Stereospecific reactions inversion

Not stereospecific racemization ac companies inversion when leaving group IS located at a chirality cen ter (Section 8 10) Stereospecific 100% inversion of configuration at reaction site Nu cleophile attacks carbon from side opposite bond to leaving group (Section 8 4)... 

Stereospecific reaction (Section 7.13) Reaction in which stereoisomeric starting materials give stereoisomeric products. Terms such as syn addition, anti elimination, and inversion of eonfiguration describe stereospecific reactions. 

Substitution of the free epoxide, which generally occurs under basic or neutral conditions, usually involves an Sn2 mechanism. Since primary substrates undergo Sn2 attack more readily than secondary, unsymmetrical epoxides are attacked in neutral or basic solution at the less highly substituted carbon, and stereospecifically, with inversion at that carbon. Under acidic conditions, it is the protonated epoxide that undergoes the reaction. Under these conditions the mechanism can be either SnI or Sn2. In S l mechanisms, which favor tertiary carbons, we might expect that attack would be at the more highly substituted carbon, and this is indeed the case. However, even when protonated epoxides react by the 8 2 mechanism, attack is... 

A stereospecific chemical reaction is one in which starting substrates or reactants, differing only in their configuration, are converted into stereoisomeric products. Note, with this definition a stereospecific reaction has to be stereoselective whereas the inverse statement (that is, with respect to a stereoselective reaction or process) is not necessarily true. 2. Referring to reactions that act on only one stereoisomer (or, have a preference for one stereoisomer). Thus, many enzyme-catalyzed reactions are stereospecific, and characterization of that stereospecificity is always an issue to be addressed for a particular enzyme. 

Walden inversion (Section 8.4) Originally, a reaction sequence developed by Paul Walden whereby a chiral starting material was transformed to its enantiomer by a series of stereospecific reactions. Current usage is more general and refers to the inversion of configuration that attends any bi-molecular nucleophilic substitution. 

The bromination of cinnamic acid leads to two intermediate enantiomeric cyclic bromonium ions which are subsequently attacked by a nucleophilic bromide ion. Since this attack can occur both at positions 2 and 3, and because the reaction proceeds stereospecifically with inversion, both bromonium ions will give rise to mutually identical pairs of enantiomeric products. This is easier to see from the Fischer projection formulae, which are readily obtained from the zigzag projections by rotation around the C2-C3 bond. 

The addition of bromine to (R) -4-chlorocyclohex- 1-ene yields initially two diastereomeric cyclic bromonium ions, which then undergo a ring opening reaction with bromide ion. The attack of bromide ion is an Sn2 process and the reaction is stereospecific with inversion. Consequently, the bromo substituents in both diastereomeric products will have a trans relationship. 

The Sn2 displacement is a good example of a stereospecific reaction one in which different stereoisomers react to give different stereoisomers of the product. To study the mechanism of a nucleophilic substitution, we often look at the product to see if the reaction is stereospecific, with inversion of configuration. If it is, the Sn2 mechanism is a good possibility, especially if the reaction kinetics are second order. In many cases (no asymmetric carbon or ring, for example), it is impossible to determine whether inversion has occurred. In these cases, we use kinetics and other evidence to help determine the reaction mechanism. 

The starting acid contains an E-alkene that gives a tram iodonium ion. Inversion occurs in the attack of the carboxylate anion on the iodonium ion and we have shown this by bringing the nucleophile in at 180° to the leaving group with both bonds in the plane of the paper. A single diastereoisomer of the iodolactone results from this stereospecific reaction. 

The reaction proceeds stereospecifically with inversion of configuration at the carbon-oxygen bond that is converted into a carbon x hlorine bond. Thus treatment of D-( — )-2-carboxy-2,4,5-trimethyl-1,3-dioxolane (10) yields i.- + )-erythro-3-ch oro-2-butyl acetate (11). Treatment of (11) with KOH in ethylene glycol yields n-(-t )-2,3-cpoxybulane (12), a reaction known to proceed with inversion at the carbon-halogen bond. 

By use of Me3SiK, some mono-, di-, and trisubstituted epoxides can be deoxygenated stereospecifically, with inversion of stereochemistry [Eq. (49)1 (96, 97). Thus, reaction of cis and trans epoxides with Me3SiK in... 

Numerous data illustrate a third order rate dependence for this type of reaction, first order with respect to the phosphonium ion concentration and second order with respect to hydroxide ion concentration. The influence of the solvent has been studied. Optically active benzylethylmethylphenylphospho-nium hydroxide was shown to decompose stereospecifically with inversion to produce optically active ethylmethylphenylphosphine oxide. On the basis of these findings, the mechanism generally admitted is indicated in Scheme 38. 

Trialkylstannyl derivatives readily cyclize to three-membered ring products by replacement of the stannyl group under conditions of 3-stannylalkyl cation formation " . It has been shown that the cyclization proceeds stereospecifically with inversion of configuration at both reaction centers (equation 4) . The precursors for this reaction... 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

The reaction of the e t/o-halo ketone, e.g. endo-, is simple, leading exclusively to the ring-contracted bicyclo[3.1.0]hex-2-ene-en(fo-6-carboxylic acid endo- l, R = H) in 97% yield, or derivatives thereof. The rearrangements occur stereospecifically with inversion of configuration at the halogen-bearing carbon. ... 

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