Stereospecific reactions examples

Bromine addition to alkenes is an example of a stereospecific reaction A stereospecific reaction is one m which stereoisomeric starting materials yield products... 

Another important use of BCl is as a Ftiedel-Crafts catalyst ia various polymerisation, alkylation, and acylation reactions, and ia other organic syntheses (see Friedel-Crafts reaction). Examples include conversion of cyclophosphasenes to polymers (81,82) polymerisation of olefins such as ethylene (75,83—88) graft polymerisation of vinyl chloride and isobutylene (89) stereospecific polymerisation of propylene (90) copolymerisation of isobutylene and styrene (91,92), and other unsaturated aromatics with maleic anhydride (93) polymerisation of norhornene (94), butadiene (95) preparation of electrically conducting epoxy resins (96), and polymers containing B and N (97) and selective demethylation of methoxy groups ortho to OH groups (98). 

Some stereospecific reactions are listed in Scheme 2.9. Examples of stereoselective reactions are presented in Scheme 2.10. As can be seen in Scheme 2.9, the starting materials in these stereospecific processes are stereoisomeric pairs, and the products are stereoisomeric with respect to each other. Each reaction proceeds to give a single stereoisomer without contamination by the alternative stereoisomer. The stereochemical relationships between reactants and products are determined by the reaction mechanism. Detailed discussion of the mechanisms of these reactions will be deferred until later chapters, but some comments can be made here to illustrate the concept of stereospecificity. 

Recall from Section 7.13 that a stereospecific reaction is one in which each stereoisomer of a particular starting material yields a different stereoisomeric form of the reaction product. In the examples shown, the product from Diels-Alder cycloaddition of 1,3-butadiene to c/s-cinnamic acid is a stereoisomer of the product from trans-cinnamic acid. Each product, although chiral, is formed as a racemic mixture. 

The hydroboration is a y -stereospecific reaction. For example reaction with 1-methylcyclopentene 11 yields the l,2-tra 5-disubstituted product 12 only ... 

This stereospecific reaction is, therefore, a rare example of stereochemical control by nucleophilic solvent assistance of an ionization process. 

However, if both maleic and fumaric acid gave the dl pair or a mixture in which the dl pair predominated, the reaction would be stereoselective but not stereospecific. If more or less equal amounts of dl and meso forms were produced in each case, the reaction would be nonstereoselective. A consequence of these definitions is that if a reaction is carried out on a compound that has no stereoisomers, it cannot be stereospecific, but at most stereoselective. For example, addition of bromine to methylacetylene could (and does) result in preferential formation of trans-1,2-dibromopropene, but this can be only a stereoselective, not a stereospecific reaction. 

Sulphoximines may also be obtained from sulphoxides by reaction with benzene- and toluenesulphonyl azides . Yields are often nearly quantitative and the reaction is usually carried out in the presence of copper. This reaction shows a high degree of stereospecificity for example, if (R)-methyl p-tolyl sulphoxide is the reactant then (S)-methyl-S-p-tolyl-iV-p-toluenesulphonylsulphoximine is the product (equation 66). It has been suggested that this reaction involves the intermediacy of a nitrene species . ... 

The most synthetically useful cheletropic elimination involves 2,5-dihydrothiophene-1,1-dioxides (sulfolene dioxides). At moderate temperatures they fragment to give dienes and sulfur dioxide.301 The reaction is stereospecific. For example, the dimethyl derivatives 22 and 23 give the E,E- and Z,E-isomers of 2,4-hexadiene, respectively, at temperatures of 100°-150°C.302 This stereospecificity corresponds to disrotatory elimination. 

Vinylic halides or vinyl-metal species couple with a very high degree of stereospecificity, cry-vinylic halides giving cis coupling products and tra/iy-vinylic halides trans coupling products (example 17, Table III). Aromatic halides give homo- or cross-coupling reactions (examples 18 and 19, Table III). 

Vinyl halides (example 17, Table VII) were first observed by Kroper to form acrylic esters by reaction with carbon monoxide under pressure and tetracarbonylnickel in methanol at 100°C. These reactions were later shown to occur under much milder conditions. Highly stereospecific reactions were observed c/s-vinyl halides gave cis-carbonylation products and trans-vinyl halides trans-carbonylation products (example 18, Table VII). Retention of configuration of alkyl substrates in carbonylation seems to be a general feature in carbon monoxide chemistry (193a). 

A reaction which leads to the exclusive or predominant formation of one of the several possible stereoisomeric compounds is called stereoselective reaction. A reaction where a given isomer leads to one product while another stereoisoimer leads to opposite product is called a stereospecific reaction. Let us illustrate this by examples. 

A cycloaddition reaction produces a ring of atoms by forming two new G-bonds, for example the formation of a cyclobutane dimer from two alkene molecules. The direct photoreaction involves the concerted reaction of the singlet Jtpt ) excited state of one alkene with the ground state of the other. Stereospecific reactions in which the dimers preserve the ground-state geometry occur when liquid cis- or trans-but-2-ene are irradiated at low temperature ... 

Hydride Transfer in NAD+- and NADP -Dependent Enzymes. The transfer of the hydride ion in redox reaction of NAD+- and NADP+-dependent enzymes can occur either to the re- or the xi-face of the pyridine ring of the coenzyme . Such stereochemistry is crucial in the characterization of these enzymes. The same enzymes from different sources can express different stereospecificities. For example, E. coli NAD(P)+ transhydrogenase expressed one form of stereospecificity whereas the Pseudomonas aeruginosa enzyme catalyzes the identical reaction with the other NAD form . 

Polymerization in which a tactic polymer is formed. However, polymerization in which stereoisomerism present in the monomer is merely retained in the polymer is not to be regarded as stereospecific. For example, the polymerization of a chiral monomer, e.g., R)-propylene oxide ((i )-methyloxirane), with retention of configuration is not considered to be a stereospecific reaction however, selective polymerization, with retention, of one of the enantiomers present in a mixture of R)- and (S)-propylene oxide molecules is so classified. 

Any reaction in which only one of a set of stereoisomers is formed exclusively or predominantly is called a stereoselective synthesis.184 The same term is used when a mixture of two or more stereoisomers is exclusively or predominantly formed at the expense of other stereoisomers. In a stereospecific reaction, a given isomer leads to one product while another stereoisomer leads to the opposite product. All stereospecific reactions are necessarily stereoselective, but the converse is not true. These terms are best illustrated by examples. Thus, if maleic acid treated with bromine gives the dl pair of 2,3-dibromosuccinic acid while fumaric acid gives the meso isomer (this is the case), the reaction is stereospecific as well as stereoselective because two opposite isomers give two opposite isomers ... 

Hydroboration of an alkene and subsequent reactions of the product trialkylborane, either with hydrogen peroxide or with acid, appear to be highly stereospecific. For example, 1-methylcyclopentene gives exclusively trans-2-methylcyclopentanol on hydroboration followed by reaction with alkaline... 

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

Treatment of 1,3,2-dioxathiolane. 7,.7-dioxides with telluride ion, generated in situ by reduction of the elemental Te, yields alkenes rapidly (10min-2h) under mild conditions (0°C to rt) <1995TL7209>. The reduction may be performed with 0.1 equiv or less of Te in the presence of a stoichiometric amount of LiEt3BH or NaH. The reaction is stereospecific, for example, / >-2,3-diphenylethane-2,3-diol produces ar-stilbene, and d,l-2,3-diphenylethane-2,3-diol gives trans-stilbene. The sulfates of m-diols are readily converted to ar-alkenes, as shown in Equation (12) <1995TL7209>. 

Fig. 3.8. A pair of stereospecific reactions (using the type of reaction in Figure 2.28 as an example).

If in a pair of analogous reactions one starting material is converted with complete stereo-selectively into a first product and a stereoisomer of the starting material is converted with complete stereoselectively into a stereoisomer of the first product, these reactions are called stereospecific. An example of this can be found in Figure 3.8. The SN2 reaction of 2 enantiomers is stereospecific too. 

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