Stereospecificity and Mechanism

For instance, in the field of elastomers, alkyllithium catalyst systems are used commercially for producing butadiene homopolymers and copolymers and, to a somewhat lesser extent, polyisoprene. Another class of important, industrial polymerization systems consists of those catalyzed by alkylaluminum compounds and various compounds of transition metals used as cocatalysts. The symposium papers reported several variations of these polymerization systems in which cocatalysts are titanium halides for isoprene or propylene and cobalt salts for butadiene. The stereospecificity and mechanism of polymerization with these monomers were compared using the above cocatalysts as well as vanadium trichloride. Also included is the application of Ziegler-Natta catalysts to the rather novel polymerization of 1,3-pentadiene to polymeric cis-1,4 stereoisomers which have potential interest as elastomers. 

Bauld, N. L., and Yang, J. "Stereospecificity and Mechanism in Cation Radical Diels-Alder and Cyclobutanation Reactions." Org. Lett, X 773-774 (1999). Gao, D., and Bauld, N. L. Mechanistic Implications of the Stereochemistry of the Cation Radical Diels-Alder Cycloaddition of 4-(cis-2-Deuteriovinyl)anisole to 1,3-Cyclopentadiene." /. Org. Chem., 65,6276-6277 (2000). Saettel, N. J., Oxsgaard, J., and Wiesl, O. "Pericyclic Reactions of Radical Cations." Eur. /. Cftem., 1429-1439 (2001). 

Origin of the stereospecificity and mechanism of isotactic propene polymerization with the initiator [Zr(r -indenyl)(Me)] -i- [MeMAO] of C2 symmetry... 

The reaction is stereospecific and can be described as a [tz2 + mechanism requires that inversion of configuration occur at C-4 as the new [Pg.760]

Although the emphasis in this chapter has been on tbe synthesis and mechanism of formation of simple enamines, brief mention will be made of the addition of amines to activated acetylenes to indicate the interest and activity in this area of substituted enamines. Since such additions tend to be stereospecific, inclusion in this section seems apropos. The addition of amines to acetylenes has been much studied 130), but the assigning of the stereochemistry about the newly formed double bond could not be done unequivocally until the techniques of NMR spectroscopy were well developed. In the research efforts described below, NMR spectroscopy was used to determine isomer content and to follow the progress of some of the reactions. 

The Pummerer reaction346 of conformationally rigid 4-aryl-substituted thiane oxides with acetic anhydride was either stereoselective or stereospecific, and the rearrangement is mainly intermolecular, while the rate-determining step appears to be the E2 1,2-elimination of acetic acid from the acetoxysulfonium intermediates formed in the initial acetylation of the sulfoxide. The thermodynamically controlled product is the axial acetoxy isomer, while the kinetically controlled product is the equatorial isomer that is preferentially formed due to the facile access of the acetate to the equatorial position347. The overall mechanism is illustrated in equation 129. 

In investigating the mechanism of addition to a double bond, perhaps the most useful type of information is the stereochemistry of the reaction. The two carbons of the double bond and the four atoms immediately attached to them are all in a plane (p. 8) there are thus three possibilities. Both Y and W may enter from the same side of the plane, in which case the addition is stereospecific and syn they may enter from opposite sides for stereospecific anti addition or the reaction may be nonstereospecific. In order to determine which of these possibilities is occurring in a given reaction, the following type of experiment is often done YW is added to the cis and trans isomers of an alkene of the form ABC=CBA. We may use the cis alkene as an example. If the addition is syn, the product will be the erythro dl pair, because each carbon has a 50% chance of being attacked by Y ... 

Kinetic inhibitors for hydrate formation may also be effective in preventing scale deposition [1627]. This may be understood in terms of stereospecific and nonspecific mechanisms of scale inhibition. 

The mechanistic aspects of nucleophilic substitution reactions were treated in detail in Chapter 4 of Part A. That mechanistic understanding has contributed to the development of nucleophilic substitution reactions as important synthetic processes. Owing to its stereospecificity and avoidance of carbocation intermediates, the Sw2 mechanism is advantageous from a synthetic point of view. In this section we discuss... 

The evidence for the mechanism is that it is a second order reaction. Further, since the reaction proceeds readily in nonpolar solvents, hence the formation of ions from perbenzoic acid, as postulated earlier, is inhibited. The final evidence is that the reaction is stereospecific and proceeds with the retention of cis or trans configurations present in the initial olefine. 

Rose n> has distinguished between substrate specificity (which may not seem stereospecific) and reaction stereospecificity (which is always present in enzyme reactions) in a thought-provoking essay, in which he also describes the usefulness of stereospecificity generally in studying enzyme mechanisms. He has done this so much better than I could, that I will not try to add to this subject. He has also pointed out how stereo-specificity may be useful in the study of evolution. This is a subject I would like to amplify. The less we know, the more we say. Nevertheless,... 

The Cope rearrangement is the conversion of a 1,5-hexadiene derivative to an isomeric 1,5-hexadiene by the [3,3] sigmatropic mechanism. The reaction is both stereospecific and stereoselective. It is stereospecific in that a Z or E configurational relationship at either double bond is maintained in the transition state and governs the stereochemical relationship at the newly formed single bond in the product.137 However, the relationship depends upon the conformation of the transition state. When a chair transition state is favored, the EyE- and Z,Z-dienes lead to anli-3,4-diastereomcrs whereas the E,Z and Z,/i-isomcrs give the 3,4-syn product. Transition-state conformation also... 

Due to the formal analogy to the classical Diels-Alder reaction, the mechanism of cyclic peroxide formation through cycloadditions of 1,3-dienes with O2 was considered for a long time to involve a concerted suprafacial [4 4- 2]-cycloaddition of a super-dienophile, namely a singlet oxygen to 1,3-dienic system In such a case, the concerted or almost concerted cycloaddition must be c -stereospecific and the stereochemical properties of the diene must be reflected in the three-dimensional structure of cyclic peroxide according to well-defined rules. Indeed, it was found in early stereochemical... 

The various kinds of growing species differ not only in their propagation but also in their stereochemical preferences. Professor Hogen-Esch will review this subject in his talk on anionic oligomerization of some vinyl monomer, and mechanisms of anionic, stereospecific polymerization of 2-vinyl pyridine will be discussed by Dr. Fontanille. In this context, the interesting paper of Schuerch et al.(12) deserves attention. Their work clearly reveals the effect of cation solvation upon the mode of monomer s approach to the growing centers. 

In support of the possible concerted nature of cycloreversion in bicyclo[2.2.0]hexane systems, the first example of a remarkably stereospecific and orbital-symmetry-predicted cleavage has been observed for the pyrolysis of ann -l,2,5,6-tetracyanotricyclo[4.2.0.02 5]octane (30) in the crystalline form to (Z, >1,2,5,6-tetracyanocycloocta-l,5-diene (32).120 This reaction should be kinetically preferred because it requires only minimal molecular motions in the solid state. However, (Z,Z)-l,2,5,6-tctracyanocycloocta-l,5-diene (31) is the only low-molecular thermolysis product when 30 is heated at 270 °C, presumably via a diradical mechanism.120... 

The addition of iodine azide to double bonds gives p-iodo azides.674 The addition is stereospecific and anti, suggesting that the mechanism involves a cyclic iodonium ion interme-... 

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