Equilibrium between η2-(o-ethynylbenzoyl)rhenium complexes and rhenium isobenzofuryl carbene complexes and subsequent reactions of isobenzofuryl carbene complexes

Article abstract of DOI:10.1021/om040063g

Thermal rearrangements of η²-(o-ethynylbenzoyl)-Re complexes 2 produced isobenzofuryl-Re carbene complexes 3. This reaction might proceed via nucleophilic attack of the carbonyl oxygen on the Re-bound alkyne or might proceed through a [1.5, 1] Re shift followed by an electrocyclic ring closure. The two species, alkyne complex 2 and carbene complex 3, were observed at equilibrium. Isobenzofuryl carbene complexes 3 were characterized by ¹H and ¹³C NMR spectroscopy and by trapping via Diels-Alder reactions with DMAD. Air oxidation of the equilibrium mixtures of 2 and 3 led to oxidation of the isobenzofuryl carbene complexes 3 to unexpected Re η²-ketone complexes 5. Studies with ¹⁷O-labeled dioxygen are consistent with oxygen addition to the isobenzofuran followed by rearrangement of the resulting endoperoxide that involves movement of the furan oxygen to the η²-ketone site in 5.

Full text of DOI:10.1021/om040063g

Organometallics 2004, 23, 4121-4130
4121
Equ ilibr iu m betw een η2-(o-Eth yn ylben zoyl)r h en iu m
Com p lexes a n d Rh en iu m Isoben zofu r yl Ca r ben e
Com p lexes a n d Su bsequ en t Rea ction s of Isoben zofu r yl
Ca r ben e Com p lexes
Charles P. Casey,* Neil A. Strotman, and Ilia A. Guzei
Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
Received April 30, 2004
Thermal rearrangements of η2-(o-ethynylbenzoyl)-Re complexes 2 produced isobenzofuryl-
Re carbene complexes 3. This reaction might proceed via nucleophilic attack of the carbonyl
oxygen on the Re-bound alkyne or might proceed through a [1.5,1] Re shift followed by an
electrocyclic ring closure. The two species, alkyne complex 2 and carbene complex 3, were
1
observed at equilibrium. Isobenzofuryl carbene complexes 3 were characterized by H and
13C NMR spectroscopy and by trapping via Diels-Alder reactions with DMAD. Air oxidation
of the equilibrium mixtures of 2 and 3 led to oxidation of the isobenzofuryl carbene complexes
3 to unexpected Re η2-ketone complexes 5. Studies with 17O-labeled dioxygen are consistent
with oxygen addition to the isobenzofuran followed by rearrangement of the resulting
endoperoxide that involves movement of the furan oxygen to the η2-ketone site in 5.
Sch em e 1
In tr od u ction
Our recent studies of rhenium alkynylcarbene com-
plexes demonstrated high reactivity at the remote
alkyne carbon center. We have observed (1) dimerization
of alkynylcarbene complexes by coupling at the remote
alkyne carbon to give enediyne complexes,1 (2) inser-
tions of the remote alkyne carbon into the CH bond of
a Cp* ligand,1a,2 and (3) intramolecular cyclopropanation
by addition of the remote alkyne carbon to a pendant
alkene.1a All three reactions involve a net [1,1.5] rhe-
nium migration to give rhenium alkyne complexes.
These three transformations suggest emerging “free
carbene” reactivity at the remote carbon of Re alkynyl
carbene complexes (Scheme 1). A [1,1.5] rhenium shift
would generate an alkyne complex adjacent to a center
with “free carbene” character and reactivity.
Sch em e 2
Sch em e 3
Strained alkynes sometimes react as formal dicar-
benes. Alkyne ring strain favors a dicarbene resonance
formulation with bent geometry at both carbons. Sup-
port for this dicarbene reactivity comes from thermal
formation of [2+2] cycloadducts from reaction of cyclo-
pentynes with alkenes, a reaction atypical of unstrained
alkynes.3 Gilbert and Laird recently observed dicarbene
reactivity of norbornyne (Scheme 2).4 When this highly
strained alkyne was generated by lithiation of 2,3-
dibromonorbornene in the presence of dihydropyran,
products were obtained that could best be explained by
dicarbene reactivity. It was proposed that one carbon
of the dicarbene added to the double bond of the
dihydropyran forming a cyclopropane adjacent to a
carbene. This carbene intermediate then reacted through
two pathways: (1) insertion into a CH bond to form a
nortricyclene and (2) ring expansion by a [1,2] shift
giving the [2+2] cycloadduct.
We wondered whether an alkyne bound to rhenium
might also be able to react as a dicarbene. A [1.5,1]
rhenium shift leading to a metal carbene complex might
provide the impetus for emergence of “free carbene”
character at the adjacent carbon (Scheme 3). This
process is the reverse of the [1,1.5] rhenium shifts of
* Corresponding author. E-mail: casey@chem.wisc.edu. Fax: (608)
262-7144.
(1) (a) Casey, C. P.; Kraft, S.; Powell, D. R. J . Am. Chem. Soc. 2000,
122, 3771. (b) Casey, C. P.; Kraft, S.; Powell, D. R. Organometallics
2001, 20, 2651. (c) Casey, C. P.; Kraft, S.; Powell, D. R. J . Am. Chem.
Soc. 2002, 124, 2584.
(2) Casey, C. P.; Kraft, S.; Kavana, M. Organometallics 2001, 20,
3795.
(3) (a) Fitjer, L.; Modaressi, S. Tetrahedron Lett. 1983, 24, 5495.
(b) Gilbert, J . C.; Baze, M. E. J . Am. Chem. Soc. 1984, 106, 1885.
(4) Laird, D. W.; Gilbert, J . C. J . Am. Chem. Soc. 2001, 123, 6704.
10.1021/om040063g CCC: $27.50 © 2004 American Chemical Society
Publication on Web 07/09/2004

4122 Organometallics, Vol. 23, No. 17, 2004
Casey et al.
Sch em e 4
Sch em e 6
Sch em e 5
Sch em e 7
alkynylcarbene complexes previously studied in our
group.
In searching for additional facile methods other than
dimerization, CH insertion, and cyclopropanation for
trapping a carbene, we turned our attention to the well-
known 6π electrocyclic ring closing rearrangements of
4-oxabutadienylcarbenes to furans (Scheme 3).5
Here we report successful isolation of Re 2′-(arylethy-
nyl)acetophenone complexes and their slow equilibra-
tion with Re isobenzofurylcarbene complexes. The isoben-
zofurylcarbene complexes were directly observed in
solution by spectroscopy and were trapped by Diels-
Alder reaction with dimethyl acetylenedicarboxylate
and by reaction with molecular oxygen.
Reaction of cis-2-penten-4-yn-1-ones with M(CO)5‚
THF led to the formation (2-furyl)carbene complexes,
which were subsequently oxidized to furfurals by di-
oxygen (Scheme 4).6,7 In related work, reaction of cis-
2-penten-4-yn-1-ones with electron-rich alkenes in the
presence of catalytic amounts of M(CO)5‚THF produced
(2-furyl)cyclopropanes.8 The authors suggested that the
(2-furyl)carbene complex intermediates were formed by
coordination of the metal to the alkyne followed by
nucleophilic attack of the carbonyl on the complexed
alkyne.6,8 However, alkyne complexes were not ob-
served, presumably due to rapid cyclization. Since weak
nucleophiles do not attack Re alkyne complexes, we
suggest consideration of an alternative pathway involv-
ing a [1.5,1] Re shift to give an intermediate with “free
carbene” reactivity which then undergoes cyclization to
a furan.
Resu lts a n d Discu ssion
Syn th esis of Alk yn e Com p lexes. 2′-(Arylethynyl)-
acetophenones (1a -c) were synthesized in very good
yield (84-98%) via palladium-catalyzed Sonogashira
couplings of 2′-iodoacetophenone with the appropriately
substituted terminal alkynes (Scheme 6). The three
alkynes were purified by flash column chromatography
and characterized by 1H and 13C NMR spectroscopy and
by HRMS.
Cp(CO)2Re-alkyne complexes 2a -c were prepared in
low yield (4-8%) by reaction of 2-6 equiv of alkynes
1a -c with CpRe(CO)2(THF), which was generated by
photolysis of CpRe(CO)3 in THF under a continuous
nitrogen purge (Scheme 7). Re-alkyne complexes 2a -c
were isolated as air-stable compounds by preparative
TLC. The low yields of Re alkyne complexes 2a -c are
attributed to incomplete photolysis (∼65% by IR spec-
troscopy) of CpRe(CO)3 and rearrangement of 2a -c to
isobenzofurylcarbene complexes both during the alkyne
complex synthesis and during TLC purification on silica
gel.
Since isobenzofurans have diminished aromaticity
relative to furans, we hypothesized that switching from
cis-2-en-4-yn-1-one complexes to 2-alkynylphenyl ketone
complexes would lead to kinetically slower cyclization
to isobenzofurylcarbene complexes and thermodynami-
cally more stable rhenium alkyne complexes (Scheme
5). We hoped that the higher energy transition state
leading to the isobenzofuran would slow conversion of
the alkyne complex to the furylcarbene complex and
that the greater relative stability of the alkyne complex
might allow its direct observation or isolation.9
Rhenium alkyne complexes were fully characterized
1
by H and 13C NMR spectroscopy and by HRMS. The
1H NMR spectra of rhenium alkyne complexes 2a -c
were similar to those of the free ligands 1a -c, except
that in each case the methyl ketone resonance was
shifted 0.4-0.5 ppm to lower frequency in the alkyne
complex. The Re(CO)2 group gave rise to two 13C NMR
resonances between δ 204 and 205 for complexes 2b and
2c and to a single broad resonance at δ 205.5 for 2a .
Slow rotation of the alkyne ligand renders the Re(CO)2
(5) (a) Tomer, K. B.; Harrit, N.; Rosenthal, I.; Buchardt, O.; Kumler,
P. L.; Creed, D. J . Am. Chem. Soc. 1973, 95, 7402. (b) Padwa, A.; Akiba,
M.; Chou, C. S.; Cohen, L. J . Org. Chem. 1982, 47, 183. (c) Nakatani,
K.; Adachi, K.; Tanabe, K.; Saito, I. J . Am. Chem. Soc. 1999, 121, 8221.
(6) Miki, K.; Yokoi, T.; Nishino, F.; Ohe, K.; Uemura, S. J . Orga-
nomet. Chem. 2002, 645, 228.
(7) Iwasawa, N.; Shido, M.; Kusama, H. J . Am. Chem. Soc. 2001,
123, 5814.
(8) (a) Miki, K.; Nishino, F.; Ohe, K.; Uemura, S. J . Am. Chem. Soc.
2002, 124, 5260. (b) Miki, K.; Yokoi, T.; Nishino, F.; Kato, Y.;
Washitake, Y.; Ohe, K.; Uemura, S. J . Org. Chem. 2004, 69, 1557.

Re Isobenzofuryl Carbene Complexes
Organometallics, Vol. 23, No. 17, 2004 4123
Sch em e 8
Sch em e 9
characterization of 3b was not possible. 3b was identi-
fied and monitored by 1H NMR resonances at δ 5.26
(Cp), 3.86 (OMe), and 2.20 (Me).
diastereotopic.10 The 13C NMR resonances of the alkyne
carbons of 2a -c (δ 79.0-91.0) are shifted 4-8 ppm to
lower frequency relative to the resonances of the free
alkynes 1a -c (δ 87.5-95.4).
We next turned to an electron-withdrawing group in
an effort to observe higher equilibrium concentrations
of an isobenzofuryl carbene complex and synthesized the
CF3-substituted alkyne complex 2c. After heating for 3
days at 38 °C in CD2Cl2, an equilibrium was established
between 2c and isobenzofuryl carbene complex 3c. The
electron-withdrawing substituent shifted the equilibri-
um toward the isobenzofuryl carbene complex isomer
(2c:3c ) 57:43, Keq ) 0.74). The 1H and 13C NMR
spectra of 3c were similar to those of 3a and provided
evidence for the isobenzofuryl carbene structure. Low-
frequency aromatic resonances at δ 7.13, 7.05 (m, 2H),
and 6.83 were observed for the four isobenzofuran
protons. The carbene carbon resonance of 3c appeared
at δ 240.9,12 and the resonances for the furyl carbons
adjacent to oxygen appeared at characteristically high
frequencies (δ 164.9 and 162.2).
Equ ilibr iu m betw een Alk yn e Com p lexes a n d
Isoben zofu r yl Ca r ben e Com p lexes. When a solution
of alkyne complex 2a was heated at 38 °C, 1H NMR
spectroscopy revealed the slow formation of a new
species which was shown to be isobenzofuryl carbene
complex 3a (Scheme 8). After 5 days (∼20 × t1/2), an
equilibrium between 2a and 3a (76:24, Keq ) 0.32) was
established. No attempt was made to isolate 3a due to
its extreme air sensitivity, its small fraction at equilib-
rium, and its relatively rapid rate of reequilibration with
2a .
1
3a was characterized by H and 13C NMR spectros-
copy of the equilibrium mixture of 2a and 3a . The
resonances of the four isobenzofuran protons appeared
at δ 7.08, 6.90 (m, 2H), and 6.80; due to lesser aroma-
ticity of the isobenzofuran ring, these resonances are
at lower frequency than typical aromatic resonances (δ
7.3-7.6). In the 13C NMR spectrum, a carbene reso-
nance was observed at δ 245.0 and two high-frequency
resonances at δ 162.7 and 161.3 were seen for the furan
carbons adjacent to oxygen.11 The 13C resonance for the
methyl carbon on the isobenzofuran appeared at δ 15.1,
which is 15 ppm to lower frequency than a methyl
ketone resonance.
The methoxy-substituted alkyne complex 2b was
synthesized in the hope that the electron-donating
methoxy group might selectively stabilize the isomeric
carbene complex 3b and give rise to more of 3b at
equilibrium. However, upon sitting at room temperature
for 3 days, 2b gave an equilibrium mixture containing
even less of isobenzofuryl carbene complex 3b (2b:3b
) 87:13, Keq ) 0.15) (Scheme 8). Due to the low
equilibrium concentration of 3b relative to 2b and to
overlap of isobenzofuryl proton resonances with reso-
nances of the protons of 2b ortho to methoxy, full
The influence of solvent polarity on the equilibrium
between alkyne and isobenzofuryl carbene complexes
was briefly investigated. The equilibrium between 2a
and 3a shifted toward isobenzofuryl carbene complex
3a in solvents of lower dielectric constant [Keq ) 0.32
in CD2Cl2 (ꢀ ) 8.9); Keq ) 0.50 in CDCl3 (ꢀ ) 4.7); Keq
)
0.75 in toluene-d8 (ꢀ ) 2.4)]. Similarly, the 2c:3c
equilibrium shifted toward 3c in less polar solvents [Keq
) 0.74 in CD2Cl2; Keq ) 1.62 in C6D6 (ꢀ ) 2.3)]. This
solvent dependence is attributed to the greater stabili-
zation of the more polar ketone functional group of 2
by solvents of higher dielectric constant than of the less
polar isobenzofuran ring of 3.
Tr a p p in g of Isoben zofu r yl Ca r ben e Com p lexes
w ith DMAD. Since isobenzofurans are very reactive
toward Diels-Alder dienophiles, we sought to trap the
isobenzofuryl carbene complex with dimethyl acetylene-
dicarboxylate (DMAD). Reaction of an equilibrium 87:
13 mixture of 2b:3b with DMAD (5 equiv)13 in CD2Cl2
1
at 40 °C was followed by H NMR spectroscopy over 10
days. Both isobenzofuryl carbene complex 3b and alkyne
complex 2b disappeared and DMAD adduct 5b formed
in 68% NMR yield. Recrystallization gave 5b in 48%
yield as a red, crystalline solid (Scheme 9). DMAD
adduct 4b was characterized by 1H and 13C NMR
spectroscopy and by HRMS. The 13C NMR showed a
(9) Although isobenzofurans are highly reactive and somewhat
unstable, they have been shown to be useful synthetic intermediates.
(a) Friedrichsen, W. Adv. Heterocycl. Chem. 1999, 73, 1. (b) Wege, D.
In Advances in Theoretically Interesting Molecules; Thummel, R. P.,
Ed.; J AI Press Inc.: Greenwich, 1998; Vol. 4, p 1.
(10) The complexed alkyne of C5Me5(CO)2Re(H3CCtCCH2OH) un-
derwent slow rotation at room temperature and showed diastereotopic
(12) The effect of aryl substituents on the 13C NMR chemical shifts
of MdC in metal carbene complexes is small and nonsystematic.1a,b
(a) Kraft, S. Ph.D. Thesis, University of Wisconsin-Madison, 2001;
pp 153-164. (b) Hafner, A.; Hegedus, L. S.; deWeck, G.; Hawkins, B.;
Do¨tz, K. H. J . Am. Chem. Soc. 1988, 110, 8413. (c) Bernasconi, C. F.;
Garc´ıa-R´ıo, L. J . Am. Chem. Soc. 2000, 122, 3821.
Re(CO)2 and CH2; dynamic NMR measurements gave ∆Gq ) 16.9
rot
kcal mol-1. The related complex C5Me5(CO)2Re(HCtCCH2OH) under-
went fast rotation at room temperature, showing equivalent CH2;
dynamic NMR measurements gave ∆Gq ) 12.2 kcal mol-1. Casey,
rot
C. P.; Selmeczy, A. D.; Nash, J . R.; Yi, C. S.; Powell, D. R.; Hayashi,
R. K. J . Am. Chem. Soc. 1996, 118, 6698.
(13) Cleaner reactions were seen when a large excess of DMAD was
employed. The large excess favors reaction of 3b with DMAD as a
(11) Furan carbons adjacent to oxygen had chemical shifts between
166 and 172 ppm in (2-(5-phenyl)furyl)carbene complexes of Cr and
W.6
dienophile over reaction of 3b with the product 4b acting as
a
dienophile.

4124 Organometallics, Vol. 23, No. 17, 2004
Casey et al.
Ta ble 1. X-r a y Cr ysta l Da ta for 4c a n d 5a
DMAD adduct 4c
ketone complex 5a
empirical formula
fw
C
31H24Cl2F3O7Re
C23H17O5Re
559.59
823.61
temperature
wavelength
cryst syst
space group
unit cell dimens
100(2) K
0.71073 Å
triclinic
P
100(2) K
0.71073 Å
monoclinic
C2/c
a ) 10.4367(6) Å
b ) 11.0323(6) Å
c ) 14.0659(8) Å
R) 69.8650(10)°
â) 80.7060(10)°
γ) 79.2990(10)°
1485.75(14) Å3
2
a ) 14.6965(17) Å
b ) 11.6450(13) Å
c ) 11.9051(14) Å
R) 90°
â) 107.666(2)°
γ) 90°
volume
Z
density (calcd)
abs coeff
F(000)
1941.4(4) Å3
4
1.841 Mg/m3
4.336 mm-1
806
1.914 Mg/m3
6.292 mm-1
1080
crystal size
0.35 × 0.35 ×
0.43 × 0.19 ×
0.20 mm3
0.10 mm3
F igu r e 1. X-ray crystal structure of 4c. Selected bond
lengths (Å) and angles (deg): RedC(8), 1.950(5); Re-C(8)-
C(16), 125.9(3); Re-C(8)-C(9), 120.3(3); C(9)-C(8)-C(16),
113.7(3); Re-C(8)-C(16)-C(28), 155.7(3).
θ range for data
collection
index ranges
2.00 to 26.37°
2.27 to 28.30°
-12ehe13,
-12eke13,
0ele17
-19ehe19,
-15eke15,
-15ele15
8621
high-frequency carbene carbon resonance at δ 288.6,
consistent with the structure of 4b. IR spectroscopy
showed two strong peaks for the metal carbonyls at 1986
no. of reflns
collected
no. of indep
reflns
completeness to
θ ) 26.37°
abs corr
12 311
5992 [R(int) )
0.0498]
98.70%
2390 [R(int) )
0.0380]
98.70%
and 1910 cm-1
.
The CF3-substituted DMAD adduct 4c was prepared
by reaction of an equilibrium 57:43 mixture of 2c:3c
with DMAD (10 equiv)13 in CD2Cl2 (Scheme 9). After 2
days at 38 °C, both starting materials 2c and 3c were
consumed and 4c was produced in 94% NMR yield.
Recrystallization gave an 81% yield of 4c as red crystals.
Spectral characterization of 4c included a 13C NMR
resonance for the carbene carbon at δ 284.3 and strong
IR stretches for the metal carbonyls at 1990 and 1913
cm-1. The X-ray crystal structure of 4c (Figure 1, Table
1) confirmed the spectral assignments.14 The structure
contained one CH2Cl2 molecule per unit cell and showed
some disorder about one of the carboxyl groups.
multiscan with
SADABS
0.4776 and 0.3122
multiscan with
SADABS
0.5719 and 0.1728
max. and min.
transmn
refinement
method
no. of data/
restraints/
params
full-matrix least-
squares on F2
5992/31/398
full-matrix least-
squares on F2
2390/0/185
goodness-of-fit
1.037
1.207
on F2
final R indices
[I>2σ(I)]
R indices
(all data)
largest diff
peak and hole
R1 ) 0.0302,
wR2 ) 0.0722
R1 ) 0.0335,
wR2 ) 0.0738
2.070 and
R1 ) 0.0339,
wR2 ) 0.0931
R1 ) 0.0343,
wR2 ) 0.0933
2.242 and
The isolation of complexes 4b and 4c offers strong
support for the structures of isobenzofuryl carbene
complexes 3b and 3c. Isolation and handling of carbene
complex 4 in air demonstrates the high stability of Re
carbene complexes toward oxidation.
-1.858 e Å-3
-1.211 e Å-3
Sch em e 10
Rea ction of Rh en iu m Isoben zofu r yl Ca r ben e
Com p lexes w ith O2. When an equilibrium mixture of
2a and 3a in CD2Cl2 was exposed to air, isobenzofuryl
carbene complex 3a disappeared over several hours,
followed by much slower disappearance of alkyne com-
plex 2a over 100 h (Scheme 10). This indicated that Re
isobenzofurylcarbene complex 3a was reacting directly
with oxygen and is consistent with depletion of Re
alkyne complex 2a by slow equilibration with 3a fol-
lowed by oxidation of 3a . 1H NMR spectroscopy showed
the formation of a single major product 5a in 65% yield
determined by NMR integration using solvent as an
internal standard.
Preparative TLC gave 5a as an air-stable, yellow
solid, which was characterized spectroscopically and by
X-ray crystallography. HRMS showed that the conver-
sion of 3a to 5a involved the addition of two oxygens.
1H NMR spectroscopy provided evidence for the forma-
tion of a methyl ketone (singlet at δ 2.74), the destruc-
tion of the isobenzofuran system (disappearance of lower
frequency resonances δ 7.08-6.80), and the generation
of a normal aromatic system (appearance of higher
frequency aromatic resonances between δ 7.42 and
(14) Re-C single bonds in the crystal structure of the metallacycle
Cp(CO)2ReCH2CH2CH2CH2 had lengths of 2.246(9) and 2.255(8) Å.14a
The Re-C single bonds in the complex cis-Cp(CO)2Re(CH2CH3)2 had
lengths of 2.262(10) and 2.263(10) Å.14b The RedC double bond distance
in the complex Cp(CO)2RedC(Tol)(CtCTol) was 2.004(5) Å.1a,12a
RedC double bond distances in the complexes Cp(CO)2RedC(C6H4-
p-SO2CF3)CtCC6H4-p-CH3,1b (η5-indenyl)(CO)2RedC(C6H4-p-CF3)-
CtCC6H4-p-CH3,1b and Cp*(CO)2RedC(Ph)CtCPh2 were 2.007(5),
1.996(4), and 2.004(5) Å, respectively. (a) Yang, G. K.; Bergman, R. G.
Organometallics 1985, 4, 129. (b) Klahn, A. H.; Manzur, C.; Toro, A.;
Moore, M. J . Organomet. Chem. 1996, 516, 51.

Re Isobenzofuryl Carbene Complexes
Organometallics, Vol. 23, No. 17, 2004 4125
Sch em e 11
7.19. The appearance of two resonances is due to slow
rotation of the phenyl group about C(13)-C(14) (Figure
2). Phenyl rotation is apparently hindered by steric
interactions with the Cp group on Re and by the
electronic requirement of breaking conjugation with the
coordinated ketone carbonyl. A rotation barrier of ∆Gq
of 12.9 ( 0.5 kcal mol-1 was estimated from the low-
temperature peak separations and the coalescence tem-
perature of 20 °C. Slow rotation of the phenyl group also
gave rise to a very broad resonance at δ 127 for the ortho
carbons in the 13C NMR spectrum.
F igu r e 2. X-ray crystal structure of 5a . Selected bond
lengths (Å) and angles (deg): Re-O(3), 2.161(10); Re-
C(14), 2.205(3); O(3)-C(14), 1.400(12); O(4)dC(15), 1.215-
(9); O(5)dC(22), 1.228 (10); Re-O(3)-C(14), 73.0(5); O(3)-
Re-C(14), 37.4(3); O(3)-C(14)-Re, 69.6(4); O(3)-C(14)-
C(13), 117.7(4); C(13)-C(14)-C(15), 118.1; O(3)-C(14)-
C(15)-O(4), 178.1(7); O(4)-C(15)-C(16)-C(17), 51.8(5);
C(16)-C(21)-C(22)-O(5), 19.4(5).
The related η2-ketone complexes 5b (66% yield) and
5c (67% yield) were prepared in a similar fashion by
air oxidation of equilibrating mixtures of 2 and 3. The
1H NMR spectra of 5b and 5c were similar to that of
5a ; very broad ortho hydrogen resonances were seen at
δ 7.69 for 5b and 7.90 for 5c, indicating hindered aryl
rotation. In the 13C NMR spectra, resonances at δ 78.9
for 5b and 76.2 for 5c were observed for the η2-ketone
carbonyl carbons. Ketone complexes 5b and 5c each
showed two strong IR absorbances for the metal carbo-
nyls (5b: 1994, 1922 cm-1; 5c: 2001, 1929 cm-1).
17O-La belin g Stu d ies of F or m a tion of η2-Keton e
Com p lexes. The more rapid disappearance of the
isobenzofuryl carbene complex 3 from the equilibrating
mixture of 3 and alkyne complex 2 requires that 3 be
the compound that reacts with oxygen. Since MdC
bonds are often subject to oxidation, we considered
mechanisms for oxidation of 3 that involved initial
attack of O2 at the metal carbene unit. This would place
one of the newly incorporated oxygens on the η2-ketone
carbonyl (the former carbene carbon of 3). Since isoben-
zofurans are easily oxidized to endoperoxides,17 we also
considered the possibility that oxidation of 3 initially
produced endoperoxide A (Scheme 11). The conversion
of isobenzofuran endoperoxides to 1,2-dialkanoylben-
zenes has been observed, though it is not clear how an
oxygen atom is lost from the endoperoxide.17 Furan
endoperoxides and isobenzofuran endoperoxides are
oxidizing agents and have been observed to transfer
oxygen atoms to alkenes to form epoxides and to
diphenylsulfides to form sulfoxides.17b,18 Consequently,
there are many possible pathways from A to the
observed η2-ketone complex 5, and it is possible to write
mechanisms that place the unique furyl oxygen on any
of the three sites in 5. To help distinguish between these
possibilities, we studied the reaction of an equilibrating
mixture of 2 and 3 with 17O-labeled dioxygen.
7.68). 13C NMR spectroscopy of 5a showed two metal
carbonyl resonances and two organic carbonyl reso-
nances (δ 205.3, 201.1, 199.1, 197.8). IR spectroscopy
showed two very strong bands at 1999 and 1925 cm-1
for the metal carbonyls and two weaker bands at 1692
and 1668 cm-1 for the organic carbonyls.
While spectroscopic data were consistent with the
structure depicted for 5a , X-ray crystallography was
required to establish its structure as an η2-ketone
complex (π-bonded to metal) (Figure 2, Table 1). η1-
Ketone complexes (lone pair σ-bonded to metal) are
more common than η2-ketone complexes (π-bonded to
metal).15 η2-Complexation is usually seen for alde-
hydes,15,16 but has only occasionally been seen for
ketones. For example, a Re-η2-ketone complex was seen
for a ketone having electron-withdrawing groups (Cl,
F) R to the carbonyl.15 The η2-ketone coordination seen
in 5a may be favored by the electron-withdrawing
nature of its aroyl substituent. There are two resonance
structures for η2-ketone complexes: π-bond coordination
to the metal and a metallaoxirane formulation. The 13C
NMR chemical shift of complexed carbonyl (δ 78.4) of
5a suggests that the metallaoxirane formulation is the
major resonance contributor.
Unusual spectral features of 5a are readily under-
stood in light of its η2-ketone structure. The unusual
13C NMR resonance at δ 78.4 is assigned to the η2-
1
complexed carbonyl of 5a . The H NMR resonance at δ
7.81, corresponding to the ortho protons of the phenyl
group, was very broad. Upon cooling to -62 °C, this
ortho resonance decoalesced to give peaks at δ 8.42 and
The two ketone carbonyls of 5 were expected to be
seen in the δ 500-600 ppm range based on comparisons
(15) (a) Klein, D. P.; Dalton, D. M.; Me´ndez, N. Q.; Arif, A. M.;
Gladysz, J . A. J . Organomet. Chem. 1991, 412, C7. (b) Delbecq, F.;
Sautet, P. J . Am. Chem. Soc. 1992, 114, 2446.
(16) (a) Schuster, D. M.; White, P. S.; Templeton, J . L. Organome-
tallics 2000, 19, 1540. (b) Garner, C. M.; Me´ndez, N. Q.; Kowalczyk,
J . J .; Ferna´ndez, J . M.; Emerson, K.; Larsen, R. D.; Gladysz, J . A. J .
Am. Chem. Soc. 1990, 112, 5146. (c) Buhro, W. E.; Georgiou, S.;
Ferna´ndez, J . M.; Patton, A. T.; Strouse, C. E.; Gladysz, J . A.
Organometallics 1986, 5, 956.
(17) Dioxygen addition to isobenzofurans, in both the presence and
absence of light, has been demonstrated to produce endoperoxides. (a)
Friedrichsen, W. Adv. Heterocycl. Chem. 1980, 26, 135. (b) Saito, I.;
Nakata, A.; Matsuura, T. Tetrahedron Lett. 1981, 22, 1697. (c)
J ohansson, E.; Skramstad, J . J . Org. Chem. 1981, 46, 3752. (d) Tobia,
D.; Rickborn, B. J . Org. Chem. 1986, 51, 3849.
(18) Adam, W.; Rodriguez, A. J . Am. Chem. Soc. 1980, 102, 404.

4126 Organometallics, Vol. 23, No. 17, 2004
Casey et al.
F igu r e 3. 17O NMR spectrum of 5a produced from reaction of 10% 17O-labeled O2 with 3a .
tive TLC. The 17O NMR spectrum of labeled 5c showed
a 51:49 ratio of two resonances at δ 564.6 and 509.1,
corresponding to two of the three 17O isotopomers. No
resonance was seen for the η2-ketone unit of 5c-17O-C.
Since two new oxygens are introduced in the conver-
sion of 3 to 5, selective routes to 5 will put equal
amounts of 17O label into two sites and leave one site
unlabeled. The nearly 50:50 mixture of 5-17O-A:5-17O-B
observed is consistent with a single mechanistic path-
way. The absence of any 5-17O-C isotopomer requires
that this pathway involve movement of the isobenzofu-
ran oxygen of 3 to the η2-ketone unit of 5. In this process,
both C-O bonds of the furan are severed, the furan
oxygen migrates to the former carbene carbon, and
dioxygen supplies the carbonyl oxygens of the noncom-
plexed ketones. Clearly, profound rearrangements are
involved.
F igu r e 4. Three singly 17O-labeled isotopomers of 5a .
with acetophenone (δ 542.5),19 acetone (δ 569),20 and 1,2-
diphenyl-1,2-ethanedione (δ 541).21 A natural abun-
dance 17O NMR spectrum of 1,2-diacetylbenzene, which
closely models the chemical environment of the methyl
ketone moiety of 5a , showed a resonance at δ 563.5.
Since no analogous η2-ketone complexes were known,
we needed to make an appropriate model compound to
assign the η2-complexed carbonyl. The η2-ketone com-
plex, Cp(CO)2Re(η2-benzil) (6), was prepared in 9.2%
yield by reaction of benzil (C6H5(CdO)(CdO)C6H5) with
Cp(CO)2Re(THF). A 13C NMR resonance at δ 78.1
established that 6 was an η2-ketone complex. The
natural abundance 17O NMR spectrum showed a broad
resonance at δ 144.3 assigned to the oxygen of the η2-
ketone unit in addition to a broad resonance at δ 523.0
for the organic carbonyl and sharp resonances at δ 379.8
and 377.2 for the diastereotopic Re(CO)2 unit.22
Discu ssion
The interesting interconversions of Re alkyne com-
plexes 2a -c and Re isobenzofuryl carbene complexes
3a -c24 can be viewed in two ways. They can be viewed
as proceeding by a nucleophilic attack of the ketone
carbonyl on the alkyne complexed to rhenium (Scheme
8). However, Re alkyne complexes are generally not
susceptible to nucleophilic attack. Another view involves
an initial [1.5,1] shift of Re to one carbon of the alkyne
in 2, which would lead to “free carbene” character at
the other alkyne carbon (Scheme 8). The “free carbene”
could then be trapped through cyclization to give
isobenzofuran 3. It is known that 4-oxabutadienyl
carbenes undergo 6π electrocyclizations to produce furan
derivatives.5 Unfortunately, we do not yet have a way
to distinguish between a nucleophilic attack pathway
and a “free carbene” pathway. We are trying to devise
other traps to test for carbene reactivity at one carbon
of a metal-alkyne complex.
Reaction of 10% 17O-enriched dioxygen with an equi-
librium mixture of 2a :3a gave 17O-labeled 5a ,which was
isolated by preparative TLC. The 17O NMR spectrum
of labeled 5a showed a 52:48 ratio of two resonances at
δ 564.6 and 510.4 corresponding to two of the three 17O
isotopomers (Figure 3).23 No 17O NMR resonance was
seen in the δ 140 range expected for an η2-ketone
complex as in 5a -17O-C (Figure 4). On the basis of
chemical shift comparisons, the δ 564.6 resonance was
assigned to the methyl ketone in 5a -17O-A and the δ
510.4 resonance was assigned to the second organic
carbonyl in 5a -17O-B (Figure 4).
In an analogous experiment, reaction of 10% 17O-
enriched dioxygen with an equilibrium mixture of 2c:
3c gave 17O-labeled 5c, which was isolated by prepara-
Earlier, 2-furylcarbene complexes of Cr and W were
generated by reacting M(CO)5(THF) with cis-2-penten-
4-yn-1-ones (Scheme 4).6,8 It was suggested that the
reaction proceeded by alkyne coordination followed by
ring-closing rearrangement. However, the metal alkyne
complex was never detected, presumably because ring-
closing rearrangement was so rapid.
(19) Chimichi, S.; Dell’Erba, C.; Gruttadauria, M.; Noto, R.; Novi,
M.; Petrillo, G.; Sancassan, F.; Spinelli, D. J . Chem. Soc., Perkin. Trans.
2 1995, 1021.
(20) Delseth, C.; Kintzinger, J . P. Helv. Chim. Acta 1976, 59, 466.
(21) Cerioni, G.; Plumitallo, A.; Frey, J .; Rappoport, Z. Magn. Reson.
Chem. 1995, 33, 874.
(22) Kintzinger, J . P. NMR of Newly Accessible Nuclei; Laszlo, P.,
Ed.; Academic Press: New York, 1983; pp 79-104.
(23) On the basis of the extent of baseline rolling and phasing error
in this 17O NMR spectrum, integrations are estimated to be accurate
to within ∼10%.
(24) 3a -c are the first spectrally characterized isobenzofuryl car-
bene complexes.

Re Isobenzofuryl Carbene Complexes
Organometallics, Vol. 23, No. 17, 2004 4127
Sch em e 12
Sch em e 13
mation by nucleophilic attack of the carbonyl on the
remote carbon of the complexed alkyne (Scheme 12).7,25
The resulting carbonyl ylide intermediates were sug-
gested to be trapped by electron-rich alkenes (n-butyl
vinyl ether) in a [3+2] cycloaddition; subsequent inser-
tion of the resulting tungsten carbene into a C-H bond
gave rise to the observed polycyclic products.7,25 Iwasa-
wa also reported reactions of o-ethynylphenyl ketones
with W(CO)5(THF) in the presence of water that appear
to proceed via tungsten alkyne complexes that lead to
five-membered-ring intermediates and products.7,25
In an attempt to trap a possible six-membered-ring
carbonyl ylide intermediate in our system, we monitored
a solution containing an equilibrium mixture of 2a and
3a and excess n-butyl vinyl ether over 3 weeks, but saw
no reaction. The origin of the preference for exclusive
five-membered-ring formation over six-membered-ring
formation in our system is not understood but may be
dependent on alkyne substitution.
To more fully characterize Re isobenzofuryl carbene
complexes 3, it was necessary to establish their struc-
tures through trapping experiments. Treatment of 3
with the electron-deficient alkyne DMAD gave Re
carbene complexes 4 through Diels-Alder cycloaddi-
tions. This confirmed the structure of 3 and demon-
strated the stability of the Re carbene moiety through
the isolation of 4 in air.
The isobenzofuryl carbene complex 3 was trapped
with dioxygen, which gave ketone complex 5. 17O-
labeling studies showed that there was a single pathway
for oxidation that requires migration of the isobenzo-
furan oxygen to the η2-ketone oxygen in 5 and cleavage
of the two C-O bonds of the isobenzofuran. We propose
that this pathway starts with endoperoxide A. We
suggest that this pathway proceeds by opening of the
endoperoxide to the carbonyl oxide-ketone intermediate
B (this is the microscopic reverse of a dipolar [3+2]
cycloaddition) (Scheme 13). Attack of the nucleophilic
oxygen of the carbonyl oxide on the ketone unit would
produce zwitterion C, which might also be formed
directly from A by opening of the five-membered ring.
Attack of the anionic oxygen of C on the electrophilic
carbene carbon would produce epoxide D, which could
then fragment to 5 (Scheme 13).
In the 2′-alkynylacetophenone systems reported here,
the diminished aromatic character of the isobenzofuran
unit relative to a furan greatly changes the relative
energies of the starting alkyne complex and the cyclized
products. This made it possible to observe both alkyne
complexes 2 and isobenzofuryl carbene complexes 3 in
equilibrium. There is presumably also significant tran-
sition state destabilization leading to the isobenzofuryl
carbene complexes, which greatly slows the rate of
conversion of 2 to 3 and makes isolation of pure alkyne
complexes possible.
The destabilizing effect of the isobenzofuran was so
great that the alkyne complexes 2 were significantly
more stable than the isomeric isobenzofuryl carbene
complexes 3. The small percentages of the carbene
complexes 3 made their identification difficult and their
isolation virtually impossible. In an effort to stabilize
the isobenzofuryl carbene complex, an electron-donating
methoxy substituent was added to the aryl ring. It was
expected that electron donation to the electron-deficient
carbene carbon of 3b would selectively stabilize it
relative to alkyne complex 2b. Surprisingly, the electron-
donating methoxy substituent had the opposite effect,
and an even less favorable equilibrium ratio of 2b:3b
was obtained than in the phenyl derivative. Use of a
strongly electron-withdrawing trifluoromethyl group
gave more of the carbene complex 3c in equilibrium with
alkyne complex 2c. We do not understand these elec-
tronic substituent effects on the relative stability of 2
and 3.
Solvent effects on the equilibrium between 2 and 3
were small, but systematic. The equilibrium was shifted
more toward the Re alkyne complexes 2 in solvents of
higher dielectric constant. The alkyne complexes possess
a polar ketone group that can be selectively stabilized
by the polar solvents.
Exp er im en ta l Section
2′-(P h en yleth yn yl)acetoph en on e (1a).26 Et3N (2.70
mL, 19.4 mmol), 2′-iodoacetophenone (2.50 g, 10.2
mmol), and phenylacetylene (1.41 mL, 12.8 mmol) were
It is interesting that five-membered-ring (isobenzo-
furan) formation was exclusively observed in our study.
This contrasts with recent work by Iwasawa, which
showed the possibility of six-membered-ring formation.
Iwasawa reported reactions of o-ethynylphenyl ketones
with W(CO)5(THF) that appear to proceed via tungsten
alkyne complexes that lead to six-membered-ring for-
(25) Iwasawa, N.; Kusama, H.; Maeyama, K. J . Synth. Org. Chem.,
J pn. 2002, 1036.
(26) Schmittel, M.; Keller, M.; Kiau, S.; Strittmatter, M. Chem., Eur.
J . 1997, 3, 807.

4128 Organometallics, Vol. 23, No. 17, 2004
Casey et al.
added to a mixture of CuI (97 mg, 0.51 mmol) and Pd-
(PPh3)4 (236 mg, 0.204 mmol) in toluene (75 mL). Upon
stirring for 24 h at 40 °C, the yellow reaction mixture
turned black. Saturated aqueous NH4Cl (70 mL) was
added at room temperature, and the mixture was
extracted with EtOAc (3 × 40 mL). The combined
organic extract was dried (MgSO4), filtered, and con-
centrated on a rotary evaporator. Flash column chro-
matography (silica gel, 100:6 pentane:ether) gave 1 as
an orange oil (1.89 g, 84%). 1H NMR (300 MHz, CDCl3)
δ 7.76 (ddd, J ) 7.7, 1.5, 0.5 Hz, 1H), 7.63 (ddd, J )
7.7, 1.5, 0.5 Hz, 1H), 7.58-7.52 (m, 2H), 7.47 (td, J )
7.5, 1.5 Hz, 1H), 7.40 (td, J ) 7.7, 1.5 Hz, 1H), 7.39-
7.35 (m, 3H), 2.80 (s, 3H). 13C{1H} NMR (75 MHz,
CDCl3) δ 200.2 (CdO), 140.6, 133.7, 131.3 (2C), 131.1,
128.6, 128.5, 128.3 (2C), 128.1, 122.7, 121.5, 94.8
(CtC), 88.3 (CtC), 29.8 (Me).
131.5, 128.98, 128.96 (2C), 128.5, 127.4, 89.2 (Cp), 87.0
(CtC), 79.9(CtC), 29.3 (Me). HRMS (EI): calcd for
C23H17O3185Re (M+) 526.0707, found 526.0706.
Cp (CO)2Re[η2-(2′-((p-m eth oxyp h en yl)eth yn yl)a c-
etop h en on e)] (2b) was prepared from 1b (1.8 g, 7.2
mmol) using the procedure described for 2a . Preparative
TLC (silica gel, 1:1 pentane/ether) gave 2b (60 mg, 6.0%,
1
Rf ) 0.40) and >85% of recovered 1b (Rf ) 0.71). H
NMR (300 MHz, CDCl3): δ 7.57 (d, J ) 7.8 Hz, 1H),
7.54-7.46 (m, 4H), 7.34 (ddd, J ) 7.5, 4.8, 3.9 Hz, 1H),
6.90 (d, J ) 9.0 Hz, 2H), 5.57 (s, Cp), 3.84 (s, OMe),
2.28 (s, Me). 13C{1H} NMR (63 MHz, CDCl3): δ 204.97
(CO), 204.95(CO), 203.2 (CdO), 159.9, 140.7, 133.0 (2C),
131.4, 131.4, 128.5, 127.0, 124.0, 114.2 (2C), 112.8, 91.0
(CtC), 88.6 (Cp), 87.7 (CtC), 55.6 (OMe), 29.2 (Me).
HRMS (EI): calcd for C24H19O4187Re (M+) 558.0841,
found 558.0839.
Cp (CO)2R e[η2-(2′-((p -(r,r,r-t r iflu or ot olyl)et h y-
n yl)a cetop h en on e)] (2c) was prepared from 1c (1.10
g, 3.8 mmol) using the procedure described for 2a .
Preparative TLC (silica gel, 3:2 pentane/ether) gave 2c
(44 mg, 4.1%, Rf ) 0.50) and 78% of recovered 1c (Rf )
2′-((p-Meth oxyph en yl)eth yn yl)acetoph en on e (1b)
was prepared from 1-ethynyl-4-methoxy benzene (1.66
mL, 12.8 mmol) by a procedure similar to that used for
1a . Flash column chromatography (silica gel, 10:1
pentane/ether) gave 1b (2.24 g, 88%) as an orange
crystalline solid, mp 51 °C. 1H NMR (300 MHz,
CDCl3): δ 7.74 (dd, J ) 7.7, 1.4 Hz, 1H), 7.61 (dd, J )
7.7, 1.4 Hz, 1H), 7.49 (d, J ) 8.7 Hz, 2H), 7.46 (td, J )
7.5, 1.5 Hz, 1H), 7.37 (td, J ) 7.5, 1.5 Hz, 1H), 6.90 (d,
J ) 8.7 Hz, 2H), 3.83 (s, 3H), 2.79 (s, 3H). 13C{1H} NMR
(75 MHz, CDCl3): δ 200.4 (CdO), 160.1, 140.6, 133.8,
133.1 (2C), 131.4, 128.8, 128.0, 122.2, 115.1, 114.2 (2C),
95.4 (CtC), 87.5 (CtC), 55.4 (OMe), 30.1 (Me). HRMS
(EI): calcd for C17H14O2 250.0994, found 250.0986.
1
0.73). H NMR (500 MHz, CD2Cl2): δ 7.67, (ddd, J )
7.8, 1.5, 0.5 Hz, 1H), 7.64 (d, J ) 8.0 Hz, 2H), 7.61 (d,
J ) 8.0 Hz, 2H), 7.56 (ddd, J ) 8.0, 7.5, 1.5 Hz, 1H),
7.49 (ddd, J ) 7.8, 1.5, 0.5 Hz, 1H), 7.41 (ddd, J ) 7.8,
7.2, 1.5 Hz, 1H), 5.61 (s, Cp), 2.36 (s, Me). 1H NMR (300
MHz, C6D6): δ 7.51 (d, J ) 8.4 Hz, 2H), 7.43 (ddd, J )
7.5, 1.5, 0.6 Hz, 1H), 7.35 (d, J ) 8.4 Hz, 2H), 7.27 (ddd,
J ) 7.8, 1.5, 0.6 Hz, 1H), 7.12 (dt, J ) 7.8, 1.5 Hz, 1H),
6.93 (td, J ) 7.8, 1.2 Hz, 1H), 4.92 (s, Cp), 2.08 (s, Me).
13C{1H} NMR (126 MHz, CD2Cl2, 4 °C): δ 205.5 (CO),
204.8 (CO), 201.7 (CdO), 139.6, 136.0 (q, J CF ) 1.5 Hz),
133.3, 131.9, 131.6 (2C), 131.1, 129.2 (q, J CF ) 33 Hz),
129.2, 127.7, 125.7 (q, J CF ) 3.8 Hz, 2C), 89.3 (Cp), 85.18
(CtC), 85.16 (CtC), 29.2 (Me), CF3 resonance not
observed. HRMS (EI): calcd for C22H16F3O1185Re (M -
2CO)+ 538.0683, found 538.0692. Molecular ion peak
was too small to be observed.
2′-((p -(r,r,r-Tr iflu or ot olyl)e t h yn yl)a ce t op h e -
n on e (1c) was prepared from 4-ethynyl-R,R,R-trifluo-
rotoluene (1.56 mL, 9.6 mmol) by a procedure similar
to that used for 1a and isolated as a brown oil (2.71 g,
98%). 1H NMR (300 MHz, CDCl3): δ 7.78 (ddd, J ) 7.5,
1.5, 0.6 Hz, 1H), 7.69-7.60 (m, 5H), 7.51 (td, J ) 7.5,
1.8 Hz, 1H), 7.44 (td, J ) 7.5, 1.5 Hz, 1H), 2.76 (s, 3H).
13C{1H} NMR (75 MHz, CDCl3): δ 199.8 (CdO), 140.9,
134.3, 132.0 (2C), 131.6, 130. 5 (q, J CF ) 33 Hz), 129.1,
129.0, 127.0 (q, J CF ) 1.5 Hz), 125.5 (q, J CF ) 3.8 Hz,
2C), 124.1 (q, J CF ) 272 Hz, CF3), 121.1, 93.2(CtC),
91.0(CtC), 29.8 (Me). HRMS (ESI): calcd for C17H11F3-
ONa (M + Na+) 311.0660, found 311.0647.
Cp (CO)2RedC(C6H5)(C9H7O) (3a ). A solution of 2a
in CD2Cl2, degassed by three freeze-pump-thaw cycles,
was heated at 38 °C for 5 days in a resealable NMR
tube to give a 76:24 equilibrium mixture of 2a :3a (Keq
) 0.32). In CDCl3, a 67:33 equilibrium mixture of 2a :
3a (Keq ) 0.50) was obtained; in toluene-d8, a 57:43
equilibrium mixture of 2a :3a (Keq ) 0.75) was obtained.
1H NMR (500 MHz, CD2Cl2) of 3a : δ 7.51 (d, J ) 8.5
Hz, 2H), 7.34 (t, J ) 7.5 Hz, 2H), 7.22 (tt, J ) 7.5, 1.5
Hz, 1H), 7.08 (ddd, J ) 9.0, 6.5, 1.0 Hz, 1H), 6.90 (m,
2H), 6.80 (ddd, J ) 8.5, 6.5, 0.5 Hz, 1H), 5.28 (s, Cp),
2.22 (s, Me). 13C{1H} NMR (126 MHz, CD2Cl2) of 3a : δ
245.0 (RedC), 206.2 (CO), 162.7, 161.3, 159.1, 129.6,
128.3, 127.8 (2C), 125.8, 125.2, 124.2, 123.4, 122.3 (2C),
120.0, 91.5 (Cp), 15.063 (Me).
C p (C O )2R e [η2-(2′-(p h e n y le t h y n y l)a c e t o p h e -
n on e)] (2a ). A solution of CpRe(CO)3 (0.600 g, 1.79
mmol) in dry degassed THF (140 mL) was photolyzed
with a Hanovia medium-pressure mercury lamp for 30
min at 0 °C with continuous N2 purging. The disap-
pearance of CpRe(CO)3 [2022 (m) and 1926 cm-1(s)] and
the appearance of CpRe(CO)2(THF) [1912 (s) and 1839
cm-1 (s)] was monitored by IR spectroscopy. 1a (1.89 g,
8.57 mmol) was added, and the solution was concen-
trated to 15 mL on a rotary evaporator. The brown
solution was stirred under N2 at 0 °C for 2 h and at
room temperature for an additional 3 h. Preparative
TLC (silica gel, 2:1 pentane/ether) gave 2a (79.8 mg,
8.5%, Rf ) 0.38) as an air-stable, brown solid and 63%
Cp (CO)2RedC(C6H4-p-OMe)(C9H7O) (3b). A solu-
tion of 2b in CD2Cl2, degassed by three freeze-pump-
thaw cycles, was monitored by NMR spectroscopy at
room temperature for 3 days (∼20 × t1/2). An 87:13
equilibrium mixture of 2b:3b (Keq ) 0.15) was obtained.
1H NMR (300 MHz, CD2Cl2): δ 5.26 (s, Cp), 3.86 (s,
OMe), 2.20 (s, Me). Aromatic and vinyl resonances were
obscured by 2b.
1
recovered 1a (Rf ) 0.62). H NMR (500 MHz, CD2Cl2):
δ 7.61 (ddd, J ) 7.5, 1.5, 0.5 Hz, 1H), 7.56-7.52 (m,
3H), 7.51 (ddd, J ) 7.5, 1.8, 0.5 Hz, 1H), 7.40-7.35 (m,
3H), 7.30 (tt, J ) 7.5, 1.3 Hz, 1H), 5.60 (s, Cp), 2.31 (s,
Me). 13C{1H} NMR (126 MHz, CD2Cl2): δ 205.5 (v br,
CO), 202.2 (CdO), 140.4, 133.6, 132.1, 131.8 (2C), 131.7,
Cp (CO)2RedC(C6H4-p-CF 3)(C9H7O) (3c). A solution
of 2c in CD2Cl2, degassed by three freeze-pump-thaw

Re Isobenzofuryl Carbene Complexes
Organometallics, Vol. 23, No. 17, 2004 4129
cycles, was heated at 38 °C for 3 days in a resealable
NMR tube to give a 57:43 equilibrium mixture of 2c:3c
(Keq ) 0.74). In benzene-d6, a 38:62 equilibrium mixture
Rh en iu m Keton e Com p lex Cp (CO)2Re[η2-OdC-
(C6H5)(C9H7O2)] (5a ). A CD2Cl2 solution of an equilib-
rium mixture of 2a :3a was exposed to air and monitored
1
1
of 2c:3c (Keq ) 1.63) was obtained. H NMR (500 MHz,
by H NMR spectroscopy. Isobenzofuran carbene com-
CD2Cl2): δ 7.62 (d, J ) 8.5 Hz, 2H), 7.54 (d, J ) 8.5 Hz,
2H), 7.13 (ddd, J ) 9.0, 6.6, 0.9 Hz, 1H), 7.08-7.02
(m, 2H), 6.83 (ddd, J ) 8.7, 6.6, 0.3 Hz, 1H), 5.28 (s,
plex 3a disappeared over several hours and alkyne
complex 2a disappeared more slowly over 100 h to give
a 65% yield of 5a . Preparative TLC (silica gel, 2:1
pentane/ether) gave 5a (Rf ) 0.06) as an air-stable
yellow solid. Recrystallization by slow diffusion of
pentane into a solution of 5a in pentane/CH2Cl2 gave
fine yellow crystals of 5a , suitable for X-ray crystal
structure analysis. Alternatively, a mixture of 2a :3a
(20.5 mg) was exposed to O2 (∼4 atm) in CH2Cl2. After
2 days, preparative TLC gave 5a (14.2 mg, 69%) as
a yellow powder. IR (THF): 1999 (s, CtO), 1925 (s,
1
Cp), 2.20 (s, Me). H NMR (300 MHz, C6D6): δ 7.31 (d,
J ) 8.1 Hz, 2H, Ph), 6.84 (dt, J ) 8.4, 0.9 Hz, 1H), 6.78
(ddd, J ) 8.7, 6.6, 0.9 Hz, 1H), 6.67 (d, J ) 7.8 Hz,
2H, Ph), 6.36 (ddd, J ) 8.7, 6.6, 0.6 Hz, 1H), 5.59 (dt, J
) 9.0, 0.9 Hz, 1H), 4.76 (s, Cp), 1.56 (s, Me). 13C{1H}
NMR (126 MHz, CD2Cl2): δ 240.9 (RedC), 205.9 (CO),
164.9, 162.2, 160.0, 130.2, 128.3, 125.5, 125.1 (q, J CF
)
3.5 Hz, 2C), 124.5, 123.1, 122.7 (2C), 120.3, 91.5 (Cp),
15.1 (Me). The resonances for CF3 and CCF3 were not
observed.
1
CtO), 1692 (m, CdO), 1668 (m, CdO) cm-1. H NMR
(300 MHz, CDCl3): δ 7.81 (br, 2H), 7.68 (dd, J ) 7.7,
1.0 Hz, 1H), 7.59 (dd, J ) 7.8, 1.2 Hz, 1H), 7.50 (td,
J ) 7.5, 1.2 Hz, 1H), 7.42 (td, J ) 7.5, 1.4 Hz, 1H), 7.33
(t, J ) 7.4 Hz, 2H), 7.18 (tt, J ) 7.4, 1.2 Hz, 1H), 5.15
(s, Cp), 2.74 (s, Me). 13C{1H} NMR (126 MHz, CDCl3):
δ 205.3 (ReCO), 201.1 (ReCO), 199.1 (CdO), 197.8
(CdO), 143.2, 139.7, 136.9, 131.9, 130.8, 129.8, 128.0
DMAD Ad d u ct Cp (CO)2RedC(C6H4-p-OMe)-
(C15H 13O5) (4b ). Dimethyl acetylenedicarboxylate
(DMAD) (18 µL, 0.15 mmol) was added to a CD2Cl2
solution of an equilibrium mixture of 2b:3b (17.0 mg,
0.030 mmol) in a resealable NMR tube. After 10 days
at 40 °C, a 68% yield of 4b was observed by NMR
spectroscopy. Evaporation of CD2Cl2 and most of the
DMAD under vacuum (3.0 × 10-2 mm), followed by
crystallization of the residue from CH2Cl2/pentane, gave
4b (10 mg, 48%) as an air-stable red crystalline solid.
IR (THF): 1986 (CtO), 1910 (CtO) cm-1. 1H NMR (300
MHz, CD2Cl2): δ 7.24 (ddd, J ) 7.2, 1.2, 0.6 Hz, 1H),
7.02 (td, J ) 7.4, 0.9 Hz, 1H), 6.97 (d, J ) 9.0 Hz, 2H),
6.89 (td, J ) 7.5, 1.2 Hz, 1H), 6.78 (d, J ) 9.0 Hz, 2H),
6.70 (dt, J ) 7.2, 0.9 Hz, 1H), 5.44 (s, Cp), 3.84 (s, OMe),
3.66 (s, OMe), 3.41 (s, OMe), 2.06 (s, Me). 13C{1H} NMR
(126 MHz, CD2Cl2): δ 288.6 (RedC), 204.1(CO), 204.0
(CO), 165.5 (CO2), 163.5(CO2), 158.8, 158.5, 155.3, 151.8,
149.9, 148.8, 125.8, 125.7, 123.8, 122.0 (2C), 119.6,
112.6, 112.2 (2C), 93.5 (Cp), 88.9, 55.8 (OMe), 52.5
(OMe), 52.3 (OMe), 14.7 (Me). HRMS (ESI): calcd for
C30H25O8185ReNa (M + Na+) 721.0971, found 721.0950.
(2C), 127.0 (v br, 2C), 126.98, 126.6, 92.6 (Cp), 78.4
185
(CORe), 27.7 (Me). HRMS (ESI): calcd for C23H17O5
-
ReNa (M + Na+) 581.0503, found 581.0492.
Rh en iu m Keton e Com p lex Cp (CO)2Re[η2-OdC-
(C6H4-p-OMe)(C9H7O2)] (5b). A CD2Cl2 solution of an
equilibrium mixture of 2b:3b was exposed to air and
monitored by 1H NMR spectroscopy. 3b disappeared
over several hours and 2b disappeared more slowly over
72 h to give a 66% yield of 5b. Preparative TLC (silica
gel, 1:1 pentane/ether) gave 5b (Rf ) 0.10) as a pale
yellow oil. IR (THF): 1994 (CtO), 1922 (CtO) cm-1
.
1H NMR (300 MHz, CD2Cl2): δ 7.69 (br d, J ) 7.8 Hz,
2H), 7.61 (dd, J ) 8.1, 1.5 Hz, 1H), 7.58 (dd, J ) 7.8,
1.5 Hz, 1H), 7.49 (td, J ) 7.5, 1.5 Hz, 1H), 7.42 (td, J )
7.5, 1.5 Hz, 1H), 6.87 (d, J ) 9.0 Hz, 2H), 5.19 (s, Cp),
3.83 (s, OMe), 2.64 (s, Me). 13C{1H} NMR (126 MHz,
CD2Cl2): δ 206.5 (ReCO), 201.3 (ReCO), 199.8 (CdO),
198.6 (CdO), 159.0, 140.1, 135.7, 132.1, 131.3, 131.0,
130.0, 129.1 (v br, 2C), 126.6, 113.6 (2C), 93.0 (Cp), 78.9
(CORe), 55.8 (OMe), 27.8 (Me). HRMS (ESI): calcd for
C24H19O6187ReNa (M + Na+) 613.0637, found 613.0663.
DMAD
Ad d u ct
Cp (CO)2RedC(C6H4-p-CF 3)-
(C15H 13O5) (4c). Dimethyl acetylenedicarboxylate
(DMAD) (82 µL, 0.67 mmol) was added to a CD2Cl2
solution of an equilibrium mixture of 2c:3c (40 mg,
0.067 mmol) in a resealable NMR tube. After 44 h at
38 °C, a 94% yield of 4c was observed by NMR
spectroscopy. Evaporation of CD2Cl2 and most of the
DMAD under vacuum (3.0 × 10-2 mm), followed by
crystallization of the residue from CH2Cl2/pentane, gave
4c (40 mg, 81%) as an air-stable red crystalline solid.
IR (THF): 1990 (CtO), 1913 (CtO) cm-1. 1H NMR (300
MHz, CD2Cl2): δ 7.54 (d, J ) 8.1 Hz, 2H), 7.25 (dt, J )
7.2, 0.9 Hz, 1H), 7.16 (d, J ) 8.1 Hz, 2H), 7.02 (td, J )
7.5, 0.9 Hz, 1H), 6.88 (td, J ) 7.5, 1.2 Hz, 1H), 6.60 (d,
J ) 7.5 Hz, 1H), 5.49 (s, Cp), 3.67 (s, OMe), 3.39 (s,
OMe), 2.078 (s, Me). 13C NMR (126 MHz, CD2Cl2): δ
284.3 (RedC), 203.5 (CO), 203.4 (CO), 169.0 (CO2), 165.6
Rh en iu m Keton e Com p lex Cp (CO)2Re[η2-OdC-
(C6H4-p-CF 3)(C9H7O2)] (5c). A CD2Cl2 solution of an
equilibrium mixture of 2c:3c was exposed to air and
monitored by 1H NMR spectroscopy. 3c disappeared
over several hours and 2c disappeared more slowly over
112 h to give a 67% yield of 5c. Preparative TLC (silica
gel, 3:2 pentane/ether) gave 5c (Rf ) 0.18) as a yellow-
brown solid. IR (THF): 2001 (CtO), 1929 (CtO) cm-1
.
1H NMR (300 MHz, CD2Cl2): δ 7.90 (br s, 2H), 7.65-
7.58 (m, 4H), 7.52 (td, J ) 7.2, 1.2 Hz, 1H), 7.45 (td, J
) 7.5, 1.5 Hz, 1H), 5.19 (s, Cp), 2.65 (s, Me). 13C{1H}
NMR (126 MHz, CD2Cl2): δ 205.7 (ReCO), 201.4 (ReCO),
199.8 (CdO), 197.8 (CdO), 148.1, 139.9, 136.9, 132.3,
130.3, 128.6 (q, J CF ) 33 Hz), 127.8 (v br, 2C), 127.0,
125.2 (q, J CF ) 3.6 Hz, 2C), 125.0 (q, J CF ) 272 Hz),
93.1 (Cp), 76.2 (CORe), 27.7 (Me). HRMS (ESI): calcd
for C24H16F3O5185ReNa (M + Na+) 649.0377, found
649.0366.
(CO2), 163.6, 154.8, 151.7, 150.8, 149.0, 128.1 (q, J CF
)
32 Hz), 126.1, 126.0, 125.0 (q, J CF ) 272 Hz), 124.1 (q,
J CF ) 3.5 Hz, 2C), 123.8, 120.3 (2C), 120.0, 93.9 (Cp),
89.4, 52.6 (OMe), 52.3 (OMe), 14.5(Me). The bridgehead
carbon adjacent to the carbene carbon was not observed
(possibly obscured by the broad resonance at δ 120.3).
HRMS (ESI): calcd for C30H22F3O7185ReNa (M + Na+)
759.0745, found 759.0735.
17O-La beled Cp (CO)2Re[η2-OdC(C6H5)(C9H7O2)]
(5a ). Dioxygen that was 10% 17O enriched was added

4130 Organometallics, Vol. 23, No. 17, 2004
Casey et al.
to a resealable NMR tube containing an equilibrium
mixture of 2a :3a in CD2Cl2. After 12 h, NMR spectros-
copy showed that 2a and 3a had been consumed.
Labeled 5a was purified by preparative TLC and was
shown to be pure by 1H NMR spectroscopy. An 17O NMR
spectrum of labeled 5a (∼30 mM in CD2Cl2) was
obtained by acquiring 1.8 × 105 scans with an acquisi-
tion time of 0.01 s and a repetition delay of 0 s. Varying
the acquisition and delay times had little effect on the
relative peak sizes. δO values are based on referencing
(0.282 g, 0.841 mmol) in dry degassed THF (140 mL)
was photolyzed with a Hanovia medium-pressure mer-
cury lamp for 30 min at 0 °C with continuous N2
purging. Benzil (C6H5(CdO)(CdO) C6H5) (1.77 g, 8.41
mmol) was added, and the solution was concentrated
to 15 mL on a rotary evaporator. The brown solution
was stirred under N2 at 0 °C and was allowed to warm
to room temperature and stirred for 22 h. Preparative
TLC (silica gel, 2:1 pentane/ether) gave 6 (40.2 mg, 9.2%,
Rf ) 0.32) as an air-stable, yellow-orange powder. IR
(THF): 2000 (s, CtO), 1928 (s, CtO), 1652 (m, CdO)
cm-1. 1H NMR (300 MHz, CD2Cl2): δ 7.95-7.89 (m, 4H),
7.45 (tt, J ) 7.2, 1.2 Hz, 1H), 7.347 (t, J ) 8.1 Hz, 2H),
7.344 (t, J ) 8.1 Hz, 2H), 7.21 (tt, J 7.2, 1.2 Hz, 1H),
5.27 (s, 5H, Cp). 13C {1H} NMR (126 MHz, CD2Cl2): δ
204.3 (ReCO), 199.3 (ReCO), 198.5 (CdO), 142.4, 137.4,
132.3, 129.2 (2C), 128.4 (2C), 128.1 (2C), 127.5, 127.3
(2C), 92.7 (Cp), 78.1 (C(O)Re). A natural abundance
(0.037%) 17O NMR spectrum of benzil (0.2 M in CD2-
Cl2) was obtained in 1.5 × 107 scans using an acquisition
time of 0.01 s and a repetition delay of 0 s. The δO value
is based on referencing to CHDCl2 in the 1H NMR
spectrum and is reported relative to H2O (δO ) 0.0 ppm).
17O NMR (67.9 MHz, CD2Cl2): δ 523.0 (br s, CdO),
379.8 (s, CtO), 377.2 (s, CtO), 144.3 (br s, C(O)Re).
HRMS (ESI): calcd for C21H15O4187ReNa (M + Na+)
541.0426, found 541.0400.
1
to CHDCl2 in the H NMR spectrum and are reported
relative to H2O (δO ) 0.0 ppm). 17O NMR (67.9 MHz,
CD2Cl2): δ 564.6 (br s, 1.00 O, MeCdO), 510.4 (br s,
0.94 O, ArCdO).
17O-La b e le d
Cp (CO)2R e [η2-OdC(C6H 4CF 3)-
(C9H7O2)] (5c). Dioxygen that was 10% 17O enriched
was added to a resealable NMR tube containing an
equilibrium mixture of 2c:3c in toluene-d8. After 36 h,
labeled 5c was purified by preparative TLC and was
shown to be pure by 1H NMR spectroscopy. An 17O NMR
spectrum of labeled 5c was obtained by acquiring 8.3
× 105 scans with an acquisition time of 0.01 s and a
repetition delay of 0 s. δO values are based on referenc-
ing to CHDCl2 in the 1H NMR spectrum and are
reported relative to H2O (δO ) 0.0 ppm). 17O NMR (67.9
MHz, CD2Cl2): δ 564.6 (br s, 1.00 O, MeCdO), 509.1
(br s, 0.95 O, ArCdO).
17O NMR Sp ectr u m of 1,2-Dia cetylben zen e. A
natural abundance (0.037%) 17O NMR spectrum of 1,2-
diacetylbenzene (1.2 M in CD2Cl2) was obtained in 5.7
× 104 scans using an acquisition time of 0.003 s and a
repetition delay of 0 s. The δO value is based on
Ack n ow led gm en t. Financial support from the Na-
tional Science Foundation is gratefully acknowledged.
1
referencing to CHDCl2 in the H NMR spectrum and is
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic data for compounds 4c and 5a . This material is
available free of charge via the Internet at http://pubs.acs.org.
reported relative to H2O (δO ) 0.0 ppm). 17O NMR (67.9
MHz, CD2Cl2): δ 563.5 (s).
Rh en iu m η2-Ben zil Com p lex Cp (CO)2Re[η2-Od
C(C6H5)(CdO(C6H5))] (6). A solution of CpRe(CO)3
OM040063G