Entirely Solvent-Free Procedure for the Synthesis of Distillable 1,3-Dithianes Using Lithium Tetrafluoroborate as a Reusable Catalyst

Abstract

Treatment of various types of aldehydes and ketones with 1,3-propanedithiol in the presence of a catalytic amount of lithium tetrafluoroborate at 25 °C under solvent-free conditions followed by direct purification by distillation of the resulting mixture affords the corresponding 1,3-dithianes in good to excellent yields. Chemoselective protection of keto aldehydes is also successfully achieved over the catalyst. The catalyst can be recovered and reused.

chemoselectivity - 1,3-dithiane - Lewis acids - lithium tetrafluoroborate - protecting groups

References

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<A NAME="RU08704ST-11">11</A>

LiBF₄ was the best catalyst among the lithium salts tested under identical conditions: LiOTf (41%), LiCl (4%), LiClO₄ (36%), LiBr (16%), LiI (39%).

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<A NAME="RU08704ST-13">13</A>

It is important to note that ketones were efficiently thioacetalized in our method, whereas LiBF₄/MeCN has been an ineffective reaction system, and the substrates remained intact after prolonged reaction times. [6f]

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<A NAME="RU08704ST-15">15</A>

Only a few methods are known in the literature for the chemoselective protection of aldehydes in the presence of ketones: ref. 6c and references cited therein.

As far as we know, the present system is the first example for the synthesis of 1,3-dithianes under entirely solvent-free conditions, although our procedure is applicable for only distillable such compounds. Though 1,3-dithiane synthesis under solvent-free conditions has been reported in some cases, unfortunately, all of these used organic solvents in the work-up processes:

<A NAME="RU08704ST-16A">16a</A>

ref. 6d,e.

<A NAME="RU08704ST-16B">16b</A>

ref. 6i.

<A NAME="RU08704ST-16C">16c</A>

ref. 7b.

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<A NAME="RU08704ST-16E">16e</A> Firouzabadi H. Iranpoor N. Kohmareh G. Synth. Commun.  2003,  33:  167 

<A NAME="RU08704ST-17">17</A>

1,2-Ethanedithiol worked equally well under the same reaction conditions. For example, treatment of benzaldehyde (6.5 mmol) with 1,2-ethanedithiol (5 mmol) in the presence of LiBF₄ (0.5 mmol) at 25 °C for 1 h afforded 2-phenyl-1,3-dithiolane in 99% yield.