Paracyclophane complexes

A unique alternative to the traditional C2 symmetric atopisomeric motif uses a paracyclophane backbone for the placement of the phosphino groups. 4,12-Bis(phosphino)-[2.2]-paracyclophane complexes, abbreviated as PhanePhos (5), have been reported to be highly active in a few classes of asymmetric hydrogenation. The synthesis is shown in Scheme 12.51.159,160... 

The reaction of ( n -arene)(ti - 2.21paracyclophane)-complexes of nithenium(II) with hydride afforded (q6-l,3-diene)(Tj -(2.2]paracyclophane)-complexes of ruthenium(0) whidi on protonation with HBF4 gave a series of highly fluxional endocyclic agostic compounds. 

As an illustrating example for the application of the Friedel-Crafts acylation in the synthesis of complex molecules, its use in the synthesis of [2.2.2]cyclophane 13 by Cram and Truesdale shall be outlined. The reaction of [2.2]paracyclo-phane 10 with acetyl chloride gives the acetyl-[2.2]paracyclophane 11, which is converted into the pseudo-geminal disubstituted phane 12 by a Blanc reaction, and further to the triple bridge hydrocarbon 13 ... 

In 2004, Bolm et al. reported the use of chiral iridium complexes with chelating phosphinyl-imidazolylidene ligands in asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee [17]. These complexes were synthesized from the planar chiral [2.2]paracyclophane-based imida-zolium salts 74a-c with an imidazolylidenyl and a diphenylphosphino substituent in pseudo ortho positions of the [2.2]paracyclophane (Scheme 48). Treatment of 74a-c with t-BuOLi or t-BuOK in THF and subsequent reaction of the in situ formed carbenes with [Ir(cod)Cl]2 followed by anion exchange with NaBARF afforded complexes (Rp)-75a-c in 54-91% yield. The chela-... 

The crystal of 2 OPr recrystallized from EtOH/H20 solution, and the mixed crystal of the same ethyl and propyl cinnamate derivatives (2 OEt and 2 OPr), on photoirradiation for 2h at room temperature with a 500 W super-high-pressure Hg lamp, afforded the highly strained tricyclic [2.2] paracyclophane (2 OEt-2 OPr-cyclo) crystal quantitatively (Maekawa et ai, 1991b). A crystal structure analysis was carried out of a single crystal of the complex of 2 OEt-2 OPr-cyclo with HFIP (recrystallization solvent) in a 1 2 molar ratio. Fig. 13 shows the molecular structure of 2 OEt-2 OPr-cyclo viewed along the phenylene planes. The short non-bonded distances and deformation of the benzene rings, as seen in Fig. 13, are common to those of [2.2] paracyclophanes, as previously reported (Hope et ai, 1972a,b). 

A variety of other highly-strained electron-rich donors also form colored complexes (similar to homobenzvalene) with various electron acceptors, which readily undergo thermal cycloadditions (with concomitant bleaching of the color).209 For example, Tsuji et al.210 reported that dispiro[2.2.2.2]deca-4,9-diene (DDD), with an unusually low ionization potential of 7.5 eV,211 readily forms a colored charge-transfer complex with tetracyanoquinodimethane (TCNQ). The [DDD, TCNQ] charge-transfer complex undergoes a thermal cycloaddition to [3,3]paracyclophane in excellent yield, i.e.,... 

More recently, the water-soluble paracyclophane (263) was demonstrated to form crystalline complexes with a range of hydrophobic substrates under acid conditions (Odashima, Itai, Iitaka Koga, 1980). For example, with durene (264), a complex of stoichiometry [host.4HCl. durene.4H20] was obtained. The X-ray structure of this species indicates... 

Another interesting asymmetric epoxidation technique using metal catalysis involves the vanadium complexes of A-hydroxy-[2.2]paracyclophane-4-carboxylic amides (e.g., 19), which serve as catalysts for the epoxidation of allylic alcohols with f-butyl hydroperoxide as... 

Polymeric pseudocrown ether networks have been generated in situ by the photopolymerization of poly(ethylene glycol) diacrylate transition metal complexes <00CM633>, and the effect of metal ion templation was evaluated. The 1,6,13,18-tetraoxa[6.6]paracyclophane-3,15-diyne (termed pyxophanes) was prepared from hydroquinone and l,4-dichlorobut-2-yne it forms size-selective 7i-complexes with alkali metal cations <00CC2377>. Dibenzo[ ]crown-m have been used in numerous elegant studies in which they were the needles that were threaded by diverse reagents the resultant... 

They have also examined the formation of the ir-complex between the silacalixarenes and silver cation by FAB mass spectrometry. Similar 77-complex formation with silver cation was observed for hexasila [2,2,2]paracyclophanes 37 (41). 

Acid-promoted aquation of the binuclear complex Cu2L of the hexaaza macrocycle L = 2,5,8,17,20,23 -hexaaza[9.9]paracyclophane, whose half-life is of the order of a second, exhibits simple one-stage first-order kinetics. This is attributed to parallel reactions at each Cu(II) center having identical rate constants (305). The kinetics of dissociation of mono- and... 

To date the structure and reactivity of numerous complexes derived from aromatic compounds and nitrosonium cation have been studied (5, 56-63). However, relatively few studies are available on the nitrosonium complexes of cyclophanes (5, 57, 59, 61, 62), cf ref. (63). The interaction of [2.2]paracyclophane with nitrosonium tetrachloroaluminate was studied by H and 13C NMR spectroscopy using deuterium isotope perturbation technique (64). It was found that the resulting nitrosonium complexes containing one (25) or two NO groups (26) are involved in fast interconversion (on the NMR time scale) (Scheme 17). 

The affinity of [2.2]paracyclophane for nitrosonium cation is much greater than that of para-xylene, presumably owing to stacking interaction between the aromatic rings in the 7i-complex. Low isotope effect on the aromatic carbon... 

Figure 2. Optimized structures of the nitrosonium complexes of [2.2]paracyclophane at DFT/3Z. Distances are in angstroms.

By comparing the UV absorptionbands of [2.2]paracyclophanequinone (30), [4.4]paracyclophanequinone (31) 39>, an equimolar mixture of 2,5-dimethylbenzoquinone and p-xylene, 2,5-dimethylbenzoquinone, and [8]paracyclophane, Cram and Day 57> were able to assign to a charge-tranfer transition the band at 340 nm found only with the intra-molecularly complex molecule 30. 

Multilayered cyclophanes having three aromatic rings fixed in parallel planes above one another exhibit properties intermediate between those of the [2.2]paracyclophanes and the above-mentioned compounds 51 and 52. A cyclic compound of this type, (53), has apparently been isolated by Hubert 77>. The tetracyanoethylene complex of... 

A number of tetracyanoethylene (TCNE) n complexes of mono-substituted [2.2]paracyclophanes were examined both spectroscopically and kinetically. The position of max of the longest-wavelength charge-transfer band in the UV spectrum of the complexes, and in some cases estimated equilibrium constants 76> were used as a measure of the relative 7t-base strengths of the substituted cycles 15>. From the results of this investigation it was at once clear that complexes of tetracyanoethylene and [2.2]paracyclophane with electron-releasing substituents (Type A,... 

The electrophile E+ attacks the unhindered side of the still unsubstituted second aromatic ring. A proton (deuteron) is transferred from this ring to the second, originally substituted ring, from which it leaves the molecule. Thus, the electrophile enters, and the proton (deuteron) leaves the [2.2]paracyclophane system by the least hindered paths. Some migration of deuterium could be detected in the bromination of 4-methyl[2.2]paracyclophane (79). The proposed mechanism is supported by the kinetic isotope effects ( h/ d) found for bromination of p-protio and p-deuterio-4-methyl[2.2]paracyclophanes in various solvents these isotope effects demonstrate that proton loss from the a complex is the slowest step. 

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