Synthesis and highly diastereoselective palladation of (η5-(S)-2-(4-methylethyl) oxazolinylcyclopentadienyl)-(η4-tetraphenylcyclobutadiene)cobalt

Article abstract of DOI:10.1021/om980812s

Summary: Starting from sodium carbomethoxycyclopentadienide, diphenylacetylene, and chlorotris(triphenyl-phosphine)cobalt(I), the title compound was obtained in four steps and 47% overall yield. On reaction with Pd(OAc)₂, a single cyclopalladated diastereoisomer of (S)-(_pR) configuration was isolated in 72% yield.

Full text of DOI:10.1021/om980812s

1346
Organometallics 1999, 18, 1346-1348
Notes
Syn th esis a n d High ly Dia ster eoselective P a lla d a tion of
(η5-(S)-2-(4-Meth yleth yl)oxa zolin ylcyclop en ta d ien yl)-
(η4-tetr a p h en ylcyclobu ta d ien e)coba lt
Andrew M. Stevens and Christopher J . Richards*,‡
Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff, CF1 3TB, U.K.
Received September 29, 1998
Sch em e 1
Summary: Starting from sodium carbomethoxycyclopen-
tadienide, diphenylacetylene, and chlorotris(triphenyl-
phosphine)cobalt(I), the title compound was obtained in
four steps and 47% overall yield. On reaction with
Pd(OAc)2, a single cyclopalladated diastereoisomer of
(S)-(pR) configuration was isolated in 72% yield.
In tr od u ction
Interest in the planar chirality displayed by differ-
ently substituted 1,2-cyclopentadienyl ligands attached
to a metal has resulted in a number of methods for the
asymmetric synthesis of ferrocene1 and cymantrene
complexes.2 Many of these involve diastereoselective
ortho-lithiation mediated by a suitable group such as
the oxazoline auxiliary. When attached to ferrocene, this
controls the selective formation of 1, in which the
oxazoline isopropyl substituent is oriented toward the
unsubstituted cyclopentadienyl ring.3 As part of a
program investigating derivatives of (η5-C5H5)(η4-C4Ph4)-
Co complexes as components of molecular gearing
devices,4 we wished to develop a method for the dia-
stereoselective ortho-metalation of the corresponding
oxazoline derivative 2. In this paper we describe the
synthesis and palladation of this complex and report on
the effect of the four phenyl substituents on the selec-
tivity and orientation of this metalation.
Resu lts a n d Discu ssion
The carbomethoxy-substituted complex 4 has previ-
ously been synthesized in 52% overall yield from 3 via
the intermediate cobalt dicarbonyl adduct.5 To avoid the
use of metal carbonyls, diphenylacetylene was combined
directly with 3 and CoCl(PPh3)3 to give a 92% yield of
ester 4.6 Subsequent hydrolysis with potassium tert-
butoxide in DMSO gave an excellent yield of the
required acid, from which the corresponding acid chlo-
ride was generated in situ with oxalyl chloride. On
addition of (S)-valinol this yielded the â-hydroxy amide
6, which was dehydrated under Appel conditions7 to give
oxazoline 2 in 47% overall yield for the four steps
(Scheme 1).
‡ RichardsCJ @cardiff.ac.uk. Fax: +44 1222 874030.
(1) (a) For a review of the recent literature until 1997, see: Richards,
C. J .; Locke, A. J . Tetrahedron: Asymmetry 1998, 9, 2377. (b) Iftime,
G.; Daran, J .-C.; Manoury, E.; Balavoine, G. G. A. Angew. Chem., Int.
Ed. Engl. 1998, 37, 1698. (c) Riant, O.; Argouarch, G.; Guillaneux, D.;
Samuel, O.; Kagan, H. B. J . Org. Chem. 1998, 63, 3511. (d) Lagneau,
N. M.; Chen, Y.; Robben, P. M.; Sin, H.-S.; Takasu, K.; Chen, J .-S.;
Robinson, P. D.; Hua, D. H. Tetrahedron 1998, 54, 7301.
(2) Loim, N. M.; Kondratenko, M. A.; Sokolov, V. I. J . Org. Chem.
1994, 59, 7485.
(4) Stevens, A. M.; Richards, C. J . Tetrahedron Lett. 1997, 38, 7805.
(5) Mabrouk, S. T.; Rausch, M. D. J . Organomet. Chem. 1996, 523,
111.
(3) (a) Richards, C. J .; Damalidis, T.; Hibbs, D. E.; Hursthouse, M.
B. Synlett 1995, 74. (b) Sammakia, T.; Latham, H. A.; Schaad, D. R.
J . Org. Chem. 1995, 60, 10. (c) Nishibayashi, Y.; Uemura, S. Synlett
1995, 79.
(6) Uno, M.; Ando, K.; Komatsuzaki, N.; Tsuda, T.; Tanaka, T.;
Sawada, M.; Takahashi, S. J . Organomet. Chem. 1994, 473, 303.
(7) Vorbru¨ggen, H.; Krolikliewicz, K. Tetrahedron Lett. 1981, 22,
4471.
10.1021/om980812s CCC: $18.00 © 1999 American Chemical Society
Publication on Web 02/27/1999

Notes
Organometallics, Vol. 18, No. 7, 1999 1347
F igu r e 2. Origin of diastereoselective palladation.
F igu r e 1. NOE connectivity and percentage enhance-
ments for 7.
room temperature) are in stark contrast to the more
vigorous conditions used in this work. Both the high
selectivity observed and the opposite orientation of this
metalation compared to that previously found for ferro-
cenyloxazolines are rationalized through the inability
of complex 2 to access rotamer A (Figure 2). This would
result in a severe repulsive interaction between the
oxazoline isopropyl substituent and the phenyl “roof”,
an effect avoided with rotamer B. This restriction of 2
to a single rotamer results in the oxazoline substituent
displaying effective planar chirality, as if it were directly
bonded to a second position of the cyclopentadienyl ring.
The implications of this arrangement and the use of
such complexes in asymmetric catalysis are currently
under investigation.
Attempts to effect diastereoselective lithiation of 2
with BuLi, sec-BuLi, and tert-BuLi all failed to result
in incorporation of lithium into the molecule, despite
prolonged reaction times and the use of additives such
as TMEDA. As a consequence, our attention turned to
palladation, prompted by previous reports on the ortho-
palladation of (η5-C5H4CH2N(CH3)2)(η4-C4Ph4)Co8 and
2-aryloxazolines.9 Accordingly, palladium acetate and
oxazoline 2 were heated at 95 °C in glacial acetic acid
for 30 min, and on cooling an orange crystalline solid
1
was isolated by filtration. Examination of the H NMR
of this material revealed the presence of only three
cyclopentadienyl proton signals, and that palladation
had occurred ortho to the oxazoline was confirmed by
the doublet/triplet/doublet coupling pattern arising from
these three adjacent hydrogens. Of greater significance
was the presence of only one set of signals in this and
all other regions of the spectra, revealing that a single
diastereoisomer had been isolated. The single peak
observed for the acetate methyl groups reveals that the
two palladated oxazolines adopt an anti relationship
about these bridging ligands.10 Examination of the
concentrated mother liquors revealed that none of the
Exp er im en ta l Section
Tetrahydrofuran was distilled from sodium benzophenone
ketyl, and toluene, dichloromethane, and acetonitrile were
distilled from calcium hydride. Petroleum ether refers to that
fraction boiling in the range 40-60 °C. Column chromatog-
raphy was performed on Matrix silica 60 (35-70 µm). All
reactions, with the exception of the palladation, were per-
formed under a nitrogen atmosphere.
(η5-Ca r b om et h oxycyclop en t a d ien yl)(η4-t et r a p h en yl-
cyclob u t a d ien e)cob a lt . 4. To a solution of chlorotris(tri-
phenylphosphine)cobalt(I)13 (7.24 g, 8.2 mmol) and diphenyl-
acetylene (3.36 g, 18.9 mmol) in toluene (56 mL) was added a
solution of sodium carbomethoxycyclopentadienide (3)14 (1.38,
9.4 mmol) in THF (14 mL). The resulting mixture was heated
at reflux for 5 h and then allowed to cool to room temperature.
The solvent was removed in vacuo and the residue suspended
in petroleum ether (50 mL) and collected by filtration to give
4 as a yellow crystalline solid (4.06 g, 92%): mp 222 °C (EtOAc/
petroleum ether) [lit.5 219-221 °C]; δH (400 MHz, CDCl3) 3.24
(3 H, s, -CH3), 4.84 (2 H, brs, CpH), 5.27 (2 H, brs, CpH),
7.22-7.32 (12 H, m, m+p-PhH), 7.42-7.48 (8 H, m, o-PhH);
δC {1H} (100 MHz, CDCl3) 51.6 (-CH3), 76.4 (C4Ph4), 84.9
(CpC), 86.8 (CpC), 87.1 (ipso-CpC), 127.1 (p-PhC), 128.3 (PhC),
129.2 (PhC), 135.5 (ipso-PhC), 167.0 (-CO2CH3).
1
product remained in this residue, nor did the H NMR
spectrum of this material indicate the presence of other
possible diastereomeric acetate dimers.
The orientation of this diastereoselective palladation
was determined by NOE difference spectroscopy (Figure
1). Irradiation of Ha (3.00 ppm) gave a 5% enhancement
of the triplet at 3.38 ppm, confirming this as arising
from Hb. The Ha irradiation also gave rise to a 5%
enhancement of the overlapping signals corresponding
to the meta/para hydrogens of the four phenyl groups,
and an enhancement of similar magnitude was observed
on irradiation of Hb. Significantly, no such enhancement
was found on irradiation of Hc (4.08 ppm) or of either
of the two diastereotopic methyl doublets (0.02 and 0.47
ppm). That Ha and Hb are in the vicinity of the “roof”
defined by the four phenyl groups, as was initially
suspected by the rather low chemical shift of these two
signals, confirms the new element of planar chirality
(η5-Ca r b oxycyclop en t a d ien yl)(η4-t et r a p h en ylcyclo-
bu ta d ien e)coba lt, 5. A mixture of 4 (0.210 g, 0.39 mmol) and
potassium tert-butoxide (0.66 g, 5.9 mmol) in DMSO (6 mL)
was stirred at room temperature for 15 h and then quenched
with 2 M HCl (5 mL). After extraction with EtOAc (80 mL),
the organic phase was separated and dried (MgSO4) and the
as R.
p
Previously reported methods for the diastereoselective
ortho-palladation of substituted metallocenes have gen-
erally resulted in incomplete stereochemical control,11
and only recently were the first fully selective examples
described.12 Furthermore, the conditions used in all of
these previous palladations (Na2PdCl4/AcOH/MeOH at
(11) (a) Sokolov, V. I.; Troitskaya, L. L.; Reutov, O. A. J . Organomet.
Chem. 1977, 133, C28. (b) Sokolov, V. I.; Troitskaya, L. L.; Reutov, O.
A. J . Organomet. Chem. 1979, 182, 537. (c) Sokolov, V. I.; Troitskaya,
L. L.; Gautheron, B.; Tainturier, G. J . Organomet. Chem. 1982, 235,
369. (d) Hollis, T. K.; Overman, L. E. Tetrahedron Lett. 1997, 38, 8837
(e) Zhao, G.; Wang, Q.-G.; Mak, T. C. W.; Organometallics 1998, 17,
3437. (f) Zhao, G.; Wang, Q.-G.; Mak, T. C. W. Tetrahedron: Asymmetry
1998, 9, 1557.
(8) Izumi, T.; Maemura, M.; Endoh, K.; Oikawa, T.; Zakozi, S.;
Kasahara, A. Bull. Chem. Soc. J pn. 1981, 54, 836.
(9) Izumi, T.; Watabe, H.; Kasahara, A. Bull. Chem. Soc. J pn. 1981,
54, 1711.
(10) Balavoine, G.; Clinet, J . C.; Zerbib, P.; Boubekeur, K. J .
Organomet. Chem. 1990, 389, 259.
(12) (a) Zhao, G.; Xue, F.; Zhang, Z.-Y.; Mak, T. C. W. Organo-
metallics 1997, 16, 4023. (b) Zhao, G.; Wang, Q.-G.; Mak, T. C. W.
Tetrahedron: Asymmetry 1998, 9, 2253.
(13) Wakatsuki, Y.; Yamazaki, H. Inorg. Synth. 1989, 26, 190.
(14) Hart, W. P.; Shihua, D.; Rausch, M. D. J . Organomet. Chem.
1985, 282, 111.

1348 Organometallics, Vol. 18, No. 7, 1999
Notes
solvent removed in vacuo. The residue was column chromato-
graphed (20% EtOAc/petroleum ether) to give 5 as an orange
crystalline solid (0.192, 94%): mp 240-244 °C (EtOAc) [lit.5
246-248 °C]; δH (400 MHz, CDCl3) 4.78 (2 H, brs, CpH), 5.22
(2 H, brs, CpH), 7.21-7.24 (12 H, m, m+p-PhH), 7.43-7.47 (8
H, m, o-PhH); δC {1H} (100 MHz, CDCl3) 77.2 (C4Ph4), 89.1
(CpC), 89.3 (CpC), 92.9 (ipso-CpC), 127.7 (p-PhC), 128.8 (PhC),
129.0 (PhC), 135.0 (ipso-PhC), 191.1 (-CO2H).
vacuo. The residue was column chromatographed (10% EtOAc/
petroleum ether) to give 2 as an orange crystalline solid (0.085
24
g, 78%): mp 160-162 °C (EtOAc/petroleum ether); [R]D
)
-
-55.2 (c 0.09, CHCl3); (found C, 78.99; H, 5.92; N, 2.23; C39H34
CoNO requires C, 79.17; H, 5.79; N, 2.37); νmax/cm-1 1654
(CdN); δH (400 MHz, CDCl3) 0.77 (3 H, d, J 6.7, -CH3), 0.97
(3 H, d, J 6.7, -CH3), 1.40 (1 H, oct, J 6.7, -CH(CH3)2), 3.41-
3.56 (3 H, m, -CHCH2-), 4.62 (1 H, brs, CpH), 4.80 (1 H, brs,
CpH), 5.09 (1 H, brs, CpH), 5.20 (1 H, brs, CpH), 7.17-7.27
(12 H, m, m+p-PhH), 7.44-7.47 (8 H, m, o-PhH); δC {1H} (100
MHz, CDCl3) 20.4 (-CH3), 21.6 (-CH3), 35.0 (-CH(CH3)2), 71.5
(-CHCH2-), 74.7 (-CHCH2-), 78.0 (C4Ph4), 84.1 (ipso-CpC),
86.4 (CpC), 87.0 (CpC), 87.1 (CpC), 88.4 (CpC), 126.8 (p-PhC),
128.3 (PhC), 129.3 (PhC), 135.8 (ipso-PhC), 162.6 (CdN); m/z
(EI) 591 (M+, 100), 415 (82), 356 (8), 178 (60).
(η5-(S)-N-2-(1-H yd r oxy-3-m et h ylb u t yl)ca r b oxa m id e-
cyclop en ta d ien yl)(η4-tetr a p h en ylcyclobu ta d ien e)coba lt,
6. To a solution of 5 (0.163 g, 0.31 mmol) in CH2Cl2 (2 mL)
was added oxalyl chloride (0.06 mL, 0.7 mmol), and the
resulting red solution was stirred at room temperature for 20
min. The solvent and excess oxalyl chloride were removed in
vacuo, and the crude acid chloride was dissolved in CH2Cl2 (2
mL) and added to a solution of (S)-valinol (0.043 g, 0.42 mmol)
in triethylamine (0.1 mL) and CH2Cl2 (1 mL). The resulting
orange reaction mixture was stirred at room temperature
overnight, quenched with water (6 mL), and extracted with
EtOAc (3 × 15 mL). The combined organic phases were dried
(MgSO4) and evaporated in vacuo, and the residue was column
chromatographed (3% MeOH/CH2Cl2) to give 6 as an orange
crystalline solid (0.133 g, 70%): mp 214-217 °C (EtOAc/
Di-µ-acetatobis[(η5-(S)-(pR)-2-(2′-(4′-m eth yleth yl)oxazol-
in yl)cyclop en t a d ien yl, 1-C, 3′-N)(η4-t et r a p h en ylcyclo-
bu ta d ien e)coba lt]d ip a lla d iu m , 7. A solution of 2 (0.314 g,
0.53 mmol) and Pd(OAc)2 (0.119 g, 0.53 mmol) in glacial acetic
acid (1 mL) was heated for 30 min at a temperature of 95 °C.
After the resultant orange mixture had cooled to room tem-
perature, it was diluted with cold glacial acetic acid (1 mL),
and the product was isolated by filtration and washed with
further glacial acetic acid (1 mL). After drying in vacuo,
complex 7 was obtained as an orange crystalline solid (0.290
24
petroleum ether); [R]D ) -16.9 (c 0.125, CHCl3); (found C,
76.83; H, 5.82; N, 2.21; C39H36CoNO2 requires C, 76.83; H, 5.95;
N, 2.30); νmax/cm-1 3364 (OH), 1666 (CdO); δH (400 MHz,
CDCl3) 0.85 (3 H, d, J 6.7, -CH3), 0.94 (3 H, d, J 6.7, -CH3),
1.59 (1 H, oct, J 6.7, -CH(CH3)2), 3.17 (1 H, brs, OH), 3.27-
3.42 (3 H, m, -CHCH2OH), 4.70 (1 H, brs, CpH), 4.74 (1 H,
brs, CpH), 4.97 (1 H, brs, CpH), 5.11 (1 H, brs, CpH), 5.35 (1
H, d, J 6.7, NH), 7.26-7.32 (12 H, m, m+p-PhH), 7.46-7.49
(8 H, m, o-PhH); δC {1H} (100 MHz, CDCl3) 19.6 (2 × -CH3),
29.4 (-CH(CH3)2), 59.0 (-CHCH2-), 64.6 (-CH2OH), 76.7
(C4Ph4), 82.5 (CpC), 82.6 (CpC), 87.0 (CpC), 87.4 (CpC), 91.0
(ipso-CpC), 127.3 (p-PhC), 128.6 (PhC), 129.2 (PhC), 135.6
(ipso-PhC), 167.0 (CdO); m/z (EI) 609 (M+, 100), 591 (27), 523
(11), 479 (4), 415 (30), 356 (8), 178 (94).
24
g, 72%): mp 189-194 °C; [R]D ) +942 (c 0.215, CHCl3);
(found C, 65.21; H, 5.01; N, 1.56; C82H72Co2N2O6Pd2 requires
C, 65.13; H, 4.80, N, 1.85); νmax/cm-1 1591 (CdN), 1498, 1408
(acetate bridge); δH (400 MHz, CDCl3) 0.02 (6 H, d, J 7.2,
-CH3), 0.47 (6 H, d, J 7.2, -CH3), 1.75-1.79 (2 H, m,
-CH(CH3)2), 1.95 (6 H, s, CO2CH3), 2.99-3.01 (2 H, m,
-CHCH2-), 3.38 (2 H, t, J 9.0, -CHH-), 4.08 (2 H, dd, J 8.5,
4.0, -CHH-), 4.23 (2 H, t, J 2.4, CpH), 4.62 (2 H, d, J 1.6,
CpH), 4.69 (2 H, d, J 2.3, CpH), 7.21-7.29 (24 H, m, m+p-
PhH), 7.65-7.67 (16 H, m, o-PhH); δC {1H} (100 MHz, CDCl3)
13.5 (-CH3), 19.0 (-CH3), 24.3 (-CH(CH3)2), 29.3 (CH3CO2),
65.3 (-CHCH2-), 71.4 (-CHCH2-), 71.4 (C4Ph4), 79.6 (ipso-
CpCCN), 85.1 (CpC), 85.6 (CpC), 87.0 (CpC), 98.3 (ipso-
CpCPd), 126.4 (p-PhC), 128.3 (PhC), 129.7 (PhC), 136.3 (ipso-
PhC), 171.0 (CdN), 181.2 (CO2); m/z (APCI) 1512 (M+ 18), 551
(100).
(η5-(S)-2-(4-Meth yleth yl)oxa zolin ylcyclop en ta d ien yl)-
(η4-tetr a p h en ylcyclobu ta d ien e)coba lt, 2. To a solution of
6 (0.113 g, 0.19 mmol) and triphenylphosphine (0.210 g, 0.80
mmol) in triethylamine (0.13 mL, 0.9 mmol) and acetonitrile
(25 mL) was added CCl4 (0.18 mL, 1.9 mmol) and the resultant
mixture stirred at room temperature for 15 h. After quenching
with water (20 mL), the mixture was extracted with petroleum
ether (5 × 60 mL), and after separation, the organic phase
was dried (MgSO4) and filtered and the solvent removed in
Ack n ow led gm en t. We wish to thank the EPSRC
for financial support (GR/K85377).
OM980812S