Ein 69‐gliedriger Ring (siehe Bild) entsteht bei einer Metathesereaktion, die ein metalliertes Tris(pinzetten)‐substituiertes Benzol als Gerüst verwendet. Das makrocyclische Produkt kann durch Zugabe von Cl− vom Templat abgespalten und das Templat quantitativ zurückgewonnen werden.

Pincer‐palladium(II) and ‐platinum(II) cations, YCY‐M (YCY=[2,6‐(YCH2)2C6H3]; Y=NMe2, SPh; M=PdII, PtII), bound to diolefin‐substituted pyridines (3,5‐ or 2,6‐substitution) were successfully synthesized, and subsequently used in olefin metathesis (RCM) as a model study for template‐directed synthesis of macrocycles. Especially a 3,5‐disubstituted pyridine bound to a NCN‐PtII‐center (5a) gave a fast metathesis reaction, while the same reaction with the PdII analogue (4a) was much slower and less selective (isomerization products were formed). Furthermore, it was found that 2,6‐diolefin‐substituted pyridines (4b, 5b, 5c) gave slow metathesis reactions, which is mainly ascribed to steric hindrance during the ring‐closing step. In all cases where prolonged reaction times were required an isomerization process, most likely assisted by cationic pincer‐MII species, was observed as a competing reaction. 1H NMR spectroscopy experiments revealed that pyridines are stronger bound to a cationic NCN‐PtII‐center than to its PdII‐analogue. This aspect is of crucial importance when these pincer‐pyridine complexes are applied in metathesis, since free pyridine in solution deactivates the Ru‐metathesis catalyst. For the templated construction of macrocycles, a strong M‐N(py) bond is also important since it determines the selectivity for the desired product. In addition, these results open a new research field in which organometallic (pincer) complexes are used as protecting groups for strong Lewis‐basic groups in catalysis. From failed attempts to prepare macrocycles using hexakis[SCS‐PdII‐(1a)] complex 14, and from the results obtained with the monometallic pincer complexes in RCM, it can be concluded that the most suitable candidate for constructing macrocycles should comprise 2,6‐diolefin‐substituted pyridines bound to a multi‐(NCN‐PtII)‐template. In such a system, intrapyridine metathesis (steric hindrance) as well as isomerization reactions (strong M‐N(py) bond) are suppressed.

Macrocyclic compounds are widely used as preorganized host molecules for the selective binding of specific guests. [1] Commonly, these guests are monometallic cations or small polar molecules, such as urea. An alternative possibility is to assemble a given set of molecules around a metal center [2] or molecular pattern [3,4] and then couple the molecules to one host±guest complex. In this reaction sequence the metal center or molecular pattern functions as template.[5] In a number of recent reports the latter strategy has been used for the synthesis of catenanes and knots, [6] and of molecular wires imbedded in an alkane double helix. [7] In these reactions, host and guest often become irreversibly integrated in an assembly with novel molecular properties.In a recent study, we prepared a series of shape-persistent multimetallic compounds which can be easily converted into the corresponding multicationic species.[8] The cationic sites in the trication of 1 (see Scheme 1) used in the present study are fixed in a two-dimensional space and are at the corners of a triangle with edges of 1.75 nm.[8b] The NCN pincer platinum cations bind new ligands exclusively trans to C ipso along the pseudo C 2 axis of the molecule (C 4 -C ipso -Pt). Accordingly, binding of pyridine ligands provides a special molecular arrangement having the planes of the tris(phenylene)benzene core and the pyridine ligands coplanar. It turns out that of the combinations ECE pincer ligand/metal/pyridine (E: N, S; metal: palladium, platinum) the NCN pincer platinum pyridine complexes are the kinetically most stable ones. [9] Moreover, the NCN-Pt complexes are the least active catalysts for the isomerization of a-olefins (see below). This makes the trication of 1 an ideal template for interconnecting the pyridine rings at the ortho or meta positions thereby forming a large tris(pyridyl) macrocyclic compound around the trication.Here we report the selective linking of 2,6-bis(dec-9-enyloxy)pyridine substituents by alkene metathesis to form a 69-membered tris(pyridyl) macrocycle. Its detachment occurs by addition of nucleophiles, for example, Cl À . The trisolefinic macrocycle could be hydrogenated and subsequently recoordinated to the tricationic template. This sequence (Scheme 1) provides a new approach to the selective synthesis of largering macrocyclic hosts which have as the only preorganization a precise atom connectivity pattern.2,6-Bisolefin-substituted pyridines 2 a,b were prepared from 9-decen-1-ol and 2,6-dibromopyridine or 2,6-bis(chloromethyl)pyridine, respectively. The template precursor 1 (1 mmol) was reacted in CH 2 Cl 2 with three equivalents of either 2 a or 2 b in the presence of suspended AgBF 4 to give the tricationic compounds 3 a or 3 b, respectively, in quantitative yields. A prolonged reaction time (30 min to 16 h) is necessary because of the poor solubility of AgBF 4 in CH 2 Cl 2 . The compounds 3 undergo alkene metathesis in the presence of the first-generation Grubbs catalyst, [Cl 2 (Cy 3 P) 2 Ru¼ CHPh] (5 mol % per pyridine ...