Y-H transfer

Scheme 11.2 Possible pathways for thermolysis of [(=SiO)2MNp2] species (i) a-H transfer (ii) y-H transfer. M = Ti, Zr [49],...

Table 1.1 clearly shows that the major pathway in the photochemistry of pentanal is the y-H transfer, followed by the C—C cleavage. The H detachment is only a minor pathway. A high percentage of trajectories are unreactive in this timescale. The relative yield of Norrish type I versus Norrish type II reaction from this table is 66% Norrish type II reaction and 34% Norrish type I reaction. This compares well to the observed experimental yield of 80% for Norrish type II reaction [16, 70]. 

To compare with. Figure 1.4 shows the first step of the Norrish type II reaction, namely, the y-H transfer to the C=0 group. 

Like other carbonyl compounds, carboxylic acids undergo /3 cleavage and y H transfer. 

Part of the driving force for the first step (rH) is provided by the formation of the extremely strong O—H bond which makes the distonic intermediate more stable than the original ketone ion. Part of the driving force for the second step is the resonance stabilization of the radical site in the product ion, which is isoelectronic with the allyl radical (see Heinrich 1986). Note that this requires -bond cleavage in the second reaction step, and thus necessitates y-H transfer to produce the reactive intermediate. 

Another example of the analogy between pyrazole and chlorine is provided by the alkaline cleavage of l-(2,4-dinitrophenyl)pyrazoles. As occurs with l-chloro-2,4-dinitrobenzene, the phenyl substituent bond is broken with concomitant formation of 2,4-dinitrophenol and chlorine or pyrazole anions, respectively (66AHC(6)347). Heterocyclization of iV-arylpyrazoles involving a nitrene has already been discussed (Section 4.04.2.1.8(i)). Another example, related to the Pschorr reaction, is the photochemical cyclization of (515) to (516) (80CJC1880). An unusual transfer of chlorine to the side-chain of a pyrazole derivative was observed when the amine (517 X = H, Y = NH2) was diazotized in hydrochloric acid and subsequently treated with copper powder (72TL3637). The product (517 X = Cl, Y = H) was isolated. 

Leconte and Basset [161-166] proposed two other possible mechanisms (Scheme 39) the first one implies a 1,2 carbon-carbon activation which invokes the de-insertion of a methylidene fragment from a surface metal-alkyl species, and the second implies a 1,3 carbon-carbon bond activation in which the key steps are the formation of a dimetallacyle by y-H activation from a metal-alkyl followed by carbon-carbon bond cleavage via a concerted electron transfer. 

Y.H. Wu and S.S. Hu, Direct electron transfer of ferritin in dihexadecylphosphate on an Au film electrode and its catalytic oxidation toward ascorbic acid. Anal. Chim. Acta 527, 37-43 (2004). 

Honciuc A, Otsuka A, Wang Y-H, McElwee SK, Woski SA, Saito G, Metzger RM (2006) Polarization of charge-transfer bands and rectification in hexadecylquinolinium 7,7,8-tricyanoquinodimethanide and its tetrafhioro analog. J Phys Chem B110 15085-15093... 

R Plaza, N. Dai Hung, M. M. Martin, Y. H. Meyer, M. Vogel, and W. Rettig, Ultrafast internal charge transfer in a donor-modified rhodamine, Chem. Phys. 168, 365-373 (1992). 

Thermal treatment of (=SiO)Hf(CH2Bu )3 at increasing temperatures leads to the successive evoluhon of neopentane, isobutene and isobutane as well as several alkanes varying from Cj to C5. Polyisobutenes are also formed on the surface. The mechanism by which such decomposition occurs suggests a succession of y-H eliminations with formahon of neopentane followed by P-methyl transfer and formation of isobutene and [Hf]-Me (Scheme 2.14). This isobutene is reinserted into [Hf]-Me with formahon of isopentene and [Hf]-H. 

As we have already noted, classical hydrogen bonds can be thermodynamically very strong but at the same time, easy to transform, due to fast proton transfer along a strong hydrogen bond. Such a dualism can also be seen in the dihydrogen bonding Y-H- H-X,... 

Mochizuki, T. Mori, Y.H. (2006). Clathrate-hydrate film growth along water/hydrate-former phase boundaries - numerical heat-transfer study. J. Crystal Growth, 290 (2), 642-652. 

Mori, Y.H. (2001). Estimating the thickness of hydrate films from their lateral growth rates application of a simplified heat transfer model. J. Crystal Growth, 223, 206-... 

K is the overall mass-transfer coefficient based on the liquid phase. A is the total interfacial area in the gas-liquid dispersion. C is the concentration in the liquid phase. C thus corresponds to equilibrium with the gas phase of composition y. H is the Henry coefficient for the gas. In the case of oxygen or a sparingly soluble compound, H is large and resistance to mass transfer is located in the liquid phase. 

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