Computational Studies of High-Oxidation State Main-Group Metal Hydrocarbon C-H Functionalization (original) (raw)
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Chem. Soc. Rev., 2018
The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.
Recent Advances on Mechanistic Studies on C–H Activation Catalyzed by Base Metals
Open Chemistry
During the last ten years, base metals have become very attractive to the organometallic and catalytic community on activation of C-H bonds for their catalytic functionalization. In contrast to the statement that base metals differ on their mode of action most of the manuscripts mistakenly rely on well-studied mechanisms for precious metals while proposing plausible mechanisms. Consequently, few literature examples are found where a thorough mechanistic investigation have been conducted with strong support either by theoretical calculations or experimentation. Therefore, we consider of highly scientific interest reviewing the last advances on mechanistic studies on Fe, Co and Mn on C-H functionalization in order to get a deep insight on how these systems could be handle to either enhance their catalytic activity or to study their own systems in a similar systematic fashion. Thus, in this review we try to cover the most insightful articles for mechanistic studies on C-H activation ca...
Comparative reactivities of hydrocarbon carbon-hydrogen bonds with a transition-metal complex
Accounts of Chemical Research, 1989
suggest such a possibility, and several CIDNP observations have been so ir~terpreted.~' Why would isc occur during rather than prior to product formation? If isc is not rapid unless the radical centers overlap, movement of the biradical termini along a reaction coordinate would induce isc and also enhance S-T splitting, such that the small activation energies for singlet biradical reaction are transformed into activation entropies for triplet biradical motion. Any additives that affect either isc or product formation then would affect both, as Scaiano has observed? The question of the separation in time of isc and product formation certainly deserves more attention. 3BR . ---lBR lis. ' ., Products (71) (a) Doubleday, C. Chem. Phys. Lett. 1981, 79,375. (b) Kaptein, R.; DeKanter, F. J. J.; Rist, G. H. J. Chem. SOC., Chem. Commun. 1981, 499.
Oxy-Functionalization of Nucleophilic Rhenium(I) Metal Carbon Bonds Catalyzed by Selenium(IV)
Journal of the American Chemical Society, 2009
Electrophilic C-H bond activation (CHA) with strong electrophiles, 1,2 such as Pt II , Pd II , Au III , and Hg II , followed by reductive oxy-functionalization (OF) of the resulting positively polarized metal alkyl (M-R δ+ ) intermediate is a well-known strategy for hydrocarbon hydroxylation. 3 However, water and alcohol inhibition of the CHA reaction renders this strategy commercially impractical. We 4 and others 5 have shown that CHA with weakly electrophilic cations, such as Ir III and Ru II , are less prone to such inhibition. However, the reductive OF reactions utilized with highly electrophilic catalysts are generally not applicable to weakly electrophilic systems because the reduced oxidation potential thermodynamically disfavors reductive functionalization and/or the reduced electrophilicity can result in M-R δpolarized bonds that exhibit high barriers for nucleophilic attack at the carbon. 6 Therefore we have set out to design new approaches for functionalization of M-R δpolarized bonds.
Structural snapshots of concerted double E–H bond activation at a transition metal centre
Nature Chemistry, 2017
Bond activation at a transition metal center is a key fundamental step in numerous chemical transformations. The oxidative addition of element-hydrogen bonds, for example, represents a critical step in a range of widely applied catalytic processes. Despite this, experimental studies characterizing intermediates along the bond activation pathway are very rare. In this work, we report on fundamental studies defining a new oxidative activation pathway: combined experimental and computational approaches yield structural snapshots of the simultaneous activation of both bonds of a β-diketiminate-stabilized GaH 2 unit at a single metal center. Systematic variation of the supporting phosphine ligands and single crystal X-ray/neutron diffraction are exploited in tandem to allow structural visualization of the activation process, from a η 2-H,H σ-complex showing little Ga-H bond activation, through species of intermediate geometry featuring stretched Ga-H and compressed M-H/M-Ga bonds, to a fully activated metal dihydride featuring a neutral (carbene-type) N-heterocyclic Ga I ligand. Main text (3050 words-not including abstract, methods, captions; 6 display items-5 figures and 1 table) Bond activation at transition metal centers is key to a wide range of societally important chemical reactions, being a fundamental step in catalytic processes underpinning synthetic, medicinal and materials science. 1 Bond cleavage can be achieved through a number of routes, with oxidative addition at late transition metal centers being among the most widely exploited and extensively studied (particularly for 'noble' metals such as ruthenium, rhodium and palladium). 2-11 Experimental and theoretical studies imply that E-H bond activation by such systems proceeds via initial formation of a η 2-E,H σ-complex, with transfer of electron density from the metal into the E-H σ* orbital bringing about bond weakening, and ultimately cleavage to give the corresponding elementyl hydride (see Fig. 1(a)). 2,9,11-18 Despite its widespread relevance, however, studies describing sequentially intercepted intermediates along this reaction trajectory for a given (single) metal system are very rare. 19,20 <Fig. 1> Simultaneous activation of two E-H bonds via an analogous η 2-H,H σ-complex leading to the formation of a metal dihydride (Fig. 1(b)) has very little precedent (even for the more polar group 13 E-H bonds which are the focus of this study), 22-26 since it requires a metal precursor with a formal electron count of ≤ 14, or an in situ source thereof. Processes for which experimental mechanistic evidence is available (e.g. the direct formation of Fischer carbene complexes [L n M=C(OR)R'] from the corresponding ether, H 2 C(OR)R') are thought to proceed via a two-step C-H oxidative addition/α-migration mechanism. 27-34 Nevertheless, such chemistry represents an attractive net transformation, given the widespread use of metal carbene and related complexes (for example in homogenous catalysis), and the importance of dehydrogenation reactions in chemical processes relevant to applications in energy, polymer synthesis, etc. 35,36 In the current contribution we demonstrate the viability of a concerted single-step double E-H activation process. Systematic variation in the ancillary metal-bound ligands and single-crystal X-ray/neutron diffraction are exploited in tandem to demonstrate sequential steps in the activation process, from a η 2-H,H σ-complex showing little E-H bond activation, through species of intermediate geometry featuring stretched E-H and compressed M-H/M-E bonds, to a fully activated metal dihydride featuring a neutral (carbene-like) element-ylidene ligand. Experimental geometric data have been analysed in the light of Atoms in Molecules and fragment orbital calculations to yield the first 'snapshot' visualization of a bond activation process of this type. The EH 2-containing system chosen for the current study is the bulky β-diketiminato supported gallane (NacNac) Dipp GaH 2 [(NacNac) Dipp = HC(MeCDippN) 2 , where Dipp = C 6 H 3 i Pr 2-2,6], 37 which has previously been shown to undergo dehydrogenation at metal carbonyl fragments to give complexes of the type M x (CO) y {Ga(NacNac) Dipp } (M = Cr, Mo, W, Fe, Co), containing the neutral two-electron (carbene analogue) gallylene ligand, :Ga(NacNac) Dipp. Spontaneous reductive loss of H 2 at M in these systems is driven by the presence of the strongly π-accepting carbonyl ligand set. 24,25 The use of ancillary phosphine donors was therefore targeted in order to stabilise higher oxidation state hydride-containing species, which might
Inorganica Chimica Acta, 1985
The activation and functionalization of carbonhydrogen bonds under homogeneous conditions continues to be an important and challenging objective. Only in recent years have a few examples of such C-H bond activation, especially at aliphatic carbon centers, been reported and these involve, for the most part, stoichiometric rather than catalytic reactions; general approaches with widespread applicability still are lacking. Drawing upon the limited insights provided by these examples, and upon the pertinent information derived from related studies on the reverse process (i.e., metal complexpromoted C-H bond formation) and on the catalytic activation of other saturated molecules (notably HZ), as well as from recent determinations of metalcarbon and metal-hydrogen bond energies, the mechanistic, kinetic and thermodynamic aspects of C-H bond activation at metal centers are analyzed. It is concluded that thermodynamic constraints, notably those associated with the characteristic weakness of metal-carbon bonds, are of dominant importance in limiting the reactivities of metal complexes toward C-H bonds.
Journal of the American Chemical Society, 2009
To capture the powerful potential of metal-mediated carbonhydrogen (C-H) bond activation, it is essential to develop compatible reactions that convert the resulting metal-alkyl (M-R) intermediates into useful functionalized products. 1 For alkane oxidation reactions, Pt, Pd, Hg, and Au metal catalysts have been exploited to break C-H bonds by electrophilic activation. 2,3 Because of the highly electrophilic C-H activation reactions, the resulting M-R intermediates react with weak O-nucleophiles to generate oxygenated products. 2 It has also recently been demonstrated that M-R intermediates can be functionalized with Oelectrophiles. 4 This type of M-R functionalization would be most useful if coupled to a nucleophilic C-H activation reaction.