A catalytic cycle for oxidation of tert-butyl methyl ether by a double C-H activation-group transfer process

Article abstract of DOI:10.1021/ja8071385

A square-planar, iridium(I) carbene complex is shown to effect atom and group transfer from nitrous oxide and organic azides, releasing the corresponding formate or formimidate and an iridium(I)-dinitrogen adduct. The dinitrogen complex performs C?H activation upon photolysis or thermolysis, regenerating the carbene from tert-butyl methyl ether with loss of H₂. Taken together, these reactions represent a net catalytic cycle for C?H functionalization by double C?H activation to generate metal?carbon multiple bonds. Additionally, the unusual group transfer from diazo reagents underscores the unique nature of the reactivity observed for nucleophilic-at-metal carbene complexes. Copyright

Full text of DOI:10.1021/ja8071385

Published on Web 11/12/2008
A Catalytic Cycle for Oxidation of tert-Butyl Methyl Ether by a Double C-H
Activation-Group Transfer Process
Matthew T. Whited and Robert H. Grubbs*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, DiVision of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125
Received September 9, 2008; E-mail: rhg@caltech.edu
Scheme 1. Proposed Cycle for MTBE Oxidation
Despite numerous advances and decades of research, unactivated
C-H bonds are still considered almost entirely inert to catalytic
cleavage and functionalization. Such unactivated C-H bonds are
ubiquitous in organic chemicals and hydrocarbon feedstocks, making
their selective transformation an important goal. Myriad transition metal
species have been reported that are capable of selective C-H
activation,1 typically by a single oxidative addition or σ-bond metath-
esis event, though other important mechanisms are known.1d,f In the
3
cases where Csp -H bonds are activated, these processes generally
produce M-Csp species, imposing limits on the types of products that
can be obtained. In contrast, we envisioned an alternate strategy where
3
organic azides should react with nucleophilic complex 1 at the
electrophilic central nitrogen.6 Subsequent cyclization and elimination
of formimidate would generate an iridium-dinitrogen complex, which
could serve as a viable precursor for C-H activation to regenerate
the iridium carbene, as depicted in Scheme 1. This reactivity would
find precedent in the work of Hillhouse, wherein organic azides and
nitrous oxide were shown to oxidize electron-rich Ni(II) dialkyls by
nitrene-group and oxygen-atom insertion, respectively.8,9
As predicted, exposure of complex 1 to organic azides resulted in
quantitative nitrene-group transfer to generate the expected formimidate
and the previously unknown dinitrogen adduct (PNP)Ir-N2 (3).10 The
transfer reaction proceeded cleanly with trimethylsilyl azide
(TMS-N3), 2,6-diisopropylphenyl azide (DIPP-N3), and 1-azidoada-
mantane (AdN3), with reaction rates reflecting the steric bulk of the
azide substituent (DIPP > Ad > TMS). In accord with the isoelectronic
analogy between CO2 and N2O, carbene 1 also reacted with nitrous
oxide to effect oxygen-atom transfer, generating tert-butyl formate and
3.
These reactions have few analogues in transition metal carbene
chemistry,11,12 presumably because Fischer carbenes are generally
electrophilic at carbon and organic azides and N2O are poor nucleo-
philes.13 The closest precedent comes from elegant studies by Collman
and co-workers on the generation of iridium(I)-dinitrogen complexes
by nitrene transfer from organic azides to bound carbonyl ligands with
release of isocyanate.14 By analogy with our previous studies, we
propose that the transformations proceed through a four-membered
iridacyclic transition state (Scheme 2), and tentatively suggest that a
similar mechanism is likely operative in the trans-Ir(Cl)(CO)(PR3)2
systems examined by Collman.14
2
multiple C-H activation events would generate MdCsp species for
further elaboration.2 In light of the rich reactivity of metal-bound
carbenes,3 generation of such a species could open a new manifold of
reactivity for catalytic C-H functionalization.
We have previously reported that pincer-type iridium complexes
supported by Ozerov’s PNP ligand (PNP ) [N(2-PiPr2-4-Me-
C6H3)2]-)4 effect the double C-H activation of tert-butyl methyl ether
(MTBE) to generate an unusual square-planar iridium(I) carbene (1)
with loss of H2.5 Unlike most carbenes, which are nucleophilic or
2
electrophilic at carbon, this complex possesses a high-lying Ir(dz )
orbital that renders it nucleophilic at iridium, effecting atom and group
transfer to the carbene from electrophilic heterocumulenes (eq 1).5a,6
In this contribution, we report that (PNP)IrdC(H)OtBu (1) promotes
analogous atom and group transfer from nitrous oxide and organic
azides, allowing recycling of the resulting iridium complexes and
2
suggesting a new catalytic cycle based on generation of MdCsp
species.7
In our previous investigations of atom and group transfer from
heterocumulenes, we have noted that the generation of a stable
(PNP)Ir-CO species (2) seems to provide the thermodynamic driving
force for the cleavage of strong CdE bonds (E ) O, S, NPh).5a
Thermolysis and photolysis of complex 2 in MTBE produces no
observable C-H activation products. Thus, the disadvantage inherent
in these reactions is the difficulty in achieving catalytic turnover due
to the reluctance of carbonyl complex 2 to serve as a precursor for
C-H activation.
Scheme 2
In light of our previous observation that C-H activation by (PNP)Ir
is not hindered by the presence of N2,5 it seemed plausible that a change
in oxidant from carbonyl reagents (EdCdO) to the isoelectronic diazo
reagents (EdNdN) could offer a related route that circumvents
unreactive carbonyl complex 2. By analogy with earlier investigations,
Dinitrogen complex 3 was characterized by NMR, IR, and single-
crystal X-ray diffraction (XRD). XRD analysis (Figure 1) revealed a
square-planar complex with metric parameters quite similar to the
previously reported (PNP)Ir-CO (2). The dinitrogen ligand exhibits
9
16476 J. AM. CHEM. SOC. 2008, 130, 16476–16477
10.1021/ja8071385 CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
Acknowledgment. Prof. Oleg Ozerov (Brandeis University) is
acknowledged for stimulating discussions. Larry Henling (Caltech)
assisted with crystallographic analysis. Generous financial support of
this research was provided by the Moore Foundation (fellowship to
M.T.W.) and BP (MC2 initiative).
Figure 1. Displacement ellipsoid (30%) representation of (PNP)Ir-N2 (3).
Selected bond lengths (Å): Ir-N1, 2.041(3); Ir-N2, 1.859(4); Ir-P1, 2.282(1);
Ir-P2, 2.283(1); N2-N3, 1.128(7).
Supporting Information Available: Detailed experimental procedures
and characterization data. Crystallographic details for 3 are provided in
CIF format. This material is available free of charge via the Internet at
http://pubs.acs.org.
an N-N bond length of 1.13 Å, indicating modest activation of the
N2 unit.15 The diagnostic IR stretch (2067 cm-1) is lower in energy
than those previously reported for N2 complexes of iridium(I),5b,14,16
probably due to the trans disposition of the dinitrogen ligand to a
strongly π-basic arylamido donor.
Dinitrogen complex 3 was found to be a suitable precursor for the
double C-H activation of MTBE. Thermolysis of 3 in neat MTBE
resulted in the slow formation of Fischer carbene 1 with loss of H2
and N2, though prolonged heating caused the gradual degradation of
this complex to the previously reported trans-(PNP)Ir(H)2(CO) (eq 2).5b
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