Planar chiral complexes

Among the numerous nickel dithiophosphates, the chiral complexes deserve to be cited.41 Unusual trinuclear Pd3(p3-E)2[S2P(OR)2]2(PPh3)2 with E = S, Se, R = Et, Pr, Pr, Bu, have been prepared and Pd3(p3-S)2[S2P(OEt)2]2(PPh3)2 revealed a structure containing three distorted square-planar Pd atoms and sharing two p3-S ligands.42... 

Prochirality Planar molecules possessing a double bond such as alkenes, imines, and ketones, which do not contain a chiral carbon in one of the side chains, are not chiral. When these molecules coordinate to a metal a chiral complex is formed, unless the alkene etc. has C2V symmetry. In other words, even a simple alkene such as propene will form a chiral complex with a transition metal. So will trans-2-butene, but cis-2-butene won t. If a bare metal atom coordinates to cis-2-butene the complex has a mirror plane, and hence the complex is not chiral. It is useful to give some thought to this and find out whether or not alkenes and hetero-alkenes form chiral complexes. One can also formulate it as follows complexation of a metal to the one face of the alkene gives rise to a certain enantiomer, and complexation to the other face gives rise to the other enantiomer. 

Metal salen complexes can adopt non-planar conformations as a result of the conformations of the ethane-1,2-diyl bridge. The conformations may have Cs or C2 symmetry, but the mixtures are racemic. Replacement of the ethylenediamine linker by chiral 1,2-diamines leads to chiral distortions and a C2 chiral symmetry of the complex due to the half-chair conformation of the 5-membered ring of the chelate. Depending on substitution at the axial positions of the salen complex, the symmetry may be reduced to Q, but as we have seen before in diphosphine complexes of rhodium (Chapter 4) and bisindenyl complexes of Group 4 metals (Chapter 10) substitution at either side leads to the same chiral complex. Figure 14.10 sketches the view from above the complex and a front view. 

The general synthesis of the Daniphos ligands starting from enantiomerically pure [(R)-l-(phenylethyl)dimethylamine]chromiumtricarbonyl 1, is depicted in Scheme 1.4.1 [15]. A directed ortho-metallation (DOM) and subsequent quench with a chlorophosphine leads to an enantiomerically pure planar-chiral complex, which after chlorination using ACE chloride (1-chloroethyl chloroformate) is transformed into the desired diphosphine by a nucleophilic substitution without any loss of optical purity (Scheme 1.4.1) [6, 10]. 

Complexes of unsymmetrically substituted conjugated dienes are chiral. Racemic planar chiral complexes are separated into their enantiomers 84 and 85 by chiral HPLC on commercially available /f-cyclodextrin columns and used for enantioseletive synthesis [25]. Kinetic resolution was observed during the reaction of the meso-type complex 86 with the optically pure allylboronate 87 [26], The (2R) isomer reacted much faster with 87 to give the diastereomer 88 with 98% ee. The complex 88 was converted to 89 by the reaction of meldrum acid. Stereoselective Michael addition of vinylmagnesium bromide to 89 from the opposite side of the coordinated Fe afforded 90, which was converted to 91 by acetylation of the 8-OH group and displacement with EtjAl. Finally, asymmetric synthesis of the partial structure 92 of ikarugamycin was achieved [27],... 

The catalytic efficiency of the diamines, 24 and 25, derived from the truncated cinchona alkaloids, quincorine and quincoridine, respectively, for the desymmetrization of meso- 1,4-diols was also investigated by Kiindig and coworkers [31a, b[. Both pseudoenantiomers 24 and 25 efficiently catalyzed the desymmetrization of the meso-complex 26 with benzoyl chloride, giving the enantiopure monobenzoylated Cr (CO)3 complexes, 27 and ent-27, respectively, with up to 99% ee (Scheme 11.17). This process will provide easy access to new planar chiral complexes. 

An asymmetric variant makes use of a planar chiral complex. This arene to substituted cyclohexenone conversion has been elegantly used in a synthesis of (-F)-ptilocaulin. (Scheme 5 see also Chap. 8) [22]. The sequence is unusual in that after nucleophilic addition, acid free ClSiMe3 is added. When a strong acid... 

A third mode of asymmetric induction is offered via planar chiral complexes. Diastereoselective hthiation and enantioselective lithiation provide efficient... 

The key step involves an electron transfer-mediated benzylic umpolung of the (central-chiral) complex 25 by means of the one electron reducing agent LiDBB (lithium di-tert-butyl diphenyl). Obviously, both the formation of the (planar-chiral) reactive intermediate 26 and the subsequent alkylation proceed in a stereo-controlled manner with overall retention of configuration. 

As demonstrated by Schmalz and coworkers, arene chromium chemistry offers unique and highly efficient entries to the aglycones of such compounds. The most successful approach [25, 26] follows the retrosynthetic analysis shown in Scheme 6, where the pseudopterosin aglycone (31) derives from a seco-com-pound of type 32. Such intermediates can be traced back (via 33) to the planar-chiral complex 34 carrying the absolute stereochemical information. 

The overall synthesis (Scheme 7) is in excellent competition in terms of yield and selectivity. The expenditure connected to the (stereoselective) introduction of the metal fragment in the beginning pays off tremendously, as both the chemical and stereochemical effects of the Cr(CO)3 unit are exploited in several key transformations. Actually, all new (lasting) stereocenters are established with virtually complete diastereoselectivity under the influence of the planar-chiral complex substructure. 

As with atropisomeric biaryls, axial chirality in atropisomeric amides maybe introduced by stereochemical control in the atroposelective reactions of planar chiral complexes [115]. Enantioselective lithiation was reported in this context by Uemura, who showed that the achiral complexes 195,198,201 and 204 are de-protonated enantioselectively by treatment with chiral lithium amide bases (Scheme 50) [116-118]. The stereogenic C-C and C-N axes in these compounds are orientated such that the larger NR2 and acyl groups, respectively, are directed away from the chromium. A range of chiral lithium amides was investigated, and by careful selection it was possible to obtain products 196,199,202 and 205... 

Lormann, M.E.P., Nieger, M. and Brase, S. (2006) Desymmetrisation of a bicy-clo[4.4.0]decadiens A planar-chiral complex proved to be most effective in an as5mmetric Heck reaction. J. Organomet. Chem., 691, 2159-61. 

Helical Chirality Induced by the introduction of helical motifs such as peptides in planar chiral complexes [20] or helical arrangements of the chelating ligands (Scheme 3.1). 

Interactions between DNA and metal complexes are the basis for the use of square-planar platinum(ll)-containing anti-cancer dmgs. (a) Explain how the interaction of right-handed DNA with chiral complexes leads to diastereoisomeric species. 

In the last decade a substantial proportion of research has been focussed on the design and implementation of efficient routes to optically active complexes possessing planar chirality. This fact, in concert with both the ability of the chromium tricarbonyl unit to block effectively one face of the aromatic nucleus and its ease of removal, has led to a rapid increase in the use of chiral complexes as intermediates in asymmetric synthesis. ... 

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