stabilization of methyl cation by alkyl substitution is a result of ... Rearrangement of carbocation .... potential energy for stable and unstable bridged ion k exo. /k.
Chap. 5 Reactive intermediates Energy surface The plot of energy (potential and kinetic) as a function of 3N6 coordinates of the chemical system (reactants, intermediates, products)
Reaction Coordinate diagram:The reaction coordinate is a oneTransition state E A+B Reaction
C+D
dimensional slice of a 3N6+1dimensional potential surface. It goes through a maximum, but along a valley (potential minimum) with respect to motion perpendicular to the reaction coordinate
Intermediate & Transition State Transition state Experimental evidences show the diradical as intermediate
intermediate
Hammond 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.
Closer in energy, closer in structure. Factors stablize the intermediate will stablize the T.S.
Reactivity 3°>2°>1°
§ Carbocations intermediates with formal charge of +1 on a carbon atom, or a collection of carbon atoms
(and those with three center-two electron bonding)
H H H
H H
planar
formation of carbenium 3° > 2° > 1° 1. alkyl gps are electron-donating. (Inductive effect ) VB description 2. hyperconjugation PMO description
QMOT version
methyl cation with 6 valence electrons is planar
A shorter C-C bond and longer C-H bond expected
stabilization of methyl cation by alkyl substitution is a result of orbital mixing
Bridged ethyl cation
Detection of carbocation, by NMR 330.0 ppm ( in SO2ClF-SbF5 )(from TMS)
25.2 ppm H3C
C
CH3
CH3
X
X OCH3 CH3 H CF3
large shift due to deshielding with low electron density δ13C 219 243 255 269
more shielded
stabilized by e--donor
However, H
H3C
CH3
C CH3 C
H3C
CH3
-125.0 ppm from CS2 charge density +0.611 -135.4 ppm from CS2 charge density +0.692
Rearrangement of carbocation CH3CH2CHCH2
CH3CH2CHCH3
H 1°
2°
NMR shows singlet for 1H at -70° in super acid one peak that couple to 9 H → 9 H become equivalent by fast rearrangement. 13C
ESCA shows 4 uncharged carbon and one positively charged carbon
The contradiction is due to different time scale of NMR and ESCA
Rearrangements possible from 2° → 1° The H can scramble in isopropyl cation, Carbon-atom also scramble.
edge-protonated
at -78℃ , t1/2 = 1hr, 13C scramble to all C corner-proronated (methyl bridged)
at -110℃ , 2 sets of peak at -40℃ , one peak only H
H
H
H
H H
H H
CH3
H
H
H
H
H
corner-protonated cycloprotane SbF5 FSO3H SO2ClF
H H
Cl
H
at -65℃
H3C
H
H H
stable at -75℃
above -30℃
H
H
H3C
CH2 H2SO4 H2C
OH
CH2
CH2 D2SO 4 H2C
CH3CHDCH2OH
CH2DCH2CH2OH + CH3CH2CHDOH +
CH2
0.46
0.17
0.38 D scrambled to all carbons
corner-protonated CH2D +
CH2 +
D
H2C
+ H2C
H2C
+ H2C
H2C
CH2
edge-protonated
CH2
H
+
CH2
CH2
CHD
CHD
+
CH3
SOH
D
CHD
CH2
CH2DCH2CH2OS +
SOH
+ H2C
CH2 H
CH3CH2CHDOS SOH
CH2DCH2CH2OS CH3CHDCH2OS
H 0~-40oC Cl
CH3 +
H
CH3 +
CH3
CH3
+
SO 2ClF - SbF5
H +
CH3 +
CH3
Nonclassical Ion Bridged structure with delocalized σ bond → a pair of e- shared by three nuclei. rate 1 HOAc KOAc
OBs
exo
OAc
350 :
HOAc KOAc OAc
1
OBs
2 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. 7 4 5 1 2
HOAc KOAc
-
OBs
OBs
≣
6
3
assisted by the C1-C6 bond to give bridged nonclassical intermediate C6:pentacoordinate
H
H
The rate enhancement was due to proper anti-alignment of the C(1)-C(6) bond and the leaving gp at C(2). The stereochemical outcome is due to a symmetric intermed.
No backside assistance in endo compound
HOAc KOAc
+
OBs
?
The endo S.M. undergoes a classical ion intermediate then nonclassical ion
HOAc KOAc
+ OBs
+
Non-classical ion
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 HOAc, → 93% racemization HCOOH → 97% racemization Observation in recovered S.M. is due to internal return of symmetrical Intermediate for exo S.M.
OBs
HOAc KOAc
82±15% retention of configuration
classical, achiral
OAc
+ OAc
H
Optically active
chiral ,non-classical
Alternative interpretation by H.C. Brown fast equilibrium of classical ions rate enhancement:release of torsional strain of leaving gp and neighboring gps. e.g.
OBs
rate
14
OBs
:
1
high exo/endo ratio:steric hindrance e.g.
kexo/kendo=140 OPNB OPNB
classical ions
product stereoselectivity:steric hindrance only
aq. acetone OH
Cl ψ
classical
ψ
little involvement of C1-C6 bond Cl
Cl
ψ
Cl ψ
rate
1
3.9
3.9×107
potential energy for stable and unstable bridged ion
A
a transition state between classical structure an intermediate in rearr. between two
B C
the only stable structure
Direct Observation of Norbornyl cation by NMR: Olah 7
13C
NMR in SO2-SbF5 (super acid )
at -70℃, 101.8 ppm C-1,2,6 162 ppm C-3,5,7 156.1 ppm C-4
4 5
3
at -60 ℃, 5.35 ppm (4H) 3.15 ppm (1H) 2.20 ppm (6H) 1H
6
H 1
2
H 7
at -150℃ and → 5°K 78.5 ppm C-1,2 171.4 ppm C-6 160.4 ppm C-4 145.8 ppm C-3,7 165.8 ppm C-5
4 5
H
6
H 2
1
7
4 5
3
6
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
3
1
2
H
H
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..
§ Carbanions: intermediates with formal charge of –1 on carbon, 8 valence electrons H H
C H
C
H
H
H
1-2 kcal
less than 109.5o
R
C
C
C
C
R
R
stability inc. (higher S-character at carbon)
Metal-carbon bond can be polar-covalent, such as C-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 NH3 ~5kcal, for NF3 ~50kcal Ph
CH3
BuLi
Br
Ph
Ph
CH3
Ph
CO2
Li
Ph Ph
CH3 CO2H
100% retention of conformation
Substituent that allows π-delocalization favors planar structure Ph Ph
CN
MeI
Li
stabilized by resonance less interaction with metal ion
CN
Ph Ph
Me racemic
a fast inverting tetrahedral carbanion center is indicated
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: Ethenemethyl 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.
§ 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: anion radical: Zn
Ph3CCl +
-e-
N
O
N
+ Na
Ph3C
O Na
t-BuOK
Ph
H
Ph
CPh3
Ph Ph
C
C
Ph
Ph CH Ph
Ph
Ph Ph Ph
Ph
Radicals are in general reactive intermediate, but can be stablized via delocalization of the radical:such as Ph
Ph
Ph
Ph Ph Cl
or by steric hindrance (H3C)3C C
C(CH3)3
Cl
Cl
Cl Cl
Cl Cl Cl
Cl
H Cl
Cl Cl
Cl Cl
Cl
PMO & EHMO description of ethyl radical
on p on methyl gp ( the one with π-symmetry ) EHMO
hyperconjugation ( VB )
Structure and bonding alkyl radical PH
or sp
C
H
is planar
2
H
Evidence of planar geometry
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
§ carbenes 6 valence electrons on a CR2
Ground state multiplicity affects reactivity (e.g. singlet has ambiphilic character; triplet reacts as diradical) Large σ-pπ separation => singlet ground state Smaller σ-pπ separation => triplet ground state. The substituents affect the ground state multiplicity Ground state for simple carbene 136°
lower energy
105°
higher energy
The substituents affect the ground state multiplicity Inductive effect: (inductively stabilize σ by increasing its S-character) σ-withdrawing group favors singlet σ-donating group favors triplet
C Li
C
Li
F
H
H
104o
129o Resonance Effect:
C F
singlet
triplet
X: +R (-F, -Cl, -Br, -I, -NR2, -PR2, -OR, -SR) Z: -R (-COR, -CN, CF3, -BR2, -SiR3, -PR3+) (X,X) –carbene => bent singlet
C X
½ σ+ X
X
C
½ σ+ X
e.g, dimethoxycarbene, dihalocarbene (Z,Z) – carbene => linear singlet carbene
Z
C
Z
Z ½ σ+
C
Z
σ_
½ σ+
e.g. diborylcarbene, dicarbomethoxycarbene
(X,Z) – carbene => linear singlet e.g. phosphinosilylcarbene
X
C
Z
X
σ+
C
Zσ-
Steric effect: increasing steric bulkness -> increase the bond angle => favor triplet Dimethylcarbene, di(t-butyl)carbene Bent singlet, 111°
diadamantylcarbene
triplet, °
triplet, 152°
To stabilize a carbene: two π-donor, σ-attractor substituents ( push push resonance, pull pull inductive substituent) NR2 R2N
Two π-attractor, σ-donor substituents (pull pull resonance, push push inductive substituent) R2B
BR2
A π-donor, π-acceptor (push pull resonance substituent) R2P
SiR3
Reaction of Carbene triplet carbene behaves as diradical singlet carbene behaves as both carbocation and carbanion. Singlet carbene inserts stereospecifically triplet carbene inserts non-stereospecifically
Addition R
R R
R
Singlet one step R
R
Triplet
R
R
Insertion CH3 CH3
+ CH2 10%
Rearrangement
H3C
+
+ 25%
+ 25%
40%