Active Ester-Forming Polymeric Reagents

The active ester methodology has been much applied mainly to form peptide bonds under mild conditions in both liquid- and solid-phase synthesis. In liquid-phase synthesis, most frequently the activation of a carboxyl group has employed HOSu, followed by esters of electron-withdrawing-containing phenols (nitrophe-nols of pentafluorophenols), the most reactive HOBt esters usually being unstable and used in solid-phase synthesis. The preparation of these active esters requires the presence of a coupling reagent such as DCC or DIC. 

When dealing with polymer-supported analogues of these active ester-forming XOH species, a lower reactivity is expected, which is attributed to a relatively slow mass transport within the polymer pores that can not keep up with the rate of the acylation reaction. Therefore, the obtained polymeric active esters usually have a long shelf-life and are rather insensitive to moisture. To overcome this lower reactivity, the supported active esters from highly reactive HOBt derivatives have been the most frequently used. This section deals with these supported active ester-forming species. 

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Esters of 2-iodoxybenzoic acid (IBX-esters) 489 have been prepared by the hypochlorite oxidation of the readily available 2-iodobenzoate esters 488 (Scheme 2.139) and isolated in the form of stable microcrystalline solids [657,658], This procedure has been used for the synthesis of IBX-esters 489 derived from various types of alcohols, such as primary, secondary and tertiary alcohols, adamantanols, optically active menthols and borneol. Single-crystal X-ray data on products 489 revealed a pseudo-benziodoxole stmcture in which the intramolecular L--0 secondary bonds partially replace the intermolecular I - O secondary bonds, disrupting the polymeric structure characteristic of Phl02 and other previously reported iodylarenes [658], This stmctural feature substantially increases the solubility of these compounds in comparison to other iodine(V) reagents and affects their oxidizing reactivity. 

One important class of anionic-polymerization initiators are the organometallic compounds, especially those of lithium and sodium (see Chapter 5 to review the general properties of the metalloalkyls). Grignard reagents can induce polymerization of acrylonitrile and acrylate esters, but not of hydrocarbons. Initiation by organosodium compounds often involves in situ generation of the active species from sodium metal. Sodium metal catalyzes the polymerization of dienes, and the mechanism has been shown to involve a dianion formed by dimerization of the diene radical anion. An alternative initiation system for anionic polymerization involves... 

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