Selective dehydrogenation of alcohols and diols catalyzed by a dihydrido iridium PCP pincer complex

Article abstract of DOI:10.1139/v01-070

The PCP pincer complex, IrH₂{C₆H₃-2,6-(CH₂P-t-Bu ₂)₂} (1) catalyzes the transfer dehydrogenation of primary and secondary alcohols. Dehydrogenation occurs across the C - O bond rather than the C - C bonds and the corresponding aldehydes or ketones are obtained as the sole products arising from the dehydrogenation reactions. Methanol is an exception to this pattern of reactivity and undergoes only stoichiometric dehydrogenation with 1 to give the carbonyl complex, Ir(CO){C₆H₃-2,6-(CH₂P-t-Bu₂) ₂} (2). The products are obtained in nearly quantitative yields when the reactions are carried out in toluene solutions. Under the same conditions, 2,5-hexanediol is converted to the annulated product, 3-methyl-2-cyclopenten-1-one which has been isolated in 91% yield in a preparative scale reaction.

Full text of DOI:10.1139/v01-070

8
23
Selective dehydrogenation of alcohols and diols
catalyzed by a dihydrido iridium PCP pincer
complex
David Morales-Morales, Rocío Redón, Zhaohui Wang, Do W. Lee, Cathleen Yung,
Kevin Magnuson, and Craig M. Jensen
Abstract: The PCP pincer complex, IrH {C H -2,6-(CH P-t-Bu ) } (1) catalyzes the transfer dehydrogenation of pri-
2
6
3
2
2 2
mary and secondary alcohols. Dehydrogenation occurs across the C—O bond rather than the C—C bonds and the cor-
responding aldehydes or ketones are obtained as the sole products arising from the dehydrogenation reactions.
Methanol is an exception to this pattern of reactivity and undergoes only stoichiometric dehydrogenation with 1 to give
the carbonyl complex, Ir(CO){C H -2,6-(CH P-t-Bu ) } (2). The products are obtained in nearly quantitative yields
6
3
2
2 2
when the reactions are carried out in toluene solutions. Under the same conditions, 2,5-hexanediol is converted to the
annulated product, 3-methyl-2-cyclopenten-1-one which has been isolated in 91% yield in a preparative scale reaction.
Key words: alcohol, dehydrogenation, ketones, iridium pincer complex, annulation.
Résumé : Le complexe PCP en forme de pince, IrH {C H -2,6-(CH P-t-Bu ) } (1) catalyse la réaction de déshydrogé-
2
6
3
2
2 2
nation par transfert des alcools primaires et secondaires. La déshydrogénation se fait à travers la liaison C—O plutôt
qu’à travers les liaisons C—C et on n’obtient que les aldéhydes et les cétones comme seuls produits de ces réactions
de déshydrogénation. Le méthanol est une exception à ce mode de réactivité et il ne subit qu’une déshydrogénation
stoechiométrique avec 1 pour conduire à la formation d’un complexe de carbonyle, Ir(CO){C H -2,6-(CH P-t-Bu ) }
6
3
2
2 2
(
2). Lorsqu’on effectue les réactions en solution dans le toluène, les produits sont obtenus en rendements pratiquement
quantitatifs. Dans les mêmes conditions, l’hexane-2,5-diol est transformé en produit cyclique, la 3-méthylcyclopent-2-
én-1-one qui a été isolée avec un rendement de 91% au cours d’une réaction à l’échelle préparative.
Mots clés : alcool; déshydrogénation, cétones, complexe de l’iridium en forme de pince, annellation.
[
Traduit par la Rédaction] Morales-Morales et al. 829
Introduction
overs/day) rates. Stoichiometric methods have been devel-
oped for the selective oxidation of alcohols to a variety of
sensitive ketones and aldehydes. However, there are increas-
ing environmental concerns about the highly toxic reagents
employed in methods such as Moffatt (DMSO, DCC, and
phosphoric acid), and Swern (oxalyl chloride) oxidations
Ketones and aldehydes are most commonly prepared
through the oxidation of alcohols (1). The large scale com-
mercial production of acetone and 2-butanone is accom-
plished through aerobic dehydrogenation of alcohols using
heterogenous catalysts such as copper chromite and silver
(
1). Alternatively, the conversion of alcohols to ketones and
(
2). The high (>250°C) temperatures that are required for
aldehydes can be accomplished under mild conditions
through transfer dehydrogenation. Oppenaurer oxidation, in
which hydrogen is transferred from an alcohol to a ketone
acceptor in the presence of a base (most commonly alumi-
num tert-butoxide in nearly stoichiometric amounts), has
been widely employed in organic synthesis (1, 4). Several
transition-metal complexes have been shown to catalyze the
transfer of hydrogen from alcohols to olefins (5), acetylenes
these processes, however, limits their application to produc-
tion of only the simplest ketones. A remarkable homoge-
neous catalyst for the aerobic oxidation of alcohols has been
discovered by James and co-workers (3) that operates at am-
bient temperature but proceeds at unpractical (1.5 turn-
Received September 20, 2000. Published on the NRC
Research Press Web site at http://canjchem.nrc.ca on July 19,
(
6), or aldehydes and ketones (7, 8) acceptors. Other com-
2
001.
plexes have been found to catalyze “acceptor-less” dehydr-
ogenation of alcohols (9, 10). Catalytic hydrogen transfer
systems have been utilized in organic synthesis for the re-
duction of oxo-steriods to hydroxy-steriods (8) and its mircr-
oreverse (10). More recently, enantioselective versions of
this reaction have been developed for asymmetric synthesis
(8d–f).
This paper is dedicated to Professor Brian James on the
occasion of his 65 birthday, in recognition of his
inspirational pioneering spirit.
th
D. Morales-Morales, R. Red\n, Z. Wang, D.W. Lee, C.
1
Yung, K. Magnuson, and G.M. Jensen. Department of
Chemistry, University of Hawaii, Honolulu, Hawaii 96822,
U.S.A.
The iridium PCP pincer complex IrH {C H -2,6-(CH P-t-
2
6
3
2
1Corresponding author (telephone: (808) 956-6721; fax: (808)
Bu ) } (1) has been found to be an efficient and robust cata-
2
2
9
56-5908; e-mail: jensen@gold.chem.hawaii.edu).
lyst for the transfer dehydrogenation of alkanes (11, 12),
Can. J. Chem. 79: 823–829 (2001)
DOI: 10.1139/cjc-79-5/6-823
© 2001 NRC Canada

8
24
Can. J. Chem. Vol. 79, 2001
Table 1. Crystallographic data for Ir(CO){C H -2,6-(CH P-t-
5890 instrument with a HP 5980A flame ionization detector
and HP-1 capillary column (25.0 m). Gas chromatographic–
mass spectral analyses were carried out using a HP 5890 Se-
ries II instrument with an 5971A mass selective detector and
HP-1 capillary column (25.0 m).
6
3
2
Bu ) } (2).
2
2
Formula
Formula weight
T (K)
C25H43OIrP2
613.73
293(2)
Crystal system
Space group
Crystal dimensions (mm)
a (Å)
b (Å)
c (Å)
Monoclinic
Catalytic reactions
P2 /n
1
Solutions of the substrates (0.26 mmol), tbe (0.20 mL,
.53 mmol), and 4 mL of toluene were charged with 1
0.4 × 0.4 × 0.3
15.443(4)
11.619(3)
30.608(8)
90
93.22
90
5483(3)
8
49.99
0.389, 1.000
0.71073
3–45
7143
5290
526
0.986
1.487
ω
1
(
22 mg, 0.037 mmol) in sealed Schlenk tubes in a Vacuum
Atmospheres glovebox under argon. The tubes were then
fully immersed in a constant temperature bath at 200°C for
the prescribed reaction times. After this time the tubes are
allowed to cool down to room temperature. The products
were identified by GC–MS analysis upon comparison to pur-
chased samples of the authentic compounds. Product yields
were calculated from the ratio of the integrated intensities of
signals produced by the products and those of the toluene
solvent after weighting the data by a predetermined relative
molar response factor.
α (deg)
β (deg)
γ (deg)
3
V (Å )
Z
–
1
µ (cm )
Transmission coeff. min, max
λ (Å) (Mo Kα radiation)
2
θ range (deg)
The attempted acceptor-less dehydrogenation reactions
were carried out with mesitylene solutions of the substrates
Independent reflections
Unique data with I >> 2((I)
Parameter refined
(
0.26 mmol) and 1 (22 mg, 0.037 mmol) that were refluxed
while passing a stream of argon above the condensor. Gas
chromatographic analysis of the reaction mixtures following
a
Goodness-of-fit
–
3
ρ calcd. (g cm )
7
2 h of rigorous reflux showed no traces of dehydrogenated
Scan type
products.
b
R (%)
3.78
8.87
c
Rw (%)
Preparative scale synthesis of 3-methyl-2-cyclopenten-1-
one
a
2
1/2
GOF = [wσ(|F | – |F |) /(N – N )]
.
o
c
o
v
b
R = σ|(F | – |F |/σ|F |.
o
c
o
c
2
2
1/2
A solution of the 2,5-hexanediol (1.0 mL, 81 mmol), tbe
2.2 mL, 17 mmol), and 4 mL of toluene was charged with 1
100 mg, 0.17 mmol) in a sealed Schlenk tube in a Vacuum
Rw = [wσ(|F | – |F |) /σwF ]
.
o
c
o
(
(
Atmospheres glovebox under argon. The tube was then fully
immersed in a constant temperature bath at 200°C for 78 h.
After this time, the reaction mixture was cooled to room
temperature and concentrated to ~1.4 mL. The product was
then separated by column chromatography (Davisil (100–
ethers (13), alkyl arenes (13), and amines (14). Thus it was
of interest to examine the reactivity of 1 towards alcohols.
We have found that 1 catalyzes the selective transfer
dehydrogenation of primary and secondary alcohols to alde-
hydes and ketones in very high yields. We have also found
that 1 catalyzes the unusual dehydrogenation and (or)
annulation of 2,5-hexanediol to 3-methyl-cyclopenten-1-one.
The results of these studies are reported herein.
2
00 mesh silica gel) by eluting first with pentane and then
with acetone. The product fraction was collected and
concentrated under vacuum. The isolated product (yield:
0
.73 mL, 91%) was identified as 3-methyl-2-cyclopenten-1-one
1
13
by MS and NMR ( H and C) analysis upon comparison to
Experimental
+
+
an authentic sample. MS (m/z): [M] 96, [M-CH3] 81.
All manipulations were carried out using standard
Schlenk and glovebox techniques under purified argon. Sol-
vents were degassed and dried using standard procedures.
The alcohols were purchased from Aldrich Chemicals Co.
and used without further purification. The complex
IrH {C H -2,6-(CH P-t-Bu ) } (1) was synthesized by the
1
H NMR (400.00 MHz, CDCl ) δ: 5.91 (m, 1H, HC-2), 2.55
3
(
m, 2H, H C-5), 2.38 (m, 2H, H C-4), 2.10 (s, 3H, CH ).
2 2 3
1
3
C NMR (100.60 MHz, CDCl ) δ: 210.15 (s, C=O), 178.82
3
(
(
s, C-3), 130.62 (s, C-2), 35.66 (s, C-5), 32.97 (s, C-4), 19.30
s, CH3).
2
6
3
2
2 2
1
literature methods (12). The H NMR spectra were recorded
on a Varian Unity Inova 400 spectrometer. Chemical shifts
are reported in ppm down field of TMS using the solvent as
internal standard (CDCl , 7.26 or cyclohexane-d , 1.38).
Synthesis of Ir(CO){C H -2,6-(CH P-t-Bu ) } (2)
6
3
2
2 2
A pentane (5 mL) solution of 1 (20 mg, 0.034 mmol) was
treated with tert-butylethylene (66 µL, 0.51 mmol) under
1 atm (1 atm = 101.325 kPa) of argon at 25°C. After 1 h of
reaction, the resulting red-purple solution was treated with
degassed methanol (0.1 mL, 5.6 mmol). Removal of the sol-
vent in vacuo yields 2 as a deep yellow-orange solid. Yield:
3
12
1
3
31
C and P NMR spectra were recorded with complete pro-
ton decoupling and are reported in ppm down field of TMS
with solvent as internal standard (CDCl , 77.0 or cyclohex-
3
ane-d , 26.43) and external 85% H PO , respectively. Infra-
1
2
3
4
1
red spectra were recorded in a PerkinElmer Paragon FT IR
spectrometer as Nujol mulls in NaCl plates. Gas chromato-
graphic analyses were performed with a Hewlett–Packard
19 mg, 90%. H NMR (400.00 MHz, cyclohexane-d ) δ:
1
2
6.93 (d, JHH = 7.3 Hz, 2H, m-H), 6.67 (t, JHH = 7.3 Hz, 1H,
p-H), 3.42 (vt, JHH = 3.0 Hz, 4H, CH ), 1.28 (vt, J
=
PH
2
©
2001 NRC Canada

Morales-Morales et al.
825
a
Table 2. Dehydrogenation of alcohols using IrH {C H -2,6-(CH P-t-Bu ) } (1).
2
6
3
2
2 2
t
P
R1
Bu2
H
R1
R2
OH
R2
O
Ir
H
+
+
t
P Bu
2
t
t
Bu
Bu
Substrate
Product
Yield (%)b
>99
OH
O
OH
O
>99
>
99
OH
OH
O
c
66
O
>
>
>
99
OH
OH
O
99
99
O
O
OH
>
>
99
99
O
OH
HO
OH
O
a
Reaction conditions: alcohol (0.26 mmol), tbe (1.53 mmol), 4 mL of benzene, and 1 (22 mg,
0
.037 mmol) at 200°C for 18 h.
b
Yields based on GC analysis.
c
Traces of aldehyde were detected in the starting material; as a result, the performance of the catalyst
was considerably reduced.
7
.3 Hz, 36H, C(CH ) ). 13C NMR (100.60 MHz, cyclohex-
Molecular structure determination of (2)
3
3
ane-d ) δ: 197.6 (s, Ir-C-O), 168.8 (s, C-1), 155.3 (vt, J =
Yellow-orange crystals of 2 that were suitable for X-ray
diffraction were obtained from slow evaporation of a
pentane solution of the complex. Centering and data collec-
tion were performed with a Nicolet P3 diffractometer, graph-
ite monochromator (λ = 0.71073 Å, Mo Kα) at room
temperature using a crystal that was mounted on a glass fiber
with epoxy. The unit cell was determined from the angular
1
2
PC
1
3
2.0 Hz, o-C), 125.9 (s, p-C), 120.3 (vt, JPC = 9.1 Hz, m-C),
9.6 (vt, JPC = 13.9 Hz, CH P), 36.5 (vt, J = 10.7 Hz,
2
PC
3
1
C(CH ) ), 30.1 (s, C(CH ) ). P NMR (161.90 MHz, cyclo-
3
3
3 3
–
1
hexane-d ) δ: 82.8 (s). IR (KBr) (cm ): ν = 1913 (s).
Anal. calcd. for C H OP Ir (613.77) (%): C 48.92, H 7.06;
found: C 48.96, H 7.01.
1
2
IrCO
2
5
43
2
©
2001 NRC Canada

8
26
Can. J. Chem. Vol. 79, 2001
Fig. 1. Thermal ellipsoid (50% probablity) drawing of
ing and gradually change color to yellow-orange during the
reaction period. Gas chromatographic analysis of the reac-
tion mixtures showed the substrates were converted to the
corresponding dehydrogenated compounds with greater than
Ir(CO){C H -2,6-(CH P-t-Bu ) } (2). Hydrogen atoms have been
6
3
2
2 2
omitted for clarity.
9
9% selectivity. However, only ≈1% yields were obtained in
preliminary experiments with the neat reaction mixture even
upon longer reaction times and increased catalyst loadings.
This finding was not unanticipated as dehydrogenation reac-
tions catalyzed by 1 have generally been found to be inhib-
ited at low concentrations of the unsaturated products (11–
1
4, 16). Apparently deactivation of the active catalyst
through a coordinative interaction with the carbonyl groups
prevents the occurrence of the slower process of catalytic
dehydrogenation of aliphatic groups. In support of this hy-
pothesis, 2-hexanone was found to be unreactive with 1 un-
der the conditions employed for the dehydrogenation of the
alcohols.
To obtain high yields, it was necessary to dilute the reac-
tion mixtures. A similar strategy has been used to achieve
high yields in the catalytic dehydrogenation of secondary
amines to imines by 1 (14). Table 2 summarizes the results
of the dehydrogenation experiments in which toluene solu-
tions of the alcohols, 1, and tbe were heated for 3 days at
2
00°C. The products were characterized by GC–MS analysis
of the reaction mixtures. Excellent selectivities and yields
were uniformly obtained without regio-restriction on the al-
cohol functionality. No traces of dehydrogenationated prod-
ucts were detected in acceptor-less experiments in which
mesitylene solutions of alcohols and 1 were refluxed while
passing a stream of argon above the condensor.
The dehydrogenation of 2,5-hexanediol produced an unex-
pected result. A single product arises upon heating dilute to-
luene solutions of the diol, 1, and tbe for 3 days at 200°C.
MS and NMR analysis showed the product to be the cyclic
ketone, 3-methyl-2-cyclopenten-1-one. This product can be
understood to arise upon dehydrogenation across both of the
C—O bonds to produce the hexadione followed by an inter-
nal aldol reaction and subsequent dehydrogenation as in the
final step of a Robinson annulation reaction (17). We are un-
aware of any previous reports of the catalytic
dehydrogenation of diols resulting in the formation of cyclic
unsaturated ketones. To probe the possible synthetic utility
of this type of reaction, the dehydrogenation of 2,5-
hexanediol was carried out on a preparative scale. Starting
with 1.00 mL (81 mmol) of the diol, we were able to isolate
coordinates of 25 reflections with 2θ values between 15 and
0°. The diffractometer autoindexing routine found a
monoclinic unit cell, which was confirmed by axial photo-
graphs.
3
Three check reflections, monitored every 100 reflections,
showed no significant decay. The data were processed using
SHELXTL program package (15), and an absorption correc-
tion was applied based upon ψ-scans of five reflections.
Since the unit cell is triclinic, the space group P2(1)/n was
assumed. The iridium was located by Patterson methods and
the remainder of the structure was easily developed via a
few cycles of least-squares refinement and difference Fourier
maps. The unit cell was found to contain two symmetry-
independent molecules of 2. Hydrogen atoms were input at
calculated positions, and allowed to ride on the atoms to
which they are attached. Two group of thermal parameters
were refined for hydrogen atoms, one each for methylene
and methyl protons. The final cycle of refinement was car-
ried out on all non-zero data using SHELXL-97 (6) and
anisotropic thermal parameters for all non-hydrogen atoms.
Crystal data and relevant information are summarized in Ta-
ble 1.
0
1
.73 mL (74 mmol, 91% yield) of 3-methyl-2-cyclopenten-
-one upon work-up using silica gel column chromatogra-
phy.
The pincer complex does not catalyze the
dehydrogenation of methanol to formaldehyde but instead
undergoes a stoichiometric reaction with methanol to yield
the carbonyl compound Ir(CO){C H -2,6-(CH P-t-Bu ) }
6
3
2
2 2
(
2). The presence of the carbonyl ligand of 2 is clearly indi-
1
3
cated by the observation of a signal at 198.23 ppm in its
C
–
1
NMR spectrum and strong absorption at 1911.8 cm in the
IR spectrum of the complex. Although carbonyl complexes
commonly result upon the reaction of late transition-metal
complexes with alcohols (for example see ref. 18; 19) it is
surprising to find this partitioning in the reactivity of 1. A
similar dichotomy was previously observed for
[Ru Cl (PMePh ) ]Cl which reacts with KOH in isopropanol
Results
The catalytic activity of 1 was initially screened using so-
lutions consisting of the saturated alcohol, and the hydrogen
acceptor, tert-butylethylene (tbe). The orange solutions were
sealed in tubes under argon and fully immersed in an oil
bath at 200°C for 18 h. The solutions became red upon heat-
2
3
2 6
©
2001 NRC Canada

Morales-Morales et al.
827
Table 3. Selected bond distances (Å) and angles (°) for Ir(CO){C H -2,6-(CH P-t-Bu ) } (2).
6
3
2
2 2
Molecule 1
Molecule 2
Bond lengths
Ir(1)—C(1A)
Ir(1)—C(25A)
Ir(1)—P(1A)
Ir(1)—P(2A)
C(25A)—O(1A)
Bond angles
2.102(8)
1.873(10)
2.298(2)
2.291(2)
1.167(10)
Ir(2)—C(1B)
2.090(9)
1.852(12)
2.304(2)
2.298(2)
1.127(12)
Ir(2)—C(25B)
Ir(2)—P(1B)
Ir(2)—P(2B)
C(25B)—O(1B)
C(1A)-Ir(1)-P(2A)
C(1A)-Ir(1)-P(1A)
P(2A)-Ir(1)-P(1A)
C(25A)-Ir(1)-C(1A)
C(25A)-Ir(1)-P(2A)
C(25A)-Ir(1)-P(1A)
O(1A)-C(25A)-Ir(1)
81.800(2)
82.900(2)
164.510(8)
178.100(3)
97.700(3)
97.700(3)
176.800(8)
C(1B)-Ir(2)-P(2B)
C(1B)-Ir(2)-P(1B)
P(2B)-Ir(2)-P(1B)
C(25B)-Ir(2)-C(1B)
C(25B)-Ir(2)-P(2B)
C(25B)-Ir(2)-P(1B)
O(1B)-C(25B)-Ir(2)
82.300(3)
82.400(3)
163.080(9)
179.400(4)
98.300(3)
97.100(3)
175.600(12)
solution to give [cis-Ru Cl (PMePh ) ] but yields
Scheme 1.
2
3
2 4
RuH (CO)(PMePh ) when the reaction is carried out in eth-
anol (19).
2
2 3
t
t
P Bu
But
But
P Bu
2
2
H
H
The molecular structure of 2 was determined through a
2
Ir
Ir
single crystal X-ray diffraction study. The structure consists
of two molecules of 2 in the asymmetric unit. A thermal el-
lipsoid drawing of one of the molecules with atomic number
scheme of the obtained structure is presented in Fig. 1. Se-
lected bond angles and distances are listed in Table 3.
A distorted square-planar coordination geometry can be
recognized about the iridium consisting of the tridentate
PCP pincer and carbonyl ligands. While the vector between
the metalated carbon in the aromatic ring and the carbonyl
ligand O(1A)-C(25A)-Ir(1) is a nearly ideal 176.800(8)°, the
vector between the two phosphorus atoms (P(2A)-Ir(1)-
P(1A), 164.510(8)°), is much more distorted due to the geo-
metric constraints imposed upon establishing the chelating
interaction of the PCP ligand. The Ir-P and Ir-metalated C
distances are comparable to those observed in similar PCP
pincer complexes (12, 20, 21).
t
t
P Bu
P Bu
2
2
HOCHRR'
R= alkyl
R'= alkyl, H
O
C
R
R'
t
t
P Bu
P Bu
2
2
H
H
H
Ir R'
Ir
C
OCHRR'
R
O
t
t
P Bu
P Bu
2
2
complexes. Dissociation of the dehydrogenated product then
regenerates 1 and completes the catalytic sequence.
Discussion
A plausible mechanism for the catalytic reaction is de-
picted in Scheme 1.
The alternate reaction pathway observed in the case of
methanol can be rationalized in terms of the differing
reactives of formyl and acyl complexes. As illustrated in
Scheme 2, such intermediates could quite plausibly result
from C-H oxidative addition of aldehydes in either: (i) an
intermolecular process involving the 14-electron intermedi-
ate in the catalytic sequence; (ii) or an intramolecular pro-
Alcohols apparently undergo O-H oxidative addition with
the 14-electron complex arising upon dehydrogention of 1
by tbe to give intermediate alkoxy hydride complexes. In
support of this hypothesis, the dehydrogenation of 1 in the
presence of water produces the analogous hydroxy hydride
complex IrH(OH)({C H -2,6-(CH P-t-Bu ) }, that has been
characterized spectroscopically and through a single crystal
X-ray structure determination (22). The pincer alkoxide
complexes would be expected to undergo the C-H β-elimina-
tion reaction that have generally been observed for metal
2
cess involving a 16-electron η -aldehyde species arising
6
3
2
2 2
2
upon reductive elimination of H from the 18-electron, η -al-
2
dehyde, dihydride intermediate in the catalytic sequence.
Hydride elimination from a formyl ligand to generate a
hydrido carbonyl complex is well known to be much more
facile than the corresponding alkyl deinsertion from an acyl
2
alkoxide complexes (23) to produce η -aldehyde or ketone
2
Copies of material on deposit (tables of crystal data, thermal parameters, bond distances, bond angles, and atomic coordinates for
Ir(CO){C H -2,6-(CH P-t-Bu ) }) may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Re-
6
3
2
2 2
search Council Canada, Ottawa, ON K1A 0S2, Canada (http://www.nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electroni-
cally). Some of this material has also been deposited with the Cambridge Crystallographic Data Centre. Copies of the data can be obtained,
free of charge, on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (Fax: 44-1223-336033 or e-mail:
deposit@ccdc.cam.ac.uk).
©
2001 NRC Canada

8
28
Can. J. Chem. Vol. 79, 2001
Scheme 2.
t
t
P Bu
P Bu
2
2
H
H
H
O
C
Ir
Ir
H
R
C
R
H2
O
2
t
t
P Bu
P Bu
2
t
t
P Bu
P Bu
2
2
O
O
C
H
R
C
Ir
Ir
R
H
t
t
P Bu
P Bu
2
2
R = H
t
t
P Bu
P Bu
2
2
H2
CO
Ir
H
Ir
CO
H
t
t
P Bu
P Bu
2
2
intermediate (24). Thus while all aldehydes are likely to un-
dergo C-H oxidative addition in this catalytic system, subse-
quent deinsertion to form a carbonyl is competitive with the
mircoreverse, reductive elimination of aldehyde only in the
case of the formyl intermediate.
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In summary, the PCP pincer complex 1 catalyzes the
highly selective dehydrogenation of a variety of primary and
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The problem of product inhibition of the catalyst is elimi-
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