Macrocycle synthesis by olefin metathesis on a nanosized, shape-persistent tricationic platinum template.

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 Cipso along the pseudo C2 axis of the molecule (C4-Cipso-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-9enyloxy)pyridine substituents by alkene metathesis to form a 69-membered tris(pyridyl) macrocycle. Its detachment occurs by addition of nucleophiles, for example, Cl . 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 2a,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 CH2Cl2 with three equivalents of either 2a or 2b in the presence of suspended AgBF4 to give the tricationic compounds 3a or 3b, respectively, in quantitative yields. A prolonged reaction time (30 min to 16 h) is necessary because of the poor solubility of AgBF4 in CH2Cl2. The compounds 3 undergo alkene metathesis in the presence of the first-generation Grubbs catalyst, [Cl2(Cy3P)2Ru1⁄4 CHPh] (5 mol% per pyridine ligand), leading to the tricationic tris-platinum heteromacrocycle complexes 4. The alkene metathesis reactions were performed under high dilution (1 i 10 3m) to prevent intermolecular olefin metathesis polymerization. Easy detachment of the newly formed macroheterocycle from the tricationic template was possible by reacting 4 with an aqueous NaCl solution, affording free macrocycle 5a or 5b and the neutral template precursor 1. In fact, pure 1 was obtained quantitatively and could be reused in subsequent experiments. [15] Crystal data for 2 : C16H23B11,Mr1⁄4 334.25, monoclinic, P21/n, a1⁄4 7.345(2), b1⁄4 17.393(4), c1⁄4 14.786(3) a, b1⁄4 99.28(2)8, V1⁄4 1864.2(8) a3, Z1⁄4 4, 1calcd1⁄4 1.191 Mgm 3, m1⁄4 0.058 mm 1, F(000)1⁄4 696. 4355 data collected, 3268 independent reflections (Rint1⁄4 0.0336), R11⁄4 0.0462, wR21⁄4 0.1081 for data with I> 2s(I), S1⁄4 1.033, largest peak 0.140 and deepest hole 0.225 ea 3. [14] [16] R. G. Parr, W. Yang, Density-Functional Theory of Atoms and Molecules, OUP, New York, 1989. Calculations used the ADF1999 program[17±19] with the BP86 functional and incorporated a treatment of relativistic effects. A double-z plus polarization STO basis set for all atoms and the frozen core approximation was applied (C, B: 1s). [17] E. J. Baerends, D. E. Ellis, P. Ros, Chem. Phys. 1973, 2, 41. [18] G. te Velde, E. J. Baerends, J. Comput. Phys. 1992, 99, 84. [19] C. F. Guerra, J. G. Snijders, G. te Velde, E. J. Baerends, Theor. Chem. Acc. 1998, 99, 391. [20] G. M. Sheldrick, SHELXTL Version 5.1, Bruker AXS Inc., Madison, Wisconsin, 1999.