalous α-fluorine effect. A correlation was identified between the C-H pKa of (phenylsulfonyl)methane derivatives and Mayr's nucleophilicity parameter (N) of the corresponding carbanions.

The Acidities of Nucleophilic Monofluoromethylation Reagents: An Anomalous α‐Fluorine Effect

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