8
934 J . Org. Chem., Vol. 65, No. 26, 2000
Burk et al.
the orange rhodium complex was completely absorbed
onto the solid support, as evidenced by transference of
color from solution to the insoluble support material. The
immobilized catalyst 4 was then filtered and extensively
washed with methanol or ethanol (Soxhlet extraction,
formylation of alkenes.11 Aldehyde reduction often is a
desirable step in this sequence which affords valuable
alcohol products from inexpensive starting materials
(e.g., alkenes, hydrogen, and carbon monoxide in the case
of hydroformylation).
1
2-16 h), dried in vacuo, and then used directly in
catalysis.
The mechanism of absorption and the exact nature
of the tethered complex remain undefined. Evidence
suggests that cationic, rather than neutral, transition
metal complexes may be required, as they are most
effectively absorbed onto the heteropoly acid-modified
supports. This notion is corroborated by the observation
that anionic species such as halides and carboxylates
effectively strip the anchored metal complex from the
support of catalysts of type 4. This presumably occurs
by coordination of the anion to the cationic metal center,
which renders the complex neutral and incapable of
interacting with the modified support. The nature of
heteropoly acids allows one to postulate that a relatively
robust linkage between the heteropoly acid and the
alumina (or silica) support may form through intercala-
tion or insertion of an oxygen atom (or atoms) of the
heteropoly acid molecule into the surface lattice of the
Despite the importance of aldehyde reduction in or-
ganic chemistry, surprisingly few generally applicable
manufacturing methods are available for this transfor-
mation. Reduction of 5 through the use of hydride
4 4
reagents (e.g., LiAlH , NaBH , etc.) often is quite facile
and selective, and arguably is the most widely practiced
12
procedure of this type. Unfortunately, hydride reducing
agents are moisture-sensitive reagents that are not
economically attractive for manufacturing since they are
employed in stoichiometric quantities. Moreover, their
use requires tedious workup procedures and generates
substantial quantities of waste (boron or aluminum
salts).
2
Numerous heterogeneous catalysts, such as PtO ,
Raney Ni, and Pd/C, can catalyze the hydrogenation of
9
support. Given the high acidity of phosphotungstic acid,
12
aldehydes. However, heterogeneous catalysts are not
one may hypothesize that the tethered heteropoly acid/
anion could play a role as a counteranion analogous to
triflate for the cationic DiPFc-Rh catalyst. In this context,
the nature of catalyst attachment may be entirely
electrostatic. However, the presence of a weak coordinate
covalent interaction between an oxygen atom of the
phosphotungstic acid component and the rhodium
atom of the pre-catalyst also is plausible (see box in
Scheme 1). A bonding interaction of this type previously
has been observed directly in solution between heteropoly
broadly applicable since they tend to be intolerant of
various organic groups, such as divalent sulfide moieties.
Moreover, heterogeneous catalysts generally do not actu-
ate hydrogenation of aldehydes with a high degree of
chemical selectivity (e.g., other sensitive groups such as
nitro, oxime, ketone, arylhalide, benzyloxy, etc. also are
reduced). Another serious problem encountered when
reducing aromatic aldehydes using heterogeneous cata-
lysts is that any formed hydroxymethyl group may be
further reduced to a methyl substituent. For example,
heterogeneous hydrogenation of benzaldehyde often af-
fords toluene due to facile hydrogenolysis of the inter-
mediate benzyl alcohol.
1
0
acids and rhodium and iridium complexes.
All tethered homogeneous catalysts used in this study
were prepared by the simple procedure outlined in
Scheme 1. Immobilized catalyst variants were fabricated
We previously demonstrated that the homogeneous
DiPFc-Rh catalyst is effective for the hydrogenation of a
range of aldehydes under mild conditions.7 We now
sought to assess the utility of immobilized catalyst 4 in
the hydrogenation of aldehydes bearing different func-
tional groups. These studies were aimed at demonstrat-
ing the combined properties of high catalytic efficiency
under mild conditions, selectivity in the reduction proc-
ess, and tolerance of the catalyst to certain functionality.
The robust nature of the catalyst system also was
important. Moreover, comparisons have been made
with the commonly employed heterogeneous catalysts
2
by changing either the support material (e.g., SiO ) or
,13
homogeneous metal complex used. It is important to note
that little tethering of homogeneous rhodium catalysts
occurs in the absence of the phosphotungstic acid com-
ponent.6
We next sought to examine the catalytic competence
of immobilized catalyst 4 and the analogous silica-
supported system. It is important to note that these
immobilized homogeneous catalysts apparently are stable
to atmospheric oxygen and moisture over extended
periods. The immobilized catalyst 4, which was used in
the hydrogenation processes described below, was stored
for over 1 year in the air, with no apparent loss of
catalytic activity.
palladium-on-carbon (Pd/C) and platinum oxide (PtO ).
2
Preliminary screening experiments using the immo-
bilized catalyst 4 were conducted using the representa-
tive substrate valeraldehyde. All hydrogenations were
performed under a standard set of mild reaction con-
ditions: hydrogen pressure ) 100 psi, temperature )
20 °C, reaction time ) 16 h, mol aldehyde/mol Rh (S/C) )
320 (based upon analysis of Rh content by atomic
Hyd r ogen a tion of Ald eh yd es. Recently, we required
a mild, efficient, and selective procedure for reducing
aldehydes 5 to alcohols 6 on commercial scale (1 kg-ton
quantities). Aldehydes are large volume products that
derive from various sources, including catalytic hydro-
(
7) Burk, M. J .; Harper, T. G. P.; Lee, J . R.; Kalberg, C. Tetrahedron
Lett. 1994, 35, 4963.
8) Appleton, T. D.; Cullen, W. R.; Evans, S.-V.; Kim, T.-J .; Trotter,
J . J . Organomet. Chem. 1985, 279, 5.
9) (a) Izumi, Y.; Hasere, R.; Urabe, K. J . Catal. 1983, 84, 402. (b)
(11) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; J ohn Wiley
& Sons: New York, 1992; pp. 106-111.
(
(12) Hudlicky, M. Reductions in Organic Chemistry; Halsted
Press: New York, 1984.
(
(13) A cationic rhodium catalyst bearing the ligand 1,3-bis(diiso-
propylphosphino)propane was also shown to promote efficient hydro-
genation of two model aldehydes under mild conditions; see: Tani, K.;
Suwa, K.; Tanigawa, E.; Yoshida, T.; Okano, T.; Otsuka, S. Chem. Lett.
1982, 261.
Izumi, Y.; Urabe, K.; Onaka, M. Zeolite, Clay, and Heteropoly Acids
in Organic Reactions; VCH: New York, 1992.
(
10) Pohl, M.; Lyon, D. K.; Mizuno, K.; Nomiya, K.; Finke, R. G.
Inorg. Chem. 1995, 34, 1413.