Room temperature operating allenylidene precatalyst [L(n)Ru=C=C=CR2]+X- for olefin metathesis: Dramatic influence of the counter anion X-

Article abstract of DOI:10.1039/a808859d

The allenylidene-ruthenium(II) complex [(p-cymene)(PCy₃)Cl- Ru=C=C=CPh₂]⁺X^-, 2a, (X^- = CF₃SO₃^-), which is produced in a one-pot reaction from commercially available [RuCl₂(p- cymene)]₂, catalyses the ring-closing metathesis of N,N-diallyltosylamide at room temperature whereas 2b (X^- = BF₄^-) favours the cyclization of the 1,6-diene 3 into the five- membered heterocycle 5 without loss of atoms. The addition of BF₃·OEt₂ to 2b restores its selectivity in the RCM reaction. Catalyst 2a also promotes the metathesis of the ene-yne 6 to produce the 3-vinyl-2,5-dihydrofuran 7.

Full text of DOI:10.1039/a808859d

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Room temperature operating allenylidene precatalyst
[L RuxCxCxCR ]‘X— for oleÐn metathesis: dramatic
n
2
inÑuence of the counter anion X—
Michel Picquet, Daniel Touchard, Christian Bruneau and Pierre H. Dixneuf*
L e t t e r
L aboratoire Organometalliques et Catalyse, UMR 6509 CNRS-Universite de Rennes, Campus de
Beaulieu, F-35042 Rennes, France
Received (in Montpellier, France) 12th November 1998, Accepted 2nd December 1998
The allenylidene-ruthenium(II) complex [(p-cymene)(PCy )Cl-
anions have been prepared. Their activity in RCM catalysis
was studied in the transformation of the simple N,N-di-
allyltosylamide 3 into the N-tosyldihydropyrrole 4 as a model
reaction:
3
RuxCxCxCPh ]‘X—, 2a, (X— = CF SO —), which is pro-
2
3
3
duced in a one-pot reaction from commercially available
[RuCl (p- cymene)] , catalyses the ring-closing metathesis of
2
2
N,N-diallyltosylamide at room temperature whereas 2b (X— =
BF —) favours the cyclization of the 1,6-diene 3 into the Ðve-
4
membered heterocycle 5 without loss of atoms. The addition of
BF Æ OEt to 2b restores its selectivity in the RCM reaction.
3
2
Catalyst 2a also promotes the metathesis of the ene-yne 6 to
produce the 3-vinyl-2,5-dihydrofuran 7.
The precursors 2a (X~ \ CF SO ~) and 2b (X~ \ BF ~)
3
3
4
Alkene metathesis has now emerged as a widely applicable
methodology for organic synthesis,1 especially via ring-closing
metathesis (RCM)2 or ring-opening cross-metathesis (ROM),3
including the generation of optically active compounds.4 This
development was made possible by the discovery of well-
deÐned, soluble molecular catalysts tolerating a variety of
functional groups, such as the ruthenium carbene complexes
Cl (PCy ) RuxCHR.5 The efficiency and advantages of these
were obtained by treatment of the complex RuCl (PCy )(p-
2
3
cymene), 1, with a slight excess of AgOTf or AgBF in
4
dichloromethane at room temperature to generate in situ the
isolatable 16-electron intermediate [(p-cymene)(PCy )-
3
ClRu]`X~. The addition to the latter of an excess of
HCy CC(Ph) OH in dichloromethane led to the isolation
2
of fully characterized 2a and 2b in 95% yields
(Scheme 1).¤ [(p-cymene)(PCy )ClRu]`[BPh ]~, 2c, was also
2
3 2
3
4
ruthenium carbene catalysts have motivated the search for
more straightforward ways to produce neutral, 16-electron
ruthenium complexes containing the secondary RuxCHR
moiety and bulky phosphines6,7 or carbene ligands.8 Recently,
it was found that the 18-electron cationic allenylidene com-
plexes of the type [(arene)(PR )ClRuxCxCxCPh ][PF ],9
obtained in 95% yield by reaction of 1 with an excess of
NaBPh in methanol in the presence of 1.5 equiv. of
4
HCy CC(Ph) OH.¤
2
3
2
6
containing only one bulky phosphine (PR \ PCy , PPri ) per
3
3
3
ruthenium site and a non-coordinating anion, constituted a
new class of RCM catalysts in the same range of activity
(80 ¡C for several hours) as the 16-electron, neutral
[Cl (PCy ) RuxCHR] complexes.5 The efficiency of these
cationic precatalysts raises, however, the question of the inÑu-
ence of the counter anion.
2
3 2
Now we report (i) the Ðrst example of an allenylidene
precatalyst able to perform the RCM reaction at room
temperature, namely the derivative [(p-cymene)(PCy )ClRux
3
CxCxCPh ][O SCF ], 2a, prepared in a one-pot reaction
2
3
3
from commercially available [RuCl (p-cymene)] and via
RuCl (PCy )(p-cymene) 1, (ii) evidence for the unexpected dra-
Scheme 1
2
2
2
3
matic inÑuence of the counter anion X~ on the catalytic activ-
ity of the [L RuxCxCxCPh ][X] salts, (iii) that the
The transformation of 3 (0.5 mmol) into 4 was performed with
2.5 mol% of isolated [RuxCxCxCPh ]`X~ catalysts 2 in
n
2
2
counter anion BF ~ in precursor 2b by contrast favours the
dilute solution (2.5 ml of toluene) at 80 ¡C. The results are
4
cyclization of a 1,6-diene with atom economy, (iv) that the
given in Table 1. The precursor 2a containing the weakly
coordinating TfO~ anion led to a complete conversion of 3
after only 1 h at 80 ¡C and a 91% isolated yield of 4 was
obtained. Moreover, when the same reaction was performed
at room temperature for 19 h, the conversion reached 90%
with exclusive formation of 4. Consequently, catalyst precur-
addition of the Lewis acid BF restores the activity and selec-
3
tivity of 2b as a RCM catalyst and (v) the ability of 2a to
perform the catalytic metathesis of ene-yne into vinyl dihydro-
furan.
In order to evaluate whether the counter anion X~ in the
catalyst precursors [(arene)(PR )ClRuxCxCxCPh ]`X~,
sor 2a is the Ðrst example of a [RuxCxCxCR ] complex
3
2
2
2, had an inÑuence on oleÐn metathesis, new complexes of
able to perform the RCM reaction at room temperature with
type 2 containing the bulky PCy ligand and various X~
an excellent efficiency. It can be noted that the original preca-
3
New J. Chem., 1999, 141È143
141

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Table 1 Activity of [(p-cymene)(PCy )ClRuxCxCxCPh ]`X~, 2,
Thus, the addition of the Lewis acid BF to 2b restores its
3
2
3
in the RCM transformation 3 ] 4a
catalytic activity towards the RCM reaction and leads to a
decrease in the formation of 5. One can make the hypothesis
that BF favours the displacement of the arene ligand allow-
T /¡C
3
ing the RCM reaction. We have shown that traces of HBF in
Heating Conversion Yield of
4
Precursor X~
period/h /%
4/%b
the reaction mixture did not favour the formation of 5, thus
the formation of 5 may arise from initial oxidative coupling of
the two CxC bonds of 3 at the RuII centre, followed by b-
elimination and reductive elimination leading to the formation
2a
2a
2b
2b
2b
2c
CF SO ~
80/1
r.t./19
80/5
80/5
80/4
80/5
99
90
90
8
99
91
99 (91)
90
31c
7.5
96
91
3
3
CF SO ~
3
BF ~
4
3
of the exocyclic HwCH bond; this oxidative coupling is of
BF ~/2 Bun NF
2
4
4
course disfavoured by the presence of a Lewis acid.
BF ~/4 Et O É BF
4
BPh ~
4
2
3
The metathesis of ene-ynes is also a powerful method for
generating conjugated alkenyl cyclooleÐn derivatives such as
cephem derivatives14 or N-containing heterocycles.15 The
interest of vinyl-2,5-dihydrofurans for access to optically
active derivatives via asymmetric catalysis16 led to the con-
sideration of their general preparation via metathesis of
ene-yne ethers in the presence of the most active triÑate salt
2a:
a 0.5 mmol of 3 in 2.5 ml of toluene with 2.5 mol% of precatalyst 2.
b Yields determined by GC or isolated. c In addition to 43% of 5.
talysts [(arene)(PR )ClRuxCxCxCPh ][PF ] containing
3
2
6
the PCy or PPri ligand and the non-coordinating anion
3
3
PF ~ exhibited poor catalytic activity below 80 ¡C.9
6
The efficiency of catalyst 2a led us to perform both its for-
mation and catalysis in one pot in order to study the ease of
the process. Thus 1.3 ] 10~2 mmol of [RuCl (p-cymene)]
2
2
and 2.8 ] 10~2 mmol of PCy were reacted in dichloro-
3
methane at room temperature for 1.5 h, then were added 1.04
equiv. of AgOTf and, after 1 h at room temperature, 1.1 equiv.
of HCy CC(Ph) OH. After 30 min of stirring at room tem-
The ene-yne 6, containing a hindered propargylic carbon
atom was reacted in toluene in the presence of 2.5 mol% of 2a
at 80 ¡C. After 3 h, its transformation was completed and the
3-vinyl-2,5-dihydrofuran 7 was produced selectively and iso-
lated in 82% yield. By contrast, the precatalyst
[RuxCxCxCPh ][PF ] is not so efficient as the total con-
2
perature the dichloromethane was eliminated under vacuum,
5 ml of toluene and 40 equiv. of 3 were successively added.
After 1 h at 80 ¡C the diene 3 was completely and selectively
transformed, and the pyrrole 4 was isolated in 89% yield. In
order to account for the activity of 2a at room temperature, it
can be suggested that the weakly coordinating TfO~ anion
should favour the dissociation of the p-cymene ligand, poss-
ibly via interaction with the allenylidene ligand, and stabilize
the 14-electron species [CF SO É É ÉRuxCxCxCPh (Cl)-
2
6
version of 6 into 7 required 24 h at 80 ¡C.17
This study not only shows that the 18-electron allenylidene
ruthenium salts are efficient precatalysts for ring-closing
metathesis of both dienes and ene-ynes, but their activity dra-
matically depends on the nature of the counter anion, which
can either control the reaction at room temperature
(CF SO ~) or create new catalytic reactions (BF ~).
3
3
2
(PCy )]. Complex 2c containing the bulky BPh ~ counter
3
4
anion showed a catalytic activity only at 80 ¡C and thus is
comparable with the precursor containing the PF ~ anion.9
6
3
3
4
It was surprising that the complex 2b with the non-
coordinating anion BF ~ led to a good conversion of 3 (90%
4
Acknowledgements
The authors wish to thank Professor A. Furstner from the
MaxÈPlanck Institut fur Kohlenforschung for stimulating dis-
cussions.
at 80 ¡C for 5 h) (Table 1) but, in contrast to 19 and 2a, a†ord-
ed a low yield of 4 (31%). Actually, the reaction was not selec-
tive and besides 4, 43% of the product 5 corresponding to the
cyclization of 3 with atom economy10 was obtained:
Notes and references
¤ Typical preparation of complex 2a. 0.65 mmol of RuCl (PCy )(p-
2
3
cymene) and 0.67 mmol of AgOTf were stirred at room temperature in
30 ml of dichloromethane for 1.5 h. After Ðltration, dichloromethane
was removed by distillation under vaccum. The cationic complex [(p-
cymene)RuCl(PCy )]`TfO~ thus formed was washed with 20 ml of a
3
(1 : 1) pentaneÈether mixture and obtained as a brown powder [95%,
This type of cyclization with a 1,6-diene has already been
observed with rhodium,11 nickel,12 and palladium11,13 cata-
lysts but in the presence of an acid (HCl, AcOH or CF SO H)
31P NMR: d 28.33 (PCy )]. To 0.183 mmol of this cationic complex
3
dissolved in 15 ml of dichloromethane was added 1.1 equiv. of HCy
CC(Ph) OH. After 1 h at room temperature, dichloromethane was
3
3
2
in order to generate a MwH bond.
eliminated under vacuum. The crude product was washed twice with
As the anion BF ~ may release F~ and BF , the inÑuence
20 ml of diethyl ether to yield 155 mg (95%) of 2a as a violet powder.
Complexes 2aÈc gave satisfactory IR and 1H and 31P NMR spec-
troscopic analyses. Selected spectroscopic data: 2a IR: m 1963 cm~1
(RuxCxCxC); 31P NMR (81.01 MHz, CDCl ): d 59.21 (PCy ); 2b
4
3
of these reagents on the precatalyst 2b has been studied in the
transformation 3 ] 4 ] 5. The addition of 2 equiv. of Bun NF
per equiv. of 2b slowed down the reaction and after 4 h at
4
3
3
IR: m 1958 cm~1 (RuxCxCxC); 31P NMR (81.01 MHz, CDCl ): d
80 ¡C only 4 was formed in 7.5% yield (Table 1). This shows
that the presence of Ñuoride dramatically inhibits the catalytic
activity of 2b towards RCM reaction. A similar inhibition was
observed in the presence of the chloride anion. The addition of
Et O É BF to the reaction mixture of 3 and 2.5 mol% of 2b
3
59.08 (PCy ); 2c IR: m 1960 cm~1 (RuxCxCxC); 31P NMR (81.01
3
MHz, CDCl ): d 57.28 (PCy ).
3
3
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2
3
increased the catalytic activity and the best results were
obtained with the use of 10 mol% of Et O É BF for 2.5 mol%
2
3
of 2b. At 80 ¡C, the reaction led to the complete conversion of
3 after 4 h and to the formation of 96% of 4 and only 3% of 5.
142
New J. Chem., 1999, 141È143

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Paper 8/08859D
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