Prochiral chromium complexes

In 2001, p-lactams have been reported to be obtained via Staudinger reaction with complete m-diastereoselection starting from prochiral imine chromium complexes (Scheme 14), [63]. 

Scheme 14 cw-Diastereoselective synthesis of (3-lactams using prochiral imine chromium complexes... 

Direct synthesis of atropisomeric benzamides and anilides from prochiral precursors has been reported using chiral-amide-mediated deprotonation of 2,6-dimethyl-substituted ben-zamide and anilide chromium complexes. A screening of amides revealed that (R,R) 3 was the most selective in the deprotonation of the benzylic methyl groups (Scheme 51)92 94. 

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). 

In a related study, Uemura and co-workers obtained planar-chiral isochromene chromium complexes by gold-catalyzed enantioselective cyclization of prochiral (l,3-dihydroxymethyl-2-alkynyl)benzene chromium complexes. " The highest selectivities in this 6-e <3to-dig-hydroalkoxylation were observed in the presence of a chiral cationic gold catalyst prepared in situ from (/ )-xylyl-BINAP(AuC1)2 and AgSbFs. With (/ )-SEGPHOS(AuCl)2 and AgBp4, the enantiomeric product was obtained, albeit with low enantioselectivity. 

Kamikawa, K., Harada, K. and Uemura, M. (2005) Catalytic asymmetric induction of planar chirality palladium catalyzed intramolecular Mizoroki-Heck reaction of prochiral (arene)chromium complexes. Tetrahedron Asymmetry, 16, 1419-23. 

Shortly after our initial report, Doyle and Jacobsen disclosed the use of prochiral tin enolates in a salen-chromium complex-catalyzed asymmetric alkylation reaction [45, 46], It is noteworthy that this reaction was the first example of an asymmetric metal-catalyzed a-alkylation of ketone enolates that utilizes a variety of alkyl halides and is not limited to allylic electrophiles. While this mechanistically interesting reaction does not proceed via a metal 7i-allyl complex, it is a rare example of a catalytic reaction that provides access to enantioenriched a-quatemary ketones. 

This work was extended to other prochiral substrates, with the chiral chromium complex of 11 catalyzing the addition of 1,3-dihalopropenes 12 to a range of aromatic aldehydes (Scheme 12.10). The best result was obtained using /t-F-CgH4CHO, which gave a syn. anti ratio of 90 10 and ee yn of 83%. The products are useful synthetic intermediates and were readily converted into their corresponding optically active vinyl epoxides. ... 

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. 

Related epoxidations of olefins with PhIO in the presence of Salen and related complexes of chromium(III), manganese(III) and cobalt(III) have been reported by Kochi and coworkers [58]. The use of nickel(II) Salen in conjunction with NaOCl was also described [59]. More recently, these systems formed the basis for the development, by Jacobsen and coworkers [60], of chiral manganese(III) Salen complexes for the enantioselective epoxidation of prochiral olefins by ArlO or NaOCl. Similarly, asymmetric epoxidations with moderate to good... 

For monosubstituted arenes, kinetically controlled discrimination between the two enantiotopic ortho hydrogens of the planar chiral benzene chromium tricarbonyl complex leads to nonracemic products. Asymmetric lithiation is more efficient when one or more oxygen atoms, such as ether linkages, are present in the starting prochiral complex (Scheme 26.14). Treatment of Cr(CO)j-anisole complex 52 with the chiral lithium amide 53, in the presence of TMSCl under ISQ conditions, affords (+)-orfho-silylated complex 54 with good chemical yield and ee value [143-145]. The isobenzofuran system 55 reacts as well to give a-sUylated product 56 [146]. 

This is also the case for the kinetically inert octahedrally coordinated chromium(III) centre (d ) found in a salene chromium(III) complex with an organoazide in close proximity to the metal site (Figure 12.10). Salene chromium(II) complexes are known to catalyze the enantioselective opening of prochiral epoxides with TMS-N3. The organoazide derivative was obtained by stoichiometric azide transfer from an azido chromium(III) salene complex to epoxycyclopentane. 

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