Reduction of (Ar L)Co II Br (Ar L = 5-mesityl-1,9-(2,4,6-Ph 3 C 6 H 2)dipyrrin) with potassium graphite afforded the novel Co I synthon (Ar L)Co I. Treatment of (Ar L)Co I with a stoichiometric amount of various alkyl azides (N 3 R) furnished three-coordinate Co III alkyl imidos (Ar L)Co(NR), as confirmed by single-crystal X-ray diffraction (R:CMe 2 Bu, CMe 2 (CH 2) 2 CHMe 2). The exclusive formation of four-coordinate cobalt tetrazido complexes (Ar L)Co(κ 2-N 4 R 2) was observed upon addition of excess azide, inhibiting any subsequent C-H amination. However, when a weak C-H bond is appended to the imido moiety, as in the case of (4-azido-4-methylpentyl)benzene, intramolecular C-H amination kinetically outcompetes formation of the corresponding tetrazene species to generate 2,2-dimethyl-5-phenylpyrrolidine in a catalytic fashion without requiring product sequestration. The imido (Ar L)Co(NAd) exists in equilibrium in the presence of pyridine with a four-coordinate cobalt imido (Ar L)Co(NAd)(py) (K a = 8.04 M −1), as determined by 1 H NMR titration experiments. Kinetic studies revealed that pyridine binding slows down the formation of the tetrazido complex by blocking azide coordination to the Co III imido. Further, (Ar L)Co(NR)(py) displays enhanced C-H amination reactivity compared to that of the pyridinefree complex, enabling higher catalytic turnover numbers under milder conditions. The mechanism of C-H amination was probed via kinetic isotope effect experiments [k H /k D = 10.2(9)] and initial rate analysis with para-substituted azides, suggesting a two-step radical pathway. Lastly, the enhanced reactivity of (Ar L)Co(NR)(py) can be correlated to a higher spin-state population, resulting in a decreased crystal field due to a geometry change upon pyridine coordination.

We report herein the improved diastereoselective synthesis of 2,5-disubstituted pyrrolidines from aliphatic azides. Experimental and theoretical studies of the C–H amination reaction mediated by the iron dipyrrinato complex (AdL)FeCl(OEt2) provided a model for diastereoinduction and allowed for systematic variation of the catalyst to enhance selectivity. Among the iron alkoxide and aryloxide catalysts evaluated, the iron phenoxide complex exhibited superior performance towards the generation of syn 2,5-disubstituted pyrrolidines with high diastereoselectivity.

Oxygen plays an important role in wound healing, as it is essential to biological functions such as cell proliferation, immune responses and collagen synthesis. Poor oxygenation is directly associated with the development of chronic ischemic wounds, which affect more than 6 million people each year in the United States alone at an estimated cost of $25 billion. Knowledge of oxygenation status is also important in the management of burns and skin grafts, as well as in a wide range of skin conditions. Despite the importance of the clinical determination of tissue oxygenation, there is a lack of rapid, user-friendly and quantitative diagnostic tools that allow for non-disruptive, continuous monitoring of oxygen content across large areas of skin and wounds to guide care and therapeutic decisions. In this work, we describe a sensitive, colorimetric, oxygen-sensing paint-on bandage for two-dimensional mapping of tissue oxygenation in skin, burns, and skin grafts. By embedding both an oxygen-sensing porphyrin-dendrimer phosphor and a reference dye in a liquid bandage matrix, we have created a liquid bandage that can be painted onto the skin surface and dries into a thin film that adheres tightly to the skin or wound topology. When captured by a camera-based imaging device, the oxygen-dependent phosphorescence emission of the bandage can be used to quantify and map both the pO2 and oxygen consumption of the underlying tissue. In this proof-of-principle study, we first demonstrate our system on a rat ischemic limb model to show its capabilities in sensing tissue ischemia. It is then tested on both ex vivo and in vivo porcine burn models to monitor the progression of burn injuries. Lastly, the bandage is applied to an in vivo porcine graft model for monitoring the integration of full- and partial-thickness skin grafts.

We report the catalytic C-H amination mediated by an isolable Co III imido complex (Tr L)Co(NR) supported by a sterically demanding dipyrromethene ligand (Tr L = 5-mesityl-1,9-(trityl)dipyrrin). Metalation of (Tr L)Li with CoCl 2 in THF afforded a high-spin (S = 3/2) three-coordinate complex (Tr L)CoCl. Chemical reduction of (Tr L)CoCl with potassium graphite yielded the high-spin (S = 1) Co I synthon (Tr L)Co which is stabilized through an intramolecular η 6-arene interaction. Treatment of (Tr L)Co with a stoichiometric amount of 1-azidoadamantane (AdN 3) furnished a threecoordinate, diamagnetic Co III imide (Tr L)Co(NAd) as confirmed by single-crystal X-ray diffraction, revealing a rare trigonal pyramidal geometry with an acute CoN imido-C angle 145.0(3)°. Exposure of 1-10 mol % of (Tr L)Co to linear alkyl azides (RN 3) resulted in catalytic formation of substituted N-heterocycles via intramolecular C-H amination of a range of C-H bonds, including primary C-H bonds. The mechanism of the C-N bond formation was probed via initial rate kinetic analysis and kinetic isotope effect experiments [k H /k D = 38.4(1)], suggesting a stepwise H−atom abstraction followed by radical recombination. In contrast to the previously reported C-H amination mediated by (Ar L)Co(NR) (Ar L = 5-mesityl-1,9-(2,4,6-Ph 3 C 6 H 2)dipyrrin), (Tr L)Co(NR) displays enhanced yields and rates of C-H amination without the aid of a cocatalyst, and no catalyst degradation to a tetrazene species was observed, as further supported by the pyridine inhibition effect on the rate of C-H amination. Furthermore, (Tr L)Co(NAd) exhibits an extremely low one-electron reduction potential (E red° = −1.98 V vs [Cp 2 Fe] +/0) indicating that the highly basic terminal imido unit contributes to the driving force for H−atom abstraction.

Scheme 1. a) Previously developed CÀN bond-forming reactions with alkyl azides. b) The reaction developed in this study: a rhodiumcatalyzed direct CÀH amination with alkyl azides. DG = directing group, LA = Lewis acid.