Orbital Symmetry and Pericyclic Reactions

Transcription

1 Paul Bracher Chem 30 ectin 12 rbital ymmetry and Pericyclic eactins ectin genda 1) Last. ectin. Ever. 2) andut: rbital ymmetry and Pericyclic eactins 3) ffice urs: Thursday, 12/ 16, 3-4PM and 8-9PM, Bauer Lbby (P#8 due Friday) 4) Exam III n Mnday, 12/20. N sectin that afternn. 5) tay tuned fr extra ffice hurs and review sessins in reading perid. Empty pace kill rill Cmplete the fllwing reactins: dis cn hν (racemic) (racemic) dis cn hν This cartn character s jb is t fill up empty handut space 1

2 Building Mlecular rbitals (Ms) We are abut t embark n a jurney thrugh the wrld f pericyclic reactins. Yu are already familiar with ne f these reactins: the iels-lder cycladditin. In all f these reactins, it is f the utmst imprtance that yu knw hw t draw the Ms f π systems. imple ules tmic rbitals must verlap if they are t interact t frm a set f mlecular rbitals. There are n exceptins rthgnal rbitals will nt interact. Yu will get n mlecular rbitals (Ms) fr every n atmic rbitals (s) that interact. The mst stable Ms have the mst bnding character. The highest energy Ms have the mst antibnding character. The energy f an M is directly prprtinal t the number f ndes. Thus, as the number f ndes increases, the antibnding character increases and the rbital is less stable. The electrn density at an atm is equal t the square f the M s cefficient at that atm. Lcatins with bigger cefficients are depicted by drawing bigger rbital lbes. The and designatins, als shwn as green and blue, have nthing t d with charge. They are designatins fr the sign f that rbital s wavefunctin. When rbitals f the same wavesign interact, there is cnstructive interference resulting in a bnding interactin. When rbitals f the ppsite wavesign interact, there is destructive interference resulting in an antibnding interactin. Ndes are lcatins in the system where the bnding interactins are cmpletely cancelled by antibnding interactins. Mlecular rbitals have zer electrn density at ndes. eactins will nt ccur at these lcatins. Mlecular rbitals fr equivalent atms are symmetric. Nte hw the Ms belw either have a mirrr plane f symmetry (a symmetric M = ) r a C 2 axis f symmetry (an antisymmetric M = ). Nte patterns in M diagrams t help yu draw them and t figure ut shrtcuts in prblem slving In the lwest energy M, all f the wavesigns are the same In the highest energy M, all f the wavesigns alternate The termini alternate between having the same wavesign and ppsite wavesigns The number f ndes increases by ne as yu climb higher in energy Fr linear systems with an dd number f s, sme ndes will lie at atms in the chain Electrnic Perturbatins f Mlecular rbitals Placing electrn dnating grups (EGs) r electrn withdrawing grups (EWGs) n a π system will shift r perturb the relative energies f the mlecular rbitals. EGs will push mre electrn density int the π system and will crrespndingly raise the energies f the mlecular rbitals EWGs will pull electrn density ut f the π system and will crrespndingly lwer the energies f the mlecular rbitals Lewis acids that cmplex with extended π systems als lwer the energies f the Ms. These metals typically serve as catalysts by lwering the energy f a reactive LUM such that it is mre accessible. etermining which atms will becme enriched r depleted in electrn density can be accmplished by using an electrn pushing mdel. 2

3 Graphical epictins f Mlecular rbitals Linear ystems Cyclic ystems Frst Circles Frst circles are the fastest methd t arrive at the mlecular rbitals fr cnjugated cyclic systems. Yu simply inscribe the plygn in a circle vertex dwn, such that ne f the crners tuches the bttm f the circle. Each vertex f the plygn represents the energy level f a mlecular rbital. Fr Chem 30, yu need nly wrry abut the relative psitin f the Ms and nt the quantitative energy difference between the Ms. wever, the reasn Frst circles are s pwerful is that they can be used t get these energies. 3

4 Pericyclic eactins Pericyclic reactins have cyclic transitin states with electrn flw that is gverned by the rules f rbital symmetry. Classificatin f Pericyclic eactins 1) Electrcyclic reactins invlve the clsure f linear π systems t rings and the reverse reactin, the pening f these rings. Yu must be able t classify an electrcyclic reactin by # f electrns, as cnrtatry/disrtatry, and as phtchemical/thermal 2) Cycladditins are reactins where tw π systems react t frm cyclic/ringed cmpunds Yu must be able t classify a cycladditin reactin by [m n] and as phtchemical/thermal. 3) igmatrpic rearrangements invlve the mvement f a σ bnd acrss a π system Yu must be able t classify a sigmatrpic reactin by [m,n] and phtchemical/thermal These reactins invlve making and breaking r shifting σ and π bnds. The key shared element is that in rder fr these reactins t ccur, the wavesigns f the mlecular rbitals have t match. When they d, the reactin is said t be symmetry allwed. When nt, symmetry frbidden. raw all f the reactins as ccurring in cyclic cnfrmatins when yu are drawing arrw pushing mechanisms. These reactins are called pericyclic fr a reasn. Beware f the reverse, retr reactins. If the frward reactin is symmetry allwed, then s is the reverse reactin. Beware f tandem reactins, where tw r mre pericyclic reactins ccur in ne pt. When yu irradiate a mixture, bth phtchemical and thermal prcesses can ccur. 4

5 Electrcyclic eactins In an electrcyclic reactin, a linear π system is clsed when a π bnd is exchanged fr a σ bnd. The reverse reactin is als electrcyclic. The simplest apprach is t analyze the frward ring clsure reactin: pprach 1) Identify the cnjugated system, cunt the number f π electrns and number f p rbitals in the chain 2) Cnstruct the M diagram, then fill in the electrns t identify the M 3) Push electrns t btain a crrect 2 Lewis structure f the prduct 4) etermine sterechemistry in the prduct by twisting the terminal atms s that the wavesigns match. 6 p rbitals 6 p electrns Φ 6 Φ 6 Φ 5 Φ 5 hν thermal M Φ 4 Φ 3 hν Φ 4 Φ 3 phtchemical M Φ 2 Φ 2 Φ 1 Φ 1 Lk at the Ms thermal case phtchemical case dis in bth cases yu twist the terminal atms such that the wavesigns match t frm the new bnding interactin cn ( / ) yu get bth enantimers, because yu can turn either way yu'd get bth enantimers here t, but the pint is mt since the prduct is achiral 5

6 Cycladditin eactins In a cycladditin reactin, tw π systems react such that fur π electrns g int frming tw σ bnds t jin the termini f the systems. The reactin will nly prceed if the wavesigns match at the termini f the π systems pprach 1) Identify the tw π systems (can be intramlecular) 2) ketch ut the mlecular rbitals fr the systems 3) Chse which system will serve as the M and LUM 4) egichemistry and terechemistry: analyze bth substitutent and secndary rbital effects diene dienephile [42] majr prduct methyl grups hinder access t the tp face f the diene, frcing the dienephile t apprach frm the bttm face, as drawn the "end" rule secndary rbital interactin (invlving carbnyl π rbitals nt shwn) is respnsible fr the end transitin state Φ 4 Φ 3 Φ 2 Φ electrn dnating alkyl substituents push up the energies f the Ms relative t butadiene M-LUM interactin 5 6 Φ 2 Φ 1 electrn withdrawing carbnyl substituents pull dwn the energies f the Ms relative t ethene Cycladditins will prceed nly if the symmetries f the M and LUM are the same s discussed previusly, substituents and Lewis acids catalysts will change the energies f the Ms. nytime the relative energy f the M and LUM are changed, the reactin rate will als change. The clser they becme in energy, the faster the reactin. The M typically has electrn dnating substituents; the LUM typically has electrn withdrawing substituents. End selectivity can lead t the mre sterically crwded prduct: [42] majr minr 6

7 igmatrpic earrangements In sigmatrpic reactins it appears as if a σ bnd has mved t a different lcatin n a π system uprafacial rearrangement: The bnd mves alng the same face f the π system ntarafacial rearrangement: The bnd crsses t the ther face f the π system Lk fr a driving frce, such as cnjugatin r the frmatin f strnger bnds ether [3,3] ketne uncnjugated [3,3] cnjugated uncnjugated [1,5] cnjugated Mlecular rbital nalysis Linear hifts [1,n] Cpe and Claisen earrangements [3,3] Lk at Ms Lk at Ms wavesigns match! [1,5] suprafacial shift is allwed! wavesigns match! [3,3] suprafacial shift is allwed! nte that wavesigns match here because yu start with a bnding interactin (in the C- σ bnd) [1,3] shift is symmetry allwed nly if antarafacial (but usually t hindered) nte that wavesigns match here because yu start with a bnding interactin (in the C-C σ bnd) 7

8 The Claisen earrangement terechemistry and [3,3] hifts The Claisen rearrangement is a [3,3] sigmatrpic shift in which an ether is cnverted int a ketne. Cncerted mechanism (ne step). Usually prceeds thrugh a chair-like transitin state, as it des in the case f the enzyme chrismate mutase, but the reactin can als (but very rarely) prceed thrugh a bat -like transitin state. The transitin state gemetry determines the sterechemistry f the prduct. The ketne is mre stable than the ether, but always remember that the reverse reactin can ccur. Be n the lkut! General Case ether ketne (mre stable) bnd rtatins chair transitin state bat transitin state tep-by-tep Prblem lving 1) ecgnize the bund allyl systems are the giveaway sign fr a [3,3] sigmatrpic rearrangement 2) Label all sterecenters as ( r and E r ) 3) raw a cyclic cnfrmatin and push arrws t give yu the bnd cnnectivity in the prduct (with ambiguus sterechemistry) 4) The mst favred transitin states fr these reactins are chairs. There are tw chair cnfrmatins which yu shuld draw. 5) uble check t ensure that yu have retained the /E/ sterechemistry n yur 3 chair drawings 6) The mst favred transitin state crrespnds t the chair with the least strain (typically places the bulkiest substituents at equatrial psitins). 7) Push arrws t yield the prduct in its 3 chair frm 8) Label all newly generated sterecenters as / r E/ 9) Transfer as wedges and dtted lines n t a Lewis structure f the prduct 8

9 Example: terechemical Issues mre strained mst bulky grup pseud-axial chair flip [3,3] [3,3] less strained mst bulky grup pseud-equatrial E minr majr 9