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Cyclophane‐Type Fullerene‐dibenzo[18]crown‐6 Conjugates with trans‐1 , trans‐2 , and trans‐3 Addition Patterns: Regioselective Templated Synthesis, X‐Ray Crystal Structure, Ionophoric Properties, and Cation‐Complexation‐Dependent Redox Behavior
Author(s) -
Bourgeois JeanPascal,
Seiler Paul,
Fibbioli Monia,
Pretsch Ernö,
Diederich François,
Echegoyen Luis
Publication year - 1999
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/(sici)1522-2675(19991006)82:10<1572::aid-hlca1572>3.0.co;2-b
Subject(s) - chemistry , selectivity , crown ether , yield (engineering) , stereochemistry , malonate , moiety , crystallography , organic chemistry , ion , catalysis , materials science , metallurgy
The fullerene‐crown ether conjugates (±)‐ 1 to (±)‐ 3 with trans‐1 ((±)‐ 1 ), trans‐2 ((±)‐ 2 ), and trans‐3 ((±)‐ 3 ) addition patterns on the C‐sphere were prepared by Bingel macrocyclization. The trans‐1 derivative (±)‐ 1 was obtained in 30% yield, together with a small amount of (±)‐ 2 by cyclization of the dibenzo[18]crown‐6(DB18C6)‐tethered bis‐malonate 4 with C 60 ( Scheme 1 ). When the crown‐ether tether was further rigidified by K + ‐ion complexation, the yield and selectivity were greatly enhanced, and (±)‐ 1 was obtained as the only regioisomer in 50% yield. The macrocyclization, starting from a mixture of tethered bis‐malonates with anti ( 4 ) and syn ( 10 ) bisfunctionalized DB18C6 moieties, afforded the trans‐1 ((±)‐ 1 , 15%), trans‐2 ((±)‐ 2 , 1.5%), and trans‐3 ((±)‐ 3 , 20%) isomers ( Scheme 2 ). Variable‐temperature 1 H‐NMR (VT‐NMR) studies showed that the DB18C6 moiety in C 2 ‐symmetrical (±)‐ 1 cannot rotate around the two arms fixing it to the C‐sphere, even at 393 K. The planar chirality of (±)‐ 1 was confirmed in 1 H‐NMR experiments using the potassium salts of ( S )‐1,1′‐binaphthalene‐2,2′‐diyl phosphate ((+)‐( S )‐ 19 ) or (+)‐(1 S )‐camphor‐10‐sulfonic acid ((+)‐ 20 ) as chiral shift reagents ( Fig. 1 ). The DB18C6 tether in (±)‐ 1 is a true covalent template: it is readily removed by hydrolysis or transesterification, which opens up new perspectives for molecular scaffolding using trans‐1 fullerene derivatives. Characterization of the products 11 ( Scheme 3 ) and 18 ( Scheme 4 ) obtained by tether removal unambiguously confirmed the trans‐1 addition pattern and the out‐out geometry of (±)‐ 1 . VT‐NMR Studies established that (±)‐ 2 is a C 2 ‐symmetrical out‐out trans‐2 and (±)‐ 3 a C 1 ‐symmetrical in‐out trans‐3 isomer. Upon changing from (±)‐ 1 to (±)‐ 3 , the distance between the DB18C6 moiety and the fullerene surface increases and, correspondingly, rotation of the ionophore becomes increasingly facile. The ionophoric properties of (±)‐ 1 were investigated with an ion‐selective electrode membrane ( Fig. 2 and Table 2 ), and K + was found to form the most stable complex among the alkali‐metal ions. The complex between (±)‐ 1 and KPF 6 was characterized by X‐ray crystal‐structure analysis ( Figs. 3 and 4 ), which confirmed the close tangential orientation of the ionophore atop the fullerene surface. Addition of KPF 6 to a solution of (±)‐ 1 resulted in a large anodic shift (90 mV) of the first fullerene‐centered reduction process, which is attributed to the electrostatic effect of the K + ion bound in close proximity to the C‐sphere ( Fig. 5 ). Smaller anodic shifts were measured for the KPF 6 complexes of (±)‐ 2 (50 mV) and (±)‐ 3 (40 mV), in which the distance between ionophore and fullerene surface is increased ( Table 3 ). The effects of different alkali‐ and alkaline‐earth‐metal ion salts on the redox properties of (±)‐ 1 were investigated ( Table 4 ). These are the first‐ever observed effects of cation complexation on the redox properties of the C‐sphere in fullerene‐crown ether conjugates.