Stereoselectivity external reagents

The structure of titanium complexes is helpfid in understanding the interaction mechanism between Lewis acidic titanium ion and substrate [4]. Some crystal structures of chelate complexes have been reported [5]. It has been shown that most of this kind of titanium complexes take in-plane coordination marmer rather than out-of-plane [6] (Scheme 14.1). The attacking nucleophile is either a ligand bonded totitanimn complex or an external reagent The stereoselectivity results solely from the bias of the chiral titanium center. 

Catalytic properties of external chiral additives such as (2S,3/ )-4-dimethyl-amino-l,2-diphenyl-3- methyl-2-butoxide (A 16) (574, 575) and 2-magnesium-3-zinc salts of dialkyl (f ,f )-tartrate (A17) were employed in the highly stereoselective addition of organozinc reagents to derivatives of 3,4-dihydro-isoquinoline-A-oxide (Scheme 2.147) (576). 

Chiral synthesis, also called asymmetric synthesis, is synthesis which preserves or introduces a desired chirality. Principally, there are three different methods to induce asymmetry in reactions. There can be either one or several stereogenic centres embedded in the substrate inducing chirality in the reaction (i.e. substrate control) or an external source providing the chiral induction (i.e. reagent control). In both cases the obtained stereoselectivity reflects the energy difference between the diastereomeric transition states. 

Finally, the allylmetal aldehyde addition can also operate under the influence of stereocontrolling reagents, in particular, chiral Lewis acids and related activators (Eq. (10.6)). In these cases the stereochemical influence on the steric course of the reaction is due to a non-covalently bound agent that is found in neither the educts or products. Thus, the term external stereoselection will be used to describe the enantiofacial outcome at the newly formed stereogenic centers. 

Transition-metal-catalyzed stereoselective reductions of vinylic sulfones with Grignard reagents are achieved with excess n-BuMgCl in tetrahydrofuran at room temperature.196,48 Better yields and selectivities are obtained with palladium catalysts [Pd(acac)2] than with nickel complexes such as [Ni(acac)2], especially if external ligands such as DABCO, triethylamine (TEA), or (n-Bu)2P are used.196,48 Nickel catalysts are used more often for reducing sulfonyl-1,3-dienes than palladium catalysts. When using this method, it is very important to remove the catalyst before isolation of the products in order to avoid isomerization of the... 

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