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jr"» II R'CHO | X
a x X
II) R -C-NH2 r* N R'
Thiocarbonyl compounds for which the C=S group is linked to two heteroatoms, as in thionocarbonates, thionocarbamates, thioureas and trithiocarbonates, have been in general less directly involved as intermediates in synthesis, and we have considered here only xanthates which intervene in the Chugaev reaction (see Section 3.1) and Barton radical reactions (see Section 3.2). They are traditionally prepared by reaction of an alkoxide anion, generated from the alcohol and a strong
base such as sodium hydride, potassium hydride or alkyl lithium, with carbon disulfide followed by alkylation with an alkyl halide [169, 170]. Phase transfer conditions have been employed with success , and a one-pot procedure under such catalysis is described .
The alcohol (10 mmol) and methyl iodide (11 mmol) were added to a two-phase system of CS2 (10 ml) and 50% aqueous NaOH (from 5 g NaOH in 5 ml H20) containing n-Bu4NHS04 (lmmol) in a 50 ml round-bottomed flask. The mixture was stirred vigorously at room temperature by a magnetic stirrer. The reaction was monitored by checking the ’H NMR spectrum of the CS2 layer. To work up the reaction, the CS2 layer was separated and the aqueous layer extracted three times with CS2 (3 X 10 ml). The combined CS2 layers were dried (Na2S04) and filtered. Evaporation of the solvent under reduced pressure afforded pure O-alkyl 5-methyl dithiocarbonate (Table 2.5). Further purification could be carried out by column chromatography on silica gel or recrystallization for solid samples.
Synthesis of O-alkyl S-methyl dithiocarbonates (xanthates)
TABLE 2.5 Synthesis of O-alkyl 5-methyl dithiocarbonates
Time (h) Yield (%)
92 (m.p. = 28°C)
From  with permission.
It is of note that no significant amounts of dimethyl trithiocarbonate were formed in the process, although similar conditions yielded trithiocarbonates from CS2 and alkyl halides in excellent yields but much
more slowly , so that, in the presence of an alcohol, their formation did not interfere with that of xanthates.
0 2 RX jf
l.»3 ------> þñ SR
For R = Me the yield was 96% after 40h. l,3-Dithiolane-3-thione (R,R = CH,CH2) was obtained in a 70% yield together with some polymeric material. The yield dropped to 30% for l,3-dithiane-3-thione (R,R=CH2CH3CH2).
2.8.6 Miscellaneous thiocarbonyl compounds
X=C=S (X=RIR2C, RN, O, S)
With a few exceptions these species dimerize readily [60, 119, 174-176], and most of the methods applicable to the synthesis of monomers involve thermolytic processes such as the flash thermolysis of 1,2,3-thiadiazoles [177,178], or of 5-silylated ketenedithioacetals .
'n 580°c » \= =s
R1. .SMe r1
-FVT- - y= =s
R2 S-SiMe3 - MeS-SiMe3 R2
Some stable thioketenes have been prepared by more conventional methods, for example di-t-butylthioketene from di-t-butylacetylchloride .
t-Bu. ft t-Bu P4SIO tBu\
* n È~ñ~ñë or A= =0 —zrT—)= =s
t Bu ^ t-Bu pyridine t Bu/
In sharp contrast to the thioketenes, carbon disulfide S=C=S, carbon oxysulfide 0=C=S and the isothiocyanates R N=C’=S are stable compounds although highly reactive, and are widely used in organosulfur chemistry. Some of their reactions with organometallics have been mentioned previously (see Section 2.8.3).
Isothiocyanates (“mustard oils") and their glucosinolate precursors are widely distributed in higher plants, especially among cruciferous vegetables, and it has been established recently that sulforaphane, found in various species and particularly in broccoli, is the major source of their anticarcinogenic action .
There are some general synthetic routes to isothiocyanates . One of the most straightforward methods is the selenium-catalysed thionation of isocyanides by elemental sulfur .
To a mixture of elemental sulfur (3mmol) and selenium (0.125 mmol, 5 mol% to isocyanide) and triethylamine (6 mmol) in THF (5 ml) was added cyclohexyl isocyanide (2.5 mmol), and the mixture was refluxed for 2h. GLC analysis showed that the isocyanide was completely consumed. Removal of the deposited selenium by filtration followed by distillation gave an 89% yield of cyclohexyl isothiocyanate.
From  with permission.
Similar selenium-catalysed syntheses of 5-alkyl thiocarbamates