Page 338 - Advanced Organic Chemistry Part B - Reactions & Synthesis

310 CH 3 SCl
CH 2 CHCH(CH ) ClCH CHCH(CH ) + CH SCH CHCH(CH )
2
3 2
3 2
2
3
3 2
CHAPTER 4 SCH 3 Cl
Electrophilic Additions 94% 6%
to Carbon-Carbon
Multiple Bonds Ref. 61a
p-ClPhSCl
CH CH CH CH 2 ClCH 2 CHCH CH + ArSCH CHCH CH 3
3
2
2
2
2
3
SAr Cl
77% 23%
Ref. 63
Terminal alkenes react with selenenyl halides with Markovnikov regioselectivity. 64
However, the -selenyl halide addition products readily rearrange to the isomeric
products. 65
ArSe
R C CH 2 + ArSeX R CCH SeAr R CCH X
2
2
2
2
2
X
4.2. Electrophilic Cyclization

When unsaturated reactants contain substituents that can participate as nucle-
ophiles, electrophilic reagents frequently bring about cyclizations. Groups that can
act as internal nucleophiles include carboxy and carboxylate, hydroxy, amino and
amido, as well as carbonyl oxygen. There have been numerous examples of synthetic
application of these electrophilic cyclizations. 66 The ring-size preference is usually
5 > 6 > 3 > 4, but there are exceptions. Both the ring-size preference and the stereo-
selectivity reactions can usually be traced to structural and conformational features of
the cyclization TS. Baldwin called attention to the role of stereoelectronic factors in
cyclization reactions. 67 He classified cyclization reactions as exo and endo and as tet,
trig, and dig, according to the hybridization at the cyclization center. The cyclizations
are also designated by the size of the ring being formed. For any given separation
(n = 1 2 3, etc.) of the electrophilic and nucleophilic centers, either an exo or endo
mode of cyclization is usually preferred. The preferences for cyclization at trigonal
centers are 5-endo >> 4-exo for n = 2; 5-exo > 6-endo for n = 3; and 6-exo >>
7-endo for n = 4. These relationships are determined by the preferred trajectory of
the nucleophile to the electrophilic center. Substituents can affect the TS structure by
establishing a preferred conformation and by electronic or steric effects.

63
G. H. Schmid, C. L. Dean, and D. G. Garratt, Can. J. Chem., 54, 1253 (1976).
64
D. Liotta and G. Zima, Tetrahedron Lett., 4977 (1978); P. T. Ho and R. J. Holt, Can. J. Chem., 60, 663
(1982).
65 S. Raucher, J. Org. Chem., 42, 2950 (1977).
66 M. Frederickson and R. Grigg, Org. Prep. Proced. Int., 29, 63 (1997).
67
J. E. Baldwin, J. Chem. Soc., Chem. Commun., 734, 738 (1976).