Metal-free processes

Et3B/02 replacement of AIBN as the radical initiator allowed the reaction to be carried out at room temperature, instead of at 80 °C, allowing milder conditions to complete the transformation. Fukuyama also found that hypophosphorous acid could be employed as an alternative radical reducing agent to tin hydride, thus, developing a metal-free process. For example, after heating a mixture of the substrate, hypophosphorous acid, triethylamine and AIBN in -PrOH at 100 °C for 20 min, 2-cyclohexylindole was prepared in 71% yield. ... 

A variety of pyrrolo[l,2-a]quinoxaline derivatives 406 were synthesized in good to excellent yields when pyrrole-2-carboxamides 405 were used instead of a-amino acid derivatives in reactions with 1-fluoro- (383) and chloro- (404) -2-nitrobenzenes under a mild transition metal-free process (Scheme 2.73) (Huang et al. 2011). 

A transition metal-free process for this reaction utilized hypervalent iodine reagents to promote the transformation [38]... 

A common oxidant is tert-butyl hydroperoxide [46-48], which can either be activated by metal salts [47] or used in an aqueous, metal-free process [48]. In this latter... 

There has been a review of the amination of azoles by copper-catalysed, acid-promoted, and metal-free processes. It has been shown that substitution of bromide by azide ions in meio-bromoporphyrins may be achieved without a catalyst. New ligands derived from quinoxaline have been found for the efficient catalysis by copper... 

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