Chirality three-coordinate

The earliest report of a reaction mediated by a chiral three coordinate aluminum species describes an asymmetric Meerwein-Poimdorf-Verley reduction of ketones with chiral aluminum alkoxides which resulted in low induction in the alcohol products [1]. Subsequent developments in the area were sparse until over a decade later when chiral aluminum Lewis acids began to be explored in polymerization reactions, with the first report describing the polymerization of benzofuran with catalysts prepared from and ethylaluminum dichloride and a variety of chiral compounds including /5-phenylalanine [2]. Curiously, these reports did not precipitate further studies at the time because the next development in the field did not occur until nearly two decades later when Hashimoto, Komeshima and Koga reported that a catalyst derived from ethylaluminum dichloride and menthol catalyzed the asymmetric Diels-Alder reaction shown in Sch. 1 [3,4]. This is especially curious because the discovery that a Diels-Alder reaction could be accelerated by aluminum chloride was known at the time the polymerization work appeared [5], Perhaps it was because of this long delay, that the report of this asymmetric catalytic Diels-Alder reaction was to become the inspiration for the dramatic increase in activity in this field that we have witnessed in the twenty years since its appearance. It is the intent of this review to present the development of the field of asymmetric catalytic synthesis with chiral aluminum Lewis acids that includes those reports that have appeared in the literature up to the end of 1998. This review will not cover polymerization reactions or supported reactions. The latter will appear in a separate chapter in this handbook. 

This thinking applies, in particular, when planning the design of a chiral three-dimensional supramolecular host-guest system, since the mutual interaction of the two distinct complementary molecular units or coordination entities is necessary. Examples of this methodology include the above-described anionic, tris-chelated transition metal oxalato complexes [Mzl(ox)3/6 which form the host system together with the cationic, tris-chelated transition metal diimino complexes, e.g. [M(bpy)3]21/31, bpy = 2,2 -bipyridine, which play the role of the guest compounds. 

The breakthrough in the stoichiometric reaction was brought about by Leyen-decker in 1983 by using hydroxyprolinol-derived sulfide 20 bearing three coordinating sites, as shown in 21 [50]. The reaction of dimethylcopper lithium with chalcone gave the product in 94% ee (Eq. (12.24)). In 1991, Alexakis introduced chiral phosphines, e.g. 22, as the ligands in the reaction of the medium order cuprate with cycloalkenones in the presence of lithium bromide to afford the product in 76-95% ee (Eq. (12.25)) [51]. 

In contrast to the compounds that form chiral three-dimensional networks, the two-dimensional framework topology implies an assembly of coordination entities with alternating chirality between nearest neighboring centers. Finally, the discrimination between the formation and crystallization of either two-dimensional or three-dimensional frameworks with analogous network stoichiometries depends on the choice of the templating counterion. In particular, pCR4]", (X = N, P R = phenyl, -alkyl) cations have been found to initiate the growth of two-dimensional... 

Helical chains such as SCoo (Figure 4.20a) may be right- or left-handed and are chiral. 6-Coordinate complexes such as [Cr(acac)3] ([acac] , see Table 7.7) in which there are three bidentate chelating ligands also possess non-superposable mirror images (Figure 4.20b). Chiral molecules can... 

A sufficient amount of oriented chiral molecules can be obtained in an induced cholesteric liquid crystal phase if the induced helical structure has been untwisted by an electric field. In the following description tensors are needed for the sake of simplicity (At least there are three tensors required the transition moment tensor (absorption tensor ,y), the rotational strength tensor (circular dichroism tensor A ,y), and the order tensor g,y33 (i,j= 1,2,3). If the molecules do not possess any symmetry, the principal axes of all of these tensors are differently oriented with respect to the molecular frame (the coordinate system in which only the three diagonal elements of a tensor are different from zero).) The only tensorial property, needed here explicitly, is the existence of three coordinates (components) of a tensor with respect to three specially chosen mutually perpendicular axes. This means that three information instead of one information about a molecule are needed instead of one CD value, namely Ae, three CD values, namely As, (i=l, 2, 3), have to be introduced. Ac is then one-third of a sum of the three so-called tensor coordinates of the CD tensor ... 

There are three main types of chiral GC stationary phases (1) chiral amino acid derivatives [8-10] (2) chiral metal coordination compounds [11] and (3) cyclodextrin derivatives [12-14]. The cyclodextrin phases have proven to be the most versatile for gas chromatography. 

The formation of an active complex 81 consisting of three components, tin(II) trifiate, chiral diamine 80, and dibutyltin acetate is assumed in these aldol reactions. The three-component complex would activate both aldehyde and silyl enolate (double activation), i.e. the chiral diamine-coordinated tin(II) trifiate activates aldehyde while oxygen atoms of the acetoxy groups in dibutyltin acetate interact with the silicon atom of the silicon enolate. Because it has been found that the reaction does not proceed via tin(II) or tin(IV) enolates formed by silicon-metal exchange, silicon enolate is considered to attack the aldehydes directly [65]. The problem of this aldol reaction is that (Z) enolates [63] react with aldehydes more slowly, consequently affording the aldols in lower yield and with lower diastereo- and enantio-selectivity. 

Then, the 3D coordinates of A are used for atom t, those of B forj, those ofC for h, and those of U for I. The first three atoms (in the order established by the ranking) define a plane if they are ordered clockwise and the fourth atom is behind the plane, the chirality signal, obtains a value of -rl for the opposite geometric arrangement, obtains a value of-1. 

Much effort has been placed in the synthesis of compounds possessing a chiral center at the phosphoms atom, particularly three- and four-coordinate compounds such as tertiary phosphines, phosphine oxides, phosphonates, phosphinates, and phosphate esters (11). Some enantiomers are known to exhibit a variety of biological activities and are therefore of interest Oas agricultural chemicals, pharmaceuticals (qv), etc. Homochiral bisphosphines are commonly used in catalytic asymmetric syntheses providing good enantioselectivities (see also Nucleic acids). Excellent reviews of low coordinate (coordination numbers 1 and 2) phosphoms compounds are available (12). 

Dipolar cydoadditions are one of the most useful synthetic methods to make stereochemically defined five-membered heterocydes. Although a variety of dia-stereoselective 1,3-dipolar cydoadditions have been well developed, enantioselec-tive versions are still limited [29]. Nitrones are important 1,3-dipoles that have been the target of catalyzed enantioselective reactions [66]. Three different approaches to catalyzed enantioselective reactions have been taken (1) activation of electron-defident alkenes by a chiral Lewis acid [23-26, 32-34, 67], (2) activation of nitrones in the reaction with ketene acetals [30, 31], and (3) coordination of both nitrones and allylic alcohols on a chiral catalyst [20]. Among these approaches, the dipole/HOMO-controlled reactions of electron-deficient alkenes are especially promising because a variety of combinations between chiral Lewis acids and electron-deficient alkenes have been well investigated in the study of catalyzed enantioselective Diels-Alder reactions. Enantioselectivities in catalyzed nitrone cydoadditions sometimes exceed 90% ee, but the efficiency of catalytic loading remains insufficient. 

The stereogenic sulfur atom in sulfoxides is usually configurationally stable at room temperature thus, sulfoxides may be chiral based on this property alone1. In fact, there are many examples of optically active sulfoxides of both synthetic and natural origin. This chapter reviews the important methods for obtaining optically active sulfoxides, and discusses some reactions at sulfur which either leave the coordination number at three or increase it to four, generally with preservation of optical activity. It also describes briefly some recent studies on the conformational analysis and chiroptical properties of sulfoxides. 

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