Rhodium-Mediated Oxygenation of Nitriles with Dioxygen: Isolation of Rhodium Derivatives of Peroxyimidic Acids

Activation of SO2F2 at a Rhodium PNP Pincer Complex: Ligand Supported S-F Bond Cleavage to Generate NSO2F Derivatives

The κ2-(P,N)-phosphine ligand precursor NH(CH2CH2PCy2)2 can be used for the synthesis of the rhodium(I) complex [Rh(CO){ĸ3-(P,N,P)-Cy2PC2H4NHC2H4PCy2}][Cl] (1). The deprotonated complex [Rh(CO){ĸ3-(P,N,P)-Cy2PC2H4NC2H4PCy2}] (2) shows a cooperative reactivity of the PNP ligand in the activation reaction of SO2F2 to yield the rhodium fluorido complex trans-[Rh(F)(CO){ĸ2-(P,P)-Cy2PC2H4N(SO2F)C2H4PCy2}]2 (3) by S-F bond cleavage. It is remarkable that no reaction was observed when 3 was treated with hydrogen sources e. g. dihydrogen, organosilicon compounds such as triethylsilane or TMS-CF3 and different fluorine sources such as SF4 or Selectfluor®. However, the treatment of complex 3 with XeF2 in the presence of CsF resulted in the formation of the unique fluorido rhodium(III) complex cis,trans-[Rh(F)3(CO){ĸ2-(P,P)-Cy2PC2H4N(SO2F)C2H4PCy2}]2 (4). In the presence of pyridine(HF)X or BF3 the fluorido complex 3 converted into the dicationic complexes [Rh(CO){ĸ2-(P,P)-Cy2PC2H4N(SO2F)C2H4PCy2}]2[XF]2, X=HF (5) or BF3 (6), respectively.

Mechanistic Insights into Nitrile Activation by Cobalt(III)-Hydroperoxo Intermediates: The Influence of Ligand Basicity

The versatile applications of nitrile have led to the widespread use of nitrile activation in the synthesis of pharmacologically and industrially valuable compounds. We reported the activation of nitriles using mononuclear cobalt(III)-hydroperoxo complexes, [CoIII(Me3-TPADP)(O2H)(RCN)]2+ [R = Me (2) and Ph (2Ph)], to form cobalt(III)-peroxyimidato complexes, [CoIII(Me3-TPADP)(R-C(=NH)O2)]2+ [R = Me (3) and Ph (3Ph)]. The independence of the rate on the nitrile concentration and the positive Hammett value of 3.2(2) indicated that the reactions occur via an intramolecular nucleophilic attack of the hydroperoxide ligand to the coordinated nitrile carbon atom. In contrast, the previously reported cobalt(III)-hydroperoxo complex, [CoIII(TBDAP)(O2H)(CH3CN)]2+ (2TBDAP), exhibited the deficiency of reactivity toward nitrile. The comparison of pKa values and redox potentials of 2 and 2TBDAP showed that Me3-TPADP had a stronger ligand field strength than that of TBDAP. The density functional theory calculations for 2 and 2TBDAP support that the strengthened ligand field in 2 is mainly due to the replacement of two tert-butyl amine donors in TBDAP with methyl groups in Me3-TPADP, resulting in the compression of the Co-Nax bond lengths. These results provide mechanistic evidence of nitrile activation by the cobalt(III)-hydroperoxo complex and indicate that the basicity dependent on the ligand framework contributes to the ability of nitrile activation.

Copper oxidation chemistry using a 19-iminopyridine-bearing steroidal ligand: (i) C5-C6 olefin difunctionalization and (ii) C1β-hydroxylation/C19-peroxidation

The Schönecker oxidation involves the 12beta-hydroxylation of 17-imino pyridine DHEA derivatives using copper and either molecular oxygen or hydrogen peroxide as the oxidant. In this study, a 19-imino pyridine DHEA derivative was synthesized and was treated with copper nitrate and hydrogen peroxide. Our results showed the difunctionalization of an olefin for delta-5 steroid substrates to yield a 5beta-hydroxylated 6alpha-nitrate ester product. In contrast, for 19-imino pyridine precursors with a 5alpha-androstane steroid backbone: a 1beta-hydroxylation and 19-peroxidation occurred to yield a 1beta-hydroxylated 19-imidoperoxoic acid product. In conclusion, new Schönecker oxidation chemistry was discovered (C5-C6 olefin difunctionalization and C1beta-hydroxylation/C19-peroxidation) when a 19-imino pyridine DHEA derivative was used as the substrate.

Controlled Regulation of the Nitrile Activation of a Peroxocobalt(III) Complex with Redox-Inactive Lewis Acidic Metals

Redox-inactive metal ions play vital roles in biological O₂ activation and oxidation reactions of various substrates. Recently, we showed a distinct reactivity of a peroxocobalt(III) complex bearing a tetradentate macrocyclic ligand, [Co^III(TBDAP)(O₂)]⁺ (1) (TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane), toward nitriles that afforded a series of hydroximatocobalt(III) complexes, [Co^III(TBDAP)(R-C(═NO)O)]⁺ (R = Me (3), Et, and Ph). In this study, we report the effects of redox-inactive metal ions on nitrile activation of 1. In the presence of redox-inactive metal ions such as Zn²⁺, La³⁺, Lu³⁺, and Y³⁺, the reaction does not form the hydroximatocobalt(III) complex but instead gives peroxyimidatocobalt(III) complexes, [Co^III(TBDAP)(R-C(═NH)O₂)]²⁺ (R = Me (2) and Ph (2^Ph)). These new intermediates were characterized by various physicochemical methods including X-ray diffraction analysis. The rates of the formation of 2 are found to correlate with the Lewis acidity of the additive metal ions. Moreover, complex 2 was readily converted to 3 by the addition of a base. In the presence of Al³⁺, Sc³⁺, or H⁺, 1 is converted to [Co^III(TBDAP)(O₂H)(MeCN)]²⁺ (4), and further reaction with nitriles did not occur. These results reveal that the reactivity of the peroxocobalt(III) complex 1 in nitrile activation can be regulated by the redox-inactive metal ions and their Lewis acidity. DFT calculations show that the redox-inactive metal ions stabilize the peroxo character of end-on Co-η¹-O₂ intermediate through the charge reorganization from a Co^II-superoxo to a Co^III-peroxo intermediate. A complete mechanistic model explaining the role of the Lewis acid is presented.

Peroxocobalt(iii) species activates nitriles via a superoxocobalt(ii) diradical state

The dioxygenation of nitriles by [CoIII(TBDAP)(O2)]+ (TBDAP = N,N-di-tert-butyl-2,11-diaza[3.3](2,6)-pyridinophane) is investigated using DFT-calculations. The mechanism proposed previously based on experimental observations, which invoked an outer-sphere cycloaddition, was found to be unreasonable. Instead, calculations suggest that an inner-sphere mechanism involving the cleavage of one of the Co-O bonds assisted by substrate uptake is much more likely. The reactively competent species is a triplet consisting of a Co(ii)-superoxo functionality, which can undergo O-C bond formation and O-O bond cleavage traversing low energy transition states. The role of the structurally rigid TBDAP ligand is to prevent the participation of the pyridyl ligand in the delocalization of the unpaired electron density.

Reactivity of rhodium and iridium peroxido complexes towards hydrogen in the presence of B(C6F5)3 or [H(OEt2)2][B{3,5-(CF3)2C6H3}4]

The peroxido complexes trans-[M(4-C5F4N)(O2)(CNtBu)(PR3)2] (1: M = Rh, R = Et; 2a: M = Ir, R = iPr) can be used in the metal-mediated hydrogenation of O2. The reaction of trans-[Rh(4-C5F4N)(O2)(CNtBu)(PEt3)2] (1) with B(C6F5)3 and H2 gave trans-[Rh(4-C5F4N)(CNtBu)(PEt3)2] (3), OPEt3 and (H2O)·B(C6F5)3, whereas treatment of [H(OEt2)2][B{3,5-(CF3)2C6H3}4] with 1 in the presence of H2 yielded trans-[Rh(4-C5F4N)(CNtBu)(PEt3)2] (3) and H2O2. The reactivity of 2a towards B(C6F5)3 and BClCy2 was also studied and an intermediate was detected which is assigned to be trans-[Ir(4-C5F4N)(Cl)(OOBCy2)(CNtBu)(PiPr3)2] (4a).

Recent advances in well-defined, late transition metal complexes that make and/or break C-N, C-O and C-S bonds

Chemical transformations that result in either the formation or cleavage of carbon-heteroatom bonds are among the most important processes in the chemical sciences. Herein, we present a review on the reactivity of well-defined, late-transition metal complexes that result in the making and breaking of C-N, C-O and C-S bonds via fundamental organometallic reactions, i.e. oxidative addition, reductive elimination, insertion and elimination reactions. When appropriate, emphasis is placed on structural and spectroscopic characterization techniques, as well as mechanistic data.