N‐Methylbenzoazacrown ethers with the nitrogen atom conjugated with the benzene ring: the improved synthesis and the reasons for the high stability of complexes with metal and ammonium cations

Abstract

An improved method for the synthesis of formyl derivatives of N‐methylbenzoazacrown ethers is proposed. They are prepared in up to 68% yields over fewer steps and with a much shorter time required for the last step. The stability constants of complexes formed by N‐methylbenzoazacrown ethers and their structural analogs with alkali metal, alkaline‐earth metal and ammonium cations were determined by 1H NMR titration in CD3CN. High stability of complexes of N‐methyl derivatives of benzoazacrown ethers is demonstrated, comparable with or even exceeding the stability of benzocrown‐ether complexes and markedly exceeding the stability of complexes of phenylazacrown ethers with the same macrocycle size. The structures of azacrown ethers and their complexes with Ba(ClO4)2 were studied by X‐ray diffraction. A high degree of pre‐organization of N‐methylbenzoazacrown ethers toward the formation of complexes with metal and ammonium cations was noted, which is due to the clear‐cut pyramidal geometry of the nitrogen atom and the orientation of the lone electron pairs (LEPs) of most heteroatoms towards the centre of the macroheterocycle. Copyright © 2009 John Wiley & Sons, Ltd.

Host-guest complexes of nitro-substituted N-alkylbenzoaza-18-crowns-6

“…The larger deviation of the N-substituents from the benzene ring plane also follows from the upfield shift (Δδ H up to -0.29 ppm) of signals from all CH 2 N and MeN groups as a result of complex formation (except for the lithium complex). Analogous pattern was observed previously while studying complex formation of benzoaza-15(18)-crowns-5(6) and their derivatives having a formyl group in the benzene ring [5,7].…”

“…Presumably, the nitrogen atom in III is essentially sp 2 -hybridized and involved in effective conjugation with the benzene ring, which prevents participation of its LEP in complex formation. This assumption is confirmed by the X-ray diffraction data for structurally related N-(4-formylphenyl)aza-18-crown-6 [7]. By contrast, the nitrogen atom in Ia-Id is largely contributed by sp 3 state as a result of spatial interactions between the N-substituents and CH 2 OAr group, so that it tends to donate its LEP for coordination of guest species.…”

“…Analogous conclusion was drawn by analysis of the NOESY spectra of N-methyl(formylbenzo)aza-18-crown-6 and crystalline structure of its complex with Ba(ClO 4 ) 2 [7]. The larger deviation of the N-substituents from the benzene ring plane also follows from the upfield shift (Δδ H up to -0.29 ppm) of signals from all CH 2 N and MeN groups as a result of complex formation (except for the lithium complex).…”

“…In this respect, benzo-fused aza crown ethers may be appreciably more advantageous; on the other hand, derivatives of 1-aza-2,3-benzocrown ethers have been studied fairly poorly, and most their functional derivatives are almost inaccessible despite simple structure [3,4]. We previously developed a procedure for the synthesis of N-methyl benzo aza crown compounds with different sizes of the macroring (both unsubstituted at the benzene ring and formyl-substituted) [5][6][7] and nitro-DOI: 10.1134/S1070428011070220 substituted N-alkylbenzoaza-15-crowns-5 via stepwise transformations of macroring in benzocrown ethers [8,9]. The developed synthetic approach to functionalized aza crown ether derivatives seems to be a promising alternative to the known methods of synthesis of 1-aza-2,3-benzocrown ethers [3,4].…”

“…We previously showed [5][6][7]13] that benzo aza crown ethers and their derivatives having one tertiary nitrogen atom conjugated with the benzene ring give rise to fairly stable host-guest complexes with alkali, DMITRIEVA et al alkaline-earth, and heavy metal ions, as well as with ammonium ions. Therefore, such crown compounds may be efficient as ionophore blocks in optical chemosensors.…”