University of Groningen Chiroptical Molecular Switches De Lange
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University of Groningen Chiroptical molecular switches de Lange, Ben IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2006 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): de Lange, B. (2006). Chiroptical molecular switches: synthesis and applications. s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 26-09-2021 CHAPTER 5 NON-CHIRQL STERICALLY OVERCROWDED ALKENES Temperature dependent NMR measurements have been successfully applied to obtain thermal isomerization barriers of chiral sterically overcrowded bistricyclic ethylenes functionalized with different bridging groups in the lower and upper part, which could not be resolved into their enantiomers via HPLC, as shown in the previous chapter. Introduction of an isopropyl group into the upper part of these alkenes allowed an accurate determination of the isomerization barriers using the coalescence of the NMR absorptions of the two methyl substituents in the isopropyl moiety at T, Interestingly, our investigations on bistricyclic ethylenes revealed that the chiml 10',10'- dimethyl substituted alkene 1 (Figure 5.1, R, = CH,) showed two well separated singlets in the 'H NMR and 13C NMR spectra for the two methyl substituents (see Experimental Section of Chapter 4).' Therefore, this "internally fixed" isopropyl group might also find application in temperature dependent NMR studies, because the two methyl groups may become equivalent in a rapidly inverting molecule at high temperatures as illustrated in Figure 5.1 (le * lb). Figwe 5.1. Interconversion of folded structures la * lb In Chapter 4, we described experiments to establish a correlation between structures and isomerization barriers of bistricyclic ethylenes. In order to determine these barriers, it was necessary to introduce substituent R, (R, = Cl-I,) in the upper part of 1 to induce chirality into the otherwise centrosymmetric structures. In contrast, application of the "internally fixed isopropyl group as a probe in dynamic NMR does not require the presence of an additional substituent R, and would therefore, theoretically, provide isomerization barriers of non-chiral alkenes (!), via interconversion of two folded structures (la * lb) as illustrated by the Newman The anisochronous properties of these two methyl groups are clearly illustrated by the X-ray stmure of IS in Section 5.3. projections in Figure 5.1. Because functionalization of bistricyclic ethylenes with one substituent (R,) will not enhance the isomerization barrier of these compounds compared to unsubstituted derivatives? the envisaged method can be used to predict quantitatively the racemization barriers of chire1 bistricyclic ethylenes via temperature dependent NMR studies on non-chiral alkenes. The synthesis and dynamic NMR study of non-chiml ethylenes might furnish valuable information on the thermal stability (i-e. racemization barriers), which can be expected for their chiral analogues, without a prior -lengthy- synthesis of these molecules with diastereotopic substituents and extensive "trial and error" experiments to resolve these molecules (see also Discussion in Section 5.5). In order to study the feasibility of this approach, we decided to prepare the unsubstituted analogue of 1 (Figure 5.1, R, = Furthermore, the synthesis of alkenes 15 - 17 has been undertaken (see Scheme 5.1), which are functionalized with a seven-, six- or five-membered central ring in the upper part of the molecule. This decrease in ring-size should also lead to a decrease in isomerization barrier, in agreement with the results described in the Chapter 4. 5.2 Synthesis The synthesis of alkenes 1 and 15 - 17 was achieved by the diazo-thioketone method and is depicted in Scheme 5.1. S NrrH, C1- 2 XIS 6 - 9 3 X-CHICH 4 X-0 5 X-- 11 X-S 1 X-s 12 X-CH-CH 15 X-CHICH 13 X-0 16 X-0 14 X-- 17 X-- cy. 6983% cY. 7845% Scheme 5.1. J)*rthesirof alk-enes I and 15 - 17. See Chapter 4, Section 4.9.1. The isomerization bamer, which can be expected for 1 (R1 = H, approximately 25 kcal.mol.', see Section 4.8) is probably slightly too high for the application of the dynamic NMR technique (A& .e 5 - 24 kcal.mol-I). 132 A Starting from hydrazones 2 - 5, which were oxidized to the corresponding diazo compounds 6 - 9 followed by addition of thioketone 10 and desulfurization of the intermediate episulfides 11 - 14, alkenes 1 and 15 - 17 were obtained in overall yields varying from 5476% see Scheme 5.1 and Table 5.1). The 'H NMR and '(3 C NMR spectra of 1, 15 and 16 showed clearly separated absorptions for the two methyl groups at position 10' and the chemical shift differences between the methyl substituents are listed in Table 5.1. In the case of the fluorene functionalized alkene 17, the methyl absorptions appeared as a single peak, indicating a rather low isomerization barrier for this compound (see Section 5.4). Table 5.1. 'H and 13c NMR Chemical Shift DiCIemces of the Methyl Grou~sin 1 and 15 - 17. compound X yielda 'H NMR AV~ 13cNMR hvb (%I (Hz) (Hz) a) Isolated yields based on the amount of added thioketone 10 (see Experimental Section). b) Av values at room temperature in CDC13. Generally, the episulfides and alkenes shown in Scheme 5.1, were obtained as slightly yellow or white solids. Crystallization of episulfide 12 and alkene 15 from ethanol, however, yielded small beautiful glittering colourless crystals for bath compounds. Because thioketone 10 was isolated as blue-purple sparkling needles (see experimental section in Chapter 3), the availability of 10, 12 and 15 as crystalline materials provides the opportunity to study the molecular structure of a thioketone, an episulfide and an alkene found in the consecutive steps of the diazo-thioketone methodology by X-ray analysis (Scheme 5.2). Apart from our desire to obtain X-ray structures of three. structural entities, namely a thiocarbonyl, a thiirane and an alkene, which are so frequently encountered throughout this thesis, several considerations have stimulated us to obtain molecular structures of 10, 12 and 15: - The opportunity to gain insight into the conformational changes of the 10',10'- dimethylanthracene part, which might occur during the thioketone + episulfide -+ alkene conversion. - The extent of folding in the upper and lower halves of the molecules both in the episulfide as well the alkene stage. - The interatomic distances in both halves of 15, which were required for the correlation study described in Chapter 4. - The molecular structure of 15 might clearly indicate why two different methyl absorptions are obtained in this centrosymmetric compound. Before the experiments with respect to the dynamic NMR behaviour of 1 and 15 -17 . are described, we will first present X-ray structures of 10,12 and 1~.~*' 12 15 Scheme 5.2 Stnudues ofthiokemne 10, episuyidc 12 and alkene 15. 5.3 MdenJor Structures of Thioketone 10, Episw 12 and Alkene 1fl~~ Suitable crystals for an X-ray investigation of 10 were grown from n-hexane and isolated as blue-purple sparkling needles. The thioketone crystallized in the Pbca space group with 8 molecules per unit cell. The molecular structure of 10 is depicted in Figure 5.2 and consists of a nearly planar conformation with one methyl substituent above and the other below the least-squares planes defined by both aromatic rings and the thiocarbonyl moiety (max deviation 0.021 A). The C=S bond length was 1.647 which can be compared to the C=S bond lengths found in thiobenzophenone (1.636 A)' and substituted derivatives thereof? Figun 5.2 Molecular smrcaue of 10 In Seetion 5.3, the molecular structures of 10, 12 and 15 will be briefly discussed, without comprehensive listings of bond distances, bond angles, et cetera.' The complete data of bond distances, bond angles, torsional angles and final fractional atomic coordinates are available on request. The X-ray analysis of 10 was performed by F. van Bolhuis and the X-rays of 12 and 15 were performed by k Meetsma of the Crystal Structure Centre of our department. ' Rindoff, G.; Carlsen, L Acm Clyst. 1979, B35, 1179. 13 a) Arjunan, P.; Ramamurthy V.; Venkatesan, K Acm Clyst. 1984, C40, 552, 556. @) Manojlovic, LM.; Edmunds, LG. Acta clyst. 1965,18,543. Slow crystallization from ethanol afforded 12 as small twinkling colourless crystals, which were suitable for an X-ray determination. Episulfide 12 crystallized in the Fdd2 space group, whereby the unit cell contains sixteen discrete molecules. Crystallization from ethanol afforded 15 as small calourless c~ystalssuitable for X-ray analysis. This compound crystallized in the Pbca space group with eight molecules of 15 in each unit cell. The structures of 12 and 15 including the adopted numbering schemes are pictured in Figure 5.3. The linear tricyclic moieties in both halves of 12 are oriented in the same direction with the central thiirane ring like a "roof' on top of these groups (ck-configuration, see also Fi re 5.5).