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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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From [315] with permission.
Cyclohexenyllithium by metallation of phenylthiocyclohexene
To a solution of LDBB (5.25 ml of a 0.40 solution in THF, 2.1 mmol) at -78C, 1-phenylthio-l-cyclohexene (0.l90g, 1.00 mmol) in THF (4 ml) was slowly added via a syringe pump over a 0.5 h period. The deep blue green reaction mixture was stirred at 78C for 2h, during which time the colour slowly changed to red.
From [315] with permission.
Cyclohexenyl cyclohexyl methanol
Cyclohexanecarboxyaldehyde (0.133 ml, 0.123 g, 1.10 mmol) in THF (2 ml) was added dropwise to the preceding mixture until it turned yellow. The reaction was quenched at 78C with brine, and the mixture was warmed to room temperature. Following extraction with ether (3 x20 ml), the combined organic layers were washed with 10% NaOH (3 X 10ml, or at
Li c-C6HnCHO
least until all the thiophenol was removed) and brine. The organic phase was dried (MgS04) and the solvent was removed by rotary evaporation. Radial chromatography with hexane was used to recover DBB from the Reaction mixture, followed by 5% ethyl acetate/hexane to give a 79% yield of l-(l'-cyclohexenyl)-l-cyclohexylmethanol.
From [315] with permission.
Under the same conditions (LDDB in THF, 78C) the reductive lithiation step occurred immediately for seven- and eight-membered rings. With a five-membered ring, cleavage of the aryl-sulfur bond was observed and phenyllithium produced.
The reductive lithiation route to carbonyl-protected 3-lithio carbonyl compounds is also valuable [317]. Two examples are given.
PhSH^ Ph!
(for the two steps)
/ \

(for the two steps) Metal-catalysed Grignard reaction with sulfides and dithioacetals
The nickel-catalysed reaction of aryl, allyl or vinyl sulfides with Grignardj reagents has been found to be an effective way for the conversion of-carbon-sulfur bonds into carbon-carbon bonds [318]. However, simple aliphatic substrates are inert.
cat. Ni(ll)
R1-S-R2 + R^MgX -----------------> R'-R3
Developments of such cross-coupling processes to dithioacetals were reported by Luh and reviewed [319]. They led to a variety of applications for carbon-carbon single and double bond formation. The gem-dialkylation of allylic and benzylic dithioacetals is a good example of their interest.
Camphene synthesis via gem-dimethylation of a dithiolane
To a THF solution of dithioacetal (198mg, l.Ommol) and NiCl2(dppe) (26.3 mg, 0.05 mmol) was added MeMgl (2.0 ml of a 2 solution in ether, 4 mmol). The mixture was refluxed for lOh. Water (15 ml) was added and the mixture was extracted with ether (2 X 20ml). The organic layer was washed with sodium hydroxide (10%, 20ml) and then dried (MgS04). The solvent was evaporated in vacuo and the residue was chromatographed on silica gel (hexane) to give camphene (112 mg, 82%).
From [320] with permission.
Another very important aspect of these transition metal-catalysed cross coupling reactions stems from the high selectivity observed with stereo-defined vinyl sulfides. Applications to the stereoselective syntheses of pheromones were straightforward (see [321] and references therein), as shown in the two-step synthesis of muscalure from (Z)-l-bromo-2-phenylthioethylene [322] involving its sequential coupling through the bromo and phenylthio substituents with the appropriate derivatives.
fl (), jMgBr, PdCl2(PPh^2
11) Me(CH2)7MgBr. NiCl2(dppe)

muscalure Reactions of a-halo sulfides with silyl enolates
We have just seen in a preceding scheme (see Section an example of alkylation of a silylated enolate with phenylthiomethyl chloride. The Lewis acid-promoted phenylthioalkylation of O-silylated enolates of ketones, aldehydes, esters and lactones has been used by Paterson and Fleming as a convenient synthetic route to a-alkylated or alkylidenated carbonyl compounds [323-325] according to the accompanying scheme.
Very good yields, mild conditions, regioselectivity, versatility in the choice of the two reacting species (easy access to thermodynamic versus kinetic silyl enol ethers as well as to the required a-chloroalkyl phenyl sulfides) give to these sequences a particularly wide scope. Neopentylation is one illustration of their utility [325].

ZnBr2 (cat)

An interesting cleavage of a carbon-sulfur bond in benzylic and allylic sulfides derived from 5-mercapto-l-phenyl-1 -tetrazole has been reported [326]. The overall reaction is an alkylation a to a carbonyl group.
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