M. Rosales et al. / Journal of Organometallic Chemistry 690 (2005) 3095–3098
3097
Rh / 2 dppe
tilled over appropriate standard agents under N2 prior
to use. RhCl3 Æ 3H2O was obtained from Pressure Chem-
icals and alkenes from Aldrich. IR spectra were recorded
in a Nicolet Magna 560 spectrophotometer and NMR
spectra were obtained in a Bruker Avance (300 MHz
OH
+ HCHO
CHO
OH
+
OHC
OH
1
for H) instrument at room temperature in CDCl3. GC
21
1
analyses were performed in a 610 Series UNICAM instru-
ment fitted with a thermal conductivity detector, a 3m
10% SE-30 on Supelcoport glass column and a UNICAM
4815 data system using helium as the carrier gas.
ð3Þ
We found that under our reaction conditions the
complex RhH(CO)(PPh3)3 reported by Ahn et al. [10],
displayed a lower catalytic activity for the hydroformy-
lation of allyl alcohol (24 TN in 4 h) than our system
Rh(acac)(CO)2/2dppe, and about 50% of the products
was propanal. Interestingly, the regioselectivity in this
case is toward the linear aldehyde (n/i > 20).
In order to gain some knowledge on the active species
involved in olefin hydroformylation with paraformalde-
hyde catalyzed by 2/2dppe, Rh(acac)(CO)2 was allowed
to react with 2 eq dppe in dioxane in the presence and in
the absence of an excess of paraformaldehyde. In both
cases, the cationic complex [Rh(dppe)2]+ was generated
(as the acac salt) as characterized by NMR spectros-
3.2. Catalytic runs
In a typical experiment, a solution of the precatalyst
(0.05 mmol), the required amount of the corresponding
phosphine, hex-1-ene (1.9 mL, 15 mmol), paraformalde-
hyde (1.125 g, 40 mmol), cycloheptane as internal stan-
dard (1.0 mL, 8.3 mmol) and dioxane (total vol 15 mL)
were placed in a glass ampoule; the system was flushed
three times with argon and then closed and heated to
the desired reaction temperature. After 4 h with constant
stirring, the ampoule was cooled in ice and the products
were analyzed by GC. Each reaction was repeated at least
twice in order to ensure reproducibility of the results.
1
copy; in the H spectrum the phenyl protons appear as
a complex multiplet at 7.7–7.2 ppm, the CH2 signals of
dppe are observed as a multiplet at 2.2 ppm and a singlet
at 2.5 is assigned to the acacꢀ counter-anion. The
31P{1H} spectrum consists of only a doublet at
58.8 ppm, with J(Rh–P) = 133 Hz. James and Mahajan
[13] have previously reported this complex as the BF4
salt. We thus believe that this cationic bis(phosphine)
species is the precursor entering the catalytic cycle.
In conclusion, olefin hydroformylation was achieved
with paraformaldehyde by use of rhodium–phosphine
catalysts in dioxane under moderate reaction conditions.
The system formed by addition of 2 eq of dppe to Rh(a-
cac)(CO)2 displayed the highest activity at 130 ꢁC.
Although the n/i ratios obtained are modest, this catalyst
could be of use in applications that do not require a high
regioselectivity, such as naphtha upgrading [11,12]. Hyd-
roformylation with paraformaldehyde offers several
advantages over the conventional process with syn-gas:
it enables the reaction to be performed under atmospheric
pressure of an inert gas in conventional glassware, not
requiring high-pressure equipment and avoiding the use
of lethal carbon monoxide. A kinetic analysis of this reac-
tion, as well as further coordination chemistry studies and
calculations are in progress, aimed at better understand-
ing this system and at improving the selectivity properties.
3.3. Stoichiometric reactions
Complex 2 (103 mg, 0.4 mmol) and dppe (320 mg,
0.8 mmol) in dioxane (10 mL) were stirred vigorously
under reflux for 2 h; the solution was evaporated under
vacuum to about 1/3 of its initial volume and the prod-
uct was precipitated by addition of n-pentane, filtered
and dried in vacuo. Yield 80%. The same complex was
obtained when this reaction was carried out in the pres-
ence of an excess of paraformaldehyde (120 mg,
4 mmol).1H RMN (CDCl3, 25 ꢁC,): 7.7–7.2 (series of
m, 40H, PPh2), 2.5 (s, 6H acac) and 2.2 ppm (m, 8H,
CH2–P); 31P{1H} RMN (CDCl3, 25 ꢁC,): 58.8 ppm (d,
2JP–Rh = 133 Hz).
Acknowledgements
Financial support from FONACIT (Project CONI-
PET 97-3777) and from CYTED (Project V.9) is grate-
fully acknowledged. We thank Consejo de Desarrollo
´
´
Cientıfico y Humanıstico (CONDES) of the Universi-
dad del Zulia (L.U.Z.) for the acquisition of a gas chro-
matograph, and Dr. Y. Alvarado and Mr. A. Fuentes
for their valuable help to record IR and NMR spectra.
3. Experimental section
References
3.1. General procedure
[1] R. Cornils, W.A. HermannApplied Homogeneous Catalysis with
Organometallic Compounds, vol. 1, VCH Publ., 1996, Chapter 2,
p. 303.
Manipulations were performed under N2 using stan-
dard Schlenk techniques. Solvents and olefins were dis-