Cyclophane ortho, para

For example, consider [2,3]-ortho,para-cyclophane (Figure 19). Before presenting the nomenclature assignment for this molecule, it is instructive to examine both the current traditional IUPAC "organic"... 

Cyclic thiourea derivatives having three different types of cyclophane structure (ortho-meta, meta-meta and meta-para) and a lariat-type thiourea have been synthesized, using reaction of isothiocyanate with amine.230 Cyclic thioureas have been obtained from formaldehyde aminals and sulfur (Sg) (Scheme 77).231... 

The photorearrangement of 7V-aryl lactams 34 to the ortho-position offers an interesting possibility for the preparation of cyclic benzo-aza-ketones of type 35, whereas the rearrangement to the para-position should lead to para-cyclophane 144 formation. 7-, 8-, and 13-Membered ring jV-aryl lactams (34, n = 5, 6, and 11) were successfully rearranged in ethanol to 35 in chemical yields of 60, 83, and 80%,13 and in quantum yields of 0.071, 0.11, and 0.082, respectively.14... 

The above successful incorporation of the tetra-substituted benzene ring to produce the fused tetracationic cyclophane units, relies on being able to react the former in a para/para fashion rather than in an ortho/ortho or meta/meta one. The present product represents the successful interlinking of three major components. If one considers that a one-step synthetic procedure was employed, then it is a total of five molecular species that are, in fact, involved in the self-assembly process. 

Aldol reaction and alkylation of ketones at the a-position via enol silyl ethers (11) under neutral conditions are good examples [8]. Ortho-quinodimethane (15) is afforded at room temperature and undergoes [4 + 2] cycloaddition [10]. In contrast, the thermal ring opening of benzocyclobutane requires a much higher temperature (180-190°C) [11]. The fluorine-mediated Si—C activation protocol was the preferred method for the formation of tetrafluoro-para-quinodimethane (19), which then dimerizes to octafluoro-para-cyclophane (20) [12]. Azomethine ylide (22) can be generated from 21 byAgF catalysis and is used for heterocycles syntheses [13]. 

In fact, high-temperature pyrolysis of both cyclophanes does give styrene (5). The principle of microscopic reversibility suggests that the xylylenes interconvert with the tolylmethylenes. Thermodynamics favor the xylylenes over the tolylmethylenes, and ort/K>-tolylmethylene certainly goes to ortho-xylylcaG (10). The loop mechanism with the interconversion of the xylylenes and the tolylmethylenes explains the minor labels in the meta and para positions of styrene (5). 

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. 

In the same catalysis, alkyne trimerization was also possible, albeit in low yield. The choice of monoyne was important 4-octyne selectively gave a para-cyclophane, but dimethyl acetylenedicarboxylate (DMAD) gave an ortho cycloadduct, which is not cyclophane (Scheme 8.10) [7b]. 

Diynes with a polyether tether were appropriate substrates, and cycloadducts were obtained in good yield by using an excess amount of DMAD (Scheme 8.13) [10b]. The ortho/meta/para ratio was altered by the tether structures, and selective meta-cyclophane synthesis could be achieved. 

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