Carbon nucleophiles intermolecular termination

For the step which limits the reaction rate (rate limiting step), three mechanisms have been proposed, two of which are intramolecular - denominated intramolecular, as such, and solvolytic - and the other intermolecular (Figure 2.1.18). The first of these implies a covalent bond between water and the atoms of carbon during the whole of the transposition. In the solvolytic mechanism there is an initial rupture from the O-C, bond, followed by a nucleophilic attack of the H2O on the C3. Whilst the intermolecular mechanism corresponds to a nucleophilic attack of H2O on the terminal carbon C3 and the loss of the hydroxyl group protonated of the C. ... 

Lewis acid complexes of p-substituted a,p-unsaturated ketones and aldehydes are unreactive toward alkenes. Crotonaldehyde and 3-penten-2-one can not be induced to undergo ene reactions as acrolein and MVK do. 34 The presence of a substituent on the p-carbon stabilizes the enal- or enone-Lewis acid complex and sterically retards the approach of an alkene to the p-carbon. However, we have found that a complex of these ketones and aldehydes with 2 equivalents of EtAlQ2 reacts reversibly with alkenes to give a zwitterion. 34 This zwitterion, which is formed in the absence of a nucleophile, reacts reversibly to give a cyclobutane or undergoes two 1,2-hydride or alkyl shifts to irreversibly generate a p,p-disubstituted-o,p-unsaturated carbonyl compound (see Figure 19). The intermolecular addition of an enone, as an electrophile, to an alkene has been observed only rarely. The specific termination of the reaction by a series of alkyl and hydride shifts is also very unusual. 35 The absence of polymer is remarkable. 

Chloroadenallene 45 gave, after refluxing with triethylphosphite for 30 min, g-2 -phosphonate 47 in 30% yield (Scheme 7). A routine dealkylation afforded phosphonic acid 48 (60%). Thus, the usual reaction at the reactive allylic terminal was suppressed in favor of a nucleophilic attack at the sp carbon atom. The mechanism may include an intramolecular dealkylation as indicated in formula 49, although an intermolecular reaction is also possible. 

The intermolecular hydroamination reactions of alkynes and alkenes occur with Markovnikov or anti-Markovnikov selectivity. The nucleophilic addition to aUenes occurs at terminal carbon of allenes not at central one. 

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