General Papers ARKIVOC 2006 (xii) 130-137
In practice, from an ecological point of view, the best solvent is without a doubt no solvent. There are of course a great many reactions that can already be carried out in the absence of solvent. Reports on solvent-free reactions have, however, become increasingly frequent and specialized over the past few years. Areas of growth include reactions between solids,10 between gases and solids,11 and on supported inorganic reagents,12 which in many cases are accelerated or even made possible through microwave irradiation.13 There are also reactions in which at least one reactant is liquid under the conditions employed, which means that the solvent that would normally be used can simply be left out. Very recently, the catalyzed conjugated addition of thiols to a,ß-unsaturated carbonyl compounds under solvent-free conditions have been reported.7
Results and Discussion
In our previous results,14 we investigated a one-pot procedure for the reductive cleavage of disulfides and Michael addition to a,ß-unsaturated carbonyl compounds mediated by the Zn/AlCl3 system in aqueous media. We have now examined the conjugate addition of thiols to a,ß-unsaturated carbonyl compounds under solvent-free conditions in the absence of any catalyst.
In a model reaction, methyl vinyl ketone was taken as a representative a,ß-unsaturated carbonyl compound; this was treated with an equimolar amount of thiophenol in the absence of a catalyst under neat conditions at room temperature (~ 30 °C) (Scheme 1); after 30 min, as monitored by TLC, the isolated yield of the Michael adduct was 76%. Under the same reaction conditions, two other experiments were also carried out with molar ratios of thiophenol : methyl vinyl ketone = 1.5:1 and 2:1; at the same reaction time (30 min), the isolated yields of the product were 83 and 93% respectively. Therefore, we decided to perform our study at thiol : a,ßunsaturated carbonyl compound of 2:1. The product isolation with very high purity was easily achieved by subjecting the crude reaction mixture to preparative TLC. The reaction was also conducted, under the same reaction conditions, in various solvents affording low to good yields of the Michael adduct (Table 1).
O
O
neat
+ PhSH 30 oC
SPh
Scheme 1
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