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

• Published in: European journal of organic chemistry 2013-06, Vol.2013 (18), p.3729-3740
• Main Authors: Tchitchanov, Boris H., Chiu, Melanie, Jordan, Markus, Kivala, Milan, Schweizer, W. Bernd, Diederich, François
• Format: Article
• Language: English
• Subjects: Alkynes; Charge transfer; Cycloaddition; Donor-acceptor systems; Platinum; Regioselectivity
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/ejoc.201300300

Formal 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. Donor‐activated triple bonds act as substrates for cycloaddition‐retroelectrocyclization reactions with TCNE or TCNQ to give cyanobutadienes. Herein we showcase the activation ability of two classes of donors: organic amines and platinum(II) centers directly bound to the acetylenic moieties and compare them directly for the first time.