Helical alkenes

Introduction of an aliphatic clamp in the ( )-position of alkenes leads to twisted double bonds and therewith to helical alkenes like 55-65. 

In a recent paper, Feringa and coworkers [11] have shown that it is possible to induce a repetitive, monodirectional rotation around a central carbon-carbon double bond in a chiral, helical alkene, with each 360° rotation involving four discrete isomerization steps activated by ultraviolet light or a change in the temperature of the system (Fig. 5). 

Photochemical E Z isomerization (Section 6.1.1) is responsible for continuous unidirectional rotary motion along a carbon carbon double bond of 433, a chiral helical alkene mounted on the surface of gold nanoparticles through two octanethiol linkers (Scheme 6.206).1245 Thanks to the considerable conformational flexibility of... 

Lautens [32, 33] andTietze (Section 3.2) have developed approaches utilizing Pd-catalyzed domino reactions to construct sterically crowded tetrasubstituted alkene structures in a highly efficient manner. A remarkable Pd-catalyzed domino reaction of aryl iodide 101 and alkyne 96 was reported by Lautens and coworkers [43], where a norbornene molecule is incorporated in the product to afford a new class of tetrasubstituted helical alkenes 140 (Scheme 3.35). A total of four C-C bonds were formed in a single operation, two of which were from C-H activations. TFP was an... 

Scheme 3.36 Proposed mechanism for the formation of tetrasubstituted helical alkenes.

Scheme 3.38 Synthesis of chiral tetrasubstituted helical alkenes.

Scheme 3.39 Synthesis of aryl-/heteroaryl-containing tetrasubstituted helical alkenes.

Chiral helical alkene-hased molecular rotors... 

A concise route to oxygen-containing helical tetrasubstituted alkenes via a palladium-catalyzed domino reaction with Sonogashira coupling/ carbopalladation/C-H functionalization sequence has been developed by Tietze and coworkers (Scheme 3.12) [30].lnthiswork, the in s/fM-generated vinyl palladium intermediate from alkyne plays a vital role in the C—H activation. The reaction is efficient, and a mixture ofZ- and -isomers of the helical alkenes was afforded under the typical condition (44—96% yields, ElZ =1.5 1 to 1 1.6). 

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. 

Figure 5 shows the molecular structures of rfs-2-nitro-7-(dimethylamino-)-9-(2,3,-dihydro-l J-f-naphtho[2,l-b]thiopyran-l -ylidene)-9J-f-thioxanthene ((P)-cis-17) and trans-dimethyl-[l-(2-nitro-thioxanthen-9-ylidene)-2,3-dihydro-lJ-f-ben-zo[f]thiochromen-8-yl]amine ((P)-trans-18). Anti-folded helical structures, in which the top and bottom parts are respectively tilted up and down relative to the plane of the central olefmic bond, are clearly observed. The extent of folding and twisting in these and related overcrowded alkenes can vary considerably. It should be emphasized that only minor deviation from planarity occurs at the central double bond (dihedral angles 5.4° (17) and 6.8° (18)), while normal bond lengths are found (1.353 A (17) and 1.338 A (18)).130,311... 

The property of the polymers in question to form nonspherical nanostructures was confirmed in experimental studies. Shih et al. [29] synthesized alternating copolymers of 1-alkenes with maleic anhydride. The maleic anhydride units were hydrolyzed to maleic acid units. Fully hydrolyzed macromolecules associated into microstructures of cylindrical and ellipselike shape. The cylindrical shape was characteristic of copolymers with octadecene and hexadecene moieties, while the copolymers with lower alkene copolymers (tetradecene, dodecene, decene, octene) formed ellipsoidal structures. Wataoka et al. [30] investigated the formation of nonspherical helices in a system of maltopentaose-carrying polystyrene (PS). The polymer was synthesized via the homopolymerization of vinylbenzyl maltopentaose amide (Scheme 3). 

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