Asymmetric catalytic reduction is the use of various chiral catalyst s to reduction chemistry reduce a prochiral organic compound to obtain a chiral product. This is one of the several techniques used in chiral synthesis . Typically, a transition metal is used with a bulky chiral ligand to such that the substrate molecule can only approach in a particular orientation. This causes an external reductant to reduce the substrate in a predictable fashion. Well known examples include William Standish Knowles Knowles ruthenium based catalyst, similar to Wilkinson s catalyst , which reduces alkenes, and Noyori asymmetric hydrogenation catalysts using chiral BINAP ligands and dihydrogen to reduce ketones and alkenes. Transition metal free catalysts, i.e. organocatalyst s such as the CBS catalyst based on proline used to ketone s are known as well. References Unreferenced date October 2007 references Chiral synthesis Category Stereochemistry Category Catalysis organic chem stub ... more details
Image Prochirality V.1.svg right thumb 240px An sp sup 2 sup hybridized carbon atom, with re and si faces. In stereochemistry , prochiral molecules are those that can be converted from achiral to Chirality chemistry chiral in a single step. ref name mcmurry cite book title Organic Chemistry author John E. McMurry John McMurry edition 6ed publisher Brooks Cole pages 301 303 ref ref http www.iupac.org goldbook P04859.pdf IUPAC Gold Book definition ref If two identical substituent s are attached to a orbital hybridisation sp sup 3 sup hybridized atom , the descriptors pro R and pro S are used to distinguish between the two. Promoting the pro R substituent to higher priority than the identical alternative would result in an R chirality center at the original sp sup 3 sup hybridized atom, and vice versa. A VSEPR trigonal planar sp sup 2 sup hybridized atom can be converted to a chiral center when a substituent is added to the re or si face of the molecule. These faces are labelled re if the substituents at the trigonal atom are disposed in decreasing Cahn Ingold Prelog priority rules Cahn Ingold Prelog priority order in a clockwise order, and si if the priorities decrease in counter clockwise order but the designation of the resulting chiral centre as s or r depends on the priority of the incoming group. References Reflist Category Stereochemistry de Prochiralit t fr Prochiralit gl Proquiralidade it Prochiralit nl Prochiraliteit ja zh ... more details
Desymmetrization in stereochemistry is the modification of a molecule that results in the loss of one or more symmetry elements. An important application involves the introduction of chirality chemistry chirality . Formally, such conversion require the loss of an improper axis of rotation mirror plane, center of inversion, rotation reflection axis . In other words, desymmetrizations convert prochiral precursors into a Chirality chemistry chiral products. ref Basic Terminology of Stereochemistry , G.P. Moss Ed. Pure Appl. Chem., Vol. 68, No. 12, pp. 2193 2222, 1996 . ref Examples One example is the conversion of cis 3,5 diacetoxycyclopentene to its the monoacetate. In this transformation, the plane of symmetry in the precursor is lost, and the product is asymmetric. The desymmetrization itself is not usually considered useful. The enantioselective desymmetrization however delivers a useful product. This particular conversion utilizes the enzyme cholinesterase . ref Donald R. Deardorff, Colin Q. Windham, and Chris L. Craney Enantioselective Hydrolysis of cis 3,5 Diacetoxycyclopentene 1R,4S 4 Hydroxy 2 cyclopentenyl Acetate Organic Syntheses, Coll. Vol. 9, p.487 1998 Vol. 73, p.25 1996 . ref File DesymDiacetate.png 300px In another example, transfer hydrogenation converts benzil PhC O C O Ph into one enantiomer of hydro benzoin ref OrgSynth author Takao Ikariya, Shohei Hashiguchi, Kunihiko Murata, and Ry ji Noyori title Preparation of Optically Active R,R Hydrobenzoin from Benzoin or Benzil vol 82 pages 10 year 2005 prep v82p0010 ref PhC O C O Ph 2 H sub 2 sub PhCH OH CH OH Ph The precursor benzil has C sub 2v sub symmetry group symmetry , and the product has C sub 2 sub symmetry. References Reflist Category Stereochemistry fr D sym trisation ... more details
Orphan date February 2009 chembox verifiedrevid 407891718 ImageFile Kelliphite structure.png ImageSize ImageFile1 ImageSize1 IUPACName 6,6 1,1 Biphenyl 2,2 diyl bis oxy bis 4,8 di tbutyl 1,2,10,11 tetramethyl dibenzo d,f 1,3,2 dioxaphosphepin OtherNames Kelliphite Section1 Chembox Identifiers CASNo PubChem SMILES Section2 Chembox Properties C 60 H 72 O 6 P 2 Appearance Density MeltingPt BoilingPt Solubility organic solvents Section3 Chembox Hazards MainHazards FlashPt Autoignition Kelliphite is an acronym for the organophosphorus compound 6,6 1,1 Biphenyl 2,2 diyl bis oxy bis 4,8 di tbutyl 1,2,10,11 tetramethyl dibenzo d,f 1,3,2 dioxaphosphepin. This Chiral synthesis chiral ligand is widely used in asymmetric synthesis . ref cite journal doi 10.1021 ja050148o last1 Clark first1 Thomas P. last2 Landis pmid 15810837 first2 CR last3 Freed first3 SL last4 Klosin first4 J last5 Abboud first5 KA title Highly Active, Regioselective, and Enantioselective Hydroformylation with Rh Catalysts Ligated by Bis 3,4 diazaphospholanes journal J. Am. Chem. Soc. year 2005 volume 127 issue 14 pages 5040 ref ref cite journal doi 10.1021 jo040128p last1 Cobley first1 Christopher J. last2 Gardner pmid 15176828 first2 K last3 Klosin first3 J last4 Praquin first4 C last5 Hill first5 C last6 Whiteker first6 GT last7 Zanotti Gerosa first7 A last8 Petersen first8 JL last9 Abboud first9 KA title Synthesis and Application of a New Bisphosphite Ligand Collection for Asymmetric Hydroformylation of Allyl Cyanide journal J. Org. Chem. year 2004 volume 69 issue 12 pages 4031 ref In one example, this ligand is used to form a rhodium complex to catalyze asymmetric hydroformylation of prochiral olefin s. It has been shown that high substrate concentrations as well as a wide variety of functional group s are tolerated. ref cite journal last1 Cobley first1 Christopher J. last2 Klosin first2 Jerzy last3 Qin first3 Cheng last4 Whiteker first4 Gregory T. title Parallel Ligand Screening on Olefin Mixtures in A ... more details
The Sch llkopf method or Sch llkopf Bis Lactim Amino Acid Synthesis is a method in organic chemistry for the asymmetric synthesis of chiral amino acid s. ref Leo A. Paquette Chiral Reagents for Asymmetric Synthesis , S.220 223, 2003, Wiley and Sons, ISBN 0470856254 ref ref name h ttermann Jan B lle, Aloys H ttermann Das Basiswissen der organischen Chemie Die wichtigsten organischen Reaktionen im Labor und in der Natur , S.310 311, 2000, Wiley VCH, ISBN 3527308474 ref The method was established in 1981 by Ulrich Sch llkopf . ref Enantioselective Syntheses of R Amino Acids Using L Valine as Chiral Agent Angewandte Chemie International Edition in English Volume 20, Issue 9, Date September 1981, Pages 798 799 Ulrich Sch llkopf, Ulrich Groth, Chuanzheng Deng DOI 10.1002 anie.198107981 ref ref Enantioselective Synthesis of Methyl aminocarboxylic Acids by Alkylation of the Lactim Ether of cyclo L Ala L Ala Angewandte Chemie International Edition in English Volume 18, Issue 11, Date November 1979 , Pages 863 864 Ulrich Sch llkopf, Wolfgang Hartwig, Ulrich Groth DOI 10.1002 anie.197908631 ref ref Asymmetric synthesis via heterocyclic intermediates XXXIX1 Asymmetric synthesis of enantiomerically and diastereomerically virtually pure methyl 2 amino 4,5 epoxy 3 hydroxy alkanoates and methyl 2 amino 3 hydroxy 4,5 methylene alkanoates by the bislactimether method Tetrahedron , Volume 44, Issue 17, 1988 , Pages 5293 5305 Ulrich Sch llkopf, Thomas Tiller, J rgen Bardenhagen DOI 10.1016 S0040 4020 01 86037 3 ref In it glycine is a substrate, valine a chiral auxiliary and the reaction taking place an alkylation. Reaction mechanism File Schoellkopf Hartwig.png thumb 500px Reaction mechanism for the Sch llkopf method Glycine and chiral R valine are converted to a cyclic dipeptide a dioxopiperazine . A twofold methylation forms the bis lactim. A proton is then abstracted from the prochiral position on glycine with the aid of BuLi . The next step decides the stereoselectivity of the meth ... more details
through study of prochiral methylene hydrogen discrimination ref cite journal author Hoye ... study of prochiral methylene hydrogen discrimination using the remote isotope method journal Org ... more details
should note that adding a chemical group to the prochiral centre from the re face will not always lead ... face it was attacked from. In the above mentioned example, if chloride Cl was added to the prochiral ... more details
Verley reduction can be performed on prochirality prochiral ketones leading to Chiral chemistry ... The Asymetric Meerwein Schmidt Ponndorf Verley Reduction of Prochiral Ketones with i PrOH Catalyzed ... more details
Transfer hydrogenation is the addition of hydrogen H sub 2 sub dihydrogen in inorganic chemistry inorganic and organometallic chemistry organometallic chemistry to a molecule from a source other than gaseous H sub 2 sub . It is applied in industry and in organic synthesis , in part because of the inconvenience and expense of using gaseous H sub 2 sub . One large scale application of transfer hydrogenation is coal liquefaction using donor solvents such as tetralin . ref Speight, J. G. The Chemistry and Technology of Coal Marcel Dekker New York, 1983 p. 226 ff. ISBN 0 8247 1915 8. ref ref Cite journal doi 10.1002 anie.200501787 volume 44 issue 41 pages 6622 6627 last Mu iz first Kilian title Bifunctional Metal Ligand Catalysis Hydrogenations and New Reactions within the Metal Di amine Scaffold13 journal Angewandte Chemie International Edition year 2005 pmid 16187395 ref Organometallic catalysts In the area of organic synthesis , a useful family of hydrogen transfer catalysts have been developed based on ruthenium and rhodium complexes with diamine and phosphine ligands. ref T. Ikariya, K. Murata, R. Noyori Bifunctional Transition Metal Based Molecular Catalysts for Asymmetric Syntheses Org. Biomol. Chem., 2006, volume 4, 393 406. ref A representative catalyst precursor is derived from cymene ruthenium dichloride dimer and the tosylated diphenylethylenediamine . These catalysts are mainly employed for the organic reduction reduction of ketones and imines to alcohols and amine s, respectively. The hydrogen donor transfer agent is typically isopropanol , which coverts to acetone upon donation of hydrogen. Transfer hydrogenations can proceed with high enantioselectivity enantioselectivities when the starting material is chirality chemistry prochiral RR C O Me sub 2 sub CHOH RR C H OH Me sub 2 sub C O where RR C H OH is a chiral product. A typical catalyst is cymene R,R HNCHPhCHPhNTs , where Ts tosyl SO sub 2 sub C sub 6 sub H sub 4 sub Me and R,R refers to the absolute co ... more details
Encyclopedia
top
