Alkene Exchange Paul R' Sharp and Endre

1
Alkene Exchange Paul R. Sharp and Endre
2
of a Platinaoxetane Szuromi, Department of
Chemistry,
3
Reversible C-O Bond University of
Missouri-Columbia,
4
Formation Columbia, Missouri
5
Introduction
1
The chemistry of transition-metal oxo complexes
has been of interest to chemists for many years.
Chemical industries use late-transition metal
catalysts in a variety of processes. Many of
these processes involve the reaction of molecules
with a metal surface, where oxygen atoms
(oxo-groups) are formed and transferred to other
species. The chemistry occurring at the surface
is poorly understood and is more easily studied
through the use of soluble model complexes. This
approach can also be useful in understanding some
important solution-state processess, such as
homogeneous catalysis (e.g. Wacker-oxidation).
Interest in late-transition metal-oxygen bonds
involving the soft transition metals is due to
the anticipated high reactivity of these bonds.
6
2
Synthesis of oxo-complex 1
21

(1)
7
3
Formation of platinaoxetane 2
(2)
100
Note Compound 2i is the only known
metallaoxetane formed in the reaction of an
oxo-complex and an alkene. Metallaoxetane species
have been postulated as intermediates in several
alkene oxidation reactions.
8
4
Crystal structure of platinaoxetane 2
9
5
The unusual chemistry of platinaoxetane 2

• Platinaoxetane 2 was found to react with alkenes
  in an unusual way. While other metallaoxetanes
  undergo alkene insertion into the oxygen-metal
  bond, platinaoxetane 2 shows C-O and C-Pt bond
  cleavage and new C-O and C-Pt bond formation by
  way of an alkene-exchange.
• It was also discovered that oxo-complex 1 and
  platinaoxetane 2 exhibit similar alkene oxidation
  chemistry.
• with ethylene both oxo-complex 1 and
  platinaoxetane 2 form acetaldehyde, CODPt(Cl)Et
  and allyl-complex CODPt(CH2CHCHCH3) (BF4).
• with propylene both oxo-complex 1 and
  platinaoxetane 2 form acetone, CODPt(Cl)(CH2C(O)CH
  3) and allyl complex CODPt(CH2CHCH2)(BF4).

• In the next slides we introduce this remarkable
  chemistry focusing on the alkene-exchange
  reaction of platinaoxetane 2 and the
  ethylene-oxidation reaction of platinaoxetane 2
  and oxo-complex 1.

10
6
Reversible alkene extrusion from a platinaoxetane
Cl

C
O
(2)
(3)
CD2Cl2
rt, overnight

(4)
(5)
100
Confirmed by 1H and 195Pt NMR
11
7
Kinetics platinaoxetane 2 norbornylene
derivative 3
2030 15 19 113 126 150
2030 110
The rate vs. initial concentration of 3 plot is
not linear. In a catalytic process this can be
consistent with first-order kinetics involving
catalyst saturation.
This result indicates that the reaction is zeroth
order in platinaoxetane 2. It suggests that the
reaction is catalytic.
12
Kinetics continued
8
2030 15 19 113 126 150
2030 110 1.510 2.510
This Lineweaver-Burk plot indicates that the
reaction is catalytic, first order in
norbornylene derivative 3, and it involves
catalyst saturation.
The rate increases with increased initial
concentration of 2, although the reaction is
zeroth-order in 2. This result suggests that the
catalyst is an impurity found in platinaoxetane
2. Identification of the catalyst is in progress.
13
9
Possible alkene-exchange mechanism
14
10
Ethylene oxidation by oxo complex 1 and
platinaoxetane 2

• 17O-labeling labeled 2 produces labeled
  acetaldehyde.

15
11
Proposed ethylene oxidation pathway based on a
metallaoxetane intermediate formation
(1)
(2)
Note Some intermediates were detected using 1H
and 13C NMR (using 13C-labeled samples at low
temperature.)
16
Acknowledgements
12
Conclusions and future plans

• Reversible alkene exchange was discovered with a
  platinaoxetane and a norbornylene derivative at
  room temperature.
• Kinetic studies showed that this reaction was a
  catalytic process zeroth-order in the
  platinaoxetane and first order in the
  norbornylene derivative with the catalyst being a
  yet unidentified impurity in the platinaoxetane.
• The chemistry exhibited by oxo-complex 1 and
  platinaoxetane 2 with alkenes demonstrates that
  late transition metal m-oxo complexes can oxidize
  alkenes possibly through formation of
  metallaoxetanes.
• Future work will be directed at (a) identifying
  the catalyst in the norbornylene-exchange
  reaction, (b) fully identifying the ethylene
  reaction intermediates and (c) investigating
  other alkene oxidation reactions of complexes 1
  and 2.

• NMR facility, Department of Chemistry,
  University of Missouri-Columbia
• X-Ray facility, Department of Chemistry,
  University of Missouri-Columbia
• U.S. Department of Energy