Page 337 - Advanced Organic Chemistry Part B - Reactions & Synthesis

PhSe 309
RCH CHR + PhSeCl + AgBF + H NCO C H RCH CHR SECTION 4.1
4
2
2 2 5
C H
NHCO 2 2 5 Electrophilic Addition to
Alkenes
Ref. 57
In the absence of better nucleophiles, solvent can be captured, as in selenenylamidation,
which occurs in acetonitrile.
PhSeCl
CH (CH ) CH CH 2 CH 3 (CH ) CHCH SePh + CH (CH ) CHCH NHCOCH 3
3
2 5
2 5
2
3
2 5
2
CN
CH 3
NHCOCH 3 SePh
85:15
Ref. 58
When reactions with phenylselenenyl chloride are carried out in aqueous acetonitrile
solution, -hydroxyselenides are formed as the result of solvolysis of the chloride. 59
PhSeCl
(CH ) C CH 2 (CH ) CCH SePh
3 2
3 2
2
CH CN H O
2
3
OH 87%
Mechanistic studies have been most thorough with the sulfenyl halides. 60 The
reactions show moderate sensitivity to alkene structure, with ERGs on the alkene
accelerating the reaction. The addition can occur in either the Markovnikov or anti-
61
Markovnikov sense. The variation in regioselectivity can be understood by focusing
attention on the sulfur-bridged intermediate, which may range from being a sulfonium
ion to a less electrophilic chlorosulfurane.
R′ R′ Cl
S + S
R C C H R C C H
H H H H
Compared to a bromonium ion, the C−S bonds are stronger and the TS for nucleophilic
addition is reached later. This is especially true for the sulfurane structures. Steric
interactions that influence access by the nucleophile are a more important factor in
determining the direction of addition. For reactions involving phenylsulfenyl chloride
or methylsulfenyl chloride, the intermediate is a fairly stable species and ease of
approach by the nucleophile is the major factor in determining the direction of ring
opening. In these cases, the product has the anti-Markovnikov orientation. 62
57 C. G. Francisco, E. I. Leon, J. A. Salazar, and E. Suarez, Tetrahedron Lett., 27, 2513 (1986).
58 A. Toshimitsu, T. Aoai, H. Owada, S. Uemura, and M. Okano, J. Org. Chem., 46, 4727 (1981).
59
A. Toshimitsu, T. Aoai, H. Owada, S. Uemura, and M. Okano, Tetrahedron, 41, 5301 (1985).
60 W. A. Smit, N. S. Zefirov, I. V. Bodrikov, and M. Z. Krimer, Acc. Chem. Res., 12, 282 (1979);
G. H. Schmid and D. G. Garratt, The Chemistry of Double-Bonded Functional Groups, S. Patai, ed.,
Wiley-Interscience, New York, 1977, Chap. 9; G. A. Jones, C. J. M. Stirling, and N. G. Bromby, J.
Chem. Soc., Perkin Trans., 2, 385 (1983).
61 W. H. Mueller and P. E. Butler, J. Am. Chem. Soc., 90, 2075 (1968); G. H. Schmid and D. I . Macdonald,
Tetrahedron Lett., 25, 157 (1984).
62
G. H. Schmid, M. Strukelj, S. Dalipi, and M. D. Ryan, J. Org. Chem., 52, 2403 (1987).