SummaryThe valence isomerization of the all-carbon and heteroelement analogues of cyclohepta-1,3,5-triene into the corresponding bicyclo[4.1.0]hepta-2,4-dienes is reviewed to show the impact of the heteroatom on the stability of both valence isomers. The focus is on the parent systems and their synthetic applications.

Phosphinidene complexes [L n M=P À R] [1] are fascinating reagents akin to the isoelectronic carbenes [L n M=CR 2 ] [2] that have made a stunning impact on the preparative and industrial scale syntheses of small molecules and polymeric materials. It is tempting to extend the relationship for lowvalent species that is based on the similar electronegativities of the element phosphorus and the diagonally related carbon to these transition-metal complexes.[3] It has already been established that phosphinidene complexes can be classified as electrophilic (Fischer-type) or nucleophilic (Schrock-type) at phosphorus depending on the nature of the ligands L; [4] electron-withdrawing ligands with strong pacceptor capacity (e.g., CO) reduce the charge concentration on the phosphorus atom and therefore enhance its electrophilicity, whereas strong s-donor ligands (e.g., Cp) increase the electron density at phosphorus and thereby increase its nucleophilic properties. Proper characterisation of these complexes assists in understanding their chemical potential.

Walking rings: At room temperature, the P‐R entity of a bicyclic phosphirane (see picture) “walks” along the perimeter of the larger ring. Although this concerted rearrangement is similar to that of its hydrocarbon analogue, the additional 1,5‐sigmatropic shift follows a different mechanism.

In a multi-component reaction (MCR) of a phosphonate, nitriles, aldehydes and isocyanates, N3-functionalised dihydropyrimidines can be synthesised efficiently via a Horner-Emmons/aza Diels-Alder pathway.

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