Addition reactions of alcohols

The results in Table 3 were explained as shown in Scheme 4. From the fact that no kinetic isotope effect was observed in the reaction of phenyl-substituted disilenes with alcohols (Table 1), it is assumed that the addition reactions of alcohols to phenyltrimethyl-disilene proceed by an initial attack of the alcoholic oxygen on silicon (nucleophilic attack at silicon), followed by fast proton transfer via a four-membered transition state. As shown in Scheme 4, the regioselectivity is explained in terms of the four-membered intermediate, where stabilization of the incipient silyl anion by the phenyl group is the major factor favoring the formation of 26 over 27. It is well known that a silyl anion is stabilized by aryl group(s)443. Thus, the product 26 predominates over 27. However, it should be mentioned that steric effects also favor attack at the less hindered SiMe2 end of the disilene, thus leading to 26. 

TABLE 4. Addition reaction of alcohols to silicon-substituted disilenes XMeSi=SiMe229... 

The addition reaction of alcohols to silenes is a strictly regiospecific process the OR group in the product is always attached to the silicon. The reaction is of special importance since silenes are easily trapped by reagents like methanol and t-butanol, and isolation of the adducts is normally taken as evidence for an intermediate silene. Furthermore, the alcohol addition to silenes is the prototypical reaction for the 1,2-addition of polar bonds across the Si=C double bond. A complete account on the alcohol addition to silenes, including the most recent mechanistic implications, is given in the chapter by Sakurai. We will therefore give only a brief survey of the reactivity of several families of silenes towards alcohols. 

Similar addition reactions occur with the Sn-0 bond of organotin alkoxides [e.g. Bu3SnOMe + PhN=CO —> Bu3SnNPhC(0)0Me], and, by recycling through the alkoxides, the bis(trialkyltin) oxides will catalyse the addition reactions of alcohols and similar compounds. These catalysed reactions are discussed in Section 14.1.2. 

Antilla and coworkers further extended their methodology to the addition of an oxygen nucleophile, instead of a nitrogen nucleophile (see Scheme 3.54), to imines (Scheme 3.56) [115]. They successfully developed the catalytic enantioselective addition reaction of alcohols as oxygen nucleophiles with N benzoyl imines using chiral phosphoric acid Ig. The method provides straightforward access to chiral hemiaminal ethers with high enantioselectivities. 

An unusual addition reaction of alcohols to olefins involving the net formation of a C-C bond between two sp -hybridized carbon atoms through the action of Wilkinson s catalyst and BF3-Et20 has been reported (eq 44). The reaction appears to be reasonably general and is interpreted in terms of a radical-based process. ... 

Alkoxysilanes are prepared by addition reactions of alcohols with organochlorosi-lanes, by reaction of organometallic reagents with lower alkoxysilanes, or by hydrosilyla-... 

When simple primary alkyl halides are permitted to react with alkoxide anions, substitution of the alkoxide for the halide occurs or, alternatively, the alkyl group has been substituted for the hydrogen which was on oxygen. Variants of this reaction, the Williamson ether synthesis, have been seen before as a substitution reaction at carbon (of the alkyl hahde) and will be seen again as an addition reaction of alcohols. 

E. ADDITION REACTIONS OF ALCOHOLS, ENOLS, AND PHENOLS I. Introduction... 

The problem is particularly acute in the subject of addition reactions of alcohols, enols and phenols that is, there are two kinds of processes that might be imagined. First, there is the addition of the electron-deficient reagent to the electron-rich oxygen of the alcohol, enol, or phenol. Enols also undergo addition reactions with reagents that add to the carbon-carbon double bond. 

Addition reactions of alcohols to alkenes and alkynes have already been provided (Chapter 6) and a few more are shown in Table 8.6. Further, although many more addition reactions will be encountered later in the text (when the functional group with which the alcohol, enol, or phenol reacts is discussed in more detail), an introduction to some of the more common reactions is provided in Table 8.6. The table presents some of the partners of addition reactions with alcohols, enols, and phenols and the products they form together. The reactions shown are typical and, as such, are briefly discussed. 

Karodia et al. reported [123] a eonvenient method for the acid-catalyzed Michael addition reactions of alcohols, thiols, and amines to methyl vinyl ketone, using the IL ethyltri- -butylphosphonium tosylate. Recently, phosphonium-based ILs have been used [124] in the degradation of phenol, esterification, Wittig reaction, Heck reactions, Suzuki cross-coupling reactions, oxidation of benzyl halides [125], etc. Phos-phonium tosylates are used as solvents in catalytic hydroformy lation reactions these catalyst systems are noncorrosive and can readily be recovered and reused [126]. 

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