We are currently working toward investigating the use of larger guests, and more interestingly, multiple guests as templates. We have prepared a large trimer carceplex, 12o(DMF)3, whose synthesis involves bis-benzylation of tetrol 1 (available in 10 gram quantities) in opposing "A,C" positions to give diol 2 (20% yield), followed by cyclization to trimer 7 (40% yield). Removal of the benzyls followed by capping using 1,3,5-tris-bromomethylmesitylene in DMF in the presence of Cs2CO3 and KI yields 12o(DMF)3 in 40% yield. (Methyls have been omitted from the mesitylenes of the drawings of 12 for clarity.) This is a very large carceplex, considering that all previous carceplexes only entrap one molecule of DMF. In addition, no carceplex or hemicarceplex has been reported that contains three guest molecules of any kind. DMF cannot escape 12 even after prolonged heating in solution; thus 12 is indeed a carceplex and contains only very small pores. Variable temperature experiments reveal that the DMFs reside in three unique environments, and the overall environment is a cross between the gas and liquid states.
In preliminary results we have found that the formation of carceplex 12 can be templated by single molecules, a pair of molecules, and sets of three molecules. Competition experiments suggest that single molecules are better templates that multiple molecules, at least at higher temperatures. It appears that multiple molecules may be favored at lower temperatures, which indicates that three molecules may be better able to complement the nooks and crannies of the cavity (better van der Waals contacts, higher enthalpy of complexation) but the entropy of organizing three molecules must be overcome (at lower T).
Chopra, N.; Sherman, J. C. Angew. Chem. Int. Ed. 1997, 26, 1727-1729.
Chopra, N.; Sherman, J. C. Angew. Chem. Int. Ed. 1999, 38, 1955-1957.