In a combined theoretical and experimental study, an efficient catalytic reaction featuring epoxide opening and tetrahydrofuran formation through homolytic substitution reactions at C-O and Ti-O bonds was devised. The performance of these two key steps of the catalytic cycle was studied and could be adjusted by modifying the electronic properties of the catalysts through introduction of electron-donating or -withdrawing substituents to the titanocene catalysts. By regarding both steps as single electron versions of oxidative addition and reductive elimination, a mechanism-based platform for the design of catalysts and reagents for electron transfer reactions evolved that opens broad perspectives for further investigations.
SummaryAn operationally simple, convenient, and mild strategy for the synthesis of triazole-substituted titanocenes via strain-driven 1,3-dipolar cycloadditions between azide-functionalized titanocenes and cyclooctyne has been developed. It features the first synthesis of titanocenes containing azide groups. These compounds constitute ‘second-generation’ functionalized titanocene building blocks for further synthetic elaboration. Our synthesis is modular and large numbers of the complexes can in principle be prepared in short periods of time. Some of the triazole-substituted titanocenes display high cyctotoxic activity against BJAB cells. Comparison of the most active complexes allows the identification of structural features essential for biological activity.
A modular approach to bench-stable titanocene enolates is described. The reaction of titanocenes containing pendent acid chlorides with activated methylene compounds in the presence of excess base results in the formation of the pivotal enolates. In all cases, the enolates are coordinated to the titanocene, which acts as a stabilizing template in an intramolecular manner, as demonstrated by NMR spectroscopy and X-ray crystallography. Upon protonation with strong acids, the C-C bond formed during the acylation is cleaved. Hence, the template effect can be reversed by adjusting the acidity of the reaction medium. § Dedicated to Prof. Uwe Rosenthal on the occasion of his 60th birthday.
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