Iteratively executed with exquisite spatial and temporal control, the selective isomerization of polarized alkenes underpins a plethora of complex biological processes ranging from natural product biosynthesis through to the mammalian visual cycle. However, nature's proficiency conceals the inherent difficulties in replicating this contra-thermodynamic transformation in the laboratory. Recently, we disclosed the first highly Z-selective isomerization of polarized alkenes, employing the cinnamoyl chromophore as a retinal surrogate under UV-irradiation (402 nm) with (-)-riboflavin (vitamin B) as an inexpensive, organic photocatalyst (J. Am. Chem. Soc. 2015, 137, 11254-11257). This study was inspired by the propensity of crystalline (-)-riboflavin in the eyes of vertebrates to invert the intrinsic directionality of retinal isomerization. Herein, we extend this methodology to include a bioinspired, catalytic E → Z isomerization of α,β-unsaturated nitriles, thereby mimicking the intermediate Opsin-derived, protonated Schiff base in the visual cycle with simple polarized alkenes. Replacement of the iminium motif by a cyano group is well tolerated and gives an additional degree of versatility for postisomerization functionalization. Broad substrate scope is demonstrated (up to 99:1 Z:E) together with evidence of mechanistic dichotomy via both singlet and triplet energy transfer mechanisms. Kinetic studies, temperature dependent photostationary state correlations and investigation of substituent-based electronic perturbation of the alkene identified polarization combined with increased Z-isomer activation barriers as the selectivity governing factors in catalysis. This investigation demonstrates the importance of internal structural preorganization on photostationary composition and explicates the augmented Z-selectivity upon hydrogen-alkyl exchange at the β-position of the alkene.

Increasing soil salinity is a major problem in global crop production. The detrimental effect of saline soils on plant growth results from ionic toxicity of the Na+ ion in combination...

No abstract

One equivalent of methanol-d 4 reacts with the iridium(III)-carbonato-NHC complex [Ir(Cp*)(NHC-theo)(O 2 CO)] [1] containing an N-heterocyclic carbene ligand with an N-tethered C8-iodotheophylline group (NHC-theo) to form the iridium(III) chelate complex [Ir(Cp*)(theophyllinato^NHC)I] [2] that bears a theophyllinato^NHC chelate ligand, an iodo ligand, and the pentamethylcyclopentadienyl ligand (Cp*). The methanol-d 4 solvent is oxidized to formaldehyde-d 2 that was identified by a detailed LC−MS study using the 2,4-dinitrophenylhydrazine (DNPH) method. A plausible rearrangement involving the unusual oxidative addition of the C8−I bond of the theophylline group to an iridium(III) center to give a seven-coordinate iridium(V) intermediate is supported by density functional theory calculations.

No abstract