Since the report of Sondheimer et al. 1 on the use of Eglinton-Glaser coupling for the preparation of unsaturated macrocyclic compounds, the oxidative homocoupling reaction between the sp-carbon centers of terminal alkynes leading to butadiyne derivatives has become one of the most useful synthetic transformations for the preparation of medium-and large-ring systems, 2 as well as for the formation of boxlike molecules. 3 As with other cyclization reactions, the variety of different cyclization products decreases as the complexity of the precursor increases. 4 We recently reported the oxidative cyclization of the rather rigid kinked phenylene-ethynylene oligomer 1a (Scheme 1) using a slurry of CuCl/CuCl 2 in pyridine as the catalyst/oxidant mixture. 3b The reactions were performed at room temperature using pseudo-highdilution conditions. According to the gel permeation chromatographic (GPC) analysis, the crude product contains the corresponding macrocycle 3a at about 60-65%. 5,6 However, the results obtained by the oxidative cyclization of the oligomers 1b and 1c differ remarkably. Under identical conditions, the crude product of the oxidative coupling of 1b contains 3b at about 75-80%.On the contrary, the crude coupling product of 1c contains 3c only at about 40-45%. 7 These results clearly indicate that the arrangement of the functional groups (the tetrahydro-2H-pyranyl (THP) ethers) has a strong influence on the yield of cyclic dimer.Although the exact mechanism of the oxidative acetylene dimerization remains unknown, a possible explanation for these results is that copper ion clusters or a copper salt surface act as a template in this reaction. 8 Coordination of the OTHP groups to the template would dramatically restrict the flexibility of the bisacetylic intermediate 2, which is formed after the first oxidative coupling reaction. Although in every case the same number of coordination sites are connected to the copper salt, the steric restriction increases as the distance of the coordination side from the reacting free acetylenes decreases in the order 2b < 2a < 2c. In other words, due to the aliphatic spacer, 2b has the highest mobility, giving it the highest probability to react intramolecularly.We also found a strong temperature dependence on the oxidative acetylene dimerization. 9 Addition of the bisacetylenes 1a and 1c to the catalyst suspension at elevated temperatures increased the amount of cyclic dimer in the crude product of the reaction (Table 1). 10 The optimum reaction temperature was found to be . (1) (a) Leznoff, C. C.; Sondheimer, F. J. Am. Chem. Soc. 1968, 90, 731. (b) Haley, M. M.; Pak, J. J.; Brand, S. C. Top. Curr. Chem. 1999, 201, 81 and references therein.(2) For macrocyclic host molecules based on the phenyl-diethynyl backbone, see, e.g.: (a) Morrison, D. L.; Höger, S. A solution of 0.2 mmol of 1 in 10 mL of dry pyridine was added to a suspension of 1.75 g of CuCl and 0.35 g of CuCl 2 in 40 mL of pyridine in 96 h (with the help of a syringe pump). After completion of the addition, the...

ring closure reactions ring closure reactions O 0130 -051Structure and Temperature Effects on the Cyclization of Rigid Bisacetylenes.-Cyclodimerization of acetylenic compounds (I), using CuCl/CuCl 2 in pyridine as catalyst/oxidant mixture, depends on the arrangement of the functional groups (suggesting that the copper salts act as template) and strongly on the temperature. Due to the aliphatic spacer, the acyclic dimeric intermediate of (Ib) has the highest mobility and, thus, the highest probability to react intramolecularly. The optimum reaction temperature is around 60 • C for optimum yields between 50 and 70%. The cyclic trimer of acetylenic compound (II) is formed in about 35% yield regardless of whether the reaction takes place at room temperature or 65