Epoxides with phenolates

ARO reaction with phenols and alcohols as nucleophiles is a logical extension of HKR of epoxides to synthesize libraries of stereochemically defined ring-opened products in high optical purity. To this effect Annis and Jacobsen [69] used their polymer-supported Co(salen) complex 36 as catalyst for kinetic resolution of epoxides with phenols to give l-aiyloxy-2-alcohols in high yield, purity and ee (Scheme 17). Conducting the same reaction in the presence of tris(trifluoromethyl)methanol, a volatile, nonnucleophilic protic acid additive accelerates KR reaction with no compromise with enantioselectivity and yield. Presumably the additive helped in maintaining the Co(III) oxidation state of the catalyst. 

Scheme 17. KR of racemic terminal epoxide with phenol using chiral catalyst 36.

Kim et al. [67], used the self-polymerized heterometallic polymeric salen complexes 26-32 as efficient catalysts for kinetic resolution of terminal epoxides with phenols to give a-aryloxy alcohols in high yields (38-43%) and ee (92-99%) (Scheme 17). These catalysts were recycled up to three times without any loss in their performance. 

Scheme 19. ARO of terminal epoxides with phenol using heterobimetallic chiral catalysts 26-32.

A number of studies of the kinetics and mechanism of the base catalysed reaction of epoxides with phenolic alcohols have served as background for the polymerization studies. These studies [14] showed that both the alcohol and the alkoxide participate in the rate determining step and subsequently a termolecular mechanism was proposed. 

The dynamic resolution of epibromohydrine provided only the bromodiol in 94% overall yield and 96% ee (sum of the five experiments). Excellent results were also obtained for the ring opening epoxides with phenol, ee could reach more than 99% and yield 98%. 

The kinetic resolution of other epoxides with phenol was also studied in enantioselective parallel synthesis with the same catalyst, providing efficient access to important precursors of pharmacologically active compounds [211]. For all the studied substrates, the catalytic system was proved to be very efficient, p-aryloxyalcohols were formed in high yield (90-100%), purity ((93-100) and ee (81->99%). 

Curing by the reaction of epoxide with phenolic OH is usually done using a phenolic novolac. These are high-temperature cures and produce very highly crosslinked polymers due to the frequency of OH groups on the novolac. 

Shechter and Wynstra were the first to examine the uncatalyzed epoxy-phenol reaction." They found that no reaction occurred when equimolar quantities of phenol and phenyl glycidyl ether (PGE, a low molecular weight epoxy) were held at 100 C. When held at 200°C, however, epoxide disappeared at a much faster rate than the phenol, with the net result that about 60% of the reaction was epoxide with phenol (Rxn. 1) and 40% was epoxide with secondary hydroxyl (Rxn. 2). The extent of the second reaction is particularly significant when one considers that the secondary hydroxyl concentration was originally zero and became finite only when some epoxide had reacted with phenol. 

A recent study of the reaction of epoxide with phenol in the presence of tertiary amines was performed by Hale. Hale formulated his analysis according to the the mechanism of Sorokin and Shode. The reaction mechanism is here described in a short-hand notation (compare to Rxns. 7 - 10) ... 

The base-catalyzed reaction of epoxide with phenol, in equimolar amounts, is somewhat slower than that of the alcohol-epoxide with the same catalyst. The mechanism of reaction is given in the following reactions ... 

Tetrakis (4-hydroxyphenyl)ethane is prepared by reaction of glyoxal with phenol in the presence of HCl. The tetraglycidyl ether [27043-37-4] (4), mp ca 80°C, possesses a theoretical epoxide functionaUty of four with an epoxy equivalent weight of 185—208 (4). 

Compounds are prepared by a fairly standard sequence which consists of condensation of an appropriate phenol with epichlorohydrin in the presence of base. Attack of phenoxide can proceed by means of displacement of chlorine to give epoxide (45) directly. Alternatively, opening of the epoxide leads to anion 44 this last, then, displaces halogen on the adjacent carbon to lead to the same epoxide. Reaction of the epoxide with the appropriate amine then completes the synthesis. 

The highest mechanical strengths are usually obtained when the fibre is used in fine fabric form but for many purposes the fibres may be used in mat form, particularly glass fibre. The chemical properties of the laminates are largely determined by the nature of the polymer but capillary attraction along the fibre-resin interface can occur when some of these interfaces are exposed at a laminate surface. In such circumstances the resistance of both reinforcement and matrix must be considered when assessing the suitability of a laminate for use in chemical plant. Glass fibres are most commonly used for chemical plant, in conjunction with phenolic resins, and the latter with furane, epoxide and, sometimes, polyester resins. 

Epoxides can react with alcohols via acidic or basic catalysed reaction mechanisms. However, since both strong acids and bases will degrade the cell wall polymers of wood, the reaction is usually catalysed via the use of amines, which are more strongly nucleophilic than the OH group. For example, whereas the production of epoxy-phenolic resins requires temperatures in the region of 180-205 °C, reaction between epoxides and primary or secondary amines takes place at 15 °C (Turner, 1967). Reaction of epoxides with wood often involves the use of tertiary amines as catalysts (Sherman etal., 1980). The sapwood is more reactive towards epoxides than heartwood (Ahmad and Harun, 1992). 

McKelvey etal. (1959) investigated the reaction of epoxides with cellulose in alkaline conditions, reporting that alkaline cellulose reacted readily once the concentration of sodium hydroxide was sufficiently high. However, no evidence was found of reaction between cotton yarn and cellulose with a range of epoxides under a variety of reaction conditions. It was concluded that the apparent reactivity of cellulose with epoxides was primarily due to alkaline swelling of the cellulose, self-polymerization of the epoxide monomers then occurring within the interior structure of the fibres. It was also noted that the reactivity with phenol OH groups was very low (e.g. only 1 % conversion of ethylene oxide with various phenols). 

Jacobsen et al. [48], in 1997 for the first time demonstrated KR of racemic terminal epoxides with water as nucleophile for the production of optically pure epoxides and corresponding 1,2-diols. Since then, various other nucleophiles viz., carboxylic acids, phenols, thiols, amines, carbamates and indols were used in KR to produce optically pure epoxides with concomitant production of corresponding enantioenriched l,2-bifimctional moieties [49-52]. 

Phenolic, (I), and naphtholic, (II), condensation polymers containing cyclopentane were previously prepared by Sue et al. (1). These materials were subsequently epoxidized with epichlorohydrin and used in electronic devices as ICs and Lumen solubility indexes (LSIs). In a subsequent investigation by Abe et al. (2) novolak resins functionalized with thiophene, (III), were prepared and used as adhesives. 

Tanaka and Kakiuchi 35,36) proposed a new mechanism of non-catalyzed copoly-merization of epoxides with anhydrides. In the presence of proton donors, they expected the formation of a transition ternary complex composed of all three components of the reaction system. The proposed mechanism (Eqs. (8-10)) is similar to the reaction of phenol with epoxide catalyzed by phenolate 38). 

The most common types of compounds with oxygen-containing functional groups are epoxides, alcohols, phenols, ethers, aldehydes, ketones, and carboxylic acids. The functional groups characteristic of these compounds are illustrated by the examples of oxygen-containing compounds shown in Figure 1.15. 

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