Alkenyl oxidation chemistry

Without doubt, structure plays a major part in determining the oxidation chemistry of alkenyl radicals. 

Secondary bromides and tosylates react with inversion of stereochemistry, as in the classical SN2 substitution reaction.21 Alkyl iodides, however, lead to racemized product. Aryl and alkenyl halides are reactive, even though the direct displacement mechanism is not feasible. With there halides the mechanism probably consists of two steps. The addition of halides to transition-metal species with low oxidation states is a common reaction in transition-metal chemistry and is called oxidative addition. An oxidative addition to the copper occurs in the first step of the mechanism, and the formal oxidation... 

Another method for activating the versatile alkenyl halide moiety was the oxidative addition of a late transition metal species, which would then facilitate insertion of the C13-C14 double bond. The Rawal,7h Bonjosch,7e and Mori7b syntheses of 1 all employed a Pd-mediated intramolecular Heck reaction to build the F ring of the molecule.71 Our system, in as much as it contained additional unsaturation, was somewhat different from those in the literature and thus had the potential for displaying new chemistry. For example, in Rawal s approach (Scheme... 

Xu et al. have further expanded the scope of Pd-catalyzed CsfP—P bond formation reactions to include a variety of different organophosphorous coupling partners [83]. For example, the coupling of 2-bromothiophene with n-butyl phenyl phosphite was effected to form //-butyl diarylphosphinate 82. This chemistry also allows for the synthesis of alkyl arylphenylphosphine oxides, functionalized alkyl arylphenylphosphinates, alkenyl arylphenylphosphinates, alkenylbenzyl-phosphine oxides, as well as chiral, nonracemic isopropyl arylmethylphosphinates [84]. Intramolecular Pd-catalyzed CspR— P bond formation has also been reported [84]. 

Selective partial hydroalumination of dienes and trienes or their mixtures is possible because of the differences in the rate of A1H addition to various sorts of C=C double bonds. Further transformations of the alkenyl alanes open the way to some interesting possibilities in preparative work. For example, by partial hydroalumination of 4-vinylcyclohexene or limo-nene, wherein the A1—H bond reacts with the terminal double bond, followed by air oxidation, it is possible to obtain cyclohexenylethanol or 18-terpineol in good yield 312). A further example from terpene chemistry is the preparation of citronellol 225). 

The Barton nitrite ester photolysis is undeniably one of the most popular and useful reactions in radical chemistry for the functionalization of remote and inactivated positions within steroids (Scheme 19). Photolysis of nitrite esters gives nitric oxide and an alkoxyl radical that abstracts an ideally positioned hydrogen atom (1,5-hydrogen atom abstraction). The resulting alkyl radical reacts with nitric oxide in a solvent cage to afford the nitroso-alcohol derivative that is finally isolated as an oxime [53]. Related cyclizations of alkoxyl radicals have been reported by Surzur photolysis of y,(5-alkenyl nitrite esters leads to alkoxyl radicals that undergo subsequent tandem 5-exo cyclization followed by NO-trapping [54, 55]. 

Palladium chemistry has been utilised to introduce aryl groups to a furan a-position by substitution of hydrogen, and via boronic acids, and in Heck-type alkenylations, again at C-2, via oxidative type palladation (cf. section 2.7.2.1). 

A variant of the chemistry just discussed involves the formation of the required alkenyl- or aryl-Pd(II) intermediate by transmetallation rather than oxidative addition (Scheme 16). 

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