General approach for template-directed synthesis of macroheterocycles by ring-closing metathesis (RCM)

A general method to prepare macroheterocycles is reported in which a rigid symmetric platinated tris-pincer tricationic complex (3) is used as a template. The system uses bisolefin functionalized pyridine ligands (1, 2) which selectively bind to the three platinum centers of the template resulting in pre-organization of the olefinic tails. By subjecting these pre-organized complexes (4) to olefin metathesis reaction conditions, the olefinic tails were interconnected affording several macroheterocycles of different compositions and sizes. Various pyridines with different substitution patterns (2,6- and 3,5-disubstituted) and different olefinic tails (aliphatic, polyether, and styryl) were synthesized and used in the cyclization reactions, showing the diversity of this method. It was found that only the use of 2,6-disubstituted pyridines resulted in the formation of the desired macrocycles while 3,5-difunctionalized pyridines gave only pyridinophane formation. Furthermore, the length of the olefinic tails needs to be at least eleven atoms (C or O) in order to suppress oligomerization in favor of macrocycle formation. Yields of up to 70% of the macrocycles were obtained using the first-generation [Cl-2(Cy3P)(2)Ru=CHPh] or second-generation [Cl-2(Cy3P)(IMes)Ru=CHPh] Grubbs' catalysts. Reactions of the platinated pincer pyridines with Schrock Mo-based metathesis catalysts results in exclusive olefin isomerization and no productive metathesis. The generated macrocycles (hosts) could effectively be separated from the template by addition of aqueous sodium chloride and reattached to the (recycled) template (guest). The X-ray structure determination of host-guest complex 10 is the definite proof for this reattachment and the perfect cyclic structure of the macrocycle.