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Transcript
INTRODUCTION Organometallic compounds are the Compounds containing a carbon –metal bond . Since lithium is a metal , so it forms an organo-metallic compound. The compounds formed by carbon– lithium bonds are called organolithium compounds e.g. CH3-Li ‘CH3’ is an organic group ‘Li’ is the metallic group Dr. HENRY GILMAN (Father of organometallic chemistry) DISCOVERY Organolithiumcompounds, discovered by Wilhelm Schlenk in 1917. But only since the 1950th, based on the pioneering work of Georg Wittig and Henry Gilman, organometallic reagents became a routinely used tool in the synthetic organic laboratory. Dr.WILHELM SCHLENK (German chemist, (1879-1943) STRUCTURE AND BONDING d- C Li d+ 57% covalent - C: 43% ionic Li+ strong base good nucleophile significant (-) charge at the carbon Carbon-metal bonds are polar bonds that can be represented by a resonance hybrid of covalent and ionic structures. C-Li are strong bases and react with acids, even weak acids like water and alcohols. STRUCTURE AND BONDING bond C-K C-Na C-Li C-Mg C-Zn C-Cd C-Cu percent ionic 51 47 43 35 18 15 9 most reactive strongest base Li is the Metal used most often best nucleophile least reactive PREPARATION OF ORGANOLITHIUM COMPOUNDS The organometallic compounds can be prepared by following methods:By Halogen Metal Exchange From Terminal Alkynes By Trans Metalation/ Metal-Metal Exchange By Directed Metalation/ Ortho Metalation PREPARATION OF ORGANOLITHIUM COMPOUNDS By Halogen Metal Exchange Normally prepared by reaction of alkyl halides with lithium R X + 2Li R Li + LiX same for Ar—X It is an oxidation-reduction reaction: carbon is reduced Examples (CH3)3CCl + 2Li diethyl ether –10°C (CH3)3CLi + LiCl (75%) diethyl ether Br + 2Li Li + LiBr 35°C (95-99%) It is usually chosen for aryl derivatives PREPARATION OF ORGANOLITHIUM COMPOUNDS From Terminal Alkynes PREPARATION OF ORGANOLITHIUM COMPOUNDS By Trans Metalation/ Metal-Metal Exchange 2Li + R2Hg 2Li + Hg In the above equation ‘Li’ metal is replaced by ‘Hg ’ This reaction is preferred to form organolithium compounds from other organometallic compound. PREPARATION OF ORGANOLITHIUM COMPOUNDS By Directed Metalation/ Ortho Metalation DMG DMG + RLi Li + R-H Metallation of an aromatic ring near a substituent, which acts as a “Directed Metallation Group”, is called “OrthoMetallation”. They have in common the ability to coordinate the approaching cation (= lithium-ion) and/or to increase the acidity of the orthohydrogen PROPERTIES OF ORGANOLITHIUM COMPOUNDS Physical properties Highly polar due to most electro positivity of lithium group . Volatile in nature, easily absorbs Polarity of Bonds moisture. Solubility in Hydrocarbons . More complex bonding. Forms covalent compounds due to vacant ‘p’ orbital SOLVENTS THAT ARE FREQUENTLY USED .. CH3CH2 .. .. .. O CH2CH3 O CH3CH2CH2CH2CH3 .. .. .. .. diethyl ether THF tetrahydrofuran These solvents do not pentane react with the metals themselves or with the organometallic benzene compounds formed. O 1,4-dioxane O PROPERTIES OF ORGANOLITHIUM COMPOUNDS Chemical properties R-CO-CH=CHR HO-CH2CH2R R1-X 9 8 R 5 6 7 R1-CO-R2 R1-C=CH R-CO-R1 R1-CO-R 4 R-Li R1-CHO R1-R CO2 epoxide 3 2 1 10 R-COOH R1-C=C-Li R1-C-OH R2 R-OH REACTIONS OF ORGANOLITHIUM COMPOUNDS Reaction with water Water or traces of moisture (water vapor) destroy the reagent ! R-Li + strong base - R H 2O + R-H H O + R H + LiOH OH - H The glassware must be dry, and we must not expose the reagent to humid air. REACTIONS OF ORGANOLITHIUM COMPOUNDS Reaction with oxygen Oxygen reacts with alkyllithium compounds to form hydroperoxides. CH3CH2CH2CH2 Li + O2 CH3CH2CH2CH2 - + OO Li H2O butyllithium CH3CH2CH2CH2 a hydroperoxide This reaction usually proceeds in low yield, meaning it is not a good synthetic method . However, it can be a annoying problem when trying to prepare alkyllithium compounds. OOH REACTIONS OF ORGANOLITHIUM COMPOUNDS Removing a Halogen The reaction with water is a way of reducing ( or dehalogenating )an alkyl halide ( converting C-X to C-H ) . H Li Br ether H2O Li CH3 C Br CH3 CH3 C - Li + C H H O H Removal of Br REACTIONS WITH THE CARBONYL (C=O) GROUP .. O: C .. - Li+ : O: d C d+ - Li + :C C new C-C bond NUCLEOPHILIC ADDITION TO CARBONYL Since oxygen is more electronegative than carbon, the carbonyl group is electron-deficient at carbon, that is, it is an electrophile. The organometallic compound (R-Li) behaves as a nucleophile with an unshared pair on the carbon. SYNTHESIS OF ALCOHOLS Reaction of RLi with aldeydes and ketones yields alcohols. H CH3CH2 Li + O H formaldehyde CH3CH2 Li + CH3 O H other aldehydes CH3CH2 Li + CH OH secondary alcohol CH3 CH3 CH3CH2 OH primary alcohol CH3 CH3CH2 CH2 O CH3 CH3CH2 C OH CH3 ketones tertiary alcohol The type of alcohol depends on whether we use formaldehyde, another aldehyde, or a ketone. SYNTHESIS OF A CARBOXYLIC ACID CH3 CH Br CH3 Li ether .. :O CH3 CH Li CH3 Cd + dry-ice :O .. d.. O: O CH3 H3O+ CH3 CH C CH C Slow step .. - + CH CH3 O : Li 3 OH .. R-Br Li R-Li CO2 R-COO- Li+ H3O+ R-COOH usually dil. HCl REACTION OF ,-UNSATURATED CARBONYL COMPOUNDS WITH ORGANOMETALLIC REAGENTS C6H5CH=CHCOC6H5 C6H5Li C6H5CH=CH-CHO When an organolithium compound react with an ,unsaturated carbonyl compounds it gives 1,2 addition product RECENT DEVELOPMENTS Lithium hexamethyldisilazide (LHMDS) lithium amide, 30 % solution in hexane Mol. Formula: C6H18LiNSi2 Application: Selective low nucleophilic base for e.g. enolisations Appearance: yellowish solution Physical Properties: Molecular weight: 167.33 Density: 0.71 g/ccm (at 20°C) Boiling point: 60 – 80 °C for Hexane LIMITATIONS OF ORGANOLITHIUM COMPOUNDS Highly volatile , so not used in mild reaction . It easily absorbs moisture , because of which it is not kept in open envoirnment. These compounds are harmful for skin. so, we should use gloves before using it