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R-N=C + S
Et3N. THF. Se cat
and thiocarbonates  from, respectively, amines or alcohols and carbon monoxide, sulfur and alkyl halides were also reported.
The facile sulfur selenium exchange from carbonyl selenide to carbonyl sulfide is noteworthy .
These are respectively the 5-oxides and 5-dioxides of thiocarbonyl compounds. The lachrymatory factor of onions was shown to be (Z)-propanethial 5-oxide (Rl = Et, R2 = H), and a good deal of the chemistry of sulfines was carried out in connection with the study of the organosulfur chemistry of alliaceous plants .
Sulfines are commonly prepared by oxidation of the corresponding thiocarbonyl compounds [187, 188]. Monosubstituted sulfines are not available by such a process, and an indirect route has been proposed .
Both sulfines and sulfenes are readily involved in cycloaddition reactions [176,187].
Thioketone 5-sulfides [190-192] and S-imides  are also attractive molecules that have been evidenced or isolated.
S + Se=C=0 -------=—*• Se + S=C=0
126.96.36.199 Sulfines and sulfenes
2.9 SULFONIUM SALTS AND SULFUR YLIDES
Sulfonium salts are available by alkylation of sulfides. They have been known for a long time, and their syntheses and general reactivity have been reviewed [194-199].
R i R3
In the context of this book we shall consider mainly the sulfonium salts as precursors of sulfonium ylides by deprotonation. Sulfur ylides can be viewed as zwitterionic species in which a carbanion is attached directly to a positively charged sulfur atom.
Stabilized sulfur ylides have been obtained very early as dimethyl sulfonium fluorenylide .
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However, their potentialities as synthetic intermediates were shown much later by the work of Corey and Chaykovsky [201, 202] on the preparations and methylene transfer reactions of dimethylsulfonium methylide (1) and dimethylsulfoxonium methylides (2).
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Reaction with carbonyl groups yields epoxides.
(1> ” ra>- />o
With activated double bonds, reaction with (2) leads to cyclopropane derivatives.
These reactions rapidly found wide use and success, and many other sulfur ylides have been prepared and exploited [194, 195, 203, 204]. Various experimental procedures are to be found in the detailed monograph by Trost and Melvin  for sulfonium salts, ylides, epoxidations and cyclopropanations.
We shall meet some examples in Chapter 4.
Apart from their fundamental role in sulfur ylide chemistry, sulfonium salts have found applications as soft electrophiles. In alkylation of ambident nucleophiles such as the enolates of [3-dicarbonyl compounds they led to selective C-alkylation .
An analogy is found in biological systems in which 5-adenosylmethionine is a universal donor of methyl groups.
Functional Modifications with Organosulfur Compounds
3.1 INTRODUCTION OF UNSATURATION
The use of elemental sulfur itself as a dehydrogenating reagent has found some utility mainly in the formation of aromatic compounds.
The Chugaev reaction , involving pyrolysis of a xanthate by an Ej mechanism, is a classical method for olefins.
XC---c' 150 250°C V ^
,\ | N ---------- ,C
í O-C-SMe - COS
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Thiocarbonate esters, obtained from aryl chlorothionoformates and hydroxyl groups, react similarly [206, 207]. This reaction was also used for the dehydration of oximes .
Formally the extrusion of sulfur from episulfides by a thiophilic reagent (phosphorus(m) compounds, organometallic compounds) introduces unsaturation, but the formation of the thiiran and the sulfur extrusion are usually coupled [209-211].
>^< “*• )=(
The cleavage of sulfoxides and sulfones bearing a (3-hydrogen, on treatment with an alkoxide  or on mild pyrolysis for a sulfoxide, allows the formation of a double bond.
, I V ----.
H S(0), - RS(0)„h
3.1.1 Creation of a double bond a to a carbonyl group
The a-sulfenylation of ketones has received much attention as it can be employed in a variety of synthetically useful reactions [38. 213]. Oxidation to the sulfoxide followed by the syn elimination just described leads to an a,p-unsaturated ketone.
An example of a mild methodology for the synthesis of a-phenylthioketones is given below . It makes use of the reagent (1) prepared by Barton  from diphenyl disulfide and chloramine T.