Photochlorination of Aromatic Isocyanates

Side-chain photo-chlorination of toluene isocyanates leads to important industrial intermediates for polyurethane synthesis, one of the most important classes of polymers [130]. Irradiated thin liquid layers in microchannels should have much higher photon efficiency (quantum yield) than given for conventional processing. 

The reaction of toluene-2,4-diisocyanate with chlorine to l-chloromethyl-2,4-diisocyanatobenzene was carried out in a falling film microreactor with a transparent window for irradiation [130]. There are two modes of reaction. The desired radical process proceeds with the photoinduced homolytic cleavage of the chlorine molecules and the chlorine radical reacts with the side chain of the aromatic compound. At too high chlorine concentrations, radical recombination becomes dominant and consecutive processes such as chlorinations of the side chain may occur as well. Another undesired pathway is the electrophilic ring substitution to toluene-5-chloro-2,4-diisocyanate, promoted by Lewis acidic catalysts in polar solvents at low temperature. Even small metallic impurities probably from corrosion of the reactor material can enhance the formation of electrophilic byproducts. 

The yield of the product l-chloromethyl-2,4-diisocyanatobenzene ranged from 24% to 54% at 130 °C (conversion 30%-81% respective selectivity 79%-67%) [130]. The content of the ring-substituted product decreases with increasing conversion 

Reaction time can also be reduced by means of microprocess technology [130]. A reaction time of 30 min was necessary for a 30 ml-batch reactor to achieve a conversion of 65% at a selectivity of 45%, whereas with the falling film microreactor only 14 s for the same performance was required. This leads also to a large space-time yield with 401 mol/(l h) forthe falling film microreactor as compared to 1.3 mol/(l h) forthe batch reactor. The impact of Lewis-acid formation on the reaction course was investigated using an iron microreactor instead of the usually used nickel microreactor. The plate surface is converted to iron chloride under the reaction conditions. The selectivity of the target product then drops from 67% to 50%. 

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Direct fluorination of aliphatics and non-C moieties Direct fluorination of benzenoid aromatics Direct fluorination of heterocyclic aromatics Oxidations of alcohols, diols and ketones with fluorine Photo-oxidation of a-terpinene and cyclopentadiene Oxidation of benzyl alcohol to benzaldehyde Homogeneously catalyzed oxidation of butyraldehyde Oxidation of sulfite to sulfate Photochlorination of aromatic isocyanates... 

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