Chiral compounds paracyclophanes

The development of ferrocene 9 was part of our studies on planar-chiral compounds, which also involved the synthesis of other scaffolds such as chromium-tricarbonyl arenes [15], sulfoximidoyl ferrocenes [16], and [2.2]paracyclophanes [17]. In aryl transfer reactions, however, ferrocene 9 proved to be the best catalyst in this series, and it is still used extensively today. 

Figure 4.3a shows a mono-substituted-[ ]paracyclophane as a typical planar-chiral compound. ... 

Thus, a slight enantiomeric imbalance in compounds induced by CPL was correlated for the first time to an organic compound with very high ee by asymmetric autocatalysis with amplification of chirality. Moreover, various chiral organic compounds such as 1,1-binaphthyl,[2.2]paracyclophanes, and primary alka-nols due to deuterium substitution have been found to serve as chiral triggers in asymmetric autocatalysis. 

The chirality of [2.2]paracyclophane derivatives has been deduced as being (—)(R) on the basis of the exciton theory of coupled oscillators 67) and confirmed by experimental results (see 2.9.1 and 2.9.3). In these compounds a negative Cotton effect at 270 nm (corresponding to the p-band) seems to be specific for the (R)-chirality 54). 

Bidentate oxazoline-imidazolylidene ligands, in which both units are linked by a chiral paracyclophane, have been studied in Bolm s group [129]. In this case, the planar chirality of the pseudo-orfho-paracyclophane is combined with the central chirality of an oxazoline (Scheme 48). Compounds 70 were tested in the asymmetric hydrogenation of olefins displaying moderate selectivity (ee s of up to 46% for dimethylitaconate in the presence of 70b). 

Apart from the role of cyclophanes as a model system for studying the electronic interaction between the aromatic moieties, chiral [2.2]paracyclophanes have also been utilized as planar chiral ligands in asymmetric catalysis. Recent advances and applications in this area have been reviewed [5, 6]. The synthesis of heterocyclic compounds based on [2.2]paracyclophane architecture, where the long-distance electronic communication and the planar chirality play significant roles in their application, has also been reported recently [7]. Although the preparation and application of chiral cyclophanes in asymmetric synthesis has attracted much attention for a long time, their chiroptical properties, especially the CD spectra, have rarely been paid attention or even completely ignored. 

The CD spectra of enantiopure pyridinophane 17 and bipyridine derivative 18 were reported. Both spectra were relatively complicated compared to those of substituted [2.2]paracyclophanes, apparently showing 6-8 positive/negative Cotton effect peaks. While the lAel value for the main band of compound 18 (20-30 M-1 cm-1) was reduced to half that of compound 17 (30-50 M 1 cm ), the spectrum of planar chiral bipyridine 18 was very sensitive to the added metal salts. The absolute configuration was determined by comparison with the theoretical spectrum at the DFT/SCI level [31]. 

Compound (35) is an example of a bipyridine ligand with planar chirality. This ligand was synthesized by reacting a cyano-substituted [2.2]paracyclophane pyridyl derivative with acetylene and cyclopentadienylcycloocta-l,5-dienecobalt in toluene by the Bonnemann reaction.133... 

The photocyclization reactions of acrylanilides have been researched for a number of years and mediation of the process by a chiral host has now been investigated. Irradiation of toluene solutions of (138) in the presence of the enantiomerically pure chiral lactam (139) produced a significant enantiomeric excess in the formation of the cis and trans cyclization products (140) and (141), respectively, at — 55°C. Under these conditions, the chemical yield is 66%, the respective ratio of (140) to (141) is 1 2.7 and the cis isomer is formed with an ee of 30%, while that of the trans is 57%. These enantiomeric excesses showed different temperature dependences and, whereas the values for (141) increased with reduction in reaction temperature, those for the cis isomer (140) increased to a maximum of 45% at — 15°C but decreased at lower temperatures. Experiments with deuterated compounds showed that the chiral lactam (139) influenced the protonation of the cis and trans forms of the reaction intermediate (142). This type of cyclization has been used to synthesize a variety of previously unknown heterophanes derived from [2.2]paracyclophane. For example, irradiation of... 

Formyl-5-hydroxy[2.2]paracyclophane (153) was used as a chiral auxiliary in the synthesis of a-amino acids [98]. The reported enantiomeric excess was in the range of 90-98%. Racemic 153 was first prepared by Hopf and Barrett [99]. To separate the enantiomers, their Schiff bases with the dipeptide (S)valyl-(S)valine was prepared. The diastereomeric copper(II) complexes of this compound show different solubility in 2-propanol. Alternatively they can be separ-... 

Recently the first use of the paracyclophane backbone for the placement of two diphenylphosphano groups to give a planar chiral C2-symmetric bisphos-phane was reported [102]. The compound 159 abbreviated as [2.2]PHANEPHOS was used as a ligand in Rh-catalyzed hydrogenations. The catalytic system is exceptionally active and works highly enantioselective [ 103]. The preparation of [2.2]PHANEPHOS starts with rac-4,12-dibromo[2.2]paracyclophane (rac-157), which was metalated, transmetalated and reacted with diphenylphosphoryl chloride to give racemic bisphosphane oxide (rac-158). Resolution with diben-zoyltartaric acid and subsequent reduction of the phosphine oxides led to the enantiomerically pure ligand 159. 

Figure 1.7. Axially chiral molecules (a)-(d) and a molecule with a chiral plane (e). (a) Dichlorallen, (b) twisted biphenyl, (c) helicene, (d) highly twisted aromatic compound [25], and (e) paracyclophane.

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