Page 515 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

496 Much less detail is available concerning the mechanism of fluorination and
iodination of alkenes. Elemental fluorine reacts violently with alkenes giving mixtures
CHAPTER 5
including products resulting from degradation of the carbon chain. Electrophilic
Polar Addition additions of fluorine to alkenes can be achieved with xenon difluoride, 81 electrophilic
and Elimination 82
Reactions derivatives of fluorine, or by use of highly dilute elemental fluorine at low temper-
ature. 83 Under the last conditions, syn stereochemistry is observed. The reaction is
believed to proceed by rapid formation and then collapse of an ß-fluorocarbocation-
fluoride ion pair. Both from the stereochemical results and theoretical calculations, 84
it appears unlikely that a bridged fluoronium species is formed. Acetyl hypofluorite,

which can be prepared by reaction of fluorine with sodium acetate at −75 Cin
halogenated solvents, 85 reacts with alkenes to give ß-acetoxyalkyl fluorides. 86 The
reaction gives predominantly syn addition, which is also consistent with rapid collapse
of a ß-fluorocarbocation-acetate ion pair.

CCF
O 2 3
CF CO F
2
3
F
There have been relatively few mechanistic studies of the addition of iodine. One
significant feature of iodination is that it is easily reversible, even in the presence
of excess alkene. 87 The addition is stereospecifically anti but it is not entirely clear
whether a polar or a radical mechanism is involved. 88
As with other electrophiles, halogenation can give 1,2- or 1,4-addition products
from conjugated dienes. When molecular bromine is used as the brominating agent
in chlorinated solvent, the 1,4-addition product dominates by ∼ 7 1 in the case of
butadiene. 89

Br 2
CH 2 CHCH CH 2 BrCH CHCH CH 2 + BrCH CH CHCH Br
2
2
2
25°C
Br 88%
12%
The product distribution can be shifted to favor the 1,2-product by use of milder
brominating agents such as the pyridine-bromine complex or the tribromide ion, Br .
−
3
It is believed that molecular bromine reacts through a cationic intermediate, whereas
81
M. Zupan and A. Pollak, J. Chem. Soc., Chem. Commun., 845 (1973); M. Zupan and A. Pollak,
Tetrahedron Lett., 1015 (1974).
82
For reviews of fluorinating agents, see A. Haas and M. Lieb, Chimia, 39, 134 (1985); W. Dmowski,
J. Fluorine Chem., 32, 255 (1986); H. Vyplel, Chimia, 39, 134 (1985).
83 S. Rozen and M. Brand, J. Org. Chem., 51, 3607 (1986); S. Rozen, Acc. Chem. Res., 29, 243 (1996).
84
W. J. Hehre and P. C. Hiberty, J. Am. Chem. Soc., 96, 2665 (1974); T. Iwaoka, C. Kaneko, A. Shigihara,
and H.Ichikawa, J. Phys. Org. Chem., 6, 195 (1993).
85 O. Lerman, Y. Tov, D. Hebel, and S. Rozen, J. Org. Chem., 49, 806 (1984).
86 S. Rozen, O. Lerman, M. Kol, and D. Hebel, J. Org. Chem., 50, 4753 (1985).
87
P. W. Robertson, J. B. Butchers, R. A. Durham, W. B. Healy, J. K. Heyes, J. K. Johannesson, and
D. A. Tait, J. Chem. Soc., 2191 (1950).
88 M. Zanger and J. L. Rabinowitz, J. Org. Chem., 40, 248 (1975); R. L. Ayres, C. J. Michejda, and
E. P. Rack, J. Am. Chem. Soc., 93, 1389 (1971); P. S. Skell and R. R. Pavlis, J. Am. Chem. Soc., 86,
2956 (1964).
89
G. Bellucci, G. Berti, R. Bianchini, G. Ingrosso, and K. Yates, J. Org. Chem., 46, 2315 (1981).