Preferred Stereochemistry

The preferred stereochemistry of addition to cyclic alkenes is anti The additions are not as highly stereoselective as hydrogen bromide addition, however. 

HOMO map for enolate B reveals preferred stereochemistry of electrophilic attack. 

It is important to note that the E isomer of the oxime predominates over the Z isomer in all the cases 21 a-f and the preferred stereochemistry for the isoxa-zolines 22a-f is trans. 

When re-alkyl ethynyl ketones were tested as the substrate of LBADH, the preferred stereochemistry and optical purity of the resulting propargylic alcohol were dependent upon the size of the alkyl group (Figure 7.26) [71]. 

The preferred stereochemistry arises from the transition state that minimizes interaction between the ethynyl and isopropyl substituents. This stereoselectivity is revealed in the rearrangement of 11 to 12. 

When recLBADH was tested with n-alkyl ethynyl ketones it was observed that the preferred stereochemistry of the resulting propargylic alcohol depends on the... 

With bidentate ligands, (Figure 14) it has been suggested that the fac isomer is preferred.167 However, structure determination of [WSCl3(mte)J (mte = l,2-bis(methylthio)ethane) has shown that it has the mer stereochemistry in the solid state.172 Whether this is indeed the preferred stereochemistry for complexes of the type [WYX3(L-L)] will have to await further studies. 

Taken together the barrier expressions describe the interplay of three effects (a) The /0G0 factor describes the energy cost due to undoing the pairing of bonding electrons in order to make new bonds, (b) The AErp factor accounts for the classical rate-equilibrium effect, (c) The quantity B is the transition state resonance energy, which involves information about the electronic structure of the transition state and the preferred stereochemistry of the reaction. 

Alkyl and aryl isothiocyantes, RCNS, parallel CS2 in their capability to insert into M—H bonds and yield products having the JV-alkyl- or JV-aryl-formamide chelate ligand RN=CH=S. The iridium(III) complexes [Ir(X)2(RN=CH=S)(PPh3)2] (207 X = Cl, Br R = Me, Et, Ph) can be prepared from trans-[Ir(H)(X)2(PPh3)2] and the appropriate alkyl or aryl isothiocyanate. Two isomeric forms of (207) were deemed plausible, with (207a) the preferred stereochemistry for the alkyl products and (207b) for the aryl products.453... 

Imines have been employed in [3+2] cycloaddition reactions with Fischer carbene complexes leading to 3-pyrroline derivatives, as shown by the conversion of the reactants 441 and 442 to the product 443 (Equation 123). The preferred stereochemistry of the resulting 3-pyrrolines is trans, with minor amounts of the m-isomers <2000JA11741 >. 

The above mono-nuclear iron(II) complex reacts with silver(I) to yield the hetero-dinuclear species, [FeAgL2]. In this, the iron(II) is bound in an octahedral site while the silver(I) ion occupies a tetrahedral one. The success of this synthesis thus depends upon a ligand design that allows programmed co-ordination of the respective metal ions such that each attains its preferred stereochemistry. 

A new synthesis" of ( )-dihydroantirhine (104) (Scheme 12) makes use of the modified Polonovski reaction for the construction of the indoloquinolizidine ring system in model experiments it had been established that the preferred stereochemistry of the product formed on closure of ring c was C-3, C-15 trans, in contrast to the other available methods of synthesis. However, the product obtained was proved to be an inseparable mixture of (104) and its C-20 epimer. 

Stereoselective A reaction in which a preferred stereochemistry is selectively produced. 

Does the "active" conformation of a cholinergic ligand correspond to its preferred stereochemistry or is an energetically less favored form bound to the receptor ... 

The preferred stereochemistry at C-22 (R) is predicted by the Cram or Felkin-Anh models for the transition state (27). The stereochemistry at C-23 appears to be determined by the approach of the anion as shown in Scheme I. It is... 

For polymeric chains, the act of coordination almost invariably requires a change in the shape of the chain as a consequence of satisfying the demands of the metal ion for a preferred stereochemistry and a set of donors with bond distances within a limited range. Thus, coordinate bond formation has consequences that clearly alter the local environment around the metal ion, but may also alter polymer chain conformation over an extended range. Since three-dimensional shape in biopolymers plays a role in function, natural complexation evolved by Nature usually plays a positive role, whereas unnatural complexation through the addition of foreign metal ions may be deleterious to function. 

---