Page 362 - Advanced Organic Chemistry Part B - Reactions & Synthesis

334 that is not stabilized by resonance, because the adjacent
bond is orthogonal to the
empty p orbital.
CHAPTER 4
Electrophilic Additions H H H H H
to Carbon-Carbon
Multiple Bonds C C C C C C
H H
H H

As a result, protonation both in solution 143 and gas phase 144 occurs at a terminal carbon
to give the 2-propenyl cation, not the allylic cation.
The addition of HCl, HBr, and HI to allene has been studied in some detail. 145 In
each case a 2-halopropene is formed, corresponding to protonation at a terminal carbon.
The initial product can undergo a second addition, giving rise to 2,2-dihalopropanes.
The regiochemistry reflects the donor effect of the halogen. Dimers are also formed,
but we have not considered them.
X X
C CH + HX CH C CH + CH CCH
CH 2 2 3 2 3 X 3

The presence of a phenyl group results in the formation of products from proto-
nation at the center carbon. 146
HCl
PhCH C CH 2 PhCH CHCH 2 Cl
HOAc

Two alkyl substituents, as in 1,1-dimethylallene, also lead to protonation at the center
carbon. 147

(CH ) C C CH 2 (CH ) C CHCH 2 Cl
3 2
3 2
These substituent effects are due to the stabilization of the carbocations that result from
protonation at the center carbon. Even if allylic conjugation is not important, the aryl
and alkyl substituents make the terminal carbocation more stable than the alternative,
a secondary vinyl cation.
Acid-catalyzed additions to terminal alkynes follow the Markovnikov rule.

+
N HBr
Et 4 2 CH
2 6
3
CH (CH ) C CH CH (CH ) C 2
2 6
3
Br 77% Ref. 148
The rate and selectivity of the reaction can be considerably enhanced by using an
added quaternary bromide salt in 1:1 TFA:CH Cl . Note that the reactions are quite
2
2
143
P. Cramer and T. T. Tidwell, J. Org. Chem., 46, 2683 (1981).
144 M. T. Bowers, L. Shuying, P. Kemper, R. Stradling, H. Webb, D. H. Aue, J. R. Gilbert, and
K. R. Jennings, J. Am. Chem. Soc., 102, 4830 (1980); S. Fornarini, M. Speranza, M. Attina, F. Cacace,
and P. Giacomello, J. Am. Chem. Soc., 106, 2498 (1984).
145
K. Griesbaum, W. Naegele, and G. G. Wanless, J. Am. Chem. Soc., 87, 3151 (1965).
146
T. Okuyama, K. Izawa, and T. Fueno, J. Am. Chem. Soc., 95, 6749 (1973).
147 T. L. Jacobs and R. N. Johnson, J. Am. Chem. Soc., 82, 6397 (1960).
148
J. Cousseau, Synthesis, 805 (1980).