Olefins quenching

The fact that plots of reciprocal of quantum yield of photoaddition vs. olefin concentration are linear with positive slope (Table 5) suggests that olefin quenches the singlet state and that all triplets formed are captured by olefin in the olefin concentration range examined in competition with unimolecular decay. Inefficiency... 

Other ions studied include the cyclohexene- and norbomylene-mercurinium ions. N.m.r. studies show deshielding of both protons and carbons, indicating charge transfer from mercury to the carbon skeleton both mercury-proton and mercury-carbon spin-spin couplings are observed. Some of the spectra are best interpreted in terms of an equilibrium between the mercurinium ion and the free olefin. Quenching experiments are consistent with the presence of mercurinium ions. 

Tsuchiya, M., Ebbesen, T. W., Nishimura, Y, Sakuragi, H., and Tokumaru, K., Kinetic studies on electron transfer photooxygenation of aromatic olefins. Quenching rate of olefin radical cation by oxygen and superoxide anion, Chem. Lett., 2121,1987. 

Other authors disagree with the results of this carbometalation sequence. They find that the initial product results stricdy from cis-carbometalation but with a trace of base, H is abstracted from the medium homolyticaHy with loss of stereochemistry (288). They show that M in the above stmcture must be Ti, not Al. If the reaction mixture is quenched with D2O containing NaOD, 95 mol % D is incorporated in the olefin. 

The reaction section consists of the high pressure reactors filled with catalyst, and means to take away or dissipate the high heat of reaction (300-500 Btu/lb of olefin polymerized). In the tubular reactors, the catalyst is inside a multiplicity of tubes which are cooled by a steam-water condensate jacket. Thus, the heat of reaction is utilized to generate high pressure steam. In the chamber process, the catalyst is held in several beds in a drum-type reactor with feed or recycled product introduced as a quench between the individual beds. 

The mechanism of the Patemo-Biichi reaction is not well understood, and while a general pathway has been proposed and widely aceepted, it is apparent that it does not represent the full scope of reactions. Biichi originally proposed that the reaction occurred by light catalyzed stimulation of the carbonyl moiety 1 into an excited singlet state 4. Inter-system crossing then led to a triplet state diradical 5 which could be quenched by olefinic radical acceptors. Intermediate diradical 6 has been quenched or trapped by other radical acceptors and is generally felt to be on the reaction path of the large majority of Patemo-Biichi reactions. Diradical 6 then recombines to form product oxetane 3. 

An olefin plant that uses liquid feeds requires an additional pyrolysis furnace, an effluent quench exchanger, and a primary fractionator for fuel oil separation. 

In studying two-component polymerization catalysts, beginning with Feldman and Perry (161), a radioactive label was introduced into the growing polymer chain by quenching the polymerization with tritiated alcohols. The use of these quenching agents is based on the concept of the anionic coordination mechanism of olefin polymerization occurring... 

However, in olefin polymerization by two-component catalysts during polymerization not only active transition metal-polymer bonds are formed, but also inactive aluminum-polymer ones, as a result of the transfer process with the participation of a co-catalyst (11, 162-164). The aluminum-polymer bonds are quenched by tritiated alcohol according to the scheme (25), so an additional tagging of the polymer occurs. The use of iodine (165, 166) as a quenching agent also results in decomposing inactive metal-polymer bonds. 

Prior literature indicated that olefins substituted with chiral sulfoxides could indeed be reduced by hydride or hydrogen with modest stereoselectivity, as summarized in Scheme 5.10. Ogura et al. reported that borane reduction of the unsaturated sulfoxide 42 gave product 43 in 87 13 diastereomer ratio and D20 quench of the borane reduction mixture gave the product 43 deuterated at the a-position to the sulfoxide, consistent with the hydroboration mechanism [10a]. In another paper, Price et al. reported diastereoselective hydrogenation of gem-disubstituted olefin rac-44 to 45 with excellent diastereoselectivity using a rhodium catalyst [10b],... 

The quantum yield for the formation of the cycloaddition product has been found to be temperature dependent, increasing by a factor of approximately three as the temperature is lowered from 65 ( = 0.24) to 5°C ( = 0.69). Photolysis of mixtures of the olefin and f/my-stilbene in the presence of sensitizers yielded no cycloaddition product (42) but rather only m-stilbene. This suggests that the cycloadduct is produced via a singlet reaction. This conclusion is supported by the fact that tetramethylethylene quenches fluorescence from the /rans-stilbene singlet. A plot of l/ (42) vs. 1/[TME] (TME = tetramethylethylene) is linear. The slope of this plot yields rate constants for cycloadduct formation which show a negative temperature dependence. To account for this fact, a reversibly formed exciplex leading to (42) was proposed in the following mechanism<82) ... 

That oxetane formation results from a singlet state reaction follows from the following evidence (a) Acetone fluorescence is quenched by addition of the olefin, (b) oxetane formation is relatively insensitive to piperylene, and (c) cis-trans isomerization of the olefin is quenched at high olefin concentrations but oxetane formation is not affected. Since oxetane formation was... 

The selective reduction of the D-ring olefin in 106 using a partially poisoned catalyst (Pd/C, 0.25 % pyridine) provided intermediate 107 (83 %), which was epimerized at -78 °C with sodium methoxide (HOAc quench at -78 °C, 89 %) (Scheme 10.9). Deoxygenation by means of tosyl hydrazone 108 and subsequent treatment with catechol borane and tetrabutylammonium acetate gave pentacyclic... 

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