Observation of Primary Reaction Intermediates

In general, the information bearing on the nature of the CO2 primary photodlssociative steps is of two kinds (i) indirect inference based upon analysis of final products following the completion of secondary reactions, and (li) direct spectroscopic observation of primary and intermediate products. Both approaches have made major contributions to the present level of understanding of the CO2 photochemistry which has been summarized recently by Welge (45), and Preston and Cvetanovlc (46) for wavelengths longer than 100 nm. 

Unlike primary kinetic isotope effects, secondary kinetic isotope effects arise due to presence of stable isotope at sites close to point of reaction and consequently influence the geometry of the reaction intermediates. Although the absolute values are much lower than those observed for primary effects, secondary kinetic isotope effects yield important information pertaining to the progress of the reaction and transition states involved therein [100,124,125],... 

With respect to the covalent activation in conjugate additions, the catalyst, usually a primary or a secondary amine, can reversibly form a chiral enamine [ 11 ] to activate the nucleophile (D, Fig. 2.2) or a chiral iminium ion [12] to activate the acceptor (E, Fig. 2.2). The detection of enamine intermediates in asymmetric oiganocatalysis has been for a long time the missing piece of evidence for the commonly accepted mechanism of enamine catalysis. This gap has been recently solved with the first detection and structnral characterization of enamine intermediates in proUne-cata-lyzed aldol reactions by real-time NMR spectroscopy [13] and the direct observation of an enamine intermediate in the crystal strnctnre of an aldolase antibody [14]. 

Nowadays silenes are well-known intermediates. A number of studies have been carried out to obtain more complex molecules having Si=C double bonds. Thus, an attempt has been made to generate and stabilize in a matrix 1,1-dimethyl-l-silabuta-l,3-diene [125], which can be formed as a primary product of pyrolysis of diallyldimethylsilane [126] (Korolev et al., 1985). However, when thermolysis was carried out at 750-800°C the absorptions of only two stable molecules, propene and 1,1-dimethylsilacyclobut-2-ene [127], were observed in the matrix IR spectra of the reaction products. At temperatures above 800°C both silane [126] and silacyclobutene [127] gave low-molecular hydrocarbons, methane, acetylene, ethylene and methylacetylene. A comparison of relative intensities of the IR... 

A moderate diastereoselectivity was observed in these reactions where a mixture of diastereomers could be generated.58 The reactivity of the halides followed the order of tertiary > secondary primary and iodide> bromide (chlorides did not react). The preferred solvent system was aqueous ethanol. The process was suggested to proceed by a free radical mechanism occurring on the metal surface under sonochemical conditions. Efforts to trap the intermediate [A] intramolecularly gave only a very low yield of the cyclization product (Scheme 10.4).59... 

The transformation of n-hexadecane was carried out in a fixed-bed reactor at 220°C under a 30 bar total pressure on bifunctional Pt-exchanged HBEA catalysts differing only by the zeolite crystallites size. The activities of the catalysts and especially the reaction scheme depended strongly on the crystallites size. Monobranched isomers were the only primary reaction products formed with the smallest crystallites, while cracking was the main reaction observed with the biggest crystallites. This was explained in terms of number of zeolite acidic sites encountered by the olefinic intermediates between two platinum particles. 

The observation of a primary solvent deuterium isotope effect (kH/fa>) = 2-4 on the specific acid-catalyzed hydrolysis of vinyl ethers provides evidence for reaction by rate-determining protonation of the alkene.69 Values of kHikD 1 are expected if alkene hydration proceeds by rate-determining addition of solvent to an oxocarbenium ion intermediate, since there is no motion of a solvent hydron at the transition state for this step. However, in the latter case, determination of the solvent isotope effect on the reaction of the fully protonated substrate is complicated by the competing exchange of deuterium from solvent into substrate (see above). 

Fahey (16) suggests that intermediate 3 dissociates formaldehyde he finds supportive evidence in the rhodium-based system by observation of minor yields of 1,3-dioxolane, the ethylene glycol trapped acetal of formaldehyde. For reasons to be discussed later, we believe the formation of free formaldehyde is not on the principal reaction pathway. (c) We have also rejected two aspects of the reaction mechanism proposed by Keim, Berger, and Schlupp (15a) (i) the production of formates via alcoholysis of a formyl-cobalt bond, and (ii) the production of ethylene glycol via the cooperation of two cobalt centers. Neither of these proposals accords with the observed kinetic orders and the time invariant ratios of primary products. 

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