Frontier Molecular Orbitals and Pericyclic Reactions

1、Frontier Molecular Orbitals and Pericyclic ReactionsThird Year Organic Chemistry CourseCHM3A2- Prof Jon A Preece -School of ChemistryUniversity of BirminghamProf Preeces Powerpoint Lecture Presentations and answers to questions can be found atwww.nanochem.bham.ac.ukUsername: UndergradchemPassword: P

2、reece57nanoTeaching ResourcesQueries on course after reading around the subjectto .Be Specific with the problem(s) in your email.Give me three times when you are free to see me. I will email you back with a time to see me.PartContents1 Pericyclic Reactions These lectures will begin with a definition

3、 of Pericyclic reactions, and will be exemplified by considering examples of cycloaddation, sigmatropic, and electrocyclic reactions. It will be highlighted how it is possible to use FMO theory (and other theories) to predict the constitution and stereochemical outcome of the products. Attention wil

4、l be drawn to the cyclic transition state and the number of electrons involved (Huckel or Mobius), highlighting that when 4n+2 electrons are involved the reaction proceeds readily under thermal conditions, and the reversibility of such reactions. The concept of Linear Combination of Atomic Orbitals

5、to form a bond(s) (and antibond(s) will be revised, and extended to the linear combination of frontier molecular orbitals. The p-molecular orbitals of ethene, butadiene and 1,3,5-hexatriene will be considered and the identities of the HOMO and LUMO will be established, as well as the FMOs of a CH bo

6、nd. 2i Electrocyclic ReactionsThis lecture will extend the predicative nature of FMO theory regarding the stereochemical outcomes to electrocyclic reactions for 4 and 6 -electron transition states (by defining the disrotatory or conrotatory movement of the termini of the HOMO in the Transition State

7、). 2iiCycloaddition ReactionsThese lectures will introduce cycloaddition reactions and the concepts of (i) phase relationships of the FMOs, (ii) geometry of approach of the FMOs (suprafacial and antarafacial will be defined), and (iii) minimum energy differences between the HOMO and LUMO. These conc

8、epts will be exemplified by several Diels-Alder and related reactions. Attention will be drawn to the nature (chemical and stereochemistry) of substituents and their stereochemistry in the product.3Photochemically Induced Pericyclic reactionsThese lecture will extend the predicative nature of FMO th

9、eory regarding the outcomes of electrocyclic reactions and cycloaddition reactions by considering how they can be induced photochemically, to give alternative stereochemical outcomes and allow reactions that did not go thermally.Course SynopsisPart 1. Frontier Molecular Orbitals Constructing molecul

10、ar orbitals and identifying the frontier molecular orbitalsPart 2.Thermal Pericyclic Reactions(i) Electrocyclic Reactions using FMO Theory(ii) Cycloaddition Reactions using FMO TheoryPart 3.Photochemical Pericyclic Reactions(i) Electrocyclic Reactions using FMO Theory(ii) Cycloaddition Reactions usi

11、ng FMO TheorySecond Year Organic Chemistry CourseCHM3A2Recommended ReadingI FlemingFrontier Orbitals and Organic Chemical Reactions, John Wiley and Sons, 1996.Part 1:Ch 1 and Ch 2Part 2 and 3:Ch 4Second Year Organic Chemistry CourseCHM3A2Frontier Molecular Orbitals and Pericyclic ReactionsPart 1(i):

12、The Questions FMO Analysis Can Answer100%0%Ionic And Radical Reactions(i)Ionic reactionsHere pairs of electrons move in one direction e.g. SN2, SN1, E2 and E1 mechnisms(ii)Radical reactionsHere single electrons move in a correlated mannere.g. chlorination of alkanesTo date you have seen two broad ca

13、tegories of reaction:Pericyclic ReactionsPericyclic reactions are the third distinct class.They involve cyclic transition statesIn which all bond breaking and bond making steps take place in commensurate mannerAnd there is no sense of the flow of electrons.Pericyclic Reactions: Electrocyclic Reactio

14、ns100%0%ClockwiseAnti-ClockwiseThere is no real senses of flow for the electrons in pericyclic reactionsStereospecific ReactionPericyclic Reactions: Cycloaddition Reactions100%0%100%0%Stereospecific ReactionRegiospecific ReactionKinetic ProductThermodynamic Product1,3-syndiaxial interactions123Revis

15、ion: 1,3Syndiaxial InteractionsaxialequitorialThermodynamic and Kinetic ControlKinetic ProductFormed in Cycloaddition ReactionThermodynamic ProductNot Formed in Cycloaddition ReactionPericyclic Reactions: Sigmatropic Reactions100%0%Stereospecific ReactionRegiospecific ReactionPericyclic Reactions: W

16、hy are they so specific?Thus, an obvious question to ask ourselves at this point is why are pericyclic reactions so selective?Pericyclic reactions show high degrees of(i) Stereoselectivity(ii) Regioselectivity, and(iii)DiastereoselectivityTo help begin to answer this question we shall briefly need t

17、o revise the SN2 reaction mechanism where YOU WILL remember that this reaction type was highly stereoselective leading to inversion of chiral centres.Revision: SN2 Reaction MechanismNucleophile attacks from behind the C-Cl s-bond.This is where the s*-antibonding orbital of the C-Cl bond is situated.

18、 The concerted flow of both pairs of electrons in the SN2 reaction mechanism leads to the transition state which allows the stereochemical information to be retained,i.e. a stereoselective reaction.This SN2 reaction mechanism should be contrasted to the SN1 reaction mechanism where the arrow-pushing

19、 is the same but the two pairs electrons do not flow in a concerted fashion. Instead, the haloalkane C-Cl bond heterolytically cleaves to give the planar sp2 hybridised carbocation reactive intermediate. Now the nucleophile can attack from either side of the carbocation leading to racemisation,i.e.

20、a non-stereoselective reaction.Revision: Transition StatesDiscussion of reaction mechanisms frequently include discussions of the nature of the transition state for each step in a reaction sequence or at least for the slowest or rate limiting step.A transition state is the point of highest energy in

21、 a reaction or in each step of a reaction involving more than one step.The nature of the transition state will determine whether the reaction is a difficult one, requiring a high activation enthalpy (DG), or an easy one.Transition states are always energy maxima, I.e. at the top of the energy hill,

22、and therefore, can never be isolated: there are no barriers to prevent them from immediately “rolling” downhill to form the reaction products or intermediates (or even reform the starting materials).A transition states structure is difficult to identify accurately. It involves partial bond cleavage

23、and partial bond formation. However, it is nigh on impossible to estimate whether the transition state is an early one (looks more like the starting materials) or a late one (looks more like the products)ProductStarting MaterialRevision: Transition StatesPericyclic Reactions: Transition StatesPericy

24、clic reactions involve concerted flow of pairs of electrons going through transition states which retains stereochemical information that was present in the starting material.Thus, now we can start to understand why pericyclic reactions are so highly stereo-, regio-, and diasteroselective.Pericyclic

25、 Reactions Involve Cyclic Transition StatesCyclic Transition StatePericyclic reactions involve ene and polyene units.Thus, the transition states involve the overlap of p-molecular orbitals in the case of electrocyclic and cycloaddition reactions, and a p-molecular orbital and s-molecular orbital in

26、the case of sigmatropic reactions.How do the orbitals overlap?In order to understand the selectivity of pericyclic reactions, we need to understand these molecular orbitals and how they overlap. Frontier Molecular OrbitalsWe will first revise some simple molecular orbitals of a C-H s-bond and a C=C

27、p-bond and then extend this analysis to highly conjugated linear polyenes and related structures/ In particular, we need to know how the Frontier Molecular Orbitals (FMOs) interact in the starting material(s) which lead to the cyclic transition states.Second Year Organic Chemistry CourseCHM2C3BFront

28、ier Molecular Orbitals and Pericyclic ReactionsPart 1(ii):Frontier Molecular OrbitalsAfter completing PART 1 of this course you should have an understanding of, and be able to demonstrate, the following terms, ideas and methods.Given a set of n p-orbitals you should be able to construct a molecular

29、orbital energy level diagram which results from their combination.(ii)In this diagram you should be able to identify for each MO nodesthe symmetric (S) or antisymmetric (A) nature of the MO towards a C2 axis or mirror planethe bonding, nonbonding or antibonding nature of it(iii)For a set of n molecu

30、lar orbitals you should be able to identify the frontier molecular orbitals.the highest occupied molecular orbital (HOMO )the lowest unoccupied molecular orbital (LUMO)(iv)The HOMO (thermal reaction) interactions are important when evaluating the probability of an unimolecular reaction occurring and

31、 the stereochemical outcome see electrocyclic reactions.The HOMO/LUMO (thermal reaction) interactions of the reacting species are important when evaluating the probability of (i) a bimolecular reaction occurring and the stereochemical outcome see cycloaddition reactions, and (ii) a unimolecular reac

32、tion occurring and the stereochemical outcome see sigmatropic reactions.The geometry, phase relationship and energy of interacting HOMOs and LUMOS is important for evaluating the probability of a reaction occurring and the stereochemical outcome. Learning Objectives Part 1 Frontier Molecular Orbital

33、sCHM2C3B Introduction to FMOs Molecular Orbitalss-BondTwo s Atomic OrbitalsMolecular Orbitalss-BondOne s Atomic Orbital and One sp3 Atomic OrbitalMolecular Orbitalsp-Bond:Two p Atomic OrbitalsThe linear combination of n atomic orbitalsleads to the formation of n molecular orbitalsCn = Coeffecient: a

34、 measure of the contribution which the atomic orbital is making to the molecular orbitalfm = Electronic distribution in the atomic orbitalsA SIMPLE Mathematical Description of a MOp = caf1 + cbf2The combination of two (or more) p-atomic orbitals (or any orbitals) to afford 2 p-molecular orbitals can

35、 be described by the following simple mathematical relationshipp* = ccf1 + cdf2The probability of finding an electron in an occupied molecular orbital is 1.p = caf1 + cbf2p* = ccf1 + cdf2Sc2 = cc2 + cd2 = 1Sc2 = ca2 + cb2 = 1Cc = 1/2Ca = 1/2Cb = 1/2Cd = -1/2NegativeThe probability of finding an elec

36、tron in an occupied molecular orbital is the Sc2Thus, for the ethene p-molecular orbitals1212So what about the combination of 3 or 4 or 5 or 6 p-atomic orbitals.That is to consider conjugated systemsThe Allyl Cation, Radical and Anion 3p AOs to give 3p MOsAllyl CationAllyl RadicalAllyl AnionThus, al

37、lyl systems result from the combination of 3 conjugated p-orbitals.Therefore, this will result in 3 p-molecular orbitals.When we constructed the p-molecular orbitals of ethene, each contributing AO was the same size, i.e. the coeffecient c were 1/2 or -1/2. When there are three or more p-atomic orbi

38、tals combining the size of each contributing p-atomic orbital will not be equal (but they will be symmetrical about the centre).Finally, we refer to the p-MOs and p*-MOs as y1, y2, y3 (yn)The Allyl p-Molecular Orbitalsy1y2y31234Nodalposition4/1 = 4Nodalposition4/2 = 2Nodalposition4/3 Nodes24We can c

39、onsider the molecular orbital (the electron density) being described by a SINE WAVE starting and finishing one bond length beyond the moleculey1 = 0 Nodesy2 = 1 Nodesy3 = 2 NodesFor our analysis of molecular orbitals we do not have to concern ourselves with the coefficients.We can draw the p-AOs tha

40、t make up the p-MOs all the same size.However, we have to always remember they are not the same size.But it is of the utmost importance that we know how to calculate where the nodes are placedBonding, Non-Bonding, and Anti-bonding LevelsAnti-bondingNon-bondingBondingWe can consider the molecular orb

41、ital (the electron density) being described by a sine wave starting and finishing one bond length beyond the moleculeLUMOs and HOMOsHOMO = Highest Occupied Molecular OrbitalLUMO = Lowest Unoccupied Molecular OrbitalLUMOAllylRadical(3e)AllylAnion(4e)HOMOLUMOHOMOLUMOHOMOAllylCation(2e)Question 1: 4 p-

42、Molecular Orbital System ButadieneConstruct the p-molecular orbitals of butadiene.Identify the number of nodes, nodal positions, HOMO and LUMO.Nodal PositionNumber of NodesynAnswer 1: 4 p-Molecular Orbital System ButadieneConstruct the p-molecular orbitals of butadiene.Identify the number of nodes,

43、nodal positions, HOMO and LUMO.Nodal Position123455/1 = 5Number of Nodes0123y1y2y3y4ynHOMOLUMOA Reminder: Sinusodal Wave FunctionCoefficients, cnn= caf1 + cbf2 + ccf3 + cnfnThat is to say the probability of finding an electron in a molecular orbital is 1Each molecular orbital is described by an equa

44、tionSc2 = 1Where c is referred to as the coefficientSuch that the3= caf1 + cbf2 + ccf3 + cdf4 We Keep FMO Analysis Simple!For the purpose of this course and the third year course (Applied Frontier Molecular Orbitals and Stereoelectronic Effects) you are expected (i)to be able to place the nodal plan

45、es in the correct place(ii)but not to be able to assign the coefficients to the molecular orbitals. That is to say you can draw the p-orbitals that make up each molecular orbital as the same size, whilst remembering that in reality they are not and for high level FMO analysis this needs to be taken

46、into account.Question 2: 5 p-Molecular Orbital System PentadienylConstruct the p-molecular orbitals of the cyclopentenyl system.Identify the number of nodes and nodal positions.Nodal PositionNumber of NodesynMolecular OrbitalsAnswer 2: 5 p-Molecular Orbital System PentadienylConstruct the p-molecula

47、r orbitals of the cyclopentenyl system.Identify the number of nodes and nodal positions.Nodal Position6/1 = 66/2 = 36/3 = 2Number of Nodes0123y1y2y3y4yn4y5123465Molecular OrbitalsQuestion 3: Pentadienyl Cation, Radical & AnionIntroduce the electrons and identify the HOMOs and LUMOsAnswer 3: Pentadie

48、nyl Cation, Radical & AnionIntroduce the electrons and identify the HOMOs and LUMOsQuestion 4: Pentadienyl Cation & AnionGenerate the cation and anion and draw the resonance structures of the above speciesAnswer 4: Pentadienyl Cation, Radical & AnionGenerate the cation and anion and draw the resonan

49、ce structures of the above species6 p-Molecular Orbital System 1, 3, 5-Hexatriene7 p-Molecular Orbital SystemQuestion 5: 6p MO SystemBy shading the p atomic orbitals, generate the molecular orbitals for hexa-1,3,5-triene .Identify the number of nodes characterising each molecular orbital. With refer

50、ence to both a mirror plane (m) and a two-fold axis, designate the orbitals as symmetric (S) or antisymmetric (A). Using arrows to represent electrons, associate the six p-electrons with the appropriate molecular orbitals of hexa-1,3,5-triene in its ground state. Finally, identify the HOMO and LUMO.

51、Answer 5: 6p MO SystemBy shading the p atomic orbitals, generate the molecular orbitals for hexa-1,3,5-triene .Identify the number of nodes characterising each molecular orbital. With reference to both a mirror plane (m) and a two-fold axis, designate the orbitals as symmetric (S) or antisymmetric (

52、A). Using arrows to represent electrons, associate the six p-electrons with the appropriate molecular orbitals of hexa-1,3,5-triene in its ground state. Finally, identify the HOMO and LUMO.Question 6: MO SystemProtonation of A affords B. Draw the three resonance structures of B in which the positive

53、 charge has formally been shifted from the oxygen atom onto three of the five carbon atoms.Considering only these three resonance structures, how many (i) carbon atoms are involved in the hybrid structure, (ii) carbon p-orbitals are there, (iii) p-electrons are associated with the carbon atoms, and

54、(iv) molecular orbitals are associated with the combination of these carbon p-orbitals In an analogous fashion to how question 1 was set out, draw out the molecular orbitals resulting from the p-orbital combination on this carbon framework, making sure you identify all of the items listed in questio

55、n 1. Answer 6: 5p MO SystemProtonation of A affords B. Draw the three resonance structures of B in which the positive charge has formally been shifted from the oxygen atom onto three of the five carbon atoms.Considering only these three resonance structures, how many (i) carbon atoms are involved in

56、 the hybrid structure, (ii) carbon p-orbitals are there, (iii) p-electrons are associated with the carbon atoms, and (iv) molecular orbitals are associated with the combination of these carbon p-orbitals In an anologous fashion to how question 5 was set out, draw out the molecular orbitals resulting

57、 from the p-orbital combination on this carbon framework, making sure you identify all of the items listed in question 5. Second Year Organic Chemistry CourseCHM2C3BFrontier Molecular Orbitals and Pericyclic ReactionsPart 1(iii):HOMO and LUMO CombinationWhat is the Driving Force for Controlling Peri

58、cyclic Reactions?The driving force which controls the product outcome in pericyclic reactions is the in phase combination of the FMOs (the HOMO and LUMO) of the reacting species in the transition state.FMO Theory is Extremely Powerful.Pericyclic Reactions Involve Conjugated Polyene SystemsPericyclic

59、 reactions involve conjugated polyene systems.Enes and Polyenes are made by the linear combination of p-AOs.Thus, we first need to construct the molecular orbitals of polyenes.Then we need to identify the Frontier Molecular Orbitals.Finally, we will need to construct the correct geometry for orbital

60、 overlap of the FMOs in the transition states of the reactions.In bimolecular reactions (like the SN2 and the Diels-Alder reaction), interaction between the two molecular components is represented by interaction between suitable molecular orbitals of each. The extent of the interaction depends upon