86 16.9 Keto-Enol Tautomerism A. Acidity of α-ydrogen α-ydrogen (α-수소) : 카보닐기의 α-탄소에결합된수소원자약한편이지만 acidic함 Table 16.5: 2-2 - - 2 =- 2 - PK a : 16 20 25 44 51 Acidity of α-hydrogen : + B 2 2 + B carbanion enolate anion 염기가 α- 수소를뽑아생성된음이온이 resonance 에의하여안정화되며, 특히 enolate anion 에서는전기음성도가큰산소가 inductive effect 에의하여음이온을 안정화시킴 예제 16.9 : + a + - Et K? B. The Position of Equilibrium in Keto-Enol Tautomerism 2 2 A 2 + 2 + A carbanion enolate anion keto form( 케토형 ) enol form( 엔올형 ) Keto enol : 실존하는구조이며, 서로 isomer 관계임 일반적으로 keto 형이 enol 형보다안정하다. Table 16.6 : The position of keto-enol equilibrium for some simple aldehyde and ketone
87 Enol 형이안정한경우 β-diketone 에서처럼 α- 탄소가두카보닐기사이에위치한경우 1,3-yclohexanedione 20 % 80 % 2,4-Pentanedione 16.10 xidation A. xidation of Aldehydes Aldehyde는 chromic acid에서부터산소에이르는다양한산화제로산화되어카복실산으로전환된다. 2 r 4 exanal exanoic acid Silver-mirror test( 은거울시험 ) Aldehyde 화합물을 Tollens 시약으로처리하면 aldehyde 는카복실산음이온으로 산화되고, Ag(I) 은금속은으로환원되어은거울이형성되는반응 Tollens 시약 : Ag 3 + 2 3 aq 3 Ag( 3 ) 2 + 3 xidation of aldehyde : R + Ag( 3 ) 2 + 3 R 4 + + Ag B. xidation of Ketones Ketone : 일반적으로산화가일어나지않음고온에서산이나염기의농도가높은조건에서 potassium dichromate (K 2 r 2 7 ), potassium permanganate (KMn 4 ) 로산화되어카복실산을생성한다. 3 heat Adipic acid
88 16.11 Reduction R 2 (1 o alcohol) RR' (2 o alcohol) R R R' R (alkane) : deoxygenation R 2 R' (alkane) A. Metal ydride Reductions Metal hydride reducing agent : lithium aluminum hydride(lial 4 ) sodium borohydride(ab 4 ) LiAl 4 : Strong nucleophilic Reduction of Rl > R, RR' > RR' > R > R Et 2 or TF 의 aprotic solvent 사용 Al 3Li + + LiAl 4 TF 0 o 3 + Acetophenone ab 4 : Weak nucleophilic Reduction of Rl > R, RR' o reduction of RR', R, R aq or Et 의 protic solvent 사용 B 3a + aq + ab 4 r. t. 3 + Acetophenone
89 B. atalytic Reduction ( 촉매환원 ) Aldehyde나 ketone을전이금속촉매 (Pt, Pd, i, o, Rh) 의분말을사용하여적절한온도 (25 o ~100 o ) 및수소압력 (1~5 atm) 에서환원시켜 alcohol을제조하는반응 Reactivity : > = > = 2, Pt 2 2, i 25 o, 2 atm Selective reduction 2, Rh i) ab 4 ii) 3 +. Reduction of a arbonyl Group to a Methylene Group = 2 : Reduction by deoxygenation 1) lemmensen reduction Aldehyde나 ketone에 Zn 아말감을첨가하여진한 l에서환류시켜 deoxygenation에의하여 =을 2 로환원하는반응 Zn(g), l reflux 진한산에민감한작용기즉, 삼차알코올이나아세탈기가있는경우는사용될수 없다. 2) Wolff-Kishner reduction Aldehyde 나 ketone 을 hydrazine( 2 2 ) 으로반응시켜생성된 hydazone 중간체 를 aq K 하에서가열하여 = 을 2 로전환하는환원반응 + 2 2 aq K reflux
90 Mechanism : + 2 2 2 K 2 2 2 - + 2 16.12 Reactions at an α-arbon A. Racemization 입체적으로순수한 (R or S) aldehyde나 ketone에산촉매나염기를첨가하여반응하면용액의광학활성은서서히감소하여결국 0에이르게된다. + aet Et a + Et + Ph Ph Ph Ph (R)-3-Phenyl-2-butanone (R) (S) B. Deuterium Exchange α- 수소를갖는 aldehyde 나 ketone 을 D + 혹은 - D 촉매하에서 D 2 로처리하면 α- 수소의교환이일어난다. + a + D a + 2 D 2 2 D D 3 D 3 + D acetone-d 6
91. α-alogenation α- 수소를갖는 aldehyde 나 ketone 을산성혹은염기성조건에서 halogen (F 2, l 2, Br 2, I 2 ) 으로반응시킬때 α- 위치에서 대신에 halogen 이치환되는반응 1) Basic condition + a 2 - a + Br Br 2 Br + abr - a + 2 + 2 1st step : - 염기가 α-를뽑아친핵체인 carbanion 혹은 enolate 음이온을생성함, K 1 < 1 2nd step : carbanion이 Br 2 와반응하여 α-bromination이일어남 K 2 > 1 전체평형을오른쪽으로치우치게하여반응을완결시켜줌단점 : 단일치환된 bromoacetone만을합성하기어려움 2) Acidic condition + + - + 2 Br Br 2 Br + Br 1st step : protonation of carbonyl 2nd step : formation of enol, rate controlling step (r.c.s) 3rd step : 의 nonbonding electron의도움에의한 =의 α-bromination, fast step 장점 : 단일치환된 bromoacetone만을합성할수있음 Rate controlling step 의증거 : + X X 2 2 X X 2 = F 2, l 2, Br 2, I 2 Rate : F 2 = l 2 = Br 2 = I 2, o element effect alogenation step 은 fast, 1st step 이 r.c.s
92 Problems 16.21 : Synthesis of 1-phenyl-2-butanol from bromobenzene and 1-butene Br + 16.24 : Wittig reaction 16.26 : Synthesis of cyclopentanecabaldehyde from cyclopentanone using l 2 l 2 Ph 3 P 3 + 16.35 : Synthetic procedure
93 16.39 : Synthesis of amphetamine 2 16.43 : Reaction of butanal 16.46 : Reaction mechanism + l 2 l + l 16.55 : Synthetic scheme + + I