Permethylcyclohexane

Conformational tuning improves the stability of spirocyclic nitroxides with long paramagnetic relaxation times

Nitroxides are widely used as probes and polarization transfer agents in spectroscopy and imaging. These applications require high stability towards reducing biological environments, as well as beneficial relaxation properties. While the latter is provided by spirocyclic groups on the nitroxide scaffold, such systems are not in themselves robust under reducing conditions. In this work, we introduce a strategy for stability enhancement through conformational tuning, where incorporating additional substituents on the nitroxide ring effects a shift towards highly stable closed spirocyclic conformations, as indicated by X-ray crystallography and density functional theory (DFT) calculations. Closed spirocyclohexyl nitroxides exhibit dramatically improved stability towards reduction by ascorbate, while maintaining long relaxation times in electron paramagnetic resonance (EPR) spectroscopy. These findings have important implications for the future design of new nitroxide-based spin labels and imaging agents.

The cyclohexasilanes Si6H11X and Si6Me11X with X=F, Cl, Br and I: a quantum chemical and Raman spectroscopic investigation of a multiple conformer problem

The conformers of the monohalocyclohexasilanes, Si(6)H(11)X (X=F, Cl, Br or I) and the haloundecamethylcyclohexasilanes, Si(6)Me(11)X (X=F, Cl, Br or I) are investigated by DFT calculations employing the B3LYP density functional and 6-31+G* basis sets for elements up to the third row, and SDD basis sets for heavier elements. Five minima are found for Si(6)H(11)X-the axial and equatorial chair conformers, with the substituent X either in an axial or equatorial position-and another three twisted structures. The equatorial chair conformer is the global minimum for the X=Cl, Br and I, the axial chair for X=F. The barrier for the ring inversion is approximately 13 kJ mol(-1) for all four compounds. Five minima closely related to those of Si(6)H(11)X are found for Si(6)Me(11)X. Again, the equatorial chair is the global minimum for X=Cl, Br and I, and the axial chair for X=F. Additionally, two symmetrical boat conformers are found as local minima on the potential energy surfaces for X=F, Cl and Br, but not for X=I. The barrier for the ring inversion is approximately 14-16 kJ mol(-1) for all compounds. The conformational equilibria for Si(6)Me(11)X in toluene solution are investigated using temperature dependent Raman spectroscopy. The wavenumber range of the stretching vibrations of the heavy atoms X and Si from 270-370 cm(-1) is analyzed. Using the van't Hoff relationship, the enthalpy differences between axial and equatorial chair conformers (H(ax-)H(eq.)) are 1.1 kJ mol(-1) for X=F, and 1.8 to 2.8 kJ mol(-1) for X=Cl, Br and I. Due to rapid interconversion, only a single Raman band originating from the "averaged" twist and boat conformers could be observed. Generally, reasonable agreement between the calculated relative energies and the experimentally determined values is found.

Chair/Twist-Boat Energy Gap in Monocyclic, Conformationally Unconstrained Polyalkylcyclohexanes

A conformational search procedure (HUNTER), in combination with the MM3(92) program, was used for the exploration of the conformational hypersurface of alkyl-substituted cyclohexanes and for the calculation of their chair/twist-boat (TB) energy gap. The systems studied were conformationally unconstrained polyalkylcyclohexanes (alkyl = methyl, ethyl, isopropyl, and tert-butyl) possessing either geminal and/or vicinal arrangements of the alkyl groups, but differing in the number of alkyl substituents and in their relative disposition (i.e., cis or trans). The calculations indicate that in 1,1,3,3,5,5-hexaisopropylcyclohexane the TB is the lowest energy form. Modification of the cis,trans relationship of vicinal alkyl groups changes the chair/TB energy gap, and in the minimum energy conformation of cis,trans,trans-1,2,3,4-tetraisopropylcyclohexane (23c) and cis,syn,cis-1,2,4,5-tetraisopropylcyclohexane (31c) the cyclohexyl ring adopts a TB conformation. The tetrasubstituted systems cis,syn,cis-1,2-diisopropyl-3,4-dimethylcyclohexane (46), cis,syn,cis-1,4-diisopropyl-2,5-dimethyl-cyclohexane (47), and cis,trans,trans-1,2-diisopropyl-3,4-dimethylcyclohexane (41) are the least crowded monocyclic unconstrained cyclohexanes found in which the TB conformation is of lower energy than the chair form. The present study indicates that two methyls and two isopropyl substituents are sufficient for stabilizing the TB form of a cyclohexyl ring relative to the chair form.