高等有機(jī)化學(xué)chapter ii

1、Chapter twoIsomer: any two molecules with the same formula but different structuresA. Constitutional isomers: differ by connectivity.B. Stereoisomers: with the same connectivity, but differ by spatial disposition.Conformational isomersConfigurational isomers Conformational isomers: can be super impo

2、sed by bond rotationConfigurational isomers: cannot be super imposed by bond rotationa. Enantiomers mirror imagerequires four different groups attached to carbonOptical purity: If a compound is 100% one enantiomer, its optical pure. If a compound is 50% : 50% two enantiomers, it is racemic. If anyth

3、ing else, enantiomeric excess = % major - % minor. If mixture: 90% one enantiomer, ee = 90% - 10% = 80%. Racemize: convert to a racemate. add chiral reagent to react with A.Asymmetric inductionAsymmetric synthesisadd chiral catalyst for this transformationAsymmetric catalysisExample of asymmetric ca

4、taysis:enantiomerdiastereoisomerendo -methylexo -methylIf there are three centers：change all “enantiomer”change one or more (not all) “diastereoisomer”epimer changing stereochemistry of one center b.Diastereoisomer:non superimposable, no mirror images differ in both physical properties and chemical

5、reactivityc. Meso compounds: contains chiral centers and elements of symmetry achiralplaneOther kinds: - planari: - point of inversionSn: - improper rotational axisIs it a meso?d. Axial dissymmetryComing in and outUp and downPPh2PPh2e. Geometric isomerEZWhat is this configuration?7C. ProchiralityIf

6、one ligand of an achiral molecule is replaced by achiral group and a stereocenter formed, the original molecule is “prochiral”.achiralchiralNon-prochiralIf replacement of a heterotopic group gives two enantiomers, it is “enantiotopic”.If replacement of a heterotopic group gives two diastereomers, it

7、 is“diastereotopic”.diastereotopicHave different chemical shiftNomenclature:pro -Spro -RKetone (prochiral):prochiral faces. re or si-facethe face above is re-faceD. Reaction classesStereoselective & stereospecific: refer to reactions where diastereomerically different compound can be made or reacted

8、.Stereoselective: a reaction where two or more diastereomers are possible, but one is preferred.Only one diastereomermeso2. StereospecificReaction which give diastereomerically different products from diastereomerically different reactants.3. Regioselective vs. regiospecificRefer to reactions where

9、bonds can be made and broken in different orientations.Regioselective: one is preferred, two or more products but one is major. Regiospecific: one is exclusive.E. Atropisomers: hindered rotationThey can be separated and will not convert to each other.F. Resolution: The process of separating a racemi

10、c mixture into its enantiomers; Resolving agent: make a racemic mixture to form a diastereomeric mixtureKinetic resolution. Conformational Analysis: Related by single bond rotations.Effecting conformations:1.Bond length2.Bond angle3.Torsional angleIs defined as two adjacent bonds4. Non-bonded intera

11、ctionsLondon dispersion forces: typically very weak, attractive forceIf atoms touch at their van der waals radius, it is weak attractive force. “ Induced dipole interaction” If atoms overlap at their van der waals radius, its very repulsive.B. Acyclic Compounds:1. ButaneThe conformation torsional an

12、gle graph for butane.G = -RT KH = -0.8 kcal/molS = -R （2/1） = -0.41 kcal/molG = H TS = -0.39 kcal/molK=1.9 K= anti/gauche =1.9=2/1anti=2/(2+1)66% anti conformerThe populations of the various conformations are related to the energy between them:How much is gauche? And anti? How to figure it out?2 gau

13、che conformers;1 anti-conformermajor isomer（%） k G(kcal/mol) 50% 1 0 60% 1.5 -0.24 70% 2.3 -0.5 90% 9.0 -1.32 99% 9.9 -2.72162. PropeneBetter overlap of electronG = - 2 kcal/mol the eclipsed conformer favored by 2 kcal/mol eclipsed is about 95%3. Butene4. Butadiene:S-Trans is favored by G = -2.3 kca

14、l/molBarrier Ea 4 kcal/molMost of butadiene is s-trans conformersChange of the geometry from s-trans to s-cis is needed prior to the reaction.5. Carbonyl compounds:a. Aldehyde and ketones favor eclipsed conformations6. Ketones:b.Kinetic control!C. Cyclic compounds:1. Strain: artificial difference be

15、tween Hf of a compound and theoretical Hf of unstrained compound. Exp: cyclopropane strain = 29 kcal/mol the bond angle is supposed to be 109 o, but in fact its 60 cyclopropene strain = 56 kcal/mol the bond angle is supposed to be 120 o, but in fact its 60 b. Cyclobutane 10990Strain = 26 kcal/molAlt

16、hough this is lower than cyclopropane, it has more “eclipsing CH2-CH2”“Torsional strain”: have to pucker to avoid the eclipsing CH2-CH2.2. Axial vs EquatorialHalf chairTwist boatBoth conformers contain too much strains!Twist boat!G = 3.6 kcal/molG = 5.5 kcal/molG = -2.6 kcal/mol29G = 1 kcal/molEa =

17、5.3 kcal/mol(ring flipping)3. Cyclohexene4 CycloalkaneCycloalkaneStrain kcal/molcycloheptane7.6cyclooctane11.9cyclononane15.5cyclodecane16.4cyclodecane11.75. Bredts RuleBridge head bonds dont exit.Anomeric effectG = 2 Kcal/molOrder of anomeric effect:NOT a ring flippingIts mutarotationExpect: G = 0.

18、9 Kcal/molAssume: 83% equatorialObserve: 86% axialG = -1.8 Kcal/mol6. HeterocyclesPut polar atom on the carbon next to the oxygen on the suguar ring.The oxygen lone pair is anti to the electron-withdrawing groupDelocalization of the lone pair electronsOverlap of an oxygen nonbonded electron pair wit

19、h the *C-X orbitalDelocalization : no to c-x * only when X is axialC-X is longer in axial than in equatorial (C-X es weaker) has weaker anomeric effect Dipoles cancelAnother Explanation:Prove: (1) The anomeric effect is smaller in more polar solvents. Because in polar solvent,It can stabilize dipole

20、 . The dipole difference is not big, anomeric effect is small(2)the anomeric effect still exists, but dipole is the major factor7. Conformational effects on reactivityaxialequ1.2 1.2D. StereoelectronicsReaction will proceed via the maximum orbital overlap with best transition state. I - goes from th

21、e back is to avoid the leaving group.In fact:As you fill *， C-Br bond breaks. The only reason why I- goes from back is that the antibond of the carbon is at the back. Full orbital will go to attack the empty orbitalThe highest filled orbital is going to react with the lowest empty orbital.Other empt

22、y orbitals have higher energy , so only *C-Br has the reactivity.In the end , all the orbital are parallel , so removal of the D is more preferred .Axial-D orbital is parallel with orbital of C-O- ; Equatorial -H orbiral is vertical with orbital of C-O- .The chair transition is preferred to the twis

23、t- boat transition stateActually, it is Nuc (filled orbital) attack the * orbital. The size of R,R can change the direction of the attack of Nuc-.3. Addition to aldehydea Cram Model - all stericsRs = H, Rm = CH3, RL= Ph, a is preferred; Rm = Et, RL = Ph, a is more preferred; Et is bigger than CH3 b.

24、 Felkin- Ahn Model - Cram Model doest do the jobOMe is not as big group as Ph, its no longer sterics.The energy of *C-O is close to *，these two orbital s can mix and overlapOxygen is electron negative, * of C-O is the lower.Overlap of *C-O and the adjacent * lower the overall energy of the * and fac

25、ilitates the reaction.*C-O*C-O*C. Chelation ControlExample:It looks like equatorial attack preferred.But not.If equitorial attack, the oxygen with down, O eclipse with the C-H bond, which is unfavored.If axial attack, the oxygen up into equaorial position.For large Nuc, 1,3-diaxial interaction makes

26、 equatorial attack preferred.Chair productTwist-boat productLittle difference between R groups boat(%) chair(%)R: H 5% 95% CH3 95% 5% stereoelectronicsteric controlR gets bigger, shields most of the top, so a is blocked.5. Baldwins rulesgovern ring closure reactionNomenclature 1) ring size 3 7 cyclo

27、propane cycloheptane. 2) endo vs exo. Breaking bond lied outside of the formed ring.Endo cyclization.3) tet, trig, digAtom attached is tetrahedral.Carbon is planar. (trigonal)digonalendo/exoNumber/3-7tet/trig/digb. Rules 1). 3- 7- exo- tet are all favored 3- 7- endo- tet are rare2). 3- 7- exo- trig are all favored 3- 5- endo- trig are unfavored 6- 7- endo- trig are favored3). 3- 4- exo- dig are all unfavored 5- 7- exo- dig are favored 3- 7- endo- dig are favoredThese rules dont say anything about thermodynamics. Just tell that a transition state can stay or not based on s