Page 322 - Advanced Organic Chemistry Part B - Reactions & Synthesis

294 Trifluoroacetic acid (TFA) is strong enough to react with alkenes under relatively mild
11
conditions. The addition is regioselective in the direction predicted by Markovnikov’s
CHAPTER 4
rule.
Electrophilic Additions
to Carbon-Carbon CF CO H
Multiple Bonds Cl(CH ) CH CH 3 2 Cl(CH ) CHCH
2 3
2
Δ 2 3 3
O CCF 3
2
Ring strain enhances alkene reactivity. Norbornene, for example, undergoes rapid
addition of TFA at 0 C. 12

4.1.3. Oxymercuration-Reduction
The addition reactions discussed in Sections 4.1.1 and 4.1.2 are initiated by the
interaction of a proton with the alkene. Electron density is drawn toward the proton and
this causes nucleophilic attack on the double bond. The role of the electrophile can also
be played by metal cations, and the mercuric ion is the electrophile in several synthet-
ically valuable procedures. 13 The most commonly used reagent is mercuric acetate,
but the trifluoroacetate, trifluoromethanesulfonate, or nitrate salts are more reactive
and preferable in some applications. A general mechanism depicts a mercurinium ion
as an intermediate. 14 Such species can be detected by physical measurements when
alkenes react with mercuric ions in nonnucleophilic solvents. 15 The cation may be
predominantly bridged or open, depending on the structure of the particular alkene.
The addition is completed by attack of a nucleophile at the more-substituted carbon.
The nucleophilic capture is usually the rate- and product-controlling step. 13 16

Hg 2+ Hg +
Nu –
RCH CH + Hg(II) RCH CH 2 or RCH CH 2 [RCHCH 2 Hg] +
2
+
Nu
The nucleophiles that are used for synthetic purposes include water, alcohols,
carboxylate ions, hydroperoxides, amines, and nitriles. After the addition step is
complete, the mercury is usually reductively removed by sodium borohydride, the net
result being the addition of hydrogen and the nucleophile to the alkene. The regio-
selectivity is excellent and is in the same sense as is observed for proton-initiated
additions. 17

11 P. E. Peterson, R. J. Bopp, D. M. Chevli, E. L. Curran, D. E. Dillard, and R. J. Kamat, J. Am. Chem.
Soc., 89, 5902 (1967).
12
H. C. Brown, J. H. Kawakami, and K.-T. Liu, J. Am. Chem. Soc., 92, 5536 (1970).
13 (a) R. C. Larock, Angew. Chem. Int. Ed. Engl., 17, 27 (1978); (b) W. Kitching, Organomet. Chem. Rev.,
3, 61 (1968).
14
S. J. Cristol, J. S. Perry, Jr., and R. S. Beckley, J. Org. Chem., 41, 1912 (1976); D. J. Pasto and
J. A. Gontarz, J. Am. Chem. Soc., 93, 6902 (1971).
15
G. A. Olah and P. R. Clifford, J. Am. Chem. Soc., 95, 6067 (1973); G. A. Olah and S. H. Yu, J. Org.
Chem., 40, 3638 (1975).
16 W. L. Waters, W. S. Linn, and M. C. Caserio, J. Am. Chem. Soc., 90, 6741 (1968).
17
H. C. Brown and P. J. Geoghegan, Jr., J. Org. Chem., 35, 1844 (1970); H. C. Brown, J. T. Kurek,
M.-H. Rei, and K. L. Thompson, J. Org. Chem., 49, 2511 (1984); H. C. Brown, J. T. Kurek, M.-H. Rei,
and K. L. Thompson, J. Org. Chem., 50, 1171 (1985).