Elementary reaction

About: Elementary reaction is a research topic. Over the lifetime, 2972 publications have been published within this topic receiving 76110 citations.

Efficient Preparation of Functionalized (E,Z) Dienes Using Acetylene as the Building Block

Abstract: Received March 10, 1998 (E,Z)-Diene structures spread widely in the scope of not only important bioactive natural products,1 but also useful chemicals applicable in the perfume industry and other fields.2 The characteristic (E,Z) double-bond configuration of the molecules, which is usually responsible for the special functions,3 meanwhile poses great challenges for their stereoselective synthesis. The advent solutions mainly reside in the range of Wittig-type olefination4 or transitionmetal-mediated coupling reactions of deliberately functionalized precursors.5 While the methods of both classes have been successfully used in a good number of laboratory syntheses, their intrinsic drawback of low atom economy causing poor mass conversion greatly limited their application in large-scale preparations. A naive retrosynthetic analysis suggests that two acetylene molecules can be added together to form the (E,Z) double bonds, provided the addition reaction occurs in a stereoselective manner. Indeed, from the well-documented organometallic elementary reactions, we could infer that a tandem addition incorporating two molecules of acetylene may lead to (E,Z)-conjugated diene structure (Scheme 1). In this sequence, the stereochemical requirements of trans-addition and cis-insertion (carbometalation) helped to establish the otherwise hard-to-access conjugate (E,Z) doublebond configuration. Despite the extreme efficiency this sequence may bring about, there was only one precedent in the literature realizing such a concept: The Pd-catalyzed cotrimerization reaction of acetylene and allyl chloride developed by Kaneda et al.,6 but the reaction gave in low yield a mixture of codimer and cotrimer and the stereochemistry of the cotrimer was not established (Scheme 2). Recently, we have developed the facile synthesis of γ,δunsaturated carbonyl compounds in which a halide-assisted protonolysis efficiently recycles Pd(II)-catalytic species,7 thus effecting the tandem addition reaction of halide, an alkyne, and an R,â-unsaturated carbonyl. In this context, we attempted the reaction of acetylene with R,â-unsaturated electron-deficient alkenes in the presence of palladium catalyst to explore the possibility of developing new methods for (E,Z)-diene synthesis. Acrolein was first selected to react with acetylene under the catalysis of Pd(OAc)2. We first used acrolein as solvent to avoid the excessive polymerization of acetylene. When acetylene was passed through a mixture of acrolein 1 (20 mL), HOAc (50 mmol), Pd(OAc)2 (0.5 mmol), and LiBr (5 mmol) at 15 °C for 2 h, rapid formation of palladium black was observed. The reaction afforded the expected dienal 2a together with the R,â-unsaturated trienal 3a in 220% and 80% yield, respectively (yields are calculated on the basis of Pd(OAc)2) (Scheme 3).8 Pure 2a could be purified carefully by column chromatography. The geometry of the two double bonds in 2a was established by the coupling constants in 1H NMR between related vinylic protons. Thus, J(H6-H7) of 14 Hz and J(H4-H5) of 11 Hz manifest the (6E) and (4Z) double bonds, respectively. Further evidence from NOESY studies of the chloro derivative 2b unambiguously shows the (E,Z)-configuration of the molecule.9 The formation of 2a and 3a could be explained by Scheme 4, which also rationalizes the stereoselectivity. First, transhalopalladation of acetylene gives the (E)-vinylpalladium intermediate 4, which is inserted by a second molecule of acetylene to form (E,Z)-dienylpalladium 5; after the insertion of acrolein, the (2-oxoalkyl)palladium intermediate undergoes protonolysis (path a) or â-hydride elimination (path b) to afford 2a or 3a, respectively. Our previous studies reveal that excess coordinating halide inhibits â-H elimination and facilitates protonolysis in acidic conditions.7 A number of reaction conditions were screened. The results were summarized in Table 1. Preliminary results showed that polar solvents, high acetylene and halide concentration, and low temperature favor the yield of 2a. In most cases, using HOAc as a solvent and passing acetylene rapidly into the reaction mixture, 2a was isolated as the main product after chromatography (entries 3-5, Table 1). A codimerization byproduct of acetylene and acrolein,10 which could be removed after purification, could be detected in some cases by 1H NMR spectra. We found that the LiBr-Pd(OAc)2 ratio has a great impact on the reaction: when the LiBr-Pd(OAc)2 ratio was in(1) (a) Lalonde, R. T.; Wong, C. F.; Hofstead, S. J.; Morris, C. D.; Gardner, L. C. J. Chem. Ecol. 1980, 6, 35. (b) Baker, R.; Bradshaw, J. W. S. In Aliphatic and Related Natural Product Chemistry; Gunstone, F. D., Ed.; Specialist Periodical Report; Royal Society of Chemistry: London, 1983; Vol. 3. (2) Goldbach, M.; Jakel, E.; Schneider, M. P. J. Chem. Soc., Chem. Commun. 1987, 1434. (3) (a) Bergmann E. D. In Pesticide Chemistry, Vol. 1, Insecticides; Tahori, A. S., Ed.; Gordon and Breach: New York, 1972; p 1. (b) Rossi, R.; Carpita, A.; Quirici, M. G.; Gaudenzi, M. L. Tetrahedron 1982, 35, 631. (c) Stille, J. K.; Simpson, J. H. J. Am. Chem. Soc. 1987, 109, 2138. (4) Crombie, L.; Fisher, D. Tetrahedron Lett. 1985, 26, 2481. (5) Alami, M.; Gueugnot, S.; Domingues, E.; Linstrumelle, G. Tetrahedron 1995, 51, 1209. (6) Kaneda, K.; Uchiyama, T.; Fujuwara, Y.; Imanaka, T.; Taranash, S. J. Org. Chem. 1979, 44, 55. (7) (a) Wang, Z.; Lu, X. Chem. Commun. 1996, 535. (b) Wang, Z.; Lu, X. J. Org. Chem. 1996, 61, 2254. (8) The ratio of 2a to 3a was determined by the 1H NMR spectra of the product mixture. The presence of the trienal product 3a was further supported by GC-MS analysis and UV spectra. (9) There is a strong NOE effect between H6 and H3 and no cross-peak between H5 and H3. Scheme 1