Page 363 - Advanced Organic Chemistry Part B - Reactions & Synthesis

slow, even under these favorable conditions, but there is clean formation of the anti 335
addition product. 149
SECTION 4.4
–
+
N Br Br Additions to Allenes and
1.0 M Bu 4
CH CH CH C CCH CH CH 3 CH CH CH 2 Alkynes
2
3
2
3
2
2
2
1:4 TFA:CH Cl 2 CH 2 CH 2 CH 3
2
144 h H 100%
–
+
N Br
1.0 M Bu 4
HC C(CH ) CH 3 CH 2 C(CH ) CH 3
2 5
2 5
Cl
1:4 TFA:CH 2 2 98%
336 h Br
Surface-mediated addition of HCl or HBr can be carried out in the presence of
silica or alumina. 150 The hydrogen halides can be generated from thionyl chloride,
oxalyl chloride, oxalyl bromide, phosphorus tribromide, or acetyl bromide. The kinetic
products from HCl and 1-phenylpropyne result from syn addition, but isomerization
to the more stable Z-isomer occurs upon continued exposure to the acidic conditions.
SOCl 2 Cl H 3 h Cl CH 3
PhC CCH 3
SiO 2 Ph CH
0.3 h 3 Ph H
The initial addition products to alkynes are not always stable. Addition of acetic
acid, for example, results in the formation of enol acetates, which are converted to the
corresponding ketone under the reaction conditions. 151
H +
C H C CC H 5 C H C CHCH CH 3 C H CCH CH CH 3
2
2 5
2
2
2 5
2 5
2
CH CO H
2
3
O CCH 3 O
2
The most synthetically valuable method for converting alkynes to ketones is by
mercuric ion–catalyzed hydration. Terminal alkynes give methyl ketones, in accor-
dance with the Markovnikov rule. Internal alkynes give mixtures of ketones unless
some structural feature promotes regioselectivity. Reactions with Hg OAc in other
2
nucleophilic solvents such as acetic acid or methanol proceed to -acetoxy- or
-methoxyalkenylmercury intermediates, 152 which can be reduced or solvolyzed to
ketones. The regiochemistry is indicative of a mercurinium ion intermediate that is
opened by nucleophilic attack at the more positive carbon, that is, the additions follow
the Markovnikov rule. Scheme 4.8 gives some examples of alkyne hydration reactions.
Addition of chlorine to 1-butyne is slow in the absence of light. When addition
is initiated by light, the major product is E-1,2-dichlorobutene if butyne is present in
large excess. 153
CH CH 2 Cl
3
CH C CH + Cl
CH 3 2 2
Cl H
149
H. M. Weiss and K. M. Touchette, J. Chem. Soc., Perkin Trans. 2, 1523 (1998).
150
P. J. Kropp and S. D. Crawford, J. Org. Chem., 59, 3102 (1994).
151 R. C. Fahey and D.-J. Lee, J. Am. Chem. Soc., 90, 2124 (1968).
152 M. Uemura, H. Miyoshi, and M. Okano, J. Chem. Soc., Perkin Trans. 1, 1098 (1980); R. D. Bach,
R. A. Woodward, T. J. Anderson, and M. D. Glick, J. Org. Chem., 47, 3707 (1982); M. Bassetti,
B. Floris, and G. Spadafora, J. Org. Chem., 54, 5934 (1989).
153
M. L. Poutsma and J. L. Kartch, Tetrahedron, 22, 2167 (1966).