Modular Approach to 9-Monosubstituted Fluorene Derivatives Using Mo(V) Reagents.

Oxidative coupling using molybdenum(V) reagents provides fast access to highly functionalized 9-monosubstituted fluorenes. This synthetic approach is highly modular, is high yielding, and tolerates a variety of labile moieties, e.g. amides or iodo groups. The established protocol leads to promising precursors for pharmacologically important analogues of melatonin.

Over-Oxidation as the Key Step in the Mechanism of the MoCl5-Mediated Dehydrogenative Coupling of Arenes.

Molybdenum pentachloride is an unusually powerful reagent for the dehydrogenative coupling of arenes. Owing to the high reaction rate using MoCl5, several labile moieties are tolerated in this transformation. The mechanistic course of the reaction was controversially discussed although indications for a single electron transfer as the initial step were found recently. Herein, based on a combined study including synthetic investigations, electrochemical measurements, EPR spectroscopy, DFT calculations, and mass spectrometry, we deduct a highly consistent mechanistic scenario: MoCl5 acts as a one-electron oxidant in the absence of TiCl4 and as two-electron oxidant in the presence of TiCl4, but leads to an over-oxidized intermediate in both cases, which protects it from side reactions. In the course of aqueous work-up the reagent waste (Mo(III/IV) species) acts as reducing agent generating the desired organic C-C coupling product.

Initial radical cation pathway in the Mo2Cl10-mediated dehydrogenative arene coupling.

Experimental (EPR) and theoretical (DFT) evidence is provided for radical cation formation as initial step in the Mo2Cl10-mediated dehydrogenative arene coupling. The initial electron transfer from methoxyarenes to molybdenum proceeds via an inner sphere mechanism.

Crystal structure of ethyl 2-(di-eth-oxy-phosphor-yl)-2-(2,3,4-tri-meth-oxy-phen-yl)acetate.

The title compound, C17H27O8P, was prepared by Michaelis-Arbuzov reaction of ethyl 2-bromo-2-(2,3,4-tri-meth-oxy-phen-yl)acetate and triethyl phosphite. Such compounds rarely crystallize, but single crystals were recovered after the initial oil was left for approximately 10 years. The bond angle of the sp (3)-hybridized C atom connecting the benzene derivative with the phospho unit is widened marginally [112.5 (2)°]. The terminal P-O bond length of 1.464 (2) Šclearly indicates a double bond, whereas the two O atoms of the eth-oxy groups connected to the phospho-rous atom have bond lengths of 1.580 (2) Šand 1.581 (3) Å. The three meth-oxy groups emerge out of the benzene-ring plane due to steric hindrance [C-C-O-C torsion angles = -179.9 (3)°, -52.9 (4)° and 115.3 (4)°]. In the crystal, inversion dimers linked by pairs of C-H⋯O=P hydrogen bonds generate R 2 (2)(14) loops. The chosen crystal was modelled as a non-merohedral twin.

Oxidative cyclization reaction of 2-aryl-substituted cinnamates to form phenanthrene carboxylates by using MoCl5.

The oxidative cyclization reaction of 2-aryl cinnamates and derivatives thereof can be easily performed with MoCl5 as the oxidant. This powerful reagent allows oxidative coupling reactions for which other reagents fail. The best results are obtained when the 2-phenyl substituent of the cinnamate is equipped with two methoxy groups. Even iodo moieties in the bay region of phenanthrene are tolerated under the reaction conditions. If naphthalene moieties are involved, a rearrangement of the skeleton occurs, providing an elegant route to highly functionalized angular arenes. The cyclization is demonstrated for 15 example substrates with isolated yields of up to 99 % for the phenanthrene derivative. The broad scope of the reaction underlines the usefulness of MoCl5 and MoCl5 /TiCl4 in the oxidative coupling reaction.

Powerful fluoroalkoxy molybdenum(V) reagent for selective oxidative arene coupling reaction.

We introduce the novel fluoroalkoxy molybdenum(V) reagent 1 which has superior reactivity and selectivity in comparison to MoCl5 or the MoCl5 /TiCl4 reagent mixture in the oxidative coupling reactions of aryls. Common side reactions, such as chlorination and/or oligomer formation, are drastically diminished creating a powerful and useful reagent for oxidative coupling. Theoretical treatment of the reagent interaction with 1,2-dimethoxybenzene-type substrates indicates an inner-sphere electron transfer followed by a radical cationic reaction pathway for the oxidative-coupling process. EPR spectroscopic and electrochemical studies, X-ray analyses, computational investigations, and the experimental scope provide a highly consistent picture. The substitution of chlorido ligands by hexafluoroisopropoxido moieties seems to boost both the reactivity and selectivity of the metal center which might be applied to other reagents as well.