Selectivity and stereochemistry

Selectivity and Stereochemistry. An important property of transition-metal complexes is that they coordinate groups in a specific manner permitting high regio-and stereoselectivity in the catalytic reaction. The migratory insertion step is a highly stereospecific transformation. The four-center transition state 16 illustrated for the Wilkinson catalyst requires a coplanar arrangement of metal, hydride, and alkene n bond  

The coplanar migratory insertion and the subsequent reductive elimination occurring with retention of configuration at the M—C bond ensure an overall syn addition 

The complexation of an alkene to a metal ion strongly affects its reactivity namely, the double bond is activated toward nucleophilic attack. Ligands decreasing the electron density on the double bond accelerate the hydrogenation reaction. Electron-withdrawing substituents similarly facilitate hydrogenation of alkenes (PhCH=CH2 RCH=CH2).29 

The hydrogenation mechanisms discussed above indicate the reversible formation of intermediates. The degree of reversibility depends on the nature of the catalyst and the alkene, and on reaction conditions. Some nonreducible internal alkenes undergo slow hydrogen exchange and isomerization, indicating that reversible steps can occur in these cases. The reversible formation of alkylmetal intermediates provides a ready explanation for the isomerization of alkenes in the presence of certain metal complexes (see Section 4.3.2). 

The regioselectivity exhibited in the hydrogenation of monoenes also holds for the hydrogenation of dienes specifically, the terminal double bond in nonconju-gated dienes is usually reduced selectively 111 

Aromatic rings are hydrogenated with a variety of catalysts. However, aromatic alkoxy and hydroxyl substituents are susceptible to hydrogenolysis under most conditions used to saturate the ring. Hydrogenolysis does not occur to any appreciable extent with ruthenium catalysts even though high temperatures and pressures are required. Thus, substituted phenols are 

The hydrogenation of ring A aromatic steroids over ruthenium occurs, almost invariably, from the a side and all substituents on the original aromatic ring are cis in the resulting cyclohexane. Estrone (62) is hydrogenated over ruthenium to 5a,10a-estrane-3/3,17j6-diol (63) in 85-90% yield. 

The hydrogenation of equilenin (64) occurs predominantly by attack at ring A over both platinum and Raney nickel. With platinum extensive cleavage of the 3-hydroxyl group takes place. 

As mentioned earlier, the particular geometric arrangement around the alkyne triple bond can also play a great part in controlling semi-hydrogenation selectivity. In molecules with both alkenic and alkynic functionality it is possible to preserve the original alkene group only if its approach to the catalyst surface can be restricted in some way. This effect was demonstrated in the reaction shown by Eqs (17) and (18) [67]  

Conjugated alkynes are further challenges to the achievement of catalyst selectivity, and performance is very much dependent on the local environment of the enyne system. The hydrogenation of ethynylcyclohexene, for example, proceeded to 94% conversion over Lindlar s catalyst with selectivities of 86% for vinyl-cyclohexene and 8 % for ethylcyclohexene [68]  

If a substituent was present on the triple bond, reaction selectivity was found to degrade as evidenced by the following process [68]  

In this case, semi-hydrogenation proceeded to 86 % conversion with 78 % yield of the diene and 8 % of the propylcyclohexene over-hydrogenated product. 

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Throughout each chapter, clear structures, schemes, and figures accompany the text. Mechanism, reactivity, selectivity, and stereochemistry are especially addressed. Special emphasis is also placed on introducing both the logic of total synthesis and the rationale for the invention and use of important synthetic methods. In particular, we amplify the most important developments in asymmetric synthesis, catalysis, cyclization reactions, and organometallic chemistry. 

First, pericyclic reactions are defined, and an example of their unusual stereochemical selectivity is presented. A theoretical treatment of pericyclic reactions requires examination of the MOs for the conjugated molecules that participate in these reactions, so MO theory for these compounds is developed next. Then a theoretical explanation for the selectivity and stereochemistry observed in each of the three classes of pericyclic reactions is presented, along with a number of common examples of reactions of each kind. 

Understanding the Mechanism, Selectivity, and Stereochemistry of Pericyclic Reactions... 

This section concerns the classical hydrodimerization of alkenes activated by electron-withdrawing substituents, as in Eq. (1). The literature in this area is extensive and this chapter cannot be exhaustive. The focus will be on typical reactions and general conclusions, which may serve as guidelines for further work. Special emphasis will be put on the effect of reaction conditions on the mechanisms, product selectivity, and stereochemistry. Section II.A deals with the monoactivated alkenes, that is, structures of the type 1 where R and R" are H, alkyl, or aryl Sec. II.B deals with intramolecular coupling reactions where two identically activated alkenes are linked together within the same molecule. The reactions of alkenes activated by two electron-withdrawing groups either in a, a- or in a,yS-positions, are treated in Sec. II.C. 

Although hydrogenation of aromatic systems requires more energetic conditions than do those of olefinic double bonds, carbocyclic aromatics are readily hydrogenated to the fully saturated product. Controlling hydrogenolysis, selectivity and stereochemistry are major problems connected with arene hydrogenations. The substituents play an important role, and their effects are not the same for all catalysts. ... 

Desulfurization. The scope, selectivity, and stereochemistry of this method (C-S - C-H) has been studied using a variety of organosulfur compounds as substrates. /3-Lactams unsubstituted at C-4 are formed in a two-step process involving photocycloaddition of chromium carbene complexes and RN=C(SMe)2 followed by desulfurization. ... 

Use the principles of pericyclic reactions to explain the mechanism, selectivity, and stereochemistry of a concerted reaction. 

Copolymers from epoxides Catalyst activity, product selectivity, and stereochemistry control 12ACR1721. 

This reaction proceeds readily over solid acids and bases. The mechanism, selectivity, and stereochemistry of dehydrochlorination and dehydrobromination have been extensively studied in relation to the acid-base properties of catalysts, as have those in homogeneous liquid phase.It has been fairly well established that the... 

Treatment of benzyl halides with dihalomethane and NaHMDS affords vinyl halides with anti-Markovnikov selectivity and -stereochemistry [207] This conversion tends to work quite well following a Wittig-type process using CX or CHX [212,214, 215], Care must be exercised with this chemistry due to a propensity to generate the haloalkyne instead of the ge/n-dihalo species. Non-Wittig approaches have also been devised [210]... 

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