Selective isomerisation of prop-2-yn-1-ols into α,β-unsaturated aldehydes catalysed by Ru[η3-CH2C(Me)CH2]2(Ph 2PCH2CH2PPh2)

Article abstract of DOI:10.1039/a701495c

The complex Ru[η³-CH₂C(Me)CH₂]₂(Ph ₂PCH₂CH₂PPh₂) is an efficient catalyst precursor for the isomerisation of prop-2-yn-1-ols into conjugated enals under mild conditions in the presence of benzoic acid via (Z)-3-hydroxyprop-1-en-1-yl benzoates.

Full text of DOI:10.1039/a701495c

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Selective isomerisation of prop-2-yn-1-ols into a,b-unsaturated aldehydes
3
catalysed by Ru[h -CH2C(Me)CH2]2(Ph2PCH2CH2PPh2)
Michel Picquet, Christian Bruneau and Pierre H. Dixneuf*
Laboratoire de Chimie de Coordination et Catalyse, UMR 6509: CNRS, Universite´ de Rennes, Campus de Beaulieu, F-35042
Rennes, France
R2
R1
R2
R1
3
(dppe)Ru(η3-CH2CMeCH2)2
The complex Ru[h -CH2C(Me)CH2]2(Ph2PCH2CH2PPh2)
is an efficient catalyst precursor for the isomerisation of
prop-2-yn-1-ols into conjugated enals under mild conditions
in the presence of benzoic acid via (Z)-3-hydroxyprop-1-en-
1-yl benzoates.
(2)
H
BzOH (1 equiv.)
O
OH
1a–f
a R1 = R2 = Me
b R1 = Me, R2 = Ph e R1 = Me, R2 = CH2CH2CH=CMe2
c R1 = Me, R2 = Pri R1 = R2 = Ph
2a–f
d R1 = Me, R2 = CH2CH2CH=CH2
The isomerisation of propynylic alcohols into a,b-unsaturated
aldehydes has the potential to offer access to a variety of
terpenoids, e.g. vitamin A derivatives, as well as perfume and
cosmetic components1,2 [eqn. (1)].
f
the crude product led to the sole a,b-unsaturated aldehyde 2a
(prenal), isolated in 56% yield (method A). Complete trans-
formation of the disubstituted prop-2-ynylic alcohols 1b and 1c
in toluene under similar conditions led after distillation to the
unsaturated aldehydes 2b (68%) and 2c (37%) (Table 1). Both
isomers were formed in the respective ratios Z/E = 2:1 (2b)
and 3:4 (2c). The unsaturated prop-2-yn-1-ols 1d and 1e were
good substrates for this isomerisation as they led to the best
yields in 2d (75%) and in the terpenoid citral 2e (73%), formed
as a mixture of the Z and E isomers in the ratio 1:2 (Table
1).
R2
R1
R2
R1
H
(1)
O
OH
Such a transformation was first performed in the Meyer–
Schuster reaction,3 which was carried out in acidic medium and
catalysed by strong acids.4 The emergence of metal complex
catalysis has allowed the use of catalytic systems based on
vanadium(v) oxides5 or the Ti(OR)4–CuCl–RCO2H associa-
tion,6 operating under milder conditions but still at 125–160 °C.
The system Bu4NReO4–MeC6H4SO3H7 gives isomerisation at
room temperature with alcohol dehydration as a side reaction. A
new system MoO2(acac)2–R2SO–ArCO2H operates at 100 °C
and appears to be efficient for the formation of prenal.8 With
these metal oxide catalysts, the suggested key steps involve the
coordination of the prop-2-ynylic alcohol oxygen atom to the
metal and addition of the metal oxide oxygen atom to the alkyne
C(1) carbon atom to generate an allenyloxy intermediate.
However, in these reactions the required presence of an acid has
never been clearly taken into account in the proposed mecha-
nisms.
We now report a new, efficient method for transformation of
prop-2-yn-1-ols into a,b-unsaturated aldehydes catalysed by a
(diphosphine)ruthenium(II) complex in the presence of benzoic
acid, and we show that the key step involves the regioselective
catalytic addition of the carboxylate to the terminal carbon atom
of the C·C triple bond, giving a (Z)-3-hydroxyprop-1-en-1-yl
benzoate intermediate, followed by thermal elimination of the
acid.
3
This reaction is also catalysed by the analogous Ru[h -CH2-
C(Me)CH2]2(dppb) complex II [dppb = Ph2P(CH2)4PPh2] but
the competitive formation of the b-oxopropyl ester 3 was
observed [eqn. (3)]. Thus, the reaction of 1e with BzOH and 1
mol% of II afforded 2e in 18% and 3e in 50% yield, and 1d gave
65% of 3d only.
1d–e
(II)
Ru
(dppb)
BzOH (1 equiv.)
PhMe
2
(1 mol%)
O
O
R
R
R
R
(3)
+
Ph
O
O
2d–e
3d–e
1e R = Me → 2e (18%) + 3e (50%)
1d R = H → 2d (traces) + 3d (65%)
The ruthenium(ii) electrophilic activation concept9 has
allowed the selective preparation of enol esters10 and furans11
from terminal alkynes, and b-oxopropyl esters from prop-2-yn-
1-ols.12 Modification of the catalyst precursors has recently
made possible control of the regio- and stereo-selective anti-
Markovnikov addition of carboxylic acids to terminal alkynes to
give (Z)-alk-1-en-1-yl carboxylates.13,14 An attempt to adapt the
latter reaction to the addition of benzoic acid to the terminal sp
carbon of prop-2-yn-1-ols in the presence of (diphosphine)-
ruthenium(ii) catalysts led us to discover the formal selective
isomerisation of prop-2-yn-1-ylic alcohols into a,b-unsaturated
aldehydes [eqn. (2)].
The formation of the keto esters 3 corresponds to the known
catalytic addition of benzoate to the C(2) carbon of the triple
bond followed by intramolecular transesterification.12 By
contrast, the mechanism of the isomerisation 1 ? 2 can be
proposed as depicted in Scheme 1. As the catalyst I is a known
precursor favouring the trans-anti-Markovnikov addition of
carboxylic acids to terminal alkynes,13,14 the intermediate A
corresponding to this first step is likely formed. Its protonolysis
leads to the (Z)-3-hydroxyprop-1-en-1-yl benzoate 4 which
gives the a,b-enal 2 via thermal elimination of benzoic acid
during distillation (Scheme 1).
This process has been demonstrated in the case of 1a with the
following experiments which were monitored by 1H NMR
spectroscopy: (i) the reaction of 1a with benzoic acid at 70 °C
without a solvent in the presence of the catalyst I (1 mol%) gave
the complete transformation of the propynol 1a after 2 h into 2a
The reaction of the prop-2-yn-1-ol 1a (R1, R2 = Me) with 1
3
equiv. of benzoic acid in the presence of 1 mol% of Ru[h -
CH2C(Me)CH2]2(dppe) I (dppe = Ph2PCH2CH2PPh2) at 70 °C
for 15 h led to the complete transformation of 1a. Distillation of
Chem. Commun., 1997
1201

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OH
R1
H
H
R2
R1
R2
+
Ru
+
Ru
O
2
I
R2
OH
R1
Ph
H+
BzOH
O
OH
H
H
H
Ru
R2
O
Ph
O
Ph
O
O
Ph
H
heat
H
O
O
R2
OH
OH
OH
R2
A
R1
R1
R1
4
Scheme 1
Table 1 Ruthenium-catalysed isomerisation of prop-2-yn-1-olsa
Yield (%) (Z/E)
(method B). Thus, 1b, 1e and 1f were converted into
3-hydroxyprop-1-en-1-yl benzoates 2 after treatment with
benzoic acid in the presence of a catalytic amount of I at 70 °C
for 2 h, and further heating in the same pot at 100 °C for 22, 15
and 32 h led to the complete transformation of (Z)-4 and the
isolation of the a,b-unsaturated aldehydes 2b, 2e and 2f in 61,
81 and 79% respective yields (Table 1). The 1H NMR
spectroscopic monitoring revealed that only the (Z)-esters 4
were converted into a,b-enals 2 on heating, whereas the
(E)-isomers remained almost unchanged except in the case of
4a. For example, from 1b, after 2 h at 70 °C, the catalytic
reaction afforded 100% of 4b (Z/E = 70:30) which led after 22
h at 100 °C to 74% of 2b (Z/E = 66:34) and 26% of (E)-4b.
The selective synthesis of a,b-unsaturated aldehydes requires
the initial regioselective anti-Markovnikov addition of the
benzoate to the triple bond, which raises the question of whether
such a key step might also be involved in the previous
isomerisations of propynylic alcohols promoted by metal
oxides.
Prop-2-yn-1-ol
Enal
Method A
Method B
56b
68b
HO
O
O
1a
2a
2b
Ph
Ph
68 (2:1)
61 (2:1)
HO
1b
37 (3:4)
—
HO
O
1c
2c
2d
75 (1:2)
73 (1:2)
—
OH
O
O
Footnote
1d
* E-mail: pierre.dixneuf@univ-rennes1.fr
81 (2:3)
OH
References
1e
2e
1 G. Ohloff, Scent and Flagrances, Springer-Verlag, Berlin, 1994.
2 C. Mercier and P. Chabarde`s, Pure Appl. Chem., 1994, 66, 1509.
3 K. H. Meyer and K. Schuster, Chem. Ber., 1922, 55, 819.
4 S. Swaminathan and K. V. Narayanan, Chem. Rev., 1971, 71, 429 and
references cited therein.
Ph
Ph
Ph
Ph
HO
79c
—
O
1f
2f
5 H. Pauling, D. A. Andrews and N. C. Hindley, Helv. Chim. Acta, 1976,
59, 1293; M. B. Erman, I. S. Aul’Chenko, L. A. Kheifits, V. G. Dulova,
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13 089.
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a General conditions: prop-2-yn-1-ol (2.5 mmol), benzoic acid (2.5 mmol),
catalyst I (1 mol%), toluene (2 ml). Method A: 70 °C, 15 h, distillation under
reduced pressure. Method B; 70 °C, 2 h, then 100 °C for 3 h (2a), 15 h (2e),
22 h (2b) and 32 h (2f), distillation under reduced pressure. Without
solvent. c Chromatography over silica gel.
b
and 4a (Z/E = 86:14) formed in 7 and 93% yields, respectively,
as determined by 1H NMR of the crude mixture before
distillation; (ii) heating at 100 °C for 3 h of the latter crude
mixture containing 2a and 4a led to 96% of prenal 2a (measured
1
by H NMR spectroscopy) and isolated in 68% yield. These
experiments show that the role of the catalyst is to generate the
3-hydroxyalk-1-en-1-yl benzoate 4a, and that heating of the
latter at 100 °C or during distillation in the presence or absence
of the ruthenium derivative is the key step for the transformation
of 4a into the unsaturated aldehyde 2a.
This sequential heating of the reaction mixture in the
presence of catalyst I, first at 70 °C (catalytic anti-Markovnikov
addition) and then at 100 °C (thermal benzoic acid elimination)
appears to be the best method to selectively produce a,b-enals
Received in Liverpool, UK, 3rd March 1997; Com. 7/01495C
1202
Chem. Commun., 1997