Other Chiral Backbones

The chirality of other structures has been used to prepare and separate P-stereogenic compounds. Some of these compounds are discussed in this section. 

Gomez and co-workers reported the preparation of tetradentate NPPN ligands 193. They were obtained as diastereomeric mixtures by reaction of diphosphides 191 with o-chlorooxazolines 192 in good yields. After careful recrystallisations of nickel complexes and decomplexation, two optically pure ligands could be obtained. Their coordination behaviour towards palladium was studied.  

Secondary phosphine 195 was prepared from bromide 194 (derived from (P)-camphor) as an equimolar mixture of epimers. Upon liberation of the ketone group under acidic conditions, nucleophilic addition of the secondary phosphine borane took place, giving (/ p)-196 as the sole compound. Interestingly, in non-polar solvents the phospholane ring opened affording 197 as a mixture of epimers. 

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The element of planar chirality plays a pivotal role in many modern ligand systems. The particularly huge success of ferrocenyl ligands has not been matched by any other chiral backbone to date. Metallocene and metal-arene-based ligand backbones exhibit the common feature that they become planar chiral only upon addition of (at least) two substituents on one ring fragment. [2.2]Paracyclophanes, however, need only one substituent (Fig. 2.1.3.1) to be chiral. 

Despite the planar conformation of nitrogens, several possibilities are available for the introduction of chirality. It is possible to prepare benzim-idazolylidenes, triazolylidenes, imidazolylidenes or unsubstituted-backbone imidazolinylidenes with a stereogenic center on one or two N-substituents (carbenes I and II). The other possibility is to relay the imidazolinylidenes backbone stereogenecity via the N-substituents or to combine stereogenic N-substituents with a chiral backbone (carbenes III or IV). It is at least possible to prepare bis-carbenes of type V with one (or two) stereogenic link between the two carbenes (Fig. 7). 

In 2004, a series of other chiral thioether-phosphine ligands based on a cyclopropane backbone were evaluated in the rhodium-catalysed hydrogenation of a dehydroamino acid by Molander el al As shown in Scheme 8.2, even if these ligands were generally active, only moderate enantioselectivities of up to 47% ee were obtained. 

Recently, a highly effective new ligand, DuPHOS122, has been introduced. With two clcctron-rich phosphorus atoms embedded in C2-symmetric phospholane rings, its structure is fundamentally different from previously developed ligands. In contrast to other diphosphanes with a chiral backbone, such as Chiraphos or Diop, the stereogenic centers of DuPHOS are located in proximity to the coordination sphere. 

Although many questions are still open, peptide nucleic acids are easier to synthesize via simple reaction routes than is natural RNA. The PNAs have another important advantage they are achiral and uncharged, i.e., they contain no chiral centres in the polymeric backbone (see Sect. 9.4). Unfortunately, however, they do not fulfil all the necessary conditions for molecular information storage and transfer. Thus, the search for other possible candidates for a pre-RNA world continues. 

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