Pioneering Work on Catenanes, Rotaxanes, and a Knotane in the University of Freiburg 1958–1988

Abstract

Research at the University of Freiburg is summarized which aimed at realizing catenanes, rotaxanes, and a molecular knot from 61–62 until 41 years ago. Taking a fresh view at ansa‐compounds – distinct from those in which he had been interested for other reasons before – Arthur Lüttringhaus began to tackle syntheses of catenanes in 1957. The first isolation of a pure catenane (37) succeeded in 1964 jointly with Gottfried Schill. Most of the progress made in the sequel was due to Schill alone or achieved under his direction; in addition, a few follow‐up studies were published without Schill or Lüttringhaus being their authors. The major feats of these endeavors were the syntheses of the following molecules: (1) the first catenanes (38 – de‐acetyl‐41, 74) without a ring in any constituent (this feature makes them unique till today; the catenane 74 holds the additional record of being the only all‐hydrocarbon catenane till now); the first [3]catenanes at all (obtained as a mixture of constitutional isomers C2v‐97 and C2h‐97); the first [3]catenanes exhibiting topological isomerism (89, iso‐89, and neo‐iso‐89); the rotaxanes 70 [which had the design peculiarity of being convertible into a catenane (74)], 113 (Schill's #1‐rotaxane), and 115; the pre‐knotane rac‐dia‐iso‐123 [provided it was obtained and not (only) its topological isomer dia‐iso‐123]. – For due perspectives, pertinent studies of third parties are included, too: 1) Wasserman's reports on the isolation of the catenane 8 are analyzed in great detail; the conclusion is that the identity of 8 was never established. 2) Schill's oscillating rotaxane 70 from 1988, which is subject to a reversible and non‐degenerate ring migration (→ iso‐70), is supplemented by Stoddart's much faster oscillating rotaxane 71 from 1991, which undergoes a reversible and degenerate ring migration (→ 71 instead of → iso‐71) and was dubbed “molecular shuttle”. 3) The first [3]catenanes from the groups of Schill (97; 1969), Sauvage (98 and 99; 1985), and Stoddart (100–102; 1991) are juxtaposed. 4) The Harrisons' synthesis of the rotaxane 106 by a most noteworthy solid‐phase approach (1967) is recalled in the context of Schill's synthesis of the rotaxane 113 (1967 or 1969). 5) Attempts at threading α‐cyclodextrin by long‐chain 1,ω‐dithiols (→ 130) and ring‐closing the latter oxidatively for obtaining catenated disulfides failed in Lüttringhaus' 1957/1958 studies; in contrast, Stoddart's threading permethyl‐β‐cyclodextrin (133) by the long‐chain 1,ω‐diamine 132 and ring‐closing the latter by a double condensation with terephthaloyl dichloride was successful by providing 3 % of the catenated bislactame 134 in 1993. The concluding section characterizes the synthetic strategies towards catenanes and rotaxanes developed in Freiburg as “template‐based”. This differs from the Nobel Prize Committee's assessment.