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Sulfur reagents in organic - Metzner P.

Metzner P., Thuiller A. Sulfur reagents in organic - Academic press, 1994. - 200 p.
Download (direct link): sulfarreagents1994.djvu
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fi /\ n-Bu2Sn(OTf)2 \/~\
\ + 22^
A remarkable chemoselectivity has been observed, and the order of reactivity is:
aldehvdes>ketones but ketone acetals>aldehyde acetals aromatic acetals>aromatic aldehydes>aliphatic acetals
Such discriminations are claimed by the authors not to be achieved by conventional methods.
2- (2-Furyl)-J,3-dithiane
A mixture of furfural (48 mg, 0.5 mmol), 2,2-di-n-butyl-2-stanna-l,3-dithiane (203 mg, 0.6mmol), n-Bu2Sn(OTf)2 (80mg, 0.15 mmol), and C1CH2CH2C1 (4 ml) was stirred at 35C for 20 h. GLC analysis of the reaction mixture indicated the formation of the corresponding 1,3-dithiane in a 100% yield relative to n-C15H32 as an internal standard. NaOH (In, 1 ml) was added to the reaction mixture, which was then diluted with CH2C12 (50ml). The organic layer was washed with In NaOH and H20. Drying and evaporation left an oil, which was purified by column chromatography on silica gel (95:5 hexane/ethyl acetate) to give pure 2-(2-furyl)-l ,3-dithiane (79 mg, 85%).
From [45] with permission.
Many dithioacetals, useful for synthetic purposes, are now commercially available, including bis(phenylthio)methane (1), dithiane itself and a number of 2-substituted dithianes (2), but also more exotic ones such as (3). (In contrast to the stench of (1), compound (3) is non-odoriferous!)
(1) (2) (3)
X-H

PPh3
SlMe3
COjEt
When using dithioacetals as intermediates the most tedious step is often their hydrolysis to carbonyl compounds. As a consequence, many methods have been developed to realize that conversion and, in their review [43], Grobel and Seebach give experimental details for most of the procedures known at that time. In view of the fundamental importance of dethioacetalization, we now describe some promising new methods that have appeared since.
Laszlo introduced an oxidative cleavage of dithioacetals by their clayfen (4) and claycop reagents under inild conditions with excellent yields [46, 47]. These reagents are convenient sources of the nitrosonium ion NO+, a soft reactive Lewis acid species, well adapted for attack of the soft sulfur atom.
^ Xj-r CH2C12. rt ^
(4) clayfen (montmorillonite K-lO/FeCNOJJ
Clayfen reagent
The reagent was prepared by dissolution of iron(in) nitrate nonahydrate in acetone (the proportions are 45 g of Fe(N03)39H20 in 750 ml of acetone for 60 g of K-10 clay). This resulted in a clear, rust-coloured solution which rapidly decayed to a muddy, light-brown suspension, to which the
montmorillonite K-10 catalyst was added. The solvent was removed with a rotary evaporator under a water pump vacuum at 50C, affording a free-flowing yellow powder (for precautions in its use see [48]: prolonged heating and bath temperatures >50C should be avoided in the preparation of the reagent). Clayfen (4) should be used freshly prepared for maximum chemical reactivity and never stored in closed containers.
From [46] with permission.
Claycop was prepared in a similar manner [47] and, unlike clayfen, showed no loss of reactivity on standing in an open box.
Cleavage of dithioacetals by clay-supported metal nitrates Example:
The dithioacetal (0.01 mol) was stirred for a few hours at room temperature with clayfen (4) (10.4 g, 11 mmol of ferric nitrate) or with claycop (12.1 g, 20 mmol of copper nitrate) in toluene, n-pentane or, preferably, dichloromethane (120ml). Evolution of nitrogen oxides occurred rapidly. Stirring was maintained until gas evolution ceased. The clay was then filtered off and washed twice with portions (50 ml) of the solvent. The resulting pale yellow or slightly green solution was filtered through a small quantity of neutral aluminium oxide and the solvent was evaporated under vacuum. In the case of dithiane and dithiolane derivatives, this afforded the pure carbonyl compound in excellent yield.
The oxidation products of dithiols are insoluble and removed with the clay residues by the filtration step, and the method appears particularly suitable for dithioacetals derived from dithiols as dithianes for instance. For other dithioacetals a further purification step was necessary.
From [47] with permission.
A t-butyl bromide (iodide)/DMSO system was used by Olah [49] as a mild, inexpensive reagent to cleave dithiolanes under neutral non-aqueous conditions.
57-100%
Cleavage of dithiolans by t-BuBr/DMSO
An ethanediyl 5,5-acetal (lmmol) was added to a solution of t-butyl bromide (3.05 g) and DMSO (1.5 g). The mixture was heated to 70-75C under a nitrogen atmosphere. The reaction was followed by thin layer chromatography (silica gel/hexane) and GLC (capillary OV-lOl column). Upon completion of the reaction the mixture was poured into water (15 ml) and extracted with hexane (3 X 20 ml). The combined hexane extracts were washed successively with aqueous sodium thiosulfate (10 ml) and water (2 X 10 ml), dried with anhydrous sodium sulfate, and the solvent evaporated. The residue was purified by column chromatography on silica gel using hexane as an eluent, followed by distillation or crystallization to give the corresponding carbonyl compound.
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