Chapter+14-1 - Peter Vollhardt University of California at...

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Unformatted text preview: Peter Vollhardt University of California at Berkeley I, too, star Ti gr e Del ta, Buenos Ai r es, 1952 te d small…….. Chapter 14 Delocalized Pi Systems Delocalized Recal l the doubl e bond Recal Tr i gonal T he π bond i s eThe r i ch: E attack , attack R∙ add ) add T he l obes of the por bi tal s: Per pendi cul ar t o the Per si gma fr ame and EtheneF 2-Propenyl (Allyl) 2-Propenyl H Questi on: What about addi ng a thi r d t hi p-or bi tal adjacent to the doubl e bond? I s ther e somethi ng speci al ? Or : I s ther e any speci al Or r eacti vi ty at the car bons adjacent to a doubl e bond? adjacent Repl aci ng one of the hydr ogens i n ethene wi th Repl another sp2-hybr i di zed car bon gi ves a pr openyl i c sp -hybr or all l yl i c system. a Al l yl i c posi ti on Obser vati ons: a. a. b. H 87 kcal mol -1: Weak! Weak! H 101 kcal mol -1 L SN 1 SN 1 r eacti vi ty of al l yl i c car bon l i ke that of RsecX , even though i t i s pr i mar y! of X, even B H B 50 c. H pK a ~ 40: Aci di c! A ci + Clearly: Allylic - are stabilized. · Why? Resonance! Why? Resonance! CH 2 H 2 C CH 2 H 2 C CH 2 H 2 C Shor t notati on: Dotted l i nes MO Picture of 2-Propenyl (Allyl) MO 3 p Or bi tal s Or 3 M ol ecul ar Or bi tal s Recal l : B onds made by over l ap Bonds of or bi tal s. L et’s l ook agai n a . + H . att H 2. H H E H +H H ++ H + H + Bondi n I n phase Out of Out phase phase Si gn of the wave functi on, Not char ge! H · H · H+ + H- +H g Anti H - bondi ng H2 Node Now l et’s l ook agai n at ethene σ bond σ* E π* π σ CH2 CH2 π bond What happens to thi s pi ctur e What w hen we i nter act wi th another p or bi tal ? Interactions of a singly occupied p-orbital -orbital with each of the π molecular orbitals 1) 1) I nter acti on of the p atomi c or bi tal w i th the π bondi ng or bi tal causes bondi t he p or bi tal l evel t o move up and up t he π bondi ng l evel t o move down . d own I nter acti on of the p atomi c or bi tal w i th the π* anti bondi ng or bi tal causes the p or bi tal l evel t o move down t o w her e i t wa s d own or i gi na l l y and that of the π* or anti bondi ng or bi tal to move up. anti up T he two effects on the p or bi tal The ca ncel each other out; the two π ca or bi tal s ar e pushed apar t: “ ener gy spl i tti ng”; i mpor tantl y, t he π bondi ng or bi tal goes down . goes Ethene π* π* 0 up N onbondi n g MO 2) p E π unchanged 3) π down Al l yl p Or bi tal Al l yl i c r adi cal i s stabi l i zed by 13.5 kcal mol -1. Resul ti ng pi ctur e: # of e d epends on +,·,- π* 0 ? p E H 2C π H C CH2 L ocati o n of +,·,of i s at ter mi ni t er Reactivity of Allylic Position Reactivity A. Radi cal H al ogenati on CH M echani sm: I ni ti ati on: 1. Br 2 h υ or Δ + Br 2 L ow conc. CH CH + H Br Faster than addi ti on! . 2 Br . + Br . CH CH . Pr opagati on: 2. CH CH CH CH . Br 2 CH CH + H Br . CH CH CH B r Br CH . Br Br Ter mi nati on: T er .+. Br 3. CH CH CH Br 2 CH CH Br CH CH Br CH . + Br . Br Br . CH CH 2 CH CH CH CH CH CH A nythi ng that tr aps r adi cal s, i ncl udi ng the “di r t” Anythi on the wal l s of the fl ask, contr i butes to ter mi nati on. t er A conveni ent sol i d br omi na ti ng a gent: conveni N -Br omosucci ni mi de, N BS Tr aces, Tr al ways pr esent i n NBS L ow conc. conc. M echani sm? :O : + :O H :: :O H : : N Br : : :O + H+ :Br :− : :N O: : Br : + N Br : :: + :B r :− : : : :: O: : :O : :O H : N: : :O :: Stoi chi ometr y: Br 2 does not show up, but i s the actual br omi nati ng speci es! d oes : + Br Br: P r oton sh if t (ta utom e ri sm ; Se ct ion 1 3-7 ) All l yl i cF11 A :: :N : :O H Pr opene gener ates a symmetr i cal al l yl i c r adi cal symmetr and onl y one pr oduct. For unsymmetr i cal one u nsymmetr systems: mii xtur es. Rati os depend on % r adi cal m Rati char acter on each car bon and TSs l eadi ng to pr oducts. B. SN 1: The Al l yl i c Cati on i s Stabi l i zed B. 2 CH CH CH CH + CH CH CH CH + CH CH CH -H l ClO C H+ CH CH CH CH CH CH CH CH OH OH Two pr oducts +H C. SN 2: F ast! T he al l yl i c TS i s stabi l i zed and C. 2: TS t he al l yl i c car bon i s r el ati vel y el ectr ophi l i c. TS del ocal i zed ‡ sp = e-wi thdr awi ng 2 Cl .. .. . . - δ δ CH CH CH 100 ti mes faster 100 than t han Cl + NaI Na CH CH CH C CH I - δ CH CH CH + Cl - D . Al l yl i c Or ganometal l i cs Or + Li Al ter nati ve pr epar ati on: al l yl i c Gr i gnar d r eagents CH 3 CH2 Li H 2C C CH 3 O RCH H 2C C OH CH 2CH R CH 3 CH 3 Br + M g M gBr We shal l encounter neutr al anal ogs of al l yl i c ani ons: CH2 X i soel ectr oni c to X : : X = OR, SR, NR2 Conjugated Double Bonds Conjugated What about CCCC CCCC ? Nomencl atur e: Ci s/ tr ans; E/ Z. Revi ew Chapter 11 N omencl Stabi l i ty: H eats of hydr ogenati on (kcal mol -1) Stabi ΔH ˚ CH 3(CH 2)3CH CH 2 + H 2 + 2 H2 1,5-H exadi ene -30.3 -60.5 But: 1,3-Butadi ene + 2 H2 -57.1 Resonance ener gy of butadi ene ~ 3.5 Structure Structure Fast Shor t r el ati ve to an al kane C―C si ngl e bond (1.54 Å). But i s thi s a good compar i son? Orbitals Orbitals Anti bondi ng Bondi ng Conjugati on stabi l i zes ther modynami cal l y, but i t al so Conjugati i ncr eases r eacti vi ty, for exampl e i n ell ectr ophi l i c addi ti ons e (r evi ew Chapter 12). (r M ar kovni kov addi ti on wi th a twi st: Fast CH CH + H Cl Reason: I nter medi ate cati on i s al so stabi l i zed Reason: 1,2-Addi ti on 1,2-Addi (“ki neti c”) (“ki Cl Cl - 1,4-Addi ti on Cl 1,4-Addi (“ther modynami c”) (“ther CH + ci s Cl M or e sta bl e H Cl AddnF11 T er mi nal al kene l ess Ter stabl e than i nter nal stabl CH Less sta bl e Kinetic vs Thermodynamic Control Kinetic Extended Conjugation Extended + H Br Qui te r eacti ve, Qui even though even stabi l i zed by conjugati on conjugati CH 3 CH 3 Br Thr ee pr oducts CH 3 Cati on al so stabi l i zed by conjugati on T her modynami c stabi l i ty does not Ther all ways equal l ack of r eacti vi ty a Cyclohexatriene is Special Benzene Benzene Cycl i c ar r ay of sii x el ectr ons h as speci al stabi l i ty, s cal l ed ar omati ci ty (Chapter 15). ar Benzene i s r el ati vel y i ner t to H 2-cat, el ectr ophi l es, -cat, oxi dants, i n compar i son wi th hexatr i ene. oxi Extended Conjugation in Natural and Unnatural Products Unnatural Or ange col or of car r ots B i ol ogi cal Bi degr adati on d egr Vi si on Organic Conductors Organic H eeger , M acDi ar mi d, Shi r akawa, Nobel Pr i ze 2000 L i ght emi tti ng di odes (L EDs) Conjugated Systems Undergo Special Transformations: Pericyclic Reactions Pericyclic T he conjugated π system can r eact as a uni t, The i nvol vi ng both ends. For exampl e, both 1. Cycl oaddi ti ons: The D i el s-Al der T he r eacti on, a [4+2] cycl oaddi ti on HC HC CH 2 CH 2 CH 2 + CH 2 Δ 20% HC HC H2 C CH 2 CH 2 Otto Di el s 1876-1954 Kur t Al der 1902-1958 4π-4C -4C Dii ene D 2π-2C -2C Dii enophi l e D C H2 N obel Pr i ze 1950 Cycl oadduct The Diels-Alder Reaction is The Chemoselective Chemoselective Dii el s-Al der r eacti ons wor k best when we pai r an D e-r i ch (push) d i ene w i th an e-poor (pul l ) dii enophi l e, e-r (push) d or an e-poor di ene w i th an e-r i ch di enophi l e e-r Depends on substi tuents: e-Donati ng: All kyl , al koxy, al kyl thi o A 3 3 3 2 H yper conjugati on O H 3 CH CH O CH CHCO, CH CH 3 S OCH 3 , Resonance Even though O i s e-negati ve (i nducti ve effect), r esonance wi ns out. O e-Wi thdr awi ng: CF , CR, C N , NO CF CR NO F I nducti ve: CF F O CR H 2C C H O CR O CR H 2C C H O CR H 2C C H O CR 90% Resonance: Exampl e: + Δ Does not compete w i th di enophi l e: r el ati vel y e-r i ch. Some Exampl es of the Tr end i n Reacti vi ty of Di enophi l es and Di enes H3 C < < F3 C < Die nophile H3 C H3 C H3 C Diene Increasing reactivity NC < NC NC < < < H 3CO Mechanism: Concerted + Δ Or bi tal descr i pti on: sp2 sp2 sp3 D i el s-Al der r eacti on r equi r es accessi ng the l ess stabl e s-ci s confor mati on CH3 CH3 H3C CH3 H s-cis s- tr ans s-cis s-tr ans When s-ci s for m i s h i nder ed or i mpossi bl e, the r eacti on sl ows or d oes not occur . When di ene i s constr ai ned sci s, the tr ansfor mati on i s accel er ated . Consequences of Concertedness Consequences Ster eospeci fi c: Retenti on of Di enophi l e Ster Ster eochemi str y (new C—C bonds gr een ) Ster O CO CH 3 O CC3 OH 80% + CH3 C3 H Ci s O COCH 3 Ci s O COCH 3 90% + H 3C CH3 Tr ans Tr ans Retenti on of Di ene Ster eochemi str y Retenti OCH 3 N C CN OCH3 + OCH 3 T r ans,tr ans Tr (same for ci s,ci s-di ene) (same N C CN CN CN CN CN OCH3 O CH3 OCH 3 N C CN + OCH 3 Ci s,tr ans N C CN CN CN CN CN H 3CO When both par tner s ar e ster eochemi cal l y defi ned: When “Endo r ul e” deter mi nes thei r appr oach. “ Endo Endo/Exo Addition Endo/Exo Substi tuents poi nt away from di ene Substi tuents poi nt towar d d i ene U sua l l y fa ster , even t hough pr oduct l ess sta bl e: Ki neti c contr ol DA F11 Another exampl e: CH 3 CH 3 H 3C Endo CN NC NC H H o H3C CN http:/ / csi .chemi e.tu-dar mstadt.de/ ak/ i mmel / Gener al l y: o i i A o i A A + A Wal baCtF Dyl an o i Reasons for endo r ul e compl ex. L i pshutzCtF Segovi a Alkynes as Dienophiles Alkynes Gener ate 1,4-cycl ohexadi enes CO2 CH 3 CO2 CH 3 OCH 3 Can r eact Can agai n agai OCH 3 CO2 CH 3 CO2 CH 3 75% + CH2 O2 CH 3 CH2 O2 CH 3 2. El ectr ocycl i c Reacti ons: I ntr amol ecul ar r i ng cl osur e and openi ngs T he Cycl obutene 1,3-Butadi ene Equi l i br i um The Δ, E a = 32.9 32.9 kcal mol -1 k cal Exother mi c (r i ng str ai n r el eased) hυ L i ght dr i ven: Can beat ther modynami cs. Wavel ength t her Wavel dependent (can go ei ther way). d ependent I mmel T he 1,3-Cycl ohexadi ene The 1,3,5-H exatr i ene Equi l i br i um L i ght dr i ven: Can beat ther modynami cs. Wavel ength t her Wavel dependent (can go ei ther way). d ependent Δ, E a = 29.9 29.9 kcal mol -1 k cal Endother mi c E ndother (C C better than C C, and no r i ng str ai n pr esent) hυ I mmel Electrocyclic Reactions are Stereospecific Stereospecific Δ CH 3 Onl y! ci s–3,4-Di methyl cycl obutene CH 3 CH 3 ci s,tr ans–2,4-H exadi ene Δ Onl y! Tr ans CH 3 Tr ans,tr ans Movement of Substituents Movement Conr otator y: T hey r otate i n the same di r ecti on same CH 3 CH 3 B oth ei ther Both cl ockwi se or counter cl ockw i se: Same pr oduct. pr H Conr otator y H CH 3 H CH 3 Δ H I mmel CH 3 H CH 3 H Conr otator y Conr (cl ockwi se) (cl CH3 H H3 C H Δ Counter cl ockwi se conr otati on i n pr i nci pl e possi bl e but ster i cal l y pr ohi bi ted : Counter ster CH3 H CH 3 H Δ H H CH 3 CH 3 Fasci nati ngl y, h υ goes dii sr otator y d (r otati on i n opposi te di r ecti ons) opposi CH 3 CH 3 hυ di s CH 3 CH 3 CH 3 CH 3 CH 3 hυ di s CH 3 E ven mor e star tl i ng: The hexatr i ene/ cycl ohexadi ene Even i nter conver si on i s al so ster eospeci fi c, but fol l ows the ster opposi te r ul es of sense of r otati on, compar ed to the opposi butadi ene/ cycl obutene system: butadi Δ = dii s d I mmel h υ = con con Rober t B. Woodwar d Rober 1917-1979 Roal d H offmann b. 1937; NP 1981 Electronic Spectroscopy Electronic (Ultraviolet-Visible or UV) Whi te (sun) l i ght i s composed Whi of the vi si bl e spectr um Remember spectr oscopy (Chapter 10): (Chapter Exci ted state E Gr ound state ΔE = h υ = h c/ λ U V-Vi s spectr oscopy r equi r es much hi gher ener gy than N M R (kcal s vs cal or i es), does not need exter nal “ condi ti on” (magnet). El ectr oni c exci tati on fr om bondi ng t o anti bondi ng l evel s, par ti cul ar l y easy for π systems, because occupi ed bu noccupi ed ΔE r el ati vel y smal l . because F or exampl e, l ook at a si mpl e π bond, as i n ethene: For N o π bond l eft! bond I .e., L i ght causes ci scauses t r ans tr i somer i zati on, r adi cal r eacti ons,…. UV Spectrum of Ethene UV Quoted as λmax Quoted B r oad, because of r otati onal and Br vi br ati onal states. El ectr oni c spectr oscopy i s fast, no “aver agi ng” “ aver A 171 nm 9 Wavel ength λ (gi ven i n nm , uni ts of 10--9 m ; not i n fr equency Wavel nm υ = c/ λ, as we di d i n NM R, wher e λ ~ 100 mm to 1m!) 100 E (kcal mol -1) = 28,600/ λ (nm) U V spectr oscopy bel ow 200 nm r equi r es vacuum, because ai r absor bs. Nor mal l y (i n atmospher e) one scans 220-400 (UV), 400-800 nm (vi si bl e). Thi s al l ows l ower ener gy tr ansi ti ons to be r ecor ded, e.g. 1,3-butadi ene: tr Rel ati vel y Rel l ow ener gy ener E λma x Peak hei ghts ar e r epor ted as Peak ε : Exti ncti on coeffi ci ent , E xti w hi ch i s absor bance absor n or mal i zed by concentr ati on : concentr Shoul der , sh λma x = 222.5 nm ( ε = 10,800) ε = A/ c Visible Absorption: Color Visible Newton N ewton L i ght enter s the pr i sm fr om the top r i ght, and i s r efr acted by the gl ass. The vi ol et i s bent mor e than the yel l ow and r ed, so the col or s separ ate. A bsor pti on Absor i n the vii si bl e v 450 nm or ange-r ed 450 or 550 nm vii ol et 550 v 650 nm bll ue-gr een 650 b Col or of Col substa nc substa e I n extended π systems many tr ansi ti ons ar e possi bl e, gi vi ng r i se to mor e compl ex and not r eadi l y i nter pr etabl e spectr a, but H OM O-L UM O gap gets smal l er : L ongest wavel ength absor pti on i s i ndi cati ve of the extent of conjugati on, e.g., extent Gr eater conjugati on: Smal l er H OM O/ L UM O gap CH 3 λ m ax = 271 nm Conjugated tr i ene λ m ax = 217 nm Unconjugated tr i ene Azul ene A zul ...
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