Carbenes, generation structural types

Enamines derived from cyclohexanone and and a-tetralones react with chloro-(phenyl)carbene [generated from dichloro(phenyl)methane and potassium tert-butoxide] to give either 1-chloro-l-phenyl-substituted cyclopropanes, or other products.The type of products formed depends on the structure of the amine moiety in the starting enamines (Table 7). 

The use of carbenes as structural components of polymers - whether as polymer building blocks (type 1, i.e., monomers) or as side-group functionalities (type 2) - has provided access to macromolecules with unique architectures. Unfortunately, however, the use of carbenes for such purposes comes with inherent challenges, due to the reactive nature of these species. Generally, the carbene monomer must be generated in situ, which often necessitates the use of heat or a catalyst, although the recent development of stable carbenes has helped to overcome this restriction. 

In contrast to carbenes of the AX2 type, which contain three atoms, generation of carbenes with a more complex structure under photolysis or vacuum pyrolysis conditions may be accompanied by intramolecular rearrangements. Thus, the matrix isolation study of the vacuum pyrolysis... 

The observed methane generation points to a plausible I —> III or II - III transformation, but it does not distinguish which of the structures (II or III) is the metathesis-active carbene. This matter is mechanistically significant with regard to the chain termination process. Type III may terminate by a bimolecular dimerization sequence as in Eq. (11), or it may convert to a 7r-olefin complex via an uncommon 1,2-hydride shift ... 

The type of the products formed in the reaction of dichlorocarbene with allenes depends on the structure of the allenes and on the method used to generate the carbene. Thus, allene afforded monoadduct 1 or diadduct 2. - ° ... 

This type of Cj-extrusion reaction has been predominantly of theoretical interest, especially with respect to the correlation of carbene reactivity with the structure of the precursor and the method of cleavage/ However, some attempts to use this Cj-extrusion method to generate certain carbenes for synthetic uses have also been reported. Thus, hexamethoxycyclopropane (5) has been probed as a potential source of dimethoxycarbene (6). 

---