Selective Protection of Carbonyl Compounds over Nano-sized Nickel Catalysts
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Selective Protection of Carbonyl Compounds over Nano-sized Nickel Catalysts Ajeet Kumar Æ Santosh Kumar Æ Amit Saxena Æ Arnab De Æ Subho Mozumdar
Received: 23 September 2007 / Accepted: 5 November 2007 / Published online: 1 December 2007 Ó Springer Science+Business Media, LLC 2007
Abstract An efficient method for the preparation of 1,3dithiolanes of aliphatic and both activated and deactivated aromatic carbonyl compounds with 1,2-ethanedithiol in the presence of a catalytic amount of inexpensive, easily recyclable, monodispersed, chemoselective Ni-nanoparticles having high TON and TOF is reported. An efficient method for the chemoselective thioacetalization of ketones in the presence of aldehydes using Ni-nanoparticles is also reported in this article. Our reaction is kinetically controlled and the method is relatively expedient, avoids the use of cost-prohibitive reagents, high temperatures and leads to excellent yield. Keywords Dithiolanes Ni-nanoparticles Monodispersed Recyclable TON TOF
1 Introduction The protection of carbonyl functionality as a dithioacetal or a thioketal is important in the multistep total synthesis of complex natural and non-natural products. Among carbonyl protecting groups, 1,3-dithiolanes, 1,3-oxathiolanes, and 1,3-dithianes are important as they are inherently stable under both mildly acidic and basic conditions [1]. In addition, these are also utilized as masked acyl anions A. Kumar S. Kumar A. Saxena S. Mozumdar (&) Department of Chemistry, University of Delhi, Delhi 110007, India e-mail: [email protected] Present Address: A. De Department of Chemistry, Indiana University, Bloomington, USA
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[2] or masked methylene functional groups [3] in carbon– carbon bond forming reactions. This ‘umpolung’ or inversion of polarity is an integral step of many multistep organic syntheses. Several methods for the preparation of thioacetals, dithiolanes, or dithianes derivatives from carbonyl compounds such as p-TsOH [4] in refluxing benzene (removing the water as an azeotrope with benzene), BF3– OEt2 [5], ZnCl2 [6], SO2 [7], TMSCl–NaI [8], TMSOTf [9], AlCl3 [10], TiCl4 [11], Cu(OTf)2–SiO2 [12], SOCl2– SiO2 [13], ZrCl4–SiO2 [14], NiCl2 [15], Bi(NO3)3 [16], Bi(OTf)3 [17], LiBr [18], LiBF4 [19], InCl3 [20], molecular I2 [21], 5 M LiClO4 [22] have been reported. Although some of these methods have been carried out under mild reaction conditions, most of them require [7, 13] reflux temperature [4, 17], long reaction times and use expensive and not readily available reagents [17]. They also suffer from a tedious work-up procedure [10, 11, 13] and require the use of stoichiometric reagents [4, 6]. Some of the methods mentioned above are incompatible with other protecting groups and fail to protect deactivated aromatic substrates [14]. Interestingly, only a few methods are known for chemoselective protection of aldehydes in the presence of ketones [12, 13, 15, 18–22]. Finally, the main disadvantage of almost all the existing methods is that the catalysts are destroyed in the work-up
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