Conformational Control of ortho-Phenylenes by Terminal Amides
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Abstract:
Control over the folding of oligomers, be it broad induction of a preferred helical handedness or subtle changes in the orientations of individual functional groups, is important for applications ranging from molecular recognition to long-range conformational communication. Here, we report a series of ortho-phenylene hexamers functionalized with achiral and chiral amides at their termini. NMR spectroscopy, taking advantage of 19F labeling, allows multiple conformers to be detected for each compound. In combination with CD spectroscopy and DFT calculations, specific geometries corresponding to each conformer have been identified and quantified. General conclusions about the effect of sterics and the amide linker on conformational behavior have been drawn, revealing some similarities to and key differences from previously reported imines. A model for twist sense control has been developed that is supported by computational models.Keywords:
Conformational isomerism
Folding (DSP implementation)
Amide
Linker
Rotamers are stereoisomers produced by rotation (twisting) about σ bonds and are often rapidly interconverting at room temperature. Xylitol-massively produced sweetener-(2R,3r,4S)-pentane-1,2,3,4,5-pentol) forms rotamers from the linear conformer by rotation of a xylitol fragment around the C2-C3 bond (rotamer 1) or the C3-C4 bond (rotamer 2). The rotamers form two distinguishable structures. Small differences in geometry of rotamers of the main carbon chain were confirmed by theoretical calculations; however, they were beyond the capabilities of the X-ray powder diffraction technique due to the almost identical unit cell parameters. In the case of rotamers of similar compounds, the rotations occurred mostly within hydroxyl groups likewise rotations in L-arabitol and D-arabitol, which are discussed in this work. Our results, supported by theoretical calculations, showed that energetic differences are slightly higher for rotamers with rotations within hydroxyl groups instead of a carbon chain.
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Arabitol
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Isopropyl
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Studies on the conformational equilibria of 2-methoxy, 2-methylthio, and 2-methylselenocyclohexanol are reported. Dynamic NMR spectroscopy experiments at 203-210 K were performed, which provided the percentages of each conformer in equilibrium. Theoretical calculations using the B3LYP method and aug-cc-pvdz basis set were applied to determine the differences in energy between the conformers. The analysis of the potential energy surface of each conformer showed the presence of two rotamers. Natural bond orbital analysis provided an explanation of which factors are driving the rotamer and conformer preferences.
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Abstract Stereoelectronic hyperconjugative interactions and the relative energies of conformers and transition states of 2‐, 3‐, and 4‐silathiacyclohexane were calculated at the B3LYP/6–311+G(d,p) level of theory. The chair conformer of 2‐silathiacyclohexane is 15.4 and 15.9 kcal mol −1 (1 kcal = 4.184 kJ), respectively, lower in energy than the chair conformers of 3‐ and 4‐silathiacyclohexane. Intrinsic reaction path calculations were used to connect the transition states between the respective chair and twist conformers and different chair–chair conformational interconversion paths were located for 3‐ and 4‐silathiacyclohexane. The energy of the transition state that connects the chair and 2,5‐twist conformers of 3‐silathiacyclohexane is 5.58 kcal mol −1 higher in energy than the chair. The transition state that connects the chair and 2,5‐twist conformers of 4‐silathiacyclohexane is 4.82 kcal mol −1 higher in energy than the chair. The energy differences (Δ E , kcal mol −1 ) between the chair conformer of 2‐silathiacyclohexane and the respective 1,4‐twist (Δ E = 4.16), 2,5‐twist (Δ E = 3.20) and 3,6‐twist (Δ E = 3.87) conformers were calculated. Small relative energy differences were calculated between the chair conformer and the respective 1,4‐twist (Δ E = 3.95), 2,5‐twist (Δ E = 4.07) and 3,6‐twist (Δ E = 3.46) conformers of 3‐silathiacyclohexane. The calculated energy differences (Δ E ) between the chair conformer and the 1,4‐twist and 2,5‐twist conformers of 4‐silathiacyclohexane were 3.50 and 4.04 kcal mol −1 , respectively. The geometric parameters and stereoelectronic hyperconjugative interactions in the silathiacyclohexanes are compared and discussed. Copyright © 2004 John Wiley & Sons, Ltd.
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Transition state
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Abstract The concept of weakening amide bonds by the rather unusual forced complexation of nitrogen to Cu ++ is not limited to tridentate ligands and was extended in this work to tetradentate ligands as well. The use of cyclic tetradentate ligands was to no avail, but an open‐chain and more‐flexible tetradentate ligand allowed mild cleavage by methanolysis after complexation. The principle was applied to the development of a new linker for solid‐phase chemistry, which was proven to be extremely robust, yet allowed mild cleavage after activation by Cu ++ complexation. Its stability and versatility was demonstrated by the successful application to a whole plethora of different types of reactions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
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Cleavage (geology)
Solid-Phase Synthesis
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Supercritical Carbon Dioxide
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The synthesis and application of a carbonyl-(13)C backbone amide linker are described. The labeled unit is conveniently mixed with commercial resins, providing a rapid means of monitoring chemistry performed with this linker on solid support using conventional (13)C NMR methods.
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Amide
Solid-Phase Synthesis
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