Regioselectivity of Insertion and Role of the Anionic Ligands in the Ruthenium Alkylidene Catalyzed Cyclopolymerization of 1,6-Heptadiynes

• Published in: Organometallics 2012-02, Vol.31 (3), p.847-856
• Main Authors: Naumov, S, Buchmeiser, M. R
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om2007216

The experimentally observed high α-addition selectivity of 1,6-heptadiynes to modified Grubbs–Hoveyda initiators was elucidated with quantum chemical calculations. For these purposes, the two possible pathways of initiation in the Ru alkylidene triggered cyclopolymerization (CP) of 1,6-heptadiynes, resulting in either five-membered (α insertion) or six-membered (β insertion) repeat units, were treated as a multistep process. The first reaction cascade entails the activation of the precatalyst RuX2(IMesH2)­(CH-2-(2-PrO-C6H4)) (1: X = F, Cl, Br, I, CF3COO; IMesH2 = 1,3-dimesitylimidazolin-2-ylidene), reaction with a 1,6-heptadiyne (π-1 complex formation), and further transformation into the first metallacyclobutene (MCB-1) followed by ring opening. The second reaction cascade entails again the formation of a π complex (π-2) through binding of the second alkyne moiety of the 1,6-heptadiyne and further transformation into MCB-2 followed by ring opening of MCB-2. The energies of the transition structures for both MCB-1 and MCB-2 formation (TS-1 and TS-2), which are considered the rate-determining steps in CP, are systematically lower for an α insertion of a monomer than for a β insertion. In addition, the geometrical parameters of the most stable structure of the βπ-2 complex are systematically less favorable for MCB-2 formation than in the case of an απ-2 complex, resulting in very high activation energies for βMCB-2 formation. Finally, the formation of βMCB-2 needs an additional step: namely, the endergonic formation of the intermediate βMCB-2*. Since a halogen exchange to pseudohalides in Grubbs–Hoveyda initiators is required to turn them into active initiators in CP, the effect of electronegativity (EN) of the X ligands on the stability of the π-1 complex was calculated for X = I, Br, Cl, CF3COO, F. There, an increase in EN results in lower energies for the α-insertion-derived π-1 complexes. For α insertion, the barriers to the MCB-1 intermediate formation, i.e. the energies of the transition states (TS-1­(α)) for MCB-1 formation, decrease in the order I > Br > Cl > CF3COO < F. All findings are consistent with the experimentally observed preference for α insertion in the cyclopolymerization of 1,6-heptadiynes with modified Grubbs–Hoveyda initiators and with the necessity for using pseudohalide variations of the Grubbs–Hoveyda initiator.