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Chem 206D. A. Evans Carbocations: Stability & Structure

Other Relevant Background Reading

March, Advanced Organic Chemistry, 4th Ed. Chapter 5, pp165-174.

Lowery & Richardson, Mech. & Theory in Org, Chem., 3rd Ed. pp 383-

412.

Arnett, Hoeflich, Schriver in Reactive IntermediatesVol 3, Wiley, 1985,

Chapter 5, p 189.

Olah, G. A. and G. Rasul (1997). Chemistry in superacids .26. From Kekule'stetravalent methane to five-, six- and seven-coordinate protonated methanes.

Acc. Chem. Res. 30(6): 245-250.

Saunders, M. and H. A. Jimenez-Vazquez (1991). Recent studies ofcarbocations. Chem. Rev. 91: 375.

Stang, P. J. (1978). Vinyl Triflate Chemistry: Unsaturated Cations andCarbenes. Acc. Chem. Res. 11: 107.

Olah, G. A. (1995). My search for carbocations and their role in chemistry(Nobel lecture). Angew. Chem., Int. Ed. Engl.34, 1393-1405

D. A. EvansMondayDecember 11, 2006

Reading Assignment for this Lecture:

Chemistry 206

Advanced Organic Chemistry

Lecture Number 33

Introduction to Carbonium Ions

! Carbocation Stabilization! Carbocation Structures by X-ray Crystallography! Vinyl & Allyl Carbonium Ions

Carey & Sundberg, Advanced Organic Chemistry, 4th Ed.Part A Chapter 5, "Nucleophilic Substitution", 263-350 .

Laube (1995). X-Ray Crystal Structures of Carbocations Stabilized by Bridgingor Hyperconjugation. Acc. Chem. Res.1995, 28,: 399 (electronic pdf)

Olah, G. A. (2001). 100 Years of Carbocations and their Significance inChemistry. J. Org. Chem.2001, 66, 5944-5957. (handout)

Walling, C. (1983). An Innocent Bystander Looks at the 2-Norbornyl Cation.Acc. Chem. Res.1983, 16, 448. (handout)

Birladeanu, L. (2000). "The Story of the Wagner-Meerwein Rearrangement.J. Chem. Ed. 2000, 77, 858. (handout)

http://www.courses.fas.harvard.edu/colgsas/1063

Problem 17: The reaction illustrated below was recently reported by Snider and co-workers(Org. Lett. 2001, 123, 569-572). Provide a mechanism for this transformation. Wherestereochemical issues are present, provide clear three dimensional drawings to supportyour answer.

Me

O

Me Me

R EtAlCl2

CH2Cl2, 0 C

Me

O

Me

R

Me

Carey & Sundberg-A, p 337: Provide mechanisms for the following reactions.

OH

NH2 NaNO2

HOAc/H2O

CHO

OH

NH2 NaNO2

HOAc/H2O

CMe3

O

CMe3


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3/13

The Gathering atThe Gathering atJDRJDRss70th70thBirthday CelebrationBirthday Celebration19881988

DervanDervan, Ireland, Evans, Bergman, Grubbs, JDR, Myers, Dougherty, Hammond, Ireland, Evans, Bergman, Grubbs, JDR, Myers, Dougherty, Hammond

Recent organic faculty at CIT, present and departedRecent organic faculty at CIT, present and departed

Chem 206D. A. Evans John D. Roberts, Institute Professor of Chemistry, Emeritus, Caltech

B.A., 1941, University of California (Los Angeles)Ph.D. 1944, University of California (Los Angeles)

John D. Roberts was born in 1918.

He became Prof. at MIT and then Prof.at Caltech where he is still active. His

work has been centered on mechanismsof organic reactions.

One of the joys of being a professoris when an exceptional studentcomes along and wants to work

with you.

J.D. Roberts, The Right Place atthe Right Time. p. 63.

John D. Roberts graduated from the University of California at Los Angeleswhere he had received A. B. (hons) degree in 1941 and the Ph. D. degree

in 1944. In 1945-1946 he was a National Research Council Fellow andInstructor at Harvard. Later on, he went to MIT in 1946 as an Instructor. Hehad introduced the terms "nonclassical" carbocations and "benzyne" intoorganic chemistry. He had won numerous awards; he is a member of theNational Academy of Sciences (1956) and the American PhilosophicalSociety (1974). He received the Welch Award (1990, with W. E. Doering),the National Medal of Science (1990), and the ACS Arthur C. Cope Award(1994). Since 1939 his research has been concerned with the mechanismsof organic reactions and the chemistry of small-ring compounds. His currentwork involves applications of nuclear magnetic resonance spectroscopy tophysical organic chemistry.

Roberts made major research and pedagogic contributions to mechanisticorganic chemistry. He pioneered the use of 14C and other isotopic labels tofollow molecular rearrangements as, for example, in the complex and subtlesolvolysis of cyclopropyl-carbinyl systems. He introduced the terms"nonclassical" carbocations and "benzyne" into organic chemistry, and usedisotopic labeling to establish the intermediacy of each. Roberts was early torecognize NMR's potential, and used 1H NMR to study nitrogen inversion,long-range spin-spin coupling and conformational isomerism, and later 13Cand 15N NMR to study other reactions, including the active sites of certainenzymes. Roberts' superb short books on "Nuclear Magnetic Resonance"(1959), "Spin-Spin Splitting in High Resolution NMR" (1961) and "Notes onMolecular Orbital Calculations" (1961) did much to popularize and clarifythese subjects for organic chemists. His highly successful text "BasicPrinciples of Organic Chemistry" (1964), written with Marjorie Caserio,introduced spectroscopy early to undergraduates. Roberts received manyawards, including the Roger Adams (1967) and Priestley (1987) Medals. Anexcellent photographer, Roberts graciously supplied several of thephotographs for the MSU collection.


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D. A. Evans. B. Breit Chem 206Carbocations: Stability

Carbocation Subclasses

R3 R2

R1

!

RR3 = alkyl or aryl

R3 R2

O!

RR3 = alkyl or aryl

R1

R3 R2

N!

RR3 = alkyl or aryl

R R

C ar bo n- sub st itu te d H et er oa tom s ta bi li ze d

The following discussion will focus on carbocations unsubstitutred with heteroatoms

C C

C!

C C

C!

C C

C

!C C

C

!

opentrivalent

hyperconjugationno bridging

unsymmetricalbridging

symmetricalbridging

classical nonclassical

increasing nonclassical character

Classical vs nonclassical carbonium ions

Stability: Stabilization via alkyl substituents (hyperconjugation)

R

R

R

H

R

R

H

H

R

H

H

H

Order of carbocation stability: 3>2>1

>> >Due to increasing number of substituents

capable of hyperconjugation

C C+H

314

276

249

231

287

386

239

Hydride ionaffinities

The relative stabilities of various carbocationscan be measured in the gas phase by theiraffinity for hydride ion.

J. Beauchamp, J. Am. Chem. Soc. 1984, 106, 3917.

+ H

Note: As S-character increases, cation stabilitydecreases due to more electronegative carbon.

+ HI

!HI increases " C(+) stability decreases

Hydride Affinity = !G

Carey & SundbergA, pp 276-

C C C C

CH3+

CH3CH2+

(CH3)2CH+

(CH3)3C+

H2C=CH+

PhCH2+

R RH

Me CH2

276 249

27

231

18Me2 CH Me3 C

Hydride ion affinities (HI)

H3C CH2

276

H2C CH

287

+21

HC C

386

+81

Ph CH2

239 276

37 20Me CH2

256

CH CH2H2C

Me CH2

276 270

7MeCH2 CH2

The effect of beta substituents: Rationalize

Hydride ion affinities versus Rates of Solvolysis

PhCH2Br CH=CHCH2Br

Relative Solvolysis rates in 80% EtOH, 80 C

100 52

0 +17

239 256HI

!-HI

A. Streitwieser, Solvolytic Displacement Reactions, p75

Conclusion:Gas phase stabilities do not always correlate with rates of

solvolysis

Me2CHBr

0.7

+10

249rel rate


5/13


6/13

D. A. Evans, B. Breit Chem 206Carbocations: Structure

+

C C

R

H

H H

HC

H

HC

H

R

Carbocation Stabilization Through Hyperconjugation

Take linear combination of ! CR (filled) and C pz-orbital (empty):

! CR

!" CR

+

! FMO Description

C HH

E

! CR

+

!" CR

Syn-planar orientation between interacting orbitals

C H

H


7/13

D. A. Evans, K. Scheidt Chem 206Carbonium Ion X-ray Structures: Bridged Carbocations

1.467

1.442

1.739 **2.092

+

+[F5SbFSbF5]

T. Laube, Angew. Chem. Int. Ed.1987, 26, 560

Me

Me

Me

H

MeMe

H

Me

Me

Me

F

**One of the longest documented CC bond lengths.

C C

C!

C C

C

!



2 SbF5

F5Sb F SbF5

1.467

+

1.855

1.503

1.495

T. Laube, JACS1989, 111, 9224

Me

Me

Ph Cl

C

Me

Me

Ph

+

AgSbF6


8/13

D. A. Evans, K. Scheidt Chem 206Carbonium Ion X-ray Structures: A Summary

1.467

1.855

1.503

1.495

1.467

1.442

1.739 2.092

+

+

+

1.408

1.432 1.371

1.446

1.439

1.442

+

98.2 1.621

1.466

+

1.551

1.608

1.622

1.421

1.432

1.422

1.725

1.668

Cl

Cl

+

1.508

1.342

(ref 1.513 )PhC(Me)=CH2

1.491

C C

C!

C C

C!

C C

C

!C C

C

!

opentrivalent



symmetricalbridging

classical nonclassical

increasing nonclassical character

Nomenclature: classical vs nonclassical


9/13

Chem 30D. A. Evans Chapter 18: Chemistry of Aryl & Vinyl Halides

Me

R

X H CMe

R

Favorable

H2C

R

XUnfavorable CC R

H

H

X

X

Substitution (SN1)

Substitution Reactions

Sp hybridized Carbonis more electronegative

CSp2 Carbonium Ions do exist!

1.221

Si

Si

1.946

Si Si

CMe3

Me

Me Me

Me

Normal CC triple bond lengths are ~1.21


10/13

D. A. Evans, B. Breit Chem 206Vinyl & Allyl Carbocations

D

R

OTf

R C CD

RR

OTf OSolv

Vinyl & Phenyl Cations: Highly Unstable

Evidence suggests that vinyl cations are linear.

As ring size decreases, the rate of hydrolysis also diminishes. Implying thatthe formation of the linear vinyl cation is disfavored due to increasing ringstrain.

Hyperconjugation

P. J. Stang J. Am. Chem Soc.1971, 93, 1513; P. J. Stang J.C.S. PT II1977, 1486.

A secondary kinetic isotope effect was measured to be KH/KD = 1.5 (quitelarge) indicating strong hyperconjugation and an orientation of the vacant p

orbital as shown above.

HOSolv

H+

Phenyl Cations

The ring geometry opposes rehybridization (top) so the vacant orbital retains

sp2 character. Additionally, the empty orbital lies in the nodal plane of the

ring, effectively prohibiting conjugative stabilization.

H3C CH2

276

H2C CH

287

+21HC C

386

+81


H2C CH

287

+11

298

Allyl & Benzyl Carbocations

R

R

R

R

Carbocation Stabilization via !-delocalization

allyl cation

! Stabilization by Phenyl-groupsThe Benzyl cation is approximately as stable as a t-Butylcation.

(CH3)3C + PhCH3 (CH3)3CH + PhCH2

!H0r[kcal/mol]

3.8

(CH3)3C + PhCH2Cl (CH3)3CCl + PhCH2 0.8

Ph CH2

239


231

Me3 C8

Hydride ion affinities versus Rates of Solvolysis

PhCH2Br Me2CHBr CH=CHCH2Br

Relative Solvolysis rates in 80% EtOH, 80 C

100 0.7 52

0 +10 +17

239 249 256HI

!-HI

A. Streitwieser, Solvolytic Displacement Reactions, p75


11/13

D. A. Evans Chem 206The Johnson Longifolene Synthesis

Volkman, Andrews, Johnson, JACS1975, 97, 4777

The plan ( According to Volkman):

Me Me

Me

CH2

H

Me Me

Me

HO

Me Me

Me

Me Me

Me

Me Me

Me

H

Me Me

Me

H

HO

longfifolene

TFA, K2CO375%

Me Me

Me

HO

Me Me

Me

H

NaBH3CN

ZnBr2

94%

Me Me

CH2

H

H

H+

91%

Me Me

Me

ZnBr2NaBH3CN

longfifolene

steps

Ho, Nouri, Tantillo, JOC2005, 70, 5139-5143

W. S. Johnson!s total synthesis of the sesquiterpenoid longifolene is a classic exampleof the power of cationic polycyclizations for constructing complex moleculararchitectures. Herein we revisit the key polycyclization step of this synthesis usinghybrid Hartree-Fock/density functional theory calculations and validate the feasibility ofJohnson!s proposed mechanism. We also explore perturbations to the 3-center 2electron bonding array in a key intermediate that result from changing the polycyclicframework in which it is embedded.

The Cationic Cascade Route to Longifolene

FIGURE 1. Relative energies (kcal/mol) of stationary points for the mechanism shown inScheme 2 (B3LYP/6-31G(d) zero-point corrected energies in italics, B3LYP/6-31G(d) freeenergies at 0 C in bold, and CPCM-B3LYP/6-31G(d) energies in water underlined).


12/13

D. A. Evans, B. Breit Chem 206Cyclopropyl-carbinyl & Bridgehead Carbocations

Carbocation Stabilization via Cyclopropylgroups

C

A rotational barrier of about13.7 kcal/mol is observed in

following example:H

Me

MeNMR in super acids

!(CH3) = 2.6 and 3.2 ppm

R. F. Childs, JACS1986, 108, 1692

1.464

1.409

1.534

1.541

1.444

24

1.302

R

O1.222

1.474

1.517

1.478

X-ray Structures support this orientation

See Lecture 5, slide 5-05 for discussion of Walsh orbitals

Solvolysis rates represent the extend of that cyclopropyl orbital overlapcontributing to the stabiliziation of the carbenium ion which is involved as a

reactive intermediate:

Me

Me

OTs

OTs

Cl

Cl

krel = 1 krel = 1

krel = 106 krel = 10

-3

OTs

OTs

krel = 1

krel = 108

Why??

CareyA, p 286

Me

Me

Me

OTs

TsO TsO TsO

Bridgehead Carbocations

1 10-7 10-13 104

Bridgehead carbocations are highly disfavored due to a strain increase inachieving planarity. Systems with the greatest strain increase upon passingfrom ground state to transition state react slowest.

why soreactive?

TsO

why so reactive?

TsO


13/13

D. A. Evans, J.Tedrow Chem 206A Stable Hypervalent Carbon Compound ?

+

2.428

2.452

1.483

2.428

2.452

+

OMe OMeCO2Me

Me3O+BF4

O OCOMeMeOMe Me

+

B2F7

"The relevant CO distances are longer than a covalent CO bond(1.43 ) but shorter than the sum of the van der Waals radii (3.25 )."

"The Synthesis and Isolation of Stable Hypervalent Carbon Compound (10-C-5) Bearing a 1,8-Dimethoxyanthrecene Ligand"

Akibe, et al. JACS1999, 121, 10644-10645

For a recent monograph on hypervalent Compounds see:"Chemistry of Hypervalent Compounds", K. Akiba, Wiley-VGH, 1999

33-carbocations-1

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