Platinum(II) Acetylides in the Formal [2+2] Cycloaddition‐Retroelectrocyclization Reaction: Organodonor Versus Metal Activation

Abstract

AbstractFormal cycloaddition‐retroelectrocyclization (CA‐RE) reactions between electron‐donor‐activated alkynes and electron‐poor alkenes yielding cyanobuta‐1,3‐dienes have recently attracted increasing interest. The transformation has been subjected to a fundamental investigation of the relative degrees of alkyne activation by organic and metallorganic donor substituents by using platinum(II) σ‐acetylides as model substrates and studying their behavior towards the cyano carbons TCNE and TCNQ. Various cyanobutadienes were obtained in good to excellent yields and four trends in reactivity were discerned: 1) The presence of an anilino substituent clearly dominates the regioselectivity of the TCNQ addition. 2) In the absence of an organodonor, the regioselectivity is inverted. 3) When platinum(II) complexes of buta‐1,3‐diynes are used, the addition always takes place at the triple bond distal to the metal center. 4) In general, trans‐bis‐acetylides are more reactive than the corresponding cis complexes. The structural parameters of the CA‐RE adducts were investigated by X‐ray crystallography and their optical properties by UV/Vis spectroscopy, giving further insights into the structural trends and degree of intramolecular charge transfer. These fundamental investigations may enable the synthesis of new Pt‐based molecular dyads in which the effect of regioselectivity on photoinduced charge separation can be studied.