Use of 1-pentafluorosulfanyl-phenylacetylenes for the preparation of SF5-substituted five-membered ring heterocycles through 1,3-dipolar cycloadditions. Isoxazoles and isoxazolines

Synthesis of Substituted Pentafluorosulfanylpyrazoles Under Flow Conditions

The development of flow conditions for the synthesis of pentafluorosulfanylpyrazoles is reported. A range of alkyl- and aryl-substituted SF5-alkynes were used in combination with different diazoacetates for this transformation. The corresponding substituted SF5-pyrazoles were obtained in up to 90% yield (average of 74% for 21 examples) as a mixture of isomers (up to 73:27 ratio). Synthetic transformations starting from an SF5-containing pyrazole were also demonstrated.

Expanding Radical Chloropentafluorosulfanylation of Alkynes

The chloropentafluorosulfanylation of alkynes is a delicate but crucial operation for accessing SF5-alkynes that serve as substrates in numerous transformations. Dolbier's procedure using Et3B/O2 was the most efficient approach, while recent efforts make use of other initiators and light activation. We found that THF, as a single stimulus, is sufficient to trigger the reaction of SF5Cl with alkynes. We determined the configuration of Cl/SF5 products and clarified the structure of side-products.

An Extrusion Strategy for On-Demand SF5Cl Gas Generation from a Commercial Disulfide

Herein we report a novel methodology for the ex situ generation of SF5Cl by employing 4,4'-dipyridyl disulfide as a safe commercial reagent, obviating the need for lecture bottles. The method is applicable to certain SF5Cl-involving transformations by using a two-chamber reactor. Moreover, easily applying SF5Cl in different solvents is rendered feasible, while avoiding the use of glovebox techniques. This report also suggests 1H-19F HOESY as a simple and fast stereochemistry indication for chloropentafluorosulfanylated olefins.

Synthesis of Pyridine-SF4-Isoxazolines Using the Functionality of trans-Tetrafluoro-λ6-sulfanyl Rodlike Linkers

The tetrafluoro-λ⁶-sulfanyl (SF₄) moiety has been relatively undeveloped since its discovery in the 1970s. In this study, we synthesized pyridine-SF₄-isoxazolines, in which the two heterocycles are connected by a rodlike trans-SF₄ linker, via the regioselective 1,3-dipolar cycloaddition of pyridine-SF₄-alkynes and nitrones in the presence of triethylamine. SF₄ linkers are a viable alternative to para-substituted benzenes, alkynes, and bicyclo[1.1.1]pentyl derivatives in drug design, and pyridine-SF₄-isoxazolines have potential applications in drug development.

Synthesis and [3,3]-sigmatropic rearrangements of 5-(pentafluorosulfanyl)-pent-3-en-2-ol, its homologues, and trifluoromethyl analogues

The synthesis of aliphatic (pentafluoro-λ6-sulfanyl)(SF5)-substituted compounds is more challenging than that of the related CF3-substituted analogues. Previous investigations of [3,3]-sigmatropic rearrangements of γ-SF5-substituted allylic alcohols failed to yield 3-SF5-substituted carboxylic acid derivatives. Herein, we present the synthesis of a series of 1-SF5-alk-1-en-3-ols and our efforts to apply them in Johnson-Claisen, ester enolate-Claisen, and Ireland-Claisen rearrangements. Unfortunately, these reactions failed to include the 1-SF5-substituted 1,2-double bond, although successful reactions of analogous CF3-allylic alcohols were reported. Further experiments revealed that bulkiness rather than electronic properties of the SF5 group prevented [3,3]-sigmatropic rearrangements. Indeed, the introduction of a competing second vinyl group into the system (1-SF5-penta-1,4-dien-3-ol) confirmed that a Johnson-Claisen rearrangement was successful (92% yield of methyl 7-SF5-hepta-4,6-dienoate) with incorporation of the unsubstituted 4,5-double bond while the 1-SF5-substituted 1,2-double bond remained unchanged. Efforts to apply 1-(SF5CF2)-substituted 1,2-double bond systems, which are similar to CF3 analogues in steric requirements, in Johnson-Claisen or ester enolate-Claisen rearrangements failed because of the instability of the SF5CF2 substituent under various reaction conditions. On the other hand, when the SF5 group was separated from the reaction center by a CH2 group instead (5-SF5-pent-3-en-2-ol), Johnson-Claisen rearrangements using six orthoesters provided the target 2-substituted 3-(CH2SF5)-hex-4-enoates in 55-76% yields as ∼1 : 1 mixtures of diastereomers. As an example to demonstrate the utility of these products, methyl 3-(CH2SF5)-hex-4-enoate was reduced, and the formed alcohol was oxidized to the aldehyde. Finally, initial experiments showed that the synthetic sequence developed for SF5 compounds is also applicable for analogous CF3-substituted allylic alcohols (5-CF3-pent-3-en-2-ol) and their Johnson-Claisen rearrangement.

Simple and Highly Efficient Synthesis of 3,5-Disubstituted Isoxazoles Using Cu/Graphene/Clay Nanohybrid as a New Heterogeneous Nano Catalyst

A facile and convenient protocol for ‘click’ cycloaddition of structurally diverse alkynes with in situ generated nitrile oxides catalysed by Cu/aminoclay/reduced graphene oxide nanohybrid (Cu/AC/r-GO nanohybrid) as a highly efficient heterogeneous nano-catalyst is described. In this method, Cu/AC/r-GO nanohybrid catalyses the 1,3-dipolar cycloaddition of alkynes and nitrile oxides in the presence of NaHCO3 in H2O/THF (50:50, V/V) to afford the corresponding 3,5-disubstituted isoxazoles. The Cu/AC/r-GO nanohybrid is a low cost, non-hygroscopic, chemically and thermally stable catalyst that can be reused for many consecutive reaction runs without significant loss in its reactivity.