A sterically driven approach to the efficient synthesis of low-symmetry 1,4-diazepinoporphyrazines

Abstract

We have investigated the general applicability of the synthetic procedure in which a carbonyl compound (in our case, 2,4-pentandione and 3-n-propyl-2,4-pentanedione) is added to a preformed TiCl4–diaminomaleonitrile complex for the preparation of 1,4-diazepine-2,3-dicarbonitriles. It has been shown that triethylamine commonly used as an auxiliary reagent (base) inhibits the formation of the TiCl4–diaminomaleonitrile complex and less basic pyridine was proved to be more suitable. The introduction of the n-propyl group into the C6 position of 5,7-bis(2'-arylethenyl)-6H-1,4-diazepine-2,3-dicarbonitrile has led to an unprecedented increase in the yield of the low-symmetry A3B-type tribenzodiazepinoporphyrazine from 5 to 40 % under Linstead cross-macrocyclization conditions. The quantum-chemical calculations at the PW6B95-D3/def2-TZVP//BP86-D3/def2-TZVP level of theory demonstrated that steric effects of substituents in 6-alkyl substituted 5,7-bis(2′-arylethenyl)-6H-1,4-diazepine-2,3-dicarbonitriles can play a key role in formation of dimeric intermediates during Linstead macrocyclization, providing high selectivity towards low symmetry porphyrazines with annulated 1,4-diazepine heterocycle(s).