More than a protecting group: The trityl group senses the chirality of an alkyl substituent in chiral trityl ethers. The CD spectra of trityl‐protected alcohols are highly sensitive to the chirality of the alcohol. The use of a trityl CD sensor provides new insight into the structure and mode of action of chiral molecular bevel gears and shows the relation between the absolute configurations of molecules and their Cotton effect patterns.

The dynamic stereochemistry of chiral molecules is one of the major challenges of contemporary structural chemistry and biochemistry.[1] Sensitive issues regarding absolute configuration and conformation can now be dealt with by applying spectroscopic (in particular, chiroptical) methods and computational methods, especially those based on density functional theory. Despite their importance, amines (secondary and tertiary) present a still unsolved problem in the determination of their absolute configuration at the nitrogen. This configuration is subject to dynamic inversion at the nitrogen, and the inversion rate approaches 10 6 s À1 .[2] Preferred relative configuration of secondary amines can be studied with the use of NMR spectroscopy.[3] X-ray diffraction studies are mainly applicable to rigid amine molecules in the solid state, such as Trçger base, [4] N-chiral 1-chloro-2,2-diphenylaziridine [5] or a chiral oxaziridine.[6]Chiroptical methods, which are sensitive to absolute configuration, have been widely used to determine the permanent point of chirality at a carbon atom by the mechanism of chirality transmission, which induces dynamic helicity in the aromatic chromophoric system attached.[7] However, chiroptical methods have only rarely been applied to chiral amines, [8] since they usually lack a chromophore suitable for electronic circular dichroism (ECD) studies. In the present study, we show that in chiral N-tritylamines permanent chirality of the carbon atom is efficiently transmitted to a nitrogen atom and to a trityl group, which plays the role of a distinct ECD chromophore. The calculated diastereoisomer population and their Boltzmann averaged ECD spectra can be compared with the experimental ones by a generally accepted verification protocol. This approach provides the distribution of amine molecules of R N and S N configuration and allows us to demonstrate that dynamical configuration change at the nitrogen atom is correlated with permanent chirality at the carbon atom and dynamic helicity of the trityl group.A number of N-tritylamines (1-12) prepared for this study, display characteristic Cotton effects in their ECD spectra in the region of 1 B transition around 192 nm (e = 0.6-0. 8 10 5 ). These Cotton effects are of the exciton type (Table 1) and are strong (except 1 and 2); this suggests a well-defined preferred helicity of the trityl chromophore.In order to determine the low-energy structures and their populations we used molecular modeling with a Monte Carlo search followed by a DFT structure optimization at the B3LYP/6-31GA C H T U N G T R E N N U N G (d,p) level and the CD spectra calculation at the M06-2X/6-311 + GA C H T U N G T R E N N U N G (2d,p) level [9] for a series of simple chiral tritylamines (1-4). In fact, 1 represents the simplest acyclic chiral structure possible, with increasing diversification in 2-4. There are several structural variables that have to be considered in the calculations: the conformation of the alkyl group, the configuration of the nitrogen center and the

Mehr als nur eine Schutzgruppe: Die Tritylgruppe wird in Tritylethern von der Chiralität eines Alkylsubstituenten beeinflusst, wie aus den CD‐Spektren solcher tritylgeschützten chiralen Alkohole hervorgeht. Das Trityl‐CD‐Verfahren gibt Einblicke in die Struktur und Funktion chiraler molekularer Getriebe und belegt den Zusammenhang zwischen der Absolutkonfiguration von Molekülen und dem Muster ihres Cotton‐Effekts.

Trityl group, Tr, is a molecular dynamic rotor of which the conformation and helicity depend on other groups in the close vicinity. Interactions with another covalently linked Tr group and with other substituents are analyzed in terms of transfer of chirality to the trityl group. Two trityl groups in a molecule can mutually interact at a distance of two, three, or five bonds. Despite its size, a Tr group attached to a cyclohexane or cyclopentane ring through an oxygen or nitrogen atom adopts either an axial or equatorial position, depending on additional stabilizing interactions, such as hydrogen bonding.

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