Page 331 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Br 303
NaBrO 3
NaHSO 3 OH SECTION 4.1
Electrophilic Addition to
H 2 O, CH 3 CN
75% Alkenes
HIO 4 I
NaHSO 3
H O, CH CN OH
3
2
80%
These reactions show the same regioselectivity and stereoselectivity as other reactions
that proceed through halonium ion intermediates.
Because of its high reactivity, special precautions must be taken with reactions
of fluorine and its use is somewhat specialized. 44 Nevertheless, there is some basis
for comparison with the less reactive halogens. Addition of fluorine to Z- and E-1-
propenylbenzene is not stereospecific, but syn addition is somewhat favored. 45 This
result is consistent with formation of a cationic intermediate.

F F
F 2
PhCH CHCH 3 Ph CH 3 + Ph CH 3
F F

In methanol, the solvent incorporation product is formed, as would be expected for a
cationic intermediate.

F 2 PhCHCHCH
PhCH CHCH 3 3
CH OH CH O F
3
3
These results are consistent with the expectation that fluorine would not be an effective
bridging atom.
There are other reagents, such as CF OF and CH CO F, that transfer an
3
3
2
electrophilic fluorine to double bonds. These reactions probably involve an ion pair
that collapses to an addition product.
PhCHCHPh
PhCH CHPh + CF OF
3
CF O F Ref. 46
3
CH (CH ) CH CH + CH CO F CH (CH ) CHCH F
2 9
3
2
2
2 9
3
3
2
CH CO 2 30% Ref. 47
3
The stability of hypofluorites is improved in derivatives having electron-withdrawing
substituents, such as 2,2-dichloropropanoyl hypofluorite. 48 Various other fluorinating
agents have been developed and used, including N-fluoropyridinium salts such as the
44 H. Vypel, Chimia, 39, 305 (1985).
45
R. F. Merritt, J. Am. Chem. Soc., 89, 609 (1967).
46
D. H. R. Barton, R. H. Hesse, G. P. Jackman, L. Ogunkoya, and M. M. Pechet, J. Chem. Soc., Perkin
Trans. 1, 739 (1974).
47 S. Rozen, O. Lerman, M. Kol, and D. Hebel, J. Org. Chem., 50, 4753 (1985).
48
S. Rozen and D. Hebel, J. Org. Chem., 55, 2621 (1990).