Nonheteroatom-substituted carbene

As will be discussed more thoroughly in Section 3.2.5, transition metal carbene complexes can mediate olefin metathesis. Because heteroatom-substituted carbene complexes are usually less reactive towards olefins than the corresponding nonheteroatom-substituted complexes, it is, e.g., possible to use enol ethers to terminate living polymerization or other types of metathesis reaction catalyzed by a non-heteroatom-substituted carbene complex. Olefin metathesis can also be used to prepare new heteroatom-substituted carbene complexes (Figure 2.15, Table 2.11). 

There are only two examples for the conversion of allyl halides to nonheteroatom-substituted vinylcyclopropanes. Firstly, vinyl- and (2-chlorovinyl)cyclopropanes can be prepared in yields of 18-77% by oxidative addition of a 3,3-dihalo-l-alkene onto a low-valent metal, e.g. Cu(0), thus forming a carbenoid which either reacts directly or after formation of the carbene with an alkene (see Houben-Weyl, Vol. E19b, pp673-674). ... 

Catalytic cydopropanation of electron-rich olefins has been realized by diazoalkanes 5 in the presence of catalytic amounts of pentacarbonyl( -ctsreacted with alkyl- and donor-substituted olefins in the presence of catalytic amounts of the pentacarbonylchromium tetrahydrofuran complex [18]. Nonheteroatom-stabilized carbene complex intermediates 6 and 8 are involved in both cases (Scheme 11.4). 

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