Carbon-selenium bonds oxidation

A photosensitized activation of carbon-selenium bonds was also used for performing phenylseleno group transfer reactions. This process involves a photosensitized electron transfer (PET) as the initial step in the reaction sequence. Fragmentation affords a radical and phenylselenolate, which is oxidized to diphenyl diselenide in the presence of oxygen. The cyclized radical is then trapped by diphenyl diselenide to afford the final product. This process is quite general for intramolecular radical reactions.70,266... 

Cyclizations by formation of carbon—selenium bonds represent a modern method with a high synthetic potential in the chemistry of cyclophanes. Selenocyanates such as 16 are accessible usually in excellent yields through the reaction of bromides with KSeCN [27], The reaction with benzylic bromides under reductive conditions using the dilution principle results in good to excellent yields of [3.3]di-selenacyclophanes which can be deselenized photochemically, pyrolytically (without previous oxidation), or by reaction with arynes, Stevens rearrangement and subsequent reaction with Raney nickel. [2.2]Metacyclophane (18), for example, is accessible in 47% total yield by using this sequence of reactions starting with... 

Other important cycloelimination procedures correspond to an elimination of H2O from cyclic ketones. Thus, the a-hydrogen atoms of semicarbazones of cyclic ketones are removed by oxidative cyclization with thionyl chloride or selenium dioxide (Scheme 8-7). The 1,2,3-thiadiazoles (71) or 1,2,3-selenadiazoles (72) which result from these reactions can be cleaved in a second step to yield cyclic alkynes (Scheme 8-8) [28]. Several fragmentation conditions are known, among them thermal decomposition and base-induced cleavage. The mechanism of these reactions has been studied in detaU [29]. It has been noted that the crucial step is the cleavage of the carbon-sulfur or carbon-selenium bond, as in this step the geometrical strain is introduced into the system. Clearly, due to the weakness of the C —Se... 

The equivalent to allylic oxidation of alkenes, but with allylic transposition of the carbon-carbon double bond, can be carried out by an indirect oxidative process involving addition of an electrophilic arylselenenyl reagent, followed by oxidative elimination of selenium. In one procedure, addition of an arylselenenyl halide is followed by solvolysis and oxidative elimination. 

A double oxidation at the 5,6-carbon-carbon double bond and N-9 by 2equiv of OT-chloroperbenzoic acid (MCPBA) was postulated as the mechanism for the oxidative rearrangement of 8-dialkylaminoxanthines to novel l-oxo-2,4,7,9-tetraazaspiro[4,5]dec-2-ene-6,8,10-triones <1999EJ02419>. The presence of an electron-donating 8-amino substituent was essential for this reaction to occur. Further oxidation of the products gave 1,3-dimethylpara-benic acid 45, which was also produced directly when selenium dioxide or hydrogen peroxide were used (Scheme 11). 

The most useful application of 3 is its use in photochemical [2+2] cycloadditions with alkenes and alkynes at the carbon-carbon double bond to afford bicyclo[4.2.0]octane-2,5-diones and bicyclo[4.2.0]oct-7-ene-2,5-diones in good to excellent yield.6 Since selenium dioxide oxidation of the resulting adducts furnishes the corresponding 3-ene-2,5-diones, diketone 3 can be regarded as a 1,4-benzoquinone equivalent leading to [2+2] cycloadducts at the carbon-carbon double bond. 

As well as thiol esters, selenol esters (1) frequently exhibit a high and selective reactivity toward nucleophiles, which is enhanced even further by activation with heavy ions or oxidizing agents. These properties make selenol esters valuable acyl transfer agents. This review deals with general methods for the synthesis of selenol esters and their reactivity as acyl transfer agents. Furthermore, selenoesters (2), isomeric compounds of selenol esters, and their derivatives selenoamides (3) are also described. These compounds show the characteristic reactivity based on the carbon-selenium double bond. 

Ozonolysis of Carbon-Heteroatom Bonds and Heterocyclie Compounds. The ozonolysis of nitrogen, phosphorus, sulfur, and selenium to form iV-oxides, phosphine oxides and phospho-nates, sulfoxides, and selenoxides that can be used to functionalize various substrates has been weU documented. A useful application of the power of ozonolysis is to cleave carbon heteroatom bonds for conversion into carbonyl compounds. A useful... 

Boron, Phosphorus, and Selenium Compounds. Oxone has been used to oxidize carbon-boron bonds during the work-up of hydroboration reactions to obtain high yields of the resultant alcohols (eq 73). Aqueous Oxone/acetone oxidizes electron-poor and electron-rich aromatic and aliphatic boronic acids and esters to the corresponding alcohols rapidly and efficiently (eq 74). A one-pot procedure for the synthesis of iweta-substituted phenols from benzenes has been developed, and a similar strategy has been devised for the synthesis of Boc-oxindoles from Boc-indoles. i3i... 

Selenium dioxide attacks allylic positions in preference to carbon-carbon double bonds and is a very useful reagent for allylic oxidation. A good deal of... 

Iv) Selenium dioxide oxidizes allylic positions of most nucleophilic double bond (trisubstituted) with -selectivity. As explained in the previous problem, trisubstituted alkenes are oxidized selectively at the more-substituted end of the carbon—carbon double bond. The position next to the unsaturated ester is not oxidized. 

Pericyclic reactions provide some of the most elegant examples of the importance of orbital symmetry in chemical reactions. Unlike in organic chemistry, however, pericyclic reactions are not of great importance in inorganic chemistry. That said, we will encounter a few significant examples in this book, including the reduction of carbon-carbon double bonds by diimide (Section 5.7a) and certain selenium dioxide oxidations (Section 6.16). 

It should not be surprising that reaction at the allylic position is not restricted to halogenation. Thus, although the carbon-carbon double bond is easily oxidized (Part I of this chapter), an oxidizing agent such as selenium dioxide (Se02) preferentially directs oxidation to the allylic position and the process by which this is thought to occur (Scheme 6.52) is also indicative of the indirect path by which vinylic substitution can be accomplished. 

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