Carbenes halogen

Alkyl groups are attractive kinetic protectors for triplet carbenes. However, they are potentially reactive toward triplet carbenes and, hence, will not be able to shield the reactive center completely. In this respect, we need to explore protecting groups that are almost completely unreactive toward triplet carbenes. Halogens are generally reactive toward the singlet state of carbene but are not reactive with the triplet. 

We will consider several types of additions to alkenes, using a wide variety of reagents water, borane, hydrogen, carbenes, halogens, oxidizing agents, and even other alkenes. Most, but not all, of these will be electrophilic additions. Table 8-1 summarizes... 

H-Pyran, 2-alkoxy-4-methyl-2,3-dihydro-conformation, 3, 630 4H-Pyran, 2-amino-IR spectra, 3, 593 synthesis, 3, 758 4H-Pyran, 4-benzylidene-synthesis, 3, 762 4H-Pyran, 2,3-dihydro-halogenation, 3, 723 hydroboration, 3, 723 oxepines from, 3, 725 oxidation, 3, 724 reactions, with acids, 3, 723 with carbenes, 3, 725 4H-Pyran, 5,6-dihydro-synthesis, 2, 91 4H-Pyran, 2,6-diphenyl-hydrogenation, 3, 777 4H-Pyran, 6-ethyl-3-vinyl-2,3-dihydro-reactions, with acids, 3, 723 4H-Pyran, 2-methoxy-synthesis, 3, 762 4H-Pyran, 2,4,4,6-tetramethyl-IR spectra, 3, 593 4H-Pyran, 2,4,6-triphenyl-IR spectra, 3, 593... 

Halogen exchange of F is usually with chloro compounds however, replacement of bromine has enabled fluorodiazirines to be obtained (83JA6513 86TL419). Diazirine (2) was previously obtained by a difficult route involving F2. The relative ease of access to (2) enables a carbene whose reactivity is intermediate between that of electrophilic ( CF2) and nucleophilic ( C(OMe)2) carbenes to be studied. 

This is not surprising, since triplet carbenes are free radicals. But singlet carbenes can also give this reaction, though in this case only halogen atoms are abstracted, not hydrogen. 

Which isomer is predominantly formed depends on R, R, and on the method by which the carbene or carbenoid is generated. Most studies have been carried out on monosubstituted species (R = H), and in these studies it is found that aryl groups generally prefer the more substituted side (syn addition) while carbethoxy groups usually show anti stereoselectivity. When R = halogen, free halocarbenes show little or no stereochemical preference, while halocarbenoids exhibit a preference for syn addition. Beyond this, it is difficult to make simple generalizations. 

More recently, such vinyl cations generated by the alkaline decomposition of 3-nitroso-2-oxazolidones have been trapped by halogens to give vinyl halides as products (108). It has been suggested that unsaturated carbenes, RjC=C , may be the intermediates in the basic decomposition of 132 (109). Indeed, when 132 (Ri=R2=CH3, R3=H) was treated with lithium ethoxide in the... 

The addition of dichlorocarbene, generated from chloroform, to alkenes gives dichlorocyclopropanes. The procedures based on lithiated halogen compounds have been less generally used in synthesis. Section D of Scheme 10.9 gives a few examples of addition reactions of carbenes generated by a-elimination. 

Section D illustrates formation of carbenes from halides by a-elimination. The carbene precursors are formed either by deprotonation (Entries 14 and 17) or halogen-metal exchange (Entries 15 and 16). The carbene additions can take place at low temperature. Entry 17 is an example of generation of dichlorocarbene from chloroform under phase transfer conditions. 

In the preparation of bis(trifluoromethyl)sulfur difluoride, explosions have occurred when chlorine mono- or tri-fluorides were treated with 2-3 mmol of the sulfide in absence of solvent. Because chlorine trifluoride is known to deprotonate solvents with formation of explosive carbene species, only fully halogenated solvents are suitable as diluents. 

---