Conjugated Systems Orbital Symmetry Organic Chemistry - Lecture notes | CHEM 238, Study notes of Organic Chemistry

Download Conjugated Systems Orbital Symmetry Organic Chemistry - Lecture notes | CHEM 238 and more Study notes Organic Chemistry in PDF only on Docsity! =~ _ . } Cxcev pr of eer CH — oe _ / _ Taken frees Orman Chant try f- [ , ff ey - + Py _€.6 Wea ; [. ; \ t Pleat (ASe te Clee up. Ong (55 ues witha Pere Class im — thao — do vat arc Ghost Concepts coupad toe and roto — a your eek — — So dp —  lo Conjugated System Orbital Symmetry, and Ultraviolet Spectroscopy 15-1 Introduction Double bonds that are separated by just one single bond interact with each’ and they are called conjugated double bends. Double bonds with two o1 single bonds separating them have little interaction and are called isolated d bonds. For example, 1,3-pentadiene has conjugated double bonds, while 1,44 diene has isolated double bonds. N a BY CH; Ne oO BL een omen Wwe Ny conjugated double bonds (more stable than isolated double bonds) 1,3-pentadiene isolated double bonds 1,4-pentadiene Because of the interaction between the double bonds, systems co: conjugated double bonds tend to be more stable than simifar systems with is double bonds. In this chapter we consider the unique properties of conju; systems, the theoretical reasons for this extra stability, and some of the chara tic reactions of molecules containing conjugated double bonds. We also stu traviolet spectroscopy, a tool for determining the structures of conjugated sy$ 15-2 Stabilities of Dienes In Chapter 7 we used heats of hydrogenation to compare the relative stabilit alkenes. For example, the heats of hydrogenation of I-pentene and trans-2-pel show that the disubstituted double bond in irans-2-pentene is 2.6 kcal/mol ( mol) more stable than the monosubstituted double bond in 1-pentene.  15-3 Figure 15-1 shows that the compound with conjugated double bonds is 3.7 kcal/mol Molecular Orbital (15 kJ/mol) more stable than a similar compound with isolated double bonds. This . Picture of a 3-7 kcal of extra stability in the conjugated molecule is called the resonance energy of the system. (Other terms favored by some chemists are conjugation energy, Conjugated System delocalization energy, and stabilization energy.) We can best explain this extra stability of conjugated systems by examining their molecular orbitals. Let’s begin with the molecular orbitals of the simplest conjugated diene, 1,3-butadiene. 15-3A Structure and Bonding of 1,3-Butadiene The heat of hydrogenation of 1,3-butadiene is about 3.6 kcal (15 kJ) less than twice that of 1-butene, showing that 1,3-butadiene has a resonance energy of 3.6 kcal. H,C=CH—CH=cCH, 22". CH,—CH,—CH,—CH, AH” = ~56.6 keal (—237 ki) 1,3-butadiene H,C=CH—CH,—CH, 2". CH,—CH,—CH,—CH, AW” = ~30.1 keal (126 KI) L-butene X 2 = ~60.2 keal (~252 kJ) resonance energy of 1,3-butadiene = 60.2 kcul — 56.6 keal = 3.6 keal (15 kJ} Figure 15-2 shows the most stable conformation of 1,3-butadiene. Note that this conformation is planar, with the p orbitals on the two pi bonds aligned. small amount of overlap f IRE 15-2 Structure of partial double utadiene in its most stable bond mation. The 1.48 A al carbon-carbon single (34a 7 ‘ 1 Kt is shorter than the 1.54 C3! Cc Bonds typical of alkanes Sef 86% Ay se of its partial double- idea Peak H character. H The C2—C3 bond in 1,3-butadiene is considerably shorter than a carbon— carbon single bond in an alkane: 1.48 versus 1.54 A. This bond is shortened slightly by the increased s character of the sp? hybrid orbitals, but the most important cause is its pi bonding overlap and partial double-bond character. The planar conforma- tion, with the p orbitals of the two double bonds aligned, allows overlap between the pi bonds. In effect, the electrons in the double bonds are delocalized over the entire molecule, creating some pi overlap and pi bonding in the C2—C3 bond. The length of this bond is intermediate between the normal length of a single bond and that of a double bond. Lewis structures are not adequate to represent delocalized molecules such as 1,3-butadiene. To represent the bonding in conjugated systems accurately, we must consider molecular orbitals that represent the entire conjugated pi system, and not just one bond at a time. 15-3B Constructing the Molecular Orbitals of 1,3-Butadiene All four carbon atoms of 1,3-butadiene are sp* hybridized, and (in the planar con- farmatian) thay all have averlanaine n arhirale Tat awe tein nenntae ated  the pi molecular orbitals (MOs) of ethylene from the p atomic orbitals of the p carbon atoms (Figure 15-3). Each p orbital consists of two lobes, with oppos; phases of the wave function in the two lobes. The plus and minus signs used drawing these orbitals indicate the phase of the wave function, not electri charges. To minimize confusion, we will color the lobes of the p orbitals to emp] size the phase difference. 3 In the pi bonding orbital of ethylene, there is overlap of lobes with the s: sign (+ with + and — with —) in the bonding region between the nuclei. We this reinforcement constructive overlap. In the antibonding orbital (marke: an *), there is canceling of opposite signs (+ with —) in the bonding region, canceling of the wave function is called destructive everlap. Electrons have | potential energy in the bonding MO than in the original p orbitals, and hi; potential energy in the antibonding MO. In the ground state of ethylene, electrons are in the bonding MO, but the antibonding MO is vacant, Stable mj cules tend to have filled bonding MOs and empty antibonding MOs, When viewing Figure 15-3, there are several important principles to ke mind. Constructive overlap results in a bonding interaction; destructive o results in an antibonding interaction. Also, the number of pi molecular orb: always the same as the number of p orbitals used to form the MOs. These mo orbitals have energies that are symmetrically distributed above and below ergy of the starting p orbitals. Half are bonding MOs, and half are antibonding M Now we are ready to construct the molecular orbitals of 1,3-butadiene. orbitals on Cl through C4 overlap, giving an extended system of four p orbitals, form four pi molecular orbitals. Two MOs are bonding, and two are antibondi represent the four p orbitals, we draw four p orbitals in a line. Although 1,3-6j ~~ 1 (antibonding) destructive overlap energy of ----------------- isolated p orbital FIGURE 15-3 Constructive overlap of unhybridized p orbitals on the sp? hybrid carbon atoms forms the pi ; bonding orbital of ethylene. 1 (bonding) Destructive overlap of these ~ two orbitals forms the antibonding pi orbital, \ Combination of two p orbitals es must give exactly two constructive molecular orbitals. overlap energy 4 . 668 Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy  ponding bonding bonding JGURE 15-4 The lowest- mergy orbital of 1,3-butadiene bonding interactions ween all adjacent carbon yms. This orbital is labeled ecause it is a pi bonding ital and it has the lowest antibonding bonding RE 15-5 The second f 1,3-butadiene has one in the center of the ule. There are bonding ictions at the Cl —C2 3 —C4 bonds, and there eaker) antibonding tion between C2 and iis 27, orbital is bonding, ot as strongly bonding nding _—_antibonding bonding i 1 G on yw ode node RE 15-6 The third lene MO is an nding orbital, and it is Tin the ground state. diene is not linear, this simple straight-line representation makes it easier to draw and visualize the molecular orbitals. okoh cf “SH represented by rr The lowest-energy molecular orbital always consists entirely of bonding in- teractions. We indicate such an orbital by drawing all the positive phases of the p orbitals overlapping constructively on one face of the molecule, and the negative phases overlapping constructively on the other face. Figure 15-4 shows the lowest- energy MO for 1,3-butadiene. This MO places electron density on all four p orbit- als, with slightly more on C2 and C3. (In these figures, larger and smaller p orbitals are used to show which atoms bear more of the electron density in a particular MO.) This lowest-energy orbital is exceptionally stable for two reasons: There are three bonding interactions, and the electrons are delocalized over four nuclei. This orbital helps to illustrate why the conjugated system is more stable than two isolated double bonds. It also shows some pi bond character between C2 and C3, which lowers the energy of the planar conformation and helps to explain the short C2— C3 bond length. As with ethylene, the second molecular orbital (ar,) of butadiene (Fig. 15-5) has one node in the center of the molecule. This MO represents the classic picture of a diene. There are bonding interactions at the C1—C2 and C3—-C4 bonds, and there is a (weaker) antibonding interaction between C2 and C3. The 2, orbital has two bonding interactions and one antibonding interaction, so we expect it to be a bonding orbital (ewo bonding — one antibonding = one bonding). It is not as strongly bonding, nor as tow in energy, as the all-bonding 7, orbital. Adding and subtracting bonding and antibonding interactions is not reliable for calculating energies of molecular orbitals, but it is useful for predicting whether a given orbital is bonding or antibonding and for ranking orbitals in order of their energy. The third butadiene MO (7¥) has two nodes (Fig. 15-6). There is a bonding interaction at the C2—C3 bond, and there are two antibonding interactions, one at the C1—C2 bond, and the other at the C3-—-C4 bond. This is an antibonding orbital (*), and it is vacant in the ground state. The fourth, and jast, molecular orbital (af) of 1,3-butadiene has three nodes and is totally antibonding (Fig. 15-7). This MO has the highest energy and is unoccupied in the molecule’s ground state. This highest-energy MO (7) is typical: For most systems the highest-energy molecular orbital has an antibonding interac- tion at each bond. Butadiene has four pi electrons (two electrons in each of the two double bonds in the Lewis structure) to be placed in the four MOs described above. Each MO can accommodate two electrons, and the lowest-energy MOs are filied first. Therefore, the four pi electrons go into 77, and 77,. Figure 15-8 shows the electronic configura- tion of 1,3-butadiene. Both bonding MOs are filled, and both antibonding MOs are empty. Most stable molecules have this arrangement of filled bonding orbitals and vacant antibonding orbitals. Figure 15-8 also gives the relative energies of the ethylene MOs to show that the conjugated butadiene system is slightly more stable than two ethylene double bonds. 15-3 Molecular Orbital Picture of a Conjugated System 669  Because of its resonance stabilization, the (primary) allyl cation is about as stable as a simple secondary carbocation such as the isopropyl cation. Substituted: allylic cations generally have at least one secondary carbon atom bearing part of the positive charge; they are about as stable as simple tertiary carbocations such as thé t-butyl cation. Stability of carbocations HC* < 1° < 2° allyl < 3°, substituted allylic be + H,C-CH==CH, is about as stable as CH,—CH—CH, CH, 6+ ot “ CH; —CH==CH==CH, is about as stable as. CH,—CZ CH, 15-5 — Electrophilic additions to conjugated dienes usually involve allylic cations as i 1,2- And 1,4-Addition mediates. Unlike simple carbocations, an allylic cation can react with a nucleophi to Conjugated Dienes at either of its positive centers. Let’s consider the addition of HBr to 1,3-butadifi an electrophilic addition that produces a mixture of two products. One pro 3-bromo-1-butene, results from Markovnikov addition across one of the dor bonds. In the other product, 1-bromo-2-butene, the double bond shifts to the C’ C3 position. H Br H | H,C=CH—CH=CH, + HBr _— H,C—CH—CH=CH, + H,C—CH=CH— 3-bromo-1-butene {-bromo-2-butene 1,2-addition 1,4-addition The first product results from electrophilic addition of HBr across a douf bond. This process is called a 1,2-addition, whether or not these two carbon at are numbered I and 2 in naming the compound. In the second product, the pr and bromide ion add at the ends of the conjugated system, to carbon atoms wi 1,4-relationship. Such an addition is called a 1,4-addition, whether or not carbon atoms are numbered 1 and 4 in naming the compound. oy a ne A woh toy iota! cec—cH=c0 AS tee eel +. tc2c=#¢ / N To \ A B A i 1,2-addition 1,4-addition The mechanism is similar to other electrophilic additions to alkenes. proton is the electrophile, adding to the alkene to give the most stable carbocat aw, he Br--H C=C. H H—C—Ct H eS Wo Ncw’ Noa” va \ HY \ H H H H Bromide can attack this resonance-stabilized intermediate at either of the t' carbon atoms sharing the positive charge, Attack at the secondary carbon giv 1,2-addition, while attack at the primary carbon gives 1,4-addition. 672 Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy  H H HC i Br oH HC i a "No : Com i B = n Ce ANF / . H H Hy 1,2-addition 1,4-addition The key to the formation of these two products is the presence of a double bond in the proper position for a stabilized allylic cation. Molecules having such double bonds are likely to undergo reactions via intermediates stabilized by the double bond’s ability to delocalize charges and radical (unpaired) electrons. PROBLEM 15-5 Treatment of an alky! halide with alcoholic AgNO, often promotes ionization. Agt + R-Cl ——> AgCl + RF When 3-chloro-1-methylcyclopentene reacts with AgNO, in ethanol, two isomeric ethers are formed. Suggest structures and give a mechanism for their formation. PROBLEM 15-6 Give a detailed mechanism for each reaction, showing explicitly how the observed mixtures of products are formed. (a) 3-methyl-2-buten-1-ol + HBr—> {-bromo-3-methyl-2-butene + 3-bromo-3-methyl-L-batene (b) 2-methyl-3-buten-2-0l + HBr —> 1-bromo-3-methyl-2-butene + 3-bromo-3-methyl-1-butene (c) L,3-butadiene + Br, — 3,4-dibromo-1-butene + 1,4-dibromo-2-butene (d) 1-chloro-2-butene + AgNO, H,O — 2-buten-1-ol + 3-buten-2-ol (e) 3-chloro-1-butene + AgNO;, H,O — 2-buten-I-ol + 3-buten-2-ol 15-6 Kinetic versus Thermodynamic Control in the Addition of HBr 0 1,3-Butadiene One of the interesting peculiarities of the reaction of 1,3-butadiene with HBr is the effect of temperature on the products. If the reagents are allowed to react briefly at —80°C, the 1,2-addition product predominates. If this reaction mixture is later allowed to warm to 40°C, however, or if the reaction itself is carried out at 40°C, the composition favors the product of 1,4-addition. (80%) HE CA —CH=CH (1,2-product) H Br a= (20%) HF — CH=CH CH (1,4-product) HBr H Br + {sore H,C=CH-—CH=CH, (15%) HE —CH CH=CH, (2,2-product) 40°c H Br (85%) HaE— CH= CH CAs (1,4-product) H Br 15-6 Kinetic versus Thermodynamic Control in the Addition of HBr to 1,3-Butadiene 673  This variation in product composition with temperature reminds us that most stable product is not always the major product. Of the two products, we e; L-bromo-2-butene (the 1,4-product) to be more stable, since it has the more hj substituted double bond. This prediction is supported by the fact that this ig¢ predominates when the reaction mixture is warmed to 40°C and allowed to ¢ brate. A potential-energy diagram for the second step of this reaction (Fig. helps to show why one product is favored at low temperatures and another at ij temperatures. The allylic cation is in the center of the diagram; it can react { the left to give the 1,2-product or toward the right to give the 1,4-product. Th product depends on where bromide attacks the resonance-stabilized allylic Bromide can attack at either of the two carbon atoms that share the positive Attack at the secondary carbon gives 1,2-addition, and attack at the primary'¢ gives 1,4-addition. delocalized allylic cation Ht + HCCH-CH=CH, —> |H,CGCH-CH=CH, <> H,C—CH—CH- :Bri- attack at attack at secondary carbon Primary carb HC CH— CH=CH H,C—CH=CH Br 1,2-addition product 1,4-addition product Kinetic Control at -80°C. The transition state for 1,2-addition has a lower than the transition state for 1,4-addition, giving the 1,2-addition a lower acti energy (E,). This is not surprising, because 1,2-addition results from bromide gi at the more highly substituted secondary carbocation, which bears more’ positive charge because it is better stabilized than the primary carbocation. Beg FIGURE 15-9 The allylic carbocation (center) formed in the addition of HBr to 1,3- butadiene can react at either of jts electrophilic carbon atoms. — CH==CH== CH, eee “Hy | | The transition state (}) leading & | r to 1,2-addition has a lower 3 intermediate — | ay? energy than that leading to the Gla cn— cH#= ‘4 1,4 product (4), so the 1,2- ay product is formed faster Br 4 (kinetic product). The 1,2- 1.2-produet -*7 ag GH product is not as stable as the (formed faster) 3 1A-produet, however. If 1,-product | (more stable) equilibrium is reached, the 1,4- product predominates (thermodynamic product). reaction coordinate 674 Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy  CH,CH,—H (CH,),CH—H (CH;);C—H © HjC=CH—CH,—H 'H,—CH,—CH=CH, + stabilization of the allylic free radical that results, The bond dissociation energies required to generate several free radicals are compared below. Notice that the allyl radical (a primary free radical) is actually 3 kcal/mol (13 kJ/mol) more stable than the tertiary-butyl radical. —_ CH;CH, + H AH = +98 kcal (+410 kJ) (primary) _ (CH;),CH’ + H- AH = +94 keal (+393 kJ) (secondary) _ (CH,);C’ + H AH = +91 kcal (+381 kJ) (tertiary) — > H,C=CH—CH, + H- AH = +88 kcal (+368 k3) (ally) The allylic 2-cyclohexeny] radical in the free-radical bromination of cyclo- hexene has its odd electron delocalized over two secondary carbon atoms, so it is even more stable than the unsubstituted allyl radical. The second propagation step may occur at either of the radical carbons, but in this symmetrical case either position gives 3-bromocyclohexene as the product. Less symmetrical compounds often give mixtures of products resulting from an allylic shift: In the product, the double bond can appear at either of the positions it occupies in the resonance structures of the allylic radical. An allylic shift in a radical reaction is similar to the 1,4-addition of an electrophilic reagent such as HBr to a diene (Section 15-5). The following propagation steps show how a mixture of products results from the free-radical allylic bromination of 1-butene. Br —> (CH,—CH—CH=CH, <> CH,—CH=CH—CH,] + HBr resonance-stabilized allylic radical |p (CH + CT; CH=CH CH, + Br Br Br (mixture) PROBLEM 15-8 When methylenecyclohexane is treated with a low concentration of bromine under irradiation by a sunlamp, two products are formed. or + Br > two substitution products + HBr methylenecyclohexane {a} Propose structures for these two products. (b) Give a mechanism to account for their formation. Bromination Using NBS. Effective use of free-radical allylic bromination requires a low concentration of bromine in the reaction mixture to enhance allylic substitu- tion over ionic addition. Simply adding bromine would make the concentration too high, and ionic addition of bromine to the double bond would result. A convenient bromine source for allylic bromination is N-bromosuccinimide (NBS), a bromin- ated derivative of succinimide, an amide of the four-carbon diacid succinic acid. 15-7 Allylic Radicals 677  succinic acid succinimide N-bromosuceinimide (NBS) NBS provides a fairly constant, low concentration of Br, because it with HBr liberated in the substitution, converting it back into Br,. This reactigg removes the HBr byproduct, preventing it from adding across the double bo: own free-radical chain reaction. Step 1: Free-radical allylic substitution (mechanism on pg. 676) hy R-H + Br —* R—Br + HBr Step 2: NBS converts the HBr byproduct back into Br, oO oO N—Br + HBr _ N—-H + Bry - ° oO NBS succinimide The NBS reaction is carried out in a clever way. The allylic compot dissolved in carbon tetrachloride, and 1 equivalent of NBS is added. NBS is than CCI, and not very soluble in it, so it sinks to the bottom of the CCL, so The reaction is initiated using a sunlamp for illumination or a radical initiatong as a peroxide. The NBS gradually appears to rise to the top of the CCL, layei actually converted to succinimide, which is less dense than CCi,. Once all thi succinimide has risen to the top, the sunlamp is tured off, the solution is filt yemove the succinimide, and the CCl, is evaporated to recover the product. PROBLEM 15-9 Devise a complete mechanism for the light-initiated reaction of |-hexene with NJ mosuccinimide in carbon tetrachloride solution. PROBLEM 15-10 Predict the product(s) of light-initiated reaction with NBS in CCI, for the followin starting materials. (a) cyclopentene (b) trans-2-pentene —_(¢) (Sa, toluene Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy  15-8 Molecular Orbitals of the Allylic System PROBLEM-SOLVING HI In drawing pi MO’s: Several p orbitals combine to give the same number of MOs, half bonding and hatf : antibonding, If there are an odd number of MOs, the middle one is nonbonding The lowest-energy MO has no nodes; each higher MO has one more node. The highest-energy MO is ntirely antibonding, with a ode at each overlap. : In a stable system, the onding MOs are filled, and ie antibonding MOs are IGURE 15-10 Geometric eestructure of the allyl cation, yl radical, and ally! anion. Let’s take a closer look at the electronic structure of allylic systems, using the allyl radical as our example. One resonance structure shows a pi bond between C2 and C3 with the radical electron on C1, and the other shows a pi bond between C1 and C2 with the radical electron on C3. These two resonance structures imply there is half a pi bond between C1 and C2 and half a pi bond between C2 and C3, with the radical electron half on Ci and half on C3. H H L, H i. le H : — Ay uetks Ne = OSS. | | | 4 | H H Cia Ki Yer S H H H H resonance structures combined representation Remember that no resonance structure has an independent existence: a com- pound has characteristics of all its resonance structures at the same time, but it does not “‘resonate’’ among them. To have pi bonding overlap simultaneously between C1 and C2 and between C2 and C3, the p orbitals of all three carbon atoms must be parallel. The geometric structure of the allyl system is shown in Figure 15-10. The allyl cation, the allyl radical, and the allyl anion all have this same geometric structure, differing only in the number of pi electrons. Just as the four p orbitals of 1,3-butadiene overlap to form four molecular orbitals, the three atomic p orbitals of the allyl system overlap to form three molec- ular orbitals, shown in Figure 15-11. These three MOs share several important features with the MOs of the butadiene system. The first MO is entirely bonding, the second has one node, and the third has two nodes and (because it is the highest- energy MO) is entirely antibonding. (An asterisk is often used to show that an orbital is antibonding, as in 23.) As with butadiene, we expect that half of the MOs will be bonding, and half antibonding; but with an odd number of MOs, they cannot be symmetrically di- vided. One of the MOs must appear at the middle of the energy levels, neither bonding nor antibonding: It is a nenbonding molecular orbital. Electrons in a nonbonding orbital have the same energy as in an isolated p orbital. The structure of the nonbonding orbital (a) may seem strange because there is zero electron density on the center p orbital (C2). This is the case because 77 must have one node, and the only symmetrical position for one node is in the center of the molecule, crossing C2. We can tell from its structure that 7, must be nonbonding, because C2’s p orbital has zero overlap with Cl and zero overlap with C3. The total is zero bonding, or a nonbonding orbital. # bonding a bondi fing 15-8 Molecular Orbitals of the Allylic System 679  15-10 Allylic halides and tosylates show enhanced reactivity toward nucleophilic S,2 Displacement placement reactions by the Sy2 mechanism, usually undergoing second-order g Reactions of Allylic stitution withow aye eure we omer rearrangements, For example, a bromj reacts with nucleophiles by the 8,2 mechanism about 40 times faster than n-proy Halides and Tosylates ide, Figure 15-13 shows how this rate enhancement can be explained by all delocalization of electrons in the transition state. The transition state for the reaction looks like a trigonal carbon atom with a p orbital perpendicular to the tf substituents. The electrons of the attacking nucleophile are forming a bond one lobe of the p orbital while the leaving group’s electrons are leaving fron! other lobe. When the substrate is allylic, the transition state receives resonance stabi tion through conjugation with the p orbitals of the pi bond. This stabilization I the energy of the transition state, resulting in a lower activation energy andi enhanced rate. The enhanced reactivity of allylic halides and tosylates makes them parti larly attractive as electrophiles for $,2 reactions. Allylic halides are so reactives they couple with Grignard and organolithium reagents, a reaction that doe: work well with unactivated halides. H,C=CH—CH,Br + CH,—(CH;);—Li —* H,C=CH—CH,—(CH,);—CH, + allyl bromide r-butyllithium 1-heptene Syy2 reaction on n-propyi bromide: Sy2 reaction on allyl bromide: FIGURE 15-13 In the transition state for the S,2 reaction of allyl bromide with H a nucleophile, the double bond Hoy, is conjugated with the p orbital HY that is momentarily present on . the reacting carbon atom. The (OX) Br resulting overlap lowers the energy of the transition state, increasing the reaction rate. transition state 682 Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy  PROBLEM 15-12 Show how you might synthesize the following compounds starting with alkyl or alkenyl halides containing four carbon atoms or fewer. (a) i-heptene —(b)_ 5-methyl-2-hexene 15-11 The Diels-Alder Reaction Jn 1928 the German chemists Otto Diels and Kurt Alder discovered that alkynes and. alkenes with electron-withdrawing groups add to conjugated dienes to form six- membered rings. The Diels-Alder reaction has proved to be a useful synthetic tool, providing one of the best ways to make six-membered rings with diverse functionality and controlled stereochemistry. In 1950 Diels and Alder were awarded the Nobel Prize for their work, The Diels—Alder reaction is also called a [4 + 2] cycloaddition, because a ring is formed. by the interaction of four pi electrons in the diene with two pi electrons on the alkene or alkyne. Since the electron-poor alkene or alkyne is prone to react with a diene, it is called a dienophile (‘‘lover of dienes”’). In effect, the Diels—Alder reaction converts two pi bonds into two sigma bonds. We can sym- bolize the Diels—Alder reaction by using three arrows to show the movement of three pairs of electrons. This electron movement is concerted, with three pairs of electrons moving simultaneously. The electron-withdrawing groups (—W) are usually carbonyl-containing (C==O) groups or cyano (—C==N) groups. H. W Oe AH A _ { A (heat) | ow a ~4 c CoH va H™ Sy H diene dienophile electron-rich electron-poor WwW nw! a A Cc 4 sil [oi 8 Cc Cw | H H diene ——_dienophile The Diels-Alder reaction is like a nucleophile—electrophile reaction. The diene is electron-rich (like a nucleophile), while the dienophile is electron-poor. Simple dienes such as 1,3-butadiene are sufficiently electron-rich to be effective dienes for the Diels— Alder reaction. The presence of electron-releasing groups such as alkyl groups or alkoxy (—OR) groups may further enhance the reactivity of the diene. Simple alkenes and alkynes such as ethene and ethyne are not good dieno- philes, however. A good dienophile generally has one or more electron-withdraw- ing groups (—W) pulling electron density away from the pi bond. Dienophiles commonly have carbonyl-containing (C=O) groups or cyano (——C IN) groups to enhance their Diels-Alder reactivity. Figure 15-14 shows some representative Diels—Alder reactions involving a variety of different dienes and dienophiles. 15-11 The Diels-Alder Reaction 683 ee  diene dienophile Diels—Alder adduct CHy C=N CH, HH a C—-C SF —~ Ltn CH, CH, SH ° \ Sc—ocn, f ; d co OCH, | O + — oy : 5 gets : C—OCH, G 3 0 oe : | FIGURE 15-14 Examples of Oo Ho 3 : the Diels-Alder reaction. Z t q i Electron-releasing substituents me + [o —_. io 3 i activate the diene; electron- — CHO C y : : withdrawing substituents CH;0 oO 3 A oO 3 i activate the dienophile. H PROBLEM-SOLVING HINT PROBLEM 15-13 Predict the products of the following proposed Diels— Alder reactions. A Diels-Alder product always contains one more ring than the reactants, The two ends of 9 the diene form new bonds to CHO the ends of the dienophile. a i oO The center (formerly single) @) K * | ) 7 bond of the diene becomes a CH; 0 double bond, The dienophile’s double bond becomes a single bond (or ifs triple bond becomes a double bond). Os, NC. CN PROBLEM-SOLVING HINT “ew ale @) + i © P+ To deconstruct a Diels-Alder aN, product, look for the double NC CN bond af the center of what 07 was the diene. Directly across the ting is the dienophile bond, usually with electron- withdrawing groups. (Ifa single bond, the dienophile PROBLEM 15-14 had a double bond; if double, i : . . . . ; . What di ive the follow: s— re ? the di i tripk hat dienes and dienophiles would react to give the follo’ ing Diels— Alder products’ bond.) Break the two bonds that join the diene and i i dienophite, and restore the C—CH, CHO. C—OCH,CH, oN two double bonds of the (a) (by © dlene and the double (or triple) bond of the dienophile, CH,O 684 Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy the diene adds to one face of the dienophile. As you can see from the transition state in Figure 15-15, there is no opportunity for any of the substituents to change their stereochemical positions during the course of the reaction. Substituents that are on the same side of the diene or dienophile will be cis on the newly formed ring. The following examples show the results of this syn addition. ° i H H C—OCH. ued Ca + LaH “s cHO-E ° COOCH, ans ans COOocH, COOCH, The Endo Rule. When the dienophile has a pi bond in its electron-withdrawing group (as in a carbonyl group or a cyano group), the p orbitals in that electron-with- drawing group approach the central carbon atoms (C2 and C3) of the diene. This proximity results in secondary overlap: an overlap of the p orbitals of the elec- tron-withdrawing group with the p orbitals of C2 and C3 of the diene (Fig. 15-17). Secondary overlap helps to stabilize the transition state. AO ain % <i secondary overlap lransition stare IGURE 15-17 In most Diels—Alder reactions, there is secondary overlap between the etron-" sedan group and those of the central carbon atoms of diene, Secondary over! lap stabilizes the transition state, and it favors products having, Ie electron withdrawing groups in endo positions 15-11 The Diels-Alder Reaction  exo endo ? Hs stereochemical positions : of norbornene > + HE endo} | Oo ~ Cc 4 or A Sy CH Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy The influence of secondary overlap was first observed in reactions usin, cyclopentadiene to form bicyclic ring systems. In the bicyclic product (called no, bornene), the electron-withdrawing substituent occupies the stereochemical pogy tion closest to the central atoms of the diene. This position is called the eng position, because the substituent seems to be inside the pocket formed by ¢f six-membered ring of norbornene. This stereochemical preference for the erg position is called the endo rule. 4 + x N77 a= ZN a \ a : exo ‘endo endo | The endo rule is useful for predicting the products of many types of Die! Alder reactions, regardless of whether they use cyclopentadiene to form norbornéff systems. The following examples show the use of the endo rule with other type: Diels—Alder reactions. ‘ e H H — H but not A H H CHG 7 B H O o0 | 0 SOLVED PROBLEM 15-1 Use the endo rule to predict the product of the following cycloaddition. OCH, imagine replacing with CH,  PROBLEM 15-15 For each proposed Diels—Alder reaction, predict the major product. Include stereo- chemistry where appropriate. H. C=N NZ : ; © - A €—OcH, o> + | ® Cy + © + ( aN Ss c-H oO I 0 15-11B Diels-Alder Reactions Using Unsymmetrical Reagents Even when the diene and dienophile are both unsymmetrically substituted, the Diels—Alder reaction usually gives a single product rather than a mixture. The product is the isomer that results from orienting the diene and dienophile so that we can imagine a hypothetical reaction intermediate with a ‘“‘push-pull’’ flow of elec- trons from the electron-donating group to the electron-withdrawing group. In the following representation, D is an electron-donating substitvent on the diene, and W is an electron-withdrawing substituent on the dienophile. Formation of 1,4 product DLA ‘ D. + IL = | but not Ss WwW WwW D WwW 14-product 1,3-product 15-11 The Diels-Alder Reaction 689