Preparation of cationic cyclopentadienyl diene molybdenum complexes through hydride abstraction effected by Di- or triarylmethyl halides in the presence of hexafluoro-2-propanol

Article abstract of DOI:10.1021/om000839n

Hydride abstraction from the model system (η⁵-cyclopentadienyl)dicarbonyl[(1-3-η)-1-cyclohexen-3-yl]mol ybdenum (1) was effected using halides such as Ph₃CCl (3b), Ph₃CBr (3a), and (4-MeOC₆H₄)₂CHBr (5), or their polymer-bound analogues, as carbocation precursors. The presence of hexafluoro-2-propanol (HFiP) as a cosolvent was critical for hydride abstraction to proceed. Good yields of (η⁵-cyclopentadienyl)dicarbonyl[(1-4-η)-cyclohexadiene]-molyb denum halides 2a,b were obtained from soluble as well as polymer-supported hydride abstractors. The influence of HFiP concentration and carbocation precursor on reaction efficiency was investigated.

Full text of DOI:10.1021/om000839n

990
Organometallics 2001, 20, 990-994
P r ep a r a tion of Ca tion ic Cyclop en ta d ien yl Dien e
Molybd en u m Com p lexes th r ou gh Hyd r id e Abstr a ction
Effected by Di- or Tr ia r ylm eth yl Ha lid es in th e P r esen ce
of Hexa flu or o-2-p r op a n ol
Emelie Bjurling,† Anders Bjo¨rkman,† and Carl-Magnus Andersson*,‡
Department of Organic Chemistry 1, Chemical Center, Lund University, P.O. Box 124,
S-221 00 Lund, Sweden, and Department of Synthetic Chemistry, ACADIA Pharmaceuticals
A/ S, Fabriksparken 58, DK-2600 Glostrup, Denmark
Received September 29, 2000
Hydride abstraction from the model system (η5-cyclopentadienyl)dicarbonyl[(1-3-η)-1-
cyclohexen-3-yl]molybdenum (1) was effected using halides such as Ph3CCl (3b), Ph3CBr
(3a ), and (4-MeOC6H4)2CHBr (5), or their polymer-bound analogues, as carbocation precur-
sors. The presence of hexafluoro-2-propanol (HFiP) as a cosolvent was critical for hydride
abstraction to proceed. Good yields of (η5-cyclopentadienyl)dicarbonyl[(1-4-η)-cyclohexadiene]-
molybdenum halides 2a ,b were obtained from soluble as well as polymer-supported hydride
abstractors. The influence of HFiP concentration and carbocation precursor on reaction
efficiency was investigated.
In tr od u ction
Cationic diene molybdenum complexes of the type (η5-
L)(diene)Mo(CO)2X constitute a valuable class of elec-
trophiles for organic synthesis.1 In particular, the
temporary complexation of carbo- or heterocyclic dienes
to molybdenum has been elegantly utilized for stereo-
selective syntheses of cis-disubstituted targets (Figure
1), as well as for the construction of quaternary centers.2
The stabilizing effect of the η5 ligand in these molyb-
denum π-complexes conveys enough chemical robust-
ness to allow sequential metal-assisted reactions and
convenient isolation and purification of the organome-
tallic intermediates. Recent development of mild and
selective decomplexation protocols3 should further en-
hance the value of these molybdenum-mediated syn-
thetic strategies.
Two routes of preparative significance are available
for the preparation of the requisite cationic diene
complexes: direct complexation and oxidation of an
allylic precursor. Stable dienes may be attached to
cationic templates such as [(η5-L)Mo(CO)2(MeCN)2]+
(η5-L ) indenyl, pentamethylcyclopentadienyl) via an
exchange reaction. Typically, (η5-L)MoMe(CO)3 in MeCN
is acidified at low temperature to afford the cationic
template and the diene introduced at room tempera-
F igu r e 1. Hydride abstraction/nucleophilic addition se-
quences involving organomolybdenum complexes.
ture.4 A similar route involving replacement of an allylic
ligand under acidic conditions has also been described.4a,5
Alternatively, oxidation of the precursor [(η5-L)Mo-
(CO)3]2 with silver ion in CH2Cl2 or MeCN can be used
to generate the cationic species.3c,4,6
† Lund University.
‡ ACADIA Pharmaceuticals.
(1) Review: Pearson, A. J . In Comprehensive Organometallic Chem-
istry II; Abel, E. W., Stone, E. G. A., Wilkinson, G., Eds.; Pergamon:
Oxford, U.K., 1995; Vol. 12, Chapter 6.3.
(2) For recent accounts, see: (a) Liu, R.-S. Adv. Met.-Org. Chem.
1998, 6, 145. (b) Li, C.-L.; Liu. R.-S. Chem. Rev. 2000, 100, 3127-
3161.
(3) (a) Yin, J .; Liebeskind, L. S. J . Am. Chem. Soc. 1999, 121, 5811.
(b) Bjurling, E.; J ohansson, M. H.; Andersson, C.-M. Organometallics
1999, 18, 5606. (c) Pearson, A. J .; Khetani, V. D. J . Am. Chem. Soc.
1989, 111, 6778. (d) Kno¨lker, H.-J .; Goesmann, H.; Hofmann, C. Synlett
1996, 737. (e) Pearson, A. J .; Khan, M. N. I. J . Am. Chem. Soc. 1984,
106, 1872.
(4) (a) Green, M.; Greenfield, S.; Kersting, M. J . Chem. Soc., Chem.
Commun. 1985, 18. (b) Bottrill, M.; Green, M. J . Chem. Soc., Dalton
Trans. 1977, 2365.
(5) (a) Giulieri, F.; Benaim, J . J . Organomet. Chem. 1984, 276, 367.
(b) Gusev, O. V.; Krivykh, V. V.; Petrovskii, P. V.; Rybinskaya, M. I.
Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.) 1987, 1532.
10.1021/om000839n CCC: $20.00 © 2001 American Chemical Society
Publication on Web 02/26/2001

Cationic Cyclopentadienyl Diene Mo Complexes
Organometallics, Vol. 20, No. 5, 2001 991
Transformation of allylic precursors of the type (η3-
allyl)MoCp(CO)2 into (η4-diene)MoCp(CO)2 cations al-
most exclusively involves abstraction of a hydride ion.
This methodology offers the most general entry into
cationic diene complexes, since the precursors are
usually obtained in high yields via oxidative addition
of readily available allylic halides or acetates. Reactive
Mo(0) species such as (MeCN)3Mo(CO)3 are conveniently
formed in situ upon refluxing molybdenum hexacarbo-
nyl in acetonitrile. Importantly, this two-step procedure
allows the preparation of synthetically useful complexes
also with labile dienes, such as cyclopentadienone7 and
2H-pyran.8
The standard protocol for hydride abstraction from
(π-allyl)molybdenum complexes involves treatment with
a trityl salt (Ph3CPF6 or Ph3CBF4) in dichloromethane,
followed by precipitaion.9 These reactions proceed un-
eventfully with most simple substrates. However, ap-
plication to more highly substituted allylic substrates
is problematic, probably as a consequence of the steric
bulkiness of the trityl ion in combination with the
stereoelectronic requirement1,9b for removal of a hydride
anti to molybdenum.10 Unfortunately, traditional trityl
ion mediated hydride abstraction has remained the only
general source of diene complexes from allylic precur-
sors. An alternative oxidation procedure, entailing
treatment of the allylmolybdenum precursor with DDQ
in the presence of HBF4‚Et2O at low temperature, has
been reported but not generally adapted. In special
cases, where the η3-organomolybdenum substrate car-
ries a suitably positioned leaving group, such as
alkoxy3c,4,6,8a,b or indolyl,3b elimination forming diene
complexes has been observed.
F igu r e 2. Reactions of model substrate 1 with trityl
halides in the presence of HFiP.
of moisture-sensitive and expensive trityl salts as well
as operating under strictly anhydrous conditions. We
also speculated that in situ generation of carbocations
might allow the use of additional carbocation precursors,
especially those presenting less sterical requirements,
which do not form isolable cation salts. Herein, we
report full details of extended studies on the use of tri-
and diarylmethane derivatives as hydride abstraction
reagents.
Resu lts a n d Discu ssion
We recently reported16 that hydride abstraction from
the allylic complex 1 proceeds smoothly in 80-86% yield
using Ph3CBr as a trityl cation precursor in 20% (v/v)
of HFiP in dichloromethane. The procedure involved
stirring at 0 °C for 1 h and simple isolation of the
cationic product by addition of degassed diethyl ether
followed by decantation. The objective of the work
reported herein was to explore the influence of variables
such as the concentration of HFiP, solvent, and car-
bocation precursor on the efficiency of the hydride
abstraction process.
We recently communicated on an alternative proce-
dure for hydride abstraction, in which trityl cation was
generated in situ from Ph3CBr. Key to this protocol was
the utilization of 1,1,1,3,3,3-hexafluoro-2-propanol11
(HFiP) as a cosolvent. HFiP is a weakly acidic (pKa )
9.4)12 and strongly hydrogen bonding13 solvent, which
has previously been used in studies of carbocation
behavior14 and also as a convenient and efficient reagent
for the cleavage of synthetic peptides from 2-chlorotrityl
resin.15 We anticipated that generating trityl ion in situ
in this fashion might provide a convenient alternative
to the traditional methodology, which requires handling
Solven t Mixtu r e Com p osition . In dichloromethane
containing 20% (v/v) HFiP, Ph3CBr presumably dissoci-
ates into free trityl cation, which, in turn, mediates
hydride abstraction from a suitable donor (Figure 2).
Evaluation of the minimum concentration of HFiP
required for the reaction was undertaken by NMR
experiments. In solutions of Ph3CBr in CD2Cl2 contain-
ing 0, 2, 5, 10, 15, or 20% (v/v) HFiP, the 1H NMR
signals from the aromatic protons changed dramatically
with increasing concentration of HFiP. The original
multiplet from Ph3CBr observed at low HFiP concentra-
tions (0, 2, and 5%) was disrupted into two triplets and
a doublet moving downfield with increasing concentra-
tions (10, 15, and 20%). In comparison, the resonances
recorded from the ionic Ph3CPF6 in the same solvent
mixtures17 were unaffected by the HFiP concentration.
In the presence of about 15% (v/v) of HFiP, the appear-
ance of the spectrum resulting from trityl bromide was
observed to approach that of Ph3CPF6. Our interpreta-
tion was that significant dissociation to form trityl
cation occurred at 10, 15, and 20% HFiP and that the
equilibrium between trityl bromide and trityl cation was
in favor of the latter at the highest HFiP concentrations.
At low HFiP content, the situation was further compli-
cated by apparent hydrolysis forming trityl alcohol.
Upon introduction of the hydride donor 1 into the
NMR samples, rapid hydride abstraction was observed
at concentrations of HFiP from 10% (v/v) and above.
(6) Pearson, A. J .; Khetani, V. D. J . Org. Chem. 1988, 53, 3395.
(7) (a) Liebeskind, L. S.; Bombrun, A. J . Am. Chem. Soc. 1991, 113,
8736. (b) Slugovc, C.; Mauthner, K.; Mereiter, K.; Schmid, R.; Kirchner,
K. Organometallics 1996, 15, 2954.
(8) (a) Rubio, A.; Liebeskind, L. S. J . Am. Chem. Soc. 1993, 115,
891. (b) Hansson, S.; Miller, J . F.; Liebeskind, L. S. J . Am. Chem. Soc.
1990, 112, 9660.
(9) (a) Faller, J . W.; Rosan, A. M. J . Am. Chem. Soc. 1977, 99, 4858.
(b) Faller, J . W.; Murray, H. H.; White, D. L.; Chao, K. H. Organome-
tallics 1983, 2, 400. (c) Pearson, A. J .; Khan, M. N. I. J . Org. Chem.
1985, 50, 5276. (d) Pearson, A. J .; Khan, M. N. I.; Clardy, J . C.; Cun-
heng, H. J . Am. Chem. Soc. 1985, 107, 2748.
(10) The same factors preclude sequential 1,2-difunctionalisation in
the analogous cyclohexadienyl iron series; e.g., see ref 1.
(11) HFiP is a sensory irritant, see: Nielsen, G. D.; Abraham, M.
H.; Hansen, L. F.; Hammer, M.; Cooksey, C. J .; Andonian-Haftvan,
J .; Alarie, Y. Arch. Toxicol. 1996, 70, 319.
(12) Arrowsmith, C. H.; Kresge, A. J .; Tang, Y. C. J . Am. Chem.
Soc. 1991, 113, 179.
(13) Abraham, M. H.; Duce, P. P.; Prior, D. V.; Barratt, D. G.; Morris,
J . J .; Taylor, P. J . J . Chem. Soc., Perkin Trans. 2 1989, 1355.
(14) (a) McClelland, R. A.; Mathivanan, N.; Steenken, S. J . Am.
Chem. Soc. 1990, 112, 4857. (b) Cozens, F.; Li, J .; McClelland, R. A.;
Steenken, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 743.
(15) Bollhagen, R.; Schmiedberger, M.; Barlos, K.; Grell, E. J . Chem.
Soc., Chem. Commun. 1994, 2559.
(16) Bjurling, E.; Andersson, C.-M. Tetrahedron Lett. 1999, 40, 1981.
(17) Resonances were as follows: δ 8.29 (tt, J ) 7.5, 1.2 Hz), 7.90
(t, J ) 7.5 Hz), 7.68 (dd, J ) 1.2, 8.4 Hz).

992 Organometallics, Vol. 20, No. 5, 2001
Bjurling et al.
Ta ble 1. In flu en ce of HF iP Con cen tr a tion on
Hyd r id e Abstr a ction a fr om Su bstr a te 1 w ith
P h 3CBr (F igu r e 2)
entry solvent CH2Cl2/HFiP product (yield, %)b
color
1
2
3
4
98/2
95/5
90/10
80/20
2a (35)
2a (66)
2a (82)
2a (86)
blue-green
green
yellow-green
yellow
a
Reactions employed 1 (100-200 mg) and 1.2 equiv of Ph3CBr
in 10 mL of the solvent mixture. After reaction at 0 °C for 1 h, the
product was isolated through precipitation with diethyl ether.
Isolated yield.
b
F igu r e 3. Carbocation precursors evaluated in the hydride
abstraction reaction.
Reaction progress was readily monitored via observation
of the replacement of the Cp resonance associated with
1 (5.3 ppm) with that of cationic complex 2 (5.8 ppm),
as well as the occurrence of a singlet originating from
Ph3CH (5.6 ppm). At HFiP concentrations under 10%,
only traces of product appeared.
Ta ble 2. Hyd r id e Abstr a ction sa Usin g Va r iou s
Ca r boca tion P r ecu r sor s
solvent
CH2Cl2/HFiP
product
entry
hydride abstractor
(yield, %)b
The results of preparative reactions employing dif-
ferent concentrations of HFiP are presented in Table
1. These reactions were run under oxygen-free condi-
tions using ordinary solvent grades. The color of the
product was useful as a measure of the performance of
the reaction and the purity of the product, which, when
pure, should be bright yellow. In the absence of HFiP,
no hydride abstraction took place. With only 2 or 5% of
HFiP present in the reaction, the isolated yields of 2a
were 35 and 66% of blue and green solids, respectively
(entries 1 and 2). We interpreted these reaction out-
comes in terms of competing hydrolysis of trityl bromide
at low HFiP content, leading to partial protolytic
decomposition of 1 or possibly 2. Although the material
isolated from these reactions gave clean NMR spectra,
the observed discoloration was indicative of the presence
of inorganic, molybdenum-containing decomposition
products. However, when the reaction mixture was
made up with 10 or 20% of HFiP (entries 3 and 4, Table
1), the isolated yields of 2a were much better, 82 and
86%, respectively, and were accompanied by improve-
ment of product purity, returning yellow-green or yellow
solids. In conclusion, the most reliable results from the
procedure are obtained using HFiP concentrations of at
least 10%. Surprisingly, replacing dichloromethane with
chloroform and using 20% HFiP, the product 2a could
be isolated in only 30% yield. Similarly, experiments
evaluating a variety of other solvents, including aceto-
nitrile, were uniformly unsuccessful.
Ca r boca tion P r ecu r sor s. An extension of the HFiP-
supported hydride abstraction protocol to additional,
readily available carbocation precursors would enhance
its applicability. In particular, our incentive for initiat-
ing the studies summarized here was a need for a less
sterically demanding alternative to trityl ion. We first
examined whether the bromine of Ph3CBr could be
exchanged for other nucleofuges. This would possibly
be interesting in order to extend availability of diene
complexes beyond the currently known hexafluorophos-
phates and tetrafluoroborates.18 The preparative results
using a few different trityl derivatives (3a -e) are
summarized in Figure 3 and Table 2 (entries 1-5).
1
2
3
4
5
6
7
Ph3CBr, 3a
Ph3CCl, 3b
Ph3COH, 3c
Ph3CCN, 3d
Ph3COCH3, 3e
Ph2CHBr, 4
(p-MeOC6H4)2CHBr, 5
80/20
80:20
75/25
80/20
95/5
2a (80-86)
2b (62)
no reacnc
no reacn
no reacn
2a (25)
80/20
80/20
2a (84)
a
Reactions employed 1 (100-200 mg) and 1.2 equiv of hydride
abstractor in 10 mL of solvent mixture. After reaction at 0 °C for
1-2 h, the product was isolated through precipitation with diethyl
b
ether. The substrate used in all cases was 1. Isolated yield.
c Addition of 1.5 equiv of HPF6 returned an 80% yield of (η5-
cyclopentadienyl)dicarbonyl[(1-4-η)cyclohexadiene]molybdenum
hexafluorophosphate.
Trityl chloride (3b) gave chloride 2b as yellow crystals
in a yield of 62%. Possibly, the lower yield returned in
this experiment, compared to analogous synthesis of the
bromide 2a , was a consequence of a more troublesome
isolation, since the precipitate did not settle very well.
As shown in entries 3-5 of Table 2, attempted use of
trityl alcohol (3c), trityl cyanide (3d ), and methyl trityl
ether (3e) as precursors met with failure. Still, the
addition of 1.5 equiv of HPF6 to the reaction employing
trityl alcohol gave an 80% yield of (η5-cyclopentadienyl)-
dicarbonyl [(1-4-η)-cyclohexadiene]molybdenum hexaflu-
orophosphate.
We were especially intrigued by the opportunity to
identify less sterically compromised hydride abstractors,
since that would potentially allow a much wider range
of organomolybdenum substrates to take part in syn-
thetically useful sequences, such as those depicted in
Figure 1. Entry 6 of Table 2 represents our first success
in this direction, employing commercially available
bromodiphenylmethane (4). Although the desired 2a
was only isolated in modest yield, this result encouraged
us to study alternative diarylhalomethanes. Further
stabilization of the presumed intermediate benzhydryl
cation by introduction of electron-donating substituents
in the phenyl rings turned out to be very productive.
Bromobis(4-methoxyphenyl)methane (5), readily avail-
able from 4,4′-dimethoxybenzhydrol,19 returned an ex-
cellent yield (84%) of 2b under the standard conditions
developed for trityl bromide. We regard this resultsto
our knowledge the first example of the use of a non-
trityl-derived carbocation precursor for hydride abstrac-
(18) Complex fluorine-containing counterions display reactivity, for
example, toward silyl ethers: (a) Baxter, J . S.; Green, M.; Lee, T. V.
J . Chem. Soc., Chem. Commun. 1989, 1595. (b) Butters, C.; Carr, N.;
Deeth, R. J .; Green, M.; Green, S. M.; Mahon, M. F. J . Chem. Soc.,
Dalton Trans. 1996, 2299.
(19) Henneuse, C.; Boxus, T.; Tesolin, L.; Pantano, G.; Marchand-
Brynaert, J . Synthesis 1996, 495.

Cationic Cyclopentadienyl Diene Mo Complexes
Organometallics, Vol. 20, No. 5, 2001 993
as well as heterogeneous diarylmethyl halides for hy-
dride abstraction from sterically hindered allylic mo-
lybdenum complexes and in enantioselective hydride
abstractions are underway in our laboratories.
Con clu sion
We have developed an alternative method for the
preparation of cationic diene molybdenum complexes,
using either trityl halides or hitherto unexplored dia-
rylmethyl halides as carbocation precursors. The pres-
ence of hexafluoro-2-propanol as a cosolvent is critical
for the hydride abstraction to proceed. Yields obtained
by the convenient homogeneous or heterogeneous pro-
cedures described herein compare favorably with those
reported for the traditional conditions. The results
observed with bromobis(p-methoxyphenyl)methane hold
promise that the methods might find further applica-
tion, especially with more highly substituted allylic
molybdenum complexes as substrates.
F igu r e 4. Abstraction of the severely hindered hydride
of 6 effected by halide 5.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All reactions were performed
under an argon atmosphere. Solvents and reagents were used
as received from commercial sources, unless otherwise indi-
cated. HFiP was purchased from Lancaster Synthesis. Trityl
chloride resin and bromo(4-methoxyphenyl)methyl polystyrene
resin were from Advanced ChemTech and NovaBiochem,
respectively. Degassing of solvents was accomplished through
ultrasound irradiation for approximately 1 h. NMR spectra
were recorded on a Bruker ARX400 instrument at 400.132
F igu r e 5. Heterogeneous hydride abstractions.
tionsto be the most significant in the series. To compare
the utility of bromide 5 with 3a , we performed hydride
abstractions employing the sterically demanding sub-
strate (η5-cyclopentadienyl)dicarbonyl[(2-4-η)-1-phenyl-
2-cyclohexen-4-yl]molybdenum (6). Experiments in the
NMR tube, utilizing 2 equiv of the carbocation precur-
sors, indicated a more rapid reaction with 5. Similarly,
in a preparative run performed under our standard
conditions, carbocation precursor 5 delivered a modest
yield of 7,20 whereas 3a provided only traces of the
product (Figure 4).21
Heter ogen eou s Reaction s u sin g P olystyr en e Res-
in s. The advent of high-throughput synthesis has
recently popularised the utilization of polymer-sup-
ported reagents, which facilitate product isolation by
simple filtration. A report15 describing the use of HFiP
for cleaving peptides from a trityl resin invited the
application of trityl chloride resin (8) in our hydride
abstraction procedure. This commercially available
reagent was found to effectively replace trityl chloride
in the preparation of 2b, which could be isolated in 85%
yield (Figure 5). The resin was carefully washed with
CH2Cl2 before the reaction solvent and allylic substrate
1 were added. Introduction of HFiP to the mixture
resulted in a sharp color change, turning the resin dark
red. Simple filtration and addition of diethyl ether to
the filtrate returned pure 2b. Similarly, when bromo-
(4-methoxyphenyl)methyl polystyrene resin 9 was em-
ployed, cationic diene complex 2a was isolated in 66%
yield. The lower yield as compared to the corresponding
homogeneous reaction involving 5 might reflect less
efficient stabilization of a cation derived from 9. Further
studies regarding the use of substituted homogeneous
MHz (1H) or 100.6 MHz (13C) or on
a Bruker ARX300
instrument at 300.135 MHz (1H) or 75.5 MHz (13C). The
chemical shifts are given relative to CD2Cl2 (δ 5.32, residual
1
1H; δ 54.0, 13C) and CD3CN (δ 1.94, residual H; δ 118.7, 13C).
IR spectra were recorded on an FT-IR Nicolet Impact 410.
Melting points were uncorrected. Elemental analyses were
obtained from H. Kolbe Mikroanalytisches Laboratorium,
Mu¨lheim a. d. Ruhr, Germany. (η5-Cyclopentadienyl)dicarbo-
nyl[(1-3-η)-1-cyclohexen-3-yl]molybdenum (1),9b bromobis(4-
methoxyphenyl)methane (5),19 trityl cyanide (3d ),22 and methyl
trityl ether (3e)23 were prepared according to the literature.
Rep r esen ta tive P r oced u r e for Hom ogen eou s Hyd r id e
Abstr a ction s. (η5-Cyclop en ta d ien yl)d ica r bon yl[(1-4-η)-
cycloh exa d ien e]m olybd en u m Br om id e (2a ). In a Schlenk
flask were placed degassed 1,1,1,3,3,3-hexafluoro-2-propanol
(2.0 mL), degassed dichloromethane (6.0 mL), and (η5-cyclo-
pentadienyl)dicarbonyl[(1-3-η)-1-cyclohexen-3-yl]molybde-
num (1; 0.21 g, 0.71 mmol). The flask was sequentially flushed
with argon and evacuated several times. The flask was cooled
to 0 °C, whereupon bromobis(4-methoxyphenyl)methane (5;
0.28 g, 0.92 mmol, 1.3 equiv), dissolved in degassed dichlo-
romethane (2 mL), was added, and the reaction mixture was
stirred at 0 °C for 1 h. Degassed diethyl ether (40 mL) was
added, and the solid was allowed to settle before the super-
natant was removed. The product precipitate was triturated
with 2 × 10 mL of diethyl ether, leaving, after drying under
vacuum, 0.32 g (85%) of (η5-cyclopentadienyl)dicarbonyl[(1-
4-η)-cyclohexadiene]molybdenum bromide (2a ). Mp: ∼140 °C
dec. Anal. Calcd for C13H13BrMoO2: C, 41.41; H, 3.48. Found:
C, 41.22; H, 3.62. 1H NMR (CD2Cl2): δ 6.50 (br s, 2H), 6.00 (s,
5H), 5.00 (br s, 2H), 2.25 (br d, J ) 13.0 Hz, 2H), 2.02 (br d, J
) 13.0 Hz, 2H). 13C NMR (CD2Cl2): δ 225.1, 94.9, 89.4, 84.8,
24.6. IR (KBr): 2000, 1934 cm-1. HRMS (FAB+): m/z calcd
for C13H13MoO2, 298.9969; found M+, 298.9963.
(20) Product isolation was compromised by incomplete precipitation
of the diene complex. However, addition of diethyl ether to the crude
reaction mixture returned a 26% isolated yield of pure 7.
(21) The corresponding tetrafluoroborate has been discussed earlier,
but no experimental data were provided; see: Wang, S.-H.; Cheng, Y.-
C.; Lee, G.-H.; Peng, S.-M.; Liu, R.-S. Organometallics 1993, 12, 3282.
(22) Zieger, H. E.; Wo, S. J . Org. Chem. 1994, 59, 3838.
(23) Stoochnoff, B. A.; Benoiton, N. L. Tetrahedron Lett. 1973, 21.

994 Organometallics, Vol. 20, No. 5, 2001
Bjurling et al.
Heter ogen eou s Rea ction s u sin g Tr ityl Ch lor id e P oly-
styr en e Resin . (η5-Cyclop en ta d ien yl)d ica r bon yl[(1-4-η)-
cycloh exa d ien e]m olybd en u m Ch lor id e (2b). Trityl chlo-
ride polystyrene resin (0.94 mmol, 1.4 equiv) was placed in a
Schlenk flask and washed with 3 × 10 mL of degassed
dichloromethane before 1,1,1,3,3,3-hexafluoro-2-propanol (de-
gassed, 2 mL) and dichloromethane (degassed, 8 mL) were
added. The polymer became dark red. The flask was cooled to
0 °C, whereupon 1 (0.20 g, 0.68 mmol) was added, and the
reaction mixture was stirred at 0 °C for 2 h.
to 0 °C, whereupon a 1 M solution of PhMgBr in THF (5 mL,
5 mmol, 7.5 equiv) was added. The reaction mixture was
stirred for 2 h at 0 °C. Saturated aqueous NH4
Cl (30 mL) was
added, and the reaction mixture was extracted with 2 × 30
mL of dichloromethane. The combined organic phases were
dried over Na2SO4, filtered, and evaporated, and the crude
product was subjected to flash chromatography (heptane/CH2-
Cl2, 4:1, Rf ) 0.15) to give 0.19 g (78%) of a yellow solid. 1H
NMR (CD3CN, 300 MHz): δ 7.44-7.48 (m, 2H), 7.30-7.36 (m,
2H), 7.21 (tt, J ) 7.3 and 1.5 Hz, 1H), 5.41 (s, 5H), 4.59 (t, J
) 7.0 Hz, 1H), 3.87-3.91 (m, 1H), 3.62 (dm, J ) 7.0 Hz, 1H),
3.01 (dm, J ) 7.0 Hz, 1H), 2.02-2.12 (m, 1H), 1.57-1.66 (m,
1H), 0.88-0.95 (m, 1H) and 0.69-0.82 (m, 1H). 13C NMR (CD3-
CN): δ 238.7, 227.8, 149.4, 129.9, 128.8, 127.5, 94.0, 59.5, 58.3,
(a ) Isola tion by P r ecip ita tion . The reaction mixture was
filtered under an inert atmosphere, and the resin was rinsed
with 2 × 10 mL of dichloromethane. Diethyl ether (200 mL)
was then added to the combined organic solutions, and the
yellow solid was allowed to settle before the supernatant was
removed. The product was triturated with 2 × 15 mL of diethyl
ether, giving, after drying under vacuum, 0.19 g (85%) of (η5-
cyclopentadienyl)dicarbonyl[(1-4-η)cyclohexadiene]molybde-
num chloride (2b). Mp: ∼ 140 °C dec. Anal. Calcd for
57.2, 40.1, 27.2, 20.2. HRMS (FAB+): m/z calcd for C19H18
-
MoO2, 376.0361; found M+, 376.0356.
(η5-Cyclop en ta d ien yl)d ica r bon yl[(1-4-η)-5-p h en ylcy-
cloh exa d ien e]m olybd en u m Br om id e (7). In a Schlenk
flask were placed degassed 1,1,1,3,3,3-hexafluoro-2-propanol
(2.0 mL), degassed dichloromethane (6.0 mL), and 6 (0.10 g,
0.27 mmol). The flask was sequentially flushed with argon and
evacuated several times. The flask was cooled to 0 °C,
whereupon bromobis(4-methoxyphenyl)methane (5; 0.17 g,
0.55 mmol, 2.0 equiv), dissolved in degassed dichloromethane
(2 mL), was added, and the reaction mixture was stirred at 0
°C. After 1 h, an additional portion of 5 (70 mg, 0.23 mmol,
0.8 equiv) was added, and stirring was continued for 45 min.
The yield of (η5-cyclopentadienyl)dicarbonyl[(1-4-η)-5-phenyl-
cyclohexadiene]molybdenum bromide (7) was determined by
NMR integration, using 1,3,5-trimethoxybenzene as the in-
ternal standard. After addition of the standard (22.5 mg), the
crude reaction mixture was concentrated under reduced pres-
sure, affording an oil. Integration of relevant peaks (CD3CN)
indicated an approximately 60% yield of the desired 7.
Subsequent addition of degassed diethyl ether (100 mL) to the
crude material gave partial precipitation of the product (NMR
analysis confirmed the presence of additional 7 in the filtrate),
C
13H13ClMoO2: C, 46.94; H, 3.94. Found: C, 47.11; H, 4.06.
1H NMR (CD2Cl2): δ 6.38 (br t, J ) 4.0 Hz, 2H), 5.95 (s, 5H),
4.82 (br s, 2H), 2.26 (br d, J ) 13.3 Hz, 2H), 2.03 (br d, J )
13.3 Hz, 2H). 13C NMR (CD2Cl2): δ 224.8, 94.8, 88.9, 84.4, 24.5.
IR (KBr): 1996, 1934 cm-1
.
(b) Isola tion by Con cen tr a tion . Trityl chloride polysty-
rene resin (0.48 mmol, 1.4 equiv) was placed in a Schlenk flask
and washed with 3 × 10 mL of degassed dichloromethane
before dichloromethane (degassed, 8 mL) and 1 (0.10 g, 0.34
mmol) were added. The flask was cooled to 0 °C, whereupon
1,1,1,3,3,3-hexafluoro-2-propanol (degassed, 2 mL) was added.
The polymer became dark red. The reaction mixture was kept
at 0 °C (ice bath) overnight without stirring and then filtered
under an inert atmosphere. The resin was rinsed with 2 × 10
mL of dichloromethane. Evaporation of the combined filtrates
gave, after drying under vacuum, 0.1 g (83%) of 2b.
Heter ogen eou s Reaction s u sin g Br om o(4-m eth oxyph e-
n yl)m eth yl P olystyr en e Resin . Bromo(4-methoxyphenyl)-
methyl polystyrene resin (1.0 mmol, 1.5 equiv) was placed in
a Schlenk flask and washed with 2 × 10 mL of degassed
dichloromethane. Degassed dichloromethane (8 mL) and 1
(0.20 g, 0.68 mmol) were added. The flask was cooled to 0 °C,
whereupon degassed 1,1,1,3,3,3-hexafluoro-2-propanol (2 mL)
was added dropwise. The reaction mixture was stirred at 0
°C for 2 h and then filtered under an inert atmosphere into
diethyl ether (250 mL). The resin was rinsed with additional
of dichloromethane (2 × 10 mL). The solid was allowed to settle
before the supernatant was removed. The product was finally
triturated with 2 × 15 mL of diethyl ether, giving, after drying
under vacuum, 0.17 g (66%) of (η5-cyclopentadienyl)dicarbonyl-
[(1-4-η)-cyclohexadiene]molybdenum bromide (2a ).
1
and inert filtration then returned 0.032 g (26%) of pure 7. H
NMR (CD3CN, 300 MHz): δ 7.30-7.35 (m, 2H), 7.19-7.25 (m,
3H), 6.18 (br s, 1H), 6.00 (br s, 1H), 5.83 (s, 5H), 4.69-4.72
(m, 1H), 4.56 (br d, J ) 4.5 Hz, 1H), 3.57 (dm, J ) 10.5 Hz,
1H), 2.62-2.72 (m, 1H), 2.3 (1H, obscured by the water signal).
13C NMR (CD3CN): δ 222.7, 222.5, 146.1, 130.4, 128.64,
128.61, 95.7, 88.2, 86.8 (br, weak), 84.4 (br, weak), 84.1, 45.3,
and 34.5 (partial decomposition complicated recording the 13C
NMR). HRMS (FAB+): m/z calcd for C19H17MoO2, 375.0283;
found M+, 375.0287.
Ack n ow led gm en t. This work was supported by the
Swedish Natural Science Research Council and Kungli-
ga Fysiografiska Sa¨llskapet i Lund. We are grateful to
the late Professor Lennart Eberson for his encourage-
ment and helpful comments. This paper is dedicated to
his memory.
(η5-Cyclop en t a d ien yl)d ica r b on yl[(2-4-η)-1-p h en yl-2-
cycloh exen -4-yl]m olybd en u m (6). In a dry, two-necked
flask were placed (η5-cyclopentadienyl)dicarbonyl[(1-4-η)-cy-
clohexadiene]molybdenum hexafluorophosphate9b,24 (0.30 g,
0.67 mmol) and dry THF (20 mL). The suspension was cooled
(24) Compound 6 may be similarly prepared from 2a or 2b.
OM000839N