Asymmetric Arene Complexes

Arene metal carbonyl complexes containing disubstituted ortho or meta arenes are asymmetric and may be resolved to give enantiomers [structures (10.3)]. 

If the metal is bonded to three different ligands in addition to the arene, then the central atom is asymmetric. Chromium, ruthenium, etc., compounds which were obtained as pure enantiomers may serve as examples [structure (10.4)]. 

Arenes, like the cyclopentadienyl ligand, may serve as bridging ligands in polylayer transition metal complexes, for instance, cpV( /-C6H6)Vcp, 

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In addition to [Hg( -toluene)2-(GaCI/ )2],168 other mercury-arene complexes of general formula [I Ig( /2-arene)2-(AlCUy have been prepared.169 These include the bis(toluene), bis(o-xylene), and bis(l,2,3-trimethylbenzene) complexes 159, 160, and 161, respectively, whose structures have all been determined (Figure 8). While the arene in 159 and 161 is coordinated in an asymmetrical -fashion, the /2-1,2,3-trim ethylbenzene ligands of 160 form two nearly equal Hg-C bonds of 2.45 and 2.46 A. DFT calculations show that the Hg-arene interactions are mostly ionic. 

Unlike the ferrocenes, the arene complexes of chromium, in particular the arenechromi-umtricarbonyls, have seen much less use in asymmetric catalysis. This is beginning to change, however , and a number of synthetic transformations of arenechromiumtricar-bonyls owe their existence to the formation of planar chiral chromium complexes by... 

Three types of reaction systems have been designed and applied for the enantioposition-selective asymmetric cross-coupling reactions so far. First example is asymmetric induction of planar chirality on chromium-arene complexes [7,8]. T vo chloro-suhstituents in a tricarhonyl("n6-o-dichlorobenzene)chromium are prochiral with respect to the planar chirality of the 7t-arene-metal moiety, thus an enantioposition-selective substitution at one of the two chloro substituents takes place to give a planar chiral monosubstitution product with a minor amount of the disubstitution product. A similar methodology of monosuhstitution can be applicable to the synthesis of axially chiral biaryl molecules from an achiral ditriflate in which the two tri-fluoromethanesulfonyloxy groups are enantiotopic [9-11]. The last example is intramolecular alkylation of alkenyl triflate with one of the enantiotopic alkylboranes, which leads to a chiral cyclic system [12], The structures of the three representative substrates are illustrated in Figure 8F.1. 

Asymmetric Synthesis using Chiral Cr(CO)3-arene Complexes... 

Cr(CO)3 coordinates from either the top or bottom side of aromatic rings, bearing two different substituents in ortho or meta position, so that the enantiomers 285 and 286 are obtained. Optical resolution of the enantiomers is carried out by recrystallization, or column chromatography. The racemic complex of benzyl alcohol derivative 287 was separated to 288 and 289 by lipase-catalysed acetylation [68]. Enzymes recognize Cr(CO)3 as a bulky group. Chiral Cr(CO)3-arene complexes are used for asymmetric synthesis [68a]. 

Asymmetric tandem additions In order to further take advantage of the high degree of stereoselectivity observed in tandem additions to [Os]-arene complexes, a chiral anisole derivative has been prepared, which has demonstrated high coordination diastereoselectivity... 

So far, chiral lithium amides for asymmetric deprotonation have found use only with a few types of substrates. The following sections deal with deprotonation of epoxides to yield chiral allylic alcohols in high enantiomeric excess, deprotonation of ketones, deprotonation of tricarbonylchromium arene complexes and miscellaneous stereoselective deprotonations. These sections are followed by sections in which various chiral lithium amides used in stereoselective deprotonations have been collected and various epoxides that have been stereoselectively deprotonated. The review ends with a summary of useful synthetic methods for chiral lithium amide precursors. 

Kiindig and coworkers have reported on the application of an azepine-derived lithium base 65 in the asymmetric desymmetrization of the carbamate arene complex 60... 

Deprotonation of benzaldimine arene complexes using (R,R) 3 gave enantioselectivities up to 92%87. A series of lithium bases derived from constrained cyclic amines did not show significant increase in asymmetric induction (Scheme 45). 

In 1997, Schinzer and coworkers (D. Schinzer, U. Abel, and P. G. Jones, Synlett 632 (1997)] reported as asymmetric synthesis of the cephalotaxine subunit (S,R)-2Si from (S)-286 (see Scheme 49). The stereochemistry of the 2-(dimethylphenylsilylmethyl)propenyl group in 286 was established through a planar chiral tj -chromium arene complex. 

Keywords Arene-chromium complexes Arene-manganese complexes Dearomatization Cyclohexadiene Organo lithium reagents Asymmetric arene transformations... 

Fig.4 The asymmetric environment of TpRe(CO)(L)(arene) complexes and the assignment of quadrants...

The synthesis of an enantiomerically enriched chromium complex via asymmetric lithiation of a prochiral tricarbonyl(ri -arene)chromium complex using a chiral lithium amide base was first demonstrated in 1994 by Simpkins [88]. Arene complex 44 was treated with C2-symmetric chiral base ent-39 in the presence of TMSCl as an internal quench and silylated complex 45 was obtained in 84% ee (Scheme 24). 

High asymmetric inductions have been achieved in the additions of 2-haloesters or 2-halonitriles (X = Cl, Br) to the aldehyde substituent on the ortho-substituted arene complexes (66), X = Me, MeO. 

Halpern has made some significant observations in the chemistry of the cationic rhodium catalysts. Because much of this relates to asymmetric induction, it is treated in Chapter 4. It is appropriate to mention here that the catalyst formed from [Rh cod) dpe)]A dpe = 1,2-diphenylphosphino-ethane) has been shown to dimerize reversibly in the absence of substrate. An aromatic ring from one of the dpe ligands of the first metal binds in an T7 -fashion to the second metal and vice versa The system is labile, in contrast to the nonlability of simple arene complexes of Ir(I). 

Chirality [17] can be introduced into organometallic arene complexes by modifications such as asymmetric substitution of the arene, introduction of chiral substituents into the ligands, the asymmetric coordination environment of the metal ion, or conformational arrangements of the ligands. Chirality in arene complexes can be classified depending on the components that generate the asymmetry in the molecule (for examples, see Figure 3.3). 

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