Chain entry

Scheme 12.10 gives some examples of these oxidations. Entry 1 is one of several aryl-conjugated alkenes that were successfully epoxidized. Entry 2 is a reaction that was applied to enantioselective synthesis of the taxol side chain. Entry 3 demonstrates... 

A number of substituted allylarenes were synthesized under mild conditions using Et3N as an additive (Table 9.5). Curiously, only products of endocyclic P-hydride elimination (from the homobenzylic position) were observed, though mixtures of E/Z isomers were obtained from substrates bearing aliphatic chains (entry 4, Equation 9.3). In entry 3, the substrate cannot undergo a 1,5-hydride shift hence, the product resulting from a 1,6-hydride shift was obtained. 

The rearrangement of the allylic acetate in the prostaglandin side chain (entries 3 and 4) demonstrates the influence of chirality and double-bond geometryl8- 24-25-39. Oxygen transposition in the (Z)-allylic acetate led to the (15.S (-acetate with the E configuration of the new double bond, whereas the ( )-acetate gave under similar conditions the (15R)-acetate, also with the E configuration of the double bond. 

On the other hand, diols were ineffective for the enhancement of decrosslinking even though they have a long alkyl chain (entries 12-14). 

The existence of this process demonstrates that the structure of the amorphous intercrystalline layers in a semicrystalline polymer is different from a polymer melt. The reason can be easily seen All the chain sequences are fixed with their ends in the crystallites and, furthermore, the concentration of entanglements is enhanced. As a consequence, the mean chemical potential of the units is higher than in a melt and varies with the layer thickness. The direction of change is obvious. The numbers of entanglements and points of chain entry into the crystallites are constant. The motional restrictions thus become diminished if the layer thickness increases, which implies a decrease in the chemical potential. Under such conditions each change in temperature leads to a new local equilibrium between crystallites and amorphous regions, via a surface crystallization or melting process. 

One of the virtues of the Fischer indole synthesis is that it can frequently be used to prepare indoles having functionalized substituents. This versatility extends beyond the range of very stable substituents such as alkoxy and halogens and includes esters, amides and hydroxy substituents. Table 7.3 gives some examples. These include cases of introduction of 3-acetic acid, 3-acetamide, 3-(2-aminoethyl)- and 3-(2-hydroxyethyl)- side-chains, all of which are of special importance in the preparation of biologically active indole derivatives. Entry 11 is an efficient synthesis of the non-steroidal anti-inflammatory drug indomethacin. A noteworthy feature of the reaction is the... 

Double bonds m the mam chain are signaled by the ending enow acid and their position IS designated by a numerical prefix Entries 6 and 7 are representative carboxylic acids that contain double bonds Double bond stereochemistry is specified by using either the cis-trans or the E-Z notation... 

This intricate mode of crystallization requires more time to accomplish than, say, the entry of small ions into growing salt crystals. This, coupled with low chain mobility due to viscous effects, makes the rate of crystallization slow and accounts in part for the fact that with rapid cooling-called quenching-the temperature drops below T without crystallization. 

Table 4.1 lists values of as well as AH and ASf per mole of repeat units for several polymers. A variety of experiments and methods of analysis have been used to evaluate these data, and because of an assortment of experimental and theoretical approximations, the values should be regarded as approximate. We assume s T . In general, both AH and ASf may be broken into contributions Ho and So which are independent of molecular weight and increments AHf and ASf for each repeat unit in the chain. Therefore AHf = Hq + n AHf j, where n is the degree of polymerization. In the limit of n AHf = n AHf j and ASf = n ASf j, so T = AHf j/ASf j. The values of AHf j and ASf j in Table 4.1 are expressed per mole of repeat units on this basis. Since no simple trends exist within these data, the entries in Table 4.1 appear in numbered sets, and some observations concerning these sets are listed here ... 

Radicals generated from water-soluble initiator might not enter a micelle (14) because of differences in surface-charge density. It is postulated that radical entry is preceded by some polymerization of the monomer in the aqueous phase. The very short oligomer chains are less soluble in the aqueous phase and readily enter the micelles. Other theories exist to explain how water-soluble radicals enter micelles (15). The micelles are presumed to be the principal locus of particle nucleation (16) because of the large surface area of micelles relative to the monomer droplets. 

Polymer crystals most commonly take the form of folded-chain lamellae. Figure 3 sketches single polymer crystals grown from dilute solution and illustrates two possible modes of chain re-entry. Similar stmctures exist in bulk-crystallized polymers, although the lamellae are usually thicker. Individual lamellae are held together by tie molecules that pass irregularly between lamellae. This explains why it is difficult to obtain a completely crystalline polymer. Tie molecules and material in the folds at the lamellae surfaces cannot readily fit into a lattice. 

Fig. 3. Polymer single crystals (a) flat lamellae and (b) pyramidal lamellae. Two concepts of chain re-entry are illustrated (6).

The addition of S—H compounds to alkenes by a radical-chain mechanism is a quite general and efficient reaction. The mechanism is analogous to that for hydrogen bromide addition. The energetics of both the hydrogen abstraction and addition steps are favorable. Entries 16 and 17 in Scheme 12.5 are examples. 

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