Inverse crown compounds

A remarkable series of inverse crown compounds featuring organic guest moieties encapsulated within host-like macrocyclic rings composed of sodium/potassium and magnesium ions together with anionic amide groups, such as TMP- (TMPH = 2,2,6,6-tetramethylpiperidine) and Pr NT (Pr NII = diisopropylamine), have been synthesized and characterized. Table 12.8.1 lists some examples of this class of inverse crown molecules. 

This section is limited to complexes which have a group 1 metal in conjunction with another, different main group metal, but also includes Cu and Cd since they exhibit properties akin to their main group analogs. It is also limited mainly to those complexes in which the metals find themselves attached to different atoms and there is a particular emphasis on compounds with alkali metal-carbon bonds of various types, except where the evolution of inverse crown complexes is discussed. There are many more heterobimetallic-heteroatom complexes (e.g., mixed metal amides), but these lie outside the scope of this current review though references may be found to them in the references for the complexes described herein. 

This compound represents an example of an inverse crown ether stmcture (Lewis acidic host-Lewis basic guest macrocyclic heterometallic alkoxides or amides/ . 

Fig. 38). In this complex, benzene has been converted to a 1,4-dianion. Comparing this complex to compounds like 57, it seems that the addition of TMEDA has resulted in the formation of a molecule akin to an open inverse crown molecule (with the extrusion of neutral NaTMP). The TMEDA ligands chelate to the Na cations. 

For crown ethers in which the H-nmr probes Ha and Hb are diastereotopic in the free crown ether too, the cation exchange process (50) becomes more complicated. For 1 1 complexes the measured free energy of activation also involves conformational inversion components (AG = AG + AG ), whereas exchange between free and complexed crown ether (2 1 ratio of crown ether to salt) only involves the free energy of activation for decomplexation (AG ). Bradshaw et al. (1979b) have recently reported AG -values for cation exchange in RNH3X complexes of crown ethers [257]. The AGf term was estimated to be 0.7, 0.5, and 1.2 kcal mol-1 for compounds [257 n = 1] with... 

That Robinson s and Ewins compounds did in fact have the trimer structures 1 and 2, respectively, was only established 50 years later by the works of Lindsey [3], Erdtman et al. [4], and Goldup et al. [5], The name cyclotriveratrylene (CTV) was given by Lindsey to 1, which was shown by NMR to adopt the locked crown conformation shown on the stereoformula and later evidenced by X-ray crystallography (Fig. 1) [6, 7], More recently [8], the inversion barrier of the crown in 1 was found to be ca. 26.5 kcal mol 1 from the racemization rate of the isotopically chiral C3-cyclotriveratrylene-d9 (3) relatively uniform values (AG 26.5 to 27.2 kcal/mol) were found [9] for a variety of chiral cyclotriveratrylenes, including the parent compound 4, C3-cyclotribenzylene-d3. Thus, in these compounds, the half-life of a crown conformer in solution is around one month at 20 °C, a few minutes at 100 °C and less than 0.1 second at 200 °C. 

A crown-family conformation also cannot explain the nmr spectrum of the acetonide of five-membered ring fused to the cyclooctane ring. 46) A temperature-independent spectrum would be predicted, because a im s-fused five-membered ring can only be located at equatorial positions in crown-family conformations, and ring inversion is therefore prohibited. The nmr spectrum of this compound is actually strongly temperature-dependent at about — 70°C, thus excluding any conformation in the crown family, at least as the sole conformation. 

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