Wheel and axle host

The DGTD belongs to a group of wheel and axle host molecules. Compared to the classical representative of this group (linear acetylenic spacer coupled with bulky end groups), DGTD can be called a four wheeled system where guest molecules can be included in the space between the front wheels (and/or rear wheels). 

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). 

Desiraju has reported a crystalline supramolecular wheel-and-axle compound with a structure based on a carboxylic acid dimer.33 Specifically, the group predicted that 4-(triphenylmethyl)benzoic acid would self-assemble to give a homodimer. The dimer was expected, owing to an inability to efficiently pack, to form inclusion compounds that host solvent molecules as guests. Such inclusion would be reminiscent of structurally similar organic molecules that serve as wheel-and-axle compounds in the solid state. The homodimer would, thus, circumvent a covalent synthesis. As predicted, the carboxylic acid formed a homodimer that produced solids that exhibited solvent inclusion (Fig. 15). The packing was dominated by... 

Wheel-and-axle organic diols are host systems well known in the solid state. The introduction of a metal into the wheel-and-axle molecular frame opens the way for the design of materials with multiple functions, joining chemical and structural properties of the metal centre with the steric and supramolecular attitudes of the organic matrix. This chapter treats the rationalization of host-guest properties of wheel-and-axle inorganic diols. 

Fig. 7. Host design according to the wheel-and-axle idea (a) sketch of the overall molecular shape 14-17 showing typical examples of molecular constitutions (b) packing diagram of a representative inclusion compound (16 with toluene, 1 1 guest molecules shaded)...

The wheel-and-axle design as source for host-guest compoimds was originally proposed by Toda and Hart in 1981 for hosts containing hydroxyl functions (see Ch. 3,... 

Figure 9 The principal intermolecular attraction in the (39)-(p3nidine)2 coordinatoclathrate structure is host-guest O-H- -N hydrogen bonding (dashed lines), and the guests occupy the concave space resulting from the wheel-and-axle moieties. Color code H, light blue O, red N, dark blue host C, green guest C, purple. ...

The last three examples emphasize just how robust the wheel-and-axle concept is, and reveal how much it can be abused while stiU retaining clathrate behavior. The complex 44, formed from two molecules each of PdCl2 and a dipyridyl tetrapropoxycalix[4]arene, yields a chloroform clathrate. The calixarene groups act here as the wheels, and the coordinated palladium functionalities provide a novel double-axle. The orthogonal view shown in Figure 10 clearly reveals the wheel-and-axle topology of this host. 

Finally, compound 46 is one of a group of chiral host molecules (TADDOLs) devised by Toda and Tanaka and synthesized from tartaric acid. These were designed and employed as chiral agents for resolution experiments. Often hydrogen-bonded complexes, rather than coordina-toclathrates, are formed using TADDOLs. However, then-molecular structure is again an excellent illustration of just how ubiquitous the wheel-and-axle motif is in inclusion chemistry. 

Draper has recently reported a study that cuts across three of the host structural systems discussed earlier, namely compounds 99a-c (axial aromatic, Section 4.1), lOOa-c (wheel and axle, Section 4.3), and lOla-c (trigonal symmetry, Section 8). The majority of these molecules exhibit inclusion properties and their crystal structures provide a revealing comparison of these host types. 

Irradiation of crystalline host-guest complexes of the nitrones 23 with the axle-wheel-type host compound (R,R)-( — )-4 gave the optically active oxaziri-dines 24 (Scheme 6) [48]. In particular, the enantioselectivities for the formation of 24d and 24e were perfect. 

Irradiation of the 2 1 host-guest crystals of cyclohex-2-enone 81 with the axle-wheel-type host compound (— )-5 as an aqueous suspension caused regio- and enantioselective [2 + 2] photodimerization to afford the (— )-anti-head-to-head dimer 82 of 48% ee in 75% chemical yield (Scheme 18) [86]. Similarly, solid-state photolysis of the 3 2 complex of cycloocta-2,4-dien-l-one 83 with (R,R)-( — )-4 gave the (-)-anti-head-to-head dimer 84 in moderate optical yield [87]. 

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