Alkenes stereospecific reactions

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

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

Cycloadditions with the Si(lOO) surface were theoretically [133] concluded to be reactions in the pseudoexcitation band. The conclusion is applicable to thermal [2+2] cycloaddition reactions of unsaturated bonds between heavy atoms. In fact, Sekiguchi, Nagase et al. confirmed that a Si triple bond underwent the stereospecific reactions with alkenes [137] along the path typical of [2+2] cycloaddition in the pseudoexcitation band. The stereospecific [2+2] cycloadditions of were designed by Inagaki et al. (Scheme 28) [138]. 

As with several other functional groups considered earlier, epoxides are most commonly found in alkaloid metabolites rather than the original compound. These epoxides may arise via oxidation of alkenes or aromatic hydrocarbons. Epoxide hydrolase catalyzes hydrolysis of epoxides to the more hydrophilic diol. As seen in Scheme 6, this is usually a stereospecific reaction that always yields a... 

This means that if a reaction is carried out on a compound that has no stereoisomers, it cannot be stereospecific but at most stereoselective. The concerted reactions, including SN2 displacements, E2 elimination of alkyl halides, anti and Syn addition to alkenes are all stereoselective. In the case of chiral or geometric substrates the nature of the product depends on the unique stereoelectronic requirement of the reaction. These are examples of stereospecific reactions. 

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 ... 

The E2 reaction is a stereospecific reaction, i.e. a particular stereoisomer reacts to give one specific stereoisomer. It is stereospecific, since it prefers the anfi-coplanar transition state for elimination. The (R,R) diastereomer gives a cis-alkene, and the (S,R) diastereomer gives a trans-alkene. 

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... 

Finally, C—C bond formation occurs through regio- and stereospecific reaction between allyl alcohols and ethyl magnesium chloride, in the presence of complex zirconium catalysts, followed by treatment with oxygen (equation 111)434. In this reaction the alkene functionality is lost. 

The heme-thiolate cytochromes P450 (P450) catalyze regio- and stereospecific reactions under physiological conditions [18-20], including the hydrox-ylation of hydrocarbons (Equation Eq. 1), alkene epoxidation, heteroatom (N,S) oxidation, dealkylation, and (anaerobic) dehalogenation [21]. 

Well before the wide use of organoselenium compounds in chemistry, it was discovered that electrophilic selenium compounds of the type RSeX add stereospecifically to alkenes.45 Since that time this reaction has been an important tool in the portfolio of organic chemists and has been used even for the construction of complex molecules. Comprehensive reviews on this chemistry have appeared46-49 and in recent times the synthesis of chiral selenium electrophiles and their application in asymmetric synthesis has emerged. As shown in Scheme 1, the addition reactions of selenium electrophiles to alkenes are stereospecific anti additions. They involve the initial formation of seleniranium ion intermediates 1 which are immediately opened in the presence of nucleophiles. External nucleophiles lead to the formation of addition products 2. The addition to unsymmetrically substituted alkenes follows the thermodynamically favored Markovnikov orientation. The seleniranium ion intermediates of alkenes with internal nucleophiles such as 3 will be attacked intramolecularly to yield cyclic products 4 and 5 via either an endo or an exo pathway. Depending on the reaction conditions, the formation of the seleniranium ions can be reversible. 

Hydroboration of alkenes is another example of a stereospecific reaction, in which different stereoisomers of the starting compound react to give different stereoisomers of the product. Problem 8-14 considers the different products formed by the hydroboration-oxidation of two acyclic diastereomers. 

Give mechanisms for these stereospecific reactions on single geometrical isomers of alkenes,... 

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. 

A lactone makes a good temporary tether because it can be hydrolysed or reduced to break the ring at the C-0 bond and reveal new stereogenic centres on the old structure. In this sequence a lactone, formed by iodolactonization, controls all the subsequent stereochemistry of the molecule in two ways it fixes the conformation rigidly in one chair form—hence forcing the iodide to be axial— and it blocks one face of the ring. The iodolactonization is very similar to one you saw on p. 872. Next, an alkene is introduced by E2 reaction on the iodide. This stereospecific reaction requires an... 

The complexes will effect oxyamination reaction with alkenes in a stereospecific reaction (Scheme 8).290 After reductive cleavage of the intermediate alkanolaminato complex (I) (see below) vicinal amino alcohols (II) are formed. The reaction is unusual in that the new C—N bond is always formed at the least substituted terminal alkenic carbon atom, and there is a clear preference for the imido complex to use its NR group for coordination to the osmium despite the steric restraints of R.299,300 However, the least sterically hindered part of the alkene moiety is attached to. the nitrogen atom.290,291,300 The yields of amino alcohols in the reaction can be improved by addition of tertiary alkyl bulkhead amines (see below). 

Aziridine formation fix>m arylnitrenes, rather than via triazolines, is known for highly fluorinated arenes. Phenyl azide with trifluoroacetic acid generates a nitrenium ion which adds stereospecifically to alkenes to give aziridines. Yields are rather low, partly due to concurrent ring opening of the aziridine by addition of trifluoroacetic acid. Similar reactions can be achieved with Lewis acids such as AlCb Enamines with aryl azides can yield either 2-aminoaziridines or amidines. ... 

Reaction of cyclic sulfates or thionocarbonates, derived from 1,2-diols, with telluride results in stereospecific alkene formation <1995TL7209>. This is illustrated by the conversion of the cyclic sulfate OTitra-l,2-diphenyl-l,2-ethanediol 49 into fif-stilbene exclusively by Te, as shown in Equation (13). Treatment of the cyclic sulfate of 47-1,2-diphenyl-1,2-ethanediol with Te produces /ra r-stilbene exclusively. These results are accounted for by intermolecular Te Sn2 displacement followed by intramolecular Sn2 displacement to form the corresponding tellurirane. The tellurirane then thermally loses tellurium stereoselectively forming alkene. Cyclic sulfates need not be used dimethanesulfonates or di-/i-toluenesulfonates prepared from 1,2-diols also, stereospecifically, provide alkenes via telluriranes <1993CC923, 1996SL655>. 

Because halogenation occurs exclusively in an anti fashion, cis and trans alkenes yield different stereoisomers. Halogenation of alkenes is a stereospecific reaction. 

Using c/s- and frans-3-hexene, demonstrate that the addition of HCI is not a stereospecific reaction. Draw the structure of the stereoisomers formed from each alkene. 

Carbene addition occurs in a syn fashion from either side of the planar double bond. The relative position of substituents in the alkene reactant is retained in the cyclopropane product. Carbene addition is thus a stereospecific reaction, since cis and trans alkenes yield different stereoisomers as products, as illustrated in Sample Problem 26.4. 

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