TransitionMetal Catalyzed Alkane Activation

1
Transition-Metal Catalyzed Alkane Activation

• Nathan Maugel 02/07/07

2
Overview

• I. Significance
• II. Challenges and Early Solutions
• Reactivity
• Selectivity Catalysis
• III. Recent Developments
• Alkane Dehydrogenation
• Alkane Metathesis
• Alkane Borylation
• IV. Summary

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Significance
Ethylene(110 million tons/year) Propylene(65
million tons/year)
Thermal Cracking
Current Oil Refining for Petrochemicals
LPG or light HCs, 850C, milliseconds
Fractional Distillation
Petrochemicals
Catalytic Reforming
Aromatics (70 million tons/year)
Light HCs, 500C/50atm Pt or Re chloride catalyst

• Problems
• Energy Intensive ()
• High Emissions
• Poor Selectivity
• Many Steps

4
Overview

• I. Significance
• II. Challenges and Early Solutions
• Reactivity
• Selectivity Catalysis
• IV. Recent Developments
• Alkane Dehydrogenation
• Alkane Metathesis
• Alkane Borylation
• V. Future Work
• VI. Summary

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Reactivity Challenges

• 5 Modes of C-H Activation
• 1.) Oxidative Addition (Isolated Intermediate)
• 2.) Electrophilic Activation (Direct
  Functionalization)
• 3.) Sigma Bond Metathesis (4-membered T.S.)

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Reactivity Challenges cont.

• 5 Modes of C-H Activation
• 4.) 1,2-Addition (Alkane MNonmetal)
• 5.) Metalloradical Activation (H-Atom
  Abstraction)

Wolczanski, JACS, 1988,110, 8731
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Reactivity Challenges cont.

• Difficulty of C-H Activation
• Inherently non-reactive (Paraffinlow affinity)
• HOMOs Deep Lying s-Bonding Orbitals
• LUMOs High Energy s Orbitals
• Sterically Hindered
• Reagent has 3 choices
• 1.) Donate e-density to C-H s (NO)
• 2.) Abstract C-H s-bonding es (LAs
  superacids)
• 3.) Do both (carbenes) CH2 CH4 C2H6
  (diamagnetic low-valent metals)

Crabtree, Chem. Rev. 1985, 85, 245 Hoffman,
JACS, 1971, 93, 6188
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Reactivity Solutions

• First SP3 C-H Activation
• Chatt and Davidson 1965
• Driven by Chelate Effect
• Gave Intermolecular C-H Addition of Arene but not
  Alkanes

Chatt, Davidson, J. Chem. Soc., 1965, 843
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Reactivity Solutions cont.

• Shilov 1972 (hydroxylation/chlorination)
• Crabtree 1979 (transfer dehydrogenation)
• Groves 1979 (biomimetic hydroxylation)

Shilov, Kinet. Katal. 1972, 13, 534 Crabtree,
JACS, 1979,101, 7738 Groves, JACS, 1979, 101,
1032
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Overview

• I. Significance
• II. Challenges and Early Solutions
• Reactivity
• Selectivity Catalysis
• III. Recent Developments
• Alkane Dehydrogenation
• Alkane Metathesis
• Alkane Borylation
• IV. Summary

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Selectivity Challenges

• Alkane Dehydrogenation
• Alkane Oxidation
• Methane Oxidation

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Selectivity Solutions

• Selective Dehydrogenation (Zakrzewski)
• Selective Carbonylation (Tanaka)
• Selective Oxidation (Periana)

Zakrzewski, Chem. Commun. 1982,1235Tanaka, Chem
Commun. 1987, 758 Periana, Science, 1993, 341
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Methane Oxidation

• Key Features
• CH3OSO3H Stability
• HgX Species

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MethaneOxidation

• Industrial 3-Step Process?

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Overview

• I. Significance
• II. Challenges and Early Solutions
• Reactivity
• Selectivity Catalysis
• III. Recent Developments
• Alkane Dehydrogenation
• Alkane Metathesis
• Alkane Borylation
• IV. Summary

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Recent Developments

• Hartwig 1997 Science, 277, 211 (1997)

None of these (Alkane C-H activation
approaches) has provided a single isomer of a
useful organic product from an alkane.
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Alkane Dehydrogenation

• n-Octane Dehydrogenation
• Observations
• Kinetic Regioselectivity Favors 1-Octene
• nbe Reacts with (PCP)IrH2 2X Faster than tbe
• nbe gives higher selectivity for 1-octene
• Higher Acceptor Concentrations Favor 1-Octene
• Ultimately
• Octene isomer distribution is determined by the
  relative rates of isomerization and
  dehydrogenation.

Jensen, Goldman, JACS, 1999, 121, 4086
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Proposed Dehydrogenation/Isomerization Cycle
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Alkane Dehydrogenation

• Test Hypothesis
• They Proposed
• 1-decene should react with (PCP)IrH2 as
  efficiently as 1-octene
• Rate of 1-octene isomerization should be slower

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Proposed Dehydrogenation/Isomerization Cycle
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Alkane Dehydrogenation

• Results
• 84 1-Octene at 94 mM (94 Turnovers)
• High Concentrations of 1-Octene?
• Selectively Remove 1-Octene?
• System Inherently Limited?

22
Overview

• I. Significance
• II. Challenges and Early Solutions
• Reactivity
• Selectivity Catalysis
• III. Recent Developments
• Alkane Dehydrogenation
• Alkane Metathesis
• Alkane Borylation
• IV. Summary

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Alkane Metathesis

• Burnett and Hughes 1973
• Solves Product Inhibition Problem?
• Significance
• Replace Fischer-Tropsch Process (CO H2
  Alkanes)?
• Tighter MW distribution (C9-C20)
• Less Energy Intensive
• Lower Emissions

Burnett, R.L. Hughes, T. R. J. Catal. 1973, 31,
55
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Alkane Metathesis

• Goldman, Brookhart 2006 Science
• Catalytic Alkane Metathesis by Tandem Alkane
  Dehydrogenation-Olefin Metathesis
• Dual Catalytic System

Goldman, Brookhart,M. Science 2006, 312, 257.
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Alkane Metathesis Cycle
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Alkane Metathesis

• Preliminary Results

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Alkane Metathesis

• 2 Questions
• 1. Pentane Heptane (Isomerization)?
• 2. Metathesis Catalyst Decomposition?

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Alkane Metathesis

• Pentane Heptane?
• Isomerization Before Metathesis?

• Solution
• Alternative (PCP)Ir Catalyst?
• Catalyze Isomerization more slowly?

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Alkane Metathesis

• Different (PCP)Ir Catalyst

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Alkane Metathesis

• 2. Metathesis Catalyst Decomposition?
• More Catalyst Reinitiates Metathesis
• 13C, 1H, and 31P NMR Reaction Monitoring
• Early Reaction Times
• 2-Methyl-2-Phenylbutane
• Late Reaction Times
• 2,6-Diisopropylaniline (Decreased Reaction Rate)

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Alkane Metathesis

• More Robust Metathesis Catalyst?

• Reaction F
• Turnover Potential of Robust Metathesis Catalyst
• 4.37M Product from 5.1M SM
• 486 Turnovers (wrt Ir)

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Alkane Metathesis

• Future Studies
• Metathesis Catalyst
• More Robust Catalysts Higher TONs
• Dehydrogenation Catalyst
• More Active Catalysts More TONs Before
  Metathesis Catalyst Decomposition
• Less Isomerization-Prone Catalysts Greater
  Selectivity

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Overview

• I. Significance
• II. Challenges and Early Solutions
• Reactivity
• Selectivity Catalysis
• III. Recent Developments
• Alkane Dehydrogenation
• Alkane Metathesis
• Alkane Borylation
• IV. Summary

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Alkane Borylation

• Hartwig 1997 Science, 277, 211 (1997)

What Does He Have To Offer????
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Terminal Selectivity

• Bergman 1982
• Jones 1983

Eliminates Alkane at 110C
Eliminates Alkane at -15C
Bergman, JACS, 1982, 104, 352 Jones,
Organometallics, 1983, 2, 562
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Alkane Borylation

• Why Borylation?
• Versatile Synthetic Intermediates
• Used Directly as Suzuki Reagents
• Boronates Can Be Converted to
• Alcohols
• Amines
• Ketones
• Boron Reagent Can be Recycled

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Alkane Borylation

• Background
• Transition-Metal Boryl Complexes
• Properties
• Empty P-Orbital Interesting Structure/Reactivity
• Strong s-Donor
• Unsaturation Metal Back-Donation

Hartwig, J. F. J. Am. Chem. Soc. 1993, 115, 4908
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Alkane Borylation

• Initial Observations
• Can X-H be C-H?

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Alkane Borylation

• First C-H Borylation
• First Example of Fp Complex C-H Activation
• Does Boron Have Special C-H Activating Properties?

Hartwig, J. F. J. Am. Chem. Soc. 1995, 117,
11357 Hartwig, Science 1997, 277, 211
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Alkane Borylation

• Selectivity
• Something Special About Boron?

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Alkane Borylation

• Thermal Catalytic Borylation

Hartwig, Science 2000, 287, 1995
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Alkane Borylation

• Mechanistic studies

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Alkane Borylation

• Computational Studies
• Sigma-Bond Metathesis Pathway?
• 10 Kcal/mol Lower In Energy Than Oxidative
  Addition Pathway

Hall, M. B. Hartwig, J. F. J. Am. Chem. Soc.
2002, 124, 858
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Alkane Borylation

• Intermediate Isolation
• Intermediates A B
• NMR Data of A B Show B-H Interaction
• 1H NMR Hydride Signal Sharpens Upon 11B
  Decoupling
• 1H-11B HMQC Shows Cross Peaks

Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 2538
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Alkane Borylation

• Crystal Structures of Intermediates A B
• Evidence For Significant
• B-H Interaction

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Alkane Borylation

• Dissociation of Borane
• Rates of dissociation directly proportional to
  rates of alkane activation (B faster than A)
• These are the 16-electron species that react with
  alkanes

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Alkane Borylation

• Conclusions
• Borylation is Catalytic and Selective Because
• Significant B-H Bonding Character
• Facilitates borane formation to generate reactive
  species.
• Stabilizes the transition state for C-H bond
  cleavage (computational)
• Stabilizes the product from C-H cleavage (comp)
• Boron P-Orbital
• Participates in C-H bond cleavage (comp)
• Thermodynamics
• Formation of B-C bond (112Kcal/mol) provides
  driving force

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Summary

• Alkane Dehydrogenation
• Excellent Kinetic Selectivity for 1-Alkenes
• Isomerization Problem
• Alkane Metathesis
• Longer Alkanes From Shorter Alkanes
• High Turnover Numbers Possible
• Metathesis Catalyst Decomposition
• Alkane Borylation
• Excellent Terminal Selectivity
• Boron is Unique
• Possible With Ruthenium
• Cost of Boronate Reagents