2 research outputs found
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)Âphenol
(<b>1b</b>, Hdmoz) were used to synthesize two sets of oxorheniumÂ(V)
complexes, namely, [ReOCl<sub>2</sub>(L)Â(PPh<sub>3</sub>)] [L = oz
(<b>2a</b>) and dmoz (<b>2b</b>)] and [ReOXÂ(L)<sub>2</sub>] [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 [ReOÂ(oz)<sub>2</sub>(NCCH<sub>3</sub>)]Â(OTf) ([<b>3a′</b>]Â(OTf)). Compound <b>4</b> is a rarely observed isomer with a <i>trans</i>-Oî—»Re–OMe
unit. Complexes <b>3a</b>, <b>3a′</b>, [<b>3a′</b>]Â(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>′]Â(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>]Â(OTf) proved the <i>N</i>,<i>N</i>-cis complexes <b>3a′</b> and [<b>3a′</b>]Â(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 tetrakisÂ(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