1.Complexes of alkenes, alkynes, and dienes
Created by Margaret L. Scheuermann, Princeton University; Abby R. O’Connor, The College of New Jersey, oconnora@tcnj.edu. Copyright Scheuermann and O’Connor, 2014. This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this license visit http://creativecommons.org/about/license/
2.2
History & Utility
W. C. Zeise, Annalen der Physik, 1831, 21, 497.
Review: Hunt. Platin. Met. Rev. 1984, 28, 76.
First example
Popular starting materials and precatalysts:
Catalytic intermediates:
Alkene hydrogenation^*
Alkene dimerization/ oligomerization/ polymerization^*
Olefin metathesis^*
Mizoroki-Heck^*
Wacker Oxidation^
Hydrofunctionalization- Hydroboration/ hydosilylation^/ hydrocyanation^/ hydroamination^/ hydroformylation^/ etc.
^ significant industrial use; * Nobel Prize
3.Orbital Overlap- Octahedral Example
3
C-C π
C-C π*
C-C σ
C-C σ*
M-olefin σ
M-olefin π
M-olefin σ*
M-olefin π*
M-olefin π*
M-olefin σ*
M-olefin π
M-olefin σ
dz2
dxz
(or dyz)
dx2-y2
Dxy (or dyz or dxz)
Olefin
Olefin along x (or y) axis
Olefin along z axis
x
y
z
4.Types of π-bound ligands
4
Extremes
Less backbonding into π*… electron poor metal center
More backbonding into π*… electron rich metal center
5.Properties
5
Rotation around metal-olefin bond
Orientation determined by steric and electronic factors
Bond lengths and angles
Electron rich metal centers tend to form more stable olefin complexes
NMR
As backbonding increases so do geometry differences
C-C bonds get longer
sp2 becomes more sp3-like
sp becomes more sp2-like
can help relieve ring strain
Buchwald. JACS 1986, 165, 7441.
late metals
Low oxidation states
high d-electron counts
low overall charge
When backbonding is minimal (electron poor metal center) 1H and 13C shifts resemble free olefin
When backbonding is significant (electron rich metal center) 1H and 13C resonances shift upfield- smaller δ
6.Synthesis & Reactivity
6
Ligand substitution*
Reduction
β-hydride elimination*
Displacement by ligand substitution*
Nucleophilic attack*
(when metal center is a poor π donor)
Insertion*
R = H, alkyl, aryl
α
β
* Often present in catalytic cycles