Resorcinarene Cavitands

2- diketones. The resulting fused pyrazines provide the deepened cavities. Diethyl 

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Fig. 40 Two resorcinarene cavitands held together by 16 hydrogen bonds with 8 bridging isopropanol molecules...

The versatility of resorcinarene-detived cavitands has generated a wealth of host molecules (Rebek 1999 Jasat and Sherman 1999 Hof et al. 2002 Rudkevich 2002 Laughrey et al. 2003 Shivanyuk et al. 2002 Barrett et al. 2002 Paek et al. 2003 Pirondini et al. 2003). Tetraquinoxaline-spanned resorcinarene cavitands 182 were introduced by Cram in the early 1980s (1983) and have been used extensively to study host-guest interactions (Moran et al. 1982 Dalcanale et al. 1989 Moran et al. 1991 Soncini et al. 1992 Careri et al. 1997 Bianchi et al. 2003). Of particular interest is therin to undergo a reversible thermal switch liom the vase to the kite conformation (Fig. 5.13). The vase exists at temperatures of 45 C and above, while the kite exists exclusively below —60 °C. 

Bridged resorcinarenes are known as cavitands (such as 31 or 32). However, cavitand 32 is not yet fully preorganized as it can give two main conformers, namely a cylindrical vessel or the open kite (Figure 3.16). 

The first approach to the formation of a ditopic cavitand complex has been reported by Sorrell et al. [56]. They identified the tri-bridged resorcinarene as a... 

This synthetic approach has been applied for the synthesis of cavitand 31b from the corresponding CH2CH2Ph-footed resorcinarene.)... 

The first example concerns the attempt to isolate a common reactive intermediate. An ordinary easily occurring reaction is the addition of primary amines to carbonyl groups to form imines. The main intermediate of this condensation reaction is a hemiaminal which can be observed only in special cases because the release of water proceeds very rapidly. A cavitand could be built which is able to increase the concentration of hemiaminal during the reaction and allows its observation by NMR spectroscopy (Scheme 10.1). Cavitand la is based on a resorcinarene-based bowl, a macrocycle easily available... 

Scheme 10.1 Stabilization of a hemiaminal in a resorcinarene-based cavitand...

Resorcinarenes have some degrees of freedom for rotation around the C—C bonds of the connecting CHa-groups, which results in five conformations, namely crown, boat, chair, diamond and saddle. The hydroxy groups at the wider rim must be linked in order to fix the receptor in the specific cavitand conformation and to improve binding properties. 

Increasing the wall of the resorcinarenes, as in IV-3, could improve the selectivity. In protic solvents, IV-3 could combine the selectivity of cavitands with the high affinity of the resorcin[4]arene. The cavitand is built up if the three diaminophenyl groups adopt an axial conformation. The cavity with an open side binds selectively alkylammonium ions complementary in size and shape (see Figure 12.1 and Table 12.4). 

The first deep-cavity cavitand to be synthesized was the 2.3-quinoxaline derivative 4 (Fig. 4). This second row of aromatic rings in this cavitand. and related cavitands formed by the condensation of resorcinarenes with 2,3-dichloropyrazines. is conformationally flexible. Thus, they exist in a range of conformers between the vaselike C4v form (shown) and a flat C21 form. These families of compounds were termed velcrands, because in the flat, "kite-like" form, they have a propensity to dimerize.When not binding themselves, these types of cal-itands also bind mono-" or di-substituted"benzene derivatives. 

An alternative to deepening the cavity in the manner of cavitand 4 comes in the form of the stereoselective bridging of resorcinarenes. The first successes in this regard were carried out with a number of phosphorus derivatives, e.g., Stereoselective bridging with... 

The bridging of vicinal arene units of resorcinarenes via their hydroxy functions leads to deeper and more rigid cavities in cavitands (23 and 24). Quinoxaline-... 

Fig. 4 Representatives of typical cyclophane host families calixarene (21), resorcinarene (22), cavitands (23, 24), cdrcerand (25). hemicarcerand (26), crown ether (27), cryptand (28), spherand (29). and hemispherand (30)...

Metal-induced capsular self-assembly 6 (Fig. 3) of two deep, resorcinarene-based cavitands was described by Dal-canale and coworkers. Two tetracyanocavitands were connected through four square-planar Pd(II) or Pt(II) entities in CH2CI2, CHCI3, and aeetone. Evidence of encapsulation of one triflate anion upon dimerization was obtained ( F-NMR, x-ray analysis). For the Pt(ll) case, the assembly process was shown to be reversible Et N dissociated the capsule 6. while the addition of trifluoro-acetic acid restored it. 

Examples of Co(II) and Fe(ll) resorcinarene-based cages 7 came from the group of Harrison (Fig. 3). These are water-soluble and formed by combining C0CI2 or FeCE in aqueous solution at pH >5 with the cavitand, functionalized with four iminodiacetic acid moieties. Upon synthesis, six water molecules initially occupy the... 

Starting from dodecyl-3,5-dihydroxyphenyl ketone, prepared in three steps in 14% overall yield, it was subjected to an acid-catalyzed condensation with pyrrole to give (19%) the calixpyrrole—resorcinarene hybrid 30, which with (1) 1,2-difluoro-4,5-dicyanobenzene in DMF at 85 °C in the presence of excess EtsN afforded (69%) the fourfold-bridged octacyanocalix[4]pyrrole 31 or (2) 2,3-dichloroquinoxaline in DMF using K CO to give (51%) cavitand 32 (14JOC5545). 

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