Optimum Design and Analysis Based on Independent Reaction Amount for Distillation Column with Side Reactors: Production of Benzyl Chloride

Recent Advances in Direct Regioselective C-H Chlorination at Aromatic and Aliphatic

Direct installation of key functionalities in a molecule through C-H bond activation is one of the thrust areas as well as challenging task in organic synthesis. Particularly, introduction of chlorine in a molecule imparts additional benefits for further functionalizations as well as improves the electronic behaviour such as lipophilicity and polarity towards drug development process. The chlorinated molecules have also been established as efficient biologically relevant scaffolds. Current manuscript has been focused on the direct installation of the chlorine atom at various aromatic and aliphatic positions to produce functional molecules. The key highlight of the manuscript belongs to the site selectivity (regioselectivity) for the installation of chlorine functionality. Manuscript describes the advanced methods developed for the direct C-H chlorination reactions and further simplified for the chlorination reactions at various positions including aromatic (o-, m-, and p-), benzylic, heteroaromatic, and aliphatic positions. Directing groups (DGs) and the coordination with the catalyst is the key for the enhancement of regioselectivities during direct C-H chlorination reactions.

Study of Cyclohexane and Methylcyclohexane Functionalization Promoted by Manganese(III) Compounds

Alkane functionalization using safe and low-energy processes is of great interest to industry and academia. Aiming to contribute to the process of saturated hydrocarbon functionalization, we have studied a set of three manganese(III) complexes as catalysts for promoting the oxidation of saturated hydrocarbons (cyclohexane and methylcyclohexane) in the presence of hydrogen peroxide or trichloroisocyanuric acid (TCCA). The mononuclear manganese(III) compounds were prepared using the ligands H2LMet4 (6,6’-((1,4-diazepane-1,4-diyl)bis(methylene))bis(2,4-dimethylphenol), H2salen (2,2’-((1E,1’E)-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))diphenol) and H2salan (2,2’-((ethane-1,2-diylbis(azanediyl))bis(methylene))diphenol). The catalytic processes were carried out in acetonitrile at 25 and 50 °C for 24 h. The increase in the temperature was important to get a better conversion. The compounds did not promote cyclohexane oxidation in the presence of H2O2. However, they were active in the presence of TCCA, employing a ratio of 1000:333:1 equivalents of the substrate:TCCA:catalyst. The best catalytic activity was shown by the compound [Mn(salen)Cl], reaching conversions of 14.5 ± 0.3% (25 °C) and 26.3 ± 1.1% (50 °C) (yield for chlorocyclohexane) and up to 12.1 ± 0.5% (25 °C) and 29.8 ± 2.2% (50 °C) (total yield for the mixture of the products 1-chloro-4-methylcyclohexane, 3-methylcyclohexene and 1-methylcyclohexene). The interaction of the catalysts with TCCA was studied using electron paramagnetic resonance (EPR), suggesting that the catalysts [Mn(LMet4)Cl] and [Mn(salan)Cl] act via a different mechanism from that observed for [Mn(salen)Cl].

Visible Light-Catalyzed Benzylic C-H Bond Chlorination by a Combination of Organic Dye (Acr+-Mes) and N-Chlorosuccinimide

By combining "N-chlorosuccinimide (NCS)" as the safe chlorine source with "Acr⁺-Mes" as the photocatalyst, we successfully achieved benzylic C-H bond chlorination under visible light irradiation. Furthermore, benzylic chlorides could be converted to benzylic ethers smoothly in a one-pot manner by adding sodium methoxide. This mild and scalable chlorination method worked effectively for diverse toluene derivatives, especially for electron-deficient substrates. Careful mechanistic studies supported that NCS provided a hydrogen abstractor "N-centered succinimidyl radical," which was responsible for the cleavage of the benzylic C-H bond, relying on the reducing ability of Acr^•-Mes.

N-Hydroxyphthalimide/benzoquinone-catalyzed chlorination of hydrocarbon C-H bond using N-chlorosuccinimide

The direct chlorination of C-H bonds has received considerable attention in recent years. In this work, a metal-free protocol for hydrocarbon C-H bond chlorination with commercially available N-chlorosuccinimide (NCS) catalyzed by N-hydroxyphthalimide (NHPI) with 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ) functioning as an external radical initiator is presented. Aliphatic and benzylic substituents and also heteroaromatic ones were found to be well tolerated. Both the experiments and theoretical analysis indicate that the reaction goes through a process wherein NHPI functions as a catalyst rather than as an initiator. On the other hand, the hydrogen abstraction of the C-H bond conducted by a PINO species rather than the highly reactive N-centered radicals rationalizes the high chemoselectivity of the monochlorination obtained by this protocol as the latter is reactive towards the C(sp3)-H bonds of the monochlorides. The present results could hold promise for further development of a nitroxy-radical system for the highly selective functionalization of the aliphatic and benzylic hydrocarbon C-H.

Mild Aliphatic and Benzylic Hydrocarbon C-H Bond Chlorination Using Trichloroisocyanuric Acid

We present the controlled monochlorination of aliphatic and benzylic hydrocarbons with only 1 equiv of substrate at 25-30 °C using N-hydroxyphthalimide (NHPI) as radical initiator and commercially available trichloroisocyanuric acid (TCCA) as the chlorine source. Catalytic amounts of CBr₄ reduced the reaction times considerably due to the formation of chain-carrying ·CBr₃ radicals. Benzylic C-H chlorination affords moderate to good yields for arenes carrying electron-withdrawing (50-85%) or weakly electron-donating groups (31-73%); cyclic aliphatic substrates provide low yields (24-38%). The products could be synthesized on a gram scale followed by simple purification via distillation. We report the first direct side-chain chlorination of 3-methylbenzoate affording methyl 3-(chloromethyl)benzoate, which is an important building block for the synthesis of vasodilator taprostene.