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84869992556
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We refer specifically to processes that rely upon an internal oxidant (cf. oxidative addition of Pd(0)-catalysts into aryl-halide bonds)
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I. P. Beletskaya A. V. Cheprakov Organometallics 2012 31 7753
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Beletskaya, I.P.1
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7
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27644519641
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Pentafluorobenzoyl oxime esters are usually employed for these processes because they are stable to Beckmann rearrangement Imino-Pd(ii) intermediates have been characterized and exploited in catalytic C-H amination
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K. Narasaka M. Kitamura Eur. J. Org. Chem. 2005 4505
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Previous studies are consistent the direct insertion of the alkene component into the N-Pd(ii) bond in a manner that is analogous to the conventional Heck reaction (see ref. 6b and c)
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44449138157
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These processes are proposed to involve oxidative addition of Cu(i) into the N-O bond. For mechanistically similar processes that involve O-acyl hydroxylamine derivatives, see
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N2 substitutions of N-O bonds. For example, see
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34247525138
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2 and provide aza-Heck-type products by an ionic mechanism that involves Lewis acid activation of the oxime ester
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M. J. Campbell J. S. Johnson Org. Lett. 2007 9 1521
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Intramolecular copper-catalyzed alkene difunctionalization reactions that use external oxidants
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Intramolecular copper-catalyzed alkene difunctionalization reactions that use internal oxidants: alkene amino-hydroxylation
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Intermolecular oxidative aza-Heck reactions that employ an external oxidant and do not rely on oxidative initiation at nitrogen
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84884290699
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The activation of oxime ester N-O bonds with catalytic Cu(i) to form new C-N bonds has been employed in various contexts. For leading references, see: aza-copper enolate generation
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T. W. Liwosz S. R. Chemler Chem.-Eur. J. 2013 19 12771
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84859096100
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The mechanism for nitrile formation is not clear. Possible pathways include (but are not limited to) Lewis acid promoted Beckmann type-II rearrangement of the oxime ester or β-hydride elimination from an imino-Cu(iii) intermediate (vide infra). See also ref. 9a Addition of molecular sieves to the reaction mixture did not suppress the formation of this byproduct. Consequently, we favor a pathway involving decomposition of the oxime ester to the corresponding NH-imine and hydrolysis to the ketone during work-up or chromatography. The NH-imine may form via either an imino-Cu(iii) intermediate or an iminyl radical (vide infra) For a review on the one electron reduction of oxime derivatives, see
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31544440977
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Iminyl radical and imino-Cu(iii) intermediates may exist in equilibrium as depicted in Scheme 2A. Imino-Cu(iii) intermediates have been proposed previously (for example, see ref. 9a and b). An alternative possibility is that a radical anion of the oxime ester is generated which then cyclizes, with concomitant loss of pivalate, to generate an alkyl radical (see ref. 11i)
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K. Narasaka M. Kitamura ARKIVOC 2006 vii 245
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Narasaka, K.1
Kitamura, M.2
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33947089830
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Carboxylate" refers to either pivalate or 2-ethylhexanoate Even though we propose a Cu(i)-initiated process, Cu(ii)-salts are preferred. Higher concentrations of Cu(ii) may increase the efficiency of oxidative elimination from secondary alkyl radical 12
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Based upon the considerations outlined in ref. 18, we favor partial disproportionation to provide small quantities of Cu(i). For the reduction potentials of copper ions in benzonitrile, see
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It is unclear whether the processes described here proceed via an imino-Cu(iii) species or the direct formation of an iminyl radical or, indeed, a radical anion of the oxime ester. To date, all attempts to isolate an imino-Cu(iii) intermediate have been unsuccessful 20a,b are formally hydrolysis and Beckmann rearrangement products of 16
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Exposure of the analogous O-pentafluorobenzoyl oxime ester to our optimized Pd-based systems (see ref. 6) did not result in the formation of 8n or products related to 34 and only formal hydrolysis to the corresponding ketone was observed. See ref. 6a for a discussion on mechanistic pathways to the ketone
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