Uncharged Catalysts The Cryptands

Herriott and Picker, in their catalyst evaluation report [19b], indicated that dicyclohexyl-18-crown-6 was as efficient a catalyst in a two-phase system (see above) as any quaternary compound. Unfortunately, 18-crown-6 was not studied. It is the authors expectation that 18-crown-6 (5) will be nearly as good a catalyst in most applications, both solid-liquid and liquid-liquid as the dicyclohexyl compound will be, but its ready availability makes it the catalyst of choice in most applications. 

The crown ether called 18-crown-6 is prepared by reaction of triethylene glycol with triethylene glycol dichloride in the presence of base [46] or by the cyclo-oligomerization of ethylene oxide [51]. Purification is effected by distillation and crystallization. The sequence is shown in equation 1.19. 

One final note regarding the use of crown ethers as phase transfer catalysts there is little literature which directly compares quaternary ammonium catalysts with crown ethers in liquid-liquid processes (see Sect. 1.10) [48]. There are examples where both have been tried and are effective. In general, however, it appears that for solid-liquid phase transfer processes, the crowns are far better catalysts than are the quaternary ammonium ions. In order for a solid-liquid phase transfer process to succeed, the catalyst must remove an ion pair from a solid matrix. The quaternary catalysts have no chelating heteroatoms with available lone pairs which would favor such a process. The combination of a quaternary catalyst and some simple coordinating amine or ether would probably succeed [28, 32, 34]. It seems likely, as mentioned above, that it is the combination of diamine and quaternary catalyst generated in situ which accounts for the success of Normanf s catalysts [28]. It is interesting to speculate on the possibility of using a quaternary ammonium compound and a drop of water as a catalytic system. 

One of the most interesting developments in recent years is the preparation of a variety of macrobicylic aminoethers by Lehn and his coworkers [49]. Their similarity to the crown ethers and to Simmons in-out bicyclic amines [50] is apparent. In a sense these cryptates combine the properties of both in that they can complex and encapsulate species within their cavities. Their binding constants with metal ions. 

The commonly used 2.2.2-cryptate is prepared by condensing triethylene glycol dichloride with tosylamide [49]. This affords the doubly N-protected macrocyclic aminoether 4. Detosylation yields the compound containing secondary nitrogen atoms (5). Double amide formation at high dilution affords the bicyclic structure 6 which can be reduced to the desired cryptate, 7. The sequence is formulated in equations 1.20-1.22. 

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