Rhodium(I)‐Catalysed Intramolecular [2+2+2] Cyclotrimerisations of 15‐, 20‐ and 25‐Membered Azamacrocycles: Experimental and Theoretical Mechanistic Studies

AbstractNumber of members makes a difference: The [2+2+2] intramolecular cyclotrimerisation of a new series of 20‐ and 25‐membered azamacrocycles catalysed by the Wilkinson’s catalyst are reported (see scheme). The 20‐ and 25‐membered azamacrocycles show different reactivity. Why? Theoretical calculations give insight into the reactivity differences observed for the 20‐ and 25‐membered macrocycles.magnified imageA new series of 20‐ and 25‐membered polyacetylenic azamacrocycles have been satisfactorily prepared and completely characterised by spectroscopic methods. Various [2+2+2] cyclotrimerisation processes catalysed by the Wilkinson’s catalyst, [RhCl(PPh3)3], were tested in the above‐mentioned macrocycles. The 25‐membered azamacrocycle (like the previously synthesised 15‐membered azamacrocyle) led to the expected cyclotrimerised compound in contrast to the 20‐membered macrocycle, which is characterised by its lack of reactivity. The difference in reactivity of the 15‐, 20‐ and 25‐membered macrocycles has been rationalised through density functional theory calculations.