Asymmetric exchange

Now, in the z-direction (the spherical component q = 0) we get the total interaction matrix, involving the isotropic exchange, asymmetric exchange and the Zeeman interaction, in the form... 

Thus, the QASes of the asymmetrical stmcture with improved steric accessibility of the exchange centre show the high selectivity to double-charged cadmium bromide complex ions relative to single-charged ions. 

If the ketone function is adjacent to a hydrogen-bearing asymmetric center, the compound can undergo epimerization. In steroids with a normal skeletal configuration (8/3, 9a, 14a) there is no detectable epimerization at C-8 or C-9 during the exchange of and 11- ketones. 

The configuration of the amine was retained, except in the case of amino acid derivatives, which racemized at the stage of the pyridinium salt product. Control experiments showed that, while the starting amino acid was configurationally stable under the reaction conditions, the pyridinium salt readily underwent deuterium exchange at the rz-position in D2O. In another early example, optically active amino alcohol 73 and amino acetate 74 provided chiral 1,4-dihydronicotinamide precursors 75 and 76, respectively, upon reaction with Zincke salt 8 (Scheme 8.4.24). The 1,4-dihydro forms of 75 and 76 were used in studies on the asymmetric reduction of rz,>S-unsaturated iminium salts. 

The influence of 1,2-asymmctric induction on the exchange of diastereotopic bromine atoms has also been investigated22,23. Thus, treatment of the / -silyloxydibromo compound 15 with butyllithium at — 110°C in the presence of 2-methylpropana led to products 17-19 after the reaction mixture was warmed to 20 °C. The distribution of the products indicates that the diastereomeric lithium compounds 16 A and 16B were formed in a ratio of 84 16, with 16A being kinetically favored by 1,2-asymmetric induction. Formation of the m-configurated epoxide (cis,anti-18) was slowed to such an extent that its formation was incomplete and a substantial amount of the parent bromohydrin 17 remained. The analogous m.yyn-configurat-ed epoxide was not observed. Presumably for sterie reasons, the parent bromohydrin did not cyclize to the epoxide but instead led to the ketone 1923. 

Alkoxyallylstannanes are also available by boron trifluoride-diethyl ether complex induced isomerization of their 1-alkoxy isomers. This isomerization proceeds in an antarafacial manner with excellent stereoselectivity to give (Z)-3-alkoxyallylstannanes possibly via an intermolecu-lar exchange process119. Coupled with the asymmetric reduction of acylstannanes (see Section 1.3.3.3.2.3.1) this provides access to 1-alkyl-3-alkoxyallylstannanes of useful optical purity106. 

This possibility can be dismissed22 because it is asymmetric. Read backward, the first step would be not the second-order axial addition of Cl" but a unimolecular rearrangement, and an improbable one at that. Aside from this, the fact that the pathway is not symmetric rules it out. If it is invalid in the special case of an exchange, then it can hardly provide the general pathway for substitution at square-planar metal centers. 

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

In 2004, Bolm et al. reported the use of chiral iridium complexes with chelating phosphinyl-imidazolylidene ligands in asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee [17]. These complexes were synthesized from the planar chiral [2.2]paracyclophane-based imida-zolium salts 74a-c with an imidazolylidenyl and a diphenylphosphino substituent in pseudo ortho positions of the [2.2]paracyclophane (Scheme 48). Treatment of 74a-c with t-BuOLi or t-BuOK in THF and subsequent reaction of the in situ formed carbenes with [Ir(cod)Cl]2 followed by anion exchange with NaBARF afforded complexes (Rp)-75a-c in 54-91% yield. The chela-... 

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