Gold(III) Complexes Catalyze Deoximations/Transoximations at Neutral pH

Abstract

Reaction of gold trihalides with reducing agents such as thiols, NaBH 4 or hydroxylamine gives gold nanoparticles (Au NPs). [1] In particular, NH 2 OH is used to produce monodisperse colloidal Au NPs by "growing" of smaller particles or to form thin films of Au on glass substrates. [2] Our interest in developing a mild catalytic method for the conversion of nitro/oxime/nitrone groups to carbonyl groups led us to examine first the interaction of gold(III) [3] with simple oximes as a potential deoximation procedure. [4] The hydrolysis of the coordination complexes of oximes and glyoximes with gold(III) [5] has not been investigated. If a partial hydrolysis of oximes (to RR'C=O and NH 2 OH) took place, NH 2 OH would be oxidized in situ by Au III , shifting the equilibrium to the right.Our first challenge was to find a water-soluble and stable gold complex that could catalyze the hydrolysis of the robust oxime group [6] at neutral pH and, as far as possible, at room temperature (RT). Under these conditions, prone-to-hydrolysis characteristic and protecting groups contained in a polyfunctional molecule would be compatible (or would have the highest chances of survival). To our delight, only AuBr 3 (99.9%) and AuCl 3 (99.99%) were capable, adjusting the pH with a standard solution of NaOH 1.000 M (99.99%), [7] of promoting the hydrolysis of 4-phenyl-2-butanone oxime (Table 1), after screening Lewis acids that are quite soluble and stable in aqueous media [Sc(OTf) 3 , LaCl 3 ·7H 2 O, FeX n , RuCl 3 , RhCl 3 , PtCl 4 , CuX 2 , AuX 3 , InBr 3 ] and related compounds. Save AuX 3 , none of these salts was a suitable initiator or catalyst at pH 4-8; [8] in fact, when the solutions of most of them were neutralized, the corresponding hydroxydes or oxide hydrates precipitated, as expected. Despite their lack of solubility in water, we also examined Pt II , see entry 8, Cu I , see entries 10-12, [9] and Au I salts, entry 17, because of their excellence as catalysts in other contexts, but no effect was observed. In short, among all these common transition metal cations, only Au III species did work at neutral pH. However, 50 mol % of Au III was required to complete the hydrolysis of 4-phenyl-2-butanone oxime (entry 21), as the active Au III species disappeared by reduction to Au 0/I NPs by NH 2 OH (as expected).Thus, the next challenge was to develop a truly catalytic version of this hydrolysis (a cheaper process). We explored the possibility of trapping NH 2 OH with carbonyl compounds capable of forming oximes more stable than the starting oxime (Table 1, entries 22-27). The best results for such a transoximation process were obtained with methyl

Brønsted acid catalyzed transoximation reaction: synthesis of aldoximes and ketoximes without use of hydroxylamine salts

“…10 In 2011, Burés and Vilarrasa et al reported de-oximation via transoximation using diacetyl as a receptor and AuBr 3 as the catalyst under neutral conditions. 11 In contrast, oximation via catalytic transoximation to prepare the oxime remains poorly developed, despite the existence of transoximases in nature. We were inspired by bio-synthetic pathways to explore catalytic transoximation for several reasons: (1) transoximation could allow the use of stable oximes instead of the dangerous hydroxylamine, (2) some oximes are inexpensively produced industrially by, for example, ammoximation, (3) carbonyl compounds generated from the oxime as byproducts are easily removed by vaporization, (4) depending on the type of byproduct generated, the target oxime can be regenerated industrially using the TS-1 catalyst, 12 and (5) the reaction mechanism of the acid-catalyzed reaction remains poorly understood (Fig.…”

Asymmetric synthesis and absolute stereochemistry of a labdane-type diterpenoid isolated from the rhizomes of Isodan yuennanensis

“…At the beginning, we tried to transform the oxime to an aldehye or a Scheme 2 Synthesis of compound 7. hemi-acetal in the presence of the free 8-OH first and then prepare the lactone by oxidation. However, all attempts with different reagents to convert this oxime in 9 to an aldehyde failed unexpectedly, such as hydrochloric acid, 11 AuBr 3 , 12 NaHSO 3 , 13 Co 2 (CO) 8 , 14 TiCl 4 , 15 and FeCl 3 . 16 (Scheme 4).…”

“…At the beginning, we tried to transform the oxime into an aldehyde or a hemi-acetal in the presence of the free 8-OH first and then prepare the lactone via oxidation. However, all of the attempts with different reagents to convert this oxime in 9 into an aldehyde failed unexpectedly, such as using hydrochloric acid, 11 AuBr 3 , 12 NaHSO 3 , 13 Co 2 (CO) 8 , 14 TiCl 4 , 15 and FeCl 3 include the oxidation of the aldoxime 9 to the nitrile 9C, 18 acid-catalyzed imidate formation, and hydration of imidate. After selective deprotection of the acetate in 10, the resultant primary alcohol was oxidized with IBX at 80 °C to produce compound 12 in a 98% yield while the application of Dess-Martin periodinane at 0 °C afforded the aldehyde in an inferior 71% yield.…”

Gold-Catalyzed Oxidation Terminal Alkyne: An Approach to Synthesize Substituted Dihydronaphthalen-2(1H)-ones and Phenanthrenols

“…3,4-Dihydronaphthalen-2(1H)-one (3a). 3a is a known compound 25 and was prepared in 83% yield (12.1 mg) according to the General Procedure C. 1 H NMR (500 MHz, CDCl 3 ) δ 7.31−7.25 (m, 3H), 7.18 (dd, J = 7.8, 4.1 Hz, 1H), 3.64 (s, 2H), 3.12 (t, J = 6.7 Hz, 2H), 2.66−2.56 (m, 2H).…”