Hydrogen abstraction complexation

The fluorination reaction is best described as a radical-chain process involving fluorine atoms (19) and hydrogen abstraction as the initiation step. If the molecule contains unsaturation, addition of fluorine also takes place (17). Gomplete fluorination of complex molecules can be conducted using this method (see Fluorine compounds, organic-direct fluorination). 

Reaction 36 may occur through a peroxy radical complex with the metal ion (2,25,182). In any event, reaction 34 followed by reaction 36 is the equivalent of a metal ion-cataly2ed hydrogen abstraction by a peroxy radical. 

Tetraphenylmolybdenocene dihydride Mo(r 5-C5HPh4)CpH2 (45) was formed by addition of diphenylacetylene to MoCpL(PhC CPh)CH3 (L = P(OMe)3) (Eq. 15), presumably via an ot-hydrogen abstraction to an intermediate methylidene hydrido complex, followed by addition of two equivalents of diphenylacetylene and C — H insertion with concomitant elimination of L [57 b],... 

Usui, Y, Hrrano, M., Fukuoka, A. and Komiya, S. (1997) Hydrogen abstraction from transition metal hydrides by gold alkoxides giving gold-containing heterodinuclear complexes. Chemistry Letters, 26, 981. 

Hydrogen Abstraction Photoexcited ketone intermolecular hydrogen atom abstraction reactions are an interesting area of research becanse of their importance in organic chemistry and dne to the complex reaction mechanisms that may be possible for these kinds of reactions. Time resolved absorption spectroscopy has typically been nsed to follow the kinetics of these reactions but these experiments do not reveal mnch abont the strnctnre of the reactive intermediates. " Time resolved resonance Raman spectroscopy can be used to examine the structure and properties of the reactive intermediates associated with these reactions. Here, we will briefly describe TR experiments reported by Balakrishnan and Umapathy to study hydrogen atom abstraction reactions in the fluoranil/isopropanol system as an example. 

The success of this reaction was ascribed to the solubility of the chlorozinc intermediate, whereas other chloramine-T derivatives (e.g. the sodium salt) are insoluble. An alternative non-nitrene pathway was not eliminated from consideration. On the other hand, no aromatic substitution or addition, characteristic of a free sulphonyl nitrene (see below), took place on treatment of jV,lV-dichloromethanesulphonamides with zinc powder in benzene in the cold or on heating. The only product isolated was that of hydrogen-abstraction, methanesulphonamide 42>, which appears to be more characteristic of the behaviour of a sulphonyl nitrene-metal complex 36,37). Photolysis of iV.iV-dichloromethanesulphonamide, or dichloramine-B, or dichloramine-T in benzene solution led to the formation of some unsubstituted sulphonamide and some chlorobenzene but no product of addition of a nitrene to benzene 19>. 

On the other hand, thermolysis of ferrocenylsulpkonyl azide (14) in aliphatic solvents may lead to the predominant formation of the amide (16) 17>. A 48.4% yield of (16) was obtained from the thermolysis in cyclohexane while an 85.45% yield of 16 was formed in cyclohexene. Photolysis of 14 in these solvents led to lower yields of sulphonamide 32.2% in cyclohexane, 28.2% in cyclohexene. This suggests again that a metal-nitrene complex is an intermediate in the thermolysis of 14 since hydrogen-abstraction appears to be an important made of reaction for such sulphonyl nitrene-metal complexes. Thus, benzenesulphonamide was the main product (37%) in the copper-catalyzed decomposition of the azide in cyclohexane, and the yield was not decreased (in fact, it increased to 49%) in the presence of hydroquinone 34>. On the other hand, no toluene-sulphonamide was reported from the reaction of dichloramine-T and zinc in cyclohexane. 

Reaction of dpp-bian with Mg in THF for 30 min reflux gives complex 87 (Ar = 2,6-diisopropylphenyl) which undergoes oxidative addition via m-bond metathesis with PhC=CH to give the black alkynyl amido complex 88. The insertion reaction of 88 with Ph2CO in EtzO yields complex 89. Unexpectedly, hydrogen abstraction to give the radical anion occurs simultaneously with ketone insertion.268... 

Imido alkylidene complexes were first prepared by a reaction analogous to that shown In equation 6. Recently they have been prepared from imido alkyl complexes by well-behaved a-hydrogen abstraction reactions (16) Imido neopentylidene complexes seem to be more stable than oxo neopentylidene complexes, possibly because the oxo ligand is sterically more accessible to Lewis acids, including another tungsten center. 

Mo and W alkylidene complexes usually are prepared from M(VI) dialkyl complexes by some variant of the a hydrogen abstraction reaction (Eq. 7 other ligands omitted) [5,41]. 

Suppression of the Pummerer reaction (Fig. 24) could also be a manifestation of the stabilization of the persulfoxide which prevents its interconversion to the hydro-peroxysulfonium ylide, HPSY (Fig. 25), which is the intermediate that has been suggested to undergo a 1,2-shift of the hydroperoxy group and ultimately produces the SC bond cleavage products.92 However, the situation is probably more complex since the intrazeolite reaction of /1-chlorosulfide, 29 (Fig. 28A), requires 7-hydrogen abstraction. The complexation motif (Fig. 28B) which favors the extended rather than folded M+-PS may also play an important role. A complete understanding of these reactions will require additional studies. 

When the reactant is cyclohexene, in the first step of Scheme 26, the direct hydrogen abstraction for the allylic oxidation (path 1) competes with the electron transfer (from the alkene to the M-oxo complex) for the epoxidation (path 2). Because the manganese complex is more readily reduced than the chromium... 

In the titanosilicate system, cyclic voltametric measurements had indicated (Section III.D) that the electron density at the tripodal sites is higher than at the tetrapodal sites. Hence, by analogy with the chromium and manganese complexes, we may expect the tripodal sites to favor hydrogen abstraction and allylic CH oxidation, although electron transfer and epoxidation occur preferentially on the tetrapodal sites. 

When considered as a part of the photochemistry of carbonyl compounds, irradiations of esters constitute a minor component. The more frequent photolyses of other carbonyl compounds, in particular ketones, is not surprising, as, even though parallels exist between ester and ketone photochemistry (for example, both experience a-cleavage and hydrogen abstraction-reactions), esters require radiation of higher energy for reaction, and typically produce more-complex mixtures of products. In addition to their similarity to other carbonyl compounds in their reactivity, esters also experience reactions that are uniquely their own. 

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