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Pt(II)SnCl2 catalysed hydromethoxycarbonylation of 1-alkynes: A remarkable example of regioselectivity control
Journal of Molecular Catalysis, 84 (1993) L141-L144 Elsevier Science Publishers B.V., Amsterdam
L141
M235
Pt (II)-SnCl, catalyzed hydromethoxycarbonylation of 1-alkynes: a remarkable example of regioselectivity control I
----
I
A. Scrivanti*, R. Chinellato and U. Matteoli Dipartimento di Chimica, Universitti di Venezia, Calle Larga S. Marta 2137, I-30123 Venezia (Italy) (Received March 28,1993; accepted June 23,1993)
Abstract Phenylacetylene and 1-heptyne have been hydrocarbomethoxylated in the presence of Pt (II) / SnCl, systems to give the corresponding a$unsaturated esters. A careful choice of the catalytic system allows a very good control of the reg-ioaelectivity of the reaction to be obtained. The complex PtHCl( PPhe )r gives carbonylation exclusively at the /3 position, whereas using PtClx (DPPB ), the substrates are almost exclusively carbonylated at the cyposition. Key words: alkynes; hydroxycarbonylation;
platinum; regioselectivity control; tin
Introduction Whereas Group VIII first and second-row metal complexes have been widely used for the catalytic or stoichiometric hydrocarbonylation of acetilenic substrates [ 1,2], there are few reports concerning the use of Pt (II) complexes as catalyst precursors for such reactions [ 3,4]. Owing to our interest in transition-metals mediated organic syntheses we thought it would be of interest to study the hydroalkoxycarbonylation of phenylacetylene la in the presence of some readily available Pt(I1) complexes such as PtHCl (PPh,), or PtCl, (DPPB ) [ DPPB = 1,4-bis- (diphenylphosphino)butane] and SnClz as promoter. The aim of the study was the selective synthesis of the corresponding acrylate 2a (see Scheme 1 ), which is an immediate precursor of optically active 2-phenylpropanoic acid, a representative of an important class of anti-inflammatory non-steroidal drugs. Results and Discussion The results obtained in phenylacetylene carbonylation together with the relevant reaction conditions are reported in Table 1 (entries l-4 ) . *Corresponding author.
0304-5102/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved.
A. Scrivanti et al. /J. Mol. Catal. 84 (1993) L141-L144
L142
RC =CH2 20 c 00CH3 RC -CH
+ CO + CH30H
+
d
1 a.b RC =CH
I H a:R=Ph; Scheme 1.
I
3 a,b
COOCH3
b: R=n-C&H11
TABLE 1 Hydromethoxycarbonylation of phenylacetylene and 1-heptyne in the presence of Pt (II) /SnCl, systemsa Run
1 2 3 4 5 6
Substrate
phenylacetylene phenylacetylene phenylacetylene phenylacetylene 1-heptyne 1-heptyne
Catalyst precursor PtI-ICl(PPh& PtHCl ( PPh3) e PtC&(DPPB) PtC&(DPPB) PtHCl(PPh& PtC&(DPPB)
Solvent
THF toluene THF toluene THF toluene
Conversion (%)
Yield (% ) 2
3
50.4 34.4 99.5 97.1 98.9 95.9
20.2 14.1 10.8 1.0 70.6
28.9 34.3 50.3
“T= lOO”C, P(C0) = 100 atm, reaction time=22 h, substrate= 10 mmol, CH,OH=20 mmol, solvent = 10 ml, Pt/ substrate= l/200, Pt/Sn = l/l, Sn as SnC12*H20.
High temperature and pressure are required to carry out the reaction with moderate yields. The substrate hydromethoxycarbonylation is accompanied by the formation of products not detected in the gas-chromatographic runs. IR analyses of some of the residues indicated the formation of poly(phenylacetylene) [mostly (2) ] and of 1,2,4, and 1,3,5,-triphenylbenzene [ 5,6] : it is noteworthy that the Pt (II) complexes catalyzed polymerization of alkynes is well documented [ 7,8]. The most attractive results are related to the regioselectivity of the reaction: when the monophosphine derivative PtHCl (PPh,)z is used as catalyst precursor in tetrahydrofurane the required methyl cw-methylene-benzeneacetate (methyl atropate) (2a) is obtained with complete regioselectivity (run 1). The use of a less polar solvent such as toluene (run 2) does not affect the regiospecificity of the reaction even if a slight lower yield of methyl atropate is attained. Looking for a higher catalytic activity and chemoselectivity we have tested as the catalyst precursor the strictly related system PtCl, (DPPB ) /SnCl, which is known to be very active in the hydroformylation of olefins. The relevant
A. Scrivanti et al, /J. Mol. Catal. 84 (1993) L141-L144
I.4143
results are reported in entries 3 and 4 of Table 1. Surprisingly, this catalytic system gives predominantly the linear regioisomer 3-phenyl-2-propenoate (methyl cynnamate) (3a). In this case the solvent also plays a role in directing the addition to the carbon-carbon triple bond: as a matter of fact the linear to branched ratio is approximately 3 : 1 in tetrahydrofurane and 30 : 1 in toluene. To establish if the peculiar nature of the substrate exerts an influence on the reaction regioselectivity, we have studied the carbonylation of 1-heptyne. The results are presented in Table 1 (entries 5 and 6). In this case also the substrate carbonylation is accompanied by the formation of high molecular by-products. Nevertheless, the regioselectivity control is complete: in THF the monophosphine catalyst precursor PtHCl ( PPh,)z gives only the branched regioisomer methyl 2-methylen-heptanoate (2b), whereas in toluene the complex PtC& (DPPB ) produces exclusively the linear isomer methyl 2-octenoate (3b). Therefore it appears that by a suitable choice of catalyst precursor and reaction solvent these systems allow the attainment of almost complete control of the regioselectivity of the 1-alkynes’ carbonylation. Little is still known about the factors influencing the regioselectivity of the addition of the H-COOR moieties to the acetylenic bond. Earlier works based on nickel catalysts suggest that this addition is mostly directed by the nature of the substituent on the alkyne and that it occurs according to Markovnikov [91. In contrast, more recent results obtained with Pd catalysts [ 3,101 indicate that the catalytic system is able to control almost completely the regiochemistry of the reaction. The present results seem to point out that with these Pt (II) /SnC!& systems the coordination mode of the phosphine ligands (monodentate mutually tram versus bidentate chelating cis) is the factor which largely prevails in directing the position of the addition of the hydro and carbalkoxy groups to the carbon-carbon triple bond. However, much more work is needed to substantiate this hypothesis and to optimize the catalytic activity. Acknowledgements
The work was carried out with the financial support of the Italian CNR (Progetto Finalizzato Chimica Fine II). References 1 2 3
P. Pino and G. Braca, in I. Wender and P. Pino (eds.), Organic Synthesis via Metal Carbonyls, Wiley, New York, 1977, Vol. II, pp. 419-515. A. Mullen, in J. Falbe (ed.), New Syntheses with Carbon Monoxide, Springer Verlag, Berlin, 1980, Ch. 3. J.F. Knifton, J. Mol. Catal., 2 (1977) 293.
L144
4 5 6 7 8 9 10
A. Scrivanti et al. /J. Mol. Catal. 84 (1993) L141-L144
Y. Tsuji, T. Kondo and Y. Watanabe, J. Mol. Catal., 40 (1987) 295. T. Masuda, N. Sasaki and T. Higashimura, Macromolecules, 8 (1975) 717. T. Masuda, T. Mouri and T. Higashimura, Bull. Chem. Sot. Jpn., 53 (1960) 1152. A. F’urlani, I. Collamati and G. Sartori, J. Organomet. Chem., 17 (1969) 463. T.G. Appleton, H.C. Clark and R.J. Puddephatt, Inorg. Chem., 2 (1972) 2074. C.W. Bird, Chem. Rev., 62 (1962) 263. B. El Ali and H. Alper, J. Mol. Catal., 67 (1991) 29.
L141
M235
Pt (II)-SnCl, catalyzed hydromethoxycarbonylation of 1-alkynes: a remarkable example of regioselectivity control I
----
I
A. Scrivanti*, R. Chinellato and U. Matteoli Dipartimento di Chimica, Universitti di Venezia, Calle Larga S. Marta 2137, I-30123 Venezia (Italy) (Received March 28,1993; accepted June 23,1993)
Abstract Phenylacetylene and 1-heptyne have been hydrocarbomethoxylated in the presence of Pt (II) / SnCl, systems to give the corresponding a$unsaturated esters. A careful choice of the catalytic system allows a very good control of the reg-ioaelectivity of the reaction to be obtained. The complex PtHCl( PPhe )r gives carbonylation exclusively at the /3 position, whereas using PtClx (DPPB ), the substrates are almost exclusively carbonylated at the cyposition. Key words: alkynes; hydroxycarbonylation;
platinum; regioselectivity control; tin
Introduction Whereas Group VIII first and second-row metal complexes have been widely used for the catalytic or stoichiometric hydrocarbonylation of acetilenic substrates [ 1,2], there are few reports concerning the use of Pt (II) complexes as catalyst precursors for such reactions [ 3,4]. Owing to our interest in transition-metals mediated organic syntheses we thought it would be of interest to study the hydroalkoxycarbonylation of phenylacetylene la in the presence of some readily available Pt(I1) complexes such as PtHCl (PPh,), or PtCl, (DPPB ) [ DPPB = 1,4-bis- (diphenylphosphino)butane] and SnClz as promoter. The aim of the study was the selective synthesis of the corresponding acrylate 2a (see Scheme 1 ), which is an immediate precursor of optically active 2-phenylpropanoic acid, a representative of an important class of anti-inflammatory non-steroidal drugs. Results and Discussion The results obtained in phenylacetylene carbonylation together with the relevant reaction conditions are reported in Table 1 (entries l-4 ) . *Corresponding author.
0304-5102/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved.
A. Scrivanti et al. /J. Mol. Catal. 84 (1993) L141-L144
L142
RC =CH2 20 c 00CH3 RC -CH
+ CO + CH30H
+
d
1 a.b RC =CH
I H a:R=Ph; Scheme 1.
I
3 a,b
COOCH3
b: R=n-C&H11
TABLE 1 Hydromethoxycarbonylation of phenylacetylene and 1-heptyne in the presence of Pt (II) /SnCl, systemsa Run
1 2 3 4 5 6
Substrate
phenylacetylene phenylacetylene phenylacetylene phenylacetylene 1-heptyne 1-heptyne
Catalyst precursor PtI-ICl(PPh& PtHCl ( PPh3) e PtC&(DPPB) PtC&(DPPB) PtHCl(PPh& PtC&(DPPB)
Solvent
THF toluene THF toluene THF toluene
Conversion (%)
Yield (% ) 2
3
50.4 34.4 99.5 97.1 98.9 95.9
20.2 14.1 10.8 1.0 70.6
28.9 34.3 50.3
“T= lOO”C, P(C0) = 100 atm, reaction time=22 h, substrate= 10 mmol, CH,OH=20 mmol, solvent = 10 ml, Pt/ substrate= l/200, Pt/Sn = l/l, Sn as SnC12*H20.
High temperature and pressure are required to carry out the reaction with moderate yields. The substrate hydromethoxycarbonylation is accompanied by the formation of products not detected in the gas-chromatographic runs. IR analyses of some of the residues indicated the formation of poly(phenylacetylene) [mostly (2) ] and of 1,2,4, and 1,3,5,-triphenylbenzene [ 5,6] : it is noteworthy that the Pt (II) complexes catalyzed polymerization of alkynes is well documented [ 7,8]. The most attractive results are related to the regioselectivity of the reaction: when the monophosphine derivative PtHCl (PPh,)z is used as catalyst precursor in tetrahydrofurane the required methyl cw-methylene-benzeneacetate (methyl atropate) (2a) is obtained with complete regioselectivity (run 1). The use of a less polar solvent such as toluene (run 2) does not affect the regiospecificity of the reaction even if a slight lower yield of methyl atropate is attained. Looking for a higher catalytic activity and chemoselectivity we have tested as the catalyst precursor the strictly related system PtCl, (DPPB ) /SnCl, which is known to be very active in the hydroformylation of olefins. The relevant
A. Scrivanti et al, /J. Mol. Catal. 84 (1993) L141-L144
I.4143
results are reported in entries 3 and 4 of Table 1. Surprisingly, this catalytic system gives predominantly the linear regioisomer 3-phenyl-2-propenoate (methyl cynnamate) (3a). In this case the solvent also plays a role in directing the addition to the carbon-carbon triple bond: as a matter of fact the linear to branched ratio is approximately 3 : 1 in tetrahydrofurane and 30 : 1 in toluene. To establish if the peculiar nature of the substrate exerts an influence on the reaction regioselectivity, we have studied the carbonylation of 1-heptyne. The results are presented in Table 1 (entries 5 and 6). In this case also the substrate carbonylation is accompanied by the formation of high molecular by-products. Nevertheless, the regioselectivity control is complete: in THF the monophosphine catalyst precursor PtHCl ( PPh,)z gives only the branched regioisomer methyl 2-methylen-heptanoate (2b), whereas in toluene the complex PtC& (DPPB ) produces exclusively the linear isomer methyl 2-octenoate (3b). Therefore it appears that by a suitable choice of catalyst precursor and reaction solvent these systems allow the attainment of almost complete control of the regioselectivity of the 1-alkynes’ carbonylation. Little is still known about the factors influencing the regioselectivity of the addition of the H-COOR moieties to the acetylenic bond. Earlier works based on nickel catalysts suggest that this addition is mostly directed by the nature of the substituent on the alkyne and that it occurs according to Markovnikov [91. In contrast, more recent results obtained with Pd catalysts [ 3,101 indicate that the catalytic system is able to control almost completely the regiochemistry of the reaction. The present results seem to point out that with these Pt (II) /SnC!& systems the coordination mode of the phosphine ligands (monodentate mutually tram versus bidentate chelating cis) is the factor which largely prevails in directing the position of the addition of the hydro and carbalkoxy groups to the carbon-carbon triple bond. However, much more work is needed to substantiate this hypothesis and to optimize the catalytic activity. Acknowledgements
The work was carried out with the financial support of the Italian CNR (Progetto Finalizzato Chimica Fine II). References 1 2 3
P. Pino and G. Braca, in I. Wender and P. Pino (eds.), Organic Synthesis via Metal Carbonyls, Wiley, New York, 1977, Vol. II, pp. 419-515. A. Mullen, in J. Falbe (ed.), New Syntheses with Carbon Monoxide, Springer Verlag, Berlin, 1980, Ch. 3. J.F. Knifton, J. Mol. Catal., 2 (1977) 293.
L144
4 5 6 7 8 9 10
A. Scrivanti et al. /J. Mol. Catal. 84 (1993) L141-L144
Y. Tsuji, T. Kondo and Y. Watanabe, J. Mol. Catal., 40 (1987) 295. T. Masuda, N. Sasaki and T. Higashimura, Macromolecules, 8 (1975) 717. T. Masuda, T. Mouri and T. Higashimura, Bull. Chem. Sot. Jpn., 53 (1960) 1152. A. F’urlani, I. Collamati and G. Sartori, J. Organomet. Chem., 17 (1969) 463. T.G. Appleton, H.C. Clark and R.J. Puddephatt, Inorg. Chem., 2 (1972) 2074. C.W. Bird, Chem. Rev., 62 (1962) 263. B. El Ali and H. Alper, J. Mol. Catal., 67 (1991) 29.