哥倫比亞大學(xué)有機(jī)化學(xué) 23 Aryl Halides (缺231237)

23.8

TheElimination-AdditionMechanism

ofNucleophilicAromaticSubstitution:

BenzyneArylHalidesUndergoSubstitutionWhen

TreatedWithVeryStrongBasesClNH2KNH2,NH3–33°C(52%)CH3NH2newsubstituentbecomesattachedtoeither

thecarbonthatboretheleavinggroupor

thecarbonadjacenttoitRegiochemistry+NaNH2,

NH3–33°CCH3BrCH3NH2newsubstituentbecomesattachedtoeither

thecarbonthatboretheleavinggroupor

thecarbonadjacenttoitRegiochemistryCH3Br+NaNH2,NH3–33°CCH3NH2CH3NH2Regiochemistry+NaNH2,NH3–33°CCH3NH2CH3NH2CH3NH2+CH3NH2Sameresultusing14ClabelCl*KNH2,NH3–33°CNH2*+NH2*(48%)(52%)Mechanism?

?NH2??–Step1HH???

?HHClH??Mechanism?

?NH2??–Step1HH???

?HHClH??HHHHNH2??H???

?Cl???

?–compoundformedinthisstepiscalledbenzyneBenzyneHHHHBenzynehasastrainedtriplebond.Itcannotbeisolatedinthisreaction,butisformedasareactiveintermediate.Mechanism?

?NH2??–Step2HHHHMechanism?

?NH2??–Step2HHHHHHHHNH2–????Anglestrainisrelieved.Thetwosp-hybridizedringcarbonsinbenzynebecomesp2hybridizedintheresultinganion.MechanismStep3HHHHNH2–????NH2??HMechanism?

?NH2??–Step3HHHHNH2–????NH2??HHHHHHNH2??HydrolysisofChlorobenzeneCl*NaOH,H2O395°COH*+OH*(54%)(43%)14Clabelingindicatesthatthehigh-temperaturereactionofchlorobenzenewithNaOHgoesviabenzyne.23.9

Diels-AlderReactionsofBenzyneOtherRoutestoBenzyneBenzynecanbepreparedasareactiveintermediatebymethodsotherthantreatmentofchlorobenzenewithstrongbases.AnothermethodinvolveslossoffluorideionfromtheGrignardreagentof1-bromo-2-fluorobenzene.OtherRoutestoBenzyne??BrF???

?Mg,THFheat??MgBrF???

?FMgBr+BenzyneasaDienophileBenzyneisafairlyreactivedienophile,andgivesDiels-Alderadduc