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.

Syn-selective silicon Mukaiyama-type aldol reactions of (pentafluoro-λ6-sulfanyl)acetic acid esters with aldehydes.

Aldol reactions belong to the most frequently used C-C bond forming transformations utilized particularly for the construction of complex structures. The selectivity of these reactions depends on the geometry of the intermediate enolates. Here, we have reacted octyl pentafluoro-λ6-sulfanylacetate with substituted benzaldehydes and acetaldehyde under the conditions of the silicon-mediated Mukaiyama aldol reaction. The transformations proceeded with high diastereoselectivity. In case of benzaldehydes with electron-withdrawing substituents in the para-position, syn-α-SF5-ß-hydroxyalkanoic acid esters were produced. The reaction was also successful with meta-substituted benzaldehydes and o-fluorobenzaldehyde. In contrast, p-methyl-, p-methoxy-, and p-ethoxybenzaldehydes led selectively to aldol condensation products with (E)-configured double bonds in 30-40% yields. In preliminary experiments with an SF5-substituted acetic acid morpholide and p-nitrobenzaldehyde, a low amount of an aldol product was formed under similar conditions.

Synthesis of α-(Pentafluorosulfanyl)- and α-(Trifluoromethyl)-Substituted Carboxylic Acid Derivatives by Ireland-Claisen Rearrangement.

Earlier studies have shown that [3,3]-sigmatropic rearrangements of allyl esters are useful for the construction of fluorine-containing carboxylic acid derivatives. This paper describes the synthesis of 3-aryl-pent-4-enoic acid derivatives bearing either a pentafluorosulfanyl (SF5) or a trifluoromethyl (CF3) substituent in the 2-position by treatment of corresponding SF5- or CF3-acetates of p-substituted cinnamyl alcohols with triethylamine followed by trimethylsilyl triflate (TMSOTf). This Ireland-Claisen rearrangement delivered approximate 1:1 mixtures of syn/anti diastereoisomers due to tiny differences (<0.5 kcal/mol) both in the energy of (Z)/(E)-isomeric ester enolates and in the alternative Zimmerman-Traxler transition states of model compounds as shown by DFT calculations. Acidic reaction conditions have to be avoided since addition of the reagents in opposite sequence (first TMSOTf then Et3N) led to oligomerization of the cinnamyl SF5- and CF3-acetates. Treatment of the corresponding regioisomeric 1-phenyl-prop-2-en-1-yl acetates under the latter conditions resulted in [1,3]-sigmatropic rearrangement and subsequent oligomerization of the intermediately formed cinnamyl esters. When Et3N was added first followed by TMSOTf, no further reaction of the formed ester was detected.

Synthesis of SF5CF2-Containing Enones and Instability of This Group in Specific Chemical Environments and Reaction Conditions.

The chemistry of the SF5CF2 moiety has been scarcely investigated. In this report, we present synthetic pathways to a variety of SF5CF2-substituted compounds starting from vinyl ethers and SF5CF2C(O)Cl. In specific chemical environments and under particular reaction conditions, the SF5CF2 moiety is unstable in downstream products resulting in the elimination of the SF5(-) anion and its decomposition to SF4 and F(-). Surprisingly, the formed F(-) can attack the intermediate difluorovinyl moiety to form trifluoromethyl substituted products. This appears to happen when an intermediate neighboring group participation involving a double bond is possible. Under slightly different conditions, the reaction stops at the stage of a difluorovinyl compound.

Anti-Selective Aldol Reactions of Pentafluorosulfanylacetic Acid Esters with Aldehydes Mediated by Dicyclohexylchloroborane.

Aldol reactions of pentafluorosulfanyl (SF5)-substituted acetic acid esters with both aromatic and aliphatic aldehydes proceeded with excellent anti-diastereoselectivity and good to high yields using dicyclohexylchloroborane/triethylamine. This methodology enabled the synthesis of hitherto unknown α-SF5-ß-hydroxy esters. Using a norephedrine-based auxiliary, high asymmetric induction was observed. The stereochemistry of products was assigned by NMR spectroscopy and proved by X-ray diffraction analysis. The intermediate enolate was identified as a highly unstable species.

Ambiphilic properties of SF5CF2CF2Br derived perfluorinated radical in addition reactions across carbon-carbon double bonds.

The extraordinary properties of the pentafluorosulfanyl (SF5) group attract attention of organic chemists. While numerous SF5-substituted compounds have been synthesized, the direct introduction of SF5(CF2)n moieties has remained almost unexplored. Our investigations revealed the ambiphilic character of the SF5CF2CF2 radical. Addition reactions to electron-rich or electron-deficient alkenes profit either from its electrophilic or nucleophilic properties. Thus, the readily available SF5CF2CF2Br proved to be a promising and versatile building block for the introduction of this perfluorinated moiety.

Pentafluorosulfanyldifluoroacetic acid: rebirth of a promising building block.

Three novel, easily scalable routes for the synthesis of pentafluorosulfanyldifluoroacetic acid, SF5CF2C(O)OH, are described. Reactions of its acid chloride with amines and alcohols led to a small library of 15 amides and five esters, respectively. The reaction of the acid chloride with phenylmagnesium bromide gave the corresponding acetophenone. Pentafluorosulfanyldifluoroacetonitrile was obtained from pentafluorosulfanyldifluoroacetamide by dehydration with diphosphorus pentoxide.