Iron-Catalyzed Isopropylation of Electron-Deficient Aryl and Heteroaryl Chlorides

Synthesis and applications of S-(trifluoromethyl)-2,8-bis(trifluoromethoxy)dibenzothiophenium triflate (Umemoto reagent IV)

A new, powerful, and easy-to-handle electrophilic trifluoromethylating agent, S-(trifluoromethyl)-2,8-bis(trifluoromethoxy)dibenzothiophenium triflate (Umemoto reagent IV), was developed. Due to the extraordinary electronic effect of trifluoromethoxy group, Umemoto reagent IV was easily synthesized by a one-pot method from readily available 3,3'-bis(trifluoromethoxy)biphenyl. It was shown that Umemoto reagent IV was more powerful than Umemoto reagent II and could trifluoromethylate many kinds of nucleophilic substrates more effectively. In addition, Umemoto reagent IV was successfully utilized for the preparation of trifluoromethyl nonaflate, a useful trifluoromethoxylating agent. The direct conversion of 2,8-bis(trifluoromethoxy)dibenzothiophene to Umemoto reagent IV with triflic anhydride was achieved, albeit in low yield.

A straightforward coupling of 4-sulfonylpyridines with Grignard reagents

A straightforward synthesis of alkyl-sulfonylpyridines and aryl-sulfonylpyridines is developed by coupling of sulfonylpyridines with the Grignard reagents. The protocol proceeds through a catalyst- and oxidant-free coupling of sulfonylpyridines as substrates via a Chichibabin-type reaction mechanism.

Iron- or Palladium-Catalyzed Reaction Cascades Merging Cycloisomerization and Cross-Coupling Chemistry

A conceptually novel reaction cascade is presented, which allows readily available enynes to be converted into functionalized 1,3-dienes comprising a stereodefined tetrasubstituted alkene unit; such compounds are difficult to make by conventional means. The overall transformation is thought to commence with formation of a metallacyclic intermediate that evolves via cleavage of an unstrained C-X bond in its backbone. This non-canonical cycloisomerization process is followed by a cross-coupling step, such that reductive C-C bond formation regenerates the necessary low-valent metal fragment and hence closes an intricate catalytic cycle. The cascade entails the formation of two new C-C bonds at the expense of the constitutional C-X entity of the substrate: importantly, the extruded group X must not be a heteroelement (X=O, NR), since activated β-C-C bonds can also be broken. This concern was reduced to practice in two largely complementary formats: one procedure relies on the use of alkyl-Grignard reagents in combination with catalytic amounts of Fe(acac)_3, whereas the second method amalgamates cycloisomerization with Suzuki coupling by recourse to arylboronic acids and phosphine-ligated palladium catalysts.

Iron-Catalyzed Cross-Couplings in the Synthesis of Pharmaceuticals: In Pursuit of Sustainability

The scarcity of precious metals has led to the development of sustainable strategies for metal-catalyzed cross-coupling reactions. The establishment of new catalytic methods using iron is attractive owing to the low cost, abundance, ready availability, and very low toxicity of iron. In the last few years, sustainable methods for iron-catalyzed cross-couplings have entered the critical area of pharmaceutical research. Most notably, iron is one of the very few metals that have been successfully field-tested as highly effective base-metal catalysts in practical, kilogram-scale industrial cross-couplings. In this Minireview, we critically discuss the strategic benefits of using iron catalysts as green and sustainable alternatives to precious metals in cross-coupling applications for the synthesis of pharmaceuticals. The Minireview provides an essential introduction to the fundamental aspects of practical iron catalysis, highlights areas for improvement, and identifies new fields to be explored.

NHC and nucleophile chelation effects on reactive iron(ii) species in alkyl-alkyl cross-coupling

While iron-NHC catalysed cross-couplings have been shown to be effective for a wide variety of reactions (e.g. aryl-aryl, aryl-alkyl, alkyl-alkyl), the nature of the in situ formed and reactive iron species in effective catalytic systems remains largely undefined. In the current study, freeze-trapped Mössbauer spectroscopy, and EPR studies combined with inorganic synthesis and reaction studies are utilised to define the key in situ formed and reactive iron-NHC species in the Kumada alkyl-alkyl cross-coupling of (2-(1,3-dioxan-2-yl)ethyl)magnesium bromide and 1-iodo-3-phenylpropane. The key reactive iron species formed in situ is identified as (IMes)Fe((1,3-dioxan-2-yl)ethyl)₂, whereas the S = 1/2 iron species previously identified in this chemistry is found to be only a very minor off-cycle species (<0.5% of all iron). Reaction and kinetic studies demonstrate that (IMes)Fe((1,3-dioxan-2-yl)ethyl)₂ is highly reactive towards the electrophile resulting in two turnovers with respect to iron (k_obs > 24 min^-1) to generate cross-coupled product with overall selectivity analogous to catalysis. The high resistance of this catalytic system to β-hydride elimination of the alkyl nucleophile is attributed to its chelation to iron through ligation of carbon and one oxygen of the acetal moiety of the nucleophile. In fact, alternative NHC ligands such as SIPr are less effective in catalysis due to their increased steric bulk inhibiting the ability of the alkyl ligands to chelate. Overall, this study identifies a novel alkyl chelation method to achieve effective alkyl-alkyl cross-coupling with iron(ii)-NHCs, provides direct structural insight into NHC effects on catalytic performance and extends the importance of iron(ii) reactive species in iron-catalysed cross-coupling.

Tuning Cross-Coupling Approaches to C3 Modification of 3-Deazapurines

A general approach to C3 modification of purine scaffold through various types of cross-coupling reactions has been established. Tuning substrate electronics and reaction conditions resulted in the development of highly efficient sp²-sp, sp²-sp², and sp²-sp³ cross-coupling conditions for modification of 3-deazaadenine to access C3-modified adenine and hypoxanthine scaffolds. The optimized methodologies to access the corresponding 3-deazaadenosine phosphoramidites for solid-phase DNA synthesis have been demonstrated.