[reaction: see text] Compact flow reactors have been constructed and optimized to perform continuous organic photochemistry on a large scale. The reactors were constructed from commercially available or customized immersion well equipment combined with UV-transparent, solvent-resistant fluoropolymer (FEP) tubing. The reactors were assessed using the [2 + 2] photocycloaddition of malemide 1 and 1-hexyne forming the cyclobutene product 2 and the intramolecular [5 + 2] photocycloaddition of 3,4-dimethyl-1-pent-4-enylpyrrole-2,5-dione 3 to form the bicyclic azepine 4. The reactors were shown to be capable of producing >500 g of 2 and 175 g of 4 in a continuous 24 h processing period. Due to the facile control of irradiation time, the continuous flow reactor was also shown to be superior to a batch reactor for performing a problematic photochemical reaction on a larger scale.

No abstract

Oligomeric ureas derived from m-phenylenediamine with chain lengths of up to seven urea linkages were made by iterative synthetic pathways. Three families were synthesized: 4 and 20, bearing a terminal chiral sulfinyl group; 24, bearing a terminal rotationally restricted amide group, and 30 bearing a terminal achiral bromophenyl group. The distal end of the oligomers was capped with an N-benzyl group to act as a diastereotopic probe. With a terminal sulfinyl group, the 1H NMR signals arising from the CH2 group of the diastereotopic probe remained anisochronous even when separated from the stereogenic center by up to 24 bonds (in 20c). With a rotationally restricted amide, anisochronicity was no longer apparent beyond 17 bond lengths (in 24c). No anisochronicity was observable with a terminal bromophenyl group. We interpret these results as indicating that the oligoureas of short lengths adopt a defined helical secondary structure in solution, but that in longer oligomers the helicity breaks down and transmission of chirality in these systems is limited to about 24 bond lengths. We propose that "apparent diastereotopicity" (anisochronicity) provides a general empirical method for identifying secondary structure in solution.