Electron deficient electroreduction

Many reductive cyclizations, including many of those that are not initiated electrochemically, correspond to variations on the electrohydrocyclization theme. The so-called electroreductive-cyclization reaction, for example, involves cyclization between the /I-carbon of an electron-deficient alkene and an aldehyde or ketone tethered to it, to form a new a-bond between these formally electron deficient centers (Scheme 2). 

As noted previously, many of the cathodic cyclizations discussed in this article are variations on the electrohydrocyclization theme developed by Baizer and coworkers [8-14,16,17,21], The next section of this article, for example, deals with what has been referred to as the electroreductive cyclization (ERC) reaction, a process that leads to cycUzation between an electron-deficient alkene and an aldehyde or ketone. With this thought in mind, several of the section titles are formulated to highlight the functional groups to be joined the following is representative. 

Electroreductive Cyclization Electron Deficient Alkene/Carbonyl... 

The utility of electroreductive cyclization (ERC) reaction is demonstrated by the formal total synthesis of the antitumor agent quadrone (120) that is outlined in Scheme 13 [35]. The reaction serves to link the -carbon of an electron deficient alkene to the carbonyl carbon of an aldehyde or ketone tethered to it. The transformation plays a pivotal role in the key carbon-carbon bond forming events leading to 124 and 122, and en route to quadrone (120). 

The electroreduction of Ni(II) porphyrins and related macrocycles was investigated by the groups of Stolzen-berg [330-332], Fajer [333], Kadish [116, 323, 328, 334, 335], Saveant [324] and more recently, by Fajer, Mansuy and coworkers [336], who showed that the very electron-deficient ((NO2 lyT (0X0-CI2) PP) Ni was reduced in three reversible one-electron reductions. Earlier work had shown that Ni(II) chlorins or isobacteri-ochlorins could be reduced to their Ni(I) state [330, 331, 333], as opposed to porphyrins, where a Ni(II) porphyrin jr-anion radicals was generally formed upon reduction [7]. However, the singly reduced products of Ni(II) porphyrins have been described as Ni(I) porphyrins, Ni(II) porphyrin r-anion radicals or Ni(II) porphyrin jr-anion radicals with some Ni(I) character [116]. The exact site of electron transfer and formulation of the reduction product will depend on several factors, two of which include the solution conditions and the nature of the porphyrin macrocycle [7]. 

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