Dedicated to Professor Rudolf Zahradník on the occasion of his 80th birthday
Electroactive paraquat 1 and diquat 2 (Figure ) are widely used herbicides produced in ton quantities every year. [1] Due to their powerful electron-accepting character they are exceptional electron-transfer quenching agents. Research into these dicationic species spearheaded the development of supramolecular science and led to the evolution of molecular devices. [2] Extensive studies into their properties in biological and photocatalytic systems, [3] and their molecular recognition capabilities further testify to the unique position of these privileged dications.
The comparatively unexplored realm of helically chiral Nheterohelicenia [4, 5] (e.g., 3, ref. [4b, c]) represents a platform on which to simultaneously expand the fields of cationic Nheteroaromatics, helicenes, and heterohelicenes. The possibility of combining the remarkably diverse application A C H T U N G T R E N N U N G potentials of these areas, is particularly attractive.
Herein, we report the synthesis, and properties of helical extended diquat 4 (helquat 4) and derivatives thereof that bear resemblance to both diquat 2, and azoniahelicene 3. In the synthesis of the parent helquat 4, we capitalize on the fact that a facile pyridine-type nitrogen quaternization protocol (5!6, Scheme 1) developed for monocation synthesis by Vaquero, Cuadro et al. [8] can be beneficially combined with a reliable entry to helical scaffolds based on [2 + 2 + 2]triyne cycloisomerization pioneered by Stary ´and Starµ. Thus, triyne 6 is prepared in high yield from the known pyridine precursor 5 (ref. [10]) and the key [2 + 2 + 2] cycloisomerization 6!4 is effected in minutes with Wilkinsons catalyst [RhA C H T U N G T R E N N U N G (PPh 3 ) 3 Cl] in acetonitrile under microwave conditions. The practicality of the protocol is underscored by the fact that a simple ethyl acetate wash of the crude product removes the catalyst affording helquat 4 as the triflate salt in good purity on milligram to gram scales. X-ray crystal structure analysis unambiguously confirms the identity of 6
[a] Dr.