Aluminum hydrogen sulfate [Al(HSO 4 ) 3 ] as an efficient catalyst for the preparation of thioacetals
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Aluminum hydrogen sulfate [Al(HSO4)3] as an efficient catalyst for the preparation of thioacetals Majid Ghashang
Received: 25 June 2012 / Accepted: 31 August 2012 / Published online: 18 September 2012 Ó Springer Science+Business Media B.V. 2012
Abstract Aluminum hydrogen sulfate, as a heterogeneous solid acid catalyst, has been used for the mild conversion of carbonyl compounds to their thioacetals using 1,2- and 1,3-dithiol under ambient conditions with short reaction times in high to excellent yield in acetonitrile as solvent. Keywords Aluminum hydrogen sulfate [Al(HSO4)3] Thioacetals Thioacetalization C–S bond formation Carbonyl compounds
Introduction Protection and de-protection of functional groups play an important role in many organic synthetic transformations. Thiols are groups of chemical compounds which used as protecting groups for carbonyl compounds in organic synthesis [1]. In this content, thioacetals, dithioacetals and oxathioacetals are some of the useful compounds in organic synthesis introduced from thiols and carbonyl compounds. These compounds are applicable as building blocks for the synthesis of acyl carbanions. They behave as blocking groups which are applicable for electrophilic substitution on the carbonyl compound. Thioacetals are stable under usual acidic or basic conditions [2–5]. Using this beneficial view of thioacetals, numerous methods have been reported for the preparation of thioacetals via condensation of carbonyl compounds with thiols or dithiols including the use of Lewis or Brønsted acid catalysts either solitarily or supported on solid materials, e.g. silica and alumina and some silicon reagents [6–32].
M. Ghashang (&) Department of Chemistry, Faculty of Sciences, Najafabad Branch, Islamic Azad University, P.O. Box 517, Najafabad, Esfaha¯n, Iran e-mail: [email protected]
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M. Ghashang
Although the disadvantages of homogeneous catalysts, such as harsh conditions, poor selectivity and yields, and also the toxicity and expense of some metal compounds, have been overcome by the use of heterogeneous catalysts and supported reagents, developments in this area demand further searches for better catalysts that could be superior to the existing ones with regard to toxicity, handling, selectivity, and recyclability. In this respect, we are interested in introducing aluminum hydrogen sulfate Al(HSO4)3 [33–38] as an efficient catalyst for the protection of carbonyl compounds and the preparation of thioacetals to overcome these limitations (Scheme 1). This protocol reports an atom-economical synthesis of a variety of thioacetals from the reaction of carbonyl compounds and thiols using Al(HSO4)3 as catalyst and acetonitrile as solvent.
Results and discussion First, the synthesis of 2-phenyl-1,3-dithiane was chosen as a model to find an appropriate solvent. Various common organic solvents such as n-Hexane, CH2Cl2, Et2O, EtOAc, EtOH, CH3CN and also solvent-free condition were examined. The results are summarized in Table 1. It can be seen that the reaction using C
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