The Fascinating World of Phosphanylphosphonates: From Acetylenic Phosphaalkenes to Reductive Aldehyde Couplings

This account highlights the versatility of phosphanylphosphonates, which can be used for the preparation of phosphorus-containing pi-systems and as reagents for the reductive coupling of carbonyl compounds to alkenes. Phosphanylphosphonates with metal fragments coordinated to the P-lone pair have been known for a long time and they have been used for the synthesis of phosphaalkenes by means of the phospha-Horner-Wadsworth-Emmons reaction. With the original aim of incorporating phosphorus heteroatoms into classical all-carbon ethynylethene scaffolds, we entered the field of phosphanylphosphonates with the discovery that these compounds engage in complex cascade reactions with acetylenic ketones, forming 1,2-oxaphospholes, cumulenes, and bisphospholes. Later, we synthesized the first metal-free phosphanylphosphonate, which reacts with aldehydes to yield phosphaalkenes, but gives phospholones when diacetylenic ketones are used as substrates. In the final part of the account, we outline our discovery and the development of an unprecedented carbonyl-carbonyl cross-coupling reaction. This protocol offers a straightforward method for the synthesis of nonsymmetric 1,2-disubstituted alkenes directly from two dissimilar aldehydes. 1Combining Acetylenes with Phosphaalkenes 2Synthetic Examples of Acetylenic Phosphaalkenes 3The Phospha-Horner-Wadsworth-Emmons Approach to Phosphaalkenes 3.1Metal-Coordinated Phosphanylphosphonates 3.2Mechanism of the Phospha-Horner-Wadsworth-Emmons Reaction 3.3The First Metal-Free Phosphanylphosphonate and Its Reactivity with Aldehydes 4Reactions with Acetylenic Ketones 4.1Metal-Coordinated Phosphanylphosphonate and Monoacetylenic Ketones 4.2Metal-Coordinated Phosphanylphosphonate and Diacetylenic Ketones 4.3Metal-Free Phosphanylphosphonate and Diacetylenic Ketones 5Metal-Free Phosphanylphosphonate as a Coupling Reagent for Aldehydes 6 E -Alkenes by the Reductive Coupling of Two Aldehydes 7Conclusions and Outlook