Dienes & Polyenes: An overview and two key reactions (Ch )

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1 Dienes & Polyenes: An overview and two key reactions (h ) 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 = bonds: H 3 H = H H = H H 3 H 2 = H H 2 H 2 H = H 2 conjugated dienes have specific reactions isolated dienes react just - cycloadditions (e.g. Diels-Alder) like regular alkenes - 1,4-addition 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) 2 == 2 Why? Hybridization and orbital overlap: -- All = double bonds have sp 2 sp 2 overlap -- single bonds in between puts sp 3 hybrids in the overlap -- The greater the s character in each hybrid, the better the overlap -- esult is greater stability in the conjugated dienes, less in the isolated. esonance stabilization and electron delocalization contributes to stability of the conjugated diene, compared to non-conjugated

2 Nomenclature of polyenes (review): 1. hains and rings are numbered and named in the usual way, starting the numbering at the end closest to a = bond, with an ending of diene or triene, etc. 2. Substituents are numbered accordingly. ommon names: H 2 = = H 2 allene isoprene (E/Z) isomerism in dienes: For any dienes except those with terminal = bonds or identical groups on a single, there will be E/Z isomerism at both = 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 3 H H 3 2 H 3 H H H

3 eactions of Dienes: Two Key Types of eaction I. eaction Type: ycloaddition (The Diels-Alder eaction) eactivity: Diene acts as e- donor and acceptor When a conjugated diene meets a dienophile, the attraction is so strong + H 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 = results in formation of a new 6-membered ring of atoms requires the diene to have both bonds in an s-cis configuration occurs spontaneously with the right dienophile Electron withdrawing groups? ertain groups of atoms can pull electron density toward themselves through σ bonds by the inductive effect. Those groups having a or N with multiple bonds to an electronegative atom withdraw e- from dienes through resonance: H H N N aldehyde acid ketone nitrile nitro This makes the reaction more favorable than with unsubstituted dienophiles

4 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

5 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.

6 II. eaction type: eactivity: Electrophilic addition to conjugated dienes When conjugated double bonds are present, the reactivity of one = affects the other Pi bonding electrons in a conjugated arrangement resonate between carbons so the single bonds have some double-bond character: H 2 H = H H 2 H 2 = H H = H 2 H 2 H = H H 2 equals resonance hybrid H 2 H H H 2 With conjugated dienes, if excess electrophile is present it reacts with both =, 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 groups) Electrophilic additions to conjugated dienes form mixtures of products due to the resonance of the allylic carbocations Ex: H 2 =H-H=H 2 + HBr H 2 =H-H-H 3 + BrH 2 -H=H-H 3 Br 1,3-butadiene 1,2 addition 1,4 addition