A chiral bicyclic guanidine was developed as a versatile Brønsted base catalyst for the enantioselective Michael reactions of dithiomalonates and b-keto thioesters using a range of acceptors including maleimides, cyclic enones, furanone and acyclic 1,4-dicarbonylbutenes.Keywords: asymmetric catalysis; dithiomalonates; guanidines; Michael addition; organic catalysis Catalytic enantioselective Michael reactions are widely used for the construction of C À C bonds.[1] Tremendous advances have been achieved using Lewis acidic, metal-based catalysts for the Michael reaction of 1,3-dicarbonyl compounds.[2] Organocatalytic approaches have also gained wide popularity in recent years.[3] Successful organocatalysts include imidazolidines, [4] bifunctional thioureas, [5] Cinchona alkaloids and derivatives, [6] quaternary ammonium salts [7] and a proline-tetrazole.[8] These catalysts are typically reported as highly effective for a particular type of Michael acceptor. Catalysts that are applicable for a range of substrates are still of high interest.Guanidines have been shown to catalyze the Strecker reaction with high enantioselectivity.[9] Chiral guanidines and guanidinium salts were also found to be useful for several other base-catalyzed reactions. [10] Recently, highly efficient axially chiral guanidine catalysts were reported.[1] We have also reported a chiral bicyclic guanidine-catalyzed anthrone Diels-Alder reaction.[12] Herein, we report the development of chiral bicyclic guanidine 1 as a versatile Brønsted base catalyst for the enantioselective Michael reactions of dithiomalonates and b-keto thioesters using a range of acceptors, including maleimides, cyclic enones, furanone and acyclic 1,4-dioxobutenes.Our initial investigation revealed that with 20 mol % of guanidine 1, dimethyl malonate 3 underwent a Michael addition to N-ethyl maleimide 2a [Eq.(1)]. The reaction was relatively slow, giving the Michael adduct 4 in only 20 % yield after 96 h and with 47 % ee. The a-proton acidity of thioesters is usually higher than that of their corresponding esters due to the poor overlap of the C(2p) and S(3p) orbitals. This enhanced acidity makes thioesters useful enol equivalents in the laboratory as well as in nature (e.g., as acyl coenzyme A demonstrates). Seeking a more active donor, we prepared a series of S,S'-dialkyl dithiomalonates and they were tested in place of the dialkyl malonates. Significant improvements in both reaction rates and ee values were achieved [Eq. (2)].With 10 mol % of catalyst 1, the reaction between S,S'-di-tert-butyl dithiomalonate 5a and maleimide 2a was completed within 1 h at room temperature, giving

The silica-supported tungsten oxo-trimethyl complex [(SiO)W(O)Me 3 ] was synthesized using a novel SOMC synthetic approach. By grafting the inexpensive stable compound WOCl 4 on the surface of silica, partially dehydroxylated at 700°C (SiO 2-700 ), a well-defined monopodal surface complex [(SiO)W(O)Cl 3 ] was produced. The supported complex directly methylated with ZnMe 2 and transformed into [(SiO)W(O)Me 3 ], which we fully characterized by microanalysis, IR, mass balance and SS NMR ( 1 H, 13 C, 1 H− 13 C HETCOR, 1 H− 1 H DQ and TQ). [(Si O)W(O)Me 3 ] has two conformational isomers on the surface at room temperature. The conversion of one to the other was observed at 318 K by variable-temperature 13 C CP/MAS and 1 H spin echo MAS solid-state NMR; this was also confirmed by NMR and DFT calculations. [(SiO)W(O)Me 3 ] was found to be active in cyclooctane metathesis and to have a wide distribution range in ring-contracted and ring-expanded products. In addition, [(SiO)W(O)Me 3 ] proved to be highly active for selective transformation of ethylene to propylene compared to other silica-supported organometallic complexes.

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