Chiral overcrowded alkene

Anabaena sensory rhodopsin (ASR, Fig. 10.2) [15, 16]—and a class [17, 18] of synthetic light-driven rotary molecular motors developed from chiral overcrowded alkenes [19], As further discussed below, verifying this analogy may have positive implications for the future design and construction of synthetic molecular motors. 

Among a variety of compounds whose molecular characteristics fall into this category, our current interests have further limited our discussion to the doublebond systems which may conveniently be classified as follows 1. chiral ( )-cyclo-alkenes, 2. chiral anti-Bredt rule compounds, 3. trans-doubly bridged ethylenes ( betweenanenes ), 4. bridged allenes, and 5. overcrowded olefins. 

Successful photoresolution and switching of enantiomers has been accomplished with two types of systems helical overcrowded alkenes and axially chiral cycloalka-nones. 

Figure 2 shows the structures of four classes of so-called overcrowded alkenes (see Section 5.3.1), designed as molecular components for a chiroptical switch based on CPL irradiation.1191 Thanks to unfavorable steric interactions around the central ole-finic bond, the molecules are forced to adopt a helical shape. The chirality in these inherently dissymmetric alkenes - denoted M and P for left-handed and right-handed helices, respectively - therefore originates from distortion of the molecular... 

The sterically overcrowded alkenes shown in Scheme 6 have been exploited in our group since, from the perspective of molecular switches design, they combine a number of attractive structural features. Steric interactions between the groups attached to the central olefmic bond force these molecules to adapt a non-planar helical shape. The chirality of these so-called inherently dissymmetric alkenes 3, is therefore the result of distortion of the entire molecular structure. Beside the heli-cene-like geometry, both a cis- and a trans-stilbene chromophore are present in the same molecule. 

The geometrical isomers and enantiomers of the overcrowded alkenes 15-18 can readily be separated using chiral HPLC. Recently, an asymmetric synthesis of overcrowded alkenes has been developed, involving chirality transfer from an axial single bond to an axial double bond (Scheme 8).32 This methodology is particularly attractive for preparation of larger quantities of enantiomerically pure chiral switches based on overcrowded alkenes. The orientation of the two xanthylidene moieties is dictated by a binaphthol template. After a coupling step and separation of the diastereomers, the bi-xanthylidene is obtained with 96 % e.e. after removal of the template. 

Scheme 8 Asymmetric synthesis route to optically active overcrowded alkenes (CB = Chiral bridging unit).

The group of Feringa developed the concept of overcrowded alkenes as potential molecular switches. These alkenes adopt a helical conformation due to steric interactions between the bulky groups attached at either end of the double bond. This approach was used in several examples of molecular switches. Compound 6 exists as two equal enantiomers. This racemic mixture can be enriched in P or M enantiomers, by irradiation at 313 nm with either left or right circular polarized light (/- or r-CPL) (Fig. This chiral bias can be removed by... 

Overcrowded alkenes are a special class of olefins with an inherently helical shape, resulting from extensive steric crowding around the central carbon-carbon double bond (Fig. 5.4b) [30]. Related molecules were first reported by Feringa and coworkers, who used the chiral molecules as chiral dopant in CLC materials [31]. The merit of these molecules as chiral dopants is that the photoisomerization between the two isomers with pseudoenantiomeric relationship often results in the handedness inversion of the cholesteric LCs. 

On the basis of 10,ll-dihydro-5//-dibenzo[a,i/]cycloheptene, the Chen group developed helical overcrowded alkenes 30a-d (Scheme 4.7) [32], which are closely related to the above switchable tetrabenzoheptafulvalene derivatives. Compounds 30a-d switch between diastereomers when irradiated with UV light of different wavelengths, and 30d was used as chiral dopant leading to a cholesteric mesophase when mixed with a nematic liquid crystal [33]. 

Molecular rotary motors are systems capable of undergoing unidirectional and repetitive rotations under the action of external energy inputs. The construction of molecular rotary motors poses several challenges, particularly because it is difficult to satisfy the unidirectional rotation requirement. Artificial rotary motors driven by light have been obtained by exploiting a-C=C-isomerization reaction in carefully designed overcrowded chiral alkenes. 

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