Restricted Rotation about Single Bonds

The progress made in the field of modern structural chemistry during the past year has consisted in the main in the determination of the structures of a number of especially interesting molecules and crystals. I have been glad to have the opportunity provided by the exhaustion of the first edition of this book to revise it by the inclusion of references to these researches and of discussion of the new structures. A few corrections have been made, and the argument ip some places has been expanded in an effort to improve the clarity of its presentation. Two new sections have been added, dealing with restricted rotation about single bonds (Sec. 14d) and the conditions for equivalence or nonequivalence of bonds (Sec. 22a). 

In a few cases, single-bond rotation is so slowed that cis and trans isomers can be isolated even where no double bond exists (see also p. 230). One example is N-methyl-A -benzylthiomesitylide (69 and 70), the isomers of which are stable in the crystalline state but interconvert with a half-life of 25 h in CDCI3 at 50°C. This type of isomerism is rare it is found chiefly in certain amides and thioamides, because resonance gives the single bond some double-bond character and slows rota-tion. (For other examples of restricted rotation about single bonds, see pp. 230-233). 

Molecules chiral due to restricted rotation about single or double bonds (atropisomers) may act as the only chiromers in stereochemical reaction cycles. Each individual reaction is presumed to occur with retention, although conceivably a reaction could be induced to occur with inversion. Such a possibility is outlined in Chart XIII, but is contrived and specialized. 

Atropisomers Stereoisomers that result from restricted rotation about single bonds where the barrier for rotation is sufficient to allow isolation of the isomers. 

There are a number of structural features which have a bearing on the value of the glass transition temperature. Since this temperature is that at which molecular rotation about single bonds becomes restricted, it is obvious that these features are ones which influence the ease of rotation. These can be divided into two groups ... 

In addition to constitution and configuration, there is a third important level of structure, that of conformation. Conformations are discrete molecular arrangements that differ in spatial arrangement as a result of facile rotations about single bonds. Usually, conformers are in thermal equilibrium and cannot be separated. The subject of conformational interconversion will be discussed in detail in Chapter 3. A special case of stereoisomerism arises when rotation about single bonds is sufficiently restricted by steric or other factors that- the different conformations can be separated. The term atropisomer is applied to stereoisomers that result fk m restricted bond rotation. ... 

In the first of four chapters in this volume of Topics in Stereochemistry, Michinori Oki presents a comprehensive review of atropisomerism with special reference to the literature of the past two decades. The review summarizes restricted rotation about sp2-sp2, sp2-sp, and sp3-sp3 bonds and it concludes with an analysis of reactions of isolated rotational isomers. It places particular emphasis on the magnitude of rotation barriers as a function of structure (incidentally identifying some of the largest barriers yet measured to conformer interconversion) and on the isolation of stable single-bond rotational diastereomers. 

The energy profile for ring inversion in cyclohexane may be rationalized given what has already been said about single-bond rotation in -butane. Basically, the interconversion of chair cyclohexane into the twist-boat intermediate via the transition state can be viewed as a restricted rotation about one of the ring bonds. 

As important to biochemists as configurations, the stable arrangements of bonded atoms, are conformations, the various orientations of groups that are caused by rotation about single bonds.5 8 In many molecules such rotation occurs rapidly and freely at ordinary temperatures. We can think of a -CH3 group as a kind of erratic windmill, turning in one direction, then another. However, even the simplest molecules have preferred conformations, and in more complex structures rotation is usually very restricted. 

The chirality of biphenyls results from restricted rotation about a single bond imposed by the bulky nature of ortho substituents. Models will help you. visualize the degree of difficulty of having the substituents pass by one another. If X = H and Y = F (Table 13-3), the enantiomers are not stable at room temperature if X = H and Y = Br, they are marginally stable if X = H and... 

Figure 9 Example of a compound with severely restricted rotation about the single bond because of the bulkiness of the substituents, Rj R6.

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