Stereo-specific addition

A stereo specific addition of BufOI to /1-methylstyrene was observed in the presence of BF3, yielding Markovnikov products. This result contrasts with the non-stereospecific addition of Bu OCl and Bu OBr. It has been suggested that the bridging in the intermediate chloronium and bromonium ion derived from PhCH=CHMe is not as symmetrical as in the iodonium ion. Consequently, charge develops on the benzylic carbon in the first two cases, and rotation occurs about the C—C bond190. By contrast, a radical mechanism is assumed in the absence of BF3 as anti-Markovnikov products are formed (both in the dark and upon UV irradiation)190. 

Both singlet and triplet nitrenes add to C=C double bonds, but by different mechanisms. The stereo specific addition of singlet aroyl nitrenes occurs in a single step. Triplet nitrenes add in two steps and therefore isomerization by rotation about the former C=C-bond may lead to a total loss of the geometrical information in the starting material. The standard... 

A new technique for a determination of spin-multiplicities is the CIDNP-method. From emission signals in the nmr-spectrum a singlet or triplet state is deduced in insertion or addition reactions. This technique has been applied recently to 3a and 3g CIDNP-measurements in cyclohexane or CCI4 demonstrate that 3a reacts as singlet whereas 3g (di-f-butyl-derivative) involves a triplet intermediate. The latter experiment is in contrast with the results of stereo-specific addition of 3g to cis- or trans-huiene However the faster rate of addition versus insertion is thought to be responsible for this descrepancy... 

In some instances the vinyl azide does not undergo the conversion to the 2/f-azirine and instead yields a nitrene, which adds to alkenes and dienes. This is seen with the azide 653 which, on irradiation through Pyrex, affords the corresponding nitrene that adds to alkenes stereospecifically to yield aziridines. Stereo specific addition to dienes is exemplified by the irradiation of the quinone azide 654 in the presence of trans,trans-hQX3,-2,4-diene. Aziridines do not result from this process and instead the stereoisomeric indole derivatives 655 and 656 are obtained. This process has been developed as a route to... 

A one-pot tandem addition/oxidative decomplexation methodology has also been developed for naphthalene which yields 1,4-disubstituted dihydronaphthalenes from osmium(ll) naphthalene complexes. The process involves the re-gio- and stereo specific addition of an electrophile and a nucleophile to generate cfs-l,4-dihydronaphthalene complexes. Oxidative decomplexation produces 1,4-dihydronaphthalenes in good overall yield (Table 5) [27]. 

Methylene transfer from lodo methylzinc iodide converts alkenes to cyclopropanes The reaction is a stereo specific syn addition of a CH2 group to the double bond... 

The reaction is easy to carry out and yields are usually high Epoxidation is a stereo specific syn addition... 

The stereochemistry of both chlorination and bromination of several cyclic and acyclic dienes has been determined. The results show that bromination is often stereo-specifically anti for the 1,2-addition process, whereas syn addition is preferred for 1,4-addition. Comparable results for chlorination show much less stereospeciftcity. It appears that chlorination proceeds primarily through ion-pair intermediates, whereas in bromina-hon a stereospecific anfi-l,2-addition may compete with a process involving a carbocation mtermediate. The latter can presumably give syn or anti product. 

Two different routes to PCs via bicyclo[3.1.0]hexane intermediates are shown. In route 1 stereo- and position-specific addition of dichloroketene to a bicyclo[3.1.0]hexene provided the framework for elaboration to prostanoids. Route 2 featured stereospecific internal cyclopropanation and stereospecific Sn2 displacement of carbon to establish the prostanoid nucleus. 

Furthermore, a neighboring group participation of a phenylthio function is observed in the Lewis acid-catalyzed nucleophilic substitution reaction of various P-nitrosulfides. Because the P-nitrosulfides are readily available, by the Michael addition of thiols to nitroalkenes (see Michael addition Chapter 4), this reaction is very useful. The P-nitrosulfides are prepared stereoselectively, and the reaction proceeds in a stereo-specific way (retention of configuration) as shown in Eqs. 31-34.35... 

While Baldwin and Barden119 found that triphenyltin deuteride added to the carbon-carbon triple bond of phenylacetylene in a stereo specific reaction, several workers120-124 have found that the addition of a trialkyltin hydride to a carbon-carbon triple bond gives a mixture of the cis and trans isomers (equation 89). The more stable trans isomer is produced in the highest yield. 

The bromination of /razw-stilbenes in methanol, trifluoroethanol and in acetic acid leads almost exclusively to the erythro adducts via a 100% anti addition, regardless the substituents and the solvent. In contrast, the stereochemistry of the reaction of c/s-stilbenes exhibits a considerable dependence on the substituents and on the solvents the reaction of p-methoxystilbenes is always stereo-convergent, that of p,p -bis(trifluoromethyl)stilbenes is stereo-specific in all investigated solvents, whereas unsubstituted stilbenes can produce variable stereochemical outcomes ranging from stereo-specific to stereo-convergent in going from methanol to trifluoroethanol as solvent. 

When acetylenes, or alkenes having an electron-withdrawing group attached to the carbon-carbon double bond, are added to the solution t-BuSOH undergoes rapid addition to the triple or double bond in the fashion shown in (4) and (5). The addition to acetylenes was shown to take place stereo-specifically in a cis fashion. 

Another classical case with respect to ort/zo-effects is found for 2-nitrostyrene78. The conceivable regio- and stereo-specifically labelled 2-nitrostyrenes have, in addition to the ring-labelled isotopomer, been studied by collision activation mass spectrometry79. Undoubtedly, the most striking result was the nearly equal contribution of both (in the neutral molecule diastereotopic) hydrogens of the fi-carbon. 

The experimental ratio of ds- to trans-cyclopropane 43 46, i.e. the stereo-specifity of the reaction cannot be considered as a simple indication of singlet or triplet percentage of RaC , since the stereochemistry of the cyclo-addition depends on many factors. Photolysis produces the exdted 5i-state of the diazoalkane 41. This compound can lose nitrogen and form the singlet carbene 42 (So-state). 42 can add directly in a stereospecific manner if ki is large. If, however, intersystem crossing 42 45 (Aisc is large) competes favorably with... 

The addition of alkenes to A -methylenium amide cations is stereo-specific in the sense that it obeys the cis principle of Alder and Stein, cis-2-butene giving a cis product (105, R = R = Me) and the trans isomer giving trans product (105, R = R = Me). The cycloaddition of unsymmetrical alkenes is highly regiospecific. Knowing how the alkene will react with a large cation allows prediction of the product (105). Thus styrene, butadiene, and vinyl acetate all react so that its substituent appears at position 6 of the dihydrooxazinium salt (105, R = Ph, vinyl, or OAc). 

The most attractive approach for the 2-deoxy-p-glycosides starts with 2-bromo-2,6-dideoxyhexopyranosyl bromides (2 ) such as These are available regio- and stereo-specifically from readily accessible and simple precursors by reaction with dibromomethyl methyl ether (DBE) (22, 2 ). In principle, the formation of related compounds may be anticipated by the addition of bromine to glycal precursors. Previous studies (24-27). however, proved these processes to yield several isomers which renders this approach of little preparative value. 

Aldolases are part of a large group of enzymes called lyases and are present in all organisms. They usually catalyze the reversible stereo-specific aldol addition of a donor ketone to an acceptor aldehyde. Mechanistically, two classes of aldolases can be recognized [4] (i) type I aldolases form a Schiff-base intermediate between the donor substrate and a highly conserved lysine residue in the active site of the enzyme, and (ii) type II aldolases are dependent of a metal cation as cofactor, mainly Zn, which acts as a Lewis acid in the activation of the donor substrate (Scheme 4.1). 

The cyclic mechanism would predict that the addition reaction would be stereo-specific with respect to the geometry of the double bond in the allylic group. This has been demonstrated to be the case. The E- and Z-2-butenyl cyclic boronate esters 1 and 2 were synthesized and allowed to react with aldehydes. The /- -boronate gave the carbinol having anti stereochemistry whereas the Z-boronate gave the syn product.34... 

A useful group of rubbers are the stereo specific poly(butadiene) rubbers formed by the polymerization of 1,3-butadiene. These rubbers have a ris-isomer content of more than 30%. They contain at least about 85% of poly(butadiene) formed by 1,4 addition. Further, the rubber should have a second order transition temperature of preferably not higher than -20°C (8). 

A second question posed by the olefin additions is one of stereochemistry. A concerted ring formation of the type shown in Equation 5.35 implies stereo-specific cis addition, a suggestion first made in 1956 by Skell and Woodworth.170... 

Such additions are very rare, apparently because the allowed ir2s + n2s process is much more accessible (see Chapter 11.3). Most photo Diels-Alder reactions that have been studied involve attack of a triplet diene on a singlet olefin and thus proceed by an intermediate biradical. Both intramolecular75 and intermolec-ular76 concerted Diels-Alder reactions have, however, recently been observed. For example, photolysis of 41, through a w4s + Tt2a transition state, stereo-specifically forms 42. Similarly, 43 is converted to 44. Nevertheless, not all intramolecular Diels-Alder reactions brought about by direct irradiation are stereospecific.77... 

Thiiranes can be formed directly and stereospecifically from 1,2-disubstituted alkenes by addition of trimethylsilylsulfenyl bromide, formed at -78 C from reaction of bromine with bis(trimethylsilyl) sulfide (Scheme 7).12 A two-step synthesis of thiiranes can be achieved by addition of succinimide-A/-sulfe-nyl chloride or phthalimide-A -sulfenyl chloride to alkenes followed by lithium aluminum hydride cleavage of the adducts (Scheme 8).13 Thiaheterocycles can also be formed by intramolecular electrophilic addition of sulfenyl chlorides to alkenes, e.g. as seen in Schemes 914 and 10.13 Related examples involving sulfur dichloride are shown in Schemes 1116 and 12.17 In the former case addition of sulfur dichloride to 1,5-cyclooctadiene affords a bicyclic dichloro sulfide via regio- and stereo-specific intramolecular addition of an intermediate sulfenyl chloride. Removal of chlorine by lithium aluminum hydride reduction affords 9-thiabicyclo[3.3.1]nonane, which can be further transformed into bicyclo[3.3.0]oct-1,5-ene.16... 

With olefins two reactions occur (a) insertion in one of the carbon-hydrogen bonds, and (6) reaction at the double bond to give an episulfide. The insertion reaction is suppressed by addition of carbon dioxide as would be expected. In the case of singlet methylenes, reactions with the double bond are stereospecific while those of the triplet methylenes are not. Here Gunning and his coworkers found the sulfur atoms to behave differently and both singlet and triplet atoms reacted stereo-specifically with double bonds in olefins. 

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