C-H Activation by Transition Metal Complexes: …chemistry.caltech.edu/courses/ch154/L11.pdf“C-H...
Transcript of C-H Activation by Transition Metal Complexes: …chemistry.caltech.edu/courses/ch154/L11.pdf“C-H...
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“C-H Activation by Transition Metal Complexes:
Fundamentals and Basic Concepts”
Chem 154 Organometallic Chemistry
Lecture 11
John Bercaw
California Institute of Technology
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There are many examples of C-H bond "activation" of alkanes (including methane) by
organometallic complexes under mild conditions
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There are many examples of C-H bond "activation" of alkanes (including methane) by
organometallic complexes under mild conditions
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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
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Thermodynamic factors: oxidative addition of C-H bonds
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Extensive work by Bergman and others reveals generally low kinetic selectivity for
these very exothermic d8 d6 oxidative additions:
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Reductive elimination of C-H bonds provides information on mechanism of reverse
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Other reductive eliminations (endothermic ones) display distinctively differing features:
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Competitive exchange between W-D and W-CH3 argues for reversible formation of a
tungstenocene-methane complex:
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complexes likely mediate all reductive eliminations
(and oxidative additions) of C-H bonds (and H2):
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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C-H bond activation without redox: bond metathesis
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Use of pentamethylcyclopentadienyl ligands simplifies metallocene chemistry by
disfavoring oligomers and adducts of these highly coordinatively unsaturated group 3
and lanthanide complexes:
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Cp*: a life-long obsession:
vanity license plate,
presented by Bercaw
research group, ca. 1983
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H/D exchange: the Cp*2Sc-R system
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Rates of bond metathesis is relatively sensitive to hybridization of reacting bonds
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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
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Experimental probes of rates and regioselectivity for reaction of Cp*2ScCH3 with
arenes provides convincing evidence for mechanism of bond metathesis
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Bonding interactions for bond metathesis
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Better total overlap leads to preferred metathetical direction for bond metathesis:
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Steric interactions with the Cp* ligands place constraints on C-H bond activation:
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Theoretical studies of bond metathesis confirm these pictures of the
bonding interactions in the transition state
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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Bergman and Wolczanski simultaneously discover 1,2-addition of C-H bonds to [Zr=NR]
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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):
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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Brad Wayland developed the cleanest and most generally reactive metalloradical
system based on (porphyrin)RhII, a stable, low spin d7, 15-electron complex:
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(porphyrin)RhII radicals are remarkably reactive for methane cleavage:
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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
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Tethered binuclear (porphyrin)RhII expands substrate scope:
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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Activation of Carbon-Hydrogen Bonds at Transition Metal Centers
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There are many examples of C-H bond "activation" of alkanes (including methane) by
organometallic complexes under mild conditions
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…
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