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1、18.11 Effects of Conjugation in a,b-Unsaturated Aldehydes and Ketones,Relative Stability,aldehydes and ketones that contain a carbon-carbon double bond are more stable when the double bond is conjugated with the carbonyl group than when it is not compounds of this type are referred to as a,b unsatur
2、ated aldehydes and ketones,Relative Stability,(17%),K = 4.8,(83%),Acrolein,Acrolein,Acrolein,Acrolein,Resonance Description,Resonance Description,a,b-Unsaturated aldehydes and ketones are more polar than simple aldehydes and ketones. a,b-Unsaturated aldehydes and ketones contain two possible sites f
3、or nucleophiles to attack carbonyl carbon b-carbon,Properties,a,b-Unsaturated aldehydes and ketones are more polar than simple aldehydes and ketones. a,b-Unsaturated aldehydes and ketones contain two possible sites for nucleophiles to attack carbonyl carbon b-carbon,Properties,b,Dipole Moments,Butan
4、al,trans-2-Butenal,m = 2.7 D,m = 3.7 D,O,O,d,d,d+,d+,d+,greater separation of positive and negative charge,18.12 Conjugate Addition to a,b-Unsaturated Carbonyl Compounds,1,2-addition (direct addition) nucleophile attacks carbon of C=O 1,4-addition (conjugate addition) nucleophile attacks b-carbon,Nu
5、cleophilic Addition to a,b-Unsaturated Aldehydes and Ketones,attack is faster at C=O attack at b-carbon gives the more stable product,Kinetic versus Thermodynamic Control,+,1,2-addition,formed faster major product under conditions of kinetic control (i.e. when addition is not readily reversible),+,1
6、,4-addition,enol goes to keto form under reaction conditions,+,1,4-addition,keto form is isolated product of 1,4-addition is more stable than 1,2-addition product,+,1,2-addition,1,4-addition,C=O is stronger than C=C,observed with strongly basic nucleophiles Grignard reagents LiAlH4 NaBH4 Sodium acet
7、ylide strongly basic nucleophiles add irreversibly,1,2-Addition,Example,1. THF 2. H3O+,(84%),observed with weakly basic nucleophiles cyanide ion (CN) thiolate ions (RS) ammonia and amines azide ion (N3) weakly basic nucleophiles add reversibly,1,4-Addition,Example,(93-96%),Example,(93-96%),via,Examp
8、le,O,CH3,C6H5CH2SH,HO, H2O,(58%),Example,via,O,CH3,C6H5CH2SH,HO, H2O,(58%),18.13 Addition of Carbanions to a,b-Unsaturated Carbonyl Compounds: The Michael Reaction,Stabilized carbanions, such as those derived from b-diketones undergo conjugate addition to a,b-unsaturated ketones.,Michael Addition,Ex
9、ample,(85%),KOH, methanol,+,The Michael reaction is a useful method for forming carbon-carbon bonds. It is also useful in that the product of the reaction can undergo an intramolecular aldol condensation to form a six-membered ring. One such application is called the Robinson annulation.,Michael Add
10、ition,Example,NaOH heat,not isolated; dehydrates under reaction conditions,Example,(85%),NaOH heat,18.14 Conjugate Addition of Organocopper Reagents to a,b-Unsaturated Carbonyl Compounds,The main use of organocopper reagents is to form carbon-carbon bonds by conjugate addition to a,b-unsaturated ket
11、ones.,Addition of Organocopper Reagents to a,b-Unsaturated Aldehydes and Ketones,Example,(98%),+,LiCu(CH3)2,18.15 Alkylation of Enolate Anions,Enolate ions are nucleophiles and react with alkyl halides. However, alkylation of simple enolates does not work well. Enolates derived from b-diketones can be alkylated efficiently.,Enolate Ions in SN2 Reactions,Example,+,CH3I,K2CO3,CH3CCHCCH3,(75-77%),
