Intrinsic stereoselectivity

With 2-butenyl- and 3-phenyl-2-propenylstannanes coupling of this Cram selectivity with the intrinsic stereoselectivity of the Lewis acid induced allylstannane-aldehyde reaction gives useful stereocontrol at three contiguous stereogenic centers66,81. 

Some care must be taken in drawing conclusions from the E/Z or syn/anti selectivity of a given catalyst/alkene combination. The intrinsic stereoselectivity may be altered in some cases by subsequent isomerizations initiated by the catalyst. For example, epimerization of disubstituted vinylcyclopropanes is effectively catalyzed by palladium compounds the cis - trans rearrangement of ethyl chrysanthemate or of chrysanthemic acid occurs already at room temperature in the presence of PdCl2 L2 (L = MeCN, EtCN, PhCN)96 Oxycyclopropane carboxylic esters undergo metal-... 

When (26) is added sufficiently slowly to this equilibrium system, it is maintained while (62) is formed via the a-adduct (67). Each of the species, or pairs of diastereomers (63)-(66) belongs to a pair of corresponding reactions whose stereoselectivity is determined by a pair of diastereomeric transition states. Thus (67), and thereby also (62), is formed by four competing pairs of corresponding reactions. Each one of these has its own intrinsic stereoselectivity Qpn.v (1 =63,...,66). The observed overall stere-... 

This approach represents a powerful method to enhance the intrinsic stereoselectivity of a given reactant and is called double or, more generally, multiple stereoselection . The reasons for the increased stereoselectivity of these processes are evident from the examples collected in Fig. 6 [33],... 

The Diels-Alder cycloaddition of acrolein with the (ft)-diene reported in reaction A affords a 4.5 1 ratio of the indicated diastereoisomers. When an enantiomerically pure (R)-dienophile is reacted with an achiral equivalent of the previous diene (Fig. 6 reaction B), two diastereoisomers are obtained in a 8 1 ratio. As the two major isomers of reactions A and B have the same configuration at the newly formed stereocenters, it can be anticipated that when the two enantiomerically pure (R)-partners are combined, the intrinsic stereoselection of each reactant is exalted to give a very stereoselective process. 

As can be seen from reaction C in Fig. 6, this is indeed the case and the expected product is obtained virtually as a single isomer (d.r. 40 1).The control experiment (reaction D) carried out with an (S)-dienophile and the (R)-diene confirms that in this case the two reactants contrast each other s intrinsic stereoselectivity, leading to poor stereocontrol. Reactions C and D are defined to occur between matched and mismatched reaction partners, respectively [33],... 

Although the [5+2] cycloadditions of alkynes and vinylcyclo-propanes service a number of objectives in synthesis, the [5+2] cycloaddition with alkenes also has enormous potential value and in addition presents a remarkable opportunity to address the intrinsic stereoselectivity of the process because two diastereoisomeric products can form. We began our investigation with substrate 38 (Table 4). Treating ene-vinylcyclopropane 38 with a catalyst derived from 0.1 mol% RhClfPPhjjj and 0.1 mol% AgOTf after 17 h at 110 °C gives cycloadduct 39 in 86% isolated yield as a single diastereomer. The... 

Stereoelectronic control also plays a role in mechanistic stereoselectivity. One such case is the very fundamental 8 2 process which proceeds rigorously with inversion of configuration at carbon. Because of that intrinsic and predictable stereoselectivity, the C-C disconnective Sn2 displacement transform is very important even though it does not directly reduce the number of stereocenters, e.g. 153 => 154. 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

Whole-cell biotransformations frequently showed insufficient stereoselectivities and/or undesired side reactions because of competing enzymatic activities present in the cells. These side reactions can modify the substrates and/or products. Furthermore, whole-cell biotransformations are limited due to the intrinsic need to grow biomass, which generates its own metabolites that are not related to the biotransformation reactions and, therefore, which need to be removed during the downstream process. Both the cells themselves and the unrelated metabolites produced are impurities that need to be removed after the biotransformation reaction. With isolated enzymes, there are no organism and unrelated metabolites to remove after the biotransformation processes. 

When the TMS group is absent and if the reaction is carried out in methanol, a platinum(ll)-catalyzed alkoxycy-clization takes place (Scheme 87).308 This cyclization catalyzed by Pt(ll) was found to be mechanistically similar to the carbohydroxypalladation reported by Genet.309 310 This process has intrinsic importance in organic synthesis since it allows the simultaneous and generally stereoselective formation of a C-O and a C-C bond to occur from an enyne system. This reaction has been applied for the synthesis of a key intermediate of podophyllotoxin.311... 

Functionalization of C-H bonds by metal carbenoid or nitrenoid insertions represents a promising alternative to the more traditional approach of direct activation by a metal center. Carbenoids and nitrenoids show unusual regio- and stereoselectivity in insertions into C-H bonds, and unlike insertions of metal centers, these are intrinsically functionalizations rather than activations, which must be followed by functionalization (although in either case, loss of the functionalized group, to regenerate the active metal complex, is still required for catalysis) [129]. The use of dimeric Rh(n) complexes in this area has been extensively developed [129]. 

Chemical reactions in the sohd state have intrinsic features different from those for reactions performed in solution or in the gaseous state. For example, sohd-state organic reactions often provide a high regio- or stereoselectivity because the reactions and the structiue of a product are determined by the crystal structure of the reactant, i.e., the reaction proceeds under crystaUine lattice control [1-8]. When the reactant molecules are themselves crystalhne (molecular crystals) or are included in host crystals (inclusion compounds), the rate and selectivity of the reaction are different from those obtained in an isotropic reaction medium. 

With respect to stereoselective synthesis, the application of CD spectroscopy is further reduced to a rather narrow range of functional (chromophoric) groups which are most frequently present in the product of synthesis in the vicinity of the newly formed stereogenic center. All this should not by any means discourage the use of CD spectroscopy for configurational studies. Rather, it is meant to show the intrinsic limitations of the method. 

Studies on stereoselective polymerization of racemic olefins also support this view.338 Polymerization of 3,7-dimethyl-l-octene (the chiral center is in a position to the double bond) took place with 90% stereoselectivity yielding an equimolar mixture of homopolymers of the two enantiomers. No stereoselectivity was observed in the polymerization of 5-methyl-1-heptene (the chiral center is in y position to the double bond). The conclusion is that the ability of a catalytic center to distinguish between the two enantiomers of a monomer required for stereoselective polymerization must arise from its intrinsic asymmetry. The first-ever chiral polypropylene synthesized using a chiral zirconium complex with aluminox-ane cocatalyst is the latest evidence to testify the role of the catalyst center in isotactic polymerization.339... 

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