Planar chiralities

Stnicture/chiroptics relationships of planar chiral and helical molecules, in particular of heteracyclophanes and heterohelicenes 98EJ01491. 

Sandorfy, C. Vibrational Spectra of Hydrogen Bonded Systems in the Gas Phase. 120,41-84 (1984). Schlogl, K. Planar Chiral Molecural Structures, 125, 27-62 (1984). 

High enantiomeric excesses for the addition of chloride to meso-epoxides were also obtained with use of a planar-chiral pyridine N-oxide 17 developed by Fu... 

Due to the inherent unsymmetric arene substitution pattern the benzannulation reaction creates a plane of chirality in the resulting tricarbonyl chromium complex, and - under achiral conditions - produces a racemic mixture of arene Cr(CO)3 complexes. Since the resolution of planar chiral arene chromium complexes can be rather tedious, diastereoselective benzannulation approaches towards optically pure planar chiral products appear highly attractive. This strategy requires the incorporation of chiral information into the starting materials which may be based on one of three options a stereogenic element can be introduced in the alkyne side chain, in the carbene carbon side chain or - most general and most attractive - in the heteroatom carbene side chain (Scheme 20). 

A bidirectional benzannulation of the axial-chiral biscarbene complex 47 affords a bis-Cr(CO)3-coordinated biphenanthrene derivative 48, which combines elements of axial and planar chirality [49] (Scheme 31). Four diastereomers are formed in moderate diastereoselectivity, two of which have been isolated as the major isomers. 

Organometallic aldehydes can be reduced enantioselectively with dehydrogenases. For example, optically active organometallic compounds having planar chiralities were obtained by biocatalytic reduction of racemic aldehydes with yeast [22c,d] or HLADH [22e] as shown in Figure 8.29. 

Figure 8.29 Reduction of organometallic aldehydes to produce alcohols with planar chiralities [22c,e].

Scheme 8 Intramolecular reductive coupling of a planar chiral diimine...

Other types of new AT-containing ligands have been described as effective chiral inductors for copper-catalyzed asymmetric cyclopropanation. Hence, Fu and Lo [42] prepared a new planar-chiral hgand, namely the C2-symmetric bisazaferrocene (structure 34 in Scheme 18), which was fbimd to be efficient for the cyclopropanation of various olefins with large diastereomeric excesses and ee values up to 95%. 

These authors further described the synthesis and resolution (by chiral HPLC) of a new C2-symmetric planar-chiral bipyridine ligand [43] (see structure 35 in Scheme 18). They obtained an X-ray crystal structure of the corresponding copper complex proving a bidentate complexation. This system led to high diastereo- (up to 94%) and enantioselectivity (up to 94%) in the... 

The synthesis of the unsymmetrical imidazolium salt 11 bearing a planar-chiral ferrocene was described by Bolm starting from (Rp)-[2-(trimethysilyl)-ferrocenyl] methanol 12 which afforded the salt in good yield after reaction with Ar,M-carbonyl diimidazole and methylation (Scheme 9) [18]. 

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-... 

It was shown that planar-chiral cyclopentadienyl-phosphine ligands were excellent chirality inducers during the synthesis of chiral-at-metal tetrahedral Ru-phosphine or -phosphite complexes (99% de). 

Mulzer J, Ohler E (2004) Olefin Metathesis in Natural Product Syntheses. 13 269-366 Muniz K (2004) Planar Chiral Arene Chromium (0) Complexes as Ligands for Asymetric Catalysis. 7 205-223 Murai S, see Kakiuchi F (1999) 3 47-79... 

Planar Chiral Ferrocenes as Lewis- or Br0nsted-Base Catalysts.163... 

Planar chirality has proven to be a very potent means in asymmetric catalysis to achieve high levels of stereocontrol (see Sect. 3) because planar chiral systems offer... 

Fig. 5 Definition of the stereodescriptors of planar chiral ferrocenes according to Schlogl...

A so far unsolved problem is the development of asymmetric procedures for the above described Fe(0)-catalyzed cycloisomerizations and cycloadditions. The option to use the element of planar chirality might allow to successfully address this issue in future applications. 

Palladacycles are defined as compounds with a Pd-C CT-bond with the Pd being stabilized by one or two neutral donor atoms, typically forming 5- or 6-membered rings [51]. Ferrocenyl palladacycles constitute a particularly attractive catalyst class partly due to the element of planar chirality. The first diastereoselective cyclopallada-tion of a chiral ferrocene derivative was reported in 1979 by Sokolov [52, 53]. 

There had been doubts about the utility of palladacycles in asymmetric catalysis, raised by the failure to achieve enantioselectivity as a result of a slow release of low ligated Pd(0) (naked Pd) [54]. However, recent success of several planar chiral palladacycles in highly enantioselective aza-Claisen reactions and in a number of other applications proves that the coordination shell of the Pd(II) species is not necessarily destroyed during the catalytic action. 

In 1997 the first asymmetric aza-Claisen rearrangement was reported by Overman et al. [55], which made use of diamines as bidentate ligands for Pd(II), allowing for moderate enantioselectivities. In the same year, Hollis and Overman described the application of the planar chiral ferrocenyl palladacycle 38 as a catalyst for the enantioselective aza-Claisen rearrangement of benzimidates 39 (Fig. 19) [56]. A related ferrocenyl imine palladacycle provided slightly inferior results, while a benzylamine palladacycle lacking the element of planar chirality was not able to provide any enantioselectivity [57]. 

In 2005, Moyano et al. [60] reported a new type of chiral dimeric ferrocene palladacycle 43 that lacked the element of planar chirality and involved three... 

A related planar chiral Co-based oxazoline palladacycle COP-X (46) was later found to be of higher synthetic utility as it permitted the use of benzimidates, [62] as well as allylic trifluoro- [63] and trichloroacetimidates [64, 65]. 46 was found to be superior to its ferrocene analogue 41 [61] in a number of aspects such as ease of... 

In 1996, Fu et al. reported the S3mthesis of the planar chiral heterocycles 64, formally DMAP fused with a ferrocene core [82]. While the original synthesis provided racemic 64a in only 2% overall yield requiring a subsequent resolution by preparative HPLC on a chiral stationary phase, a recently improved synthesis furnished the racemic complexes 64 in 32-40% yield over seven steps. A subsequent resolution with di-p-toluoyltartaric or dibenzoyltartaric acid gave access to the enantiomers with >99% ee (28 14% yield for each isomer in this step) [83]. 

The [Fe-Cp]-fragment does not only play the role of an additional steric element introducing planar chirality into the otherwise flat pyridine system. Substitution at the pyridine 2-position usually cuts the nucleophilicity of the nitrogen atom thus limiting the possibilities to achieve efficient chirality transfer using nucleophilic pyridine catalysts [84]. Ferrocene, however, functions as a strong electron donor (see Sect. 1) and thus restores the nucleophilicity impaired by substitution. 

Extending the same concept of a planar chiral nucleophilic or basic heterocyclic Fe-sandwich complex, aza-ferrocenes 65 were prepared. The latter have also been successfully applied as bidentate ligands in transition metal catalysis [85]. 

Enantioenriched alcohols and amines are valuable building blocks for the synthesis of bioactive compounds. While some of them are available from nature s chiral pool , the large majority is accessible only by asymmetric synthesis or resolution of a racemic mixture. Similarly to DMAP, 64b is readily acylated by acetic anhydride to form a positively charged planar chiral acylpyridinium species [64b-Ac] (Fig. 43). The latter preferentially reacts with one enantiomer of a racemic alcohol by acyl-transfer thereby regenerating the free catalyst. For this type of reaction, the CsPhs-derivatives 64b/d have been found superior. 

Other reactions not described here are formal [3 -i- 2] cycloadditions of a,p-unsaturated acyl-fluorides with allylsilanes [116], or the desymmetrization of meso epoxides [117]. For many of the reactions shown above, the planar chiral Fe-sandwich complexes are the first catalysts allowing for broad substrate scope in combination with high enantioselectivities and yields. Clearly, these milestones in asymmetric Lewis-base catalysis are stimulating the still ongoing design of improved catalysts. 

Cohen F, Overman LE (1998) Planar-chiral cyclopalladated ferrocenyl amines and imines as enantioselective catalysts for allylic imidate rearrangements. Tetrahedron Asymmetry 9 3213-3222... 

Jautze S, Diethelm S, Frey W, Peters R (2009) Diastereoselective bis-cyclopalladation of ferrocene-1,1 -diyl bis-imidazolines translation of central via axial into planar chirality. Organometallics 28 2001-2004... 

Huang H, Peters R (2009) A highly strained planar chiral platinacycle for catalytic activation of internal olefins in the Eriedel-Crafts alkylation of indoles. Angew Chem Int Ed 48 604-606... 

Wurz RP, Lee EC, Ruble JC, Fu GC (2007) Synthesis and resolution of planar-chiral derivatives of 4-(dimethylamino)pyridine. Adv Synth Catal 349 2345-2352... 

Fu GC (2006) Application of planar-chiral heterocycles as ligands in asymmetric catalysis. Acc Chem Res 39 853-860... 

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