A NMR, X-ray, and DFT combined study on the regio-chemistry of nucleophilic addition to platinum(II) coordinated terminal olefins

The series of platinum complexes [PtCl(η2-CH2CH–C6H4–X)(tmeda)](ClO4) (X = H, 1b; 4-OMe, 1c; 3-OMe, 1d; 4-CF3, 1e; 3-CF3, 1f; 3-NO2, 1g; tmeda = N,N,N′,N′-tetramethyl-1,2-ethanediamine) has been considered. In the styrene complex (1b) both solution (NMR) and solid state (X-ray) data indicate a significant difference in the Pt–C bond lengths (the longer bond being that involving the olefin carbon atom carrying the phenyl ring). Such a difference increases when X is an electron donor group (EDG, 1c) and decreases when X is an electron withdrawing group (EWG, 1d–g). The attack of a nucleophile (MeO−) to the substituted carbon (Markovnikov type, M) is by far the most favoured in the case of unsubstituted (1b) or EDG-substituted (1c) styrenes. The presence of an EWG (compounds 1d-g) levels off the probability of M and anti-M type of attack. DFT calculations on 1b,c and 1e were also performed. The NLMO analysis reveals the crucial role of the interaction between the filled π orbital of the olefin and the empty d orbital of platinum; the carbon with greater electron density becoming less susceptible of nucleophilic attack.

The Markovnikov vs. anti-Markovnikov nucleophilic addition to coordinated styrene in the complexes [PtCl(η2-CH2CH–C6H4–X)(tmeda)](ClO4) has been evaluated. The presence on the phenyl ring of an electron donor group produces a nearly 100% Markovnikov isomer (); an electron withdrawing substituent reduces the selectivity (→) without reversing it.Figure optionsDownload as PowerPoint slideFigure optionsDownload as PowerPoint slide