Wheel and axle

Well-packpapier, -papier, n. corrugated paper, -pappe, /. corrugated (paper)board. -rad, n. wheel and axle arbor wheel, -rohr, n. corrugated tube or pipe. 

There is generally considered to be five distinct simple machines lever, wedge, wheel and axle, pulley, and screw. The transmission of energy by these simple machines is so basic that people use them with little understanding of the physical principles involved. Most learn their use intuitively, through experience, and consider their application just plain common sense. 

Scheme 24 Electrochemical Polymerization of wheel-and-axle type phosphorus porphyrins...

Fig. 8. A rotaxane dendrimer with dendron units attached to both wheel and axle...

Wheel-and-Axle Type of Host Molecules in Clathrate Inclusion... 

Table 1. Crystal data for structures of inclusion compounds with wheel-and-axle type hosts containing large aromatic end groups Jl)...

The wheel-and-axle design as source for host-guest compounds was originally proposed by Toda and Hart in 1981 for hosts containing hydroxyl functions 481 (see Ch. 3, Sect. 2.1 of Vol. 140). The l,l,6,6-tetraphenylhexa-2,4-diyne-l,6-diol (24) provides a representative compound. It forms 1 2 crystalline inclusion complexes with a large number of small guest molecules, including a variety of ketones, amines, amides and a sulfoxide 48). 

The term rotamne derives (G. Schill, Histoiy , In Catenaries, Rotaxanes, and Knots, Academic, New York, 1971, pp. 1-4) from the Latin words rota, meaning wheel, and axis, meaning axle. In these molecular compounds, several wheel and axle components are constrained to be bound to one another mechanically, Le., without the aid of any valence forces, since the axle(s) is/are endowed with bulky stopper groups that prevent the extrusion of the wheel(s). 

Assembling [2] rotaxanes (wheel and axle) can involve three basic processes (Scheme 13). One of these (93CC1269) involves slippage, in which the axle (115) is linked by a 4,4 -bipyridine, and the wheel (116) is a bisparaphenylene-34-crown-10 ether. Heating the two components in acetonitrile at 60°C yields the rotaxane, which can be characterized by FABMS and H and l3C NMR, but extrusion of the wheel occurs at 100°C. In other developments (94NAT(369)133), the pyridine component may be incorporated in the wheel, as in structure (117), where two bipyridinium units are connected by p-xylyl groups, and here the rotaxane acts as molecular switch. At room temperature in acetonitrile the wheel... 

Figure 23. Key-lock interactions assist the molecular recognition of wheel and axle in the threading approach.

The sulfonamide macrocycle proved to be a hospitable template, and at the same time the sulfonamide functionality bears the possibility of farther derivatiza-tion of [2]rotaxanes (see Section 8.4 Chemistry with Amide-Based Catenanes and Rotaxanes). Furthermore the expansion of the cavity did not afford a change of the blocking group - no disassembly of wheel and axle was detected. 

These promising results led us to believe that rotaxanes, which do not have attractive interactions between wheel and axle and cannot therefore benefit from a template effect, should also be accessible by a slipping process performed in the melt. Indeed the first tetralactam rotaxane 94 containing a pure hydrocarbon dumbbell 93 was synthesized in 6% yield (Figure 36) [53]. 

As expected, NMR experiments revealed that the extrusion process of 94 does not depend on the polarity of the solvent as found for 82. Because no appreciable attractive interactions between wheel and axle are effective, the disassembly process depends only on the size ratio of the components. 

Figure 34. A new slipping process of tailor-made wheels and axles in the melt enables easy access to rotaxanes and pseudoiotaxanes.

Figure 35. Schematic representation depicting the self organization of rotaxane-like entities using the slipping approach in the melt (high concentrations of wheel and axle). R Rotax-ane where the hydrogen bonds between wheel and axle have not yet been formed. R Rot-axane with hydrogen bonds between wheel and axle. Entropy loss upon formation of the mechanical bond is not considered in the graph.

Our vision is to use the knotanophanes and the linear oligomeric knotanes as chiral wheel and axle components, respectively, in future giant rotaxanes which would mimic naturally occurring enzyme complexes [60],... 

The -electron-deficient pyridinium ring has played an important role in template-directed synthesis in which the interaction of a -electron acceptor and a -electron donor facilitate self-assembly. This has led to the construction of a variety of supramolecular assemblies including catenanes (linked rings) and rotaxanes (wheel and axle) . 

Because of their wheel and axle structure, much of the interest in rotaxane chemistry is focused on the mechanical movements of the ring around and along the dumbbell-shaped component. Of particularly interest is the possibility to control... 

A second variation of the lever is the simple machine known as a wheel and axle. A wheel and axle cm-sists of two circular pieces of drffermt si2BS attached to each other. The larger circular piece is the wheel in the stenr, and the mraller circular piece is the axle. One of the circular pieces can be considered as the effort arm of the lever and the second, the resistance arm. The place at whichthe two are joined is the fulcrum of the stem. 

Some examples of the wheel and axle include a door knob, a screwdriver, an egg beater, a water wheel, the... 

The mechanical advantage of a wheel and axle machine can be found by dividing the radius of the wheel by the radius of the axle. For example, suppose that the crank on a water well turns through a radius of 2 ft (61 cm) and the radius of the axle around which the rope is wrapped is 4 in (10 cm). Then the mechanical advantage of this wheel and axle system is 2 ft divided by 4 in, or 6. 

On the basis of the hypothesis of the intertwining mechanism described above, the syntheses of the first amide-based rotaxanes (Structures 20a, 20b) were successful as well [21], Their (supramolecular template) syntheses were strikingly simple. The central part of the axle 13a is added to a tetralactam wheel and subsequently capped with triphenylmethane stoppers. The macromonocycle ( wheel ) provides the receptor cavity for the axle and the stoppers confirm the mechanical bond between wheel and axle . 

---