Page 361 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Scheme 4.7. (Continued) 333

a. B. M. Trost, T. N. Salzmann, and K. Hiroi, J. Am. Chem. Soc., 98, 4887 (1976). SECTION 4.4
b. P. G. Gassman, D. P. Gilbert, and S. M. Cole, J. Org. Chem., 42, 3233 (1977).
c. P. G. Gassman and R. J. Balchunis, J. Org. Chem., 42, 3236 (1977). Additions to Allenes and
d. G. Foray, A. Penenory, and A. Rossi, Tetrahedron Lett., 38, 2035 (1997). Alkynes
e. P. Magnus and P. Rigollier, Tetrahedron Lett., 33, 6111 (1992).
f. A. B. Smith, III, and R. E. Richmond, J. Am. Chem. Soc., 105, 575 (1983).
g. H. J. Reich, J. M. Renga, and I. L. Reich, J. Am. Chem. Soc., 97, 5434 (1975).
h. J. M. Renga and H. J. Reich, Org. Synth., 59, 58 (1979).
i. T. Wakamatsu, K. Akasaka, and Y. Ban, J. Org. Chem., 44, 2008 (1979).
j. H. J. Reich, I. L. Reich, and J. M. Renga, J. Am. Chem. Soc., 95, 5813 (1973).
k. I. Ryu, S. Murai, I. Niwa, and N. Sonoda, Synthesis, 874 (1977).

unsaturation. The products can subsequently be oxidized to sulfoxides and selenoxides
that readily undergo elimination (see Section 6.8.3), generating the corresponding , -
unsaturated carbonyl compound. Sulfenylations and selenenylations are usually carried
out under conditions in which the enolate of the carbonyl compound is the reactive
species. If a regiospecific enolate is generated by one of the methods described in
Chapter 1, the position of sulfenylation or selenenylation can be controlled. 140 Disul-
fides are the most common sulfenylation reagents, whereas diselenides or selenenyl
halides are used for selenenylation.
Scheme 4.7 gives some specific examples of these types of reactions. Entry
1 shows the use of sulfenylation followed by oxidation to introduce a conjugated
double bond. Entries 2 and 3 are -sulfenylations of a ketone and lactam, respectively,
using dimethyl disulfide as the sulfenylating reagent. Entries 4 and 5 illustrate the
use of alternative sulfenylating reagents. Entry 4 uses N-phenylsulfenylcaprolactam,
which is commercially available. The reagent in Entry 5 is generated by reaction of
diphenyldisulfide with chloramine-T. Entries 6 to 10 are examples of reactions of
preformed enolates with diphenyl diselenide or phenylselenenyl chloride. As Entries
11 and 12 indicate, the selenenylation of ketones can also be effected by reactions of
enol acetates or enol silyl ethers.

4.4. Additions to Allenes and Alkynes

Both allenes 141 and alkynes 142 require special consideration with regard to mecha-
nisms of electrophilic addition. The attack by a proton on allene might conceivably
lead to the allyl cation or the 2-propenyl cation.

H + H + +
+ CH 2 CH CH 2 CH 2 C CH 2 CH 3 C CH 2

An immediate presumption that the more stable allyl ion will be formed overlooks
the stereoelectronic facets of the reaction. Protonation at the center carbon without
rotation of one of the terminal methylene groups leads to a primary carbocation

140
P. G. Gassman, D. P. Gilbert, and S. M. Cole, J. Org. Chem., 42, 3233 (1977).
141 H. F. Schuster and G. M. Coppola, Allenes in Organic Synthesis, Wiley, New York, 1984 ; W. Smadja,
Chem. Rev., 83, 263 (1983); S. Ma, in Modern Allene Chemistry, N. Krause and A. S. K. Hashmi, eds.,
Wiley-VCH, Weinheim, 2004, pp. 595–699.
142
W. Drenth, in The Chemistry of Triple Bonded Functional Groups, Supplement C2, Vol. 2, S. Patai,
ed., John Wiley & Sons, New York, 1994, pp. 873–915.