Conjugation & Resonance Are Different Ways of Describing the SAME Electron Delocalization in Pisystems! resonance contributors

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1 onjugated Systems Return to M.. Theory 1 onjugated Systems and Resonance Requirements for onjugation: Same as resonance! Parallel array of atomic p orbitals on adjacent atoms (adjacent sp2 or sp hybridized atoms) allene conjugated non-conjugated NT parallel NT conjugated cumulated The two pi-bonds in allene (the lower structure above) are NT conjugated because the formal p atomic orbitals do not overlap across both double bonds, the 2 p A..s on the central carbon are at RIGT ANGLES to each other, this situation is called cumulative Significance of onjugation Allows reactions to be explained which cannot using simple Lewis structures, since under these conditions simple two-atom Lewis bonding does not occur, pi-bonding can occur over the entire length of the conjugated system, bonds including multiple atoms onjugation & Resonance Are ifferent ays of escribing the SAME Electron elocalization in Pisystems! sp2 sp2 sp2 conjugated, p orbitals parallel and adjacent = sp2 sp2 sp2 sp2 sp2 sp2 resonance contributors ~1/2 ~1/2 "actual" structure All conjugated systems involve resonance delocalization, and vice versa; you can draw resonance contributors for any conjugated system ur discussion of conjugated systems is simply a more detailed and complete look at systems that have resonance, their underlying structural properties, and their chemical reactions, AN a look at some reactions that can NT be explained using only the resonance model (remember chemical models!) Examples of onjugated Systems: the allyl radical: BE (kcal/mol) ~98 ~ allylic, more stable than a 3 radical lower BE means higher radical stability onjugated Systems : Page 1 ~93 ~86 lowest BE

2 RESNANE ELALIZATIN in the NJUGATE SYSTEM stabilizes the radical due to delocalization of electrons (the electrons see more nuclei) Example: dienes starts lower in energy non-conjugated conjugated catalyst all the same - all pentane reaction (kcal/mol) the LESS ETERMI the reaction, the MRE STABLE the diene (since it starts lower in energy) the conjugated diene has overall lower energy electrons due to delocalization compared to the non-conjugated diene (-30 x 2) -53 addition 2 Reactions of Allyl Systems : onjugated Intermediates l allyllic position 3 allyl group = 2 allyl chloride allyl methyl ether Allyl systems often react via conjugated (resonance stabilized) intermediates Allylic Substitution via S N 2 (Review, seen before!) Recall: S N 2 reactivity: 3 < 2 < 1 < allylic (fastest of all, because the transition state is conjugated!) R Nu Nu Nu Nu R = = primary allylic conjugation (resonance) stabilizes the electrons in the transition state for SN2 reactions at an ALLYLI carbon. ne onsequence conjugated slow reaction, Grignard not a R' Mg R' strong enough nucleophile primary halide Grignard R' Mg R' new - bond, important! but!! allylic halide So this is a NE REATIN, Grignard reagents will do SN2 reactions at allylic carbons to displace good leaving groups, often bromide, and make new - bonds, but NLY at allylic carbons Example NBS hν PhMg Ph onjugated Systems : Page 2

3 omination via Allyl Radicals (Mainly Review!) NBS hν unexpected here does this "unexpected product come from? The answer is in the mechanism, and the resonance contributors to the essential radical intermediate Partial chanism: explains why T products are actually formed remember the resonance contributors are telling us what the real structure of the radical is, i.e. that the nonbonding electron is not localized on a single carbon atom, but that it is delocalized over two carbon atoms, the two atoms where the substitution takes place, which is why we get two products the resonance contributors are not two different things, but the same thing drawn two different ways the "other" product was always formed in allylic brominations, until now we have always tried to simply give examples where reaction at the two carbons with radical electron density gave the same product Substitution via S N 1 (Mainly Review) Recall LB/BB LA/BA 1 carbon SN2 2 substitution occurs at the PRIMARY carbon by an SN2 mechanism owever: unexpected chanism: LB/BB LA/BA allylic carbon SN1 2 LA LB LA LB recall, protonation of an - group converts it from a poor to a good leaving group in this case, substitution occurs via an SN1 mechanism at the ALLYLI carbon (bromide is a weak nucleophile and a resonance stabilized (conjugated) cation intermediate can be formed via SN1) onjugated Systems : Page 3

4 Another Example: SN1 eat minor major 3 These are classic SN1 reaction conditions, an allylic bromide, a polar protic solvent, a weak nucleophile, heat Q. hich is the MAJR and which is the MINR product? A. This depends upon the conditions, i.e. whether the reaction is under kinetic or thermodynamic control 3 Reactions of ienes : Kinetic versus Thermodynamic ontrol Addition of : recall Analogously: major >50% minor <50% faster slower 1,2 addition product at -80 Major product Minor product but at 40 Minor product Major product 1,4 addition product The charge is not evenly distributed in the resonance stabilized cation intermediate, shown above are two IFFERENT ways of indicating the differing amounts of positive charge on the carbons, either in terms of the "actual" resonance hybrid cation structure, or as an unequal resonance mixture (both models say the same thing) Reaction is SLER at the primary carbon, there is less positive charge there, but reaction there forms the more substituted (more stable) alkene isomeric product Reaction is FASTER at the secondary carbon, there is more positive charge there, but reaction there forms the less substituted (less stable) alkene isomeric product YU AN MAKE G ARGUMENTS FR BT F TE PRUTS, a KINETI argument says that the 1,2-product is formed faster and should be the major product, a TERMYNAMI argument says that the 1,4- product is more stable and should be the major product = 1 less charge <1/2 >1/2 2 more charge "actual" resonance hybrid onjugated Systems : Page 4

5 Another Example: major minor >50% <50% faster slower = 1 less charge <1/2 >1/2 2 more charge at % at % formed slower formed faster: kinetic product less substituted (stable) more stable thermodynamic product NTE, a bromonium ion intermediate is NT formed here, as it would have been for 2 addition to a simple alkene, a more stable, resonance stabilized allyl cation intermediate is formed instead as in the previous example, the FASTER reaction at the carbon that has a larger partial positive charge forms the LESS STABLE 1,2-addition product, the KINETI product, and the SLER reaction carbon that has a larger partial positive charge forms the LESS STABLE 1,2-addition product and the SLER reaction at the carbon that has a smaller partial positive charge forms the MRE STABLE 1,4-addition product, the TERMYNAMI product hat etermines Kinetic and Thermodynamic ontrol of Products? This is best explained in terms of a reaction energy diagram Energy δ δ 1,2 E a 1,4 E a both reactions AN reverse at high temperature 2 1,2-adduct, smaller activation energy, faster return reaction coordinate 1,4-adduct, larger activation energy, slower return At all temperatures, the less stable 1,2-adduct is formed FASTER from the primary resonance stabilized intermediate At low temperatures, the initially formed 1,2-product and 1,4-products "stay there", reverse reaction is not possible, there is not enough thermal energy to return (the reactions are IRREVERSIBLE), more 1,2-product is formed because this product is formed faster At high temperatures, the 1,2-product and the 1,4-products can return to the intermediate (REVERSIBLE), although return of the 1,2-product is faster (requires less energy) and return of the 1,4-product is slower (requires more energy) Eventually the more stable 1,4-product accumulates and becomes the major product onjugated Systems : Page 5

6 when the reactions are IRREVERSIBLE, the entire process is under KINETI NTRL, i.e. IRREVERSIBLE at lower temps major product formed faster when the reactions are REVERSIBLE, the entire process is under TERMYNAMI NTRL, i.e. major product more stable REVERSIBLE at higher temps MST RGANI REATINS ARE UNER KINETI NTRL, EAMPLES F TERMYNAMI NTRL ARE RARE An example in reactions of alkenes: thermodynamically controlled product kinetically controlled product 4 iels-alder Reaction : ycloaddition = pi-donating group; = pi-withdrawing group donating NT Lewis acid/base reaction cyclic loop of electrons in 2 withdrawing 2 2 iene ienophile 4 electrons 2 electrons cycloaddition This is an Addition reaction that makes two new sigma-bonds and a ring: a cycloaddition reaction All bonds made and broken at the same time: A oncerted reaction (like SN2 and E2) 4 Electrons from diene and 2 electrons from dienophile, thus 4 2 cycloaddition reaction TIS IS NT A LEIS AI/BASE ELETRPILE/NULEPILE REATIN, both reactants provide a pair of electrons to make a new bond to each other, AN, the reaction has an interesting "electrons in a loop" transitions state that is important The reaction goes faster (has a lower activation energy) with an electron donating group (-) on the diene and an electron withdrawing group (-) on the dienophile relative energy () E () a () groups make reaction faster, lower E a onjugated Systems : Page 6 E a Exothermic Reaction- breaks 2 π-bonds makes 2 σ-bonds () reaction coordinate

7 REMINER of Electron onating and ithdrawing Groups on pi-systems Recall: donating and withdrawing ability of substituents is measured relative to hydrogen increasing electron withdrawing ability increasing electron donating ability F 3 N 3 S 2 N R 3 N strong R R R 2 N moderate N 2 R N R NR 2 Ar 2 R R weak moderate strong Remember, distinguishing the donating and withdrawing groups is fairly easy - N morization! ith the exception of the simple alkyl and aryl substituents, all of the donating groups all have non-bonding electrons on the atom that is attached to the pi-system Just about everything else is a withdrawing substituent due to the inductive effect iels-alder reactions are increasingly faster with increasingly stronger donating groups () on the diene iels-alder reactions are increasingly faster with increasingly stronger withdrawing groups () on the dienophile Some More on onating Groups: N weaker donor because of adjacent = bond strong even stronger N less electronegative increasing donating ability Non-bonding electrons on the less electronegative element N are higher in energy and more "available" for resonance donation than those on oxygen Non-bonding electrons that are resonance delocalized into other pi-systems, e.g. part of an ester, above left, are lower in energy and less "available" for resonance donation into a diene The iene needs to be in the correct conformation to react: "s-trans", more stable (heat) F 3 F 3 "s-cis", less stable "s-cis" and "s-trans" refer to the different NFRMERS corresponding to rotation around the ENTRAL - bond, they are related to cis-/trans- isomers of alkenes, but the restricted rotation is around a single bond the iels-alder reaction needs an "s-cis" conformation of the diene for reaction reaction is ALS STERESPEIFI, it is concerted, and generally needs (some) heat onjugated Systems : Page 7

8 Explanation for the Stereochemistry: oncerted reactions are often STERESPEIFI!! Recall: the following NERTE reactions (all bonds broken and made at the same time) SN2 (S) Et t-bu E2 Et (R)- (cis)- NLY! The stereochemistry of the iels-alder reaction is centered around the substituents, specifically.. "outside" "outside" group F 3 "inside" both dashed F 3 both wedged F 3 (pair of enantiomers) both the "outside" substituents on the diene end on the same side in the cyclohexene product (i.e. both have wedged or dashed bonds) both "inside" groups on the diene end on the opposite side to the "outside" groups on the diene The groups on the dienophile end on the same side as the "outside" groups on the diene, the EN RULE The oncerted Reaction: the p A..s at the end of the diene and the dienophile have to transform into sp3 atomic orbitals as the new sigma-bonds are formed, the substituents on these atoms will "move" in response to the change in hybridization This is illustrated below for the case of dienophile addition from the "bottom" substituents on diene go "up" substituents on dienophile go "down" new σ-bond sp2 sp2 sp3 new σ-bond product in boat-type conformation = "outside" = "inside" new σ-bond down down "relaxation" to chair-type conformation down down down down onjugated Systems : Page 8

9 both new bonds made at the same time on same side of the diene (the bottom in this example), the reaction is thus SUPRAFAIAL with respect to the diene both new bonds made at the same time on same side of the dieneophile (the top in this example), the reaction is ALS SUPRAFAIAL with respect to the dieneophile the iels-alder reaction is a 4(s) 2(s) cycloaddition, where (s) refers to suprafacial Suprafacial Reaction: efinition Both bonds are made to the same "face" of the reactant, i.e. for a flat pi-system this means both bonds made to the bottom face or both bonds made to the top face suprafacial on the diene AN both bonds to bottom of diene suprafacial on the dienophile SYN- SYN- both bonds to top of the dienophile 4(s) 2(s) cycloaddition reaction suprafacial same as SYN-addition If a reaction is SUPRAFAIAL on one of the reactants, then this is the same as SYN-AITIN on that reactant rigin of the Endo Rule EN means "in", as opposed to E which means "out" The dienophile can approach the diene in one of two different ways, i.e. with the withdrawing groups "in", towards the diene (EN), or with the withdrawing group "out", away from the diene (E). The EN reaction is shown below, you can see that this case the withdrawing group will either be below or above the diene. F 3 F3 -group EN (in) lower energy F 3 F 3 enantiomers The EN approach of the dienophile usually leads to the lower energy transition state because the electronegative elements in the -group can stabilize the relatively high energy electrons in the diene, this is called a secondary orbital interaction. The EN approach results in the - group and the substituents in the "outside" of the diene (both - in the example above) being on the same side of the cyclohexene ring in the product, i.er. these bonds are both dashed (when reaction occurs from below) or they are both wedged if reaction occurs from above. In the alternative E approaches, the -group is "out" with respect to the diene (it points "away" from the diene, and the electronegative elements in the - group cannot stabilize the higher energy electrons in the diene. Reaction E would proceed via a higher energy transition state, I.e., slower. F 3 F 3 -group E, "out" or away from the diene IGER energy F 3 F 3 onjugated Systems : Page 9

10 Visualize Stereochemical onsequences of the NERTE iels-alder Reaction Examples: on dienophile outside cyclopentadiene outside N N cis- N N N cis- N (meso) FUR chiral/asymmetric centers are generated, but the structure has a mirror plane of symmetry, thus it is an achiral meso compound FUR chiral/asymmetric centers are generated, the structure has N mirror plane of symmetry, it is thus formed as a racemic mixture Examples: on diene outside N N cyclopentadiene outside N trans- N trans- N N both outside (meso) same side FUR chiral/asymmetric centers are generated, but the structure has a mirror plane of symmetry, thus it is an achiral meso compound outside inside FUR chiral/asymmetric centers are generated, the structure has N mirror plane of symmetry, it is thus formed as a racemic mixture onjugated Systems : Page different sides

11 Mixed Examples 3 N N i.e. rientation of addition: the Regiochemistry The reaction is also REGISPEIFI (one structural isomer is favored over others) 2 N 2 2 N N 1 3 1,3 product 4 not formed 1,4 product MAJR There is only NE chiral center formed (), and so there is not need to draw the bond as wedged or dashed, in fact doing so would have no meaning in this case, we don't need to specify that this bond is wedged or dashed with respect to any other bond in the product structure, therefore this bond should be drawn as PLAIN, with the racemic mixture symbol to show that we made two enantiomers the 1,4-product is formed NT the 1,3-product, these are structural isomers and shows that the reaction has REGISPEIFIITY - Remember, the way that we draw the molecules on paper isn t necessarily the way that they are when undergoing a reaction, don t be fooled by the way the structures are drawn on paper! N 2 N 2 N 2 1,3 product not formed 1,2 product MAJR these reactions can be explained using the following MINR RESNANE NTRIBUTRS. Formation of the 1,3-product is always disfavored due to electron repulsion Let's look at this for GENERI donating (-) and withdrawing (-) groups 1,2-Addition: product formed faster, MAJR organic product δ δ δ δ 1,2 product lower energy transition state FASTER REATIN onjugated Systems : Page 11

12 1,3-Addition: product formed slower, minor organic product δ δ δ δ 1,3 product electron repulsion higher energy transition state SLER REATIN Example: with actual donating and withdrawing groups N 2 N 2 1,2 product N 2 Example Reactions with stereo- and regiochemistry: ,4 1,3 NE chiral center, no wedged/dashed bonds (only ±), wedged/dashed bonds have no meaning if a racemic mixture is specified, AN, 1,4-product is major, not the 1,3-product 1,2 1,3 T chiral centers, need BT dashed bonds to illustrate the relative stereochemistry of the - and the - group, AN the racemic mixture symbol to indicate the other enantiomer, 1,2-product is major, not 1,3-product onjugated Systems : Page 12

13 = (meso) 2 chiral centers in the product (), but the product has a mirror plane of symmetry, it is a meso compound stronger weaker N 2 N N 2 N is 1,2 to the STRNGEST N 2 N the product that has the - group 1,2- with respect to the strongest donating group is major 5 pi-molecular rbitals : Pericyclic Reactions Molecular orbitals - here ARE the electrons in a molecule? build molecular orbitals by combining appropriate atomic orbitals build pi-molecular orbitals by combining p atomic orbitals the number of molecular orbitals we must make is equal to the number of p atomic orbitals that we combine First, we are concerned with linear systems, cyclic systems come later Need to (briefly) revisit concept of orbital wavefunction: example of a p-a.. Ψ node at change of sign (zero value) shown above is a graphical representation of waveform of an atomic p orbital the waveform is a mathematical function which evaluates to positive value, negative value, or zero only the relative sign of the wavefunction is important ( or ), we distinguish using shading (black/white) Revisit concept of bonding and making Molecular rbitals by ombining Atomic rbitals For the case of two p atomic orbitals (As) on two adjacent carbon atoms, overlap of the orbital wavefunctions results in simultaneous constructive (in-phase) AN destructive (out of phase) interference of the wavefunctions to generate two new wavefunctions overlap of the wavefunctions of atomic orbitals of same sign (in-phase, constructive interference) generates a new bonding wavefunction, the new binding molecular orbital the new bonding wavefunction builds electron density between the nuclei, allows electrons to "see" both nuclei an electron in the new bonding wavefunction is lower in energy compared to being in the original atomic orbital bonding combine the Ψ two p As this way makes horizontal node Ψ for new bonding M.. the wave nature of orbital wavefunctions means that if the atomic wavefunctions can overlap in-phase (as above), then they must simultaneously overlap out of phase (whether we like it or not(!), shown below) which onjugated Systems : Page 13

14 results in ESTRUTIVE interference of the atomic orbital wavefunctions between the atoms, which generates a new ANTI-bonding wavefunction the new anti-bonding wavefunction eliminates electron density between the nuclei an electron in the new anti-bonding wavefunction would be IGER in energy compared to being in the original atomic orbital anti-bonding combine the Ψ two p As this way 5.1 Rules for onstructing Molecular rbitals You will not memorize these rules, you will learn them very quickly by using them 1) The number of M..'s = the number of A..'s, BEAUSE, orbitals can't "disappear" 2) 2 electrons per M.., BEAUSE, of the Pauli principle 3) Lowest energy M.. has 0 vertical nodes, BEAUSE, it is most "bonding" 4) Increasing vertical nodes means increasing energy, BEAUSE, they become less "bonding" 5) Symmetry of nodes determined by molecular symmetry, BEAUSE, they are standing waves 5.2 pi-molecular rbitals of Ethylene makes vertical node horizontal node Ψ for new anti-bonding M.. Energy 1 vertical Node (2 nodes total) antibonding π 2 π -bonding M.. horizontal node π 2 nonbonding energy level p A.. on carbon magnitude of Ψ p A.. on carbon π 1 horizontal node electrons bonding π-bonding M.. graphical representation of the Ms π 1 accurate computed Ψ for the Ms 1) 2 A..'s, therefore, 2 M..'s must be formed when they combine (pi 1 and pi 2) 2) 2 Electrons (max.) per M electrons in bonding pi 1 M.. 3) pi 1 has N VERTIAL nodes 4) pi 2 has NE VERTIAL node (wavefunction changes sign, goes through zero) 5) Node position determined by molecular symmetry (in the middle!), wavefunctions are STANING AVES the p-a..'s do not actually exist in the molecule, the M..s are drawn using a "cartoon" or "digital" approximate p A.. method because it is too hard to draw the actual orbitals accurately The wavefunctions really look like waves! 5.3 pi-molecular rbitals of Butadiene 4 atomic p orbitals 4 π electrons here we consider the PI-MLEULAR RBITALS NLY (ignore the sigma-ms) there are 4 p A..'s, therefore 4 pi-m..'s must be formed when they combine, pi 1, pi 2, pi 3, pi 4 2 electrons in each M.. (pi 1, pi 2) Lowest, pi 1, has no vertical nodes (all have one horizontal node) Increasing vertical nodes = Increasing energy onjugated Systems : Page 14

15 Nodes are symmetrical because the wavefunctions of the M..s describe STANING AVES vertical nodes π 4 π 4 3 LUM1 Energy π 3 π 3 2 LUM non-bonding energy level "frontier" M..'s π 2 π 2 1 M magnitude 4 electrons π 1 of Ψ π 1 0 M-1 pictures of "actual" wavefunctions of the orbitals M = ighest ccupied Molecular rbital representations of Ψ LUM = Lowest Unoccupied Molecular rbital The avefunction Squared give the probability of finding the electrons in the orbitals = = Ψ 2 for π2 M.., gives probability of finding the electrons, hard to draw! "representation" of Ψ for π2 M.. using A..'s what E draw accurate computed Ψ for π2 M.. too hard to draw! electrons are distributed over all 4 carbon atoms, but never found at a node (horizontal in this case). 5.4 pi-molecular rbitals of The Allyl System The allyl system describes the simplest resonance stabilized ion and radical intermediates in organic chemistry, shown below are the simplest allyl anion, allyl cation and allyl radical, each are conjugated systems, each have 3 carbon atoms involved in the conjugated system (resonance), but they have different numbers of electrons associated with the conjugated system (involved in resonance). allyl cation allyl anion allyl radical 3 A..'s 2 electrons 3 A..'s 4 electrons 3 A..'s 3 electrons onjugated Systems : Page 15

16 node on central carbon atom Energy Vertical Nodes allyl cation allyl anion allyl radical π 3 2 LUM nonbonding energy level magnitude of Ψ π 2 π LUM M M SM The avefunction Squared give the probability of finding the electrons in the M of the allyl anion = = Ψ 2 for π2 M.., gives probability of finding the electrons, hard to draw! "representation" of Ψ for π2 M.. using A..'s what E draw accurate computed Ψ for π2 M.. too hard to draw! 5.5 Relative Energies of Pi-Molecular rbitals Energy "most" Anti-bonding Anti-bonding M..s non-bonding energy level non-bonding M. Bonding M..s "most" bonding, most delocalized Example problem: hat is the M & LUM of the pentadienyl anion? pentadiene Base 1. ow many p A..'s do we have to work with? Answer = five 2. ow many M..'s will we have? Answer = five 3. ow many electrons do we have? Answer = six onjugated Systems : Page 16

17 4. ow many nodes will the M have? Answer = two 5. ow many nodes will the LUM have? Answer = three Energy π M.. 5 π M.. 4 nonbonding energy level π M.. 3 LUM M π M.. 2 π M.. 1 representations of Ψ of M.. using A..'s what E draw 6 Frontier Molecular rbital Theory : Pericyclic Reactions All pericylic reactions are NERTE and involve cyclic transition states and we need M..'s to understand There are several kinds of pericyclic reaction, we look at only two: 1. ycloadditions 2. Electrocyclic reactions There are three different theories for pericyclic reactions, we will look at two of them: 1. Frontier Molecular rbitals (Today) 2. Aromatic Transition State (Later) The other theory is conservation of orbital symmetry, it is more complete the other two, but is beyond the scope of a General rganic hemistry course FM Theory: (Approximate, but works!) fundamental Principle for Understanding Pericyclic Reactions... donated electrons come from the highest energy occupied orbital, i.e. M electrons are accepted into the lowest energy unoccupied orbital, i.e. LUM LUM LUM E M diene E M electrons flow from the Ms to the LUMs the smaller the M/LUM energy gap, E the faster the reaction dienophile onjugated Systems : Page 17

18 The smaller the energy gap between where the electrons come from (M) and are going to (LUM), the faster the reaction A donating substituent on the diene raises the energy of the electrons, specifically in the M A withdrawing substituent on the dienophile lowers the energy of electrons, specifically in the LUM ompare: M/LUM Energy Gap in iels-alder reaction with the addition of - and - Substituents M LUM E larger E, larger Ea slower rate M - raises M energy electron flow from M to LUM is determined by the M/LUM energy "gap", elta E there is a smaller M/LUM energy gap with - and - substituents, elta-e becomes smaller, faster reaction FM thery explains the substituents effects on the rate of the iels-alder reaction Revisit iels-alder Reaction ycloaddition Reaction E LUM - lowers LUM energy smaller E, smaller Ea slower rate diene () 4 electrons suprafacial dienophile () 2 electrons suprafacial 4(s) 2(s) cycloaddition when atomic wavefunctions overlap in phase - they make a bond when molecular wavefunctions overlap in phase - they make a bond! we now ask the following question: can overlap the molecular orbitals of BT REATANTS, SUPRAFAIAL/SUPRAFAIAL occur IN-PASE and therefore make BT BNS at the same time? If we can then the 4(s) 2(s) NERTE REATIN is allowed FIRST: consider suprafacial/suprafacial overlap of the M of the diene the LUM of the dienophile Recall: SUPRAFAIAL IS TE SAME AS SYN-AITIN Ψ of M of diene suprafacial = SYN wavefunctions in phase, makes a bond! wavefunctions in phase, makes a bond! suprafacial = SYN Ψ of LUM of dieneophile M π4 π3 π2 π1 diene vertical nodes 1 0 alkene π π LUM verlap of the M of the diene SUPRAFAIAL and the LUM of the dienophile SUPRAFAIAL results in IN-PASE and BNING interactions to make BT of the new sigma-bonds at the same time, Therefore, a concerted iels-alder reaction 4(s) 2(s) cycloaddition IS ALLE, according to F.M.. theory onjugated Systems : Page 18

19 N: onsider reaction "other way round", M of dienophile & LUM of diene Ψ of LUM of diene node wavefunctions in phase, makes a bond! node wavefunctions in phase, makes a bond! Ψ of M of dieneophile LUM π4 π3 π2 π1 diene vertical nodes 1 0 alkene π π M verlap of the LUM of the diene SUPRAFAIAL and the M of the dienophile SUPRAFAIAL ALS results in IN-PASE and BNING interactions to make BT of the new sigma-bonds at the same time, Therefore, the concerted iels-alder reaction 4(s) 2(s) cycloaddition IS ALLE according to F.M.. theory using either analysis, i.e. M/LUM R LUM/M hen considering whether a concerted cyclodaddtion reaction is allowed or not it doesn't matter which M/LUM combination we consider hat About a 2(s) 2(s) ycloaddition Reaction? 2 electrons Y 2 electrons Z Y Z??? does this work too? verlap M and LUM of the two alkenes SUPRAFAIAL/SUPRAFAIAL (add to both sides of both molecules at the same time) vertical nodes 1 0 alkene π LUM π M wavefunctions UT F PASE, anti-bn Ψ alkene LUM Y Z Ψ alkene M wavefunctions in phase, bonding! Z Y Ψ alkene M wavefunctions UT F PASE, anti-bn Ψ alkene LUM This results in NE ANTI-BNING interactions for both M/LUM combinations A suprafacial/suprafacial 2(s) 2(s) cycloaddition reaction is therefore FRBIEN according to F.M.. theory Example: a different 4 2 cycloaddition F 3 4 electrons 2 electrons allowed F 3 allowed or forbidden? 4(s) 2(s) cycloaddition onjugated Systems : Page 19

20 F 3 1 node Ψ of M for allyl anion ALLE!! when this reaction is SUPRAFAIAL/SUPRAFAIAL, 4(s) 2(s), then reaction is allowed NLY suprafacial/suprafacial reaction is possible in this case because the cyclic system is not large enough to have bonding occurring any other way, so we find that the possible reaction is allowed Example: 6 2 cycloaddition reaction suprafacial for allyl anion wavefunctions in phase, makes two bonds! Ψ of M for alkene suprafacial for alkene LUM M π3 π2 π1 allyl anion vertical nodes N N electrons per arrow! what is the product??? Ψ of M node node Ψ of LUM suprafacial for triene N N N N suprafacial/suprafacial suprafacial for gives cis-product alkene FRBIEN!! π6 π5 π4 π3 π2 π nodes M 6(s) 2(s) cycloaddition reaction is forbidden Ψ of M suprafacial (SYN-) for triene Ψ of LUM N N SUPRAFAIAL/ANTARAFAIAL to give the trans-product N ALLE!! ANTARAfacial (ANTI-) bonds made on PPSITE face N ANTARAFAIAL IS TE PPSITE F SUPRAFAIAL - it means making the two bonds to PPSITE side 6(s) 2(a) cycloaddition reaction is allowed (suprafacial/antarafacial) a (6(a) 2(s) cycloaddition would also be allowed (antarafacial/suprafacial) antarafacial cycloadditions are possible when there are enough atoms involved in the cycloaddition, usually at least seven or eight atoms are required (above there were eight), and in fact, (s) (a) cycloadditions are rare! onjugated Systems : Page 20

21 Electrocyclic Reactions trans- is only isomer! BUT e need to consider T ways that the ring can be closed: NRTATRY: the sigma-bonds and orbitals rotate in the SAME direction (in NERT) ISRTATRY: the sigma-bonds and orbitals rotate in PPSITE directions 3 Example: Show how you would determine the product of the reaction below using F.M.. theory 3 cis- is only isomer! 3 product In this case the reaction is UNIMLEULAR (not bimolecular), electron flow is not from one molecule to another, we are concerned with the highest energy (most reactive) electrons in the starting structure only, these are contained in the ighest ccupied Molecular rbital, the M NRTATRY R ISRTATRY M 3 3 rotate in same direction 3 M 3 rotate in opposite directions 3 bonding interaction antibonding interaction new σ bond in ALLE trans-product antibond, cisproduct is FRBIEN As the ring closes, the important orbitals are those at the end of the conjugated system, they transform from formal p atomic orbitals into sp3 hybridized atomic orbitals, and at the same time they rotate 90 degrees so that they are pointing towards each other so that they can overlap and make the new sigma-bond. onrotatory ring closure results in a bonding (in-phase) interaction between these orbitals, both in the transition state and in the final product, conrotatory ring closure is ALLE isrotatory ring closure results in anti-bonding (out of phase) orbital overlap, disrotatory ring closure results in a higher energy transition state, this reaction is FRBIEN in the language of F.M.. theory onjugated Systems : Page 21

22 isrotatory and onrotatory Electrocyclic Reactions using FM Theory Electrocyclic Reactions using FM Theory and Fingers and Thumbs Examples outside outside which isomer, cis- or trans-?? up 3 outside Ψ for M 3 disrotatory closure leads to up bonding interaction in the 3 outside transition state outside Ψ for M same as above (meso) 3 3 inside disrotatory ring closure allowed onjugated Systems : Page 22 cis-product is ALLE trans-product is allowed

23 outside Ψ for M same as above F 3 outside the enantiomer is the product that is obtained by doing disrotatory ring closure in the "other direction", which results in the product with the upper 3 wedged and the lower one dashed disrotatory ring closure allowed F 3 trans-product is allowed hat we just did is AMAZING (you should be amazed!) mind = blown these reactions can NLY be explained and understood by taking the AVE PRPERTIES of the electrons in the molecular orbitals into account understanding the wave nature of electrons is difficult, but without this there is no understanding of pericyclic reactions The wavefunctions AVE to overlap in phase in order to get the reactions to go, at least as the concerted reactions discussed here These reactions represent QUANTUM MEANIS in RGANI EMISTRY the quantum/wave nature of electrons is difficult to understand, BUT: Explain the presence of orbitals any other way! Explain pericyclic reactions any other way! onjugated Systems : Page 23

24 8 Summary of onjugated Reaction Types NBS, hν PhMg Ph 3 3 heat thermodynamic product kinetic product 3 kinetic product thermodynamic product The iels-alder reaction is stereochemical and regiochemical controlled N N N N N N N N F 3 F F 3 F 3 onjugated Systems : Page 24