Cyclophane conversion

In the synthesis of l,3-dithiolan-2-ones from spirocyclic intermediates, via episulfides, substituted tetrathiacyclododecane and the related pentathiacyclopentadecane were isolated in good yields <96JCS(P1)289>. Preparation and molecular dynamics studies of 2,5,8,17,20,23-hexathia[9.9]-p-cyclophane have been reported <96P4203>. The syntheses and properties of thiocrowned l,3-dithiole-2-thiones and their conversion to tetrathiafiilvenes via treatment with triethylphosphine have been described <96LA551>. 

A copper-mediated cyclization of metallated thiophenes has been utilized to prepare polycyclic thiophenes and thiophene cyclophanes. Treatment of dibromide 106 in succession with M-butyllithium (halogen-metal exchange), zinc chloride (transmetallation), and copper chloride gave 7//-cyclopcnta[ 1,2-fc 4,3-6 dithiophene (107) <00H(52)761>. This conversion has also been achieved using a palladium-mediated cyclization performed in the presence of hexamethylditin . Copper-mediated cyclizations have also been applied to the syntheses of cyclopenta[2,l-6 3,4-A ]dithiophen-4-one (108) (three steps from 73) <00S1253> and cyclophane 109 <00CC2329>. 

Scheme 1.4 Conversion of cyclophane 3 into in the gas phase in laser desorption mass spectrometry.

The parent compound (92a) has been synthesized from the tetra-ketone (90). Addition of hydrazine afforded the locked metahetero-cyclophane (91a). Conversion of 91a to 91b could be accomplished by palladium-on-charcoal, further dehydrogenation to 92a requiring palladium-on-charcoal in nitrobenzene or DDQ in toluene.110,111... 

Table 2 Kinetic and thermodynamic parameters for the conversion of compound 93 + into cyclophane 104+ in CH3CN at 335.0 K60...

The scope of this new approach to extended cyclophanes is further demonstrated by the conversion of the tetraethynyl derivatives 126 to the [10.10]cyclophane 128, the reactive [5]cumulene 127 formally being passed en route. The yields for the latter three reactions are low (between 20 and 30 %), but are still acceptable considering what has been accomplished [74]. 

A Merck group reported an interesting kinetic resolution of a racemic di-bromocyclophane via Pd-catalyzed amination [91]. While BINAP was a poor ligand for the reaction in terms of selectivity, the C2-symmetric cyclophane-derived PHANEPHOS (17) proved to be optimal. Reaction of the cyclophane derivative with benzylamine afforded the unreacted dibromide in 45% ee after 37% conversion, corresponding to a selectivity factor of 12, Eq. (78). 

An interesting reaction that has been developed over the past decade is the application of (2 + 2)-cycloaddition reactions to the synthesis of cyclophanes . One of the earliest examples of this is the selective conversion of the bis(arylalkenes) 112 into the adducts 113. The yield of product is dependent to some extent on the chain length separating the aryl groups and the best yield of 41% is obtained when the separation includes four methylene units (n = 4). Lower yields are recorded with the other derivatives. Mixtures of products are formed when the m-isomers 114 are used. This affords 115 and 116. The yields of these are better than those obtained from the p-isomers lll l. Nishimura and coworkers have examined the ease with which such cyclobutanes, e.g. 115, n = 2,... 

Unexpected results were obtained when chromenone-based cyclophanes was treated with DBU to achieve HNO elimination, followed by DDQ oxidation to give benzo[fo]furan-based cyclophanes as shown below <05TL8789>. DDQ oxidative conversion of (Q-p-[2-hydroxyphenylethylene]benzeneethanol into 2-phenylbenzofuran was also reported . In addition, an unusual rearrangement of substituted 2-phenylbenzo[d]pyrrolo[3,2-fo]pyrylium perchlorate to 2-phenylfuro[23-c]isoquinoline was observed <05SL1036>. 

Fig. 16. A photo- and electrochemically controllable molecular shuttle. The unperturbed rotaxane 116+ exists preferentially in the translational isomer in which the BPP34C10 crown ether resides around the bipyridinium unit, a Photochemical excitation of the Ru(bipy)3 unit results in PET to the bipyridinium site, and consequent translation of the crown ether to the 3,3dimethylbipyridinium unit, which is a less efficient recognition site for the cyclophane CBPQT4+ than a bipyridinium system. This process occurs only in the presence of a sacrificial reductant which reduces the Ru(III) center back to its Ru(II) state in order to prevent charge recombination, b Conversely, upon electrochemical reduction of the bipyridinium unit, the crown ether takes up residency around the 3,3 -dimethylbipyridi-nium site. This process is reversed through electrochemical oxidation of the bipyridinium radical cation back to the dication...

A further account of the intramolecular hydrogen abstraction processes within the cyclophanes of the type shown as (70) with a variety of linkers has been published. The irradiation brings about the conversion into the products such as (71) by a 1,6 hydrogen transfer. The yields are variable and are shown below the structures. Other studies by Park and his co-workers have reported other cyclizations using excitation at 350 nm in benzene. These results are shown in Scheme 2. As can be seen, excitation results in 6-hydrogen abstraction from the side chains, and the resultant 1,5-biradicals undergo ring closure to yield the diols. These products are readily dehydrated to afford the difuran derivatives in 40% overall yield. The latter compounds were used in reactions to synthesize novel cyclophanes. ... 

Non-benzoid phanes have always played an important role in cyclophane chemistry and many structures have been prepared with other Hiickel aromatic rings such as azulene or tropone [49]. A cyclophane with two bridged annulene units was recently synthesized by Mitchell [50,51 ]. Dimethyldihydropyrene (76), an excellent NMR probe first introduced by Boekelheide [52], was converted into the dialcohol 77 in three steps. Reaction with adipoyl chloride afforded the large [10.10]cyclophane 78. Unfortunately the conversion of the dialcohol into the corresponding dibromide 79, an obvious precursor to the interesting phane 80, has failed so far. 

Among the first to exploit totally synthetic water soluble host compounds to catalyze chemical reactions were Tabushi et. al. who found an accelerating influence of their newly developed polyammonium cyclophane 1 on the hydrolysis of aromatic chloroacetates These authors showed conclusively that a rapid association of substrate and 1 proceeds the rate limiting attack of solvent on the ester group. This step is amenable to buffer catalysis, too. Some relevant rate data are given in Table 1. The evaluation of these data now depends largely on definitions. Tabushi et al. chose to view their results in terms of a kinetic sul trate specificity manifested in ratios. As a corollary they state a marked specificity in the conversion of substrates 2-4. This is formally corr t but it bears the danger of misinterpretation and is certainly seductive to draw faulty conclusions. 

We could easily test for para-xylyltnt and mera-xylylene conversion to styrene, which would imply conversion of the xylylenes to the corresponding tolylmethylenes. Pyrolysis of [2.2]para-cyclophane gives para-xylylcnc and by inference [2.2]meto-cyclophane pyrolysis should give meta-xylylene. At 930 C, both cyclophanes ought to give styrene if the xylylenes convert to the tolylmethylenes. 

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