anion binding in catalysis - princeton university · chiral anion mediated asymmetric chemistry!...

Anion binding in Catalysis

Asymmetric and Non-Asymmetric Examples

Andy McNally

MacMillan Group Meeting

2/04/09

Chiral Anion Mediated Chemistry

! The use of 'cationic' counter-ion chemsitry far outweighs anion-binding

Introduction

SubstrateCounter-ion

!+ !–

SubstrateCounter-ion

!– !+

! Ion-pair catalysis has been known for a very long time - phase transfer catalysis

! Ion-pairing involving a catalytic anionic component has only recently emerged

! New strategy for catalysis:

! Unexplored reactivity

! Enhanced reactivity vs ligand based approaches

! New asymmetric variants and entirely new transformations

Chiral Anion Mediated Asymmetric Chemistry

! The chiral pool provides a convenient source of enantiomerically pure anions

From Humble Beginings to New Strategies for Catalysis

–O2CCO2

Ph CO2–

–O2CCO2

Review: Lacour, J. Chem. Soc. Rev. 2003, 32, 373-382.

! Uses of chira anions: determination of enantiomeric purity

Mn(CO3

Pht-Bu

! Uses of chira anions: purification and enantiomeric separation of cations

(X = H or NMe2)

MeO OMe

MeO OMe N N

(X = H or Br)

! First transition metal-catalysed enantioselective transformation using chiral anions

PhPhINTs

OCu(MeCN)4

Benzene

Arndstsen, B. A. Org. Lett. 2000, 2, 4165-4168.

! Aziridination under 'classical' conditions displays a pronounced counteranion effect

PhPhINTs

10 mol% CuX, 0 ºC N

t-Bu t-Bu

(11 mol%)

X % ee (benzene) % ee (MeCN)

OTf 66 2

ClO4 57 2

Cl 26 2

PF6 33 2

! Synthesis of copper-boronate catalyst

Cu(MeCN)4

1) H2BBrSMe2

2) Ag2CO3

! Ion-pair crystal structure

Cu-O, 2.16 A

! Enantiocontrol from chiral boronate is low

PhPhINTs

OCu(MeCN)4

(1-3 mol% Cu-B(OR*)4)

1-3 mol% L

t-Bu t-Bu

(S)-Cu-B(OR*)4

13% ee (R)

t-Bu t-Bu

(R)-Cu-B(OR*)4

12% ee (R)

no ligand

(S)-Cu-B(OR*)4

7% ee (S)

no ligand

(R)-Cu-B(OR*)4

7% ee (R)

(R)-Cu-B(OR*)4

10% ee (R)

! Enantiocontrol from chiral boronate is low

PhPhINTs

OCu(MeCN)4

(1-3 mol% Cu-B(OR*)4)

1-3 mol% L

t-Bu t-Bu

(S)-Cu-B(OR*)4

13% ee (R)

t-Bu t-Bu

(R)-Cu-B(OR*)4

12% ee (R)

no ligand

(S)-Cu-B(OR*)4

7% ee (S)

no ligand

(R)-Cu-B(OR*)4

7% ee (R)

(R)-Cu-B(OR*)4

10% ee (R)

Cu-O, 2.55A

! Crystal structure of (bipy)Cu(H2C=CHPh)+(R)-B(OR*)4–

! Peptide derived chiral boranate cannot provide useful levels of enantioselectivity

Cu(MeCN)

1 mol%, CH2Cl2N2

max ee's 19% 34%

1:1 dr 3% yield

Arndstsen, B. A. Tetrahedron: Asymmetry 2005, 16, 1789-1799.

Braun, M. Angew. Chem. Intl. Ed 2004, 43, 514-517.

Titanium (IV) Catalysed Allylation

! Braun allylation of ethers is an overlooked reaction within this class

Anion Binding With Useful Levels of Selectivity

Me OTMS TMS Ti-cat. 10 mol%

–TMSOTMS

96% 98.9% ee

Me OTMS(±)

Me3SiOTiL*F2

Substrate

Gold Mediated Counteranion Catalysis

! Gold Catalysed enantioselective transformations were predicted to be difficult

The Toste Story

Au+Ligand

Large Distance

Short Reviews: Krause. Angew. Chem. Intl. Ed. 2008, 47, 2178-2181; Widenhoefer. Chem. Eur. J. 2008, 14, 5382-5391.

! Solution one: Development of bis(gold)-phosphine complexes

The Toste Story

(R)-DTBM-SEGPHOS-(AuCl)2

(S)-DTBM-MeOBIPHEP-(AuCl)2

2.5 mol% cat.

5 mol % AgSbF6

2.5 mol% cat.

5 mol % AgSbF6

2.5 mol% cat.

5 mol % AgSbF6

PhPh n

n = 0, 67%, 93% een = 1, 96%, 88% ee

NHTs Me

99%, 99% ee

60-85%

76-94% ee

Substrate

! Chiral anion strategy may also overcome inherent proximity problem

The Toste Story

Au+Ligand

Large Distance

Counter-ion ShortDistance

Toste, D. F. J. Am. Chem. Soc. 2007, 129, 2452-2453.

! Counter-ion effects had already been identified in ligand controlled reactions

The Toste Story

(R)-xylyl-BINAP-(AuCl)2

TsNNHTs

P Au Cl

+ BF4–

2+ 2BF4–

3 mol% cat, 3 mol% AgBF4

81% 51% ee

3 mol% cat, 6 mol% AgBF4

82% 1% ee

Toste, D. F. Science 2007, 317, 496-499

! Initial investigations: ligand vs anion control

The Toste Story

OOH H3 mol% L(AuCl)2, 3 mol% AgX

Ag-CO2-4-(NO2)-C6H3

0-8% ee

CH2Cl2

Toste, D. F. Science 2007, 317, 496-499

The Toste Story

OOH HLAuCl or L(AuCl)2, AgCat

L = PPh3 (5 mol%), 5 mol% (R)-Agcat

L = dppm (2.5 mol%), 5 mol% (R)-Agcat

CH2Cl2

i-Pr i-Pr

i-Pri-Pr

L = dppm (2.5 mol%), 5 mol% (R)-Agcat

89% 48% ee

76% 65% ee

90% 98% ee

(benzene)

dppm = Ph2P PPh2

Toste, D. F. Science 2007, 317, 496-499

The Toste Story

OH H2.5 mol% dppm(AuCl)2

benzene, 23 ˚C

79-96% 90-99% ee

5 mol% Agcat.

SO2MesN

NHSO2Mes H5 mol% Ph(CH3)2PAuCl

benzene, 23 ˚C

73-97% 96-99% ee

5 mol% Agcat.

Toste, D. F. Science 2007, 317, 496-499

Substrate

! Chiral amplification via ligand-counter ion relay

The Toste Story

Au+Ligand

Large Distance

Counter-ion ShortDistance

Toste, D. F. Science 2007, 317, 496-499

Substrate

! Chiral amplification via ligand-counter ion relay

The Toste Story

Au+Ligand

Large Distance

Counter-ionInteraction

ChiralAmplification?

Toste, D. F. Science 2007, 317, 496-499

! Chiral amplification vis ligand-counterion relay

The Toste Story

benzene, 23 ˚C

5 mol% Agcat.OH OH

i-Pr i-Pr

i-Pri-Pr

Ph2P PPh2

2.5 mol% dppm(AuCl)2

96% 80% ee

The Toste Story

benzene, 23 ˚C

5 mol% Agcat.OH OH

i-Pr i-Pr

i-Pri-Pr

(S,S)-DIPAMP

2.5 mol% [(S,S)-DIPAMP](AuCl)2

96% 92% ee

Toste, D. F. Science 2007, 317, 496-499

The Toste Story

benzene, 23 ˚C

5 mol% Agcat.

i-Pr i-Pr

i-Pri-Pr BINAP

dppm (R)-AgCat.

H5 mol% L(AuCl)2

(R)-BINAP (R)-AgCat.

(S)-BINAP (R)-AgCat.

89% 12% ee (S)

91% 3% ee (R)

88% 82% ee (S)

Norton, J. R. J. Am. Chem. Soc. 2005, 127, 7805-7814

Chiral Couneranion-Aided Asymmetric Hydrogenation

! Imine hydrogenation via an ionic mechanism

Asymmetric Imine Reduction

M–H N

Norton, J. R. J. Am. Chem. Soc. 2005, 127, 7805-7814

! Imine hydrogenation via an ionic mechanism

M–H N

BF4–

2 mol %

CpRu(P-P)HPPh2

(R,R)-Norphos50 psi H2

74% 54% ee

Xiao, J. J. Am. Chem. Soc. 2008, 130, 14450-14451

! Ir(III)-Phosphoric acid system leads to high enantioselectivities via ligand-anion control

HX*–

[Ir] (1mol%)

H2 (20 bar), toluene, 20 ºC

0% 0% ee

30% 81% ee

27% –3% ee

(R = 2,4,6-(2-C3H7)3C6H2)

Pacid (6mol%)

(Ar = 4-CH3C6H4)

HX*–

[Ir] (1mol%)

76% 97% ee 92% 97% ee

(R = 2,4,6-(2-C3H7)3C6H2)

Pacid (x mol%)

(Ar = 2,3,4,5,6-(CH3)5C6)

+ 1 mol%Pacid

92-95% 84-97% ee

(R = 2,4,6-(2-C3H7)3C6H2)

(Ar = 2,3,4,5,6-(CH3)5C6)

+ 1 mol%Pacid

MeNHPMB

Me NHPMB

93% 91% ee 88% 95% ee

90% 92% ee 91% 94% ee

List, B. J. Am. Chem. Soc. 2007, 129, 11336-11337.

Chiral Counteranions in Transition Metal Catalysis

! Pd/Brønsted acid-catalysed direct allylation of aldehydes

Chiral Co-Catalysts - List

i-Pr i-Pr

i-Pr i-Pr(1.5 mol%)

5A MS, MTBE, 40 ºC, 8-24 hr

R1 = Ar: 40-89% 83-97% ee

(1.0 equiv)

Pd(PPh3)4 3 mol%

R1 = Alkyl: 45-65% 80-90%ee

Iminium Catalysis - An Alternative Approach

! Proposed mechanism

(R)-TRIP

OR*O OR*

OR**RO

OR*O*RO

(R)-TRIP

Chiral Anion Phase Transfer Catalysis

! Reverse of roles commonly associated in phase transfer systems

Chiral Borate Anions

NucAnion cat. Nuc

Et3NH+

i-Pr i-Pr

2008 - Toste >90% ee2003 - Nelson <15% ee

ASYMMETRIC RING OPENING OF MESO-AZIRIDINIUM IONS

NucAnion cat. Nuc

Et3NH+

i-Pr i-Pr

*B(OR)4–

50 mol% cat.

THF-Toluene100 ˚C

H2N PhPh

A+ Yield % ee %

Et3NH+

Et2NH2+

Ph NH3+

3% (R, R)

8% (S, S)

Nelson, A. Tetrahedron: Asymmetry 2003, 14, 1995-204.

! Ion-pairing NMR studies

Nelson, A. Tetrahedron: Asymmetry 2003, 14, 1995-204.

Ph NH3+

! Borate salt was found to be soluabalised but the aziridinium triflate in CDCl3

! Proton and carbon shifts vary linearly with amount of borate salt present

! Lack of spliting of enatiotopic protons in the aziridium salt indicates poor discrimination from the chiral anion

Chiral Phosphonate Anions

NucAnion cat. Nuc

Et3NH+

i-Pr i-Pr

! Halide abstraction method requires regeneration of chiral silver salt

Cat.–

NucCat.Ag Nuc

–AgCl –Cat.H

! Addition of an achiral Ag(I) source must not introduce an interfereing counteranion

AgHalAgB

NucHR-Nuc

AgCat.

R+ Cat.–HCat.

Halide

Abstraction

Cat.–

NucCat.Ag Nuc

–AgCl –Cat.H

AgHalAgB

NucHR-Nuc

AgCat.

R+ Cat.–HCat.

Nucleophilic Addition

Cat.–

NucCat.Ag Nuc

–AgCl –Cat.H

AgHalAgB

NucHR-Nuc

AgCat.

R+ Cat.–HCat.

PhaseTransfer

Use solid phase Ag(I)

solid-liquid PTC

Toste, F. D. J. Am. Chem. Soc. 2008, 130, 14984-14986.

! Suitability of silver(I) sources is crucial for the catalyst control

Cat.–

O15 mol% cat.

i-Pr i-Pr

Toluene, 50˚C

Ag2CO3

88% 56% ee

77% 94% ee

84% 94% ee(4A MS)

! Utility - alcohol and amine fucntionality

81% 92% ee

Me OMe

MeMeMe

Me OMe

4-NO2-Ph

Me OMe

67% 94% ee 85% 97% ee 50% 92% ee

86% 92% ee 70% 99% ee 76% 94% ee 80% 94% ee, 94% de

! Utility - alcohol and amine fucntionality

81% 92% ee

Me OMe

MeMeMe

Me OMe

4-NO2-Ph

Me OMe

67% 94% ee 85% 97% ee 50% 92% ee

86% 92% ee 70% 99% ee 76% 94% ee 80% 94% ee, 94% de

! Extention to meso-episulfonium ions uses a modified system

Ph SMe

Cat.–

Ph SMe

OR15 mol% cat.

i-Pr i-Pr

ASYMMETRIC RING OPENING OF MESO-EPISULFONIUM IONS

Toluene, 23˚C

90-98% yield

CCl3HN

87-92%ee

! Mechanistically distinct from other phosphoric acid catalysed reactions enantioselectivity results from H-bonding to the electrohile

! Ring opening of the meso-episulfonium is the enantiodetermining step. Ion-pairing is responsible for the stereoselectivity

Hydrogen Bonding Mediated Counterion Catalysis

! Ureas and thioureas are proven motifs towards anion-binding using hydrogen bonding

Anion Recognition

Review: Schmidtchen, F. P. Chem. Rev. 1997, 97, 1609-1646.

! The stereoselectivity achieved by thiourea catalysts appears unusual

The Jacobsen Story

1) R'CHO, 3A MS˚

2) AcCl, 2,6-lutidine

Cat. (5-10 mol%)

Et2O, –30 to –60 ºC

65-81%, 85-95%ee

! The stereoselectivity achieved by thiourea catalysts appears unusual

The Jacobsen Story

1) R'CHO, 3A MS˚

2) AcCl, 2,6-lutidine

Cat. (5-10 mol%)

Et2O, –30 to –60 ºC

N(i-Bu)2

65-81%, 85-95%ee

! "The ability to activate a weakly Lewis basic N-acyliminium ion towards enantioselective transformations presents new opportunities for catalysis and raises intriguing questions as to the nature of this interaction."

Jacobsen, E. N. J. Am. Chem. Soc. 2004, 126, 10558-10559.

! H-Bond donor - anion binding catalysis is proposed in 2007

The Jacobsen Story

Cat. (10 mol%)

–55 ºC or –78 ºC

52-94%, 81-99%ee

! Alkylated derivatives react significantly faster than reduced compounds. SN1 mechanism.

R3 n=1, 2NH

R4n=1, 2

TMSCl, TBME

! SN2 Mechanism

Jacobsen, E. N. J. Am. Chem. Soc. 2008, 127, 13404-13405..

! Two SN1-type mechanisms are likely

The Jacobsen Story

Cl–N

spiro pathway

direct pathway

The Jacobsen Story

Cl–N

spiro pathway

direct pathway

! DFT calculations of fully ionised N-acyliminium ions interacting with thiourea derivatives failed to converge on any ground state structure.

The Jacobsen Story

Cl–N

spiro pathway

direct pathway

! Pronounced counter ion effect:

X– ee

Cl– 97%

Br– 68%

I– <5%

The Jacobsen Story

Cl–N

spiro pathway

direct pathway

! Pronounced solvent effect

solv ee

TBDME 97%

THF 34%

DCM <5%

see also: Jacobsen, E. N. Org. Lett,2008, 10,1577-1578.

! A versatile method for other highly reactive cationic species?

The Jacobsen Story

ACYL-IMINIUM ION STABILISATION OXOCARBENIUM ION STABILISATION?

! A versatile method for other highly reactive cationic species?

The Jacobsen Story

ACYL-IMINIUM ION STABILISATION OXOCARBENIUM ION STABILISATION?

! Cyclic oxo-carbenium ions are extremely unstable species (lifetime ~ 10-12 s)

! Enantioselective addtions to oxo-carbenium ions are extememly rare with only two examples reported (Braun: Angew. Intl. Ed. 2004, 43, 514-517 and Evans: JACS, 2005, 127, 10506-10507).

! Importance of catalyst structure

The Jacobsen Story

10 mol% cat.

ENANTIOSELECTIVE ADDITIONS TO OXOCARBENIUM IONS

TBDME, –78 ˚CO

R2 CO2R1R2

70-96% 74-97%ee

Reactivity at low temp

3º amide important forreactivity and selectivity

Aryl group crucial forselectivity - engagesoxocarbenium ion in TS?

Jacobsen, E. N. J. Am. Chem. Soc. 2008, 130 ,7198-7199.

The Jacobsen Story

10 mol% cat.

TBDME, –78 ˚CO

R2 CO2R1R2

70-96% 74-97%ee

87% 87% ee 71% 90% ee 70% 90% ee 96% 74% ee

The Jacobsen Story

10 mol% cat.

TBDME, –78 ˚CO

R2 CO2R1R2

70-96% 74-97%ee

92% 92% ee 84% 94% ee 85% 92% ee 87% 93% ee

MeMe O

! Explotation of ion-pair intermediate in iminium catalysis

NH HX*

Amine ControlledStereoselectivity

Anion ControlledStereoselectivity

'Matched' Ion-PairControl

! Reaction works across a range of aromatic aldehydes

i-Pr i-Pr

(20 mol%)

CO2MeMeO2C

(1.1 equiv) O

MeArDioxane, 50 ˚C, 24 hr

63-90% 96-99% ee

List, B. Angew. Chem. Intl. Ed. 2006, 45, 4193-4195.

! Achieving previously unatainable results - sterically unhindered aliphatic aldehydes

i-Pr i-Pr

(20 mol%)

CO2MeMeO2C

(1.1 equiv)

THF, r.t., 24 hr

71% 90% ee

Me Me Me Me

(E)-Citral (R)-Citronellal

58% 40% ee 82% 40% ee

! Extention to enone hydrogenation - matched-mismatched effects

i-Pr i-Pr

(20 mol%)

CO2MeMeO2C

(1.1 equiv)

Bu2O, 60 ºC, 24 hr

66% 54% ee 81% 94% ee

CO2t-Bu

CF3COO–

CO2t-Bu

(R)-TRIP

CO2t-Bu

(R)-TRIP

CO2t-Bu

(S)-TRIP

66% 48% ee 45% 16% ee

! Extention to enone hydrogenation - substrate tolerance

i-Pr i-Pr

(5 mol%)

CO2EtEtO2C

(1.2 equiv)

Bu2O, 60 ºC, 48 hr

68-78% 96-98% ee 99% 96% ee

CO2t-Bu

89-99% 90-98% ee R = CO2Et: 99% 90% ee

R = Ph: 81% 70% ee

! Extention to enal epoxidation

i-Pr i-Pr

i-Pr i-Pr(10 mol%)

(1.1 equiv)

Dioxane, 35 ºC, 72 hr

R = Ar: 60-84%, 98:2->99:1 dr90-96% ee

t-BuOOH

R = n-hex: 67%, 94:6 dr70% ee (maj), 92% ee (min)

! Tri-substituted enals have proven to be elusive substrates

i-Pr i-Pr

i-Pr i-Pr(10 mol%)

(1.1 equiv)

TBDME, 0 ºC, 24 hr

t-BuOOH

83% 94%ee 85% 94%ee 75% 90% ee85% 72:28 dr

76% ee (trans), 92% ee (cis)

! Proposed mechanism - unusual enantiodetermining step

t-BuOOH

TRIPMe

O Ot-Bu

t-BuOH

Ar NH2

Weakly Coordinating Anions in Chemistry

! Non-coordinating anions – Fact or fiction?

Background to Carboranes

ClO4 BF4 PF6B(C6H5)4

decreasing coordinating ability

! Weakly coordinating anions or 'superweak anions': considerations

F– abstraction

Accessible

O– coordination

Prone to hydrolysis,and oxidation

B(C6H5)4

Krossing, I. Angew. Chem. Intl. Ed. 2004, 43, 2066-2090.

! What should we look for in a weakly coordinating anion?

(Or, how to make a very strong Brønsted acid)

! Low nucleophilicity

! Chemical inertness (especially to protonation!)

! Low redox activity

! Charge delocalisation

! Large size

! Peripheral atoms non-basic

! Solubility in nonpolar solvents

Krossing, I. Angew. Chem. Intl. Ed. 2004, 43, 2066-2090.

! What should we look for in a weakly coordinating anion? Candidates:

CF3 CF3

[B(C6F5)4]

[CB11H12]

Reviews: Strauss, S. H. Chem. Rev. 1993, 93, 927-942; Seppelt, K. Angew. Chem. Intl. Ed. 1993, 33, 1025-1027.

! A brief introduction to carborane chemistry

! Constructed of 2c-2e- and 3c2e- bonds

! Delocalised !-bonding (!-aromaticity)

! 3D-analogue of benzene

! Aromatic properties: delocalised bonding unusual stablilty propensity for substitution reactions

! Other idications: resonance energies geometries (bond order indices etc.) magnetic properties

! Very strong B-H bonds (103 kcal/mol)

Michl, J. Chem. Rev. 2006, 106, 5208-5249.

! Carborane charge distribution

Michl, J. Chem. Rev. 2006, 106, 5208-5249.

! Carborane synthesis: B-H Insertion route

B10H14 - commerciallyavailable but expensive

Michl, J. Chem. Rev. 2006, 106, 5208-5249.

NaB11H14 - 50% yield

! Carborane synthesis: B-H Insertion route

NaBH4 BF3 (OEt)+

! Carbon insertion route

NaH CHCl3

Michl, J. Chem. Rev. 2006, 106, 5208-5249.

! Typical reactivity of carboranes

! Electrophilic substitution

n-BuLi MeI

ICl, DME

65 ºC

I2, AcOH

25 ºC

ICl, 200ºC

Michl, J. Chem. Rev. 2006, 106, 5208-5249.

! H(CHB11Cl11) - The strongest known Brønsted acid

! "Strong yet gentle": Can protonate C60 and benzene to form isolable salts.

! Acidity only measurable by indirect (NMR) methods. Outranks HFSO3,

! HFSO3/SbF6 decomposes such molecules

! Exceptional anion stability (c.f. HSbF6 & [B(C6F5)4]–

! Stability due to weakly basic anion - large size, s-delocalisation, Cl-shielding

Reed, C. A. Angew. Chem. Intl. Ed. 2004, 43, 5352-5355 J. Am. Chem. Soc. 2006, 128, 3160-3161.

Reed, C. A. Chem. Commun. 2006, 7, 767-769 Chem. Commun. 2005, 13, 1669-1677..

! Solution to an old problem: existence of a trialkylsilylium ion (R3Si+)

! Three coordinate silicon prone to coordination by anions, solvent and even argon!

! Si-Cl bond length 2.334 Å

! Ave. C-Si-C = 116.5º

! 29Si NMR (115 ppm) therefore 'ion like'.

! Highly Lewis acidic silicon group

Reed, C. A. Chem. Commun. 2006, 7, 767-769 Chem. Commun. 2005, 13, 1669-1677..

! Solution to an old problem: i-Pr3Si!+(CB11H6Cl6)!–

! Three coordinate silicon prone to coordination by anions, solvent and even argon!

! Si-Cl bond length 2.334 Å

! Ave. C-Si-C = 117.6º

! 29Si NMR (115 ppm) therefore 'ion like'.

! Highly Lewis acidic silicon group

! Uses of a R3Si+ cation: Hydrodefluorination reactions

R F H H R H H F

catalyst

R F SiR3 H R H SiR3 F

catalyst

! Challenges:

C-F bonds are amongst the most passive functonalities in chemistry (C-F is the strongest single bond to carbon)

C-F bond strengths increase as the degree of flourination increases

C-F bonds are poor ligands and poor substrates for nuc substitution and oxidative addition

Short Review: Braun, T. Angew. Chem. Intl. Ed. 2009, 48, 2-6.

R F H H R H H F

catalyst

R F SiR3 H R H SiR3 F

catalyst

! Why do we need a hydrodefluorination reaction?

Despite the utility of fluorinated compounds (refridgerants, anesthetics, polymers, solvents, ligands, catalysts),

the high persistence of fluorocarbons is responsible for their contribution to global warming

favourable by~ 190 kJ / mol

Oserov, O. V. J. Am. Chem. Soc. 2005, 127, 2852-2853.

Max TON ~ 100

perfluorinated alkanes 0% conv

Oserov, O. V. J. Am. Chem. Soc. 2005, 127, 2852-2853.

Max TON ~ 100

Max TON ~ 30

alkane solvents only

Krossing, I. Tetrahedron Lett. 2007, 48, 8900-8903.

[i-Bu2-Al]+ [Al(C6F5)4]

[i-Bu2-Al]+ [Al{OC(CF3)3}4]

ori-Bu2AlH + Ph3C+M–

Ph3C[HCB11H5Cl6] Et3SiH Et3Si[HCB11H5Cl6] Ph3CH

Et3SiH

–Et3SiF

86%, 0.08 mol%TON = 1250

0%, 5% indaneremainder higher MW

28% 13%

10% <3%

>97% convTON = 200

>97% convTON = 780

Oserov, O. V. Science 2008, 321, 1188-1190.

Ph3C[HCB11H5Cl6] Et3SiH Et3Si[HCB11H5Cl6] Ph3CH

Et3SiH

–Et3SiF

28% 13%

10% <3%

>97% conv

TON = 200

in o-C6H4Cl2 (>97% conv. TON = 1250)

in benzene (>97% conv. TON = 780)

Anion Binding in Chemistry

! Area has promise for a general catalytic manifold

Conclusions

! Much work is needed towards mechanistic understanding

! Many new reactions to be discovered

! 4-years is a very short amount of time since breakthrough reactions

! Lots of scope for the design of new catalysts

anion binding in catalysis - princeton university · chiral anion mediated asymmetric chemistry!...

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