An unexpected SN′ reaction on cobaltocenium triflates: Substitution on one ring and elimination on the other

Article abstract of DOI:10.1021/om030182g

CpCo-capped tetra-n-propylcyclopentadienone complexes were O-alkylated and O-acylated. Reactions of the O-triflated product with organolithium compounds afford a substitution of the Cp ring in 40-80% yield; as products the RCpCo-capped tetra-n-propyl-cyclopenta-dienone complexes were isolated.

Full text of DOI:10.1021/om030182g

2370
Organometallics 2003, 22, 2370-2372
An Un exp ected SN′ Rea ction on Coba ltocen iu m Tr ifla tes:
Su bstitu tion on On e Rin g a n d Elim in a tion on th e Oth er
Mareile von der Gruen, Carsten Schaefer, and Rolf Gleiter*
Organisch-Chemisches Institut der Universita¨t Heidelberg, Im Neuenheimer Feld 270,
D-69120 Heidelberg, Germany
Received March 11, 2003
Sch em e 1
Sch em e 2
Summary: CpCo-capped tetra-n-propylcyclopentadienone
complexes were O-alkylated and O-acylated. Reactions
of the O-triflated product with organolithium compounds
afford a substitution of the Cp ring in 40-80% yield; as
products the RCpCo-capped tetra-n-propyl-cyclopenta-
dienone complexes were isolated.
The aromatic nucleophilic substitution reaction is a
well-studied process which proceeds with second-order
kinetics (Scheme 1). Most of these reactions require
some activation by electron-withdrawing groups to
stabilize the anionic intermediate 2, also sometimes
called the Meisenheimer intermediate.1
In organometallic π-systems the role of the electron-
withdrawing groups can be taken over by a metal in a
higher oxidation state. The nucleophilic substitution is
accompanied by a change of hapticity of the metal and
reduction of the metal at the same time. The role of the
leaving groups can be taken by a ligand at the metal or
by hydride abstraction at the π-system if we are dealing
with metallocenes. Although this view seems to be a
logical extension of the aromatic SN2 process shown in
Scheme 1, to the best of our knowledge only a few
processes are known which fit into this concept. Accord-
ing to Scheme 2, the nucleophile adds to a CpFeL3 (eq
1)2 and a CpOsL4 system (eq 2),3 respectively, or to a
cobaltocenium salt (eq 3).4 All reactions can be rational-
ized by assuming that the negative charge introduced
by the nucleophile is taken over by the metal (intramo-
lecular redox process). In the final step the original
formal charge on the metal is reestablished by proto-
nating a ligand (e.g. eq 1), by splitting off a leaving
group and protonating the metal3 (e.g. eq 2), or by
hydride abstraction4 (eq 3).
Ch a r t 1
ligand. We furthermore demonstrate the usefulness of
this process in synthesis.
During our endeavor to synthesize superphanes with
two CpCo-capped cyclopentadienone rings,5 we explored
the electronic properties of CpCo-stabilized cyclopenta-
dienones.6 The electronic structure of the cyclopentadi-
enone complexes can be described by the resonance
structures A and B (Chart 1). The complexation by the
metal fragment goes along with a higher electron
density at the cyclopentadienone ring, as expressed by
valence structure B. Consequently, a high-field shift of
the 13C signal at the CO group by about 40-50 ppm
has been encountered, in contrast to the uncomplexed
cyclopentadienone. A further consequence is the higher
electron density at the oxygen atom. As anticipated, we
were able to protonate, to alkylate, and to acylate the
oxygen center and to create the corresponding cobalto-
cenium salts in good yields (Scheme 3). The protonation,
In this paper we present an example where the
nucleophile attacks at one ring of a cobaltocenium ion
whereas the leaving group is at the other ring. This
probably goes along with a change of hapticity of the
(1) J ones, R. A. Y. Physical and Mechanistic Organic Chemistry;
Cambridge University Press: Cambridge, U.K., 1979.
(2) Reger, D. L. Acc. Chem. Res. 1988, 21, 229-235.
(3) Baya, M.; Crochet, P.; Esteruelas, M. A.; On˜ate, E. Organome-
tallics 2001, 20, 240-253.
(4) Wildschek, M.; Rieker, C.; J aitner, P.; Schottenberger, H.;
Schwarzhans, K. E. J . Organomet. Chem. 1990, 396, 355-361.
(5) Roers, R.; Rominger, F.; Nuber, B.; Gleiter, R. Organometallics
2000, 19, 1578-1588.
(6) Gleiter, R.; Roers, R.; Rominger, F.; Nuber, B.; Hyla-Kryspin, I.
J . Organomet. Chem. 2000, 610, 80-87.
10.1021/om030182g CCC: $25.00 © 2003 American Chemical Society
Publication on Web 05/10/2003

Communications
Organometallics, Vol. 22, No. 12, 2003 2371
Sch em e 3
to the substituted cyclopentadienone complex 15. The
elimination of SO2CF3- was detected by mass spectrom-
etry during the synthesis of 15c.
This kind of nucleophilic attack at cyclopentadienyl
ligands of alkoxy-substituted cobaltocenium salts, which
were synthesized by alkylation of CpCo-cyclopentadi-
enone complexes, was known, but the formation of the
cyclopentadienone complex could not be observed.12
To check the net charges at the cyclopentadienyl
ligands, we carried out density functional theory (DFT)
calculations on the CpCo-capped cyclopentadienone
complex and its O-protonated and O-triflated species.13
A comparison between the net charges at the cyclopen-
tadienyl ligands of the complexes A, C, and D (Chart 2
and Table 1) shows that the triflate group changes the
charge distribution dramatically, which can be ex-
plained by interaction between the cyclopentadienyl and
the cyclopentadienyloxy fragments, strictly speaking by
a transfer of electron density from the Cp fragment to
the cyclopentadienyloxy fragment. The charge at the Co
the alkylation, and the acylation of the cyclopentadi-
enone complex 137 have been carried out with CF3-
COOH,8 (Et3O)BF4,8 CH3SO2Cl,8 CH3C6H4SO2Cl,8 and
(CF3SO2)O,9 respectively, in methylene chloride.
By reacting 14e with various C-nucleophiles such as
MeLi, n-BuLi, and PhLi we observed a substitution in
the unsubstituted Cp ring in yields between 40 and
80%.10 Even the lithium salt of (trimethylsilyl)acetylene
gave the corresponding alkyne derivative (Scheme 4).
The yield of the ethynyl-substituted complex could be
increased to 41% by using (triethylsilyl)acetylene,11
followed by removal of the triethylsilyl group. The latter
process is the simplest way of introducing an alkyne
unit into the Cp ring. In the case of 15c we were able
to introduce a second n-butyl group in the â-position of
the first one in 80% yield in two further steps: triflation
and reaction with n-BuLi (Scheme 5).11
(10) Procedure for the RCpCo-capped cyclopentadienone complexes
15a -d and 17: a solution of 1.68 mmol of the lithium salt was slowly
added to a solution of 1.53 mmol of the cobaltocenium salts of 14e and
of 16, respectively, in 100 mL of THF at -30 °C. After the mixture
was warmed overnight, the mixture was hydrolyzed with 30 mL of
saturated NH4Cl solution, the layers were separated, and the aqueous
layer was washed three times with ether. After drying (Na2SO4) of
the combined organic layers, the solvent was removed and the crude
mixture was purified by column chromatography on silica gel with 30:1
CH2Cl2/MeOH. Data for 15a are as follows. Yield: 77%. Red solid.
Mp: 104 °C. 1H NMR (500 MHz, CD2Cl2; δ/ppm): 0.97 (t, 6H, CH3),
1.00 (t, 6H, CH3), 1.31-1.49 (m, 6H, CH2), 1.59-1.67 (m, 2H, CH2),
1.83 (s, 3H, Cp-CH3), 1.82-1.88 (m, 2H, CH2), 2.23-2.29 (m, 2H, CH2),
2.35-2.42 (m, 4H, CH2), 4.32-4.33 (m, 2H, Cp H), 4.42-4.43 (m, 2H,
Cp H). 13C NMR (125.76 MHz, CD2Cl2; δ/ppm): 10.1 (Cp-CH3), 14.9
(CH3), 15.0 (CH3), 23.0 (CH2), 25.0 (CH2), 27.8 (CH2), 28.6 (CH2), 81.8
(Cp C), 82.6 (Cp-CCH3), 83.4 (Cp C), 92.1 (C), 95.5 (C), 156.2 (CdO).
The nucleophile attacks the cyclopentadienyl ligand
at the exo position. Elimination of the HSO2CF3 leads
(7) Procedure for the CpCo-capped cyclopentadienone complex 13:
a solution of 6.1 g (34 mmol) of CpCo(CO)2 in 150 mL of decalin was
heated at 190 °C. During 8 h 3.3 g (30 mmol) of 4-octyne dissolved in
150 mL of decalin was added. When the addition was completed, the
heating was continued for an additional 5 days. The reaction mixture
was cooled to room temperature and then filtered through alumina
(neutral, grade III). With petroleum ether as eluent the solvent decalin,
unreacted CpCo(CO)2, and the benzoic byproducts could be extracted.
The cyclopentadienone complex was extracted with 50:1 CH2Cl2/MeOH
as a broad orange band. After removal of the solvent the crude product
was purified by column chromatography on alumina (neutral, grade
III, 20:1 CH2Cl2/MeOH) to yield 3.01 g (54%) of 13: Red solid. Mp:
113 °C. 1H NMR (500 MHz, CDCl3; δ/ppm): 0.96-1.00 (m, 12H, CH3),
1.41-1.45 (m, 6H, CH2), 1.67-1.68 (m, 2H, CH2), 1.88-1.93 (m, 2H,
CH2), 2.25-2.49 (m, 6H, CH2), 4.60 (s, 5H, Cp). 13C NMR (125.76 MHz,
CDCl3; δ/ppm): 15.5 (CH3), 15.6 (CH3), 23.4 (CH2), 25.2 (CH2), 28.5
(CH2), 29.1 (CH2), 82.6 (C), 83.0 (Cp C), 92.9 (C), 158.7 (CdO). Exact
MS: m/z calcd for C22H34CoO [M+ + H] 373.1941, found 373.1946. Anal.
Calcd for C22H33CoO: C, 70.95; H, 8.93. Found: C, 70.84; H, 8.95.
(8) For analytical details see the Supporting Information.
(9) Procedure for the synthesis of the cobaltocenium salts 14e and
16: to a cooled (-50 °C) solution of 2.8 g (10.1 mmol) of trifluo-
romethanesulfonic anhydride in 70 mL of methylene chloride was
added over a period of 15 min 2.5 g (6.7 mmol) of the cyclopentadienone
complex dissolved in 20 mL of methylene chloride. The mixture was
warmed slowly and stirred for 12 h at room temperature. After
evaporation of the solvent 100 mL of ether was added. The precipitate
was filtered and recrystallized from CH2Cl2/Et2O. Data for 14e are as
follows. Yield: 85%. Yellow solid. Mp: 161 °C. 1H NMR (300 MHz,
CD2Cl2; δ/ppm): 1.05 (t, 6H, CH3), 1.08 (t, 6H, CH3), 1.43-1.62 (m,
8H, CH2), 2.47-2.68 (m, 8H, CH2), 5.54 (s, 5H, Cp). 13C NMR (75.47
MHz, CD2Cl2; δ/ppm): 14.4 (CH3), 14.7 (CH3), 24.2 (CH2), 25.2 (CH2),
26.4 (CH2), 27.7 (CH2), 88.2 (Cp), 95.9 (CdC), 99.1 (CdC), 118.4 (C-
OTf). Exact MS: m/z calcd for [C23H33CoF3O3S]+ [M+] 505.1434, found
505.1455. Anal. Calcd for C24H33CoF6O6S2: C, 44.04; H, 5.08. Found:
C, 44.21; H, 5.26. Data for 16 are as follows. Yield: 76%. Yellow solid.
Mp: 146 °C. 1H NMR (500 MHz, CD2Cl2; δ/ppm): 0.90 (t, 3H, CH3),
1.06 (t, 6H, CH3), 1.08 (t, 6H, CH3), 1.30-1.37 (m, 2H, CH2), 1.48-
1.57 (m, 10H, CH2), 2.25 (t, 2H, CH2), 2.42-2.61 (m, 8H, CH2), 5.23
(m, 2H, Cp-H), 5.46 (m, 2H, Cp-H). 13C NMR (125 MHz, CD2Cl2,
δ/ppm): 13.7 (CH3), 14.4 (CH3), 14.7 (CH3), 22.5 (CH2), 24.2 (CH2), 25.2
(CH2), 26.1 (CH2), 26.3 (CH2), 27.5 (CH2), 32.5 (CH2), 86.7 (Cp C), 88.0
(Cp C), 95.5 (C), 98.6 (C), 108.2 (Cp-CBu), 117.7 (C-OTf). Exact MS:
m/z calcd for [C27H41CoF3O3S]+ [M+] 561.2060, found 561.2078. Anal.
Calcd for C28H41CoF6O6S2: C, 47.32,; H, 5.81. Found: C, 47.46; H, 5.83.
Exact MS: m/z calcd for
C
23H36CoO [M+ + H] 387.2098, found
387.2107. Anal. Calcd for C23H36CoO: C, 71.48; H, 9.13. Found: C,
71.35; H 9.02. Data for 15b are as follows. Yield: 58%. Dark red solid.
Mp: 156 °C. 1H NMR (500 MHz, CD2Cl2; δ/ppm): 0.90 (t, 6H, CH3),
0.91 (t, 6H, CH3), 1.25-1.40 (m, 8H, CH2), 1.55-1.67 (m, 4H, CH2),
1.95-2.01 (m, 2H, CH2), 2.08-2.15 (m, 2H, CH2), 2.19-2.25 (m, 2H,
CH2), 4.76-4.77 (m, 2H, Cp H), 4.98-4.99 (m, 2H, Cp H), 7.28-7.31
(m, 1H, Ph H), 7.37-7.42 (m, 4H, Ph H). 13C NMR (125.76 MHz, CD2-
Cl2; δ/ppm): 14.5 (CH3), 14.6 (CH3), 22.4 (CH2), 24.3 (CH2), 27.2 (CH2),
27.7 (CH2), 78.6 (Cp), 81.4 (C), 83.8 (Cp), 92.1 (C), 97.0 (C), 125.9 (Ph
CH), 127.4 (Ph CH), 128.6 (Ph CH), 132.2 (C), 157.8 (CdO). Exact
MS: m/z calcd for C28H38CoO [M+ + H] 449.2255, found 449.2280. Anal.
Calcd for C28H37CoO: C, 74.98; H, 8.31. Found: C, 74.8; H, 8.31. Data
for 15c are as follows. Yield: 79%. Red solid. Mp: 81 °C. 1H NMR
(500 MHz, CDCl3; δ/ppm): 0.77 (t, 3H, CH3), 0.85 (t, 6H, CH3), 0.88 (t,
6H, CH3), 1.14-1.22 (m, 2H, CH2), 1.24-1.38 (m, 8H, CH2), 1.54-1.65
(m, 2H, CH2), 1.70-1.76 (dt, 2H, CH2), 2.06-2.16 (m, 4H, CH2), 2.21-
2.27 (m, 2H, CH2), 2.30-2.36 (dt, 2H, CH2), 4.24-4.25 (m, 2H, Cp H),
4.34-4.35 (m, 2H, Cp H). 13C NMR (125.76 MHz, CDCl3; δ/ppm): 14.0
(CH3), 15.0 (CH3), 15.1 (CH3), 22.3 (CH2), 22.8 (CH2), 24.7 (CH2), 25.7
(CH2), 27.7 (CH2), 28.4 (CH2), 33.1 (CH2), 81.8 (Cp C), 82.1 (Cp C),
91.6 (C), 100.4 (C), 156.4 (CdO). Exact MS: m/z calcd for C26H42CoO
[M+ + H] 429.2567, found 429.2595. Anal. Calcd for C26H42CoO: C,
73.0; H, 9.43. Found: C, 72.8; H, 9.50. Data for 15d are as follows.
Yield: 65%. Red solid. Mp: 149 °C. 1H NMR (500 MHz, CD2Cl2;
δ/ppm): 0.97 (t, 6H, CH3), 1.01 (t, 6H, CH3), 1.23 (s, 9H, CH3), 1.28-
1.36 (m, 2H, CH2), 1.40-1.48 (m, 4H, CH2), 1.61-1.69 (m, 2H, CH2),
1.88-1.94 (m, 2H, CH2), 2.29-2.35 (m, 2H, CH2), 2.43-2.54 (m, 4H,
CH2), 4.33 (m, 2H, Cp H), 4.50 (m, 2H, Cp H). 13C NMR (125.76 MHz,
CD2Cl2; δ/ppm): 14.9 (CH3), 15.1 (CH3), 23.0 (CH2), 25.0 (CH2), 28.4
(CH2), 29.2 (CH2), 30.6 (CH3), 31.8 (C), 79.0 (CH), 81.9 (CH), 92.1 (C),
157.6 (C). Exact MS: m/z calcd for C26H42CoO [M+ + H] 429.2568,
found 429.2547. Anal. Calcd for C26H42CoO: C, 72.87; H, 9.64. Found:
C, 72.75; H, 9.62. Data for 17 are as follows. Yield: 82%. Red solid.
Mp: 58 °C. 1H NMR (300 MHz, CDCl3, δ/ppm): 0.87 (t, 6H, CH3), 0.95
(t, 6H, CH3), 0.98 (t, 6H, CH3), 1.22-1.46 (m, 14H, CH2), 1.68-1.76
(m, 4H, CH2), 2.09-2.41 (m, 10H, CH2), 4.10 (m, 1H, Cp H), 4.18 (m,
2H, Cp H). 13C NMR (75 MHz, CDCl3, δ/ppm): 14.0 (CH3), 15.0 (CH3),
15.1 (CH3), 22.4 (CH2), 22.7 (CH2), 24.6 (CH2), 26.0 (CH2), 27.4 (CH2),
28.2 (CH2), 33.1 (CH2), 81.3 (Cp C), 81.7 (C), 81.8 (Cp C), 90.6 (C),
99.6 (C), 154.8 (CdO). Exact MS: m/z calcd for C30H50CoO [M+ + H]
485.3194, found 485.3168. Anal. Calcd for C30H49CoO: C, 74.35; H,
10.19. Found: C, 74.20; H, 9.93.

2372 Organometallics, Vol. 22, No. 12, 2003
Communications
Sch em e 4
Sch em e 5
Ch a r t 2
Ta ble 1. Mu llik en Net Ch a r ges Ca lcu la ted for A,
C, a n d D
A
C
D
∑C1-C5
-0.362
+0.330
-0.586
-0.371
+0.989
-0.023
+0.255
-0.133
-0.064
+0.964
+0.028
+0.204
-0.017
-0.486
+0.964
atom remains nearly constant. In A the sum of the
negative net charges at C1-C5 amounts to -0.362. For
the cyclopentadienyl ligand in C these net charges sum
C6
∑C7-C10
O
Co
(11) Procedure for the substituted complexes 15e,f: 2.29 mmol of
n-butyllithium (1.6 M solution) was slowly added to a solution of 3.06
mmol of (trialkylsilyl)acetylene in 60 mL of THF at -30 °C. After the
mixture was stirred at this temperature for 90 min, a solution of 1.53
mmol of the cobaltocenium salt 14e in 30 mL of THF was added over
a period of 10 min. After it was warmed overnight and stirred further
at room temperature for 48 h, the mixture was hydrolyzed with 30
mL of saturated NH4Cl solution, the layers were separated, and the
aqueous layer was washed three times with ether. After drying
(Na2SO4) of the combined organic layers, the solvent was removed and
the crude mixture was purified by column chromatography on silica
with 30:1 CH2Cl2/MeOH. Data for 15e are as follows. Yield: 15%. Dark
red oil. 1H NMR (500 MHz, CDCl3; δ/ppm): 0.25 (s, 9H, Si(CH3)3),
0.96-1.04 (m, 12H, CH3), 1.45-1.49 (m, 6H, CH2), 1.63-1.65 (m, 2H,
CH2), 1.90-1.93 (m, 2H, CH2), 2.27-2.43 (m, 6H, CH2), 4.61 (m, 2H,
Cp H), 4.73 (m, 2H, Cp H), 13C NMR (75.47 MHz, CDCl3; δ/ppm): 0.0
(Si(CH3)3), 15.0 (CH3), 15.0 (CH3), 22.9 (CH2), 24.5 (CH2), 27.5 (CH2),
27.5 (CH2), 77.5 (C), 78.8 (C), 84.5 (Cp C), 85.2 (Cp C), 92.9 (C), 94.7
(C), 99.1 (C), 158.6 (CdO). Exact MS: m/z calcd for C27H42CoOSi
[M+ + H] 469.2337, found 469.2339. Data for 15f are as follows.
Yield: 41%. Dark red oil. 1H NMR (500 MHz, CD2Cl2; δ/ppm): 0.87
(q, 6H, SiCH2), 1.15 (t, 9H, SiCH2CH3), 1.20 (t, 6H, CH3), 1.24 (t, 6H,
CH3), 1.56-1.70 (m, 6H, CH2), 1.78-1.87 (m, 2H, CH2), 2.08-2.14 (m,
2H, CH2), 2.45-2.52 (m, 2H, CH2), 2.56-2.64 (m, 4H, CH2), 4.85-4.86
(m, 2H, Cp H), 4.92-4.93 (m, 2H, Cp H). 13C NMR (125.76, CD2Cl2;
δ/ppm): 4.7 (SiCH2CH3), 7.7 (SiCH2), 14.9 (CH3), 15.0 (CH3), 22.9
(CH2), 24.6 (CH2), 27.5 (CH2), 27.6 (CH2), 79.1 (C), 82.3 (C), 84.5 (Cp
C), 85.2 (Cp C), 92.3 (C), 92.9 (C), 100.2 (C), 158.9 (CdO). Exact MS:
m/z calcd for C30H47CoOSi [M+ + H] 511.2806, found 511.2823. Anal.
Calcd for C30H47CoOSi: C, 70.55; H, 9.28. Found: C, 70.49; H, 9.30.
(12) Vollhardt, K. P. C.; Tane, J . P. Angew. Chem. 1982, 94, 642-
643; Angew. Chem. Int. Ed. Engl. 1982, 21, 617-618; see also Angew.
Chem. Suppl. 1982, 1360-1372.
up to -0.023. For the O-triflated species D the sum of
the net charges shows even a value of +0.028. These
theoretical investigations can be used to rationalize the
nucleophilic substitution of Li-organyl species on cobal-
tocenium ions with subsequent elimination of the sul-
finate anion (SO2CF3-).
Ack n ow led gm en t. We are grateful to the Deutsche
Forschungsgemeinschaft (Grant No. SFB 623), the
Fonds der Chemischen Industrie, and the BASF Ak-
tiengesellschaft, Ludwigshafen, Germany, for financial
support.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details of the synthesis and characterization of 14a -
d . This material is available free of charge via the Internet at
http://pubs.acs.org.
OM030182G
(13) (a) Kohn, W.; Sham, L. J . Phys. Rev. A 1965, 140, 1133-1138.
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Wiley-VCH: Weinheim, Germany, 2000.