Vol.4, No.1 (2011), 73-85 ISSN: 0974-1496 CODEN: RJCABP http://www.rasayanjournal.com
RECENT DEVELOPMENTS ON BISMUTH (III) IN CARBONCARBON BOND FORMATION CHEMISTRY Suresh and Jagir S. Sandhu* Department of Chemistry, Punjabi University, Patiala, Patiala-147 002, Punjab, India *E-mail: j_sandhu2002@yahoo.com ABSTRACT The present mini account overview describes briefly prosperous usage of Bismuth (III) as an attractive, green alternative to strong, toxic and hazardous, catalysts in some selected organic named reactions. Scope of this catalyst system is too broad and developments have been too fast only recent account is presented on very important carbon-carbon bond forming named reactions like Knoevenagel, Michael, Doebnor modification of Knoevenagel, Biginelli and Hanstzch wherein both these reactions are also proven combination of Knoevenagel and Michael reactions. Keywords: bismuth (III) compounds, C-C bond forming name reactions, Bi (OTf)3.xH2O © 2011 RASĀYAN. All rights reserved.
INTRODUCTION Over the past decade synthetic organic chemistry have been focusing on increasing the use of environmentally benign/eco-friendly catalysts and reagents for the use of compounds which follow twelve principle of green chemistry.1-3 Bismuth compounds have been employed as green and effective catalysts in many synthetic reactions to replace toxic catalysts e.g. compounds of mercury, lead, thallium, arsenic and antimony etc. Bismuth: General Information Now it is relevant to discuss certain general aspects of this element i.e. Bismuth is the 83rd element in the periodic table, which is placed in Group V of the Periodic Table with other members like nitrogen, phosphorus, arsenic and antimony and it is a brittle white metal with a pinkish tinge having an atomic mass of 209.980 amu. Bismuth resembles antimony in its mode of occurrence but relatively it is less common.4 Abundance of bismuth in the earth’s crust has been estimated (229) to be 0.00002 weight %, i.e. approximately same as silver and is produced as a by-product of copper and tin refining. Bismuth being a radioactive element, it is stable because of it’s an extremely long half life (t1/2~2×1018years) makes it practically stable.5-7 Despite its heavy metal status, bismuth and its compounds are relatively non-toxic and can be utilized for a variety of different medicinal as well as catalysts purposes.8-14 Lewis Acid and Eco-friendly Nature Several of the heavy metals have a relatively high toxicity and if used in reactions can be present in byproducts. One of the principal factors contributing to the low toxicity of bismuth is the poor solubility of it in aqueous solutions or in bio-fluids at moderate pH values which is found in plasma/fluids of living system.15 From table 1, it is clear that many bismuth compounds are less toxic than common salt (NaCl) and many other metallic salts.16 Since Bi-compounds are comparatively non-toxic and have been employed as eco-friendly mild Lewis acid catalysts system in synthetic green chemistry.17-27 Lewis acidity of bismuth (III) compounds are a result of poor shielding of the f-orbital electrons (electronic configuration of Bi: [Xe]4f145d106s26p3) allowing bismuth to accept an electron pair readily as well as availability of unoccupied orbital’s elevate its affinity to extend coordination. Electron pair demand of Bi further augmented by strong electron withdrawing groups (EWGs) such as halides, triflate etc. Bismuth exists in two oxidation states, +3
BISMUTH (III) IN CARBON-CARBON BOND
Suresh and Jagir S. Sandhu