Protonation of cobalt-allene constitutional isomers: Highly selective formation of cobalt-allyl and oxacobaltacyclopentadiene complexes

Article abstract of DOI:10.1021/om100701e

The first reactivity studies of mononuclear cobalt-allene complexes are reported. The isomeric cobalt-allene complexes (RS,RS,Z)-[(η⁵- C₅H₅)Co(1,2-η²-CH(SO₂Ph)=C= CH(CO₂Et))] (2-Z) and (SR,

Full text of DOI:10.1021/om100701e

Organometallics 2010, 29, 6161–6164 6161
DOI: 10.1021/om100701e
Protonation of Cobalt-Allene Constitutional Isomers: Highly Selective
Formation of Cobalt-Allyl and Oxacobaltacyclopentadiene Complexes
Joseph M. O’Connor,*,† Ming-Chou Chen,*,†,‡ and Ryan L. Holland†
†Department of Chemistry and Biochemistry (0358), University of California, San Diego, 9500 Gilman Drive,
La Jolla, California 92093-0358, United States , and ‡Department of Chemistry, National Central University,
Chung-Li, Taiwan 32054, Republic of China
Received July 15, 2010
Scheme 1. Protonation of Metal-η2-Allene Complexes
Summary: The first reactivity studies of mononuclear cobalt-
allene complexes are reported. The isomeric cobalt-allene
complexes (RS,RS,Z)-[(η5-C5H5)Co(1,2-η2-CH(SO2Ph)d
CdCH(CO2Et))] (2-Z) and (SR,RS,E)-[(η5-C5H5)Co(2,
3-η2-CH(SO2Ph)dCdCH(CO2Et))] (3-E) undergo protonation
with HBF4 to give the cationic π-allyl complex (η5-C5H5)Co-
(PPh3)[exo-η3-CH(CO2Et)CHCH(SO2Ph)](BF4) (4) and the
first structurally characterized oxacobaltacyclopentadiene
complex, (η5-C5H5)Co(PPh3)[κ2-OC(OEt)CHdC(CH2SO2Ph)]-
(BF4) (5-BF4), respectively. Deprotonation of 5-BF4 regener-
ates cobalt-allene complex 3-E, as well as 2-Z.
Protonation of metal-η2-allene complexes (I) has been
demonstrated to give η3-allyl (II),1 κ1-alkenyl (III),1a,2 or 1-me-
tallacyclopropene (IV)3 complexes, depending on the nature of
the metal, ancillary ligands, reagents, and allene substituents
(Scheme 1).4 For example, Werner reported that (η5-C5H5)Rh-
(PiPr3)(η2-H2CdC=CHMe) underwent reaction with HBF4
(followed by NH4PF6) to give the η3-allyl rhodium complex
[(η5-C5H5)Rh(PiPr3)(η3-H2CCHCHMe)]PF6, whereas, proton-
ation with CF3CO2H in the presence of NaI led cleanly to the
κ1-vinyl complex (η5-C5H5)Rh(PiPr3)I(κ1-CMedCHMe).1a
Pombeiro observed that protonation of (dppe)2ReCl-
(η2-H2CdCdCHPh) gave a 1-metallacyclopropene complex,3a
and Casey found that treatment of (η5-C5Me5)Re(CO)2-
(η2-H2CdCdCHMe) with HBF4 at low temperature generated
Scheme 2. Conversion of Cobaltacyclobutene 1 to Isomeric Cobalt-
Allene Complexes
*Corresponding authors. E-mail: jmoconnor@ucsd.edu; mcchen@
ncu.edu.
a 1-rhenacyclopropene, which underwent rearrangement to a
rhenium-allyl at -16 °C.3b
(1) (a) Wolf, J.; Werner, H. Organometallics 1987, 6, 1164. (b) Gibson,
D. H.; Vonnahme, R. L.; McKiernan, J. E. J. Chem. Soc. (D) 1971, 720.
(2) Bowden, F. L.; Giles, R. Coord. Chem. Rev. 1976, 20, 81.
(3) (a) Pombeiro, A. J. L.; Hughes, D. L.; Richards, R. L.; Silvestre,
J.; Hoffmann, R. J. Chem. Soc., Chem. Commun. 1986, 1125. (b) Casey,
C. P.; Brady, J. T.; Boller, T. M.; Weinhold, F.; Hayashi, R. K. J. Am. Chem.
Soc. 1998, 120, 12500. (c) Kuznetsov, M. L.; Pombeiro, A. J. L.; Dement'ev,
A. I. J. Chem. Soc., Dalton Trans. 2000, 4413.
(4) For the reactions of metal hydrides with allenes to give allyl or vinyl
complexes see: Bai, T.; Ma, S.; Jia, G. Coord. Chem. Rev. 2009, 253, 423.
(5) Cobalt-allene complexes appear to serve as key intermediates in a
number of cobalt-mediated reactions of allenes: (a) Greenfield, H.;
Wender, I.; Wotiz, J. H. J. Am. Chem. Soc. 1956, 21, 875. (b) Nakamura,
A.; Kim, P. J.; Hagihara, N. J. Organomet. Chem. 1965, 3, 7. (c) Nakamura,
A. Bull. Chem. Soc. Jpn. 1966, 39, 543. (d) Furukawa, J.; Kiji, J; Ueo, K.
Makromol. Chem. 1973, 170, 247, and references therein. (e) van Ommen, J.
G; Stijntjes, J.; Mars, P. J. Mol. Catal. 1979, 5, 1. (f ) Caffyn, A. J. M.; Mays,
M. J.; Solan, G. A.; Conole, G.; Tiripicchio, A. J. Chem. Soc., Dalton Trans.
1993, 2345. (g) Bates, R. W.; Devi, T. R. Tetrahedron Lett. 1995, 36, 509.
(h) Hayes, B. L.; Adams, T. A.; Pickin, K. A.; Day, C. S.; Welker, M. E.
Organometallics 2000, 19, 2730, and references therein. (i) Wu, M. S.;
Shanmugasundaram, M.; Cheng, C. H. Chem. Commun. 2003, 718. ( j) Petit,
M.; Aubert, C.; Malacria, M. Tetrahedron 2006, 62, 10582. (k) Chang, H. T.;
Jayanth, T. T.; Cheng, C. H. J. Am. Chem. Soc. 2007, 129, 4166. (l) Park,
J. H.; Kim, E.; Kim, H. M.; Choi, S. Y.; Chung, Y. K. Chem. Commun. 2008,
2388.
To date there have been only two reports of isolated mono-
nuclear cobalt-η2-allene complexes.5 In 1965 Nakamura and
co-workers reported [(η5-C5H5)Co(CO)(η2-Ph2CdCdCPh2)],
which was isolated in 2% yield.5b More recently, we reported
the conversion of cobaltacyclobutene complex 16 to three
isomeric allene complexes: (RS,RS,Z)-[(η5-C5H5)Co(1,2-η2-
CH(SO2Ph)dCdCH(CO2Et))] (2-Z), (RS,RS,Z)-[(η5-C5H5)-
Co(1,2-η2-CH(SO2Ph)dCdC(CO2Et)H)] (2-E), and (SR,RS,
E)-[(η5-C5H5)Co(2,3-η2-CH(SO2Ph) dCdCH(CO2Et))] (3-E)
(Scheme 2).7
This paper describes the first reaction chemistry of char-
acterized cobalt-η2-allene complexes. Allene 2-Z undergoes
protonation to form an π-allyl complex, whereas protonation
of 3-E generates an oxacobaltacyclopentadiene complex. De-
protonation of the oxacobaltacyclopentadiene regenerates 3-E,
(6) O’Connor, J. M.; Ji, H.; Iranpour, M.; Rheingold, A. L. J. Am.
Chem. Soc. 1993, 115, 1586.
(7) O’Connor, J. M.; Chen, M. C.; Fong, B. S.; Wenzel, A.; Gantzel,
P.; Rheingold, A. L.; Guzei, I. A. J. Am. Chem. Soc. 1998, 120, 1100.
r
2010 American Chemical Society
Published on Web 11/11/2010
pubs.acs.org/Organometallics

6162 Organometallics, Vol. 29, No. 23, 2010
Scheme 3. Protonation of Cobalt-Allene Constitutional Isomers
O’Connor et al.
thereby demonstrating that oxametallacyclopentadienes serve
as precursors to η2-allene complexes.
OCH2CH3); δ 4.87 (1H, d, JHH = 16.8 Hz, CH2SO2Ph) and
5.45 (1H, d, JHH = 17.1 Hz, CH2SO2Ph). Thus, protonation
of 3-E delivered a hydrogen to the sulfone-bearing carbon of
the allene ligand, rather than to the central allene carbon, as
was the case for 2-Z. A 1-cobaltacyclopropene structure (IV,
Scheme 1) for 5-BF4 is excluded on the basis of the 13C{1H}
NMR (CD2Cl2) spectrum, which exhibited no resonances
downfield of the ester carbonyl carbon signal at 188.4 ppm
(s). For comparison, the 13C NMR resonances for the car-
bene carbons in the Pombeiro and Casey 1-rhenacyclopro-
penes were observed at δ 258.2 and 302.6, respectively.3
Two types of spectroscopic evidence suggest that the ester
oxygen is coordinated to cobalt in 5-BF4. In the 13C NMR
spectrum, the 188.4 ppm carbonyl carbon resonance is signifi-
cantly downfield of the 176 ppm ester resonance observed in 3-
E, and no carbonyl ν(CdO) absorption is observed at wave-
numbers higher than 1647 cm-1 in the IR spectrum (thin film)
of 5-BF4.
Access to 2-Z and 3-E afforded a unique opportunity to
examine protonation selectivity for a pair of isomeric allene
complexes. When HBF4 (1.2 equiv) was added to a -78 °C
diethyl ether solution of 2-Z, followed by warming of the solu-
tion to room temperature, the allyl complex 4 precipitated as an
orange solid. In the 1H NMR spectrum (CDCl3) of 4, diaste-
reotopic hydrogen resonances were observed at δ 4.45 (1H, m,
OCH2CH3) and 4.57 (1H, m, OCH2CH3), and resonances attri-
buted to the three allyl ligand hydrogens (CoCH) were observed
at δ 2.00 (t, 1H, JPH = JHH = 10 Hz), 2.29 (t, 1H, JPH = JHH
=
10 Hz), and 6.65 (t, 1H, JHH=10 Hz). For comparison, in the 1H
NMR spectrum (acetone-d6) of [(η5-C5H5)Co(PPh3)(η3-CH2-
CMeCH2)](BF4) (6), the Hanti resonance of the allyl ligand is
observed at δ 1.4 (d, JPH=13 Hz) and the Hsyn resonance at
4.43 (s).8 The relatively downfield Hanti chemical shift for 4 is
attributed to the presence of the electron-withdrawing sulfone
and ester substituents. The assignment of the allyl hydrogen
resonances in the 1H NMR spectrum of 4 was further supported
by a series of NMR experiments. Saturation of the δ 6.65 reso-
nance resulted in the collapse of the δ 2.00 and 2.29 triplets to
doublets, and saturation of either the 2.00 or the 2.29 resonance
led to collapse of the 6.65 triplet to a doublet. The exo allyl
orientation for 4 is supported by the observation of a NOE at
the δ 6.65 resonance;but not at the 2.00 or 2.29 resonances;
upon irradiation of the C5H5 hydrogen resonance. The allyl
ligand chemical shift assignments were confirmed by a 2D-
HSQC-NMR experiment (CDCl3).9
In addition to the above data, the R-sp2-carbon resonance
of the metallacycle ring in 5-BF4 is observed at δ 174.6 (d,
JPC = 25 Hz), which is significantly downfield of the corre-
sponding R-sp2-carbon resonance (159.5, d, JPC = 4.4 Hz)
for the cobaltacyclopentene (η5-C5H5)Co(PPh3)(κ2-CRd
CRCHRCHR) (7-CO2Me, R=CO2Me, Figure 1).11 We attri-
bute the downfield nature of the R-carbon resonance to its
electron-deficient identity as the β-carbon of an R,β-unsatu-
rated ester and possibly to a contribution from the cobaltafuran
resonance structure 5-BF4-R (Scheme 3).
The conversion of 3-E to 5-BF4 was demonstrated to be
reversible by the observation that addition of n-BuLi (2 equiv)
to a -78 °C THF solution of 5-BF4 (0.026 mmol, 8.7 mM) led
to the formation of 3-E and 2-Z in a 9:1 ratio (78% combined
yield).
Protonation of 3-E takes a much different course than in
the case of 2-Z. Addition of HBF4 (1.2 equiv) to a diethyl
ether solution of 3-E led to formation of oxacobaltacyclo-
pentadiene complex 5-BF4, which was isolated as a brown
powder (Scheme 3).10 In the 1H NMR spectrum (acetone-d6)
of 5-BF4, two sets of diastereotopic hydrogen resonances
were observed: δ 3.70 (1H, m, OCH2CH3) and 4.10 (1H, m,
The structure of 5-BF4 was confirmed by a single-crystal X-ray
diffraction study on (η5-C5H5)Co(PPh3)[κ2-OC(OEt)CHd
C(CH2SO2Ph)](CF3CO2) (5-CF3CO2), which was prepared by
protonation of 3-E with trifluoroacetic acid (Figure 2). The
oxametallacycle ring defined by Co-C2-C3-C4-O1 is essen-
tially planar, with the largest displacements of the ring atoms
ꢀ
(8) Aviles, T.; Barroso, F.; Royo, P.; Noordik, J. H. J. Organomet.
Chem. 1982, 236, 101.
(9) 1H-13C correlations (1H, δ; 13C, δ): (1.42; 14.44), (2.00; 51.50),
(2.29; 76.91), (4.45; 63.52) (4.57; 63.52), (5.70; 92.96), (6.65; 86.47), (7.52;
128.88), (7.63; 130.07), (7.67; 132.82), (7.69; 134.59). See Supporting
Information for the spectrum.
˚
˚
from the mean plane at C3 (þ 0.017(2) A) andC4(-0.017(2) A).
The conformation about the C1-C2 bond places the phenyl
sulfone group away from the bulky PPh3 ligand with S1
˚
(10) We have not found a literature example of an oxacobaltacyclo-
pentadiene complex for comparison to 5. For oxairidacyclopentadiene
complexes, see: (a) Bleeke, J. R.; New, P. R.; Blanchard, J. M. B.; Haile,
T.; Beatty, A. M. Organometallics 1995, 14, 5127. (b) O'Connor, J. M.;
Hiibner, K.; Merwin, R.; Pu, L. J. Am. Chem. Soc. 1995, 117, 8861.
positioned 1.17 A above the plane of the metallacycle ring.
(11) Wakatsuki, Y.; Aoki, K.; Yamazaki, H. J. Am. Chem. Soc. 1979,
101, 1123.

Communication
Organometallics, Vol. 29, No. 23, 2010 6163
Figure 1. Cobaltacyclopentene complexes.
Scheme 4. Mechanistic Hypotheses Involving Protonation at C-3 of the Allene Ligand
˚
C-Co-C angle in 7-Ph (82.0(4)°). The 1.914(2) A Co-C2
bond distance in 5-CF3CO2 is significantly shorter than the
2
˚
corresponding Co-C(sp ) distance in 8 (1.954(6) A), which
is consistent with a contribution from resonance structure
5-BF4-R.
By analogy to Casey’s mechanism for the conversion of
rhenium-allenes to allyl complexes,3b 1-cobaltacyclopro-
pene intermediates may be involved in the formation of both
4 and 5. Protonation at C-3 of the allene ligand in 2-Z and
3-E would give cationic 1-cobaltacyclopropene intermedi-
ates V and VI, respectively (Scheme 4). The observed reac-
tion outcomes then require a formal 1,2-hydride shift in V
and a carbon to oxygen tautomerization in VI. In the case
of intermediate VI, either the tautomerization process
must be faster than a 1,2-hydride shift or the 1,2-shift
must be reversible. A similar tautomerization in V would
be less favorable in that it would require binding of the
sulfone oxygen to cobalt.
A fundamentally distinct mechanism involves initial pro-
tonation at the metal center (Scheme 5). It is well estab-
lished that metal-hydrides undergo reaction with allenes
to give either allyl or vinyl complexes.1,4 The allyl pro-
ducts result from reductive elimination of the hydride
and C(2) of the allene ligand, whereas the formation of
vinyl complexes results from reductive elimination of
hydride and C-1 of the allene. In the case of 2-Z, proton-
ation at the metal would generate hydride intermediate
VII, from which a reductive elimination at C-2 of the
allene ligand leads directly to η3-allyl 4 with the correct
stereochemistry. However, in the case of 3-E, protonation
at cobalt generates hydride intermediate VIII, from which
there is no simple direct path to 5, since a reductive
elimination at C-1 in VIII would give a vinyl complex,
and elimination at C-2 would give a propargyl intermedi-
ate. In either case, a subsequent hydrogen shift is required
to produce 5.
Figure 2. ORTEP drawing of (η5-C5H5)Co(PPh3)[κ2-OC(OEt)-
CHdC(CH2SO2Ph)](CF3CO2) (5-CF3CO2) at the 50% prob-
ability level (hydrogen atoms and CF3CO2- counterion omitted
˚
for clarity). Selected bond distances (A) and angles (deg): Co-
C2 1.914(2), Co-O1 1.950(2), Co-P1 2.279(1), C1-C2 1.508(3),
C2-C3 1.338(4), C3-C4 1.434(4), C4-O1 1.239(3), C2-Co-O1
84.4 (1), Co-C2-C1 126.7(2), Co-C2-C3 111.0(2), Co-O1-
C4 110.9(1).
˚
The C2-C3 bond distance of 1.338(4) A and the C3-C4 bond
˚
distance of 1.434(4) A indicate localized carbon-carbon dou-
ble and single bonds, respectively.
Unfortunately, the large standard deviations for the
bond distances in cobaltacyclopentene complex 7-Ph10 and
cobaltaoxanorbornadiene complex 812 (Figure 1) do not
permit many useful comparisons of bond distance data to
those for 5-CF3CO2. However, the Co-P bond distance in
˚
5-CF3CO2 (2.279(1) A) is significantly longer than in cobalt-
˚
acyclopentene 7-Ph (2.232(3) A), and the C2-Co-O1 angle
in 5-CF3CO2 (84.4(1)°) is larger than the corresponding
The protonation reactions of cobalt-allene complexes 2-Z
and 3-E proceed in high yield (93%) yet display very distinct
outcomes. Allyl complex 4 is generated from 2-Z, whereas
(12) Stolzenberg, A. M.; Scozzafava, M.; Foxman, B. M. Organome-
tallics 1987, 6, 769.

6164 Organometallics, Vol. 29, No. 23, 2010
Scheme 5. Mechanistic Hypotheses Involving Protonation at Cobalt
O’Connor et al.
the first structurally characterized cobaltafuran, 5, is formed
from 3-E. The available experimental data do not allow us
to rule out pathways that involve protonation at the metal
or protonation at C-3 of the allene ligand. Future experi-
ments will employ isotopic labeling studies and an examina-
tion of the less readily available allene stereoisomer 2-E in
the hopes of clarifying the mechanistic details. Deprotona-
tion of the cobaltafuran complex regenerates cobalt-allene
complexes, thus providing a new synthetic route toward
allene complexes.
Acknowledgment. The support of the National Science
Foundation is gratefully acknowledged (Grants CHE0518707
and CHE0911765 and Instrumentation Grant CHE0741968).
The authors thank Dr. Peter Gantzel for assistance with the
X-ray crystal structure determination.
Supporting Information Available: CIF files giving details of
the crystal structure determination of 5-CF3CO2 and character-
ization data for 4 and 5-BF4. This material is available free of
charge via the Internet at http://pubs.acs.org.