Design of new hemilabile (P,S) ligands for the highly diastereoselective coordination to alkyne dicobalt complexes: Application to the asymmetric intermolecular Pauson-Khand reaction

Article abstract of DOI:10.1021/om021008f

Here we synthesized a new generation of hemilabile P,S ligands designed for diastereoselective coordination to terminal alkyne-hexacarbonyldicobalt complexes. The camphor-derived CamPHOS (2) and MeCamPHOS (12) ligands were obtained in 68% and 59% yield in their borane-protected form starting from readily available 1,3-oxathiane 4. Thermally induced coordination to dicobalt-hexacarbonyl-alkyne (HCCR, R = Ph, Bu^t, SiMe₃, C(CH₃)₂OH) complexes was studied. MeCamPHOS (12) provided an unprecedented diastereoselectivity on coordination to terminal complexes (up to 90% de). The solid state structure of a coordinated CamPHOS ligand with [Co₂(μ-Bu^tSO₂C)₂(CO)₆] was determined and confirms the preference to coordinate the bimetallic cluster in a bridged manner. The resulting adducts were submitted to the intermolecular Pauson-Khand cyclization. Remarkably, CamPHOS and MeCamPHOS provided Pauson-Khand products of opposite absolute configurations. This behavior could be explained on the basis that 2 and 11 ligands lead to pseudoenantiomeric tetracarbonyl complexes, as confirmed by circular dichroism analysis. Introduction of a CH₃- group on the carbon bridge between P and S increased CamPHOS selectivity dramatically from 33% to 90% de of the opposed sign with an overall destabilizing effect of 2.15 kcal/mol for 7b.

Full text of DOI:10.1021/om021008f

1
868
Organometallics 2003, 22, 1868-1877
Design of New Hem ila bile (P ,S) Liga n d s for th e High ly
Dia ster eoselective Coor d in a tion to Alk yn e Dicoba lt
Com p lexes: Ap p lica tion to th e Asym m etr ic
In ter m olecu la r P a u son -Kh a n d Rea ction
,
†
†
,†
Xavier Verdaguer,* Miquel A. Peric a` s, Antoni Riera,*
‡
‡
Miguel A. Maestro, and J os e´ Mah ´ı a
Unitat de Recerca en S ´ı ntesi Asim e` trica (URSA-PCB), Parc Cient ı´ fic de Barcelona,
and Departament de Qu ı´ mica Org a` nica, Universitat de Barcelona, J osep Samitier 1-5,
E-08028 Barcelona, Spain, and Servicios Xerais de Apoio a´ Investigaci o´ n,
Campus Zapateria, s/ n, Universidade da Coru n˜ a, E-15071 A Coru n˜ a, Spain
Received December 16, 2002
Here we synthesized a new generation of hemilabile P,S ligands designed for diastereo-
selective coordination to terminal alkyne-hexacarbonyldicobalt complexes. The camphor-
derived CamPHOS (2) and MeCamPHOS (12) ligands were obtained in 68% and 59% yield
in their borane-protected form starting from readily available 1,3-oxathiane 4. Thermally
t
induced coordination to dicobalt-hexacarbonyl-alkyne (HCCR, R ) Ph, Bu , SiMe
3
, C(CH
3
)
2
-
OH) complexes was studied. MeCamPHOS (12) provided an unprecedented diastereoselec-
tivity on coordination to terminal complexes (up to 90% de). The solid state structure of a
t
coordinated CamPHOS ligand with [Co
2
(µ-Bu SO
2
C)
2
(CO)
6
] was determined and confirms
the preference to coordinate the bimetallic cluster in a bridged manner. The resulting adducts
were submitted to the intermolecular Pauson-Khand cyclization. Remarkably, CamPHOS
and MeCamPHOS provided Pauson-Khand products of opposite absolute configurations.
This behavior could be explained on the basis that 2 and 11 ligands lead to pseudo-
enantiomeric tetracarbonyl complexes, as confirmed by circular dichroism analysis. Introduc-
tion of a CH
3
- group on the carbon bridge between P and S increased CamPHOS selectivity
dramatically from 33% to 90% de of the opposed sign with an overall destabilizing effect of
2
.15 kcal/mol for 7b.
In tr od u ction
Sch em e 1
Diastereoselective ligand exchange reaction on hexa-
carbonyl complexes of terminal alkynes is an elusive
goal. Cobalt atoms from dicobalt hexacarbonyl com-
plexes of terminal alkynes are enantiotopic. Therefore,
substitution of one cobalt atom for another metal or
simply substitution of one CO for another ligand in one
of the metal centers leads to chiral bimetallic complexes
coordination of an appropriate chiral bidentate phos-
phine. However, none of these methodologies has found
(Scheme 1). To date only phosphines and phosphites
4
have been used for this purpose. Optically pure phos-
phine dicobalt hexacarbonyl complexes have been se-
lectively synthesized in a number of ways: (a) using
wide application because of several drawbacks; for
example, (a) coordination selectivities are low, usually
close to 1:1, and separation of diastereomeric complexes
is often difficult, (b) thermally induced phosphine
isomerization can take place even at low temperatures,
and (c) phosphine- or phosphite-substituted dicobalt
complexes show lower reactivity than their parent
1
acetylenes with an adjacent chiral center, (b) using
2
monophosphines that contain chiral appendages, (c)
enantioselective coordination of an achiral phosphine
3
induced with an external chiral promoter, or (d)
4
a,5
†
complexes.
Universitat de Barcelona.
Universidade da Coru n˜ a.
‡
Optically pure dicobalt hexacarbonyl complexes have
synthetic applications, mainly in the development of
(
1) (a) Bradley, D. H.; Khand, M. A.; Nicholas, K. M. Organometallics
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1
(
(4) (a) Derdau, V.; Laschat, S.; Dix, I. Organometallics 1999, 18,
3859-3864. (b) Hiroi, K.; Wantanabe, T.; Kawagishi, R.; Abe, I.
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1
Asymmetry 1990, 1, 711-714. (c) Hay, A. M.; Kerr, W. J .; Kirk, G. G.;
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1
0.1021/om021008f CCC: $25.00 © 2003 American Chemical Society
Publication on Web 03/27/2003

Design of New Hemilabile (P,S) Ligands
Organometallics, Vol. 22, No. 9, 2003 1869
6
enantioselective versions of the Pauson-Khand and
7
Nicholas reactions. Accordingly, when a preparative
use of these compounds is in mind, the reactivity toward
these reactions is crucial. The use of chiral auxiliaries
in the Pauson-Khand reaction has been extensively
8
studied by our group. We have previously introduced
highly stereoselective inductors (Figure 1, II) capable
of accelerating cyclopentenone formation by coordinat-
ing a hemilabile sulfide ligand.9 To find easy-to-handle
systems that can be used as promoters, we recently
developed a new chiral bidentate (P,S) ligand derived
from natural pulegone.11 This approach allowed us to
obviate the cumbersome steps needed for the introduc-
tion and elimination of a chiral auxiliary and demon-
strate that a hemilabile P,S ligand accelerates the
Pauson-Khand reaction. Nevertheless, despite achiev-
ing a significant level of selectivity (up to 60% de), the
coordination diastereoselectivity to dicobalt complexes
still has a major practical limitation.
F igu r e 1.
,10
F igu r e 2.
Sch em e 2
Here we report the synthesis of a new generation of
P,S ligands, their coordination behavior toward terminal
alkyne dicobalt carbonyl complexes, and the structural
features that lead to a highly diastereoselective coor-
dination. Finally, the application of these ligands to the
intermolecular asymmetric Pauson-Khand reaction is
also discussed.
Resu lts a n d Discu ssion
Design , Syn th esis, a n d Coor d in a tion Beh a vior
of Ca m P HOS. An important feature for the design of
a new hemilabile bidentate ligand is to ensure coordina-
tion in a bridged (III) rather than chelated (IV) disposi-
tion in the dicobalt-alkyne cluster (Figure 1). We
envisaged that the coordination of the sulfur and
phosphorus to different cobalt atoms is crucial for the
tern, the length between the two coordinative atoms is
a key point. One carbon link between phosphorus and
sulfur guarantees bridged type coordination (III).12 In
PuPHOS ligand (1), the sulfur atom is contained in an
1
,3-oxathiane ring and the diaryl phosphine unit is
desired directing and accelerating effect in the Pauson-
Khand cyclization.10 To achieve this coordination pat-
linked to the thioacetal carbon in an equatorial posi-
1
1
tion. This structural motif provides the bridged tet-
racarbonyldicobalt complexes desired, but with low
diastereoselectivities. We therefore assumed that the
flat bicyclic backbone of pulegone was not the optimum
scaffold for the differentiation of diastereotopic cobalt
atoms in the bimetallic cluster. In an effort to find a
more selective ligand, we reasoned that the camphor
skeleton, bearing an out-of-the-plane gem-dimethyl
group, could provide an extended diasteremeric bias
compared to pulegone, and we therefore synthesized
CamPHOS (2).The synthesis of CamPHOS (Scheme 2)
started from commercially available (+)-camphorsul-
fonic acid. Thus, 3 was transformed into 1,3-oxathiane
4 in three steps, following procedures described in the
(
6) (a) Buchwald, S. L.; Hicks, F. A. In Comprehensive Asymmetric
Catalysis I-III; J acobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
Springer: Berlin, 1999; pp 491-510. (b) Geis, O.; Schmalz, H.-G.
Angew. Chem., Int. Ed. 1998, 37, 911-914. (c) Sugihara, T.; Yamagu-
chi, M.; Nishizawa, M. Chem. Eur. J . 2001, 7, 1589-1595. (d)
Brummond, K. M.; Kent, J . L. Tetrahedron 2000, 56, 3263-3283.
(
7) (a) Bradley, D. H.; Khand, M. A.; Nicholas, K. M. Organometallics
992, 11, 2598-2607. (b) Schreiber, S. L.; Sammakia, T.; Crowe, W.
E. J . Am. Chem. Soc. 1986, 108, 3128-3130.
8) (a) Verdaguer, X.; Moyano, A.; Peric a` s, M. A.; Riera, A.; Greene,
A. E.; Piniella, J . F.; Alvarez-Larena, A. J . Organomet. Chem. 1992,
33, 305-310. (b) Tormo, J .; Moyano, A.; Peric a` s, M. A.; Riera, A. J .
1
(
4
Org. Chem. 1997, 62, 4851-4856. (c) Montenegro, E.; Moyano, A.;
Peric a` s, M. A.; Riera, A.; Alvarez Larena, A.; Piniella, J . E. Tetrahe-
dron: Asymmetry 1999, 10, 457-471. (d) Montenegro, E.; Poch, M.;
Moyano, A.; Peric a` s, M. A.; Riera, A. Tetrahedron 1997, 53, 8651-
8
664. (e) Fonquerna, S.; Moyano, A.; Peric a` s, M. A.; Riera, A. J . Am.
Chem. Soc. 1997, 119, 10225-10226. (f) Castro, J .; Sorensen, H.; Riera,
A.; Morin, C.; Moyano, A.; Peric a` s, M. A.; Greene, A. E. J . Am. Chem.
Soc. 1990, 112, 9388-9389.
13
literature. Metalation of 4 at the 2 position was carried
out with sec-BuLi at -78 °C since n-BuLi is not
1
4
(9) (a) Verdaguer, X.; Moyano, A.; Peric a` s, M. A.; Riera, A.; Ber-
appropriate in this case. Phosphinylation of 2-lithio
camphorly-1,3-oxathiane with chlorodiphenylphosphine
at -78 °C afforded a single 2-phosphino-1,3-oxathiane.
On the basis of related alkylation of 1,3-oxathianes, we
assumed that the absolute configuration was S (equato-
nardes, V.; Greene, A. E.; Alvarez-Larena, A.; Piniella, J . F. J . Am.
Chem. Soc. 1994, 116, 2153-2154. (b) Montenegro, E.; Poch, M.;
Moyano, A.; Peric a` s, M. A.; Riera, A. Tetrahedron Lett. 1998, 39, 335-
3
38. (c) Bernardes, V.; Kann, N.; Riera, A.; Moyano, A.; Peric a` s, M.
A.; Greene, A. E. J . Org. Chem. 1995, 60, 6670-6671. (d) Marchueta,
I.; Montenegro, E.; Panov, D.; Poch, M.; Verdaguer, X.; Moyano, A.;
Peric a` s, M. A.; Riera, A. J . Org. Chem. 2001, 66, 6400-6409.
(
10) For ligand-accelerated Pauson-Khand reactions see: (a) Krafft,
(12) Edwards, A. J .; Mack, S. R.; Mays, M. J .; Mo, C. I.; Raithby, P.
R.; Rennie, M. A. J . Organomet. Chem. 1996, 519, 243-252.
(13) DeLucchi, O.; Lucchini, V.; Marchiorio, C. J . Org. Chem. 1986,
51, 1457-1466.
(14) (a) Eliel, E. L.; Frazee, W. L. J . Org. Chem. 1979, 44, 3598-
3599. (b) Fuji, K.; Ueda, M.; Sumi, K.; Kajiwara, K.; Fujita, E.;
Iwashita, T.; Miura, I. J . Org. Chem. 1985, 50, 657-661.
M. E. J . Am. Chem. Soc. 1988, 110, 968-70. (b) Krafft, M. E.; J uliano,
C. A.; Scott, I. L.; Wright, C.; McEachin, M. D. J . Am. Chem. Soc. 1991,
1
13, 1693-703. (c) Krafft, M. E.; Scott, I. L.; Romero, R. H. Tetrahedron
Lett. 1992, 33, 3829-32.
11) Verdaguer, X.; Moyano, A.; Peric a` s, M. A.; Riera, A.; Maestro,
M. A.; Mahia, J . J . Am. Chem. Soc. 2000, 122, 10242-10243.
(

1
870 Organometallics, Vol. 22, No. 9, 2003
Verdaguer et al.
Ta ble 1. Rea ction s of Ca m P HOS w ith Ter m in a l Alk yn e Dicoba lt Hexa ca r bon yl Com p lexes
entry
R
starting complex
T (°C)
time (h)
yield (%)
dra
productc
1
2
8
Ph
6a
6b
65
60
90
80
24
20
2
85
98
96
90
1.5:1
1:1
1.5:1
2:1
7a /8a
7b/8b
7b/8b
7b/8b
t-Bu
t-Bu
t-Bu
b
7b/8b (1:1)
7b/8b (1.5:1)
b,d
4
66
a
Established by 1H MNR spectroscopy of the resulting mixture of complexes. Isomerization experiments were run with no added
b
extra base or ligand. Depicted absolute configurations of major/minor isomers are arbitrary. d Isomerization was conducted under CO.
c
Sch em e 3
rial position). As a result of the electron-releasing
oxathiane group, the new phosphine was highly prone
to oxidation. Consequently, the new bidentate ligand
was more conveniently isolated as its borane complex.
Quenching the reaction mixture with borane-dimethyl-
sulfide afforded BH3-CamPHOS (5) in 68% yield (from
4
) as a white shelf-stable crystalline solid (Scheme 2).
Removal of the borane-phosphine protecting group
can be done either by treatment with a nucleophilic
amine or with a strong acid (TfOH, HBF4).15 In the case
of BH3-CamPHOS, reaction with 2 equiv of diazabicyclo-
Cr ysta l Str u ctu r e of Coor d in a ted Ca m P HOS. In
contrast to our initial expectations, CamPHOS showed
a lower diastereomeric excess than its PuPHOS prede-
cessor. To gain further insight into the parameters that
favor the formation of one complex with respect to its
diastereomer, analysis of complexes 7a /8a and 7b/8b
by X-ray diffraction was attempted. Unfortunately,
suitable single crystals of these compounds for X-ray
analysis could not be obtained. We therefore used the
dicobalt hexacarbonyl complex of bis-tert-butylsulfonyl-
ethyne 9, which, in our experience, tends to yield highly
crystalline solids. Accordingly, the reaction of BH3-
CamPHOS with 9 in the presence of DABCO provided
the desired tetracarbonyl adduct in 87% yield as a sole
isomer (Scheme 3). Layering hexane over a toluene
solution of 10 provided appropriate crystals for X-ray
analysis. The unit cell contained four molecules of 10
and four of toluene. The solvent fragment and carbonyl
O(3) are plagued with severe disorder. The correspond-
ing ORTEP drawing of 10 is shown in Figure 3.
Relevant crystal and structure refinement data are
shown in Table 2. Selected atomic distances and angles
are displayed in Table 3.
[3.3.0]octane (DABCO) in toluene at 50 °C provided the
free phosphine within 30 min. This allowed the depro-
tection and complexation steps to be performed in one
pot. When the dicobalt carbonyl complex of phenylacety-
lene 6a , 1.0 equiv of BH3-CamPHOS (5), and 1.5 equiv
of DABCO were mixed in toluene at 65 °C, after 30 min,
a brown intermediate was observed by TLC analysis
(SiO2), probably caused by the formation of a monoden-
tate complex. Upon further reaction, this brown complex
led to the emergence of two new purple spots. Further
heating resulted in the purple complexes being the only
visible products in the reaction mixture. Simple filtra-
1
tion on silica afforded a purple oil (85% yield). H NMR
analysis showed that the product consisted of a diaster-
eomeric mixture of complexes 7a /8a in a 1.5:1 ratio; this
was established by integration of the terminal alkyne
(doublet, H-CCCo2) and acetal (singlet, H-CPSO) proton
resonances (δ 4.70-6.00 ppm). IR analysis of the car-
bonyl region showed four absorption bands with an
average frequency (ν) of 1992 cm- , which confirmed
bridged double coordination of CamPHOS.16
1
Analogous reaction conditions (60 °C, 24 h) with tert-
butylacetylene dicobalt complex (6b) resulted in no
selectivity, yielding a 1:1 mixture of diastereomeric
tetracarbonyl complexes
First, the X-ray structure of 10 allowed us to confirm
that the phosphino group in CamPHOS is in an equato-
rial position in the 1,3-oxathiane chair ring. As expected,
coordination of CamPHOS to the bimetallic cluster
occurred in a bridged manner. The phosphorus and the
sulfur were placed in two eclipsed pseudo-equatorial
coordination sites. The cyclic sulfide was coordinated to
cobalt through its equatorial lone pair. Both metal
atoms, the two donor atoms, and the methyne in the
ligand (Co-Co-P-C-S) formed a five-membered ring
with an envelope-like conformation. It is important to
note that the hydrogen in the methyne group is close
to that of the tert-butylsulfonyl groups on the alkyne.
The P,S ligand probably adopted this coordination in
order to avoid unfavorable steric interactions between
the gem-dimethyl bridge (CMe2) in the camphor scaffold
and the bulky tert-butyl groups in the alkyne backbone.
Similarly, the tert-butylsulfonyl groups adopted a syn-
(Table 1, entry 2). This mixture, however, was isomer-
ized: raising the temperature to 90 °C for 2 h shifted
the equilibrium to a biased mixture of 1.5:1 (Table 1,
entry 3). Further heating of the 1.5:1 mixture at 80 °C
for 66 h under CO allowed the complexes to attain a
final 2:1 ratio of 7b/8b (Table 1, entry 4). Isomerization
experiments with complexes 7b/8b were run in toluene
with no added DABCO or extra ligand.
(
15) (a) Yang, H.; Lugan, N.; Mathieu, R. Organometallics 1997, 16
(
6), 2089-2095. (b) Chen, Z.; Zhu, G.; J iang, Q.; Xiao, D.; Cao, P.;
Zhang, X. J . Org. Chem. 1998, 63, 5631-5635. (c) Imamoto, T.;
Tsuruta, H.; Wada, Y.; Masuda, H.; Yamaguchi, K. Tetrahedron Lett.
1
995, 36, 8271-8274.
16) Varadi, G.; Vizi-Orosz, A.; Vastag, S.; Palyi, G. J . Organomet.
Chem. 1976, 108, 225-223.
(

Design of New Hemilabile (P,S) Ligands
Organometallics, Vol. 22, No. 9, 2003 1871
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(
d eg) for Str u ctu r e 10
Co(1)-C(2)
Co(1)-S(3)
Co(2)-C(2)
Co(2)-P(1)
S(3)-C(15)
C(1)-C(2)
Co(1)-C(1)
Co(1)-Co(2)
Co(2)-C(1)
P(1)-C(15)
1.954(2)
2.27712(9)
1.1.946(2)
2.2260(8)
1.839(3)
1.361(3)
1.914(3)
2.4665(6)
1.943(3)
1.842(3)
S(3)-Co(1)-Co(2)
95.29(2)
C(2)-Co(2)-P(1)
105.21(8)
106.79(8)
96.42(2)
105.09(9)
107.22(9)
105.4(1)
3.01(3)
1.9(2)
12.9(3)
48.0(6)
33.06(9)
C(2)-Co(1)-S(3)
P(1)-Co(2)-Co(1)
C(15)-S(3)-Co(1)
C(15)-P(1)-Co(2)
S(3)-C(15)-P(1)
S(3)-Co(1)-Co(2)-P(1)
C(13)-Co(1)-Co(2)-C(1)
C(14)-Co(1)-Co(2)-C(12)
S(1)-C(1)-C(2)-S(2)
Co(2)-Co(1)-S(3)-C(15)
F igu r e 3. ORTEP drawing of 10 showing 30% ellipsoids.
Solvent inclusion and hydrogen atoms have been omitted
for clarity. Carbonyl O(3a) corresponds to half occupancy.
Ta ble 2. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
for 10
empirical formula
cryst syst
space group
a, Å
b, Å
c, Å
C44H58Co2O9PS3
orthorhombic
P212121
15.357(2)
15.940(3)
19.197(3)
90
F igu r e 4.
R, deg
â, deg
90
90
led us to reason that increasing the steric bulk in the
carbon bridge between the sulfur and phosphorus would
have a major impact on stereoselectivity (Figure 4). The
methyne in the 1,3-oxathiane ring is in close proximity
to the diastereotopic bimetallic cluster, and the ex-
change of the small hydrogen for a larger group could
affect the selectivity of the overall process. To test this
hypothesis, we synthesized a methyl-substituted Cam-
PHOS ligand.
γ, deg
volume, Å
3
4699.4 (13)
4
1.379
Z
3
density(calcd), mg/m
F(000)
2044
cryst size, mm
θ range, deg
index ranges
0.45 × 0.20 × 0.20
1.70-26.39
-16 e h e 19
-19 e k e 12
23 e l e 23
-
29 169
no. of reflns collected
no. of ind reflns
abs corr
max. and min. transmn
refinement method
9612 [Rint ) 0.0336]
semiempirical
0.8367 and 0.6811
full-matrix least-squares on F2
9612/0/531
Borane-protected dimethylarylphosphines and dia-
rylmethylphosphines are good substrates for base-
induced deprotonation of the methyl group with n-
1
5c,18
BuLi.
Similarly, deprotonation of 5 with sec-BuLi
no. of data/restraints/params
goodness-of-fit on F2
at -78 °C led to the corresponding anion, which, in the
presence of 12-crown-4, was alkylated with methyl
iodide to furnish BH3-MeCamPHOS (11) as a white
crystalline solid in 87% yield. Alkylation with MeI
produced a single isomeric compound. We assumed that
the configuration of 11 is that depicted in Scheme 4,
where the methyl group is placed in the axial position
and the bulky boranodiphenylphosphine remains equa-
1.000
final R indices [I>2σ(I)]
R indices (all data)
abs struct param
R(F) ) 0.0332
2
Rw(F ) ) 0.0759
R(F) ) 0.0462
2
Rw(F ) ) 0.0816
-0.002(10)
-
3
largest diff peak/hole, e Å
0.269/-0.187
clinal conformation instead of an antiperiplanar disposi-
tion with respect to the cluster C-C bond. This twisted
conformation has been described for dicobalt pentacar-
bonyl complexes of 9 with sulfide ligands.17
1
9
torial in the 1,3-oxathiane ring.
Thus, we next studied the coordination behavior of
the new MeCamPHOS (12) ligand toward several dico-
balt hexacarbonyl complexes. After extended heating at
A Mod ified Ca m P HOS Liga n d : Design , Syn th e-
sis, a n d Coor d in a tion Beh a vior . To improve ligand
performance, an analysis of the crystal structure of 10
6
5 °C, the reaction of 11 with the dicobalt complex of
(18) (a) Muci, A. R.; Campos, K. R.; Evans, D. A. J . Am. Chem. Soc.
1
995, 1998, 9075-9076.
(19) Mikolajczyk, M.; Graczyk, P.; Wieczorek, M. W.; Bujacz, G.
(17) Verdaguer, X.; Moyano, A.; Peric a` s, M. A.; Riera, A.; Alvarez
Larena, A.; Piniella, J . F. Organometallics 1999, 18, 4275-4285.
Tetrahedron Lett. 1988, 29, 6801-6804.

1
872 Organometallics, Vol. 22, No. 9, 2003
Verdaguer et al.
Sch em e 4
Ta ble 4. Rea ction of MeCa m P HOS w ith Ter m in a l
Alk yn e Dicoba lt Hexa ca r bon yl Com p lexes
F igu r e 5. Subtituted dicobalt hexacarbonyl complexes.
5
0 °C. On the other hand, switching from a phenyl to a
tert-butyl group on the alkyne moiety resulted in
decreased reactivity (Table 5, entry 2). Cycloaddition of
the 2:1 mixture of 7b/8b yielded the Pauson-Khand
adduct with a loss of optical purity from a theoretical
33% to a mere 6% ee. MeCamPHOS showed a similar
trend, and a reaction of a 20:1 (90% de) mixture of 13b/
starting
complex (°C) (h)
T
time yield
entry
R
(%)
dra productc
1
2
3
4
Ph
6a
6b
6c
6d
65
65
80
65
48
18
19
18
13a /14a
20:1 13b/14b
12:1 13c/14c
13:1 13d /14d
t-Bu
SiMe3
CMe2OH
80
64
99
c
1
4b afforded 15b with only 40% ee. However, unexpect-
a
1
Established by H MNR spectroscopy of the resulting mixture
edly, the resulting cyclopentenone obtained from the
MeCamPHOS complex held the opposite configuration
of that obtained using CamPHOS. Similarly, complexes
b
of complexes. Depicted absolute configurations of major/minor
c
isomers are arbitrary. The reaction was first heated at 80 °C (3
h) and then at 70 °C (16 h).
1
3c/14c and 13d /14d , containing the MeCamPHOS
ligand, led to the corresponding levorotatory cyclopen-
tenones (Table 5, entries 4-7). Thermal activation
induced unwanted isomerization during cyclization.
Thus the reaction of 13c/14c and 13d /14d with norbor-
nadiene at 60-65 °C resulted in cyclopentenones with
a lower enantiomeric excess than their parent dicobalt
complexes. This drawback was minimized by the use of
N-methylmorpholine N-oxide (NMO) (entries 5 and 7,
Table 5). For instance, the reaction of 13c/14c (12:1 dr)
with NMO in CH2Cl2 at room temperature led to 15c
with 79% ee and practically no loss of the original
stereochemical integrity.
phenylacetylene in the presence of DABCO yielded
1
(Table 4, entry 1) a mixture of several species. H NMR
analysis of the reaction mixture revealed four main
resonances caused by the terminal alkyne hydrogen.
These were assigned as follows: one triplet to a diphos-
phino adduct, two doublets to two diasteromeric mono-
phosphino complexes in a 1:1 ratio, and a doublet to a
single bridged complex. Further heating did not lead to
the desired isomerization and caused decomposition.
However, under the same conditions, the reaction of tert-
butylethyne complex 5b with MeCamPHOS after 18 h
at 65 °C gave a mixture of bridged adducts 13b/14b in
a 20:1 (90% de) ratio (Table 4, entry 2). To our
knowledge this is the highest diastereoselectivity ob-
served in the coordination of a chiral phosphine to a
dicobalt hexacarbonyl complex of a terminal acetylene.
Moreover, the reaction of MeCamPHOS with the com-
plexes 5c and 5d bearing, respectively, a trimethylsilyl
and dimethylhydroxymethyl groups, yielded their cor-
responding tetracarbonyl complexes with remarkable
selectivity: 85% and 86% de (Table 4, entries 3 and 4).
None of the isomers for 13/14 mixtures could be isolated
by either crystallization or chromatography.
Ch ir op t ica l P r op er t ies of (µ-R CtCH )Co2(µ-P -
S)*(CO)4 Com p lexes. Having observed that CamPHOS
(2) and MeCamPHOS (12) led to intermolecular Pau-
son-Khand adducts of opposite configuration, we next
examined the absolute configuration of the bridged
complexes that arose from 2 and 12. Circular dichroism
(
CD) is an excellent tool for the study of chirally
2
0
perturbed dicobalt-alkyne clusters. We recently stud-
ied a series of alkyne complexes bearing chiral mono-
dentate phosphines and found that the absolute con-
figuration of the bimetallic cluster (i.e., whether the
phosphine was coordinated to either a pro-(S) or pro-
In ter m olecu la r P a u son -Kh a n d Rea ction s of (µ-
RCtCH)Co2(µ-P -S)*(CO)4 Type Com plexes. As pre-
viously established with PuPHOS, hemilabile ligands
are excellent reagents for the highly enantioselec-
tive intermolecular Pauson-Khand reaction.1 To test
whether the new generation of P,S ligands could serve
as inductors in the aforementioned cyclization, we
studied the reaction of a diastereomeric mixture of
complexes with norbornadiene. CamPHOS (2) adducts
were first examined (Table 5, entries 1 and 2). Dia-
steromeric-bridged complexes derived from phenylacet-
ylene (7a /8a ) were separated by flash chromatography.
The reaction of major isomer 7a with norbornadiene was
highly stereospecific and provided the corresponding
(R) cobalt atom (Figure 5, Ia and Ib)) could be directly
ascertained by means of the positive or negative CD
2
d
absorptions in the 450-650 nm region.
In this case, the C Co chromophore bearing bridged
1
2
2
P,S ligands exhibited a different coordination pattern
Figure 5, IIIa and IIIb), which should give rise to a
(
distinct CD spectra. Therefore we studied whether this
technique could be used to determine the absolute
configuration of the present tetracarbonyl complexes.
The CD spectrum of a diasteriomerically pure sample
of major 7a complex holding a coordinated CamPHOS
ligand displayed several bands between 250 and 600 nm
(Figure 6), most of which were in the positive region of
(
+)-cyclopentenone with 97% ee and 93% yield. Fur-
(
20) Kajt a´ r, M.; Kajt a´ r-Mikl o´ s, J .; Giacomelli, G.; Ga a´ l, G.; V a´ radi,
thermore, CamPHOS exhibited a great accelerating
effect since the reaction was complete within 30 min at
G.; Horov a´ th, I. T.; Zucchi, C.; P a´ lyi, G. Tetrahedron: Asymmetry 1995,
6, 2177-2194.

Design of New Hemilabile (P,S) Ligands
Organometallics, Vol. 22, No. 9, 2003 1873
Ta ble 5. P a u son -Kh a n d Cycliza tion s w ith Ca m p h or -Der ived Br id ged P ,S Liga n d s
starting
complex
entry
dra
R
conditionsb
time
yield (%)
eec
productc
1
2
3
4
5
6
7
7a
7b/8b
-
Ph
t-Bu
t-Bu
Si(CH3)3
Si(CH3)3
CMe2OH
CMe2OH
toluene/50 °C
toluene/90 °C
toluene/50 °C
C6D6/65 °C
CH2Cl2/NMO/r.t.
toluene/60 °C
CH2Cl2/NMO/rt
30 min
3 h
18 h
3 h
2 d
18 h
4 d
93
75
80
68
72
99
90
97
6
(+)-15a
(+)-15b
(-)-15b
(-)-15c
(-)-15c
(-)-15d
(-)-15d
2/1
13b/14b
13c/14c
13c/14c
13d /14d
13d /14d
20/1
12/1
12/1
13/1
13/1
40
56
79
42
50
a
Diastereomeric ratio of starting complexes. b All reactions were conducted under nitrogen with 10 equiv of norbornadiene. c Enantiomeric
excess (%) was determined either by HPLC (Chiracel-OD) or by GC (â-DEX).
F igu r e 6. CD spectra of camphor-derived P,S bridged complexes.
ꢀ. Two characteristic absorption maximums were
Sch em e 5
∆
located at 348 nm (∆ꢀ ) +38.7) and 500 nm (∆ꢀ ) +5.9).
By analogy, circular dichroism of a diastereomeric
mixture of complexes 7b/8b (2:1 ratio) displayed a
parallel CD spectrum with 7a , with all CD absorptions
in the positive area. Interestingly, the CD of a dicobalt
tetracarbonyl complex with the MeCamPHOS ligand
(13b/14b in a 20:1 ratio) showed a totally pseudo-
enantiomeric spectrum (Figure 6). Four negative ab-
sorption bands were identified at λ ) 290 nm (∆ꢀ ) -
3
4
4.1), 336 nm (∆ꢀ ) - 32.9), 386 nm (∆ꢀ ) - 9.1), and
95 nm (∆ꢀ ) - 4.6). From this chiroptical behavior,
we conclude that MeCamPHOS and CamPHOS led to
pseudo-enantiomeric C2Co2 clusters with a coordination
pattern as in IIIa and IIIb (Figure 5). Furthermore,
since the lowest energy absorption between 450 and 600
nm can be clearly attributed to transitions in the C2-
Co2(CO)4 chromophore, we propose that it can be
considered a diagnostic band to assign the configuration
of P,S bridged complexes (vide infra).
bridged ligands, the positions of P and S on each cobalt
atom determine the stereochemical course of the cy-
clization (Scheme 5). Consistent with the hypothesis
that the labile sulfur ligand leaves a vacant coordination
site for the incoming olefin (norbornadiene), the alkene
coordination is directed to the metal center which is not
bonded to phosphorus, as occurs for monodentate ligands.
According to this model, dextrorotatory cyclopentenones
produced when CamPHOS is used entail attachment of
the phosphorus to the pro-R cobalt center (Scheme 5).
Conversely, MeCamPHOS led to levorotatory Pauson-
Khand products that arise from phosphine coordination
at the pro-S metal. In further agreement with this
model, the sign of the CD bands in the 450-600 region
and the topicity of phosphorus coordination coincided
Esta blish m en t of th e Absolu te Con figu r a tion of
Com p lexes 7/8 a n d 13/14. The absolute configuration
of the dicobalt tetracarbonyl complexes with P,S ligands
can be ascertained from the corresponding Pauson-
Khand products. As we previously demonstrated, dex-
trorotatory tricyclic pentenones arise from complexes
where monophosphine ligands coordinate to the pro-(R)
cobalt atom in the bimetallic cluster.2d Likewise, for
2d
with that observed for monodentate phosphines. Thus,

1
874 Organometallics, Vol. 22, No. 9, 2003
Verdaguer et al.
Sch em e 6. P ostu la ted Equ ilibr iu m P r ocess for
Coor d in a tion of P ,S Liga n d s
F igu r e 7. PM3 optimized structures of major diastereo-
mers for CamPHOS and MeCamPHOS.
a negative (positive) CD absorption at around 500 nm
implies phosphorus coordination at the pro-S (pro-R)
cobalt atom. On the basis of these observations, we
confirm that circular dichroism provides a simple and
direct method to assess the absolute configuration of
dicobalt complexes with bridged P,S ligands.
Chemical correlation with the Pauson-Khand prod-
ucts and the CD spectra permitted us to establish the
absolute configuration of the major diastereomer in each
diasteromeric pair of complexes 7/8 and 13/14. When
using CamPHOS, phosphorus is attached at the pro-R
metal atom in the major diastereomeric complex 7
(Scheme 5 and Figure 7). On the other hand, the
opposite configuration was obtained when MeCam-
PHOS was used and the major 13 bore the phosphine
on the pro-S cobalt atom (Scheme 5 and Figure 7).
Likewise, minor diastereomers 8 and 14 had the op-
posite configuration with respect to their major coun-
terparts.
ence to bind either metal atom can be expected at this
point, and an approximately 1:1 mixture of diastereo-
mers Ia -a x and Ib-a x was found. From axial complexes,
direct formation of a bridged structure like III is not
feasible; the phosphorus atom must reach a pseudo-
equatorial site as in Ia -eq and Ib-eq. Hence, from I-a x
complexes, ligand pseudo-rotation would provide the
needed pseudo-equatorially coordinated complexes I-eq,
which can easily exchange a CO with sulfur to yield the
Remarkably, a relatively small methyl group had a
huge effect on the coordination selectivity of P,S ligands.
Introducing a CH3- in the carbon bridge between
phosphorus and sulfur produced a switch on selectivity.
This change was significant, since diastereselectivity
increased from 33% de for CamPHOS in 7b/8b to 90%
de of opposite sign for 13b/14b when using MeCam-
PHOS. This observation implies that the overall desta-
bilizing effect of the methyl group for major isomer 7b
was 2.15 kcal/mol. PM3-optimized structures of major
anti-isomers for CamPHOS and MeCamPHOS are
shown in Figure 7. CamPHOS in 7b yielded a configu-
ration analogous to that observed by X-ray crystal-
lography for PuPHOS.11 The camphor skeleton was too
far from the bimetallic cluster to offer any efficient
discrimination. Alternatively, the methyl group in 13b
was closer to the alkyne moiety and exerted a high
diastereomeric bias.
R a t ion a liza t ion of t h e Coor d in a t ion P a t h w a y
Obser ved . Experimental observations made during the
reactions of P,S ligands with dicobalt hexacarbonyl
complexes are consistent with the sequence of events
depicted in Scheme 6. First, exchange of a CO ligand
by the phosphine in CamPHOS should provide diaster-
eomeric pentacarbonyl complexes Ia -a x and Ib-a x, in
which the phosphorus atom reacts first because it is a
better donor with respect to the sulfide. At this stage,
the incoming phosphine is more likely to occupy a
pseudo-axial coordination site on cobalt, as in other
monodentate systems.21 Little selectivity in the prefer-
17,22
final IIIa and IIIb tetracarbonyl complexes.
This behavior was experimentally confirmed by moni-
toring the reaction mixture of MeCamPHOS with 5c by
1
H NMR early in the reaction. Thus, after heating the
1
reaction mixture at 55 °C for 1 h the H NMR spectrum
showed, in an approximately 1:1 ratio, two resonances
at δ 5.28 and 5.47 ppm, which corresponded to mono-
dentate structures Ia -a x and Ib-a x. Moreover, in the
same spectrum, additional signals belonging to the final
bridged tetracarbonyl species IIIa and IIIb were al-
ready observed in a 6:1 ratio.
In an equilibrium process like the one proposed in
Scheme 6, the relative energies of the end complexes
IIIa and IIIb determine the final diastereomeric ratio
observed. Isomerization between diasteromeric com-
plexes is more likely to occur at the monodentate I-a x
and I-eq structure level than at the bridged ones (III),
in which concomitant migration of both donor atoms
would be necessary. Isomerization between these com-
plexes during the Pauson-Khand reaction would ac-
count for the decrease observed in stereospecificity
during cyclization. Therefore, the appropriate choice of
reaction conditions (NMO, CH2Cl2) could minimize the
loss of stereochemical purity. In our case amine N-oxide
did not have a clear effect on the reaction rate and
probably prevented the presence of free CO in the
reaction medium.
(
21) (a) Bonnet, J . J .; Mathieu, R. Inorg. Chem. 1978, 17, 1973-
976. (b) J effrey, J . C.; Pereira, R. M. S.; Vargas, M. D.; Went, M. J .
J . Chem. Soc., Dalton Trans. 1995, 1805.
1
(22) Amouri, H. E.; Gruselle, M. Chem. Rev. 1996, 96, 1077-1103.

Design of New Hemilabile (P,S) Ligands
Organometallics, Vol. 22, No. 9, 2003 1875
Con clu sion s
Purification by flash chromatography (SiO
0%), and crystallization from ether/hexane mixtures yielded
1.35 g (68%) of 5 as a colorless crystalline solid. Mp: 148-
150 °C. [R] -92.5° (c 1.4, CHCl ). IR (KBr): νmax 688, 1043,
437, 2348, 2386, 2967 cm . H NMR (300 MHz, CDCl ): δ
.40-1.80 (br m, BH ), 0.85-1.06 (m, 2H), 0.84 (s, 3H), 1.03
2
, hexane/AcOEt,
1
Here we have designed a new family of camphor-
derived P,S ligands for bridged coordination to dicobalt
hexacarbonyl complexes. To our knowledge, MeCam-
PHOS (12) exhibits the highest diastereoselectivity ever
observed (up to 90% de) in the coordination process to
hexacarbonyl dicobalt complexes. Experimental obser-
vations indicate that this is the result of thermodynamic
equilibration that yields the more stable isomer. The
absolute configuration of the new bridged complexes was
established by chemical correlation from their intermo-
lecular Pauson-Khand products. Along these lines, we
also demonstrate the usefulness of circular dichroism
to assign the absolute configuration of dicobalt clusters
that bear bridged P,S ligands.
D
3
-
1
1
1
0
(
3
3
s, 3H), 1.40-1.50 (m, 1H), 1.62-1.74 (m, 3H), 1.78-1.88 (m,
1
1
7
H), 2.73-2.79 (dd, J ) 3 and 8 Hz, 1H), 3.09 (d, J ) 14 Hz,
H), 3.59-3.63 (dd, J ) 3 and 8 Hz, 1H), 5.56 (s, 1H), 7.38-
.55 (m, 6H), 7.54-7.88 (m, 4H) ppm. 13C NMR (75 MHz,
CDCl
5.2, 46.6, 79.2 (J
Hz), 127.3 (J P ) 55 Hz), 128.1 (J P ) 10 Hz), 128.5 (J P ) 10
3
): δ 20.2, 22.5, 27.1, 29.4 (J
P
) 7 Hz), 34.1, 37.5, 42.9,
4
P
) 44 Hz), 86.7 (J
P
) 6 Hz), 125.7 (J ) 55
P
Hz), 131.3 (J
P
) 2 Hz), 131.5 (J
) 9 Hz) ppm. 31P NMR (121 MHz, CDCl
br s) ppm. Anal. Calcd for C23
P
) 2 Hz), 133.2 (J
P
) 9 Hz),
): δ 25.6
134.2 (J
(
P
3
H
30BOPS: C 69.70, H 7.63, S
.09. Found: C 69.79, H 7.70, S 7.80.
+)-(1S,4S,6R ,8R )-4-Dip h e n ylp h osp h in o-4,11,11-t r i-
8
(
1
,6
m eth yl-5-oxa -3-th ia tr icyclo[6.2.1.0 ]u n d eca n e Bor a n e
Com p lex, 11. To a cooled (-78 °C) solution of borane-
phosphine complex 5 (1.0 g, 2.5 mmol) in THF (12 mL) was
added dropwise 1.2 M sec-BuLi in cyclohexane (2.2 mL, 2.7
mmol), and this yielded an orange solution. 12-Crown-4 (0.99
mL, 5.5 mmol) was then added with a syringe, and the
temperature was allowed to rise to -50 °C. At this tempera-
ture, methyl iodide (127 µL, 2.7 mmol) was added via syringe.
This resulted in a change of color from orange to pale yellow.
The reaction temperature was next allowed to reach room
temperature. Solvent removal under vacuum afforded a single
crude spot (TLC) of the desired boraphosphine complex.
Exp er im en ta l Section
Gen er a l P r oced u r es. All reactions were conducted under
nitrogen or argon atmosphere using standard Schlenk tech-
niques. Nuclear magnetic resonance (NMR) spectra were
1
13
recorded on a Varian Unity-300 spectrometer. H and C NMR
spectra were referenced relative to residual solvent peaks. 31
were referenced with an external H PO sample. In most
alkyne dicobalt complexes the carbon signals corresponding
P
3
4
1
3
to the cluster carbonyls do not appear in the C NMR
spectrum and have been omitted. The major part of alkyne-
chelated cobalt complexes solidified to give amorphous solids,
from which solvent could not be removed completely. This
precluded satisfactory elemental analysis of these compounds.
Infrared (IR) spectra were recorded on a Nicolette 510 FT
spectrometer. Melting points were determined by differential
scanning calorimetry (DSC) on a Mettler DSC-30 under
nitrogen. Elemental analyses were performed at “Servei de
microan a` lisi del CSIC de Barcelona”. High-resolution mass
spectroscopy (HRMS) was conducted at “Unidade de Espec-
trometr ´ı a de Masas de la Universidad de Santiago de Com-
postela”. All anhydrous solvents were distilled under nitrogen,
THF was distilled from sodium benzophenone ketyl, toluene
Purification by filtration on SiO
crystallization from hexane yielded 0.9 g (87%) of 11 as a
colorless crystalline solid. Mp: 120 °C. [R] +74.0° (c 1.0,
CHCl ). IR (KBr): νmax 698, 1105, 1437, 2392, 2954 cm . H
NMR (300 MHz, CDCl ): δ 0.40-1.90 (br m, BH ), 0.66-0.80
m. 1H), 0.79 (s, 3H), 0.82-1.01 (m, 1H), 1.08 (s, 3H), 1.22-
2
(hexane/AcOEt, 10%) and
D
-
1 1
3
3
3
(
1
1
.32 (m, 2H), 1.52-1.66 (m, 2H), 1.76 (d, J ) 12 Hz, 3H), 1.74-
.83 (m, 1H), 2.58-2.63 (dd, J ) 3 and 14 Hz, 1H), 2.85-2.89
(
7
d, J ) 14 Hz, 1H), 3.10-3.17 (m, 1H), 7.37-7.59 (m, 6H),
.89-7.96 (m, 2H), 8.11-8.18 (m, 2H) ppm. 13C NMR (75 MHz,
CDCl
3
)
9
1
2
3
): δ 19.9, 20.4, 26.2 (J
1.1, 36.8, 44.2, 46.6, 46.8, 81.6, 84.8 (J
P
) 4 Hz), 26.6, 27.3 (J
) 32 Hz), 126.6, (J
) 9 Hz), 128.5 (J
) 8 Hz),
): δ 25.9-
6.3 (br d, J ) 48 Hz) ppm. HRMS (FAB+): calcd for C24
P
) 11 Hz),
P
P
was distilled from molten sodium, and CH
from CaH . Silica gel used for filtration and flash chromatog-
raphy of cobalt complexes was previously washed through with
Et O. (+)-10-Thioisoborneol oxathiane was prepared according
2 2
Cl was distilled
51 Hz), 127.5 (J
Hz), 131.1 (J
P
) 51 Hz), 127.0 (J
P
P
)
2
P
) 2 Hz), 131.6 (J
P
) 2 Hz), 133.9 (J
P
34.6 (J
P
) 8 Hz) ppm. 31P NMR (121 MHz, CDCl
3
2
32
H -
2
3
to a published literature procedure. Complexes 6a , 6b, 6c,
and 6d were prepared according to a standard procedure (1.0
BOPS-H 409.1926, found 409.1918.
Gen er a l P r oced u r e for th e Syn th esis of Tetr a ca r bon yl
Com p lexes. Co (µ-P h CCH)(CO) (µ-C23 27OP S), 7a a n d 8a .
equiv of Co
2 8
(CO) , hexane, rt) from the corresponding com-
mercially available alkynes.
2
4
H
Dicobalt hexacarbonyl complex of phenylethyne 6a (100 mg,
0.25 mmol), phosphine-borane complex 5 (102 mg, 0.25 mmol),
DABCO (42 mg, 0.37 mmol), and toluene (1 mL) were charged
in a Schlenk flask under nitrogen. The reaction mixture was
heated to 65 °C for 24 h, removing periodically the CO by
means of vacuum and nitrogen refilling. Complexation was
monitored by TLC. Upon reaction completion the solvent was
removed in vacuo. The residue was then purified by flash
(
-)-(1S,4S,6R,8R)-4-Diph en ylph osph in o-11,11-dim eth yl-
1,6
5
5
-oxa -3-th ia tr icyclo[6.2.1.0 ]u n d eca n e Bor a n e Com p lex,
. To a cooled (-78 °C) solution of (+)-10-thioisoborneol
oxathiane (1.0 g, 5.0 mmol) in THF (10 mL) was added
dropwise 1.3 M sec-BuLi in cyclohexane (4.0 mL, 5.3 mmol).
The temperature was allowed to rise to -30 °C, and next, the
reaction was cooled back to -78 °C. Chlorodiphenylphosphine
0.99 mL, 5.5 mmol) was then added with a syringe, and the
temperature was kept at -78 °C for 1 h and then allowed to
reach room temperature. After 3 h the reaction was quenched
at 0 °C by the addition of BH
The reaction mixture was diluted with Et
added (CAUTION! bubbling occurs). The aqueous layer was
washed with Et O, and the combined organic layers were
washed with brine and dried (MgSO ). Solvent removal under
(
2
chromatography on SiO to yield 156 mg (85%) of 7a /8a as a
1
red-purple viscous oil. H NMR analysis revealed the existence
of a 1.5:1 mixture of diastereomeric complexes. A pure fraction
of the major diastereomer 7a was purified by flash chroma-
3
-S(CH
3
)
2
(0.62 mL, 6.5 mmol).
2
O, and water was
tography (SiO
2
, hexane/AcOEt, 3%). IR (KBr): νmax 1962, 1991,
-
1 1
2
7
2
024 cm . H NMR (300 MHz, C ) major: δ 0.30-1.60 (m,
H), 0.46 (s, 3H), 0.83 (s, 3H), 2.63 (d, J ) 14 Hz, 1H), 2.69-
.76 (dd, J ) 6 and 14 Hz, 1H), 2.94-2.98 (dd, J ) 3 and 8
6 6
D
2
4
vacuum afforded a mixture of the desired boraphosphine
complex and starting material (75% conv by H NMR).
1
Hz,1H), 4.73 (s, 1H), 5.97 (d, J ) 7 Hz, 1H), 7.00-7.30 (m,
H), 7.73-7.79 (m, 2H), 7.86-7.93 (m, 4H) ppm; distinct
signals from minor: δ 0.73 (s, 3H), 5.01 (s, 1H), 5.64 (d, J ) 7
9
(23) (a) Aggarwal, V. K.; Coogan, M. P.; Stenson, R. A.; J ones, R. V.
13
Hz, 1H) ppm. C NMR (75 MHz, C
7.0, 33.8, 37.1, 42.4 (J ) 8 Hz), 44.4, 45.6, 46.5, 73.7 (J
7 Hz, Co CCH), 86.9 (C OCH), 97.8 (J ) 22 Hz, OPSCH),
6 6
D ) major: δ 19.9, 22.5,
H.; Fieldhouse, R.; Blacker, J . Eur. J . Org. Chem. 2002, 319-326. (b)
Aggarwal, V. K.; Ford, J . G.; Fonquerna, S.; Adams, H.; J ones, R. V.
H.; Fieldhouse, R. J . Am. Chem. Soc. 1998, 120, 8328-8339.
2
P
P
)
2
2
P

1
1
)
3
876 Organometallics, Vol. 22, No. 9, 2003
Verdaguer et al.
26.8, 128.8, 129.9, 130.0 (J
12 Hz), 132.5 (J ) 35 Hz), 135.1 (J
5 Hz), 142.4, 203.0-206.2 (br, 4CO) ppm; distinct signals from
minor: 69.4 (Co CCH), 87.3 (C OCH), 91.2 (J ) 27 Hz,
OPSCH) ppm. P NMR (121 MHz, C ): δ -27.4maj, -30.4min
33Co PS-CO 686.0501,
P
) 3 Hz), 130.2, 130.4, 132.5 (J
P
(J
P
) 30 Hz), 135.3 (J
P
) 11 Hz), 135.5 (J
P
) 12 Hz), 203.5-
3
1
P
P
) 12 Hz), 135.6 (J
P
)
207.5 (4 CO) ppm. P NMR (121 MHz, C D ): δ -20.0maj (br
6 6
s) ppm. HRMS (FAB+): calcd for C34
found 680.0978.
H
39Co
2
O
5
PS-CO 680.0971,
2
2
P
3
1
6
D
6
Co
(µ-Me SiCCH)(CO) (µ-C24H29OP S), 13c a n d 14c. Ac-
2
3
4
ppm. HRMS (FAB+): calcd for C35
found 686.0448.
H
2 5
O
cording to the general procedure, dicobalt hexacarbonyl com-
plex 6c (51 mg, 0.13 mmol), phosphine-borane 11 (50 mg, 0.12
mmol), DABCO (21 mg, 0.18 mmol), and toluene (2 mL) were
used. The reaction mixture was heated at 80 °C for 3 h and
then at 70 °C for 16 h. Purification by flash chromatography
(hexane/AcOEt, 1%) afforded 57 mg (64%) of 13c/14c as a
t
Co
2
(µ-Bu CCH)(CO)
4
(µ-C23
H
27OP S), 7b a n d 8b. Accord-
ing to the general procedure, dicobalt hexacarbonyl complex
b (100 mg, 0.27 mmol), phosphine-borane 5 (107 mg, 0.27
6
mmol), DABCO (45 mg, 0.4 mmol), and toluene (1 mL) were
used. The reaction mixture was heated at 60 °C for 20 h.
Purification by flash chromatography yielded 185 mg (98%)
of 7b and 8b as a red viscous oil, which solidified upon
1
viscous red oil. H NMR analysis showed the corresponding
diastereomeric complexes were found in a 1:12 ratio. IR
-
1 1
(KBr): νmax 1960, 1989, 2022 cm . H NMR (300 MHz, C D )
6
6
1
standing. H NMR analysis showed a 1:1 ratio of diastereo-
major: δ 0.12-1.70 (m, 7H), 0.45 (s, 3H) 0.57 (s, 9H), 0.83 (s,
3H), 1.75 (d, J ) 13 Hz, 3H), 2.06-2.65 (AB, J ) 14 Hz, 2H),
2.90-2.98 (m, 1H), 6.33 (d, J ) 5 Hz, 1H), 6.95-7.25 (m, 6H),
8.12-8.18 (m, 2H), 8.31-8.37 (m, 2H) ppm; distinct signals
meric complexes. Further equilibration of the diastereomeric
mixture at 80 °C under CO atmosphere for 66 h yielded a 2:1
mixture of diastereomers (90% yield). IR (KBr): νmax 1958,
-
1
1
13
1
985, 2020 cm . H NMR (300 MHz, C
.6 (m, 7H), 0.45 (s, 3H), 0.77 (s, 3H), 1.47 (s, 9H), 2.68 (d, J
12 Hz, 1H), 2.75-2.86 (dd, J ) 6 and 11 Hz, 1H), 2.96-3.04
dd, J ) 3 and 9 Hz, 1H), 4.71 (d, J ) 2 Hz, 1H), 5.66 (d, J )
Hz, 1H), 7.01-7.25 (m, 6H), 7.71-7.92 (m, 4H) ppm; distinct
signals from minor: δ 0.66 (s, 3H), 0.44 (s, 3H), 4.99 (s, 1H),
6
D
6
) major: δ 0.30-
from minor: δ 5.72 (d, J ) 5 Hz, 1H) ppm. C NMR (75 MHz,
1
)
(
C D ) major: δ 1.0, 19.7, 20.3, 23.8 (J ) 14 Hz), 26.6, 31.9,
6
6
P
32.6, (J P ) 10 Hz), 36.9, 44.5, 47.1, 49.6, 82.2 (C OCH), 88.7
2
(Co CCH), 103.3, (J P ) 16 Hz, CH OPSC), 128.8 (J ) 9 Hz),
2
3
P
7
130.3, 130.8, 133.8 (J P ) 31 Hz), 134.0 (J ) 30 Hz), 135.3 (J P
P
) 13 Hz), 135.5 (J P ) 13 Hz) ppm. 31P NMR (121 MHz, C D ):
6
6
1
3
5
.19 (d, J ) 9 Hz, 1H) ppm. C NMR (75 MHz, C
δ 19.9, 22.4, 27.0, 33.3, 33.8, 36.8, 42.3 (J ) 11 Hz), 44.4,
OCH), 97.1 (OPSCH, J . )
1 Hz), 120.5 (br), 128.8, 129.6, 130.0, 130.4, 132.3 (J ) 11
) 13 Hz) ppm; distinct signals from minor: δ
6
D
6
) major:
δ -14.2maj (br s) ppm. HRMS (FAB+): calcd for C H Co O -
33
39
2
5
P
PSSi-CO 696.0740, found 696.0761.
4
2
5.7, 46.5, 72.8 (Co
2
CCH), 86.7 (C
2
P
Co (µ-HOMe CC H)(CO) (µ-C24H29OP S), 13d a n d 14d .
2
2
2
4
P
According to the general procedure, dicobalt complex 6d (18
mg, 0.048 mmol), phosphine-borane 11 (20 mg, 0.048 mmol),
DABCO (8 mg, 0.072 mmol), and toluene (1 mL) were used.
The reaction mixture was heated to 65 °C for 18 h. Purification
by flash chromatography (hexane/AcOEt, 10%) afforded 35 mg
(99%) of 13d /14d as a red viscous oil, which solidifies upon
Hz), 135.4 (J
P
6
7.0 (Co
P NMR (121 MHz, C
2
CCH), 87.2 (C
2
OCH), 89.6 (OPSCH, J
P
) 26 Hz) ppm.
3
1
6
D
37Co
6
): δ -27.8maj, -29.1min ppm. HRMS
(FAB+): calcd for C33
H
2
O
5
PS 694.0763, found 694.0747.
(µ-C23 27OP S), 10. Accord-
t
t
Co
2
(µ-Bu SO
2
C
2
SO
2
Bu )(CO)
4
H
1
ing to the general procedure, dicobalt hexacarbonyl complex
9
standing. H NMR analysis showed a 13:1 ratio of diastereo-
1
7
-1 1
(65 mg, 0.11 mmol), phosphine-borane 5 (46 mg, 0.11
meric complexes. IR (KBr): νmax 1962, 1991, 2024 cm . H
mmol), DABCO (19 mg, 0.17 mmol), and toluene (2 mL) were
used. The reaction mixture was heated at 60 °C for 18 h.
Filtration through a pad of silica (hexane/AcOEt, 20%) yielded
NMR (300 MHz, C D ) major: δ 0.0.40-1.80 (m, 7H), 0.45 (s,
6
6
3H) 0.82 (s, 3H), 1.75 (s, 3H), 1.78 (s, 3H), 1.79 (d, J ) 12 Hz,
3H), 2.06-2.64 (AB, J ) 14 Hz, 2H), 2.82-2.91 (m, 1H), 5.74
(d, J ) 5 Hz, 1H), 6.95-7.20 (m, 6H), 8.06-8.14 (m, 2H), 8.27-
8.35 (m, 2H) ppm; distinct signals from minor: δ 5.00 (d, J )
9
[
2
0
0 mg (87%) of 10 as a red cristalline solid. Mp: 181.4 °C.
R] 104.1° (c 0.02, CH Cl ). IR (KBr): νmax 1995, 2018, 2031,
056 cm . H NMR (300 MHz, C ): δ 0.46-1.72, (m, 7H),
.43 (s, 3H), 0.61 (s, 3H), 1.43 (s, 9H), 1.64 (s, 9H), 2.72-2.79
D
2
2
-
1
1
13
6
D
6
5 Hz, 1H) ppm. C NMR (75 MHz, C D ) major: δ 19.7, 20.2,
6
6
23.9 (J P ) 15 Hz), 26.5, 31.8, 32.3, (J P ) 10 Hz), 33.0, 33.2,
36.9, 44.4, 47.0, 49.3, 73.3, 74,0 (Co CCH), 82.2 (C OCH), 102.7
(
dd, J ) 6 and 13 Hz, 1H), 3.62 (d, J ) 13 Hz, 1H), 3.67-3.75
2
2
(m, 1H), 6.98-7.18 (m, 7H), 7.72-7.51 (m, 2H), 7.53-7.63 (m,
(J P ) 16 Hz, CH OPSC), 106.5 (J P ) 15 Hz), 128.8 (J P ) 9
3
1
3
2
2
8
J
H) ppm. C NMR (75 MHz, C
6.9, 33.6, 37.0, 38.2, (J ) 9 Hz), 44.7, 45.7, 46.6, 62.1, 62.2,
6.6 (C OCH, J ) 4 Hz), 88.4 (OPSCH, J ) 28 Hz), 128.8,
) 9 Hz), 129.4 (J ) 36 Hz), 130.3, 131.0, 132.3, (J ) 36
) 10 Hz) ppm. 31P NMR
6
D
6
): δ 19.9, 22.3, 24.8, 25.1,
Hz), 130.4, 130.9, 133.3 (J P ) 31 Hz), 133.7 (J ) 28 Hz), 135.3
P
P
(J P ) 11 Hz), 135.5 (J P ) 12 Hz) ppm. 31P NMR (121 MHz,
2
P
P
C D ): δ -17.0 (br s) ppm. HRMS (FAB+): calcd for C H37-
6
6
maj
33
P
P
P
Co O PS+H 711.0790, found 711.0719.
2
6
Hz), 132.6 (J
121 MHz, C
for C37
Co (µ-Bu CCH)(CO)
P
) 10 Hz), 134.3 (J
P
(
+)-(3a S,4R,7S,7a S)-2-P h en yl-3a ,4,7,7a -tetr a h yd r o-4,7-
(
6
D
6
): δ -33.3 (br s) ppm. HRMS (FAB+): calcd
24
m eth a n oin d en -1-on e, 15a . Diastereomerically pure major
H
45Co
2
t
O
9
PS
3
+H 879.0705, found 879.0702.
(µ-C24 29OP S), 13b a n d 14b. Ac-
7
a (49 mg, 0.068 mmol), norbornadiene (68 µL, 0.68 mmol),
2
4
H
and toluene (2 mL) were charged in a Schlenk flask under
nitrogen and heated to 50 °C with stirring. TLC monitoring
showed the reaction was complete after 30 min. Purification
cording to the general procedure, dicobalt hexacarbonyl com-
plex 6b (97 mg, 0.26 mmol), phosphine-borane 11 (100 mg,
0
.24 mmol), DABCO (40 mg, 0.36 mmol), and toluene (2 mL)
by flash chromatography on SiO
4 mg (93%) of (+)-15a as a white solid (97% ee). HPLC
analysis: CHIRALCEL OD (25 cm), 2% IPA-98% hexane, 0.5
mL/min, λ ) 254 nm. t (+) isomer ) 16.8 min. t (-) isomer
20.6 min.
+)-(3a S,4R,7S,7a S)- a n d (-)-(3a R,4S,7R,7a R)-2-ter t-
2
(hexane/AcOEt, 2.5%) yielded
were used. The reaction mixture was heated at 65 °C for 18
h. Filtration through a pad of silica (hexane/AcOEt) yield-
ed 141 mg (80%) of 13b/14b as a red viscous oil, which
1
R
R
1
solidified upon standing. H NMR analysis showed that
)
diastereomeric complexes were found in a 20:1 ratio. IR
(
-
1 1
(
KBr): νmax 1958, 1987, 2020 cm . H NMR (300 MHz, C
6
D
6
)
25
Bu tyl-3a,4,7,7a-tetr ah ydr o-4,7-m eth an oin den -1-on e, 15b.
From CamPHOS: A 2:1 diastereomer mixture of 7b/8b (55
mg, 0.079 mmol), norbornadiene (79 µL, 0.79 mmol), and
toluene (2 mL) was charged in a Schlenk flask under nitrogen
and heated initially at 60 °C for 1 h and then 90 °C for 3 h.
major: δ 0.35-1.70 (m, 7H), 0.46 s, 3H), 0.84 (s, 3H), 1.52 (s,
9
1
5
8
H), 1.83 (d, J ) 12 Hz, 3H), 2.07-2.12 (dd, J ) 2 and 14 Hz,
H), 2.61-2.66 (dd, J ) 1 and 14 Hz, 1H), 2.84-2.92 (m, 1H),
.86 (d, J ) 5 Hz, 1H), 6.95-7.21 (m, 6H), 8.09-8.19 (m, 2H),
.30-8.40 (m, 2H) ppm; distinct signals from minor: δ 5.06
2
Purification by flash chromatography on SiO (hexane/AcOEt,
1
3
(d, J ) 5 Hz, 1H) ppm. C NMR (75 MHz, C
6
D
6
) major: δ
) 10 Hz),
1
3
8
9.7, 20.3, 23.9 (J
3.3, 36.9, 37.1, 44.4, 47.0, 49.3, 75.4 (J
2.2 (C OCH), 102.5 (J ) 15 Hz, CH OPSC), 127.6 (J
) 9 HZ), 130.2, 130.7, 133.6 (J ) 31 Hz), 134.0
P
) 14 Hz), 26.6, 31.8, 32.3 (J
P
(
24) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E.; Foreman,
M. I. J . Chem. Soc., Perkin Trans. 1 1973, 977-981.
25) Lee, B. Y.; Han, J . W.; Chung, Y. K.; Lee, S. W. J . Organomet.
Chem. 1999, 587, 181-190.
P
) 6 Hz, Co
2
CCH),
) 9
2
P
3
P
(
Hz), 128.8 (J
P
P

Design of New Hemilabile (P,S) Ligands
%) yielded 12 mg (75%) of (+)-15b (6% ee). From MeCam-
PHOS: A 20:1 diastereomer mixture of 13b/14b (79 mg, 0.1
mmol), norbornadiene (100 µL, 1.0 mmol), and toluene (2 mL)
were charged in a Schlenk flask under nitrogen and heated
initially at 50 °C for 18 h and then 80 °C for 1 h. Purification
Organometallics, Vol. 22, No. 9, 2003 1877
1
(1 mL) was charged in a Schlenk flask under nitrogen and
heated at 60 °C for 18 h. This resulted in complete conversion
of the starting complex (TLC) and yielded 7 mg (99%) of (-)-
15d in 42% ee as calculated by HPLC analysis. N-Oxide
conditions: A 13:1 diastereomer mixture of 13d /14d (27 mg,
0.038 mmol), norbornadiene (38 µL, 0.38 mmol), N-methyl-
by flash chromatography on SiO
6 mg (80%) of (-)-15b (40% ee). HPLC analysis: CHIRALCEL
OD (25 cm), 0.5% IPA-95.5% hexane, 1.0 mL/min, λ ) 254
nm. t (+) isomer ) 8.4 min. t (-) isomer ) 10.3 min.
-)-(3a S,4S,7R,7a R)-2-Tr im et h ylsila n yl-3a ,4,7,7a -t et -
2
(hexane/AcOEt, 5%) yielded
1
2 2
morpholine N-oxide (26 mg, 0.22 mmol), and CH Cl (3 mL)
was charged in a Schlenk flask tube under nitrogen and stirred
at room temperature for 4 days to yield 7 mg (90%) of (-)-15d
in 50% ee as calculated by HPLC analysis. HPLC analysis:
CHIRALCEL OD (25 cm), 5% IPA-95% hexane, 1.0 mL/min,
R
R
(
2
d
r a h yd r o-4,7-m eth a n oin d en -1-on e, 15c. Thermal condi-
tions: A 12:1 diastereomer mixture of 13c/14c (15 mg, 0.02
λ ) 254 nm. t
R
(-) isomer ) 6.5 min. t (+) isomer ) 7.3 min.
R
mmol), norbornadiene (20 µL, 0.2 mmol), and C
was charged in a NMR tube under nitrogen and heated at 65
C for 3 h. This resulted in complete conversion of the starting
6 6
D (0.75 mL)
Ack n ow led gm en t. We gratefully thank CIRIT-
001SGR-00050 and BQU2002-02459 for financial sup-
port. X.V. thanks the MCYT for a Ram o´ n y Cajal re-
search position.
°
2
complex (TLC) and yielded 3 mg (68%) of (-)-15c in 56% ee
as calculated by GC analysis. N-Oxide conditions: A 12:1
diastereomer mixture of 13c/14c (40 mg, 0.054 mmol), nor-
bornadiene (50 µL, 0.5 mmol), N-methylmorpholine N-oxide
Su p p or tin g In for m a tion Ava ila ble: Tables of complete
2 2
(38 mg, 0.32 mmol), and CH Cl (2 mL) was charged in a
X-ray crystal data: ORTEP drawings, refinement parameters,
Schlenk flask under nitrogen and stirred at room temperature
for 2 days to yield 8 mg (72%) of (-)-15c in 79% ee as
calculated by GC analysis. GC analysis: Supelco â-DEX 120,
1
atomic coordinates, and bond distances and angles for 10. H
NMR of ligand 11 and complexes 7a , 7b/8b, 13b/14b, 13c/
4c, and 13d /14d . This material is available free of charge
via the Internet at http://pubs.acs.org.
1
3
0m, 150 °C. t
R
(-) isomer ) 31.7 min. t (+) isomer ) 33.4
R
min.
(
-)-(3aR,4S,7R,7aR)-2-(1-Hydr oxy-1-m eth yleth yl)-3a,4,7,-
2c,26
OM021008F
7
a -tetr a h yd r o-4,7-m eth a n oin d en -1-on e, 15d .
Thermal
conditions: A 13:1 diastereomer mixture of 13d /14d (25 mg,
.035 mmol), norbornadiene (35 µL, 0.35 mmol), and toluene
(
26) Lee, B. Y.; Chung, Y. K.; J eong, N.; Lee, Y.; Hwang, S. H. J .
0
Am. Chem. Soc. 1994, 116, 8793-4.