Selective Oxidation of the Alkyl Ligand in Rhenium(V) Oxo Complexes

Abstract

Rhenium(V) oxo alkyl triflate compounds (HBpz3)ReO(R)OTf [R = Me (4), Et (5), n-Bu (6); HBpz3 = hydrotris(1-pyrazolyl)borate; OTf = OSO2CF3, triflate] are formed on sequential reaction of (HBpz3)ReOCl2 with dialkyl zinc reagents and AgOTf. These triflate compounds are rapidly oxidized at ambient temperatures by oxygen atom donors pyridine N-oxide (pyO) and dimethyl sulfoxide (DMSO) to give (HBpz3)ReO3 (7) and the corresponding aldehyde. In the cases of 5 and 6 this transformation is quantitative. The addition of 2,6-lutidine to a low-temperature oxidation of 5 by DMSO redirects the reaction to form cis-2-butene instead of acetaldehyde. These oxidation reactions do not proceed through alkoxide intermediates, as shown by independent studies of alkoxide oxidations. Reaction of 5 with pyO or DMSO at −47 °C results in the formation of intermediates which are assigned as ylide or “trapped-carbene” complexes [(HBpz3)ReO(OH){CH(L)CH3}]OTf (L = py (8) or SMe2 (9), respectively). Mechanistic studies and analogies with related systems suggest that oxygen atom transfer to 4-6 forms [(HBpz3)ReO2R]+. Transfer of an α-hydrogen from the alkyl group to an oxo ligand then forms an alkylidene complex which is trapped by SMe2 or py to give the observed intermediates. Further oxidation of the ylide complex gives the aldehyde.

Rhenium(V) Dioxo Complexes with Dihydrobis(pyrazolyl)borates: Synthesis and Reactivity toward Electrophilic Substrates

“…The O(1)−Re−O(2) bond angle (178.8(2)°) indicates that the oxygen from the siloxide is almost trans to the oxo group. The Re−O(1) bond distance (1.688(4) Å) is comparable to the values found in other rhenium(V) complexes with poly(pyrazolyl)borates and is in the usual range for rhenium(V) monooxo complexes. − , The coordination of the siloxide ligand is characterized by a Re−O(2) distance of 1.886(3) Å, which is comparable to and shorter than the values described for [ReOCl 2 (OSiMe 3 )(py) 2 ] (1.889(4) Å) and [(η 5 -C 5 Me 5 )ReCl(OSiMe 3 )(NCH 2 -2-C 5 H 4 N)] (2.020(6) Å), respectively. , These data are consistent with some competition of the siloxide for π bonding with the metal, the Re−OSiMe 3 having bond some multiple character. Each dihydrobis(pyrazolyl)borate defines a six-membered ReN 4 B chelating ring in a typical boat conformation, with the apexes of each boat conformation pointing alternatively toward the oxo and siloxide ligands.…”

“…Each dihydrobis(pyrazolyl)borate defines a six-membered ReN 4 B chelating ring in a typical boat conformation, with the apexes of each boat conformation pointing alternatively toward the oxo and siloxide ligands. The average Re−N bond length of 2.113(5) Å is comparable to the values found in other rhenium monooxo complexes with poly(pyrazolyl)borate ligands. − The siloxide ligand bends (Re−O(2)−Si angle of 160.7(2)°) toward the bidentate ligand which has the boat conformation directed toward the oxo group, certainly to minimize repulsive interactions.

2 ORTEP view of [ReO(OSiMe 3 ){H 2 B(pz) 2 } 2 ] ( 3 ).

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“…With hydrotris- and tetrakis(pyrazolyl)borates a rich and versatile chemistry has been developed for Re(V), Re(VI), and Re(VII). − Some recent examples are the Re(V) and Re(VI) dioxo complexes trans -[ReO 2 {η 2 -RB(pz) 3 }(py) 2 ] (R = H, pz) 24 and cis -[ReO 2 X{η 3 -HB(pz) 3 }] (X = Cl, Br, I) . In contrast, with dihydrobis(pyrazolyl)borates ([H 2 B(pz*) 2 ] - , pz* = pz or substituted pz) the chemistry of rhenium remains rather unexplored.…”

Synthesis and Reactions of Rhenium(V) Oxo−Hydride Complexes

“…The mechanism of the oxidation reactions likely proceeds by initial oxygen-atom transfer to give a reactive rhenium(VII) dioxo cation, [Tp*Re(O) 2 H][OTf] ( A in Scheme ), by analogy with previous studies on TpReO(X)OTf compounds (X = alkyl, aryl, alkoxy, and halide 9c ). Intermediate A could then be deprotonated directly by NPh 3 , py, or OTf - (the lower pathway in Scheme ), but it seems unlikely that A would be such a strong acid .…”

Synthesis, characterization and structural investigation of new rhenium-oxo complexes containing bidentate phosphine ligands: an exploration of chirality and conformation in chelate rings of small and large bite angle ligands

“…Oxorhenium(V) complexes of the type [ReOX 3 L 2 ] (X = halogen; L = triaryl or alkyl phosphine, arsine or stibine ligands) are versatile precursors in the synthesis of a wide variety of rhenium complexes [11][12][13][14][15][16] and also display activity as oxygen transfer agents. [17][18][19][20][21][22][23][24][25][26] Organophosphorus compounds continue to gain in popularity as ligands in synthetic inorganic chemistry since, by varying the substituents on the phosphorus, the chemical behavior of both the ligand and the resultant complex can be altered. [27][28][29][30][31][32] The number of different phosphines available to the coordination chemist is vast and continues to increase.…”