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,
J. N. Novikov and M. E. Vol’pin, Tetrahedron Lett., 1976, 34, 2984;
P. Chabarde`s, E. Kuntz and J. Varagnat, Tetrahedron, 1977, 33,
1775.
6 P. Chabarde`s, Tetrahedron Lett., 1988, 48, 6253.
7 K. Narasaka, H. Kusama and Y. Hayashi, Chem. Lett., 1991, 1413.
8 C. Y. Lorber and J. A. Osborn, Tetrahedron Lett., 1996, 37, 853.
9 C. Bruneau and P. H. Dixneuf, Chem. Commun., 1997, 507.
10 C. Bruneau, M. Neveux, Z. Kabouche and P. H. Dixneuf, Synlett, 1991,
755.
11 B. Seiller, C. Bruneau and P. H. Dixneuf, Tetrahedron, 1995, 51,
13 089.
12 C. Bruneau, Z. Kabouche, M. Neveux, B. Seiller and P. H. Dixneuf,
Inorg. Chim. Acta., 1994, 222, 155.
13 H. Doucet, B. Martin-Vaca, C. Bruneau and P. H. Dixneuf, J. Org.
Chem., 1995, 60, 7247.
14 H. Doucet, J. Ho¨fer, N. Derrien, C. Bruneau and P. H. Dixneuf, Bull.
Soc. Chim. Fr., 1996, 133, 939.
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