Methylenic protons, electrophilic substitution

Cyanoheptafulvene has been made by removal of a proton from the exocyclic methylene groip of a (cyanomethyl)tropylium salt [365], It undergoes electrophilic substitution at the 8-position, in accord with there being some electronegative character at this site [366]... 

Electrophilic substitution of methylenic protons in sulfides and sulfoxides. Alkaloidal / -sulfoxides could be chlorinated in -position by means of sulfurylchloride in the presence of pyridine. 

Novolacs are prepared with an excess of phenol over formaldehyde under acidic conditions (Fig. 7.6). A methylene glycol is protonated by an acid from the reaction medium, which then releases water to form a hydroxymethylene cation (step 1 in Fig. 7.6). This ion hydroxyalkylates a phenol via electrophilic aromatic substitution. The rate-determining step of the sequence occurs in step 2 where a pair of electrons from the phenol ring attacks the electrophile forming a car-bocation intermediate. The methylol group of the hydroxymethylated phenol is unstable in the presence of acid and loses water readily to form a benzylic carbo-nium ion (step 3). This ion then reacts with another phenol to form a methylene bridge in another electrophilic aromatic substitution. This major process repeats until the formaldehyde is exhausted. 

Electrophiles other than protons were indeed shown to react with lignin model compounds at the 2- and 6-positions in acidic media Kratzl and Wagner investigated the reaction of paraphenolic benzyl alcohols in alkaline and acidic solutions (10). When 4-hydroxy-3-methoxybenzyl alcohol 4 was reacted with the 4-alkyl substituted phenol 5 under alkaline conditions, the expected ortho linked product (6) was isolated. However, under acidic conditions the methylene linkage formed meta to the phenolic hydroxy group (Fig. 2). 

One of the subjects of our investigations involved the interaction of allenes with the P=E derivatives. This work provided some very interesting, unexpected results that may well be of use in synthetic organophosphorus chemistry. Thus, in attempting to study the (2+2)-cycloadditions of the phosphinimine and the (methylene)phosphine systems with the allenic C=C bond, we established that the reactions take a different and more interesting course — that of an ene" reaction (eq 1). In the first step of the reaction, the electrophilic phosphorus center apparently attacks the nucleophilic central carbon of the allene system. Then, instead of undergoing nucleophilic attack on the incipient carbonium ion, the anionic center (E) abstracts a proton from the terminal C-H bond, leading to the formation of a new double bond in the phosphorus-substituted 1,3-butadiene derivatives (3). 

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