Molecular turnstiles

This first example inspired the design of many other similar systems, most famously those investigated in the 1970s and 1980s by Mislow and Iwamura [19]. Later, the structurally complex but conceptually similar molecular turnstiles 3-5 (Fig. 2) were introduced by Moore [20]. Variable temperature XH NMR studies on the methylene protons, ffa and H, showed that the central aromatic ring of 4 spins rapidly on the NMR timescale at room temperature, while spindle rotation does not occur in turnstile 5 even at 85 °C due to steric constraints. 

Fig. 2 Molecular turnstiles reported by Bedard and Moore [20]. The variable temperature NMR of protons Ha and provides evidence of spindle rotation in 4 and a locked spindle in 5...

Another example of rotary motion in a molecular system is provided by the molecular turnstile prepared by Moore and Bedard [30]. The turnstile architecture consists of a hexa(phenylacetylene) macrocyclic frame with a... 

The molecular turnstile was synthesized by regioselective and sequential palladium-catalyzed cross-coupling reactions between aryl halides and terminal alkynes [32]. The successful strategy incorporates the spindle in the first step of the synthesis and the last step is a macrocyclization. Three molecular systems (12-14) with different spindles have been prepared. The compounds, particularly 12 and 13, show low solubility, perhaps due to their relative planarity. 

Homeomorphs of the two-point bicycle show up in an interesting collection of structures, ranging from the simple cage-type bicyclophane (73) [93, 94] to the extended homeomorphs by Lehn et al. [95], Vogtle et al. [96], and Moore et al. [97]. Other molecular representations are Moore and Bedard s flat molecular turnstile (74) [98], Hart and Vinod s cuppedophanes (75) [99-102], and Okazaki et al. s bowl-shaped bicyclic cyclophane (76) [103,104]. 

Scheme 46 Organometallic molecular turnstiles ruthenium[penta(4-halogenophenyl)-cyclopentadienyI] [hyd rotrls i ndazolyI) borato] com plexes.

Permutational isomerization of pentavalent phosphorus compounds, 9, 25 Phosphorous compounds, pentavalent, turnstile rearrangement and pseudorotation in permutational isomerization, 9, 26 Planar and non-planar aromatic systems, 1, 203 Polarizability, molecular refractivity and, 3,1 Polarography and reaotion kinetios, 5, 1 Polypeptides, calculations of conformations of, 6, 103... 

Molecular systems that exhibit controlled or coordinated rotary motion are discussed in this chapter. These systems represent a reproduction of a variety of macroscopic mechanical devices on the molecular scale. From gears to a motor, passing through a turnstile, a brake and a ratchet, we describe their design, synthesis and dynamic behavior. The importance of molecular motors in the biological realm and possible applications in nanotechnology are also discussed. 

In summary, the turnstile represents a new example of rotation in a molecular system. The fact that phenylacetylene macrocycles form discotic liquid crystals suggests the possibility that turnstiles might function as discotic ferroelectric liquid crystals [34]. A dipole appropriately incorporated on the spindle might allow for its own rotation to be controlled rapidly and reversibly by an external field. 

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