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. 2017 May 19;82(10):5027-5033.
doi: 10.1021/acs.joc.7b00852. Epub 2017 May 1.

Asymmetric Synthesis of Second-Generation Light-Driven Molecular Motors

Thomas van Leeuwen  1 , Wojciech Danowski  1 , Edwin Otten  1 , Sander J Wezenberg  1 , Ben L Feringa  1
Affiliations

Asymmetric Synthesis of Second-Generation Light-Driven Molecular Motors

Thomas van Leeuwen et al. J Org Chem. .

Abstract

The enantiomeric homogeneity of light-driven molecular motors based on overcrowded alkenes is crucial in their application as either unidirectional rotors or as chiral multistate switches. It was challenging to obtain these compounds as single enantiomers via the established synthetic procedures due to loss of optical purity in the key step, i.e., the Barton-Kellogg olefination reaction. Searching for strategies to avoid racemization, a new class of light-driven molecular motors was designed, synthesized, and studied. The stereochemical integrity was fully preserved throughout the synthesis, and on the basis of photochemical and kinetic studies using UV/vis, CD, and 1H NMR spectroscopy, it was established that they still function properly as unidirectional molecular motors.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Four-Step Operating Mechanism Leading to Unidirectional Rotation of the Rotor (Red Upper Half) Relative to the Stator (Black Lower Half) Powered by Light (Depicted for the R Enantiomer
)
Scheme 2
Scheme 2. Synthetic Strategy toward Enantiopure Molecular Motors
Figure 1
Figure 1
Energy diagram for the THI of 2.
Scheme 3
Scheme 3. Synthesis of Ketone 9(18)
Scheme 4
Scheme 4. Synthesis of Motors 2 and 3
Figure 2
Figure 2
X-ray structure of (M)-(S,S)-15. Ellipsoids set at 50% probability. Hydrogen atoms are omitted for clarity. (Left) Front view. (Right) Side view.
Figure 3
Figure 3
(a) Changes in UV/vis spectrum of a solution of 2 in CH2Cl2 upon irradiation at 365 nm. (b) Changes in CD spectrum of a solution of 2 in CH2Cl2 upon irradiation at 365 nm.
Figure 4
Figure 4
Changes in the 1H NMR spectrum of a solution of 2 in CD2Cl2 upon irradiation at 365 nm at −40 °C.
Scheme 5
Scheme 5. Photochemical and Thermal Isomerization Behavior of 18
See this image and copyright information in PMC

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