Solvent effect stereochemistry

Discussions and studies of reaction mechanisms attempt to analyse the way in which a compound A is transformed into a compound B. Varying degrees of sophistication are attached to the phrase reaction mechanism but the aim is generally to define the reaction in terms of elementary steps and stereochemistry. In solution chemistry, the structures of compounds A and B will be known and mechanistic information may be deduced from kinetic studies, solvent effects, stereochemistry, isotopic labelling, and other slight structural modifications. 

Most Diels-Alder reactions, particularly the thermal ones and those involving apolar dienes and dienophiles, are described by a concerted mechanism [17]. The reaction between 1,3-butadiene and ethene is a prototype of concerted synchronous reactions that have been investigated both experimentally and theoretically [18]. A concerted unsymmetrical transition state has been invoked to justify the stereochemistry of AICI3-catalyzed cycloadditions of alkylcyclohexenones with methyl-butadienes [12]. The high syn stereospecificity of the reaction, the low solvent effect on the reaction rate, and the large negative values of both activation entropy and activation volume comprise the chemical evidence usually given in favor of a pericyclic Diels-Alder reaction. 

We have discussed in this chapter the thermal pyrolyses of a number of strained ring compounds. In most of the cases considered there is good evidence that the processes are unimolecular. Where possible we have tried to suggest plausible transition complexes, and reaction paths, based on a consideration of such factors as the kinetic parameters, stereochemistry of the reaction and effect of substituents. In reactions of this type, the description of the transition complex is fraught with difficulties, since the absence of such things as solvent effects (which can be so helpfrd in bimolecular reactions) limit the criteria on which such descriptions may be based. Often two types of transition complex may be equally good at accounting for the observed data. Sometimes one complex will explain some of the data while another is better able to account for the remainder. It is probable that in many cases our representation... 

Also known are 2 + 2 cycloadditions proceeding by way of a bipolar ion, path (b) of Scheme l.28 These reactions occur in situations such as that depicted in Equation 12.14, where the intermediate zwitterion (10) is strongly stabilized. Tetracyanoethylene adds by this mechanism to /Mnethoxyphenyl-,29 alkoxyl-,30 and cyclopropyl-31 substituted olefins. The additions show large solvent effects.32 Partial loss of stereochemistry occurs as in the biradical cases, but it is much less pronounced. 

The mechanism of insertion of 2-alkylphenylnitrenes into a 1,5-related CH bond was studied by three methods 80 determination of isotope effects, stereochemistry, and radical clock. During the formation of indolines, a kn/kD of 12.6-14.7 was observed coupled with complete loss of stereochemical integrity at the CH carbon. When the CH insertion carbon bore a cyclopropane group, ring-opening products were observed. These observations suggest a mainly radical H-atom abstraction mechanism. The sensitivity of the isotope effects to solvent was taken to imply a small concerted nitrene insertion contribution. 

Various mechanisms for the insertion reaction are conceivable (a) ionic stepwise, (b) radical (chain or nonchain), and (c) concerted. Generally, ionic or radical mechanisms give a mixture of products with cis and trans stereochemistry. In some special cases of the ionic reaction, however, exclusive formation of a trans product has been observed (62, 66). Therefore, stereoselectivity does not necessarily imply a concerted mechanism other evidence, e.g., regioselectivity, kinetic data, solvent effects, and substituent effects, must be sought out. 

Even with these limitations, nuclear magnetic resonance has made significant contributions to four areas of the chemistry of the platinum group metals bonding problems, molecular stereochemistry, solvation and solvent effects, and dynamic systems—reaction rates. Selected examples in each of these areas are discussed in turn. Because of space limitations, this review is not meant to be comprehensive. 

Both the nature of solvent and the temperature influenced the stereochemistry of boron enolates, with the solvent effect being greater than that of the temperature. Aliphatic and alicyclic hydrocarbon solvent favored the formation of yy/z-aldols at — 78 °C, whereas aromatic and chlorinated aliphatic solvents favored the formation of anti-dXdoh at 0-25 °C (Equation (179)).682 683... 

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