S-H bonds, activation

So far, progress on the late transition metal-catalyzed reactions utilizing S-H bond activation has been surveyed. Finally, the recent advancement of chiral Lewis... 

The ruthenium hydride complex can be readily converted to a thiolate-containing product through S-H bond activation. ... 

The reactions for C-H, 0-H and S-H bond activation typically involve metal atom insertion reactions. In the presence of solution, the hydrogen that forms can be directly transferred into solution as proton. The site dependence for these reactions, which are at the heart of many electrochemical processes, may not be very strong. The reactivity of terraces, steps and kinks may be quite similar. This is different to the activation of the molecules over a metal in the gas phase, which is structure sensitive. The electrochemical behavior will, of course, be strongly dependent upon the potential. 

In analogy to phenol oxidations (see discussion above), Wenger has demonstrated that thiophenol may also undergo oxidative PCET upon treatment with photoexcited [Ru(bpz)3] (Fig. 26) [190]. In this case, the substituents on the thiophenol were found to have a profound effect on the mechanism of S-H bond activation. For thiol 43, no KIE is observed upon luminescence decay of the photocatalyst excited state, suggesting the quenching event is not accompanied by a... 

The progress on S-C bond activation, which covers the reduction of a C-S bond to a C-H bond, cross coupling reaction of sulfides with main group organometaUic nucleophiles, ring opening reactions of thietanes and thiiranes, and desulfurization of thiols, sulfides, and thiophenes has already been reviewed elsewhere [6-10], and... 

In 1999, it was reported that the palladium catalyzed azathiolatiori, that is, the addition of the S-N bond of sulfenamide 115 to carbon monoxide can be catalyzed by palladium(O) complexes in pyridine to provide the thiocarbamate 116 in good yields (Eq. 7.69) [67]. Contrary to the other S-X bond activations described so far, where X has the same electronegativity as S (i.e. X = S) or lower (X = H, B, Si, Ge, and P), the S-N bond has a strong S -N bond character and shows unique reactivity. 

Murahashi S-I, Nakae T, Terai H, Komiya N (2008) Ruthenium-catalyzed oxidative cyana-tion of tertiary amines with molecular oxygen or hydrogen peroxide and sodium cyanide sp3 C-H bond activation and carbon-carbon bond formation. J Am Chem Soc 130 11005-11012... 

From chemical point of view, efficient free radical scavengers must contain substituents with the very weak C—H, O—H, or S—H bonds, from which reactive free radicals are able to abstract a hydrogen atom. It can be seen that the antioxidants discussed above (ascorbic acid, a-tocopherol, ubihydroquinones, glutathione, etc) fall under this category. However, many other compounds manifest free radical scavenging activity in in vitro and in vivo systems. 

Facile C-H bond activation by Pt(II) metal centers seems to require at least one labile ligand in the coordination sphere of platinum. One of the earliest intermolecular examples of this is the activation of C-D bonds in benzene-f/, by 0 an.S -(PAIe .) Pt(neopentyl)(OTf) at 133 °C, where trifluoromethanesulfonate (triflate, OTf) provides the labile group (Scheme 7, A) (26). 

The direct silylation of arenes through C—H bond activation provides an attractive route for the synthesis of useful aromatic compounds [64]. Vaska s complex was the first of the iridium catalysts to be reported for activation of the C—H bond in benzene by Si—H of pentamethyldisiloxane to yield phenylsubstituted siloxane [65]. However, a very attractive method for the aromatic C—H silylation with disilanes has been recently reported by the groups of Ishiyama and Miyaura [66-68]. 

Some of these intermediates are analogous to those proposed by Chauvin in olefin metathesis ( Chauvin s mechanism ) [36]. They can be transformed into new olefins and new carbene-hydrides. The subsequent step of the catalytic cycle is then hydride reinsertion into the carbene as well as olefin hydrogenation. The final alkane liberation proceeds via a cleavage of the Ta-alkyl compounds by hydrogen, a process already observed in the hydrogenolysis [10] or possibly via a displacement by the entering alkane by o-bond metathesis [11]. Notably, the catalyst has a triple functionality (i) C-H bond activation to produce a metallo-carbene and an olefin, (ii) olefin metathesis and (iii) hydrogenolysis of the metal-alkyl. 

Asymmetric activation of the C—H bonds in benzyl silyl ethers was achieved by using Hashimoto s A-phthaloyl-based Rh2((5)-PTTL)4 catalyst (Figure 5.6) in high diastereoselectivities and enantioselectivities (Scheme 5.15). The well-established dirhodium tetraprolinates such as Rh2((5)-DOSP)4 and Rh2((R)-DOSP)4 catalysts, which generally are excellent catalysts for asymmetric C—H bond activation, were not suitable catalysts in these reactions. 

Asymmetric allylic oxidation and benzylic oxidation (Kharasch-PSosnovsky reaction) are important synthetic strategies for constructing chiral C—O bonds via C—H bond activation.In the mid-1990s, the asymmetric Kharasch-Sosnovsky reaction was first studied by using chiral C2-symmetric bis(oxazoline)s. " Later various chiral ligands, based mainly on oxazoline derivatives and proline derivatives, were used in such asymmetric oxidation. Although many efforts have been made to improve the enantioselective Kharasch-Sosnovsky oxidation reaction, most cases suffered from low to moderate enantioselectivities or low reactivities. 

As a true testament to the potential long-term impact of H-bonding activation, a number of ureas, thioureas, and acid catalysts are now finding broad application in a large number of classical and modem carbon-carbon bond-forming processes. On one hand, Johnston s chiral amidinium ion 28 was elegantly applied to the asymmetric aza-Henry reactions (Scheme 11.12d). On the other hand, chiral phosphoric acids (e.g., 29 and 30), initially developed by Akiyama and Terada, have been successfully employed in Mannich reactions, hydrophosphonylation reac-tions, aza-Friedel-Crafts alkylations (Scheme 11.12e), and in the first example... 

Bicydic products were obtained from C—C-reductive elimination of a presumed rhodacyde intermediate (see 33). Both of Uemura s reactions constitute significant contributions to the catalysis literature because they involve (i) vinylidenes, (ii) catalysis of a pericyclic process, and (iii) C—H bond activation. 

Not surprising, the insertion of silylene into the S-H bond of hydrogen sulfide shows qualitatively the same features as the reaction with water. However, the complexation energy (36.0 kJ mol-1) and the activation energy (56.1 kJ moL1) of the rearrangement are lower at the MP4SDTQ/6-31G // 3-21G level.14 A compi increases to 55.3 kj mol-1 when calculated at the MP2/CEP-31g(2d,p)//MP2/CEP-31g(2d,p) level.83... 

In 1982, Curtis and co-workers reported that Vaska s complex promotes the formation of phenylsiloxanes from the reaction of hydridosiloxanes and benzene in a catalytic, albeit low-yield, process.93 Catalytic arylsilane formation has also been reported by Tanaka and co-workers. Under photo-lytic conditions, RhCl(CO)(PMe3)2 catalyzes the C-H bond activation of arenes in reactions with hydrosilanes or disilanes, leading to the formation of new silicon-carbon bonds.94 More recently, C-H bond activation of arenes resulting in arylsilane formation has been observed in the... 

Ye S, Neese F. Nonheme oxo-iron(IY) intermediates form an oxyl radical upon approaching the C-H bond activation transition state. Proc Natl Acad Sci USA. 2011 108 1228-33. 

The fact that the final product 3-Tp Rh(CO)(H)(R) does not appear on the ultrafast time scale (<1 ns,) (Fig. 4) indicates a free energy barrier greater than 5.2 kcal/mol for the alkane C-H bond activation. Nanosecond step-scan FTIR experiments on the 3-Tp Rh(CO)2/cyclohexane system show that the remnant of the 2-Tp Rh(CO)(S) peak persists for 280 ns after photoexcitation, while the product CO stretch at 2032 cm-1 rises with a... 

A different type of C—H bond activation has been observed upon substitution of MeCN in the raft Osg(CO)2o(MeCN) with phenylacetylene (70). X-ray analysis of Osg(CO)2o CC(H)Ph has revealed that a 12 hydrogen shift has occurred in this system to give a vinylidine ligand with a coordination mode similar to the one exhibited by the CCHj fragment on Pt (1 1 1) surfaces (431). This process was accompanied by reorganization of the raft Osg framework. Ejection of Os(CO)s from this cluster was found to occur upon heating to afford Os5(CO)i5 CC(H)Ph (70) (Scheme 35). 

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