Carbon halocarbenes

Cyclopropanation of carbon-carbon double bonds with methylene can be achieved in several ways. Aside from the carbene or carbenoid additions, activated multiple bonds may be cyclopropanated by reaction with ylides or by a synthetic route via 1-pyrazolines (see Sections III. B and III. D in this chapter, and also Chapter 9). Addition of halocarbenes followed by hydrogenolysis provides a further route for the cyclopropanes (Section V.A). 

O. R. XIII-3, C. Walling, and E. S. Huyser, Free Radical Additions to Olefins and Acetylenes to Form Carbon-Carbon Bonds XI11-1, G. Zweifel and H. C. Brown, Hydration of Olefins, Dienes, and Acetylenes via Hydroboration XIII-2, W. E. Parham and E. E. Schweizer, Halocyclopropanes from Halocarbenes. ... 

This reaction constitutes a method for the transformation of saturated cyclic or acyclic carbonyl compounds in three steps (silyl enol ether formation, halocarbene addition, rearrangement) to a,) -unsaturated carbonyl compounds with one-carbon ring enlargement or chain elongation, respectively. The rearrangement can be induced under acidic, basic, or thermolytic conditions, or with silver(I) salts. 

The second column consists of halide-substituted carbene complexes [14]. In these cases, the bonding scheme is similar to Fischer carbenes in that the singlet halocarbene fragments interact with singlet metal centers, but the halide substituents do not contribute as much electron density into the carbon p orbital as the more strongly jt-donating alkoxide substituents. Presumably, this electronic deficit... 

Others have reported analogous photocatalyzed polymerizations when W(CO)6 is irradiated in halocarbon solvents in the presence of alkynes and halocarbene complexes have been suggested as the key intermdiates [110]. Halocarbene intermediates have also been implicated in the photocatalysis of olefin metathesis reactions by the group VI carbonyls M(CO)6 in carbon tetrachloride [111,112]. 

---