Octahedral bipyridine and bipyrimidine dioxomolybdenum(VI) complexes: characterization,application in catalytic epoxidation,and density functional mechanistic study

Complexes of the general formula [MoO(2)X(2)L(2)] (X=Cl, Br, Me; L(2)=bipy, bpym) have been prepared and fully characterized, including X-ray crystallographic investigations of all six compounds. Additionally, the highly soluble complex [MoO(2)Cl(2)(4,4'-bis(hexyl)-2,2'-bipyridine)] has been synthesized. The reaction of the complexes with tert-butyl hydroperoxide (TBHP) is an equilibrium reaction, and leads to MoV(I) eta(1)-alkylperoxo complexes that selectively catalyze the epoxidation of olefins. Neither the Mo-X bonds nor the Mo-N bonds are cleaved during this reaction. These experimental results are supported by theoretical calculations, which show that the attack of TBHP at the Mo center through the X-O-N face is energetically favored and the TBHP hydrogen atom is transferred to a terminal oxygen of the Mo=O moiety. After the attack of the olefin on the Mo-bound peroxo oxygen atom, epoxide and tert-butyl alcohol are formed. The latter compound acts as a competitive inhibitor for the TBHP attack, and leads to a significant reduction in the catalytic activity with increasing reaction time.     

Enantioselective [4+2]-cycloaddition reaction of a photochemically generated o-quinodimethane: mechanistic details, association studies, and pressure effects

1,2,3,4-Tetrahydro-2-oxoquinoline-5-aldehyde (2) was prepared from m-aminobenzoic acid and 3-ethoxyacryloyl chloride (4) in 19 % overall yield. Compound 2 underwent a photochemically induced [4+2]-cycloaddition reaction with various dienophiles upon irradiation in toluene solution. The exo product 10 a was obtained with acrylonitrile (9 a) as the dienophile, whereas methyl acrylate (9 b) and dimethyl fumarate (9 c) furnished the endo products 11 b and 11 c (69-77 % yield). The reactions proceeded at -60 degrees C in the presence of the chiral complexing agent 1 (1.2 equiv) with excellent enantioselectivity (91-94 % ee). The enantiomeric excess increases in the course of the photocycloaddition as a result of the lower product association to 1. The intermediate (E)-dienol 8 was spectroscopically detected at -196 degrees C in an EPA (diethyl ether/isopentane/ethanol) glass matrix. The association of the substrate 2 to the complexing agent 1 was studied by circular dichroism (CD) titration. The measured association constant (K(A)) was 589 M(-1) at room temperature (25 degrees C) and normal pressure (0.1 MPa). An increase in pressure led to an increased association. At 400 MPa the measured value of K(A) was 703 M(-1). Despite the stronger association the enantioselectivity of the reaction decreased with increasing pressure. At 25 degrees C the enantiomeric excess for the enantioselective reaction 2 + 9 a-->10 a decreased from 68 % ee at 0.1 MPa to 58 % ee at 350 MPa. This surprising behavior is explained by different activation volumes for the diastereomeric transition states leading to 10 a and ent-10 a.     

Grafting of [Mn(CH2tBu)2(tmeda)] on silica and comparison with its reaction with a silsesquioxane

The reaction of [Mn(CH2tBu)2(tmeda)] (1) and a silica partially dehydroxylated at 700 degrees C (SiO(2/700)) yields a single surface species [([triple bond]SiO)Mn(CH2tBu)(tmeda)] (2a; tmeda = tetramethylethylendiamine), while a mixture of 2a and [([triple bond]SiO)2Mn(tmeda)] (2b) is obtained by using SiO(2/200), SiO(2/300), or SiO(2/500) as evidenced by mass balance analysis, and IR and EPR spectroscopy. The reaction of 1 and (c-C5H9)7Si7O12SiOH (3), a soluble silanol that is a molecular model for a silica support, generates the bis-siloxy complex 4, [[(c-C5H9)7Si7O12SiO]2Mn(tmeda)2], in a quantitative yield; compound 4 was characterized by single-crystal X-ray diffraction. These reactions exemplify the limitation of considering molecular silanol derivatives as straightforward and reliable homogeneous models for silica, and address the need for thorough characterization of surface species by the use of surface-science techniques. These studies show the possibility of preparing coordinatively and geometrically unique surface species that would be difficult to prepare by solution chemistry methods; insights into the chemical and physical properties of these surface species are also gained.     

Glycolate and Thioglycolate Complexes of Rhenium and Their Oxidative Elimination of Ethylene and of Glycol

Selenium dioxide and osmium tetroxide are effective reagents and catalysts for olefin oxidation, although, owing to their toxicity, reservations remain as to their applicability.^[1] We are therefore seeking more easily handled metal oxides that are soluble in organic solvents and that are as effective as osmium tetroxide in carrying out stereospecific cis hydroxylation of olefins. The rhenium(VII ) oxide 1 , which has meanwhile become readily accessible, is a favorable candidate.^[2]