Electrophilic additions protic acids

In addition, protic acids also cleave organomercurials to yield the proton-substituted products analogous to those for halogenation above. Finally, mercuric salts also act as electrophilic reagents towards organomercurials. 

Hydroboration is highly regioselective and stereospecific. The boron becomes bonded primarily to the less-substituted carbon atom of the alkene. A combination of steric and electronic effects works to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value (-0.5).156 Compared with bromination (p+ = -4.3),157 this is a small substituent effect, but it does favor addition of the electrophilic boron at the less-substituted end of the double bond. In contrast to the case of addition of protic acids to alkenes, it is the boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is reinforced by steric factors. Hydroboration is usually done under conditions in which the borane eventually reacts with three alkene molecules to give a trialkylborane. The... 

An unexpected reaction occurs when 2-alkyl-4(5)-nitroimidazoles (27 R = alkyl) are reduced in protic solvents [92JCS(P1)2779]. Catalytic hydrogenation of 2-methyl-4(5)-nitroimidazole (27 R = Me) in a solution of acetic anhydride and acetic acid gave 4,4 -diacetamido-2,2 -dimethyl-5,5 -diimidazole (32 yield 10%) in addition to the expected 4-acetamido-l-acetyl-2-methylimidazole (28%). Similarly, reduction of the 2-alkyl-4(5)-nitroimidazoles (27 R = Me, Et, iPr) in ethanol solution in the presence of diethyl ethoxymethylenemalonate [EMME (135)] gives predominantly the 5,5 -diimidazole adducts (33). The formation of these products (33) is believed to involve an electrophilic addition of the starting material (27) to the electron-rich aminoimidazoles (25) [92JCS(P1)2779]. Interestingly, replacement of ethanol by dioxane suppressed diimidazole formation. 

The addition of thiols to C—C multiple bonds may proceed via an electrophilic pathway involving ionic processes or a free radical chain pathway. The main emphasis in the literature has been on the free radical pathway, and little work exists on electrophilic processes.534-537 The normal mode of addition of the relatively weakly acidic thiols is by the electrophilic pathway in accordance with Markovnikov s rule (equation 299). However, it is established that even the smallest traces of peroxide impurities, oxygen or the presence of light will initiate the free radical mode of addition leading to anti-Markovnikov products. Fortunately, the electrophilic addition of thiols is catalyzed by protic acids, such as sulfuric acid538 and p-toluenesulfonic acid,539 and Lewis acids, such as aluminum chloride,540 boron trifluoride,536 titanium tetrachloride,540 tin(IV) chloride,536 540 zinc chloride536 and sulfur dioxide.541... 

The PTOC protocol for the preparation of nitrogen centered radicals is also compatible with a variety of Lewis acids that apparently complex with the aminyl radicals to give reactive, electrophilic species. Lewis acids offer potentially milder reaction conditions than protic acids for sensitive compounds. Efficient intermolecular addition reactions have... 

Some alkene monomers can be polymerized by a cationic initiator, as well as by a radical initiator. Cationic polymerization occurs by a chain-reaction pathway and requires the use of a strong protic or Lewis acid catalyst. The chain-carrying step is the electrophilic addition of a carbocation intermediate to the carbon-carbon double bond of another monomer unit. Not surprisingly, cationic polymerization is most effective when a stable, tertiary carbocation intermediate is involved. Thus, the most common commercial use of cationic polymerization is for the preparation of polyisobutylene by treatment of isobutylene (2-mcthylpropene) with BF3 catalyst at -80 C. The product is used in the manufacture of inner tubes for truck and bicycle tires. 

A-Acyliminium ions are suitable partners for electrophilic addition reactions of allylstannanes. Although the corresponding silanes have been more widely studied in these cases/ allylic stannanes are fully competent reaction participants, as illustrated by formation of the l-azabicyclo[3.1.0]pentane 159 via intra-molecular cyclization of the fi-allylic stannane 157 (Scheme 5.2.33). The reaction produces the vinyl cyclopropane under protic acid conditions with complete stereocontrol. s... 

Reaction of DCT with styrene in polar protic solvents (water, acetic acid, and methanol) proceeds with the electrophilic addition of the halogen and the solvent moieties according to the Markovnikov rule. In carbon tetrachloride in the presence of an initiator (AIBN) addition of DCT and TBT to styrene proceeds to form anti-Markovnikov adducts and a small amount of 1,2-dihalo derivatives (69ZC325) (Scheme 72). 

In this chapter, we discuss reactions that either add adjacent (vicinal) groups to a carbon-carbon double bond (addition) or remove two adjacent groups to form a new double bond (elimination). The discussion focuses on addition reactions that proceed by electrophilic polar (heterolytic) mechanisms. In subsequent chapters we discuss addition reactions that proceed by radical (homolytic), nucleophilic, and concerted mechanisms. The electrophiles discussed include protic acids, halogens, sulfenyl and selenenyl reagents, epoxidation reagents, and mercuric and related metal cations, as well as diborane and alkylboranes. We emphasize the relationship between the regio-and stereoselectivity of addition reactions and the reaction mechanism. 

Addition of carboxylic acids to alkynes generally requires the presence of strong protic acids or other electrophilic catalysts. It was, however, observed that carboxylic acids having pAa < 4.5 do add to phenylacetylene in the absence of a strong acid catalyst to give enol esters In case of formic acid, addition occurred by simply heating the alkyne in formic acid at 100 C. A variety of diaryl, dialkyl, aralkyl and terminal alkynes react, but dimethyl acetylenedicarboxylate was found to be unreactive. The final product is a ketone, while a stoichiometric amount of carbon monoxide is released. 

Electrophilic Addition This can be considered as an acid—base reaction, where the reagent acts as an acid, whether a protic one (e.g., hydriodic acid or iodine monochloride) or a Lewis acid (e.g., molecular iodine, which can be polarized by electrophilic solvents or catalysts), and the double-bond acts as abase (Argentini, 1982). 

Electrophilic attack on olefin ligands coordinated to electron-rich, strongly backbonding metals is illustrated by the reactions of (P group 4 olefin and alkyne complexes, as well as some electron-rich olefin complexes. Zirconocene- and and hafnocene-olefin complexes generated by reaction of zirconocene dichloride with two equivalents of alkyl lithium and isolated upon addition of a phosphine ligand react with carbonyl compounds and weak protic acids to form insertion products and alkyl complexes. Several examples of the reactions of these complexes with electrophiles are shown in Equations 12.65-12.66. Zirconocene-alkyne complexes prepared by thermolysis of vinyl alkyl complexes and titanium-alkyne complexes generated by the reduction of Ti(OPr ) also react with electrophiles, such as aldehydes and acid, to form products from insertion into the M-C bond and protonation of the M-C bond respectively. 

Few reports on the reactions of alkyne complexes with electrophiles other than protic acids have appeared. Bromine has been found to react with a few systems to afford vicinal dibromoalkenes [e.g., Eqs. (41) and (42)] (Kruerke and Hubei, 1961 Hiibel and Merenyi, 19M). These reactions may, in fact, involve bromine addition subsequent to oxidative release of the ligand from the metal. The platinum dicyanoacetylene complex behaves differently in forming a product in which addition has occurred but the complex has remained intact (McClure and Baddley, 1970). 

Apparently, Eq. (29) represents a polar nonradical addition. If a two-step mechanism is conceived, intermediates of the type [XB=NRH] will be reasonable, though such cations proved to be rather unstable as isolated species (unless X represents a x-electron donating group) (33). Intermediates of the type HY—B(X)=NR would explain the fast reaction with protic bases of vanishing Bronsted acidity. The results, however, mentioned in Sections V, A, and V, C, favor to some extent the picture of iminoboranes as preferring electrophilic to nucleophilic attack. The high activity of amines can also be rationalized in terms of a concerted process, with a transition state of type VI. 

To the extent that the enolate resulting from conjugate addition at the (3-carbon can be stabilized, the rate of this reaction pathway is enhanced. For example, (3-Michael additions are observed for MVK, acrolein, and acetylenic electrophiles even without the presence of a Lewis acid. Furthermore, MVK reacts with the 2,5-dimethylpyrrole complex (22) to form a considerable amount of (3-alkylation product, whereas only cycloaddition is observed for methyl acrylate. The use of a Lewis acid or protic solvent further enhances the reactivity at the (3-position relative to cycloaddition. While methyl acrylate forms a cycloadduct with the 2,5-dimethylpyrrole complex (22) in the absence of external Lewis acids, the addition of TBSOTf to the reaction mixture results in exclusive conjugate addition (Tables 3 and 4). 

Another instructive example of electrophilic or H-bonding assistance of protic solvents (or co-solvents) in SnI reactions is the accelerated acetolysis rate of 2-bromo-2-methylpropane upon the addition of phenols to a tetrachloromethane/acetic acid solution of the reactant [582] see reference [582] for further examples. The usefulness of phenol as a solvent for SnI solvolysis reactions, in particular phenolysis of 1-halo-l-phenylethanes, has been stressed by Okamoto [582], In spite of its low relative permittivity (fir = 9.78 at 60 °C), its low dipolarity fi = 4.8 10 Cm = 1.45 D), and its low nucleophihcity, it represents a solvent of high ionizing power due to its electrophilic driving force. 

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