Summary The reaction of carboxylic acids with propadiene or propyne catalysed by [Ru(m- O2C H ) ( C O )2( P P h3) ]2 affords isopropenyl esters via regioselective addition of the carboxylate to the central carbon atom of the allene. Introduction The C3 mixture resulting from cracking of hydrocarbon feedstocks mainly contains propene and propane, together with propyne and propadiene. The possibility of direct utilization of this C3 mixture without separation of its constituents to produce high added-value compounds via a clean and selective catalytic transformation would increase the synthetic value of this hydrocarbon fraction and be of major interest from an economical point of view.The ability of ruthenium complexes to activate terminal alkynes in a stoichoimetric manner is well documented1 and new ruthenium-catalysed transformations of alkynes with high regioand stereoselectivity and atom economy have been developed.2–4 By contrast, the analogous catalytic activation of allenes by ruthenium catalysts has not been performed yet.Isopropenyl esters5 and diisopropenyl diesters,6 respectively prepared from carboxylic acids and oxalic acid, and rutheniumactivated propyne have been shown to be useful acylating and 1,2-diacylating reagents under very mild and neutral conditions releasing only acetone as by-product. The presence of allene together with propyne in the industrial hydrocarbon C3 mixture provided impetus to study the addition of carboxylic acids to allene in the presence of a ruthenium catalyst, as we had previously shown that the addition of acids to alkenes did not take place under these catalytic conditions. The palladium-catalysed additions of carbonucleophiles,7 amines,8 tosylhydrazine9 and carboxylic acids10 to unactivated allenes have recently been reported, all of them afford allylic derivatives resulting from an intermediate allylpalladium species generated via insertion of the terminal double bond of the allene into a Pd–H bond.We report here that ruthenium catalysis offers the first example of metal-catalysed activation of propadiene towards the addition of carboxylic acids to selectively produce isopropenyl esters v i a a d d ition of the carboxylate to the central C-2 carbon atom (Scheme 1). Results and discussion In the presence of (p-cymene)RuCl2(PPh3) as catalyst, which is known to give the Markovnikov addition of carboxylic acids to The conversion of small hydrocarbon feedstocks to more functional materials is of vital importance to the chemical industry.The ability to carry out such transformations without the need for pre-separation of components in a complex mixed feedstock is an additional benefit, as it reduces the complexity of the process and eliminates wasteful and energy-intensive separations.This article describes the conversion of a C3 feedstock containing a range of compounds (including propadiene and propyne) to isopropenyl esters. Reaction involves catalytic hydrocarboxylation of both propyne and propadiene with high regioselectivity to C-2. The products are excellent acylating agents.DJM Green Context Utilization of an industrial feedstock without separation Ruthenium-catalysed hydrocarboxylation of propadiene and propyne Christian Bruneau,* Muriel Neveux-Duflos and Pierre H. Dixneuf* UMR 6509: CNRS - Université de Rennes, Organométalliques et Catalyse : Chimie et Electrochimie Moléculaires, Campus de Beaulieu, 35042 Rennes, France Received 29th April 1999 Scheme 1 terminal alkynes,3–5 the addition of benzoic acid to propyne at 100 °C for 20 h led to 87% of esters containing 91% of isopropenyl benzoate 1 (R = Ph).Under similar conditions, the activation of allene itself in the presence of (p-cymene)RuCl2(PPh3) was less efficient as only 44% of the acid was converted after 20 h at 100 °C and the total conversion required 40 h of heating.[Ru(m-O2CH)(CO)2(PPh3)]2 11 appeared to be a much better catalyst and the treatment of 20 mmol of propadiene with 10 mmol of benzoic acid in 10 mL of toluene at 100 °C for 15 h in the presence of 0.5 mol% of [Ru(m-O2CH)(CO)2(PPh3)]2 as catalyst precursor led to the complete conversion of the carboxylic acid into propenyl benzoates. An overall yield of 81% of ester was isolated by distillation under reduced pressure, which contained 90% of isopropenyl benzoate 1 and 10% of the two stereoisomers of prop-1-en-1-yl benzoate PhCO2CHNCHMe.Moreover, when a 1+1 mixture of propyne and allene was reacted with benzoic acid at 100 °C for 20 h in the presence of the same catalyst, complete conversion was observed and after distillation 86% of benzoates were collected containing isopropenyl benzoate as the major Green Chemistry August 1999 183 C Gcompound (>90%) with a small amount of prop-1-en-1-yl benzoate.Isopropenyl benzoate has a boiling point of 80 °C under 2 mm Hg and is easily separated from the gaseous starting C3 reactants. These first results clearly showed that in the presence of the binuclear ruthenium complex as catalyst, the regioselective addition of the carboxylate to the central carbon atom of both the allene and propyne leading to isopropenyl ester could be achieved.Among the C3 components, as only allene and propyne—and not propene and propane—are activated towards the addition of carboxylic acids in the presence of a ruthenium catalyst and as the reaction is complete, this catalytic transformation followed by a classical distillation allows an easy separation of the enol esters from the other hydrocarbons.From another viewpoint, this catalytic reaction actually makes possible the elimination of allene and propyne from the C3 mixture. The ruthenium-catalysed addition to allene was extended to other carboxylic acids. p-Chlorobenzoic acid led to the corresponding isopropenyl aromatic ester 2 (R = p-ClC6H4) in 80% yield and more than 90% regioselectivity after reaction for 20 h at 100 °C in toluene in the presence of [Ru(m- O2CH)(CO)2(PPh3)]2 as catalyst.Under the above conditions, the addition of N-protected Z-alanine gave the optically pure isopropenyl amino ester 3 in 86% yield with a high regioselectivity of 90% (Scheme 2).Scheme 2 Similarly, mandelic acid led to 77% of the dioxolanone 5 in the presence of [Ru(m-O2CH)(CO)2(PPh3)]2. We have shown11 that the dioxolanone 5 resulted from the formation of the intermediate isopropenyl mandelate 4 without racemization, and that the ruthenium catalyst promoted as well the intramolecular addition of the OH group to the activated CNC bond of 4 to produce 5 with a high stereoselectivity (Scheme 3).Scheme 3 It must be noted that the presence of a ruthenium catalyst precursor had no influence on the reaction of carboxylic acid with an activated allene such as methoxyallene. Indeed, the addition of benzoic acid to methoxyallene at 100 °C for 20 h with or without catalyst led to a mixture of the allylic esters 5 and (E) -6 in the ratio 5 7+43 (84% total yield) resulting from the addition of the carboxylate to both C-1 and C-3 carbons of the allene (Scheme 4).Scheme 4 The results in Table 1 show that the reactivity of allene in the presence of [Ru(m-O2CH)(CO)2(PPh3)]2 which provides the most efficient catalytic system is very similar to that of propyne. The addition of carboxylic acids to propadiene takes place with the same regioselectivity as the addition of thiols to allenes in the presence of Pd(OAc)2 12 but dramatically contrasts with the Pd0-catalysed addition of other pronucleophiles which leads to allylic derivatives via (p-allyl)Pd species resulting from hydroor carbopalladation of the allene.7–10 The reactivity of ruthenium catalysts corresponds to an electrophilic activation of one CNC double bond of the allene, whereas protonation of the precursor to give an allylruthenium intermediate is not favoured as is the insertion of allenes into a Pd–H bond (Scheme 5).Scheme 5 In conclusion, we have shown that the catalytic activation of propadiene towards the addition of carboxylic acids to form isopropenyl esters is very efficient in the presence of the binuclear ruthenium catalyst [Ru(m-O2CH)(CO)2(PPh3)]2.As propyne and propadiene present the same reactivity towards carboxylic acids under similar catalytic conditions, the above results have potential for the direct utilization of the industrial C3 mixture without previous separation of its constituents for the catalytic preparation of isopropenyl esters. The use of a recoverable and reusable ruthenium catalyst such as [Ru(O2CH)(CO)2(Ph2P(CH2CH2)nCH2CH3)]2 which is also very efficient in this type of catalytic addition13 might present advan - tages from economical, industrial and environmental viewpoints. The overall process requires the catalytic transformation followed by a simple distillation and can be achieved on the spot where C3 is produced, thus avoiding inflammable products transportation.References 1 (a) M. I. Bruce, Chem. Rev., 1991, 91, 197; (b) H. Le Bozec, K. Ouzzine and P. H. Dixneuf, Organometallics, 1991, 10, 2768; (c) D. Touchard, P. Haquette, S. Guesmi, L. Le Pichon, Table 1 Ruthenium-catalysed formation of isopropenyl esters from propyne and propadienea Yield from Yield from Me–CáCH H2CNCNCH2 cat. [Ru]1 cat. [Ru]2 Acid Ester (%) (%) Benzoic acid 1 873 90 p-Chlorobenzoic acid 2 583 80 Z-Alanine 3 723 76 (R)-Mandelic acid 5 7011 77 a Allene or propyne (30 mmol), carboxylic acid (20 mmol), toluene, 100 °C, 20 h.Z = PhCH2OCO; [Ru]1 = (pc y m e n e ) R u C l2( P P h3); [Ru]2 = [Ru(m- O2C H ) ( C O )2( P P h3) ]2. 184 Green Chemistry August 1999A. Daridor, L. Toupet and P. H. Dixneuf, Organometallics, 1997, 16, 3640. 2 (a) B. M. Trost, Chem. Ber., 1996, 1 2 9, 1313; (b) F. Kakiuchi, Y. Yamamoto, N. Chatani and S. Murai, Chem. Lett., 1995, 681; (c) T. Kondo, N. Suzuki, T. Okada and T. Mitsudo, J. Am. Chem. Soc., 1997, 1 1 9, 6187; (d) S. Dérien, D. Jan and P. H. Dixneuf, Tetrahedron, 1996, 52, 5511; (e) Y. Wakatsuki and H. Yamazaki, J. Organomet. Chem., 1995, 500, 349. 3 C.Bruneau, M. Neveux, Z. Kabouche, C. Ruppin and P. H. Dixneuf, Synlett, 1991, 755. 4 C. Bruneau and P. H. Dixneuf, Chem. Commun., 1997, 507. 5 C. Ruppin, P. H. Dixneuf and S. Lécolier, Tetrahedron Lett., 1988, 29, 5365. 6 M. Neveux, C. Bruneau, S. Lécolier and P. H. Dixneuf, Tetrahedron, 1993, 49, 2629. 7 (a) Y. Yamamoto, M. Al-Masum and N. Asao, J. Am. Chem. Soc., 1994, 116, 6019; (b) B.M. Trost and V. J. Gerusz, J. Am. Chem. Soc., 1995, 117, 5156; (c) L. Besson, J. Goré and B. Cazes, Tetrahedron Lett., 1995, 36, 3853; (d) Y. Yamamoto, Pure Appl. Chem., 1996, 68, 9. 8 (a) L. Besson, J. Goré and B. Cazes, Tetrahedron Lett., 1995, 36, 3857; (b) M. Al-Masum, M. Meguro and Y. Yamamoto, Tetrahedron Lett., 1997, 38, 6071. 9 S. Kamijo, M. Al-Masum and Y. Yamamoto, Tetrahedron Lett., 1998, 39, 691. 10 M. Al-Masum and Y. Yamamoto, J. Am. Chem. Soc., 1998, 120, 3809. 1 1 M. Neveux, B. Seiller, F. Hagedorn, C. Bruneau and P. H. Dixneuf, J. Organomet. Chem., 1993, 451, 133. 1 2 A. Ogawa, J.-I. Kawakami, N. Sonoda and T. Hirao, J. Org. Chem., 1996, 61, 4161. 13 O. Lavastre, P. Bebin, O. Marchaland and P. H. Dixneuf, J. Mol. Catal. A, 1996, 108, 29 Paper 9/03452H Green Chemistry August 1999 185A.Daridor, L. Toupet and P. H. Dixneuf, Organometallics, 1997, 16, 3640. 2 (a) B. M. Trost, Chem. Ber., 1996, 1 2 9, 1313; (b) F. Kakiuchi, Y. Yamamoto, N. Chatani and S. Murai, Chem. Lett., 1995, 681; (c) T. Kondo, N. Suzuki, T. Okada and T. Mitsudo, J. Am. Chem. Soc., 1997, 1 1 9, 6187; (d) S. Dérien, D. Jan and P. H. Dixneuf, Tetrahedron, 1996, 52, 5511; (e) Y.Wakatsuki and H. Yamazaki, J. Organomet. Chem., 1995, 500, 349. 3 C. Bruneau, M. Neveux, Z. Kabouche, C. Ruppin and P. H. Dixneuf, Synlett, 1991, 755. 4 C. Bruneau and P. H. Dixneuf, Chem. Commun., 1997, 507. 5 C. Ruppin, P. H. Dixneuf and S. Lécolier, Tetrahedron Lett., 1988, 29, 5365. 6 M. Neveux, C. Bruneau, S. Lécolier and P. H. Dixneuf, Tetrahedron, 1993, 49, 2629. 7 (a) Y. Yamamoto, M. Al-Masum and N. Asao, J. Am. Chem. Soc., 1994, 116, 6019; (b) B. M. Trost and V. J. Gerusz, J. Am. Chem. Soc., 1995, 117, 5156; (c) L. Besson, J. Goré and B. Cazes, Tetrahedron Lett., 1995, 36, 3853; (d) Y. Yamamoto, Pure Appl. Chem., 1996, 68, 9. 8 (a) L. Besson, J. Goré and B. Cazes, Tetrahedron Lett., 1995, 36, 3857; (b) M. Al-Masum, M. Meguro and Y. Yamamoto, Tetrahedron Lett., 1997, 38, 6071. 9 S. Kamijo, M. Al-Masum and Y. Yamamoto, Tetrahedron Lett., 1998, 39, 691. 10 M. Al-Masum and Y. Yamamoto, J. Am. Chem. Soc., 1998, 120, 3809. 1 1 M. Neveux, B. Seiller, F. Hagedorn, C. Bruneau and P. H. Dixneuf, J. Organomet. Chem., 1993, 451, 133. 1 2 A. Ogawa, J.-I. Kawakami, N. Sonoda and T. Hirao, J. Org. Chem., 1996, 61, 4161. 13 O. Lavastre, P. Bebin, O. Marchaland and P. H. Dixneuf, J. Mol. Catal. A, 1996, 108, 29 Paper 9/03452H Green Chemistry August 1999 185