C-H Activation by Transition Metal Complexes: …chemistry.caltech.edu/courses/ch154/L11.pdf“C-H Activation by Transition Metal Complexes: Fundamentals and Basic Concepts” Chem

“C-H Activation by Transition Metal Complexes:

Fundamentals and Basic Concepts”

Chem 154 Organometallic Chemistry

Lecture 11

John Bercaw

California Institute of Technology

There are many examples of C-H bond "activation" of alkanes (including methane) by

organometallic complexes under mild conditions



Oxidative addition/reductive elimination of H2 and C-H bonds:

relative to C-H bonds, oxidative addition of H2 is common, reversible, and facile

H

L

Thermodynamic factors: oxidative addition of C-H bonds

Extensive work by Bergman and others reveals generally low kinetic selectivity for

these very exothermic d8 d6 oxidative additions:

Reductive elimination of C-H bonds provides information on mechanism of reverse

Other reductive eliminations (endothermic ones) display distinctively differing features:

Competitive exchange between W-D and W-CH3 argues for reversible formation of a

tungstenocene-methane complex:

complexes likely mediate all reductive eliminations

(and oxidative additions) of C-H bonds (and H2):

Activation of Carbon-Hydrogen Bonds at Transition Metal Centers



C-H bond activation without redox: bond metathesis

Use of pentamethylcyclopentadienyl ligands simplifies metallocene chemistry by

disfavoring oligomers and adducts of these highly coordinatively unsaturated group 3

and lanthanide complexes:

Cp*: a life-long obsession:

vanity license plate,

presented by Bercaw

research group, ca. 1983

H/D exchange: the Cp*2Sc-R system

Rates of bond metathesis is relatively sensitive to hybridization of reacting bonds

Faster rates of bond metathesis for sp2 and sp hybridized C-H bonds might

suggest electrophilic Cp*2Sc-R attacks at bonds of substrate:

• HOMO of arene attacked by electrophilic

Cp*2ScR

• sterically worst approach and TS

• C-H bond attacked (lower energy

than orbitals) by Cp*2ScR

…as for H2, CH4, etc.

• sterically best approach and TS

Experimental probes of rates and regioselectivity for reaction of Cp*2ScCH3 with

arenes provides convincing evidence for mechanism of bond metathesis

Bonding interactions for bond metathesis

Better total overlap leads to preferred metathetical direction for bond metathesis:

Steric interactions with the Cp* ligands place constraints on C-H bond activation:

Theoretical studies of bond metathesis confirm these pictures of the

bonding interactions in the transition state




Bergman and Wolczanski simultaneously discover 1,2-addition of C-H bonds to [Zr=NR]

Jordan Bennett and Peter Wolczanski take a careful and in-depth look at mechanism for

1,2 addition of C-H to [Ti=NR] (and by inference to his [Zr=NR], [V=NR] and [Ta=NR] systems):




Brad Wayland developed the cleanest and most generally reactive metalloradical

system based on (porphyrin)RhII, a stable, low spin d7, 15-electron complex:

(porphyrin)RhII radicals are remarkably reactive for methane cleavage:

Mechanism deduced from rate laws, kH/kD, small H‡, and Ho:

Termolecular TS with backside attack restricts substrates to H2 and unhindered sp3 C-H bonds

Tethered binuclear (porphyrin)RhII expands substrate scope:





Unfortunately, thus far most of the organometallic complexes that react with C-H bonds

of alkanes are decomposed by O2 and/or ROH…

with the exceptions of some of those complexes that react via “electrophilic displacement” of

H+…we shall consider whether this mechanism is, in fact, a different mechanism…

END

Documents

C-H Activation by Transition Metal Complexes: …chemistry.caltech.edu/courses/ch154/L11.pdf“C-H Activation by Transition Metal Complexes: Fundamentals and Basic Concepts” Chem