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Transcript
Dienes & Polyenes: An overview and two key reactions (Ch. 14.1-14.5) Polyenes contain more than one double bond and are very common in natural products (ex: carotene). Diene chemistry applies to trienes, tetraenes, etc. The chemistry of polyenes depends on the relative positions of the C=C bonds: CH3 – HC = CH – HC = CH – CH3 H2C = CH – CH2 – CH2 – HC = CH2 conjugated dienes have specific reactions - cycloadditions (e.g. Diels-Alder) - 1,4-addition isolated dienes react just like regular alkenes Electrophilic additions to isolated dienes resemble those of regular alkenes. -- Addition of H – X produces alkyl halides with Markovnikov orientation -- Acid-catalyzed addition of water and oxymercuration produce alcohols with Markovnikov orientation, alkoxymercuration produces ethers -- Hydroboration produces alcohols with “anti-Markovnikov” orientation -- Hydrogenation produces at least some saturated bonds Stability: Based on heats of hydrogenation, the trend in relative stabilities is: conjugated dienes > isolated dienes > cumulated dienes (allenes) R2C=C=CR2 Why? Hybridization and orbital overlap: -- All C = C double bonds have sp2 – sp2 overlap -- C – C single bonds in between puts sp3 hybrids in the overlap -- The greater the “s” character in each hybrid, the better the overlap -- Result is greater stability in the conjugated dienes, less in the isolated. Resonance stabilization and electron delocalization contributes to stability of the conjugated diene, compared to non-conjugated Nomenclature of polyenes (review): 1. Chains and rings are numbered and named in the usual way, starting the numbering at the end closest to a C = C bond, with an ending of “diene” or “triene”, etc. 2. Substituents are numbered accordingly. Common names: H2C = C = CH2 “allene” “isoprene” (E/Z) isomerism in dienes: • For any dienes except those with terminal C = C bonds or identical groups on a single C, there will be E/Z isomerism at both C = C bonds. • The E or Z configuration at each double bond, preceded by its bond position number, appears at the beginning of the name Draw and name all the possible configurations of: H 3C H2 C CH3 C C H C H C H CH3 Reactions of Dienes: Two Key Types of Reaction I. Reaction Type: Reactivity: Cycloaddition (The Diels-Alder Reaction) Diene acts as e- donor and acceptor When a conjugated diene meets a dienophile, the attraction is so strong … O CH + …that the pi-bonding electrons rearrange themselves into new bonds, joining the diene and the dienophile together in a pericyclic, concerted reaction! The Diels-Alder cycloaddition results in 2 new σ bonds, with one π bond moving to a new position occurs when a diene meets an alkene or alkyne, particularly one with an electron-withdrawing group attached to the C = C results in formation of a new 6-membered ring of C atoms requires the diene to have both bonds in an s-cis configuration occurs spontaneously with the right dienophile Electron withdrawing groups? Certain groups of atoms can pull electron density toward themselves through σ bonds by the inductive effect. Those groups having a C or N with multiple bonds to an electronegative atom withdraw e- from dienes through resonance: O O C R O C H aldehyde R acid O C OH R R ketone R C N nitrile N O nitro This makes the reaction more favorable than with unsubstituted dienophiles If the diene and dienophile are asymmetrical, two structural isomers form. Example: 1-chlorobutadiene + propenal: The Diels-Alder reaction is stereospecific: Any geometric isomers (e.g. cis/trans or E/Z-alkenes) maintain the relative positions of substituents in the product: Note that in the product, one additional ring is formed. If you start with a cyclic compound: cyclic diene + dienophile = bridged bicyclic compound When the diene approaches the dienophile, proper overlap of pi-bonding orbitals may favor specific stereoisomers. For example, the reaction you will perform in the lab could produce "endo" or "exo" orientation: But the predominant product is the endo isomer. II. Reaction type: Electrophilic addition to conjugated dienes Reactivity: When conjugated double bonds are present, the reactivity of one C=C affects the other Pi bonding electrons in a conjugated arrangement “resonate” between carbons so the single bonds have some double-bond character: .. + H2C– CH = CH – CH2 .. H2C – HC = CH – CH2 + H2C = CH – HC = CH2 equals “resonance hybrid” H2C C H C H CH2 With conjugated dienes, if excess electrophile is present it reacts with both C=C, BUT when the electrophile is present in limited supply, it will add preferentially to the more reactive bond (the one leading to the more stable carbocation, or surrounded by more R groups) Electrophilic additions to conjugated dienes form mixtures of products due to the resonance of the allylic carbocations Ex: CH2=CH-CH=CH2 + HBr 1,3-butadiene H2C=CH-CH-CH3 + BrCH2-HC=CH-CH3 Br 1,2 addition 1,4 addition