Stereoselective control

A particularly challenging situation in catalysis in general and in olefin oligomerization in particular is that of stereoselectivity control, including diastereoselectivity and enantioselectivity (6). 

Stereoselective Control in Phase-transfer Catalysed Reactions... 

A degree of stereoselective control of the course of a reaction, which is absent or different from that prevalent when the reaction is conducted in the absence of quaternary ammonium salts, may be achieved under standard phase-transfer catalysed reaction conditions. The reactions, which are influenced most by the phase-transfer catalyst, are those involving anionic intermediates whose preferred conformations or configurations can be controlled by the cationic species across the interface of the two-phase system. For example, in the base-catalysed Darzens condensation of aromatic aldehydes with a-chloroacetonitriles to produce oxiranes (Section 6.3), the intermediate anion may adopt either of the two conformations, (la) or (lb) which are stabilized by interaction across the interface by the cations (Scheme 12.1) [1-4]. 

Once regioselective and stereoselective controls have been exerted, the cis-decalins must be isomerised to rrans-decalins, the configuration present in the target molecules. Since frans-decalins are thermodynamically more stable than the corresponding cis-decalins, it is possible to isomerise the latter through enolisation, a process that can be favored by the presence of a carbonyl group near to the centre to be inverted. 

In Heading 1.4 we have already seen that three different kinds of control elements [3] may be considered 1) chemoselective control elements (controlling chemical reactivity), 2) regioselective control elements (controlling the orientation of reactants) and 3) stereoselective control elements (controlling the spatial arrangement of atoms within the molecule), which may control either the relative (diastereoselective) or the absolute spatial arrangement (enantioselective control elements). 

Regarding stereoselective control elements, some of the most important and updated methods and strategies, have been already discussed in Chapters 8 and 9 (see also the Summary given below). 

Caryophyllenes, as an example of two naturally occurring isomeric sesquiterpenes containing a medium-sized ring, in which the success of the total syntheses lies in the stereoselective control of a chiral centre, in a common synthetic key intermediate, which governs the configuration (JE or Z) of the double bonds present in each one of the two isomers. In this context, a brief reference to Cecropia Juvenile Hormone synthesis by the Syntex group, as well as to Johnson s cationic cyclisation of unsaturated polyolefins to fused polycyclic compounds, is made. 

Natural chiral essential oil components generally have a characteristic enantiomeric distribution that is attributable to stereoselectivity — controlled biogenetic formation mechanisms. An excess of one enantiomer or the other occurs and can be detected, in a variety of essential oils and oleoresins. The authentic enantiomeric ratio of some essential oil components can be modified by the addition of synthetic racemic or natural ingredients (adulteration). 

Lithium 2-p-tolylsulfinylbenzyl carbanions have been reacted with different ALsubsLi Luted imines affording 1,2-diarylamines with high stereoselectivity control at both benzylic (only dependent on the sulfur configuration) and iminic carbons.51 (g) The anttsyn ratio was found to be dependent on the electronic density at the nitrogen atom. 

SCHEME 3.27 Stereoselective control of glycosylation by using glycosyl epoxide. TBAF, tetrabutylammonium fluoride. 

Figure 35 Different stereoselectivity controlled by Me3AI dimer...

Shichi, T., Takagi, K. and Sawaki, Y. (1996). Stereoselectivity control of [2 + 2] photocycloaddition by changing the site distances of hydrotalcite interlayers. J. Chem. Soc., Chem. Commun., 2027. 

Many transition metals and their compounds with organic ligands initiate the polymerization of alkenes and/or dienes. Some of them do not need any special treatment to this end while others require the presence of some organic or mineral compound or a special physical modification. In contrast to ZN catalysts, they are active without an organometal of Groups I—III. They are commonly known as metal alkyl free (MAF) catalysts. Many of their features are, of course, in common with ZN catalysts. MAF catalysts initiate stereoselectively controlled polymerization. Even less is known of their operating mechanism than that of ZN catalysts. It is assumed that propagation also occurs on the transition metal-carbon bond. 

Similarly, reversed-phase HPLC can be used as an Eilternative to the racemization test for amino acids as developed by Manning and Moore (115). Rivier and Burgus (109) have suggested the use of L-phenylalanine, coupled via the N-carboxyanhydride method to a hydrolysate, to monitor racemization during synthesis, although other hydrophobic L-amino acids should also prove equally effective. The use of /eri-butyloxycarbonyl-L-amino acid-Af-hydroxysuccinimide esters in the separation of enantiomeric amino acids and diastereoisomeric peptides has been described (110). Ultimately, these methods may not prove as versatile as the use of chiral stationary phases made by stereoselective control of the bonding process or, alternatively, with surface-active reagents similar to the D-... 

Thus far, we have considered the case for radical polymerization for the conversion of unsaturated monomers into polymer chains. However, none of the aforementioned techniques offer stereoselective control over the growing polymer chain, resulting in purely atactic polymers. In order to gain such control, it is necessary to spatially... 

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