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CHEM 3750 (7)

Chapter 3

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University of Guelph
CHEM 3750
Adrian Schwan

3 - CONJUGATION 1. Terminology and Nomenclature (SF10 13.1 13.6) A compound containing a double bond is called an alkene, olefin or maybe simply "ene". There are often other names associated with double bonded compounds depending on the use of the compound (monomer, dienophile). Often, in more complicated systems, "ene" will be the only label attached to the name that indicates the presence of a double bond. More than one double bond can be in a given compound: isolated double bonds: n=1 or more (CH2)n conjugated double bonds: (CH ) n=0 2n -called a diene, a term usually used in the case of conjugation and rarely in other circumstances such as with isolated double bonds. C cumulated double bond: - the carbon centre is sp hybridized Examples: H2C C CH 2 1,3,5-hexatriene 1,3-pentadiene 1,3-butadiene 1,2-propadiene (allene) CHEM*3750 SCHWANC OURSE NOTES F13 Chapter 3 Page 1 2. AllySF10 13.2-13.4) The above three systems that are conjugated are termed "even systems" and as such are isolable molecules. Note that each possesses an even number of linearly attached atoms. The odd systems are not stable molecules but are assumed to exist as short-lived, reaction intermediates, or simply reactive intermediates. H H H C H H C H allyl C C+ +C C cation H H H H H H H C H H C H C C. .C C allyl reactive radical intermediates H H H H H H H C H H C H C C- -C C allyl anion H H H H *he odd systems containing three atoms and one double bond are called ALLYL compounds. Whenever the 2-propenyl system is attached to anything else the term allyl is used. It is not formal terminology, but is one of the most commonly used designations. OH Br allyl alcohol allyl bromide *The name probably arises from allium which is the genus for vegetables such as garlic, onions, shallots and chives. Some of these compounds, particularly garlic, contain several compounds which have the allyl fragment attached to sulfur atoms. CHEM*3750 SCHWANC OURSEN OTESF13 Chapter 3 Page 2 The three atom unit may also contain atoms other than carbon. H H H H O - O the simplest enolate - H H When one wishes to identify something attached to the non-double bonded carbon of the allyl fragment, we say it is at the allylic position. H + Br Br allylic hydrogen allylic carbocation allylic bromide Allylic carbocations are a special class of carbocation. Carbocations are transient species that exist as intermediates during organic reactions. If one wishes to investigate the properties of these carbon cations, one should study organic reactions that involve carbocation intermediates. a) Structure and Stability of Allylic Cations Allylic carbocations can be generated for study through a number of reactions. Two common methods include the following: CH3 Cl CH 3 CH2+ Ag 2, H2O room temp. C CH3 CH 3 CH 3 CH3 CH 2 HBr C + CH 3 0 °C OH CH3 Products in these reactions arise from nucleophilic capture of the cation. CHEM*3750 SCHWANC OURSE NOTES F13 Chapter 3 Page 3 allyl cations valence-bond (VB) picture: H H H C H H C H C C + +C C H H H H equal contributions H H C H summarized by C C ½+ ½+ H H orbital picture: H 10 ° H C H C C+ 120 10° H H p l a , lbond ang ls = 120° 2 π electo s sha edo e r 3 2 orbi ls otho o a lt σ o n ra e or k pm r ng b d f the oleculap l e Csp2-C p2, Csp2-H 1s The delocalization of the π electrons leads to greater stabilization of the cationic intermediate. Therefore any reaction going through a carbocation intermediate with a carbocation-like transition state (T.S.) will be faster for an allylic system than for the corresponding saturated system. Note that with allyl cation, the positive charge is shared equa1ly by C C3, with no charge on 2 . Placement of alkyl or aryl groups1on C 3r C stabilizes o o allylic cations further. Allyl cation itself is a 1 /1 cation, while the 1-methylallyl and 1,1-dimethylallyl are 1 /2 and 1 /3 cations, respectively. Substitut2on on C has little effect. stability:3 /3 > 2 /2 > 1 /1o CHEM*3750 SCHWANC OURSE N OTES F13 Chapter 3 Page 4 With respect to reactivity, generally, the rate of attack of a nucleophile on two conjugated cationic centres within the same cation decreases in the order: 3 centre > 2 centre > 1 centre The result is faster attack where more of the positive charge is. Geometric Isomerism. Since allylic cations have partial double bond character, it should be recognized that double bond isomerism may be possible. For instance: H H H C H CH C H C C and 3 C C δ+ δ+ δ+ δ+ CH 3 H H H rotational barriers: H H H H H CH 3 C CH3 H C C H C C δ+C Cδ+ H H H H H H ca. 18-20 kcal/mol 3 kcal/mol ca. 60 kcal/mol in Sb5- SOClF b) Allylic Radicals allyl radicCH 2CH-CH •2 ↔ •CH 2CH=CH 2 -stabilized over a simple alkyl radical by ca. 11 kcal/mol. This causes the C-H bonds in the allylic position of compounds like propene to have lower C-H bond dissociation energies. Hence they are activated toward radical halogenation reactions. e.g., radical bromination of allylic position: conditions: N-bromosuccinimide (NBS)4in CCllly with a thermal (benzoyl peroxide or AIBN) or photochemical (hν) initiator. CHEM*3750 SCHWANC OURSE NOTESF13 Chapter 3 Page 5 O O CH =CH-CH + inert solventCH =CH-CH Br + N H 2 3 N Br 2 2 initiator O O NBS succinimide inert solvents might include benzene, carbon tetrachloride, toluene The active reagent in this reaction is believed to b2 Br which is present in very low concentrations. The radical mechanism shown below is favoured over the ionic addition of Br2 across the double bond because of three factors: 1. high temperature 2. non-polar solvent 3. low Br2concentration O O HBr is produced in the reaction, but is also present in the early N Br + HBr N H + Br2 stages as an impurity in the NBS. O O Usually a thermal initiator is used initiator 2 R• to promote radical formation. CH 3-CH=CH 2 + R• •CH2-CH=CH 2 CH 2CH-CH 2• + HR CH3-CH=CH 2 + Br• •CH 2CH=CH 2 CH 2=CH-CH 2 + HBr [1] BrCH -CH=CH + Br• [2] •CH2-CH=CH 2 + Br 2 2 2 and repeat [1] and [2] CHEM*3750 SCHWANC OURSE N OTES F13 Chapter 3 Page 6 More complex alkenes afford mixtures. Reactions of2Br with an allylic radical having two non-equivalent radical sites shows a modest preference (e.g., 2 or 3:1) for reaction leading to the more stable product. CH CHCH=CH 3• 2 CH 3HCH=CH 2 NBS CH 3H C2=CH 2 Br CCl4 • + hν CH 3CH=CH-CH 2 CH 3CH=CH-CH 2B major (E & Z) However, when the alkene reactant has two or more nonequivalent allylic hydrogens, the hydrogen abstraction step is more selective. The rate of abstraction by Br• decreases in the order 3° H> 2° H> 1° H. H H most of the products arise from initial abstraction H of this allylic H (3° vs. 2°) Me c) Allylic Anions -can be generated as follows THF n-BuLi + CH 2=CH-CH 3 CH 2=CH-CH 2Li + n-butane -slow reaction -accelerated by amines such as TMEDA [(Me) NCH CH N(Me) ] 2 2 2 2 -allylic H has aK of ca. 42; alkyl H has pK of ca. 50 Since Grignard reagents are a form of carbon anion, then as an alternative, these reagents are preferred over the allylic lithium procedure. CHEM*3750 SCHWANC OURSE NOTES F13 Chapter 3 Page 7 Grignard reagents are similar to lithiated species in that they have considerable carbanionic character. Consequently, allylic Grignards are prone to an allylic rearrangement via an allylic carbanionic intermediate. B r M g MgBr or eth r M Br g Br equibi i m it e 1 CO + +2 H3O 2.H 3 + H + CO 2H H CO H 57% Z 27% 2 E 16% Other typical Grignard reactions also give a mixture of products. The major product in all cases is the product of substitution at the 3-position. These experimental results can be explained by allylic resonance, this time with an anion. The charge is delocalized by the resonance. The following equilibrium is rapid at room temperature. MgBr δ- δ- MgBr +MgBr 3. Conjugated Dienes ( SF10 13.7, 13.8) There is extra stability associated with a conjugated system. There are methods available to quantify the amount of stabilization. CH 2CH-CH 2-CH=CH 2 CH 3-CH=CH-CH=CH 2 1,4-pentadiene 1,3-pentadiene (isolated double bonds) (conjugated double bonds) ∆H for the reaction = ca. -6 kcal/mol. The extra stability is due to the overlap of the π-electrons of the sp carbons of the conjugated system. For maximum benefit of overlap, all carbons must lie in one plane and two planar conformations are possible. CHEM*3750 SCHWANC OURSE N OTES F13 Chapter 3 Page 8 pla a o e lp Two planar configurations of 1,3-butadiene: H H H H H H H H H H H H s-trans conformation (180º) s-ci conformation (0º) The s-trans conformation is an energy minimum. A second, higher energy minimum lies close to the s-cis conformation at about 40º. The s-cis conformation does not correspond to an energy minimum because of steric repulsion. The barrier to interconversion of the two minimum energy conformations by rotation about the 2-C3bond is about 3.9 kcal/mol. The energy maximum (transition state) is around 90º. This barrier is a measure of conjugative stabilization (ca. 4 kcal/mol). CH 2 40º H CH2 H CH 2 H CH2 H 180º (trans) 40º (skew) 0 kcal/mol 2.8±0.3 kcal/mol CHEM*3750 SCHWANC OURSE NOTES F13 Chapter 3 Page 9 Electrophilic AdditionSF10 13.10) HBr -80 ºC CH2=CH-CH=CH 2 + HBr CH 3CH=CH-CH B2 + CH 3-CHBr-CH=CH 2 1-bromo-2-butene 3-bromo-1-butene (mostly E 80% 20% +40 ºC product of kinetic control +40 ºC CH 3-CH=CH-CH B2 + CH -C3Br-CH=CH 2 85% 15% equilibrium mixture (product of thermodynamic control) Many electrophilic addition reagents are possible. A short list includes HBr, HCl, C2, Br2,2I , ICl. Organic substrates will also add across double bonds. These usually contain atoms from elsewhere in the periodic table. e.g., S, Se, Hg. Br2 -15 ºC CH 2=CH-CH=CH 2 + Br2 BrCH 2-CH=CH-CH 2r + BrCH -C2Br-CH=CH E-1,4-dibromo-2-butene 3,4-dibromo-1-butene 46% 54% product of kinetic control 60 ºC BrCH 2CH=CH-CH Br2+ BrCH -CH2r-CH=CH ca. 90% ca. 10% product of thermodynamic control CHEM*3750 SCHWANC OURSE N OTES F13 Chapter 3 Page 10 Explanation for B2 Addition: + + CH 2CH-CH=CH 2 + Br 2 BrCH -2H=CH-CH 2 BrCH 2CH-CH=CH 2 Br- BrCH -CH=CH-CH Br + BrCH -CHBr-CH=CH +CH 2CHBr-CH=CH 2 + Br- 2 2 2 2 major product of primary, non-allylic cation; major product of not formed thermodynamic kinetic control since control attacks of Br occurs more rapidly at the 2- position Remember: ‡ -∆G /RT -∆G o/RT Rate: k = k T e Equilibrium: Keq = e rate h ‡ ∆∆G For attack of Br at the two positions allylic cation CH 2CH-CH=CH 2 + Br2 o ∆G BrCH 2CHBr-CH=CH 2 BrCH -CH=CH-CH Br formed faster, 2 2 smaller∆G ‡ more stable, o smaller G CHEM*3750 SCHWANC OURSE NOTES F13 Chapter 3 Page 11 4. The Diels Alder ReactSF10(13.11) -the reaction (cycloaddition) of a 1,3-diene with an alkene (dienophile) that results in a cyclohexene as a product. H H H H H H H H H H + ∆ H H H H pressure H H H H
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