The-Art-of-writing-reasonable-organic-mechanism-4有

4.1AnswersToChapter4Problems.1.(a)Aneight-electron[4+4]cycloaddition.Itproceedsphotochemically.(b)Afour-electronconrotatoryelectrocyclicringopening.Itproceedsthermally.(c)Asix-electronenereaction.(Notethetranspositionofthedoublebond.)Itproceedsthermally.(d)Asix-electron[1,5]sigmatropicrearrangement.Itproceedsthermally.(e)Aten-electron[8+2]cycloaddition.Itproceedsthermally.(f)Asix-electron[2,3]sigmatropicrearrangement.Itproceedsthermally.(g)Asix-electrondisrotatoryelectrocyclicringopening.Itproceedsthermally.(h)Afour-electrondisrotatoryelectrocyclicringclosing.Itproceedsphotochemically.(i)Asix-electrondisrotatoryelectrocyclicringclosing.Itproceedsthermally.(j)Asix-electron[3+2](dipolar)cycloaddition.Itproceedsthermally.(k)Afour-electron[2+2]cycloaddition.Itproceedsphotochemically.(l)Asix-electronconrotatoryelectrocyclicringopening.Itproceedsphotochemically.2.(a)Regio:RNHandCHOare1,2.Stereo:CHOandCH3remaintrans;NHRisout,CHOisendo,sotheyarecisinproduct.BnO2CHNCHOCH3(b)ThetwoCH3groupsarebothoutgroups,sotheyarecisinproduct.OOH3CCH34.2(c)Regio:C4ofdieneisnucleophilic,soitmakesabondtoelectrophilicCofdienophile.Stereo:EtOisout,CO2Etgroupisendo,sotheyarecisinproduct.OEtOCO2Et(d)Regio:CHOandOSiMe3are1,4.Stereo:theCH2CH2bridgeisinatbothendsofthediene,CHOisendo,sotheyaretransinproduct.Me3SiOH3CCHOCH3(e)Dienophileaddstolesshinderedfaceofdiene.C(sp3)offive-memberedringisin,NO2isendo,sotheyaretransinproduct.NO2HEtOEtO(f)Regio:NucleophilicOaddstoelectrophilicβCofunsaturatedester.Stereo:alkylandCO2Megroupsremaintrans;Hisin,CO2Meisendo,sotheyaretransinproduct.NOCO2MeHOMs(g)Stereo:CO2Megroupsremaintrans.Argroupisprobablyoutforstericreasons,CO2Meisendo,sothetwoarecisintheproduct.NCO2MeMeO2CArPh(h)The[14+2]cycloadditionmustbeantarafacialwithrespecttoonecomponent.Thetwoingroupsofthe14-atomcomponentbecometransintheproduct.CNCNNCNCHH4.33.1,3,5,7-Cyclononatetraenecantheoreticallyundergothreedifferentelectrocyclicringclosures.HH8e–electrocyclicringclosingconrotatoryHH≡HHH6e–electrocyclicringclosingdisrotatoryHHH≡HHH4e–electrocyclicringclosingconrotatoryHHH≡HHWhensmallringsarefusedtootherrings,thecisringfusionisalmostalwaysmuchmorestablethanthetransringfusion.Theoppositeistrueonlyforsaturated6-6orlargerringsystems.(Makemodelstoconfirmthis.)Theorderofstabilityofthethreepossibleproductsshownaboveis:cis-6-5trans-7-4trans-8-3.4.(a)ChairTS,withtheMeontheC(sp3)equatorial.OPhOPhCH3H3CH3CCH3≡OPhH3CCH3(b)ChairTS,withthePhequatorial.OPhOPh≡OSiR3OBnOSiR3OPhOSiR3BnOBnO(R)H(S)4.4(c)ChairTS,withbothsubstituentsequatorial.PhCH3PhH3CHH≡PhH3CHH(d)Twodifferentchairsarepossible,butone(Phequatorial)islowerinenergythantheother.PhCH3PhHCH3H≡PhHCH3HPhHHCH3(e)AchairTSisnotpossible,soitgoesthroughaboatTS.HH3CO2CCH3OHH3CHCH3H3CO2CCH3OHchairTSnowayJoséHH3CO2CCH3OHH3CH≡H3COHH3CO2CCH3HboatTSmuchbetter(f)Again,aboatTSisnecessary.HHH60°CHHHH≡4.5(g)AchairTSwouldproduceatransdoublebondintheseven-memberedring,sotheboatTSisoperative,andtheHandOSiR3groupsonthetwostereogenicatomsarecistooneanother.OSiR3HOSiR3HHHOSiR3HOSiR3HHHtransdoublebondOSiR3HHHOSiR3HHHOSiR3OSiR3H≡OSiR3HOSiR3H(h)ThechairTSisenforcedinthismacrocycliccompound.CH3OH3CCH3H3CHOHCH3OOHHH3C≡CH3CH3OOHHH3C5.(a)Firststep:hetero-Diels–Alderreaction(six-electron,[4+2]cycloaddition).Secondstep:Claisenrearrangement(six-electron,[3,3]sigmatropicrearrangement).(b)Thedieneiselectron-rich,soitrequiresanelectron-poordienophileforanormalelectrondemandDiels–Alderreaction.TheC=Cbondofketenesisprettyelectron-rich,duetooverlapwiththelonepairsonO:H2C=C=O¨↔H2C––C≡O+.OnlytheC=Obondoftheketeneisofsufficientlylowenergytoreactwiththedieneatareasonablerate.(c)First,itisimportanttorememberthatinketenes,theporbitalsoftheC=Obondarecoplanarwiththe4.6substituentsontheterminalC.RSRLOBecauseoftheketene’sgeometry,intheTSofthehetero-Diels–Alderreaction,eitherRSorRLmustpointdirectlyatthediene.ThelowerenergyapproachtowardsRSischosen,andtheproductinwhichRSpointsbacktowardtheformerdieneportionofthecompoundisobtained.CORSRLORSRL+CORSRLSecondstep:Thenewσbondformsbetweenthebottomfaceofthedoublebondontheleftandthebottomfaceofthedoublebondontheright,givingtheobserved,lessthermodynamicallystableproduct.ORSRLHHHORSRLHHHHORSRLHHHH≡ORSRLHHHH6.(a)NumbertheC’s.C1,C2,C5andC6areclearinbothstartingmaterialandproduct.Therestfollows.4.7H3COOSiMe3ΔH3CMe3SiOOH123456789563214789WebreaktheC4–C6bond,andweformC3–C8andC4–C9.Theformationofthelattertwobondsandthefactthatwe’reformingacyclobutanonesuggestsa[2+2]cycloadditionbetweenaketeneatC3=C4=OandtheC8=C9πbond.WecangeneratetherequisiteC3=C4πbondbyelectrocyclicringopeningofthecyclobuteneringintheS.M.H3COOSiMe3ΔMe3SiOCOH3CMe3SiOH3CCO(b)Electrocyclicringclosingfollowedbybase-catalyzedtautomerization(bothstartingmaterialandproductarebases)givestheproduct.NHNMeNSMeNONHNMeNSMeNHOBNHNMeNSMeNOHNHNMeNSMeNO–HB(c)Diels–AlderreactionfollowedbyspontaneouseliminationofMe3SiO–andaromatizationgivestheproduct.LossofMe3SiO–occurssoreadilybecausetheMe3Sigroupisaπelectronwithdrawerlikeacarbonylgroup.4.8OSiMe3ArOMe3SiOCO2MeCO2MeOSiMe3A