Manganese-catalyzed selective oxidation of aliphatic C-H groups and secondary alcohols to ketones with hydrogen peroxide.

An efficient and simple method for selective oxidation of secondary alcohols and oxidation of alkanes to ketones is reported. An in situ prepared catalyst is employed based on manganese(II) salts, pyridine-2-carboxylic acid, and butanedione, which provides good-to-excellent conversions and yields with high turnover numbers (up to 10 000) with H2 O2 as oxidant at ambient temperatures. In substrates bearing multiple alcohol groups, secondary alcohols are converted to ketones selectively and, in general, benzyl C-H oxidation proceeds in preference to aliphatic C-H oxidation.

The unexpected role of pyridine-2-carboxylic acid in manganese based oxidation catalysis with pyridin-2-yl based ligands.

A number of manganese-based catalysts employing ligands whose structures incorporate pyridyl groups have been reported previously to achieve both high turnover numbers and selectivity in the oxidation of alkenes and alcohols, using H(2)O(2) as terminal oxidant. Here we report our recent finding that these ligands decompose in situ to pyridine-2-carboxylic acid and its derivatives, in the presence of a manganese source, H(2)O(2) and a base. Importantly, the decomposition occurs prior to the onset of catalysed oxidation of organic substrates. It is found that the pyridine-2-carboxylic acid formed, together with a manganese source, provides for the observed catalytic activity. The degradation of this series of pyridyl ligands to pyridine-2-carboxylic acid under reaction conditions is demonstrated by (1)H NMR spectroscopy. In all cases the activity and selectivity of the manganese/pyridyl containing ligand systems are identical to that observed with the corresponding number of equivalents of pyridine-2-carboxylic acid; except that, when pyridine-2-carboxylic acid is used directly, a lag phase is not observed and the efficiency in terms of the number of equivalents of H(2)O(2) required decreases from 6-8 equiv. with the pyridin-2-yl based ligands to 1-1.5 equiv. with pyridine-2-carboxylic acid.

Manganese catalyzed cis-dihydroxylation of electron deficient alkenes with H(2)O(2).

A practical method for the multigram scale selective cis-dihydroxylation of electron deficient alkenes such as diethyl fumarate and N-alkyl and N-aryl-maleimides using H(2)O(2) is described. High turnovers (>1000) can be achieved with this efficient manganese based catalyst system, prepared in situ from a manganese salt, pyridine-2-carboxylic acid, a ketone and a base, under ambient conditions. Under optimized conditions, for diethyl fumarate at least 1000 turnovers could be achieved with only 1.5 equiv. of H(2)O(2) with d/l-diethyl tartrate (cis-diol product) as the sole product. For electron rich alkenes, such as cis-cyclooctene, this catalyst provides for efficient epoxidation.

An enantioselective synthetic route toward second-generation light-driven rotary molecular motors.

Controlling the unidirectional rotary process of second-generation molecular motors demands access to these motors in their enantiomerically pure form. In this paper, we describe an enantioselective route to three new second-generation light-driven molecular motors. Their synthesis starts with the preparation of an optically active alpha-methoxy-substituted upper-half ketone involving an enzymatic resolution. The subsequent conversion of this ketone to the corresponding hydrazone by treatment with hydrazine led to full racemization. However, conversion to a TBDMS-protected hydrazone by treatment with bis-TBDMS hydrazine, prepared according to a new procedure, proceeds with nearly full retention of the stereochemical integrity. Oxidation of the TBDMS-protected hydrazone and subsequent coupling to a lower-half thioketone followed by recrystallization provided the molecular motors with >99% ee. As these are the first molecular motors that have a methoxy substituent at the stereogenic center, the photochemical and thermal isomerization steps involved in the rotary cycle of one of these new molecules were studied in detail with various spectroscopic techniques.

A trimer of ultrafast nanomotors: synthesis, photochemistry and self-assembly on graphite.

Lightning quick! A new ultrafast light-driven molecular motor was developed, which was readily incorporated into a larger trimeric system. The trimer of these motors was studied with STM and at the interface of highly oriented pyrolytic graphite and 1-phenyloctane the molecules form stable arrays in which the chirality of the trimer is expressed on both the molecular and the supramolecular level (see figure).

On the effect of donor and acceptor substituents on the behaviour of light-driven rotary molecular motors.

Light-driven rotary molecular motors based on overcrowded alkenes can be substituted with electron-donating and electron-withdrawing substituents (R = OMe, Cl and CN) in direct conjugation with the central double bond (the axis of rotation) without having a significant influence on the rate-limiting, thermal isomerisation step of their rotary cycle. This indicates that in this system, it is predominantly steric factors that determine the barrier to the thermal helix inversion. In contrast, the quantum yield and photoequilibria in the photochemical step were found to be quite sensitive to the combination of substituent and solvent employed.

Light-controlled supramolecular helicity of a liquid crystalline phase using a helical polymer functionalized with a single chiroptical molecular switch.

Control over the preferred helical sense of a poly(n-hexyl isocyanate) (PHIC) by using a single light-driven molecular motor, covalently attached at the polymer's terminus, has been accomplished in solution via a combination of photochemical and thermal isomerizations. Here, we report that after redesigning the photochromic unit to a chiroptical molecular switch, of which the two states are thermally stable but photochemically bistable, the chiral induction to the polymer's backbone is significantly improved and the handedness of the helical polymer is addressable by irradiation with two different wavelengths of light. Moreover, we show that the chiral information is transmitted, via the macromolecular level of the polyisocyanate, to the supramolecular level of a lyotropic cholesteric liquid crystalline phase consisting of these stiff, rodlike polymers. This allows the magnitude and sign of the supramolecular helical pitch of the liquid crystal film to be fully controlled by light.

Control of dynamic helicity at the macro- and supramolecular level.

In this review, various systems developed in recent years which aim to control dynamic helicity at the macro- and supramolecular level are discussed. The strong interactions between the individual molecular components in these controlled helical assemblies, ranging from columnar aggregates to helical polymers and cholesteric liquid crystals, result in stereoinduction from the molecular level to the level of these macro- and supramolecular helical architectures. Therefore these systems are potentially useful for various applications, including responsive materials and chirality sensors and amplifiers.

Acceleration of a nanomotor: electronic control of the rotary speed of a light-driven molecular rotor.

A new second-generation light-driven molecular motor was designed, in which the presence of a potential electronic push-pull system leads to a significant increase of the rate of rotation compared to previous motor systems, without disturbing its overall unidirectionality.