Chap. 5 Reactive intermediates

Transcription

1 Energy surface hap. 5 Reactive intermediates The plot of energy (potential and kinetic) as a function of 3N- 6 coordinates of the chemical system (reactants, intermediates, products) Reaction oordinate diagram: The reaction coordinate is a onedimensional slice of a 3N- Transition state 61dimensional potential surface. E It goes through a maximum, but AB Reaction D along a valley (potential minimum) with respect to motion perpendicular to the reaction coordinate

2 Intermediate & Transition State Transition state intermediate Experimental evidences show the diradical as intermediate ammond Postulate If two states, as for example, a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their inter conversion will involve only a small reorganization of the molecular structure. ( close in energy, close in structure ) endothermic rxn thermoneutral rxn exothermic rxn T.S. oser in energy, closer in structure. Factors stablize the intermediate will stablize the T.S. Reactivity 3 >2 >1

3 arbocations intermediates with formal charge of 1 on a carbon atom, or a collection of carbon atoms (and those with three center-two electron bonding) planar formation of carbenium 3 > 2 > 1 1. alkyl gps are electron-donating. (Inductive effect ) VB description 2. hyperconjugation PMO description

4 QMOT version methyl cation with 6 valence electrons is planar A shorter - bond and longer - bond expected stabilization of methyl cation by alkyl substitution is a result of orbital mixing Bridged ethyl cation

5 Detection of carbocation, by NMR 25.2 ppm X 3 3 X ppm ( in SO 2 F-SbF 5 )(from TMS) large shift due to deshielding with low electron density δ 13 O F stabilized by e - -donor more shielded owever, ppm from S 2 charge density ppm from S 2 charge density Rearrangement of carbocation NMR shows singlet for 1 at -70 in super acid 13 one peak that couple to 9 9 become equivalent by fast rearrangement. ESA shows 4 uncharged carbon and one positively charged carbon The contradiction is due to different time scale of NMR and ESA 2

6 Rearrangements possible from 2 1 The can scramble in isopropyl cation, arbon-atom also scramble. edge-protonated at -78, t 1/2 = 1hr, 13 scramble to all corner-proronated (methyl bridged) at -110, 2 sets of peak at -40, one peak only corner-protonated cycloprotane 3 SbF 5 FSO 3 SO 2 F stable at -75 above -30 at

7 SO 4 O D 2 SO 4 2 D 2 2 O 3 2 DO 3D 2 O D scrambled to all carbons corner-protonated D 2 2 D 2 2 D 2 2 D edge-protonated D SO D 2 2 SO 2 D 2 2 OS 2 D 2 2 OS 3 2 DOS 2 2 SO 3 D 2 OS 0~-40o SO 2 F - SbF

8 Nonclassical Ion Bridged structure with delocalized bond a pair of e - shared by three nuclei. exo OBs OAc KOAc OAc rate OBs OAc KOAc OAc Optically active exo racemic exo 3 Optically active endo 93% racemic exo 4 For chiral exo S.M., recovered S.M. partially racemized. 5 For chiral endo S.M., recovered S.M. did not racemized OBs OAc KOAc - OBs assisted by the 1-6 bond to give bridged nonclassical intermediate 6 pentacoordinate The rate enhancement was due to proper anti-alignment of the (1)-(6) bond and the leaving gp at (2). The stereochemical outcome is due to a symmetric intermed.

9 No backside assistance in endo compound OBs OAc KOAc The endo S.M. undergoes a classical ion intermediate then nonclassical ion? OBs OAc KOAc assical ion Non-classical ion Two steps, the classical ion may be intercept by solvent to retain configuration, depending on the nuceophilicity of the solvent. Thus, in acetone, 87% racemization OAc, 93% racemization OO 97% racemization Observation in recovered S.M. is due to internal return of symmetrical Intermediate for exo S.M. OBs OAc KOAc OAc 82 15% retention of configuration OAc Optically active classical, achiral chiral,non-classical

10 Alternative interpretation by.. Brown fast equilibrium of classical ions rate enhancement release of torsional strain of leaving gp e.g. and neighboring gps. OBs high exo/endo ratio steric hindrance e.g. rate 14 1 OPNB classical ions OPNB product stereoselectivity steric hindrance OBs k exo /k endo =140 classical aq. acetone rate potential energy for stable and unstable bridged ion a transition state between classical structure A an intermediate in rearr. between two B O only little involvement of 1-6 bond the only stable structure

11 Direct Observation of Norbornyl cation by NMR: Olah 13 NMR in SO 2 -SbF 5 (super acid ) at -70, ppm -1,2,6 162 ppm -3,5, ppm at -60, 5.35 ppm (4) 3.15 ppm (1) 2.20 ppm (6) at -150 and 5 K 78.5 ppm -1, ppm ppm ppm -3, ppm at -150, the hydride shift frozen the NMR confirm the existence of norbornyl cation as non-classical. or a equilibrating intermediate with barrier < 0.2kcal 1 2 Summarize Nonclassical or bridged structure are readily attainable intermediate or transition state for many cations. For norbornyl cation, the bridged structure is an intermediate. tertiary cation is nearly always classical cation. primary cation can rearrange to more stable secondary or tertiary carbocation, with bridged structure as T.S..

12 arbanions: intermediates with formal charge of 1 on carbon, 8 valence electrons 1-2 kcal less than 109.5o R R R stability inc. (higher S-character at carbon) Metal-carbon bond can be polar-covalent, such as -Li Organometallic species can be associated species(oligomeric) The barrier of inversion at carbanion carbon can be increased by incorporation of small ring, attaching electronegative substituent, cyclopropyl anion is stable pyramidal Inversion barrier for N 3 ~5kcal, for NF 3 ~50kcal 3 BuLi 3 O 2 3 Br Li O 2 N MeI 100% retention of conformation Substituent that allows π-delocalization favors planar structure N Li Me stabilized by resonance less interaction with metal ion racemic a fast inverting tetrahedral carbanion center is indicated

13 Generation of carbanion reduction of carbon-halogen bond by metal deprotonation (acid/base rxn) by strong base reduction/addition of alkene. Stability of carbanion Acidity 3º < 2º < 1º in solution (due to destabilization of e-donating alkyl groups?) But in gas phase, measured acidity: Ethene<propane(2º-)<methane<isobutane(3º-) The solution acidity is complicated by solvation The order of acidity of alcohol In gas phase t-butyl > isopropyl>ethyl>methyl In aqueous solution methyl>ethyl>isopropyl>t-butyl The alkyl group can polarize the electron away from the center of negative charge and stabilize it.

14 Free Radicals Structure with unpaired electron and zero formal charge 7 valence electrons (for neutral radical ) They are paramagnetic, attracted by magnetic field. cation radical N -e - N anion radical 3 Zn 3 O Na O Na 3 t-buok Radicals are in general reactive intermediate, but can be stablized via delocalization of the radical such as or by steric hindrance ( 3 ) 3 ( 3 ) 3

15 PMO & EMO description of ethyl radical on p EMO on methyl gp ( the one with -symmetry ) hyperconjugation ( VB ) Structure and bonding alkyl radical P - or sp 2 is planar Evidence of planar geometry

16 Methyl radical is planar, with very small barrier for pyramidalization. All other radicals are nonplanar. Electronegative substituent shifts the shape to pyramidal. staggered bridgehead radical are less strained, as compare to planar carbonium ion stability of radical methyl < 1 < 2 < 3 due to hyperconjugation

17 carbenes 6 valence electrons on a R 2 Ground state multiplicity affects reactivity (e.g. singlet has ambiphilic character; triplet reacts as diradical) Large -p separation => singlet ground state Smaller -p separation => triplet ground state. The substituents affect the ground state multiplicity Ground state for simple carbene lower energy higher energy

18 The substituents affect the ground state multiplicity Inductive effect: (inductively stabilize by increasing its S-character) -withdrawing group favors singlet -donating group favors triplet Li Li Resonance Effect: 129 o triplet X: R (-F, -, -Br, -I, -NR2, -PR2, -OR, -SR) Z: -R (-OR, -N, F3, -BR2, -SiR3, -PR3) (X,X) carbene => bent singlet F 104 o singlet F ½ X X X e.g, dimethoxycarbene, dihalocarbene (Z,Z) carbene => linear singlet carbene ½ X Z Z Z Z ½ ½ e.g. diborylcarbene, dicarbomethoxycarbene (X,Z) carbene => linear singlet _ X Z X Z - e.g. phosphinosilylcarbene

19 Steric effect: increasing steric bulkness -> increase the bond angle => favor triplet Dimethylcarbene, di(t-butyl)carbene diadamantylcarbene Bent singlet, 111 triplet, triplet, 152 To stabilize a carbene: two π-donor, -attractor substituents ( push push resonance, pull pull inductive substituent) R 2 N Two π-attractor, -donor substituents (pull pull resonance, push push inductive substituent) A π-donor, π-acceptor (push pull resonance substituent) NR 2 R 2 B BR 2 R 2 P SiR 3

20 Reaction of arbene triplet carbene behaves as diradical singlet carbene behaves as both carbocation and carbanion. Addition R Singlet R one step R R Singlet carbene inserts stereospecifically triplet carbene inserts non-stereospecifically Triplet R R R R Insertion Rearrangement 10% 25% 25% 40%