Electrophilic attack by protons

Furans with a free a-position are susceptible to electrophilic attack by protonated aldehydes and ketones. The yields in these reactions are often poor because of the... 

Although the reports of chemical reactivity of coordinated C02 ligand in well-characterized complexes are still very limited, several examples have been described showing that the oxygen of the r 2- or ri -bonded coordinated C02 can undergo an electrophilic attack by protons or other similar reagents. 

In solution, propargyl cations may in principle be generated by electrophilic addition to vinylacetylenes (equation 34). However, by all the evidence, the electrophilic attack by protons occurs more readily at the terminal acetylenic carbon than at the terminal vinyl carbon, and cation 176 is generated instead (de la Mare and Bolton, 1966). 

The imido groups of transition metal complexes undergo a wide range of reactions including electrophilic attacks by protons, NR transfer, and many other reactions.154... 

The Ti=0 group is more basic and more susceptible to electrophilic attack by protons than the corresponding vanadyl complexes leading to Ti—O—Ti19 and [Ti=0—Ti—O—Ti—0=Ti] 20 species. Another interesting complex with a Ti—O—Ti motif, [(NH3)5Ti—O—Ti(NH3)5]I4,21 is prepared by reaction of Til3 with ammonia in the presence of traces of water or oxygen. 

Crucial to the structure determination was the acid-catalysed rearrangement of rhazinilam to a mixture of compounds (69) all having the distinctive u.v. absorption of a pyrrolo[2,3-c]quinoline. This transformation can be seen to involve [arrows on (66) in Scheme 19] electrophilic attack by protonated amide carbonyl on the pyrrole a-position, C-21, followed by a reverse alkylation of the pyrrole ring. Loss of water and proton (in one of four possible ways) then leads to the observed mixture (69) of pyrroloquinolines. 

While several examples demonstrate that coordinated CO2 undergoes electrophilic attack by protons or other similar reagents at the -bonded oxygen, there is little evidence [15, 25 b, 41] that coordination promotes the formation of a C-C bond, e.g. between CO2 and an olefin. In the latter case, it is more likely that CO2 interacts with a M(olefin)-adduct Alternatively, a three-molecular mechanism involving the metal center, the olefin and CO2 may operate (see Section 1.4.1.1). Scheme 1.2 gives an overview of the documented reactions of coordinated CO2. 

It is reasonable to assume that the initial step in the cycloaddition reaction is an electrophilic attack by the carbene on the nitrogen atom to form the A -ylid. Where proton shift is possible, cyclization does not occur and the A-ylid produces the N-formyl compound (Scheme 7.30) [36]. 

Both mechanisms discussed above involve an acidic species, and differ primarily in whether that species behaves as a Bronsted or Lewis acid. The vanadic acid approach treats the Y species as a proton donor whereas the oxygen abstraction implies electrophilic attack by V. In either case, it is easy to see how oxygen rich, basic oxides such as MgO function as Y passivators. 

The nucleophiles that are used for synthetic purposes include water, alcohols, carboxylate ions, hydroperoxides, amines, and nitriles. After the addition step is complete, the mercury is usually reductively removed by sodium borohydride. The net result is the addition of hydrogen and the nucleophile to the alkene. The regioselectivity is excellent and is in the same sense as is observed for proton-initiated additions.16 Scheme 4.1 includes examples of these reactions. Electrophilic attack by mercuric ion can affect cyclization by intramolecular capture of a nucleophilic functional group, as illustrated by entries 9-11. Inclusion of triethylboron in the reduction has been found to improve yields (entry 9).17... 

Formation of an ylide, which can react further with an electrophile or by proton abstraction, is defined as Type E behaviour (Scheme 51)- The ylide may arise either by nucleophilic attack at a hydrogen atom (equation 74) or by, f°r instance, an a-elimination of carbon dioxide, the Hammick reaction (Scheme 52) (70LA(732)43). 

In the absence of a substituent at the 1-position, 3-hydroxymethylindoles are also unstable under basic conditions, due to the activation of the heterocyclic ring to electrophilic attack by the initial removal of the proton from the 1-position. Under these conditions the bis(3-indolyl)methanes are formed. However, it has been noted that under neutral conditions 3-hydroxymethyl-2-phenylindole is converted into 2-phenylindole with the extrusion of formaldehyde <79HC(25-3)l). It has been suggested that the bulky 2-phenyl group inhibits the alternative formation of the indolylmethane. 

The acid-catalyzed dimerization of pyrroles and indoles also involves electrophilic attack by the 2H- or 3//-protonated species upon the non-protonated heterocycles (Schemes 6, 7 and 8, Section 3.05.1.2.2), and 3,3-dimethyl-3//-indole has been reported to react with 7r-electron-rich aromatic compounds to yield the 2-ary.l-3,3-dimethyl-2,3-dihydroindoles (77S343). In the absence of a nucleophile strong acids promote the interchange of substituents at the 2- and 3-positions of 2,3,3-trisubstituted 3//-indoles, e.g. (510) (511) (62JOC1553). 

Protonation of 21 yields H2Os,0C(CO)24 (5 7). The crystal structure of the dihydride has not been determined, but analysis of the vibrational spectrum of the cluster in the region associated with motion of the interstitial carbon atom has led to the conclusion that the symmetry of 21 is reduced on protonation, probably by protonation of the central Os6 octahedron (see Section VI,A) (57). Cluster 21 also reacts with iodine to yield sequentially [Osl0C(CO)24I]", 22, and Os.oCfCO), 23, the result of electrophilic attack by 1+ on the dianion [Eq. (16)] (55). 

The isomerization is triggered by protonation of the aromatic ring, an electrophilic attack by HC1 catalyzed by A1C13. 

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