Oxorhenium(V) Complexes with Phenolate–Oxazoline Ligands: Influence of the Isomeric Form on the O‑Atom-Transfer Reactivity

The bidentate phenolate–oxazoline ligands 2-(2′-hydroxyphenyl)-2-oxazoline (<b>1a</b>, Hoz) and 2-(4′,4′-dimethyl-3′,4′-dihydrooxazol-2′-yl)­phenol (<b>1b</b>, Hdmoz) were used to synthesize two sets of oxorhenium­(V) complexes, namely, [ReOCl₂(L)­(PPh₃)] [L = oz (<b>2a</b>) and dmoz (<b>2b</b>)] and [ReOX­(L)₂] [X = Cl, L = oz (<b>3a</b> or <b>3a′</b>); X = Cl, L = dmoz (<b>3b</b>); X = OMe, L = dmoz (<b>4</b>)]. Complex <b>3a′</b> is a coordination isomer (<i>N</i>,<i>N</i>-cis isomer) with respect to the orientation of the phenolate–oxazoline ligands of the previously published complex <b>3a</b> (<i>N</i>,<i>N</i>-trans isomer). The reaction of <b>3a′</b> with silver triflate in acetonitrile led to the cationic compound [ReO­(oz)₂(NCCH₃)]­(OTf) ([<b>3a′</b>]­(OTf)). Compound <b>4</b> is a rarely observed isomer with a <i>trans</i>-ORe–OMe unit. Complexes <b>3a</b>, <b>3a′</b>, [<b>3a′</b>]­(OTf), and <b>4</b> were tested as catalysts in the reduction of a perchlorate salt with an organic sulfide as the O acceptor and found to be active, in contrast to <b>2a</b> and <b>2b</b>. A comparison of the two isomeric complexes <b>3a</b> and <b>3a′</b> showed significant differences in activity: 87% <b>3a</b> vs 16% <b>3a′</b> sulfoxide yield. When complex [<b>3a</b>′]­(OTf) was used, the yield was 57%. Density functional theory calculations circumstantiate all of the proposed intermediates with <i>N</i>,<i>N</i>-trans configurations to be lower in energy compared to the respective compounds with <i>N</i>,<i>N</i>-cis configurations. Also, no interconversions between <i>N</i>,<i>N</i>-trans and <i>N</i>,<i>N</i>-cis configurations are predicted, which is in accordance with experimental data. This is interesting because it contradicts previous mechanistic views. Kinetic analyses determined by UV–vis spectroscopy on the rate-determining oxidation steps of <b>3a</b>, <b>3a′</b>, and [<b>3a′</b>]­(OTf) proved the <i>N</i>,<i>N</i>-cis complexes <b>3a′</b> and [<b>3a′</b>]­(OTf) to be slower by a factor of ∼4

6,6-Dicyanopentafulvenes: Electronic Structure and Regioselectivity in [2 + 2] Cycloaddition–Retroelectrocyclization Reactions

We present an investigation of the electronic properties and reactivity behavior of electron-accepting 6,6-dicyanopentafulvenes (DCFs). The electron paramagnetic resonance (EPR) spectra of the radical anion of a tetrakis­(silylalkynyl) DCF, generated by Na metal reduction, show delocalization of both the charge and unpaired electron to the nitrogens of the cyano moieties and also, notably, to the silicon atoms of the four alkynyl moieties. By contrast, in the radical anion of the previously reported tetraphenyl DCF, coupling to the four phenyl rings is strongly attenuated. The data provide physical evidence for the different conjugation between the DCF core and the substituents in both systems. We also report the preparation of new fulvene-based push–pull chromophores via formal [2 + 2] cycloaddition–retroelectrocyclization reaction of DCFs with electron-rich alkynes. Alkynylated and phenylated DCFs show opposite regioselectivity of the cycloaddition, which can be explained by the differences in electronic communication between substituents and the DCF core as revealed in the EPR spectra of the radical anions