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Ethers,Ethers, SulfidesSulfides (omit), andand EpoxidesEpoxides Chapter 11 1 StructureStructure • The functional group of an ether is an oxygen atom bonded to two carbon atoms. – In dialkyl ethers, oxygen is sp3 hybridized with bond angles of approximately 109.5°. – In dimethyl ether, the C-O-C bond angle is 110.3°. H H •• HOC CH •• H H 2 StructureStructure – In other ethers, the ether oxygen is bonded to an sp2 hybridized carbon. – In ethyl vinyl ether, for example, the ether oxygen is bonded to one sp3 hybridized carbon and one sp2 hybridized carbon. Ethoxyethene CH3 CH2 -O- CH= CH2 (Ethyl vinyl ether) 3 Nomenclature •IUPAC: the longest carbon chain is the parent. Name the OR group as an alkoxy substituent. • Common names: name the groups bonded to oxygen in alphabetical order followed by the word ether. OH CH3 CH OCCH CH3 CH2 OCH2 CH3 3 3 OCH2 CH3 CH3 Ethoxyethane trans- 2-Ethoxy- 2-Methoxy-2- (Diethyl ether) cyclohexanol methylpropane (tert- Butyl methyl ether) 4 Nomenclature • Although cyclic ethers have IUPAC names, their common names are more widely used. – IUPAC: prefix ox- shows oxygen in the ring. – The suffixes -irane,irane -etane,etane -olane,olane and -ane show three, four, five, and six atoms in a saturated ring. 23 O O 1O O O O Oxirane Oxetan e Oxolane Oxane 1,4-Dioxane (Ethylene oxide) (Tetrahydrofuran) (Tetrahydropyran) 5 Physical Properties • Although ethers are polar compounds, only weak dipole-dipole attractive forces exist between their molecules in the pure liquid state. Note bene: there is NO intramolecular H-bonding 6 Physical Properties • Boiling points of ethers are – lower than alcohols of comparable MW. – close to those of hydrocarbons of comparable MW. • Ethers are hydrogen bond acceptors. – They are NOT soluble in H2O, but – They are more soluble in H2O than are hydrocarbons.. Note bene: there IS intermolecular H-bonding 7 Preparation of Ethers • Williamson ether synthesis: Ether synthesis by the SN2 displacement of halide, tosylate, or mesylate by alkoxide ion. CH3 CH3 - + SN 2 + - CH3 CHO Na + CH3 I CH3 CHOCH3 + Na I Sodium Iodomethane 2-Methoxypropane isopropoxide (Methyl iodide) (Isopropyl methyl ether) OH OR' 1) Nao R R(H) 2) R'X R R(H) R(H) 1o or 2o R(H) ROH Ether 8 Preparation of Ethers –Yields are highest with methyl and 1° halides, –lower with 2° halides (competing β- elimination) CH3 CH3 - + SN2 + - CH3 CO K + CH3 Br CH3 COCH3 + K Br CH3 CH3 Potassium Bromomethane 2-Methoxy-2-methylpropane tert-butoxide (Methyl bromide) (tert-Butyl methyl ether) 9 Preparation of Ethers reaction fails with 3° halides (β-elimination only). CH3 CH 3 - + E2 + - CH3 CBr+ CH3 O Na CH 3 C= CH2 + CH3 OH + Na Br CH3 2-Bromo-2- Sodium 2-Methylpropene methylpropane methoxide 10 Preparation of Ethers • Acid-catalyzed dehydration of alcohols – Diethyl ether and several other ethers are made this way on an industrial scale. – A specific example of an SN2 reaction in which a poor leaving group (OH-) is converted to a better one (H2O). H SO 2CH CH OH 2 4 CH CH OCH CH + H O 3 2 140°C 3 2 2 3 2 Ethanol Diethyl ether 11 Preparation of Ethers – Step 1: Proton transfer gives an oxonium ion. fast and O + O reversib le - CH3 CH2 -O-H + H-O-S-O-H CH3 CH2 -O-H + O- S- O-H O H O An oxonium ion – Step 2: Nucleophilic displacement of H2O by the OH group of the alcohol gives a new oxonium ion. + + SN2 CH3 CH2 -O-H+ CH3 CH2 -O-H CH3 CH2 -O-CH2 CH3 + O-H H H H A new oxonium ion 12 Preparation of Ethers Step 3: Proton transfer to solvent completes the reaction. proton + transfer + CH 3 CH 2 -O-CH2 CH3 + O-H CH3 CH 2 -O-CH2 CH3 + HO-H H H H 13 Preparation of Ethers • Acid-catalyzed addition of alcohols to alkenes – Yields are highest using an alkene that can form a stable carbocation and – using methanol or a 1° alcohol that is not prone to undergo acid-catalyzed dehydration. 2 4 2 2 14 Preparation of Ethers 1. Use an alkene that can form a stable carbocation (2o or 3o) 2. Use a 1° alcohol (that is not prone to undergo acid-catalyzed dehydration). CH acid CH 3 catalyst 3 CH3 C= CH2 + CH3 OH CH3 COCH3 CH3 2-Methoxy-2-methyl propane 15 Preparation of Ethers – Step 1: Protonation of the alkene gives a carbocation. CH 3 + CH3 CH C= CH + HOCH CH CCH + O CH 3 2 3 3 + 3 3 H H – Step 2: Reaction of the carbocation (an electrophile) with the alcohol (a nucleophile) gives an oxonium ion. CH 3 CH 3 CH CCH + HOCH CH CCH 3 + 3 3 3 + 3 O H CH3 16 Preparation of Ethers Step 3: Proton transfer to solvent completes the reaction. CH3 + CH 3 CH3 OH+ CH3 CCH3 CH3 O H + CH3 CCH3 + O H O CH3 HCH3 17 Epoxides • Epoxide: A cyclic ether in which oxygen is one atom of a three-membered ring. – Simple epoxides are named as derivatives of oxirane. – Where the epoxide is part of another ring system, it is shown by the prefix epoxy-.- H 1 23 O H2 C CH2 1 O 2 H Oxirane 1,2-Epoxycyclohexane (Ethylene oxide) (Cyclohexene oxide) 18 Epoxides • Epoxide: A cyclic ether in which oxygen is one atom of a three-membered ring. – Common names are derived from the name of the alkene from which the epoxide is formally derived. H H 1 23 H CH H3 C 3 H2 C CH2 C C O 1 O O 2 H Oxirane cis- 2,3-Dimethyloxirane 1,2-Epoxycyclohexane (Ethylene oxide) (cis- 2-Butene oxide) (Cyclohexene oxide) 19 Synthesis of Epoxides • Ethylene oxide, one of the few epoxides manufactured on an industrial scale, is prepared by air oxidation of ethylene. Ag 2CH2 =CH2 + O2 2 H2 C CH2 O Oxirane (Ethylene oxide) procaine 20 Synthesis of Epoxides • The most common laboratory method is oxidation of an alkene using a peroxycarboxylic acid (a peracid). O COOH O CH3 COOH Cl Peroxyacetic acid (Peracetic acid) meta- Chloroperoxy- benzoic acid (MCPBA) 21 Synthesis of Epoxides • Epoxidation of cyclohexene H O O + RCOOH O + RCOH CH 2 Cl2 H Cyclohexene A peroxy- 1,2-Epoxycyclohexane A carboxylic carboxylic acid (Cyclohexene oxide) acid 22 Synthesis of Epoxides • Epoxidation is stereospecific: – Epoxidation of cis-2-butene gives only cis-2,3- dimethyloxirane. – Epoxidation of trans-2-butene gives only trans-2,3-dimethyloxirane. O HCH H CH 3 3 H C H CC RCO H 3 + CC 3 CC H CH H 3 H3 C H O H3 C trans-2-Butene trans-2,3-Dimethyloxirane (a racemic mixture) 23 Synthesis of Epoxides • A mechanism for alkene epoxidation. R R O C 3 OC O H 2 H O O 4 1 O CC CC 24 Synthesis of Epoxides • Epoxides are also synthesized via halohydrins. OH O Cl2 , H2 O NaOH, H2 O CH 3 CH= CH2 CH CH- CH CH CH CH 3 2 S 2 3 2 Cl N PropeneA chlorohydrin Methyloxirane (racemic) (racemic) 2 2 25 Synthesis of Epoxides The second step is an internal SN2 reaction. O O internal S N 2 CC+ Cl CC Cl An epoxide 26 Synthesis of Epoxides – Halohydrin formation is both regioselective and stereoselective; for alkenes that show cis,trans isomerism, it is also stereospecific. – Conversion of a halohydrin to an epoxide is stereoselective: H H CH H H 1. Cl , H O H3 C 3 CC 2 2 CC H C CH 3 3 2. NaOH, H2 O O cis- 2-Butene cis- 2,3-Dimethyloxirane 27 Synthesis of Epoxides • Problem: Account for the fact that conversion of cis-2-butene to an epoxide by the halohydrin method gives only cis-2,3-dimethyloxirane. H H CH H H 1. Cl , H O H3 C 3 CC 2 2 CC H C CH 3 3 2. NaOH, H2 O O cis- 2-Butene cis- 2,3-Dimethyloxirane 28 Reactions of Epoxides • Ethers are not normally susceptible to attack by nucleophiles. • Because of the strain associated with the three-membered ring, epoxides readily undergo a variety of ring-opening reactions. Nu CC + HNu: C C O HO 29 Reactions of Epoxides • Acid-catalyzed ring opening – In the presence of an acid catalyst, such as sulfuric acid, epoxides are hydrolyzed to glycols. H+ OH O + H2 O HO Oxirane 1,2-Ethanediol (Ethylene oxide) (Ethylene glycol) 30 Reactions of Epoxides Step 1: Proton transfer to oxygen gives a bridged oxonium ion intermediate. Step 2: Backside attack by water (a nucleophile) on the oxonium ion (an electrophile) opens the ring. Step 3: Proton transfer to solvent completes the reaction. H H O HH O 3 2 H H +O 3 OH + H2 CCH2 H2 CCH2 CH2 CH2 CH2 CH2 + H3 O O O 2 OH + + OH 1 HHO H (1) H 31 Reactions of Epoxides Attack of the nucleophile on the protonated epoxide shows anti stereoselectivity. – Hydrolysis of an epoxycycloalkane gives a trans- 1,2-diol. OH OH H+ O + H2 O + OH OH 1,2-Epoxycyclopentane trans- 1,2-Cyclopentanediol (Cyclopentene oxide) (a racemic mixture) (achiral) 32 O OH + H3O CCH 2 CCH R 2 R R R OH 33 Reactions of Epoxides The value of epoxides is the variety of nucleophiles that will open the ring and the combinations of functional groups that can be prepared from them. CH 3 CH 3 CH 3 HOCH 2 CHOH A glycol HC CCH CHOH HSCH2 CHOH 2 A β-mercaptoalcohol A β-alkynylalcohol + H2 O/ H3 O + - 1.
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  • Tricyclic Oxonium Tamed

    Tricyclic Oxonium Tamed

    RESEARCH HIGHLIGHTS Nature Reviews Chemistry | https://doi.org/10.1038/s41570-019-0150-y | Published online 13 November 2019 NATURAL PRODUCT SYNTHESIS could only be characterized at low temperature. However, 1H and 13C NMR spectra are indicative of its Tricyclic oxonium tamed formation with large downfield shifts of the signals for the three Natural products or secondary formal positive charge on oxygen carbons adjacent to the oxygen and metabolites have hugely diverse (a trialkyloxonium ion) proposed for the protons attached to them. We decided structures. Simple-looking linear for other biosyntheses have proved In addition, a new cross-peak was to try to precursors can, via highly reactive far more elusive. observed in the 1H–13C correlation intermediates, fold up into a variety “The structures of a number spectrum, indicating the formation investigate of complex 3D structures. Writing of Laurencia natural products had of a new transannular bond. 1H and whether these in the Journal of the American originally been misassigned,” explains 13C chemical shifts calculated somewhat Chemical Society, Jonathan Burton Burton. “Our work with Rob Paton on using density functional theory also exotic looking and co-workers from the University calculating the NMR spectra for the favoured the assigned structures and of Oxford, in collaboration with proposed structures of these natural formation of the oxonium ions. species might Robert Paton at Colorado State products led us to earlier proposals of Four different oxonium ions really exist University, describe the synthesis and tricyclic oxonium species. We decided derived from the diastereomers characterization of tricyclic oxonium to try to investigate whether these of laurediol were prepared and intermediates and their conversion somewhat exotic looking species characterized — the formation of into ten different natural products might really exist.” one of these, from 3Z, S,S-laurediol originally isolated from algae of the In the proposed biosyntheses, is pictured.