Nonselective carbene insertion

Figure 4 Darvon Synthesis, Nonselective Carbene Insertion...

The highly reactive species methylene inserts into C—H bonds,both aliphatic and aromatic,though with aromatic compounds ring expansion is also possible (see 15-62). This version of the reaction is useless for synthetic purposes because of its nonselectivity (see p. 248). This contrasts with the metal carbene insertion reaction, which can be highly selective, and is very useful in synthesis. Alkylcarbenes usually rearrange rather than give insertion (p. 249), but, when this is impossible. 

These reactions have very low activation energies when the intermediate is a free carbene. Intermolecular insertion reactions are inherently nonselective. The course of intramolecular reactions is frequently controlled by the proximity of the reacting groups.113 Carbene intermediates can also be involved in rearrangement reactions. In the sections that follow we also consider a number of rearrangement reactions that probably do not involve carbene intermediates, but lead to transformations that correspond to those of carbenes. 

Indeed the carbene-alkyl insertion mechanism in Scheme 45 neatly explains why the rates of dehydrocyclization of 1, 2, and 3 are so similar. However, since 2-methylhexane also undergoes 1-5 dehydrocyclization, involvement of methylenic carbon atoms and not simply terminal carbon atoms must also be possible. The pathway for the C7-alkanes must be the reverse of nonselective hydrogenolysis of methylcyclopentane (Mechanism A), since it also results in isomerization to 2,4-dimethylpentane and 3-methylhexane, most likely via adsorbed 1,3-dimethylcyclopentane (scheme 46). It is... 

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