Form optical activity

The first example of a stereoselective substitution at tin was the reaction of (—)- -butylneophylphenyltin hydride (65) ([with diazomethane in the presence of copper in diethyl ether to form optically active methylneophylphenyl-t-butyltin (84) ([o g5 - 1.5) 20 44- >. 

Either the reaction is not stereoselective and yields a racemic mixture, or the racemization of the formed optically active (5) is very rapid in the presence of HC179) (cf. Fig. 2)... 

Chiral amines have also been used to prepare the quaternary ammonium salts, forming optically active polysilanes (see Section 3.11.6.1.3).173... 

The carbon atom is not the only atom which produces chiral centres in organic compounds. Silicon, germanium, tin, or N (in quaternary salts or N-oxides) whose bonds point towards the comers of a tetrahedron also form optically active compounds provided the groups are different. 

Inoue s cyclic dipeptide 17 had been shown to catalyze the addition of HCN to aldehydes to form optically active cyanohydrins [70]. Lipton found that this same catalyst was unable to catalyze the mechanistically related Strecker reaction... 

Ruthenium may therefore be placed among the elements w hich form optically active compounds due to asymmetry of the molecule. 

Asymmetric synthesis of fi-hydroxy carboxylic estersEsters of 1 in the presence of f-butylmagnesium bromide as base react with aldehydes and ketones to form optically active a-sulfinyl-jS-hydroxy carboxylic esters, which are desulfurized by aluminum amalgam in aqueous THF (equation I). Chemical yields are 75% of... 

Scheme 68 illustrates cyclopolymerization of 1,5-hexadiene catalyzed by a homogeneous chiral zirconocene complex to form optically active poly(methylenecyclopentane), whose chirality derives from configurational main-chain stereochemistry (757). This polymer is predominantly isotactic and contains predominantly trans cyclopentane rings. 

A binaphthol-modified Ti(IV) complex effects enantioselective addition of trimethylsilyl cyanide to aldehydes to form optically active cyanohydrin derivatives (Scheme 124). The highest ee value of 82% is achieved in the reaction of isovaleraldehyde with 20 mol % of the catalyst (286a). Use of a tartrate-derived modifier in combination with molecular sieves 4A is also effective for this type of addition and results in... 

Asymmetric [1,3 dipolar cycloaddition.2 This sulfoxide reacts with C-phenyl-N-methylnitrone (2) to form optically active 4-(p-tolylsulhnyl)-isoxazoIiilines (3a and 3b), which differ only in chirality of the S=0 group. The optical purity was shown to be no less than 80% by conversion to the amino alcohol 4. The absolute configuration... 

The Kiliani-Fischer synthesis lengthens the carbon chain of an aldose by one carbon at the aldehyde end and forms a new aldose with its corresponding epimers. When glucose and its epimer are produced from the corresponding pentose via the Kiliani-Fischer synthesis, and then both epimers are reacted with dilute nitric acid, both form optically active compounds. 

The asymmetric catalytic nitroaldol reaction, also known as the asymmetric Henry reaction, is another example of an aldol-related synthesis of high general interest. In this reaction nitromethane (or a related nitroalkane) reacts in the presence of a chiral catalyst with an aldehyde, forming optically active / -nitro alcohols [122], The / -nitro alcohols are valuable intermediates in the synthesis of a broad variety of chiral building blocks, e.g. / -amino alcohols. A highly efficient asymmetric catalytic nitroaldol reaction has been developed by the Shibasaki group, who used multifunctional lanthanoid-based complexes as chiral catalysts [122-125],... 

Logically, it makes sense that optically inactive reagents and catalysts cannot form optically active products. If the starting materials and reagents are optically inactive, there... 

It is of interest that each Pt in [Pt4(CH3COO)4(en)4] + (15) is in a chiral environment (Fig. 13). However, the compound is optically inactive as a whole, because chelate ring configurations around the four Pt atoms are of meso(AAAA) form. Optically active Pt4 clusters [Pt4(CH3COO)4(L-aa)4] have been prepared by introducing optically active a-amino acid (aa) at the in-plane sites (L-aa = L-alanine, L-proline, L-hydroxyproline, L-phenylalanine, L-valine) (37, 38). As described in the previous section, these a-amino acids take the chelating coordination mode in Pt4 clusters. Therefore, as far as the chelate ring... 

The final aspect of selectivity is related to forming optically active mirror image isomers (enantiomers). The problem of enantioselectivity is extremely important in organic synthesis, but will not be addressed in this text since this is an independent topic in its own right (see, however, discussion of some aspects of this problem in Chapter 4). 

Intramolecular asymmetric allylic substitution reactions have been applied to the synthesis of optically active cyclic compounds. The high efficiency of the dihydroxylated ferrocenylphosphine 8b has been shown in the cyclization of 2-butenylene dicarbamates 52 to form optically active 4-vinyl-2-oxazolidones 53, which are... 

In catalytic processes with enzymes such as D-oxynitrilase and (R) xynitrilase (mandelonitrilase) or synthetic peptides such as cyclo[(5)-phenylalanyl-(5)-histidyl], or in reaction with TMS-CN pro-mot by chiral titanium(IV) reagents or with lanthanide trichlorides, hydrogen cyanide adds to numerous aldehydes to form optically active cyanohydrins. The optically active Lewis acids (8) can also be used as a catalyst. Cyanation of chiral cyclic acetals with TMS-CN in the presence of titanium(IV) chloride gives cyanohydrin ethers, which on hydrolysis lead to optically active cyanohydrins. An optically active cyanohyrMn can also be prepared from racemic RR C(OH)CN by complexation with bru-... 

Use Organic synthesis. Note The above are properties of the dl form. Optically active forms are also known. 

Palladium-catalyzed hydrosilylation of 1,3-dienes is one of the important synthetic methods for allylic silanes, and considerable attention has been directed to the asymmetric synthesis of the latter by catalytic methods [9]. Optically active allyhc silanes have been used as chiral allylating reagents in S reactions with electrophiles, typically aldehydes [38,39]. In the presence of Pd catalysts the reaction with hydrosilanes containing electron-withdrawing atoms or substituents on sihcon usually proceeds in a 1,4-fashion giving allyHc silanes [40,41]. Asymmetric hydrosilylation of cyclopentadiene (29) forming optically active 3-silylcyclopentene (30) has been most extensively studied (Scheme 13). In the first report, hydrosilylation of cyclopentadiene (29) with methyldichlorosilane in the presence of 0.01 mol % of palladium-(l )-(S)-PPFA (15a) as a catalyst gave... 

Asymmetric hydroboration of norbornene (27) is a synthetically useful transformation forming optically active norbornanol (28) which is an important chiral synthon. The catalytic enantioselective hydroboration with catecholborane was examined using rhodium complexes coordinated with several chiral phosphine ligands (Scheme 7 and Table 4) [14,15,17,23,24,27]. For this reaction, DIOP (10) and its derivatives 21 and 22, which are modified on the diphenyl-phosphino group, are more enantioselective Hgands than BINAP (7) or chira-phos (9). The highest enantioselectivity was observed in the reaction at -25 °C... 

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