1.C-H activation of ethers by pyridine tethered PCsp3P-type iridium complexes.
Cui P1, Babbini DC1, Iluc VM1. Dalton Trans. 2016 Apr 7. [Epub ahead of print]
Iridium PCsp3P complexes featuring a novel bis(2-diphenylphosphinophenyl)-2-pyridylmethane ligand (PCPyHP) are reported. C-H activation reactions between the dihydride complex [(PCPyP)Ir(H)2] and tetrahydrofuran or methyl tert-butyl ether in the presence of a hydrogen acceptor, norbornene (NBE), at ambient temperature led exclusively to the hydrido oxyalkyl complexes, [(PCPyP)IrH(C4H7O)] and [(PCPyP)IrH(CH2OtBu)], respectively. The internal pyridine donor is important and stabilizes these species by coordination to the iridium center. The coordination of pyridine to the iridium center is labile, however, and its dissociation occurs in the presence of a suitable substrate, as demonstrated by the intramolecular nucleophilic attack of pyridine on a vinylidene intermediate generated from PhC[triple bond, length as m-dash]CH.
2.Asymmetric Syntheses of (+)-Preussin B, the C(2)-Epimer of (-)-Preussin B, and 3-Deoxy-(+)-preussin B.
Buchman M, Csatayová K, Davies SG, Fletcher AM, Houlsby IT, Roberts PM, Rowe SM, Thomson JE. J Org Chem. 2016 Apr 14. [Epub ahead of print]
Efficient de novo asymmetric syntheses of (+)-preussin B, the C(2)-epimer of (-)-preussin B, and 3-deoxy-(+)-preussin B have been developed, using the diastereoselective conjugate addition of lithium (S)-N-benzyl-N-(alpha-methylbenzyl)amide to tert-butyl 4-phenylbut-2-enoate and diastereoselective, reductive cyclisation of gamma-amino ketones as the key steps to set the stereochemistry. Conjugate addition followed by enolate protonation generated the corresponding beta-amino ester. Homologation using the ester functionality as a synthetic handle gave the corresponding gamma-amino ketone. Hydrogenolytic N-debenzylation was accompanied by diastereoselective, reductive cyclisation in situ; reductive N-methylation then gave 3-deoxy-(+)-preussin B as the major diastereoisomeric product. Meanwhile, the same conjugate addition but followed by enolate oxidation with (+)-camphorsulfonyloxaziridine (CSO) gave the corresponding anti-alpha-hydroxy-beta-amino ester.
3.Ligand-centred oxidative chemistry in sterically hindered salen complexes: an interesting case with nickel.
Thomas F1. Dalton Trans. 2016 Apr 14. [Epub ahead of print]
Salen ligands are ubiquitous ligands because they can be readily prepared by condensation of a diamine with two equivalents of salicylaldehyde. They form stable complexes with a great variety of metal ions and find applications in various fields, especially catalysis. The introduction of chirality at the bridge and the adjunction of sterically demanding tert-butyl groups in ortho and para positions of the phenols allow for efficient enantioselective catalysis. On the other hand, early investigations on the oxidation chemistry of phenols highlighted that the incorporation of tert-butyl groups in ortho and para positions can stabilize enormously the one-electron oxidized product e.g. the phenoxyl radical. The redox-activity of sterically hindered salen ligands will be discussed in this perspective article. We will focus our attention on nickel salen complexes since both the metal and the ligand are potentially redox-active, while the oxidized products are stable enough to be characterized by EPR and NIR spectroscopies.
4.Fluorescent Brighteners as Visible LED-Light Sensitive Photoinitiators for Free Radical Photopolymerizations.
Zuo X1,2, Morlet-Savary F1, Graff B1, Blanchard N3, Goddard JP2, Lalevée J1. Macromol Rapid Commun. 2016 Apr 13. doi: 10.1002/marc.201600103. [Epub ahead of print]
The photochemical and electrochemical investigations of commercially available, safe, and cheap fluorescent brighteners, namely, triazinylstilbene (commercial name: fluorescent brightener 28) and 2,5-bis(5-tert-butyl-benzoxazol-2-yl)thiophene, as well as their original use as photoinitiators of polymerization upon light emitting diode (LED) irradiation are reported. Remarkably, their excellent near-UV-visible absorption properties combined with outstanding fluorescent properties allow them to act as high-performance photoinitiators when used in combination with diaryliodonium salt. These two-component photoinitiating systems can be employed for free radical polymerizations of acrylate. In addition, this brightener-initiated photopolymerization is able to overcome oxygen inhibition even upon irradiation with low LED light intensity. The underlying photochemical mechanisms are investigated by electron-spin resonance-spin trapping, fluorescence, cyclic voltammetry, and steady-state photolysis techniques.