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46543-B4
Ion Sensitive Push-Pull Pyridoannulenes

Keith C. Russell, Northern Kentucky University

The goal of this work is for undergraduates to synthesize a series of annulenes and test their spectroscopic properties in the presence of various metal cations.  Towards this goal, annulene 6 was successfully produced.  The synthesis is shown in Scheme 1.   

First, commercially available benzo-18-crown-6 was treated with periodic acid in methanol.  The result of this reaction was the facile isolation of diiodide 2 in nearly quantitative yield and in sufficient purity that no contaminants could be detected by either ¹H or ¹³C NMR spectroscopy.  This compound underwent Sonogashira coupling with two equivalents of 3 to yield 4 in 71 % yield.  Compound 3 was prepared from commercially available 2-bromo-3-pyridinol in three steps using a procedure previously published from our lab.  Several attempts were made at this reaction. In the first attempts at this reaction, using just slightly more than two equivalents of 3, monocoupled proucts were identified by NMR spectroscopy, yet no free alkyne, from unreacted 3 was detected.  It is likely that under the reaction conditions small amounts of 2 are being oxidatively homocoupled to form a bispyridyl butadiyne byproduct. This would not be a problem, except for the fact that it is not possible to separate the monocoupled material from the desired decoupled product, 4.  It was possible to take the mixture on to product by reacting the mixture with additional 3.  It is likely that this will interfere with our ability to synthesize asymmetric analogues of 6.  Deprotection of intermediate 4 was achieved with TBAF to give compound 5. After workup, 5 was found to be of satisfactory purity by ¹H NMR so that no further purification was necessary.   Oxidative coupling of the terminal alkynes afforded annulene 6 in 30 % yield from 4.  This reaction was somewhat problematic in the isolation of the final product, 6.  After the column was complete, it was clear that the mass balance was not close to that expected.  Flushing the column recovered more of the product, although less pure than the first.  It is clear that more work is needed on the purification of these compounds.  

This research has had a significant impact on all who participated in the project, as well as the PI, and other students who were not specifically involved in this area.  Through presentations and the experiments themselves, we all learned a great deal with respect to handling these crown compounds.  The summer had a significant impact on the PI with respect to the desired to continue working and expanding the research in this area.  Students involved in the project benefited from the experience and were motivated to seek post graduate training in chemistry, one now having gone to graduate school and a second who has indicated that she now intends to go to graduate school.

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