Chiral arrangement

To create a chiral center at an sp3-hybridized carbon requires a chiral environment to stereodirect the reaction. This chiral environment may exist as a chirally substituted sp3-hybridized carbon, on which appropriate substitution creates a new molecule with the same or inverted chirality at the former chiral center or as a chiral arrangement near a prochiral (prechiral) sp2 carbon. 

Enantioselective catalytic surfaces may be conceived of being synthesized three basic ways (Fig. 3.3) (1) attach a catalyst to, or embed a catalyst in, a chiral matrix (2) create a chiral arrangement of active sites on a catalytic surface and (3) adsorb or attach chiral molecules onto a catalytic surface. We discuss each in turn. 

Create chiral arrangement of catalytic surface atoms... 

Create a Chiral Arrangement of Active Sites on the Catalytic Surface... 

An excellent enantioselective heterogeneous metal catalyst should be one in which the active metal sites are set in a chiral arrangement fulfilling the Ogston principles. Assuming no surface atom rearrangement, such a catalyst should furnish high enantioselectivities. 

A naturally occurring chiral metal structure is a screw dislocation (Fig. 3.4),11 which is a chiral arrangement observed in metal crystals but never resolved and tested for enantioselective heterogeneous catalysis. A possible method of making chiral arrangements like screw dislocations is by the glancing angle deposition technique, which can produce chiral sculptured thin films.12... 

There exists the possibility that one can imprint a chiral arrangement on a catalytic surface. Although chiral imprinting is now occurring on silica surfaces14 and in polymers,15 it has not been used to induce chirality in metal surfaces. This is difficult to explore. 

At least one group suggests that tartaric acid leaches Ni in a chiral arrangement Keane, M. A., and Webb, G., Chem. Comm. 1619 (1991). 

Very recently, an exciting approach to control the chiral ordering in optically active polythiophenes by a doping process has been reported [130]. It was found that the addition of Fe(C104)3, NaS03CF3, or AgS03CF3 to chiral polythiophenes had a dramatic effect on the chiral arrangement of the polymer chains. No detailed description of the nature of the helical... 

Their isolation by flash chromatography on silica gel was comparatively easy. The CD spectra of related pairs of diastereomers whose addition pattern represent pairs of enantiomers, reveal pronounced Cotton effects and mirror image behavior. It is the chiral arrangement of the conjugated Jt-electron system within the fullerene core that predominantly determines the chiroptical properties. Adducts with a C2-... 

The first successful experiments were reported by Schwab [16] Cu, Ni and Pt on quartz HI were used to dehydrogenate racemic 2-butanol 23. At low conversions, a measurable optical rotation of the reaction solution indicated that one enantiomer of 23 had reacted preferentially (eeright-handed quartz gave the opposite optical rotation it was deduced that the chiral arrangement of the crystal was indeed responsible for this kinetic resolution (for a review see [8]). Later, natural fibres like silk fibroin H5 (Akabori [21]), polysaccharides H8 (Balandin [23]) and cellulose H12 (Harada [29]) were employed as chiral carriers or as protective polymer for several metals. With the exception of Pd/silk fibroin HS, where ee s up to 66% were reported, the optical yields observed for catalysts from natural or synthetic (H8, Hll. H13) chiral supports were very low and it was later found that the results observed with HS were not reproducible [4],... 

If two (or more adjacent) distinct chromophores exist in a chiral arrangement with respect to one another in a dendrimer molecule, the CD spectrum shows two intense Cotton effects of opposite signs which merge with each other [80]. This phenomenon, known as an exciton couplet, arises from the interaction of two electronic transition moments in a mutually chiral orientation, such as oc-... 

By x-ray analysis, the chiral arrangement of lc molecules and the achiral arrangement of If and li molecules in their crystals have been proven [6], The photochemical conversion of Id in its chiral crystal to the optically active (3-lactam 2d was monitored by continuous measurements of CD spectra of Nujol mulls (Fig. 

Even for the oxoamides which do not form chiral crystals, enantioselective photoconversion to optically active (3-lactams can easily be accomplished by photoirradiation in their inclusion crystals with an optically active host compound. For example, irradiation of a 1 1 inclusion complex crystal of 9 with 11 [8] gave (-)-10 of 100% ee in a quantitative yield [9], The host compound 11 is recovered and can be used again. The chiral arrangement of 9 molecules in the inclusion complex was studied by x-ray analysis [9], A schematic stereoview of the inclu-... 

In the case of 18, however, photoreaction in its chiral crystals gave 19 (22% yield, 32% ee) in the low chemical and optical yields as indicated, together with the side product 20 (17% yield, 46% ee) [15]. The reason for the low optical yield is not clarified. As the photoreaction in the chiral crystal proceeds, chiral arrangement of the achiral molecules of 18 in their crystal would be disturbed and racem-ized. 

The chiral arrangement of 80a molecules in the complex with (—)-12c is also easily detected by measurement of CD spectra as Nujol mulls. For example, the inclusion crystal of 80a with (—)-12c showed a (—)-Cotton effect, and that of 80a with (+)-12c showed a (+)-Cotton effect at around 260 and 310 nm, respec-... 

The efficient enantioselective dimerization reaction of 97 in the inclusion complex (100) with 12a suggests that two coumarin molecules are arranged in chiral positions in 100 and their [2 + 2] photodimerization occurs by keeping the chirality in 100. The chiral arrangement of 97 was detected by CD spectral measurement of the inclusion complex in Nujol mulls. The 1 1 complex of 97 with 12a and 13a showed CD spectra with a mirror-imaged relation (Fig. 17). After photoirradiation, the CD absorptions of the complex (100) at 225, 275, 300, and 330 nm disappeared, and the new CD absorption due to the inclusion complex (101) of 98 at 240 nm appeared. The photodimerization of 97 was also followed by measurement of infrared (IR) spectra as Nujol mulls. Upon photoirradiation, the nCO absorption of 97 in 100 at 1700 cm 1 decreased gradually and finally disappeared after 4 hr, and new vCO absorption due to 98 in 101 appeared at 1740 cm-1. 

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