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1、10ReactionsInvolvingCarbocations,Carbenes,andRadicalsasReactiveIntermediatesIntroductionTrivalentcarbocations,carbanions,andradicalsarethemostfundamentalclassesofreactiveintermediates.ThebasicaspectsofthestructuralandreactivityfeaturesoftheseintermediateswereintroducedinChapter3ofPa
2、rtA.DiscussionofcarbanionintermediatesinsynthesisbeganinChapter1ofthepresentvolumeandcontinuedthroughseveralfurtherchapters.Thefocusinthischapterisonelectron-deficientreactiveintermediates,includingcarbocations,carbenes,andcarbon-centeredradicals.Bothcarbocationsandcarbeneshaveacarb
3、onatomwithsixvalenceelectronsandarethereforeelectron-deficientandelectrophilicincharacter,andtheyhavethepotentialforskeletalrearrangements.Wealsodiscusstheuseofcarbonradicalstoformcarbon-carbonbonds.Radicalsreactthroughhomolyticbond-breakingandbond-formingreactionsinvolvingintermedi
4、ateswithsevenvalenceelectrons.+.CC:CcarbocationcarbeneradicalAcommonfeatureoftheseintermediatesisthattheyareofhighenergy,comparedtostructureswithcompletelyfilledvalenceshells.Theirlifetimesareusuallyveryshort.Bondformationinvolvingcarbocations,carbenes,andradicalsoftenoccurswithlowa
5、ctivationenergies.Thisisparticularlytrueforadditionreactionswithalkenesandothersystemshavingbonds.Thesereactionsreplaceabondwithabondandareusuallyexothermic.861862++.C+CCCCCorC+CCCCC.CHAPTER10ReactionsInvolvingCarbocations,Carbenes,Owingtothelowbarrierstobondformation,reactantcon
6、formationoftenplaysaandRadicalsasReactiveIntermediatesdecisiveroleintheoutcomeofthesereactions.Carbocations,carbene,andradicalsfrequentlyundergoveryefficientintramolecularreactionsthatdependontheproximityofthereactioncenters.Conversely,becauseoftheshortlifetimesoftheintermediates,re
7、actionsthroughunfavorableconformationsareunusual.Mechanisticanalysesandsyntheticdesignsthatinvolvecarbocations,carbenes,andradicalsmustpayparticularlycloseattentiontoconformationalfactors.10.1.ReactionsandRearrangementInvolvingCarbocationIntermediatesInthissection,theemphasisisoncar
8、bocationreactionsth
