Journal of Organometallic Chemistry, 464 (1994) C33-C34 c33 JOM 24108PC zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Preliminary Communication Palladium catalyzed reactions of cr-bromo-cr &unsaturated carbonyl compounds with 1-t E)-alkenylboronic acids Neudo Urdaneta, Jacqueline Ruiz and Antonio J. Zapata Departamento de Q&mica, Universidad Sim6n BoKvar, Apartado Postal 89000, Caracas 108OA (Venezuela) (Received June 3, 1993) Abstract A new synthesis of functionalized 1,3-dienes is described based on the palladium catalyzed coupling reaction of l_(E)-alkenylboronic acids with cr-bromoa,&unsaturated carbonyl compounds. Key words: Palladium; Alkenyl; Roronic acid; Catalysis In a previous communication we reported the first example of the coupling reaction between 3-bromo- acrylates and 1-(E)-alkenylboronic acids, catalyzed by complexes of palladium(O) [l]. The utility of this reac- tion was later demonstrated by its application to the synthesis of functionalized 2,4-dienes [2]. More re- cently, other groups have reported examples of this type of catalytic coupling using different reaction con- ditions [3,4]. For instance, a combination of palladium acetate and triphenylphosphine as catalyst, potassium carbonate or sodium bicarbonate as bases, and solvent systems such as ethanol-water, anhydrous ethanol or methanol, and dimethoxyethane were investigated in these reports. In this communication we report the palladium cat- alyzed coupling reactions between trans-alkenylboronic acids and a-bromo-cu,&unsaturated carbonyl com- pounds. To our knowledge, this is the first example of the use of these functionalized vinylic bromides in this type of coupling reactions. truns-1-Hexenylboronic acid (1.0 mmol, > 98% E) [5] was treated with methyl a-bromocinnamate (1.1 mmol > 95% Z) in the presence of tetrakis(triphenyl- Correspondence to: Professor N. Urdaneta. phosphinel-palladium(O) (3-5% mol) and potassium carbonate (2.0 mmol), using a mixture of tetrahydrofu- ran-water (3 : 1,4 ml) as solvent (eqn. (1)). The reaction H -R(OH), + Ph CO&H, Br CO&H, PdfPPh,),, K&O3 THF*H,O, reflux, 3 h ’ “+ (1) Ph mixture was refhtxed until no further change was ob- served, as indicated by TIC analysis (3 h). After purifi- cation of the crude product by flash chromatography [6], the desired diene was obtained in 74% yield as an oily liquid which exhibited spectroscopic (IR and NMR) and analytical data (MS) in accord with the assigned structure. Retention of the double bond geometry was observed affording the stereoisomerically pure ( > 90%) E,E-diene as indicated by ‘H NMR analysis. Applica- tion of this procedure to a series of representative cy-bromo-@-unsaturated carbonyl compounds af- forded the results shown in Table 1. Retention of double bond geometry was observed in all cases, as estimated by ‘H NMR spectroscopy. The stereochemical purity of the products was better than 90%. With the exception of methyl 2-bromo-2-octade- cenoate (Table 1, entry 3), the yields of products de- rived from truns-hexenylboronic acid were generally good. The lower yield obtained with the long chain ester can be attributed to an steric effect. The reactions using truns-styrylboronic [5] acid gave lower yields of the expected dienes. This has been observed before by other authors in this type of cou- pling reactions [7]. Perhaps, the stabilizing effect of the P-phenyl ring in the alkenylboronic acid makes it less reactive towards the nucleophilic attack by the hydroxopalladium(II1 complex in the transmetallation step of the proposed catalytic mechanism [7], thus lowering the yield of the corresponding product. The reaction of truns-alkenyl boronic acids with cY-bromo-o&unsaturated carbonyl compounds repre- sents a convenient method for the preparation of stereodefined dienes in good yields. These functional- ized organic compounds can be transformed into more complicated molecules. Currently we are exploring the 0022-328X/94/$7.00 SSDZ 0022-328X(93)24108-H 0 1994 - Elsevier Sequoia. All rights reserved