Dehydrogenation of ketones by pincer-ligated iridium: Formation and reactivity of novel enone complexes

Article abstract of DOI:10.1016/j.ica.2010.11.016

The transfer dehydrogenation of several ketones by (PCP)IrH₂ (PCP = κ³-C₆H₃-2,6-(CH₂P ^tBu₂)₂) (1) has been observed. Catalytic turnover was inhibited in most cases by

Full text of DOI:10.1016/j.ica.2010.11.016

Inorganica Chimica Acta 369 (2011) 253–259
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Dehydrogenation of ketones by pincer-ligated iridium: Formation and reactivity
of novel enone complexes
⇑
Xiawei Zhang, David Y. Wang, Thomas J. Emge, Alan S. Goldman
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, USA
a r t i c l e i n f o
a b s t r a c t
The transfer dehydrogenation of several ketones by (PCP)IrH2 (PCP = j
3-C6H3-2,6-(CH2PtBu2)2) (1) has
been observed. Catalytic turnover was inhibited in most cases by the formation of stable metallacycles
Article history:
Available online 20 November 2010
or the O–H oxidative addition of phenolic products. Catalytic transfer dehydrogenation of 3,3-dimethyl-
cyclohexanone was achieved, giving the corresponding a,b-enone. The transfer dehydrogenation reaction
Dedicated to Bob Bergman, an exemplary
scientist and human being.
of cycloheptanone with 1 was found to generate a surprisingly stable PCP-iridium troponyl hydride (9),
which is stabilized by conjugation and possibly represents an unusual bicyclo[5.2.0]troponyliridium
metalloaromatic structure. Complex 9 was found to catalyze the dimerization of tropone to give a fused
tricyclic dihydrodicycloheptafuranol. A mechanism for this reaction is proposed wherein the coordinated
troponyl group nucleophilically attacks a free tropone molecule.
Keywords:
Transfer dehydrogenation of ketones
PCP pincer-ligated iridium complexes
Organometallic homogeneous catalysis
Tropone dimerization
Ó 2010 Elsevier B.V. All rights reserved.
X-ray crystal structures
1. Introduction
are the most successful class of catalysts for the dehydrogenation
of aliphatic linkages reported to date [11,12]. In addition to the
a
,b-Unsaturated ketones are of great utility in a wide variety of
dehydrogenation of alkanes, the catalytic dehydrogenation of THF
[13] and tertiary amines [14] has been reported with selectivity
organic transformations, arising from the ability to activate and
functionalize the carbonyl moiety and positions
carbonyl group. Classically, ,b-unsaturated ketones can be pre-
pared via aldol condensation [1], allylic oxidation [2–4], and elim-
ination reactions from -substituted ketones [5]. These methods
often require several steps or may be complicated by low yields
and unwanted byproducts. A simple one-step reaction to introduce
the double bond into an aliphatic molecular framework would be a
desirable synthetic tool.
a
, b, and
c
to the
for the a,b-position. In this contribution, we report the results of
a
attempts to effect (PCP)Ir-catalyzed dehydrogenation of ketones.
We find that ketone dehydrogenation occurs readily but in general
the resulting enones, or products of further dehydrogenation, tend
to strongly inhibit catalysis. In the case of a simple acyclic enone, a
conjugated metallacyclic species is formed which is remarkably
stable. In the particular case of cycloheptanone, a very unusual
conjugated [5.2.0] metallabicyclic species is formed, apparently
resulting from the triple dehydrogenation (transfer of 6H atoms)
of the ring to give tropone, followed by C–H addition and
O-coordination. Accordingly this complex can be generated
directly by the reaction of ‘‘(PCP)Ir’’ with tropone. Remarkably, this
species acts as a catalyst for the dimerization of tropone to give a
fused tricyclic dihydrodicycloheptafuranol, a reaction which to
our knowledge is without reported precedent.
a
Direct dehydrogenation of ketones has been one strategy to
approach
a,b-unsaturated ketones. Dehydrogenation of ketone
substrates by palladium complexes in both stoichiometric and
catalytic amounts have been reported, although efforts at cata-
lytic reactions have been hampered by low conversions and yields
[6–9]. The use of substituted iodoxybenzoic acids with Oxone has
been reported to effect the formation of
a,b-unsaturated ketones
from alcohols, presumably via initial alcohol oxidation and
subsequent ketone dehydrogenation; however, the formation of
2. Results and discussion
a
,b-unsaturated ketones by this method is limited to cyclic sub-
strates [10].
Pincer-ligated iridium complexes, specifically derivatives of
The reaction of complex 1 (17 mM) with 3-pentanone (51 mM)
and tert-butylethylene (TBE; 153 mM), after 24 h at room temper-
ature, resulted in the near-quantitative formation of metallacycle
complex 2 as determined by 31P and 1H NMR spectroscopy
(Scheme 1). In the 1H NMR spectrum, complex 2 displays a hydride
signal as a triplet at d À9.04 ppm, which is indicative of a
‘‘(PCP)Ir’’ (PCP = j
3-C6H3-2,6-(CH2PR2)2; R = t-Bu in this report),
⇑
Corresponding author.
E-mail address: alan.goldman@rutgers.edu (A.S. Goldman).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.11.016

254
X. Zhang et al. / Inorganica Chimica Acta 369 (2011) 253–259
6-coordinate iridium metal center with a hydride positioned trans
to a hydrocarbyl carbon [15]. Signals in the 1H NMR spectrum that
appear at d 12.27 ppm and d 7.68 ppm were assigned to the vinylic
protons.
Presumably, the formation of 2 results from the transfer dehy-
drogenation of 3-pentanone to give ethyl vinyl ketone, which is fol-
lowed by terminal vinylic C–H addition to ‘‘(PCP)Ir’’ and carbonyl
oxygen coordination.
Upon heating at 90 °C for 2 h, complex 2 isomerized quantita-
tively to complex 3, in which the hydride and the coordinating
carbonyl moiety are mutually trans (Scheme 1). The 31P NMR spec-
trum of complex 3 displayed a doublet resonance at d 62.3 ppm.
The 1H NMR spectrum displayed resonances at d 12.60 ppm and
d 7.81 ppm, attributable to the vinyl protons, and a signal at d
À24.51 ppm, which is indicative of a hydride ligand trans to a car-
bonyl oxygen [15]. Yellow crystals were obtained in 90% yield from
a pentane solution. X-ray crystallographic analysis confirmed the
assignment of 3 (Fig. 1). It should be noted that formation of the
thermodynamic product 3 from the kinetic product 2 indicates that
coordination of the carbonyl moiety occurs subsequent to – rather
than prior to – vinylic C–H activation; C–H activation ‘‘directed’’ by
the carbonyl oxygen would be expected to give isomer 3 directly.
This result is consistent with results that we have previously re-
ported regarding addition of the ortho C–H bonds of acetophenone
and nitrobenzene [15].
Fig. 1. Crystal structure of complex 3. Selected bond distances (Å) and angles (°):
Ir1–C1, 2.126(3); Ir1–O1, 2.257(2); Ir1–C25, 2.043(3); C25–C26, 1.372(5); C26–C27,
1.440(5); O1–C27, 1.262(4); Ir1–C25–C26, 117.8(3); Ir1–O1–C27, 111.8(2); O1–Ir1–
C25, 75.77(12); C1–Ir1–O1, 101.48(11); C25–C26–C27, 114.7(3); C26–C27–O1,
119.9(3).
The X-ray crystal structure of 3 (Fig. 1) suggests that both reso-
nance forms I and II (Scheme 2) contribute to its structure. The vi-
nyl C–C bond distance is 1.372(5) Å, and the C–O double bond
distance is 1.262(4) Å, both of which are longer than the corre-
sponding C–C and C–O double bonds of a reported
a,b-enone
(1.325 and 1.222 Å, respectively) [16]. The C(vinyl)-C(carbonyl)
bond distance is 1.440(5) Å, which is shorter than the distance of
1.474 Å found in the reported
a,b-enone. The 5-membered
Ir-containing ring is rigorously planar, with internal angles totaling
540.0°. Analogous metallacyclic complexes with iridium and
osmium that display similar downfield vinyl NMR shifts and
X-ray crystal structures have been reported by Werner co-workers,
Crabtree co-workers, and Esteruelas co-workers [17–21].
Scheme 2. Resonance forms of 3.
Because formation of a stable cyclometallated species presum-
ably prevents catalytic turnover, we considered that the catalytic
dehydrogenation of cyclic ketones, which cannot form such 5-
membered rings, might be achievable. Complex 1 (17 mM) was re-
acted with cyclohexanone (51 mM) and TBE (170 mM) at 90 °C,
yielding a red solution. After 3 h, the reaction was found to be com-
plete by 31P and 1H NMR spectroscopy. Complex 4 was isolated
from solution and characterized by NMR spectroscopy and X-ray
crystallography. The hydride signal in the 1H NMR spectrum was
found at d À39.26 ppm, which is strongly indicative of a 5-coordi-
nate iridium(III) hydride [22–24]. The X-ray crystal structure re-
vealed complex 4 to be a PCP-iridium phenoxy hydride (Scheme
3) [25].
oxidative addition product of the enol tautomer of cyclohexanone.
Upon heating at 90 °C for 2 h, complex 5 was quantitatively con-
verted to complex 4 (Scheme 3). Thus, while the reaction of 1 with
a cyclic ketone resulted in more than one dehydrogenation event,
the dehydroaromatization of the cyclohexanone ring followed by
the formation of a stable phenoxy hydride inhibited catalytic
turnover.
Dehydrogenation of 3-methylcyclohexanone (51 mM) was at-
tempted with complex 1 (17 mM) and excess norbornene (NBE;
170 mM) at 90 °C for 3 h (Scheme 3). Analogously to the reaction
of 1 with cyclohexanone, the reaction of 1 with 3-methylcyclo-
hexanone yielded the substituted phenoxy hydride complex 6.
Notably, the methyl group on the cyclohexanone ring had no major
effect on the dehydrogenation reaction; the sterically hindered ter-
When the same reaction of 1 with cyclohexanone and excess
TBE was performed at room temperature, complex 5 was formed
in 98% yield after 2 h (Scheme 3). Complex 5 was characterized
by 1H and 31P NMR spectroscopy and may be viewed as the O–H
PtBu2
PtBu2
Ir
PtBu2
O
Ir
p-xylene
O
rt
tBu
+
+
IrH2
O
90 ºC
H
H
PtBu2
PtBu2
PtBu2
tBu
1
2
3
Scheme 1. Reaction of 3-pentanone with 1.

X. Zhang et al. / Inorganica Chimica Acta 369 (2011) 253–259
255
O
PtBu2
PtBu2
O
Ir
O
Ir
90 ºC
H
H
TBE
PtBu2
PtBu2
PtBu2
IrH2
p-xylene, rt
5
4
O
PtBu2
PtBu2
1
O
Ir
NBE
H
PtBu2
p-xylene, 90 ºC
6
Scheme 3. Reaction of cyclohexanone and 3-methylcyclohexanone with 1.
tiary hydrogen was removed efficiently to give the substituted
phenolic product.
In a further effort to prevent aromatization, 3,3-dimethylcyclo-
hexanone was employed as a dehydrogenation substrate. Gratify-
ingly, the reaction of 3,3-dimethylcyclohexanone (150 mM) and
TBE (300 mM) was catalyzed by complex
1 (7.5 mM) and
proceeded to completion after 10 h at 120 °C (Scheme 4). By
NMR spectroscopy and gas chromatographic analysis, the expected
a,b-unsaturated cyclic ketone 7 was formed in 85% yield. Further
heating did not yield more product. With the use of the para-
methoxy-PCP iridium catalyst 8 [26], which has been previously
reported to be a more robust catalyst [27], the product was
obtained in 99% yield at 1% catalyst loading.
Cycloheptanone was next investigated as a ketone substrate.
We hypothesized that the possible dehydrogenation products of
cycloheptanone should not form alcohols, and, as a result, cyclo-
heptanone might undergo catalytic dehydrogenation. Treatment
of complex 1 (17 mM) with cycloheptanone (51 mM) and NBE
(170 mM) at 120 °C for 12 h and subsequent recrystallization
yielded an orange solid. The product was identified as complex 9
through NMR spectroscopy and X-ray crystallography (Fig. 2,
Scheme 5).
The X-ray crystal structure of 9 features an unusual [5.2.0] met-
allabicyclic unit that includes the 6-coordinate iridium atom in the
4-membered ring (Fig. 2). The 4-membered ring is planar, with
internal angles totaling 360.0°. Similarly, the 7-membered ring is
also rigorously planar, with internal angles totaling 899.9°. The
sums of the three bond angles around C25 (359.9°) and C26
(360.0°) indicate that the 7-membered ring and 4-membered rings
are co-planar. Analogously to metallacyclic complex 3, different
resonance forms may also contribute significantly to the electronic
structure of complex 9. The bond distances between the seven car-
bons range from 1.374 to 1.452 Å; these values are between the
normal bond distances of C–C single and double bonds (1.54 and
1.34 Å, respectively). The high stability of metallacyclic complexes
3 and 9 coupled with the NMR spectroscopic data and X-ray crystal
Fig. 2. Crystal structure of complex 9. Selected bond distances (Å) and angles (°):
Ir1–C1, 2.095(2); Ir1–O1, 2.2866(17); Ir1–H1, 1.548(17); Ir1–C26, 2.060; O1–C25,
1.288(3); C25–C26, 1.452(3); C26–C27, 1.385(3); C27–C28, 1.411(4); C28–C29,
1.375(4); C29–C30, 1.405(4); C30–C31, 1.374(4); C25–C31, 1.420(3); Ir1–O1–C25,
90.63(14); Ir1–C26–C25, 95.76(15); Ir–C26–C27, 139.15(19); O1–Ir1–C26, 62.46(8);
O1–C25–C26, 111.1(2); C25–C26–C27, 126.1(2); C26–C27–C28, 130.1(3); C27–
C28–C29, 129.9(3); C28–C29–C30, 127.6(3); C29–C30–C31, 129.8(3); C25–C31–
C30, 127.9(3); C26–C25–C31, 129.5(2); O1–C25–C31, 119.3(2).
structures suggest that they might exhibit metalloaromatic charac-
ter; the possible metalloaromatic nature of these complexes is cur-
rently under investigation.
Reaction of complex 1 (26 mM) with NBE (39 mM) and a small
excess of tropone (32 mM) at room temperature yielded the ki-
netic product 10 (Scheme 5), which was characterized by NMR
spectroscopy. Complex 10 was found to be much less stable than
PtBu2
O
O
X% [M]
tBu
+
tBu
+
1
,
X% [M] = 5%
1% MeO
IrH2
º
120 C
PtBu2
10 h
7
8
Scheme 4. Catalytic transfer dehydrogenation of 3,3-dimethylcyclohexanone.

256
X. Zhang et al. / Inorganica Chimica Acta 369 (2011) 253–259
PtBu2
IrH2
PtBu2
PtBu2
O
Ir
O
p-xylene
rt
+
+
Ir
O
rt
H
H
PtBu2
PtBu2
PtBu2
1
10
9
Scheme 5. Addition of cycloheptanone to 1.
the analogous kinetic C–H adduct 2; it isomerizes to complex 9
within 1 h at room temperature. This difference in stability may
be attributable to the greater strain in the 4-membered metalla-
cyclic ring as compared to the 5-membered metallacyclic unit in
2. In analogy to the reaction of pentanone (Scheme 1), initial for-
mation of 10, despite the greater thermodynamic stability of 9,
suggests that coordination of the carbonyl moiety to the iridium
atom does not precede C–H bond activation.
troponyl moiety lengthen from 1.405, 1.411, 1.420, and 1.452 Å
in complex 9 to 1.415, 1.422, 1.461, and 1.476 Å for the corre-
sponding bonds in complex 11. The C–C double bonds of the trop-
onyl moiety shorten upon coordination of CO from 1.374, 1.375,
and 1.385 Å in complex 9 to 1.351, 1.360, and 1.375 Å in complex
11. Likewise, the troponyl C–O bond shortens upon coordination of
CO, from 1.288 Å in complex 9 to 1.248 Å in complex 11. These
changes in bond distances suggest the possibility of delocalized
resonance forms that contribute significantly to the electronic
structure of complex 9, including at least one that could promote
the formation of tropone dimer 12 as detailed below.
A possible mechanism for dimerization is given in Scheme 6.
Complex 9 has a resonance form that suggests that C31 (Fig. 2)
may be nucleophilic. Nucleophilic attack by complex 9 on free tro-
pone in a 1,8-fashion yields intermediate 13 after proton transfer.
The ability of tropone to undergo nucleophilic 1,8-addition reac-
tion is well precedented [29–31], but to our knowledge, no exam-
ples of tropone serving as a nucleophile have been reported in the
literature. A formal 1,7-hydride shift would yield iridium alkyl hy-
dride intermediate 14. Reductive elimination and hemiketal for-
mation would then afford dimerization product 12. In this
reaction mechanism, reductive elimination of the alkyl hydride
intermediate 14 should be facile because the metallacycle lacks
resonance stabilization.
When excess tropone was reacted with complex 1 and nor-
bornene, a dark-colored solution was formed in contrast to the
orange-colored solution that was obtained from the reaction with
only one equivalent of tropone. Addition of CO to the resulting
solution yielded the
g
1-troponyl iridium CO complex 11, and
dark amber-colored crystals were crystallized from the solution.
Surprisingly, the expected CO adduct 11 was found to have co-
crystallized with organic compound 12 (Fig. 3). Compound 12 is
a previously unreported dimerization product of tropone [28].
Upon further investigation, when a large excess of tropone (26
equivalents) was reacted with complex 1 and a small excess of
norbornene, tropone was consumed completely to form primarily
compound 12. Addition of tropone to a solution containing a cat-
alytic amount of complex 9 also yielded tropone dimer 12, indi-
cating that the tropone adduct 9 is a probable intermediate in
the dimerization mechanism.
Coordination of CO to complex 9 eliminates possible electron-
delocalization between the two rings of 9 and possible resonance
forms consistent with such a delocalized system, as well as the
possibility of enhanced stability engendered by a bicyclic metalloa-
romatic system. The effect of metallacyclic resonance forms can be
seen in the comparison of the X-ray crystal structures of complexes
9 and 11. Upon coordination of CO, the C–C single bonds of the
3. Conclusion
The transfer dehydrogenation of several ketones by complex 1
has been observed. Catalytic turnover was inhibited in most cases
by the formation of stable metallacycles or the O–H oxidative
Fig. 3. Crystal structures of CO adduct 11 and tropone dimer 12. Selected bond distances (Å): Ir1–C1, 2.098(3); Ir1–C27, 2.141(3); O2–C26, 1.248(3); C26–C27, 1.476(4); C27–
C28, 1.375(4); C28–C29, 1.422(4); C29–C30, 1.360(4); C30–C31, 1.415(4); C31–C32, 1.351(4); C26–C32, 1.461(4).

X. Zhang et al. / Inorganica Chimica Acta 369 (2011) 253–259
257
HO
H
PtBu2
O
Ir
PtBu2
O
Ir
PtBu2
O
Ir
O
1,8-addition
H
H
H
H
PtBu2
9
PtBu2
PtBu2
13
formal
1,7-hydride shift
O
PtBu2
Ir
PtBu2
HO
O
Ir
H
HO
PtBu2
PtBu2
O
14
12
Scheme 6. Possible mechanism for tropone dimerization.
addition of phenolic products. Catalytic transfer dehydrogenation
of 3,3-dimethylcyclohexanone was achieved, giving the corre-
sponding a,b-enone.
The transfer dehydrogenation reaction of cycloheptanone with
1 was found to generate a surprisingly stable product apparently
derived from dehydrogenation to give tropone, followed by
(d, JPH = 18.7 Hz). 1H NMR (300 MHz, benzene-d6): d 12.27 (d,
JHH = 9.6 Hz, 1H, vinyl CH), 7.68 (d of vt, JPH = 1.2 Hz, JHH = 9.6 Hz,
1H, vinyl CH), 6.97 (d, JHH = 5.8 Hz, 2H, PCP m-H), 6.96 (t,
JHH = 5.8 Hz, 1H, PCP p-H), 3.10 (d of vt, JPH = 3.0 Hz, JHH = 16.0 Hz,
2H, CH2), 3.01 (d of vt, JPH = 4.0 Hz, JHH = 16.0 Hz, 2H, CH2), 2.51
(q of vt, JHH = 7.4 Hz, JPH = 1.6 Hz, 2H, CH2), 1.31 (t, JPH = 6.4 Hz,
18H, C(CH3)3), 1.12 (t, JHH = 7.4 Hz, 3H, CH3), 0.97 (t, JPH = 6.4 Hz,
18H, C(CH3)3), À9.04 (t, JPH = 19.4 Hz, 1H, Ir-H). 13C NMR
a-C–H addition and O-coordination. Complex 9 is stabilized by
conjugation and possibly represents an unusual bicyclo[5.2.0]tro-
ponyliridium metalloaromatic structure.
(300 MHz, benzene-d6):
d 249.38 (t, JCP = 4.4 Hz), 211.99 (s),
The formation of the novel tropone dimer 11 appears to be cat-
alyzed by complex 9. A possible mechanism for the dimerization
reaction is suggested in which tropone, normally an electrophile,
acts as a nucleophile upon activation by C–H oxidative addition
to the iridium(I) center. The possibility that C–H addition to a tran-
sition metal complex can serve to induce nucleophilic character at
a remote site of a substrate molecule is currently under further
investigation.
146.54 (t, JCP = 7.5 Hz), 143.05 (s), 136.39 (s), 121.28 (s), 121.11
(t, JCP = 7.3 Hz), 38.56(t, JCP = 8.3 Hz), 37.46 (t, JCP = 14.6 Hz), 36.49
(t, JCP = 12.0 Hz), 32.36(s), 30.649 (t, JCP = 2.0 Hz), 30.52 (t,
JCP = 2.3 Hz), 11.01 (s).
4.2. (PCP)Ir(H)(j-C,O-C2H2C(O)C2H5), (3)
Twenty milligrams of complex 2 was dissolved in 1 mL of p-xy-
lene solution and heated at 90 °C for 2 h. Complex 3 was formed in
quantitative yield (31P{1H} NMR). The solvent was removed in va-
cuo, and the resulting pale yellow solid was redissolved and recrys-
tallized in pentane solution at room temperature. Yellow crystals
were obtained. 31P{1H} NMR (121.4 MHz, benzene-d6): d 62.97 (d,
JPH = 16.0 Hz). 1H NMR (300 MHz, benzene-d6): 12.60 (d,
JHH = 9.2 Hz, 1H, vinyl CH), 7.81 (d, JHH = 9.2 Hz, 1H, vinyl CH),
7.16–7.24 (m, 3H, PCP H), 3.53 (d of vt, JPH = 3.0 Hz, JHH = 16.0 Hz,
2H, CH2), 3.20 (d of vt, JPH = 4.0 Hz, JHH = 16.0 Hz, 2H, CH2), 2.53
(q of vt, JHH = 7.6 Hz, JPH = 1.6 Hz, 2H, CH2), 1.13 (t, JHH = 7.6 Hz,
3H, CH3), 1.11 (t, JPH = 6.2 Hz, 18H, C(CH3)3), 0.96 (t, JPH = 6.2 Hz,
18H, C(CH3)3), À24.51 (t, JPH = 16 Hz, 1H, Ir-H). 13C NMR
4. Experimental
All reactions, recrystallizations, and routine manipulations were
conducted under argon using an argon-filled glove box or by using
standard Schlenk techniques. Reagent grade solvents were used
and dried according to established methods, then degassed with
argon. All NMR solvents were dried, vacuum-transferred, and
stored in an argon-filled glove box. 1H, 31P{1H}, and 13C{1H} NMR
spectra were obtained on 300-MHz or 400-MHz Varian Spectrom-
eters. 1H and 13C NMR chemical shifts are reported in ppm down-
field from tetramethylsilane and were referenced to residual
protiated (1H) or deuterated solvent (13C). 31P{1H} NMR chemical
shifts are referenced to an external standard, Me3P in mesity-
lene-d12 solvent (d À62.4 ppm), in a capillary tube. GC–MS analyses
were carried out with a Varian GC–MS (3900 GC and 2100T MS).
Complex 1 and 8 were prepared as described in the literature
[11,27].
(300 MHz, benzene-d6):
d 232.22 (t, JCP = 6.0 Hz), 211.59 (t,
JCP = 1.4 Hz), 165.26 (t, JCP = 3.4 Hz), 148.90 (t, JCP = 8.4 Hz),
136.35 (s), 122.63 (t, JCP = 1.0 Hz), 119.70 (t, JCP = 7.5 Hz), 41.03 (t,
JCP = 14.9 Hz), 38.96 (t, JCP = 8.7 Hz), 35.51 (t, JCP = 12.8 Hz), 32.81
(s), 29.92 (t, JCP = 2.0 Hz), 29.51 (t, JCP = 2.2 Hz), 10.08 (s). IR (pen-
tane):
m .
CO = 1723.0 cmÀ1
4.1. (PCP)Ir(H)(j-C,O-C2H2C(O)C2H5), (2)
4.3. (PCP)Ir(H)(OPh), (4)
Ten milligrams of complex 1 (0.017 mmol) was dissolved in a
1 mL of p-xylene solution containing 20 L of TBE (0.153 mmol)
at room temperature. To the resulting solution was added 4.6
of 3-pentanone (0.051 mmol); after stirring for 24 h, complex 2
was formed in 98% yield (31P{1H} NMR). Solvent and volatile com-
pounds were removed in vacuo, and the product was obtained as
orange powder. 31P{1H} NMR (121.4 MHz, benzene-d6): d 65.24
Ten milligrams of complex 1 (0.017 mmol), 4.6
lL of cyclohex-
l
anone (0.051 mmol), and 22 L of TBE (0.17 mmol) were dissolved
l
lL
in 1 mL of p-xylene. The solution was heated at 90 °C for 3 h, a red
solution was obtained. Complex 4 was formed in 98% yield (31P{1H}
NMR). The solvent was removed in vacuo, and the resulting solid
was redissolved in and recrystallized from pentane; red crystals
were obtained [25]. Complex 4 can also be made by the addition

258
X. Zhang et al. / Inorganica Chimica Acta 369 (2011) 253–259
of a small excess of phenol into a solution of 10 mg of (PCP)IrH2
vt, JPH = 4.2 Hz, JHH = 16.8 Hz, 2H, CH2), 1.15 (t, JPH = 6.4 Hz, 18H,
C(CH3)3), 1.11 (t, JPH = 6.3 Hz, 18H, C(CH3)3), À29.53 (t,
JPH = 14.4 Hz, 1H, Ir-H).
(0.017 mmol) and 4.4 lL of TBE (0.034 mmol) at room tempera-
ture. 31P{1H} NMR (121.4 MHz, benzene-d6):
JPH = 12.8 Hz). 1H NMR (300 MHz, benzene-d6):
d
d
67.26 (d,
7.36 (dd,
JHH = 7.5 Hz, JHH = 7.2 Hz, 2H, OPh, m-H), 6.97–7.10 (m, 3H, PCP
H), 6.92 (d, JHH = 7.5 Hz, 2H, OPh, o-H), 6.76 (t, JHH = 7.2 Hz, 1H,
OPh, p-H), 3.05 (d of vt, JPH = 3.8 Hz, JHH = 17.4 Hz, 2H, CH2), 2.97
(d of vt, JPH = 3.8 Hz, JHH = 17.4 Hz, 2H, CH2), 1.20 (t, JPH = 6.5 Hz,
18H, C(CH3)3), 1.10 (t, JPH = 6.7 Hz, 18H, C(CH3)3), À39.26 (t,
JPH = 12.8 Hz, 1H, Ir-H).
4.8. Cis and trans (PCP)Ir(H)(j-O,C-OC7H5), (troponyl hydride complex
10 and 9)
Fifteen milligrams of (tBuPCP)IrH2 (0.026 mmol) was dissolved
in 1 mL of p-xylene solution containing 3.7 mg of norbornene
(0.039 mmol) at room temperature. To the resulting solution was
added 3.1 lL of tropone (0.032 mmol); after stirring for ca. 1 min,
4.4. (PCP)Ir(H)(OC6H9), (5)
the red solution turned orange. The reaction was monitored by
NMR. Complex 10 was formed in 95% yield (31P{1H} NMR). 31P
NMR (162 MHz, p-xylene-d10): d 62.54 (d, JPH = 12.8 Hz). 1H NMR
(400 MHz, p-xylene-d10): 1.34 (t, JPH = 6.4 Hz, 18H, C(CH3)3), 0.90
(t, JPH = 6.2 Hz, 18H, C(CH3)3), À10.19 (t, JPH = 18.2 Hz, 1H, Ir-H).
Complex 10 isomerized to 9 at room temperature in about 1 h.
Complex 9: 31P{1H} NMR (121.4 MHz, benzene-d6): d 58.68 (d,
Ten milligrams of complex 1 (0.017 mmol), 4.6
lL of cyclohex-
anone (0.051 mmol), and 22 L of TBE (0.17 mmol) were dissolved
l
in 1 mL of p-xylene. The solution was left at room temperature for
2 h. A red solution was obtained. Complex 5 was formed in 98%
yield (31P{1H} NMR). 31P{1H} NMR (121.4 MHz, benzene-d6): d
68.00 (d, JPH = 9.11 Hz). 1H NMR (300 MHz, benzene-d6): d 7.34
(d, JHH = 7.2 Hz, 2H, PCP, m-H), 7.16 (t, JHH = 7.2 Hz, 1H, PCP, p-H),
4.42 (t, JHH = 6.2 Hz, 1H, cyclohexenol CH), 3.06 (d of vt,
JPH = 3.9 Hz, JHH = 16.8 Hz, 2H, CH2), 2.98 (d of vt, JPH = 5.1 Hz,
JHH = 16.8 Hz, 2H, CH2), 2.53 (t,d, JHH = 6.2 Hz, JHH = 3.3 Hz, 2H,
cyclohexenol CH2), 2.46 (t, JHH = 6.2 Hz, 2H, cyclohexenol CH2),
1.93–2.00 (m, 2H, cyclohexenol CH2), 1.78–1.83 (m, 2H, cyclohex-
enol CH2), 1.32 (t, JPH = 6.5 Hz, 18H, C(CH3)3), 1.29 (t, JPH = 6.5 Hz,
18H, C(CH3)3), À38.68 (t, JPH = 13.05 Hz, 1H, Ir-H).
JPH = 14.0 Hz). 1H NMR (300 MHz, benzene-d6):
d 8.54 (d,
JHH = 8.4 Hz, 1H, tropone CH), 7.26 (d, JHH = 7.2 Hz, 2H, PCP, m-H),
7.18 (t, JHH = 7.2 Hz, 1H, PCP, p-H), 6.73–6.40 (m, 4H, tropone
C4H4), 3.60 (d of vt, JPH = 3.0 Hz, JHH = 16.8 Hz, 2H, CH2), 3.25 (d of
vt, JPH = 4.2 Hz, JHH = 16.8 Hz, 2H, CH2), 1.15 (t, JPH = 6.4 Hz, 18H,
C(CH3)3), 1.11 (t, JPH = 6.3 Hz, 18H, C(CH3)3), À29.53 (t,
JPH = 14.4 Hz, 1H, Ir-H).
4.9. (PCP)Ir(H)(CO)(C7H5O), (11)
4.5. (PCP)Ir(H)(OPh-3-CH3), (6)
Fifteen milligrams of 9 (0.020 mmol) was dissolved in 1 mL of
p-xylene. The solution was freeze-pump-thawed, and 800 torr of
CO was immediately added. The dark solution turned dark yellow
slowly, and the reaction was complete after 12 h at room temper-
ature. The solvent was removed in vacuo, and a yellow solid was
obtained (95% yield). X-ray crystallography revealed that complex
11 co-crystallized with compound 12. 31P NMR (121.4 MHz,
benzene-d6): d 52.62 (d, JPH = 10.9 Hz). 1H NMR (300 MHz, ben-
zene-d6): d 8.57 (d, JHH = 8.4 Hz, 1H, CH), 7.10–7.03 (m, 3H, PCP aro-
matic), 6.67–6.26 (m, 4H, tropone C4H4), 3.28 (d of vt, JPH = 3.2 Hz,
JHH = 16.5 Hz, 2H, CH2), 3.19 (d of vt, JPH = 4.2 Hz, JHH = 16.5 Hz, 2H,
CH2), 1.19 (t, JPH = 6.8 Hz, 18H, C(CH3)3), 1.07 (t, JPH = 6.6 Hz, 18H,
C(CH3)3), À9.65 (t, JPH = 17.0 Hz, 1H, Ir-H).
Ten milligrams of complex 1 (0.017 mmol), 6.0 lL of 3-methyl-
cyclohexanone (0.051 mmol), and 16 mg of norbornene (0.17
mmol) were dissolved in 1 mL of p-xylene. The solution was heated
at 90 °C for 3 h. A red solution was obtained. Complex 6 was
formed in 98% yield (31P{1H} NMR). 31P{1H} NMR (121.4 MHz, ben-
zene-d6): d 67.26 (d, JPH = 12.9 Hz). 1H NMR (300 MHz, benzene-
d6): d 7.25-6.59 (m, 7H, aromatic H), 3.05 (d of vt, JPH = 3.9 Hz,
JHH = 17.1 Hz, 2H, CH2), 2.97 (d of vt, JPH = 3.9 Hz, JHH = 17.1 Hz,
2H, CH2), 2.32 (s, 3H, CH3), 1.21 (t, JPH = 6.5 Hz, 18H, C(CH3)3),
1.12 (t, JPH = 6.6 Hz, 18H, C(CH3)3), À39.06 (t, JPH = 12.9 Hz, 1H, Ir-
H).
4.6. 4,4-Dimethylcyclohex-2-enone, (7)
3,3-Dimethylcyclohexanone (20.8
lL, 0.150 mmol) in 1 mL of p-
4.10. 5,5-Dihydrodicyclohepta[b,d]furan-5a-ol, (12)
xylene-d10 was reacted with complex 1 (4.4 mg, 0.0075 mmol) and
TBE (38.7
l
L, 0.30 mmol) at 120 °C. The reaction was monitored by
Fifteen milligrams of (PCP)IrH2 (0.026 mmol), 3.1 lL of tropone
31P and 1H NMR spectroscopy and was found to be complete after
10 h in 85% yield by GC chromatographic analysis. 1H NMR
(300 MHz, benzene-d6): 0.80 (s, 6H, CH3), 1.72 (dd, JHH = 4.05 Hz,
JHH = 2.1 Hz, 2H, CH2), 1.99 (s, 2H, CH2), 5.92 (dt, JHH = 10.2 Hz,
JHH = 2.1 Hz, 1H, CH), 6.27 (dt, JHH = 10.2 Hz, JHH = 4.05 Hz, 1H, CH).
(0.032 mmol), and 6.2 mg of norbornene (0.065 mmol) was dis-
solved in 0.8 mL of p-xylene-d10 to yield complex 9 after 3 h at
room temperature. To this reaction mixture, additional tropone
(100 lL, 1.00 mmol) was added, and the reaction mixture was
heated at 80 °C. After 11 h, no further change in the tropone con-
centration was observed by NMR spectroscopy. Column chroma-
tography (20:1 ? 1:1 hexanes:ethyl acetate) afforded the product
as a dark red oil (35.6 mg, 32% isolated yield). Addition of excess
4.7. (PCP)Ir(H)(
j-O,C-OC7H5), (9)
Ten milligrams of (PCP)IrH2 (0.017 mmol), 6.0
lL of cyclohepta-
D2O (25 lL, 1.4 mmol) to a solution of 12 (35.6 mg, 0.168 mmol)
none (0.051 mmol), and 16 mg of norbornene (0.17 mmol) was dis-
solved in 1 mL of p-xylene. The solution was heated at 120 °C for
12 h. A red solution was obtained. Complex 9 was formed in 98%
yield. The solvent was removed in vacuo, and the resulting solid
was redissolved in and recrystallized from pentane; orange crys-
tals were obtained. 31P{1H} NMR (121.4 MHz, benzene-d6): d
58.68 (d, JPH = 14.0 Hz). 1H NMR (300 MHz, benzene-d6): d 8.54
(d, JHH = 8.4 Hz, 1H, tropone CH), 7.26 (d, JHH = 7.2 Hz, 2H, PCP, m-
H), 7.18 (t, JHH = 7.2 Hz, 1H, PCP, p-H), 6.73–6.40 (m, 4H, tropone
C4H4), 3.60 (d of vt, JPH = 3.0 Hz, JHH = 16.8 Hz, 2H, CH2), 3.25 (d of
in methylene chloride-d2 (0.8 mL) resulted in the disappearance
of the singlet at d 9.69 ppm in the 1H NMR spectrum. 1H NMR
(400 MHz, methylene chloride-d2):
d 9.69 (s, 1H), 7.71 (dd,
J = 8.6 Hz, 1.2 Hz, 1H), 7.30 (ddd, J = 12.0 Hz, 7.6 Hz, 1.7 Hz, 1H),
7.23–7.10 (m, 5H), 6.85 (dd, J = 8.1 Hz, 1.2 Hz, 1H), 6.82 (td,
J = 7.4 Hz, 1.3 Hz, 1H), 3.89 (s, 2H). 13C NMR (100 MHz, methylene
chloride-d2): d 188.64, 155.91, 154.52, 141.06, 138.72, 138.18,
135.61, 135.00, 130.91, 128.87, 125.80, 120.54, 118.14, 37.18.
GC–MS (EI) calculated for
found: 194.00.
C
14H10O+ (C14H12O2–H2O): 194.07,

X. Zhang et al. / Inorganica Chimica Acta 369 (2011) 253–259
259
[15] X. Zhang, M. Kanzelberger, T.J. Emge, A.S. Goldman, J. Am. Chem. Soc. 126
(2004) 13192.
Acknowledgment
[16] T. Nishimura, Y. Yasuhara, T. Hayashi, J. Am. Chem. Soc. 129 (2007) 7506.
[17] B. Papenfuhs, T. Dirnberger, H. Werner, Can. J. Chem. 84 (2006) 205.
[18] T. Dirnberger, H. Werner, Organometallics 24 (2005) 5127.
[19] X. Li, P. Chen, J.W. Faller, R.H. Crabtree, Organometallics 24 (2005) 4810.
[20] X. Li, C.D. Incarvito, R.H. Crabtree, J. Am. Chem. Soc. 125 (2003) 3698.
[21] M.L. Buil, M.A. Esteruelas, K. Garces, M. Olivan, E. Onate, Organometallics 27
(2008) 4680.
We thank the National Science Foundation (Grant CHE-0719307)
for support of this work.
Appendix A. Supplementary material
[22] C.J. Moulton, B.L. Shaw, J. Chem. Soc., Dalton Trans. (1976) 1020.
[23] M. Kanzelberger, B. Singh, M. Czerw, K. Krogh-Jespersen, A.S. Goldman, J. Am.
Chem. Soc. 122 (2000) 11017.
[24] D. Morales-Morales, D.W. Lee, Z. Wang, C.M. Jensen, Organometallics 20
(2001) 1144.
[25] We have subsequently published the crystal structure of complex 4 in the
context of a different study: J. Choi, Y. Choliy, X. Zhang, T.J. Emge, K. Krogh-
Jespersen, A.S. Goldman, J. Am. Chem. Soc. 131 (2009) 15627.
[26] K. Krogh-Jespersen, M. Czerw, K. Zhu, B. Singh, M. Kanzelberger, N. Darji, P.D.
Achord, K.B. Renkema, A.S. Goldman, J. Am. Chem. Soc. 124 (2002) 10797.
[27] K. Zhu, P.D. Achord, X. Zhang, K. Krogh-Jespersen, A.S. Goldman, J. Am. Chem.
Soc. 126 (2004) 13044.
[28] Tropone photodimerization, to give cycloaddition products very different from
11, has been well studied; for examples, see: (a) T. Mukai, T. Tezuka, Y.
Akasaki, J. Am. Chem. Soc. 88 (1966) 5025;
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2010.11.016.
References
[1] R. Mahrwald (Ed.), Modern Aldol Reactions, Wiley-VCH, Berlin, 2004.
[2] Y. Zhao, Y.-Y. Yeung, Org. Lett. 12 (2010) 2128.
[3] T.K.M. Shing, Y.-Y. Yeung, P.L. Su, Org. Lett. 8 (2006) 3149.
[4] A.J. Catino, R.E. Forslund, M.P. Doyle, J. Am. Chem. Soc. 126 (2004) 13622.
[5] H.J. Reich, J.M. Renga, I.L. Reich, J. Am. Chem. Soc. 97 (1975) 5434.
[6] For a recent mini review, see: J. Muzart, Eur. J. Org. Chem. (2010) 3779.
[7] M. Tokunaga, S. Harada, T. Iwasawa, Y. Obora, Y. Tsuji, Tetrahedron Lett. 48
(2007) 6860.
[8] Y. Shvo, A.H.I. Arisha, J. Org. Chem. 63 (1998) 5640.
[9] R.J. Theissen, J. Org. Chem. 36 (1971) 752.
[10] M. Uyanik, M. Akakura, K. Ishihara, J. Am. Chem. Soc. 131 (2009) 251.
[11] M. Gupta, C. Hagen, R.J. Flesher, W.C. Kaska, C.M. Jensen, Chem. Commun.
(1996) 2083.
(b) A.S. Kende, J. Am. Chem. Soc. 88 (1966) 5026;
(c) T. Tezuka, Y. Akasaki, T. Mukai, Tetrahedron Lett. (1967) 1397;
(d) I.D. Reingold, J.A. Kowalski, G.C. Cummings, R. Gleiter, H. Lange, S. Lovell, B.
Kahr, K. Aflatooni, P.D. Burrow, G.A. Gallup, J. Phys. Org. Chem. 19 (2006) 642.
[29] V. Nair, M. Poonoth, S. Vellalath, E. Suresh, R. Thirumalai, J. Org. Chem. 71
(2006) 8964.
[12] For
a recent lead reference see: B. Punji, T.J. Emge, A.S. Goldman,
Organometallics 29 (2010) 2702.
[13] M. Gupta, W.C. Kaska, C.M. Jensen, Chem. Commun. (1997) 461.
[14] X. Zhang, A. Fried, S. Knapp, A.S. Goldman, Chem. Commun. (2003) 2060.
[30] K. Kumar, A. Kapur, M.P.S. Ishar, Org. Lett. 2 (2000) 787.
[31] J.H. Rigby, J.Z. Wilson, J. Am. Chem. Soc. 106 (1984) 8217.