New organometallic reagents for olefin synthesis


The synthesis, structural characterization, and reactivity of zirconium metalloazines are presented. A mechanism for olefination of aldehydes or ketones by metalloazines is discussed based on a series of mechanistic studies concerning this reaction. INTRODUCTION To take advantage of reactivity of transition metal complexes in order to develop new systems for use in organic synthesis is a topic of considerable interest in modern organic chemistry. We are studying the creation of methodology which complements olefin synthesis by the "classical" Wittig reaction, and we have focused our attention on developing procedures which obviate the formation of by-products (such as triorganophospine oxides) which are not only expensive to discard or recycle, but also which present problems with regard to separation from reaction mixtures. CAR B E N E COMPLEXES Our first approach to the development of olefination reagents was based on observations originally made for the "Tebbe" reagent that low-valent titanium methylene complexes could methylenate a wide variety of organic carbonyls under mild conditions'. Several tantalum' and tungsten3 alkylidene analogs have also been prepared. are, however, without drawbacks. The Tebbe reagent is easy to make, but its chemistry is limited to methylene transfer. In the niobium and tungsten cases, the starting materials are not easily accessible to synthetically oriented organic chemists. In every case metallic by-products have not been shown to be recyclable, and control of E,Z stereochemistry has not yet been demonstrated. Our first efforts involved the synthesis of long alkyl chain anaologs of the Tebbe reagent: CpzZr-C(H)R. These compounds could be made from an acetylene via a tedious synthesis involving conversion of the acetylene to a metal alkenyl and thence to a heterobimetallic alkylidene-bridged complex4 which could be selectively cleaved to give the carbene complex5. We were able to demonstrate that these unsymmetrically substituted carbene complexes could react with imine or imidate derivatives of a carbonyl group containing material, and through variation in the size of the organic group on nitrogen, the E,Z stereochemistry of olefination could be affected6. However, the tedious synthesis of the zirconium carbene complexes precludes this approach as a viable method from the point of view of practical organic synthesis. None of these reagent systems METALLOAZINES We next considered the possibility that transition metal derivatives could be prepared from readily available organic starting materials through procedures familiar to synthesisoriented organic chemists. Our intent was to create a family of compounds M-X-C(H)R, in which the [-X-C(H)R] group could be obtained in straightforward fashion and in which the reactivity of the [X-C(H)R] group paralleled that of its carbene complex analogs, M-C(H)R.

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@inproceedings{Schwartz2005NewOR, title={New organometallic reagents for olefin synthesis}, author={Jeffrey J Schwartz and Georgia M. Amanitis and John A. Smegel}, year={2005} }