Supplementary Topic Pericyclic Reactions C

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SupplementaryTopicCPericyclic ReactionsAlthough most organic reactions take place by way of ionic or radical intermediates, a number ofuseful reactions occur in one-step processes that do not form reactive intermediates. A pericyclic reaction is a concerted reaction that proceeds through a cyclic transitionstate.Stereospecific reactions werefirst discussed in Chapter 10.Pericyclic reactions require light or heat and are completely stereospecific; that is, a singlestereoisomer of the reactant forms a single stereoisomer of the product. We will consider twocategories of pericyclic reactions: electrocyclic reactions and cycloadditions.An electrocyclic reaction is a reversible reaction that can involve ring closure or ring opening. An electrocyclic ring closure is an intramolecular reaction that forms a cyclic productcontaining one more σ bond and one fewer π bond than the reactant. new σ bond1,3,5-hexatriene3 π bonds1,3-cyclohexadiene2 π bonds An electrocyclic ring-opening reaction is a reaction in which a σ bond of a cyclicreactant is cleaved to form a conjugated product with one more π bond.Cleave this σ bond. cyclobutene1 π bond1,3-butadiene2 π bonds A cycloaddition is a reaction between two compounds with π bonds to form a cyclicproduct with two new σ bonds.new σ bond new π bondnew σ bondThe Diels–Alder reaction in Chapter 16 is one example of a cycloaddition.Two features determine the course of the reactions: the number of π bonds involved andwhether the reaction occurs in the presence of heat (thermal conditions) or light (photochemicalC-1smi75624 topicC.indd C-16/1/10 12:16 PM

C-2Topic CPericyclic Reactionsconditions). These reactions follow a set of rules based on orbitals and symmetry first proposedby R. B. Woodward and Roald Hoffmann in 1965, and derived from theory described by KenichiFukui in 1954.To understand pericyclic reactions we must review and expand upon what we learned about themolecular orbitals of systems with π bonds in Chapter 17.Problem C.1Classify each reaction as an electrocyclic reaction or a cycloaddition. Label the σ bonds that arebroken or formed in each reaction.c.a.OO b.H2CCH2C.1 Background on Molecular OrbitalsIn Section 17.9 we learned that molecular orbital (MO) theory describes bonds as the mathematical combination of atomic orbitals that forms a new set of orbitals called molecular orbitals (MOs). The number of atomic orbitals used equals the number of molecular orbitalsformed.Since pericyclic reactions involve π bonds, let’s examine the molecular orbitals that result from porbital overlap in ethylene, 1,3-butadiene, and 1,3,5-hexatriene, molecules that contain one, two,and three π bonds, respectively. Keep in mind that the two lobes of a p orbital are opposite inphase, with a node of electron density at the nucleus.EthyleneThe π bond in ethylene (CH2 –– CH2) is formed by side-by-side overlap of two p orbitals on adjacent carbons. Two p orbitals can combine in two different ways. As shown in Figure C.1, whentwo p orbitals of similar phase overlap, a π bonding molecular orbital (designated as ψ1) results.Two electrons occupy this lower-energy bonding molecular orbital. When two p orbitals of opposite phase combine, a π* antibonding molecular orbital (designated as ψ2*) results. A destabilizing node occurs when two orbitals of opposite phase combine.Figure C.1nodeThe π and π* molecularorbitals of ethylene Energylike phasesinteractingψ 2 π antibonding MOopposite phasesinteractingp atomic orbitalsmi75624 topicC.indd C-2 energy ofp atomic orbital ψ1The electrons residein the bonding MO.π bonding MO6/1/10 12:16 PM

C.1 Background on Molecular OrbitalsFigure C.2Ground stateThe four π molecularorbitals of 1,3-butadieneC-3Excited stateψ4 excited state LUMOψ3 excited state HOMOEnergyground state LUMOhνp atomic orbitalψ2ground state HOMOψ11,3-ButadieneThe two π bonds of 1,3-butadiene (CH2 –– CH – CH –– CH2) are formed by overlap of four p orbitals on four adjacent carbons. As shown in Figure C.2, four p orbitals can combine in four different ways to form four molecular orbitals designated as ψ1–ψ4. Two are bonding molecularorbitals (ψ1 and ψ2), and two are antibonding molecular orbitals (ψ3* and ψ4*). The two bondingMOs are lower in energy than the p orbitals from which they are formed, whereas the two antibonding MOs are higher in energy than the p orbitals from which they are formed. In the ground-state electronic arrangement, the four π electrons occupy the twobonding molecular orbitals.Also recall from Section 17.9: The highest energy orbital that contains electrons is called the highest occupiedmolecular orbital (HOMO). In the ground state of 1,3-butadiene, ψ2 is the HOMO. The lowest energy orbital that contains no electrons is called the lowest unoccupiedmolecular orbital (LUMO). In the ground state of 1,3-butadiene, ψ3* is the LUMO.The thermal reactions discussed in Section C.2 utilize reactants in their ground state electronicconfiguration.When 1,3-butadiene absorbs light of appropriate energy, an electron is promoted from ψ2 (theHOMO) to ψ3* (the LUMO) to form a higher energy electronic configuration, the excited state. Inthe excited state, the HOMO is now ψ3*. In the photochemical reactions in Section C.2, the reactant is in its excited state. As a result, the HOMO is ψ3* and the LUMO is ψ4* for 1,3-butadiene.All conjugated dienes can be described by a set of molecular orbitals that are similar to thosedrawn in Figure C.2 for 1,3-butadiene.Problem C.2smi75624 topicC.indd C-3For each molecular orbital in Figure C.2, count the number of bonding interactions (interactionsbetween adjacent orbitals of similar phase) and nodes. (a) How do these two values compare for abonding molecular orbital? (b) How do these two values compare for an antibonding molecular orbital?6/1/10 12:16 PM

C-4Topic CFigure C.3Pericyclic ReactionsThe six π molecular orbitals of 1,3,5-hexatrieneGround stateExcited stateψ6 ψ5 excited state LUMOψ4 excited state HOMOEnergyground state LUMOhνp atomic orbitalψ3ground state HOMOψ2ψ11,3,5-HexatrieneThe three π bonds of 1,3,5-hexatriene (CH2 –– CH – CH –– CH – CH –– CH2) are formed by overlapof six p orbitals on six adjacent carbons. As shown in Figure C.3, six p orbitals can combine insix different ways to form six molecular orbitals designated as ψ1–ψ6. Three are bonding molecular orbitals (ψ1–ψ3), and three are antibonding molecular orbitals (ψ4*–ψ6*).In the ground state electronic configuration, the six π electrons occupy the three bonding MOs,ψ3 is the HOMO, and ψ4* is the LUMO. In the excited state, which results from electron promotion from ψ3 to ψ4*, ψ4* is the HOMO and ψ5* is the LUMO.smi75624 topicC.indd C-4Problem C.3(a) Using Figure C.2 as a guide, draw the molecular orbitals for 2,4-hexadiene. (b) Label the HOMOand the LUMO in the ground state. (c) Label the HOMO and the LUMO in the excited state.Problem C.4(a) How many π molecular orbitals are present in 1,3,5,7,9-decapentaene– CH – CH –– CH – CH –– CH – CH –– CH – CH –– CH2)? (b) How many are bonding MOs and how(CH2 –many are antibonding MOs? (c) How many nodes are present in ψ1? (d) How many nodes arepresent in ψ10*?6/1/10 12:16 PM

C.2C-5Electrocyclic ReactionsC.2 Electrocyclic ReactionsAn electrocyclic reaction is a reversible reaction that involves ring closure of a conjugatedpolyene to a cycloalkene, or ring opening of a cycloalkene to a conjugated polyene. Forexample, ring closure of 1,3,5-hexatriene forms 1,3-cyclohexadiene, a product with one more σbond and one fewer π bond than the reactant. Ring opening of cyclobutene forms 1,3-butadiene,a product with one fewer σ bond and one more π bond than the reactant. Ring-closing reaction1,3,5-hexatrienenew σ bond1,3-cyclohexadiene Ring-opening reactioncyclobutene1,3-butadieneTo draw the product in each reaction, use curved arrows and begin at a π bond. Move the π electronsto an adjacent carbon–carbon bond and continue in a cyclic fashion. In a ring-forming reaction, thisprocess forms a new σ bond that now joins the ends of the conjugated polyene. In a ring-openingreaction, this process breaks a σ bond to form a conjugated polyene with one more π bond.Whether the reactant or product predominates at equilibrium depends on the ring size of thecyclic compound. Generally, a six-membered ring is favored over an acyclic triene at equilibrium. In contrast, an acyclic diene is favored over a strained four-membered ring.Problem C.5Use curved arrows and draw the product of each electrocyclic reaction.a. b.c. Stereochemistry and Orbital SymmetryElectrocyclic reactions are completely stereospecific. For example, ring closure of(2E,4Z,6E)-2,4,6-octatriene yields a single product with cis methyl groups on the ring. Ringopening of cis-3,4-dimethylcyclobutene forms a single conjugated diene with one Z alkene andone E alkene.CH3 CH3(2E,4Z,6E -cyclohexadieneNOT formedcis product onlyCH3CH3CH3cis-3,4-dimethylcyclobutene CH3(2E,4Z )-2,4-hexadiene(2E,4Z ) diene onlysmi75624 topicC.indd C-5CH3CH3NOT formed6/1/10 12:16 PM

C-6Topic CPericyclic ReactionsMoreover, the stereochemistry of the product of an electrocyclic reaction depends on whetherthe reaction is carried out under thermal or photochemical reaction conditions—that is, with heator light, respectively. Cyclization of (2E,4E)-2,4-hexadiene with heat forms a cyclobutene withtrans methyl groups, whereas cyclization with light forms a cyclobutene with cis methyl groups.CH3Electrocyclic ring closuregenerally forms either anachiral meso compound or amixture of chiral enantiomers.When enantiomers form, onlyone enantiomer is drawn inthese reactions. trans product onlyCH3CH3CH3CH3hνcis product only(2E,4E )-2,4-hexadieneCH3To understand these results, we must focus on the HOMO of the acyclic conjugated polyene thatis either the reactant or product in an electrocyclic reaction. In particular, we must examine the porbitals on the terminal carbons of the HOMO, and determine whether like phases of the orbitalsare on the same side or on opposite sides of the molecule.like phases on the same sidelike phases on opposite sides An electrocyclic reaction occurs only when like phases of orbitals can overlap to form abond. Such a reaction is symmetry allowed. An electrocyclic reaction cannot occur between orbitals of opposite phase. Such areaction is symmetry forbidden.To form a bond, the p orbitals on the terminal carbons must rotate so that like phases can interactto form the new σ bond. Two modes of rotation are possible. When like phases of the p orbitals are on the same side of the molecule, the twoorbitals must rotate in opposite directions—one clockwise and one counterclockwise.Rotation in opposite directions is said to be w σ bond When like phases of the p orbitals are on opposite sides of the molecule, the twoorbitals must rotate in the same direction—both clockwise or both counterclockwise.Rotation in the same direction is said to be conrotatory.conrotatoryclockwiseclockwisenew σ bondThermal Electrocyclic ReactionsTo explain the stereochemistry observed in electrocyclic reactions, we must examine the symmetry of the molecular orbital that contains the most loosely held π electrons. In a thermal reac-smi75624 topicC.indd C-66/1/10 12:16 PM

C.2Electrocyclic ReactionsC-7tion, we consider the HOMO of the ground state electronic configuration. Rotation occurs ina disrotatory or conrotatory fashion so that like phases of the p orbitals on the terminal carbonsof this molecular orbital combine. The number of double bonds in the conjugated polyene determines whether rotation isconrotatory or disrotatory.Two examples illustrate different outcomes.Thermal electrocyclic ring closure of (2E,4Z,6E)-2,4,6-octatriene yields a single product with cismethyl groups on the ring. disrotatoryCH3H hyl-1,3-cyclohexadienecis product(2E,4Z,6E )-2,4,6-octatrieneCyclization occurs in a disrotatory fashion because the HOMO of a conjugated triene has likephases of the outermost p orbitals on the same side of the molecule (Figure C.3). A disrotatoryring closure is symmetry allowed because like phases of the p orbitals overlap to form the newσ bond of the ring. In the disrotatory ring closure, both methyl groups are pushed down (or up),making them cis in the product.This is a specific example of the general process observed for conjugated polyenes with an oddnumber of π bonds. The HOMO of a conjugated polyene with an odd number of π bonds has likephases of the outermost p orbitals on the same side of the molecule. As a result: Thermal electrocyclic reactions occur in a disrotatory fashion for a conjugated polyenewith an odd number of π bonds.In contrast, thermal electrocyclic ring closure of (2E,4E)-2,4-hexadiene forms a cyclobutenewith trans methyl groups.The conrotatory ring closure of(2E,4E)-2,4-hexadiene is drawnwith two clockwise rotations.The conrotatory ring closurecould also be drawn with twocounterclockwise rotations,leading to the enantiomer ofthe trans product drawn. Bothenantiomers are formed inequal diene ans productCyclization occurs in a conrotatory fashion because the HOMO of a conjugated diene has likephases of the outermost p orbitals on opposite sides of the molecule (Figure C.2). A conrotatoryring closure is symmetry allowed because like phases of the p orbitals overlap to form the newσ bond of the ring. In the conrotatory ring closure, one methyl group is pushed down and onemethyl group is pushed up, making them trans in the product.This is a specific example of the general process observed for conjugated polyenes with an evennumber of π bonds. The HOMO of a conjugated polyene with an even number of π bonds haslike phases of the outermost p orbitals on opposite sides of the molecule. As a result: Thermal electrocyclic reactions occur in a conrotatory fashion for a conjugated polyenewith an even number of π bonds.Since electrocyclic reactions are reversible, electrocyclic ring-opening reactions follow the samerules as electrocyclic ring closures. Thus, thermal ring opening of cis-3,4-dimethylcyclobutene—smi75624 topicC.indd C-76/1/10 12:16 PM

C-8Topic CPericyclic Reactionswhich ring opens to a diene with an even number of π bonds—occurs in a conrotatory fashion toform (2E,4Z)-2,4-hexadiene as the only product.CH3CH3 se(2E,4Z )-2,4-hexadienecis-3,4-dimethylcyclobuteneSample Problem C.1HDraw the product of each thermal electrocyclic ring closure.CH3 a.b. CH3O2CCH3 CO2CH3B(2E,4Z,6Z )-2,4,6-octatrieneSolutionCount the number of π bonds in the conjugated polyene to determine the mode of ring closure in athermal electrocyclic reaction. A conjugated polyene with an odd number of π bonds undergoes disrotatory cyclization. A conjugated polyene with an even number of π bonds undergoes conrotatory cyclization.a. (2E,4Z,6Z)-2,4,6-Octatriene contains three π bonds. The HOMO of a conjugated polyene withan odd number of π bonds has like phases of the outermost p orbitals on the same side of themolecule, and this results in disrotatory cyclization. disrotatoryCH3 methyl-1,3-cyclohexadienetrans product(2E,4Z,6Z )-2,4,6-octatrieneb. Diene B contains two π bonds. The HOMO of a conjugated polyene with an even number of πbonds has like phases of the outermost p orbitals on opposite sides of the molecule, and thisresults in conrotatory cyclization.CH3O2CHCO2CH3HclockwiseCH3O2C conrotatoryCO2CH3Hclockwisetrans productBProblem C.6HWhat product is formed when each compound undergoes thermal electrocyclic ring opening or ringclosure? Label each process as conrotatory or disrotatory and clearly indicate the stereochemistryaround tetrahedral stereogenic centers and double bonds.C6H5a.b.c.d.C6H5Problem C.7What cyclic product is formed when each decatetraene undergoes thermal electrocyclic ringclosure?a.smi75624 topicC.indd C-8b.6/1/10 12:16 PM

C.2C-9Electrocyclic ReactionsPhotochemical Electrocyclic ReactionsPhotochemical electrocyclic reactions follow similar principles as those detailed in thermalreactions with one important difference. In photochemical reactions, we must consider theorbitals of the HOMO of the excited state to determine the course of the reaction. Theexcited state HOMO has the opposite orientation of the outermost p orbitals compared tothe HOMO of the ground state. As a result, the method of ring closure of a photochemicalelectrocyclic reaction is opposite to that of a thermal electrocyclic reaction for the samenumber of π bonds.Photochemical electrocyclic ring closure of (2E,4Z,6E)-2,4,6-octatriene yields a cyclic productwith trans methyl groups on the ring.hνconrotatoryCH3H HCH3CH3HHCH3clockwise 4Z,6E )-2,4,6-octatrienetrans productCyclization occurs in a conrotatory fashion because the excited state HOMO of a conjugatedtriene has like phases of the outermost p orbitals on the opposite sides of the molecule (FigureC.3). In the conrotatory ring closure, one methyl group is pushed down and one methyl group ispushed up, making them trans in the product. This is a specific example of the general processobserved for conjugated polyenes with an odd number of π bonds. Photochemical electrocyclic reactions occur in a conrotatory fashion for a conjugatedpolyene with an odd number of π bonds.Photochemical electrocyclic ring closure of (2E,4E)-2,4-hexadiene forms a cyclobutene with cismethyl groups.CH3HclockwiseHCH3counterclockwise(2E,4E hylcyclobutenecis productCyclization occurs in a disrotatory fashion because the excited state HOMO of a conjugateddiene has like phases of the outermost p orbitals on the same side of the molecule (Figure C.2).In the disrotatory ring closure, both methyl groups are pushed down (or up), making them cis inthe product. This is a specific example of the general process observed for conjugated polyeneswith an even number of π bonds. Photochemical electrocyclic reactions occur in a disrotatory fashion for a conjugatedpolyene with an even number of π bonds.smi75624 topicC.indd C-9Problem C.8What product is formed when each compound in Problem C.6 undergoes photochemicalelectrocyclic ring opening or ring closure? Label each process as conrotatory or disrotatoryand clearly indicate the stereochemistry around tetrahedral stereogenic centers and double bonds.Problem C.9What cyclic product is formed when each decatetraene in Problem C.7 undergoes photochemicalelectrocyclic ring closure?6/1/10 12:16 PM

C-10Topic CPericyclic ReactionsSummary of Electrocyclic ReactionsTable C.1 summarizes the rules, often called the Woodward–Hoffmann rules, for electrocyclic reactions under thermal or photochemical reaction conditions. The number of π bondsrefers to the acyclic conjugated polyene that is either the reactant or product of an electrocyclicreaction.Table C.1 Woodward–Hoffmann rules for electrocyclic reactionsSample Problem C.2Number of π bondsThermal reactionPhotochemical nrotatoryIdentify A and B in the following reaction sequence. Label each process as conrotatory ordisrotatory.C6H5CH3C6H5 hνABCH3SolutionRing opening of a cyclohexadiene forms a hexatriene with three π bonds. A conjugated polyenewith an odd number of π bonds undergoes a thermal electrocyclic reaction in a disrotatory fashion(Table C.1). The resulting hexatriene (A) then undergoes a photochemical electrocyclic reaction in aconrotatory fashion to form a cyclohexadiene with cis methyl groups (B).C6H5C6H5HCH3CH3HC6H5C6H5C6H5conrotatoryCH3H CH3AHCH3H CH3Hexcited state HOMOC6H5C6H5hνdisrotatorytrans CH3 groupsProblem C.10C6H5 HHC

C-6 Topic C Pericyclic Reactions Moreover, the stereochemistry of the product of an electrocyclic reaction depends on whether the reaction is carried out under thermal or photochemical reaction conditions—that is, with heat or light, respectively.

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