Rakesh Kumar Sharma

   

1. Rakesh Kumar Sharma and T. V. RajanBabu,
   Department of Chemistry, Ohio State University, Columbus OH-43210
   
   Cobalt catalyzed asymmetric hydrovinylation of alkenes and 1,3-dienes: development of new catalytic system and mild reaction conditions for  sensitive reaction¹
   

   Cobalt catalyzed asymmetric hydrovinylation of alkenes and 1,3-dienes: development of new catalytic system and mild reaction conditions for  sensitive reaction1 Development of new air stable bidentate cobalt based catalysts for asymmetric hydrovinylation of alkenes is the mail goal of these studies. The present successful nickel based catalyst are not air stable and required drybox to perform every single experiment. Also the nickel based catalysts are monodentate that have limited control on stereochemistry and design. A series of new chiral and achiral cobalt phosphine complexes are synthesized and screened for various alkenes and 1,3-dienes. Under optimized reaction conditions cobalt complexes work tremendously well and gave 100% conversion for styrenes and its derivatives. More interesting results were obtained when 1,3-dienes are used as substrate, a ligand directing 1,2-A (>99%) or 1,4-B hydrovinylation (>99%) reactions were observed with excellent enantioselectivity( >99%).

   
   
   
2. Rakesh Kumar Sharma and A. G. Samuelson,
   Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore
   
   New bisphosphinites from naturally occurring chiral sources: modified template method for direct synthesis of metal complexes from diols²
   

   New bisphosphinites from naturally occurring chiral sources: modified template method for direct synthesis of metal complexes from diols2 The objective of this research is to develop new chiral catalysts derived from natural chiral pool for application as chiral auxiliaries in several important asymmetric catalytic reactions. In this respect synthesis of  new chiral bisphosphinites  and their palladium and platinum complexes with framework I, from inexpensive natural chiral compounds was intended.  The modular construction of these ligands enable one to study the effect of  (a) steric and electronic effect of substituents R1-R5 in order to improve selectivity of these reactions (b) Hydrogen bonding effects on selectivity if the substituents are  hydrogen bond donors or acceptors.     Most complexes are synthesized with a modified template procedure. In this method, alcohols are directly treated with M(PPh2Cl)2Cl2, generated in situ from [M(COD)Cl2] (M = Pd/Pt) and PPh2Cl2. Forty two new complexes are prepared with different ring sizes and functional group tolerance. Synthetically difficult bimetallic complexes are synthesized with ease using the modified template procedure. Seventeen complexes are found to be suitable for X-ray diffraction studies. Crystal structures show distorted square planar geometry around the metal with a twist half boat conformation. The diols are chosen based on the backbone functionalities such as hydrogen bond donors/acceptors and steric demand. Particularly, ascorbic and isoascorbic acid derived bisphosphinites are systematically edited to understand the influence of hydrogen bonding and steric interactions in asymmetric transformations (II).

   
   Allylation of imines: key role of water in the allylic activation catalyzed by Pd (II) bisphophinites²
   

   Allylation of imines: key role of water in the allylic activation catalyzed by Pd (II) bisphophinites2 In initial attempts nucleophilic allylation reaction and the effect of promoters on chemical and optical conversion were investigated. An important role of water is observed  in allylation of imines using palladium bisphosphinite complexes  (Scheme I)  derived from mandelic acid. It is found that one equivalent of water reduces the reaction time and enhances the yield. Allylation of diverse imines were carried out in essentially neutral condition in contrast to the Lewis acid catalyzed reaction.

   
   Allylation of aldehydes: Acetic acid promoted asymmetric induction in synthesis of homoallylic alcohols³
   

   Allylation of aldehydes: Acetic acid promoted asymmetric induction in synthesis of homoallylic alcohols3 In contrast to imines nucleophilic allylation of aldehydes is catalyzed by Pt-bisphosphinites (Scheme II). Acetic acid is found to be the best promoter among the promoters tested. High yield  and enantioselectivity (> 88% ee) are observed with complex A derived from chloramphenicol. Ligands based on ascorbic acid and isoascorbic acid show good potential for enantioselective allylation of carbonyl compounds.

   
   Tsuji-Trost reaction: effect of ligand design on enantioselectivity⁴
   

   Tsuji-Trost reaction: effect of ligand design on enantioselectivity4 In another study of asymmetric allylic alkylation (AAA) reaction of rac-1,3-diphenyl-3-acetoxy-1-ene with a series of  Pd-bisphosphinite complexes under the standard Tsuji-Trost condition (Scheme III). The Pd-bisphosphinites are having good potential for catalyzing AAA reaction. Bulky and  hydrogen bond donor groups are having a positive effect on enantioselectivity. Among the catalysts screened, the 3-O-benzyl  substituted ascorbic acid derived complex B gives highest enantioselectivity 97%.

   
   Hydrosilylation of alkenes: a novel solvent free highly enantioselective reaction for synthesis of chiral silicon masked compounds⁵
   
   

   Hydrosilylation of alkenes: a novel solvent free highly enantioselective reaction for synthesis of chiral silicon masked compounds5 To see the activity of bisphosphinites in addition reactions, Pt-bisphosphinite catalyzed hydrosilylation of styrene with trichlorosilane in solvent free conditions (Scheme IV). The results show that enantioslectivities of  product depend strongly on the type of functional groups that exist on the backbone of the ligand.  The enatioselectivities are high with complexes that contain a hydrogen bond donor group and those with sterically demanding groups. Ascorbic acid based complexes show superior catalytic activity among the complexes tested, dibenzylated ascorbic acid derived platinum complex C gives highest enantiomeric excess as high as 98 %.

   
   
   
3. Rakesh Kumar Sharma and Bipin Pandey,
   Zydus Research Center, Zydus Cadila Healthcare, Ahmadabad
   
   A catalytic phenylation and hydrolysis process for the preparation of 4-[(un)substituted phenyl]-3,4-dihydro-1-(2H)-naphthalene-1-ones from 1-alkoxynaphthalenes and 1,2-disubstituted benzenes⁶
   
   

   A catalytic phenylation and hydrolysis process for the preparation of 4-[(un)substituted phenyl]-3,4-dihydro-1-(2H)-naphthalene-1-ones from 1-alkoxynaphthalenes and 1,2-disubstituted benzenes6

   (S)-(+)-clopidogrel: Development of new aqueous Strecker reaction⁷
   

   (S)-(+)-clopidogrel: Development of new aqueous Strecker reaction7

   
   
   
4. Rakesh Kumar Sharma and R.V. Jasra,
   Central Salt & Marine Chemicals Research Institute, Bhavnagar
   
   Green catalysts for Friedel-Craft acylation⁸
   
   

   Green catalysts for Friedel-Craft acylation8

References:

1. Rakesh K Sharma and T. V. RajanBabu* J. Am. Chem. Soc. 2010, 132, 3295.
2. Rakesh K Sharma and Ashoka. G. Samuelson* J.Chem. Sci  2006, 118, 569.
3. Rakesh K Sharma and Ashoka G. Samuelson* Tetrahedron: Asymmetry 2007, 18, 2387 
4. Rakesh K Sharma, Munirathnam Nethaji, Ashoka G. Samuelson* Tetrahedron: Asymmetry 2008, 19, 655.
5. Unpublished results.
6. Indian Patent IN 194118, 2004.
7. PCT Int. Appl.  WO 2007144895, 2007.
8. Unpublished results.