Planar chiral compounds hydrogenation

Because a comprehensive review on the catalytic performance of Josiphos ligands has been published,20 we restrict ourselves to a short overview on the most important fields of applications. Up to now, only the (7 )-(S)-family (and its enantiomers) but not the (R)-(R) diastereoisomers have led to high enantioselectivities (the first descriptor stands for the stereogenic center, and the second stands for the planar chirality). The most important application is undoubtedly the hydrogenation of C = N functions, where the effects of varying R and R1 have been extensively studied (for the most pertinent results see Table 15.5, Entries I—4). Outstanding performances are also observed for tetrasubstituted C = C bonds (Entry 5) and itaconic and dehydroamino acid derivatives (Entries 6 and 7). A rare example of an asymmetric hydrogenation of a heteroaromatic compound 36 with a respectable ee is depicted in Scheme 15.6.10b... 

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). 

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. 

So far, no example of reduction has been reported in the literature using a catalytic system relying on planar chirality. However, ansa-derivatives of Hantzsch esters were developed by Kanomata and Nagata [58] as NADH model compounds (Scheme 8.20). These reagents displayed excellent hydrogen transfer abilities-reduction of carbonyl compounds was achieved with almost perfect enantioselec-tivily. There is no doubt that, coupled with a proper metal-based hydride-transfer system, such NADH models could perform well in a catalytic version. 

Mainly (i ) Deuterium compounds with axial or planar chirality, (ii) Chirality due to isotopes of atoms other than hydrogen. (See also Z5 l)... 

Molecules with shapes analogous to screws are also chiral, since they can be right-handed or left-handed. There are several kinds of molecules in which steric factors impose a screwlike shape. A very important case is 1, T-binaphthyl compounds. Steric interactions between the 2 and 8 hydrogens prevent these molecules from being planar, and as a result, there are two nonsuperimposable mirror image forms. 

If there is a chiral center bearing a hydrogen atom adjacent to a carbonyl group, racemization is often easy and rapid. Consider the enolization of compound 17.19. The enol is planar, and cannot remember which enantiomer it used to be, so it may be reprotonated on either face of the double bond to give either enantiomer. If this compound is subjected to acidic or basic conditions, it will be rapidly racemized. This type of racemization will be of concern to us again when we come to look at peptide synthesis. 

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