Organometallics Study Meeting

2011/08/28 Kimura

Chapter 14. Principles of Catalysis
14. 1. General Principles

14.1.1. Definition of a Catalyst
14.1.2. Energetics of Catalysis
14.1.3. Reaction Coordinate Diagrams of Catalytic Reactions
14.1.4. Origins of Transition State Stabilization

14.1.5. Terminology of Catalysis
14.1.6. Kinetics of Catalytic Reactions and Resting States
14.1.7. Homogeneous vs. Heterogeneous Catalysis

114. 2. Fundamentals of Asymmetric Catalysis

14.2.1. Importance of Asymmetric Catalysis
14.2.2. Classes of Asymmetric Transformations
14.2.3. Nomenclature
14.2.4. Energetics of Stereoselectivity

14.2.5. Transmission of Asymmetry
14.2.6. Alternative Asymmetric Processes:
Kinetic Resolution and Desymmetrizations2

14.2.4. Energetics of Stereoselectivity• ΔΔG‡= 1.38 kcal/mol => 10:1 ratio of product (at rt.)
• ΔΔG‡= 2 kcal/mol => 90%ee3

14.2.4.1.1 Reaction with a Single Enantioselectivity-Determining Step
• simplest case:>direct reaction of catalyst. and prochiral substrate.
>without coordination of subst. to cat. before enantioselectivity-determining step
• atom and group-transfer reactions (epoxidation, aziridination etc.)414.2.4.1.1 Reaction with Revesibility Prior to the Enantioselectivity-Determining
Step: The Curtin-Hammett Principle Applied to Asymmetric Catalysis• Prochiral substrates bind to catalyst in a separate step from enantioselectivitydetermining step (EDS)•1) interconversion of I and I’ is slow relative to conversion to the product (Scheme 14.12.A)
EDS = binding to the prochiral olefin faces to the metal•2) interconversion of I and I’ is significantly fast: (Scheme 14.12.B)
EDS = reaction to form the product (Curtin-Hammett conditions)5

14.2.4.1.1 The Curtin-Hammett Principle• when competing reaction pathways begin from rapidly interconverting isomers,⇒ product ration is determined by the relative heights of the highest
barriers leading to the two different products (DDG ‡= GI‡- GI’ ‡)•DDG‡ exp( )RT[I ]
K eq 
[I ' ]
 DDG ‡RS

enantioselectivity is controlled by the relative energy of the two diastereomericTSs (rather than the stabilities of the two diastereomeric intermediates) 6

14.2.4.1.3.2.1 Curtin-Hammett : Example 1: Asymmetric Hydrogenation 1

Figure 14.13.
Mechanism of the asymmetric hydrogenation, illustrating a reaction meeting
the Curtin-Hammett conditions

714.2.4.1.3.2.1 Curtin-Hammett : Example 1: Asymmetric Hydrogenation 28

14.2.4.1.3.2.2 Curtin-Hammett : Example 2: Asymmetric Allylic Alkylation 1

Interconversion occurs within thecoordination sphere of the metal center.
Figure 14.15.
Interconversion of the diastereomeric p-allyls I and I’ occurs via an h1-allyl. The enantioselectivitydetermining step depends on the relative rates of psp isomerization and nucleophilic attack.

• dilute conditions will help to achieve Curtin-Hammett conditions(unimolecular v.s. bimolecular)• Halide ions catalyze the isomerization• reversed enantioselectivity in the presence/absence of additives

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14.2.4.1.3.2.2 Curtin-Hammett : Example 2: Asymmetric Allylic Alkylation 2

B. M. Trost, F. D. Toste JACS, 1999, 121, 4545

• Halide anion + diluted condition => Curtin-Hammett conditions• Ammonium cation lowers phenol nucleophilicity?

1014.2.5.1 Effect of C2 Symmetry• it was often observed that C2-symmetric catalyst were most effective• Kagan: smaller number of metal-substrate adducts and TSs available

Figure 14.15.
Interconversion of the diastereomeric pallyls I and I’ occurs via an h1-allyl. The
enantioselectivity-determining step
depends on the relative rates of pspisomerization and nucleophilic attack.
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14.2.5.2 Quadrant Diagrams• generic model for steric biasingof chiral metal-ligand adducts• shaded: hindered• white: less hindered• stereogenic centers close to the metal: e.g.. Pybox (Fig. 14.18.A)
more distant from metal: e.g. Chiraphos (Fig. 14.18.B)• chiraphos) Me: pseudo-equatorial
two phenyls: pseudo-axial (edge) + pseudo-equatorial (face)1214.2.6 Alternative Asymmetric Processes:
Kinetic Resolutions and Desymmetrizations

14.2.6.1. Kinetic Resolutions
14.2.6.2. Dynamic Kinetic Resolution
14.2.6.3. Dynamic Kinetic Asymmetric Transformations
14.2.6.4. Asymmetric Desymmetrizations

1314.2.6.1. Kinetic Resolutions
Kinetic Resolution (KR)• reactions that occur at different rates with two enantiomers of a chiral substrate• do not usually generate additional stereochemistry• distinguish one enantiomer from another by creating new functionality• maximum yield: 50%• best option when racemate is inexpensive,
no practical enantioselective route is available

1414.2.6.1.3. Examples of Kinetic Resolutions

Figure. 14.26.
Kinetic resolution in theasymmetric allylic substitution
Trost, B. M. et al. TL 1999, 40, 219

Schrock, R. R.. et al.
JACS 1999 121 8251

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14.2.6.2. Dynamic Kinetic Resolutions
Dynamic Kinetic Resolution (DKR)• KR in a fashion that allows the conversion of both enantiomers of the reactant
into a single enantiomer of the product• KR with a rapid racemization of the chiral substrate thorough an achiralintermediate (=I) or transition state•In a typical DKR: krac ≥ kfast• if substrate fully equbriuming andkfast /kslow ~ 20 => ee ~ 90%

1614.2.6.2.1. Examples of Dynamic Kinetic Resolutions

Noyori, R. et al. BCSJ 1995, 68, 36
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14.2.6.3. Dynamic Kinetic Asymmetric Transformations (DyKAT)• Mechanism of stereochemical interconversions distinguishes DKR and DyKAT• DKR: catalyst that promotes racemization is achiralunrelated to resolution step
• DyKAT: interconversion of subst. stereochemistry occurs on asymmetric cat.
(epimerization)
DyKAT

KR

DKR

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14.2.6.3. Examples of DyKAT

D. S. Glueck et al. JACS 2002 124 13556

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14.2.6.4. Desymmetrization Reactions• differential reactivity of enantiotopic FGs of subst. with chiral reagent or cat.
• catalyst differentiates between enantiotopic groups within single substrate(cf. KR: differentiate between enantiomers of a racemic substrate)

Figure. 14.34.
Desymmetrization of dienes by catalytic asymmetric hydrosilylation.
Oxidation of the product provides a valuable 1,3-diol

Ito, Y. et al.
TL 1990, 31, 7333

(14.19)Shibasaki, M. et al.
TL 1993, 34, 4219

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