Electrophilic atoms

Regioseleaiivitij A slightly better ivay (than the method on p T137) to work out the regio-chcmlstry of many DieIs-Alder reactions is to identify the most nucleophilic atom on the diene and the most electrophilic atom on the dienophile and join them. 

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... 

Then the second arene undergoes electrophilic aromatic substitution, with the terminal N of the diazonium salt as the electrophilic atom. When nucleophilic arenes are added to diazonium salts, electrophilic aromatic substitution tends to take place instead of SnI substitution of the diazonium salt. 

Figure 4.73 Examples of electro-philic metabolites. The small arrow Httxtfip bo-uoqjryjwprw indicates the electrophilic atom.

We can extend Markownikoff s rule to cover additions of substances of the general type X—Y to unsymmetrically substituted alkenes when a clear-cut decision is possible as to whether X or Y is the more electrophilic atom of X—Y. If the polarization of the X—Y bond is such that X is positive, 6 X—Y 0, then X will be expected to add as X to the alkene to form the more stable carbocation. This step will determine the direction of addition. For example, if we know that the O-Br bond of HOBr is polarized as 60 6 ... 

Transferring the results, obtained from the reactions of RSC1 with H20, from parachlorine to chlorine we get a suggestive mechanism which remains speculative till further results on this subject are available. One piece of support for this mechanism can already be provided. Kinetic studies on the reaction Cl2 + H20 — H+ + Cl" + HCIO show that the most probable first step is the nucleophilic addition of H20 at the electrophile atom in Cl2, forming the intermediate... 

Decide which is the nucleophilic and which the electrophilic atom... 

Each of these molecules is electrophilic. Identify the electrophilic atom and draw a mechanism for reaction with a generalized nucleophile Nil-, giving the product in each case. 

The most nucleophilic atom on the diene adds to the most electrophilic atom on the ketene and the cis geometry at the ring junction comes from the cis double bond of cyclopentadiene. It is impressive that even this excellent diene undergoes no Diels-Alder reaction with ketene as dienophile. The [2 + 2] cycloaddition must be much faster. 

The alkene s HOMO interacts with the isocyanate s LUMO, and the most electrophilic atom is the carbonyl carbon so this is where the terminal carbon atom of the alkene attacks. The chlorosulfonyl group can be removed simply by hydrolysis under mild conditions via the sulfonic acid. 

Hydrolysis. Compounds that possess an electrophilic atom (electron-poor) have a tendency to undergo hydrolysis reactions with electron-rich water or hydroxide ion (OH-). The reaction... 

Some heterocycles have both nucleophilic and electrophilic atoms in their molecule. Thus they can be opened and polymerized by the anionic, cationic or coordination mechanisms. Examples are lactams, lactones, and cyclic siloxanes. Investigations of the mechanism of lactam propagation are complicated by the occurence of side reactions. In principle, the mechanism described in Chap. 3 by the schemes (55)—(57) and (71) is accepted. Anionic polymerization of cyclic esters consists, in most cases (see Chap. 4, Sect. 2.2) of repeated reversible attacks on the carbonyl carbon by the anion 0]-. From e-caprolactone, polyester chains grow according to [315]... 

The redox potentials for the electron acceptors that react with HO (Table 15) are such that a pure outer-sphere single-electron transfer (SET) step would be endergonic (the HO /HO redox potential is more positive than the redox potential of the electron acceptor). Hence, the observed net reactions must be driven by coupled chemical reactions, particularly bond formation by the HO to the electrophilic atom of the acceptor molecule that accompanies a singleelectron shift. (The formation of the bond provides a driving force sufficient to make the overall reaction thermoneutral or exergonic 1.0 V per 23.1 kcalmol of bond energy.) The effect of various transition metal complexes on the oxidation potential for HO in MeCN illustrates some of these effects the results are summarized in Table 16. ... 

In a proton transfer to an aromatic carbon atom, a so-called sigma complex is formed in which the configuration of the valence electrons of the carbon has been changed from sp2 to sp3. In the next step, the other electrophilic atom or group bonded to the same carbon may be split off. This leads to an electrophilic aromatic substitution. Examples are aromatic hydrogen isotope exchange, aromatic decarboxylation, deboro-nation, and deiodination (see Sect. 9 Chap. 2, and Vol. 13, Chap. 1). 

A third mechanism may be postulated if it is assumed that the 1,3-hydrogen shift of the imino group in I to its enamino form, which is nucleophilic at the i3-vinyl carbon, occurs at an early stage. The electrophilic atom to which that... 

The last two molecules are forced to break a bonds. In the case of chlorine, the two ends are the same so you can attack either Cl atom. In MeSCl, the S-Cl bond is weaker than the C-S bond and Cl is the most electrophilic atom in the molecule. The S-Cl bond is broken and the negative charge ends up as chloride ion. 

Use SpartanView to examine electrostatic potential maps of trifluoroacetic acid, 3-chloropropene, ter -butyl cation, and protonated dimethyl ether. Assuming that the most positive atom is also the most electrophilic, identify the most electrophilic atom in each molecule. (Identity the most electrophilic carbon atom in 3-chloropropene.)... 

An electrophilic atom is one that has an empty orbital that is relatively low in energy. (Electrophilicity is discussed in more detail later in this chapter.)... 

Lone-pair nucleophiles contain atoms with lone pairs. The lone pair is used to make anew bond to an electrophilic atom. Alcohols (ROH), alkoxides (RO ), amines (R3N), metal amides (R2N ), halides (X ), thiols (RSH), sulfides (R2S), and phosphines (R3P) are all examples of lone-pair nucleophiles, as are the O atoms of car-... 

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