Hydroxy complexes of palladium(II) and platinum(II) as catalysts for the acetalization of aldehydes and ketones

Article abstract of DOI:10.1016/S1381-1169(98)00299-4

The acetalization of aldehydes or ketones is a reaction of synthetic interest in organic chemistry that is commonly catalyzed by Bronsted or Lewis acids. We have found that a class of Pd(II) and Pt(II) complexes of the type [(P-P)M(μ-OH)]²⁺₂ (P-P = a series of diphosphines; M = Pd, Pt) are effective catalysts for the acetalization of a variety of aldehydes and ketones in the presence of alcohols or glycols. The use of epoxides instead of alcohols resulted in lower rates and yields. The rate and yield of the reaction is not affected by the size of the diphosphine in the series dppe, dppp, dppb. Attempts to perform the reaction in a stereoselective fashion showed no difference depending on whether regular achiral catalysts or catalysts modified with chiral diphosphines were used.

Full text of DOI:10.1016/S1381-1169(98)00299-4

Journal of Molecular Catalysis A: Chemical 142 Ž1999. 305–316
ž /
ž /
Hydroxy complexes of palladium II and platinum II as catalysts
for the acetalization of aldehydes and ketones
Maurizio Cataldo, Enrico Nieddu, Roberta Gavagnin, Francesco Pinna,
Giorgio Strukul )
Department of Chemistry, UniÕersity of Venice, Dorsoduro 2137, 30123 Venice, Italy
Received 30 April 1998; accepted 28 August 1998
Abstract
The acetalization of aldehydes or ketones is a reaction of synthetic interest in organic chemistry that is commonly
catalyzed by Brønsted or Lewis acids. We have found that a class of PdŽII. and PtŽII. complexes of the type
2
2
q
wŽP–P.MŽm-OH.x ŽP–Psa series of diphosphines; MsPd, Pt. are effective catalysts for the acetalization of a variety of
aldehydes and ketones in the presence of alcohols or glycols. The use of epoxides instead of alcohols resulted in lower rates
and yields. The rate and yield of the reaction is not affected by the size of the diphosphine in the series dppe, dppp, dppb.
Attempts to perform the reaction in a stereoselective fashion showed no difference depending on whether regular achiral
catalysts or catalysts modified with chiral diphosphines were used. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Hydroxy complexes; Catalysts; Aldehydes
1
. Introduction
The acetalization reaction is a process that is widely used in organic synthesis to protect the
carbonyl group of aldehydes and ketones w1x and for the synthesis of end products Ž .
acetals , including
enantiomerically pure compounds w2–7x, that find practical application in the field of synthetic
carbohydrate w8x, steroids w9x, pharmaceuticals and fragrances w10,11x, and as polymers and copoly-
mers w12x. This reaction is normally acid catalyzed, although the use of transition metal complexes as
catalysts has been occasionally reported in the literature w13–17x. Recently, Gorla and Venanzi w18x
observed for the first time the possibility to use cationic, solvent complexes of Rh
Pt II as catalysts for the acetalization of a variety of aldehydes and ketones. According to these
III , Pd II and
Ž . Ž .
Ž
.
authors, the basic requirements necessary to achieve a high catalytic activity are the presence of an at
least 2q positive charge on the complex to ensure a sufficient Lewis acidity, and the existence of two
adjacent vacant coordination sites in order to have the reactants
present in a mutually cis position. These principles led to the design of complexes such as
aldehyderketone and alcohol
Ž .
)
Corresponding author. Tel.: q39-41-257-8551; Fax: q39-41-257-8517; E-mail: strukul@unive.it
1
381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S1381-1169Ž98.00299-4

3
06
M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
2
q
wŽ
dppe
.
M
Ž
H O.2
x
Ž
MsPd, Pt
.
that proved very active as catalysts w18x. However, these complexes
2
q
are relatively unstable and, according to the mechanism of reaction suggested, they produce H in the
system, so that acid catalysis cannot be unambiguously excluded. In this paper we report the use of a
Ž . Ž .
class of complexes of Pd II and Pt II shown in Chart 1, that are stable in air both in the solid state
and in solution and proved also efficient catalysts for the Baeyer–Villiger oxidation of ketones with
hydrogen peroxide w19x. These dimeric compounds easily react with protic molecules leading to
condensation compounds in which a vacant coordination site may be present. For example, the
Baeyer–Villiger oxidation of ketones with hydrogen peroxide occurs via prior bridge-splitting and
condensation according to Eq. 1 . Therefore, they seem particularly suited for testing as catalysts for
Ž .
the acetalization reaction avoiding the possible ambiguity arising from proton generation in the
system.
1
Ž .
2
. Experimental section
2
.1. Apparatus
IR spectra were taken on a Nicolet FTIR Magna 750 and on a Digilab FTS 40 interferometers
1
31
1
either in solid
Ž
KBr pellets
.
or in CH Cl solution using CaF windows. H and
P H NMR
Ä 4
2
2
2
spectra were recorded on a Bruker AC 200 spectrometer operating in FT mode, using as external

M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
307
references TMS and 85% H PO , respectively. Conductivity measurements were performed on a
3
4
y
3
Radiometer instrument using 10 M solutions in MeOH at 258C. GC measurements were taken on a
Hewlett-Packard 5790A gas chromatograph equipped with a 3390 automatic integrator. GC-MS
measurements were performed on a Hewlett-Packard 5971 mass selective detector connected to a
Hewlett-Packard 5890 II gas chromatograph. Identification of products was made with GC or GC-MS
by comparison with authentic samples.
2
.2. Materials
Solvents were dried and purified according to standard methods. Aldehyde and ketone substrates,
alcohols, glycols and epoxides employed in the acetalization reactions were commercial products
purum or puriss quality used without purification.
The following complexes were prepared according to literature procedures: w
OH BF 1b dppp Pd m-OH BF 1c dppb Pd m-OH BF
m-OH m-OH
w22,23x, w BF . Ž2b w22x, w
dppb R-binap m-OH
w22,23x, w BF . Ž2d w22,23x, w BF . Ž2e
Ž
.
Ž
dppe
1d
m-OH
x2
w24x.
.
Pd
w21x,
BF
Ž
m-
.
x
Ž
.
Ž
.
w19x, w
Ž
.
Ž
.
x
Ž
.
Ž
.
.
w20x, w
Ž
.
Ž
.
x
Ž
Pt
.
Ž
.
2
4 2
2
4 2
2
.
4 2
wŽ
dppm
.
.
Pt
Ž
.
x2
Ž
.
BF . Ž2a
.
Ž
dppe
.
.
Pt
Ž
x2
Ž
.
Ž
dppp
Ž
.
Ž
.
4 2
Ž
4 2
4 2
Ž
2c
Ž
Pt
m-OH
.
x2
Ž
Ž
.
Pt
Ž
.
x
Ž
.
4 2
2
4 2
2
.3. [(R-binap)PdCl2 ]
The complex w
Ž
MeCN.2 PdCl2 x
Ž
0.10 g, 0.38 mmol
.
and R-binap
Ž
0.24 g, 0.38 mmol
.
were placed
10 ml were
solid in a round bottomed flask under nitrogen atmosphere. Dry, nitrogen saturated DCE
Ž
.
added and the orange solution was heated under reflux for 3 days. The solution was concentrated in
vacuo to small volume and addition of Et O led to the formation of an orange precipitate, that was
2
filtered washed several times with Et O and dried in vacuo
Ž
yield 91%
.
.
.
Ž
2
Anal. Calcd.1Žfound
.
for C H P PdCl : C, 59.52
Ž
59.75 ; H, 3.43
3.27
.
. IR
Ž
KBr pellet
.
: 303,
32
22
2
2
y
1
31
2
91 cm .
P
Ä
H
4
NMR
Ž
CD Cl2.: d 28.8 ppm
Ž
s
.
.
2
2
.4. [(R-binap)Pd(m-OH)] (BF ) (1e)
2 4 2
The complex w
Ž
R-binap
.
PdCl2 x
Ž
0.24 g, 0.30 mmol
.
was placed in a round-bottomed flask, to
were added. After saturating with N the
which CH Cl
Ž
30 ml
.
and reagent grade acetone
Ž
55 ml
.
2
2
2
pale yellow solution, 0.613 ml 0.60 mmol of a AgBF solution in acetone were added. The mixture
Ž .
4
was stirred in the dark for 1 h and then AgCl was filtered off. The resulting yellow solution was
concentrated to small volume under reduced pressure and the addition of Et O resulted in the
2
precipitation of a pale yellow solid that was filtered, washed with Et O and dried in vacuo yield
Ž
2
8
7% .
.
Anal. Calcd.1Žfound
.
for C H O P Pd B F : C, 56.63
Ž
56.31
.
; H, 3.42
3.38 . IR C H Cl
Ž . Ž
2 4 2
64
46
2
4
2
2
8
y
y
1
31
1
sln
.
: 3590 cm
Ž
OH
.
. IR
Ž
KBr pellet
.
: 1101–997 cm .
P
Ä
H
4
NMR s .
Ž
CD Cl2.: d 29.3 ppm Ž .
2
y
3
y
1
y1
2
Conductivity
Ž
10 M in MeOH
.
: 173 V mol cm .
2
.5. Synthesis of acetals
Acetals used as standards for gas chromatography in the individual catalytic reactions were
20 mmol and the stoichiometric amount of
synthesized from the starting aldehyderketone
Ž
.
alcoholrglycol in 25 ml DCE, to which 0.2 mmol PTSA were added under N with stirring. The
solution containing the acetal was used for qualitative identification in the GC analysis.
2

3
08
M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
2
.6. Catalytic reactions
These were carried out in a 25 ml round-bottomed flask equipped with a stopcock for vacuumrN2
operations and a side-arm fitted with a screw-capped silicone septum to allow sampling. Constant
temperature "0.18C was maintained by water circulation through an external jacket connected with
Ž
.
a thermostat. For reactions carried out at temperatures )258C, the reaction vessel was equipped with
a reflux condenser and heating was ensured by a thermostatted external oil bath. Stirring was
performed by a teflon-coated bar driven externally by a magnetic stirrer.
The following general procedure was followed: The required amount of catalyst was placed solid in
the reactor which was evacuated and filled with N . Nitrogen saturated alcoholrglycol was added
2
under N flow, followed, if necessary, by the required amount of solvent. After thermostatting at the
2
required temperature for a few minutes, the aldehyderketone in the appropriate amount was injected
through the septum and time was started.
All reactions were monitored with GC by direct injection of samples taken periodically from the
reaction mixtures with a microsyringe. Separation of the products was performed on 25 m capillary
columns using a flame ionization detector. Prior quenching of the catalyst using an excess of LiCl
showed no differences in randomly selected analyses.
3
. Results and discussion
3
.1. Acetalization of aldehydes and ketones
The first aspect considered was the search for the best operating conditions in the catalytic reaction.
To this purpose, complex 1d was selected as the catalyst for the acetalization of acetaldehyde with
EtOH used both as reactant and as solvent at 258C, taken as a test reaction. The complex was found
Ž
.
very active and the results of the effect of a different catalyst to substrate ratio is shown in Fig. 1. The
reaction conditions indicated in Fig. 1 and a 1:1000 catalyst to substrate ratio were chosen as the
standard for all the reactions reported hereafter. This ratio is a good compromise between the
Fig. 1. Effect of different catalystrsubstrate ratios in the acetalization of acetaldehyde with ethanol catalyzed by 1d. Reaction conditions: T
2
58C; acetaldehyde 10 mmol; EtOH 100 mmol; 1d 55.6 mmol Žcircles., 10 mmol Žsquares., 5 mmol Žtriangles..

M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
309
Fig. 2. Effect of the diphosphine in the acetalization of propanal with ethanol catalyzed by 1. Reaction conditions: T 258C; propanal 10
mmol; EtOH 100 mmol; 1 10 mmol; catalyst: 1b Žcircles., 1c Ždiamonds., 1d Žtriangles..
observation of a high rate and a good catalyst productivity in terms of turnover number. Where a
solvent was used, an amount of alcoholrglycol in the stoichiometric ratio with respect to the
aldehyderketone was employed.
The effect of the diphosphine on the catalytic activity was tested both in the case of Pd and in the
case of Pt complexes. At variance with what has been recently observed in the Baeyer–Villiger
oxidation of ketones with the same class of catalysts w21x, in the present case the activity is practically
unaffected see for example the case of Pd complexes reported in Fig. 2 by the size of the
Ž .
metal-diphosphine ring, a factor that has been related to the ease with which the hydroxy bridge opens
up to produce monomeric, coordinatively unsaturated metal species w21x.
A summary of the reactivity of these complexes as catalysts in the acetalization of aldehydes is
reported in Table 1. The reaction profiles are in all cases similar to those shown in Figs. 1 and 2,
Table 1
Acetalization of different aldehydes at 258C catalyzed by hydroxy complexes of palladium and platinuma
Entry
Aldehyde
Alcohol
Catalyst
Acetal yield Ž%.
3
h
24 h
1
2
3
4
5
6
7
8
9
acetaldehyde
propanal
propanal
EtOH
EtOH
EtOH
eth glycol
eth glycol
eth glycol
eth glycol
eth glycol
1,3-propandiol
EtOH
1d
1d
2d
1b
1b
2d
2d
2d
1b
1d
1d
2d
1d
1d
75
60
25
89
33
15
76
4
86
33
47
30
18
32
97
91
62
97
74
55
96
21
98
72
50
40
24
66
propanal
propanal
propanal
propanal
b
b
b,c
d
propanal
propanal
octanal
benzaldehyde
benzaldehyde
m-anisaldehyde
p-Cl-benzaldehyde
10
11
12
13
14
EtOH
EtOH
EtOH
EtOH
a
b
Experimental conditions: catalyst 10 mmol, aldehyde 10 mmol, alcohol 100 mmol, T 258C, T 258C. Glycol 10 mmol, 5.0 ml DCE as
c
d
solvent. T 808C. 5 ml THF as solvent, T 658C.

3
10
M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
hence the yields after 3 h and 24 h are reported in Table 1 as representative of the reactivity in the
individual reactions. The selectivity observed was in all cases )99%, since acetals were the only
detectable reaction products.
As can be seen in Table 1
general more reactive than Pt complexes in reactions performed at room temperature, while, on the
contrary, they are far less stable for reactions performed at relatively higher temperature 838C . For
example, the reactivity observed in entry 6 with complex 2d as catalyst at 258C can be significantly
increased entry 7 by heating at 838C. The use of 1b under the same conditions results in the rapid
decomposition of the catalyst with loss of any catalytic activity. A solvent effect is evident by
comparison of entry 7 and entry 8. The use of a more polar, coordinating solvent such as THF entry
results in a much slower reaction compared to DCE entry 7 under similar conditions.
With aromatic aldehydes, the reactivity seems to be influenced by the presence of substituents on
the aromatic ring capable to alter the electrophilicity of the carbonyl carbon. For example, benzal-
dehyde entry 11 reacts more efficiently than m-anisaldehyde entry 13 , while o- or p-anisaldehyde
entry 14 in the case of p-chlorobenzalde-
compare for example entries 5 and 6 or 11 and 12 , Pd complexes are in
Ž .
Ž
.
Ž
.
Ž
8
.
Ž
.
Ž
.
Ž
.
do not react at all. Conversely, a higher yield is observed
Ž
.
hyde.
The acetalization of ketones
However, it is generally slower although a better degree of efficiency can be observed by using 2d at
Table 2 occurs with features similar to those reported for aldehydes.
Ž .
8
38C andror by increasing the catalyst to substrate ratio entries 10, 12 .
Ž .
3
.2. Acetalization with epoxides
The possibility to perform acetalization with epoxides instead of glycols has been recently reported
by Zhu and Espenson w16x using methylrhenium trioxide
MTO as the catalyst. Being the formation
Ž
.
of cyclic acetals from glycols an equilibrium reaction, the alternative use of epoxides has the intrinsic
advantage of avoiding the formation of water as coproduct, thereby leading, in principle, to more
favorable reaction conditions.
The use of epoxides as reactants has been tested with the Pd and Pt catalysts here considered in the
case of benzaldehyde and is summarized in Table 3. As can be seen, compared to the use of glycols
Table 2
Acetalization of different ketones at 258C catalyzed by hydroxy complexes of palladium and platinuma
Entry
Ketone
Alcohol
Solvent
Temp. Ž8C.
Catalyst
Acetal yield Ž%.
3
h
24 h
1
2
3
4
5
6
7
8
9
cyclohex
cyclohex
cyclohex
cyclohex
cyclohex
cyclohex
2-hex
EtOH
EtOH
s
25
25
25
25
25
83
25
83
83
83
83
83
1d
2d
1b
2d
2d
2d
1b
2d
2d
2d
2d
2d
2
3
30
96
97
26
79
58
51
24
33
43
54
s
16
65
45
5
75
10
50
18
24
40
52
eth glycol
eth glycol
eth glycol
eth glycol
eth glycol
eth glycol
eth glycol
eth glycol
eth glycol
eth glycol
s
s
DCE
DCE
s
DCE
DCE
DCE
DCE
DCE
2-hex
acetoph
acetoph
b
1
1
1
0
1
2
cyclopent
cyclopent
b
a
Experimental conditions: catalyst 10 mmol, ketone 10 mmol, alcohol 100 mmol, T 258C; in DCE Ž5 ml. as solvent, alcohol 10 mmol.
Catalyst 50 mmol.
b

M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
311
Table 3
Acetalization of benzaldehyde with epoxides in DCE catalyzed by Pd and Pt complexes
a
Catalyst
Epoxide
Temp. Ž8C.
Acetal yield Ž%.
4 h
16
1.4 q1.7
21
8
cisrtrans
2
1b
1b
2d
1b
propene oxide
glycidol
propene oxide
propene oxide
25
25
83
25
1.1
b
c
1.1
1.1
d
a
b
c
Experimental conditions: catalyst 10 mmol, benzaldehyde 1 mmol, epoxide 1 mmol, DCE 3 ml, T 258C. Dioxane product. Dioxolane
d
product. Benzaldehyde 10 mmol, epoxide 10 mmol, DCE 5 ml.
under similar conditions the reaction is much slower and less productive. In fact, the yields reported
in Table 3 after 24 h are also the maximum yields that can be obtained under the conditions used.
According to Eq.
in Table 3 indicates that the stereoselectivity of the system is modest. Furthermore, when using
glycidol, the substituted 1,3-dioxane and 1,3-dioxolane are formed Eq.
.., albeit in low yields. The
formation of two regioisomers from glycidol requires the existence of a common intermediate
probably 3 which allows the alternative 1,2- or 1,3-addition Eq.
... Compared to MTO the present
2 , the reaction produces two different stereoisomers. The cisrtrans ratio reported
Ž .
Ž
Ž
3
Ž
.
Ž
Ž
3
system is less productive in the acetalization of benzaldehyde, probably reflecting a higher difficulty
of the bridging hydroxy complexes to interact with epoxides with respect to MTO. In fact, the latter is
Ž . Ž .
known to interact readily with epoxides to yield bis alcoxy rhenium VII complexes in excellent
yields w24x and these intermediates were the access key to the synthesis of acetals w16x. The formation
of 3 from 2 requires the protonation of the epoxidic oxygen from the bridging hydroxy ligand, i.e., the
existence of an acid character that has been evidenced only in reactions with strong bases such as Li
amides w22,25,26x. On the contrary, most of the reactivity of complexes 2 is associated with their
basic behavior, as is known from their reactivity and as will be clear from the higher catalytic activity
observed for 1,2-propanediol and glycerol reported in Section 3.3.
2
Ž .
3
Ž .

3
12
M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
3
.3. StereoselectiÕe reactions
The reaction between most carbonyl compounds and epoxides or glycols such as propene oxide,
,2-propanediol, glycidol, or glycerol entails some stereochemical aspects that were considered
1
worthwhile of investigation. In fact, depending on the orientation with which the glycol epoxide
Ž .
binds the carbonyl compound different stereoisomers can be formed. Hereafter, only the case of
glycols will be considered, because, being more reactive, they allow an easier characterization of the
stereoisomers formed in the reaction. The same considerations apply also to epoxides. The case of
benzaldehyde and 1,2-propanediol is shown as an example in Scheme 1. The situation is complicated
by the fact that 1,2-propanediol is a chiral molecule, so that four different stereoisomers
of enantiomers may form. The process consists of two parallel diastereoselective reactions in which
Ž
two couples
.
either 1,2-propanediol enantiomer induces the formation of a new stereocenter leading to the
formation of diastereoisomers.
The use of racemic 1,2-propanediol led to the formation of all four predicted stereoisomers
.
Scheme 1 . Analysis of the reaction mixture was performed by GC using a chiral column.
Ž
Unfortunately, the separation of the enantiomers was largely incomplete, preventing an assessment of
the stereochemistry of the reaction.
The study of the system was simplified by using
were performed at 258C and the results are summarized in Table 4. GC analysis on the chiral column
allowed the separation of the diastereoisomers and the calculation of the diastereomeric excess d.e.
As can be seen compare entries 1 and 2 of Table 4 , the use of DCE as solvent improves the d.e.
R -1,2-propanediol as the reactant. All reactions
Ž .
Ž
.
.
Ž
.
significantly, while only modest differences are observed on going from Pd to Pt complexes. We
thought also of interest to see whether the use of a chiral catalyst had an influence on the
Ž . Ž .
diastereoselectivity of the reaction and to this purpose the binap derivatives of Pd 1e and Pt 2e
were tested. As is clear from entries 3 and 5, no difference is observed, suggesting that the
stereochemistry of the reaction is controlled mainly by the stereocenter in the glycol reactant.
Finally, the reaction between benzaldehyde and glycerol was studied. Although glycerol is an
achiral molecule the same mixture of stereoisomers shown in Scheme 1 is obtained, because two
stereocenters originate during the course of the reaction. However, glycerol entails also a problem of
regioselectivity
Ž
see above the case of glycidol
.
, since 1–2 and 1–3 addition products
Ž
dioxolanes and
Scheme
dioxanes, respectively
.
can be formed, the latter existing either in cis or trans configuration
Ž
2 .
.
The catalysts tested were 1b, 1e, 2d, and 2e. In these experiments, GC analysis of the reaction
mixtures with the chiral column allowed the separation of all six isomers. Best results are obtained in
Scheme 1.

M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
313
Table 4
Catalytic acetalization of benzaldehyde with R-propene glycol: yields and diastereomeric excess of the product
a
Entry
Catalyst
Acetal yield Ž%.
d.e. Ž%.
3
h
24 h
1
2
3
4
5
1bb
1b
1e
2d
2e
87
41
23
26
33
91
62
45
50
54
32
60
60
51
50
a
b
Reaction conditions: catalyst 10 mmol, benzaldehyde 10 mmol, glycol 10 mmol, DCE 6.6 ml, T 258C. No solvent, glycol 100 mmol.
the absence of solvent
favored with respect to 1,3-dioxanes. However, a reaction profile
reaction times, when the acetalization reaction is finished and the conversion is constant, part of the
,3-dioxolanes tend to be converted into 1,3-dioxanes. This seems to suggest the occurrence of a
consecutive reaction in which the kinetically favored dioxolanes are slowly reconverted into the
thermodynamically more stable dioxanes. Other features of this reactions are the following: the
transrcis 1,3-dioxanes ratio is always close to 2 and in favor of the more thermodynamically stable
trans isomer, ii the four 1,3-dioxanes are present in virtually identical amounts ;25% each . These
Ž
yields in the range of 90%
.
and the formation of 1,3-dioxolanes is generally
Ž
Fig. 3 indicates that for longer
.
1
i
Ž .
Ž
.
Ž
.
results are observed with all catalysts including the chiral Pd and Pt complexes, which again seem to
have no control on the isomeric composition of the 1,3-dioxolanes and 1,3-dioxanes mixtures.
3
.4. Mechanistic considerations
A possible mechanism for the acetalization of carbonyl compounds using bis aquo -Pd II and
Ž . Ž .
Ž . complexes as catalysts has been suggested by Gorla and Venanzi w18x. This mechanistic
-Pt II
suggestion can easily apply to the present system with some minor modifications and is shown in
Scheme 3. The only difference with the proposal of Gorla and Venanzi is in the initial steps that lead
to the formation of 5.
The genesis of 4 requires the protonation of the bridging hydroxy ligand by an incoming alcohol
2
q
molecule. Evidence for the basic character of the bridging –OH ligand in wŽ . Ž .x
P–P Pt m-OH
2
complexes has been found by us w27x and other groups w18,23,28,29x in the reaction with a variety of
strong and weak acids leading to condensation products. Experimental evidence for the existence of a
3
1
1
species like 4 was found by analyzing the
P H NMR spectrum of 2d in CD OD. The spectrum
Ä 4
3
Scheme 2.

3
14
M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
Fig. 3. Acetalization of benzaldehyde with glycerol catalyzed by 1b. Reaction conditions as in Fig. 2. Conversion Žtriangles., yield in
dioxolane Ždiamonds., yield in dioxane Žcircles..
1
shows the existence of a mixture of two products: 2d
Ž
d 5.84 ppm; J
3514 Hz
. w21,22x and a new
2
P – Pt
P – Pt
1
2
1
species
Ž
d 2.84 ppm; J
3409 Hz, J
24.6 Hz; d 5.87 ppm; J
3574 Hz, J
24.6 Hz in
.
P – Pt
P – P
P – P
an approximately 1:5 ratio. The existence of two independent P nuclei and the extent of the coupling
Scheme 3.

M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
315
constants suggest for the new species a formulation such as in 4, which is homologous to that found
in reactions with carboxylic acids w27x. Addition of an excess of propanal, to mimic the catalytic
reaction conditions, reverted the initial equilibrium
d as the major product, leaving only minor amounts of the species similar to 4, and showing the
water is produced by the catalytic reaction with
Ž .
2
formation of a weak unresolved multiplet around 4 ppm, probably a mixture of different reaction
intermediates.
The formation of 5 by displacement of water results in the increase of the electrophilic character of
the aldehyderketone making it more reactive towards the nucleophilic attack by the alcohol. Species
q
homologous to 5, such as w
Ž
dppb
.
Pt
Ž
ketone.ŽOOH
.
x , have been evidenced as the key intermediate in
a mechanistic study of the Baeyer–Villiger oxidation of ketones with hydrogen peroxide catalyzed by
d w21x. The transformation of 5 into 7 and 7 into 9 are suggested to occur via the formation of cyclic
structures 6 and 8 in which hydrogen bonding forms between the free alcohol and the coordinated
2
Ž
.
alcoxide. Structures of this type have been already described in the case of Pd w30x. On the other hand,
the coordination chemistry of Pt complexes with bidentate O-donor ligands, forming 4-membered
2
y
rings, such as CO , RCO y is well established w31,32x, and thus, the direct transformation of 5 into
3
2
7
by a direct intramolecular reaction cannot, in principle, be excluded.
The reaction sequence from 5 to 9, which is speculative and identical to the proposal of Gorla and
Venanzi w18x, may seem exceedingly complex. However, the process going from 5 to 7 is responsible
for the genesis of the chiral center in the reactant when certain glycols are used see above . Since the
degree of control of the chiral catalyst in asymmetric transformations is practically negligible, this
Ž
.
Ž
.
finding can be more easily explained by an external attack of the alcohol such as in 6 or 8 and is an
indirect but important piece of support to the suggestions of Gorla and Venanzi. Under this
hypothesis, the metal behaves mainly as a Lewis acid increasing the electrophilicty of the carbonyl
carbon by coordination, while, at the same time, improving the reactivity of the alcohol via hydrogen
bonding with the coordinated alcoxide.
4
. Conclusions
The results here reported represent one of the few examples of transition metal catalysis applied to
q
the acetalization of aldehydes and ketones with alcohols or glycols. Ambiguities arising from H
generated in the system seem to be excluded by the chemistry known for this class of bridging
hydroxo complexes of palladium and platinum which behave as bases capable of reacting with weak
Ž .
acids with formation of water. Apart from this intrinsic advantage, the main features of the
acetalization reaction are rather disappointing. The lack of any effect on the reactivity induced by the
steric properties of different diphosphines and the lack of stereochemical control by the catalyst in
stereoselective reactions, seem to suggest for the cationic metal centers a role very close to that of a
simple Lewis acid. The notable reactivity of the catalysts and the possibility to use platinum
complexes at moderately high temperatures recommend for the system the use towards aldehydes or
ketones which cannot be acetalized under acid catalysis, that are currently being tested.
Acknowledgements
This work was supported jointly by MURST and CNR both in Rome . Thanks are expressed to
Ž .
Professor O. De Lucchi of this Department for helpful discussions.

3
16
M. Cataldo et al.rJournal of Molecular Catalysis A: Chemical 142 (1999) 305–316
References
w1x T.W. Greene, Protective Groups in Organic Synthesis, Wiley-Interscience, New York, 1981.
w2x W.S. Johnson, C.A. Harbert, R.D. Stipanovic, J. Am. Chem. Soc. 90 Ž1968. 5279.
w3x W.S. Johnson, C.A. Harbert, B.E. Ratcliffe, R.D. Stipanovic, J. Am. Chem. Soc. 98 Ž1976. 6188.
w4x P.A. Bartlett, W.S. Johnson, J.D. Elliott, J. Am. Chem. Soc. 105 Ž1983. 2088.
w5x A. Mori, H. Yamamoto, J. Org. Chem. 50 Ž1985. 5444.
w6x K. Narasaka, M. Inoue, T. Yamada, J. Sugimori, N. Iwasawa, Chem. Lett., 1987, p. 2409.
w7x D. Seebach, R. Imwinkelried, T. Weber, in: R. Scheffold ŽEd.., Modern Synthetic Methods, Vol. 4, Springer, Berlin, 1986.
w8x D.M. Clode, Chem. Rev. 79 Ž1979. 491.
w9x J.R. Bull, J. Floor, G.J. Kruger, J. Chem. Res. Synop., 1979, p. 224.
w10x K.A.M. Walker, U.S. Patent 4150153, 1979.
w11x K. Bruns, J. Conrad, A. Steigel, Tetrahedron 35 Ž1979. 2523.
w12x A.J. Elliot, in: A.J. Elliot ŽEd.., 1,3-Dioxolane Polymers in Comprehensive Heterocyclic Polymers, Vol. 6, Pergamon, Oxford, UK,
1
984.
w13x J. Ott, G.M. Ramos Tombo, B. Schmid, L.M. Venanzi, G. Wang, T.R. Ward, Tetrahedron Lett. 30 Ž1989. 6151.
w14x W. Voelter, C. Djerassi, Chem. Ber. 101 Ž1986. 1154.
w15x R.V. Hoffman, Tetrahedron Lett., 1974, p. 2415.
w16x J.F. Biellmann, M.J. Jung, W.R. Pilgrim, Bull. Soc. Chim. Fr. 7 Ž1971. 2720.
w17x Z. Zhu, J.H. Espenson, Organometallics 16 Ž1997. 3658.
w18x F. Gorla, L.M. Venanzi, Helv. Chim. Acta 73 Ž1990. 690.
w19x G.W. Bushnell, K.R. Dixon, R.G. Hunter, J.J. McFarland, Can. J. Chem. 50 Ž1972. 3694.
w20x C. Pisano, G. Consiglio, A. Sironi, M. Moret, J. Chem. Soc. Chem. Commun., 1991, p. 421.
w21x R. Gavagnin, M. Cataldo, F. Pinna, G. Strukul, Organometallics 17 Ž1998. 661.
w22x J.J. Li, P.R. Sharp, Inorg. Chem. 35 Ž1996. 604.
w23x A.L. Bandini, G. Banditelli, F. Demartin, M. Manassero, G. Minghetti, Gazz. Chim. Ital. 123 Ž1993. 417.
w24x Z. Zhu, A. Al-Ajlouni, J.H. Espenson, Inorg. Chem. 35 Ž1996. 1408.
w25x J.J. Li, P.R. Sharp, Inorg. Chem. 33 Ž1994. 183.
w26x J.J. Li, C.L. Barnes, P.R. Sharp, J. Chem. Soc., Chem. Commun., 1990, p. 1634.
w27x G. Strukul, A. Varagnolo, F. Pinna, J. Mol. Catal. 117 Ž1997. 413.
w28x B. Longato, B. Corain, G.M. Bonora, G. Pilloni, Inorg. Chim. Acta 137 Ž1987. 75.
w29x A.L. Bandini, G. Banditelli, M.A. Cinellu, G. Sanna, G. Minghetti, F. Demartin, M. Manassero, Inorg. Chem. 28 Ž1989. 404.
w30x Y.-J. Kim, K. Osakada, A. Takenaka, A. Yamamoto, J. Am. Chem. Soc. 112 Ž1990. 1096.
w31x B. Hathaway, in: G. Wilkinson, R.D. Gillard, J. McCleverty ŽEds.., Comprehensive Coordination Chemistry, Vol. 2, Chap. 15.5,
Pergamon, Oxford, 1987, p. 413.
w32x C. Oldham, in: G. Wilkinson, R.D. Gillard, J. McCleverty ŽEds.., Comprehensive Coordination Chemistry, Vol. 2, Chap. 15.6,
Pergamon, Oxford, 1987, p. 435.