Reactive intermediate-mediated species

Fe. Iron metal was found to be able to mediate the allylation reactions of aryl aldehydes with allyl bromide using sodium fluoride as the promoter. The formation of an allyliron species was proposed as the reactive intermediate in the reaction (Scheme 8.20).187 In view of... 

The aldol reactions introduced thus far have been performed under basic conditions where enolate species are involved as the reactive intermediate. In contrast to the commonly accepted carbon-anion chemistry, Mukaiyama developed another practical method in which enol species can be used as the key intermediates. He is the first chemist to successfully demonstrate that acid-catalyzed aldol reactions using Lewis acid (such as TiCU) and silyl enol ether as a stable enol equivalent can work as well.17 Furthermore, he developed the boron tri-fluoromethane sulfonate (triflate)-mediated aldol reactions via the formation of formyl enol ethers. 

Several specialized cyclization strategies should not be dismissed. A novel rhodium acetate-mediated cyclization was employed for the first synthesis of 3-methyl-4,6-diphenylfuro[3,4-tf]is-oxazole (9) from the 5-(a-diazobenzyl)isoxazole (185) (Equation (56)). The reactive intermediate is believed to be a carbenoid species <9iCB248i>. Another strategy exploits the reactivity of 4-halo-pyrazol-5-ones (e.g., 186) with stabilized anions (e.g., cyanoacetate esters and nitriles) to afford the 4-cyanofuro[2,3-c]pyrazol-5-ones (187) and 5-aminofuro[2,3-c]pyrazoles (188) (Scheme 30) <84H(22)2523>. Also of interest is the trichloroacetonitrile cyclization of aminopyrazole ketones (189) to the pyrrolo[2,3-c]pyrazoles (190) (Equation (57)) <86S74>. The generality of this cyclization is not known. 

NATs are also involved in bioactivation reactions via O-acetylation of Y-hydroxylamines formed from CYP-mediated N-hydroxylation of arylamines. These bioactivation reactions form unstable acetoxy esters that decompose to highly reactive species, which bind to cellular DNA [83], The O-sulfonation of compounds catalyzed by SULTs can also result in the formation reactive intermediates. Recently, it has been shown that a-hydroxytamoxifen (derived from CYP-mediated hydroxy-lation of tamoxifen) is bioactivated by SULTs [177],... 

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