Metallated chiral

Variations and Improvements on Alkylations of Chiral OxazoUnes Metalated chiral oxazolines can be trapped with a variety of different electrophiles including alkyl halides, aldehydes,and epoxides to afford useful products. For example, treatment of oxazoline 20 with -BuLi followed by addition of ethylene oxide and chlorotrimethylsilane yields silyl ether 21. A second metalation/alkylation followed by acidic hydrolysis provides chiral lactone 22 in 54% yield and 86% ee. A similar... 

Ligand11 Metal Chirality of Propene A E enant A tenant Favored Eout - inw... 

Figure 2-2. Electrophilic substitution to the carbonyl group of aldehydes and ketones via metalated (chiral) hydrazones. 

Sundaram RM, Koziol KKK, Windle AH. Continuous direct spinning of fibers of single-walled carbon nanotubes with metallic chirality. AdvMater. 2011 Nov 16 23(43) 5064-8. 

TABLE 16. Epoxidation of (ii)-enones catalyzed by different metal/chiral ligand systems (yields are given ee values are given in parentheses)... 

The principal question addressed, is there any kind of chiral recognition in electron transfer reactions involving GO or HRP and enantiomerically pure metal complexes. The chirality of optically active metal complexes may be different. Examples include central carbon chirality, when a complex has a side chain with an asymmetric sp3 carbon (Chart 2A), planar chirality as in the case of asymmetrically 1,2-substituted ferrocenes (Chart 2B,C), and central metal chirality when an octahedral central metal itself generates and enantiomers (Chart 2D) (202). These three types are discussed in this section. 

The detailed pathways of asymmetric induction during catalysis are not well understood even for the more widely studied hydrogenations (8, 22), and matching substrates with the most suitable transition-metal chiral catalyst remains very much an empirical art. We are not aware of kinetic studies except our own on catalyzed asymmetric hydrogenations, although they usually are assumed to follow well-studied nonchiral analogs for example, RhClP3 systems, where... 

The examples of metal-chiral structures are mainly cationic mixed-ligand cobalt(III) complexes of ethylenediamine and its monodimethylphosphine analogue [Co(H2NCH2CH2NH2)3(H2NCH2CH2PMe3)3 J3+. Their synthesis, separation to enantiomers, and establishment of absolute configuration have been carried out for these compounds [276]. The binuclear cobalt(III) complexes 924 possess similar optical properties [277] ... 

In some instances, for example the reactions involving complexes 7 and 8, the metal-chiral ligand complex alone does not suffice for good performance and addition of external base in either catalytic or (over)stoichiometric quantity, or even trimethylsilyl triflate, is required [13, 14]. Apparently, in Scheme 6, although the metal center(s) is (are) fundamental for a well-ordered transition state and for acceptor activation, the external base is required for deprotonation leading to formation of a transient metal enolate. 

There are (at least) four ways a Cp-metal fragment can assume a chiral character (a) the Cp ring can carry an optically active substituent (R, 19) (b) with two (or more) different substituents a Cp-metal combination can exist in two enantiomeric forms (20) (c) if free rotation around a Cp-substituent bond is no longer possible (see also Section IV,C,2) a Cp-metal chirality arises as well (21) (d) if the rotation about the metal-ring vectors... 

Various metalated chiral organosilicon compounds bearing the SMP moiety have been alkylated to synthesize chiral alcohols. Excellent regio- and stereoselectivities have been observed in the alkylation of chiral silylpropargyl anions (eq 8). ... 

Fig. 4-27. Reaction of metalated chiral ferrocene derivatives with halogenes, boron and carbon electrophiles.

Figure 24 Structure of a carbon nanotube, (a) the unraveled (4,2) tube, where the indices (m,n) refer to the vector (in terms of a and 32) which spans the circumference of the tube. The vector (4,-5) indicates the size of the unit cell. The angle q is the chiral angle of the tube, (b) Positions of the (m,n) vectors on a graphite sheet. Metallic chiralities are indicted with a full circles and semiconducting tubes are indicated with open circles.

Enders, D., Eichenauer, H. Enantioselective alkylation of aldehydes via metalated chiral hydrazones. Tetrahedron Lett. 1977, 191-194. 

Enders, D., Eichenauer, H., Baus, U., Schubert, H., Kremer, K. A. M. Asymmetric syntheses via metalated chiral hydrazones. Overall enantioselective a-alkylation of acyclic ketones. Tetrahedron 1984,40, 1345-1359. 

Metalated chiral salen ligands were first introduced during the 1990s by Jacobsen and Katsuki as highly enantioselective catalysts for the asymmetric... 

Asymmetric protonation of a metal enolate basically proceeds catalytically if a coexisting achiral acid A-H reacts with the deprotonated chiral acid A -M faster than with the metal enolate, a concept first described by Fehr et al. [44]. A hypothesis for the catalytic cycle is illustrated in Scheme 2. Reaction of the metal enolate with the chiral acid A -H produces (R)- or (S)-ketone and the deprotonated chiral acid A -M. The chiral acid A -H is then reproduced by proton transfer from the achiral acid A-H to A -M. Higher reactivity of A -M toward A-H than that of the metal enolate makes the catalytic cycle possible. When the achiral acid A-H protonates the enolate rapidly at low temperature, selective deprotonation of one enantiomer of the resulting ketone by the metallated chiral acid A -M is seen as an alternative possible mechanism. 

However, soluble metal chiral complex catalysts are better, even though ketone hydrogenation under these conditions is more difficult to run than olefin hydrogenation. Notable achievements have been made using Rh and Ru metals. Phosphine ligands are often used, which contain either an asymmetric alkyl group have the phosphorous atom as the asymmetric center, or have an axial element of chirality (atropisomerism) . Other chiral ligands commonly used are derived from amino acids, from L-hydroxyproline and from ferrocene. 

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