Solvent/base effects in the selective domino synthesis of phenanthridinones that involves high-valent palladium species: experimental and theoretical studies.

The domino reaction of o-bromobenzamides 1a-m in the presence of K(2)CO(3) and the [PdCl(2)(PPh(3))(2)] catalyst granted a selective access to phenanthridinones 2 or to the new 1-carboxamide phenanthridinones 3 depending on the solvent, DMF or 1,4-dioxane, respectively. Investigations of the reaction parameters provided the first example of a direct correlation between the base dissociation and the solvent polarity on the selectivity observed. Moreover, mechanistic studies (NMR spectroscopy and ESI-MS monitoring) allowed us to characterize Pd(II) palladacycle 4 and biaryl species as common intermediates for these two domino processes. On that basis, C(sp(2))-C(sp(2)) bond formation is envisaged by generation of a Pd(IV) complex after oxidative addition of 1 into Pd(II) palladacycle 4, a rationale that is supported by DFT calculations. A general catalytic cycle is proposed to account for these observations.

Selective unusual Pd-mediated biaryl coupling reactions: solvent effects with carbonate bases.

A one-step Pd-catalyzed reaction performed on an o-bromobenzamide permitted the selective formation of either phenanthridinones 2 via an ipso substitution or new phenanthridinone-1-carboxamides 3 through a direct N-arylation. A direct correlation between the solvent polarity and the carbonate base on the selectivity has been observed. The proposed catalytic cycle involves the initial formation of a common intermediate and depends on the base assistance.

Passage from stepwise to concerted dissociative electron transfer through modulation of electronic states coupling.

Reductive cleavage of the three cyanobenzyl chloride isomers in N,N-dimethylformamide gives new insights into the factors that control the mechanism during dissociative electron transfer. Within the family of investigated compounds, electrochemical reduction leads to expulsion of the chloride ion. While electron transfer is concerted with breaking of the C-Cl bond and acts as the rate-determining step in the case of both the ortho and para isomers, an intermediate anion radical is formed before rapid fragmentation in the case of the meta isomer. Such an unexpected mechanistic shift (all key thermodynamic parameters are very similar for the three chlorides) is interpreted in the framework of a modified version of the dissociative electron-transfer model that includes electronic coupling effects between the diabatic states of the products. These effects appear to control the very existence of a transient species along the reaction pathway.