Homogenous Hydrogenation - Lecture 6-8

activation of dihydrogen by TM, mechanisms, chemoselective and stereoselective reactions

Hydrogenation reaction

• favourable dynamics

• alkene → alkane

• carbonyl → alcohol

The direct addition of H2 is forbidden why?

• symmetry

• the HOMO and the LUMO of the dihydride are of the wrong symmetry to overlap with the HOMO and LUMO of the alkene

How to do a hydrogenation reaction?

1. stoichiometric addition of hydride and proton

   • LiAlH4 or NaBH4 reduction of ketones and aldehydes (H- then H+)

2. catalytic activation of hydrogen using transition metals

   • metal nanoparticles / metal surfaces = heterogenous

   • single TM centre = homogenous

3. transfer hydrogenation - extraction of hydrogen from sacrificial molecules

   • take it from an alcohol

Activation of dihydrogen

• very stable molecule

• strong and completely non polar H-H bonds

• typically require activation with a (reactive) TM

  • no reaction

  • stable n2-H2 complex (L)

  • dihydride complex (X2) - most common

  • mono hydride complex

A coordinated H₂ has ….

• weaker bonds

bonding in dihydrogen complexes

• formed by both HOMO and LUMO of the H2 with the frontier MOs of TM

• results in population of the sigma* orbital of the HH and H-H bond elongation

H-H bond length elongation

• pi back donation leads to reduction of H2 bond order

• complete donation results in OA of H2

Reactivity of dihydrogen complexes

1. low coordinate metal centre with d(pi) electrons (free coordination site)

2. stable intermediate or TS (of metal and dihydrogen)

3. either

   • homolytic cleavage via oxidative addition change in VN = +2

   • heterolytic cleavage via deprotonation by base, change in VN = 0

Reactivity of dihydrogen complexes - when there is a x type ligand cis to the free coordination site

1. x ligand cis to the free coordination site

2. sigma bond metathesis

3. concerted transition state

Wilkinson’s pre catalyst

• involves OA of H₂

• common for group 9 metals - before coordination of the olefin

Explain this kinetic pathway of a Wilkinson’s catalyst

wilkinson’s pre catalyst - deactivation pathway

• lone pair on chlorine forms bonds

• forms an inactive dimeric species

Mono hydride mechanism

• single hydride ligand present prior to the coordination of the alkene

• no OA

Explain this Mono hydride mechanism

Asymmetric hydrogenation catalysis

• ability to control stereochemistry has very important applications

• selectivity is reported as enantiomeric excess ee(%) = %major - %minor

Re vs Si face

chiral chelating ligands

• when you have the product - not enantiomers they are distereoisomers

Control of enantioselectivity

• use chiral, enantiopure metal catalyst [m]

Curtin-Hammett principle

• If A + B are in fast equilibrium (relative to product formation), then the product composition is determined by the difference G at Ts A and TS B

enamide hydrogenation - why is there such different barriers

• major diastereomer gives minor product (S) due to forced steric clash

• minor diastereomer gives the major product (R) due to minor changes in conformation, lowest barrier

Hydrogenation of carbonyls - Inner sphere

• coordination of the unsaturated substrate (mono hydride mechanism - no change in OS or VN)

• Alkenes > ketones

• strongly coordinating double bond

Hydrogenation of carbonyls - Outer sphere

• no direct coordination of the unsaturated substrate

• ketones > alkenes

• polar double bond

Explain the outer sphere hydrogenation - ionic mechanism

Explain the outer sphere hydrogenation - bifunctional mechanism