Page 564 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

enormous difference, 265 which is partially compensated by the higher ground state 545
energy of the alkynes. Reactions that proceed through TSs leading to bridged interme-
SECTION 5.9
diates typically show much greater rate retardation for the alkyne addition. Bromination
is the best studied example of this type. The lower rate reflects the greater strain of Additions to Alkynes and
Allenes
bridged species in the case of alkynes. Bridged intermediates derived from alkynes
must incorporate a double bond in the three-membered ring. 266 The activation energies
for additions to alkynes through bridged intermediates are thus substantially greater
than for alkenes.

5.9.5. Additions to Allenes

Electrophilic additions to allenes represent an interesting reaction type that is
related to additions to both alkenes and alkynes. 267 An allene could, for example,
2
conceivably be protonated at either a terminal sp carbon or the central sp carbon.
H + + +
RCH C CHR RCH CH CHR versus RCH 2 CH CHR
The allylic carbocation resulting from protonation of the center carbon might seem the
2
obvious choice but, in fact, the kinetically favored protonation at a sp carbon leads
to the vinyl cation intermediate. The reason for this is stereoelectronic. The allene
structure is nonplanar, so that a protonation of the center carbon leads to a twisted
structure that lacks of allylic conjugation. This twisted cation is calculated to be about
36–38 kcal/mol higher in energy than the cation formed by protonation at a terminal
carbon. 268

R H R + H
C C C C C
H H C
R H R
H +

Consistent with this generalization, addition of hydrogen halides to terminal allenes
initially gives the vinyl halide; if the second double bond reacts, a geminal dihalide is
formed. 269 The regioselectivity of the second step is consistent with Markovnikov’s rule
because a halogen atom can stabilize a carbocation by resonance (see Section 3.4.1).

HX X
RCH C CH 2 RCH 2 CCH 2 RCH 2 C CH 3
X X

265 Z. Rappoport, in Reactive Intermediates, Vol. 3, R. A. Abramovitch, ed., Plenum Press, New York,
1985, Chap. 7; Y. Apeloig and T. Muller, in Dicoordinated Carbocations, Z. Rappoport and P. J. Stang,
eds., John Wiley & Sons, New York, 1997, Chap. 2.
266
G. Melloni, G. Modena, and U. Tonellato, Acc. Chem. Res., 8, 227 (1981).
267 For a review of electrophilic additions to allenes, see W. Smadja, Chem. Rev., 83, 263 (1983).
268 K. B. Wiberg, C. M. Breneman, and T. J. Le Page, J. Am. Chem. Soc., 112, 61 (1990); A. Gobbi and
G. Frenking, J. Am. Chem. Soc., 116, 9275 (1994).
269
T. L. Jacobs and R. N. Johnson, J. Am. Chem. Soc., 82, 6397 (1960); R. S. Charleston, C. K. Dalton,
and S. R. Schraeder, Tetrahedron Lett., 5147 (1969); K. Griesbaum, W. Naegle, and G. G. Wanless,
J. Am. Chem. Soc., 87, 3151 (1965).