3 - CONJUGATION. More than one double bond can be in a given compound: n=0

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1 3 - JUGATIN 1. Terminology and Nomenclature (SF ) 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). ften, 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: (C 2 ) n n=1 or more conjugated double bonds: (C 2 ) n n=0 -called a diene, a term usually used in the case of conjugation and rarely in other circumstances such as with isolated double bonds. cumulated double bond: C - the carbon centre is sp hybridized Examples: 2 C C C 2 1,3,5-hexatriene 1,3-pentadiene 1,3-butadiene 1,2-propadiene (allene) CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 1

2 2. Allyl (SF ) 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. C C C C C C allyl cation C C C.. C C C allyl radical reactive intermediates C C C - - C C C allyl anion The 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. 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. CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 2

3 The three atom unit may also contain atoms other than carbon. - - the simplest enolate 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. 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: C 3 C 3 Cl Ag 2, 2 room temp. C 3 C 2 C C 3 C 3 C 3 0 C C 3 C 2 C C 3 Products in these reactions arise from nucleophilic capture of the cation. CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 3

4 allyl cations valence-bond (VB) picture: C C C C C C equal contributions summarized by ½ C C C½ orbital picture: C C C planar, all bond angles = σ bond framework: C s p 2 - C s p 2, C s p 2-1s 2 π electrons shared over 3 2p orbitals orthogonal to the molecular plane 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 equally by C 1 and C 3, with no charge on C 2. Placement of alkyl or aryl groups on C 1 or C 3 stabilizes allylic cations further. Allyl cation itself is a 1 o /1 o cation, while the 1-methylallyl and 1,1-dimethylallyl are 1 o /2 o and 1 o /3 o cations, respectively. Substitution on C 2 has little effect. stability: 3 o /3 o > 2 o /2 o > 1 o /1 o CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 4

5 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 o centre > 2 o centre > 1 o 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: and δ C C C 3 C C δ δ C C δ C 3 rotational barriers: C C C C C 3 C C C 3 δ Cδ 3 kcal/mol ca. 60 kcal/mol ca kcal/mol in SbF 5 - SClF b) Allylic Radicals allyl radical: C 2 =C-C 2 C 2 -C=C 2 -stabilized over a simple alkyl radical by ca. 11 kcal/mol. This causes the C- bonds in the allylic position of compounds like propene to have lower C- bond dissociation energies. ence they are activated toward radical halogenation reactions. e.g., radical bromination of allylic position: conditions: N-bromosuccinimide (NBS) in CCl 4, usually with a thermal (benzoyl peroxide or AIBN) or photochemical (hν) initiator. CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 5

6 C 2 =C-C 3 N inert solvent initiator C 2 =C-C 2 N NBS succinimide inert solvents might include benzene, carbon tetrachloride, toluene The active reagent in this reaction is believed to be 2 which is present in very low concentrations. The radical mechanism shown below is favoured over the ionic addition of 2 across the double bond because of three factors: 1. high temperature 2. non-polar solvent 3. low 2 concentration N initiator N 2 Usually a thermal initiator is used 2 R to promote radical formation. is produced in the reaction, but is also present in the early stages as an impurity in the NBS. C 3 -C=C 2 R C 2 -C=C 2 C 2 =C-C 2 R C 3 -C=C 2 C 2 -C=C 2 2 C 2 -C=C 2 C 2 =C-C 2 [1] C 2 -C=C 2 [2] and repeat [1] and [2] CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 6

7 More complex alkenes afford mixtures. Reactions of 2 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. C 3 C 2 C=C 2 NBS CCl 4 hν C 3 CC=C 2 C 3 C=C-C 2 C 3 CC=C 2 C 3 C=C-C 2 B major (E & Z) owever, when the alkene reactant has two or more nonequivalent allylic hydrogens, the hydrogen abstraction step is more selective. The rate of abstraction by decreases in the order 3 > 2 > 1. most of the products arise from initial abstraction of this allylic (3 vs. 2 ) c) Allylic Anions -can be generated as follows n-buli C 2 =C-C 3 TF C 2 =C-C 2 Li n-butane -slow reaction -accelerated by amines such as TMEDA [() 2 NC 2 C 2 N() 2 ] -allylic has pk a of ca. 42; alkyl has pk a 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. CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 7

8 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. or Mg ether Mg equilibrium mixture Mg 1. C C 2 C 2 57% Z 27% E 16% ther 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. Mg δ- δ- Mg Mg 3. Conjugated Dienes (SF , 13.8) There is extra stability associated with a conjugated system. There are methods available to quantify the amount of stabilization. C 2 =C-C 2 -C=C 2 1,4-pentadiene (isolated double bonds) C 3 -C=C-C=C 2 1,3-pentadiene (conjugated double bonds) for the reaction = ca. -6 kcal/mol. The extra stability is due to the overlap of the π-electrons of the sp 2 carbons of the conjugated system. For maximum benefit of overlap, all carbons must lie in one plane and two planar conformations are possible. CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 8

9 planar overlap Two planar configurations of 1,3-butadiene: s-trans conformation (180º) s-cis 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 C 2 -C 3 bond 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). C 2 C 2 C 2 180º (trans) 0 kcal/mol C 2 40º (skew) 2.8±0.3 kcal/mol 40º CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 9

10 Electrophilic Additions (SF ) C 2 =C-C=C 2-80 ºC C 3 -C=C-C 2 C 3 -C-C=C 2 40 ºC 1-bromo-2-butene (mostly E) 20% product of kinetic control 3-bromo-1-butene 80% 40 ºC C 3 -C=C-C 2 C 3 -C-C=C 2 85% 15% equilibrium mixture (product of thermodynamic control) Many electrophilic addition reagents are possible. A short list includes, Cl, Cl 2, 2, I 2, ICl. rganic substrates will also add across double bonds. These usually contain atoms from elsewhere in the periodic table. e.g., S, Se, g. 2 C 2 =C-C=C ºC C 2 -C=C-C 2 C 2 -C-C=C E-1,4-dibromo-2-butene 3,4-dibromo-1-butene 46% 54% product of kinetic control 60 ºC C 2 -C=C-C 2 C 2 -C-C=C ca. 90% ca. 10% product of thermodynamic control CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 10

11 Explanation for 2 Addition: C 2 =C-C=C 2 2 C 2 -C=C-C 2 C 2 -C-C=C 2 - C2 -C-C=C 2 - C 2 -C=C-C 2 C 2 -C-C=C 2 primary, non-allylic cation; not formed Remember: major product of thermodynamic control major product of kinetic control since attacks of - occurs more rapidly at the 2- position Rate: k rate = kt h e - G /RT Equilibrium: Keq = e - Go /RT G For attack of - at the two positions allylic cation C 2 =C-C=C 2 2 G o C 2 -C-C=C 2 formed faster, smaller G C 2 -C=C-C 2 more stable, smaller G o CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 11

12 4. The Diels Alder Reaction (SF ) -the reaction (cycloaddition) of a 1,3-diene with an alkene (dienophile) that results in a cyclohexene as a product. pressure The simplest possible example, the reaction of 1,3-butadiene with ethene (ethylene) requires high temperatures and pressures. Many other examples such as cyclopentadiene and maleic anhydride, a reaction often performed in undergraduate laboratories, will proceed rapidly in solution. diene (4π electrons) alkene (2π electrons) product (cycloadduct) The reaction is said to be a concerted π 2 π 4 (or 2π 4π) cycloaddition meaning the two new C-C bonds are formed at the same time in a single step. There are no reaction intermediates. probable transition state CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 12

13 rbital picture will be a full π bond diene ( butadiene ) now a full π bond sp 3 will be a σ bond will be a σ bond sp 3 full σ bond full σ bond sp 3 sp 3 dienophile ( ethene ) cycloadduct ( cyclohexene ) Some alkenes (dienophiles): C 3 C C C 3 The more reactive alkenes have one or more electron withdrawing groups attached to their double or triple bond. Some dienes: Some non-olefinic compounds can also react as dienophiles: N N N Ph N Ph There are some stereochemical aspects that are pertinent to the Diels-Alder reaction. a) The relative geometry of the dienophile substituents is maintained in the cycloadduct. CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 13

14 C, 20 h Dimethyl maleate C 2 C 2 cis product, 68% Dimethyl fumarate C, 3.5 h C 2 C 2 trans product, 95% b) The relative geometry of the dienophile substituents is maintained in the cycloadduct. NC NC NC NC c) The diene must able to obtain the s-cis (or cisoid) conformation so that the alkene and diene terminal carbons are in the proper proximity for bond making. Anything hindering or preventing formation of this conformation hinders or prevents the reaction. s-trans (transoid) s-cis (cisoid) This compound will not undergo the cycloaddition. It is held in the transoid conformation. CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 14

15 Compounds whose two double bonds are held firmly in the cisoid arrangement can be very reactive in Diels-Alder reactions. Examples include cyclopentadiene and 1,3-cyclohexadiene. d) A cycloaddition where at least one addend is cyclic will produce a bicyclic product. Thus the substituents on one of the reactants will end up exo or endo in the bridged product. The following diagram helps to define exo and endo. smallest bridge EX FACE largest bridge R R is exo is endo. END FACE The relative arrangement of the two reactants in the transition state for the cycloaddition defines the geometry of the products. NC NC endo cyanos endo 's When the non-hydrogen substituents are in the endo position, the result is called the endo product. With cyclopentadiene, the endo product tends to be the major product. 80 C thyl propenoate endo addition C 2 91%, endo product CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 15

16 From a Schwan publication in Diels-Alder chemistry: Diels-Alder Cycloadditions of Ethyl 2- Carbomethoxyethenesulfinates with Cyclopentadiene. Lewis Acid Enhancement and Adduct Identification with the Assistance of Competitive Stereodifferentiating Iodolactonization and Iodosultinization Reactions Yvonne Lear and Adrian L. Schwan* J. rg. Chem. 1999, 64, Scheme 2 S Et 1 C() S Et C() S Et C() S Et 3a (syn/endo) 3b (anti/endo) 4 S Et 2 S Et C() S Et C() S Et ()C 5a (syn/s-endo) 5b (anti/s-endo) 6a/b (S-exo) CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 16

17 Scheme 3 3b C() S Et i I 7, 47% S Et I 3a C() S Et i C() S 8, 71% 5a S Et C() i I a 9a, 58% C() b S a : 3.68 ppm b : 2.55 ppm 5b S Et C() i I a 9b, 86% C() b S a : 3.17 ppm b : 3.75 ppm 6a/b S Et C() i I 10a/b, 88% S Et Reagents: i) AgC()CF 3, I 2, DME, rt. CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 17

18 TE FLLWING SECTIN [5] IS A READING SECTIN AND SULD BE ADDRESSED IN JUNCTIN WIT TER REFERENCE(S) 5. Molecular rbital Description f Conjugation (SF , 13.3A, 13.7C, ) a) Molecular rbitals Consider a conjugated system consisting of a σ bond skeleton of n trigonally hybridized carbons, each with a 2p atomic orbital overlapping with adjacent 2p atomic orbitals in a π fashion. verlap of the n 2p atomic orbitals leads to n π molecular orbitals. Each of the π molecular orbital wave functions can be described mathematically as a linear combination of the n atomic orbitals wave functions, φi, one centred on each carbon atom: Ψ n = c 1 φ 1 c 2 φ 2 c 3 φ 3... Ψ n = nth molecular orbital wave function φi = A.. wave function centred on atom i. c 1, c 2... are constants The form of these π M..'s and their relative energies depends on the relative arrangement of the p orbitals (i.e., on the σ bond skeleton) In ethene, two p orbitals produce a bonding and an antibonding M.. that can be represented as follows: Ψ 2 1 n ode (a nti bo ndi n g) Ψ 1 0 n ode s (bond i ng) Ψ 1 = c(φ 1 φ 2 ); Ψ 2 = c(φ 1 - φ 2 ) Each M.. can accommodate 2 electrons so that the two π electrons occupy a bonding M.. CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 18

19 In allyl overlap of 3 2p A..'s leads to 3 M..'s: (SF ) allyl nodes 2 antibonding E allyl cation allyl radical allyl anion 1 non-bonding 0 bonding π electrons: ,3-butadiene (SF 13.7) nodes Characteristics of π M..'s of Linear Conjugated Systems 1. Lowest level has zero nodes; successive levels have 1,2...up to (n-1) nodes. 2. Even systems: n/2 bonding and n/2 antibonding levels. 3. dd systems: (allyl, pentadienyl, etc.) one non-bonding level, (n-1)/2 bonding, and (n-1)/2 antibonding levels. 0 CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 19

20 PRACTICE PRBLEMS 7 You are capable of doing Questions , , 13.18, 13.19, 13.25, 13.26, 13.36, and in SF ow would you convert cyclohexene to 1,3-cyclohexadiene? 2. Rank the following carbocations from most stable to least stable. C 2 C 3 C 3 C 3 3. Draw all the allylic isomers that allylic cation 2 might change to, if it is created in an environment that provides sufficient heat for isomerization. Et 2 Pr (Pr = n-propyl) 4. What is the structure of the Grignard reagents that will react with allyl bromide to form: a) 4,4-dimethyl-1-pentene; b) 4-methyl-1-hexene 5. Draw all the dibromo products that may form from the ionic bromination of 1,3,5-hexatriene with one equivalent of 2. (ignore stereochemical and cistrans isomers). 6. In the reaction of 1,3-cyclohexadiene with dry Cl at 0 C, no significant amount of 4-chlorocyclohexene is formed. Why not? 7. Keeping in mind the reason why s-cis-1,3-butadiene is not entirely happy in a planar conformation, suggest the structure of a molecule containing a conjugated diene unit that is not entirely happy in a planar s-trans form. 8. Identify the four products (two pairs of stereoisomers) that form under the radical reaction conditions indicated. (The thermal initiator breaks to make the radicals that begin the chain reaction.) C 3 C 3 C 3 NBS, C 6 6 thermal initiator SLUTINS T PRACTICE PRBLEMS 7 CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 20

21 Q1. NBS benzene intiator heat K 2 Q2. most stable to least stable. C 3 C 3 C 3 > > > C 2 Q3. Pr Et 2 Et Pr Et Pr Et Pr Q4. a) 4,4-dimethyl-1-pentene; Mg b) 4-methyl-1-hexene CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 21

22 Mg Q won't form Q6. CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 22

23 Cl Cl allylic cation, forms K primary cation won't form 4-chlorocyclohexene only forms from this Cl Cl - Q7.. Many ideas would work, here are two possibilities: tbu tbu Ph Ph Q8. C 3 C 3 C 3 C3 C 3 C 3 NBS, C 6 6 thermal initiator C 3 C 3 C 3 and C 3 C 3 C 3 C 3 C 3 C 3 CEM*3750 SCWAN CURSE NTES F13 Chapter 3 Page 23