Efficient Organic Reactions by Dynamic Control of Early Transition Metal Complexes

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1. Selective Transformations with Novel Organochromium Species : The family of trans-iodocyclo-propanes are good precursors for constructing cyclopropyl-cyclopropyl and-vinyl carbon skeletons. We have found that iodocyclopropanation of terminal alkenes proceeds with organochromium species generated by chromium(II)-reduction of iodoform in the presence of TEEDA (N,N,N',N'-tetraethylethylenediamine) under high stereocontrol. The transformation proceeds smoothly without the presence of a hydroxy or an alkoxy group near the double bond, which is necessary for the cyclopropanation of iodoalkenes with zinc carbenoids.2. Synthesis and Reactions of Tantalum η^2-Bis(trimethylsilyl)acetylene Complex : A tantalum-alkyne complex TaCl_3(Me_3SiC≡CSiMe_3)(dme) (1, DME=1,2-dimethoxyethane) is synthesized by treatment of the corresponding alkyne with a low-valent tantalum derived by reduction of TaCl_5 with zinc in a mixed solvent of toluene and DME according to our previously reported procedure. The t … More antalum complex 1 reacts with an internal alkyne to give a corresponding alkyne complex via ligand exchange reaction. The mechanism of the ligand exchange is clarified by kinetic study of the formation of diphenylacetylene complex from 1 and diphenylacetylene.3. Carbon-Hydrogen Bond Activation with Rhenium Catalysts and Its Application to Organic Synthesis : A rhenium complex, [ReBr(CO)_3(thf)]_2, catalyzes the reaction of an aromatic aldimine with an isocyanate and an acetylene to give a phthalimidine and an indene derivative in a quantitative yield, respectively. The reactions proceed via C-H bond activation, insertion of the isocyanate or the acetylene, intramolecular nucleophilic cyclization to the aldimine of the generated amido-or alkenyl-rhenium species, and reductive elimination. In contrast to ruthenium and rhodium catalysts, which are usually employed in this type of reaction, the rhenium catalyst promotes the insertion of a polar unsaturated molecule. The rhenium-catalyzed reaction of an aromatic ketimine with ethyl acrylate gave an indene derivative in good yield via C-H bond activation. The indene derivatives could also be obtained by the reactions of aromatic ketones with α,β-unsaturated esters in the presence of catalytic amounts of the rhenium complex andp-anisidine. Less