Phosphetes via Transition Metal Free Ring Closure - Taking the Proper Turn at a Thermodynamic Crossing.

Fabian Roesler,Zsolt Kelemen,Rudolf Pietschnig,Clemens Bruhn,Máté Kovács

doi:10.1002/chem.202101298

Fabian Roesler, Zsolt Kelemen + Show 3 more

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https://doi.org/10.1002/chem.202101298

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Abstract

A transition metal free route to phosphetes featuring an exocyclic alkene unit is presented. In this approach phosphanides are added to a variety of diynes generating phosphaallylic intermediates which depending on the reaction conditions transform either to phosphetes or the corresponding phospholes. Investigation of the reaction mechanism by combined quantum chemical and experimental means identifies phosphole formation as thermodynamically controlled reaction path, whereas kinetic control furnishes the corresponding phosphetes. Structural and luminescence properties of the rare class of phosphetes are explored, as well as for selected key intermediates.

Highlights

Phosphorus heterocycles emerged as useful class of π-conjugated luminophores for light emitting or light harvesting applications.[1]
With experimental and computational measures, we explore a synthetic pathway giving selectively access to either the ring contracted four-membered ring heterocyclic phosphetes or to phospholes, their five-membered ring heterocyclic isomers, depending on the reaction conditions, with identification of key intermediates with spectroscopic and structural means
It needs to be pointed out that for all isolated phosphetes shown in Scheme 2 only the Z isomers with respect to the exocyclic alkene unit are obtained exclusively

Summary

Introduction

Phosphorus heterocycles emerged as useful class of π-conjugated luminophores for light emitting or light harvesting applications.[1] Among the organophosphorus materials, phospholes are probably the most prominent examples for such purposes,[2] besides λ5-phosphinines[3] and more sophisticated molecular scaffolds[4] or systems with additional heteroatoms[5] (Scheme 1). Phosphetes with an exocyclic methylene unit are constitutional isomers of phospholes, and first examples containing the four-membered heterocyclic motif have been obtained via rearrangement in the coordination sphere of transition metal fragments.[6] Their potential revolving around their optoelectronic properties has been discovered only recently.[6h,7] A peculiar feature of such strained heterocycles is their structural flexibility in terms of ring opening and closure which led to the description of phosphetes as masked 1phosphabutadienes.[6b] Somewhat related is the addition of neutral phosphane units to an adjacent alkyne leading to phosphanylidene bridged stilbenes[4e] or cyclic P-ylidic mesoionic carbenes,[8] which attracted attention as ligand system towards transition metal centers.

Results and Discussion

Conclusion

Conflict of Interest