Cyclophanes extended

The Cahn-Ingold-Prelog system is unambiguous and easily applicable in most cases. Whether to call an enantiomer (R) or (S) does not depend on correlations, but the configuration must be known before the system can be applied, and this does depend on correlations. The Cahn-Ingold-Prelog system has also been extended to chiral compounds that do not contain chiral atoms.A series of new rules have been proposed to address the few cases where the rules can be ambiguous, as in cyclophanes and other systems. ... 

Macrocycles containing isoxazoline or isoxazole ring systems, potential receptors in host—guest chemistry, have been prepared by multiple (double, triple or quadruple) 1,3-dipolar cycloadditions of nitrile oxides, (prepared in situ from hydroxamoyl chlorides) to bifunctional calixarenes, ethylene glycols, or silanes containing unsaturated ester or alkene moieties (453). This one-pot synthetic method has been readily extended to the preparation of different types of macrocycles such as cyclophanes, bis-calix[4]arenes and sila-macrocycles. The ring size of macrocycles can be controlled by appropriate choices of the nitrile oxide precursors and the bifunctional dipolarophiles. Multiple cycloadditive macrocy-clization is a potentially useful method for the synthesis of macrocycles. 

In ( )-49 it-electron conjugation between the two free poles is maintained through two tran.v-stilbcnc-likc bridges. As a result of this extended conjugation, the compound is red in solution, with an end absorption extending to 600 nm. This contrasts with the light-yellow color (end adsorption around 450 nm) of the hexakis adducts with a pseudo-octahedral addition pattern, in which the residual tt-electron chromophore is reduced to a benzenoid cubic cyclophane -type substructure [15,54],... 

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]. 

Cyclophane ruthenium complexes are of interest as possible monomeric components in the construction of multilayered organometallic polymers such as 262, which might show extended ir-electron delocalization (Scheme 25). 

The substituted [2.2]paracyclophanes prepared by the present procedure have proven to be useful starting materials for the synthesis of cyclophanes with extended aromatic ring systems,16 additional ethano bridges,17-18 and chromium tricarbonyl complexes.19... 

In the field of hydrocarbons, radical cations of extended 7t systems tend to form 7t dimers under appropriate reaction conditions. This was established for classical aromatics such as naphthalene or anthracene. In cyclophanes, the geometric restrictions force the aromatic constituents to interact through space. 

In an extension of the previous studies involving 13, it has been demonstrated that a number of other [2]-catenanes can be synthesised using self-assembly procedures in which the donor macrocyclic polyethers incorporate both hydroquinone and 1,5-dioxynaphthalene units, while the acceptor tetracationic cyclophanes contain bipyridinium and/or its extended analogue, bis(pyridinium)ethylene. Although the trans carbon-carbon double bond in the bis(pyridinium)ethylene units are... 

These happy coincidences meant that, if this unsymmetrical tetracationic cyclophane is concatenated with a symmetrical crown ether, there is every chance that the bipyridinium unit of the cyclophane will reside preferentially inside the cavity of the 71-electron-rich macrocyclic polyether. With a lower reduction potential than its extended counterpart, the bipyridinium unit will be reduced first in a voltammetric experiment, and hence it is expected that the cyclophane will circumrotate in order that the vinylogous bipyridinium recognition site - now a much better 7t-electron-receptor system than the radical cation of the bipyridinium unit - resides within the cavity of the crown ether. 

The rationale behind this design was justified upon electrochemical investigation of the [2]catenane 184+. This catenane - synthesized in 43% yield (Fig. 26) from crown ether BPP34C10, the bipyridinium dibromide derivative 192+ and ( )-l,2-bis(4,4 -bipyridyl)ethylene - was demonstrated to consist, in solution, of mainly co-conformer A, with the more powerful n-electron-accepting bipyridinium unit located inside the cavity of the crown ether. Upon electrochemical reduction of this bipyridinium unit, the cyclophane undergoes a circumrotational movement with respect to the crown ether such that the profoundly more electron-deficient 7t-extended bipyridinium unit resides inside the cavity of the crown ether, affording co-conformer B. When the bipyridinium radical cation is oxidized back down to its dicationic state, the opposite circumrotational process occurs and the system reverts back to co-conformer A, its ground state [49]. 

Homeomorphs of the two-point bicycle show up in an interesting collection of structures, ranging from the simple cage-type bicyclophane (73) [93, 94] to the extended homeomorphs by Lehn et al. [95], Vogtle et al. [96], and Moore et al. [97]. Other molecular representations are Moore and Bedard s flat molecular turnstile (74) [98], Hart and Vinod s cuppedophanes (75) [99-102], and Okazaki et al. s bowl-shaped bicyclic cyclophane (76) [103,104]. 

More than 90 years after the first reported synthesis of [2.2]metacyclophane [1] and nearly 50 years after the synthesis of [2.2]paracyclophane, [2] cyclophane chemistry is still a field of ongoing research. In the beginning, work had been focused on the development of new synthetic methods yielding cyclophanes and ansa-compounds and the investigation of their physical properties. Later, the scope was extended to the incorporation of heterocycles into phanes and the more sophisticated techniques allowed the synthesis of multibridged and multilayered phanes. All this has been extensively reviewed [3]. 

Over the last 10 years more complex cyclophane systems have become available by modern synthetic methods. Such extended systems make use of the special electronic and steric effects of cyclophanes in physical organic and coordi-... 

---