Cyclophane, tripodal

Interesting planar chiral tripodal cyclophanes have also been synthesized with high enantioselectivity by the cationic rhodium(I)/Me-Duphos complex-catalyzed [2+2+2] cycloaddition of branched triynes (Scheme 21.25) [29]. 

SCHEME 21.25 Enantioselective synthesis of planar chiral tripodal cyclophanes. 

Cyclotriveratrylene, by analogy with calixarenes, cyclodextrins and a few other cyclophane-type compounds, can be used as a matrix to preorganize coordination sites for transition metals, either at the edge of the rigid structure, or around an expanded CTV s cavity. Achiral 2- and 3-pyridine tripod ligands have been synthesized together with a bipyridine multichelate. Studies of these new ligands, both by... 

Patterns of [2 - - 2 - - 2] cycloaddition for the synthesis of cyclophane are depicted in Scheme 8.1. Intermolecular reaction of diyne and monoyne can provide ortho, meta, and para isomers as dipodal cyclophanes (pattern A). Linear triyne can be transformed into ortho-ortho and ortho-meta isomers by an intramolecular reaction (pattern B). In the reaction of branched triyne, symmetrical 1,3,5- and unsymmetrical 1,2,4-isomers can be obtained as tripodal cyclophanes (pattern C). The choice of catalyst and tether is very important for induction of the aforementioned regioselectivities. 

Shibata et al. disclosed the reaction of branched triynes in which a 1,6-diyne moiety and alkyne are connected by a rigid 2-aminophenyl tether. An intramolecular [2 + 2 + 2] cycloaddition gave tripodal cyclophanes in high yield with excellent ee (Scheme 8.18) [13a]. It is noteworthy that the [15]cyclophane skeleton (n = 10) can be constructed efficiently without racemization. This is the first example of the enantioselective synthesis of tripodal cyclophanes. 

Enantioselective reaction was achieved by cationic rhodium/chiral diphosphine complexes. The intramolecular reaction of linear triynes gave planar-chiral meta-cyclophanes in excellent ee. The intermolecular reaction of diynes with alkyne afforded planar-chiral para-cyclophanes with strained benzene ring in moderate to excellent ee. The intramolecular reaction of branched triynes gave planar-chiral tripodal-cyclophanes in excellent ee. 

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