Aldehyde decarbonylation catalysis under mild conditions

Article abstract of DOI:10.1021/om9905106

Reaction of [RhCl(NBD)]₂ with 2.0 equiv of triphos (triphos = bis(2-diphenylphosphinoethyl)phenylphosphine; NBD = bicyclo[2.2.1]hepta-2,5-diene) in THF solution at room temperature affords [Rh(NBD)(triphos)][Cl] (4a), which was isolated as [Rh(NBD)(triphos)]-[SbF₆] (4b) in 67% yield. Treatment of 4b with aqueous formaldehyde in THF solution at 80 °C forms [Rh(CO)(triphos)][SbF₆] (2a), which reversibly binds a second equivalent of CO_(g) to give [Rh(CO)₂(triphos)][SbF₆] (2b). The complex [Rh(CO)(triphos)][SbF₆] has been found to be an effective aldehyde decarbonylation catalyst for primary and aryl aldehydes at temperatures as low as that of refluxing dioxane, with little or no undesirable side products resulting from β elimination or radical rearrangement.

Full text of DOI:10.1021/om9905106

Organometallics 1999, 18, 5311-5317
5311
Ald eh yd e Deca r bon yla tion Ca ta lysis u n d er Mild
Con d ition s
Christopher M. Beck, Scott E. Rathmill, You J ung Park, J unyi Chen, and
Robert H. Crabtree*
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06511
Louise M. Liable-Sands and Arnold L. Rheingold*
University of Delaware, Department of Chemistry, Newark, Delaware 19716
Received J uly 1, 1999
Reaction of [RhCl(NBD)]2 with 2.0 equiv of triphos (triphos ) bis(2-diphenylphosphino-
ethyl)phenylphosphine; NBD ) bicyclo[2.2.1]hepta-2,5-diene) in THF solution at room
temperature affords [Rh(NBD)(triphos)][Cl] (4a ), which was isolated as [Rh(NBD)(triphos)]-
[SbF6] (4b) in 67% yield. Treatment of 4b with aqueous formaldehyde in THF solution at 80
°C forms [Rh(CO)(triphos)][SbF6] (2a ), which reversibly binds a second equivalent of CO(g)
to give [Rh(CO)2(triphos)][SbF6] (2b). The complex [Rh(CO)(triphos)][SbF6] has been found
to be an effective aldehyde decarbonylation catalyst for primary and aryl aldehydes at
temperatures as low as that of refluxing dioxane, with little or no undesirable side products
resulting from â elimination or radical rearrangement.
In tr od u ction
However, attempts to make this system catalytic failed
because 1 does not react with aldehydes at temperatures
below 200 °C. Complex 1 is believed to be inactive
because the electronically depleted Rh(I) center prohib-
its the necessary oxidative addition of aldehyde C-H
bonds and loss of CO is difficult. One approach has been
to introduce a reagent that abstracts a CO ligand. In
imaginative work, O’Connor5 has shown that the pres-
ence of stoichiometric amounts of the CO abstraction
reagent P(O)(OPh)2N3 allows Wilkinson’s catalyst to be
catalytic for the decarbonylation of aldehydes at room
temperature; however, the process was limited to pri-
mary aldehydes. In addition, intriguing results by
Goldman6 suggest that the dimeric species Rh2(PMe3)2-
(CO)2Cl2 is an active aldehyde decarbonylation catalyst
by direct oxidative addition of the aldehyde C-H bond,
without prior dissociation of any ligands from the
complex.
Despite much effort, transition metal catalysts for
aldehyde decarbonylation1 still require forcing condi-
tions such as elevated temperature. This is exemplified
by the demanding conditions (190-210 °C) required for
the decarbonylation of various campholenal derivatives
with several transition metal compounds reported by
Chapuis.2
Although not all catalysts require such high temper-
atures, even the more effective catalysts for the decar-
bonylation of aldehydes, Ru(II), Rh(I), and Ir(I) com-
plexes with bis (bidentate phophines),1a still require
temperatures of 150-180 °C. The synthetic utility of
this process would be much enhanced if more active
catalysts, effective at lower temperatures, could be
found. Transition metal porphyrin complexes3 have also
been shown to be effective decarbonylation catalysts;
however, these proceed by a radical mechanism and, as
a consequence, are unselective and can give unwanted
rearranged products.
Reported here is the development of a new catalyst,
[Rh(CO)(triphos)][SbF6] (2a ), which is capable of selec-
tive aldehyde decarbonylation at temperatures ap-
proaching 100 °C.
As early as 1965, Tsuji4 reported that the decarbon-
ylation of aldehydes by stoichiometric quantities of
Wilkinson’s catalyst (RhCl(PPh3)3) under ambient con-
ditions generates mixtures of alkanes, alkenes, and the
stable inorganic product trans-RhCl(CO)(PPh3)2 (1).
Resu lts a n d Discu ssion
Ca ta lyst Design a n d Syn th esis. In designing a
catalyst for aldehyde decarbonylation, we sought to
meet three criteria. The first requirement was to achieve
a more electron-rich Rh (I) center than complex 1, to
facilitate oxidative addition of the aldehyde C-H bond.
The second objective was to construct a system that
would disfavor â elimination and thus prevent the
formation of unwanted alkene side products previously
(1) (a) Doughty, D. H.; Pignolet, L. H. In Homogeneous Catalysis
with Metal Phosphine Complexes; Pignolet, L. H., Ed.; Plenum: New
York, 1983; Chapter 11, pp 343-375. (b) Alyea, E. C.; Meek, D. W.
Catalytic Aspects of Metal Phosphine Complexes; American Chemical
Society: Washington, D.C., 1980; Chapter 4, pp 65-83.
(2) Chapuis, C.; Winter, B.; Schulte-Elte, K. H. Tetrahedron Lett.
1992, 33, 6135.
(3) (a) Domazetis, G.; Tarpey, B.; Dolphin, D.; J ames, B. R. J . Chem.
Soc., Chem. Commun. 1980, 939. (b) Belani, R. M.; J ames, B. R.;
Dolphin, D.; Rettig, S. J . Can. J . Chem. 1988, 2072.
(5) (a) O’Connor, J . M.; Ma, J . J . Org. Chem. 1992, 57, 5075. (b)
O’Connor, J . M.; Ma, J . Inorg. Chem. 1993, 32, 1866.
(6) Abu-Hasanayn, F.; Goldman, M. E.; Goldman, A. S. J . Am. Chem.
Soc. 1992, 114, 2520.
(4) (a) Tsuji. J .; Ohno, K. Tetrahedron Lett. 1965, 3969. Tsuji, J .;
Ohno, K. J . Am. Chem. Soc. 1968, 90, 94. (b) Tsuji, J .; Ohno, K. J .
Am. Chem. Soc. 1968, 90, 99.
10.1021/om9905106 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 11/17/1999

5312 Organometallics, Vol. 18, No. 25, 1999
Beck et al.
observed with other decarbonylation catalysts. Finally,
any potential catalyst must be able to expel CO from
its coordination sphere in order to prevent catalyst
poisoning.
In light of these criteria, we felt a cationic carbonyl
complex of Rh(I) that incorporated a tridentate phos-
phine might offer the best hope of a synthetically useful
catalyst, and to accomplish this we chose the com-
mercially available phosphine triphos. We felt that a
complex such as [Rh(CO)(triphos)]+ might offer several
advantages over both Rh(CO)Cl(PPh3)2 and the known1a
high-temperature decarbonylation catalysts such as
those reported by Pignolet. First, the three-point coor-
dination of this phosphine would create a more basic
metal center than complex 1. Second, the chelate-
enforced coordinative saturation of the metal center
following oxidative addition of the aldehyde C-H bond
might well serve to prevent â elimination and subse-
quent formation of alkene byproducts. Last, similar
chelating phosphines7 have been shown to form cationic
carbonyl complexes with Rh(I) that undergo reversible
coordination of a second equivalent of CO, which might
serve as a mechanism to regenerate the catalyst under
appropriate reaction conditions (e.g., vigorous reflux or
argon purge).
(1)
1.90 and 2.95 ppm, integrating to eight protons. The 31
P
NMR spectrum of 4b shows a doublet of triplets at 94.9
ppm arising from the central phosphorus atom (P2)
coupling to the rhodium center (1J P-Rh ) 121 Hz) and
two magnetically equivalent wingtip phosphorus atoms
(P1 and P3, 2J P-P ) 22 Hz). The wing phosphorus atoms
appear as a doublet of doublets resonating at 56.2 ppm
(1J P-Rh ) 125 Hz).
Figure 1 shows an ORTEP diagram of the [Rh(NBD)-
(triphos)] cation, with crystallographic data given in
Table 1 and selected bond lengths and angles given in
Table 2. The structure is a distorted trigonal bipyramid,
with the central phosphorus atom (P2) and the C1-C2
alkene double bond occupying the axial positions and
the two wingtip phosphorus atoms (P1, P3) and the C6-
C7 alkene double bond occupying the equatorial posi-
tions. A similar geometry was reported by Kubiak et
al.10 for [Ir(COD)(triphos)][Cl]. This geometry is prob-
ably preferred so as to allow the equatorial alkene
ligand to participate fully in back-bonding from Rh(I),
which is always strongest in the equatorial position of
a trigonal bipyramidal structure. This effect is evidenced
clearly in the bond lengths of the equatorial and axial
alkenes; that for the equatorial alkene (C6-C7), which
participates the most in back-bonding, is 1.418(4) Å,
while that for the axial alkene is only 1.378(4) Å long.
On heating in toluene solution at 90 °C for 2 h,
[Rh(NBD)(triphos)][Cl] (4a ) was quantitatively con-
verted into the known compound RhCl(triphos) (3a ),
which was spectroscopically identical to 3a , as previ-
ously reported9 (eq 2). This result suggests that 4a is
Although [Rh(CO)(triphos)][Cl] has been previously
reported,8 we sought to prepare complex 2 by metathesis
of the chloride ligand of RhCl(triphos) in the presence
of CO. To prepare RhCl(triphos), we turned to the
previous work by Marder et al.,9 who reported the
reaction of [RhCl(COD)]2 (COD ) 1,5-cyclooctadiene)
with 2 equiv of triphos in either THF at ambient
temperatures or refluxing toluene yielded RhCl(triphos)
(3a ). However, our attempts to prepare this compound
by treatment of a THF solution of the norbornadiene
analogue [RhCl(NBD)]2 with 1.0 equiv of triphos per Rh
produced a yellow precipitate of the unexpected kinetic
product [Rh(NBD)(triphos)][Cl] (4a ). We recognized that
this kinetic product might be a useful intermediate in
the synthesis of our target catalyst, 2, because of the
presence of the chloride ligand in the outer coordination
sphere. Clean metathesis of the chloride anion was
achieved by treating 4a with excess NaSbF6, and
subsequent purification by silica gel column chroma-
tography afforded the hexafluoroantimonate salt, 4b, as
an analytically pure solid in 67% isolated yield (eq 1).
The tetraphenyl borate analogue of this complex (4c)
was prepared in a similar manner in 75% isolated yield
for crystallographic studies.
1
The H NMR spectrum of 4b shows the presence of
coordinated norbornadiene, which resonates as broad
singlets at 3.47, 3.01, and 1.11 ppm, integrating for 4:2:2
protons, respectively. The resonance at 3.47 ppm was
assigned to the vinylic protons on the basis of their
integration. The ethylene linkages of the triphos ligand
appear as the expected overlapping multiplets between
(2)
(7) Nappier, T. E.; Meek, D. W. J . Am. Chem. Soc. 1972, 94, 306.
(8) Khan, M. M. T.; Martell, A. E. Inorg. Chem. 1974, 13, 2961. These
authors reported the synthesis by a different route of a compound they
suggested was [Rh(CO)(triphos)][Cl]. However, the color, elemental
analysis, and νCO values (νCO ) 1935 cm-1) are all inconsistent with
the data reported here. Upon treatment with CO gas, the authors
reported the formation of [Rh(CO)2(triphos)][Cl] with νCO ) 1940, 1975
cm-1, again inconsistent with our data.
the kinetic product in the reaction of triphos with [RhCl-
(NBD)]2 to give 3 as the thermodynamic product.
Indeed, the norbornadiene ligand of 4b exchanged with
other halides to generate neutral complexes of the type
(triphos)RhX, where X ) Cl, Br, and I. When 4b was
(9) Westcott, S. A.; Stringer, G.; Anderson, S.; Taylor, N. J .; Marder,
T. B. Inorg. Chem. 1994, 33, 4589.
(10) Gull, A. M.; Fanwick, P. E.; Kubiak, C. P. Organometallics 1993,
12, 2121.

Aldehyde Decarbonylation Catalysis
Organometallics, Vol. 18, No. 25, 1999 5313
(3)
F igu r e 1. ORTEP diagram of [Rh(NBD)(triphos)][BPh4]
(4c), shown with 30% thermal ellipsoids for non-hydrogen
atoms.
Ta ble 1. Cysta llogr a p h ic Da ta for
[Rh (NBD)(TRIP HOS)][BP h 4], 4c
formula
fw
space group
a, Å
b, Å
c, Å
C65H61BP3Rh
1048.77
P1
(toluene, sealed tube), whereas reactions with bromide
and iodide required only 48 and 1 h, respectively, at 80
°C in THF (sealed tube). The reactions appeared quan-
titative by 31P NMR, and the spectroscopic data for these
products are shown in Table 3. Despite attempted
purification, (triphos)RhCl could not be obtained in
analytical purity by this synthesis due to contamination
14.5339(2)
14.6240(2)
15.3691(2)
106.5730(8)
104.7022(2)
113.5455(8)
2606.77(5)
2
R, deg
â, deg
γ, deg
V, Å3
1
of both water and 18-crown-6; however, both H and 31P
Z
NMR spectral data are in agreement with those data
previously reported for this compound.9 Related com-
pounds have been synthesized by Nappier and Meek7
by reaction of PhP[CH2CH2CH2PPh2]2 with [Rh(C8H12)X]2
(X ) Cl, Br, I) in refluxing ethanol.
In attempting to prepare our target catalyst 2a , we
were initially disappointed to find that 4b fails to react
directly with CO in THF solution. However, treatment
of 4b with excess aqueous formaldehyde in THF solution
at 80 °C for 49 h (sealed tube) resulted in the formation
of [Rh(CO)(triphos)][SbF6], 2a (eq 4). The 31P NMR
D(calc), g cm-3
µ(Mo KR), cm-1
temp (°C)
radiation
R(F), %a
R(wF2), %a
1.336
4.61
-100(2)
Mo KR (λ ) 0.71-73, Å)
4.19
8.91
Quantity minimized ) R(wF2) ) ∑[w(Fo2 - Fc2)2]/∑[(wFo2)2]1/2
R ) ∑∆/Σ(Fo), ∆ ) |(Fo - Fc)|.
;
a
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 4c
Rh(1)-C(6)
Rh(1)-C(7)
Rh(1)-C(1)
Rh(1)-C(2)
C(1)-C(2)
C(2)-C(3)
C(5)-C(6)
C(3)-C(4)
2.155(3)
2.163(2)
2.271(3)
2.279(3)
1.378(4)
1.528(4)
1.537(4)
1.542(4)
Rh(1)-P(1)
Rh(1)-P(2)
Rh(1)-P(3)
C(1)-C(5)
C(3)-C(7)
C(4)-C(5)
C(6)-C(7)
2.3558(6)
2.2428(7)
2.3461(7)
1.531(4)
1.523(4)
1.536(4)
1.418(4)
C(6)-Rh(1)-C(7)
C(7)-Rh(1)-P(2)
C(7)-Rh(1)-C(1)
C(6)-Rh(1)-C(2)
P(2)-Rh(1)-C(2)
C(6)-Rh(1)-P(3)
P(2)-Rh(1)-P(3)
C(2)-Rh(1)-P(3)
C(7)-Rh(1)-P(1)
C(1)-Rh(1)-P(1)
P(3)-Rh(1)-P(1)
38.35(9)
99.41(8)
76.03(10) P(2)-Rh(1)-C(1)
76.06(11) C(7)-Rh(1)-C(2)
160.08(7)
150.10(7)
84.19(3)
92.68(7)
147.13(7)
87.41(7)
101.02(2)
C(6)-Rh(1)-P(2)
C(6)-Rh(1)-C(1)
97.03(8)
64.29(10)
155.59(7)
63.49(10)
35.26(9)
111.84(7)
119.95(7)
108.84(7)
84.04(2)
C(1)-Rh(1)-C(2)
C(7)-Rh(1)-P(3)
C(1)-Rh(1)-P(3)
C(6)-Rh(1)-P(1)
P(2)-Rh(1)-P(1)
C(2)-Rh(1)-P(1)
C(9)-P(1)-Rh(1)
115.84(7)
108.15(8)
(4)
C(11)-P(2)-Rh(1) 110.25(10) C(10)-P(2)-Rh(1) 109.63(9)
heated with alkali halide salts in THF or toluene with
stoichiometric quantities of 18-crown-6, complexes 3a -c
were obtained in 37-71% isolated yield (eq 3). The
substitution of halide for norbornadiene appears to be
more facile for the series I > Br > Cl, as is evidenced
by the conditions necessary for their synthesis. The
reaction of 4b with chloride required 46 h at 116 °C
spectrum of 2a shows the expected doublets of doublets
for the two magnetically equivalent wingtip phosphorus
2
atoms at 55.0 ppm (1J P-Rh ) 127 Hz, J P-P ) 28 Hz)

5314 Organometallics, Vol. 18, No. 25, 1999
Beck et al.
Ta ble 3. 31P NMR Da ta for th e Com p ou n d s
(tr ip h os)Rh Xa
effective than for secondary aldehydes. For example,
yields of 57% and 65% were obtained after 40 h for
dodecylaldehyde and 4-biphenylcarboxaldehyde to gen-
erate undecane and biphenyl, respectively, while the
decarbonylation of 2-phenylpropanal generated ethyl-
benzene in only 15.7% yield under identical conditions.
In all cases, only mixtures of unreacted aldehyde and
product were observed, and reactions performed in the
absence of catalyst under identical conditions showed
only starting aldehyde. The absence of products such
as undecene and styrene suggests that â elimination
does not occur in this system. Examination of the
inorganic products by 31P NMR spectroscopy revealed
that only compound 2a is present. This suggests that
2a is regenerated under the reaction conditions, and this
might allow for decarbonylation to become catalytic at
modest temperature.
Catalytic decarbonylation of 4-biphenylcarboxalde-
hyde was achieved in refluxing dioxane, but longer
reaction times were needed. Treatment of a 64.2 mM
solution of 4-biphenylcarboxaldehyde with 5 mol % 2a
resulted in a 33.7% yield of biphenyl after 94 h (∼7
turnovers). Although the rate of decarbonylation in this
system is slow, it provides a rare example of a catalyst
capable of decarbonylating aldehydes below 110 °C.
Attempts to increase the efficiency of this catalytic
reaction by moving to higher temperatures, such as
refluxing diglyme (bp 161 °C), were hampered by the
partial decomposition of the catalyst. However, despite
decomposition, synthetically reasonable yields (43%-
100%) were obtained for the decarbonylation of primary
and aryl aldehydes in refluxing diglyme after 48 h.
Again, as can be seen from the data in Chart 1, yields
were lower for secondary aldehydes, with 2-phenyl-
propanal yielding ethylbenzene in only 18%, with traces
(1%) of the â elimination product, styrene.
trans
cis
compound
δP2 J (P-Rh) δP1,P3 J (P-Rh) J (P-P)
(triphos)RhCl 110.8
(triphos)RhBr 112.5
164
167
167
42.4
44.4
47.8
143
142
140
32
31
30
(triphos)RhI
113.0
a
Chemical shifts are reported in ppm relative to external 85%
H3PO4, and coupling constants are reported in hertz.
and a doublet of triplets for the central phosphorus atom
at 104.5 ppm (1J P-Rh ) 116 Hz). The presence of a
carbonyl ligand was established from the strong CO
stretching absorption at 2029 cm-1 in the IR spectrum
of 2a . When a THF solution of 2a was treated with
carbon monoxide gas at room temperature, a new
product was observed by 31P NMR showing a doublet
2
of doublets at 65.4 ppm (1J P-Rh ) 124 Hz, J P-P ) 22
Hz) and a doublet of triplets at 105.7 ppm (1J P-Rh ) 89
Hz). The IR spectrum of this solution shows two distinct
carbonyl stretching absorptions at 2020 and 1968 cm-1
,
suggesting the presence of a second carbonyl group
arising from the coordinatively saturated species
[Rh(CO)2(triphos)][SbF6] (2b). When a solution of 2b,
prepared as described above, was treated with a stream
of nitrogen gas, a 31P NMR spectrum identical to 2a was
obtained, suggesting that the coordination of a second
equivalent of carbon monoxide is reversible (eq 5). The
To investigate the possibility that a radical mecha-
nism could account for the observed decarbonylation,
we chose an aldehyde that would serve as a radical trap,
citronellal.6a,11 Under similar catalytic conditions, one
product, 2,6-dimethyl-2-heptene, was obtained. The
absence of cyclic species such as menthone or isomen-
thone provides strong support for a nonradical mecha-
nism, such as illustrated in Scheme 1. Such a mecha-
nism would be likely to involve formal oxidative addition
of the aldehyde C-H bond at the metal center to
generate a metal acyl complex similar to 5. Complex 5
is then expected to undergo rapid retro-migratory inser-
tion of the acyl R group to generate an alkyl/aryl hydride
complex, generically described as compound 6 in Scheme
1. Subsequent reductive elimination would then yield
alkane or arene and complex 2b, in equilibrium with
2a , completing the catalytic cycle.
(5)
IR data reported here are in disagreement with those
previously reported for [Rh(CO)(triphos)][Cl],8 but are
in close agreement with those reported for [Rh(CO)(PhP-
{CH2CH2CH2PPh2}2)][PF6] by Meek,7 who also reports
reversible CO coordination for this compound.
Ald eh yd e Deca r b on yla t ion s b y [R h (CO)(t r i-
p h os)][SbF 6]. We now find that 2a is active for the
decarbonylation of primary alkyl or aryl aldehydes at
temperatures as low as that of refluxing 1,4-dioxane (bp
103 °C). Yields ranging from 15% to 65% were obtained
for the stoichiometric (Rh:RCHO, 1:1) decarbonylation
of several aldehydes (eq 6). The decarbonylation of
Con clu sion
Treatment of [RhCl(NBD)]2 with 2.0 equiv of triphos
at room temperature results in the kinetic product [Rh-
(NBD)(triphos)][Cl]. Treatment of [Rh(NBD)(triphos)]-
[SbF6] with aqueous formaldehyde afforded [Rh(CO)-
(triphos)][SbF6], which exhibits reversible coordination
of a second equivalent of CO in solution. The complex
(6)
(11) Kampmeier, J . A.; Harris, S. H.; Wedegaertner, D. K. J . Org.
Chem. 1980, 45, 315.
primary and aryl aldehydes was found to be more

Aldehyde Decarbonylation Catalysis
Organometallics, Vol. 18, No. 25, 1999 5315
Ch a r t 1
Sch em e 1
[Rh(CO)(triphos)][SbF6] was found to be an effective
catalyst for the decarbonylation of primary and aryl
aldehydes under mild conditions with little or no
production of unwanted side products resulting from â
elimination or radical rearrangement. Preliminary evi-
dence suggests this catalyst functions by formal oxida-
tive addition of the aldehyde C-H bond.
Exp er im en ta l Section
Cr ysta llogr a p h ic Str u ctu r a l Deter m in a tion . Crystal,
data collection, and refinement parameters are given in Table
1. A crystal suitable for single X-ray diffraction was obtained
by the slow diffusion of pentane into a concentrated sample of
4c in dichloromethane and mounted on the tip of a capillary

5316 Organometallics, Vol. 18, No. 25, 1999
Beck et al.
with epoxy cement. The data were collected on a Siemens P4
diffractometer equipped with a SMART/CCD detector.
No evidence of symmetry higher than triclinic was observed
in the diffraction data. E-statistics suggested the centrosym-
metric space group option, P1, which yielded chemically
reasonable and computationally stable results of refinement.
The structure was solved by direct methods, completed by
subsequent difference Fourier syntheses, and refined with
anisotropic displacement parameters, and hydrogen atoms
were treated as idealized contributions. All software and
sources of the scattering factors are contained in the SHELXTL
(5.1) program library.12
Gen er a l P r oced u r es. All manipulations were performed
in an atmosphere of dry nitrogen or argon using flame-dried
glassware and standard techniques unless otherwise noted.
Tetrahydrofuran and hexane were freshly distilled from potas-
sium metal prior to use. Dichloromethane was distilled from
CaH2 and stored over activated 4 Å molecular sieves. Deionized
water and reagent grade pentane were degassed by three
freeze-pump-thaw cycles prior to use. [RhCl(NBD)]2 was
synthesized according to literature methods.13 Triphos was
purchased from Aldrich Chemical Co. and used without further
purification. NMR spectra were recorded on a GE Omega-300
MHz spectrometer and referenced to residual solvent peaks;
31P NMR (121.6 MHz) spectra were referenced to external 85%
H3PO4. Chemical shifts are reported in ppm, and coupling
constants are reported in hertz. Elemental analyses were
performed by Robertson Microlit Laboratories Inc., Madison,
NJ .
7.6-6.8 (m, 45H), 3.48 (s, 4H), 2.98 (s, 2H), 2.85-1.8 (m, 8H),
1.08 (s, 2H). 31P NMR (CD2Cl2): δ 94.7 (dt, 1J P-Rh ) 121, 2J P-P
1
2
) 23), 56.3 (dd, J P-Rh ) 126, J P-P ) 23). Anal. Calcd for
C
65H61BP3Rh: C, 74.37; H, 5.82. Found: C, 74.42; H, 6.01.
H a lo-η3-(b is(d ip h e n ylp h osp h in oe t h yl)p h e n ylp h os-
p h in e)r h od iu m (I) H exa flu or oa n t im on a t e (3). Ch lor o
Com p lex (3a ). To a 100 mL bulb equipped with a Kontes
valve and magnetic stirrer was added 4b (300. mg, 0.31 mmol),
KCl (150. mg, 2.00 mmol), and 18-crown-6 (150. mg, 0.56
mmol). The solids were then suspended in toluene (30 mL),
and the flask was sealed under nitrogen and heated in an oil
bath at 116 °C for 46 h. The solids were allowed to settle, and
the solution was decanted via cannula into a separate flask
while warm. The solution volume was then reduced in vacuo
to 10 mL, and solids were obtained by the slow addition of
pentane (20 mL), which were then isolated by filtration and
vigorously washed with 5 × 5 mL portions of water and 4 × 5
mL portions of pentane. The solids were dried in vacuo at room
1
temperature for 72 h: yellow solids, yield 75 mg (37.3%). H
NMR (CD2Cl2): δ 8.2-7.3 (m, 25H), 2.7 (m, 4H), 1.75 (m, 4H).
31P NMR (CD2Cl2): δ 110.8 (dt, 1J P-Rh ) 164, 2J P-P ) 32), 42.4
1
2
(dd, J P-Rh ) 143, J P-P ) 32).
Br om o Com p lex (3b). To a 25 mL bulb equipped with a
Kontes valve and magnetic stirrer was added 4b (300. mg 0.31
mmol), KBr (150. mg, 1.26 mmol), and 18-crown-6 (150. mg,
0.56 mmol). The solids were suspended in THF (15 mL), and
the flask was sealed under argon before heating in an oil bath
at 80 °C for 72 h. The yellow solution was then decanted via
cannula into a separate flask before reducing the solvent
volume to ca. 12 mL in vacuo. Yellow solids were obtained by
the slow addition of water (25 mL) and isolated by filtration.
The solids were then washed with 2 × 3 mL of water and 8
mL of hexane before being dried in vacuo at room temperature
for 20 h, and then at 60 °C for 48 h: yellow solids, yield 159
η4-(Bicyclo[2.2.1]h epta-2,5-dien e)-η3-(bis(diph en ylph os-
p h in oeth yl)p h en ylp h osp h in e)r h od iu m (I) Hexa flu or oa n -
tim on a te (4b). To a 500 mL round-bottomed flask, equipped
with a sidearm, was added [RhCl(NBD)]2 (1.3 g, 2.82 mmol).
The solids were then dissolved in THF (30 mL), and triphos
(3.02 g, 5.65 mmol), dissolved in THF (45 mL), was added over
10 min via an addition funnel. As the addition was completed,
a yellow precipitate formed, and the solution was allowed to
stir at room temperature for 4 h, at which point the solution
was evaporated to dryness in vacuo. The remainder of the
procedure was carried out in air using unpurified, reagent
grade solvents. To the solid residue, dissolved in acetone (900
mL) in a 2 L Erlenmeyer flask, was added NaSbF6 (9.60 g,
37.1 mmol) in acetone (200 mL). The solution was then
evaporated to dryness in vacuo at room temperature, and the
solids were suspended in CH2Cl2 (45 mL). The solution was
then filtered, and the solids were washed with 3 × 45 mL
portions of CH2Cl2. The combined CH2Cl2 solution was then
evaporated to dryness to afford orange solids. The compound
was further purified by silica gel column chromatography, first
loading the complex in benzene, then washing with benzene
(400 mL) to remove organic impurities, and finally eluting with
CH2Cl2 (ca 1.5 L), until the elutant was colorless. The CH2Cl2
fractions were then collected and evaporated to dryness to
afford bright yellow solids, which were dried in vacuo over-
mg (71.4%). 1H NMR (CD2Cl2): δ 8.1-7.3 (m, 25H), 3.0-2.4
1
(m, 4H), 1.8 (m, 4H). 31P NMR (CD2Cl2): δ 112.2 (dt, J P-Rh
)
2
1
2
167, J P-P ) 31), 44.4 (dd, J P-Rh ) 142, J P-P ) 32). Anal.
Calcd for C34H33BrP3Rh: C,56.93; H,4.60. Found: C, 56.62;
H, 4.83.
Iod o Com p lex (3c). A similar procedure was employed as
described above but with the following quantities: 4b (300.
mg. 0.31 mmol); NaI (140. mg, 0.93 mmol); 18-crown-6 (50 mg,
0.19 mmol); THF (15 mL). Heating was continued for 1 h at
80 °C to afford an orange solution. An orange solid was isolated
as described for the bromo complex. Yield: 0.165 g (69.5%).
1H NMR (CD2Cl2): δ 7.9-7.3 (m, 25H), 2.7 (m, 4H), 1.8 (m,
4H). 31P NMR (CD2Cl2): δ 113.0 (dt, 1J P-Rh ) 167, 2J P-P ) 30),
1
2
47.8 (dd, J P-Rh ) 139, J P-P ) 29). Anal. Calcd for C34H33IP3-
Rh: C, 53.34; H, 4.35; I, 16.60; P, 12.16. Found: C, 53.57; H,
4.66; I, 16.32; P, 12.27.
(B is (d ip h e n y lp h o s p h in o e t h y l)p h e n y lp h o s p h in e )-
ca r bon ylr h od iu m (I) Hexa flu or oa n tim on a te (2a ). To a 100
mL glass bulb equipped with a Kontes valve was added 1b
(0.500 g, 0.524 mmol). The solids were dissolved in 15 mL of
THF, and to this yellow solution was added formaldehyde (12.5
mL of a 37% aqueous solution). The flask was then sealed
under argon and heated at 80 °C for 49 h. The solution volume
was then reduced in vacuo to ca. 15 mL. Yellow solids, obtained
by the slow addition of reagent grade pentane (10 mL), were
then filtered and washed with pentane (3 × 10 mL) in air and
dried in vacuo for 1 h. The compound was then recrystallized
from anhydrous THF (ca 3 mL) by the slow addition of reagent
grade pentane (10 mL) and then dried in vacuo at 110 °C for
84 h to afford a yellow solid. Yield: 0.170 g (36%). 1H NMR
(CD2Cl2): δ 7.8-7.4 (m, 25H), 3.2 (m, 4H), 2.9 (m, 2H), 2.2
1
night. Yield: 3.65 g (67.1%). H NMR (CD2Cl2): δ 7.65-6.90
(m, 25H), 3.47 (s, 4H), 3.01 (s, 2H), 2.95-1.90 (m, 8H), 1.11
1
2
(s, 2H). 31P NMR (CD2Cl2): δ 94.9 (dt, J P-Rh ) 121, J P-P
)
1
2
22), 56.15 (dd, J P-Rh )125, J P-P ) 23). Anal. Calcd for
41H41F6P3RhSb: C, 51.03; H, 4.25; F, 11.81. Found: C, 50.88;
C
H, 4.45; F, 11.84.
η4-(Bicyclo[2.2.1]h epta-2,5-dien e)-η3-(bis(diph en ylph os-
p h in oeth yl)p h en yl-p h osp h in e)r h od iu m (I) Tetr a p h en yl-
bor a te (4c). A procedure similar to that described above was
employed, except NaBPh4 was substituted for NaSbF6, and the
following quantities were used: [RhCl(NBD)]2 (0.112 g, 0.242
mmol), triphos (0.260 g, 0.486 mmol), THF (8.5 mL), acetone
(35 mL), NaBPh4 (0.860 g, 2.51 mmol), CH2Cl2 (45 mL) to give
a yellow solid. Yield: 0.190 g (75.1%). 1H NMR (CD2Cl2): δ
(m, 2H). 31P NMR (CD2Cl2): δ 104.5 (dt, 1J P-Rh ) 116, 2J P-P
)
2
28), 55.0 (dd,1J P-Rh ) 127, J P-P ) 28). IR (NaCl, film, neat):
νCO ) 2029 cm-1. Anal. Calcd for C35H33F6OP3RhSb: C, 46.64;
H, 3.66; P, 10.32. Found: C, 46.43; H, 3.84; P, 10.31.
Stoichiometric decarbonylations were performed in a 40 mL
flask equipped with a sidearm and charged with 2a (0.150 g,
(12) Sheldrick, G. Siemens XRD; Madison, WI.
(13) Abel, E. W.; et al. J . Chem. Soc. 1959, 3178.

Aldehyde Decarbonylation Catalysis
Organometallics, Vol. 18, No. 25, 1999 5317
0.166 mmol), p-xylene (as internal standard, 20 µL, 0.163
mmol; in the case of 2-phenylpropanal, 9,10-dihydroanthracene
was employed as the standard), 0.17 mmol aldehyde, and 15.0
mL of dioxane to afford a yellow solution. The flask was then
fitted with a reflux condenser and an active argon purge
established by needle inlet at the top of the condenser and
exit gases vented into an oil bubbler. The flask was then
lowered into an oil bath and vigorously refluxed for 41.5 h.
The flask was then cooled, and an aliquot removed and
percolated through silica gel before being analyzed by GC/MS.
Product identity was established by comparison of retention
time and mass spectra to authentic sample. Reported yields
were determined by integration of GC peak area in comparison
to internal standard with calibrated detector responses.
Catalytic experiments were performed in a 10 mL flask
equipped with a sidearm and charged with 2a (0.050 g, 0.056
mmol), aldehyde (1.11 mmol), 9,10-dihydroanthracene (as
internal standard, 0.100 g, 0.559 mmol), and 5.0 mL of
diglyme. A procedure was employed as in the stoichiometric
reactions, with the reflux being carried out for 45-47 h. Data
for the catalytic decarbonylation of various aldehydes are
reported in Chart 1.
Ack n ow led gm en t. The authors are grateful to the
U.S. Department of Energy for funding.
Su p p or tin g In for m a tion Ava ila ble: Tables of X-ray
crystallographic data for complex 4c. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM9905106