Other Stereochemical Considerations

Migration to stereogenic centers in alkenes also routinely occurs with retention of stereochemistry. Eq. 12.50 shows the insertion of an isotopically-labeled -alkene into a M-D bond from Cp2ZrDCl. The product retains the stereochemistry of the alkene, thereby supporting a syn addition of the Zr-D bond. The syn addition leads to a picture of the insertion involving attack of the Zr and Don the same face of the alkene—that which is coordinated to the metal. 

The stereochemistry for migratory insertion of alkynes has led to effective methods for creating each of the different stereoisomers of deuterated terminal alkenes. Eq. 12.51 shows a sequence of reactions involving f-butylacetylene. Due to the 100% stereoselective syn addition of the Zr-D bond, only one product is obtained. 

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There are other stereochemical aspects to the reduction of aldehydes and ketones. If there is a chiral center to the carbonyl group, even an achiral reducing agent can give more of one diastereomer than of the other. Such diastereoselective reductions have been carried out with considerable success. In most such cases Cram s rule (p. 147) is followed, but exceptions are known. ... 

Moving on to some wider stereochemical considerations, just as enantiomers are indistinguishable as far as their physical and chemical properties are concerned (except, of course, as regards their reactions with other optically active reagents) so their spectra, acquired under normal conditions, are identical. The NMR spectrometer does not differentiate between optically pure samples and racemic ones. Note there is a way of differentiating between enantiomers by NMR but it involves using certain chiral reagents which we ll discuss in detail later. 

Stereochemical considerations in the E2 reactions The E2 follows a concerted mechanism, where removal of the proton and formation of the double bond occur at the same time. The partial 7t bond in the transition state requires the parallel alignment or coplanar arrangement of the p orbitals. When the hydrogen and leaving group eclipse each other (0°), this is known as the 5 yn-coplanar conformation. 

There are other stereochemical features which have nothing to do with the symmetry of the orbitals, and are much less powerfully controlled. In many cycloadditions, there are two possible all-suprafacial approaches one having what is called the extended transition structure 2.102, in which the conjugated systems keep well apart, and the other called the compressed 2.103, where they lie one above the other. Both are equally allowed by the rules that we shall see in Chapter 3, but one will usually be faster than the other. This type of stereochemistry applies only when the conjugated systems have at least three atoms in each component it is therefore only rarely a consideration. It shows up in the cycloadditions of allyl cations to dienes, where the two adducts 2.56 and 2.57 on p. 13 are the result of the compressed transition structure 2.104 and the extended 2.105, respectively, with the former evidently lower in energy. 

J. W., Mass Spectral Techniques in Heterocyclic Chemistry Applications and Stereochemical Considerations in Carbohydrates and Other Oxygen Heterocycles, 42, 335. 

Carbohydrates and other oxygen heterocycles, applications of mass spectral techniques and stereochemical considerations in, 42, 335 Carbolines, 3, 79... 

Mass spectral techniques in heterocyclic chemistry applications and stereochemical considerations in carbohydrates and other oxygen heterocycles, 42, 335 Mass spectrometry of heterocyclic compunds, 7, 301 of nucleic acids, 39, 79 Medium-large and large 7r-excessive heteroannulenes, 23, 55 Meso-ionic compounds, 19, 1 Metal catalysts, action on pyridines, 2, 179 Metalation, directed, of pyridines, quinolines, and diazines, 52, 187... 

Applications and Stereochemical Considerations in Carbohydrates and Other Oxygen Heterocycles ... 

One of the most successful and widely used methods for diastereoselective aldol addition reactions employs Evans imides 17 and the derived dialkyl boryleno-lates [8J. The 1,2-svn aldol adducts are typically isolated in high diastereoisomeric purity (>250 1 dr) and useful yields. More recent investigations of Ti(IV) and Sn(II) enolates by Evans and others have considerably expanded the scope of the aldol process [9], In 1991, Heathcock documented that diverse stereochemical outcomes could be observed in the aldol process utilizing acyl oxazolidinone imides by variation of the Lewis acid in the reaction mixture [10]. Thus, for example, in contrast to the, l-syn adduct (21) isolated from traditional Evans aldol addition, the presence of excess TiCL yields the complementary non-Evans 1,2-syn aldol diastereomer. This and related observations employing other Lewis acids were suggested to arise from the operation of open transition-state structures wherein a second metal independently activates the aldehyde electrophile. 

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