Spectroscopic Analyses on Reaction Intermediates Formed during Chlorination of Alkanes with NaOCl Catalyzed by a Nickel Complex

Apparao Draksharapu†, Zoel Codolà‡, Laura Gómez‡§, Julio Lloret-Fillol‡, Wesley R. Browne*†, and Miquel Costas*‡

† University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands
‡ Institut de Química Computacional i Catalisi (IQCC) and University of Girona, Spain

Inorg. Chem. 2015,
DOI: 10.1021/acs.inorgchem.5b01463

Abstract

The spectroscopic, electrochemical, and crystallographic characterization of [(Me,HPyTACN)NiII(CH3CN)2](OTf)2 (1) (Me,HPyTACN = 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane,

OTf = CF3SO3) is described together with its reactivity with NaOCl. 1 catalyzes the chlorination of alkanes with NaOCl, producing only a trace amount of oxygenated byproducts. The reaction was monitored spectroscopically and by high resolution electrospray-mass spectrometry (ESI-MS) with the aim to elucidate mechanistic aspects. NaOCl reacts with 1 in acetonitrile to form the transient species [(L)NiII-OCl(S)]+ (A) (L = Me,HPyTACN, S = solvent), which was identified by ESI-MS. UV/vis absorption, electron paramagnetic resonance, and resonance Raman spectroscopy indicate that intermediate A decays to the complex [(L)NiIII–OH(S)]2+ (B) presumably through homolytic cleavage of the O–Cl bond, which liberates a Cl• atom. Hydrolysis of acetonitrile to acetic acid under the applied conditions results in the formation of [(L)NiIII-OOCCH3(S)]2+ (C), which undergoes subsequent reduction to [(L)NiII-OOCCH3(S)]2+ (D), presumably via reaction with OCl– or ClO2–. Subsequent addition of NaOCl to [(L)NiII-OOCCH3(S)]+ (D) regenerates [(L)NiIII-OH(S)]2+ (B) to a much greater extent and at a faster rate. Addition of acids such as acetic and triflic acid enhances the rate and extent of formation of [(L)NiIII-OH(S)]2+ (B) from 1, suggesting that O–Cl homolytic cleavage is accelerated by protonation. Overall, these reactions generate Cl• atoms and ClO2 in a catalytic cycle where the nickel center alternates between Ni(II) and Ni(III). Chlorine atoms in turn react with the C–H bonds of alkanes, forming alkyl radicals that are trapped by Cl• to form alkyl chlorides.

ニッケル錯体とハイポクロライトで、基質のクロロ化が進行したという報告です。
酢酸を加えることで、反応性と選択性が高まります。
もっともよいデータで、シクロヘキサンのクロロ化が酸化剤ベースでyeild 37%で進行し、R–Cl：R–OH が15:1、TONが24回だそうです。
反応機構としては、Ni(II)－OClが生成したのち、O–Cl結合がホモリティックに開裂して生成するCl•が活性種としています。
EPRとrRamanで検出される化学種は、Ni(III)–OH、もしくは、Ni(III)–OAcであるとしています。これらのNi三価錯体は、どこからか電子をとって、二価にもどるそうです。