Functionalization of Non-activated CH Bonds of Alkanes: An Effective and Recyclable Catalytic System Based on Fluorinated Silver Catalysts and Solvents (original) (raw)
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Journal of Organometallic Chemistry, 2015
Trispyrazolylborate-containing copper and silver complexes are known to promote the catalytic transfer of CHCO 2 Et groups from ethyl diazoacetate onto carbon-hydrogen bonds of alkanes C 2 H 2n+2. In this account the knowledge compiled for this transformation regarding the design of the catalyst to enhance activity and selectivity is presented, aiming researchers to continue the development of better catalytic systems for this simple reaction employing the readily available alkanes. Keywords: alkane C-H activation-alkane-C-H functionalization-copper catalysis-silver catalysis-ethyl diazoacetate-carbene insertion Dedicated to the memory of Prof. Alexander Shilov, a pioneer in the area of C-H bond activation and functionalization
Angewandte Chemie International Edition, 2012
Catalyzed hydrofunctionalizations of unsaturated substrates are of tremendous interest, primarily because of their atom efficiency. Intramolecular catalytic cyclohydroamination of aminoalkenes can be efficiently promoted by d 0 complexes of the heavy alkaline-earth (Ae) metals, typically Ca and in rare cases Sr, as recently exemplified by the groups of Hill, [4] Ward, and Roesky. [6] The catalytic activity of these complexes, which are based on large, electropositive elements (ionic radii: Ca 2+ (6), 1.00 ; Sr 2+ (6), 1.18 ), compares well with that of isoelectronic trivalent rare-earth catalysts. Through their seminal studies, Hill and co-workers have shown that the stable b-diketiminate compound [{L 3 }CaN-(SiMe 3 ) 2 (THF)] [9][10] ({L 3 }H = H 2 C{C(Me)N-2,6-(iPr) 2 C 6 H 3 } 2 ; THF = tetrahydrofuran) is highly versatile and effective not only for intramolecular hydroamination, [4] but also for other reactions 12] such as the intermolecular hydrophosphination of alkynes and activated alkenes, [12a] a transformation which has not been catalyzed by trivalent rare-earth complexes thus far. [8a, 13] Few examples of intermolecular hydroamination reactions catalyzed by Ae complexes are known, and they involve activated alkenes, that is, vinyl arenes and conjugated dienes. Very recently, Emge and Hultzsch reported a heteroleptic chiral magnesium phenolate complex which displayed an outstanding performance in the enantioselective intra-and intermolecular hydroamination of terminal aminoalkenes and styrene derivatives, respectively. [3d] Prior to this, Hill and coworkers had employed the homoleptic precursors [{M[N-(SiMe 3 ) 2 ] 2 } 2 ] (M = Ca, Sr) to illustrate theoretical calculations on related, yet heteroleptic, systems. In the original study, the authors showed that the activity of Ae catalysts (M = Mg, Ca, Sr, Ba) does not increase linearly with the size of the metal (Mg 2+ (6), 0.72 ; Ba 2+ (6), 1.35 ). Calculations showed that a model Sr heteroleptic complex should be more active in the amination of ethylene with ammonia than its Ca derivative (which in turn should be far more active than the Mg analogue), but they also suggested that the trend should not be respected with Ba. Experimental data obtained in the thorough study of the hydroamination of activated alkenes catalyzed by the homoleptic complexes [{Ae[N-(SiMe 3 ) 2 ] 2 } 2 ] (Ae = Mg, Ca, Sr, Ba) and [Ae{CH(SiMe 3 ) 2 } 2 -(THF) 2 ] (Ae = Ca, Sr) demonstrated that the Sr complex was indeed superior to that of Ca, and the Mg and Ba derivatives displayed very poor activities. Unfortunately, no experimental data were available for the series of heteroleptic complexes [{L 3 }AeN(SiMe 3 ) 2 (THF) n ], as the Sr and Ba species are not stable in solution. [10b] As part of our ongoing program aimed at implementing Ae-based catalysts for a diversity of transformations, we report herein the use of three families of heteroleptic complexes of the large Ae metals supported by various ancillary ligands for the anti-Markovnikov intermolecular hydroamination of vinyl arenes and isoprene. In all cases, the activity trend varies in the order (Mg <)Ca < Sr < Ba, that is, the activity increases linearly with the size of the metal. Also, the catalytic activity in the intermolecular hydrophosphination of styrene follows the same order. The Ba complexes are not only the most active in these series, but also represent the first examples of complexes of this metal that are capable of promoting the intermolecular hydrofunctionalizations of alkenes.
Nature chemistry, 2018
C-H functionalization represents a promising approach for the synthesis of complex molecules. Instead of relying on modifying the functional groups present in a molecule, the synthetic sequence is achieved by carrying out selective reactions on the C-H bonds, which traditionally would have been considered to be the unreactive components of a molecule. A major challenge is to design catalysts to control both the site- and stereoselectivity of the C-H functionalization. We have been developing dirhodium catalysts with different selectivity profiles in C-H functionalization reactions with donor/acceptor carbenes as reactive intermediates. Here we describe a new dirhodium catalyst capable of the functionalization of non-activated primary C-H bonds with high levels of site selectivity and enantioselectivity.
Metal-free, selective alkane functionalizations
Advanced Synthesis & Catalysis, 2003
The present overview of alkane functionalization reactions presents comparisons between radical and metal-initiated (sometimes metal-catalyzed) methodologies. While metal-catalyzed processes are excellent approaches to this problem, metal-free alternatives are equally if not, at least from an environmental and cost perspective, more useful. This conclusion is supported by the fact that many so-called metal-catalyzed reactions also work without the metal present, and the large variety of metals showing the same product distributions emphasizes that the metal often just aids in the generation of the active species, i.e., the metal itself is not participating in the crucial CH activation step. Highly selective alkane functionalization reactions such as those derived from nitroxyl and related radicals as well as through radical reactions conducted in phase-transfer catalyzed systems are available but generally underutilized. These systems, in contrast to typical metal-catalyzed approaches, are also applicable to highly strained alkanes and offer the highest 3°/2° CH selectivities reported to date in a radical reaction. The article closes with representative experimental protocols for the PTC bromination of cubane as an example of the applicability of this method to strained hydrocarbons and the direct iodination of cyclohexane as well as adamantane as typical alkanes bearing secondary and tertiary CH bonds.
Current Organic Chemistry, 2011
Heterocyclic compounds are of paramount importance in essentially all fields of chemistry and in our daily life. Therefore, the synthesis and modification of such compounds is an ever expanding field in synthetic chemistry. In recent years, all synthetic efforts were guided increasingly by the search for more atom efficient and economical methods. Transition metal catalyzed C-H activation reactions have proved to provide both: i) an increased atom efficiency since the number of functional groups required to form a desired bond can be reduced due to substitution of at least one functional group usually required in cross-coupling reactions by a C-H bond, and ii) shorter overall reaction times and a reduced number of synthetic steps which ultimately leads to more economical processes. Additionally, the C-H activation approach can be considered as an effort towards a greener chemistry, since the reduction in the number of synthetic steps in the majority of cases leads to a decrease in the amount of waste produced (silica gel, solvents, etc.) and energy consumed. The focus of this review is on the direct functionalization of both saturated and unsaturated heterocycles. Methods for the formation of CC and carbon-heteroatom bonds will be discussed. In the case of saturated heterocycles, another focus will be put on the stereospecific functionalization of such building blocks. Dedicated to Emeritus Prof. Fritz Sauter on the occasion of his 80 th birthday.
Organometallics, 2005
The complexes [Tp Br3 Ag] 2 ‚CH 3 COCH 3 (1) and Tp Br3 Ag(thf) (2) catalyze the insertion of the :CHCO 2 Et group from ethyl diazoacetate (EDA) into the saturated C-H bonds of several C5, C6, and C8 linear and branched alkanes. In addition to secondary and/or tertiary sites, the unprecedented insertion into the primary C-H bonds of the substrates studied has been observed with variable regioselectivity, toward the methyl groups, within the range of 40-60% for branched and nearly 25% for linear alkanes.
Easy Alkane Catalytic Functionalization
Organometallics, 2008
The carbon-hydrogen bonds of alkanes, R-H, can be converted in high yields into amino functionalities (R-NHTs; Ts ) p-toluensulfonyl), with the aid of silver-based catalysts in a reaction implying the thermal (80°C) insertion of a nitrene NTs unit into the C-H bond of the hydrocarbon. Complexes Tp x Ag (Tp x ) hydrotris(pyrazolyl)borate ligand) serve as catalysts and PhIdNTs serves as the nitrene source.
Advances in Development of C–H Activation/Functionalization Using a Catalytic Directing Group
The transition metal catalyzed direct site‐selective carbon –hydrogen functionalization is ubiquitous in organic synthesis and has reached an impressive level of sophistication and efficiency emerging as a powerful synthetic strategy for C–C bond and C–X bond formation to access various useful arylated organic molecules. For the past two decades, directing group strategy has been used for selective activation and functionalization of certain inert C–H bonds. Despite the enormous development in this field, still, the majority of systems require two redundant steps, i. e. installation and removal of DGs. To overcome these limitations, recently, traceless and multitasking groups were invented as a partial solution to DG release however installation still remains unsolved. Ideally, use of the catalytic directing group, which can be reversibly linked to the substrate and can serve as an efficient directing role would circumvent this problem and is considered as one of the most efficient and powerful strategies for the non‐activated C–H functionalization (C(sp2)–H or C(sp3)–H). In this review, we describe the enormous advances in this field for direct selective C–H functionalization without involving additional steps, i. e. installation and removal of directing groups and also discuss less explored but significant non‐covalent interactions such as hydrogen bonding or ion pairing, which helps to control the selectivity of a substrate in a catalytic organic reaction.
Review: active homogeneous reagents and catalysts in n -alkane activation
Journal of Coordination Chemistry, 2013
The development of selective, efficient, and direct routes for activation and functionalization of naturally abundant n-alkanes could lead to a new paradigm in materials and energy technologies. In this context, the use of homogeneous catalysts to functionalize C-H bonds of unactivated hydrocarbons is of particular interest from a scientific as well as an economic viewpoint. Despite the large body of work on stoichiometric C-H activation reactions produced over the last three decades, relatively few systems have been developed to allow catalytic functionalization of hydrocarbons. This review deals with homogeneous catalytic processes available in the literature for paraffin activation and functionalization. The key intermediates involved in catalytic systems are highlighted, providing important information in the design of new and efficient catalysts. Also, some of the key challenges and approaches to rational development of the next generation of organometallic catalysts will be highlighted.
Organometallics, 2009
Carbon-halogen (C-X) bonds (X=Cl, Br) can be easily functionalized with ethyl diazoacetate (N 2 CHCO 2 Et) in the presence of silver-based catalysts containing the Tp x Ag core (Tp x = hydrotrispyrazolylborate ligand). Polyhalomethanes are converted into products derived from the formal insertion of the carbene CHCO 2 Et units into the C-X bond. In the case of monohaloalkanes (C 4 -C 6 ), cleavage of the C-X bond is observed, with formation of XCH 2 CO 2 Et and the corresponding olefin. Experimental evidence and theoretical calculations have led to the proposal of a novel mechanism to account for these transformations, in which the metal participates along the pathway in all the reaction steps. Among the experimental data, the first example of a metal-induced, asymmetric functionalization of a C-Cl bond by carbene insertion is included (ee=14 ( 2%). (36) Drudis-Sole, G.; Maseras, F.; Lledos, A.; Vallribera, A.; Moreno-Manas, M. Eur. J. Org. Chem. 2008, 5614.