C-H Oxygenation

M benzhydrol, 27° 3°C. h Oxygen absorption had become very slow. 

Steele, B.C.H., Oxygen-transport and exchange in oxide ceramics. Journal of Power Sources, 1994, 49, 1-14. 

C-H Oxygenation. The combination of PhI(OAc)2 and catalytic Pd(OAc)2 can be used with functionalized aromatic and aliphatic hydrocarbons for the directed oxygenation of C-H bonds (eq 64). 66 6 Substrates containing pyridine, azole, imine, and oxime groups fuction under these reaction conditions to afford acetoxy lated or methoxylated compounds. Oxidation of 1 ° methyl centers is strongly preferred over 2° and 3° C-H sites. 

Sanford and coworkers have reported the use of Oxone in combination with palladium acetate for the oxime-ether directed C-H oxygenation of arenes (eq 111). A range of substituted arenes are tolerated under the reaction conditions and the use of Oxone rather than periodate-based oxidants afforded A -oxidized products in some cases. 

Oxadiazoles and 1,2,4-oxadiazoles are heterocyclic aromatic compounds that appear in many bioactive molecules. Previous methods for the synthesis of 1,2,4-oxadiazoles include the coupling of amidoximes with carboxylic acid derivatives, aerobic C—H oxygenation of amidoximes, or a cyclization of nitrile oxides to nitriles. Telvekar and Takale developed the preparation of 1,2,4-oxadiazoles from substituted diketone derivatives through a Beckmann rearrangement process tScheme S.3S1. When treated with diphosphorus tetraiodide in dichloromethane at room temperature, dioximes 150 formed the Beckmann products, 1,2,4-oxadiazoles 151, in excellent yields. 

In addition to the most common C-H oxygenation reactions of alkanes employing peroxides as oxidants, novel reactivity for these substrates has emerged in the past few years. Remarkable examples of this emerging reactivity involves the a-oxygenation of electron-rich substrates employing O2 as terminal oxidant, and desaturation of alkane C-H bonds. 

Aekermann described the first example of C-H oxygenation of aryl Weinreb amides under mild conditions thereby giving access to ortho-hydro>ylated aldehydes by reduction (Scheme 17). ° ... 

Chelate-assisted C-H bond aminations often require the presence of an oxidant in analogy to the previously discussed C-H oxygenations. Therefore, many protocols use a mixture of amine source and oxidant as reagents in order to achieve C-N bond formation. Intermolecular directed C-H aminations have been realized with this approach by Che and coworkers (Scheme 23.36). Remarkably, this methodology is capable of employing a variety of different directing groups as well as primary amides as amine sources [129]. 

In various C-H oxygenation reactions, organometallic complexes can play the role of precatalyst. Compounds bearing carbon-metal bonds can also be some of intermediate compounds in the catalytic cycles. In this chapter, we discuss reactions in which an organometallic complex catalyzes the insertion of oxygen atoms into C-H bonds of hydrocarbons or other organic compounds. The focus will be made on the author s own works. 

Remarkably, C—H oxygenations can also be performed with O2 as terminal oxidant, with and without the use of co-catalysts. Variations of the catalytic conditions result in efficient protocols for introducing C—O bonds in allylic (24), aromatic (25), benzylic (27), and aliphatic positions (28) with high atom economy. 

Carter S., Selcuk A., Chater R.J., Kajda J., Kilner J.A., Steele B.C.H. Oxygen transport in selected nonstoichiometric perovskite-structure oxides. Solid State Ion. 1992 53-56 597-605... 

Considering the similarity of the hydroxyl group in alcohols and silanols, Gevorgyan developed a silanol-directed palladium-catalyzed C-H activation/C-O cyclization of phenoxy silanols [61]. The obtained oxasilacycles were further desilylated with TBAF to give catechols. In a follow-up work, the same group disclosed a palladium-catalyzed C-H oxygenation with benzylsilanols to achieve oxasilacycles under similar conditions (Scheme 3.32) [62]. The obtained oxasilacycles proved to be valuable synthetic intermediates and could be converted to a variety of useful compounds. 

In the CuBr-SMe2/2,2 -bipyridine catalytic system under an O2 atmosphere, dihydrooxazoles could be obtained through direct C-H oxygenation of C(sp )-H bonds of AT-aikyiamidines (Scheme 8.72). The proton could be abstracted via 1,5-H radical shift under this Cu/02-promoted radical process [119]. 

Scheme 8.72 Synthesis of dihydrooxazoles from W-alkylamidines through direct C-H oxygenation.

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