A Mild and Regiospecific Synthesis of Pyrazoleboranes

Polymer-supported strong Lewis acid phosphonium cation catalysis applied to sydnone synthesis

Organochlorophosphonium P(V) species are strong Lewis acids deriving from a low-lying σ* orbital at P opposite to Cl. Herein, applying this strong acidity to heterogenous reactivity, we introduce polymer-supported phosphorus(V)-mediated Lewis acid catalysis. Key to this innovation is the use of a recyclable solid support Merrifield resin P(V) Lewis acid catalyst with demonstrated utility for the one-pot synthesis of sydnones. This concept of pnictogen P(V) Lewis acid catalysis is simple to apply, selective, and benefits from mild conditions with short reaction times. Further, experimental and computational findings reveal the crucial mechanistic role of phosphonium cation P(V) species in these reactions.

A Dynamic Thermodynamic Resolution Strategy for the Stereocontrolled Synthesis of Streptonigrin

We report that axially chiral biaryl boronic esters can be generated with control of atroposelectivity by a Binol-mediated dynamic thermodynamic resolution process. These intermediates can be progressed to enantioenriched products through stereoretentive functionalization of the carbon-boron bond. Finally, we have exploited this method in the first highly stereoselective total synthesis of P-streptonigrin.

A Mild and Regioselective Route to Fluoroalkyl Aromatic Compounds via Directed Cycloaddition Reactions

The synthesis of perfluoroalkyl-substituted (hetero)arenes by benzannulation strategies is complementary to ring functionalization approaches as it obviates the need for pre-existing functionality and innate regiocontrol. We report a mild and regiospecific boron-directed benzannulation method as a vehicle for accessing a range of perfluoroalkyl-substituted (hetero)aromatic building blocks that can be readily elaborated through established C–B bond functionalization processes.

Synthesis and reactivity of alkynyl boron compounds

Over the last century, there have been considerable developments in organoboron chemistry due to the stability, non-toxicity, and easy commercial availability of various boronic esters. Several organoboron reagents have emerged and play an increasingly important role in everyday organic synthesis. Among them, alkynyl boron compounds have attracted significant attention due to their easy synthesis and diverse reactivity. In this review, we summarize the advancement of research on alkynyl boron compounds, highlighting their importance in the synthesis of valuable compounds.

Arylethynyltrifluoroborate Dienophiles for on Demand Activation of IEDDA Reactions

Strained alkenes and alkynes are the predominant dienophiles used in inverse electron demand Diels–Alder (IEDDA) reactions. However, their instability, cross-reactivity, and accessibility are problematic. Unstrained dienophiles, although physiologically stable and synthetically accessible, react with tetrazines significantly slower relative to strained variants. Here we report the development of potassium arylethynyltrifluoroborates as unstrained dienophiles for fast, chemically triggered IEDDA reactions. By varying the substituents on the tetrazine (e.g., pyridyl- to benzyl-substituents), cycloaddition kinetics can vary from fast (k₂ = 21 M^–1 s^–1) to no reaction with an alkyne-BF₃ dienophile. The reported system was applied to protein labeling both in the test tube and fixed cells and even enabled mutually orthogonal labeling of two distinct proteins.

Click and Bio-Orthogonal Reactions with Mesoionic Compounds

Click and bio-orthogonal reactions are dominated by cycloaddition reactions in general and 1,3-dipolar cycloadditions in particular. Among the dipoles routinely used for click chemistry, azides, nitrones, isonitriles, and nitrile oxides are the most popular. This review is focused on the emerging click chemistry that uses mesoionic compounds as dipole partners. Mesoionics are a very old family of molecules, but their use as reactants for click and bio-orthogonal chemistry is quite recent. The facility to derivatize these dipoles and to tune their reactivity toward cycloaddition reactions makes mesoionics an attractive opportunity for future click chemistry development. In addition, some compounds from this family are able to undergo click-and-release reactions, finding interesting applications in cells, as well as in animals. This review covers the synthetic access to main mesoionics, their reaction with dipolarophiles, and recent applications in chemical biology and heterocycle synthesis.

Exploiting Synergistic Catalysis for an Ambient Temperature Photocycloaddition to Pyrazoles

Sydnone-based cycloaddition reactions are a versatile platform for pyrazole synthesis, however they operate under harsh conditions (high temperature and long reaction times). Herein we report a strategy that addresses this limitation utilizing the synergistic combination of organocatalysis and visible-light photocatalysis. This new approach proceeds under ambient conditions and with excellent levels of regiocontrol. Mechanistic studies suggest that photoactivation of sydnones, rather than enamines, is key to the successful implementation of this process.

Mechanosynthesis of sydnone-containing coordination complexes

Mechanochemistry provides a powerful approach to the multistep synthesis of novel coordination complexes, featuring sydnones as ligands, starting with N-arylglycines.

[3 + 2]-Cycloaddition reaction of sydnones with alkynes

This review covers all known examples of [3 + 2]-cycloaddition between sydnones and both terminal as well as internal alkynes/cycloalkynes taken from literature since its discovery by Huisgen in 1962 up to the current date. Except enumeration of synthetic applications it also covers mechanistic studies, catalysis, effects of substituents and reaction conditions influencing reaction rate and regioselectivity.

Sydnone-alkyne cycloaddition: applications in synthesis and bioconjugation

Sydnones are among the most popular mesoionic compounds studied so far for cycloaddition reactions. However, despite their good chemical stability and versatility, only a limited number of research groups have worked on their chemistry and use in organic synthesis. This feature article aims at providing an overview of the most recent developments in sydnone-alkyne cycloadditions, with particular attention on the strategies that allow us to achieving high regiocontrol and milder reaction conditions. The recent discovery that this dipole is able to undergo click and biorthogonal reactions with cycloalkynes may stimulate renewed interest from the scientific community. Given the high potential and flexibility of this family of mesoionics, we believe that major developments are to be expected both in terms of organic synthetic methodologies and biorthogonal chemistry applications in the field of chemical biology.

Transition metal-free controlled synthesis of bis[(trifluoromethyl)sulfonyl]ethyl-decorated heterocycles

The metal- and irradiation-free C–H bis(triflyl)ethylation reactions of a variety of heterocycles including marketed drugs have been accomplished.

Influence of Endo- and Exocyclic Heteroatoms on Stabilities and 1,3-Dipolar Cycloaddition Reactivities of Mesoionic Azomethine Ylides and Imines

The geometries, stabilities, and 1,3-dipolar cycloaddition reactivities of 24 mesoionic azomethine ylides and imines were investigated using density functional theory calculations at the M06-2X/6-311+G-(d,p)/M06-2X/6-31G-(d) level. The computed structures highlight how the commonly used "aromatic" resonance form should be replaced by two more accurate resonance structures. Stabilities of the dipoles were assessed by various homodesmotic schemes and are consistent with these compounds being nonaromatic. The activation free energies with ethylene or acetylene range from 11.8 to 36.6 kcal/mol. Within each dipole type, the predicted cycloaddition reactivities correlate with the reaction energies and the resonance stabilization energies provided by the various substituents. Endocyclic (X) heteroatoms increase the reactivity of the 1,3-dipoles in the order of O > NH ≅ S, whereas exocyclic (Y) substituents increase it in the order of CH₂ > NH > O > S. Distortion/interaction analysis indicated that the difference in reactivity between differently substituted 1,3-dipoles is driven by distortion, whereas the difference between azomethine ylides and imines is related to lower interaction energies of imines with the dipolarophiles.