Dirhodium tetracarboxylates are versatile catalysts for the reactions of donor/acceptor carbenes. They catalyze a variety of transformations, including enantioselective intermolecular cyclopropanations. This study is focused on understanding the kinetics of the rhodium-catalyzed cyclopropanation, and this information was used to develop conditions for conducting the reactions with very low catalyst loadings. The enantioselective cyclopropanation of styrenes can be conducted with a catalyst loading of 0.001 mol % and still maintain high levels of enantioselectivity (86−99% ee). A triarylcyclopropanecarboxylate (TPCP) catalyst, Rh 2 (p-Ph-TPCP) 4 , was the optimum catalyst for maintaining high enantioselectivity with very low catalyst loading. The reaction also benefited from using dimethyl carbonate as the solvent, an environmentally benign and nontoxic material.

Methods for the synthesis of cyclopropanes are critical for drug discovery, chemical biology, total synthesis, and other fields. Herein, we report the use of the strong sterically encumbered Lewis acid tris­(pentafluorophenyl)­borane as a catalyst for the cyclopropanation of unactivated alkenes using aryldiazoacetates. The cyclopropane products are synthesized using 10 mol % of the catalyst under mild conditions in up to 90% yield (8:1 to >20:1 dr). We propose that the reaction proceeds via a Lewis acid-activated carbene.

The enantioselective intermolecular sp3 C–H functionalization at allylic and benzylic positions was achieved using rhodium-catalyzed reactions with 4-phenyl-N-methanesulfonyl-1,2,3-triazole. The optimum dirhodium tetracarboxylate catalyst for these reactions was Rh2(S-NTTL)4. The rhodium-bound α-imino carbene intermediates preferentially reacted with tertiary over primary C–H bonds in good yields and moderate levels of enantioselectivity (66-82% ee). This work demonstrates that N-sulfonyltriazoles can be applied to the effective C–H functionalization at sp3 C–H bonds of substrates containing additional functionality.

No abstract

The synthesis of functionalized azepanes was accomplished through the palladium-mediated cross-coupling of α-halo eneformamides with mostly unactivated nucleophiles under mild conditions. Alkenylations proceeded with excellent stereoselectivitiy. In most cases, high yields of the coupling products were obtained.

A W(CO)(5)·THF-catalyzed cycloisomerization of bicyclo[4.1.0] substrates to afford mono C4-substituted 4,5-dihydro-benzo[b]furans and -indoles is reported. The title compounds are versatile intermediates that lead to a range of fused bicycles including the cores of various furan-, benzofuran-, and indole-containing natural products. In many cases, the functionalization of the dihydro-benzo[b]furans and -indoles is orthogonal to that of the corresponding benzofurans and indoles and, thus, offers complementary approaches for synthesis.

A mechanistic study of a new heterocycloisomerization reaction that forms annulated aminopyrroles is presented. Density functional theory calculations and kinetic studies suggest the reaction is catalyzed by trace copper salts and that a Z- to E-hydrazone isomerization occurs through an enehydrazine intermediate before the rate-determining cyclization of the hydrazone onto the alkyne group. The aminopyrrole products are obtained in 36-93% isolated yield depending on the nature of the alkynyl substituent. A new automated sampling technique was developed to obtain robust mechanistic data.

Herein we report the divergent reactivity of 2,2-dialkyl-3-(E)-alkenyl N-tosylhydrazones using Pd-catalyzed crosscoupling conditions, which enable the Z-selective synthesis of 3-aryl-1,4-dienes and gem-dialkyl vinylcyclopropanes. We found that the dialkylbiaryl phosphine ligand SPhos was the optimal ligand for this transformation producing skipped dienes in up to 83% isolated yield. The ratio of skipped diene to vinylcyclopropane is dependent on both the structure of the α,α-disubstituted hydrazones and the aryl halide partner. Using sterically encumbered aryl bromides provided the trans-cyclopropane products selectively in up to 69% yield. The reaction is stereospecific and stereoselective and occurs alongside a competing 1,2-alkenyl group migration pathway.