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

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Milton Lewis
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1 Dienes & Polyenes: An overview and some 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: 3 = = 3 2 = 2 2 = 2 conjugated dienes have specific reactions isolated dienes behave like regular alkenes 3 = = 2 3 cumulated dienes are less common Stability: Based on heats of hydrogenation, the trend in relative stabilities is: conjugated dienes > isolated dienes > cumulated dienes Why should the positions of the pi bonds matter? 1) ybridization 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 -- an also be explained by Ms as shown in fig esult is greater stability in the conjugated dienes, less in the isolated. 2) esonance stabilization: Electron delocalization contributes to stability of the conjugated diene: Pi bonding electrons in a conjugated arrangement may resonate between carbons so the single bonds have some double-bond character: = 2 2 = = 2 2 = 2 equals resonance hybrid 2 2

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. 3 l ommon names: 2 = = 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 ow many configurations are possible for: A side issue: naming compounds with other functional groups in addition to = Such compounds are named as members of the functional group class having priority. Naming and numbering follows this priority order: = > > N 2 > = > = ketones, aldehydes alcohols amines alkenes alkynes The suffix of the name is the ending for the highest priority group. The position number is inserted between the root and the suffix. Alkene bonds must also be indicated as part of the parent chain (or ring) name, with their position number preceding the parent name. Example: = penten-1-ol The compound is named as an alcohol, but the presence of the = bond must be indicated by using en in the parent name. Below, the compound is named as an alkene Example: l 2-chloro-5-hexyn-1-ene

3 eactions of Dienes: Two Key Types of eaction I. eaction type: Electrophilic addition eactivity: Dienes are nucleophilic just like alkenes & alkynes Two situations are possible when more than one = is present in a molecule: A) If they are far apart, both groups may react independently of each other B) If they are conjugated, the reactivity of one will affect the other (A) Electrophilic additions to isolated dienes resemble those of regular alkenes. -- Addition of X produces alkyl halides with Markovnikov orientation -- Acid-catalyzed addition of water and oxymercuration produce alcohols with Markovnikov orientation, alkoxymercuration produces ethers -- ydroboration produces alcohols with anti-markovnikov orientation -- ydrogenation produces at least some saturated bonds If enough electrophile is present it will react with all the =, but when electrophile is present in short supply, it will add preferentially to the more reactive bond (the one leading to the more stable carbocation, or surrounded by more groups) (B) Electrophilic additions to conjugated dienes form mixtures of products due to the resonance of the allylic carbocations Ex: 2 =-= 2 + Br 2 = Br 2 -=- 3 Br 1,3-butadiene 1,2 addition 1,4 addition

4 II. eaction Type: eactivity: ycloaddition (The Diels-Alder eaction) Diene acts as donor and acceptor When a conjugated diene meets a dienophile, the attraction is so strong + 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 is actually a 1,4-addition reaction 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: N N aldehyde acid ketone nitrile nitro This makes the reaction even more favorable than with unsubstituted dienophile!

5 If the diene and dienophile are asymmetrical, two structural isomers form. The Diels-Alder reaction is also stereospecific: that means that any geometric isomers (for example, cis or trans-alkenes) maintain their configuration: Note that in the product, one additional ring is formed. If you start with a cyclic compound: cyclic diene + dienophile = bridged bicyclic compound In some situations, approach of the dienophile may occur in two orientations, leading to an exo or endo configuration (exo has the substituent pointing toward the sp 3 bridge ) Product formation relies on the correct overlap of orbitals in the diene & dienophile. An example is this reaction, which you will perform in the lab. Proper positioning of pi-bonding orbitals is key! The predominant product is the endo isomer: cis-norbornene-5,6-endo-dicarboxylic anhydride.

Conjugation is broken completely by the introduction of saturated (sp3) carbon:

Conjugation is broken completely by the introduction of saturated (sp3) carbon: Chapter 16 Conjugation, resonance, and dienes Conjugation relies on the partial overlap of p-orbitals on adjacent double or triple bonds. A common conjugated system involves 1,3-dienes, such as 1,3-butadiene.