Synthesis of planar chiral cobalt metallocenes by microwave-assisted diastereoselective complexation

Article abstract of DOI:10.1021/om060015r

Ether- and ester-linked diynes were readily synthesized in two steps by starting from secondary terminal propargylic alcohols. On either heat- or microwave-assisted reaction of the diynes with cyclopentadienylcobalt dicarbonyl, new planar chiral (η^5<

Full text of DOI:10.1021/om060015r

Organometallics 2006, 25, 2899-2902
2899
Notes
Synthesis of Planar Chiral Cobalt Metallocenes by
Microwave-Assisted Diastereoselective Complexation
Caroline J. Taylor, Majid Motevalli, and Christopher J. Richards*
School of Biological and Chemical Sciences, Queen Mary, UniVersity of London, Mile End Road,
London E1 4NS, U.K.
ReceiVed January 6, 2006
Scheme 1. General Approach to Planar Chiral Cobalt
Metallocenes
Summary: Ether- and ester-linked diynes were readily synthe-
sized in two steps by starting from secondary terminal propar-
gylic alcohols. On either heat- or microwaVe-assisted reaction
of the diynes with cyclopentadienylcobalt dicarbonyl, new planar
chiral (η5-cyclopentadienone)(η5-cyclopentadienyl)cobalt met-
allocenes were formed with diastereomeric ratios of up to 5:1.
For both the ether and ester series the identity of the major
isomer was established by X-ray crystallography. The complex-
ation method was shown to be applicable to the synthesis of
scalemic planar chiral metallocenes. Preliminary inVestigations
were carried out to transform the new complexes into other
functionalized cobalt metallocenes.
Scheme 2. Synthesis of Diynes
Introduction
Planar chiral metallocenes, and in particular ferrocene deriva-
tives, have found widespread application as ligands and catalysts
in asymmetric synthesis.1 They have also been extensively
employed as building blocks for the synthesis of new materials.2
The vast majority of such enantiopure derivatives are generated
by a three-step sequence involving (a) metallocene synthesis,
(b) introduction of a chiral auxiliary or a fixed stereogenic
center, and (c) diastereoselective attachment of a second
substituent leading to a new element of planar chirality.3 An
alternative and potentially more attractive strategy is to generate
the planar chirality simultaneously with the metallocene. One
way in which this has been achieved, for example for the
synthesis of group 4 metallocenes, utilizes the diastereofacial
complexation of auxiliary substituted cyclopentadienyl anions.4
In contrast, we were attracted by the potential of employing
chiral diynes 1 for the stereoselective synthesis of cobalt
metallocenes 2 and 3 (Scheme 1). The use of achiral diynes in
such complexation reactions is well precedented,5 and the
unwitting isolation of chiral cyclobutadiene cobalt metallocenes
has also been reported.6 In this note we report on our initial
results exploring this strategy for the stereoselective synthesis
of planar chiral cyclopentadienone complexes 3.
Scheme 3. Synthesis of 5-Ring Ether Fused Metallocenes
Results and Discussion
Two different diynes of general structure 1 were employed,
with X ) CH2, o-C6H4CO. The former were used to provide a
new stable five-membered-ring ether and the latter to give a
seven-membered-ring lactone with the potential for ring opening
and further synthetic manipulation. Two examples of both class-
es of diyne were synthesized from the propargylic alcohols 4a,b
employing a straightforward two-step procedure (Scheme 2).
* To whom correspondence should be addressed. E-mail: c.j.richards@
qmul.ac.uk. Fax: +44-(0)20 7882 7427.
(1) (a) Colacot, T. J. Chem. ReV. 2003, 103, 3101. (b) Richards, C. J.;
Locke, A. J. Tetrahedron: Asymmetry 1998, 9, 2377.
(2) See for example: (a) Bunz, U. H. F. J. Organomet. Chem. 2003,
683, 269. (b) Chuard, T.; Deschenaux, R. Chimia 2003, 57, 597.
(3) See for example: (a) Riant, O.; Samuel, O.; Kagan, H. B. J. Am.
Chem. Soc. 1993, 115, 5835. (b) Rebie`re, F.; Riant, O.; Ricard, L.; Kagan,
H. B. Angew. Chem., Int. Ed. Engl. 1993, 32, 568.
A solution of diyne 6a and cyclopentadienylcobalt dicarbonyl
in toluene was heated at reflux overnight. After filtration through
a silica plug to remove paramagnetic impurities, the ratio of
(4) Schumann, H.; Stenzel, O.; Dechert, S.; Halterman, R. L. Organo-
metallics 2001, 20, 1983.
(5) Gleiter, R.; Kratz, D.; Ziegler, M. L.; Nuber, B. Tetrahedron Lett.
1990, 31, 6175.
1
the two new dark red complexes was established by H NMR
spectroscopy as 4:1 (Scheme 3, Table 1, entry 1). Following
10.1021/om060015r CCC: $33.50 © 2006 American Chemical Society
Publication on Web 04/27/2006

2900 Organometallics, Vol. 25, No. 11, 2006
Notes
Table 1. Synthesis of 5-Ring Ether Cyclopentadienone
Complexes
Table 2. Synthesis of 7-Ring Lactone Cyclopentadienone
Complexes
entry
substrate
R
conditions
yield, %
9:10a
entry
substrate
R
conditions
yield, %
11:12a
1
2
3
4
5
6a
6a
6b
6b
6b
Me
Me
i-Pr
i-Pr
i-Pr
Ab
Bc
A
61
68
47
49
34
4:1
3.2:1
5:1
5:1
3.5:1
1
2
3
4
5
8a
8a
8b
8b
8b
Me
Me
i-Pr
i-Pr
i-Pr
Ab
Bc
A
21
29
33
57
11
3:1
3.4:1
4:1
3:1
5:1
B
B
Cd
Cd
a Ratio determined by 1H NMR spectroscopy. b Conditions A: Cp-
Co(CO)2, toluene, 120 °C, N2, 18 h. c Conditions B: CpCo(CO)2, decalin,
190 °C, N2, microwave, 10 min. d Conditions C: Co2(CO)8 (0.5 equiv) C5H6
(2 equiv), decalin, 5 min at 40 °C, 10 min at 190 °C.
a Ratio determined by 1H NMR spectroscopy. b Conditions A: Cp-
Co(CO)2, toluene, 120 °C, N2, 18 h. c Conditions B: CpCo(CO)2, decalin,
190 °C, N2, microwave, 10 min. d Conditions C: Co2(CO)8 (0.5 equiv),
C5H6 (2 equiv), decalin, 5 min at 40 °C, 5 min at 190 °C.
Scheme 4. Synthesis of 7-Ring Lactone Fused Metallocenes
Co2(CO)8, respectively. However, we are not aware of any
previous reports on the application of microwave conditions to
the synthesis of an organometallic species from a metal carbonyl
starting material.
Figure 1. Representation of the X-ray crystal structure of 9a.
Selected bond lengths (Å): C(6)-O(1) ) 1.247(6), C(7)-C(6) )
1.482(8), C(8)-C(7) ) 1.415(7), C(9)-C(8) ) 1.423(8), C(10)-
C(9) ) 1.438(7), C(6)-C(10) ) 1.504(7).
The isopropyl-substituted diyne 6b gave improved diaste-
reoselectivity under both sets of conditions (entries 3 and 4),
the identity of the major isomer 9b being confirmed by the
similarity of the 1H NMR spectrum to that of 9a. As cyclopen-
tadienylcobalt dicarbonyl is generated from the reaction between
dicobalt octacarbonyl and cyclopentadiene,8 combination of
these last two reagents with 6b followed by microwave
irradiation also resulted in the formation of the cyclopentadi-
enone metallocenes (entry 5). Following isolation of the minor
diastereoisomer 10b by column chromatography, microwave
irradiation of a solution of this in decalin at 190 °C resulted in
no change into 9b, a result which suggests that the diastereo-
selectivity observed is a consequence of kinetic control of
metallocene formation.
separation by chromatography, the identity of the major product
as a cyclopentadienone metallocene was established by a X-ray
crystal structure determination (Figure 1).7
The minor product was assigned as the diastereoisomer 10a,
due to the similarity of both the color and the 1H NMR spectrum
to those of 9a and the presence of a band at 1581 cm-1 in the
infrared spectrum indicative of a complexed cyclopentadienone
carbonyl group. The corresponding cyclobutadiene complexes
were not observed, even when the reaction was repeated while
being irradiated with light, conditions which have previously
been used to isolate cyclobutadiene complexes.6a,8
These complexation procedures were also applied to the ester-
linked diynes 8a,b (Scheme 4). With a methyl R substituent
the same major diastereoisomer was obtained from both the heat-
and microwave-assisted complexation reactions, albeit in rather
modest yield (Table 2, entries 1 and 2). Following purification
of the major product by recrystallization, its identity as the
cyclopentadienone complex 11a was established by an X-ray
crystal structure determination (Figure 2).11 As observed in the
previous example, the preferred diastereoisomer is that with the
methyl group oriented away from the cobalt-cyclopentadienyl
moiety. Again the corresponding cyclobutadiene complexes
were not observed. Better yields were obtained with the
isopropyl-substituted derivative (entries 3 and 4), and as before,
the major diastereoisomer 11b was assigned by comparison of
NMR data. This complex was also synthesized in low yield
using dicobalt octacarbonyl as the starting material (entry 5).
To illustrate the viability of this protocol for the synthesis of
nonracemic metallocenes, we employed the propargylic alcohol
(S)-4b12 of 72% ee, obtained in three steps following the
The reaction time was shortened to only 10 min on microwave
irradiation of a reaction mixture in decalin at 190 °C. Signifi-
cantly, these conditions resulted in little erosion of diastereo-
selectivity (entry 2). In contrast, microwave irradiation of
otherwise identical decalin, toluene, THF, or acetonitrile solu-
tions up to 140 °C for 10 min resulted in yields of less than
10% and, for the last three solvents, in essentially no diaste-
reoselectivity. Microwave irradiation has previously been
employed to shorten the reaction times of the Do¨tz9 and
Pauson-Khand10 annulation reactions employing Cr(CO)6 and
(6) (a) Bradley, A.; Motherwell, W. B.; Ujjainwalla, F. Chem. Commun.
1999, 917. (b) Phannarath, P.; Aubert, C.; Malacria, M. Tetrahedron Lett.
1998, 39, 1561. (c) Eckenberg, P.; Groth, U. Synlett 2003, 2188.
(7) Crystal data for 9a: C25H21CoO2‚1/6CH3OH, Mr ) 417.69, trigonal,
a ) 33.155(5) Å, b ) 33.155(5) Å, c ) 9.4226(7) Å, R ) 90.00°, â )
90.00°, γ ) 120.00°, V ) 8970(2) Å3, space group R3 (No. 146), Z ) 18,
Dc ) 1.392 Mg/m3, µ ) 0.880 mm-1, 13 654 reflections measured, 3277
unique reflections with Rint ) 0.0826, T ) 120(2) K, final R indices for
I > 2σ(I) R1 ) 0.0630, wR2 ) 0.1483 and for all data R1 ) 0.1380,
wR2 ) 0.1829.
(8) The reaction between cyclopentadienylcobalt dicarbonyl and diphen-
ylacetylene under thermal conditions results in the formation of both the
corresponding cyclobutadiene (46%) and the cyclopentadienone (10%)
complexes; see: Rausch, M. D.; Genetti, R. A. J. Org. Chem. 1970, 35,
3888.
(9) Hutchinson, E. J.; Kerr, W. J.; Magennis, E. J. Chem. Commun. 2002,
2262.
(11) Crystal data for 11a: C31H23CoO3, Mr ) 502.42, triclinic, a )
10.512(3) Å, b ) 10.734(3) Å, c ) 11.473(3) Å, R ) 68.257(3)°, â )
71.825(3)°, γ ) 81.176(4)°, V ) 1141.4(5) Å3, space group P1 (No. 1),
Z ) 2, Dc ) 1.462 Mg/m3, µ ) 0.785 mm-1, 8280 reflections measured,
3868 unique reflections with Rint ) 0.0674, T ) 120(2) K, final R indices
for I > 2σ(I) R1 ) 0.0537, wR2 ) 0.1146 and for all data R1 ) 0.1060,
wR2 ) 0.1375.
(10) Fischer, S.; Groth, U.; Jung, M.; Schneider, A. Synlett 2002, 2023.

Notes
Organometallics, Vol. 25, No. 11, 2006 2901
cobalt metallocenes. Furthermore, the method is applicable to
the synthesis of nonracemic complexes. In view of the variety
of methods now available for the enantioselective synthesis of
propygylic alcohols,14 and the potential modularity of subsequent
diyne syntheses, this methodology is applicable to the synthesis
of a variety of scalemic planar chiral metallocenes for applica-
tion in asymmetric catalysis and materials chemistry.
Experimental Section
Petroleum ether refers to that fraction boiling in the range 40-
60 °C. THF was distilled from sodium benzophenone ketyl and
toluene from sodium wire. Silica gel (40-63 µm) was used for
chromatography. Butyllithium was used as a 2.5 M solution in
hexanes. Microwave reactions were carried with a CEM Discover
instrument (300 W). Thin-layer chromatography was performed on
silica gel 60 F254 plates (Merck).
Figure 2. Representation of the X-ray crystal structure of 11a.
Selected bond lengths (Å): C(1)-O(1) ) 1.233(5), C(1)-C(2) )
1.486(6), C(2)-C(3) ) 1.431(6), C(3)-C(4) ) 1.437(6), C(4)-
C(5) ) 1.434(6), C(5)-C(1) ) 1.476(6).
General Procedure for the Synthesis of 9-12 via Thermal
Complexation using CoCp(CO)2: Conditions A. To a stirred
solution of the required ether (6a or 6b) or ester (8a or 8b) (1.2
equiv) in dry toluene (5 mL/mmol of 6a,b) under nitrogen was
added CoCp(CO)2 (1 equiv) via syringe, and the mixture was then
heated at reflux for 18 h. After it was cooled, the reaction mixture
was filtered through a plug of silica gel, washing through with 30%
EtOAc/70% petroleum ether. The solvent was removed in vacuo
Scheme 5. Representative Transformations of the
Cyclopentadienone Complexes 9b and 11a
1
and the ratio of diastereoisomers determined by H NMR spec-
troscopy.
General Procedure for the Synthesis of 9-12 via Microwave
Complexation using CoCp(CO)2: Conditions B. To a dry
microwave tube containing a magnetic bean was added CoCp(CO)2
(1 equiv), the required ether (6a or 6b) or ester (8a or 8b) (1.2
equiv), and decalin (5 mL/mmol of 6a,b). The tube was sealed
and the reaction mixture microwaved for 10 min at 190 °C (ramp
time ca. 5 min, hold time 10 min). The reaction mixture was then
filtered through a short plug of silica gel, first with petroleum ether
as eluent, followed by 5-10% MeOH/90-95% ethyl acetate. The
solvent was removed in vacuo and the ratio of diastereoisomers
enantioselective addition of 2-methyl-3-butyn-2-ol to isobu-
tyraldehyde.13 Following conversion into (S)-8b, as summarized
in Scheme 2, microwave-assisted complexation with CpCo(CO)2
gave a 3.8:1 ratio of diastereoisomers in 38% yield. The major
and minor isomers were shown by HPLC to have enantiomeric
excesses of 74% and 72%, respectively. This proves that no
erosion of absolute stereochemistry occurs during conversion
to the metallocene, making this procedure suitable for the
synthesis of scalemic complexes containing both central and
planar elements of chirality.
Finally, two aspects of the chemistry of these new cyclopen-
tadienone complexes were explored in the context of making
new metallocene derivatives. First, addition of gaseous HCl to
9b dissolved in ether resulted in precipitation of 13 (Scheme
5). As far as we are aware, this is the first example of a planar
chiral hydroxycobaltocenium salt, which in contrast to most
isoelectronic hydroxyferrocene derivatives is stable on exposure
to atmospheric oxygen. Second, reaction of a 3:1 mixture of
11a and 12a with methanol gave the ring-opened ester 14,
unchanged as a 3:1 mixture of diastereoisomers. Related motifs
containing a hydroxy-substituted stereogenic center R to a
metallocene have proven very valuable in ferrocene chemistry
for the synthesis of a wide variety of ligands.
1
determined by H NMR spectroscopy.
General Procedure for the Synthesis of 9-12 via Microwave
Complexation using Co2(CO)8 and Cyclopentadiene: Conditions
C. In a dry microwave tube containing a magnetic bean was added
Co2(CO)8 (1 equiv), freshly cracked cyclopentadiene (4 equiv), and
decalin (2.5 mL/mmol of Co2(CO)8). The tube was sealed and the
reaction mixture microwaved for 5 min at 40 °C. The required ether
(6a or 6b) or ester (8a or 8b) (2.4 equiv) was placed into the
reaction mixture, which was then microwaved for a further 10 min
at 190 °C. The reaction mixture was then filtered through a short
plug of silica gel, first with petroleum ether as eluent, followed by
5-10% MeOH/90-95% ethyl acetate. The solvent was removed
in vacuo and the ratio of diastereoisomers determined by 1H NMR
spectroscopy.
9a/10a: conditions A, use of 6a (0.160 g, 0.61 mmol) gave a
4:1 ratio of 9a and 10a isolated as a red crystalline solid (0.155 g,
61%); conditions B, use of 6a (0.160 g, 0.61 mmol) gave a 3.2:1
ratio of 9a and 10a (0.173 g, 68%). Subsequent chromatography
(30% EtOAc/70% petroleum ether; Rf(major isomer) ) 0.32,
Rf(minor isomer) ) 0.26) resulted in separation of the two
diastereoisomers.
Conclusion
We have demonstrated that the reaction between ether- or
ester-linked chiral diynes and cyclopentadienylcobalt dicarbonyl
results in a one-pot diastereoselective synthesis of planar chiral
1
9a: mp 126-127 °C; IR (CH2Cl2) νmax 1581 cm-1; H NMR
(δ, 270 MHz, CDCl3) 1.36 (3 H, d, J ) 6.2 Hz), 4.58 (5 H, s),
4.86 (1 H, d, J ) 12.4 Hz), 5.14 (1 H, d, J ) 12.4 Hz), 5.68 (1 H,
(12) Addition of 2-methyl-3-butyn-2-ol to isobutyraldehyde in the
presence of Zn(OTf)2, NEt3, and (-)-N-methylephedrine (lit.13 ee ) 98%),
was followed by benzoylation, fragmentation to the primary acetylene, and
Sonagashira cross-coupling with iodobenzene. See the Supporting Informa-
tion.
(13) Boyall, D.; Lo´pez, F.; Sasaki, H.; Frantz, D.; Carreira, E. M. Org.
Lett. 2000, 2, 4233.
(14) (a) Parker, K. A.; Ledeboer, M. W. J. Org. Chem. 1996, 61, 3214.
(b) Matsumura, K.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem.
Soc. 1997, 119, 8738. (c) Tao, B.; Ruble, J. C.; Hoic, D. A.; Fu, G. C. J.
Am. Chem. Soc. 1999, 121, 5091. (d) Schubert, T.; Hummel, W.; Kula,
M.-R.; Mu¨ller, M. Eur. J. Org. Chem. 2001, 4181. (e) Li, X.; Lu, G.; Kwok,
W. H.; Chan, A. S. C. J. Am. Chem. Soc. 2002, 124, 12636.

2902 Organometallics, Vol. 25, No. 11, 2006
Notes
q, J ) 6.7 Hz), 7.36-7.39 (6 H, m), 8.05 (2 H, d, J ) 8.2 Hz),
8.17 (2 H, d, J ) 7.9 Hz); 13C NMR (δ, 100 MHz, CDCl3) 19.7,
67.1, 70.7, 72.5, 76.4, 84.4, 91.7, 96.3, 127.4, 127.4, 127.7, 127.8,
128.8, 128.9, 134.5, 134.5, 159.0; high-resolution MS (m/z, FAB)
found for MH+ 413.0949, calcd for C25H22CoO2 413.0946.
7.31-7.47 (7 H, m), 7.80-7.94 (5 H, m); 13C NMR (δ, 100 MHz,
CDCl3) 18.0, 20.9, 29.3, 74.8, 80.7, 81.7, 84.6, 85.3, 92.6, 128.1,
128.2, 128.6, 128.8, 129.0, 130.0, 130.1, 130.4, 131.3, 131.6, 131.7,
131.9, 132.7, 134.0, 157.9, 170.2; high-resolution MS (m/z, EI)
found for MH+ 531.1365, calcd for C35H28CoO3 531.1365.
12b: 1H NMR (δ, 270 MHz, CDCl3) 0.32 (3 H, d, J ) 6.2 Hz),
0.92 (3 H, d, J ) 6.2 Hz), 2.38-2.50 (1 H, m), 4.05 (1 H, d, J )
9.7 Hz), 4.92 (5 H, s), 6.97 (1 H, d, J ) 7 Hz), 7.15-7.95 (13 H,
m).
10a: IR (CH2Cl2) νmax 1581 cm-1 1H NMR (δ, 270 MHz,
;
CDCl3) 1.75 (3 H, d, J ) 6.2 Hz), 4.68 (1 H, d, J ) 12.9 Hz), 4.73
(5 H, s), 4.94 (1 H, q, J ) 6.2 Hz), 5.13 (1 H, d, J ) 12.6 Hz),
7.31-7.42 (6 H, m), 7.77 (2 H, dd, J ) 7.4, 1.5 Hz), 8.09 (2 H,
dd, J ) 7.9, 1.7 Hz).
Synthesis of 13. Hydrogen chloride gas was bubbled through a
red solution of 9b (0.050 g, 0.11 mmol) in ether, resulting in the
immediate precipitation of a yellow solid that was filtered, washed,
and dried to give 13 (0.054 g, >99%): mp 160-164 °C; IR (Nujol)
9b/10b: conditions A, use of 6b (0.350 g, 1.21 mmol) gave a
5:1 ratio of 9b and 10b isolated as a red crystalline solid (0.252 g,
47%); conditions B, use of 6b (0.160 g, 0.55 mmol) gave a 5:1
ratio of 9b and 10b (0.120 g, 49%); conditions C, use of 6b (0.160
g, 0.55 mmol) gave a 3.5:1 ratio of 9b and 10b (0.082, 34%).
Subsequent chromatography (30% EtOAc/70% petroleum ether;
Rf(major isomer) ) 0.55, Rf(minor isomer) ) 0.49) resulted in
separation of the two diastereoisomers.
ν
max 3425 cm-1; 1H NMR (δ, 400 MHz, DMSO) 0.60 (3 H, d, J )
6.7 Hz), 0.81 (3 H, d, J ) 6.9 Hz), 1.65-1.78 (1 H, m), 4.80 (1 H,
d, J ) 12.9 Hz), 5.05 (5 H, s), 5.32 (1 H, d, J ) 13.4 Hz), 5.56 (1
H, d, J ) 3.9 Hz), 7.41-7.51 (6 H, m), 7.99-8.20 (4 H, m); 13C
NMR (δ, 100 MHz, DMSO) 16.5, 19.1, 32.2, 67.4, 82.6, 85.0, 85.8,
94.0, 95.9, 128.1, 128.2, 128.3, 128.5, 128.6, 128.7, 131.9, 132.0
(2 signals coincident); high-resolution MS (m/z, EI) found for
M - Cl+ 441.1259, calcd for C27H26CoO2 441.1259.
1
9b: mp 65-67 °C; IR (CH2Cl2) νmax 1581 cm-1; H NMR (δ,
270 MHz, CDCl3) 0.65 (3 H, d, J ) 6.7 Hz), 0.97 (3 H, d, J ) 6.7
Hz), 1.86-1.93 (1 H, m), 4.59 (5 H, s), 4.83 (1 H, d, J ) 12.3
Hz), 5.20 (1 H, d, J ) 12.6 Hz), 5.42 (1 H, d, J ) 3.0 Hz), 7.24-
7.39 (6 H, m), 8.05-8.07 (4 H, m); 13C NMR (δ, 100 MHz, CDCl3)
16.5, 19.8, 33.0, 60.4, 69.6, 84.7, 85.4, 92.2, 95.2, 127.4, 127.7,
127.8, 127.9, 128.1, 128.6, 128.9, 134.5, 171.2 (1 signal coincident);
high-resolution MS (m/z, EI) found for MH+ 441.1261, calcd for
C27H26CoO2 441.1259.
10b: 1H NMR (δ, 270 MHz, CDCl3) 0.74 (3 H, d, J ) 6.9 Hz),
1.24 (3 H, d, J ) 6.4 Hz), 1.99-2.10 (1 H, m), 4.54 (1 H, d, J )
8.2 Hz), 4.70 (1 H, d, J ) 12.9 Hz), 4.80 (5 H, s), 5.19 (1 H, d,
J ) 12.9 Hz), 7.24-7.36 (6 H, m), 7.64-7.67 (2 H, m), 8.07-
8.10 (2 H, m).
Synthesis of 14. A solution of 11a and 12a (3:1 ratio, 0.054 g,
0.11 mmol) in methanol (15 mL) containing triethylamine (0.10
mL) was stirred at 50 °C for 96 h; the solution was cooled to room
temperature and poured into ether (15 mL) and dilute HCl(aq) (15
mL). The yellow aqueous layer was separated and made basic with
the addition of small portions of sodium hydrogen carbonate until
effervescence subsided. Upon addition of sodium hydrogen carbon-
ate there was a color change form yellow back to red. The basic
solution was then extracted with dichloromethane (10 mL), dried
(MgSO4), filtered, and evaporated to give 14 as a red-orange solid
as a 3:1 ratio of diastereoisomers (0.050 g, 87%): νmax 3369, 1595
1
cm-1; H NMR (δ, 270 MHz, CDCl3) 1.28 (3 H, d, J ) 6.7 Hz),
11a/12a: conditions A, use of 8a (0.190 g, 0.54 mmol) gave a
3:1 ratio of 11a and 12a isolated as a red crystalline solid (0.056
g, 21%); conditions B, use of 8a (0.190 g, 0.54 mmol) gave 11a
and 12a as a 3.4:1 ratio of diastereiosomers (0.079 g, 29%); TLC
(30% EtOAc/70% petroleum ether) Rf ) 0.25.
1.31* (3 H, d, J ) 6.7 Hz), 2.43* (1 H, brd, J ) 8 Hz), 2.54 (1 H,
brs), 3.47 (3 H, s), 3.48* (3 H, s), 4.76-4.90 (1 H, m), 5.01 (5 H,
s), 7.05-7.20 (3 H, m), 7.33-7.51 (6 H, m), 7.60-7.68 (2 H, m),
7.96-8.01 (2 H, m), 8.18* (1 H, d, J ) 8.4 Hz), 8.22 (1 H, d, J )
7.7 Hz) (asterisks denote distinct signals for minor diastereoisomer);
13C NMR (δ, 100 MHz, CDCl3) 22.3, 52.3, 64.9, 77.7, 81.4, 84.5,
94.0, 95.4, 127.1-135.5, 158.1, 167.5 (major diastereoisomer only);
high-resolution MS (m/z, EI) found for MH+ 535.1318; calcd for
C32H28CoO4 535.1314.
1
11a: mp 137-138 °C; IR (CH2Cl2) νmax 1705, 1595 cm-1; H
NMR (δ, 400 MHz, CDCl3) 1.42 (3 H, d, J ) 6.9 Hz), 4.93 (5 H,
s), 5.60 (1 H, q, J ) 6.9 Hz), 7.22-7.91 (14 H, m); 13C NMR (δ,
100 MHz, CDCl3) 21.0, 71.1, 85.5, 127.8, 128.2, 128.6, 129.0,
129.1, 130.5, 131.2, 131.3, 131.7, 131.9, 132.1, 132.3, 169.9 (7
signals coincident); m/z (ES) found for MH+ 503.1053, calcd for
C31H24CoO3 503.1052.
12a: 1H NMR (δ, 400 MHz, CDCl3) 0.92 (3 H, d, J ) 6.9 Hz),
4.98 (6 H, brs) 7.22-7.91 (16 H, m). The ratio 11a to 12a may
also be determined by 13C NMR (11a δ 85.50 (C5H5), 12a δ 85.25
(C5H5)).
Acknowledgment. We thank the EPSRC and StylaCats for
financial support (C.J.T.) and the EPSRC National Crystal-
lography Service (Southampton University) for data collection.
We are also indebted to Greg Coumbarides for assistance with
NMR experiments and the EPSRC National Mass Spectrometry
Service Centre, University of Wales, Swansea, U.K.
11b/12b: conditions A, use of 8b (0.200 g, 0.53 mmol) gave a
4:1 ratio of 11b and 12b isolated as a red crystalline solid (0.093
g, 33%); conditions B, use of 8b (0.200 g, 0.53 mmol) gave a 3:1
ratio of 11b and 12b (0.16 g, 57%); conditions C, use of 8b (0.200
g, 0.53 mmol) gave a 5:1 ratio of 11b and 12b (0.03, 11%); TLC
(30% EtOAc/70% petroleum ether) Rf(major isomer) ) 0.24,
Rf(minor isomer) ) 0.20).
Supporting Information Available: Text giving details of the
synthesis of 4a,b-8a,b, (S)-4b, and (S)-8b and the subsequent
reaction of the last species with CpCo(CO)2, together with text and
figures giving details of the X-ray crystal structure determinations
of 9a and 11a; crystallographic data for 9a and 11a are given as
CIF files. This material is available free of charge via the Internet
at http://pubs.acs.org.
11b: mp 214-216 °C; IR (CH2Cl2) 1705, 1595 cm-1; 1H NMR
(δ, 270 MHz, CDCl3) 0.21 (3 H, d, J ) 6.7 Hz), 0.85 (3 H, d, J )
6.7 Hz), 1.50-1.65 (1 H, m), 4.86 (1 H, d, J ) 11.1 Hz), 4.90 (5
H, s), 6.93 (1 H, d, J ) 7.4 Hz), 7.18 (1 H, td, J ) 7.4, 1.5 Hz),
OM060015R