Page 335 - Advanced Organic Chemistry Part B - Reactions & Synthesis

RCH CHR RCH CHR RC CHR 307
N X N X
SECTION 4.1
O HO
Electrophilic Addition to
Alkenes
4.1.6. Addition Reactions with Electrophilic Sulfur and Selenium Reagents
Compounds having divalent sulfur and selenium atoms bound to more electroneg-
ative elements react with alkenes to give addition products. The mechanism is similar
to that in halogenation and involves of bridged cationic intermediates.

R′
S + SR′
Cl –
R′S Cl + RCH CHR RCH CHR RCHCHR
R′ Cl
SeR′
+ Se Cl –
R CH CHR
R′Se Cl + RCH CHR RCH CHR
Cl

In many synthetic applications, the sulfur or selenium substituent is subsequently
removed by elimination, as is discussed in Chapter 6.
A variety of electrophilic reagents have been employed and several examples are
given in Scheme 4.3. The sulfenylation reagents are listed in Section A. Both aryl and
alkyl sulfenyl chlorides are reactive (Entries 1 and 2). Dimethyl(methylthio)sulfonium
fluoroborate (Entry 3) uses dimethyl sulfide as a leaving group and can be utilized
to effect capture of hydroxylic solvents and anionic nucleophiles, such as acetate
and cyanide. Entries 4 and 5 are examples of sulfenamides, which normally require
a Lewis acid catalyst to react with alkenes. Entry 6 represents application of the
Pummerer rearrangement for in situ generation of a sulfenylation reagent. Sulfoxides
react with acid anhydrides to generate sulfonium salts. When a t-alkyl group is present,
fragmentation occurs and a sulfenylium ion is generated. 55 TFAA is the preferred
anhydride in this application.
O O CCF 3
2
(CF CO) O
2
3
R S C(CH ) R S C(CH ) RS + + (CH ) CO CCF 3
3 3
3 3
3 3
2
+
The selenylation reagents include the arylselenenyl chlorides and bromides
(Entries 7 and 8), selenylium salts with nonnucleophilic counterions (Entry 9), and
selenenyl trifluoroacetates, sulfates, and sulfonates (Entries 10 to 13). Diphenyl-
diselenide reacts with several oxidation reagents to transfer electrophilic phenylsele-
nenylium ions (Entries 14 to 16). N-Phenylselenenylphthalimide is a useful synthetic
reagent that has the advantage of the nonnucleophilicity of the phthalimido leaving
group (Entry 18). The hindered selenenyl bromide in Entry 19 is useful for selenylcy-
clizations (see Section 4.2.2).
Selenylation can also be done under conditions in which another nucle-
ophilic component of the reaction captures the selenium-bridged ion. For
55
M.-H. Brichard, M. Musick, Z. Janousek, and H. G. Viehe, Synth. Commun., 20, 2379 (1990).