Substitution, nucleophilic structure

The general approaches for the synthesis of poly(arylene ether)s include electrophilic aromatic substitution, nucleophilic aromatic substitution, and metal-catalyzed coupling reactions. Poly(arylene ether sulfone)s and poly(arylene ether ketone)s have quite similar structures and properties, and the synthesis approaches are quite similar in many respects. However, most of the poly(arylene ether sul-fone)s are amorphous while some of the poly(arylene ether)s are semicrystalline, which requires different reaction conditions and approaches to the synthesis of these two polymer families in many cases. In the following sections, the methods for the synthesis of these two families will be reviewed. 

Figure 8.4. Optimal structure of the transition state for addition of an X -substituted nucleophile to a carbonyl group according to orbital interaction considerations.

The source of nitric oxide was diethylenetriamine/nitric oxide, DETA/ NO (Scheme 20.4) a compound that has been used in studies of the cytostatic, vasodilatory and other pharmacological properties of NO [65-67]. DETA/NO is a 1-substituted diazen-l-ium-l,2-diolate containing the [N(0)N0] functional group that has been proved to be useful for the reliable generation of nitric oxide in homogenous solutions [68]. When dissolved in blood, cell culture medium or buffer this compound dissociates to generate NO leaving the nucleophilic structure as a by-product. 

Sato/Kaneko [104] and Carreira [105] have independently employed acetoacetate-derived O-silyl dienolates as Si-substituted nucleophiles in asymmetric catalytic aldol reactions. The aldol products, d-hydroxy-/3-ketoesters, and the derived syn- and anti-yS,d-diol esters are ubiquitous structural subunits in biologically active natural products such as the polyene macrolide antibiotics. These structural subunits are also found in chemotherapeutics, most notably compactin analogs [106] that have been studied as... 

Other cases in which second-order kinetics seemed to require an associative mechanism have subsequently been found to have a conjugate base mechanism (called S ICB, for substitution, nucleophilic, unimolecular, conjugate base in Ingold s notation ). These reactions depend on amine, ammine, or aqua ligands that can lose protons to form amido or hydroxo species that are then more likely to lose one of the other ligands. If the structure allows it, the ligand Irons to the amido or hydroxo group is frequently the one lost. 

Okamoto, K., I. Nitta, T. Imoto, and H. Shingu (1967), Kinetic Studies of Bimolecular Nucleophilic Substitution. II. Structural Effects of Alkyl Halides on the Rate of SN2 Reactions—A Reinvestigation of the Linear Free-energy Relationships for the Structural Variation of the Alkyl Groups, Bull. Chem. Soc. Jpn., 40, 1905-1908. 

As mentioned, the reaction may proceed by substitution nucleophilic unimolecular (Sjjl) or substitution nucleophilic bimolecular(Sj ). The conditions and structure of the ether will determine which way the reaction goes, and Sp are outlined below. 

Table 6.2 should be helpful. It summarizes what we have said so far about the two substitution mechanisms, and it compares them with respect to two other variables, solvent and nucleophile structure, which we will discuss here. 

Sulfur ylides represent another family of nucleophiles structurally coupled with a leaving group. It is therefore to be expected that they will react with imines to give aziridines via an addition-substitution sequence. In fact, the sulfur ylid obtained by metallation c trimethylsulfonium iodide with n-butyl lithium reacts at -30 C in THF with both silylimines and p-methoxyphenyl imines to give the corresponding aziridines. Again a single isomer is obtained in the case of (S)-lactaldehyde imines (Scheme 22). 

The addition of a chiral allenyl metal to an aldehyde generating a 2-substituted butynyl structure is named Marshall-Tamaru MT) reaction (Scheme 5-13). An allenyl palladium species is generated via a formal Sn2 substitution of the mesylate, which in turn undergoes a transmetalation with diethylzinc yielding a nucleophilic species (Scheme 5-13). The reaction of the electrophile proceeds via a similar -ester enolate transition state as depicted in Scheme 5-13. Corresponding allenylindium reagents can also be used instead of allenylzinc intermediates. ... 

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