Alkoxyl group reactions with alcohols

Reactions with Alcohols. The tendency of titanium(IV) to reach coordination number six accounts for the rapid exchange of alkoxy groups with alcohols. Departure of an alkoxy group with the proton is the first step in the ultimate exchange of all four alkoxyls. The four-coordinated monomer is expected to react... 

Alkoxylation. The reaction of alcohols with ethylene oxide gives polymeric products in which many units of the ethoxy group are incorporated (Reaction XXII). The reaction can be controlled... 

An example of an addition reaction is the reaction of aldehydes with alcohols in the presence of catalytic amoimts of acid. In this reaction, the hydrogen of the alcohol adds to the carbonyl oxygen. The alkoxyl group of the alcohol (—OR) adds to the carbonyl carbon. The predicted product, having the structure just described, is a hemiacetal. 

Reaction 6 representing /1-scission of alkoxyl radicals leading to the reduction of molar mass competes with transfer of a free radical centre to surrounding groups with consequent formation of alcoholic groups (reaction 7), which subsequently loose water and C = C unsaturation appears randomly along the polymer chain. 

Reactions of Esters Esters are much more stable than acid chlorides and anhydrides. For example, most esters do not react with water under neutral conditions. They hydrolyze under acidic or basic conditions, however, and an amine can displace the alkoxyl group to form an amide. Lithium aluminum hydride reduces esters to primary alcohols, and Grignard and organolithium reagents add twice to give alcohols (after hydrolysis). 

Catalysts from Group VIII metals have given unsatisfactory results. In the polymerization of butadiene with soluble cobalt catalysts tritium is not incorporated when dry active methanol is employed [115], although it is combined when it has not been specially dried [117, 118]. Alkoxyl groups have been found when using dry alcohol [115, 119] but the reaction is apparently slow and not suited to quantitative work [119]. Side reactions result in the incorporation of tritium into the polymer other than by termination of active chains [118], probably from the addition of hydrogen chloride produced by reaction of the alcohol with the aluminium diethyl chloride [108], Complexes of nickel, rhodium and ruthenium will polymerize butadiene in alcohol solution [7, 120], and with these it has not been possible to determine active site concentrations directly. 

These molecules scavenge radicals and vitamin E can react with an alkoxyl radical by donating a hydrogen atom (from the OH group) to give an alcohol and a vitamin E radical, which is much less harmful. Vitamin E is believed to be regenerated by reaction with vitamin C at the cell membrane (water-lipid) surface. 

Pasto has reported the generation of alkoxyl radical by the photo-induced ho-molytic dissociation of alkyl 4-nitrobenzenesulfenate (52]. Cekovic has examined the photolytically induced decomposition of benzenesulfenates in the presence of hexabutylditin [53]. This reaction allows selective introduction of a phenylthio group at the -position via a 1,5-hydrogen transfer. The overall reaction is an O to C transfer of a phenylthio group. This reaction has been applied for the synthesis of acetyl scopine starting from (Af-ethoxycarbonyl)nortropine benzenesulfenate (Scheme 8) [54]. The sulfenate esters are stable compounds and are easily prepared by reaction of alcohols with PhSCl/EtyN. 

Claisen condensation (Section 19.1) A reaction in which an enol ate anion from one ester attacks the carbonyl function of another ester, forming a new carbon-carbon cr-bond. A tetrahedral intermediate is involved that, with expulsion of an alkoxyl group, collapses to a )S-ketoester. The two esters are said to condense into a larger product with loss of an alcohol molecule. 

As shown in Table 26.2, the general formula of an ester is RCCXTR. Comparing the general formula of an ester with that of a carboxylic acid, we see that an ester is a molecule in which the hydroxyl group (—OH) of a carboxylic acid functional group is replaced by an alkoxyl group (—OR ). In the lab, esters can be prepared by the reaction of a carboxylic acid and an alcohol. The products of the reaction are an ester and a water molecule. 

Alkoxyl tion. The nucleophilic replacement of an aromatic halogen atom by an alkoxy group is an important process, especially for production of methoxy-containing iatermediates. Alkoxylation is preferred to alkylation of the phenol wherever possible, and typically iavolves the iateraction of a chloro compound, activated by a nitro group, with the appropriate alcohol ia the presence of alkaU. Careful control of alkaU concentration and temperature are essential, and formation of by-product azoxy compounds is avoided by passiag air through the reaction mixture (21). 

The photolysis of organic nitrites of appropriate constitution and conformation in solvents such as benzene or acetonitrile transforms them into A-nitroso alcohols via the sequence (1) a homolytic fission of the O-N bond of their nitrosoxy group (2) an intramolecular d-hydrogen abstraction of the resulting alkoxyl radicals to generate a d-carbon radical and (3) formation of d-nitroso alcohols by combining of the d-carbon radical with the generated nitric oxide. The nitroso alcohols are isolated as d-hydroxyimino alcohols as a result of spontaneous thermal isomerization or as nitroso-dimers [1] (Scheme 1). This transformation has been named the Barton reaction [2, 3],... 

A functionalization that converts C—H bonds to C— NO bonds occurs when nitrite esters are photo-lyzed (the Barton reaction Scheme 4). Again an alkoxyl radical abstracts a 8-hydrogen, and the resulting carbon radical picks up NO. The product nitroso compounds convert easily to oximes. Particularly valuable examples have bMn studied in the steroid field. If the photolysis is performed in the presence of copperfll) acetate the intermediate carbon radical can be oxidized to an alkene, rather than capture NO. If the alcohol whose nitrite ester is photolyzed is part of a cyanohydrin, then the Heusler-Kalvoda reaction occurs, and the product is a ketone with a migrated cyano group (Scheme 5) ... 

Unfortunately, it is a difference between a starter, such as nonylphenol, or a fatty alcohol, which have only one type of hydroxyl group and polyols. Some polyols used as starters for rigid polyether polyols have in the same molecule various types of hydroxyl groups (for example, primary hydroxyls and secondary hydroxyls) which do not have equivalent reactivities in the alkoxylation reactions. For example, sorbitol has two primary hydroxyls and four secondary hydroxyls, sucrose has three primary hydroxyls and five secondary hydroxyls. In both polyols, the secondary hydroxyls have different substituents and they are not totally equivalent. TMP, pentaeriythritol and dipentaerythritol have only one type of equivalent primary hydroxyl group. Thus, the initiation reaction (reaction of PO with hydroxyl groups of starter) is in fact the sum of the reactions of PO with each type of hydroxyl group of the starter ... 

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