Carbon Free radicals

Aromatic Substitution by Free Radicals. Carbones, and Nitrenes... 

An interesting free radical carbon-carbon bond formation with concomitant elimination of a /5-thio substituent was achieved during the course of Boger s impressive synthesis of CC-1065.26-27 In the event, treatment of aryl bromide 70 (see Scheme 13) with tri-n-... 

II), and its formation therefore is more probable. If the substituent X possesses unsaturation conjugated with the free radical carbon, as for example when X is phenyl, resonance stabilization may be fairly large. The addition product (I) in this case is a substituted benzyl radical. Comparison of the C—I bond strengths in methyl iodide and in benzyl iodide, and a similar comparison of the C—H bond strengths in methane and toluene, indicate that a benzyl radical of type (I) is favored by resonance stabilization in the amount of 20 to 25 kcal. 

A one-electron reduction of the bond between an aliphatic carbon and a halogen leads to a halogen anion and a carbon-free radical. A good example is the reduction of carbon tetrachloride as discussed earlier in this chapter. The first product in the reduction is the trichloromethyl-free radical. Carbon-centered radicals are not very reactive with biological molecules, but they react very rapidly with molecular oxygen (a diradical) to form a peroxy-free radical (Fig. 5.15), which is quite toxic (10). 

D. J. Hart, Free radical carbon-carbon bond formation in organic synthesis, Science 223 883 (1984). 

Fonnylcarbene ( CHCHO), 116 Free Radicals, carbon, 110-114 RSE, table of, 114 structure, 110 substituents on, 111 Frequency analysis, 32-33 scale factors, 33 Fulvene, 268 SHMO, 268 Furan, 267 SHMO, 267... 

There was a lot of discussion about the carbon monoxide in the atmosphere. You pointed out that through free radicals carbon monoxide is oxidized to carbon dioxide. Do you believe that this is the real sink for carbon monoxide, in other words, that the scheme you described is really representative for sink for carbon monoxide ... 

Methyl radical, see also Free Radicals, carbon... 

Balczewski, P., Pietrzykowski, W.M., and Mikolajczyk, M., Studies on the free radical carbon-carbon bond formation in the reaction of a-phosphoryl sulfides and selenides with alkenes. Tetrahedron, 51, 7727, 1995. 

As mentioned in an earlier section in this Report, free-radical carbon-carbon bond-forming processes are becoming increasingly important in synthesis, and this year they have proved themselves particularly useful for the synthesis of pyrrolizidine alkaloids. Thus, Hart and his group have now applied their intramolecular tin hydride generated ot-acylamino radical to alkyne cyclization [viz. (97) - (98)] and to the synthesis of (-)-dehydrohastanecine (99), (+)-heliotridene (100), and (+)-hastanecine (101). In addition, free radical in mechanism is the photochemical cyclization of the -acylpyrrolidine (102) to the pyrrolizidene (103), a key intermediate in a synthesis of ( )-isoretronecanol (104). 

The radical A is a stable unreactive free radical which is not capable of propagating the chain reaction. However, a disadvantage for hindered phenols and aromatic amines is that they do not trap carbon free radicals. Carbon free radical trapping reduces hydroperoxide formation to a great extent and is preferable to peroxy radical trapping. Hindered amines, which are well known as photostabilizers, can trap alkyl radicals and constitute a new class of radical trapping antioxidants. 

Wu JH, Radinov R, Porter NA. Enantioselective free radical carbon-carbon bond forming reactions chiral Lewis acid promoted acyclic additions. J. Am. Chem. Soc. 1995 117 (44) 11029-11030. 

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